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Microporous and Mesoporous Materials 81 (2005) 305–312

FTIR spectroscopic study of CO adsorption on Pt–H–ZSM-5

Kristina Chakarova, Mihail Mihaylov, Konstantin Hadjiivanov *

Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria

Received 31 August 2004; received in revised form 25 January 2005; accepted 26 January 2005

Available online 31 March 2005

Abstract

This work reports the formation of some new non-classical carbonyls of platinum formed after CO adsorption on Pt–H–ZSM-5.

These are: (i) Pt3+(CO)2 dicarbonyls (2211 and 2175 cm�1), which are decomposed without producing monocarbonyls; (ii)

Pt2+(CO)2 (2165 and 2150 cm�1), decomposed via Pt2+–CO species (2113 cm�1); and (iii) Pt+(CO)3 complexes (2162, 2150 and

2110 cm�1) which easily loose one CO ligand even at room temperature being thus converted to Pt+(CO)2 (2120 and 2091 cm�1).

The latter are thermally decomposed without producing monocarbonyls.

� 2005 Elsevier Inc. All rights reserved.

Keywords: Adsorption; Carbon monoxide; FTIR spectroscopy; Non-classical carbonyls; Platinum; Pt-ZSM-5

1. Introduction

The coordination chemistry of cations in zeolites has

created an increasing interest. It has been reported that

two [1–9], three [10–14] or even four [15–17] small mol-ecules can be simultaneously coordinated to one cation

located in a suitable position in some zeolites or related

materials. Now, it is generally accepted that the phe-

nomenon is due to the low coordination of such cations.

Many systematic investigations have been performed on

CO or N2 adsorption on alkali or alkaline-earth cations

in zeolites [1–5,13]. In these cases the adsorption is weak

and the probe molecule provides information on thenumber of the effective coordinative vacancies of the ex-

changed cation. It has been established that the ability

of a cation exchanged in zeolites to coordinate more

than one CO or N2 molecule depends on the cationic ra-

dius: small cations penetrate the O-ring where they are

located and, because of steric reasons, can coordinate

one guest molecule only [4,9,13,18]. The critical cationic

1387-1811/$ - see front matter � 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2005.01.033

* Corresponding author. Fax: +359 2 705024.

E-mail address: kih@svr.igic.bas.bg (K. Hadjiivanov).

radius depends on the dimension of the O-rings and is

therefore different for different zeolites or cationic posi-

tions in one zeolite [9]. In a series of works we have

called the geminal species which are produced due to

the low coordinaton of the cation, site specified

[4,9,19]. In contrast, complex-specified geminal species

are produced whatever the support is and are more sta-

ble than the corresponding linear species. Typical exam-

ples are the well known Rh+(CO)2 gem-dicarbonyls

[15,16,20,21]. They are stabilised because of the achieve-

ment of 16- or 18-electron configuration [20,21]. As a re-

sult, they are always produced after CO adsorption on

Rh+-containing systems.The most frequently used IR probe molecule is CO

[22–24]. Although the results obtained with CO adsorp-

tion on d0 cations in zeolites are very important from a

theoretical point of view, the coordination chemistry of

the transition metal cations is definitely more interest-

ing. However, here the situation is more complicated

due to several reasons. One of them is the ability of

many of these cations to form complex-specified geminalspecies. Another complication of CO as a probe mole-

cule arises from the fact that many transition metal cat-

ions react with CO, thus changing their oxidation state

306 K. Chakarova et al. / Microporous and Mesoporous Materials 81 (2005) 305–312

during the experiments [22,24]. In particular, the situa-

tion with platinum in zeolites is even more complicated

due to the willingness of platinum to form Chini-com-

plexes, i.e. anionic platinum carbonyls [25,26].

The aim of this work is to investigate the species pro-

duced after CO adsorption on cationic platinum sites inPt–H–ZSM-5. We expected that the large cationic radius

of Pt2+ and/or Pt+ cations would favour the simulta-

neous coordination of more than one CO molecule to

one cationic site.

2. Experimental

The Pt–H–ZSM-5 sample was prepared by a conven-

tional ion-exchange procedure. The starting H–ZSM-5

material was supplied by Degussa. 1.00 g of H–ZSM-5

was suspended in 100 ml 0.0025 M solution of

Pt(NH3)4Cl2 and stirred for 1 week at ambient tempera-

ture. Then the precipitate was thoroughly washed with

distilled water and dried at 353 K for 10 h. The platinum

concentration in the sample thus obtained was 2.3 wt.%.For comparison purposes another sample (Pt/H–

ZSM-5) was prepared by impregnation. 1.0 g of the H–

ZSM-5 sample was impregnated with 10 ml 0.0156 M

solution of Pt(NH3)4Cl2. The nominal platinum concen-

tration in this sample was 3.0 wt.% Pt.

The IR spectra were recorded on a Nicolet Avatar

360 spectrometer at a spectral resolution of 2 cm�1

and accumulation of 64 scans. Self-supporting pellets(ca. 10–20 mg cm�2) were prepared from the sample

powder and treated directly in the purpose-made IR

cells. The latter were connected to a vacuum-adsorption

apparatus with a residual pressure below 10�3 Pa. Prior

to the adsorption measurements, the samples were

heated in fresh oxygen (20 kPa for 15 min) in steps of

50 K up to 723 K (1 h at this temperature) and then

evacuated for 1 h at 723 K.Carbon monoxide (>99.997) was supplied by Linde

AG. Nitrogen monoxide (>99.0) was purchased from

Messer Griesheim GmbH. Before use, carbon monoxide

and oxygen were passed through a liquid nitrogen trap.

3. Results

3.1. Initial characterization of the samples

The IR spectrum of the activated Pt–H–ZSM-5 sam-

ple displays, in the O–H stretching region, two bands

with maxima at 3745 and 3610 cm�1 (spectra not

shown). These two bands have also been recorded with

the pure H–ZSM-5 sample and are assigned to the

O–H stretching modes of silanol groups and zeolitebridged hydroxyls, respectively. The band at 3610 cm�1

is less intense with the Pt–H–ZSM-5 sample, which is

consistent with the exchange of some acidic protons with

Ptn+ cations.

The background spectrum of the Pt/H–ZSM-5 sam-

ple is very similar to that of Pt–H–ZSM-5. In particular,

the intensity of the OH band at 3610 cm�1 is almost the

same, which evidences that some of the platinum cationshave occupied cationic positions. The higher platinum

concentration in this sample indicates, however, that

some of the deposited platinum has not been exchanged

with the acidic zeolite protons.

3.2. Study of the possibility of formation of Chini

complexes

The Chini complexes are anionic platinum carbonyls

[25]. Since there are many papers [26] reporting the for-

mation of ½Pt3ðCOÞ6�2�2 species after CO interaction with

platinum-containing zeolites, we initially studied the

possibility of formation of such complexes with our

samples. The ½Pt3ðCOÞ6�2�2 species are characterized by

two main IR bands, one around 2050–2025 cm�1 (linear

CO) and another at ca. 1800–1750 cm�1 (bridged CO)[26]. The formation of these species requires a special

treatment of non-activated samples in CO atmosphere

at moderate temperatures (ca. 373 K). However, our

experience has shown that some of them are produced

on Pt/NaY zeolite merely after CO adsorption. That is

why we studied the possibility of formation of Chini

complexes with our samples.

Carbon monoxide (2.3 kPa) was introduced to thesamples which were previously evacuated at ambient

temperature. Than the samples were heated in the CO

atmosphere for 24 h at 373 K. Evidence of Chini com-

plexes was found neither with Pt–H–ZSM-5, nor with

Pt/H–ZSM-5. This is explained by the fact that the pores

of the ZSM-5 zeolite are smaller than the ½Pt3ðCOÞ6�2�2

complexes.

3.3. Adsorption of CO on activated Pt–H–ZSM-5

Adsorption of CO on activated Pt–H–ZSM-5 leads to

the appearance of a series of bands in the IR spectrum,

their maxima being at 2211, 2175, 2155 and 2101 cm�1

(Fig. 1, spectrum a). In addition, two weak features at

2195 and 2118 cm�1 are also discernible. Evacuation

hardly affects the bands above 2110 cm�1: only the bandat 2195 cm�1 disappears (Fig. 1, spectra b–e). However,

the spectrum changes strongly in the lower frequency re-

gion. The band at 2101 cm�1 initially increases in inten-

sity and is red shifted to 2099 cm�1 (Fig. 1, spectrum b)

then declines and is additionally shifted to 2096 cm�1

(Fig. 1, spectra b–e). Simultaneously, a new band at

2077 cm�1 develops at the expense of the 2101 cm�1

band.In order to check whether the conversion of the band

at 2100 cm�1 into the band at 2077 cm�1 was reversible,

2250 2200 2150 2100 2050 2000

- 219

5

g

ea

- 211

8

f

- 215

5

- 217

5

- 2099

0.05- 2

211

2101 -

- 2077

e

a

Abso

rban

ce, a

.u.

Wavenumber, cm-1

Fig. 1. FTIR spectra of CO adsorbed on activated Pt–H–ZSM-5:

Equilibrium CO pressure of 1600 Pa (a) and after evacuation for 5 (b),

10 (c), 15 (d) and 20 min (e), subsequent introduction of CO (1330 Pa

equilibrium pressure) (f) and evacuation for 10 min (g). The spectra are

background corrected.

2220 2200 2180

- 221

1

0.01

- 217

5

d

a

Abso

rban

ce, a

.u.

Wavenumber, cm-1

Fig. 2. FTIR spectra of CO and H2O co-adsorbed on activated Pt–H–

ZSM-5: Equilibrium CO pressure of 1600 Pa followed by 30 min

evacuation (a) and development of the spectra in the presence of water

vapour (133 Pa equilibrium pressure): spectrum taken immediately (b),

after 10 (c) and 15 min (d).

K. Chakarova et al. / Microporous and Mesoporous Materials 81 (2005) 305–312 307

we added again CO to the IR cell. This led to a weak in-

crease in intensity of the 2096 cm�1 band and a shift of

its maximum to 2100 cm�1 (Fig. 1, spectrum f). Theband at 2077 cm�1 also showed a slight shift (to

2078 cm�1) accompanied by a rise in intensity. These re-

sults demonstrate that the conversion is not reversible,

i.e. it is due either to reduction, or to structural changes.

Subsequent evacuation at ambient temperature leads

again to a decrease in intensity of the band at

2100 cm�1 and additional development of the band at

2077 cm�1 (Fig. 1, spectrum g). We assign the bandsat ca. 2100 and at 2077 cm�1 to two kinds of Pt0–CO

species. It is logical to suppose that initially highly dis-

persed platinum is formed (carbonyl band at

2100 cm�1) and then the platinum particles agglomerate

(carbonyl band at 2077 cm�1). However, there is no

agreement on the dependence of m(CO) on the size of

platinum particles. According to Davydov [24], the

smaller the platinum particle, the lower the stretchingfrequency of CO adsorbed on it. In a recent publication

[27] another possibility was supposed, namely: CO

adsorbed on small (2–3.5 nm) platinum particles mani-

fests a band around 2100 cm�1 while CO adsorbed on

large (10–50 nm) particles produces a band at ca.

2155 cm�1. It is not excluded that in our case the high

frequency of the band at 2100 cm�1 can be due to partial

positive charge of the corresponding platinum particles.The two bands at 2211 and 2175 cm�1 change in con-

cert (see also what follows). Hence, they can be attrib-

uted to one species. Although both bands have similar

intensities, the lower frequency band is more intense.

Such an intensity ratio is typical of dicarbonyl species.

The results allow us to assign the bands at 2211 and

2175 cm�1 to ms and mas modes, respectively, of Ptn+(CO)2species. No band assignable to the corresponding linearcomplexes is produced during the decomposition of the

Ptn+(CO)2 species, hence they are complex-specified

[19]. Additional experiments have shown that the

Ptn+(CO)2 bands easily disappear in the presence of

water in the gas phase (see Fig. 2). This shows that

H2O replaces CO as a ligand of the Ptn+ cations.

In order to check the possibility of insertion of one or

more CO ligands into the Ptn+(CO)2 species, we haveperformed CO adsorption at low temperature. Introduc-

tion of CO (400 Pa equilibrium pressure) at 100 K does

not affect the Ptn+(CO)2 bands. Hence, these species are

not able to accommodate any additional CO molecule.

The bands at 2195 and 2155 cm�1 will be discussed

below.

3.4. Adsorption of CO on partially reduced Pt–H–ZSM-5

In order to obtain more information about the carbo-

nyls of partly reduced platinum sites we activated a fresh

sample and during the activation introduced, for 30 min,

CO (ca. 10 Pa equilibrium pressure). Adsorption of CO

(1600 Pa equilibrium pressure) on the sample thus trea-

ted resulted in the appearance of a series of bands at

2250 2200 2150 2100 2050 2000

-212

0

2152

-

- 209

1

- 220

0

- 216

2

- 212

2

c

b

0.1

- 215

0

- 211

0

- 2088

f

aAbso

rban

ce, a

.u.

Wavenumber, cm-1

Fig. 3. FTIR spectra of CO adsorbed on partially reduced Pt–H–

ZSM-5 (for details see text): Equilibrium CO pressure of 1600 Pa (a),

10 min evacuations at ambient temperature (b), at 473 (c) 573 (d) 673

(e) and 723 K (f). The spectra are background corrected.

2250 2200 2150 2100 2050 2000

- 205

5d

0.1

- 215

2

- 209

1

2118 -

b

a

Abso

rban

ce, a

.u.

Wavenumber, cm-1

Fig. 4. FTIR spectra of CO adsorbed on partially reduced Pt–H–

ZSM-5 (for details see text). Introduction of CO (1600 Pa equilibrium

pressure), followed by evacuation of the sample for 10 min at ambient

temperature (a), short (5 s) evacuations at 673 (b) and 723 K (c, d). The

spectra are background corrected.

308 K. Chakarova et al. / Microporous and Mesoporous Materials 81 (2005) 305–312

2200 (vw), 2162, 2150, ca. 2110 and 2091 cm�1 (Fig. 3,

spectrum a). Note that no bands due to Ptn+(CO)2 spe-cies were detected with this reduced sample. After evac-

uation three bands at 2152, 2122 and 2088 cm�1 were

seen in the spectrum (Fig. 3, spectrum b). Evacuations

at higher temperatures (Fig. 3, spectra c–f) left a set of

two other bands at 2120 and 2091 cm�1 which decreased

in concert. A weak band at 2152 cm�1 was also visible.

The above results can be rationalized assuming that

one kind of carbonyl species (A-species, bands at 2162,2150 and 2110 cm�1) looses one CO ligand and is con-

verted into another type of carbonyls (B-species, bands

at 2120 and ca. 2091 cm�1). As for the weak band at

2200 cm�1, it is probably a satellite of the bands at

2162, 2150 and 2110 cm�1. The existence of a number

of bands for each individual species indicates that they

are polycarbonyls.

3.5. Impulse desorption experiments

Impulse desorption is a useful technique for decom-

position of polyligand species [28]. In another set of

experiments we partially reduced the sample as already

described. Than CO was adsorbed on it and pumped

out. Three bands, at 2152, 2122 and 2098 cm�1, were

discernible in the spectrum (Fig. 4, spectrum a). Thespectrum is similar to the spectrum ‘‘b’’ from Fig. 3,

however, the lowest-frequency band is less intense which

indicates a lower concentration of metallic platinum.

Then we evacuated the sample for very short times at

673 and 723 K. As a result, three principal bands re-

mained in the spectrum, their maxima being at 2118,

2091 and 2055 cm�1 (Fig. 4, spectra b–d). A weak band

at ca. 2152 cm�1 was also visible. The bands at 2118 and

2091 cm�1 changed in concert, which confirmed the

above interpretation that they characterize B-species.

The band at 2055 cm�1 abruptly followed the intensities

of the bands at 2118 and 2091 cm�1. However, the spec-tra presented on Fig. 4 confirm that this band is not re-

lated to the higher-frequency bands (for instance,

compare spectra c and d). We infer that this band is

due to carbonyls of metallic platinum that are produced

via reduction at the high temperature of cationic plati-

num sites by the CO evolved during desorption. This

mechanism explains the almost parallel intensity change

of the band and those of the bands at 2118 and2091 cm�1.

3.6. Adsorption of CO on Pt–H–ZSM-5 oxidized in a

NO + O2 mixture

It is well known that a NO + O2 mixture is a stronger

oxidizing agent than is oxygen itself. In order to obtain

additional information on the oxidation state of the cat-ionic platinum sites, we treated the sample in a NO (1

kPa partial pressure) + O2 (7 kPa partial pressure) mix-

ture for 1 h at 723 K and then evacuated the gas phase at

the same temperature.

Adsorption of CO (1600 Pa equilibrium pressure) on

the sample thus treated results in the appearance of six

2250 2200 2150 2100 2050 2000

fe

d

c

2150

-

b

- 209

0a

- 2065

- 2097

- 2094

2113

- 212

2

2155

- 217

5

- 219

50.05

- 221

1

Abso

rban

ce, a

.u.

Wavenumber, cm-1

Fig. 5. FTIR spectra of CO adsorbed on Pt–H–ZSM-5 oxidized in a

NO + O2 mixture (for details see text): Equilibrium CO pressure of

1600 Pa (a), after 10 min evacuation at ambient temperature (b),

10 min evacuation at 473 (c), 573 (d) 673 (e) and 723 K (f). The spectra

are background corrected.

2250 2200 2150 2100 2050 2000

2174

2143

- 215

2

- 219

5

2163

2122

2091

- 221

0

d

0. 01

a

Abso

rban

ce, a

.u.

Wavenumber, cm-1

Fig. 6. FTIR spectra of CO adsorbed on activated Pt/H–ZSM-5:

Equilibrium CO pressure of 900 Pa (a), after 10 min evacuation at

ambient temperature (b), at 423 (c) and 673 K (d). The spectra are

background corrected.

K. Chakarova et al. / Microporous and Mesoporous Materials 81 (2005) 305–312 309

principal IR bands at 2211, 2195, 2175, 2155, 2122 and

2097 cm�1 (Fig. 5, spectrum a). Note that the bands as-

signed to Ptn+(CO)2 species are much more intense thanin the previous experiments. Evacuation hardly affects

the spectrum: only the band at 2174 cm�1 decreases in

intensity (which could be explained by destruction of

the OH–CO complexes formed with H–ZSM-5 even at

room temperature [22]) and the band at 2097 cm�1 shifts

to 2094 cm�1 (Fig. 5, spectrum b). Evacuation at 473 K

leads to a ca. twofold decrease in intensity of the bands

at 2211 and 2175 cm�1 (Fig. 5, spectrum c). The band at2195 cm�1 almost disappears and the band at 2155 cm�1

decreases in intensity and is shifted to 2150 cm�1. These

results suggest the existence of another kind of

Ptn+(CO)2 species which are characterised by bands at

2195 and ca. 2155 cm�1. The band at 2094 cm�1 shows

a strong intensity decrease. This confirms that the band

(or part of it) characterises an individual species. Evac-

uation at 573 K leads to disappearance of all bandsabove 2150 cm�1 and almost full disappearance of the

band at 2094 cm�1 (Fig. 5, spectrum d). A weak band

at 2065 cm�1 becomes visible. The band at 2150 cm�1

slightly decreases and a new band at 2115 cm�1 devel-

ops. After further evacuation at 673 K the band at

2150 cm�1 additionally decreases in intensity, whereas

the band at 2115 cm�1 develops (Fig. 5, spectrum e).

The latter is the only band (settled at 2113 cm�1) that re-mains in the spectrum after evacuation at 723 K (Fig. 5,

spectrum f).

3.7. Adsorption of CO on Pt/H–ZSM-5

The spectra obtained after CO adsorption on theimpregnated sample resemble those recorded with Pt–

H–ZSM-5 but are somewhat more complicated. Here

we shall describe briefly only the main results. Adsorp-

tion of CO on the sample results in the appearance of

the bands assigned to: Ptn+(CO)2 species (2210 and

2174 cm�1); Ptn+–CO or Ptn+(CO)2 (2195 cm�1), A-spe-

cies (2163, 2152 and 2130 cm�1) and CO adsorbed on

metallic platinum (2106 and 2091 cm�1) (Fig. 6, spec-trum a). In addition, a band at 2143 cm�1 is also visible

and is assigned to CO adsorbed on oxidized platinum

sites that are not in cationic positions.

Evacuation at elevated temperatures (Fig. 6, spectra

b–d) leads to a gradual disappearance of the (di)carbo-

nyls of Ptn+ cations and conversion of the A-species into

B-species (2120 and 2091 cm�1). Here the bands are

somewhat blue shifted as compared to the case of Pt–H–ZSM-5. These results show that there is no essential

difference between the platinum species in the samples

prepared by ion-exchange and by impregnation.

4. Discussion

The carbonyl chemistry of platinum has been subjectof a steady interest. Platinum is among the elements that

form the so-called non-classical carbonyls, i.e. carbonyls

Table 1

Assignment of the IR carbonyl bands observed in this study

Bands Assignment Comments

2211 and 2175 cm�1 ms and mas of Pt3+(CO)2 species Complex-specified geminal species

2195 cm�1 mCO of Pt3+–CO or ms of Pt3+(CO)2 Less stable than the above species

2165 and 2150 cm�1 ms and mas of Pt2+(CO)2 species Site-specified geminal species

2113 cm�1 mCO of Pt2+–CO The most stable species

2162, 2150 and 2110 cm�1 Pt+(CO)3 Easily loose one CO ligand

2120 and 2091 cm�1 ms and mas of Pt+(CO)2 species Complex-specified geminal species

Bands below 2100 cm�1 mCO of Pt0–CO Different types

310 K. Chakarova et al. / Microporous and Mesoporous Materials 81 (2005) 305–312

with the participation of cations [29]. Due to the great

importance of platinum in catalysis, there are many pa-

pers on the characterisation of different platinum-con-

taining samples using CO as an IR probe molecule.

However, most of the studies have been performed with

reduced samples and the work dealing with CO adsorp-tion on oxidized platinum sites are restricted [22]. Very

often carbonyl bands at higher frequencies have been

simply assigned as due to Ptd+–CO species. The most

typical oxidation states of platinum are 2+ and 4+,

but compounds of Pt3+ and Pt+ are also known [30].

In this work we have obtained evidence of the formation

of so far unknown non-classical platinum carbonyls. Be-

cause of the lack of literature data, the assignments ofthe observed bands are rather difficult. In what follow

we shall discuss the different carbonyl species. For con-

venience the main observed bands and their assignments

are summarised in Table 1.

4.1. Ptn+(CO)2 species (bands at 2211 and 2175 cm�1)

These two bands were already assigned to the sym-metric and antisymmetric modes of Ptn+(CO)2 dicarbo-

nyls. Analysis of literature data shows that similar

bands have already been described in the literature but

assigned in other ways. Thus, Solomennikov and

Davydov [31] observed bands at 2205 and 2175 cm�1

after CO adsorption on PtY (oxidized at 673 K).

Although the bands changed in concert, the authors

assigned them to two kinds of Pt2+–CO species. Veryrecently, these bands were detected with Pt–H–ZSM-5

[32], but the authors focused their attention on the

lower-frequency bands and did not propose any assign-

ment of the bands under consideration. Analysis of the

published spectra shows that these two bands change

indeed in parallel in the different experiments.

Water was found to replace CO as a ligand from the

Ptn+(CO)2 species. In principle, water can replace COfrom carbonyls in which the M–CO bond has mainly a

r-character and the p-back donation is very weak [33].

When the p-back donation is significant, as in the case

of the carbonyls of Pt2+ and Pt+, it is CO that replaces

H2O as a ligand [33]. This strongly suggests that plati-

num cations in the Ptn+(CO)2 species are in a high oxi-

dation state: 3+ or 4+. The initial oxidation state of

platinum during deposition was 2+. However, the oxi-

dation treatment at 723 K in oxygen evidently oxidized

some of these cations. The fraction of the oxidized cat-

ions increased when the oxidation treatment was carried

out under a NO + O2 mixture.

Similar dicarbonyls (2221 and 2200 cm�1) have re-cently been observed with Pd–H–ZSM-5 samples and

assigned to Pd3+(CO)2 species [34,35]. Taking into ac-

count the analogous electron configuration of palladium

and platinum, as well as their similar carbonyl chemis-

try, we infer that the species under consideration are

Pt3+(CO)2 dicarbonyls. Evidently, the Pt3+ cations are

formed during the oxidative treatment of the sample.

The Pt3+(CO)2 species are stable up to 473 K and aredecomposed without producing Pt3+–CO linear com-

plexes. However, in a series of experiments a band at

2195 cm�1 has been registered. The CO stretching fre-

quency lies well between the ms and mas modes of the

Pt3+(CO)2 species. Analysis of the results shows that

both species change independently. Therefore, we assign

the band at 2195 cm�1 to another kind of carbonyls of

Pt3+ cations. It was already suggested that the band at2195 cm�1 could characterise the symmetric modes of

another kind of Pt3+(CO)2 geminal species. Unfortu-

nately, the corresponding antisymmetric modes (sup-

posed to be around 2155 cm�1) are not unambiguously

detected due to the superposition with other strong

bands in the regions. Therefore, a possible alternative

interpretation of the 2195 cm�1 band is that it arises

from linear Pt3+–CO species, the corresponding Pt3+

sites, for steric reasons, being not able to form dicar-

bonyls.

4.2. A-species (bands at 2162, 2150 and 2110 cm�1)

and B-species (bands at 2120 and 2091 cm�1)

Start the discussion with the B-species. In fact, the set

of bands at 2120 and 2091 cm�1 has already been re-ported (at 2123 and 2092 cm�1) in a study of CO

adsorption on reduced Pt–H–ZSM-5 [36]. The intensity

ratio of the two bands is comparable to the ratio ob-

served in this study. The authors have assigned the

bands to dicarbonyls of monoatomic Pt0 species. Similar

conclusions have been drawn concerning analogous

bands observed with other Pt-zeolitic systems [37,38].

K. Chakarova et al. / Microporous and Mesoporous Materials 81 (2005) 305–312 311

However, one can impeach such an assignment because

usually the band due to the antisymmetric mode of

dicarbonyls is more intense [22]. For those reasons we

initially favoured the hypothesis of formation of tricar-

bonyls. However, very recent experiments we performed

with 12CO–13CO isotopic mixtures indicated that thespecies under consideration were dicarbonyls. This con-

tradiction can be explained by the fact that the angle be-

tween the two CO molecules is smaller than 90�. Note

that the B-species are complex-specified, i.e. they are

decomposed without producing a measurable fraction

of di- or mono-carbonyls.

To the best of our knowledge, the bands due to A-

species (2162, 2150 and 2110 cm�1) have not been re-ported so far. Since the A-species are obtained from

the B-species by addition of CO ligand(s), we assign

them to tricarbonyls.

It is well known that the stretching frequency of the

carbonyls generally increases with the oxidation state

of the cation [24]. It has already been stated that the

dicarbonyls of Pt3+ absorb at 2211 and 2175 cm�1.

Hence, the oxidation state of platinum in the A- andB-species is 2+ or 1+. The cationic radius of Pt2+ is

0.80 A, similarly to the radius of Mn2+ [39]. It has been

reported that Mn2+ cations are big enough to form

dicarbonyl species when exchanged in ZSM-5 zeolite,

but they cannot adsorb three CO molecules [9]. How-

ever, the A-species have three CO ligands. Tricarbonyls

have been reported with Cu+ cations (ionic radius of

0.96 A) in Cu+-ZSM-5 [10] and with other big univalentcations in ZSM-5 (Co+ [17], Ni+ [14], Rh+ [15,16]).

Although the exact location of different cations in

ZSM-5 is not established, one can, by analogy, infer that

the tricarbonyls formed with our sample are of Pt+(CO)3type. This assumption is also supported by the fact that

similar bands have been reported for CO adsorbed on

hydrogen-reduced Pt–H–ZSM-5 [36].

4.3. Species characterized by bands at 2155 and

2113 cm�1

An intense band at 2155 (shifted to 2150 cm�1 after

evacuation at 473 K) was registered after CO adsorption

on activated samples and on the samples oxidized by a

NO + O2 mixture, but not with the partially reduced

samples. During evacuation at elevated temperaturesthis band is converted into another band at 2113 cm�1.

The position of both bands is between the stretching

parameters of the carbonyls formed with the participa-

tion of Pt3+ and Pt+ cations, respectively. Consequently,

we assign the bands at 2150 and 2113 cm�1 to carbonyls

of Pt2+ cations. The band at 2113 cm�1 is the only band

present in the spectra after evacuation at 723 K (see Fig.

5, spectrum f) and is therefore attributed to linear Pt2+–CO carbonyls. The band at 2150 cm�1 declines when the

band at 2113 cm�1 develops, hence it characterizes spe-

cies with more CO ligands. As was already discussed,

the cationic radius of Pt2+ cations is above the critical

one for formation of dicarbonyl species in ZSM-5. In

this case it is normal to expect the appearance of

two IR bands. Indeed, a careful inspection of the spec-

tra shows a weak band around 2165 cm�1, which couldbe assigned to the symmetric modes of Pt2+(CO)2species.

Note that in this case the dicarbonyls are site-speci-

fied, i.e. they are decomposed via production of mono-

carbonyls. This is quite the contrary to the case of the

Pt3+(CO)2 and Pt+(CO)2 species which are complex-

specified.

Finally, a few words on the existence of metallic plat-inum on our sample. Evidently, some platinum particles

were formed during the sample activation and/or the

interaction with CO. However, no anionic carbonyls

were detected in the samples, which is explained by the

small volume of the pores in ZSM-5 and the steric hin-

drance to the formation of the relatively large Chini

complexes.

5. Conclusions

• Platinum cations in different oxidation states andexchanged in ZSM-5 zeolite form polycarbonyl spe-

cies after CO adsorption. These are: (i) Pt3+(CO)2dicarbonyls (2211 and 2175 cm�1) decomposed with-

out producing monocarbonyls; (ii) Pt2+(CO)2 (2165

and 2150 cm�1) decomposed via Pt2+–CO species

(2113 cm�1); (iii) Pt+(CO)3 complexes (2162, 2150

and 2110 cm�1) which easily loose one CO ligand

even at room temperature, being thus convertedto Pt+(CO)2 (2120 and 2091 cm�1). The latter are

thermally decomposed without producing monocar-

bonyls.

• The preparation technique (ion-exchange, impregna-

tion) hardly affects the state of platinum in ZSM-5.

Some amount of platinum cations not in exchanged

positions was found with the impregnated sample.

Acknowledgement

This work was supported by the Bulgarian National

Research Foundation (project X-1205) and the Alexan-

der von Humboldt Foundation.

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