Progress in Surface Science 76 (2004) 71–146

www.elsevier.com/locate/progsurf

Review

Carbon monoxide MgO fromdispersed solids to single crystals: a review and

new advances

G. Spoto, E.N. Gribov, G. Ricchiardi, A. Damin, D. Scarano,S. Bordiga, C. Lamberti, A. Zecchina *

Department of Inorganic, Physical and Materials Chemistry, University of Turin, Via Pietro Giuria 7,

I-10125 Turin, Italy

NIS Centre of Excellence, Turin, Italy

Abstract

In this review we describe 30 years of research on the surface properties of magnesium oxide,

considered as the model prototype oxide of cubic structure. The surface properties of single

crystals, thin films and powdered samples (sintered at progressive higher temperatures) are con-

sidered and compared, with the aim of demonstrating that the gap between ‘‘believed perfect’’

single crystal surfaces, typical of ‘‘pure’’ Surface Science, and high surface area samples, typical

of Catalysis Science, can be progressively reduced. The surface features considered in this

review are the structural (morphological), optical, absorptive and reactive properties. As the

carbon monoxide molecule is able to probe the surface properties of both anions and cations,

it can give a complete information of the surface structure of MgO samples. For this reason the

adsorption and spectroscopy of this molecule is preferentially considered in this review. Partic-

ular emphasis is given in reviewing results obtained by high resolution transmission microscopy

and in situ IR spectroscopy of adsorbed species (in both reflection and transmission modes),

but also UV–Vis diffuse reflectance, photoluminescence, TDS, EPR, electron based techniques

are mentioned. Reviewed experimental results are also commented in view of the important the-

oretical literature available on this topic and are complemented by new transmission IR data

concerning CO adsorbed, down to 60 K, on powdered MgO samples with increasing surface

0079-6816/$ - see front matter � 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.progsurf.2004.05.014

* Corresponding author. Tel.: +39 11 6707860; fax: +39 11 6707855.

E-mail address: adriano.zecchina@unito.it (A. Zecchina).

72 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

area. These innovative experiments allow us to perform, on powdered samples, the adsorption

experiments typical of single crystals (or films) Surface Science, with an increase of the S/N of

the vibrational features higher than two order of magnitude.

As far the new results (never published before) are concerned, we report IR spectra of CO

dosed at 60 K on polycrystalline MgO samples with different surface area obtained by

Mg(OH)2 decomposition and progressive sintering at high temperature. The samples morphol-

ogy of each sintering stage has been controlled by high resolution TEM. The decomposition of

the hydroxide to oxide is shown to occur with partial retention of the long range order, with

formation of layers of compenetrated cubes oriented according to the original brucite planes.

The CO adsorption experiments have been carried out using a new apparatus developed ad hoc

to perform in situ mid-IR experiments, in transmission mode, on an activated (up to 1100 K)

powdered samples in the desired atmosphere at any defined temperature in the 300–20 K

interval.

The influence of the surface area on the IR features characterizing the MgO/CO system at

60 K have been investigated by increasing the sintering treatment of the native Mg(OH)2 and

by preparing a low area MgO smoke sample, obtained by Mg combustion. New results have

been compared with literature data obtained on powdered MgO at higher temperature and

on MgO single crystals and thin films. A decrease of about 40 K with respect to the classical

IR experiments reported in the literature results in a remarkably detailed evolution of the

spectra as a function of CO pressure, allowing us to better understand the complex interac-

tion of the CO molecule with the different cationic and anionic sites of the MgO surface. In

particular, it has been possible to observe the precursors of the polymeric species, formed on

the basic coordinatively unsaturated O2�sites, which dominate the room temperature spec-

tra. Ab initio calculations, on simple models, have been used for the vibrational assignment

of surface species. A qualitative agreement has been obtained between computed and exper-

imental IR modes. The evolution of the spectra at decreasing MgO surface area (i.e. upon

decreasing the surface defectivity) results in spectra whose features are well comparable with

those obtained by IRAS on vacuum cleaved single crystals, but characterized by a much bet-

ter signal/noise ratio. The temperature evolution of the intensity of the IR features of CO

adsorbed on individual adsorption sites allows, unlike microcalorimetric experiments, the

determination of site-specific adsorption enthalpies. The adsorption enthalpy of

Mg2+� � �(CO) adducts on 5- and 4-fold coordinated magnesium cations are �12 and �22kJmol�1 respectively. This relevant amount of new experimental data allows us to critically

review experimental and theoretical works appeared in the literature on this case study of

Surface Science.

� 2004 Elsevier Ltd. All rights reserved.

PACS: 68.37.Lp; 68.43.�h; 68.43.Bc; 68.43.Pq; 78.40.�q; 78.55.�m; 81.05.Je; 81.07.Wx; 82.65.+rKeywords: MgO; CO; FTIR; TEM; TDS; Reflectance spectroscopy; Photoluminescence spectroscopy;

Adsorption at surfaces; Adsorption enthalpy; In situ spectroscopy; Ab initio computations

Contents

1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

1.1. The historical role played by high surface area oxides of cubic structure

in surface science. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

1.2. Outline of the topics treated in the work . . . . . . . . . . . . . . . . . . . . . . . . 82

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 73

2. Samples preparation (from single crystals to powders) and experimental details

concerning new advances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

2.1. MgO samples preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

2.1.1. Single crystals and thin MgO films . . . . . . . . . . . . . . . . . . . . . . 86

2.1.2. Powdered materials: a mean to tune the surface area . . . . . . . . . 86

2.2. Experimental details concerning new advances . . . . . . . . . . . . . . . . . . . . 87

2.2.1. Sample synthesis and thermal pretreatments . . . . . . . . . . . . . . . 89

2.2.2. Characterization techniques (IR and TEM) . . . . . . . . . . . . . . . . 89

2.2.3. Ab initio calculations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

3. Effect of thermal treatments and of the synthesis procedure on the habit of the

MgO microcrystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

4. The evolution of the IR spectra of CO adsorbed at 60 K as function of the

crystallites dimension and perfection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

5. The IR spectra of Mg2+(CO)n (n = 1,2) complexes at 60 K and their evolution

with CO pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

5.1. Mg2þ3c ðCOÞ species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

5.2. The Mg2þ4c (CO) complexes on edges and steps and their evolution with

CO pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

5.2.1. The 2180–2160 cm�1 absorption . . . . . . . . . . . . . . . . . . . . . . . . 101

5.2.2. The 2150–2145 cm�1 absorption . . . . . . . . . . . . . . . . . . . . . . . . 101

5.3. The Mg5c(CO) complexes on (100) terraces and facelets: comparison

with the results obtained on (100) faces of single crystals . . . . . . . . . . . . 102

6. The IR spectra of CO species formed at 60 K on low coordinated O2� sites:

comparison between ab initio and experimental results . . . . . . . . . . . . . . . . . . . 105

6.1. Ab initio calculations on simple cluster models . . . . . . . . . . . . . . . . . . . 105

6.2. CO2�2 ‘‘carbonites’’ species: doublet at 1316 and 1279 cm�1. . . . . . . . . . . 106

6.3. (C3O4)2� trimeric species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

6.4. The evolution at 60 K of CO2�2 (A species) at intermediate PCO:

formation of dimeric C3O2�3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

6.5. The evolution of the C3O2�4 trimeric species into polymeric entities at the

highest PCO. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

7. Comparison with literature results obtained at higher temperatures. . . . . 111

7.1. CO on MgO: 100 K experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

7.2. CO on MgO: room temperature experiments . . . . . . . . . . . . . . . . . . . . . 112

7.3. CO on CaO and SrO: room temperature experiments . . . . . . . . . . . . . . . 116

8. The intensity of the stretching bands of CO adsorbed on 4- and 5-fold

coordinated Mg2+ ions as function of T at constant pressure: thermodynamic

implications on the CO bonding energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

8.1. Comparison with CO bonding energies obtained by TDS on single

crystals and with other experimental results . . . . . . . . . . . . . . . . . . . . . . 118

8.2. Comparison with CO bonding energies obtained by ab initio calculations 121

74 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

8.2.1. Interaction of CO with regular Mg2þ5c surface sites . . . . . . . . . . . . 121

8.2.2. Interaction of CO with Mg2þ4c and Mg2þ3c defective surface sites . . . 126

9. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

10. Note added in Proofs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

Appendix A. List of acronyms and symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

1. Introduction

The idea to write a concise review article on one of the most classical topic of Sur-

face Science (CO on MgO) was stimulated by new results obtained by means of an

unique experimental set-up (vide infra Section 2.2.2) realized in our laboratory

allowing to perform IR experiments in transmission mode down to 20 K on materi-

als activated in situ up to 1100 K. These recent results have added new information

on the interaction of the CO probe with surface cations and anions; so a much more

complete view on the topic has been reached. Furthermore, as the experimental set-up allows to perform experiments by controlling both the CO equilibrium pressure

and the adsorption temperature, new site-specific thermodynamic data have been ob-

tained that could not be obtained before by classical calorimetric experiments where

integrated (on all adsorption sites) values are accessed. By investigating fully dehy-

drated MgO samples, characterized by surface areas in the 400–10 m2g�1 range,

we were able to bridge the gap between single crystal [1–4] (or thin films [5–8])

and highly dispersed powders [9–11], typical of ‘‘pure’’ surface science and of catal-

ysis, respectively. All these temperature controlled IR experiments, on samples withdifferent morphologies, are supported by parallel high resolution TEM investigations

and will be compared with literature data on: (i) IR spectroscopy obtained on single

crystal, thin films and powdered MgO samples; (ii) thermodynamic experiments; (iii)

ab initio calculations. Due to the wide literature on the subject; this comparison give

to this work a rather interdisciplinary review character which can be of interest for a

wide community of surface scientists.

1.1. The historical role played by high surface area oxides of cubic structure in surface

science

Many oxidic systems belong to the class of rock-salt solids, and among them we

mention MgO, CaO, SrO, BaO, NiO and CoO. These solids can be prepared in form

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 75

of very high surface area polycrystalline samples, which exhibit large number of very

reactive step, edges and corner sites [11]. Due to their simple crystal structure and to

the fact that the most commonly exposed face is the (001) one, they represent an

ideal family of solids for the investigation of the surface properties of both cations

(Mg2+, Ni2+, Ca2+, Ba2+, Ni2+, and Co2+) and oxygen anions in different local envi-ronments. For both cation (M2+) and anion (O2�) we can distinguish among regular

five coordinated sites (M2þ5c and O2�

5c ) on flat (001) faces, four coordinated sites (M2þ4c

and O2�3c ) on step and edge and three coordinated sites (M2þ

3c and O2�3c ) on corner

(c = coordinatively unsaturated).

Such surface heterogeneity results in the appearance of characteristic electronic

transitions (up to three) which differs markedly form the bulk electronic transitions.

In the seventies such ‘‘non-bulk’’ transitions observed, on high surface area alkaline

earth monoxides, have been investigated by scientists like Coluccia, Garrone, Hale,Henrich, Nelson, Pott, Stone, Tench and Zecchina [12–26]. The use of high surface

area oxides has been mandatory in order to highlight the presence of the new elec-

tronic transitions, which have been attributed by the authors to ‘‘surface-specific’’

transitions.

As for MgO, UV–Vis reflectance and photoluminescence spectroscopies have re-

vealed surface transitions involving oxygen valence electrons [12–15,17,18]. Using

the energy and angle-of-incidence dependence of the EELS spectra of MgO, Henrich

et al. have resolved the excitonic transitions from the Mg core levels to the excitedstates into those of bulk and surface origin [25,26]. The bulk transitions have been

found to be very close to those of the free Mg2+ ion. The surface-state transitions

have been described by Stark splitting of the energy levels of the surface Mg2+ ions

in the intense Madelung electric fields at the crystal surface. 1

As for CaO, SrO and BaO the experimental evidences comes from UV–Vis reflect-

ance [12,13,15,16,18,27] and photoluminescence [17,19,28] experiments.

Fig. 1a summarizes the UV–Vis reflectance spectra of the four isostructural (fcc)

alkaline earth monoxides (MgO, CaO, SrO and BaO) obtained by Zecchina et al.[13,15,16,18] on activated high surface area samples. The adsorption edges due to

bulk exciton transitions were well known from previous studies [29–35] and are sum-

marized here in Table 1 (second row); these edges are well visible in the high energy

tail of the spectra reported in Fig. 1a. It is evident that for each oxide there are

absorptions at energies below the bulk adsorption: three features (labeled as I, II

and III in the original works) are observed for MgO, CaO and SrO cases, while only

two (I and II) are appreciable for BaO. Zecchina et al. [13,15,16,18] immediately no-

ticed that the intensities of features (I, II and III) decreases with decreasing the sur-face area. For a given oxide, the progressive decrease of the surface area resulted in

the progressive diminishing (up to the extinguishment) of the III, II and I features,

in the given order. On the basis of these evidences, Zecchina et al. [13,15,16,18] as-

cribed the components I, II and III to exciton charge transfer transitions where an

1 Please note that whereas the bulk excitons are delocalized electron–holes couples that can move freely

in the lattice, surface excitons are bounded to the surface defects and can be considered as an excited

electronic state of the defect.

Fig. 1. Part (a): DRS UV–Vis spectra of polycrystalline of MgO, CaO, SrO and BaO activated at 1073 K.

The spectra have been vertically shifted for clarity. Feature III, ascribed to charge transfer from O2�3cus

corner anions, has not been observed for BaO, owing to the insufficient surface area of the investigated

sample. Adapted from Refs. [15,16]: A. Zecchina, M.G. Lofthouse, F.S. Stone, J. Chem. Soc., Faraday

Trans. I, 71 (1975) 1476 and A. Zecchina, F.S. Stone, J. Chem. Soc., Faraday Trans. I, 72 (1976) 2364,

with permission. Copyright (1975 and 1976) by the Royal Society of Chemistry. Part (b): Effect of a first

(dot-dashed line) and of a second (dashed line) water dosage, from the gas phase, on the DRS UV–Vis

spectrum of polycrystalline of MgO activated at 1073 K (full line). Feature III disappears first, followed by

feature II. This experiment probes the higher reactivity of surface sites responsible of feature III. The inset

of part (b) schematizes the location of the surface O2� anions responsible of the UV–Vis features I, II and

III. Adapted from Ref. [18]: E. Garrone, A. Zecchina, F.S. Stone, Phil. Mag. B 42 (1980) 683, with

permission. Copyright (1980) by Taylor & Francis.

76 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

electron is, at least partially, transferred from a surface oxygen (O2�5c , O

2�4c and O2�

3c ,

respectively) to its immediate surroundings. On a simple theoretical ground this attri-

bution is supported by the fact that surface excitons require less energy then bulk

ones because of the reduced local Madelung constant of the coordinatively unsatu-

rated oxygen ions, as clearly predicted by the model developed by Levine and Mark

[36,37]. The trend Eex < EI < EII < EIII observed for all investigated oxides (Table 1,

Figs. 1a and 2) agrees with this model. On the other hand, the trend EX(MgO) > EX-(CaO) > EX(SrO) > EX(BaO) for (X = ex, I, II and III) reflects the increased lattice

parameter a, by moving from MgO to BaO, and agrees with the basic LCAO theory

applied to the determination of the band-gap of solids.

As I, II and III transitions are associated with surface oxygen ions, characterized

by decreasing Madelung contributions, which react in different way when contacted

with gaseous molecules, they are supposed to undergo a different perturbation as a

Table 1

Energy gap (Egap), bulk exciton (Eex), surface excitons (EI, EII and EIII) of the investigated alkaline earth

oxides (Panel A). Bulk values (Egap and Eex), are reported from Refs. [29–35]. Lattice parameter (a, and its

inverse) and surface area of the investigated alkaline earth oxides (Panel C). Surface values (EI, EII, EIII,

and surface area), summarizes Zecchina et al. works [12,13,15,16,18] and refers to the values obtained from

the spectra reported in Fig. 1. Room temperature PL excitation and emission components summarizes the

results obtained by Coluccia et al. in Refs. [17,19,28] and refer to the spectra reported in Fig. 3 (Panel B).

Feature III, ascribed to charge transfer from O2�3c corner anions, has not been observed for BaO, (neither in

the UV–Vis DRS nor in the photoluminescence excitation spectra) owing to the insufficient surface area of

the investigated sample. PL values for the BaO oxide have to be considered with care as BaO particles have

been supported on MgO to improve the surface area of the sample

Oxide MgO CaO SrO BaO

Panel A: Room temperature UV–Vis DRS spectra

Egap (eV) 8.7 7.7 6.7 4.4

Eex (eV) 7.7 6.8 5.8 4.1

EI (eV) 6.6 5.50 4.62 3.50

EII (eV) 5.75 4.43 3.99 3.31

EIII (eV) 4.62 3.75 3.50 Not observeda

Panel B: Room temperature photoluminescence excitation spectra

EI (eV) Not observedb Not observedb 4.38 (EIII) 3.76

EII (eV) Not observed b 4.46 3.98 (EIII) 3.69

EIII (eV) 4.52 �4.0 (shoulder) �3.5 (shoulder) Not observed a

Room temperature photoluminescence emission spectra

From O2�3c sites 3.18 3.06 2.67 2.64

Panel C: Structural and superficial data

a (A) 2.106 2.405 2.581 2.760

a�1 (A�1) 0.4748 0.4158 0.3874 0.3623

Surface area (m2g�1) 210 110 6 < 1

a Not observed due to the insufficient surface area of the investigated BaO sample.b Not observed due to the high energy cut-off of the PL instrument.

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 77

function of the gas equilibrium pressure. This is verified when small amounts of a

probe molecule like H2O [18], for instance, is dosed on the samples as shown in

Fig. 1b for the MgO case: feature III disappears first, followed by component II,

while component I vanishes at higher coverages only. The trend observed for water

adsorption on MgO is general and holds for all oxides showing that the sensitivity of

the bands toward surface reactions increases in the sequence: III > II > I. The same

holds when other molecules, like CO [23,38], CO2 [16], O2 [16], N2O [16], NH3 [21],

H2 [24] or pyridine [23] are adsorbed on the surface.These experiments described in the last two paragraphs have thus demonstrated

that reflectance UV–Vis spectroscopy is able to probe the coordination state and

reactivity of oxygen atoms at the surface of alkali earth oxides.

UV–Vis reflectance results and attributions are supported by parallel photolumi-

nescence spectroscopic investigations [14,17,19,28,39]. Alkaline earth oxide powders

(activated at high temperature) are photoluminescent when excited with near UV

photons, as clearly shown by the full line spectra reported in Fig. 3. For each oxide,

Fig. 2. Energies of surfaces and bulk electronic transitions as a function of the anion coordination number

and of the inverse of the lattice parameter, parts (a) and (b), respectively. Features I, II and III refer to

surfaces sites schematized in the inset of Fig. 1b. Adapted from Ref. [18]: E. Garrone, A. Zecchina, F.S.

Stone, Phil. Mag. B 42 (1980) 683, with permission. Copyright (1980) by Taylor & Francis.

78 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

the maxima in the excitation spectra (high energy region in Fig. 3) appear almost at

the same energies of the I, II and III bands observed in the UV–Vis reflectance spec-

tra 2 [13,15,16,18] (see Fig. 1 and Table 1) i.e. at energies definitively lower than

those of the band-gap edges and of the bulk excitons [29–35] (Table 1 secondrow). Such bands have therefore been assigned to radiative excitation process asso-

ciated with surface anions and cations in low coordination geometries

[14,17,19,28,39]. In particular, the strong and well defined band at 4.52 eV (274

nm) observed in the excitation spectra of MgO (Fig. 3a) has been ascribed to O2�3c

sites. This assignment has been supported by the evidence that the intensity of the

4.52 eV excitation band mirrors the evolution of the abundance of defective surface

O2�3c sites as clearly testified by the experiments collected in Fig. 4a–c. A remarkable

reduction of the 4.52 eV band is observed by moving from an high surface area MgOsample (ex hydroxide), part a, to a low surface area MgO smoke, part b (see section

2.1.2). The latter photoluminescence spectrum, typical of well defined MgO cubes

(vide infra the TEM micrographs in Fig. 9b), can be transformed into a spectrum

typical of high surface area samples by increasing the population of O2�3c sites by

2 Taking SrO as example (because all the three surface components are observed in the excitation

spectra reported in Fig. 3c, see also Table1), the energies of the components observed in the excitation PL

spectra are: 4.38, 3.98 and 3.5 eV, in fair agreement with the values extracted from the UV–Vis spectra:

4.62, 3.99 and 3.50 eV, for EI, EII and EIII respectively. In this regard, please note that, when observed,

excitation PL spectra allow a better determination of the surface excitons energy as the corresponding

feature is a rather narrow band and not an edge as in the case of UV–Vis DRS spectroscopy.

Fig. 3. Photoluminescence excitation (higher energy curves) and emission (lower energy curves) spectra of

alkaline earth oxides activated at 1200 K: MgO, CaO, SrO and BaO, parts (a), (b), (c) and (d), respectively.

BaO particles have been supported on MgO to improve the surface area of the sample. Full line spectra

refer to samples measured in vacuo conditions while dashed lines testify the quenching of the

photoluminescence by adsorption of water molecules. Adapted from Ref. [19]: S. Coluccia, Stud. Surf. Sci.

Catal. 21 (1985) 5, with permission. Copyright (1985) by Elsevier.

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 79

water attack (Fig. 4c). Using photoluminescence, TEM and EPR, Coluccia et al.

[19,39,40] have in fact observed that etching the MgO smoke cubes by water vapor(and subsequent evacuation) leads to an increase of the number of reactive O2�

3c sites

of low coordination on the surface, without a parallel increase of the surface area as

schematically depicted in Fig. 4e,f.

Coming to the emission spectra (low energy region in Fig. 3), the seminal work of

Coluccia and Tench [14,17,19,28,39] proved that the luminescent sites are the same

surface O2� sites involved in the light adsorption processes. Room temperature photo-

luminescence spectra showed that on high surface area oxides activated at high tem-

peratures only the photon emission from O2�3c sites can be observed, whatever is the

exciting wavelength (i.e. whatever is the original excited center). This implies remark-

ably efficient energy transfer mechanisms from O2�5c and O2�

4c sites to O2�3c corner sites.

In agreement with the different mobility of the surface excitons [18], the radiative

Fig. 4. Photoluminescence excitation spectra obtained on different magnesium oxide samples: high surface

area MgO (part a); low surface area MgO (smoke, part b); water attacked MgO smoke (part c). These

spectra represents the direct measurement of the amount of surface O2�3c anions in defective positions. The

bottom parts depicts schematically the effect crystal erosion by progressive water attack: parts (d)–(f).

Adapted from Ref. [19]: S. Coluccia, Stud. Surf. Sci. Catal. 21 (1985) 5, with permission. Copyright (1985)

by Elsevier.

80 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

decay pathways are thus more efficient for the 3-coordinated ions than for the 4- and

5-coordinated ones. Luminescence by less coordinatively unsaturated O2�5c and O2�

4c

surface sites can be observed only by quenching the O2�3c corner sites. This can be

done either by activating the high surface area samples at lower temperatures (i.e.by removing the surface OH groups only from O2�

5c and O2�4c sites) or by dosing small

amounts of adsorbates (which are preferentially coordinated on the most reactive

O2�3c corner sites) on high temperature activated samples. In this respect, it is worth

noticing that the photoluminescence bands, observed on high surface area MgO in

vacuo are quenched by the admission of O2 [14,41], CO [41] or H2O [19]. As an

example of the quenching effect obtained in huge equilibrium pressure of water,

the reader is referred to the dashed line spectra in Fig. 3.

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 81

Two different mechanisms have been proposed to explain the observed quenching

[15,16,38,41]. In the first one the quenching is due to collisions between molecules in

the gas phase and the surface luminescent centers, or to molecules weakly adsorbed

on them, whereby probe molecules interact with the light-emitting sites in one of

their metastable excited states. In the second one the quenching is due to the forma-tion of an adsorbed complex. These two mechanisms operate in the quenching proc-

esses to give non-radiative decay pathways i.e. to enhance the inter system crossing

to the ground state [19].

A different picture emerges from photoluminescence spectra performed at liquid

nitrogen temperature [19,28]. As an example we cite here the SrO case, where the

emissions coming from O2�5c , O2�

4c and O2�3c surface sites occurs at 2.02 eV (400

nm), 1.83 eV (440 nm) and 1.72 eV (470 nm), respectively. The emission bands from

O2�5c and O2�

4c sites appear when either of those sites are excited, whereas the emissionfrom O2�

3c corner sites is observed exclusively when they are directly excited. The fact

that the energy transfer mechanisms from O2�5c and O2�

4c sites to O2�3c corner sites (ex-

tremely efficient at room temperature) is almost totally absent at liquid nitrogen tem-

perature implies that the corresponding mechanisms have a relatively high activation

energy.

Summarizing the pioneering works reviewed in this subsection, it is clear that

they represented a real break-through in Surface Science, giving the first experi-

mental evidences of surface-specific electronic transitions in cubic oxides, associ-ated with O2� ions of the surfaces. The different surface oxygen anions singled

out by the above reviewed experiments are those who are responsible for the

complex chemistry observed when molecules interacts on activated, high surface

area alkaline earth oxides powders [11,42–44]. This is also valid for CO

[9,11,45–48], which is the reference surface probe molecule of this review. The

IR spectroscopy of CO adsorbed on MgO is deeply discussed in this review

(see Section 6) and briefly compared with that obtained on CaO and SrO in Sec-

tion 7.3. For the remaining oxides the reader is referred to the following litera-ture: [9,11,45–48].

Among all cubic oxides mentioned above, MgO has been the most investigated,

being considered as a model system for both computational and experimental sur-

face studies. This fact together with its high ionicity and structural simplicity jus-

tifies the enduring interest in this solid of the surface science and heterogeneous

catalysis communities, and why it has been used as a playground for the testing

of increasingly accurate computational methods by the theoretical chemistry com-

munity [49–84]. Sophisticated experimental methods have been applied on bothsingle crystals or thin films [2,5,6,8,82,85–108] and high surface area powdered

MgO [9–11,19–23,41,45,80,83,109–154]. Inter alia, these studies have produced a

tremendous amount of experimental and theoretical data concerning the adsorp-

tion of small molecules, like CO, H2, N2, NO, O2, O3, CO2, H2O, NH3, CH4,

etc. . . As carbon monoxide is able to probe both surface anion and cation sites,

in this work we deeply review the MgO–CO interaction, while for the remaining

molecules we refer to the recent review of Zecchina et al.: see Ref. [11] and refer-

ences therein.

82 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

1.2. Outline of the topics treated in the work

Let us now focus our attention on carbon monoxide, which has been the most

used probe. The spectroscopy of CO adsorbed on polycrystalline MgO has been

studied since the seventies [155–157]. Pioneering studies have been performed onsamples activated at moderate temperatures and still covered by a fair number of

OH groups [155,156]. Under these conditions CO is adsorbed as carbonate-like spe-

cies only in presence of O2. Successive studies performed on totally dehydrated sam-

ples [9,45,48,110,113,117,126,128,131,134,141,157–159] and in absence of O2, have

shown that CO chemisorption leads to formation of peculiar, highly colored, anionic

polymeric species. Experimental data on powdered materials were mainly obtained

by adsorbing CO in the 300–100 K temperature range. At first sight, vibrational

spectra of CO adsorbed on single crystal MgO(001) (vide infra Fig. 15b) look so dif-ferent from those obtained on high surface area powdered crystals (vide infra Figs.

12a and 13a), to be interpretable as originating from entirely different systems. This

hypothesis is reasonable because the surface chemistry of dispersed samples is dom-

inated by the activity of a distribution of ill-defined surface defects, while the surface

properties of single crystals are determined by defect-free low index faces.

However, as demonstrated for a-Cr2O3 [160], NiO [161] and other oxides or ha-

lides [11,162,163] the gap between the single crystals and polycrystalline samples is

not so profound as it has been depicted in early studies and in particular it shouldbe easily bridged for MgO [163]. In fact, the particles of this oxide, even when pre-

pared in highly dispersed form (200–300 m2g�1), show a great tendency to assume a

cubic habit and to expose low index (100) faces and terraces [11,162]. In other

words, these particles are intrinsically possessing the properties of single crystals

(100) faces, the only difference being represented by the larger proportion of sites

located on the edges and on the corners of the cubelets. As a consequence it can

be safely hypothesized that, as far as MgO is concerned, the above mentioned gap

of understanding could be diminished and rationalized by studying samples with spe-cific surface areas gradually varying from 200–300 to a few m2g�1 (see Section 2.1.2).

In order to achieve this goal, appropriate preparation procedures with an accurate

morphological control must be adopted. To proof the validity of this approach is

the first scope of this investigation.

It is well known that on high surface area MgO systems (200–300 m2g�1), a great

number of surface species are formed upon interaction with CO at RT, through a

complex sequence of surface reactions involving the most basic surface O2� ions.

Some of these reactions are activated and, at 300 K, they need considerable timeto be completed, with formation of oligomeric pink colored species [11,48,126,128,

131,141,157–159,162,164–166]. It has been hypothesized that all these species are

originated from a common CO2�2 precursor generated via the primary attack of

CO on the low coordinated O2�3c and O2�

4c oxygen ions present at defect sites like cor-

ners, edges, kinks etc. . . [11,141,162]. At low temperature (T � 100 K), the hypoth-

esized reaction sequence is:

O2�3c þ CO�!CO2�

2 ð1Þ

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 83

CO2�2 þ CO! C2O

2�3 ð2Þ

C2O2�3 þ CO! C3O

2�4 ð3Þ

leading to the forma tion of a complex and time dependent population of charged

monomers, dimers, trimers, and oligomers. Only at ambient temperature dispropor-

tionation products (carbonates, squarate or rhodizonate anions) are slowly formed

(reaction (4)):

Cnþ1O2�nþ2 þO2� �!CnO

2�n þ CO2�

3 ðdisproportionationÞ; ð4Þ

which can be the intermediates of the Bouduart reaction [128]

2CO! CO2ðas surface carbonate speciesÞ þ C ð5Þ(which is known to occur on MgO at high temperature). The structure of the species

formed following reaction (1)–(4) and the activation barrier of the individual steps

have not been yet fully elucidated. It can be easily foreseen that the relative propor-

tion of carbonite CO2�2 , olygomeric and final disproportionation products observed

by IR spectroscopy, at a given temperature and for a given contact time, is depend-

ing upon many factors: (i) the activation barriers of the (1)–(4) sequential reactions;

(ii) the stability of the various species on the surface; and (iii) the pressure of CO. It is

also expected that some of the intermediates (i.e. the less surface-stabilized and mostreactive) can have transient character and be therefore hardly observable by IR. As

far as the surface stabilization is concerned it can be noticed that the species formed

in reactions (1)–(4) incorporate the pristine highly basic O2�3c and O2�

4c centers into

more complex structures where the negative charge is delocalized on a larger set

of carbon and oxygen atoms. The stability of these structures on the surface will con-

sequently depend very much on the Coulombic interactions with the surface ions and

more specifically on the distribution of the positive centers interacting with the neg-

ative parts of the admolecule. Therefore, it is expected that the structure of these spe-cies and the structure of the adsorbing centers should be closely complementary and

connected via a surface-molecule recognition relation. On this basis, it is evident that

the detailed knowledge of the structure of the species formed at lowest temperatures

(where surface rearrangements and migrations are suppressed) give indirect informa-

tion on the structure of the adsorbing centers. The investigation of the structural

relations between the structure of negative species and the structure of the O2� sites

where they are generated, represents the second scope of this investigation. To this

end, experiments where CO is dosed at temperatures lower than 100 K are manda-tory, because surface re-arrangement upon interaction with CO is minimized and be-

cause disproportionation reactions are suppressed.

In addition to the negative species formed on low-coordinated basic O2� centers,

the properties of the CO/MgO system are also characterized by the presence of spe-

cies formed by interaction of CO with the positive Mg2+ surface centers. Indeed

Mg2þ3c � � �CO adducts are formed by interaction of CO with Mg2þ3c located at corner

sites are clearly observed in the IR spectrum of CO adsorbed at RT, already reported

in early studies [128,157,158] (vide infra Figs. 19 and 20). It is also known that at

84 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

T 6 300 K, CO also forms unstable adducts with Mg2+ ions emerging on steps

(Mg2þ4c � � �CO) and (100) terraces (Mg2þ5c � � �CO) [11,131,141,162]. The stability of

the Mg2+(CO) species is in the order: corners (Mg3c � � �CO) > steps and edges

(Mg2þ4c � � �CO) > (100) faces and terraces (Mg2þ5c � � �CO). At temperatures lower than

300 K and under appropriate pressure conditions, the Mg2þ3c � � �CO adducts are alsotransformed into Mg2þ3c � � � ðCOÞ2 species [71,141]. On the basis of all these results it is

concluded that by using appropriate pressure and temperature conditions, CO selec-

tively probes the vast majority of the positive centers of the surface and hence gives

comprehensive information on the structure and distribution of the Mg2+ centers.

However, it is worth to underline that despite the extensive work carried out in this

field, not all of the numerous species formed by interaction of CO with positive cen-

ters have been unambiguously assigned so far.

Consequently, a third scope of this review is to obtain a more complete assign-ment of the Mg2+(CO) species formed on extended faces, terraces and on the great

variety of defects present on high surface area materials. This goal can be achieved

only by systematically comparing the literature data collected on powdered MgO

samples (prepared at different sintering stages, see Section 7), on single crystals

and thin films (see Sections 5.3 and 8.1) at different adsorption temperatures and

CO equilibrium pressures, with the abundant literature on theoretical studies (Sub-

section 8.2) and with the complex, but highly informative, experimental data pre-

sented here for the first time (Sections 3 and 4).Contrary to what observed on high surface area materials, on single crystals faces

the number of structurally different CO species detected so far is significantly re-

duced. In particular, the formation of negatively charged species discussed before

has never been reported. As far as the species formed by interaction with positive

centers located on (100) faces are concerned, the dominant species described so

far are the Mg2þ5c � � � ðCOÞ adducts [87,92,93]. The same holds for low surface area

MgO polycrystalline samples obtained by combustion of Mg in air

[11,131,162,163]. These weakly adsorbed species can be observed only at T < 100K. The CO molecules adsorbed on (100) faces initially form a two-dimensional layer

of parallel CO species adsorbed through the carbon end on positive centers and ori-

ented perpendicularly to the (100) plane [87,92]. The completion of this two-dimen-

sional structure corresponds to the saturation of only half of the available Mg2+

centers. At temperatures in the 60–30 K interval and under appropriate pressure con-

ditions, also the remaining half of the magnesium centers are gradually filled by CO.

However, the formation of this compact layer is accompanied by tilting of CO and

profound modification of the vibrational properties [93,163], see Section 5.Due to the reduced number of surface atoms exposed on the faces of single crys-

tals, the intensity of the CO spectra is low or very low [93]. This fact usually pre-

cludes the possibility to investigate the adsorptive properties of surface defects,

certainly present also on single crystal faces (for instance in forms of steps and ter-

races). This has generated the widespread persuasion that the surfaces of single crys-

tals are more perfect than they actually do. In our opinion, the absence in single

crystal experiments of the typical manifestations of CO adsorbed on low coordinated

O2� and Mg2+ ions is not only the consequence of the low concentration of surface

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 85

defects but is also due to the low sensitivity of the reflection techniques, which allow

only to investigate the vibrational properties of the most abundant species. In this

respect, let us notice that, even on low surface area MgO samples, the number of

atoms effectively involved in transmission experiment is at least 102 times greater

than that involved in reflection experiments on single crystals. Therefore, a fourthscope of this investigation is the spectroscopic analysis of the less abundant surface

species, which can be analyzed by IR transmission spectroscopy on samples consti-

tuted by microcrystals of well defined shape (Section 6).

For the fulfillment of the scopes discussed so far, we take advantage of a new

home-made apparatus allowing to collect FTIR spectra of adsorbed species in trans-

mission mode as a function of both equilibrium pressure and adsorption temperature

(in the 10�4–10+2 Torr and 300–20 K ranges) on powdered samples previously ther-

mally activated in situ in vacuo up to 1100 K (Section 2.2.2). It will be shown that theexperiments carried out in such a large temperature and pressure interval can be of

extreme utility for complementing the available experimental information on the

structures which are formed by interaction of CO with MgO surfaces containing con-

trolled amounts of surface defects. To this end, the surface properties of MgO sam-

ples with decreasing surface area from �250 (high surface area) to �10 m2g�1

(smoke) and increasing definition of the crystalline habit are studied. In particular,

as the MgO smoke sample is constituted by nearly perfect cubelets, the vibrational

properties of CO adsorbed on it can be usefully compared with those of CO ad-sorbed on the (100) faces of single crystals. It will be shown that the study of the

evolution of adsorptive properties with the increase of the size and perfection of

the crystals sheds light on the debated role of surface defects and contributes to de-

crease the gap between the surface chemistry of high surface area and that of single

crystal systems (Sections 4–6). This experimental strategy has very high sensitivity

and hence it permits the detection not only of the vibrational properties of the most

abundant species, but also of the manifestations associated with defects sites, includ-

ing the defects present on nominally flat faces (Section 8).Finally, our variable temperature apparatus allows the measurement of the inten-

sity of the single peaks associated with the various adsorbed species as a function of

temperature, resulting (in some favorable cases) in the precise determination of the

adsorption enthalpy of species adsorbed on single surface sites [113,117,163,

167,168]. A fifth scope of this investigation is consequently the determination of

the thermodynamic parameters of the individual adsorbed species present on the

MgO surface.

2. Samples preparation (from single crystals to powders) and experimental details

concerning new advances

2.1. MgO samples preparation

The different techniques adopted to produce MgO samples used in surface sci-

ence studies will be now briefly summarized. In Section 2.1.1 we will deal with single

86 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

crystals and thin MgO films typical of ‘‘pure’’ surface science, while the method used

to prepare MgO powders with different surface area are discussed in Section 2.1.2.

2.1.1. Single crystals and thin MgO films

In situ, vacuum cleaved, MgO single crystals exhibits the regular (001) face with anegligible density of surface defects [2,4,25,87,93,97,99,169]. Ultrathin MgO films are

usually synthesized by evaporating the metallic component in a moderate oxygen

atmosphere on an adequate metallic substrate. The lattice mismatch plays a crucial

role in epitaxial growth, in fact both the detailed nature of the oxide–metal bonding

and the planarity of the overlayer depend on the extent of interface strain [170,171].

In this respect, both Mo(001) [5,88–91,170,172–175] and Ag(001) [82,96,98,100–

108,176,177] substrates represent good candidates for the preparation of MgO epi-

taxial layers, because of the reduced lattice misfit. Stoichiometry, morphology anddefectivity of epitaxial MgO layers reactively grown on Ag(001) have been recently

investigated. Although highly ordered, stoichiometric films were obtained, deviation

from the 1:1 composition was suggested for the very outermost layer [176,177] and a

larger concentration of defects than the surface of single crystals has been proposed

[177]. A mosaic structure has been suggested for very thin (less than seven monolay-

ers) MgO layers [96].

Both single crystals and thin films MgO sample require the typical UHV condi-

tions of pure ‘‘Surface Science’’; this requests that the sample has to be in situ cleavedor growth. In that ambient the sample will then be investigated with the typical sur-

face sciences techniques: AES, LEED, XPS (ESCA), UPS, EELS, IRAS, TDS,

PDME, SEXAFS. . .etc. Recently the local structure of thin (3–10 monolayer thick)

MgO/Ag(001) films has been investigated by ex situ Mg–K edge EXAFS spectro-

scopy using a few monolayers of NiO acting as capping layer and compared with

in situ O–K edge EXAFS data [107,108,171]. These studies complements previous

ones on the complementary NiO/Ag(001) system capped with MgO [171,178–181].

In all studies the differences in the in and out of plane lattice parameter inducedby epitaxy with the Ag(001) substrate has been evidenced performing polarization

dependent EXAFS studies. An high crystalline perfection of both MgO and NiO

thin films emerges from such EXAFS studies, as contributions up to the seventh

coordination shell around the absorbing atom have been experimentally observed.

However, the refined first shells coordination numbers significantly deviate from

the ideal values for 3 monolayer thick samples, suggesting a non-negligible surface

roughness [171,179,181].

2.1.2. Powdered materials: a mean to tune the surface area

MgO cubes of high crystallographic quality, characterized by well defined low

index [001], [010] and [100] faces, by low density of surface defects and by a surface

area in the order of some m2g�1 can be obtained by direct combustion of Mg ribbon

in air [9–11,131,141,163].

High surface area (200–500 m2g�1) MgO powders can be prepared using non-

equilibrium techniques such as: (i) decomposition in vacuo of Mg(OH)2; (ii) precip-

itation from liquid solutions in autoclave, following an hypercritical drying aerogel

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 87

procedure; (iii) chemical vapor deposition (CVD) method. The Mg(OH)2 decompo-

sition procedure (typically performed in vacuo around 530 K) was largely used by

our group in Torino (Italy) since the seventies [9,11,13,15,17,18,128,131,141]. It re-

sults in MgO samples having a surface area in the 200–250 m2g�1 range. The main

advantages of this technique consists in the fact that Mg(OH)2 powders can be di-rectly converted into dehydrated MgO nanocrystals inside cells conceived for per-

forming in situ IR, UV–Vis, TEM, EPR volumetric or calorimetric experiments

(vide infra Section 3). The undesired re-hydration of the samples upon contact with

the atmosphere can thus be easily avoided.

The aerogel preparation of MgO nanoparticles has been developed in the nineties

by Klabunde et al. in the Kansas State University (USA) [182–198]. It involves a sol–

gel approach where methoxides are converted to hydroxide gels followed by hyper-

critical drying and vacuum dehydration. This procedure results in ultra fine MgOparticulates characterized by a surface area as high as 350–500 m2g�1.

CVD method, for the MgO synthesis, has been optimized in the nineties by the

group of Erich Knozinger in Vienna (Austria) [129,134,135,138–140,148–

150,154,199–201]. With this technique, MgO powders with surface area in the

300–400 m2g�1 range have been obtained. Two TEM micrographs of MgO nano-

crystals synthesized with the CVD technique are shown in Fig. 5.

As the high surface area MgO powders prepared by aerogel or CVD methods are

synthesized in an ambient (autoclave or CVD reactor) that is not suitable for per-forming in situ IR, UV–Vis, EPR, TEM investigations, they have to be exposed

to air before being transferred in the cells (chambers) used for the experimental

investigations. This makes a reactivation process at high temperature mandatory

to clean the surface of samples (removal of adsorbed impurities and surface hydrox-

yls). Activation processes at high temperatures implies a progressive sintering of the

native MgO powders with parallel decrease of the surface area. As an example, Kno-

zinger et al. [201] report that the extremely high surface area of the freshly prepared

CVD MgO samples (400 m2g�1) falls down below 300 m2g�1 after the activationprocedure in vacuo at 900 �C. This implies sintering and interpenetration of the orig-

inal cublets with formation of larger particles, which are not so different to those pre-

pared by Mg(OH)2 decomposition. This fact explains why the chemistry of fully

dehydroxylated MgO polycrystalline samples is largely dependent upon the prepara-

tion and activation procedure.

This means that the thermal sintering procedure represents a reliable mean to tune

the surface area (and thus the surface defectivity) of oxide particles. Severely acti-

vated samples show thus predominant [001] faces, characterized by a low concentra-tion of surface defects. The gap between MgO smoke (vide supra at the beginning of

this Subsection) and high surface area MgO powders can be filled by subjecting high

surface area samples to progressively more severe sintering conditions [9–11,202].

2.2. Experimental details concerning new advances

The new advances presented in this work refers to three different MgO polycrys-

talline samples characterized by significant different surface areas: 230, 40 and 10

Fig. 5. HRTEM images of MgO nanocubes prepared by CVD technique and subsequent thermal

treatment at 1173 K under high vacuum conditions as described in Refs. [135,201]. Unpublished

micrographs kindly supplied by the group of Prof. E. Knozinger, Institut fur Materialchemie der TU-

Wien, Austria.

88 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 89

m2g�1, as detailed in Section 2.2.1. Such samples have been investigated by TEM

and by an innovative low temperature IR apparatus, described in Section 2.2.2. Fi-

nally, details on the ab initio calculation performed to support the interpretation of

the IR data are reported in Section 2.2.3.

2.2.1. Sample synthesis and thermal pretreatments

The high surface area samples were obtained by decomposition of Mg(OH)2in vacuo and will be hereafter named as hsa MgO. The low surface area samples

(hereafter MgO smoke) was obtained by direct combustion of Mg ribbon in air. Sam-

ples with intermediate surface area were obtained from the hsa ones by sintering at

1073 K according to the well known procedure described in Refs. [9,11,45,128,

131,141], and will be hereafter named as MgO sintered. All the samples were oxidized

at 623 K for 30 0, outgassed and annealed in vacuo at 1073 K for 2 h before dosingCO. This thermal treatment is sufficient to eliminate the vast majority of surface hyd-

roxyl groups as testified by the very weak m (OH) band at �3730 cm�1 (less then 0.02

a.u. on the hsa pellet), due to few residual (isolated) OH groups located on corner

positions or on (111) facets. The total elimination of these OH groups can be

achieved by outgassing at 1150 K. Despite the residual presence of this (very weak)

m(OH) band, the samples treated in this way can be safely considered as substantially

‘‘clean’’.

2.2.2. Characterization techniques (IR and TEM)

The apparatus used for performing in situ FTIR experiments at temperature var-

iable in the 20–300 K range, is presented in Fig. 6. It can be divided into three parts:

(i) The cryogenic part (a in the figure), which consists of cryogenic head where a de-

fined temperature can be set in the 20–300 K range exploiting a commercial cryostat

apparatus (Oxford instrument); (ii) The activation part (b in the figure), where the

sample can be heated either in high vacuum conditions (P < 10�3 Torr: 1

Torr � 133.3 Pa) or in the desired atmosphere up to 1073 K. A magnetic manipula-tor (c in the figure, see also its magnification in the right part of the figure) allows us

to move the sample holder (sh in the figure) from the oven to the cryogenic cell under

high vacuum conditions; (iii) The vacuum system (d in the figure), which allows to

maintain the dynamical vacuum during the activation procedure as well as to dose

the desired amount of the probe gas on the sample maintained at the desired temper-

ature and to reduce (or increase) progressively the coverage. At each state of the

experiment, the equilibrium pressure is monitored in the different parts of the instru-

ment using both Pirani (relative pressure in the 10�3–10 Torr interval) and mem-brane gauges (Varian, absolute pressure in the 0.2–200 Torr interval). The

cryogenic cell, equipped with four IR transparent windows, is hosted inside an

ad hoc modified Bruker Equinox-55 FTIR-spectrometer (e in the figure) allowing

the IR beam to pass through the activated sample (in form of self supporting pellet)

for transmission measurements.

This instrument allow to monitor the modification of the IR spectra of molecules

adsorbed on clean surfaces according to two main procedures. Following the first

one, the sample temperature is kept fixed, and the equilibrium pressure of the probe

Fig. 6. Low-temperature FTIR apparatus: (a) cryogenic head; (b) electric furnace for sample activation;

(c) magnetic manipulator for the transfer of the sample holder (sh) from the furnace to the cryostat; (d)

vacuum line for sample evacuation and gas admission; (e) FTIR instrument (unpublished).

90 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

gas is changed. According to the second procedure, a known amount of probe gas is

introduced into the cell and the different spectra are then recorded by changing thesample temperature in the desired range (between 300 and 20 K). The measurement

of the integrated area of a specific IR band, upon changing the sample temperature,

allows us to calculate the adsorption energy of the corresponding surface adduct

[110,113,117,126,163,168,203]. In both procedures, time invariance of the spectra

is used as a criterium to prove that the system has reached the equilibrium at the

given thermodynamic condition (pressure and temperature). In this work CO has

been used as probe. The spectrum collected before CO dosage has been used as back-

ground. All the spectra reported in this work are background subtracted and havebeen acquired at a resolution of 1 cm�1 by averaging 128 interferograms. Experi-

ments performed according to procedure 1 have been carried out at 60 K, while dur-

ing those performed according to procedure 2, the whole 60–300 K interval has been

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 91

explored. 60 K has been chosen because it is the minimum temperature avoiding con-

densation of CO on the metallic walls of the cryostat.

HRTEM experiments were performed with a JEOL 2000EX microscope equipped

with a top-entry stage and operating at 200 Kv. The thermal decomposition of

Mg(OH)2 has also been carefully studied in situ under the electron beam in orderto understand the structural relationship between the precursor and the final

material.

2.2.3. Ab initio calculations

A very basic model of an active oxygen site was adopted, consisting of a bare

unconstrained Na–O–Na neutral cluster (see discussion in Subsection 6.1 for the rea-

sons of this choice). The reaction of this cluster with 1 to 5 CO molecules was stud-

ied, in order to evaluate the vibrational frequencies of CO polyadducts and theirrelative stability.

All calculations were done with density functional methods using the B3-LYP

functional [204–206] and a standard 6–311+G(d,p) basis set, as coded in the Gaus-

sian 98 (Rev A.7) program [207]. Geometry was optimized without constrains. The

Cs symmetry, which was exploited during calculations did not result in any con-

strain, as verified during frequencies calculations. The binding energies of the

adducts were not corrected neither for basis set superposition error (BSSE) [208],

nor for thermal energies. 3

3. Effect of thermal treatments and of the synthesis procedure on the habit of the MgO

microcrystals

High surface area MgO (250–400 m2g�1) can be prepared either by decomposi-

tion in vacuo of Mg(OH)2 at about 530 K [9,11,128,131,141], by sol-gel methods

[182–198] or by chemical gas phase deposition [134,135,201], see Subsection 2.1.2.For the sake of brevity, we illustrate only the case of the first procedure, where

the formation of MgO microcrystals by loss of water from Mg(OH)2 seems to occur

in a topotactic way with initial formation of a fully hydroxylated (111) surface. In

Fig. 7a, a micrograph of the starting Mg(OH)2 sample is shown. The presence of

thin platelets, variably oriented with respect to the electron beam, is evident. Some

of them are oriented perpendicularly (regions 1 in the plate) and some are parallel

(regions 2). From this observation it can be deduced that: (i) the platelets have

irregular contours, (ii) they have diameters in the 200–500 nm range and (iii)their thickness is in the 5–25 nm range. It must be underlined that under the effect

of vacuum and of the electron beam, the flat Mg(OH)2 microcrystals readily

3 Simply speaking, the BE of the A + B ! AB reaction is defined as: BE = E(A) + E(B)�E(AB). Now,

as E(A) is computed with the A basis set, E(B) with that of B and E(AB) with the union of the two basis

sets, the product AB is computed with a more complete basis set that each if the reactants. This results in a

systematic overestimation of BE, which is called BSSE and which has to be corrected when quantitative

values are needed.

Fig. 7. Low (Part a) and high magnification (parts b and c) TEM images of Mg(OH)2. Platelets oriented

perpendicular (1) and parallel (2) to the electron beam are recognizable. Fragmentation of the platelets (b)

and appearance of very small cubic structures (c) are a consequence of partial Mg(OH)2 decomposition to

MgO in the HRTEM experimental conditions (UHV and heating effect of the electron beam), see text for

further details. Unpublished micrographs.

92 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

decompose, with loss of water and formation of MgO. However as the

Mg(OH)2!MgO transformation is topotactic [209] the gross contours illustrated

in Fig. 7a are still informative on the shape of the original hydroxide particles. The

transformation of Mg(OH)2 into MgO is accompanied by: (i) fragmentation of the

original laminae into parallel foils of MgO with 1–1.5 nm thickness developed

along the (111) plane (Fig. 7b); (ii) clear appearance of aggregates of interpene-trated cubelets with 1–1.5 nm edges, well visible at the border of the laminae.

The corners of the cubes intersection of (100), (010) and (001) terraces are ori-

ented parallel to the plate (Fig. 7c) i.e. along the [111] direction. This is the conse-

quence of faceting of the unstable (111) face originally formed by topotactic

Mg(OH)2!MgO transformation [209,210].

In order to obtain highly active samples, the complete or nearly complete elimi-

nation of the surface OH groups must be achieved. This result is obtained by heating

the sample in vacuo at 1073 K. This thermal treatment is accompanied, by a substan-tial increase of the dimension of the interpenetrated MgO cubelets (from 1–1.5 to 2–3

nm) as illustrated in Fig. 8a (treatment at 1073 K for 2 hours in vacuo), The resulting

Fig. 8. Low (part a) and high magnification (Parts b and c) TEM images of MgO, (ex hydroxide) activated

at 1073 K for 2 h in vacuo. Substantial increase of the dimension of the interpenetrated MgO cubelets is

evident. Two views of similar aggregates observed along a perpendicular direction are reported in parts (b

and c). Unpublished micrographs.

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 93

aggregates are however still maintaining the gross shape of the original Mg(OH)2microcrystals. In Fig. 8b,c two views of similar aggregates observed along a perpen-

dicular direction are reported.

The effect of successive annealing at higher temperature (1173 K) causes a further

increment of the MgO terraces from 2–3 to 20 nm (Fig. 9a). Samples constituted by

nearly perfect separated cubelets can be obtained only by combustion of metallic Mgto give MgO ‘‘smoke’’, see Fig. 9b.

A schematic representation of the process leading from brucite to hsa MgO is re-

ported in Fig. 10 from part (a) to part (c). The morphologies of hsa and smoke MgO

are schematically compared in Fig. 11. Fig. 10a reports the layered structure of the

brucite. Activation in vacuo at moderate temperatures results in the elimination of

water molecules at the interfaces of the Mg(OH)2 layers yielding to needle-like

MgO crystals whose surface are still hydroxylated, Fig. 10b. Activation at higher

temperature results in the total removal of surface hydroxyls, as tentatively sche-matized in Fig. 10c. It is however evident that structures like that hypothesized in

Fig. 10c are highly instable and subjected to severe surface reconstruction; this is

Fig. 9. TEM images of heavily sintered MgO (ex hydroxide) and of MgO smoke, parts (a) and (b),

respectively. Unpublished micrographs; part (a) has been kindly supplied by Prof. Coluccia�s group.

94 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

the reason why, when more severe sintering conditions are adopted, the cubic-like

structures schematized in Fig. 11b are largely dominant. Fig. 11a is representingan array of interpenetrated cubelets with corners approximately pointing along the

(111) plane, which is obtained by faceting a (111) surface of MgO, following Schon-

nenbeck et al. [210]. From this figure the presence of inverse steps and inverse corner

sites is clearly emerging. Similar structures are obviously absent on the separated,

nearly perfect single, crystals with cubical shape, typical of MgO smoke (schematized

in Fig. 11b).

In conclusion, three main points merit consideration. (i) The Mg(OH)2 micropar-

ticles decompose under vacuum with formation of layers of compenetrated cubelets.These cubes are aligned along a preferential direction. They cannot be considered as

separate particles because they appear to be connected continuously across the (100)

faces. Indeed, individual cubic particles are rare in these samples. A consequence of

this morphology is that, beside the typical sites characteristic of individual cubic

nanocrystals (corners, edges and faces) steps and inverse steps sites of variable

height, formed by the intersection of (100) terraces, are very abundant. All inverse

sites could play a role in chemisorption of hydrogen and CO as recently hypothesized

by Ricci et al. [83]. In this regard, please see the related point in the Note added in

Fig. 10. A schematic representation of the progressive evolution from the layered structure of Mg(OH)2(Part a), to that of MgO by progressive water elimination (parts b and c.) Part (b) reports a still

hydroxylated MgO sample, while a hypothetical structure of a completely dehydroxylated MgO sample is

reported in Part c. Compare this scheme with the area labeled with 2 in the micrographs reported in Fig. 7.

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 95

Proofs section for further discussion. It is worth noting that inverse corners and

edges are a peculiar features deriving from the reminiscence of the Mg(OH)2 layered

structure. (ii) The progressive sintering affords a way to increase the dimension of

terraces and to decrease the concentration of low coordinated sites present on steps

and other defects. Inverse edge and corner sites are preferentially affected. (iii) Only

MgO smoke is constituted by individual, nearly perfect, separated cubic crystals,

with low concentration of defects.

Fig. 11. A schematic representation of the morphologies of ‘‘hsa’’ and of ‘‘smoke’’ MgO samples: parts (a)

and (b), respectively. The cubic crystals of (b) are separated, nearly perfect single crystals (see Fig. 9b),

while in (a) compenetrated cubes are present (see Fig. 9a). Inverse edges and corners are present on the

latter.

96 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

4. The evolution of the IR spectra of CO adsorbed at 60 K as function of the crystallites

dimension and perfection

The IR spectra of CO adsorbed at 60 K on hsa, sintered and smoke MgO samples

are shown, as a function of CO coverage h, in parts a–c of Fig. 12, respectively. These

spectra are substantially different from those published in previous papers and ob-

tained at higher T (about 100 K), both in the 2250–2100 cm�1 and in the 1800–1100 cm�1 ranges [9–11,128], vide infra Section 7.1 and Figs. 17 and 18. This is due

to the fact that at 60 K higher CO coverages can be obtained with respect to those

obtained at similar PCO in experiments performed around 100 K. The most intense

spectra of the three sequences correspond to PCO = 40 Torr. The other spectra have

been obtained by decreasing the pressure in steps at T = 60 K. The spectra with lowest

intensity have been obtained after prolonged pumping at 60 K and correspond to CO

equilibrium pressures lower than 10�3 Torr. As the initial spectrum can be restored by

redosing CO at 60 K, it can be concluded that the species responsible for the pressuredependent IR bands illustrated in Fig. 12 are completely reversible and involve sur-

face processes characterized by very low or negligible activation barriers.

The three sequences reported in Fig. 12 allow us to appreciate directly how

the decrease of the specific surface area is accompanied by a dramatic decrease

Fig. 12. Coverage dependence of the IR spectra of CO dosed at 60 K on hsa, sintered and smoke MgO

samples: parts (a), (b) and (c), respectively. The left parts refer to the chemistry of the Mg2+ � � � COadducts (2250–2050 cm�1 region) while the right parts refer to the chemistry of CO interacting with low

coordinated O2� basic centers (1700–1125 cm�1 region). All spectra have been vertically shifted for sake of

clarity. The decrease of the band intensity by moving from high (a) to low (c) surface area samples is

remarkable: note the nearly total extinction of the O2� chemistry on the smoke sample (c). Unpublished

results.

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 97

of the overall intensity of the IR bands and by a drastic spectral simplification.

In particular all bands in the 2140–1100 cm�1 interval are heavily affected by sin-

tering and are nearly totally absent on smoke MgO. According to previous re-

ports [11,128,157,162,164] they belong to species formed at very reactive 3-fold

and 4-fold coordinated oxygen sites located on edges and steps (O2�4c ) and on cor-

ner (O2�3c ). The nearly complete absence of these bands on smoke MgO indicates

that the concentration of O2�4c and O2�

3c sites is below 0.1%, which can be consid-ered as the minimum concentration leading to detectable bands in IR transmis-

sion experiments. We shall discuss their detailed attribution in the following

(Section 6).

Unlike the bands in the 2140–1100 cm�1 range, the complex absorption in the

2220–2140 cm�1 interval (characterized by several components) is not completely

disappearing on passing from hsa sample to smoke MgO. The intensity decrement

is accompanied by a great simplification. This result is in agreement with the

Fig. 13. Magnification of the C–O stretching region (2210–2075 cm�1) of the IR spectra of carbon

monoxide dosed at 60 K on hsa (Part a), sintered (Part b) and smoke (Part c) MgO samples, already

reported in the left parts of Fig. 12: parts (a), (b) and (c) respectively. All spectra have been vertically

shifted for sake of clarity. Unpublished results.

98 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

accepted view that CO molecule is probing all positive Mg2+ centers and that these

bands are associated with CO adsorbed on 5-, 4- and 3-fold Mg2+ sites [11].

Due to their complexity, the spectra illustrated in Fig. 12 do not allow a satis-

factory appreciation and assignment of all the numerous components of the spec-

tra. For this reason an exploded view of the sequence of spectra in the 2210–

2075 cm�1 region is reported separately in Fig. 13a–c. Similarly an exploded viewof the 2120–1125 cm�1 region of CO adsorbed on the hsa sample is reported in Fig.

14.

5. The IR spectra of Mg2+(CO)n (n = 1,2) complexes at 60 K and their evolution with

CO pressure

The spectroscopy of CO adsorbed on Mg2+ sites of powdered MgO has beenalready widely investigated. We shall here briefly summarize well established assign-

ments of the main features observed in the literature spectra (collected at about 100

K) [9,11,45,110,113,117,126,128,131,134,141,211], vide infra Fig. 18.

Fig. 14. Magnification of the IR spectra of CO dosed at 60 K on hsa MgO sample, already reported in

Fig. 12a in the 2120–2050 cm�1 and 1700–1125 cm�1 regions. All spectra have been vertically shifted for

sake of clarity. Labels A, D, C (and C 0) and P refer to bands ascribed to CO2�2 carbonites, (C2O3)

2�

dimers, (C3O4)2� trimers, and polymeric (CnOn+1)

2� species, respectively (see text for a more detailed

discussion). Unpublished results.

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 99

In the quoted papers, it has been shown that CO is able to distinguish among sur-

face Mg2+ sites of different coordinative unsaturation. In particular, at very low

coverage (h < 0.1) the frequency of CO (singleton) adsorbed through the carbon

end on Mg2þ3c , Mg2þ4c , and Mg2þ5c , is upward shifted with respect to the frequency of

the CO gas of +60, +27 and +14, cm�1 respectively (�mðCOÞ ¼ 2203, 2170 and 2157

cm�1). This upward shift is the typical result of the Stark effect associated with the

positive electric field of the cation. Following Hush and Williams [212] and Pacchioniet al. [53], when no d-electrons are involved, the shift is, for moderate fields, propor-

tional to the strength of the electric field sensed by CO. This agrees with the intuitive

concept that, to a first approximation, the effective field sensed by CO adsorbed on a

cationic site is the result of the contribution of cation and of the O2� anions of the first

coordination sphere. When the number of anions surrounding a givenMg2+ decreases

(as on corners, edges and steps) the negative contribution to the electric field decreases

and hence the ‘‘effective’’ positive field sensed by CO increases.

100 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

In the following we shall highlight the fine spectroscopic details that can be appre-

ciated by lowering the sample temperature down to 60 K. Sections 5.1, 5.2 and 5.3

are devoted to the discussion of the CO complexes formed on Mg2þ3c , Mg2þ4c , and

Mg2þ5c surface sites, respectively.

5.1. Mg2þ3c (CO) species

The weak band at 2205–2200 cm�1, already assigned to Mg2þ3c (CO) complexes, is

well evident on the hsa material (Fig. 13a), very weak on sintered samples (Fig. 13b)

and absent on smoke (Fig. 13c). The frequency is shifted upwards of about +60

cm�1, indicating that the involved sites are associated with a strong polarizing field

[11,53,212,213]. The high quality of the spectra of Fig. 13 allows us to observe that

the 2205–2200 cm�1 band is complex, showing a distinct tail on the low frequencyside. This evidence can be explained on the basis of the presence of many surface

configurations containing 3-fold coordinated ions, differing in the second coordina-

tion sphere. According to this explanations, Mg2þ3c ðCOÞ complexes, formed at corner

positions of cubelets and at corners of monoatomic steps are expected to result in

slightly different m(CO).

Upon increasing the PCO the peak observed at 2203 cm�1 first increases, than it

saturates and finally it disappears with formation of a shoulder at �2185 cm�1. This

fact is well known and has been attributed to the formation of dicarbonylic species[214]. We shall not return on this point because it can be considered as fully under-

stood. At higher pressures, also the shoulder at 2185 cm�1 gradually disappears,

plausibly because the responsible peak is shifting to lower frequency and becomes

obscured by the extremely strong bands associated with CO adsorbed on more abun-

dant sites (Mg2þ4c and Mg2þ5c ). This shift could be associated with formation of tricar-

bonylic species. An alternative explanation requires coupling effects between

Mg2þ3c ðCOÞ2 complexes and adjacent CO molecules adsorbed on neighbouring steps

sites. In other words at high coverage the stretching of CO on corner sites cannot beconsidered any more as a localized vibration.

At the highest PCO four weak bands are emerging in the 2380–2210 cm�1 region

(see inset in Fig. 13a). The absorption at 2310 and 2225 cm�1 are assigned to com-

bination modes of m(CO) with m(Mg–CO) and d(Mg–CO) respectively of CO species

formed on (100) terraces. The two other bands at 2365 and 2260 cm�1 are attributed

to the same combination modes for CO molecules adsorbed on Mg2þ4c sites of (110)

planes. On the basis of this hypothesis the m(Mg–CO) and the d(Mg–CO) have fre-

quencies of 155(195) and 70(90) cm�1 for CO adsorbed on Mg2þ5c ðMg2þ4c Þ sites. Similarmetal–CO stretching frequencies were observed upon the interaction of CO with

alkaline metals in zeolites by Otero Arean et al. [215].

5.2. TheMg2þ4c (CO) complexes on edges and steps and their evolution with CO pressure

By increasing PCO, after formation of Mg2þ3c ðCOÞ adducts at 3-fold sites, also the

Mg2þ4c sites begin to be populated. Two new strong and complex absorptions in the

2180–2160 cm�1 and 2150–2145 cm�1 intervals appear in the IR spectra of CO

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 101

adsorbed on hsa samples (Fig. 13a). The absorption in the 2180–2160 cm�1 range is

well observable (although with reduced intensity) on the sintered samples (Fig. 13b)

and is still present also in the spectra of CO on smoke (Fig. 13c). The effect of the

increase of the microcrystal perfection on the intensity of 2180–2160 and 2150–

2145 cm�1 absorption well agrees with the attribution to Mg(CO) adducts formedat 4-fold Mg2+ sites of edges and steps [9–11,158,202,211]. On both hsa and sintered

materials the intensity of these peaks is at least one order of magnitude higher than

that of Mg2þ3c ðCOÞ species. The 2150–2145 cm�1 component dominates the low and

the intermediate PCO spectra in the sintered sample, the intermediate PCO spectra in

the hsa sample and is totally absent on smoke. This component looks slightly more

affected by sintering than the 2180–2160 cm�1 one, which is scarcely appreciable in

the low PCO spectra of smoke.

5.2.1. The 2180–2160 cm�1 absorption

From the spectra of Fig. 13a–c it is clear that this absorption is constituted by a

few distinct components, the most intense being that at 2170 cm�1. The composite

character of the 2180–2160 cm�1 absorption is due to the heterogeneity of the

Mg2þ4c sites (located on edges and steps of variable high). The dominant component

at 2170 cm�1 (Fig. 13a) is assigned to CO adsorbed on sites located on the edges of

cubelets or on multiatomic steps, where the interaction with the underlying (001)

surface can be neglected. This assignment is suggested by the fact that this peakcan be clearly observed also on smoke (Fig. 13c) An important feature which can

be observed clearly only in Fig. 13a (hsa sample) is that the 2170 cm�1 peak reaches

a maximum and than disappears upon gradually increasing PCO with simultaneous

formation of a new narrow component at 2167 cm�1. This is likely due to the forma-

tion of Mg2þ4c (CO)2 dicarbonylic entities. Notice that, at the highest coverages (see in

particular parts (b) and (c) of Fig. 13), this peak is totally merging into the extremely

intense band of CO on (100) facelets and terraces. Also in this case we think that at

high coverage the localized nature of the absorption is lost because of coupling ef-fects with the modes of adjacent molecules. This band seems then to follow the same

fate of that of the Mg2þ3c ðCOÞ2 complexes (high PCO in part a) and already discussed

in Section 5.1.

It is worth noticing that, even on vacuum cleaved single crystal (100) faces, an

absorption in the same frequency region can be appreciated, although with very

weak intensity [93]. In our opinion this means that also on the nominally ‘‘perfect’’

faces of single crystals a relevant number of terraces (and hence steps) is still present.

This observation also implies that a clear cut between ‘‘surface science quality’’ re-sults obtained on single crystals and results obtained on sintered dispersed materials

cannot be made. This again suggests that the gap between the two fields can be filled

when appropriate experimental conditions are adopted.

5.2.2. The 2150–2145 cm�1 absorption

The absorption in this region is constituted by a strong component at 2150 cm�1

and by a weaker shoulder at 2140 cm�1. On passing from hsa sample (Fig. 13a) to the

sintered material (Fig. 13b) this absorption undergoes a strong decrement. Moreover

102 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

it is totally absent on smoke. As observed before, this behavior allows to assign it to

the stretching mode of CO adsorbed on a specific fraction of Mg2þ4c sites. From the

spectroscopic point of view, the attribution of this band to linear species adsorbed

through the carbon end is troublesome. In fact, being the frequency lower than that

of Mg2þ5c ðCOÞ species on (100) faces and only slightly higher than that of CO gas(even lower for the shoulder) it suggests that weaker polarization fields and hence

lower adsorption enthalpies are involved. This is however in contrast with the obser-

vation that these species are present at the lowest equilibrium pressures and with the

measured adsorption enthalpy (vide infra Section 8). To overcome this problem it

has been hypothesized [48,158,211] that this peculiar spectroscopic manifestation

can be explained in terms of CO adsorbed on Mg2þ4c pairs at monoatomic steps either

(i) through both the carbon and oxygen ends (parallel species: Scheme 1a) or (ii)

through the carbon end only (bridged species: Scheme 1b). On the basis of thecomputed �mðCOÞ, ab initio studies by Soave et al. [211] favors this second

interpretation.

The sequence of spectra of Fig. 13b shows that, upon increasing PCO, the 2150

cm�1 band gradually grows up to a maximum and then disappears at the highest

coverages. This behavior finds explanation only if it is assumed that CO is initially

adsorbed on pairs (one Mg2þ4c and one Mg2þ5c ) while, upon increasing the CO cover-

age, an additional CO molecule is adsorbed and each of the two magnesium sites

interacts with its own CO molecule. This results in the formation of ‘‘conventional’’linear complexes whose frequency can be hardly distinguished from that of the linear

Mg2þ4c ðCOÞ and Mg2þ5c ðCOÞ previously discussed (Scheme 1c). In conclusion the 2150

cm�1 band is the fingerprint of monoatomic steps (or of inverse edge sites of mono-

atomic height).

5.3. The Mg5c(CO) complexes on (100) terraces and facelets: comparison with the

results obtained on (100) faces of single crystals

The narrow and dominant peak observed on smoke in the 2157–2149 cm�1 inter-

val (Fig. 13c) is attributed to the m(CO) mode of molecules perpendicularly adsorbed

through the carbon end on 5-fold coordinated Mg2+ ions of (100) terraces and face-

lets. The same peak can be clearly observed also on sinteredMgO (Fig. 13b). On high

surface area material (Fig. 13a) this peak cannot be clearly observed because, at high

Mg

Mg

Mg

Mg

O

OO

O

O5c

4c

Mg

MgO

OMg

O

Mg

C

O

(a) (b) (c)

Mg

Mg

Mg

Mg

O

OO

O

O

Mg

Mg

O

O

Mg

O

Mg

5c

CO

4cMg

OMg

Mg

Mg

O

OO

O

Mg

O

O

Mg

Mg

O

Mg

5c

4c C

OC

O

Scheme 1.

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 103

h, its intensity is exceedingly high, while the low h spectra are dominated by the

bands of CO interacting with Mg2+ sites exhibiting a higher coordinative unsatura-

tion. As the same peak has been observed on the (100) face of MgO single crystals

by Heidberg et al. [93] (see Fig. 15b), this demonstrates that regular and extended

(100) faces and terraces are present on both samples. As discussed by Spoto et al.[163] this result is in total agreement with the HRTEM data and once more confirms

that the gap between the surface properties of high surface area oxides and the surface

properties of single crystals can be completely filled through the adoption of suitable

preparation and sintering methods (Fig. 15a). Furthermore, the full width at half

maximum of CO peak observed onMgO smoke (2.5 cm�1), is well comparable to that

Fig. 15. Comparison of the IR spectra of CO adsorbed at 60 K and equilibrium pressure ranging from

10�3 (bottom spectrum in part a) up to 60 Torr (top spectrum in part a) on MgO smoke (part a, adapted

from Ref. [163]: G. Spoto, E. Gribov, A. Damin, G. Ricchiardi, A. Zecchina, Surf. Sci. 540 (2003) L605,

with permission. Copyright (2003) by Elsevier.) and at constant pressure and variable temperature on the

(001) surface of UHV cleaved single crystal MgO (p-polarized spectra; part b adapted from Ref. [93]:

J. Heidberg, M. Kandel, D. Meine, U. Wildt, Surf. Sci. 331-333 (1995) 1467, with permission. Copyright

(1995) by Elsevier.).

104 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

observed on single crystals, indicating that the two type of samples expose faces and

terraces of comparable regularity and perfection. A significant fraction of surface de-

fects, interrupting the surface periodicity and causing the pronounced broadening of

the m(CO) peaks observed on several polycrystalline systems [11,162,216–218], is not

present on MgO smoke. Notice also that the intensity of the bands due to Mg2þ5c ðCOÞadducts on smoke is still very high, (0.9 in optical density for hmax), and that the ob-

served spectra are characterized by a remarkably good signal/noise ratio. This result

differentiates the IR data obtained in transmission experiments on finely divided

materials from those obtained by reflection on single crystals faces, where the optical

density is typically in the 0–0.01 interval, with a consequent lower signal/noise ratio.

The detailed comparison of the spectra obtained on smoke and on single crystals

(100) faces, evidenced in Fig. 15, indicates that the differences are negligible and that

both samples are of single crystal quality (although the peak intensity is two order ofmagnitude higher on smoke because of the higher number of surface atoms exposed

to the IR beam).

The negative shift of the peak with increasing PCO (in the 10�3–4 · 10+1 Torr

interval, Fig. 13b,c), is due to the building up of lateral dynamic and static interac-

tions between parallel oscillators as thoroughly discussed in Refs. [9–11,131,162,

163,216–218]. At the end of this process the surface is covered by an adlayer of par-

allel oscillators perpendicularly oriented with respect to the (100) plane and the cor-

responding peak must be considered as a collective vibration where all the COmolecules are undergoing an in phase stretching mode perpendicular to the surface.

At the highest h (PCO > 20 Torr) two narrow peaks at 2137 and 2132 cm�1 also ap-

pears on the low frequency side of the main band (smoke and sinteredMgO samples).

Simultaneously to the progressive growth of the doublet, the main band undergoes a

shift in opposite direction (+2 cm�1) being finally observed at 2151 cm�1. Following

Heidberg et al. [93] and Spoto et al. [163] these features can be explained by consid-

ering that further adsorption of CO on the vacant Mg2+ ions is necessarily accom-

panied by the building up of repulsive effects between the molecules. In fact,because of this repulsive interactions, a fraction of the CO molecules in the increas-

ingly dense layer assume a tilted orientation with respect to the surface. According to

Heidberg et al. [93] this explains the appearance of the two narrow peaks at 2137 and

2132 cm�1, which are due to the in phase and out of phase excitations involving the

two tilted oscillators present in the unit cell. In favor of this assignment is the exper-

imental evidence that the 2132 cm�1 band is the only one observed with the incident

beam polarized parallel to the surface plane [93]. This explanation does not appar-

ently explain why both bands are occurring at lower frequencies. As observed bySpoto et al. [163], the sequence of spectra illustrated in Fig. 13b,c can help to clarify

this problem. In fact, the appearance of the doublet is also accompanied by a distinct

inversion of the main peak shift with coverage. This means that the bands assigned

to vibrational modes with electric moment perpendicular to the surface are coupled

and must be considered together. On this basis, the baricenter of the two perpendic-

ular bands on one side and of the parallel band on the other side are positively

shifted with respect to the �mðCOÞ gas, in agreement with the accepted model of

CO adsorbed on positive centers [53,77,212].

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 105

6. The IR spectra of CO species formed at 60 K on low coordinated O2� sites:

comparison between ab initio and experimental results

6.1. Ab initio calculations on simple cluster models

As it is known from previous studies [9–11,128], the anionic species formed by

interaction of CO with O2� ions located at corners, edges and step sites originate

a complex IR spectrum whose detailed assignment has not been completely achieved

in the past. This failure has to be attributed to the exceedingly large variety of species

formed in the 100–373 K interval. Scope of this contribution is to obtain a more ad-

vanced analysis and assignment based on a new set of simpler IR spectra obtained

under controlled pressure at a temperature (60 K) where activated reactions are sup-

pressed (Fig. 14). Due to the still persisting complexity of the spectra obtained at 60K, we have taken the view to compare the experimental results with the spectra ob-

tained from quantum calculations performed on suitably chosen molecular models

of the oligomers. The adopted molecular models have been obtained by interacting

CO with a bare Na+O2�Na+ cluster, i.e. a cluster where the O2� is sufficiently basic

to simulate the reactive oxygen ions present at surface defects of MgO. Although the

limitations of this trivial cluster are evident when compared to the more realistic

structures used by Lu et al. [75], the advantages are also conspicuous because the

vibrational and thermodynamic properties of large CnO2�nþ1 species can be more easily

calculated and compared with the experimental results. The validity of the whole

procedure must be evaluated only on the basis of its ability to explain previously

unexplained IR details.

Fig. 16 presents the calculated infrared spectra for a series of CnO2�nþ1 species with

n comprised in the 1–5 range and the corresponding optimized structures. For sake

Fig. 16. Calculated vibrational spectra and optimized structures of Na+O2�Na+ Æ (CO)n models, with

n = 1�5 (unpublished).

106 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

of comparison also the spectrum of the free CO molecule is reported, resulting in a�mðCOÞ of 2213 cm�1 under the adopted computational method, to be compared with

the experimental value of 2143 cm�1.

The ‘‘carbonite’’ CO2�2 ion has a bent structure, chelating the Na+ ion (Fig. 16a). It

is stabilized by a remarkable binding energy: 218 kJmol�1 with respect to the isolatedNa+O2�Na+ and CO molecules. The dimer (Fig. 16b) has non-planar geometry. The

overall binding energy is 226 kJmol�1 only, which implies that the binding of the sec-

ond COmolecule to the carbonite is negligible. The trimer Na+O2�Na+ Æ (CO)3 on the

contrary (Fig. 16c) is a very stable molecule (BE = 385 kJmol�1) also in accord with

cited models [75]. For more than three CO molecules, the adducts may take different

structures, which are likely to be stabilized to a different extent by different surface

environments. For example, the tetramer can be an open chain (not shown), a four

membered ring (not shown) or a five-membered ring (Fig. 16d). Among the investi-gated tetramers, the latter structure is the most stable (BE = 245 kJmol�1), anyhow

less stable than the trimer. Considering the spontaneous planarity of the structures,

the extension to larger rings by insertion of the carbon atom of CO into the ring is

straightforward. It is remarkable that the pentamer (Fig. 16e) is again a stable adduct

(BE = 400 kJmol�1), while the hexamer (not reported for brevity) is not. Summariz-

ing, we observe that the addition of a CO molecule to a Na+O2�Na+ Æ (CO)n cluster is

favored when n is an even number and isoenergetic or disfavored when n is odd. Fol-

lowing Lu et al. [75] we have suspected that this low stability could derive from awrong spin state assumption. In the case of the dimer, we have repeated the calcula-

tions assuming a triplet ground state, but contrary to what observed by Lu et al., the

low stability was confirmed in our model.

The infrared spectra reported in Fig. 16 were obtained from the calculated fre-

quencies and intensities, arbitrarily assuming a Gaussian shape with 10 cm�1 width.

They will be discussed in the following paragraphs, together with the experimental

spectra.

6.2. CO2�2 ‘‘carbonites’’ species: doublet at 1316 and 1279 cm�1

The peaks at 1316 cm�1 (very sharp and intense) and at 1279 cm�1 (broader and

weaker) are one of the most important IR manifestations observed at lowest CO cov-

erages (see A doublet in Fig. 14). This result does not differ from that obtained at RT

and at 100 K [9,11,128,164]. The nearly immediate formation of these species (here-

after A species) at a temperature as low as 60 K indicates that the involved reaction,

Eq. (1), is substantially not activated. This experimental evidence well agrees with thehigh stability of the calculated adducts (218 kJmol�1, vide supra Section 6.1). In

agreement with Refs. [128,164] the two bands are assigned to the asymmetric and

symmetric stretching modes of the CO2�2 structure and find close analogy with the

spectra of the chelating form of the isoelectronic NO�2 species adsorbed on MgO

[219]. This assignment is confirmed by our calculations which predict a correct bar-

center of the IR modes and a correct relative intensity (see Fig. 16). Conversely, the

splitting between the asymmetric and symmetric stretching modes is overestimated in

this simple model. This is not surprising because on MgO the CO2� is interacting

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 107

with more than a two positive centers, a fact certainly having influence on its struc-

ture. It has been suggested [128] that these chelating ‘‘carbonite’’ CO2�2 species are

formed at 3-fold coordinated O2� sites belonging to the family of O2�3c ions located

on corners, following Scheme 2.

This assignment is in agreement with the results shown in Fig. 12c for the smoke

MgO sample, where the carbonite bands have intensity lower than the detection limit

owing to the very low O2�3c population (less than 0.1% of the surface anions).

6.3. (C3O4)2� trimeric species

Four doublets at (�m1 ¼ 2108, 2093 cm�1 strong), (�m2 ¼ 1566, 1545 cm�1 medium),

(�m3 ¼ 1376, 1355 cm�1 strong) and (�m4 ¼ 1166, 1157 cm�1 weak) appear simultane-

ously to the carbonite peaks, i.e. already at the lowest PCO. These four doubletsevolve together upon changing PCO. These peaks have been assigned [9,67,128] to

the stretching modes of trimeric C3O2�4 structures (hereafter C-type species) formed

by further reaction with CO following Eq. (3). The fact that two markedly separated

bands are present for each of the four modes has been interpreted as the proof of the

presence of two main families of slightly different trimeric C3O2�4 structures (C and

C 0). On the basis of the band intensity C and C 0 species represent about 60% and

40% of the C3O2�4 structures present on MgO hsa (represented as full and dotted line

quadruplets respectively in Fig. 14). In particular the four modes of the more abun-dant C family appear at: �m1 ¼ 2108, �m2 ¼ 1566, �m3 ¼ 1355 and m4 = 1157 cm�1. As for

C 0, the quadruplet is observed at �m1 ¼ 2093, �m2 ¼ 1545, �m3 ¼ 1376 and (m4 = 1166

cm�1). The reported frequencies refer to the lowest coverages. All components are

pressure independent in the low and medium PCO ranges, while they undergo a per-

turbation at the highest PCO, where the formation of polymeric species occurs, vide

infra Section 6.5. Components m1 and m2 appear at higher frequency in C complexes

than in the C 0 ones, while the opposite holds for modes m3 and m4. As for the most

intense m1 and m3 modes, a third weak component is visible at 2084 cm�1 (near to�m1) and 1398 cm�1 (near to �m3). The new components are tentatively assigned to

the m1 and m3 modes of a less abundant (less than 5% on hsa MgO) third family

C00 of trimeric C3O2�4 species. Being �m1ðC00Þ < �m1ðC and C0Þ and being

�m3ðC00Þ > �m3ðC and C0Þ the m2 and m4 components of C00 species are expected to appear

CO

Mg

Mg

Mg

Mg

O

OO

O

O

Mg

Mg

O

O

Mg

O

Mg

Mg Mg OO

3c

Mg

Mg

Mg

Mg

O

O

O

O

Mg

Mg

O

O

Mg

O

Mg

Mg Mg OO

(CO2)2-

Scheme 2.

108 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

below and above those observed for C and C 0 complexes. Owing to the much weaker

extinction coefficient of m2 and m4 modes (with respect to m1 and m3) and owing to the

much lower abundance of C00 species (with respect to C and C 0) m2 and m4 components

of C00 species are not observed.

The ketenic mojety present in these species explains the m1 and m3 (strong) compo-nents in terms of the symmetric and antisymmetric stretching modes of the C@C@O

group. The other two components m2 (medium) and m4 (weak) are supposed to be

mainly associated with the stretching modes of the remaining C–O and C–C groups

present in the same structure. This assignment is in agreement with previous ones

[9,67,75,128]. To support this picture new ab initio calculations have been performed

using a cluster approach (vide supra Section 6.1).

The calculated spectrum of the trimeric species (Fig. 16) shows five bands at 2122

cm�1 (strong), 1492 cm�1 (medium), 1368 cm�1 (strong), 1306 cm�1 (medium) and1161 cm�1 (weak). The presence of an additional mode is in clear conflict with the

previous assignment. Notwithstanding this fact, by comparing the frequencies and

the relative intensities of the IR bands reported in Fig. 14, ð�m1 � �m2 > �m3 > �m4Þ andI(m1) � I(m3) > I(m2)� I(m4), with those of the calculated spectrum (Fig. 16) an agree-

ment can be found assuming that the fifth component (of medium intensity in the

computated spectrum) lies between the experimental m3 and m4 bands (closer to m3).Indeed in this region an unassigned band of medium intensity is well present at

1324 cm�1, (see � in Fig. 14). The twin component, expected for the co-presenceof C and C 0 complexes, is not clearly observed at low PCO but it becomes evident

around 1318 cm�1 at medium PCO, when the strong asymmetric mode of the carb-

onite A moiety disappears. This new component will be labeled as m3a.Calculations allows to assign the band as follows: �m1 (calc.: 2122 cm�1, exp.: 2108,

and 2093 cm�1 strong) C5–O6 stretching; �m2 (calc.: 1492 cm�1, exp.: 1566, 1545 cm�1

medium) Os–C1–O2 antisymmetric stretching; �m3 (calc.: 1368 cm�1, exp.: 1376, 1355

cm�1 strong) C1–C3 stretching; �m3a (calc.: 1306 cm�1, exp.: 1324, and 1318 cm�1 med-

ium) O4–C3–C5 antisymmetric stretching; �m4 (calc.: 1161 cm�1, exp.: 1166, 1157 cm�1

weak) collective mode.

The C3O2�4 surface compounds described so far are the first oligomeric species ob-

served on the surface as result of a non-activated attack of CO on anionic basic sites.

As far as the coordination state of the O2� sites responsible for the formation of tri-

meric C species is concerned, we can only say that they are affected by sintering, sug-

gesting that C (C 0 and C00) species are likely formed on the edge and step sites. All

these IR manifestations are totally absent on MgO smoke, while they can be still

appreciated in the sintered sample, although much less intense (see low frequency re-gion of Fig. 12b for the m1 component).

At this stage the question may be raised why the formation of such trimeric C, C 0

and C00 species is not preceded by the formation of a transient monomeric ‘‘carbon-

ite’’ precursors and by dimeric intermediates. On the basis of the ab initio study (vide

supra Section 6.1) this experimental evidence is explained by the different variation

of the BE observed upon further CO addition. Evolution from monomeric to dimeric

species is almost isoenergetic, being DBE1!2 = 3 kJmol�1 only, while a considerable

energetic gain is observed upon adsorption of CO on the dimer: DBE2!3 = 164

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 109

kJmol�1. Summarizing, we are dealing with two types of monomeric families, the

former so reactive to escape detection and evolving immediately to trimeric species

while the latter (species A) is clearly observed at the lowest PCO (bands at 1316

and at 1279 cm�1) and is less prone to further CO insertion, evolving into more com-

plex structures only at higher PCO (vide infra Section 6.4). We think that the differentreactivity of the two types of monomeric precursors initially formed at low coordi-

nated O2� sites is related to the structure of their immediate surroundings (which has

influence on the stabilization of the oligomeric species). The limitation of our model

in considering the relations between the structure of the adsorbing sites and the

structure of the adsorbed species is clearly emerging here.

6.4. The evolution at 60 K of CO2�2 (A species) at intermediate PCO: formation of

dimeric C3O2�3

Upon further CO dosage at 60 K, see Fig. 14, the doublet at 1316–1279 cm�1

(carbonites A) disappears and a new triplet at 1635 cm�1 (medium), 1476 cm�1

(strong) and 1344 cm�1 (weak) grows in a parallel way. This triplet is peculiar of

experiments performed at 60 K, and it has not been observed in conventional IR

experiments, performed at around 100 K or at higher temperatures. These bands

can be tentatively explained in term of addition of further CO, on preformed A spe-

cies. We will hereafter label these species as D species.The calculated spectrum (Fig. 16) of the dimer results in a triplet of modes in the

investigated frequency region. The high frequency one occurs at 1544 cm�1, (exp.:

1635 cm�1 medium) and is due to the out of phase coupling between m(C1–O2)

and m(C3–O4). The intermediate one occurs at 1484 cm�1, (exp.: 1476 cm�1 strong)

and is due to the in phase coupling of the same modes. The lower frequency mode

appears at 1190 cm�1 (exp.: 1344 weak) and is ascribed to the coupling of m(Os–

C1) with m(C1–C2) and d(Os–C1–C2).

The agreement between experimental and theoretical spectra is less good than inthe case of the trimer, for two main reasons. Firstly, the nature of the adsorption site

(O2� on the MgO surface in the experiment and Na+O2�Na+ cluster in the model) is

expected to affect more the vibrations of the smaller adducts than those of larger

ones. Secondly, we are clearly dealing with two different dimeric species. The calcu-

lated one is an unstable transient species in presence of CO in the gas phase, like the

precursor of C, C 0 and C00 species. The experimentally observed dimer is conversely a

species which is stable in a large PCO interval. The stabilization of the experimentally

observed dimers can be due to particular local environments, like e.g. in the proxim-ity of corners or edges, not included in the model.

6.5. The evolution of the C3O2�4 trimeric species into polymeric entities at the highest

PCO

As the equilibrium pressure of CO is further increased, the bands of the trimeric

species (C, C 0 and C00) decreases simultaneously without disappearing completely,

see Fig. 14, while several new bands grow up. Among them the most intense are

110 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

observed at 1668 cm�1 (strong), 1580 cm�1 (very strong) and 1266 cm�1 (strong),

labeled ‘‘P’’ in Fig. 14. This process is reversible because a successive decrement of

the PCO restores the initial situation. This reversibility implies that both the enth-

alpy and the activation energy for the formation of polymeric species are modest.

The calculated instability of the tetramer (less stable than the trimer by 140kJmol�1 vide supra Section 6.1) suggests that the new bands observed are due to

pentamers or to oligomers of higher nuclearity. It must be underlined that the

agreement between the calculated and the experimental frequencies is quite poor.

This can be due to the limitations of the model (as underlined before) or, more

probably, to the presence of non-cyclic oligomers of the type hypothesized in

Ref. [128].

The polymeric CnOx�nþ1 species formed at 60 K are stable only in presence of high

PCO. This means that their stability is poor and that the process of CO insertionand release at 60 K is associated with a remarkably small activation energy. It is

worth mentioning that if, at constant PCO, the temperature of the system is in-

creased up to 100 K (not reported for brevity), the IR spectrum of adsorbed species

changes dramatically and becomes similar to that observed and already discussed in

previous contributions [11,128,157,158,164], vide infra Fig. 17. The species formed

Fig. 17. IR spectra of CO dosed at liquid nitrogen temperature (around 100 K) on an high surface area

MgO sample at two different equilibrium pressures: PCO = 1 Torr (full line); PCO = 4 Torr (dotted line).

These spectra are to be compared with the new ones, collected at 60 K (Figs. 12a, 13a and 14). Adapted

from Ref. [128]: A. Zecchina, S. Coluccia, G. Spoto, D. Scarano, L. Marchese, J. Chem. Soc. Faraday

Trans. 86 (1990) 703, with permission. Copyright (1990) by The Royal Society of Chemistry.

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 111

at higher temperature (100 K) are irreversible and cannot be removed by pumping.

This means that the formation of polymeric species at 100 K is associated to higher

activation energies. We hypothesize that this increment is due to an activated sur-

face rearrangement. In other words, in the 60 K experiment we have the opportu-

nity of studying monomeric, oligomeric and polymeric precursors of the polymericspecies stable at higher temperatures, whose spectra have been described in Refs.

[11,128,158]. As these species are formed without substantial activation barrier,

they directly reflect the surface topology. This result is well illustrating the utility

of the low temperature experiments and the extreme complexity of the processes

occurring at the surface of hsa MgO when a large temperature interval is

considered.

As a final observation, let us stress that the detailed description of these processes

is of high interest because it represents one of the best examples of how highly basicoxygen species present at defect sites can attack the relatively unreactive CO mole-

cule with formation of chemically interesting species. In other words they are good

examples for inspiring the design of new CO activation routes.

7. Comparison with literature results obtained at higher temperatures

7.1. CO on MgO: 100 K experiments

Fig. 17, from Zecchina et al. [128], reports the IR spectra of CO dosed at about

100 K on an high surface area MgO sample at two different PCO: 1 and 4 Torr, full

and dotted lines, respectively. These literature spectra are to be compared with the

new ones obtained at 60 K, see Figs. 13a and 14 for the high and low frequency

regions, respectively. Beside the remarkable improvement of the spectra quality,

the bands observed, at the highest PCO, at 1668 cm�1 (strong), 1580 cm�1 (very

strong) and 1266 cm�1 (strong) and labeled as ‘‘P’’ in Fig. 14 could not be observedin the previous experiments carried out at higher temperature (Fig. 17). This fact

confirms the attribution to polymeric CnOx�nþ1 species, with n = 4, 5 performed in

Section 6.5. In fact, the species with highest n values are found only at 60 K because

the CO coverage is larger than that obtained (at similar PCO) in the experiment per-

formed at 110 K. This confirms once more that in the 60–110 K interval reversible

polymerization–depolymerization processes are induced by increment–decrement of

PCO.

The effect of sintering, as monitored by IR spectroscopy of CO dosed at 100 K, isreported in Fig. 18, to be compared with the same experiment performed at 60 K

(Fig. 13). Also in this case, the improved quality of the spectra (in all parts) is evi-

dent. Moreover, the doublet at 2137 and 2132 cm�1, typical of the low temperature

IRAS spectra obtained on MgO(001) single crystals [93], can be observed (at high

PCO) on both smoke and sintered MgO samples only by lowering the temperature

down to 60 K. This means that the use of the new experimental set-up described

in Section 2.2.2 has been fundamental in definitively bridge the gap between single

crystals and powdered MgO materials.

Fig. 18. IR spectra, in the C–O stretching region, of CO dosed at liquid nitrogen temperature (around 100

K) on polycrystalline MgO samples with different surface area surface (increasing coverages). Part (a):

high surface area MgO (200 m2g�1). Part (b): sintered MgO (35 m2g�1); the apparently horizontally

shifted spectrum represents the high coverage spectrum obtained by dosing a 12CO/13CO (15/85) isotopic

mixture. Part (c): MgO smoke (10 m2g�1). These spectra are to be compared with the new ones, collected

at 60 K (Fig. 13). Adapted from Ref. [9]: A. Zecchina, D. Scarano, S. Bordiga, G. Ricchiardi, G. Spoto,

F. Geobaldo, Catal. Today 27 (1996) 403, with permission. Copyright (1996) by Elsevier.

112 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

7.2. CO on MgO: room temperature experiments

When the surface of highly dispersed MgO samples, carefully activated under high

vacuum at high temperatures (T P 1100 K), is probed by CO at room temperature

(RT) a very complex IR spectrum is observed in the 2200–1100 cm�1 range, which

has been the subject of many detailed investigations [48,126,128,131,157–159,164–166].

For short contact times the reaction path is represented by a series of steps already

discussed in section 1.2, see Eqs. (1)–(4), with formation of negatively charged mono-

meric, dimeric and polymeric (conjugated) species characterized by a very complex

IR spectra, see Fig. 19 where the evolution of the spectra of adsorbed CO with

decreasing surface area of the samples is also reported (spectra from a to d). The

CO2�2 (carbonite which has a bi-dentate structure) being the precursor of the dimeric

and oligomeric species, has a transient character and its concentration is maximumonly in the initial stages of the chemisorption process. Fig. 20 reports the effect of

increasing contact times (t) and PCO on the IR spectra of CO adsorbed at room tem-

perature on high surface area MgO. Parts (A) and (B) refer to low (PCO = 1 Torr)

and high (PCO = 8 Torr) equilibrium pressures respectively.

Fig. 19. Effect of sintering process on the IR spectra of CO adsorbed at room temperature on: high surface

area MgO (curve a); progressively sintered MgO samples (curves b and c); MgO smoke (d). Bands labeled

with dots (d) belong to oxidized carbonate-like groups; bands labeled with asterisks (�) refer to the

reduced counterparts. For a more detailed assignment, the reader is referred to the original article.

Adapted from Ref. [158]: S. Coluccia, M. Baricco, L. Marchese, G. Martra, A. Zecchina, Spectrochimica

Acta A 49 (1993) 1289, with permission. Copyright (1993) by Pergamon.

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 113

The surface species formed at RT derives only from the interaction of CO with theMg2þ3c cations and with the O2�

3c and O2�4c anions, while the remaining, more coordi-

nated surface sites are not active. The interaction of CO with Mg2þ3c cations located

on corners can be clearly seen in Figs. 19 and 20, where a peak at 2003 cm�1 is clearly

evident. The bands due to CO adsorbed on Mg2þ4c and Mg2þ5c sites are completely ab-

sent because the corresponding adsorption energies (vide infra Section 8) are too low

to allow a sufficient coverage of these sites at room temperature.

Coming to the anionic sites, the comparison between the species formed at low

temperature and those formed at RT is more complex and needs a detailed discus-sion. Some species are observed at liquid nitrogen temperatures only, other appear

exclusively in the RT experiments, while a third subset is visible in both type of

experiments, as discussed hereafter. Species, observed at room temperature only

are evident in Figs. 19 and 20B and have been labeled with asterisks and dots. As

the formation of these species occurs at room temperature and only after prolonged

contact times (Fig. 20B) they are associated with a substantial activation energy.

The C3O2�4 species discussed in the experiments at 60 and at 100 K are also clearly

visible at RT, where the C2O2�3 and the (CnO2n+1)

2� species with n = 4,5 are con-versely absent. From Fig. 20B it can be also observed that the CO2�

2 species, initially

formed, rapidly evolves into new entities labeled with asterisks and dots. Following

Zecchina et al. [38,128], this evolution is not the simple addition of CO to CO2�2 spe-

cies with formation of (CnO2n+1)2� species, but is a complex reaction giving rise to

oxidized (carbonate-like species: bands labeled with dots in Figs. 19 and 20B) and

reduced species (bands labeled with stars).

Fig. 20. Effect of increasing contact times (t) and CO equilibrium pressures (PCO) on the IR spectra of CO

adsorbed at room temperature on high surface area MgO. Part (A): PCO = 1 Torr and t=0 min (curve a), 2

min (curve b), 5 min (curve c), 15 min (curve d), 45 min (curve e), 80 min (curve f). Part (B): PCO = 8 Torr

and t = 0 min (curve a), 30 min (curve b) and 1440 min (curve c). Spectra collected at t = 0 min are to be

intended as immediately (few seconds) after CO contact. Symbols (d,�), as in Fig. 19. Adapted from Ref.

[128]: A. Zecchina, S. Coluccia, G. Spoto, D. Scarano, L. Marchese, J. Chem. Soc. Faraday Trans. 86

(1990) 703, with permission. Copyright (1990) by The Royal Society of Chemistry.

114 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

The reduced species are highly colored (as testified by the UV–Vis spectra re-

ported in Fig. 21) as they are resonance-stabilized and are characterized by an exten-sive p conjugation. These species are oxygen sensitive and can be oxidized to

Fig. 21. UV–Vis DRS spectra of CO chemisorbed at room temperature on an high surface area MgO

previously activated in vacuo at 1073 K. Spectrum 1: before CO dosage. Spectra 2–6 are collected after 10

min of contact at increasing PCO (0.06, 0.25, 3, 10 and 20 kPa). Spectrum 8 has been collected after 30 min

of contact time at PCO = 20 kPa. Adapted from [38]: A. Zecchina, F.S. Stone, J. Chem. Soc. Faraday

Trans. I 74 (1994) 2278, with permission. Copyright (1990) by The Royal Society of Chemistry.

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 115

carbonates by exposing the surface to O2. Note that the disproportionation reaction

leads to a small erosion of the UV-bands of O2�4c sites (Fig. 21). The formation of oxi-

dized and reduced species can be considered as a disproportionation reaction as de-

scribed here below:

2CO2�2 þ ðn� 1ÞCO ! CO2�

3 þ ðCOÞ2�n ð6Þ

Beside the erosion of the exciton components related to O2�3c and O2�

4c sites surface

sites (see Section 1.1), CO adsorption at RT causes the appearance of two well de-fined bands in the near UV, at 34,000 cm�1 (4.2 eV), and in the visible 21,500

cm�1 (2.7 eV). The former is due to the C3O2�4 trimeric species [38]. The latter, con-

ferring the pink color to the powder, is due to the ðCOÞ2�n oligomers [38]. Whether

this reaction is involving only the CO2�2 precursor species or also the CnO

2�2nþ1 oligo-

mers is not possible to ascertain. The disproportionation chemistry described in Eq.

(6) is appreciable only for samples characterized by a high concentration of basic O2�3c

sites, even if the additional participation of some O2�4c sites cannot be ruled out. This

is clearly shown in Fig. 19, where the spectra of CO adsorbed at RT on progressivelysintered samples and on MgO smoke are compared. From all the data collected at

116 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

60, 100 and 300 K, it is inferred that the formation of oligomeric structures, either of

the (CnO2n+1)2� or of the reduced (CnO2n)

2� type, is the manifestation of the pres-

ence of highly basic O2�3c sites. Comparison with the chemistry of the CO interaction

with the surfaces of the more basic CaO and SrO oxides (see next Subsection) totally

confirms this conclusion.The negatively charged polymeric and conjugated species have also been charac-

terized by EPR [133,220–222] and UV–Vis [13,38,223] spectroscopies. Due to the

extensive p-type conjugation, some of the oligomeric compounds are highly colored

[13,38,223] a fact which is directly visualizing the high reactivity of low-coordinated

O2� anions, see Fig. 21. The presence of a very strong interaction of CO with few

very reactive surface sites has been evidenced by Huzimura et al., [224,225] who have

observed the isotopic exchange between CO and MgO. Some of the species originate

from the interaction of CO with O2�3c pairs in corner position, a situation which is

likely associated with vicinal O2�3c ions of reconstructed (111) faces. For longer con-

tact times a disproportionation reaction also occurs, leading to oxidized (carbonate-

like) and to reduced CnO2�n species, as reported in Fig. 21.

7.3. CO on CaO and SrO: room temperature experiments

The chemistry of CO dosed on both CaO and SrO oxides has been studied by

means of IR spectroscopy by Coluccia et al. [27] and compared with that observedon MgO. It has been concluded that the CO interacts irreversibly with O2� surface

anions with formation of CO2�2 (carbonite) and oligomeric species and that a strong

electrostatic interaction between the negative species and the surface cations ac-

counts for the marked dependence of the IR signals upon the lattice parameter of

the solid aMgO < aCaO < aSrO (see Table 1). The increasing basicity along the series

(MgO < CaO < SrO) causes a marked increase of the total adsorptive capacity, an

increase of the relative population of negatively charged CO polymers with respect

to dimers and an increase in importance of a Bounduart-like reaction, see Eq. (5),upon desorption (leading to oxidized and reduced species).

8. The intensity of the stretching bands of CO adsorbed on 4- and 5-fold coordinated

Mg2+ ions as function of T at constant pressure: thermodynamic implications on the

CO bonding energy

As already detailed in Section 2.2.2, the experimental apparatus allows to collectthe spectra of adsorbed species working at constant equilibrium pressure by chang-

ing gradually the temperature. In this case the isobaric experiment (PCO = 60 Torr)

has been carried out on the MgO/CO system (hsa sample) in the 300–100 K interval.

The intensity (hereafter A) of bands due to linear (adsorption on edges or on poly-

atomic steps, see Section 5.2.1) and bridged (adsorption on monoatomic steps, see

Section 5.2.2) Mg2þ4c (CO) complexes and of the band due to linear Mg2þ5c (CO) com-

plexes (adsorption on regular faces or terraces, see Section 5.2.2) has been measured

as a function of the temperature (T). In all cases the intensities increase gradually

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 117

with decreasing T, reaching asymptotically a maximum at different T, which reflects

the saturation of the corresponding sites. The saturation allow us to know quantita-

tively, at any T, the fraction of sites covered by CO, defined as h(T) = A(T)/Amax,

and thus the fraction of empty sites [1�h(T)]. The equilibrium constant (Kads) of

the adsorption process for any given temperature can be described, under the Lang-muir approximation, as follows [113,117,163,167,203]:

Kads ¼ hðT Þ=ðð1� hðT ÞÞ � PCOÞ ð7ÞApplying the Vant–Hoff equation, the angular dependence of the ln(Kads) versus 1/T

gives the adsorption energy. This model, introduced originally by Paukshtis et al.

[113,117], has more recently been adopted to describe the interaction of adsorbed mol-

ecules on the external surfaces of oxides by Dulaurent [167], onMgO smoke by Spotoet al. [163] and on the internal surfaces of zeolites by Otero Arean et al. [203]. Fig. 22

shows the dependence of ln(Kads), for each species, plotted against 1/T. We observe a

surprisingly good linear correlation in all cases, reflecting the validity of the adopted

model. The slopes give the adsorption enthalpies, which are very close for CO ad-

sorbed on edge species: 21.9 and 22.6 kJmol�1 for the bands due to CO absorbed

on Mg2þ4c sites (linear and bridged cases). As for the band due to CO adsorbed on reg-

ular (100) terraces, the measured adsorption enthalpy is much lower: 12.5 kJmol�1.

The energy associated with the species linearly absorbed on both Mg2þ4c and Mg2þ5csites fit the characteristic enthalpy/frequency correlation [11,226,227] of the CO spe-

cies linearly adsorbed through the carbon end on non-d, d10 or d0 cationic sites, see

open star and triangle symbols respectively in Fig. 23, data taken from Refs. [11,226–

231]. Conversely, the species absorbing at 2150 cm�1 (open circle symbol in Fig. 23)

0.004 0.005 0.006 0.007 0.008 0.009 0.010

-2

0

2

4

6

- Hads

= 12.5 ± 0.1 kJ/moland - H

ads= 22.6 ± 0.8 kJ/mol

band due to linear CO on Mg4c

2+

band due to bridged CO on Mg4c

2+

band due to linear CO on Mg5c

2+

Ln

Kad

s

1/T (K-1)

Fig. 22. The dependence of the ln(Kads), denoted in Eq. (7), on the 1/T for CO adsorbed on Mg2þ4c in linear

form (w– symbols), bridged form (s – symbols) and on Mg2þ5c (M – symbols). Unpublished results

collected on MgO hsa sample. The preparation of an extremely thin pellet has been a determinant point in

this experiment, to maintain the intensity of the IR bands within 1.5 a.u., which guarantees the linear

response of the technique.

0 20 40 60 80 100

-20

0

20

40

60

80

100

(CO

) (c

m-1)

-ads

Ho (kJ mol -1)∆

∆ν

Fig. 23. C–O stretching frequency shift vs. molar standard enthalpies of adsorption for CO adsorbed on

various d, d0 and d10 metal ions hosted on different oxidic surfaces. Full squares data have been taken from

refs: [11,226–231]; open circle, triangle and star are the data obtained from the temperature dependent IR

study summarized in Fig. 22 where the same symbols have been adopted (unpublished data).

118 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

is anomalous, showing a much higher DHads with respect to the observed D�mðCOÞ.This is due to the fact that CO molecules contributing to this component do not

interact with a single cationic site, as hypothesized in parts a or b of Scheme 1.

Unfortunately the intensity of the 2203 cm�1 band, due to Mg2þ3c (CO) complexesformed on corners is not changing substantially in the 60–300 K interval upon PCO

because the corresponding sites are almost fully saturated even at room temperature

and higher temperatures are required to modify the coverage. We are so unable to

extract from our data the DHads for the Mg2þ3c ðCOÞ complexes. On the basis of the

linear correlation between DHads and D�mðCOÞ values, from the fit reported in Ref.

[11], the adsorption energy of CO on Mg2þ3c sites is estimated to be 63 kJmol�1. Sum-

marizing, IR spectroscopy at variable T allows to estimate separately the DHads val-

ues of individual species. This result differentiates this techniques from calorimetrywhich always give average values for all species.

In a, recently published, previous work Spoto et al. [163] performed a similar,

temperature dependent IR study on a MgO smoke sample, deriving an adsorption

energy for CO on regular Mg2þ5c sites of 11 kJmol�1. We believe that the discrepan-

cies between the two experiments (here we have obtained +12.5 kJmol�1) is related

to the errors in the calculations of the integrated area of the CO bands and has to be

considered within the accuracy of this method. The new results described in this Sec-

tion will now be compared with previous experimental and theoretical results in Sec-tions 8.1 and 8.2 respectively.

8.1. Comparison with CO bonding energies obtained by TDS on single crystals and with

other experimental results

In 1999 the group of Freund in Berlin reported an accurate TDS study on the

interaction of CO and NO molecules on MgO(100) and NiO(100) surfaces obtained

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 119

by UHV cleavage of single crystals [97,169]. As for NiO also thin films grown by the

oxidation of Ni(100) surface have been investigated. The experimental results ob-

tained on the CO/MgO(100) system are reported in Fig. 24 and can be summarized

as follows. For small coverages, the data exhibit a desorption peak in the region

around T = 57 K which corresponds to a bonding energy of about 0.13 eV (12.5kJmol�1) as estimated by Freund et al. according to the Redhead model [232].

On one hand, the temperature T = 57 K of the TSD study reported in Fig. 24, well

agrees with the temperature-resolved LEED study, performed down to 30 K on

MgO(100) single crystal surfaces cleaved in situ, by Audibert et al. [87]. Authors

found that in the 30–40 K interval CO forms a 2 · 4 commensurate bi-dimensional

solid phase. A sharp uniaxial transition occurs above this temperature, along the [10]

surface direction which locks the monolayer into a new commensurate 2 · 3 phase

stable over a temperature range of 8 K. Above 50 K, this second commensurate

Fig. 24. Thermal desorption spectra of CO on MgO(100) cleaved in UHV performed with an heating rate

of 0.2 K/s being the mass spectrometer was set to the mass of the CO molecule (28 amu). The coverage

values h are given relative to the coverage of a full monolayer. Adapted from Ref. [97]: R. Wichtendahl, M.

Rodriguez-Rodrigo, U. Hartel, H. Kuhlenbeck, H.J. Freund, Surf. Sci. 423 (1999) 90, with permission.

Copyright (1999) by Elsevier.

120 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

phase expands itself uniaxially in a sharp transition toward a solid with disorder

increasing with temperature. No further ordered CO superstructures have been ob-

served by Audibert et al. at temperatures higher than 55 K [87].

On the other hand, as far as the CO binding energy with regular Mg2þ3c sites is con-

cerned, our datum of 12.5 kJmol�1 (extrapolated from the temperature dependentIR data on sintered sample, as described in Section 8) is in impressive agreement with

the value of 0.13 eV (12.54 kJmol�1) previously estimated by Freund et al. [97,169]

with TDS experiments and with the value of 11 kJmol�1 found by us with the same

method applied to MgO smoke [163]. Comparable values (in the 13–22 kJmol�1

range) have been reported by Zaki and Helmut Knozinger [126] and by Furuyama

et al. [110], both working on MgO powders. These values, obtained with independent

techniques, disagree with that obtained by the group of Goodman (41.4 kJmol�1) on

few monolayers thick MgO films grown on a Mo(100) surface [88,89] estimatedaccording to the isothermal adsorption method. This disagreement can be partially

understood by considering that Goodman et al. investigated the interaction of car-

bon monoxide on MgO/Mo(001) in the 100–180 K range, observing by IRAS�mðCOÞ values at 2178 cm�1 and at 2201 cm�1, which are close to those observed

by us for CO adsorbed on Mg2þ4c (2170 cm�1) and on Mg2þ3c (2203 cm�1) sites and def-

initely much higher than that observed on regular Mg2þ5c sites (2157 cm�1, vide supra

Section 5). Notwithstanding these consideration the value of 41.4 kJmol�1 is still

about twice that obtained by us for CO adsorbed on Mg2þ4c (22.6 kJmol�1) or for thatattributed to CO molecules bridged on a monoatomic step (21.9 kJmol�1 and 2150

cm�1, see Scheme 1) and can be tentatively ascribed to CO adsorbed on Mg2þ3c corner

sites.

Summarizing all these data, we can conclude that the values reported by Good-

man et al. [88,89] can be compatible with those reported by Freund et al. [97,169],

by Audibert et al. [87] and by us here only assuming that the few monolayers thick

MgO/Mo(001) films prepared by the Goodman group are characterized by a non-

negligible density of surface defective sites, which dominate the process of COadsorption in the relatively high temperature (100–180 K) and low pressure ranges

investigated in works [97,169].

In this regard, also the earlier work of Henry et al. [233] has to be commented.

This work is focused on the characterization of the physisorption process of CO mol-

ecules on a vacuum cleaved MgO(100) single crystal, supporting epitaxially grown

Pd particles, however also the interaction of CO with the bare MgO surface (prior

to Pd growth) has been briefly discussed. The adsorption rate of CO on the Pd par-

ticles is measured, at zero coverage by a molecular beam technique, as a function ofthe substrate temperature (in the 400–540 K interval) and for different particle sizes.

A kinetic model describing the adsorption, desorption, diffusion, and capture by the

metallic clusters of CO molecules is given. Comparison with the experimental data

gives an adsorption probability of 0.5 ± 0.05 and a saddle energy for surface diffu-

sion: 0.25 ± 0.05 eV for CO molecules on MgO. The angular distribution and the

time evolution of CO scattered from the MgO substrate at 553 K as well as the

adsorption rate of CO on the MgO(100) surface have been measured. Authors con-

cluded that the adsorption energy of CO on MgO is smaller than 0.4 eV (38.6

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 121

kJmol�1). The results of this experiment have been sometime mis-interpreted by

other authors in the successive years as they attributed the value of 0.4 eV (38.6

kJmol�1) to the binding energy of CO on regular Mg2þ5c sites, while IR experiments

performed on powdered materials have shown since several years

[48,126,128,131,157–159,164–166] that at such temperatures CO can interact withdefective sites only (vide supra Section 7.2).

8.2. Comparison with CO bonding energies obtained by ab initio calculations

Cubic oxides have been important not only for the experimental studies but also

for the role of case systems that they have played in theoretical works. In fact, the

high symmetry of adsorbing sites on regular (001) faces, has made such solids ideal

systems for ab initio calculations with both cluster and periodic approaches. Amongthem, the ‘‘case study’’ role played by MgO is explained by considering that among

all the binary oxides with cubic structure (all strongly basic and with definite ionic

character) MgO exhibits the smallest number of electrons in its asymmetric unit,

the lighter BeO crystallizing in the hexagonal structure [234]. Even when focusing

the attention on the MgO/CO system only, the theoretical literature is still very abun-

dant. Therefore only a selection of the seminal works and of the most recent high-

lights will be made here. For an exhaustive description of the theoretical works

published so far, the readers are referred to the important reviews of Colbourn[55], Sauer et al. [235] and to the very recent one by Pacchioni [76]. In this regard,

please note that a brief, but authoritative overview of the whole subject has appeared

in 1999 [72].

To face the problem of CO adsorbed on regular Mg2þ5c sites with ab initio meth-

ods, early studies [49,50,236] used the cluster approach. In such studies, the com-

puted binding energies (around 40 kJmol�1) were systematically overestimated

with respect to the correct experimental value because of the intrinsic limitations

of the bare cluster approach (due to the boundary and size effects greatly affectingMadelung potential).

8.2.1. Interaction of CO with regular Mg2þ5c surface sites

With the aim to overcome the limitations of the bare cluster approach, Pacchioni

et al. have performed a Hartree–Fock study on a (MgO5)8� cluster embedded in a

large array of fixed point charges [237] of nominal value (± 2 jej) mimicking the

MgO(001) surface [53,54,238] and giving a converging Madelung potential in the re-

gion where CO is bonded, i.e. 2–4 A above the Mg(001) surface. Constrained SpaceOrbital Variation (CSOV) scheme was adopted as a technique for portioning both

the BE and the C–O frequency shift. The analysis revealed that electrostatic and Pau-

li exchange repulsion where the main two contributions, whereas the charge transfer

was negligible. The obtained BE ranged between 23 and 32 kJmol�1, at HF and CI

levels, respectively. The computed C–O blue shift was rather high (D�mðCOÞ = +31

cm�1).

The ideal way to overcome the problem related to a correct evaluation of the

Madelung potential is the use of a periodic approach. In this regard, the Theoretical

122 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

Chemistry Group of the University of Turin (I) in collaboration with the Computa-

tional Group of the Daresbury Laboratories (UK), has developed the CRYSTAL

code [239]. The first versions of this software were based on the periodic Hartree–

Fock (HF) approach. Using this code, they have performed a systematic and detailed

study on magnesium oxide: the bulk was studied first [240,241], than the bareMgO(001) surface [242] and finally the interaction of CO [51]. Successively, the same

approach was extended to the study of the MgO(110) surface, before [243] and after

interaction with CO [244]. The CO binding energies obtained with these periodic

studies are 17 and 26 kJmol�1 for the MgO(001) and MgO(110) surface respec-

tively. In relation to this, it is worth recalling that Pisani et al. have proposed the

‘‘perturbed cluster’’ approach, as in the EMBED code [245–249], aimed to describe

with high precision a MgO cluster containing the adsorbed CO molecule, embedded

in a bi-dimensional infinite MgO hosting slab [52], which is able to take into accountthe Madelung corrections. Nowadays it is evident that the results reported in Refs.

[51,52,244] cannot be trusted on a quantitative ground owing to the modest compu-

tational resources available at the end of the eighties: calculations were run at the HF

level only, with rather poor basis sets, particularly for the description of the CO mol-

ecule. Notwithstanding these fact, papers [51,52,244] have represented a pioneering

example of the application of periodical codes to the study of adsorption phenomena

at surfaces, and the main conclusions can be summarized as follows: (i) The BE

ranges between 18 and 33 kJmol�1; (ii) CO can be adsorbed on Mg2+ sites throughboth carbon and oxygen ends, being the former slightly preferred; (iii) No charge

transfer occurs between the molecule and the surface; (iv) CO polarization is sizable.

More than 10 years after the appearance of the first periodic studies on the topic

(1998), Chen et al. reported a periodic study based on plane wave basis, using an

LDA Hamiltonian in the context of the full potential linearized augmented plane

wave (FLAPW) method [250]. Authors found a BE of 27 kJmol�1, accompanied

by a blue shift of the C–O stretching frequency as high as +33 cm�1. As clearly

pointed out, in a comment published one year after (1999) and signed by the mostauthoritative theoretical scientists [72], such high values are the consequence of the

well known inadequacy of LDA to compute the BE for intermolecular complexes,

which are usually grossly overestimated. In 2000, Snyder et al. [251] reports a peri-

odic density functional LDA and GGA study performed with a Gaussian basis sets,

using the NWChem code [252]. As expected, calculations at the LDA level were una-

ble to reproduce the experimental values, resulting in a BE of 30 kJmol�1 and in a

batochromic frequency shift of �5 cm�1. Conversely, calculation at the GGA level

obtained reasonable values: BE = 8.0 kJmol�1 and D�mðCOÞ = +4 cm�1.Coming back to 1994, Pacchioni et al. [61] compared the SCF results for CO

adsorbed on several clusters of different size with those obtained with CRYSTAL

code, showing that by using neutral clusters of increasing size [(MgO)13 and

(MgO)21] the CO binding energy decreases rapidly and becomes comparable to the

values obtained with CRYSTAL code. The same holds for the computed Dm(CO)

which drops down from +31 cm�1 to a more reasonable values of few cm�1. Nygren

et al. [62] reports an exhaustive study on the CO/Mg(001) system, computing the CO

interaction energies as a function of the cluster size, (from a single Mg2+ up to

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 123

Mg14O5) by using ab initio model potentials (AIMP) to embed the cluster in a point

charge distribution. This approach is able to avoid the huge polarization of the

atoms at the border of the quantum zone due to the unscreened charges, typical

of the first works of Pacchioni et al. [53,54,237,238]. The obtained BE values (6–9

kJmol�1) have been considered by the authors as significantly underestimated withrespect to the experimental values reported in the works of Goodman�s group

[88,89]. In the light of the discussion performed at the end of Section 8.1, the BE val-

ues estimated by Nygren et al. [62] are only slightly underestimated. In a successive

work Nygren and Pettersson obtained a remarkable agreement with the experiment

as D�mðCOÞ values as good as 10 cm�1 [66]. Finally, also the group of Illas. [64] has

investigated the CO/MgO system, with a point charge embedded cluster approach,

concluding that the interaction is weak (BE = 0.17 eV, i.e. 16.4 kJmol�1) and mainly

of electrostatic origin.Local density functional approximation (LDA) formalism has also been applied

to the cluster models, giving a somewhat different description of the bond

[57,59,70,253–255]. The first LDA studies by Neyman and Rosch, found a rather

high BE (35–54 kJmol�1) and a non-negligible charge transfer from CO to the sur-

face Mg2þ5cus (about 0.1jej) [57,253,254]. In a successive work [65] the same group

underlined that the well known differences between the local density approximation

and the exchanged correlation functional [256] are probably the cause of the over-

estimation of the BE found at the level of the theory adopted in the previous works[57,253,254]. To overcome this problem they have performed an improved density

functional study employing a gradient-correlated potential [65]: in this way the

CO binding energy dropped to about 10 kJmol�1, in much better agreement with

the experimental results. Despite these improvements the method gave a still rather

high estimation of the frequency shift (D�mðCOÞ = +34 cm�1). Neyman and Rosch, in

a joint paper with Pacchioni [61], report a detailed discussion about the origin of the

differences between the results obtained with Hartree–Fock and LDA approaches.

Among the whole set of theoretical results summarized above only the more re-cently reported BE values were corrected by the basis set superposition error (BSSE)3

[208]. As far as periodic approaches are concerned, also the surface coverage must be

considered. These facts must of course be considered when the BE coming from dif-

ferent studies are compared. Table 2 reports, for the most refined works published so

far, the BE and the D�mðCOÞ. For comparison, the experimental reference values, dis-

cussed in Section 8.1, are also reported.

In the upgraded version named CRYSTAL-98, the periodic code developed in

Torino was able to run calculations at both HF and DFT levels [249,257]. Exploitingthese new capabilities, in 2001 Damin et al. [77] reported a periodic study, at both

HF and B3-LYP [204–206] levels, of the interaction of CO on the perfect

Mg(001) surface as a function of the CO coverage. Two different basis sets A and

B (being the latter the more accurate one) have been adopted in the calculations per-

formed at the B3-LYP level (see the original work for more details). With the aim to

single out the CO–CO lateral interactions, three different coverages have been inves-

tigated: 1 · 2 (CO · Mg); 1 · 4 and 1 · 8 [77], see Fig. 25. The most diluted system

(1 · 8) could be studied with the less accurate A basis set only. Authors reached

Table 2

Selection of theoretical works investigating the interaction of CO on regular Mg2þ5c of MgO(001) surface

(Panel A); Selected experimental works (Panel B)

Approach Method Coveragea BE

(kJ/mol)

D�mðCOÞ(cm�1)

First author

(year) [Ref.]

Panel A

Cluster B3-LYP 1.0b �7 Pacchioni (2000) [211]

Embedded cluster B3-LYP 5.8b +15 Xu (2003) [277]

Embedded cluster MCPF-AE 7.7b +7 Nygren (1994) [62]

Embedded cluster CI-AE 30.9 + 43 Pacchioni (1992) [54]

Periodic LDA-FLAPW (1 · 1) 27.0c +33 Chen (1998) [250]

Periodic LDA-AE (1 · 2) 30.9b �5 Snyder (2000) [251]

Periodic PBE96-AE (1 · 2) 8.0b +4 Snyder (2000) [251]

Periodic B3-LYP/A (1 · 2) 1.2b,d +6 Damin (2001) [77]d

Periodic B3-LYP/A (1 · 4) 2.6b,d 0 Damin (2001) [77]

Periodic B3-LYP/A (1 · 8) 1.9b,d �1 Damin (2001) [77]

Periodic B3-LYP/B (1 · 2) 0.4b,d +8 Damin (2001) [77]

Periodic B3-LYP/B (1 · 4) 3.3b,d +5 Damin (2001) [77]

ONIOM2/periodic MP2:B3-LYP (1 · 4) 12.7b –e Ugliengo (2002) [81]

Panel B

TDS (exp.) – – 12.5 +14 Freund (1999) [97,169]

T dependent IR (exp) – – 11.0 +13.5 Spoto (2003) [163]

T dependent IR (exp) – – 12.5 +14 Spoto (2004) this work

a The coverage applies to periodic studies only.b BSSE corrected BE values.c BSSE correction not needed in the FLAPW method.d Damin (2001) et al. [77] performed periodic calculations at the B3-LYP level using two different basis

sets A and B, being the latter the more accurate one. The most diluted system (1 · 8) could be studied with

the less accurate basis set A only. See the original work for details.e No frequency calculations has been performed in the ONIOM2/periodic study of Ugliengo and

Damin [81].

124 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

the following conclusions: (i) The surface reconstruction is negligible; (ii) The electric

field in the CO bonding region is almost insensitive to the thickness of the MgO(001)

slab as it is the resulting surface energy; (iii) The electric field outside the MgO sur-face computed at HF level is indistinguishable from that at B3-LYP level; (iv) The

BE is very small and for its correct evaluation the BSSE should be correctly ac-

counted for, being almost half of the uncorrected BE value; (v) Electron correlation

is important for reducing the exchange repulsion which, in turn, allows a tighter con-

tact between the surface and the adsorbate; (vi) The trend of the computed C–O

stretching frequencies as a function of the dilution of CO on the MgO(001) surface

(1 · 2)! (1 · 8) was not able to reproduce the experimental blue shift; this holds

also for the calculations performed at HF level (not reported in Table 2); (vii) TheBE computed with the most accurate level (B3-LYP/B), on the (1 · 4) coverage, is

3.3 kJmol�1, a value that results from a delicate balance between the attractive elec-

trostatic interaction and the repulsive exchange contribution, playing both the polar-

ization and the charge transfer contributions a negligible role. Damin et al. [77]

concluded their work by observing that the CO/MgO(001) BE, computed at the

Fig. 25. Pictorial view of the CO/MgO(001) surface at different CO coverages. Ionic radii have been used

for the sphere representing Mg2+ and O2� ions, and van der Waals radii for CO. The unit cell a is based on

the experimental Mg–O distance of 2.109 A. Adapted from. Ref. [77]: A. Damin, R. Dovesi, A. Zecchina,

P. Ugliengo, Surf. Sci. 479 (2001) 255, with permission. Copyright (2001) by Elsevier’’.

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 125

most refined level of theory so far published, is still a value which is considerably

underestimated with respect of the experimental value of 12.5 kJmol�1 found bythe Freund group [97,169]. Damin et al. [77] suggested that this discrepancy should

be due to dispersive interactions that cannot be accounted for by the adopted B3-

LYP level of theory [258–260]. Among more refined methods [261–263] (MP2,

MP3, MP4, MP5, CI, CCSDT etc.), the less computationally expensive MP2 level

[261], is able to account for most of the dispersive interaction, as documented in

the study of the Ne–Ne dimer.

The crucial point is that, at the moment, the MP2 level of theory is not generally

available in quantum mechanical periodic programs, even if Scuseria and coworkers[264] have recently reported promising MP2 results for periodic systems obtained

with a development version of the GAUSSIAN suite of programs [207]. In 2002,

Ugliengo and Damin [81] proposed a computational recipe based on the ONIOM2

scheme for molecules [265,266] or to the closely related QMpot method [267,268] for

crystalline materials.

It has been shown that the ONIOM2 method can be successfully employed in the

study of physisorption processes at different surfaces [269–273]. The ONIOM2

scheme allow to treat at high level only a central model zone of the system, beingthe remaining part of the system treated at lower level of theory. The total energy

of the embedded system, E, is obtained from three distinct self consistent field energy

calculations which are combined according to:

E ¼ EHighðmodel zoneÞ þ ½ELowðwhole systemÞ � ELowðmodel zoneÞ�; ð8Þ

being in that case: (i) ELow (whole system) the energy of MgO/CO system computed

with the periodic approach of CRYSTAL-98 code at the B3-LYP level [204–206],

with a 1 · 4 surface coverage; (ii) ELow (model zone) the energy of the model zone(the Mg9O9/CO cluster), computed at the low level of theory (B3-LYP); (iii) EHigh

(model zone) the energy of the model zone computed at the high level of theory

(MP2 [261]). See Fig. 26 for a representation of the whole system and of the model

Fig. 26. Pictorial view of the real Mg(001)/CO (left) and Model Mg9O9/CO (right) systems. Ionic radii

have been used for the spheres representing Mg2+ and O2� ions, and van der Waals radii for CO. For the

Real system the unit cell borders have been highlighted. Adapted from. Ref. [81]: P. Ugliengo, A. Damin,

Chem. Phys. Lett. 366 (2002) 683, with permission. Copyright (2001) by Elsevier.

126 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

zone, left and right part of the Figure, respectively. Of course the same approach hasbeen used to model the bare MgO(001) surface too.

The scheme adopted by Ugliengo and Damin [81] allows to correct the periodic

B3-LYP binding energy for the dispersive contributions by means of an MP2 calcu-

lation [261] run on the Mg9O9 and Mg9O9/CO clusters. Once dispersive interactions

have been taken into account, the obtained BE increases to 12.7 kJmol�1 (being the

dispersive contribution of 6.6 ± 1.2 kJmol�1), a value which is now in excellent

agreement with the experimental value of 12.5 kJmol�1 found by the Freund group

[97,169] and by us in this work.

8.2.2. Interaction of CO with Mg2þ4c and Mg2þ3c defective surface sites

As far as the interaction of CO with edge, step and corner sites is concerned, we

mention the pioneering contribution of Colbourn et al., [274] who extended previous

calculations concerning carbon monoxide adsorbed on regular Mg(001) surface site

[49,50,236,275] (based on cluster approach) to CO adsorbed on Mg2þ4c , and Mg2þ3csites. Despite the small cluster size, a significant increase of the binding energies

on passing from Mg2þ5c � � �CO, to Mg2þ4c � � �CO to and Mg2þ3c � � �CO was obtained.The same philosophy was followed about 10 years later by Pacchioni et al. in Ref.

[54], to study the adsorption of CO on Mg2þ4c , and Mg2þ3c sites, using (MgO4)6� and

(MgO3)4� clusters respectively, both embedded in a large array of fixed point charges

suitably chosen in order to stabilize the Madelung potential. In agreement with the

experimental results, a consistent increase in both CO binding energy and of the

D�mðCOÞ was computed on passing from 5- to 4-, to 3- coordinated Mg2+ sites.

The same trend was also found by Neyman and Rosch [59]. The interaction between

a defect cluster of (MgO)3 and a CO molecule is studied by using the ab initio molec-ular-orbital method by Matsumara et al. [276]. The calculated energetics demon-

strate that the CO molecule can be trapped stably in irregularities such as steps,

taking the intermediate form of MgCO3 species. Authors proposed that CO oxida-

tion is caused when a low-coordination oxygen ion is removed from the step sites.

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 127

Successively, Pelmenschikov et al. [214] have reported a computational study on bare

Mg4O4 and Mg6O6 neutral clusters which simulate Mg2þ3c and Mg2þ4c sites respectively.

In a subsequent paper [71], the same authors studied the effect of the cluster size:

CO � � �MgnOn clusters (n = 6, 10, 12 and 20) for Mg2þ4c sites and CO � � �MgnOn clus-

ters (n = 4,10,13 and 19) for Mg2þ3c sites have been considered. In these MgO clustersonly the position of the adsorbing Mg2þ4c (or Mg2þ3c ) ion was optimized, while the

remaining Mg and O atoms have been fixed to their bulk positions. The authors

found that the computed CO binding energy and C–O frequency shift are nearly

independent on the cluster size, and are reproducing in a rather good way the exper-

imental values. Finally, the adsorption of CO on regular and defect sites of

MgO(001) surface has been studied by Xu et al. [277] using embedded cluster models

by DFT/B3LYP method. The value of embedded point charges is determined by the

charge self-consistent technique. The calculated results indicate that CO adsorp-tion energy on the regular site of MgO(001) surface can agree well with the recent

experimental data. The frequency shifts of CO for regular 5-coordinated terrace,

low 4-coordinated edge and 3-coordinated corner sites, via C bound down on cati-

onic centers of MgO(001) surface are: +15, +35 and +66 cm�1, respectively, in good

agreement with the experimental values (+14, +27 and, +60 cm�1 respectively).

From the description made so far concerning the evolution with time of the

accuracy of the computational methods applied to the CO/MgO(001) system, it

can be safely concluded that MgO has played a fundamental role in favoringthe improvements of the theoretical methods applied to surface science and that sys-

tems with rock-salt structure represent an ideal playground for this type of

calculations.

9. Conclusions

More than 30 years of studies on the surface science properties of MgO have beenreviewed. Starting from the pioneering works of the seventies, reporting (for high

surface area samples) the first examples of spectroscopic evidences of surfaces states

and their reactivity, we progressively move to the more recent results where complex

UHV experiments have been devoted to the study of the interaction of probe mole-

cules on clean MgO single crystal surface or on, in situ grown, thin MgO films. Due

to its ability to interact with both Mg2+ and O2� surface sites, carbon monoxide has

been chosen as case study probe molecule, while IR spectroscopy has been selected

as key technique to investigate in situ the CO/MgO interaction.Reporting temperature and pressure controlled IR experiments on fully dehy-

drated polycrystalline MgO samples, characterized by surface areas in a range as

wide as the 400–10 m2g�1 interval, which are systematically compared with experi-

mental and theoretical literature data, we have been able to show that the gap be-

tween single crystal or thin films (typical of ‘‘pure’’ surface science) and highly

dispersed powders (typical of catalysis) can be progressively bridged. The tremen-

dous complexity of the IR spectra obtained by dosing CO on high surface area sam-

ples is progressively reduced by decreasing the surface area of the MgO samples. The

128 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

apparent simplicity of the vibrational spectra observed on single crystals is well com-

parable to that obtained on MgO smoke samples.

Although the interaction of CO molecule with regular Mg2þ5c cationic sites of the

MgO(001) surface was thought to be a classical and simple example in Surface Sci-

ence, the agreement of C–O stretching frequency and the binding energy of theMg2þ5c � � �CO adduct obtained by (i) experiments on powdered materials; (ii) experi-

ments on thin films or single crystals and (iii) ab initio calculations, has been reached

in the years 2000 only.

To confirm or revise literature data on the CO/MgO systems, this review work has

been complemented by unpublished TEM and IR spectra of the CO/MgO system at

increasing PCO, collected at 60 K on powdered MgO samples with different surface

area: 10, 40 and 230 m2g�1. The evolution of the size and morphology of the crystals

upon the Mg(OH)2!MgO transformation and successive outgassing and sinteringtreatments in vacuo has been investigated by HRTEM. It is concluded that in high

surface area materials the aggregates are constituted by interpenetrated cubelets

characterized by the presence not only of (100) terraces, edges and corners but also

of inverse step and corner sites. These structures can explain most the peculiar

adsorptive properties of high surface area MgO. Upon CO dosage formations of

Mg2+ � � � (CO) adducts on 3-, 4- and 5-fold coordinated Mg2+ sites has been ob-

served, whose relative population (Mg2þ3c =Mg2þ4c and Mg2þ4c =Mg2þ5c ) increases by

increasing the surface area. At higher PCO, polyaddition of CO on the same Mg2+

sites occurs. The evolution of the spectra as a function of the decreasing MgO sur-

face area (i.e. upon decreasing the surface defectivity) results in spectra whose fea-

tures are well comparable with those obtained on vacuum cleaved single crystals

by IRAS but characterized by a much better signal/noise ratio. The experiments per-

formed on hsa MgO at fixed PCO, by decreasing the sample temperature from 300 to

100 K have allowed us to measure the increase of the intensity of the IR bands

ascribed to Mg2þ4c � � � ðCOÞ and Mg2þ5c � � � ðCOÞ adducts. From the corresponding

ln(Kads) vs. 1/T plots we have calculated an energy of formation of the carbonyl ad-ducts of �12 and �22 kJmol�1 for the Mg2þ5c and Mg2þ4c sites respectively. These two

couples of DHads, Dm(CO) data fits remarkably well with the empirical enthalpy/fre-

quency correlation, reported in Ref. [11,278], of the CO species linearly adsorbed

through the carbon end on non-d, d10 or d0 cationic sites.

Coming to the IR manifestations of CO molecules adsorbed on the basic oxygen

of the MgO surface the new experiments here reported have been performed about

40 K below the temperatures used in the classical IR experiments reported in the lit-

erature on MgO powders. Under these conditions all activated adsorption processesare suppressed and the number and complexity of adsorbed species with (CnOn+1)

2�

formula is consequently reduced. This has allowed us to better understand the first

stages of the complex interaction of the CO molecules with the different basic sites of

the MgO surface. In particular, it has been possible to observe the precursors of the

polymeric species which dominate the room temperature spectra. Ab initio calcula-

tions, on simple models, have allowed to establish a stability scale for the (CnOn+1)2�

species, explaining why IR spectroscopy reveals prevalently complexes with an odd

number of CO molecules. A qualitative agreement has been obtained between

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 129

computed and experimental IR frequencies and intensities of some of the formed

structures.

10. Note added in Proofs

The Present Section is devoted to briefly discuss the most recent works, appeared

between the submission of the review (end of February 2004) and the proofs correc-

tion (end of August 2004). Also few less recent (but relevant) works, that have been

overlooked by us in the first draft are here mentioned.

The same set of MgO samples (hsa, sintered and smoke, see SubSection 2.1.1) used

here to investigate the interaction of CO probe with the different surface sites of mag-

nesium oxide has been used to study the adsorption of H2 in the 300–20 K range[279] with the same IR apparatus described in SubSection 2.2.2. The new IR results

on CO adsorption reported in this review, together with those of H2 adsorption re-

ported in Ref. [297] have to be considered as part of an unique scientific project

aimed to better understand the surface properties of MgO and their evolution upon

modifying the sample surface area. This is clearly evident when the spectra reported

in Fig. 12 (CO adsorbed at 60 K) are compared with those reported in Fig. 27 (H2

adsorbed at 20 K).

5000 4500 4000 3500 1500 1400 1300 1200 11000.0

0.3

0.6

0.9 (a)

Wavenumber (cm-1)

(b)

(c)

Fig. 27. Pressure dependence of the IR spectra of the H2 adsorbed at 20 K on (a) hsa, (b) sintered and (c)

smoke MgO samples previously outgassed in vacuo at 1073 K. The upper curve of each series is the

spectrum at maximum coverage (H2 equlibrium pressure 10 mbar), the bottom curve that recorded after

prolonged outgassing at 20 K (residual equlibrium pressure <10�3 mbar). All spectra have been vertically

shifted for sake of clarity. The ordinate scale is the same in the three parts. The striking quenching of the

surface reactivity towards H2 of MgO observed by reducing the surface area (parts a to c) perfectly mirrors

what observed for the reactivity towards CO, see Fig. 12 (parts a to c).

130 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

The main achievements obtained in Ref. [279] are now briefly summarized. On hsa

MgO dissociative adsorption of H2 has been observed with formation of reversible

(modes at 3454 and 1325 cm�1) and irreversible (modes at 3712 and 1125 cm�1)

OH and MgH species already reported in previous studies at 300 K. Cooling the

MgO/H2 system down to 20 K results in the irreversible formation, at about 200 K,of new OH (modes at 3576–3547 cm�1) andMgH (modes at 1430–1418 cm�1) surface

groups never observed before (Fig. 27a). The spectra recorded at 20 K in H2 atmos-

phere also show absorptions in the 4800–4000 cm�1 frequency interval, evidencing

the presence of molecularly adsorbed species [279]. Decreasing the MgO surface area

(sintered and smoke MgO samples, Fig. 27b and c) results in the disappearance of all

the spectroscopic manifestations due to the hydride and hydroxyl groups formed upon

dissociative adsorption of hydrogen, while those due to H2 adsorbed in molecular

form are maintained (although with much reduced intensity). This behavior is theobvious consequence of the reduction, revealed by HRTEM andAFM, of the concen-

tration of surface defects (cationic and anionic sites located on edges, corners, steps,

inverse edges and inverse corners) discussed here in Section 3 (see also Ref. [280]),

and mirrors the reduction (see Fig. 12) of the formation, upon CO dosage, of the neg-

atively charged monomeric, dimeric and polymeric species discussed in Section 6. On

the basis of the morphological characterization and of the IR spectroscopic studies,

the authors of Ref. [279] concluded that the sites responsible for the H2 dissociative

adsorption are mainly inverse steps ‘‘coupled’’ with edges and corners (see Fig.11a). In particular, three different families of O2�H+ and Mg2+H� surface species

are formed by heterolytic splitting of H2 (one of them being never observed in previous

studies). Conversely, more usual ‘‘isolated’’ defects, like edges, steps and corners, are

able to adsorb H2 in molecular form only. Following the procedure described here in

Section 8, the authors of Ref. [279] calculated the specific adsorption energy for the

formation of molecular Mg2+ � � �H2 adducts on Mg23cþ corners (7.5 kJmol�1), Mg2þ4cedges (4.6 kJmol�1) and Mg2þ5c regular (100) sites (3.6 kJmol�1).

The interaction of H2 on MgO nanoparticles has been very recently investigatedby UV–Vis spectroscopy by Berger et al. [281] (Vienna�s group). Polychromatic UV

light has been used to for color center formation, the MgO nanoparticles. Authors

observed essentially two absorption features at 1.8 and 2.4 eV, with their relative

intensities depending on the applied H2 pressure during UV irradiation. Authors ex-

plained these observation by changes in the relative abundance of hydride groups

stemming from H2 chemisorption at different surface defects, a conclusion wich is

in perfect agreement with the presence of, three different families of O2�H+ and

Mg2+H� surface species claimed in Ref. [279] by the Zecchina�s team. The photolysisof irreversibly formed hydride groups results in the low-energy absorption at 1.8 eV,

whereas the UV irradiation of a second type of hydride contributes to the appear-

ance of a broad feature around 2.4 eV, which is composed of more than one band

[281]. It is worth underlying that Ref. [281] is just the more recent work of the Erich

Knozinger group in the investigation of the MgO/H2 interaction and in the surface

OH groups characterization [134,138,139,154,282,283].

An interesting review describing the characterization with metastable impact elec-

tron and ultraviolet photoelectron spectroscopies of several thin films, including the

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 131

MgO(001) one, has recently appeared by the Goodman group [284]. Extended defect

sites on MgO are identified by a narrowing of the O(2p) features with a reduction in

the density of extended defect sites. Authors underline that CO is an appropriate

probe molecule for estimating the defect density of MgO surfaces [284].

Coming finally to the most recent theoretical studies, we want mention the letterby Wang et al. [285] and the review authored by some out of the scientists that have

developed the CRYSTAL code for periodic ab initio calculations [286]. Wang et al.

[285] have investigated the initial decay of electron-hole excitations in a molecular

layer of CO adsorbed on the MgO(001) insulator surface using ab initio many-body

perturbation theory. Authors found very fast decay processes with lifetimes that are

about 5 times shorter than the transfer of single charge carriers, attributed to a

strong coupling of the molecular excitations to charge-transfer-exciton states be-

tween the adlayer and the substrate [285]. In the review of Dovesi et al. [286] the basisof the periodic ab initio methods used in the simulation of 3D crystals is firmly de-

scribed. Then authors came to the discussion of the method used to simulate 2D sur-

faces and interfaces. In that section, the case study of the MgO surface and of its

interaction with CO is treated. There the problem investigating the interaction of

CO with the MgO(001) surface ad different coverages (see SubSection 8.2.1) is dis-

cussed. Also the modeling techniques required to simulate defects are reported. Fi-

nally, in the appendices, Dovesi et al. [286] discuss the performances of the

CRYSTAL periodic code and report a complete list of the other available periodiccodes.

Acknowledgments

We are deeply indebted to Prof. S. Coluccia (University of Turin) of enlightening

discussions and for a critical reading of the manuscript. We also acknowledge Prof.

S. Coluccia and his group and Prof. E. Knozinger (University of Vienna) for havingkindly supplied us the unpublished TEM micrograph reported in Fig. 9a and c

respectively. The interesting discussions with Profs. E. Giamello, G. Martra, R. Do-

vesi, B. Civalleri, C. Pisani, P. Ugliengo (University of Torino), G. Pacchioni (Uni-

versity of Milano), S. Valeri, P. Luches, S. D�Addato, (University of Modena), and

H.J. Freund (Max Planck Inst, Berlin) are acknowledged. This study has been par-

tially supported by INFM project ISADORA and by COFIN 2003/04, both coordi-

nated by G. Pacchioni.

Appendix A. List of acronyms and symbols

A(T) integrated intensity of an IR band at the given temperature T

Amax integrated intensity of an IR band at saturation

AE all electrons

AED Auger electron diffraction

AES Auger electron spectroscopy

132 G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146

AIMP ab initio model potentials

amu atomic mass unit

B3-LYP hybrid Hamiltonian developed by Becke, Lee, Yang and Parr

BE binding energy

BSSE basis set superimposition errorCCSDT coupled cluster single and double theory (post-SCF computational

method)

CSOV constrained space orbital variation

CVD chemical vapor deposition

CI configuration interaction (based on a poly-determinant approach)

DFT density functional theory

DOS density of occupied electron states

DRS diffuse reflectance spectroscopyd bending mode

m stretching mode�mðCOÞ C–O stretching frequency

D�mðCOÞ variation of the C–O stretching frequency with respect to that in the gas

phase

DHads enthalpy of adsorption

Eex energy of the bulk exciton

EI energy of the surface exciton localized on O2�5c site

EII energy of the surface exciton localized on O2�4c site

EIII energy of the surface exciton localized on O2�3c site

EELS electron energy loss spectroscopy

ESCA electron spectroscopy for chemical analysis

EPR electron paramagnetic resonance

EXAFS extended X-ray absorption fine structure

FLAPW full potential linearized augmented plane wave

FTIR Fourier-transform infra-redGCF Guassian centered functions

GCF-AE Guassian centered functions all electrons

GGA generalized gradient approximation

GIXRD grazing incidence X-ray diffraction

HF Hartree–Fock

HRTEM high resolution TEM

hsa high surface area

IR infra-redIRAS infrared reflection absorption spectroscopy

Kads equilibrium constant of the adsorption process

LDA local density functional approximation (sometime LDF)

LDF local density functional (sometime LDA)

LEED low energy electron diffraction

MCPF modified coupled pair functional

Mg2þ5c regular, 5-coordinated, surface magnesium cation

G. Spoto et al. / Progress in Surface Science 76 (2004) 71–146 133

Mg2þ4c 4-coordinated surface magnesium cation (step site)

Mg2þ3c 3-coordinated surface magnesium cation (corner site)MP2 second order Moeller–Plesset perturbation theory (post-SCF computational

method)

MP3 third order Moeller–Plesset perturbation theory (post-SCF computational

method)

MP4 fourth order Moeller–Plesset perturbation theory (post-SCF computational

method)

MP5 fifth order Moeller–Plesset perturbation theory (post-SCF computational

method)O2�

5c regular, 5-coordinated, surface oxygen anion

O2�4c 4-coordinated surface oxygen anion (step site)

O2�3c 3-coordinated surface oxygen anion (corner site)

PDMEE primary beam diffraction modulated electron emission

PCO CO equilibrium pressure

PL photoluminescence

PES photoelectron spectroscopy

SCF self consistent fieldSEXAFS surface extended X-ray absorption fine structure

STM scanning tunneling microscopy

T temperature

TEM transmission electron microscopy

TDS thermal desorption spectroscopy

h surface coverage

hmax maximum surface coverage

UHV ultrahigh vacuumUPS ultraviolet photoelectron spectroscopy

UV ultraviolet

UV–Vis ultraviolet–visible

XPS X-ray photoelectron spectroscopy

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