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An ultraviolet–visible–near infrared study of the electronic structure of oxide-supported vanadia–tungsta and vanadia–molybdena
Materials Chemistry and Physics 72 (2001) 337–346
An ultraviolet–visible–near infrared study of the electronic structure of oxide-supported vanadia–tungsta and vanadia–molybdena M.A. Larrubia1 , G. Busca∗ Dipartimento di Ingegneria Chimica e di Processo “G.B. Bonino” and INFM, Università di Genova, P.le J.F. Kennedy 1, I-16129 Genova, Italy Received 29 May 2000; received in revised form 28 July 2000; accepted 16 September 2000
Abstract The ultraviolet–visible–near IR spectra of a series of catalysts have been recorded and discussed. In particular bulk and alumina-, titania- and silica-supported V, W, and Mo oxides have been considered. Additionally mixed V–W and V–Mo-supported oxides have been investigated. The data show that, in agreement with vibrational data and other literature data, the oxides supported on silica are similar to the corresponding bulk oxides. In contrast, the spectra of the oxides supported on alumina and titania correspond to surface oxide species where the metal stays in a lower overall coordination with respect to the bulk oxides. Finally, the spectra show that only in the case of titania-supported oxides an electronic interaction between the supported metal oxide centers through the support conduction band is possible. This allows to justify on electronic bases the activating effect of titania for vanadia, vanadia–tungsta and vanadia–molybdena catalysts used in the hydrocarbon selective oxidation catalysis and for the selective catalytic reduction of nitrogen oxide by ammonia. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ultraviolet spectroscopy; Electronic spectra of catalysts; Supported vanadium oxide catalysts; Vanadium oxide catalysts; Mixed oxide catalysts
1. Introduction Oxide-supported vanadium oxides find several applications as industrial catalysts. In particular, vanadia–titania catalysts are applied industrially for the phthalic anhydride synthesis from both o-xylene and naphthalene [1]. Titania-supported vanadia–tungsta [2,3] and/or vanadia–molybdena [4] constitute the industrial catalysts for the abatement of nitrogen oxides using ammonia or urea as the reducing agent. This process, called SCR (selective catalytic reduction), which consists actually in a selective oxidation of ammonia to nitrogen, by NO and oxygen [5], is nowadays widely applied to clean up the waste gases from thermal power stations [6,7], but could be extended to diesel-engine trucks. It has been demonstrated that titania–anatase is used as the best catalyst support in both cases because it shows an activating effect with respect to vanadia in oxidation catalysis as well as because it is stable to sulphation in SO2 containing atmospheres [5]. The reasons of the activating ef∗ Corresponding author. Tel.: +39-010-353-6024; fax: +39-010-353-6028. E-mail address: [email protected] (G. Busca). 1 Present address: Departamento de Ingenier´ıa Qu´ımica, Universidad de M´alaga, Spain.
fect have been the object of many studies and different explanations have been offered. According to Courtine et al. [8,37] titania–anatase and V2 O5 present a remarkable crystallographic fit and this could explain the activating effect. However, vibrational studies [9] (including O16 /O18 exchange experiments [10]), and 51 V nuclear magnetic resonance (NMR) studies [11] have clearly shown that the vanadium-oxide centers in vanadia–titania catalysts are not constituted by V2 O5 crystals. On the contrary, at least at low coverage (where the activating effect is already evident) they are constituted by vibrationally isolated mono-oxo vanadyl centers [9–11]. Non-activating supports such as silica actually produce supported V2 O5 particles, without a relevant dispersing effect. For this reason it is possible to suppose that the activating effect of titania is due to the ability to disperse the vanadium oxide centers just in a particular more active structure. However, infrared (IR), Raman and NMR spectroscopies showed that the structures of vanadium oxide centers on titania are nearly the same as those on alumina [11,12], which however gives rise to a much weaker activating effect. For this reason, electronic effects could be invoked to explain the activating effect of titania. On the other hand, it has been established that the addition of WO3 and/or MoO3 to V2 O5 –TiO2 catalysts further strongly enhances the activity and also the selectivity to nitrogen (with respect to the main byproduct which is N2 O) in
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the SCR catalysis. This is in spite of the by far lower activity of WO3 –TiO2 and MoO3 –TiO2 with respect to V2 O5 –TiO2 . This synergetic effect has been explained, based on electron spin resonance (ESR) and Raman experiments, with electronic effects [13]. Other authors invoked the necessity of a two atom sites, i.e. the effect of an acid site (W or Mo) together with a redox site (V) for SCR catalysis [14]. On the other hand, it has also been shown that W oxide has an activating effect not only with respect to V2 O5 –TiO2 , but also with respect to V2 O5 –Al2 O3 [15]. To have a more precise picture of the electronic structure of the materials of interest in the SCR catalysis we have reinvestigated the ultraviolet–visible–near infrared (UV–Vis–NIR) spectra of V, Mo and W oxides and their mixtures as such or supported onto titania, alumina and silica.
ple was prepared by precipitation starting from TiCl4 . The catalyst samples, both single and mixture oxides, were prepared by dry impregnation with hot water solutions of the corresponding salts (NH4 VO3 , 5(NH4 )2 O·12WO3 ·5H2 O, (NH4 )6 Mo7 O24 ·4H2 O), followed by drying and calcination at 773 K for 3 h. So all materials studied here are in the oxide state. Some characteristics of the supports and of the catalysts are summarized in Table 1. Diffuse reflectance spectra in the range 2500–200 nm were obtained with a Jasco V-570 spectrophotometer at room temperature and the samples expend at air. The Jasco instrument standard sample has been taken as reference sample to the UV–Vis–NIR analysis.
2. Experimental
3.1. Oxide-supported vanadia catalysts
The supports used for the catalysts preparations were mainly commercial products (see Table 1). One titania sam-
The UV–Vis–NIR spectra of the pure supports used in this study are compared in Fig. 1. The spectrum of
3. Results
Table 1 Summary of the samples under study Material
Sample
Origin
XRD
Surface area (m2 /g)
% supported oxide (w/w)
TiO2
A B C A B A A A A C B B B B B B Com3 C C Com1 Com2 A A A A A B B B B B
Ti oxide Rhone Poulenc Homemade Degussa Akzo Degussa Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Commercial Homemade Homemade Commercial Commercial (Eurocat) Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade
Anatase Anatase Anatase Gamma Gamma Amorphous Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Amorphous Amorphous Amorphous Amorphous Amorphous Gamma Gamma Gamma Gamma Gamma V 2 O5 MoO3 WO3 WO3 WO3
117 71 90 100 190 200 115 108 100 90 70 62 57 72 22 21 78 87 64 70 47 185 190 190 n.m.a n.m. n.m. n.m. n.m. n.m. n.m. 18 15 15 n.m. n.m.
1 4 8 9 1 3 10 1/10 2/10 5/10 1.2/6.2 0.78/9 4/9 0.56/11.04 4.25/10.57 1.21 1.21 1.21 1.21/1.9 1.21/3 0.5 5 17 10 5/15 Bulk Bulk Bulk 40%/60% bulk 27%/73% bulk
Al2 O3 SiO2 V/TiO2 V/TiO2 V/TiO2 W/TiO2 Mo/TiO2 Mo/TiO2 Mo/TiO2 V–Mo/TiO2 V–Mo/TiO2 V–Mo/TiO2 V–Mo/TiO2 V–W/TiO2 V–W/TiO2 V–W/TiO2 V–W/TiO2 V/SiO2 Mo/SiO2 W/SiO2 V–Mo/SiO2 V–W/SiO2 V/Al2 O3 V/Al2 O3 V/Al2 O3 Mo/Al2 O3 V–W/Al2 O3 V2 O5 MoO3 WO3 V–W V–W a
Not measured.
M.A. Larrubia, G. Busca / Materials Chemistry and Physics 72 (2001) 337–346
Fig. 1. UV–Vis–NIR spectra of three support materials: TiO2 –anatase (sample A), amorphous silica SiO2 and Al2 O3 (A) (K/M: Kubelka munk function).
titania–anatase shows an absorption edge with an onset near 400 nm and an inflection point near 360 nm. The absorption maximum is found at 320 nm while a shoulder is very evident near 260 nm. The edge is due 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) [16,17]. The position of this absorption and the corresponding energy gap (Eg ≈ 3 eV) characterizes stoichiometric TiO2 –anatase as an intrinsic semiconductor [18]. The presence of two components in the absorption is due to the splitting of the 3d levels of titanium ions [16,17]. The alumina samples are (see Fig. 1 for sample A) almost totally transparent in our available spectral region (wavelength λ > 200 nm), although very weak absorptions, possibly associated to impurity ions, can be found in the range 200–350 nm. These observations are well in line with the strongly insulating character of alumina polymorphs, including ␥-Al2 O3 , that is reported to have an Eg value as high as 7.2 eV [19]. Also silica presents only very weak absorption in the region 200–300 nm, according to its insulating character (Eg > 10 eV) [18]. The UV–Vis spectra of the bulk oxides V2 O5 , MoO3 and WO3 are compared in Fig. 2. In all cases strong absorptions are observed in the UV region. This absorption largely expands to the visible region for V2 O5 (which is deep orange colored, accordingly) while only a tail is observed in the visible region for WO3 and MoO3 , which in fact are very light yellow. These absorptions are associated to charge transfer transitions from the O 2p valence band of all oxides to the empty V 3d, Mo 4d and W 5d orbitals. In fact, all these ions have empty d orbitals. Previous UV studies focused the attention on the relations between the position of the UV–Vis absorptions and the overall coordination and of the oxidation state of the metal element. This has been in particular studied for vanadium oxide based materials [20,38]. Actually, the pure oxides whose UV–Vis spectra are reported in Fig. 2 all have
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Fig. 2. UV–Vis–NIR spectra of the pure oxides V2 O5 , WO3 and MoO3 .
an overall coordination 6 for the corresponding cation. On the other hand, mixed oxides of V, W and Mo can also have lower coordination, such as 4 or 5 [21]. The UV–Vis spectra of two vanadate compounds characterized by coordination 4 for V5+ are reported in Fig. 3. In the case of Mg divanadate Mg2 V2 O7 the divanadate ion is present which is constituted by couples of corner-shared tetrahedral vanadate units. In the case of ammonium metavanadate virtually infinite chains of corner-shared tetrahedral vanadate units are present. Both compounds absorb in the region 400–200 nm, i.e. at definitely lower wavelengths than V2 O5 . This is a quite general rule, i.e. the higher the coordination, the higher the absorption wavelength. On the other hand, the extent of the polymeric chains of vanadate tetrahedral shifts perhaps a little the maximum absorption towards higher wavelengths. The spectra of three different V2 O5 –TiO2 catalysts, with increasing vanadia content, are compared in Fig. 4. The impregnation of small amounts of vanadium oxide onto the anatase surface causes a partial decrease of the main absorption intensity and also the formation of a tail at higher wavelengths with a consequent shift of the absorption onset up to near 500 nm. This shows that the vanadium oxide centers differ from V2 O5 which, in contrast, shows a strong absorption
Fig. 3. UV–Vis–NIR spectra of Mg2 V2 O7 and NH4 VO3 .
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Fig. 4. UV–Vis–NIR spectra of V2 O5 –TiO2 samples A.
also in the region 500–600 nm. This fully agrees with vibrational and NMR spectroscopic data that show that the supported vanadium oxide centers are associated to mono-oxo vanadyl centers [9–11]. Further increase of the impregnated vanadium oxide amount results in a important increase of the absorption in the region 400–500 nm and a further shift of the absorption onset up to near 550 nm. However, the absorption seems to be always different from that of V2 O5 . The structure for supported vanadyl species as proposed on the basis of IR, Raman and NMR spectroscopies is shown in Fig. 5. At high coverage polymeric metavanadate-type species can also be formed. Interestingly, also the intensity of the absorption in the O2− → Ti4+ charge transfer transition region seems to decrease further. This behavior can be interpreted assuming that the V 3d orbitals lie a little below the lower energy limit of the Ti 3d conduction band but perturb the TiO2 conduction band. The UV–Vis spectra of V2 O5 –Al2 O3 samples are reported in Fig. 6. The sample with 0.5% V2 O5 w/w presents a band centered 280 nm which is necessarily assigned to the O2− → V5+ charge transfer transitions of vanadyl centers. By increasing the vanadium oxide content the absorption increases and a new component apparently forms centered at higher wavelengths (305 nm) with a tail extending to near 490 nm. At coverage approaching those of the theoretical “monolayer” further absorption grows at higher wavelengths. However, the absorption onset is still not higher than at 560 nm, so well below that observed for
Fig. 5. Proposed structures for the isolated surface vanadyl species of vanadia–titania and vanadia–alumina catalysts in their anhydrous form (left) and hydrated form (right) based on IR, Raman and 51 V NMR spectroscopies. Note that the lines represent coordinative valencies, not oxidative valencies.
Fig. 6. UV–Vis–NIR spectra of V2 O5 –Al2 O3 (B) and V2 O5 –WO3 –Al2 O3 (B).
bulk V2 O5 . The impression is that three different species are successively formed by increasing the coverage, characterized by absorptions at progressively higher wavelengths, although always lower than that typical of bulk V2 O5 . The comparison of the absorption spectra of V2 O5 –TiO2 (Fig. 4) and V2 O5 –Al2 O3 (Fig. 6) catalyst on one side, and of V2 O5 (Fig. 2), Mg2 V2 O7 and NH4 VO3 (Fig. 3) samples on the other side, suggest that vanadium oxide centers on the supported catalysts have lower coordination than on bulk vanadia, possibly 4 or 5. This fully agrees with IR, Raman and NMR data that suggest coordination 4 for vanadyls on both titania and alumina in dry conditions and possibly coordination 5 in the hydrated form (see Fig. 5) [9–12]. On the other hand, it is worth noticing that the absorption energies of vanadium oxide species are, in the case of V2 O5 –Al2 O3 , far lower than those corresponding to the energy gap of alumina. In contrast for V2 O5 –TiO2 the absorption energy due to surface vanadium oxide species is only a little lower than that of the bulk valence band–conduction band transition of titania. The spectrum of a very low loading V2 O5 –SiO2 catalyst is shown in Fig. 7. In spite of the small amount of vanadium
Fig. 7. UV–Vis–NIR spectra of SiO2 , V2 O5 –SiO2 , WO3 –SiO2 and V2 O5 – WO3 –SiO2 .
M.A. Larrubia, G. Busca / Materials Chemistry and Physics 72 (2001) 337–346
Fig. 8. UV–Vis–NIR spectra of MoO3 –TiO2 samples (B).
oxide loaded here (see Table 1), the absorption is very strong and is centered at 390 nm with the tail extending up to above 600 nm. This means that, already at low loading, on silica the absorption is in the region typical of bulk V2 O5 (Fig. 2). This agrees with the well-known behavior of silica which is unable to disperse vanadium oxide and that, consequently, gives rise to V2 O5 particles already at very low coverage. Also in this case the absorption of the supported phase is far at lower energies than that of the highly insulating support bulk. 3.2. Oxide-supported molybdena and tungsta catalysts The UV–Vis–NIR spectra of MoO3 –TiO2 samples are reported in Fig. 8. Here the spectra are all very similar to each other in the region below 500 nm, although only an extremely weak tail grows by increasing Mo content. This shows that the O2− → Mo6+ charge transfer transitions of molybdenyl centers (which have been well characterized by vibrational spectroscopy on different MoO3 –TiO2 catalysts [22,23] and look just the same as for vanadyls, Fig. 5) are just superimposed to the valance band–conduction band transition of the bulk TiO2 . This means that the Mo 4d levels lie in the conduction band or just a little lower. In any case, the effect of Mo oxide species on the absorption of titania is much less than that of vanadium oxide species. This means that the Mo 4d levels of the surface molybdenyl species lie at higher energy than the V 3d levels of surface vanadyls species. In parallel, the absorption of bulk vanadia is centered at lower energy than that of bulk molybdena (Fig. 2). Actually, also the O2− → Mo6+ charge transfer transitions of the surface molybdenum oxide on alumina and on silica lie in the range 200–400 nm (see Fig. 9 for a molybdena–silica sample). In all cases, additionally, when molybdenum oxide loading is relatively high an absorption grows in the region centered near 1000 nm. This absorption, which contributes to the appearance of a blue color, is at least in part associated to d–d transitions of reduced molybdenum oxide species.
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Fig. 9. UV–Vis–NIR spectra of MoO3 –SiO2 , SiO2 , MoO3 –Al2 O3 (B) and Al2 O3 (B).
Titania- and alumina-supported WO3 have been the object of a recent UV–Vis–NIR study [24]. It has been shown that the impregnation with WO3 does not modify at all the absorption spectrum of titania. This is confirmed here (see below Fig. 12). On the contrary, impregnation of alumina and silica with WO3 (Figs. 6 and 7, respectively) results in the appearance of an absorption in the region 200–400 nm. In the case of tungsta silica (Fig. 7) we can find two main absorption maxima (both weak) at 260 and 300 nm, with an onset just above 400 nm. These components fall not far from those observed for bulk WO3 . On alumina (Fig. 6) the absorption is at a distinctly lower wavelength with the maximum at 215 nm and a pronounced shoulder at 250 nm [22]. These data show that the O2− → W6+ charge transfer transitions of wolframyl centers (which have been well characterized by vibrational spectroscopy on different WO3 –TiO2 catalysts [25,26] and are again similar to those depicted in Fig. 5 for V centers) are perfectly superimposed to the valence band-to-conduction band transition of bulk TiO2 –anatase. The comparison of the data arising from the UV–Vis spectra of V2 O5 –TiO2 (Fig. 4), MoO3 –TiO2 (Fig. 8) and WO3 –TiO2 (Fig. 12) suggest, consequently, that while the W 5d levels of the surface wolframyl species lie into the TiO2 conduction band (as discussed elsewhere [22]), the Mo 4d levels of the surface molybdenyl centers lie just at the higher energy limit of the gap and, finally, the V 3d levels of the surface vanadyl centers is at lower energy into the gap, although also not far from the lower energy limit of the conduction band (see Fig. 10). The empty d levels of V, Mo and W are at similar position on alumina but in this case they are much far from the supported conduction band. 3.3. Bulk mixed V, W oxides The UV–Vis spectra of some bulk solids composed by V and W oxides are reported in Fig. 11. In both cases
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Fig. 10. Schematic representation of electronic bands and of energy levels in pure vanadia and in TiO2 - and Al2 O3 -supported vanadia, tungsta and molybdena.
the X-ray diffraction spectra are those of WO3 . UV spectra clearly show absorption in the region 200–500 nm where both O2− → Wn+ charge transfer transitions and O2− → Vn+ charge transfer transitions are expected. In fact in this range the spectrum appears to be somehow intermediate between those of V2 O5 and WO3 (Fig. 2). However, additionally two medium-strength absorptions with maximum at 730 and 1000 nm are observed. They are certainly due to transitions of reduced V or W centers. These materials, which are dark colored, are “bronzes” and necessarily contain reduced metal centers.
Fig. 11. UV–Vis–NIR spectra of bulk V2 O5 –WO3 powders.
3.4. Oxide-supported V, W oxides The UV–Vis spectra of different V2 O5 –WO3 –TiO2 catalysts are reported in Fig. 12. The homemade materials belong to the same set of powders, all prepared starting from the same TiO2 support and with the same WO3 amount. In all cases we can find the usual absorption edge 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) of the anatase support, in the
Fig. 12. UV–Vis–NIR spectra of V2 O5 –WO3 –TiO2 samples (C), WO3 – TiO2 (C) and TiO2 (C). In the insert: commercial V2 O5 –WO3 –TiO2 catalyst.
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range below 400 nm. As evident in the spectrum, and as already cited above, the addition of 9% WO3 w/w does not modify the spectrum of titania, according to the location of the W 5d orbitals just in the conduction band of titania–anatase. The further addition of vanadium oxide only causes the growth of a tail at higher wavelengths, as observed already for tungsta-free vanadia–titania. This agrees with the IR spectroscopic data that show vanadyl and wolframyl species are essentially independent structures anchored to the titania surface [27]. The absence of relevant absorption in the visible and NIR region evidences that there is no formation of V, W bronzes. The spectra of two commercial V–W–Ti catalysts are also reported in the insert in Fig. 12. They are also characterized by the absorption edge of anatase with an additional absorption at higher wavelengths which can be associated to surface vanadyl species. However, for the sample denoted com1 this tail is very weak but an additional broad absorption in the visible region also exists, centered near 580 nm. This sample, which is used in several power plants, has been characterized and described in detail [2]. Actually, this commercial powder does not contain only the true catalyst but also glass particles to improve mechanical resistance. The chemical analysis evidences the presence of a number of elements besides V, W, Ti and oxygen, including iron. It is consequently reasonable to suppose that the broad absorption in the visible region could be at least in part associated to these additional materials. The other commercial sample com2, which has also been the object of a deep characterization study [28] is definitely richer in vanadium (see Table 1). Accordingly, a stronger absorption is observed in the region 400–500 nm as usual for high-vanadium content V2 O5 –TiO2 (see above Fig. 4). It has already been shown that UV–Vis spectroscopy is a good technique for the characterization of commercial V–W–Ti SCR catalysts [29]. The spectrum of a V2 O5 –WO3 –Al2 O3 sample is reported and compared with that of the WO3 -free V2 O5 –Al2 O3 sample in Fig. 6. In spite of the fact that WO3 species absorb at lower wavelengths than V oxide species, the addition of WO3 causes a shift to higher wavelengths of the absorption bands. In spite of the lower overall coverage (in terms of fraction of the monolayer) with respect to the V2 O5 –WO3 –TiO2 (Fig. 12) samples described above, and to the absence of any contribution of alumina to the absorption, the absorption of V2 O5 –WO3 –Al2 O3 is shifted at higher wavelengths with respect to V2 O5 –WO3 – TiO2 . The above spectra can be compared with those of V2 O5 –WO3 –SiO2 (Fig. 7). In spite of the very low surface coverage here (in terms of fraction of the theoretical monolayer) we have an absorption even more shifted to higher wavelengths. The spectrum corresponds to that of V2 O5 –SiO2 also taking into account that WO3 –SiO2 has a very weak absorption (Fig. 7).
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Fig. 13. UV–Vis–NIR spectra of TiO2 , MoO3 –TiO2 and V2 O5 –MoO3 – TiO2 samples (B). In the insert: commercial V2 O5 –MoO3 –TiO2 catalyst.
3.5. Oxide-supported V, Mo oxides The UV–Vis spectra of homemade and commercial V2 O5 –MoO3 –TiO2 samples are compared in Fig. 13. Again we observe that the addition of 1% V2 O5 causes the formation of a tail just above 400 nm. This is also what is observed for the commercial sample (see in the insert) that actually contains a small amount of vanadium. The addition of higher amounts of V2 O5 causes a deeper modification of the spectrum and also the appearance of an absorption in the visible and NIR region. In parallel, the absorption in the region below 400 nm, due to the bulk titania absorption, strongly decreases. The UV–Vis spectra of a low coverage V2 O5 –MoO3 –SiO2 sample is shown in Fig. 14. We observed a strong absorption in the region 300–550 nm which is similar to that observed for V2 O5 (Fig. 2) and V2 O5 –SiO2 (Fig. 6) samples and must be consequently mainly be assigned to O2− → V5+ charge transfer transitions. However, additionally a strong broad absorption grows in the visible and NIR region, which should be associated to reduced centers.
Fig. 14. UV–Vis–NIR spectra of SiO2 , V2 O5 –SiO2 , MoO3 –SiO2 and V2 O5 –MoO3 –SiO2 .
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4. Discussion The data reported here confirm that UV–Vis spectroscopy is an useful method for the characterization of oxide-supported oxide catalysts and, in particular, SCR and oxidation catalysts. The UV–Vis spectrum of V2 O5 –SiO2 (Fig. 7) already at very low loading is closely similar to that of bulk V2 O5 (Fig. 2) and shows that, as well known indeed, silica has a poorly dispersing effect on vanadia. The additions of WO3 (Fig. 7) and MoO3 (Fig. 9) do not modify very much the spectrum except that they cause the increase of the absorption in the visible and NIR region. This gives rise to a spectrum which is similar to those of bulk mixed V, W oxides which could have the character of bronzes and imply transitions involving reduced centers (Fig. 11). This again agrees with the non-dispersing character of silica as a support. This suggests that, according to the strong covalent nature of both silica and vanadia, tungsta and molybdena, the chemical interaction between the supported phase and the silica support is very weak [30]. Also from the point of view of the electronics, the interaction of silica with vanadia and vanadia-containing mixed oxides is certainly weak, according to the strong insulating character of silica, which only absorbs in the far UV region. The spectra of alumina-supported single V, Mo or W (Figs. 6 and 9) oxides are characterized by charge transfer transitions located at relatively low wavelengths, definitely lower than that typical for the corresponding bulk V, Mo and W oxides (Fig. 2). In particular, the absorptions of V2 O5 –Al2 O3 powders are located at a position which is typical for V5+ centers in low coordination. This fully agrees with IR, Raman and NMR data that showed such species are low coordination vanadyls, likely in a trigonal-pyramidal or square-pyramidal coordination (Fig. 5). This is due to the ionicity and acido-basic character of alumina [29]. In any case the absorptions of vanadyl, molybdenyl and wolframyl centers on alumina are located very far from the absorption edge of alumina which is a typical insulating material and absorbs consequently only in the far UV region (Fig. 10). This supports the idea that vanadyl, molybdenyl and wolframyl centers on alumina cannot interact electronically with the bulk support and, when well dispersed, they are also electronically isolated to each other. The addition of WO3 to V2 O5 –Al2 O3 (Fig. 6) actually causes a growth of the absorption in the region near 400 nm, in spite of the typical absorption of WO3 –Al2 O3 which is near 250 nm. This is probably due to a competition between WO3 (more acidic) and V2 O5 for the adsorption on the basic sites of alumina [26]. WO3 tends to displace V2 O5 for the basic sites, allowing it to condense probably in polymeric chains, although still with quite a low coordination. In other words, the addition of WO3 has a similar effect than the addition of more V2 O5 , with the result of forming condensed vanadate units. In fact, also the increase of the V2 O5 loading on alumina causes the growth of additional absorption in the
region near and above 400 nm. In any case, we cannot observe features suggesting the formation of mutual electronic interactions between vanadyl and wolframyl centers on alumina, in contrast with what happens on silica and in the bulk mixed oxides. Quite a different situation (see also Fig. 10) is found for the oxides supported on titania–anatase. Like alumina, titania is an ionic acido-basic highly dispersing oxide support [29]. This oxide, however, is an intrinsic semiconductor and, accordingly, shows the absorption edge due to the valence band–conduction band transition (O2− → Ti4+ charge transfer transition) in the region near 400 nm. The addition of WO3 to anatase does not cause any modification in the spectrum (Fig. 12). This means that the O2− → W6+ charge transfer transitions of the surface tungsten oxide species just are superimposed to the O2− → Ti4+ charge transfer transition of the bulk. This agrees with the dispersion of tungsten oxide species on titania in the form of low coordination wolframyl sites, as deduced by IR and Raman studies. In fact, WO3 (where W6+ ions are in an overall coordination 6) absorbs significantly above 400 nm. This also means that the W 5d levels fall in the conduction band of the overall solid. This agrees with the observation that it is not possible to detect the ESR spectrum of W5+ in the case of slightly reduced WO3 –TiO2 [13], due to the fact that the (formally) d1 electron of W5+ is actually delocalized in the titania conduction band. The addition of MoO3 at the anatase surface (Fig. 8) actually causes the detection of a weak tail just near 400 nm in the UV spectrum. This suggests that the Mo 4d levels fall just below the lower energy limit of the conduction band of the overall solid. This agrees with the observation that it is possible to detect the ESR spectrum of Mo5+ at 77 K in the case of slightly reduced MoO3 –TiO2 [31], due to the fact that the d1 electron of Mo5+ is, at low temperature, actually localized in the Mo 4d level. However, at slightly higher temperature this electron can easily reach the conduction band and, accordingly, the ESR signal vanishes. The addition of V2 O5 at the anatase surface (Fig. 4) actually causes the detection of quite important absorption in the 400–500 nm region. The difference with respect to the spectrum of bulk vanadia (which absorbs up to 600 nm) (Fig. 2) suggests that vanadium oxide species on titania are in the form of low coordination centers, as already deduced from Raman, IR and NMR data. This also shows that the V 3d levels of surface vanadyls fall well below the lower energy limit of the conduction band of the overall solid. This agrees with the observation that it is possible to detect the ESR spectrum of V4+ in the case of slightly reduced V2 O5 –TiO2 [32], due to the fact that the d1 electron of V4+ is, at low temperature, actually localized in the V 3d level. However, at 573–673 K (where catalysis actually occurs) the gap to the conduction band (≈0.5 eV) is overcome and the electron is delocalized in the overall solid. In fact, V2 O5 –TiO2 materials have definitely higher conductivity than TiO2 [33,34].
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The UV spectra of V2 O5 –WO3 –TiO2 (Fig. 10) catalysts show nearly the same spectra of V2 O5 –TiO2 (Fig. 4) catalysts with the same or a little higher V2 O5 content. This suggests that vanadyl and wolframyl centers do not interact directly from the point of view of electronics. These data agree (Fig. 10) with the observation that it is possible to detect the ESR spectrum of V4+ in the case of slightly reduced V2 O5 –WO3 –TiO2 catalysts [13,35], due to the fact that the d1 electron of V4+ is, at low temperature, actually localized in the V 3d levels. In any case the closeness between these levels and the conduction band of the overall solid allows easily (at the working temperature of SCR catalysts which is near 573 K) the migration of electrons from V atoms to the conduction band and vice versa. This allows to explain the effect of addition of WO3 in activating the V2 O5 –TiO2 catalysts for the SCR reaction. In fact, WO3 give rise to reducible wolframyl centers which certainly increase the electron density in the conduction band of the overall solid. These electrons tend to fall into the lower-energy V 3d levels, so giving rise to reduced vanadyl centers. It has been previously proposed that VO2+ centers are the active sites for SCR reaction. This possibly increases the concentration and, actually, the reducibility of the surface vanadyl centers. On the other hand, it is evident that the active sites for an oxidation catalyst working with a redox or Mars–Van Krevelen mechanism also need to be rapidly reoxidized by oxygen. Wolframyl centers also allow the electrons, produced by reduction of vanadium centers, to be rapidly and easily given to oxygen producing oxide species. In other words, the presence of wolframyl centers increases the electron density in the solid and fastens the redox cycles needed to perform the SCR reaction. In agreement with these V2 O5 –MO3 –WO3 show high electrical conductivity [36]. A similar effect could concern V2 O5 –MoO3 –TiO2 (Fig. 13) catalysts with low V content, such as the industrial catalyst investigated here. Mo 4d levels, which seem to stay just below the lower energy limit of the conduction band, can bridge between the conduction band and the V 3d levels, so also favoring the movements of the electrons between the bulk and V centers. In the case of MoO3 based catalyst with high loading absorption grows in the visible region showing a tendency of molybdenum oxide species to stay in a reduced state. This effect seems to not involve directly vanadium oxide species, because it happens also for V-free catalysts. So, it is probably not relevant for SCR catalysis where V plays a key role.
5. Conclusions The main conclusion from this work is that titania–anatase has very likely a double activating effect for vanadia catalysts. One is a result of its ionicity and acido-basic character, allowing to disperse vanadium, molybdenum and
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tungsten oxide species in the form of low coordination nearly isolated centers. This, however, is more or less performed also by other ionic oxide supports such as alumina. Additionally, titania being a semiconductor, allows the electronic contact between the metal oxide centers supported on it. This electronic contact, through the conduction band of titania, explains both the activating effect of titania on vanadia, and the activating effects of molybdena and tungsta on vanadia–titania in SCR catalysis.
Acknowledgements M.A.L. acknowledges Ministerio de educacion y ciencia (MEC, Madrid) for an FPI grant. References [1] V. Nikolov, D. Klissurski, A. Anastasov, Catal. Rev. Sci. Eng. 33 (1991) 319. [2] L.J. Alemany, F. Berti, G. Busca, G. Ramis, D. Robba, G.P. Toledo, M. Trombetta, Appl. Catal. B 10 (1996) 299. [3] G.W. Spitznagel, K. Huttenhofer, J.K. Beer, in: J.N. Armor (Ed.), Environmental Catalysis, ACS Symposium Series 552, American Chemical Society, Washington, DC, 1994, p. 172. [4] L. Lietti, I. Nova, G. Ramis, L. Dell’Acqua, G. Busca, E. Giamello, P. Forzatti, F. Bregani, J. Catal. 187 (1999) 419. [5] G. Busca, L. Lietti, G. Ramis, F. Berti, Appl. Catal. B 18 (1998) 1. [6] S.M. Cho, Chem. Eng. Prog. 90 (1) (1994) 39. [7] P. Forzatti, L. Lietti, Heter. Chem. Rev. 3 (1996) 33. [8] A. Véjux, P. Courtine, J. Solid State Chem. 23 (1978) 93. [9] C. Cristiani, P. Forzatti, G. Busca, J. Catal. 116 (1990) 586. [10] G. Ramis, C. Cristiani, P. Forzatti, G. Busca, J. Catal. 124 (1990) 574. [11] H. Eckert, I.E. Wachs, J. Phys. Chem. 96 (1989) 6796. [12] G. Busca, Mater. Chem. Phys. 19 (1988) 157. [13] L.J. Alemany, L. Lietti, N. Ferlazzo, P. Forzatti, G. Busca, E. Giamello, F. Bregani, J. Catal. 155 (1995) 117. [14] G. Deo, I.E. Wachs, J. Catal. 146 (1994) 323 and 336. [15] A. Andreini, M. de Boer, M.A. Vuurman, G. Deo, I.E. Wachs, J. Chem. Soc., Faraday Trans. 92 (1996) 3267. [16] H. Bevan, S.V. Dawes, R.A. Ford, Spectrochim. Acta 13 (1958) 43. [17] J.K. Burdett, T. Hughbanks, G.J. Miller, J.W. Riochardson, J.V. Smith, J. Am. Ceram. Soc. 109 (1987) 3639. [18] J.B. Torrance, J. Solid State Chem. 96 (1992) 59. [19] R.H. French, J. Am. Ceram. Soc. 73 (1990) 477. [20] H. Praliaud, M. Mathieu, J. Chim. Phys. 73 (6) (1976) 689. [21] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, 4th Edition, Wiley, New York, 1980. [22] G. Ramis, G. Busca, V. Lorenzelli, Zeit. Phys. Chem. Neue Folge 153 (1987) 189. [23] D.S. Kim, I.E. Wachs, K. Segawa, J. Catal. 146 (1994) 268. [24] A. Gutiérrez-Alejandre, J. Ram´ırez, G. Busca, Catal. Lett. 56 (1998) 29. [25] G. Ramis, G. Busca, C. Cristiani, L. Lietti, P. Forzatti, F. Bregani, Langmuir 8 (1992) 1744. [26] D.S. Kim, M. Ostromecki, I.E. Wachs, J. Mol. Catal. A 106 (1996) 93. [27] G. Ramis, G. Busca, F. Bregani, Catal. Lett. 18 (1993) 299. [28] J.C. Vedrine, Catal. Today 56 (4) (2000). [29] G. Ramis, G. Busca, P. Forzatti, Appl. Catal. B 1 (1992) 9. [30] G. Busca, PCCP 1 (1999) 723.
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[31] G. Busca, L. Marchetti, J. Chem. Res. (S) (1986) 174; (M) (1986) 1546. [32] G. Busca, E. Giamello, Mater. Chem. Phys. 25 (1990) 475. [33] E. Pereira, L.A. Gambaro, H.J. Thomas, Transition Met. Chem. 5 (1980) 139. [34] J.M. Herrmann, Catal. Today 3 (1994) 423. [35] M.C. Paganini, L. Dell’Acqua, E. Giamello, L. Lietti, P. Forzatti, G. Busca, J. Catal. 166 (1997) 195.
[36] V.I. Marshneva, E.M. Slavinskaya, O.V. Kalinkina, G.V. Odegova, E.M. Moroz, G.V. Lavrova, A.N. Salanov, J. Catal. 155 (1995) 171. [37] P. Courtine, E. Bordes, in: R.K. Grasselli, S.T. Oyama, A.M. Gaffney, J.E. Lyons (Eds.), Proceedings of the Third World Congress on Oxidation Catalysis, Elsevier, Amsterdam, 1997, p. 177 [38] G. Catana, R. Ramachandra, B.M. Weckhuysen, P. Van der Voort, E. Vansant, R.A. Schoonheydt, J. Phys. Chem. 102 (1998) 8005.
An ultraviolet–visible–near infrared study of the electronic structure of oxide-supported vanadia–tungsta and vanadia–molybdena M.A. Larrubia1 , G. Busca∗ Dipartimento di Ingegneria Chimica e di Processo “G.B. Bonino” and INFM, Università di Genova, P.le J.F. Kennedy 1, I-16129 Genova, Italy Received 29 May 2000; received in revised form 28 July 2000; accepted 16 September 2000
Abstract The ultraviolet–visible–near IR spectra of a series of catalysts have been recorded and discussed. In particular bulk and alumina-, titania- and silica-supported V, W, and Mo oxides have been considered. Additionally mixed V–W and V–Mo-supported oxides have been investigated. The data show that, in agreement with vibrational data and other literature data, the oxides supported on silica are similar to the corresponding bulk oxides. In contrast, the spectra of the oxides supported on alumina and titania correspond to surface oxide species where the metal stays in a lower overall coordination with respect to the bulk oxides. Finally, the spectra show that only in the case of titania-supported oxides an electronic interaction between the supported metal oxide centers through the support conduction band is possible. This allows to justify on electronic bases the activating effect of titania for vanadia, vanadia–tungsta and vanadia–molybdena catalysts used in the hydrocarbon selective oxidation catalysis and for the selective catalytic reduction of nitrogen oxide by ammonia. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Ultraviolet spectroscopy; Electronic spectra of catalysts; Supported vanadium oxide catalysts; Vanadium oxide catalysts; Mixed oxide catalysts
1. Introduction Oxide-supported vanadium oxides find several applications as industrial catalysts. In particular, vanadia–titania catalysts are applied industrially for the phthalic anhydride synthesis from both o-xylene and naphthalene [1]. Titania-supported vanadia–tungsta [2,3] and/or vanadia–molybdena [4] constitute the industrial catalysts for the abatement of nitrogen oxides using ammonia or urea as the reducing agent. This process, called SCR (selective catalytic reduction), which consists actually in a selective oxidation of ammonia to nitrogen, by NO and oxygen [5], is nowadays widely applied to clean up the waste gases from thermal power stations [6,7], but could be extended to diesel-engine trucks. It has been demonstrated that titania–anatase is used as the best catalyst support in both cases because it shows an activating effect with respect to vanadia in oxidation catalysis as well as because it is stable to sulphation in SO2 containing atmospheres [5]. The reasons of the activating ef∗ Corresponding author. Tel.: +39-010-353-6024; fax: +39-010-353-6028. E-mail address: [email protected] (G. Busca). 1 Present address: Departamento de Ingenier´ıa Qu´ımica, Universidad de M´alaga, Spain.
fect have been the object of many studies and different explanations have been offered. According to Courtine et al. [8,37] titania–anatase and V2 O5 present a remarkable crystallographic fit and this could explain the activating effect. However, vibrational studies [9] (including O16 /O18 exchange experiments [10]), and 51 V nuclear magnetic resonance (NMR) studies [11] have clearly shown that the vanadium-oxide centers in vanadia–titania catalysts are not constituted by V2 O5 crystals. On the contrary, at least at low coverage (where the activating effect is already evident) they are constituted by vibrationally isolated mono-oxo vanadyl centers [9–11]. Non-activating supports such as silica actually produce supported V2 O5 particles, without a relevant dispersing effect. For this reason it is possible to suppose that the activating effect of titania is due to the ability to disperse the vanadium oxide centers just in a particular more active structure. However, infrared (IR), Raman and NMR spectroscopies showed that the structures of vanadium oxide centers on titania are nearly the same as those on alumina [11,12], which however gives rise to a much weaker activating effect. For this reason, electronic effects could be invoked to explain the activating effect of titania. On the other hand, it has been established that the addition of WO3 and/or MoO3 to V2 O5 –TiO2 catalysts further strongly enhances the activity and also the selectivity to nitrogen (with respect to the main byproduct which is N2 O) in
0254-0584/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 3 2 9 - 7
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the SCR catalysis. This is in spite of the by far lower activity of WO3 –TiO2 and MoO3 –TiO2 with respect to V2 O5 –TiO2 . This synergetic effect has been explained, based on electron spin resonance (ESR) and Raman experiments, with electronic effects [13]. Other authors invoked the necessity of a two atom sites, i.e. the effect of an acid site (W or Mo) together with a redox site (V) for SCR catalysis [14]. On the other hand, it has also been shown that W oxide has an activating effect not only with respect to V2 O5 –TiO2 , but also with respect to V2 O5 –Al2 O3 [15]. To have a more precise picture of the electronic structure of the materials of interest in the SCR catalysis we have reinvestigated the ultraviolet–visible–near infrared (UV–Vis–NIR) spectra of V, Mo and W oxides and their mixtures as such or supported onto titania, alumina and silica.
ple was prepared by precipitation starting from TiCl4 . The catalyst samples, both single and mixture oxides, were prepared by dry impregnation with hot water solutions of the corresponding salts (NH4 VO3 , 5(NH4 )2 O·12WO3 ·5H2 O, (NH4 )6 Mo7 O24 ·4H2 O), followed by drying and calcination at 773 K for 3 h. So all materials studied here are in the oxide state. Some characteristics of the supports and of the catalysts are summarized in Table 1. Diffuse reflectance spectra in the range 2500–200 nm were obtained with a Jasco V-570 spectrophotometer at room temperature and the samples expend at air. The Jasco instrument standard sample has been taken as reference sample to the UV–Vis–NIR analysis.
2. Experimental
3.1. Oxide-supported vanadia catalysts
The supports used for the catalysts preparations were mainly commercial products (see Table 1). One titania sam-
The UV–Vis–NIR spectra of the pure supports used in this study are compared in Fig. 1. The spectrum of
3. Results
Table 1 Summary of the samples under study Material
Sample
Origin
XRD
Surface area (m2 /g)
% supported oxide (w/w)
TiO2
A B C A B A A A A C B B B B B B Com3 C C Com1 Com2 A A A A A B B B B B
Ti oxide Rhone Poulenc Homemade Degussa Akzo Degussa Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Commercial Homemade Homemade Commercial Commercial (Eurocat) Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade Homemade
Anatase Anatase Anatase Gamma Gamma Amorphous Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Anatase Amorphous Amorphous Amorphous Amorphous Amorphous Gamma Gamma Gamma Gamma Gamma V 2 O5 MoO3 WO3 WO3 WO3
117 71 90 100 190 200 115 108 100 90 70 62 57 72 22 21 78 87 64 70 47 185 190 190 n.m.a n.m. n.m. n.m. n.m. n.m. n.m. 18 15 15 n.m. n.m.
1 4 8 9 1 3 10 1/10 2/10 5/10 1.2/6.2 0.78/9 4/9 0.56/11.04 4.25/10.57 1.21 1.21 1.21 1.21/1.9 1.21/3 0.5 5 17 10 5/15 Bulk Bulk Bulk 40%/60% bulk 27%/73% bulk
Al2 O3 SiO2 V/TiO2 V/TiO2 V/TiO2 W/TiO2 Mo/TiO2 Mo/TiO2 Mo/TiO2 V–Mo/TiO2 V–Mo/TiO2 V–Mo/TiO2 V–Mo/TiO2 V–W/TiO2 V–W/TiO2 V–W/TiO2 V–W/TiO2 V/SiO2 Mo/SiO2 W/SiO2 V–Mo/SiO2 V–W/SiO2 V/Al2 O3 V/Al2 O3 V/Al2 O3 Mo/Al2 O3 V–W/Al2 O3 V2 O5 MoO3 WO3 V–W V–W a
Not measured.
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Fig. 1. UV–Vis–NIR spectra of three support materials: TiO2 –anatase (sample A), amorphous silica SiO2 and Al2 O3 (A) (K/M: Kubelka munk function).
titania–anatase shows an absorption edge with an onset near 400 nm and an inflection point near 360 nm. The absorption maximum is found at 320 nm while a shoulder is very evident near 260 nm. The edge is due 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) [16,17]. The position of this absorption and the corresponding energy gap (Eg ≈ 3 eV) characterizes stoichiometric TiO2 –anatase as an intrinsic semiconductor [18]. The presence of two components in the absorption is due to the splitting of the 3d levels of titanium ions [16,17]. The alumina samples are (see Fig. 1 for sample A) almost totally transparent in our available spectral region (wavelength λ > 200 nm), although very weak absorptions, possibly associated to impurity ions, can be found in the range 200–350 nm. These observations are well in line with the strongly insulating character of alumina polymorphs, including ␥-Al2 O3 , that is reported to have an Eg value as high as 7.2 eV [19]. Also silica presents only very weak absorption in the region 200–300 nm, according to its insulating character (Eg > 10 eV) [18]. The UV–Vis spectra of the bulk oxides V2 O5 , MoO3 and WO3 are compared in Fig. 2. In all cases strong absorptions are observed in the UV region. This absorption largely expands to the visible region for V2 O5 (which is deep orange colored, accordingly) while only a tail is observed in the visible region for WO3 and MoO3 , which in fact are very light yellow. These absorptions are associated to charge transfer transitions from the O 2p valence band of all oxides to the empty V 3d, Mo 4d and W 5d orbitals. In fact, all these ions have empty d orbitals. Previous UV studies focused the attention on the relations between the position of the UV–Vis absorptions and the overall coordination and of the oxidation state of the metal element. This has been in particular studied for vanadium oxide based materials [20,38]. Actually, the pure oxides whose UV–Vis spectra are reported in Fig. 2 all have
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Fig. 2. UV–Vis–NIR spectra of the pure oxides V2 O5 , WO3 and MoO3 .
an overall coordination 6 for the corresponding cation. On the other hand, mixed oxides of V, W and Mo can also have lower coordination, such as 4 or 5 [21]. The UV–Vis spectra of two vanadate compounds characterized by coordination 4 for V5+ are reported in Fig. 3. In the case of Mg divanadate Mg2 V2 O7 the divanadate ion is present which is constituted by couples of corner-shared tetrahedral vanadate units. In the case of ammonium metavanadate virtually infinite chains of corner-shared tetrahedral vanadate units are present. Both compounds absorb in the region 400–200 nm, i.e. at definitely lower wavelengths than V2 O5 . This is a quite general rule, i.e. the higher the coordination, the higher the absorption wavelength. On the other hand, the extent of the polymeric chains of vanadate tetrahedral shifts perhaps a little the maximum absorption towards higher wavelengths. The spectra of three different V2 O5 –TiO2 catalysts, with increasing vanadia content, are compared in Fig. 4. The impregnation of small amounts of vanadium oxide onto the anatase surface causes a partial decrease of the main absorption intensity and also the formation of a tail at higher wavelengths with a consequent shift of the absorption onset up to near 500 nm. This shows that the vanadium oxide centers differ from V2 O5 which, in contrast, shows a strong absorption
Fig. 3. UV–Vis–NIR spectra of Mg2 V2 O7 and NH4 VO3 .
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Fig. 4. UV–Vis–NIR spectra of V2 O5 –TiO2 samples A.
also in the region 500–600 nm. This fully agrees with vibrational and NMR spectroscopic data that show that the supported vanadium oxide centers are associated to mono-oxo vanadyl centers [9–11]. Further increase of the impregnated vanadium oxide amount results in a important increase of the absorption in the region 400–500 nm and a further shift of the absorption onset up to near 550 nm. However, the absorption seems to be always different from that of V2 O5 . The structure for supported vanadyl species as proposed on the basis of IR, Raman and NMR spectroscopies is shown in Fig. 5. At high coverage polymeric metavanadate-type species can also be formed. Interestingly, also the intensity of the absorption in the O2− → Ti4+ charge transfer transition region seems to decrease further. This behavior can be interpreted assuming that the V 3d orbitals lie a little below the lower energy limit of the Ti 3d conduction band but perturb the TiO2 conduction band. The UV–Vis spectra of V2 O5 –Al2 O3 samples are reported in Fig. 6. The sample with 0.5% V2 O5 w/w presents a band centered 280 nm which is necessarily assigned to the O2− → V5+ charge transfer transitions of vanadyl centers. By increasing the vanadium oxide content the absorption increases and a new component apparently forms centered at higher wavelengths (305 nm) with a tail extending to near 490 nm. At coverage approaching those of the theoretical “monolayer” further absorption grows at higher wavelengths. However, the absorption onset is still not higher than at 560 nm, so well below that observed for
Fig. 5. Proposed structures for the isolated surface vanadyl species of vanadia–titania and vanadia–alumina catalysts in their anhydrous form (left) and hydrated form (right) based on IR, Raman and 51 V NMR spectroscopies. Note that the lines represent coordinative valencies, not oxidative valencies.
Fig. 6. UV–Vis–NIR spectra of V2 O5 –Al2 O3 (B) and V2 O5 –WO3 –Al2 O3 (B).
bulk V2 O5 . The impression is that three different species are successively formed by increasing the coverage, characterized by absorptions at progressively higher wavelengths, although always lower than that typical of bulk V2 O5 . The comparison of the absorption spectra of V2 O5 –TiO2 (Fig. 4) and V2 O5 –Al2 O3 (Fig. 6) catalyst on one side, and of V2 O5 (Fig. 2), Mg2 V2 O7 and NH4 VO3 (Fig. 3) samples on the other side, suggest that vanadium oxide centers on the supported catalysts have lower coordination than on bulk vanadia, possibly 4 or 5. This fully agrees with IR, Raman and NMR data that suggest coordination 4 for vanadyls on both titania and alumina in dry conditions and possibly coordination 5 in the hydrated form (see Fig. 5) [9–12]. On the other hand, it is worth noticing that the absorption energies of vanadium oxide species are, in the case of V2 O5 –Al2 O3 , far lower than those corresponding to the energy gap of alumina. In contrast for V2 O5 –TiO2 the absorption energy due to surface vanadium oxide species is only a little lower than that of the bulk valence band–conduction band transition of titania. The spectrum of a very low loading V2 O5 –SiO2 catalyst is shown in Fig. 7. In spite of the small amount of vanadium
Fig. 7. UV–Vis–NIR spectra of SiO2 , V2 O5 –SiO2 , WO3 –SiO2 and V2 O5 – WO3 –SiO2 .
M.A. Larrubia, G. Busca / Materials Chemistry and Physics 72 (2001) 337–346
Fig. 8. UV–Vis–NIR spectra of MoO3 –TiO2 samples (B).
oxide loaded here (see Table 1), the absorption is very strong and is centered at 390 nm with the tail extending up to above 600 nm. This means that, already at low loading, on silica the absorption is in the region typical of bulk V2 O5 (Fig. 2). This agrees with the well-known behavior of silica which is unable to disperse vanadium oxide and that, consequently, gives rise to V2 O5 particles already at very low coverage. Also in this case the absorption of the supported phase is far at lower energies than that of the highly insulating support bulk. 3.2. Oxide-supported molybdena and tungsta catalysts The UV–Vis–NIR spectra of MoO3 –TiO2 samples are reported in Fig. 8. Here the spectra are all very similar to each other in the region below 500 nm, although only an extremely weak tail grows by increasing Mo content. This shows that the O2− → Mo6+ charge transfer transitions of molybdenyl centers (which have been well characterized by vibrational spectroscopy on different MoO3 –TiO2 catalysts [22,23] and look just the same as for vanadyls, Fig. 5) are just superimposed to the valance band–conduction band transition of the bulk TiO2 . This means that the Mo 4d levels lie in the conduction band or just a little lower. In any case, the effect of Mo oxide species on the absorption of titania is much less than that of vanadium oxide species. This means that the Mo 4d levels of the surface molybdenyl species lie at higher energy than the V 3d levels of surface vanadyls species. In parallel, the absorption of bulk vanadia is centered at lower energy than that of bulk molybdena (Fig. 2). Actually, also the O2− → Mo6+ charge transfer transitions of the surface molybdenum oxide on alumina and on silica lie in the range 200–400 nm (see Fig. 9 for a molybdena–silica sample). In all cases, additionally, when molybdenum oxide loading is relatively high an absorption grows in the region centered near 1000 nm. This absorption, which contributes to the appearance of a blue color, is at least in part associated to d–d transitions of reduced molybdenum oxide species.
341
Fig. 9. UV–Vis–NIR spectra of MoO3 –SiO2 , SiO2 , MoO3 –Al2 O3 (B) and Al2 O3 (B).
Titania- and alumina-supported WO3 have been the object of a recent UV–Vis–NIR study [24]. It has been shown that the impregnation with WO3 does not modify at all the absorption spectrum of titania. This is confirmed here (see below Fig. 12). On the contrary, impregnation of alumina and silica with WO3 (Figs. 6 and 7, respectively) results in the appearance of an absorption in the region 200–400 nm. In the case of tungsta silica (Fig. 7) we can find two main absorption maxima (both weak) at 260 and 300 nm, with an onset just above 400 nm. These components fall not far from those observed for bulk WO3 . On alumina (Fig. 6) the absorption is at a distinctly lower wavelength with the maximum at 215 nm and a pronounced shoulder at 250 nm [22]. These data show that the O2− → W6+ charge transfer transitions of wolframyl centers (which have been well characterized by vibrational spectroscopy on different WO3 –TiO2 catalysts [25,26] and are again similar to those depicted in Fig. 5 for V centers) are perfectly superimposed to the valence band-to-conduction band transition of bulk TiO2 –anatase. The comparison of the data arising from the UV–Vis spectra of V2 O5 –TiO2 (Fig. 4), MoO3 –TiO2 (Fig. 8) and WO3 –TiO2 (Fig. 12) suggest, consequently, that while the W 5d levels of the surface wolframyl species lie into the TiO2 conduction band (as discussed elsewhere [22]), the Mo 4d levels of the surface molybdenyl centers lie just at the higher energy limit of the gap and, finally, the V 3d levels of the surface vanadyl centers is at lower energy into the gap, although also not far from the lower energy limit of the conduction band (see Fig. 10). The empty d levels of V, Mo and W are at similar position on alumina but in this case they are much far from the supported conduction band. 3.3. Bulk mixed V, W oxides The UV–Vis spectra of some bulk solids composed by V and W oxides are reported in Fig. 11. In both cases
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Fig. 10. Schematic representation of electronic bands and of energy levels in pure vanadia and in TiO2 - and Al2 O3 -supported vanadia, tungsta and molybdena.
the X-ray diffraction spectra are those of WO3 . UV spectra clearly show absorption in the region 200–500 nm where both O2− → Wn+ charge transfer transitions and O2− → Vn+ charge transfer transitions are expected. In fact in this range the spectrum appears to be somehow intermediate between those of V2 O5 and WO3 (Fig. 2). However, additionally two medium-strength absorptions with maximum at 730 and 1000 nm are observed. They are certainly due to transitions of reduced V or W centers. These materials, which are dark colored, are “bronzes” and necessarily contain reduced metal centers.
Fig. 11. UV–Vis–NIR spectra of bulk V2 O5 –WO3 powders.
3.4. Oxide-supported V, W oxides The UV–Vis spectra of different V2 O5 –WO3 –TiO2 catalysts are reported in Fig. 12. The homemade materials belong to the same set of powders, all prepared starting from the same TiO2 support and with the same WO3 amount. In all cases we can find the usual absorption edge 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) of the anatase support, in the
Fig. 12. UV–Vis–NIR spectra of V2 O5 –WO3 –TiO2 samples (C), WO3 – TiO2 (C) and TiO2 (C). In the insert: commercial V2 O5 –WO3 –TiO2 catalyst.
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range below 400 nm. As evident in the spectrum, and as already cited above, the addition of 9% WO3 w/w does not modify the spectrum of titania, according to the location of the W 5d orbitals just in the conduction band of titania–anatase. The further addition of vanadium oxide only causes the growth of a tail at higher wavelengths, as observed already for tungsta-free vanadia–titania. This agrees with the IR spectroscopic data that show vanadyl and wolframyl species are essentially independent structures anchored to the titania surface [27]. The absence of relevant absorption in the visible and NIR region evidences that there is no formation of V, W bronzes. The spectra of two commercial V–W–Ti catalysts are also reported in the insert in Fig. 12. They are also characterized by the absorption edge of anatase with an additional absorption at higher wavelengths which can be associated to surface vanadyl species. However, for the sample denoted com1 this tail is very weak but an additional broad absorption in the visible region also exists, centered near 580 nm. This sample, which is used in several power plants, has been characterized and described in detail [2]. Actually, this commercial powder does not contain only the true catalyst but also glass particles to improve mechanical resistance. The chemical analysis evidences the presence of a number of elements besides V, W, Ti and oxygen, including iron. It is consequently reasonable to suppose that the broad absorption in the visible region could be at least in part associated to these additional materials. The other commercial sample com2, which has also been the object of a deep characterization study [28] is definitely richer in vanadium (see Table 1). Accordingly, a stronger absorption is observed in the region 400–500 nm as usual for high-vanadium content V2 O5 –TiO2 (see above Fig. 4). It has already been shown that UV–Vis spectroscopy is a good technique for the characterization of commercial V–W–Ti SCR catalysts [29]. The spectrum of a V2 O5 –WO3 –Al2 O3 sample is reported and compared with that of the WO3 -free V2 O5 –Al2 O3 sample in Fig. 6. In spite of the fact that WO3 species absorb at lower wavelengths than V oxide species, the addition of WO3 causes a shift to higher wavelengths of the absorption bands. In spite of the lower overall coverage (in terms of fraction of the monolayer) with respect to the V2 O5 –WO3 –TiO2 (Fig. 12) samples described above, and to the absence of any contribution of alumina to the absorption, the absorption of V2 O5 –WO3 –Al2 O3 is shifted at higher wavelengths with respect to V2 O5 –WO3 – TiO2 . The above spectra can be compared with those of V2 O5 –WO3 –SiO2 (Fig. 7). In spite of the very low surface coverage here (in terms of fraction of the theoretical monolayer) we have an absorption even more shifted to higher wavelengths. The spectrum corresponds to that of V2 O5 –SiO2 also taking into account that WO3 –SiO2 has a very weak absorption (Fig. 7).
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Fig. 13. UV–Vis–NIR spectra of TiO2 , MoO3 –TiO2 and V2 O5 –MoO3 – TiO2 samples (B). In the insert: commercial V2 O5 –MoO3 –TiO2 catalyst.
3.5. Oxide-supported V, Mo oxides The UV–Vis spectra of homemade and commercial V2 O5 –MoO3 –TiO2 samples are compared in Fig. 13. Again we observe that the addition of 1% V2 O5 causes the formation of a tail just above 400 nm. This is also what is observed for the commercial sample (see in the insert) that actually contains a small amount of vanadium. The addition of higher amounts of V2 O5 causes a deeper modification of the spectrum and also the appearance of an absorption in the visible and NIR region. In parallel, the absorption in the region below 400 nm, due to the bulk titania absorption, strongly decreases. The UV–Vis spectra of a low coverage V2 O5 –MoO3 –SiO2 sample is shown in Fig. 14. We observed a strong absorption in the region 300–550 nm which is similar to that observed for V2 O5 (Fig. 2) and V2 O5 –SiO2 (Fig. 6) samples and must be consequently mainly be assigned to O2− → V5+ charge transfer transitions. However, additionally a strong broad absorption grows in the visible and NIR region, which should be associated to reduced centers.
Fig. 14. UV–Vis–NIR spectra of SiO2 , V2 O5 –SiO2 , MoO3 –SiO2 and V2 O5 –MoO3 –SiO2 .
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4. Discussion The data reported here confirm that UV–Vis spectroscopy is an useful method for the characterization of oxide-supported oxide catalysts and, in particular, SCR and oxidation catalysts. The UV–Vis spectrum of V2 O5 –SiO2 (Fig. 7) already at very low loading is closely similar to that of bulk V2 O5 (Fig. 2) and shows that, as well known indeed, silica has a poorly dispersing effect on vanadia. The additions of WO3 (Fig. 7) and MoO3 (Fig. 9) do not modify very much the spectrum except that they cause the increase of the absorption in the visible and NIR region. This gives rise to a spectrum which is similar to those of bulk mixed V, W oxides which could have the character of bronzes and imply transitions involving reduced centers (Fig. 11). This again agrees with the non-dispersing character of silica as a support. This suggests that, according to the strong covalent nature of both silica and vanadia, tungsta and molybdena, the chemical interaction between the supported phase and the silica support is very weak [30]. Also from the point of view of the electronics, the interaction of silica with vanadia and vanadia-containing mixed oxides is certainly weak, according to the strong insulating character of silica, which only absorbs in the far UV region. The spectra of alumina-supported single V, Mo or W (Figs. 6 and 9) oxides are characterized by charge transfer transitions located at relatively low wavelengths, definitely lower than that typical for the corresponding bulk V, Mo and W oxides (Fig. 2). In particular, the absorptions of V2 O5 –Al2 O3 powders are located at a position which is typical for V5+ centers in low coordination. This fully agrees with IR, Raman and NMR data that showed such species are low coordination vanadyls, likely in a trigonal-pyramidal or square-pyramidal coordination (Fig. 5). This is due to the ionicity and acido-basic character of alumina [29]. In any case the absorptions of vanadyl, molybdenyl and wolframyl centers on alumina are located very far from the absorption edge of alumina which is a typical insulating material and absorbs consequently only in the far UV region (Fig. 10). This supports the idea that vanadyl, molybdenyl and wolframyl centers on alumina cannot interact electronically with the bulk support and, when well dispersed, they are also electronically isolated to each other. The addition of WO3 to V2 O5 –Al2 O3 (Fig. 6) actually causes a growth of the absorption in the region near 400 nm, in spite of the typical absorption of WO3 –Al2 O3 which is near 250 nm. This is probably due to a competition between WO3 (more acidic) and V2 O5 for the adsorption on the basic sites of alumina [26]. WO3 tends to displace V2 O5 for the basic sites, allowing it to condense probably in polymeric chains, although still with quite a low coordination. In other words, the addition of WO3 has a similar effect than the addition of more V2 O5 , with the result of forming condensed vanadate units. In fact, also the increase of the V2 O5 loading on alumina causes the growth of additional absorption in the
region near and above 400 nm. In any case, we cannot observe features suggesting the formation of mutual electronic interactions between vanadyl and wolframyl centers on alumina, in contrast with what happens on silica and in the bulk mixed oxides. Quite a different situation (see also Fig. 10) is found for the oxides supported on titania–anatase. Like alumina, titania is an ionic acido-basic highly dispersing oxide support [29]. This oxide, however, is an intrinsic semiconductor and, accordingly, shows the absorption edge due to the valence band–conduction band transition (O2− → Ti4+ charge transfer transition) in the region near 400 nm. The addition of WO3 to anatase does not cause any modification in the spectrum (Fig. 12). This means that the O2− → W6+ charge transfer transitions of the surface tungsten oxide species just are superimposed to the O2− → Ti4+ charge transfer transition of the bulk. This agrees with the dispersion of tungsten oxide species on titania in the form of low coordination wolframyl sites, as deduced by IR and Raman studies. In fact, WO3 (where W6+ ions are in an overall coordination 6) absorbs significantly above 400 nm. This also means that the W 5d levels fall in the conduction band of the overall solid. This agrees with the observation that it is not possible to detect the ESR spectrum of W5+ in the case of slightly reduced WO3 –TiO2 [13], due to the fact that the (formally) d1 electron of W5+ is actually delocalized in the titania conduction band. The addition of MoO3 at the anatase surface (Fig. 8) actually causes the detection of a weak tail just near 400 nm in the UV spectrum. This suggests that the Mo 4d levels fall just below the lower energy limit of the conduction band of the overall solid. This agrees with the observation that it is possible to detect the ESR spectrum of Mo5+ at 77 K in the case of slightly reduced MoO3 –TiO2 [31], due to the fact that the d1 electron of Mo5+ is, at low temperature, actually localized in the Mo 4d level. However, at slightly higher temperature this electron can easily reach the conduction band and, accordingly, the ESR signal vanishes. The addition of V2 O5 at the anatase surface (Fig. 4) actually causes the detection of quite important absorption in the 400–500 nm region. The difference with respect to the spectrum of bulk vanadia (which absorbs up to 600 nm) (Fig. 2) suggests that vanadium oxide species on titania are in the form of low coordination centers, as already deduced from Raman, IR and NMR data. This also shows that the V 3d levels of surface vanadyls fall well below the lower energy limit of the conduction band of the overall solid. This agrees with the observation that it is possible to detect the ESR spectrum of V4+ in the case of slightly reduced V2 O5 –TiO2 [32], due to the fact that the d1 electron of V4+ is, at low temperature, actually localized in the V 3d level. However, at 573–673 K (where catalysis actually occurs) the gap to the conduction band (≈0.5 eV) is overcome and the electron is delocalized in the overall solid. In fact, V2 O5 –TiO2 materials have definitely higher conductivity than TiO2 [33,34].
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The UV spectra of V2 O5 –WO3 –TiO2 (Fig. 10) catalysts show nearly the same spectra of V2 O5 –TiO2 (Fig. 4) catalysts with the same or a little higher V2 O5 content. This suggests that vanadyl and wolframyl centers do not interact directly from the point of view of electronics. These data agree (Fig. 10) with the observation that it is possible to detect the ESR spectrum of V4+ in the case of slightly reduced V2 O5 –WO3 –TiO2 catalysts [13,35], due to the fact that the d1 electron of V4+ is, at low temperature, actually localized in the V 3d levels. In any case the closeness between these levels and the conduction band of the overall solid allows easily (at the working temperature of SCR catalysts which is near 573 K) the migration of electrons from V atoms to the conduction band and vice versa. This allows to explain the effect of addition of WO3 in activating the V2 O5 –TiO2 catalysts for the SCR reaction. In fact, WO3 give rise to reducible wolframyl centers which certainly increase the electron density in the conduction band of the overall solid. These electrons tend to fall into the lower-energy V 3d levels, so giving rise to reduced vanadyl centers. It has been previously proposed that VO2+ centers are the active sites for SCR reaction. This possibly increases the concentration and, actually, the reducibility of the surface vanadyl centers. On the other hand, it is evident that the active sites for an oxidation catalyst working with a redox or Mars–Van Krevelen mechanism also need to be rapidly reoxidized by oxygen. Wolframyl centers also allow the electrons, produced by reduction of vanadium centers, to be rapidly and easily given to oxygen producing oxide species. In other words, the presence of wolframyl centers increases the electron density in the solid and fastens the redox cycles needed to perform the SCR reaction. In agreement with these V2 O5 –MO3 –WO3 show high electrical conductivity [36]. A similar effect could concern V2 O5 –MoO3 –TiO2 (Fig. 13) catalysts with low V content, such as the industrial catalyst investigated here. Mo 4d levels, which seem to stay just below the lower energy limit of the conduction band, can bridge between the conduction band and the V 3d levels, so also favoring the movements of the electrons between the bulk and V centers. In the case of MoO3 based catalyst with high loading absorption grows in the visible region showing a tendency of molybdenum oxide species to stay in a reduced state. This effect seems to not involve directly vanadium oxide species, because it happens also for V-free catalysts. So, it is probably not relevant for SCR catalysis where V plays a key role.
5. Conclusions The main conclusion from this work is that titania–anatase has very likely a double activating effect for vanadia catalysts. One is a result of its ionicity and acido-basic character, allowing to disperse vanadium, molybdenum and
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tungsten oxide species in the form of low coordination nearly isolated centers. This, however, is more or less performed also by other ionic oxide supports such as alumina. Additionally, titania being a semiconductor, allows the electronic contact between the metal oxide centers supported on it. This electronic contact, through the conduction band of titania, explains both the activating effect of titania on vanadia, and the activating effects of molybdena and tungsta on vanadia–titania in SCR catalysis.
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