A Vibrational and Spectroscopic Study of WO3 ... - ACS Publications

630

Langmuir 1998, 14, 630-639

A Vibrational and Spectroscopic Study of WO3/TiO2-Al2O3 Catalyst Precursors Aı´da Gutie´rrez-Alejandre,† Jorge Ramı´rez,*,† and Guido Busca‡ UNICAT, Departamento de Ingenierıá Quı´mica, Facultad de Quı´mica, Universidad Nacional Auto´ noma de Me´ xico, Ciudad Universitaria, 04510 Me´ xico, DF, Me´ xico, and Istituto di Chimica, Facolta` di Ingegneria, Universita` di Genova, P. le J. F. Kennedy, I-16129 Genova, Italy Received September 3, 1997. In Final Form: November 14, 1997 WO3/TiO2-Al2O3 catalyst powders with titania-alumina mixed oxides synthesized by a sol-gel procedure have been prepared by dry impregnation. The surface structure of the resulting materials has been investigated by IR and Raman spectroscopies in the skeletal region and IR spectra of adsorbed water and ammonia. It is concluded that the largely predominant tungsten species, when these are below the monolayer coverage and in dry conditions, on all the supports studied, are constituted by monooxo wolframyl species which are coordinatively unsaturated and act as strong Lewis acid sites. The overall coordination around tungsten is consequently 4 and/or 5. By adsorption of water, the overall coordination of tungsten grows and the site behaves as a strong Brønsted acid site. The vibrational behavior of such species suggests that the WdO vibrators are uncoupled, which means that they belong to isolated molecular units anchored to the surface by W-O-(Ti,Al) bonds, without significant extent of W-O-W bridges. The nature of the support surface significantly modifies the strength of the WdO bond, which is indicative of the electronic state of tungsten. This is attributed to the different basicity of the surface oxide ions that act as the ligands of the wolframyl ion. The support surface hydroxy groups apparently do not play an important role in anchoring the wolframyl species. The “monolayer capacity” of the supports apparently depends quite strongly on the support nature, and it seems that care should be taken with its calculation, to compare on the same basis the results arising from different laboratories.

Introduction Oxide-supported tungsten oxide-containing powders are the object of increasing interest in the field of heterogeneous catalysis. Tungsten oxide supported on different metal oxides gives rise to strongly acidic materials1 which are active in reactions such as olefin methathesis2 and ethene homologation.3 WO3-ZrO2 has been defined as a “superacid” solid,4 WO3-TiO25 and WO3-Al2O36 have been used for n-butene to isobutene skeletal isomerization, and WO3-Al2O3 is active in the hydrocarbon synthesis from methanol.7 WO3-TiO2 is a commercial product8 and has been investigated9 because, after impregnation with small amounts of V2O5, it constitutes the industrial catalyst for the reduction of NOx by ammonia.10 † ‡

Universidad Nacional Autonoma de Me´xico. Universita` de Genoa.

(1) Murrell, L. L.; Dispenziere, N. C., Jr. Catal. Lett. 1990, 4, 235. (2) Mol, J. C.; Moulijn, J. C. In Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer Verlag: Berlin, 1987; Vol. 8, p 69. (3) Yamaguchi, T.; Nakamura, S.; Tanabe, K. J. Chem Soc., Chem. Commun. 1982, 621. (4) Arata, K. Adv. Catal. 1990, 37, 165. (5) Patrono, P.; LaGinestra, A.; Ramis, G.; Busca, G. Appl. Catal, A: Gen. 1994, 107, 247. (6) Meijers, S.; Gielgens, L. H.; Ponec V. J. Catal. 1995, 156, 147. (7) Hutchings, G. J.; van Rensburg, J. L.; Pickl, W.; Hunter, R. J. Chem. Soc., Faraday Trans. 1 1988, 84 (5), 1311. (8) (a) A-DW-1 Bayer Titan 10% WO3-TiO2 from Bayer A. G., Leverkusen, Germany; (b) DT-52 10% WO3-TiO2 from Thann & Mulhouse, France. (9) (a) Vuurman, M. A.; Wachs, I. E.; Hirt, A. M. J. Phys. Chem. 1991, 95, 9982. (b) Ramis, G.; Busca, G.; Cristiani, C.; Lietti, L.; Forzatti, P.; Bregani, F. Langmuir 1992, 8, 1744. (c) Hilbrig, F.; Gobel, H. E.; Kno¨zinger, H.; Schmeltz, H.; Lengeler, B. J. Phys. Chem. 1991, 95, 6974. (d) Engweiler, J.; Harf, J.; Baiker, A. J. Catal. 1996, 159, 259. (10) (a) Wood, S. C. Chem Eng. Prog. 1994, 90 (1), 32. (b) Alemany, L.; Berti, J. F.; Busca, G. G.; Ramis, G. J.; Robba, D.; Toledo, G. P.; Trombetta, M. Appl. Catal. in press.

WO3-TiO2 and WO3-Al2O3 have also been the focus of increased interest because they are the precursors of the tungsten-sulfide based catalysts for oil hydrotreating processes.11 It has been shown that TiO2-supported hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) catalysts are more active but less thermally stable than those supported on Al2O312 and that they are more easily sulfided than their alumina-supported counterparts.13 For this reason, catalysts supported on aluminacontaining titania could in principle combine the good properties of those supported on the two pure oxides. After a study on the preparation and the characterization of mixed alumina-titania supports,14 we have undertaken an investigation of tungsten oxide supported on them, as precursors for sulfide HDS catalysts. It is in fact quite possible that the state of supported tungsten sulfide species somehow reflects the state of the supported tungsten oxide precursors. Although a great amount of data are available today on oxide-supported metal oxides, mainly based on spectroscopic measurements,15,16 several problems still remain without proper clarification on such systems. For example, according to Wachs,16 the nature and the dispersion of the metal oxide species on alumina and titania are nearly the same while, while according to Vermaire and van Berge17 (11) Topsøe, H.; Clausen B. S.; Massoth, F. E. Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1996; Vol. 11, p 1. (12) Ramı´rez, J.; Fuentes, S.; Dıáz, G.; Vrinat, M.; Breysse, M.; Lacroix, M. Appl. Catal. 1989, 52, 211. (13) Grimblot, J.; Gengembre, L.; D’Huysser, A. J. Electron. Spectrosc. Rel. Phenom. 1990, 52, 485. (14) Gutie´rrez, A.; Trombetta, M.; Busca, G.; Ramirez, J. Microporous Mater. 1997, 12 (1-3), 79. (15) Wachs, I. E. Characterization of Catalytic Materials, 1st de.; Butterworth-Heinemann: Boston, MA, 1992. (16) Wachs, I. E. Catal. Today 1996, 27 (No. 3-4), 437.

S0743-7463(97)00993-1 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/16/1998

WO3/TiO2-Al2O3 Catalyst Powders

Langmuir, Vol. 14, No. 3, 1998 631 Table 1. Summary of Samples under Study

sample

TiO2/TiO2 + Al2O3

W atoms/nmsupp2

Al-Ti(1.0) W/Al-Ti(1.0)-1.4 W/Al-Ti(1.0)-2.8 W/Al-Ti(1.0)-3.5 W/Al-Ti(1.0)-4.2 Al-Ti(0.95) W/Al-Ti(0.95)-1.4 W/Al-Ti(0.95)-2.8 W/Al-Ti(0.95)-3.5 W/Al-Ti(0.95)-4.2 Al-Ti(0.9) W/Al-Ti(0.9)-1.4 W/Al-Ti(0.9)-2.8 W/Al-Ti(0.9)-3.5 W/Al-Ti(0.9)-4.2 Al-Ti(0) W/Al-Ti(0)-1.4 W/Al-Ti(0)-2.8 W/Al-Ti(0)-3.5 W/Al-Ti(0)-4.2

1 1 1 1 1 0.95 0.95 0.95 0.95 0.95 0.90 0.90 0.90 0.90 0.90 0 0 0 0 0

0 1.4 2.8 3.5 4.2 0 1.4 2.8 3.5 4.2 0 1.4 2.8 3.5 4.2 0 1.4 2.8 3.5 4.2

their dispersion should be significantly different. In any case, it appears that WO3-TiO2 and WO3-Al2O3 catalysts behave in a quite different way. On the other hand, little data exist on oxides supported on mixed metal oxides. Our previous characterization study showed that some properties of titania are significantly modified by the addition of small amounts of alumina,14 giving rise to solids with peculiar properties. This made necessary a study of tungsten oxide catalysts supported on such titania-rich alumina-titania mixed oxides, whose results are presented here. According to previous studies12 the addition of 10 mol % alumina to the titania support reduces the HDS activity of the catalyst to nearly that of catalysts supported on pure alumina. For this reason, we will compare here samples with a TiO2/TiO2 + Al2O3 molar ratio not lower than 0.9 to those based on pure alumina. Experimental Section (a) Preparation Procedure. The supports were prepared following the same method described previously.18 They were impregnated with a solution containing the tungsten precursor ((NH4)6H2W12O40) at room temperature by the pore volume method. After that, they were aged for 2 h at room temperature and then dried at T ) 373 K for 18 h. Finally they were calcined at T ) 773 K for 3 h. The tungsten contents used in the samples were those needed to have a virtual coverage of 1.4, 2.8, 3.5, and 4.2 atoms/nm2, calculated on the basis of the support area. Accordingly, the maximum tungsten oxide content in the samples investigated here corresponds to that estimated by Vermaire and van Berge17 and by Wachs11 as the monolayer capacity of TiO2 for WO3, i.e., near 7 µmol/m2 tungsten or tungsten oxide, corresponding just to 4.2 tungsten atoms/nm2. In the case of tungsten supported on alumina, while Vermaire and van Berge17 estimated a monolayer capacity of WO3 on alumina of 9.88 µmol/ m2, corresponding to 5.9 atoms/nm2, according to Wachs16 this value should be, for tungsten oxide supported on alumina and also on titania, near 4.2 tungsten atoms/nm2. However, while Vermaire and van Berge17 estimated a monolayer capacity of WO3 on alumina of 9.88 µmol/m2, corresponding to 5.9 atoms/ nm2, according to Wachs,16 this value should be for tungsten oxide supported on alumina and also on titania near 4 tungsten atoms/nm2. However, impregnation, drying, and calcination of the tungsten salt precursors, result in a lowering of the surface area of the catalyst with respect to that of the support. Consequently, the true coverage calculated on the basis of the catalyst area is higher than the tungsten coverage calculated on (17) Vermaire, D. C.; van Berge, P. C. J. Catal. 1989, 116, 309. (18) Ramı´rez, J.; Ruiz-Ramı´rez, L.; Cedenõ, L.; Harle, V.; Vrinat, M.; Breysse, M. Appl. Catal., A: General 1993, 93 163.

% WO3 (w/w) 1.48 2.82 3.47 4.21 4.82 9.20 11.24 13.19 5.98 11.28 13.72 16.02 10.34 18.75 22.38 25.71

Sg (m2/g) 27 27 28 27 25 94 91 82 79 78 118 103 92 87 77 214 185 162 153 151

atoms W/nmcat2

detect WO3

1.38 2.62 3.38 4.36

+ + + + +

1.38 2.91 3.70 4.39 1.5 3.2 4.0 5.4 1.45 3.0 3.8 4.4

the basis of the support area. As mentioned before, the coverages reported here for our samples were calculated on the basis of the support surface area. (b) Characterization Techniques. Supports and catalysts surface area measurements were made by nitrogen physisorption at 77 K using a Micromeritics ASAP 2000. Raman spectra were recorded with a Nicolet 950 FT-Raman spectrometer equipped with an InGaAs detector and a Nd:YAG laser source with a resolution of 4 cm-1. The Infrared spectra were recorded with Nicolet model Magna 750 and a model 5ZDX Fourier transform spectrometers, with a resolution of 4 cm-1, and 200 scans The supports and catalysts wafers, made by pressing the pure solids, were activated at T ) 373, 473, 623, and 723 K under outgassing for the OH analysis. For the ammonia adsorption studies, these samples were previously outgassed into the IR cell connected with a conventional gas-manipulation-evacuation apparatus at 623 K except for the catalysts supported on alumina that were outgassed at 773 K.

Results (a) Porosity and Surface Area. Table 1 shows the compositions of supports and catalysts, the tungsten coverages (W atoms per nm2), the surface areas, and the indication if WO3 was detected (+) or not (-) by X-ray diffraction. (b) Surface Area. The surface areas (Sg) of the tungsten oxide-containing powders (Table 1) are in most cases lower than those of the corresponding pure supports. This surface area decrease is quite important for WO3/ Al2O3 and the alumina-containing mixed oxide supports, and almost negligible for tungsten oxide/titania. However, the surface area expressed as m2 of catalyst is, in this case, affected by the relatively large weight of tungsten, in relation to titanium and/or aluminum. To avoid this effect, we can instead calculate a catalyst surface area value per gram of support (Ss), i.e., m2cat/gsupp, that will allow evaluation of how the surface area of the support is modified by the impregnation of WO3. The result of this calculation shows that this effect is in most cases not greater than 10% (usually near 5%), although always in defect, indicating a good dispersion of the tungsten phases on most of the support samples. (c) Skeletal IR and Raman Spectra. The skeletal Raman spectra of the catalysts under study, recorded at ambient conditions, are reported in Figures 1 and 2. The spectrum of the TiO2 support (Figure 1, bottom) shows the typical pattern of anatase,19 with peaks at 640, 515, 398, 198 (weak), and 146 cm-1, due to the fundamental modes, and an overtone mode at 798 cm-1. As discussed

632 Langmuir, Vol. 14, No. 3, 1998

Figure 1. Skeletal FT-Raman spectra of the samples prepared with the Al-Ti(1.0) support (top) and with the Al-Ti(0.95) support (bottom), all recorded at ambient conditions.

previously,14 the intensity of these peaks diminishes by the addition of Al to the support. The pure Al2O3 support, not shown in Figure 1, only shows a very broad and intense feature assigned previously to the fluorescence phenomena.20 The addition of 1.4 atoms/nmsupp2 of tungsten oxide to the pure titania (x ) 1) results, as already shown,21 in a severe weakening of the scattering peaks of anatase. However, with an increase in the tungsten oxide coverage, no further significant weakening of the anatase peaks is observed. The incorporation of tungsten oxide gives also rise to the appearance of a broad feature at ca. 960 cm-1. A similar result is obtained by loading tungsten oxide over the Al-Ti(0.95) support (Figure 1, bottom), although in this case the above band now observed at 970 cm-1 is more evident as a result of the higher absolute amount of tungsten species according to the higher surface area of the Al-Ti(0.95) support with respect to the Al-Ti(1.0) one and, consequently, the higher WO3 loading needed to obtain the same surface coverage. The situation changes with the Al-Ti(0.9) support (Figure 2, top). In this case, besides the peak at ca. 965 cm-1, that grows like in the previous cases with tungsten content, quite strong peaks appear at 808, 716, and 273 cm-1 for the Al-Ti(0.9)-3.5 sample and, even more intense peaks for the Al-Ti(0.9)-4.2 sample. These peaks correspond to the strongest Raman peaks of monoclinic tungsten oxide.22 In the case of the samples prepared with the Al-Ti(0) support (Figure 2, bottom), a weak peak is observed at (19) Busca, G.; Ramis, G. J.; Gallardo Amores; Sańchez Escribano, M. V.; Piaggio, P. J. Chem. Soc., Faraday Trans. 1994, 90, 3181. (20) Chan, S. S.; Wachs, I. E.; Murrell, L. L.; Dispenziere, N. C. J. Catal. 1985, 92, 1. (21) Alemany, L. J.; Lietti, Ferlazzo, L. N.; Forzatti, P.; Busca, G.; Giamello, E.; Bregani, F. J. Catal. 1995, 155, 117.

Gutie´ rrez-Alejandre et al.

Figure 2. Skeletal FT-Raman spectra of the samples prepared with the Al-Ti(0.90) support (top) and with the Al-Ti(0) support (bottom), all recorded at ambient conditions.

962 cm-1 with 1.4 atoms/nm2 of tungsten, which then grows with increasing W content. For a W content of 2.8 atoms/ nm2, in addition to the peak at 972 cm-1, a second peak at 808 cm-1 also appears, indicating the presence of WO3. From the above data it could be concluded that the dispersion of tungsten oxide in our samples is complete on the pure titania and on the Al-Ti(0.95) supports, but it strongly decreases by the addition of more aluminum. In line with this, the Al-Ti(0.9) sample shows the largest drop in surface area with respect to the W-free support. In contrast, for high-titania content powders, such a surface area decrease is much less. This is likely due to the higher surface area and pore volume of the highalumina supports (with respect to the rich titania samples), in which there is a great contribution of small pores that have been likely blocked upon impregnation. So, with our support samples and our preparation procedure, the support surface area is only partially available to impregnation with WO3. As a consequence, for high aluminacontaining samples, the tungsten loading expressed as W atoms/nm2cat is definitely higher than if expressed as W atoms/nmsupp2. In any case, even considering the W atoms/nmcat2 values for evaluating the coverage and the dispersion, the detection of WO3 by Raman spectroscopy indicates that, on our samples, the tungsten dispersion on the pure alumina and the Al-Ti(0.9) supports is definitely worse than those on the pure titania and the Al-Ti(0.95) supports. This is not in agreement with the results of both Vermaire and van Berge17 and of Wachs,16 which also, when expressed in the same units, differ among them. However, Wachs16 gives much lower monolayer capacities (22) Daniel, M. F.; Desbat, B.; Lassegues, J. C.; Gerand, B.; Figlarz, M. J. Solid State Chem. 1987, 67, 235.


Figure 3. FT-IR spectra of pressed disks of the (a) Al-Ti(1.0) supports outgassed at room temperature (top), 473 K (middle), and 623 K (bottom), and of W/Al-Ti(1.0) catalysts in the OH stretching region, after outgassing at 473 K (top) and 623 K (bottom) with tungsten loading of (b) 1.4, (c) 2.8, and (d) 4.2 tungsten atoms per square nanometer.

for alumina with respect to titania in the case of several other oxides like niobia and chromia. These discrepancies in the estimation of the monolayer coverages suggest that the data of monolayer capacities reported by different authors, using different impregnation procedures and different precursors of the supported phases, can be fairly different, and consequently, it is likely that the results obtained by one author cannot be generalized. The skeletal IR spectra of the catalysts with different W loading (1.4, 2.8, 3.5, and 4.2 W atoms/nm2) also show a weak broad feature in the region 980-950 cm-1 for all the W-containing samples; however, it was not possible to clearly detect WO3 particles, mostly because of the breadth of the IR bands. This result confirms the higher sensitivity of Raman spectroscopy with respect to IR at the coverage used here. On the other hand, this indicates that the species formed by dispersion of tungsten oxide over such oxide supports is characterized by a (broad) IR band which is nearly at the same position as the corresponding Raman peak. The assignment of this band will be discussed below when the evidence for wolframyl groups is presented. (d) FT-IR Characterization of the Surface Hydroxy Groups. The spectra of the powders pressed into disks in the OH stretching region, after outgassing at 473 and 623 K, are reported in Figures 3 to 6. We can distinguish in all cases the existence of relatively sharp bands in the region 3800-3600 cm-1 and a broad absorption in the region 3600-2500 cm-1. The former spectral region is due to bands which are invariably observed on the surface of oxide powders after outgassing and are due to the OH stretching of surface hydroxy groups free from hydrogen bonding.23 In the latter region hydrogen-bonded OH groups absorb. In all cases the broad absorptions due to H-bonded hydroxy groups markedly decrease in intensity with respect to that of the free OHs by increasing the outgassing temperature. Also, it is observed that as the W content is increased, the absorption intensity of H-bonded OHs (23) Boehm, H. P.; Kno¨zinger, H. In Catalysis Science and Tecnology; Anderson, J. R., Boudart, M., Eds.; Springer Verlag: Berlin, 1983; Vol. 4, p 39.

Langmuir, Vol. 14, No. 3, 1998 633

Figure 4. FT-IR spectra of pressed disks of the samples prepared with the Al-Ti(0.95) support in the OH stretching region, after outgassing at 473 K (top) and 623 K (bottom) with tungsten loading of (a) 0.0, (b) 1.4, (c) 2.8, and (d) 4.2 tungsten atoms per square nanometer.

Figure 5. FT-IR spectra of pressed disks of the powders prepared with the Al-Ti(0.90) support in the OH stretching region, after outgassing at 473 K (top) and 623 K (bottom) with tungsten loading of (a) 0.0, (b) 1.4, (c) 2.8, and (d) 4.2 tungsten atoms per square nanometer.

becomes relatively stronger, at least up to an outgassing temperature of 473 K. Moreover, by comparison of the spectra of the samples outgassed at 623 K, it is observed that the absolute absorption intensity in the region 38003600 cm-1 decreases, although not always linearly, with the tungsten loading. However, in all cases, the part of the spectrum corresponding to the free surface OHs does not disappear completely, even at tungsten oxide loadings virtually allowing the complete coverage of the surface. In fact, the absolute intensity of the free surface OHs never decreases below one-third of the intensity measured on the free supports, even at virtual monolayer coverage. The spectra of the free OHs on the W-containing samples prepared with the Al-Ti(0.95) and Al-Ti(0.9) supports (Figures 4 and 5) are constituted by the same bands observed on the pure supports near 3780, 3720, and 3680 cm-1, which were assigned predominantly to Al-OH groups.14 However, the intensity of the two highest

634 Langmuir, Vol. 14, No. 3, 1998


Figure 7. FT-IR spectra of the surface species arising from ammonia adsorption at room temperature on the pure supports Al-Ti(1.0) (a), Al-Ti(0.90) (b), and Al-Ti(0) (c), before (top) and after (bottom) outgassing at room temperature.

Figure 6. FT-IR spectra of pressed disks of the powders prepared with the Al-Ti(0) support in the OH stretching region, after outgassing at 473 K (top) and 623 K (bottom) with tungsten loading of (a) 0.0, (b) 1.4, (c) 2.8, and (d) 4.2 tungsten atoms per square nanometer.

frequency bands seems to decrease with tungsten loading more than that at 3680 cm-1. A more complex situation is observed for the samples supported on pure titania, Al-Ti(1.0). This support shows a complex spectrum (Figure 3) with a number of sharp components typical of the OH region in anatase preparations.24,25 After outgassing at room temperature a sharp peak at 3630 cm-1 predominates, with a weaker maximum at 3660 cm-1 and shoulders at 3672 and near 3680 cm-1. Under outgassing at higher temperature, the sharp peak at 3630 cm-1 decreases markedly and almost disappears after outgassing at 473 K. At these conditions the main maximum is at 3670 cm-1, with definite components at 3715 cm-1 (weak maximum) and unresolved shoulders at 3735, 3660, and 3640 cm-1. The shape of this spectrum is almost unchanged after further outgassing up to 723 K, although all bands are further weakened. The spectra of the W/Al-Ti(1.0) samples at different outgassing temperatures and with different tungsten loading are also shown in Figure 3. The W/Al-Ti(1.0)4.2 sample (not shown in Figure 3) presents several components with that at 3660 ( 5 cm-1 being always the strongest. In the higher frequency region a shoulder near 3675 cm-1, which is clearly evident at room temperature, disappears quite fast under outgassing at higher temperatures. In contrast, the weak maximum near 3720 cm-1 is more stable and remains even at outgassing temperatures of 723 K. In the lower frequency region at low outgassing temperature we detect a shoulder near 3635 cm-1, which also disappears under outgassing at higher temperature. At tungsten loading lower than 4.2 W at/nm2 the spectra of the titanium supported samples (Figure 3), after outgassing at 623 K, show main weak bands at around 3735, 3680, and 3650 cm-1. The above data indicate that several different hydroxy groups of titania behave in a different and complex way with respect to WO3 loading. In the case of the samples with different tungsten loading prepared on the pure alumina support (Figure 6), (24) Busca, G.; Saussey, H.; Saur, O.; Lavalley, J. C.; Lorenzelli, V. Appl. Catal. 1985, 14, 245. (25) Oliveri, G.; Ramis, G.; Busca, G.; Sańchez Escribano, V. J. Mater. Chem. 1993, 3, 1239.

the spectra of the free surface hydroxy groups (38003600 cm-1) are only still detectable at the loading corresponding to the W/Al-Ti(0)-1.4 sample, while they appear to be canceled at higher tungsten contents. However, due to the large surface area of the alumina support, we must mention that these samples contain a very high amount of tungsten oxide per gram of catalyst. This makes them quite opaque to IR in the highest frequency range, rendering therefore very noisy spectra in the OH stretching region. The data shown here suggest that the completion of the tungsten oxide monolayer does not result in the complete elimination of the free surface hydroxy groups of the support, except perhaps on pure alumina. This contrasts with the current opinion that the carrier surface OH groups are the adsorption sites for tungsten oxide species or, in a more general way, for metal oxide species on oxide carriers.26,16 (e) FT-IR Study of Ammonia Adsorption. The FTIR spectra of ammonia adsorbed on the pure titania support Al-Ti(1.0) are reported in Figure 7a. After contact and outgassing at room temperature the observed bands can be assigned to NH3 molecules molecularly coordinated over Lewis acid sites. These bands can be assigned to the NH stretching, with the bands at 3380, 3350, 3240, and 3100 cm-1, due to the two asymmetric stretchings, the symmetric stretching, and the overtone of the asymmetric deformation mode, respectively. The band at 1602 cm-1 due to the asymmetric NH3 deformation and the band with two split components at 1222 and 1155 cm-1 are due to the symmetric NH3 deformation. This spectrum agrees with that previously observed for ammonia adsorbed on anatase.24,27-29 The splitting of the δsymNH3 mode has been interpreted as evidence for the presence of two different coordinated species, due to the adsorption of ammonia on two different Lewis acid sites, i.e., coordinatively unsaturated Ti4+ ions. Before outgassing, a weak absorption is found at about 1445 cm-1 and also weak and broad absorption in the region near 2800 cm-1. This shows that in these conditions few ammonium cations NH4+ are formed by ammonia protonation, as already observed.24 However, the weakness of the absorptions due to the asymmetric NH4 (26) Millman, W. S.; Crespin, M.; Cirillo, A C.; Abdo, S.; Hall, W. K. J. Catal. 1979, 60, 404. (27) Tsyganenko, A. A.; Pozdnyakov, D. V.; Kilimonov, V. N. J. Mol. Struct. 1975, 29, 299. (28) Ramis, G.; Busca, G.; Lorenzelli, V.; Forzatti, P. Appl. Catal. 1990, 64, 243. (29) Hadjiivanov, K.; Saur, O.; Lamotte, J.; Lavalley, J. C. Z. Phys. Chem. Bd. 1994, 187 S. 281.


Langmuir, Vol. 14, No. 3, 1998 635

deformation, 1445 cm-1, and to the N-H stretching (30002700 cm-1), and their disappearance by simple evacuation at room temperature show that the Brønsted sites on titania are extremely weak. The addition of alumina to titania (Figure 7) results in some important modifications in the spectra. In the NH stretching region the spectra become definitely less resolved and more noisy, due to the increased light absorption in this region, according to a much higher population of H-bonded surface hydroxy groups that partly obscure the spectra. In the lower frequency region, the spectra of coordinated molecular ammonia also change significantly with respect to the pure titania sample. In particular, the δasymNH3 mode shifts to higher frequencies (now 1620-1625 cm-1), while for the δsymNH3 mode we observe a progressive decrease of the component at 1155 cm-1 and a progressive growth of a band at 1240 cm-1. These results can be interpreted assuming that the abundance of coordinatively unsaturated Ti4+ cations progressively decreases while coordinatively unsaturated Al3+ progressively appears. Coordinatively unsaturated Al cations are stronger as Lewis acid sites than Ti cations, mainly due to their smaller ionic radius. This results, according to the literature,27,30 in an upward shift of the δsymNH3 mode of coordinated ammonia. Additionally, the presence of alumina in the support also causes the formation of quite strong Brønsted acid sites, detected by the presence of the band near 1470 cm-1, assigned to the corresponding asymmetric deformation mode. In our case, and in agreement with previous literature data,27,31 this band is also observed on pure alumina. This datum apparently contradicts our previous conclusion that the OH groups on titania are stronger, as Brønsted acids, than those of alumina and of mixed oxide samples. However, the previous conclusion was reached on the basis of the shift of the OH stretching frequency upon hydrogen bonding with pivalonitrile.14 Also, it is obvious that, on metal oxide surfaces, different populations of different types of OH groups can occur. It is possible then that most of the OHs of alumina are weaker as acids than most of the OHs of titania (according to our results on pivalonitrile adsorption14), but it is also possible that few strongly acidic OHs occur on the alumina surface (unlike on titania) and that these few fairly strong OHs are those which protonate ammonia. According to Kno¨zinger,31 outgassing at higher temperatures than 773 K causes these sites to disappear. The addition of tungsten oxide to the support samples causes again some modifications on the spectra of adsorbed ammonia. Figure 8 shows the spectra of ammonia adsorbed on the W/Al-Ti(0.90)-4.2 and W/Al-Ti(0.95)4.2 samples at different outgassing temperatures. In all cases both coordinated and protonated ammonia are observed. The asymmetric deformation mode of protonated ammonia is observed shifting from 1460-1470 to ca. 1440 cm-1 by outgassing. Furthermore, a comparison of the protonated/coordinated relative intensity ratio of the bands in the W-free samples (supports) versus those of the W-containing samples shows that this ratio is definitely higher on W-containing samples. Also, the absorptions of coordinated ammonia are sensitive to the addition of tungsten. In particular, the δsymNH3 mode is observed (on W-containing samples) shifted at 1280 and 1230 cm-1. The observed frequencies are similar to those reported previously by Ramis et al. for ammonia adsorbed on WO3-

TiO2 samples9b and on pure WO3.32 These results strongly suggest that the adsorption sites for ammonia on the sample with a “formal” tungsten oxide monolayer (4.2 atoms/nmsupp2) are WOH Brønsted acid sites and Lewis acidic coordinatively unsaturated W6+ cations. (f) Evidence of Surface Wolframyl Centers. The FTIR spectra of most of the pure W-containing samples after outgassing show features that can be assigned to vibrational modes of W-containing surface species. In particular, several samples show sharp weak bands in the region near 2000 cm-1 and stronger bands in the region just above 1000 cm-1. These bands can be better observed in the subtraction spectra. To illustrate this point, in Figure 9 we show the features observed in the subtraction spectra of the samples W/Al-Ti(1.0)-3.5, W/Al-Ti(0.95)3.5 and W/Al-Ti(0.9)-3.5. The positions of the observed features for all samples are reported in Table 2. As discussed previously, the features near 2000 cm-1 are the overtones and/or the combination bands of those near 1000 cm-1, which are due to the fundamental WdO stretchings of surface W-containing species.9b,33 Unfortunately, the latter region is very noisy, due to several reasons: (i) the transmittance in this region is very low, because it falls

(30) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; Wiley: New York, 1985. (31) Kno¨zinger, H. Adv. Catal. Relat. Phenom. 1976, 25, 184.

(32) Ramis, G.; Cristiani, C.; Elmi, A. S.; Villa, P. L.; Busca, G. J. Mol. Catal. 1990, 61, 319. (33) Busca, G.; Lavalley, J. C. Spectrochim. Acta 1986, 42A, 443.

Figure 8. FT-IR spectra of the surface species arising from ammonia adsorption at room temperature on the powders W/Al-Ti(0.95)-4.2 (top) and W/Al-Ti(0.9)-4.2 (bottom), before outgassing (a) and after outgassing at room temperature (b), 373 K (c), and 473 K (d).

Figure 9. FT-IR spectra of the surface species detected on the surface of the powders W/Al-Ti(0.95)-3.5 (a, b), W/Al-Ti(0.9)3.5 (c, d), and W/Al-Ti(0)-3.5 (e, f) after outgassing at 473 (a, c), 723 K (b, d, f), and 773 K (e).

636 Langmuir, Vol. 14, No. 3, 1998


Table 2. Position (wavenumber, cm-1) of the IR Absorption Bands of W)O Stretchings of Surface Wolframyl Species on WO3/TiO2-Al2O3 Catalysts sample notation

atoms of W/nm2

Al-Ti(1.0) W/Al-Ti(1.0)-1.4 W/Al-Ti(1.0)-2.8 W/Al-Ti(1.0)-3.5 W/Al-Ti(1.0)-4.2 Al-Ti(0.95) W/Al-Ti(0.95)-1.4 W/Al-Ti(0.95)-2.8 W/Al-Ti(0.95)-3.5 W/Al-Ti(0.95)-4.2 Al-Ti(0.9) W/Al-Ti(0.9)-1.4 W/Al-Ti(0.9)-2.8 W/Al-Ti(0.9)-3.5 W/Al-Ti(0.9)-4.2 Al-Ti(0) W/Al-Ti(0)-1.4 W/Al-Ti(0)-2.8 W/Al-Ti(0)-3.5 W/Al-Ti(0)-4.2

0 1.4 2.8 3.5 4.2 0 1.4 2.8 3.5 4.2 0 1.4 2.8 3.5 4.2 0 1.4 2.8 3.5 4.2

outgassed at 473 K overtone

fundamental

outgassed at 723 K overtone

fundamental

(2015) 2013 2015

1010

(2004) 2006 2004

1006

(2020) 2005 2005 (2018) 2007 2007

1005, 998 1008 1014, 1007 1005

2021 2025 2018 2016

1011 1019 1014 1013

1007 1010 1011 1014

2018 2029 2025 2027

1016 1016 1020

2029 2032 2034

1022 1025 1029

2007 2012 2003 2010

just above the cutoff limit due to the bulk absorptions of alumina, observed near 990 cm-1; (ii) in these regions there are also absorptions due to the deformation modes of hydrogen bonded OHs; (iii) absorptions due to the stretchings of surface Al-O bonds also occur in this region. Therefore, the analysis of this spectral region even in subtraction spectra is highly complex due to the superposition of negative and positive bands. In cases like this one should attempt the analysis of the overtone region which is easier and equally as informative as the fundamental region. In the overtone region (also shown in Figure 9), we observe essentially a single band, although with very small splittings observed only in some samples. As discussed previously,9b,33 a single overtone band points to the existence of a single vibrating bond, which in our case implies that these features are due to monooxo wolframyl species. This is also supported by the virtual coincidence of the fundamental stretching mode observed here by IR spectroscopy on dry outgassed samples with those reported in the literature34,35 for the Raman spectra of dry samples (1011 cm-1 for WO3-TiO2 and 1020 cm-1 for monolayertype WO3-Al2O3). In fact, the coincidence of the Raman and IR stretching frequency strongly supports the identification of these species again as due to monooxo wolframyl species. The data reported in Table 2 show that there is a slight dependence of the position of these bands with (i) the total W loading (the stretching frequencies tend to increase by increasing the tungsten loading), (ii) the nature of the support since the stretching frequencies tend to slightly increase by increasing the aluminum content in the support. This effect is definitely higher on alumina than on titania, as already observed by Wachs16 and by some of us,9b,33 and (iii) the outgassing temperature (the stretching frequencies increase with the outgassing temperature). Moreover, these WdO bands due to wolframyl species only appear on outgassed samples, being undetectable on wet samples. This can be rationalized by assuming that the adsorption of water causes a strong

perturbation of the corresponding W-oxide species, with a consequent strong broadening and shift down of these bands which become almost undetectable. (g) Diffuse Reflectance UV-vis Spectroscopy. The UV-vis spectra of the samples prepared with the AlTi(0) support, Figure 10a, show the formation of an absorption likely constituted by two components, one centered near 250 nm and the other near 220 nm, in agreement with literature reports.36,37 According to the literature, these absorptions can be assigned to the O2f W6+ charge transfer transitions with W in a relatively low coordination state (lower than 6-fold). The W-free supports do not present any absorption in this region, according to its insulating character. The intensity trend of such absorptions is quite complex and difficult to interpret. It is possible that the very strong intensity for the sample 1.4 at is due to the fact that the isolated wolframyls are very strong absorbers, while the “polymeric” wolframate species and WO3 are weaker absorbers. By increase of W content, the amount of the latter species grows possibly at the expense of the monomeric species. The spectra of the samples supported on pure and rich titania supports present a strong absorption in the region 200-380 nm with maxima near 225 and 325 nm, due to TiO2 anatase. The addition of W does not cause shifts in these absorption, but only intensity modifications. This is certainly due to the superimposition and mixing of the features due to the O2- f Ti4+ and O2- f W6+ charge transfer transitions. These data show that Al3+ cations cannot interfere with the electronic state of W oxide centers, due to the much higher energy of the Al 3p levels with respect to the W 5d levels. On the contrary, Ti4+ and W6+ cations strongly interact with each other because the Ti 3d and the W 5d levels have nearly the same energy. From these data we can conclude that the W 5d levels stay in the gap of the alumina insulating support while they stay in the Ti 3d derived conduction band of the semiconducting support anatase. This can explain the different behavior and reducibility of tungsten oxide when supported on alumina and on titania.

(34) Kim, D. S. Ostromecki, M.; Wachs, I. E. J. Mol. Catal. 1996, 106, 93. (35) Ouafi, D.; Mauge, F.; Lavalley, J. C.; Payen, E.; Kasztelan, S.; Houari, M.; Grimblot, J.; and Bonnelle, J. P. Catal. Today 1988, 23.

(36) Tittarelli, P.; Iannibello, A.; Villa, P. L. J. Solid State Chem. 1981, 37, 95. (37) Iannibello, A.; Villa, P. L.; Marengo, S. Gazz. Chim. Ital. 1979, 109, 521.


Langmuir, Vol. 14, No. 3, 1998 637

Figure 10. UV-vis DRS spectra of W/Al-Ti(x) samples with tungsten loading of 1.4, 2.8, 3.5, and 4.2 tungsten atoms per square nanometer. For the W/Al-Ti(0.95)-1.4, 3.5, and 4.5 overlap almost completely. For the W/Al-Ti(1.0)-2.8 and 3.5 overlap completely.

Discussion The results presented above show that, using IR and Raman spectroscopies, two tungsten species can be observed at the surface of WO3/TiO2-Al2O3 powders. At relatively low tungsten coverages, a species is observed which is characterized, in wet atmospheres, by a single band nearly coincident in IR and Raman spectra near 965 cm-1. In dry atmospheres (IR adsorption experiments, under outgassing at high temperature) the observed species are characterized by a sharp fundamental WdO stretching mode in the region 1010-1025 cm-1 and by a single overtone in the region 2000-2030 cm-1. Our IR data of previously outgassed WO3 supported on pure TiO2 and alumina supports fully agree with those of Wachs16,34 obtained by Raman spectroscopy with samples dried in pure oxygen. As already discussed, the lack of any multiplicity in the 1010-1025 cm-1 IR band and its overtone at about 20002030 cm-1 definitely support that this species has in its “molecular structure” only one WdO short double bond. In fact, if two WdO bonds were present, either on the same tungsten atom or on near tungsten atoms, their stretching mode would couple and give rise to multiple fundamental stretching modes, with different relative intensities in IR and in Raman, as well as to split overtone bands. This is clearly found in dioxo species like in O2WCl2,38 where the two stretching WdO modes are separated by 40 cm-1, and for CdWO439 with the wolframite structure, where a polyoxoanion with 8-fold W coordination occurs and where every W atom has two terminal WdO (38) Levason, Narayanaswamy, W. R.; Ogden, J. S.; Rest, A. J.; Turff, J. W. J. Chem. Soc., Dalton Trans. 1982, 2009. (39) Daturi, M.; Busca, G.; Borel, M. M.; Leclaire, A.; Piaggio, P. J. Phys. Chem. 1997, 101, 4358.

bonds whose asymmetric and symmetric stretchings are split off near 80 cm-1. Also in the case of short WdO bonds whose W atoms are bridged by an oxygen, giving rise to polymeric species, the stretching modes should couple, giving rise to more than one fundamental and overtone bands. An accidental superimposition of the strongest Raman and IR fundamental stretching modes is only possible when coupling does not occur for geometrical reasons (O-W-O angle of 90°), which is very unlikely. However, although the above observations point out to the existence of only monomeric W monooxo species, the definitive proof can only arise from the study of partially 18O/16O isotopically exchanged samples. In fact, if two (or n) wolframyls are “vibrationally coupled” although their stretching modes accidentally coincide, they should give rise to three (n + 1) components for partially 18 O/16O isotopically exchanged samples. These experiments have been performed for supported V2O5-TiO2,40 whose spectroscopic behavior is similar to WO3-TiO2 and to WO3-Al2O3,33,41 and unequivocally showed that the corresponding surface species are monoxo and monomeric. Also, the previously published Raman spectra of Stencel et al.42 on 18O exchanged WO3-Al2O3 also somehow exclude the existence of more than one WdO stretching mode on the “molecular structure” of the surface tungsten oxide species. Consequently, the vibrational behavior of such a species strongly suggests that they primarily interact with the (40) Ramis, G.; Cristiani, C.; Forzatti, P.; Busca, G. J. Catal. 1990, 124, 574. (41) Ramis, G.; Busca, G.; Lorenzelli, V. Structure and Reactivity of Surfaces; Morterra, C., Zecchina, A., Costa, G., Eds.; Elsevier: Amsterdam, 1989; p 777. (42) Stencel, J. M.; Makovsky, L. E.; Diehl, J. R.; A Sarkus, T. J. Raman Spectrosc. 1984, 15, 282.

638 Langmuir, Vol. 14, No. 3, 1998

Figure 11. Tungsten species present on the catalysts surface.

oxide supports via W-O-(Al,Ti) bonds and that the presence of polymeric species with W-O-W bonds is, if any, definitely not predominant. The ammonia adsorption data as well as the water desorption (and adsorption) data indicate that, on dry surfaces, these species are Lewis acidic; i.e., they are coordinatively unsaturated. This implies that the overall coordination at tungsten is lower than the usual overall coordination of hexavalent tungsten, i.e., six or four. Adsorption of water and/or ammonia can give rise to a Lewis acid-base interaction allowing the completion of the overall coordination sphere at tungsten. So, it seems reasonable to propose that the overall coordination at tungsten on dry surfaces is 4 and/or 5, in agreement with the extended X-ray adsorption fine structure and X-ray absorption near edge structure (EXAFS-XANES) data of Hilbrig et al.9c The interaction upon water adsorption/desorption can be explained with a structure similar to the one shown in Figure 11 where the “dry” wolframyl species I are responsible for the sharp bands in the region 1010-1025 (fundamental WdO stretching) and 2000-2025 cm-1 (first overtone), whose exact position, however, depends on the actual structure of the surface sites to which they are linked. The wet species II and III (see Figure 11) have a decreased WdO bond order, according to the higher overall coordination at tungsten and the interaction of water with the terminal oxygen. So, the “terminal” WdO stretching mode is significantly decreased down to the observed value of 965 cm-1, by both IR and Raman, on wet samples, before any activation procedure. In fact, the coincidence of the IR and Raman features suggests that, also for wet samples, the WdO bond is single. Our data show that such species are also associated to the generation of strong Brønsted acidity (as shown by ammonia adsorption experiments) and to the formation of broad OH stretching bands. This can be interpreted assuming that the partly hydrated species II (Figure 11) is still present in incompletely dehydroxylated surfaces and acts as a strong Brønsted acid. Its OH group is hydrogen bonded as observed. The overall picture for the tungsten species we propose here seems substantially consistent also with most of the conclusions of Hilbrig et al.9c based on EXAFS-XANES data on tungsten oxide-titania and tungsten oxidealumina. However, as already discussed, we cannot fully support the conclusion of Hilbrig et al.9c and of Engweiler et al.,9d that the above tungsten species are polymeric, with W-O-W bonds. In fact, we do not find, looking also at the cited references, any experimental datum that actually supports the “polymeric nature” of such species. We also disagree with the recent conclusions of Kim et al.34 that on alumina and titania such tungsten species are “highly distorted, octahedrally coordinated” ones. Our data, in agreement with Wachs,16,34 show that although the structure of the species is essentially the same on titania, alumina, and mixed oxides, the stretching of the


WdO bond is definitely modified both by the support and by the coverage. The WdO bond strength and bond order depend inversely on the basic strength of the oxide ligands, so that it is stronger if the oxide ligands are weaker as bases. This explains why on a more acidic (with less basic surface oxide ions) surface like alumina the WdO stretching of these species is found at slightly higher frequencies than on less acidic surfaces like titania, whose oxide ions are likely more basic. As a result of this, the two structures, although very similar, can have quite a different chemical behavior. Also, the observed upward shift of the WdO stretching by increasing surface coverage on all samples agrees with this interpretation. In fact, the WO4+ wolframyl cation is Lewis acidic, as demonstrated by the studies of ammonia adsorption. Its interaction with the surface can be supposed to be acidobasic in nature; i.e. the wolframyl cation will interact with the basic anions on the support surface. However, at low W coverage, wolframyl species will be free to interact with the strongest basic centers on the oxide surface. By an increase of the coverage, the wolframyl cations will adapt to interact also with weaker basic centers. So, at lower coverage the WdO stretching is expected to be at lower frequency than at higher coverage, just like observed here for wolframyl, and previously for surface vanadyl centers on vanadia-based catalysts.43 Our data and the above interpretation can be coupled with the result that even at nearly full coverage loading, the free surface hydroxy groups of the supports have not disappeared (except perhaps those of alumina) but are only decreased, at most, to one-third of their original intensity. This makes doubtful the widespread conclusion that the anchoring sites for the oxide-supported oxides are the supports OHs. In fact, in a dehydrated surface (like that obtained after outgassing) the surface hydroxy groups are expected to be definitely less basic than the surface oxide anions. In other words, protons will interact with basic oxygens “neutralizing” them, so that oxide anions not neutralized by protons are expected to retain their basicity. In this respect, it seems reasonable to propose that, in samples like ours, the interaction with a virtually completely dehydroxylated surface, WO4+ cations and H+ ions should be competitive. Moreover, according to our results, the expected anchoring sites for wolframyl cations are preferentially oxide anions rather than hydroxyls. Our data apparently disagree, at least partially, with some literature data concerning the dispersion behavior of supported oxides like tungsten oxide on oxide carriers. In effect, our data show that on titania and titania-rich supports tungsten oxide is much better dispersed than on alumina. We have already discussed above this point and we have concluded that the dispersion data are certainly very sensitive to the textural properties of the support used, as well as to the details of the impregnation procedure and the nature of the precursor species. For this reason, the reported values of monolayer coverage cannot be generalized to all cases. Another point that should be well taken into account when comparing monolayer coverage values is the basis taken for the calculation of such data. For example, Kim et al.34 showed that the Raman spectrum of a 25% WO3-Al2O3 catalyst already contains the features of WO3. However, these authors later in the same publication gave as a monolayer loading for WO3-Al2O3 28 wt % of tungsten oxide. (43) (a) Ramis, G.; Busca, G.; Bregani, F. Catal. Lett. 1993, 18, 299. (b) Lietti, L.; Forzatti, P.; Ramis, G.; Busca, G.; Bregani, F. Appl. Catal. B: Environ. 1993, 3, 13.


Moreover, apparently they did not measure the surface areas of their supported WO3 catalysts. Finally, they calculated the surface density of supported WO3 catalysts at monolayer coverage (Table I in their paper) using the surface area of the support instead of that of the catalyst. This gave rise, in our opinion, to tungsten monolayer capacity data, which have inconsistent units and, therefore, cannot be generalized. We recommend then, for the benefit of all, that the basis of calculation of monolayer coverage should be quite clearly stated and also, enough detail on the catalyst preparation procedures and precursor salts used be given. Conclusions From the results of the present study the following conclusions can be drawn: 1. “Mononuclear” wolframyl species are found on low coverage WO3/Al2O3-TiO2 catalysts. 2. The WdO bond order of such species, in their dried form, is sensitive to the support composition in the sense that it increases by increasing Al content. 3. The WdO bond order of the above wolframyl species, in their dried form, is also sensitive to the coverage, in the sense that it increases by increasing the tungsten coverage.

Langmuir, Vol. 14, No. 3, 1998 639

4. Such wolframyl species are strongly Lewis acidic. 5. The tungsten surface species, in a partly hydrated form, are also strongly Brønsted acidic and are associated to hydrogen bonded hydroxyls. 6. The tungsten wolframyl species are anchored to the support surface preferentially through the more basic oxide anions, while the role of hydroxy groups in their anchoring is a secondary one, if at all. 7. The tungsten “monolayer capacity” data estimated for our samples according to our preparation procedures, differs from those obtained in the literature, at least for alumina-supported ones. However, it is evident that the “monolayer capacity” data available in the literature are affected by the particular basis of calculation used in each case and are, sometimes, calculated incorrectly. Acknowledgment. The authors acknowledge exchange grants in the frame of the CNR (Rome, Italy) CONACYT (Me´xico, D.F., Me´xico) cooperation program. We also acknowledge financial support from DGAPAUNAM, PEMEX-Refinacioń, and The IMP-FIES program. LA970993N

A Vibrational and Spectroscopic Study of WO3 ... - ACS Publications

Recommend Documents