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Microporous and Mesoporous Materials 94 (2006) 40–49

Ga-promoted sulfated zirconia systems.II. Surface features and catalytic activity q

G. Cerrato a,*, C. Morterra a, M. Rodrıguez Delgado b, C. Otero Arean b,M. Signoretto c, F. Somma c, F. Pinna c

a Department of Chemistry IFM, University of Turin, and Consortium INSTM, UdR Turin University, via P. Giuria, 7-10125 Turin, Italyb Department of Chemistry, Universidad de las Islas Baleares, 07122 Palma de Mallorca, Spainc Department of Chemistry, University of Venice, and Consortium INSTM, UdR Venice, Italy

Received 15 December 2005; received in revised form 7 March 2006; accepted 8 March 2006Available online 27 April 2006

Abstract

Sulfated zirconia samples having a variable Ga2O3 content (in the 1–15% molar range) were synthesized. The promoting effect of gal-lium was studied in the catalytic isomerization of n-butane at 523 K, by feeding n-butane and H2 (with a 1:4 ratio). Catalytic activity wasfound to be greatly dependent on gallium loading. Catalysts containing 3–5 mol% Ga2O3 doubled the activity of sulfated zirconia,whereas for a sample with 15 mol% Ga2O3 the catalytic activity was completely lost. Surface chemistry of these materials was studiedby means of FTIR spectroscopy and adsorption microcalorimetry, using selected probe molecules (CO and 2,6-dimethylpyridine). IRspectroscopy showed that gallium-containing sulfated zirconia samples exhibit both Lewis and Brønsted acidity. Lewis acidity is attrib-uted to coordinatively unsaturated Zr4+ ions located in defective surface sites, whereas Brønsted acidity is associated to surface sulfategroups. Samples with a Ga2O3 content between 1 and 9 mol% show a combination of Lewis and Brønsted acidity that quantitativelydecreases with increasing gallium oxide content. Samples having a Ga2O3 content equal to 9 mol% or greater show the following specificfeatures: (i) these samples are much more difficult to dehydrate than those with smaller gallium content, and the hydroxy groups interactby hydrogen bonding, (ii) the sulfate groups progressively lose their covalent character, and (iii) both Lewis and Brønsted acidity of thesamples decreases drastically. Couples of Lewis and Brønsted acid sites appear to be needed for catalytic activity in n-butane isomeri-zation, and they present an optimum ratio when the catalyst is brought to a medium-high dehydration degree and when its Ga2O3 con-tent is of about 3–5 mol%.� 2006 Elsevier Inc. All rights reserved.

Keywords: Sulfated zirconia; Gallium oxide promotion; n-butane isomerization; Physico-chemical characterization; Surface acidity

1. Introduction

Catalytic systems based on sulfated zirconia (SZ) are ofhigh interest in the petrochemical industry, since these cat-alysts play an important role in the isomerization of lightlinear alkanes, such as n-butane [1], n-pentane and n-hex-ane [2]. Catalysts currently employed in the abovementioned reactions consist of Pt supported on chlorinated

1387-1811/$ - see front matter � 2006 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2006.03.018

q Part I: Ref. [9b].* Corresponding author. Tel.: +39 011 6707534; fax: +39 011 6707855.

E-mail address: giuseppina.cerrato@unito.it (G. Cerrato).

Al2O3 or similar systems, such as Pt supported on morde-nite [3]. However, some of these systems operate at an ele-vated temperature and require constant addition of alkylchlorides to recover acid functionalities. Owing to increas-ingly strict environmental regulations, researchers arepaying increasing attention to solid acids in the searchfor stable and environmentally friendly catalysts. Amongsolid acids, SZ systems attract considerable interest becausethey are almost environmentally benign, highly active, andquite selective for the transformation of hydrocarbons. Inorder to overcome the fast deactivation affecting plain SZsystems, the addition of small amounts of Pt was tested

G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49 41

[4], and was found to be of great help in enhancing bothactivity and stability of SZ. Successively, in order to avoidthe use of Pt for economical reasons, promotion with otherelements, such as Fe [5], Ni [6] and, more recently, either Al[7,8] or Ga [2,8a,9] was investigated.

Aims of the present contribution are: (i) to investigatethe physico-chemical aspects of the introduction ofGa2O3 as a promoting agent in SZ systems; (ii) to findout the range of Ga2O3 concentration that leads to themost effective promotion role; and (iii) to see whether acorrelation can be found between surface acidity featuresand promoting effects.

2. Experimental

2.1. Catalyst preparation

Ga-promoted SZ systems, termed SZGn (where n standsfor the mol% Ga2O3) were prepared by a co-precipitationmethod, starting from solutions of ZrOCl2 Æ 8H2O (AlfaAesar, 99.9%) and Ga(NO3)3 Æ xH2O (Alfa Aesar, 99.9%)mixed in the appropriate proportion to obtain nominal nvalues of 1%, 3%, 5%, 9%, and 15%, after precipitationwith an ammonia solution at pH 8 (±0.1), as described indetail elsewhere [9b]. The precipitate was dried at 383 Kfor 20 h and then sulfated by incipient wetness impregna-tion with a 1.8 M aqueous solution of (NH4)2SO4 (0.5 mlsolution per gram of solid). The amorphous sulfated sam-ples thus obtained were dried at 383 K overnight followedby calcining for 3 h at 923 K, leading to crystallization ofamorphous SZ precursors (mostly) in the tetragonal (t-ZrO2) form, which is known to have enhanced catalyticactivity [10]. Table 1 reports relevant data for calcinedSZ (reference) and SZGn samples.

2.2. Catalyst characterization

The effect of gallium content on catalytic activity of theSZGn samples was studied in the n-butane isomerizationreaction, as described elsewhere [9b].

FTIR spectra were obtained at 2 cm�1 resolution with aBruker spectrophotometer equipped with a MCT criode-tector. All powders were inspected in the form of a thinlayer deposited on a pure Si platelet, starting from aqueoussuspensions. The Si platelet was the same for all samples,so that the geometrical area of samples was constant.

Table 1Specific surface area, Ga2O3 and sulfate content in SZ and SZGn samples

Sample Specific surfacearea (BET, m2 g�1)

SO2�4

(wt%)SO2�

4

(nm�2)Ga2O3

(mol%)

SZ 98 3.7 2.4 –SZG1 109 3.9 2.3 1.1SZG3 113 4.3 2.4 3.4SZG5 112 4.1 2.3 5.6SZG9 102 3.9 2.4 9.7SZG15 94 3.2 2.1 15.5

The thickness of deposited samples was kept as constantas possible (�10 mg cm�2), in order to allow an acceptablenormalization of band intensities against a virtually con-stant sample weight. Home-made quartz infrared cells,equipped with KBr windows, suitable for either beam tem-perature or low (liquid nitrogen) temperature measure-ments, were connected to a conventional vacuum line(residual pressure <1.3 · 10�5 mbar) and allowing to per-form, in strictly in situ conditions, both thermal treatmentson the samples, and probe molecules adsorption/desorp-tion cycles on the activated samples. All IR spectra wererecorded at either beam temperature (BT), i.e., the temper-ature reached by samples in the IR beam (BT is estimatedto be some 30 K higher than the actual room temperature[11]), or at approximately liquid nitrogen temperature, i.e.,�100 K.

XPS measurements were performed with an M-ProbeInstrument (SSI) equipped with a monochromatic Al Kasource (1486.6 eV) with a spot size of 200 · 750 lm and apass energy of 25 eV, providing a resolution of 0.74 eV.The energy scale was calibrated with reference to the4f7/2 level of a freshly evaporated gold sample, at 84.00 ±0.1 eV, and with reference to the 2p3/2 and 3s levels of cop-per at 932.47 ± 0.1 and 122.39 ± 0.15 eV, respectively.With a monochromatic source, an electron flood gun wasused to compensate the build up of positive charge onthe insulating samples during the analyses: a value of10 eV was selected to perform measurements on these sam-ples. For all samples, the C1s peak level was taken as inter-nal reference at 284.6 eV. The accuracy of the bindingenergies is approximately ±0.2 eV.

Microcalorimetric measurements were carried out at303 K using a heat-flow microcalorimeter (standardTian-Calvet type, Seratam, France) connected to a gas-vol-umetric apparatus, which enabled the simultaneous deter-mination of evolved heats (integral adsorption heats, qint)and adsorbed amounts (na). The two sets of experimentaldata, converted into intensive parameters by normalizationper unit surface area, led to the calculation of differentialmolar adsorption heats (qdiff). To check for the possiblepresence of irreversible CO adsorption processes, the firstadsorption run (primary isotherm) of each sample was fol-lowed by overnight outgassing at the adsorption tempera-ture and then by a second adsorption run at the sametemperature and up to the same final pressure as before (sec-ondary isotherm). Note that the possible occurrence of irre-versible adsorption or surface reactions would lead to thenon-coincidence of primary and secondary adsorption runs.

3. Results and discussion

3.1. Catalytic activity

The effect of gallium content on the catalytic activity forn-butane isomerization at 523 K was investigated for allsamples. The profile of activity vs. time on stream is shownin Fig. 1. After an initial decrease, the activity stabilizes

0 200 400 600 800 1000 12000

2

4

6

8

10

12

14 SZ SZG1 SZG3 SZG5 SZG9 SZG15

Activ

ity (m

mol

h-1 m

-2)

Time (min.)

Fig. 1. Catalytic activity vs. time on stream for the reaction of n-butaneisomerization at 523 K over SZGn catalysts. Reaction conditions: 0.5 gcatalyst, T = 523 K, feed n-C4H10/H2 = 1/4, total flux = 25 ml min�1.

Table 2Surface atomic composition (external 40–50 A layer) of SZ and SZGn

samples as determined from XPS spectroscopy

Atomic ratio Zr/S Zr/Ga Ga/S

SZ 8.3 – –SZG1 8.4 50.2 0.2SZG5 8.2 8.6 1.0SZG9 6.0 4.6 1.3SZG15 3.0 2.8 1.0

3200340036003800

Abso

rban

ce

3760

3650

0.5

1

6

5

4

3

2

Wavenumber (cm-1)

Fig. 2. Absorbance FTIR spectra in the 3850–3000 cm�1 range (mOH

stretching region) of SZGn samples activated in vacuo at 673 K. 1, Non-promoted SZ reference material; 2, SZG1; 3, SZG3; 4, SZG5; 5, SZG9;and 6, SZG15.

42 G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49

and no further deactivation is observed within 20 h.Steady-state activity was found to be dependent on galliumloading: a promoting effect is clearly evident for Ga2O3

loadings between 1 and 9 mol%. SZGn samples withn = 3 or 5 showed the best conversion output, whereasfor the SZG15 sample the catalytic activity is lost. Thisshows that an appreciable promoting effect is obtained onlywith relatively low Ga2O3 concentrations. The selectivity toiso-butane was found to be higher than 95%, with onlyminor amounts of propane and methane being formed.

A thermal treatment in air at 723 K restores completelythe catalytic activity of all active samples, but the steep ini-tial decrease is still present in a second catalytic run. Thissuggests that the initial deactivation may be due to the for-mation of a carbonaceous deposit on a fraction of veryactive sites, which can be eliminated by thermal treatmentin an oxidizing atmosphere. The positive effect of hydrogenon the catalytic activity is probably ascribable to the for-mation, in the presence of hydrogen, of a more limitedamount of carbonaceous deposits. However, it can alsobe considered that this gas (at the catalytic test tempera-ture) could selectively interact with Ga3+ ions, increasingtheir catalytic activity.

3.2. XPS spectroscopy

XPS data reported in Table 2 show that, with increasingGa loading, Zr/Ga ratio decreases much more thanexpected on the basis of pure stoichiometric figures. More-over, up to 5% of Ga2O3 content, the Zr/S ratio remainsvirtually unchanged with respect to the unpromoted SZ ref-erence sample, whereas it decreases fast for Ga2O3 contentsP9 mol%, even when the overall SO2�

4 concentrationremains almost constant (Table 1), and it is known thatin crystalline ZrO2-based systems sulfates are located atthe surface [12]. These observations indicate that the outer-

most layers of ZrO2 crystallites tend to loose part of theirZr component and become enriched in the Ga componentwith increasing gallium loading. When Ga2O3 content isP9 mol%, the formation of an amorphous overlayer coat-ing the ZrO2 particles has been observed [9b].

It is also worth noting that the best catalytic activitytowards n-butane isomerization (in mild conditions), whichhas been observed for SZGn samples containing a medium-low Ga2O3 amount, seems to be related to: (i) a high andnearly unchanged Zr/S ratio; (ii) a low Ga/S ratio; and(iii) absence of a Ga-rich phase over-coating the highlycrystalline materials [9b].

3.3. FTIR spectroscopy—surface functionalities

In situ FTIR spectroscopy indicates that both, intrinsicfunctionalities (mainly OH groups, absorbing in the 3850–3000 cm�1 range; Fig. 2) and added functionalities (sulfate

1400 1200 1000

933

910

1077

νS-OνS=O

1015

1390

0.5

1

65

4

3

2

Abso

rban

ce

Wavenumber (cm-1)

*

Fig. 3. Absorbance FTIR spectra in the 1500–850 cm�1 range (mSO

stretching region) of SZGn samples activated in vacuo at 673 K. 1, Non-promoted SZ reference material; 2, SZG1; 3, SZG3; 4, SZG5; 5, SZG9;and 6, SZG15. Broken-line inset: mS–O spectral region of sulfates on asulfated m-ZrO2 sample.

G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49 43

species, absorbing in the 1500–850 cm�1 range; Fig. 3) arepresent at the surface of all SZGn samples. It is clear thatthe presence of Ga does not suppress surface functionalitiesnor does it introduce new ones, since in both spectralranges examined the spectral features of SZGn sampleslook quite similar to those typical of all active SZ-basedcatalysts [13–15]. However, some differences are noticeablefor what concerns the relative intensity of the IR absorp-tion bands of surface functionalities, as reported below.

3.3.1. Intrinsic functionalities

All catalysts, activated in a dynamic vacuum at 673 K,were brought to a medium-high dehydration degree, inorder to simulate the gas-flow activation step carried outbefore catalytic tests. When Ga2O3 content is not higherthan 9 mol%, it can be noted that the spectral region of sur-face OH groups (Fig. 2, spectra 1–5) shows a two-bandspattern, with spectral positions and relative intensities typ-ical of all catalytically active SZ-based systems and charac-teristic of a mainly tetragonal ZrO2 phase [15]. Two H-bondfree OH species are present, as revealed by a prominent low-m band (�3650 cm�1) due to tri-coordinated OH groups,and a weaker high-m band (�3760 cm�1) due to terminal(or mono-coordinated) OH species [15–18]. With increasingGa2O3 content, the tailing on the low-m side of the free OHbands progressively increases, while both intensity and

sharpness of the free OH bands decline. For the highestGa2O3 loading (spectrum 6 of Fig. 2), the whole OH stretch-ing range becomes deeply modified: the two IR bands due tofree OH species are either absent or hardly resolved at all,whereas a broad and unresolved envelope centred atm < 3600 cm�1 becomes dominant. This spectral behaviouris indicative of the existence of an extended system of nonwell-defined OH-containing species mutually interactingby H-bonding, in spite of the relatively high vacuum activa-tion temperature to which samples were subjected. It shouldbe noted that, in these conditions (i.e., when n > 9), theSZGn system presents an overall amorphous habit, asshown by structural and morphological data reported else-where [9b], and the sulfated ZrO2 particles exhibit a thinamorphous over-layer that, on the basis of the broad mOH

band discussed above, should be ascribed to a sort of amor-phous hydroxide-like species. A higher structural disordercould thus justify the observed gradual broadening of theIR absorption bands ascribed to m(O–H) stretching modes.

3.3.2. Added functionalities

The IR spectra of SZGn samples in the 1500–850 cm�1

spectral range are shown in Fig. 3. When gallium oxidecontent is lower than 9 mol% (Fig. 3, spectra 2–4) the spec-tra exhibit two main IR absorption bands centred at�1390 cm�1 (relatively sharp and asymmetric, with a tailon the low-m side) and �1015 cm�1 (broader and complex).A large spectral separation characterizes these two well-resolved absorption bands, and this spectral behaviourindicates that the bands are typical of non-coupled mS=O

and mS–O vibrational modes of isolated (i.e., with no mutualperturbations) and highly covalent poly-dentate surfacesulfates [19,20].

In the case of the unpromoted reference SZ sample(Fig. 3, spectrum 1), the presence of a sharp IR absorptionband at �1077 cm�1 and of two weak bands at m <950 cm�1 clearly indicates the existence of some amount ofa monoclinic ZrO2 phase [21] (see, for comparison, the bro-ken-line spectra at the top of Fig. 3, relative to the IR patternof sulfated m-ZrO2). The absence of these absorption bandsin SZGn samples with n > 2 (Fig. 3, spectra 3–6) confirmsthat one of the main roles played by Ga-doping on SZ sys-tems is the stabilisation of the tetragonal ZrO2 phase in thesurface layer, as also suggested by Raman spectra reportedelsewhere [9b].

For Ga2O3 loading P9 mol%, the spectral features ofsurface sulfates become deeply modified (Fig. 3, spectra 5and 6), even if the overall sulfates loading remains nearlyunchanged (Table 1). The bands due to covalent poly-dentate species are less intense, broader and somewhat lessseparated in frequency, while new strong, broad and ill-resolved spectral components, typical of poly-nuclear sul-fate species, grow in the 1300–1100 cm�1 range. This aspectis ascribed to the presence of increasing amounts of anamorphous phase coating the particles, as reported above.

It can be concluded that the spectral features of thesurface functionalities considered (i.e., intrinsic surface

0 20 40 60 80 1000

2

4

6

2220 2200 2180 2160

a

Wavenumber (cm-1)

2194

2201b

2160218022002220

5

4

1

Abs

orba

nce

0.1

1

23456

A (In

tegr

ated

are

a)

PCO (Torr)

1 2 3 4 5 6

Fig. 4. (a) Differential absorbance FTIR spectra (i.e., backgroundspectrum subtracted) in the mCO stretching region of CO adsorbed(pCO = 130 mbar) at BT on SZGn samples activated in vacuo at 673 K. 1,Non-promoted SZ reference material; 2, SZG1; 3, SZG3; 4, SZG5; 5,SZG9; and 6, SZG15. Inset: optical adsorption isotherms (integratedabsorbance vs. pCO) corresponding to BT CO uptake on the samplespresented in this figure. (b) Differential absorbance FTIR spectra in themCO stretching region of CO adsorbed (pCO = 130 mbar) at BT on SZGn

samples activated in vacuo at 673 K. 1, Non-promoted SZ referencematerial; 4, SZG5; and 5, SZG9. Individual curves (broken lines) showsome of the results of a spectral deconvolution carried out in the 2250–2150 cm�1 range.

44 G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49

OH species and added sulfate groups) are influenced in anon-uniform way by the amount of added Ga2O3. Up toa certain Ga2O3 content, virtually no differences are noted,except that of a stabilisation of the tetragonal ZrO2 phase,whereas for Ga2O3 contents above 5–6 mol% the surface ofSZGn samples becomes modified. In order to ascertainwhether any relationship exists between the spectralfeatures of surface species and surface activity, a systematicstudy of both qualitative and quantitative aspects ofsurface acidity was carried out by IR spectroscopy andadsorption microcalorimetry, as described below.

3.4. FTIR spectroscopy—surface acidity features

FTIR spectroscopy of basic probe molecules (CO and2,6-dimethylpyridine, 2,6-DMP) adsorbed onto Ga-pro-moted SZ systems has been resorted to in order to test sur-face acidity. Useful information can thus be obtainedabout: (i) strong Lewis acidity, as revealed by CO adsorp-tion at BT; (ii) total Lewis acidity and protonic (Brønsted)acidity, as revealed by CO adsorption at �100 K, andby 2,6-DMP adsorption at BT and desorption in mildconditions.

3.4.1. Carbon monoxide adsorption

Fig. 4a reports the IR spectra of CO (130 mbar)adsorbed at BT on SZGn systems, activated in vacuo at673 K. In both reference and Ga-doped systems only oneIR absorption band is present, centred at �2197 cm�1,and tailing on the low-m side. If we perform some band fit-tings on the mCO absorption band centred at �2197 cm�1,and starting from the plain SZ system (curve 1 ofFig. 4b), we can observe the presence of a high-m CO com-ponent (�2201 cm�1), and a low-m CO component(�2194 cm�1). On the basis of this spectral behaviour andof previously reported data [22,23], the �2201 cm�1 bandcan be ascribed to the C–O stretching mode of carbon mon-oxide adsorbed on a family of strong Lewis acid sites, rep-resented by coordinatively unsaturated (cus) Zr4+ cationslocated in crystallographic defective positions, whereas the�2194 cm�1 component can be ascribed to mCO mode ofCO molecules interacting with cus Zr4+ present on extendedordered patches of the ZrO2 crystallites. It is important torecall that on t-ZrO2 the presence of surface sulfate speciestends to reduce the amount of strong Lewis acid centres andto modify only to a rather limited extent (if at all) thestrength of strong Lewis acidity [21,23]. In fact, the strengthof Lewis acidity remains virtually unchanged with respect tosulfate-free t-ZrO2 and rather modest if compared, forinstance, with that of transition aluminas, which are knownto possess strong Lewis acidic sites (and mCO frequencies ashigh as �2220–2240 cm�1) [24].

In the experimental conditions adopted, no evidence wasobserved for specific carbonyl-like bands ascribable to COinteracting with strongly acidic cus Ga3+ species, sincethese species are expected to give a mCO band centred at�2225 cm�1 [25–27]. This indicates that, in spite of a high

Ga/Zr surface ratio, there is no significant amount of Ga3+

ions having a high coordinative unsaturation. On the con-trary, Fig. 4a and b indicate that as Ga loading increases,the (area normalised) intensity of the broad band due toZr4+–CO adducts declines sharply, loosing preferentiallythe low-m component. This suggests that a substantialdecrease of (at least one type of) coordinatively unsatu-rated Zr4+ ions having strong Lewis acidity is graduallybrought about by Ga loading.

Considering the IR optical adsorption isotherms (insetof Fig. 4a) corresponding to the BT interaction of CO withSZ and SZGn samples, it can be further seen that:

(i) In the case of the lowest Ga2O3 loading (n = 1, curve2), the very close similarity of spectroscopic profilewith the unpromoted SZ system (curve 1) seems tobe confirmed also on a semi-quantitative spectro-scopic ground, since the two optical adsorption iso-therms are nearly coincident in the whole pCO range

G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49 45

explored. However, quantitative messages derivingfrom purely spectroscopic data must be regarded withcaution.

(ii) In the case of medium Ga2O3 loadings (n = 3–5,curves 3 and 4), the amount of cus Zr4+ ions still pres-ent at the surface is very similar and seems to corre-spond to some 30–50% of the initial value.

(iii) For very high Ga2O3 loadings (n P 9, curves 5 and6), the amount of Lewis acidic sites substantiallydecreases (apparently, only some 10–15% of the start-ing spectral intensity remains).

Comparing the above data, relative to the adsorption ofCO at BT, with those relative to CO uptake at a low tem-perature (�100 K; some of the spectra are shown in Fig. 5),it can be added that: (i) even at a low temperature there isno evidence for the formation of carbonyl-like speciesinvolving strongly acidic Ga3+ centres; (ii) the mCO compo-nent at �2190 cm�1, due to weaker Lewis acid sites, isbroad and complex (i.e., it comprises several un-resolvedcomponents) and is far dominant, meaning that at low tem-perature many more families of sites can be involved in theadsorption process. However, as the Ga loading increases,this spectral component declines in intensity much fasterthan the high-m component, confirming the preferential

210021502200

2199 2191

2166

2x

2x

0.5

Abs

orba

nce

5

4

3

2

1

Wavenumber (cm-1)

Fig. 5. Differential absorbance FTIR spectra (i.e., background spectrumsubtracted) in the mCO stretching region of CO adsorbed (pCO in the 1–60 mbar range) at �100 K on SZGn samples activated in vacuo at 673 K.1: Non-promoted SZ reference material; 2: SZG1; 3: SZG5; 4: SZG9; and5: SZG15.

elimination of weaker Lewis acid sites. In fact, a weakmCO component at �2200–2205 cm�1 remains visible alsoon high-loaded systems; (iii) there is a spectral componentat �2165 cm�1, ascribable to the H-bonding interaction ofCO with surface OH species (energetically possible at lowtemperature). For Ga2O3 loadings up to 5 mol% this com-ponent becomes progressively weaker, indicating that oncrystalline ZrO2 surface OH species already involved inH-bonding (see in Fig. 2 the OH band tailing at low fre-quencies) cannot interact by H-bonding with CO mole-cules. However, for high Ga loadings (n P 9 mol%), the2165 cm�1 band becomes much stronger and much broader(extending down to �2140 cm�1). This suggests that in thehydroxide-like phase forming on the ZrO2 particles at highGa contents, formation of H-bonded complexes is the mainform of interaction with adsorbed CO.

3.4.2. 2,6-Dimethylpyridine adsorption

The unpromoted SZ system and all SZGn samples werealso characterized using 2,6-DMP as a probe molecule forIR spectroscopy; procedures were similar to thosedescribed in detail elsewhere [28]. Fig. 6a reports, as anexample, some spectra corresponding to the adsorptionof 2,6-DMP on the SZG3 sample. Protonic (i.e., Brønsted)acidity, witnessed by a characteristic bands envelope atm > 1620 cm�1 is always present, as expected in the pres-ence of surface sulfates. In fact, it is known that surfacesulfation of oxides induces protonic acidity of variablestrength [29]. At m < 1620 cm�1, other bands are alsoevident; in particular, when the spectrum is obtained inthe presence of an excess of 2,6-DMP (Fig. 6a, spectruma), two bands can be observed at �1580 and �1594cm�1, they are ascribed to physisorbed and H-bonded2,6-DMP species, respectively [28]. In addition, a med-ium-strength and broad shoulder is observed at�1610 cm�1 that is ascribable (on the basis of its spectro-scopic behaviour) to the 8a mode of Lewis-coordinated2,6-DMP species interacting with cus Zr4+ cations [30].

Upon BT outgassing of the 2,6-DMP excess (Fig. 6a,spectra b and c) some important features can be seen: (i)the overall intensity of the two-bands complex located atm < 1600 cm�1 is drastically decreased, as expected forweakly bound H-bonded and physisorbed species. As forthe band at �1608–1610 cm�1, due to the 8a mode ofLewis-coordinated 2,6-DMP, it becomes now quite evidentas a well resolved and complex component; (ii) the broadenvelope located at m > 1620 cm�1 (due to Brønsted-bound2,6-DMP species) turns out to be little affected by outgas-sing. It remains strong and is better resolved than it wasat the beginning, due to selective removal of adjoiningcomponents ascribable to vibrational combination modesof the more labile 2,6-DMP species. The latter effect is evenmore evident when evacuation of the base is carried out at423 K: spectrum d in Fig. 6a, the bands due to protonated2,6-DMP remain almost unaltered, whereas also the banddue to 2,6-DMP interacting with Lewis acid sites(�1610 cm�1) reaches a negligible residual concentration.

1650 1600 1550 155016001650

b

cd

b

a

1645

1630

1608

Lu-L

Lu-B

0.2

Abso

rban

ce

1580

1594

a

1580

1611

1628

654321

Wavenumber (cm-1)

0.2

1643

Fig. 6. (a) Differential absorbance FTIR spectra (i.e., background spectrum subtracted) in the range of the 8a–8b ring stretching modes of 2,6-DMPadsorbed/desorbed on SZG3 activated in vacuo at 673 K. Curve a: 2,6-DMP adsorption (�2.5 mbar) at BT; curves b and c: after evacuation at BT (1 and15 min, respectively); curve d: after evacuation at 423 K. (b) Differential absorbance FTIR spectra of 2,6-DMP desorbed at BT (15 min) on SZGn samplesactivated in vacuo at 673 K. 1: Non-promoted SZ reference material; 2: SZG1; 3: SZG3; 4: SZG5; 5: SZG9; 6: SZG15.

46 G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49

In fact, for several acidic oxide systems, it has beenreported that, if 2,6-DMP adsorption/desorption is carriedout at a temperature slightly above BT, steric hindrancefrom the two methyl groups of the 2,6-DMP molecules ren-ders the Lewis-coordinated species far more labile than theBrønsted-bound species [31,23b].

Fig. 6b reports some spectra relative to desorption at BT(15 min) of 2,6-DMP for the unpromoted and all the SZpromoted systems of interest. In these experimental condi-tions the fractions of physisorbed and H-bonded 2,6-DMPare not present anymore, and some differences among thevarious samples can be seen. In particular, as Ga2O3 con-tent increases: (i) the overall amount of 2,6-DMP thatremains adsorbed at the surface is gradually decreased.(ii) The complex 8a mode of Lewis-coordinated 2,6-DMPspecies (�1610 cm�1) decreases and becomes virtuallyabsent for n P 9 mol%. This is in agreement with the dataobtained using CO adsorption at both BT and low temper-ature (see above). (iii) Protonic (Brønsted) acidity is alwayspresent, but when the amount of Ga2O3 is maximum (spec-tra 6) its residual surface concentration is only marginal,even if the overall OH content in the particles over-layeris very high (as also witnessed by the strong H-bondinginteraction with CO at low temperature).

Since the catalytic activity of SZGn systems was foundto reach a maximum for n = 3–5, the simultaneous pres-ence of Lewis and Brønsted acid sites is likely to be a nec-essary condition for such an activity, as also proposed fornon-promoted SZ catalysts [32]. This suggests that catalyt-ically active Lewis acid sites belong to the family of cusZr4+ ions characterized by the highest coordinative unsat-

uration and charge-withdrawing power, and that activeBrønsted acid sites are associated to surface sulfate groups.

3.5. Adsorption microcalorimetry

In order to analyse CO adsorption at ambient tem-perature on more quantitative grounds, adsorption micro-calorimetry was used. This approach demonstrated to beparticularly successful in providing information on popula-tion and energetics of the sites present at the surface ofzirconia-based and other oxide systems [20,33–35]. Carbonmonoxide adsorption microcalorimetry was performed at303 K after activation of the samples at 673 K, i.e. afterthe same activation treatment used in the case of in situFTIR measurements.

Fig. 7a reports the volumetric adsorption isotherms (na

vs. pCO) for the reference SZ and for some SZGn systems.The following can be observed:

(i) CO adsorption turned out to be fully reversible, sincethe curves relative to primary isotherms (solid sym-bols) and secondary isotherms (open symbols) areclosely overlapping.

(ii) In the presence of gallium oxide, the overall popula-tion of Lewis acid sites active at 303 K becomes rap-idly lower the higher the amount of Ga2O3 species:compare curves b–d, for SZGn with n = 1, 3, and9 mol%, respectively, with curve a for the unpro-moted SZ sample. Note, in particular, that thesequantitative data indicate that 1% Ga2O3 reducesCO uptake by some 25–30%, whereas IR data

0 20 40 60 800.0

0.2

0.4

0.6

0.8

1.0a

d

c

a

b

n a (μm

ol m

-2)

PCO (Torr)

0 20 40 60 800.00.20.40.60.81.01.2

c q

int (

Jm

-2)*

102

PCO (Torr)

0.0 0.2 0.4 0.6 0.8 1.00

10

20

30

40

50

60 b

d c ba

q diff (

kJ m

ol-1)

na (μmol m-2)

Fig. 7. (a) Volumetric adsorption isotherms for CO uptake at 303 K onSZGn samples vacuum activated at 673 K. Inset: calorimetric adsorptionisotherm for CO uptake at 303 K on SZG3 vacuum activated at 673 K. (b)Differential heats of adsorption as a function of surface coverage for COuptake at 303 K on SZGn samples vacuum activated at 673 K. Curves a:the reference non-promoted SZ system. Curves b–d: SZGn systems, withn = 1, 3 and 9, respectively. Solid symbols: primary adsorption isotherm;open symbols: secondary adsorption isotherm.

G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49 47

(Fig. 4a) gave for the CO band of SZ and SZG1 vir-tually unchanged spectral profiles and intensity. Bycontrast, a reduction of CO uptake by some 50%upon loading 3 mol% Ga2O3, as suggested by IR datain Fig. 4a, is roughly confirmed by quantitative volu-metric data.

(iii) In the pCO range examined, the completion of themonolayer (h = 1) is not easily attained (especiallyin the case of SZ and of low-loaded SZGn samples),as indicated by curves still growing at the highestequilibrium pressure reached (�100 mbar). Thisshows that, especially in the case of reference SZand of low-loaded SZGn samples, a broad range ofLewis acid strength is likely to exist, which is increas-ingly probed by CO at BT upon increasing the equi-librium pressure.

(iv) Also, on the energetic ground, the process of COadsorption/desorption is fully reversible (see the insetto Fig. 7a). Moreover, the spectrum of energiesinvolved in CO uptake must be quite broad, sincethe qint vs. pCO plot increases in the first adsorptionstep much faster than does the corresponding na plot.This aspect becomes clearer by using qdiff plots, asreported below.

Fig. 7b reports the differential adsorption heat (qdiff,kJ mol�1) as a function of the amount of adsorbed CO(na, lmol m�2), as obtained from volumetric and calori-metric adsorption isotherms by graphic differentiation.These curves seem to present two different parts: the firstsection of the curves, corresponding to low coverages,shows a gradual and limited heat-versus-coverage decreasefor SZ and SZG1 samples (curves a and b), and a muchsharper and large decrease for the samples with 3% and9% Ga2O3 (curves c and d), respectively. The second regionof the curves, that is significantly present only in SZ, SZG1and (to a minor extent) in SZG3 samples (curves a-c),exhibits a differential adsorption heat that is approximatelyconstant or just slowly decreasing with increasing CO cov-erage. The presence of two regions in the qdiff curves con-firms that the CO adsorption process is heterogeneous. Inparticular, it indicates that there is a relatively small groupof highly energetic species (qdiff values between �50 and�35 kJ mol�1) and another group of less energetic spe-cies (qdiff values between �40 and �30 kJ mol�1). Theindications coming from FTIR spectroscopy data are thusconfirmed. SZGn catalysts exhibit the same type of adsorb-ing sites as those shown by non-promoted SZ systems (i.e.,cus Zr4+ cations), even in the presence of very high Galoadings. However, the surface population of these sitesdepends on Ga2O3 loading in a complex way: (i) there isa fraction of highly energetic sites (corresponding to thehigh mCO IR band) that, although decreasing when Galoading is increased, remains always present and coversthe same energy range regardless of the amount of Galoading; (ii) a second fraction of lower energy sites (corre-sponding to the mCO component at lower wavenumbers)nearly disappears for samples with high Ga2O3 loading.

4. Conclusions

Sulfated zirconia samples having a variable Ga2O3 con-tent (from 1 to 15% molar) were synthesized, with the aimof studying the promoting effect of gallium in the isomer-ization of n-butane. FTIR spectroscopy was used to studysurface functionalities (OH and sulfate groups) of thesamples; the adsorption of carbon monoxide and 2,6-dim-ethylpyridine on gallium promoted sulfated zirconia mate-rials showed that these catalysts combined Brønsted withLewis acidity. Lewis acidity was attributed to coordin-atively unsaturated Zr4+ ions located in defective crystal-lographic sites, whereas Brønsted acidity was associated tosurface sulfate groups. CO adsorption microcalorimetry

48 G. Cerrato et al. / Microporous and Mesoporous Materials 94 (2006) 40–49

showed that the total acid strength considerably decreaseswhen Ga2O3 content goes over 5 mol%. All these featurescorrelate with catalytic activity in n-butane isomerization.This catalytic activity was shown to reach a maximumprecisely at 3–5% Ga2O3, i.e., for those materials whichwere shown to possess an adequate Lewis/Brønsted acid-ity ratio.

As a first hypothesis, the promoting role of gallia on SZcan be attributed to stabilisation of the tetragonal crystalphase of zirconia. As shown in the first part of this series[9b], addition of gallia delays crystal growth of zirconiaparticles, thus favouring formation of the tetragonal poly-morph. This effect might be sufficient on its own for pro-motion; note that promotion with Al species causes asimilar effect and generates a positive response on catalyticperformance of SZ materials [7]. However, the presentstudy on gallia-promoted SZ suggests that an optimumratio of Lewis/Brønsted acid sites is also a relevant factorfor catalytic activity. Finally, it should be noted that Kloseet al. [36] have recently proposed that a main role of thepromoter (manganese and iron oxides in their study) inSZ is to enhance the oxidation potential of sulfate (SVI)surface species. These species would facilitate alkane isom-erization through an initial step involving oxidative dehy-drogenation. Such an effect could also be a further sourceof promotion in gallia-containing SZ, since previouslyreported TPR-MS data [9b] show that for moderate Galoadings reduction of SVI species occurs at a temperaturesignificantly lower than in the absence of gallia.

Acknowledgments

This research was partly financed with funds fromMIUR (Project FIRB 2001, code RBAU01X7PT_001)and from INSTM Consortium (Project PRISMA 2002).M.R.D. thanks the Spanish Ministry of Education for aPh.D. fellowship. Thanks are also due to Dr. C.L. Bianchi(University of Milan, Italy) for XPS measurements.

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