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ZrO2 catalysts
Applied Catalysis A: General 240 (2003) 119–128
WOx /ZrO2 catalysts Part 3. Surface coverage as investigated by low temperature CO adsorption: FT-IR and volumetric studies Giovanni Ferraris a,∗ , Sergio De Rossi a , Delia Gazzoli a , Ida Pettiti a , Mario Valigi a , Giuliana Magnacca b , Claudio Morterra b a
Istituto di Metodologie Inorganiche e dei Plasmi (IMIP) del CNR (ex “Centro di Studio su Struttura ed Attività Catalitica di Sistemi di Ossidi”) e Dipartimento di Chimica, Università “La Sapienza”, Piazzale Aldo Moro 5, 00185 Rome, Italy b Dipartimento di Chimica IFM, Università di Torino, Via Pietro Giuria 7, 10125 Torino, Italy Received 28 March 2002; received in revised form 18 July 2002; accepted 23 July 2002 Paper in honor of Professor Alessandro Cimino in the occasion of his 75th birthday.
Abstract The adsorption of CO at 77 K on zirconia-supported tungsten oxide (ZW) samples prepared by equilibrium adsorption or by impregnation and with tungsten content up to 12.4 W atoms nm−2 was studied with the aim of determining the coverage of ZrO2 surface by the supported tungsten oxospecies. As an essential background for quantitative volumetric measurements, ZW samples were investigated by FT-IR spectroscopy to identify the surface sites responsible for the CO adsorption. FT-IR showed 6+ that CO adsorbed selectively and irreversibly on Zr 4+ cus centers of the support surface, but not on Wcus sites. By volumetric determinations, the coverage of the zirconia surface by tungsten oxospecies was about 50% at maximum, irrespective of the preparation procedure and of the support crystallographic modification (monoclinic or tetragonal). Zirconia coverage remained significantly unchanged after leaching with a NH3 solution that removed tungsten oxospecies not directly interacting with the zirconia surface. This finding indicates that ZrO2 support coverage depends mainly on the tungsten species strongly anchored to the zirconia surface. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Zirconia-supported tungsta; Surface coverage; Low temperature CO chemisorption; FT-IR spectroscopy
1. Introduction The rationalization of the catalytic activity of supported catalysts requires a knowledge of the dispersion of the active species in order to draw a correlation ∗ Corresponding author. Tel.: +39-06-49913759; fax: +39-06-490324. E-mail address: [email protected] (G. Ferraris).
between active metal content and catalytic behavior. Many studies have therefore investigated the surface coverage of the support by the active species for various catalytic reactions. The surface coverage can be studied by various techniques, notably spectroscopy and volumetric adsorption analysis. These two methods, based on the chemisorption of probe molecules either on the free portion of the support surface or on the supported
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 4 1 2 - X
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species, are complementary in that the former identifies the surface adduct whereas the latter provides overall quantitative data. Among the probe molecules, carbon monoxide as well as carbon dioxide have been extensively used for titrating acid or basic surface sites by virtue of their basic or acid nature, respectively [1]. Titration by carbon dioxide relies upon the formation of carbonate and bicarbonate species by reaction of CO2 with oxo/hydroxo groups at the support surface. During the last decade, the technique has been successfully applied to titania-supported systems, such as MoO3 /TiO2 [2] and WO3 /TiO2 [3], because in these systems CO2 chemisorption allowed monitoring the free titania surface. By contrast, on alumina supported Cr [4] and Mo [5] oxides and on zirconia-supported Mo species [6], the room temperature chemisorption of CO2 significantly overestimated the support coverage. More recently, the selective irreversible chemisorption of CO at 77 K on coordinatively unsaturated (cus) Lewis acid sites of support has been used in catalytic systems in which CO2 chemisorption overestimates the coverage [5,7]. Because CO adsorption requires highly dehydroxylated samples care must be taken to control modifications induced by the high temperatures (up to 1200 K, for alumina) needed for pre-treatment. Moreover, it is necessary to check that the chemisorption sites belong only to the support phase. Among supported catalysts, those based on tungsten oxide are active for reactions such as isomerization, hydrogenation and alkene metathesis [8–10]. Recent attention has focussed on zirconia-supported tungsten oxide systems, usually prepared starting from hydrous zirconium oxide and calcined at high temperature to develop the activity [11–16]. We have recently reported a detailed characterization of this system and the catalytic behavior for the isomerization of n-butane [17,16]. These studies showed that the high activation temperature (1073 K) causes an interaction between tungsten oxide and zirconia. This interaction affects some support behaviors, such as crystallization and sintering, and the dispersion of tungsten species. By leaching the catalysts with an ammonia solution, we removed crystalline WO3 and tungsten species not directly anchored to the support, and determined the fraction of the tungsten interacting with ZrO2 . This raised the question of whether ZrO2 coverage depends on the various supported species.
In this study, we extended our investigation on surface coverage for zirconia-supported tungsten oxide system. We studied surface coverage by volumetric adsorption analysis and IR spectroscopy, using of CO at 77 K as the probe molecule. 2. Experimental methods 2.1. Sample preparation and characterization Hydrous zirconium oxide, designated as ZrO2 (383), was obtained by precipitation with a stream of ammonia-saturated nitrogen from a ZrOCl2 solution and dried at 383 K in air for 24 h [17]. Two sets of tungsten-containing samples were prepared. The first one was obtained by equilibrium adsorption by suspending a known amount of ZrO2 (383) in a large volume of aqueous ammonium metatungstate [(NH4 )6 H2 W12 O40 ·nH2 O, Fluka] solution at pH 10. The suspension was shaken for 72 h in a closed glass container and the solid was then separated by filtration and dried at 383 K for 24 h [17]. The second set was prepared by an impregnation procedure using ZrO2 (383) previously heated in air at 823 K for 5 h, ZrO2 (823). In particular, a known amount of ZrO2 (823) was contacted with a standardized aqueous ammonium metatungstate solution (liquid volume comparable to that of the solid) and dried at 383 K, 24 h. Both series of samples and a fraction of ZrO2 (383) were calcined in air at 1073 K for 5 h. Samples are designated as ZWx when prepared by equilibrium adsorption and ZWx(i) when obtained by impregnation where x stands for the analytical tungsten metal content (wt.%). Tungsten-free zirconium oxide, prepared by the same procedure outlined above and used as a first reference material, is indicated in the text as ZrO2 (1073). Fractions of some ZWx samples were submitted to a leaching treatment with an ammonia solution to remove WO3 and tungsten–oxygen species not directly interacting with the zirconia surface [18]. In particular, 0.25 g of sample were suspended in 20 ml of a 1 M NH3 solution and gently heated at 333 K for half an hour under stirring. After separation from the liquid fraction, the solid was washed by NH3 solution, dried at 343 K and then analyzed for tungsten. The leached samples are indicated by ZWxLy where L stands for
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leached and y is the tungsten content (wt.%) remaining in the solid residue. As a second reference plain zirconia system, we used an yttria-stabilized zirconia system, containing 3 mol% of Y2 O3 in solid solution. It is a pure tetragonal ZrO2 sample, prepared by the sol–gel method and designated TO3(1073), as in [19]. Pure WO3 was obtained, for comparison purposes, by heating ammonium metatungstate in air at 773 K for 5 h. All the samples were characterized by X-ray diffraction (XRD) for phase analysis [17]. Still for comparison purposes, a silica-supported tungsten oxide sample was prepared by impregnation of SiO2 (Aerosil 300, Degussa) with an aqueous ammonium metatungstate solution and subsequent calcination in air at 1073 K for 3 h. The sample, containing 16 wt.% of tungsten, is designated as SW16. XRD showed that it contains mostly monoclinic WO3 in agreement with literature data [20]. The tungsten content was determined spectrophotometrically by the thiocyanate method at λ = 400 nm [21,22]. Details are given elsewhere [17]. The specific surface area of all the samples and CO adsorption were measured in a conventional BET all glass volumetric apparatus. Surface area was measured after evacuating at 473 K for 0.5 h, using nitrogen as adsorbate (σN2 = 16.2 Å2 ). For some samples, the specific surface area per gram of ZrO2 , SZrO2 , was
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been calculated as follows: SZrO2 =
S (1 − (%W/100)(231.94/183.94))
where S is the specific surface area of a given sample, %W the tungsten content as weight percent, 231.94 g mol−1 the WO3 molecular molar mass and 183.94 g mol−1 the W atomic molar mass. The results of the characterization of the samples are reported in Table 1. 2.2. FT-IR spectroscopy FT-IR spectroscopic data were obtained using a Bruker spectrophotometer (Mod. Equinox 55) equipped with a MCT detector (resolution = 4 cm−1 ) and a cryogenic apparatus (Oxford Instrument) allowing the in situ activation of the samples at temperatures up to 1100 K under high-vacuum conditions, and allowing the sample to cool down to ca. 12 K. Samples were prepared as self-supporting pellets (ca. 40 mg cm−2 ), activated in 1.33 kPa of O2 at 1000 K for 1 h, cooled down to 400 K and outgassed in vacuo during cooling to room temperature. The irreversible CO uptake was qualitatively evaluated by observing the residual CO band left after the following operations: contacting the sample with 1.33 kPa CO at room temperature, decreasing the temperature
Table 1 XRD, BET specific surface areas and CO adsorption at 77 K results on WOx /ZrO2 Samples
ZrO2 ZrO2 (TO3) ZW0.62 ZW0.68 ZW1.15 ZW1.15L0.72 ZW5.5(i) ZW10(i) ZW7.78 ZW18.2 ZW18.2L5.2 ZW21.5 ZW21.5L6.1
XRD phases
m t m m m m m m m m m m m
+ ta +t +t +t +t +t +t +t +t +t +t +t
+ WO3 + WO3 + WO3 + WO3
m: monoclinic zirconia; t: tetragonal zirconia. a Very weak lines.
SBET (m2 g−1 )
18 53 12 19 21 21 33 35 70 48 62 58 67
W (atoms nm−2 )
– – 1.69 1.18 1.79 1.12 5.51 9.36 3.64 12.41 2.77 12.12 2.98
CO (irreversible) mlSTP g−1
molecules nm−2
3.2 9.0 1.3 2.3 2.4 2.6 2.9 3.6 7.0 4.6 6.9 5.8 7.0
4.78 4.59 2.91 3.25 3.07 3.33 2.36 2.77 2.69 2.57 2.99 2.69 2.81
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to 77 K, and eventually outgassing CO for 30 min at the same temperature. 2.3. Carbon monoxide adsorption For CO adsorption at 77 K, samples (0.3–1 g) were first heated in a pure dry oxygen (Matheson, 5 N) flow at 773 K for 2 h and then at 1000 K for 2 h. Samples were then cooled down to room temperature and evacuated for 1 h under high vacuum. Total CO uptakes were determined by zero pressure extrapolation of the adsorption isotherm in the PCO range 2.63–13.15 kPa. Samples were subsequently evacuated at the adsorption temperature for 0.5 h, and a second isotherm due to the reversible part of adsorption was determined. The difference between the total (first isotherm) and the reversible (second isotherm) uptakes in the parallel and linear part of the isotherms gave the irreversible part of adsorption (double isotherm method). Because repeated experiments on ZW21.5 sample using liquid nitrogen or liquid oxygen as thermostatic bath have demonstrated that the CO irreversible uptake can diminish by >30% for a temperature increase of only 12 K, in volumetric and FT-IR determinations the actual adsorption temperature must be carefully checked. The surface fractional coverage of ZrO2 by tungsten oxospecies, θ , were obtained as follows: first the specific uptake of CO on zirconia, fZrO2 , was calculated as fZrO2 = VZrO2 /SZrO2 , where SZrO2 is the surface area (m2 g−1 ) and VZrO2 the CO uptake (mlSTP g−1 ) on pure ZrO2 (1073) support. Then the coverage, θ , was obtained applying the equation (VZW /fZrO2 ) (1) θ =1− SZW where SZW and VZW are the specific surface area and the CO uptake values (mlSTP g−1 ) for the ZW samples. 3. Results 3.1. FT-IR characterization It is known that the low temperature adsorption of CO on vacuum activated microcrystalline non-transition metal oxidic systems brings about the formation of various adspecies, among which some
Lewis-coordinated carbonyl-like surface complexes. The IR spectral features of the latter species imply the formation of an usually complex absorption located in the range 2170–2220 cm−1 , i.e. at wavenumbers appreciably higher than in the case of the free CO molecule (2143 cm−1 ), as a consequence of molecular polarization and/or of a partial σ -charge release from CO to the coordinatively unsaturated (cus) surface cationic sites. The complex absorption may be made up of several ill-resolved components of odd and variable intensity, corresponding to families of cus cationic sites characterized by different crystallographic location and/or different degree of coordinative unsaturation. Spectra for the mainly monoclinic ZrO2 support, used here as a first reference system, activated at 1000 K and exposed to 1.33 kPa of CO at 77 K, exhibited two broad and complex absorptions centred at ∼2143 and ∼2168 cm−1 , respectively (see curve a, Fig. 1A): (i) the low-frequency absorption is due to the weak H-bonding interaction between CO and polar OH groups still present on the surface of ZrO2 [23]. Accordingly, the spectra for the hydroxyl region before and after CO adsorption (inset of Fig. 1, curves a and b) showed that this interaction mainly modified the bridged hydroxyl group at ∼3680 cm−1 . In addition, after evacuation, the ν CO band at 2143 cm−1 disappeared, and the ν CO signals recovered its original profile (curve a in the inset); (ii) the high-frequency absorption yielded band(s) due to CO adsorbed onto Zr 4+ cus sites [24]. In particular, a prolonged evacuation at the adsorption temperature left two absorptions (see curves e and f of Fig. 1A): a dominant low-ν component centred at ∼2175 cm−1 , and a far weaker high-ν component centred at ∼2190 cm−1 , indicating CO irreversibly bonded to two families of cus surface cationic sites. As this reference ZrO2 system, though prepared with the same procedure of the ZW catalysts, is structurally quite different from the ZW systems (in which the presence of tungsta is known to stabilize the tetragonal ZrO2 modification [17]), a second reference ZrO2 system stabilized by Y2 O3 in the tetragonal form (termed TO3, see Section 2), has been resorted to. sSimilar high-frequency ν CO absorptions have been found on this tetragonal ZrO2 , as already reported [19]. In particular, curve f of Fig. 1B shows that, on TO3
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Fig. 1. Spectra in the ν CO region relative to the adsorption of 1.33 kPa CO at 77 K, followed by outgassing at the same temperature for up to 30 min on: (section A) ZrO2 activated at 1000 K under oxidizing conditions (ZrO2 (1073)); (section B) tetragonal ZrO2 activated at 1000 K under oxidizing conditions (TO3(1073)). (a) 1.33 kPa CO; (b) 0.65 kPa CO; (c) 0.13 kPa CO; (d) 0.01 kPa CO; (e) 1 Pa CO; (f) 0.1 Pa CO; (f ) when present: under direct outgassing, the temperature of the cryogenic apparatus was allowed to raise to ∼100 K. In the inset: ν OH spectral region relative to the ZrO2 (1073) system before CO adsorption (solid-line curve a) and after maximum CO uptake (broken-line curve b).
vacuum activated at 1073 K, the fraction of CO irreversibly adsorbed at 77 K on cus Zr4+ sites presents a strong low-ν component at ∼2183 cm−1 , and a high-ν component of almost comparable intensity with apparent maximum at ∼2195 cm−1 . When band-resolved, the latter component is found to correspond to a discrete band centred at ν as high as 2197 cm−1 , as shown by the broken-line traces in Fig. 1B. If the CO evacuation temperature is allowed to raise somewhat above 77 K (∼100 K; see curve f of Fig. 1B), the high-ν CO species remains the sole irreversible component, centred at a frequency as high as 2200 cm−1 . The
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differences of both spectral position and spectral relative intensity observed for CO Lewis-coordinated at cus Zr4+ sites on the two different reference ZrO2 systems is ascribable to several factors, among which the crystalline phase and, even more important, the morphology of the ZrO2 crystallites. In fact, the high-ν component has been shown [25] to be ascribable to carbonyl-like species formed at cus Zr4+ sites located in crystallographically defective terminations (e.g. corners, edges, and high-index crystal planes): the TO3 reference system presents surface area higher than monoclinic ZrO2 (see Table 1), and its high-resolution electron micrographs (reported and commented elsewhere [23]) confirm a rather irregular particles morphology. To check the formation and stability of carbonyl-like 6+ centers, we studied by species possibly formed at Wcus IR spectroscopy the CO interaction at 77 K with plain WO3 and with SiO2 -supported WO3 (the SW16 sample). In the former case, no irreversible carbonyl-like bands ascribable to a strong interaction with cus cationic sites could be observed. As for the low temperature interaction of CO with SiO2 -supported WO3 , it deserves some comments for which some data are reported in Fig. 2. CO adsorption at 77 K on pure SiO2 yields two bands (Fig. 2A). The first band, centred at ∼2157 cm−1 , corresponds to CO weakly interacting by H-bonding with surface silanols [26]. Accordingly, in the ν OH spectral region the typical band of “free” silanols (inset of Fig. 2, curve a) changed in shape and frequency after CO adsorption (dotted curve b), and it was completely restored after outgassing at 77 K. The second band, broad and complex, centred at ∼2140 cm−1 , corresponds to CO forming a liquid-like physisorbed phase. After a brief outgassing at the adsorption temperature, both absorptions (2157 and 2145 cm−1 ) disappeared. The spectra for the SW16 sample (Fig. 2, section B) showed, in addition to the bands found for the plain SiO2 support, a weak signal at ∼2215 cm−1 , that can be ascribed to the adsorption of CO onto cus W6+ centers. Also this sample gave no irreversible uptake (curve e of Fig. 2B shows that no CO bands remain after sample evacuation at 77 K), confirming that the possible adsorption of CO onto cus W6+ sites does not give rise to a strong interaction, in agreement with Scheithauer et al. [27]. On spectroscopic analysis of typical samples of high-loading zirconia-supported tungsten oxide
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Fig. 2. Spectra in the ν CO region relative to the adsorption of 1.33 kPa CO at 77 K followed by outgassing at the same temperature for up to 30 min on SiO2 (section A) and on SW16 (section B) samples activated at 1000 K under oxidizing conditions. (a) 1.33 kPa CO; (b) 0.65 kPa CO; (c) 0.13 kPa CO; (d) 0.01 kPa CO; (e) if present: 1 Pa CO. In the inset: ν OH spectral region relative to SiO2 before CO adsorption (solid-line curve a) and after maximum CO uptake (broken-line curve b).
(ZW21.5 and ZW21.5L6.1), the overall spectral behavior does not seem to change much with respect to what reported above for the reference ZrO2 systems and, in particular, for the tetragonal TO3(1073). The high-pressure CO adsorption at 77 K on ZW21.5 yielded a broad absorption encompassing at least three components (see curve a of Fig. 3A). The signals at ∼2143 cm−1 (liquid-like phase) and ∼2164 cm−1 arose from weakly adsorbed species. In particular, the latter component (2164 cm−1 ) is ascribed to H-bonding interactions with surface residual OH groups, on the basis of a very weak perturbation observed in the ν OH spectral region (inset of Fig. 3A, curves a and b). The background spectrum, obtained from this sample treated at high temperature (inset of Fig. 3A, curve a), showed no evident signals of residual OH groups, owing to the low absorption coefficients usually exhibited by H-bonding free OH groups at the surface of metal oxides [28]. As for the last complex component, apparently centred (under a high CO pressure) at ∼2190 cm−1 , it clearly corre-
Fig. 3. Spectra in the ν CO region relative to the adsorption of 1.33 kPa CO at 77 K followed by outgassing at the same temperature for up to 30 min on: ZW21.46 (section A) and ZW21.46L6.09 samples (section B) activated at 1000 K under oxidizing conditions. (a) 1.33 kPa CO; (b) 0.65 kPa CO; (c) 0.13 kPa CO; (d) 0.01 kPa CO; (e) 0.1 Pa CO. In the inset: ν OH spectral region relative to ZW21.46 system before CO adsorption (solid-line curve a) and after maximum CO uptake (broken-line curve b).
sponds to CO more strongly bonded to cus surface cationic sites. Upon outgassing at the adsorption temperature, the broad bands at ∼2143 and ∼2164 cm−1 completely disappeared, as expected of weakly adsorbed species. The complex band initially centred at ∼2190 cm−1 progressively diminished and shifted to higher ν (Fig. 3A, curves a–d; note that the band intensity decrease is largely apparent, due to the initial overlap with lower-ν components), but an irreversible component remained clearly visible at ∼2200 cm−1 , even after a prolonged outgassing at 77 K (curve e of Fig. 3A). This behavior, quite similar to that reported above for the reference ZrO2 systems (and, in particular, for the more defective TO3 sample), together with the total absence of either reversible or stable bands at ν as high as ∼2215 cm−1 , indicate that the irreversible uptake of CO is not related to W6+ species, but must be still ascribed to the sole interaction with surface cus Zr4+ sites. The irreversible fraction of CO uptake can be thus thought to indicate the WOx -free fraction of the ZrO2 surface.
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Turning now to the ammonia-leached ZW21.5L6.1 sample (see Fig. 3B), it is noted that: (i) also in this sample there is a labile CO component at ∼2165 cm−1 , related to an H-bonding interaction between CO and residual OH groups, although also in this case the interaction is hardly visible at all in the ν OH spectral region (not shown); (ii) the complex absorption at 2190–2205 cm−1 , ascribed to the interaction of CO with surface Zr 4+ cus sites, is still made up of two closely overlapped components, with a high-ν one that is now dominant and centred at 2200–2202 cm−1 , as in the case of the most defective ZrO2 reference systems (note that the surface area of high-loading ZW systems, both before and after the leaching treatment, is quite similar to that of the TO3 reference sample; Table 1). Also the low-ν CO component reproduces the behavior previously reported for the high-area ZrO2 specimens, although its intensity is definitely lower than on pure tetragonal ZrO2 , especially in the case of the non-leached system shown in Fig. 3A. This decrease of one of the Lewis-coordinated CO components may possibly lead to some hypotheses on the most favorable location(s) of the WOn units when loading WO3 on ZrO2 , but this aspect goes far beyond the scope of the present note; (iii) again, a contribution of cus W6+ species to the irreversible adsorption of CO can be ruled out, because no bands can be observed at ∼2215 cm−1 .
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Fig. 4. Irreversible carbon monoxide uptake (expressed as CO molecules adsorbed nm−2 ) at 77 K on WOx /ZrO2 as a function of the surface tungsten density (expressed as W atoms nm−2 ). (䊉) ZrO2 ; (䊊) ZrO2 (TO3). Samples obtained by equilibrium adsorption: ( ) ZW0.68; ( ) ZW0.62; ( ) ZW7.78; (䊏) ZW18.2; (䊐) W18.2L5.2; (䉱) ZW1.15; () ZW1.15L0.72; (䉬) ZW21.5; (䉫) ZW21.5L6.1. (䉲) Samples obtained by impregnation.
The irreversible CO adsorption on ZW samples decreased from 4.78 to about 2.6 CO molecules nm−2 , as the surface tungsten density increases from 0 to 4 atoms nm−2 and then it leveled off at about 2.4 CO molecules nm−2 for higher tungsten loadings
3.2. Carbon monoxide adsorption at 77 K No appreciable irreversible CO uptake was measured for polycrystalline WO3 . This result was verified on the SW16 sample at the same temperature and agrees with literature infrared data on WO3 /ZrO2 system [27], showing that no carbonyl bands form on cus W+6 after exposure to CO at 85 K. The irreversible CO adsorption on our monoclinic ZrO2 (4.78 molecules nm−2 , column 4 of Table 1) and on the tetragonal ZrO2 (TO3) sample (4.59 molecules nm−2 ) compares well with the value (4.60 molecules nm−2 ) reported by Vaidynathan et al. [29] for ZrO2 (Degussa) calcined at 873 K. Monoclinic ZrO2 leached with NH3 yielded a close value (4.59 molecules nm−2 ), thus confirming that the uptake ability of zirconia depends neither on the crystalline phase nor on the leaching.
Fig. 5. Surface coverage of ZrO2 by WOx , determined from CO uptake (Eq. (1)), as a function of the surface tungsten density (expressed as W atoms nm−2 ) (䊉) ZrO2 . Samples obtained by equilibrium adsorption: ( ) ZW0.68; ( ) ZW0.62; ( ) ZW7.78; (䊏) ZW18.2; (䊐) W18.2L5.2; (䉱) ZW1.15; () ZW1.15L0.72; (䉬) ZW21.5; (䉫) ZW21.5L6.1. (䉲) Samples obtained by impregnation. (×) Data from [29].
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(Fig. 4). The plot for coverage, Eq. (1) showed that after an initial steep rise to about 40% for the addition of about 2 W atoms nm−2 , coverage reached saturation around 50% (Fig. 5). Coverage remained practically unchanged before and after leaching, although the overall W contents distinctly differed (ZW18.2 and ZW18.2L5.2, 46% versus 39%; ZW21.5 and ZW21.5L6.1, 44% versus 42%; Fig. 5). Our data agree well with those by Vaidynathan et al. [29], obtained by the adsorption of CO at 77 K for zirconia-supported tungsten oxide samples prepared by impregnation of monoclinic ZrO2 and calcined at 813 K (Fig. 5).
4. Discussion One of the main findings of this work is that we identified carbonyl species strongly held at 77 K on the free portion of the support in tungsten-containing zirconia samples. Exposing pure zirconia or tungstencontaining zirconia samples to CO at 77 K leads to FT-IR bands between ∼2175 and ∼2200 cm−1 which are resistant to evacuation. (Figs. 1 and 3) and can be unambiguously attributed to carbon monoxide strongly adsorbed on Zr 4+ cus centers located in various crystallographic configurations. On the other hand, we detected no stable carbonyls on cus W6+ species of the tungsten-containing zirconia samples before or after the removal of WO3 by leaching. These results suggest that the free ZrO2 surface can be obtained by quantitatively volumetric measuring the irreversibly adsorbed CO at 77 K on the surface of tungsten-containing zirconia samples. Another important finding is that the effective coverage does not depend on the preparation method, i.e. equilibration on amorphous hydrous zirconia or impregnation on calcined monoclinic zirconia. Moreover, coverage remains practically unchanged if the leachable part of the supported tungsten species is removed. In fact, the points related to concentrated samples (full symbols) shift to lower concentration after leaching (open symbols), but their ordinate is not appreciably changed (Fig. 5). This result indicates that the effective ZrO2 coverage depends mainly on the tungsten species strongly anchored to the zirconia surface. As the plateau in Fig. 5 also shows, no more than about 50% of the support surface, regardless of its crystalline phase, is engaged in the interaction with
tungsten species. Interestingly, this value is close to that reported by Pfaff et al. [7] for CO adsorption at 77 K on WO3 /Al2 O3 (55%). To investigate the saturation limit for surface coverage, we sought a relation of coverage with CO chemisorption and catalytic behavior of the WO3 /ZrO2 system. According to Vaidynathan et al. [29], a full close-packed monolayer of WO3 on ZrO2 (100% coverage) would require a W surface density of about 4.7 atoms nm−2 . If the W-species were deposited full-packed in a uniform way, without interaction hindrance, a linear relation should be found between the total W deposited (atoms nm−2 ) and coverage up to the maximum permitted value (4.7 atoms nm−2 ). This behavior, represented by the dashed line in Fig. 5, can be visualized as deposition of “tiles” of fixed area up to the maximum allowable number. Our data and those of Vaidynathan et al. [29] (Fig. 5) agree with the dashed line up to ca. 40% coverage. The marked deviation thereafter leads to a maximum experimental coverage of about 50%, i.e. ca. 4.7/2 = 2.35 atoms nm−2 . We can interpret this coverage limit as due to the saturation value of fully isolated species. As shown in Fig. 6, taking the plane (1 0 1) (Teufer [30] indexing), as the most frequently ex-
Fig. 6. Idealized (1 0 1) plane for tetragonal zirconia containing the maximum number of isolated tungsten (VI) species and the corresponding unit cell (dotted line). (䊐) oxygen atoms above the Zr atoms not shown in the vertices of triangles (side length = 0.35 nm).
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posed for tetragonal zirconia, fully isolated W species would cover only 50% of the surface, giving a W surface density of 2.35 atoms nm−2 (see Appendix A). This value is substantially lower than the average value of 3.1 atoms nm−2 we previously reported by determining the analytical W content and the BET areas for leached samples ([17], Fig. 3). The discrepancy implies that, as shown by Raman and X-ray absorption data [17], the tungsten atoms directly interacting with ZrO2 are not isolated but form clusters containing few W atoms. Interestingly, studying n-butane isomerization on zirconia-supported tungsten samples, we found no detectable activity for a W surface density lower than about 3 W atoms nm−2 ([16], Fig. 10), whereas starting from a W surface density higher than about 3 activity sharply increased. Leached samples showed similar behavior, thus suggesting an important role of W small clusters in n-butane isomerization. The similar saturation coverage we found in ZWx and ZWx(i) samples suggests that in both catalysts an interaction between the tungsten oxospecies and the zirconia surface takes place during heating. This interaction induces a protective effect towards zirconia sintering [17]. Heating at 1073 K decreased the surface area of ZrO2 from 42 to 18 m2 g−1 whereas in the ZWx(i) samples, a similar thermal treatment left the specific surface area of zirconia, SZrO2 , practically unchanged (for ZW5.5(i), 36 m2 g−1 and for ZW10(i), 40 m2 g−1 ). As a final comment, the low temperature adsorption of CO on the ZW system has a notable technical advantage, because the measurement of CO adsorption also gives the total surface area of the sample. In fact, the calculation of coverage by Eq. (1) involves adsorption measurement at 77 K of two adsorbates: nitrogen, to obtain the total area (BET method) and carbon monoxide, to determine the support surface area, owing to its selective irreversible adsorption. Total CO uptake is always determined, to calculate the irreversible part of the adsorption. In strict analogy with the BET method (N2 adsorption), on highly dehydroxylated ZrO2 surfaces and for CO relative pressure between 0.04 and 0.22 (CO saturation pressure at 77.3 K = 449 Torr [31]), the total CO uptake involves both the selective titration of cus Zr4+ sites and the development of a multilayer of physisorbed gas over the whole surface. When we compared the monolayer
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Fig. 7. Monolayer volume, Vm , on WOx /ZrO2 samples, obtained with the BET method using CO (ordinate) or N2 (abscissa) as adsorbate at 77 K.
volume values obtained by applying the BET method with the two adsorbates, we found a good linear correlation between these two quantities for the whole series of samples, namely for all coverages (Fig. 7). The best fit (correlation coefficient = 0.997) of the line passing through the experimental points, gives a slope of 0.95. Assuming that the BET method with nitrogen (σN2 = 16.2 Å2 ) gives the actual area of the samples, the carbon monoxide molecular cross-sectional area is 17.0 Å2 . This value is close to that calculated from CO liquid density or from physical adsorption data at 77.3 K (16.0 and 14.7 Å2 [32]). Hence the CO adsorption measurement at 77 K provides both total BET area and free support area. Using these values in Eq. (1) gives the absolute coverage for ZW samples.
5. Conclusions This FT-IR spectroscopy study of the adsorption of CO at 77 K on ZrO2 -supported tungsten oxide samples, heated at 1073 K in air, shows that CO irreversibly adsorbs selectively on Zr 4+ cus centers of the support surface. Under these conditions, because no 6+ sites, the covCO is irreversibly adsorbed on Wcus erage of the zirconia surface by tungsten oxospecies, determined by volumetric method, turns out to be
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about 50% at maximum, irrespective of the support crystallographic phase (monoclinic or tetragonal). ZrO2 coverage remains unchanged after a leaching treatment that removes tungsten oxospecies not directly interacting with the zirconia surface and therefore depends mainly on the tungsten oxospecies strongly anchored to the support surface. These species help to prevent sintering of zirconia both in the tetragonal and in the monoclinic form. Instead of spreading and reacting with the whole zirconia surface, tungsten species in our experimental conditions tend to form polymeric structures.
Acknowledgements The present research was partly financed by the Italian MURST, project COFIN 2000. Some of the low temperature CO spectra were carried out by Dr. E. Gribov, and his contribution is gratefully acknowledged.
Appendix A Fig. 6 shows isolated W(VI) species on an (1 0 1) idealized plane of tetragonal ZrO2 [33]. The dotted area contains one W isolated atom and consists of eight equilateral triangles, each having an area of 0.053 nm2 (i.e. (0.35 nm)2 × 0.866/2). The tungsten surface density is therefore: 1 W atoms/(8 triangles × 0.053 nm2 triangle−1 ) = 2.35 W atoms nm−2 . An idealized (1 1 1) plane of monoclinic zirconia yields a similar surface density. References [1] H. Knözinger, in: G. Ertl, H. Knözinger, J. Weitkamp (Eds), Handbook of Heterogeneous Catalysis, vol. 2, VCH, Weinheim, 1997, p. 707. [2] D.S. Kim, Y. Kurusu, I.E. Wachs, F.D. Hardcastle, K. Segawa, J. Catal. 120 (1989) 325. [3] N. Vaidyanathan, M. Houalla, D.M. Hercules, Catal. Lett. 43 (1997) 209. [4] S.J. Scierka, M. Houalla, A. Proctor, D.M. Hercules, J. Phys. Chem. 99 (1995) 1537. [5] V. Sazo, L. Gonzales, J. Goldwasser, M. Houalla, D.M. Hercules, Surf. Interf. Anal. 23 (1995) 367. [6] K. Stoppek-Langner, J. Goldwasser, M. Houalla, D.M. Hercules, Catal. Lett. 32 (1995) 263.
[7] C. Pfaff, M.J. Pérez Zurita, C. Scott, P. Patiño, M.R. Goldwasser, J. Goldwasser, F.M. Mulcahy, M. Houalla, D.M. Hercules, Catal. Lett. 49 (1997) 13. [8] T. Yamaguchi, Y. Tanaka, K. Tanabe, J. Catal. 65 (1980) 442. [9] W. Grunert, R. Feldhaus, K. Anders, E.S. Shpiro, K.M. Minachev, J. Catal. 120 (1989) 444. [10] D.G. Barton, S.L. Soled, E. Iglesia, Topics in Catal. 6 (1998) 87. [11] M. Hino, K. Arata, J. Chem. Soc. Chem. Commun. (1987) 1259. [12] G. Larsen, E. Lotero, R.D. Parra, in: J.W. Hightower, W.N. Delgass, E. Iglesias, A.T. Bell (Eds.), Proceedings of the 11th International Congress on Catalysis—40th Anniversary, Studies in Surface Science and Catalysis, vol. 101, Elsevier, Amsterdam, 1996, p. 543. [13] S.R. Vaudagna, R.A. Comelli, N.S. F´ıgoli, Appl. Catal. A: Gen. 164 (1997) 265. [14] J.C. Yori, C.R. Vera, J.M. Parera, Appl. Catal. A: Gen. 163 (1997) 165. [15] R.A. Boyse, E.I. Ko, J. Catal. 171 (1997) 191. [16] S. De Rossi, G. Ferraris, M. Valigi, D. Gazzoli, Appl. Catal. A.: Gen. 231 (2002) 173. [17] M. Valigi, D. Gazzoli, I. Pettiti, G. Mattei, S. Colonna, S. De Rossi, G. Ferraris, Appl. Catal. A.: Gen. 231 (2002) 159. [18] CRC Handbook of Chemistry and Physics, D.R. Lide (Ed.), 2nd ed., CRC Press, Boca Raton, 1991–1992, p. 109. [19] C. Morterra, G. Cerrato, V. Bolis, C. Lamberti, L. Ferroni, L. Montanaro, J. Chem. Soc., Faraday Trans. 91 (1) (1995) 113. [20] L.L. Murrel, D.C. Grenoble, R.T.K. Baker, E.B. Prestridge, S.C. Fung, R.R. Chianelli, S.P. Cramer, J. Catal. 79 (1983) 203. [21] C.E. Crouthamel, C.E. Johnson, Anal. Chem. 26 (1954) 1284. [22] D.F. Wood, R.T. Clarke, Analyst 83 (1958) 326. [23] C. Morterra, G. Cerrato, L. Ferroni, A. Negro, L. Montanaro, Appl. Surf. Sci. 65/66 (1993) 257. [24] G. Cerrato, S. Bordiga, S. Barbera, C. Morterra, Surf. Sci. 377/379 (1997) 50. [25] C. Morterra, G. Cerrato, F. Pinna, Spectrochim. Acta Part A 55 (1999) 95. [26] A. Zecchina, G. Ghiotti, L. Cerruti, C. Morterra, J. Chim. Phys. 68 (1971) 1479. [27] M. Scheithauer, R.K. Grasselli, H. Knözinger, Langmuir 14 (1998) 3019. [28] L.H. Little, Infrared Spectra of Adsorbed Species, Academic Press, London, 1966. [29] N. Vaidyanathan, M. Houalla, D.M. Hercules, Surf. Interf. Anal. 26 (1998) 415. [30] G. Teufer, Acta Crystallogr. 15 (1962) 1187. [31] D. Goodwin, J. Phys. Chem. Ref. Data 14 (4) (1985) 849. [32] H.L. Pickering, H.C. Eckstrom, J. Am. Chem. Soc. 74 (1952) 4775. [33] J. Livage, D. Vivien, C. Mazières, in: J.W. Mitchell, R.C. De Vries, R.W. Roberts, P. Cannon (Eds.), Proceedings of the 6th International Symposium on Reactivity of Solids, Schenectady, 25–30 August 1968, Wiley/Interscience, New York, 1969, p. 271.
WOx /ZrO2 catalysts Part 3. Surface coverage as investigated by low temperature CO adsorption: FT-IR and volumetric studies Giovanni Ferraris a,∗ , Sergio De Rossi a , Delia Gazzoli a , Ida Pettiti a , Mario Valigi a , Giuliana Magnacca b , Claudio Morterra b a
Istituto di Metodologie Inorganiche e dei Plasmi (IMIP) del CNR (ex “Centro di Studio su Struttura ed Attività Catalitica di Sistemi di Ossidi”) e Dipartimento di Chimica, Università “La Sapienza”, Piazzale Aldo Moro 5, 00185 Rome, Italy b Dipartimento di Chimica IFM, Università di Torino, Via Pietro Giuria 7, 10125 Torino, Italy Received 28 March 2002; received in revised form 18 July 2002; accepted 23 July 2002 Paper in honor of Professor Alessandro Cimino in the occasion of his 75th birthday.
Abstract The adsorption of CO at 77 K on zirconia-supported tungsten oxide (ZW) samples prepared by equilibrium adsorption or by impregnation and with tungsten content up to 12.4 W atoms nm−2 was studied with the aim of determining the coverage of ZrO2 surface by the supported tungsten oxospecies. As an essential background for quantitative volumetric measurements, ZW samples were investigated by FT-IR spectroscopy to identify the surface sites responsible for the CO adsorption. FT-IR showed 6+ that CO adsorbed selectively and irreversibly on Zr 4+ cus centers of the support surface, but not on Wcus sites. By volumetric determinations, the coverage of the zirconia surface by tungsten oxospecies was about 50% at maximum, irrespective of the preparation procedure and of the support crystallographic modification (monoclinic or tetragonal). Zirconia coverage remained significantly unchanged after leaching with a NH3 solution that removed tungsten oxospecies not directly interacting with the zirconia surface. This finding indicates that ZrO2 support coverage depends mainly on the tungsten species strongly anchored to the zirconia surface. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Zirconia-supported tungsta; Surface coverage; Low temperature CO chemisorption; FT-IR spectroscopy
1. Introduction The rationalization of the catalytic activity of supported catalysts requires a knowledge of the dispersion of the active species in order to draw a correlation ∗ Corresponding author. Tel.: +39-06-49913759; fax: +39-06-490324. E-mail address: [email protected] (G. Ferraris).
between active metal content and catalytic behavior. Many studies have therefore investigated the surface coverage of the support by the active species for various catalytic reactions. The surface coverage can be studied by various techniques, notably spectroscopy and volumetric adsorption analysis. These two methods, based on the chemisorption of probe molecules either on the free portion of the support surface or on the supported
0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 8 6 0 X ( 0 2 ) 0 0 4 1 2 - X
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species, are complementary in that the former identifies the surface adduct whereas the latter provides overall quantitative data. Among the probe molecules, carbon monoxide as well as carbon dioxide have been extensively used for titrating acid or basic surface sites by virtue of their basic or acid nature, respectively [1]. Titration by carbon dioxide relies upon the formation of carbonate and bicarbonate species by reaction of CO2 with oxo/hydroxo groups at the support surface. During the last decade, the technique has been successfully applied to titania-supported systems, such as MoO3 /TiO2 [2] and WO3 /TiO2 [3], because in these systems CO2 chemisorption allowed monitoring the free titania surface. By contrast, on alumina supported Cr [4] and Mo [5] oxides and on zirconia-supported Mo species [6], the room temperature chemisorption of CO2 significantly overestimated the support coverage. More recently, the selective irreversible chemisorption of CO at 77 K on coordinatively unsaturated (cus) Lewis acid sites of support has been used in catalytic systems in which CO2 chemisorption overestimates the coverage [5,7]. Because CO adsorption requires highly dehydroxylated samples care must be taken to control modifications induced by the high temperatures (up to 1200 K, for alumina) needed for pre-treatment. Moreover, it is necessary to check that the chemisorption sites belong only to the support phase. Among supported catalysts, those based on tungsten oxide are active for reactions such as isomerization, hydrogenation and alkene metathesis [8–10]. Recent attention has focussed on zirconia-supported tungsten oxide systems, usually prepared starting from hydrous zirconium oxide and calcined at high temperature to develop the activity [11–16]. We have recently reported a detailed characterization of this system and the catalytic behavior for the isomerization of n-butane [17,16]. These studies showed that the high activation temperature (1073 K) causes an interaction between tungsten oxide and zirconia. This interaction affects some support behaviors, such as crystallization and sintering, and the dispersion of tungsten species. By leaching the catalysts with an ammonia solution, we removed crystalline WO3 and tungsten species not directly anchored to the support, and determined the fraction of the tungsten interacting with ZrO2 . This raised the question of whether ZrO2 coverage depends on the various supported species.
In this study, we extended our investigation on surface coverage for zirconia-supported tungsten oxide system. We studied surface coverage by volumetric adsorption analysis and IR spectroscopy, using of CO at 77 K as the probe molecule. 2. Experimental methods 2.1. Sample preparation and characterization Hydrous zirconium oxide, designated as ZrO2 (383), was obtained by precipitation with a stream of ammonia-saturated nitrogen from a ZrOCl2 solution and dried at 383 K in air for 24 h [17]. Two sets of tungsten-containing samples were prepared. The first one was obtained by equilibrium adsorption by suspending a known amount of ZrO2 (383) in a large volume of aqueous ammonium metatungstate [(NH4 )6 H2 W12 O40 ·nH2 O, Fluka] solution at pH 10. The suspension was shaken for 72 h in a closed glass container and the solid was then separated by filtration and dried at 383 K for 24 h [17]. The second set was prepared by an impregnation procedure using ZrO2 (383) previously heated in air at 823 K for 5 h, ZrO2 (823). In particular, a known amount of ZrO2 (823) was contacted with a standardized aqueous ammonium metatungstate solution (liquid volume comparable to that of the solid) and dried at 383 K, 24 h. Both series of samples and a fraction of ZrO2 (383) were calcined in air at 1073 K for 5 h. Samples are designated as ZWx when prepared by equilibrium adsorption and ZWx(i) when obtained by impregnation where x stands for the analytical tungsten metal content (wt.%). Tungsten-free zirconium oxide, prepared by the same procedure outlined above and used as a first reference material, is indicated in the text as ZrO2 (1073). Fractions of some ZWx samples were submitted to a leaching treatment with an ammonia solution to remove WO3 and tungsten–oxygen species not directly interacting with the zirconia surface [18]. In particular, 0.25 g of sample were suspended in 20 ml of a 1 M NH3 solution and gently heated at 333 K for half an hour under stirring. After separation from the liquid fraction, the solid was washed by NH3 solution, dried at 343 K and then analyzed for tungsten. The leached samples are indicated by ZWxLy where L stands for
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leached and y is the tungsten content (wt.%) remaining in the solid residue. As a second reference plain zirconia system, we used an yttria-stabilized zirconia system, containing 3 mol% of Y2 O3 in solid solution. It is a pure tetragonal ZrO2 sample, prepared by the sol–gel method and designated TO3(1073), as in [19]. Pure WO3 was obtained, for comparison purposes, by heating ammonium metatungstate in air at 773 K for 5 h. All the samples were characterized by X-ray diffraction (XRD) for phase analysis [17]. Still for comparison purposes, a silica-supported tungsten oxide sample was prepared by impregnation of SiO2 (Aerosil 300, Degussa) with an aqueous ammonium metatungstate solution and subsequent calcination in air at 1073 K for 3 h. The sample, containing 16 wt.% of tungsten, is designated as SW16. XRD showed that it contains mostly monoclinic WO3 in agreement with literature data [20]. The tungsten content was determined spectrophotometrically by the thiocyanate method at λ = 400 nm [21,22]. Details are given elsewhere [17]. The specific surface area of all the samples and CO adsorption were measured in a conventional BET all glass volumetric apparatus. Surface area was measured after evacuating at 473 K for 0.5 h, using nitrogen as adsorbate (σN2 = 16.2 Å2 ). For some samples, the specific surface area per gram of ZrO2 , SZrO2 , was
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been calculated as follows: SZrO2 =
S (1 − (%W/100)(231.94/183.94))
where S is the specific surface area of a given sample, %W the tungsten content as weight percent, 231.94 g mol−1 the WO3 molecular molar mass and 183.94 g mol−1 the W atomic molar mass. The results of the characterization of the samples are reported in Table 1. 2.2. FT-IR spectroscopy FT-IR spectroscopic data were obtained using a Bruker spectrophotometer (Mod. Equinox 55) equipped with a MCT detector (resolution = 4 cm−1 ) and a cryogenic apparatus (Oxford Instrument) allowing the in situ activation of the samples at temperatures up to 1100 K under high-vacuum conditions, and allowing the sample to cool down to ca. 12 K. Samples were prepared as self-supporting pellets (ca. 40 mg cm−2 ), activated in 1.33 kPa of O2 at 1000 K for 1 h, cooled down to 400 K and outgassed in vacuo during cooling to room temperature. The irreversible CO uptake was qualitatively evaluated by observing the residual CO band left after the following operations: contacting the sample with 1.33 kPa CO at room temperature, decreasing the temperature
Table 1 XRD, BET specific surface areas and CO adsorption at 77 K results on WOx /ZrO2 Samples
ZrO2 ZrO2 (TO3) ZW0.62 ZW0.68 ZW1.15 ZW1.15L0.72 ZW5.5(i) ZW10(i) ZW7.78 ZW18.2 ZW18.2L5.2 ZW21.5 ZW21.5L6.1
XRD phases
m t m m m m m m m m m m m
+ ta +t +t +t +t +t +t +t +t +t +t +t
+ WO3 + WO3 + WO3 + WO3
m: monoclinic zirconia; t: tetragonal zirconia. a Very weak lines.
SBET (m2 g−1 )
18 53 12 19 21 21 33 35 70 48 62 58 67
W (atoms nm−2 )
– – 1.69 1.18 1.79 1.12 5.51 9.36 3.64 12.41 2.77 12.12 2.98
CO (irreversible) mlSTP g−1
molecules nm−2
3.2 9.0 1.3 2.3 2.4 2.6 2.9 3.6 7.0 4.6 6.9 5.8 7.0
4.78 4.59 2.91 3.25 3.07 3.33 2.36 2.77 2.69 2.57 2.99 2.69 2.81
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to 77 K, and eventually outgassing CO for 30 min at the same temperature. 2.3. Carbon monoxide adsorption For CO adsorption at 77 K, samples (0.3–1 g) were first heated in a pure dry oxygen (Matheson, 5 N) flow at 773 K for 2 h and then at 1000 K for 2 h. Samples were then cooled down to room temperature and evacuated for 1 h under high vacuum. Total CO uptakes were determined by zero pressure extrapolation of the adsorption isotherm in the PCO range 2.63–13.15 kPa. Samples were subsequently evacuated at the adsorption temperature for 0.5 h, and a second isotherm due to the reversible part of adsorption was determined. The difference between the total (first isotherm) and the reversible (second isotherm) uptakes in the parallel and linear part of the isotherms gave the irreversible part of adsorption (double isotherm method). Because repeated experiments on ZW21.5 sample using liquid nitrogen or liquid oxygen as thermostatic bath have demonstrated that the CO irreversible uptake can diminish by >30% for a temperature increase of only 12 K, in volumetric and FT-IR determinations the actual adsorption temperature must be carefully checked. The surface fractional coverage of ZrO2 by tungsten oxospecies, θ , were obtained as follows: first the specific uptake of CO on zirconia, fZrO2 , was calculated as fZrO2 = VZrO2 /SZrO2 , where SZrO2 is the surface area (m2 g−1 ) and VZrO2 the CO uptake (mlSTP g−1 ) on pure ZrO2 (1073) support. Then the coverage, θ , was obtained applying the equation (VZW /fZrO2 ) (1) θ =1− SZW where SZW and VZW are the specific surface area and the CO uptake values (mlSTP g−1 ) for the ZW samples. 3. Results 3.1. FT-IR characterization It is known that the low temperature adsorption of CO on vacuum activated microcrystalline non-transition metal oxidic systems brings about the formation of various adspecies, among which some
Lewis-coordinated carbonyl-like surface complexes. The IR spectral features of the latter species imply the formation of an usually complex absorption located in the range 2170–2220 cm−1 , i.e. at wavenumbers appreciably higher than in the case of the free CO molecule (2143 cm−1 ), as a consequence of molecular polarization and/or of a partial σ -charge release from CO to the coordinatively unsaturated (cus) surface cationic sites. The complex absorption may be made up of several ill-resolved components of odd and variable intensity, corresponding to families of cus cationic sites characterized by different crystallographic location and/or different degree of coordinative unsaturation. Spectra for the mainly monoclinic ZrO2 support, used here as a first reference system, activated at 1000 K and exposed to 1.33 kPa of CO at 77 K, exhibited two broad and complex absorptions centred at ∼2143 and ∼2168 cm−1 , respectively (see curve a, Fig. 1A): (i) the low-frequency absorption is due to the weak H-bonding interaction between CO and polar OH groups still present on the surface of ZrO2 [23]. Accordingly, the spectra for the hydroxyl region before and after CO adsorption (inset of Fig. 1, curves a and b) showed that this interaction mainly modified the bridged hydroxyl group at ∼3680 cm−1 . In addition, after evacuation, the ν CO band at 2143 cm−1 disappeared, and the ν CO signals recovered its original profile (curve a in the inset); (ii) the high-frequency absorption yielded band(s) due to CO adsorbed onto Zr 4+ cus sites [24]. In particular, a prolonged evacuation at the adsorption temperature left two absorptions (see curves e and f of Fig. 1A): a dominant low-ν component centred at ∼2175 cm−1 , and a far weaker high-ν component centred at ∼2190 cm−1 , indicating CO irreversibly bonded to two families of cus surface cationic sites. As this reference ZrO2 system, though prepared with the same procedure of the ZW catalysts, is structurally quite different from the ZW systems (in which the presence of tungsta is known to stabilize the tetragonal ZrO2 modification [17]), a second reference ZrO2 system stabilized by Y2 O3 in the tetragonal form (termed TO3, see Section 2), has been resorted to. sSimilar high-frequency ν CO absorptions have been found on this tetragonal ZrO2 , as already reported [19]. In particular, curve f of Fig. 1B shows that, on TO3
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Fig. 1. Spectra in the ν CO region relative to the adsorption of 1.33 kPa CO at 77 K, followed by outgassing at the same temperature for up to 30 min on: (section A) ZrO2 activated at 1000 K under oxidizing conditions (ZrO2 (1073)); (section B) tetragonal ZrO2 activated at 1000 K under oxidizing conditions (TO3(1073)). (a) 1.33 kPa CO; (b) 0.65 kPa CO; (c) 0.13 kPa CO; (d) 0.01 kPa CO; (e) 1 Pa CO; (f) 0.1 Pa CO; (f ) when present: under direct outgassing, the temperature of the cryogenic apparatus was allowed to raise to ∼100 K. In the inset: ν OH spectral region relative to the ZrO2 (1073) system before CO adsorption (solid-line curve a) and after maximum CO uptake (broken-line curve b).
vacuum activated at 1073 K, the fraction of CO irreversibly adsorbed at 77 K on cus Zr4+ sites presents a strong low-ν component at ∼2183 cm−1 , and a high-ν component of almost comparable intensity with apparent maximum at ∼2195 cm−1 . When band-resolved, the latter component is found to correspond to a discrete band centred at ν as high as 2197 cm−1 , as shown by the broken-line traces in Fig. 1B. If the CO evacuation temperature is allowed to raise somewhat above 77 K (∼100 K; see curve f of Fig. 1B), the high-ν CO species remains the sole irreversible component, centred at a frequency as high as 2200 cm−1 . The
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differences of both spectral position and spectral relative intensity observed for CO Lewis-coordinated at cus Zr4+ sites on the two different reference ZrO2 systems is ascribable to several factors, among which the crystalline phase and, even more important, the morphology of the ZrO2 crystallites. In fact, the high-ν component has been shown [25] to be ascribable to carbonyl-like species formed at cus Zr4+ sites located in crystallographically defective terminations (e.g. corners, edges, and high-index crystal planes): the TO3 reference system presents surface area higher than monoclinic ZrO2 (see Table 1), and its high-resolution electron micrographs (reported and commented elsewhere [23]) confirm a rather irregular particles morphology. To check the formation and stability of carbonyl-like 6+ centers, we studied by species possibly formed at Wcus IR spectroscopy the CO interaction at 77 K with plain WO3 and with SiO2 -supported WO3 (the SW16 sample). In the former case, no irreversible carbonyl-like bands ascribable to a strong interaction with cus cationic sites could be observed. As for the low temperature interaction of CO with SiO2 -supported WO3 , it deserves some comments for which some data are reported in Fig. 2. CO adsorption at 77 K on pure SiO2 yields two bands (Fig. 2A). The first band, centred at ∼2157 cm−1 , corresponds to CO weakly interacting by H-bonding with surface silanols [26]. Accordingly, in the ν OH spectral region the typical band of “free” silanols (inset of Fig. 2, curve a) changed in shape and frequency after CO adsorption (dotted curve b), and it was completely restored after outgassing at 77 K. The second band, broad and complex, centred at ∼2140 cm−1 , corresponds to CO forming a liquid-like physisorbed phase. After a brief outgassing at the adsorption temperature, both absorptions (2157 and 2145 cm−1 ) disappeared. The spectra for the SW16 sample (Fig. 2, section B) showed, in addition to the bands found for the plain SiO2 support, a weak signal at ∼2215 cm−1 , that can be ascribed to the adsorption of CO onto cus W6+ centers. Also this sample gave no irreversible uptake (curve e of Fig. 2B shows that no CO bands remain after sample evacuation at 77 K), confirming that the possible adsorption of CO onto cus W6+ sites does not give rise to a strong interaction, in agreement with Scheithauer et al. [27]. On spectroscopic analysis of typical samples of high-loading zirconia-supported tungsten oxide
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Fig. 2. Spectra in the ν CO region relative to the adsorption of 1.33 kPa CO at 77 K followed by outgassing at the same temperature for up to 30 min on SiO2 (section A) and on SW16 (section B) samples activated at 1000 K under oxidizing conditions. (a) 1.33 kPa CO; (b) 0.65 kPa CO; (c) 0.13 kPa CO; (d) 0.01 kPa CO; (e) if present: 1 Pa CO. In the inset: ν OH spectral region relative to SiO2 before CO adsorption (solid-line curve a) and after maximum CO uptake (broken-line curve b).
(ZW21.5 and ZW21.5L6.1), the overall spectral behavior does not seem to change much with respect to what reported above for the reference ZrO2 systems and, in particular, for the tetragonal TO3(1073). The high-pressure CO adsorption at 77 K on ZW21.5 yielded a broad absorption encompassing at least three components (see curve a of Fig. 3A). The signals at ∼2143 cm−1 (liquid-like phase) and ∼2164 cm−1 arose from weakly adsorbed species. In particular, the latter component (2164 cm−1 ) is ascribed to H-bonding interactions with surface residual OH groups, on the basis of a very weak perturbation observed in the ν OH spectral region (inset of Fig. 3A, curves a and b). The background spectrum, obtained from this sample treated at high temperature (inset of Fig. 3A, curve a), showed no evident signals of residual OH groups, owing to the low absorption coefficients usually exhibited by H-bonding free OH groups at the surface of metal oxides [28]. As for the last complex component, apparently centred (under a high CO pressure) at ∼2190 cm−1 , it clearly corre-
Fig. 3. Spectra in the ν CO region relative to the adsorption of 1.33 kPa CO at 77 K followed by outgassing at the same temperature for up to 30 min on: ZW21.46 (section A) and ZW21.46L6.09 samples (section B) activated at 1000 K under oxidizing conditions. (a) 1.33 kPa CO; (b) 0.65 kPa CO; (c) 0.13 kPa CO; (d) 0.01 kPa CO; (e) 0.1 Pa CO. In the inset: ν OH spectral region relative to ZW21.46 system before CO adsorption (solid-line curve a) and after maximum CO uptake (broken-line curve b).
sponds to CO more strongly bonded to cus surface cationic sites. Upon outgassing at the adsorption temperature, the broad bands at ∼2143 and ∼2164 cm−1 completely disappeared, as expected of weakly adsorbed species. The complex band initially centred at ∼2190 cm−1 progressively diminished and shifted to higher ν (Fig. 3A, curves a–d; note that the band intensity decrease is largely apparent, due to the initial overlap with lower-ν components), but an irreversible component remained clearly visible at ∼2200 cm−1 , even after a prolonged outgassing at 77 K (curve e of Fig. 3A). This behavior, quite similar to that reported above for the reference ZrO2 systems (and, in particular, for the more defective TO3 sample), together with the total absence of either reversible or stable bands at ν as high as ∼2215 cm−1 , indicate that the irreversible uptake of CO is not related to W6+ species, but must be still ascribed to the sole interaction with surface cus Zr4+ sites. The irreversible fraction of CO uptake can be thus thought to indicate the WOx -free fraction of the ZrO2 surface.
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Turning now to the ammonia-leached ZW21.5L6.1 sample (see Fig. 3B), it is noted that: (i) also in this sample there is a labile CO component at ∼2165 cm−1 , related to an H-bonding interaction between CO and residual OH groups, although also in this case the interaction is hardly visible at all in the ν OH spectral region (not shown); (ii) the complex absorption at 2190–2205 cm−1 , ascribed to the interaction of CO with surface Zr 4+ cus sites, is still made up of two closely overlapped components, with a high-ν one that is now dominant and centred at 2200–2202 cm−1 , as in the case of the most defective ZrO2 reference systems (note that the surface area of high-loading ZW systems, both before and after the leaching treatment, is quite similar to that of the TO3 reference sample; Table 1). Also the low-ν CO component reproduces the behavior previously reported for the high-area ZrO2 specimens, although its intensity is definitely lower than on pure tetragonal ZrO2 , especially in the case of the non-leached system shown in Fig. 3A. This decrease of one of the Lewis-coordinated CO components may possibly lead to some hypotheses on the most favorable location(s) of the WOn units when loading WO3 on ZrO2 , but this aspect goes far beyond the scope of the present note; (iii) again, a contribution of cus W6+ species to the irreversible adsorption of CO can be ruled out, because no bands can be observed at ∼2215 cm−1 .
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Fig. 4. Irreversible carbon monoxide uptake (expressed as CO molecules adsorbed nm−2 ) at 77 K on WOx /ZrO2 as a function of the surface tungsten density (expressed as W atoms nm−2 ). (䊉) ZrO2 ; (䊊) ZrO2 (TO3). Samples obtained by equilibrium adsorption: ( ) ZW0.68; ( ) ZW0.62; ( ) ZW7.78; (䊏) ZW18.2; (䊐) W18.2L5.2; (䉱) ZW1.15; () ZW1.15L0.72; (䉬) ZW21.5; (䉫) ZW21.5L6.1. (䉲) Samples obtained by impregnation.
The irreversible CO adsorption on ZW samples decreased from 4.78 to about 2.6 CO molecules nm−2 , as the surface tungsten density increases from 0 to 4 atoms nm−2 and then it leveled off at about 2.4 CO molecules nm−2 for higher tungsten loadings
3.2. Carbon monoxide adsorption at 77 K No appreciable irreversible CO uptake was measured for polycrystalline WO3 . This result was verified on the SW16 sample at the same temperature and agrees with literature infrared data on WO3 /ZrO2 system [27], showing that no carbonyl bands form on cus W+6 after exposure to CO at 85 K. The irreversible CO adsorption on our monoclinic ZrO2 (4.78 molecules nm−2 , column 4 of Table 1) and on the tetragonal ZrO2 (TO3) sample (4.59 molecules nm−2 ) compares well with the value (4.60 molecules nm−2 ) reported by Vaidynathan et al. [29] for ZrO2 (Degussa) calcined at 873 K. Monoclinic ZrO2 leached with NH3 yielded a close value (4.59 molecules nm−2 ), thus confirming that the uptake ability of zirconia depends neither on the crystalline phase nor on the leaching.
Fig. 5. Surface coverage of ZrO2 by WOx , determined from CO uptake (Eq. (1)), as a function of the surface tungsten density (expressed as W atoms nm−2 ) (䊉) ZrO2 . Samples obtained by equilibrium adsorption: ( ) ZW0.68; ( ) ZW0.62; ( ) ZW7.78; (䊏) ZW18.2; (䊐) W18.2L5.2; (䉱) ZW1.15; () ZW1.15L0.72; (䉬) ZW21.5; (䉫) ZW21.5L6.1. (䉲) Samples obtained by impregnation. (×) Data from [29].
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(Fig. 4). The plot for coverage, Eq. (1) showed that after an initial steep rise to about 40% for the addition of about 2 W atoms nm−2 , coverage reached saturation around 50% (Fig. 5). Coverage remained practically unchanged before and after leaching, although the overall W contents distinctly differed (ZW18.2 and ZW18.2L5.2, 46% versus 39%; ZW21.5 and ZW21.5L6.1, 44% versus 42%; Fig. 5). Our data agree well with those by Vaidynathan et al. [29], obtained by the adsorption of CO at 77 K for zirconia-supported tungsten oxide samples prepared by impregnation of monoclinic ZrO2 and calcined at 813 K (Fig. 5).
4. Discussion One of the main findings of this work is that we identified carbonyl species strongly held at 77 K on the free portion of the support in tungsten-containing zirconia samples. Exposing pure zirconia or tungstencontaining zirconia samples to CO at 77 K leads to FT-IR bands between ∼2175 and ∼2200 cm−1 which are resistant to evacuation. (Figs. 1 and 3) and can be unambiguously attributed to carbon monoxide strongly adsorbed on Zr 4+ cus centers located in various crystallographic configurations. On the other hand, we detected no stable carbonyls on cus W6+ species of the tungsten-containing zirconia samples before or after the removal of WO3 by leaching. These results suggest that the free ZrO2 surface can be obtained by quantitatively volumetric measuring the irreversibly adsorbed CO at 77 K on the surface of tungsten-containing zirconia samples. Another important finding is that the effective coverage does not depend on the preparation method, i.e. equilibration on amorphous hydrous zirconia or impregnation on calcined monoclinic zirconia. Moreover, coverage remains practically unchanged if the leachable part of the supported tungsten species is removed. In fact, the points related to concentrated samples (full symbols) shift to lower concentration after leaching (open symbols), but their ordinate is not appreciably changed (Fig. 5). This result indicates that the effective ZrO2 coverage depends mainly on the tungsten species strongly anchored to the zirconia surface. As the plateau in Fig. 5 also shows, no more than about 50% of the support surface, regardless of its crystalline phase, is engaged in the interaction with
tungsten species. Interestingly, this value is close to that reported by Pfaff et al. [7] for CO adsorption at 77 K on WO3 /Al2 O3 (55%). To investigate the saturation limit for surface coverage, we sought a relation of coverage with CO chemisorption and catalytic behavior of the WO3 /ZrO2 system. According to Vaidynathan et al. [29], a full close-packed monolayer of WO3 on ZrO2 (100% coverage) would require a W surface density of about 4.7 atoms nm−2 . If the W-species were deposited full-packed in a uniform way, without interaction hindrance, a linear relation should be found between the total W deposited (atoms nm−2 ) and coverage up to the maximum permitted value (4.7 atoms nm−2 ). This behavior, represented by the dashed line in Fig. 5, can be visualized as deposition of “tiles” of fixed area up to the maximum allowable number. Our data and those of Vaidynathan et al. [29] (Fig. 5) agree with the dashed line up to ca. 40% coverage. The marked deviation thereafter leads to a maximum experimental coverage of about 50%, i.e. ca. 4.7/2 = 2.35 atoms nm−2 . We can interpret this coverage limit as due to the saturation value of fully isolated species. As shown in Fig. 6, taking the plane (1 0 1) (Teufer [30] indexing), as the most frequently ex-
Fig. 6. Idealized (1 0 1) plane for tetragonal zirconia containing the maximum number of isolated tungsten (VI) species and the corresponding unit cell (dotted line). (䊐) oxygen atoms above the Zr atoms not shown in the vertices of triangles (side length = 0.35 nm).
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posed for tetragonal zirconia, fully isolated W species would cover only 50% of the surface, giving a W surface density of 2.35 atoms nm−2 (see Appendix A). This value is substantially lower than the average value of 3.1 atoms nm−2 we previously reported by determining the analytical W content and the BET areas for leached samples ([17], Fig. 3). The discrepancy implies that, as shown by Raman and X-ray absorption data [17], the tungsten atoms directly interacting with ZrO2 are not isolated but form clusters containing few W atoms. Interestingly, studying n-butane isomerization on zirconia-supported tungsten samples, we found no detectable activity for a W surface density lower than about 3 W atoms nm−2 ([16], Fig. 10), whereas starting from a W surface density higher than about 3 activity sharply increased. Leached samples showed similar behavior, thus suggesting an important role of W small clusters in n-butane isomerization. The similar saturation coverage we found in ZWx and ZWx(i) samples suggests that in both catalysts an interaction between the tungsten oxospecies and the zirconia surface takes place during heating. This interaction induces a protective effect towards zirconia sintering [17]. Heating at 1073 K decreased the surface area of ZrO2 from 42 to 18 m2 g−1 whereas in the ZWx(i) samples, a similar thermal treatment left the specific surface area of zirconia, SZrO2 , practically unchanged (for ZW5.5(i), 36 m2 g−1 and for ZW10(i), 40 m2 g−1 ). As a final comment, the low temperature adsorption of CO on the ZW system has a notable technical advantage, because the measurement of CO adsorption also gives the total surface area of the sample. In fact, the calculation of coverage by Eq. (1) involves adsorption measurement at 77 K of two adsorbates: nitrogen, to obtain the total area (BET method) and carbon monoxide, to determine the support surface area, owing to its selective irreversible adsorption. Total CO uptake is always determined, to calculate the irreversible part of the adsorption. In strict analogy with the BET method (N2 adsorption), on highly dehydroxylated ZrO2 surfaces and for CO relative pressure between 0.04 and 0.22 (CO saturation pressure at 77.3 K = 449 Torr [31]), the total CO uptake involves both the selective titration of cus Zr4+ sites and the development of a multilayer of physisorbed gas over the whole surface. When we compared the monolayer
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Fig. 7. Monolayer volume, Vm , on WOx /ZrO2 samples, obtained with the BET method using CO (ordinate) or N2 (abscissa) as adsorbate at 77 K.
volume values obtained by applying the BET method with the two adsorbates, we found a good linear correlation between these two quantities for the whole series of samples, namely for all coverages (Fig. 7). The best fit (correlation coefficient = 0.997) of the line passing through the experimental points, gives a slope of 0.95. Assuming that the BET method with nitrogen (σN2 = 16.2 Å2 ) gives the actual area of the samples, the carbon monoxide molecular cross-sectional area is 17.0 Å2 . This value is close to that calculated from CO liquid density or from physical adsorption data at 77.3 K (16.0 and 14.7 Å2 [32]). Hence the CO adsorption measurement at 77 K provides both total BET area and free support area. Using these values in Eq. (1) gives the absolute coverage for ZW samples.
5. Conclusions This FT-IR spectroscopy study of the adsorption of CO at 77 K on ZrO2 -supported tungsten oxide samples, heated at 1073 K in air, shows that CO irreversibly adsorbs selectively on Zr 4+ cus centers of the support surface. Under these conditions, because no 6+ sites, the covCO is irreversibly adsorbed on Wcus erage of the zirconia surface by tungsten oxospecies, determined by volumetric method, turns out to be
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about 50% at maximum, irrespective of the support crystallographic phase (monoclinic or tetragonal). ZrO2 coverage remains unchanged after a leaching treatment that removes tungsten oxospecies not directly interacting with the zirconia surface and therefore depends mainly on the tungsten oxospecies strongly anchored to the support surface. These species help to prevent sintering of zirconia both in the tetragonal and in the monoclinic form. Instead of spreading and reacting with the whole zirconia surface, tungsten species in our experimental conditions tend to form polymeric structures.
Acknowledgements The present research was partly financed by the Italian MURST, project COFIN 2000. Some of the low temperature CO spectra were carried out by Dr. E. Gribov, and his contribution is gratefully acknowledged.
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