Redox and acid reactivity of wolframyl centers on oxide carriers: Brønsted, Lewis and redox sites

Applied Catalysis A: General 216 (2001) 181–194

Redox and acid reactivity of wolframyl centers on oxide carriers: Brønsted, Lewis and redox sites A´ıda Gutiérrez-Alejandre a , Perla Castillo a , Jorge Ram´ırez a,∗ , Gianguido Ramis b , Guido Busca b a

b

UNICAT, Departamento de Ingenier´ıa Qu´ımica, Facultad de Qu´ımica, UNAM, Cd. Universitaria, Coyoacán, México D.F., C.P. 04510, Mexico Laboratorio di Chimica delle Superfici e Catalisi Industriale, Dipartimento di Ingegneria Chimica e di Processo, Università, P. le J.F. Kennedy, I-16129 Genova, Italy Received 13 October 2000; received in revised form 28 February 2001; accepted 2 March 2001

Abstract Catalysts prepared by impregnating tungsten oxide on alumina, titania and zirconia and their mixed oxides have been characterized by skeletal IR, IR of adsorbed ammonia, Raman and UV–VIS–NIR spectroscopies and by temperature programmed reduction. In all cases catalysts with W loading well below the monolayer have been taken into consideration. Surface mono-oxo wolframyl species with similar low coordination structure have been found to largely predominate in all the supports. However, the W=O bond length, the Lewis acidity, the charge transfer transition energies and the reducibility of the WOx species strongly depend on the support nature. In particular, the wolframyls on alumina are most acidic, have higher charge transfer transition energies and are less easily reducible than those on titania. The wolframyls on zirconia show intermediate properties. Evidence is given for the different behavior of wolframyl centers in spite of their similar geometric “molecular” structure. The different properties of wolframyl centers on the supports used here explain the different behavior of these materials for hydrocarbon conversion, in the selective catalytic reduction of NO by ammonia and as precursors of hydrodesulphurization catalysts. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Tungsten oxide; Alumina; Titania; Zirconia; Alumina–titania; Titania–zirconia; Redox sites; Brønsted acid sites; Lewis acid sites

1. Introduction Tungsten oxide supported on oxide carriers such as alumina, zirconia and titania represent relevant materials in heterogeneous catalysis. Most of them display strong surface acidity [1] and have been proposed as catalysts for hydrocarbon conversion and synthesis of hydrocarbons from methanol [2]. Zirconia supported ∗ Corresponding author. Fax: +52-5622-5366. E-mail addresses: [email protected] (J. Ram´ırez), [email protected] (G. Busca).

tungsten oxide is reported to display “superacidic” behavior and to be very active in paraffins isomerization [3]. Additionally, these materials can be used as precursors for sulfided hydrodesulphurization (HDS) or hydrotreating catalysts [4,5], as well as for reduced olefin methathesis catalysts [6]. Moreover, tungsten oxide on titania is an active catalyst for the reduction of NOx by ammonia (SCR process [7]), and is used industrially as the support for vanadia based SCR catalysts. Tungsten oxide supported on titania–zirconia mixed oxides has been used as catalyst for the decomposition of chlorofluorocarbons [8]. Finally, tungsten

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oxides [9] and metal tungstates [10] are active in catalytic oxidation. Several investigations have appeared in the open literature concerning the characterization of oxide-supported tungsten oxides. In particular, Wachs investigated several oxide-supported tungsten oxide systems and reviewed their results [11]. More recently, the same group reported data on the effects of dopants on the nature and vibrational properties of surface species on WO3 -Al2 O3 systems [12,13]. Ram´ırez and co-workers investigated tungsten oxides supported on alumina, titania and their mixed oxides as precursors for HDS catalysts [14–16]. Data on the structural and acidic properties of WO3 -Al2 O3 and WO3 -TiO2 systems have also been reported by other research groups [17–20]. The reducibility of these systems has also been investigated [21,22]. The WO3 -ZrO2 has also been deeply characterized in relation to their alkane isomerization activity [23,24]. Although many data have been published on these systems, the origin of their strong acid reactivity and of their oxidation activity have not been fully ascertained so far. In particular, the origin of the supposed “superacidity” of WO3 -ZrO2 systems is largely unclear and under debate. Some authors suggest that paraffin isomerization is, on such kind of catalysts, not simply acid catalyzed [25]. Redox cycles, possibly with a reduction of tungsten oxide centers and radical-like intermediates, can be involved. Additionally, the nature of the promoting effect of TiO2 used

as support, in particular in sulfided HDS catalysts, has been already pointed out. For these reasons, we undertook a comprehensive study of the surface acidity, the electronic structure and the redox behavior of tungsten oxide supported on different carriers such as Al2 O3 , ZrO2 and TiO2 , and some of their mixed oxides.

2. Experimental In Table 1, some data on the samples used in this study are summarized. The different support samples, ZrO2 , Al2 O3 , TiO2 , TiO2 -Al2 O3 and TiO2 -ZrO2 , were prepared starting from Zr(NO3 )4 (MEL Chemicals, solution 40%), Al(OCH(CH3 )2 )3 and Ti(OCH(CH3 )2 )4 (both from Aldrich, 97%). All the supports were prepared by precipitation or co-precipitation, as reported elsewhere [26–28]. The samples will be referred as (A) for alumina, (T) for titania, (AT) for the alumina–titania 50:50 atomic ratio mixed oxide, (Z) for zirconia, and (TZ) for the titania–zirconia 50:50 atomic ratio mixed oxide. The preparation of the tungsten-containing catalysts was accomplished by impregnating the supports with an aqueous solution of ammonium metatungstate ((NH4 )6 H2 W12 O40 ) in the appropriate concentration to yield the composition values given in Table 1. The catalyst precursors were subsequently dried for 24 h at 373 K and calcined for 3 h at 773 K. Hereafter, the catalysts will be referred as W followed by the support

Table 1 Phases and textural characteristics of the materials under study Sample

WO3 (wt.%)

W surface density (WO3 /nm2 )

XRD phases

SBET (m2 g−1 )

Dp a (Å)

Vp b (cm3 g−1 )

A WA AT WAT T WT TZ WTZ Z WZ

– 18 – 18 – 10 – 10 – 10

– 2.8 – 2.8 – 3.07 – 1.42 – 3.07

Gamma Gamma Amorphous Amorphous Anatase Anatase Amorphous Amorphous Monoclinic + tetragonal Monoclinic + tetragonal

214 162 254 203 94 92 223 182 94 85

68 67 89 84 113 103 25 24 79 80

0.38 0.28 0.59 0.44 0.26 0.24 0.14 0.11 0.18 0.17

a b

Dp : average pore diameter. Vp : pore volume.

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notation (i.e. WT for tungsten oxide supported on titania). The BET surface area, pore volume and pore size distribution of supports and catalysts were obtained from nitrogen adsorption–desorption isotherms measured at 77 K with a Micromeritics ASAP 2000 automatic analyzer. Prior to the physisorption measurements, all samples were outgassed for 6 h at 543 K. In general, the errors found in repeated measurements of surface area determinations were within 2–3% of the total surface area. The X-ray diffraction patterns were recorded in the range 3◦ ≤ 2Θ ≤ 90◦ in a Philips PW 1050/25 diffractometer, using Fe-filtered Cu K␣ radiation (λ = 1.5418 Å) and a goniometer speed of 1◦ (2Θ) min−1 . The UV–VIS diffuse reflectance spectra were recorded at ambient conditions in a Cary 5E UV–VIS–NIR spectrometer using polytetrafluoroethylene as reference. Some spectra were also acquired at dehydrated conditions (heating under vacuum, 1 h at increasing temperatures), using a Jasco V570 instrument equipped with a special quartz cell. The infrared spectra were recorded with a Nicolet Magna 550 FT instrument with a 4 cm−1 spectral resolution and 100 scans. The samples were analyzed as KBr pressed disks at room temperature and also using a special IR cell connected to a conventional gas-manipulation-evacuation apparatus that allows heating and evacuation of the sample wafer. The supports and catalysts wafers, made by pressing the pure solids, were outgassed at T = 373, 473, 623 and 723 K for the OH analysis. For the ammonia adsorption studies, the samples were pretreated by outgassing at 723 K and then cooled to room temperature before ammonia adsorption (48 Torr NH3 ). Raman spectra at ambient conditions were recorded with a Nicolet 950 FT instrument equipped with an InGaAs detector and a Nd:YAG laser source with a resolution of 4 cm−1 and 200 scans. The temperature-programmed reduction (TPR) of the samples was performed in an ISRI RIG-100 automated characterization system, equipped with a thermal conductivity cell. The pre-treatment of the samples (125 mg in all cases) consisted of in situ calcination at 773 K under airflow (30 ml min−1 , 2 h). The samples were cooled in an Argon stream. The reduction step was performed with an Ar/H2 mixture (30/70 v/v, 25 ml min−1 ), with a heating rate of 10 K min−1 . After reaching 1273 K the sample was maintained

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at this temperature until the trace reached the base line.

3. Results 3.1. Morphological and phase characterization of oxide carriers and W-containing catalysts In Table 1, the surface areas and XRD data (phase analysis) of the support oxides and catalysts used in this study are reported. The XRD crystal phase of alumina is poorly crystallized ␥-alumina. The TiO2 sample shows only anatase crystal structure while zirconia is a mixture of tetragonal and monoclinic ZrO2 . The AT and TZ mixed oxides are essentially amorphous. From the data in Table 1 is possible to see that the observed drop in surface area and pore volume between supports and catalysts is close to the expected value, according to the weight of WO3 present in the sample, indicating the absence of important pore blocking effects. It is only in the case of the WAT and WTZ catalysts where some pore blockage might have taken place. However, no crystalline WO3 was detected by XRD and therefore, if some pore blockage occurred in the WAT and WTZ samples it occurred in the pores with small diameters (<30 Å). The pore size distribution curves of the WAT and WTZ catalysts and their corresponding supports (not shown) indicate that pore blockage is more likely in the WTZ sample. In agreement with this, the FT-Raman results (see below) insinuate the presence of polytungstates only in the case of the WTZ sample. Table 1 also reports the tungsten surface density expressed as WO3 /nm2 . The tungsten oxide loading to reach the monolayer coverage on such kind of catalysts has been reported to be in the range 4–7 WO3 /nm2 [11,12,13,22], which means that our materials contain less than monolayer coverage. 3.2. FT-IR and FT-Raman studies of the surface tungsten oxide species The IR spectra of KBr catalysts pressed disks (not shown) present a broad band in the 1000–950 cm−1 region. The Raman spectra of the pure catalysts also present a band in that region. This is shown in Fig. 1, where the Raman spectra of the samples WZ, WTZ and WT are reported and compared with those of the

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Fig. 1. FT-Raman spectra of the samples Z, WZ, TZ, WTZ, T and WT, where exp. is the expanded spectrum.

corresponding supports, Z, TZ and T. In Fig. 1, it is possible to observe that besides the bands related to the supports, which appear beyond 900 cm−1 , there is one band at 959 cm−1 for WZ and at 969 cm−1 for WT. According to previous studies [20] these bands are assigned to W=O stretching vibrations in surface wolframyl species, which are in a hydrated form at ambient conditions. In the case of the WTZ catalyst besides the band at 951 cm−1 there is an additional weak one at ∼800 cm−1 , which indirectly argues for the presence of polytungstate species in this sample

[29,30]. In a previous publication with the same WA sample, we showed that also in this case a small amount of polytungstates or WO3 is present. The features near 1000 cm−1 are detectable also in the IR spectra of the pure pressed disks after evacuation, when the wolframyl species are converted into a dried form. However, if the support is alumina or titania the detection of these bands is difficult because in that region most of the IR light is absorbed by the supports so that light transmittance is low or very low. In the case of WZ, where the zirconia support shows the cut-off limit at lower IR frequencies, this band can be detected easily (Fig. 2a). The main band for WZ is detected at 1002 cm−1 with shoulders at 1012 and 1005 cm−1 . On the other hand, the IR band around 1000 cm−1 shifts down and broadens upon adsorption of water, ammonia or other basic molecules, due just to the adsorption of these molecules on the wolframyl sites. After ammonia adsorption on the WZ sample, the band shifts down and is now well split with components at 975 and 955 cm−1 (Fig. 2b). The same occurs on the other W-containing catalysts. The band just above 1000 cm−1 in dried samples becomes well apparent in the subtraction spectra, when the spectrum of the activated surface is subtracted from that with the basic molecule adsorbed, even when the supports cut most of the IR radiation. This is shown in Fig. 3 for the samples WT and WTZ. In these subtraction spectra, we can observe a strong negative and sharp feature

Fig. 2. FT-IR spectra of WZ (a) after activation at 723 K and (b) after adsorbing and outgassing ammonia at RT.

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185

Fig. 3. Subtraction FT-IR spectra (spectrum of activated sample subtracted from the one after NH3 adsorption at RT) of the WT and WTZ catalysts.

centered at 1008 cm−1 for WTZ and near 1012 cm−1 for WT. This band is due in all cases to the W=O stretching of surface wolframyl species in their dried form. Additionally, in the spectra of the activated samples a small band is observed in the region near 2000 cm−1 , which also disappears upon ammonia or water adsorption and is consequently visible as a negative feature in the subtraction spectra (Fig. 3). As discussed before [20] this feature is due to the first overtone of the above W=O stretching mode. The presence of one single band in particular in the overtone region is evidence for the presence of one single W=O bond, so of “mono-oxo” wolframyl species. Therefore, the IR and Raman spectra of the catalysts under study show that wolframyl species exist in all our W-containing catalysts, in a dry form after activation and in a hydrated form at ambient conditions. Consistent with the low tungsten loading used in our catalysts, our IR and Raman spectra show, in general, the absence of clear bands at 808, 720 and 270 cm−1 , which can be attributed to other more polymerized W-containing species like polytungstates or WO3 , observed at higher loading. Only in the WA and WTZ samples we observe a small evidence of the presence of polytungstates. It has been reported for the different pure supports that polytungstate species and WO3 are normally detected at loadings higher than the ones used here, i.e. 4.5 W atoms/nm2 for WOx -ZrO2

catalysts [29,30]. Therefore, in our relatively low loading region, wolframyl species must be the only or, at least, the largely predominant W-containing surface species. The existence of some polytungstate species cannot be however completely ruled out because it is well known that the Raman scattering cross section of W–O–W in polytugstate species is very small making difficult their detection. The characteristic IR bands of wolframyl species (dried form) observed in our samples shift a little with the outgassing temperature and also with W loading. Table 2 summarizes the observed positions of these bands at all the outgassing temperatures. The frequency intervals show quite a clear trend in relation to the support. In fact they are located definitely at the highest frequency when the support is alumina, Table 2 Position (cm−1 ) of the bands of W=O stretching frequency of surface wolframyl species Sample

WA WAT WT WTZ WZ

First overtone IR

2028–2035 2015–2030 2010–2015 2008–2012 2005–2010

Fundamental IR

Raman

1022–1030 1010–1020 1010–1018 1005–1012 1000–1010

956 951 969 951 959

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and at the lowest when the support is zirconia. For the mixed oxide supports a shift is also evident between the position observed for the mixed oxide system and the two corresponding pure oxide carriers. The above data can be discussed in relation to the known inorganic chemistry of wolframyl compounds. It is in fact evident, and has been discussed elsewhere too [31], that the existence of one single band in both IR and Raman spectra is indicative of mono-oxo wolframyl species. The W=O bond length of these species depends, however, on the basic strength of the ligands to which the W atom is also coordinated. The stronger the basic strength of the “equatorial” or in plane oxide ligands, the larger and weaker the W=O “axial” (out of plane) W=O bond, and the lower the W=O frequency. The observed shifts in the vibration frequency of the W=O bond are in line with the trend of the basicity of the support oxides ligands, which is expected to be ZrO2 > TiO2 > Al2 O3 . This agrees well with the result arising from the measure of the trend of the Lewis acid strength that we will present below. In fact, the acidity trend of the cationic sites is expected to be the reverse of the basicity trend of the oxide anions. As a conclusion, these data show that similar tungsten species are formed (at low coverages) on all the supports considered here. However, although the geometry of the tungsten oxide species formed is similar, the shift of the W=O stretching bands is significant, suggesting that the chemical reactivity and the electronic distribution of these species is dependent on the acid–base properties of the support. 3.3. FT-IR study of the surface acidity The surface acidity of the samples under study was investigated by IR spectroscopy using ammonia as the molecular probe. The adsorption of ammonia on the pure oxides Al2 O3 , TiO2 and ZrO2 has been the object of several previous studies [32]. In all cases ammonia coordinatively adsorbed on Lewis acid sites can be detected through the N–H stretching bands in the region 3500–3000 cm−1 , the asymmetric NH3 deformation near 1610 cm−1 and the NH3 symmetric deformation in the range 1350–1100 cm−1 . The position of the last band is sensitive to the Lewis acid strength of the adsorbing site. The position of this band observed on the samples under study is reported in Table 3. The data show that the symmetric

Table 3 Summary of observed IR-NH3 bands after ammonia adsorption on the samples under study Sample

Lewis bonded ammonia (δ sym NH3 )

Brønsted bonded ammonia

A WA AT WAT T WT TZ WTZ Z WZ

1240 1280, 1230 1240, 1155 1280, 1230 1222, 1155 1250–1230 1230, 1210, 1175 1250–1230 1210, 1170 1250–1230

Traces Yes Traces Yes Traces Yes No Yes No Yes

deformation band is shifted at higher frequency for alumina (1240 cm−1 ) with respect to the other oxides, according to the well known strong Lewis acidity of surface Al3+ cations and to its very high polarizing power [33]. In the case of titania and zirconia, this band is split, likely due to the presence of two sites with different overall coordination and Lewis acidity. However, for both oxides, the two components lie nearly in the same region (1220–1150 cm−1 ), suggesting that surface Zr4+ and Ti4+ ions have similar Lewis acidity. In the case of the alumina–titania mixed oxides the spectra show both a band near 1240 cm−1 (like for alumina) and another one near 1155 cm−1 , like the main band found for titania. This suggests that both very acidic Al3+ and less acidic Ti4+ ions are exposed at the surface of the supports. The addition of WO3 causes in all cases a relevant shift upwards of the ammonia symmetric deformation band. The spectra recorded for the WTZ and WT samples are shown in Fig. 3. The ammonia symmetric deformation band is now centered in the region 1250–1230 cm−1 when the support is T or TZ (Fig. 3) or Z (Fig. 2b), and up to 1280 cm−1 when the support is A. This provides evidence of the even greater Lewis acidity of wolframyl species with respect to Al3+ cations. Note that, as shown in Figs. 2 and 3, upon ammonia adsorption, the W=O stretching band and its overtone shift strongly to lower frequencies and broaden. This shows that the wolframyl sites are actually involved directly in the adsorption of ammonia and that they have now an additional strong basic ligand (ammonia) that allows the W=O bond to relax,

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Fig. 4. FT-IR spectra of WA, A, WZ and Z, after outgassing at 723 K in the OH stretching region.

causing its stretching frequency to shift down. These observations allow us to conclude that wolframyl sites are, on the dry surface with which we are working, coordinatively unsaturated and hence Lewis acidic. The Brønsted acidity can be revealed by adsorption of ammonia when ammonium ions, typically absorbing near 1450 cm−1 , δ as NH4 + , and in the region 3000–2500 cm−1 (νNH), are formed. The IR spectra of adsorbed ammonia show that on the oxide carriers this band is either not present or very weak (Table 3). This means that the Brønsted acidity of the support oxides is poor or nil. On the contrary, when W-oxide species are present, ammonium ions are always formed and their bands are very intense (Fig. 3). This suggests that at least part of wolframyl species either carries one acid proton or involves the presence of acid protons nearby. In any case, wolframyl species give rise to Brønsted acidity too. On the other hand, W-containing catalysts present a very different spectrum in the OH stretching region with respect to the corresponding supports. This is shown for the WA, WZ, WTZ and WT catalysts in Figs. 4 and 5. The IR spectra of the supports after activation under outgassing show sharp bands in the region 3800–3600 cm−1 . These bands are due to the different types of surface hydroxy groups free from H-bondings. The corresponding W-containing catalysts show only a small residual absorption in this

region but also a strong broad absorption in the region 3600–3000 cm−1 . This occurs for all W-containing catalysts under study and shows that after our activating pretreatment, hydroxy groups originated from surface wolframyl species still exist. The spectra show that these groups, absorbing at such low frequency, are H-bonded, as discussed previously for similar catalysts [18]. The higher stability of ammonium ions on WA upon increasing outgassing temperature indicates that the strength of Brønsted sites is the highest on this catalyst. It seems likely that the Brønsted sites are constituted by the fraction of hydrated wolframyls that have not been dried upon activation. 3.4. UV–VIS–DRS study of the electronic structure Fig. 6 presents the diffuse reflectance UV–VIS spectra, recorded in air, of some of the samples under study. Almost no absorption is observed for the alumina sample in our available spectra region (λ > 200 nm), although a weak peak, associated to impurity absorptions, can be found centered near 265 nm. These observations agree with the very high Eg energy of 7.2 eV reported for ␥-Al2 O3 [34] which determines it to be an insulating oxide. On the contrary, both titania-anatase and zirconia show an absorption edge in our available region. For titania the absorption onset is near 450 nm and an

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Fig. 5. FT-IR spectra of WTZ, TZ, WT and T, after outgassing at 723 K in the OH stretching region.

inflection point is observed near 370 nm. An absorption plateau is also found in the region between 330 and 230 nm. The absorption is associated to the O2− → Ti4+ charge transfer transitions corresponding to the excitation of electrons from the valence band (having the O 2p character) to the conduction band (having the Ti 3d character) [35,36]. This corresponds to the energy gap (E g = 3 eV) that characterizes stoichiometric TiO2 -anatase as an intrinsic semiconductor [37]. For zirconia the absorption edge is located at much lower wavelength. The inflection point of the

absorption edge is near 240 nm, although an absorption tail can also be observed in the 400–250 nm region. The main edge is due to O2− → Zr 4+ charge transfer transitions, corresponding to the excitation of electrons from the valence band (having O 2p character) to the conduction band (having Zr 4d character). The edge position agrees well with literature data, reporting an Eg value for zirconia of near 5 eV [37], showing that zirconia is almost an insulating material. The mixed oxide supports containing titania (TZ and TA) show a strong absorption with an edge located at only slightly higher energies than that of titania.

Fig. 6. UV–VIS–DR spectra of WA and A, WZ and Z and of WT and T.

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Regarding the W-containing samples, the UV spectrum of WA shows a strong absorption with a main maximum near 215 nm, and a pronounced shoulder near 250 nm, which is not present in the spectrum of the pure alumina sample. This absorption is certainly associated to O2− → W6+ charge transfer transition involving surface tungsten oxide species. In the case of WZ, a more intense absorption is evident at the lower energy side of the zirconia edge, with an onset near 400 nm and an inflection point near 300 nm. This indicates that the energy of the O2− → W6+ charge transfer transition of surface tungsten oxide species on zirconia is lower than the energy gap of zirconia (as for WA with respect to A). On the other hand, the position of such O2− → W6+ charge transfer transition is at higher energy for alumina-supported catalysts than for zirconia-supported catalysts. In both the cases, this absorption is located at definitely lower energies with respect to the corresponding absorption observed for bulk WO3 [38,39], corresponding to the transition of electrons from the O 2p valence band to the W 5d conduction band of WO3 . This shows definitely that the electronic state of tungsten oxide species on alumina and zirconia is different from each other and from that of bulk tungsten oxide, in spite of the similar structure deduced from the infrared spectra on the supported catalysts. The lower W=O stretching frequency observed for WZ with respect to WA, arising from a larger W=O bond length or a weaker bond strength, may be reflected by the lower energy needed to perform the O2− → W6+ charge transfer transition in WZ. On the contrary, the incorporation of tungsten oxide to titania, titania–zirconia and titania–alumina supports does not modify at all the spectra of the corresponding supports. This means that the empty orbitals of hexavalent tungsten (W 5d) lie into the Ti 3d conduction band so that the O2− → W6+ charge transfer transitions are superimposed to or, more likely, mixed with the O2− → Ti4+ charge transfer transitions, in the three cases. We have also recorded the DRS spectra under outgassing at increasing temperatures. This is shown in Fig. 7 for WZ. Only slight or no changes are found in the position of the O2− → W6+ charge transfer transitions, associated to the conversion of the tungsten oxide centers from hydrated to dehydrated wolframyl forms. This means that the energy of the O2− → W6+

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Fig. 7. UV–VIS–DR spectra of WZ recorded after outgassing at different temperatures.

charge transfer transition is not very sensitive to the overall coordination of tungsten in isolated centers. In the case of W-oxide species supported on TiO2 -containing supports (WT, WTZ and WTA), upon outgassing at high temperatures, we observe a progressive increase of the absorption baseline in the region above the edge, i.e. below the energy of the gap. This is associated to a reduction of the catalyst in such conditions. It is well known that Ti4+ can be reduced to Ti3+ by heating under vacuum between 470–570 K [40]. 3.5. Temperature programmed reduction study The TPR spectra of the samples under study are reported in Figs. 8 and 9. As reported previously [15], no hydrogen consumption is found for alumina, showing that this material is not reducible in these conditions. On the contrary, both TiO2 and ZrO2 samples show some reduction. The TPR profile of the T sample shows two reduction zones: one broad and small low temperature peak that begins at 730 K and a second one beginning at 1000 K and not resolved till 1273 K, near the final temperature of the experiment. The TPR profile for Z shows mainly one reduction peak with maximum at 939 K. The TZ sample presents two reduction peaks with maxima at 815 and 977 K. Finally, AT samples show also weak reduction peaks in the region above 770 K. When W is incorporated to the different supports, some interesting differences in the reduction

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Fig. 8. TPR of the T, WT, Z, WZ, A and WA samples.

behavior of the catalysts are observed. In agreement with previous reports [21,22,41,42], the reduction of the WO3 -Al2 O3 system showed a single strong reduction peak at high temperature (1313 K), associated to the single step reduction of W6+ → W0 . The temperature at which the reduction occurs indicates the difficulty to reduce the W-oxide species in this system. The TPR trace of the WT system shows, besides the features due to the reduction of pure TiO2 , the

appearance of a strong reduction peak (maximum at 1033 K). This peak can be assigned, according to previous studies that corroborate the stabilization of the W4+ species on TiO2 [22], to the reduction of W6+ to W4+ . The second reduction step from W4+ to W0 cannot be clearly observed, although it is evident that begins around 1200 K, because of the final temperature of the experiment. Unlike TiO2 , the reduction of ZrO2 is clearly modified by the presence of W oxide.

Fig. 9. TPR of the TZ, WTZ, AT and WAT samples.

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The TPR pattern of the WZ sample shows two reduction peaks, one at 778 K and another at 1060 K. Additional experiments performed with different W loading confirm that the peak at 778 K can be assigned to the reduction of ZrO2 . This peak appears shifted to lower temperature with respect to the reduction of pure Z. The downwards shift in the reduction temperature of ZrO2 has been observed before in Mo-ZrO2 [43] systems. This behavior can be explained by a change in the electronic characteristics induced by the presence of W-oxide species on the ZrO2 support, according to the proximity of the Zr 4d and W 5d electron levels, as evidenced by UV spectroscopy. The shift in the reduction temperature of Z is also observed in the reduction profile of the TZ sample, suggesting that TiO2 may induce the same behavior on ZrO2 . Therefore, the peak at 1060 K, is the only one that can be assigned to the reduction of surface wolframyl species. The features of this peak (broad and symmetric) resemble those recorded on WA. This suggests that tungsten supported on Z is reduced in one single step from W6+ to W0 . Additional TGA/DTA experiments in the 293–1723 K temperature range confirmed the sole existence of the two reduction peaks observed by TPR. The reduction of the WTZ sample shows that the mayor reduction of the tungsten species on this support occurs at about the same temperature as on WZ. The smaller reduction peaks at lower temperature are mainly assigned to the reduction of the different support contributions. With respect to the WAT sample, the TPR profile shows three peaks with maxima at 1073, 1185 and 1269 K, the first one has been assigned to the reduction of the titania in the support, and to the first-step reduction (W6+ to W4+ ) of tungsten species in octahedral coordination. The second reduction peak is associated to the second reduction step (W4+ to W0 ) of octahedral tungsten species. The third reduction peak at higher temperature is associated to the reduction of tetrahedral Mo species in strong interaction with the support [15]. The above TPR results indicate that W-oxide species behave differently upon reduction depending on the nature of the support. Clearly, it is more difficult to reduce such species on Al2 O3 than on ZrO2 or TiO2 , as evidenced from the reduction peak maxima (1313 K for Al2 O3 , 1060 K for ZrO2 and 1033 K for TiO2 ).

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4. Discussion The results described above show, in agreement with previous studies [14,30], that on the surface of alumina, titania, zirconia and their mixed oxides, the impregnation of tungsten oxide gives rise, at relatively low loadings (below the monolayer), to mono-oxo wolframyl species (at least predominantly). These species are characterized by a single W=O stretching band which is sharp above 1000 cm−1 for dried forms and shifts below 1000 cm−1 and broadens for their wet hydrated forms. Our data show, however, that although apparently similar, these surface WOx species present differences in each case, in the sense that the W=O bond strength and length differ significantly from carrier to carrier. In particular, this bond is stronger on alumina and alumina containing supports because of the lower basicity of the surface oxide anions of this support, which corresponds well with the higher acidity of its surface cationic sites. So, the basicity of the oxide anions to which wolframyl species are anchored determines the length and strength of the W=O bonds. Therefore, the W=O bond is the weakest for tungsten on zirconia, intermediate for tungsten on anatase and definitely the strongest for tungsten on alumina. The ammonia adsorption data show that surface wolframyl sites in the dried state are Lewis acidic and can coordinate ammonia. The IR spectra show that the surface wolframyl centers on alumina are more acidic than those observed on the other supports, and that this is probably again a consequence of the lower basicity of the oxide ligands to which the tungsten atoms are bonded. So, in spite of their geometric similarity the Lewis acidity of the surface wolframyl species is definitely different from carrier to carrier. In any case, the Lewis acidity of these sites implies that they are, at least on the dried surface, coordinatively unsaturated so that a four-fold or five-fold coordination can be proposed. The WOx species observed on titania are similar to those observed on alumina (wolframyl species). However, they are characterized by a longer W=O bond (lower W=O stretching frequency) and weaker Lewis acidity. The W species formed on zirconia also appear to be similar to those formed on titania but with even lower W=O bond length and Lewis acidity. For WOx supported on the mixed oxides intermediate situations

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Scheme 1. Proposed structure for anhydrous (left) and hydrated (right) wolframyls on oxide supports.

are observed. As observed in the IR experiments, in all cases the presence of such tungsten oxide species induces Brønsted acidity. It is possible to suppose that this Brønsted acidity is due to the incomplete dehydration of the wolframyl sites upon outgassing. This, on the other hand, is what likely occurs in most practical conditions upon catalysis where water is frequently present in the gas phase and, in spite of the relatively high temperature at which catalysis typically occurs (∼573 K), it likely hydrates part of the surface. So we can propose that Brønsted acidity most likely is due to surface wolframyl centers still existing in the hydrated form. The proposed structures for the Lewis acidic (anhydrous form) and Brønsted acidic (hydrated form) surface wolframyls are shown in Scheme 1. Our data show that the electronic transitions, and hence, the electronic state, of the wolframyl species are also different from support to support. In particular, the energy of the O2− → W6+ charge transfer transitions is higher on alumina than on zirconia. In the case of titania, the absorption of this transition is obscured by the support absorption. The difference in the charge transfer transition energy for wolframyl centers on zirconia and alumina points out a remarkable effect of the interaction with the support on the electronic structure of the surface species, in spite of their similar geometric structure. This likely would result in a different chemical behavior in catalytic cycles that involve redox behavior. The UV spectra also show that there is a strongly different relationship between the energy of the O2− → W6+ charge transfer transitions and the support energy gaps. In fact, alumina is an insulating material and does not show significant absorption above 200 nm. This means that the 3s, 3p conduction band of alumina lies very far from the W 5d orbitals of surface wolframyls species. Thus, the electronic interaction between wolframyl centers and the bulk

of alumina is certainly negligible. Additionally, the insulating ␥-Al2 O3 will isolate electronically from each other the surface wolframyl centers that can undergo reduction in a reducing atmosphere. In contrast, the energy of the O2− → W6+ charge transfer transitions are near the value of the energy gap of zirconia. However, in this case the W 5d orbitals fall again in the gap. This means that these orbitals would behave at low temperature as traps for electrons that are produced by reduction. However, at higher temperatures electrons can go easily from the W 5d orbitals to the Zr 4d conduction band, allowing the wolframyl centers to become in electronic contact with each other through the support conduction band. As for WT, WTZ and WAT catalysts, W 5d orbitals fall in the support conduction band. This justifies the hypothesis of much stronger electronic interactions for tungsten oxide supported on titania than for tungsten oxide supported on zirconia, and even more, on alumina. These interactions are probably those governing the reducibility of supported tungsten oxide catalysts, investigated using the TPR technique. It is in fact evident that surface wolframyl species are most easily reduced on titania than on zirconia, while they are reduced with more difficulty on alumina. This is reflected on the TPR profiles where the maxima of the reduction of the W-oxide species are at 1033 K for WT, 1060 K for WZ and 1313 K for WA. On the other hand, our TPR data indicates that the reduction of the T and Z supports starts before that of the W-oxide species. Again for mixed oxide carriers the reducibility behavior is intermediate. The surface WOx species observed here are similar to those reported earlier, W mono-oxo species [14,29,30]. However, although the surface tungsten oxide species on these carriers are similar, mono-oxo wolframyl species, their reactivity is remarkably different in terms of both their acidity and their redox behavior. Our results also suggest that in the case of tungsten oxide-alumina catalysts surface wolframyl species are very acidic as Lewis sites, can display strong Brønsted acidity, are isolated from each other electronically, are in contact with an non-reducible insulating support, and are hardly reducible. This suggests that, in the case of WA, wolframyl centers operate as isolated species in their redox behavior, for example during reduction and sulfidation to produce WS2 /alumina HDS

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catalysts. This can be the origin of the previously reported difficulty in their sulfidation [44]. On the contrary, tungsten oxide-titania catalysts carry surface wolframyls that are less strongly acidic as Lewis sites, also give rise to Brønsted acidity, and are in electronic contact through the conduction band of titania, which is also a reducible oxide and because of this can pump electrons towards them. Additionally, wolframyl sites on titania are more easily reducible as shown by TPR. This behavior agrees with the easier reduction and sulfidation of surface W-oxide centers to convert them into WS2 in HDS catalysts [44]. The role of titania in activating WS2 (and MoS2 ) HDS catalysts supported on titania, which are definitely more active than those based on alumina [15,45], can also be explained by the reducibility and the electronic conductivity of titania that allows to pump electrons towards the supported metal sulfide, as discussed previously for molybdenum-based catalysts [46]. The electronic interaction between W-oxide species and the titania bulk can also be the reason for the activity of tungsten oxide-titania catalysts in the selective catalytic reduction of ammonia, where the mechanism involves the reduction of the active sites by ammonia and its reoxidation by NO and/or oxygen [7]. The catalysts based on V–W–Ti ternary oxides are even better materials and electronic interactions are certainly involved in this case between the tungsten oxide-titania support and the vanadia active site [47]. In the skeletal isomerization of small alkanes alumina- and titania-supported tungsten oxide catalysts are less active than tungsten oxide-zirconia. We have shown that tungsten oxide-zirconia is less easily reducible than tungsten oxide-titania, but much more than tungsten oxide-alumina. The role of redox cycles in this catalytic reaction, that has already been described as non-purely acidic [25], is most likely. Although, the catalysts we studied here have much less tungsten oxide loading than those used for skeletal isomerization, it is most likely that the support reducibility should have a role on the redox cycles that occur on real skeletal isomerization catalysts. The optimal redox and acid balance of WO3 -ZrO2 can be at the origin of its behavior as a good catalyst for alkanes skeletal isomerization. On the other hand, when only the acid functionality is needed, such as for gas oil cracking [48] and for methanol conversion to hydrocarbons [2], WO3 -Al2 O3

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catalysts appear to be the best among the systems studied here. Our data confirm that WO3 -Al2 O3 catalysts are most acidic from the point of view of both Lewis and Brønsted acidity. The stronger acidity of WO3 -Al2 O3 seems to arise from the strong acidity of the support, which provides weak basic oxide anions as ligands for wolframyl sites. Mixed oxide as supports have more or less intermediate behavior with respect to the corresponding pure oxides. However, their acidic and redox properties can vary in a non-linear way with support composition as shown previously [16,48]. Finally, we want to emphasize that the same or strongly related species like the hydrated and anhydrous forms of wolframyl centers can carry, simultaneously, Lewis and Brønsted acidity and redox properties. This seems quite interesting because it is not infrequent to find in the literature that such sites are necessarily separated on different surface structures. On the other hand, it seems obvious that when one of these properties is perturbed (i.e. the acidity by adding basic dopants) you will certainly also perturb to some extent the others.

5. Conclusions The results discussed above show that mono-oxo wolframyl species are formed when tungsten oxide is impregnated at low loading on alumina, titania, zirconia and their mixed oxides. However, our TPR and surface acidity study show that in spite of the similar geometrical “molecular” structure of the WOx surface species, the reactivity of these species varies strongly from carrier to carrier. In particular, these sites are strongly acidic and hardly reducible on alumina, less acidic and definitely more easily reducible on titania and zirconia, as well as on their mixed oxides. These differences in the behavior of the WOx surface species arise in turn from differences in the properties of the carriers, such as reducibility, electrical conductivity and acid–base properties. Among the supports studied here, titania is the smaller gap semiconductor when stoichiometric and the most easily reducible, and makes the corresponding wolframyl species to be the most easily reducible too. So, the different properties of the supports can justify the different behavior of supported tungsten oxides observed previously

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in alkane isomerization, in the reduction of NO by ammonia and as precursors for hydrodesulfurization catalysts.

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