TiO2− ZrO2 Catalysts by XPS and Other

10176

J. Phys. Chem. B 1998, 102, 10176-10182

Characterization of V2O5/TiO2-ZrO2 Catalysts by XPS and Other Techniques Benjaram M. Reddy,* Biswajit Chowdhury, and Ibram Ganesh Inorganic Chemistry DiVision, Indian Institute of Chemical Technology, Hyderabad 500 007, India

E. P. Reddy, T. C. Rojas, and A. Ferna´ ndez Instituto de Ciencia de Materiales de SeVilla, Centro de InVestigaciones Cientificas “Isla de la Cartuja”, AVda. Americo Vespucio s/n, 41092-SeVilla, Spain ReceiVed: June 15, 1998; In Final Form: August 1, 1998

A high surface area titania-zirconia mixed oxide support was prepared by the technique of precipitation from homogeneous solutions. Vanadia (12 wt %) was impregnated on TiO2-ZrO2 support by using an oxalic acid solution of NH4VO3. The TiO2-ZrO2 binary oxide support and the V2O5/TiO2-ZrO2 catalyst were then subjected to thermal treatments from 500 to 800 °C. The influence of thermal treatments on the dispersion and stability of the catalyst was investigated by X-ray diffraction (XRD), FT infrared (FTIR), UV-vis absorption, and X-ray photoelectron spectroscopy (XPS) techniques. The characterization results suggest that the TiO2-ZrO2 binary oxide support is thermally quite stable up to 800 °C. Calcination of the coprecipitated titanium-zirconium hydroxides at 500 °C result in the formation of an amorphous phase, and further heating at 600 °C converts this amorphous phase into a crystalline ZrTiO4 compound. Impregnation of V2O5 and heating of the V2O5/TiO2-ZrO2 catalyst beyond 600 °C results in the formation of ZrV2O7, with the simultaneous presence of the TiO2 rutile phase. However, the vanadia is in a highly dispersed state on the TiO2-ZrO2 mixed oxide support when calcined at less than 600 °C.

Introduction Supported vanadium pentoxide is a well-known catalyst for the selective oxidation and ammoxidation of hydrocarbons as well as for the selective catalytic reduction (SCR) of NOx with NH3.1-6 The activity and selectivity of these catalysts is known to be sensitive to the composition of the support and, in the case of TiO2-supported catalysts, the phase of the support.7,8 Prior research has clearly shown that the preferred phase of TiO2 is anatase and that the activity per gram of catalyst increases with increasing vanadia loading up to the point where the surface of the support is covered by a theoretical monolayer of vanadia.7-12 This behavior is attributed to the fact that the vanadium oxide structure changes with loading. Many theories have been proposed to explain the unique effect of anatase as a support in V2O5/TiO2 catalysts.13-15 Vejux and Courtine16 have proposed that a close match between the structure of the (010) plane of V2O5 and the (001) plane of anatase (the predominant morphological plane exposed) leads to the epitaxial growth of V2O5 as platelets. Exposure of the (010) plane leads to an increase in the number of surface VdO groups, thus accounting for the greater activity of monolayer-loaded anatasesupported catalysts compared with other supported or unsupported ones. Thus, titania has been employed as a support or a component in a number of commercially important heterogeneous catalyst systems.17,18 However, a great disadvantage associated with titania support is its low specific surface area, poor mechanical strength, lack of abrasion resistance, and high price. In addition, the anatase phase of titania has poor thermal stability at high temperatures. Thermal stability of the catalysts is one of the important factors in catalyst selection, since in * Author to whom correspondence should be addressed. Fax: (91)40 7173387. E-mail: [email protected].

high-temperature oxidations or SCR, long-term thermal stability dictates the catalyst life. Therefore, several efforts are being made to develop various kinds of new oxide supports in the place of titania to satisfy the needs of practical application.19-23 Research by many groups has shown that high activity and selectivity are achieved when the vanadia is present on the support surface in the form of highly dispersed, amorphous species, rather than as crystallites of V2O5.7,8 Several groups have investigated the structure of the immobilized vanadia by using various spectroscopic and nonspectroscopic techniques.7,8,19-29 Most of the papers concerning SCR reaction have dealt with zeolite or single metal oxide-based catalysts. However, binary metal oxides are expected to exhibit better catalytic activity for this reaction due to their solid acid or base properties.30 In fact, Haneda et al.31 examined the catalytic performance of a group of binary metal oxides for NO reduction by propane and found that some of them show good catalytic activity. Among various binary oxides, the TiO2-ZrO2 exhibited very good catalytic activity. The TiO2-ZrO2 binary oxide has also been reported to exhibit high surface acidity by a charge imbalance based on the generation of Ti-O-Zr bonding.32 Further, recent studies also reveal that TiO2-ZrO2 is an active catalyst for dehydrocyclization of n-paraffins to aromatics33 and hydrogenation of carboxylic acids to alcohols,34 and also is an effective support for MoO3-based catalysts for hydroprocessing applications.35 Thus, the combined TiO2-ZrO2 mixed oxide has attracted attention recently as a catalyst and support for various applications. The aim of the present study is to provide basic insights into the structure of the V2O5/TiO2-ZrO2 catalyst, shedding light on the influence of thermal treatments on the surface structure and dispersion of these catalysts. In this investigation the TiO2ZrO2 binary oxide support obtained by a homogeneous copre-

10.1021/jp9826165 CCC: $15.00 © 1998 American Chemical Society Published on Web 11/20/1998

Characterization of V2O5/TiO2-ZrO2 Catalysts

J. Phys. Chem. B, Vol. 102, No. 50, 1998 10177

cipitation method, and the monolayer V2O5/TiO2-ZrO2 catalyst were subjected to thermal treatments at different temperatures in order to understand the dispersion of vanadium oxide and the temperature stability of these catalysts. In this study thermal effects are examined by different characterization techniques including XRD, FTIR, UV-vis, and XPS. Experimental Section Catalyst Preparation. The TiO2-ZrO2 mixed oxide (1:1 mole ratio) support was prepared by a homogeneous coprecipitation method using urea as the precipitation reagent.36 An aqueous solution containing the requisite quantities of TiCl4 (Fluka, AR grade), ZrOCl2 (Fluka, AR grade), and urea (Loba Chemie, GR grade) were heated together to 95 °C with vigorous stirring. In about 6 h of heating, as decomposition of urea progressed to a certain extent, the formation of precipitate gradually occurred and the pH of the solution increased. The precipitate was heated for 6 h more to facilitate aging. The coprecipitate was then filtered off, washed several times with deionized water until no chloride ions were detected by the addition of AgNO3 to the filtrate, and then dried at 110 °C for 12 h. The oven-dried precipitate was finally calcined at 500 °C for 6 h in an open-air atmosphere. The resulting TiO2ZrO2 mixed oxide support had a N2 BET surface area of 160 m2 g-1 and was found to be uniform throughout the bulk. Some portions of this support were once again heated at 600, 700, and 800 °C for 6 h in a closed electrical furnace in open-air atmosphere. The V2O5/TiO2-ZrO2 catalyst, containing 12 wt % V2O5 was prepared by a standard wet impregnation method. To impregnate V2O5, the requisite quantity of ammonium metavanadate (Fluka, AR grade) was dissolved in 1 M oxalic acid, and the finely powdered calcined (500 °C) mixed oxide support was added to this solution. The excess water was evaporated on a water bath with continuous stirring. The resultant solid was then dried at 110 °C for 12 h and calcined at 500 °C for 6 h in a closed electrical furnace in a flowing oxygen atmosphere. Some portions of the finished catalyst were once again heated at 600, 700, and 800 °C for 6 h in a closed electrical furnace in an open-air atmosphere. The rate of heating (as well as cooling) was always held at 10 °C min-1. X-ray Diffraction. X-ray powder diffraction patterns have been recorded on a Siemens D-500 diffractometer by using a CuKR radiation source and a scintillation counter detector. The XRD phases present in the samples were identified with the help of ASTM Powder Data Files. Infrared Spectra. The FTIR spectra were recorded on a Nicolet 740 FTIR spectrometer at ambient conditions, using KBr disks, with a nominal resolution of 4 cm-1 and averaging 100 spectra. UV-Vis Absorption Spectra. UV-vis spectra were recorded in the diffuse reflectance mode (R∞) and transformed to a magnitude proportional to the extinction coefficient (K) through the Kubelka-Munk function (F(R∞)).37 To evaluate the absorption behavior of different catalyst samples (F(R∞)) was plotted against hν, where hν is the photon energy. X-ray Photoelectron Spectroscopy. The XPS spectra were recorded with a VG-ESCALAB 210 spectrometer working in the constant analyzer energy mode with a pass energy of 50 eV and MgKR radiation as the excitation source. The binding energy (BE) reference was taken at the Ti 2p3/2 peak of Ti4+ at 458.5 eV. An estimated error of (0.1 eV can be assumed for all the measurements. Quantification was accomplished by determining the elemental peak areas, following a Shirley

Figure 1. X-ray diffraction patterns of the TiO2-ZrO2 support calcined at different temperatures: (b) characteristic lines due to ZrTiO4.

background subtraction. Quantitative analysis was carried out using the sensitivity factors supplied with the instrument. The modified Auger parameter of Ti was calculated according to the following equation:

RI ) R + hν ) BE of the Ti 2p peak + KE of the Ti L3M23V Auger peak where BE is the binding energy and KE is the kinetic energy. The Auger parameter is considered to give information about the electronic properties of the oxides dispersed on metal or metal oxide supports.38 Results and Discussion The XRD patterns of the TiO2-ZrO2 support calcined at various temperatures from 500 to 800 °C are shown in Figure 1. As can be noted from this figure, the TiO2-ZrO2 mixed support is in an amorphous or a poorly crystalline state up to its 600 °C calcination temperature. However, formation of the crystalline ZrTiO4 compound can be noted from 700 °C and above temperatures, and the intensity of lines due to this compound also increase with increase of calcination temperature. Recently, Fung and Wang33 also reported the formation of the ZrTiO4 compound at 650 °C and above temperatures coinciding with our XRD observations. Noguchi and Mizuno39 reported that tetragonal and monoclinic ZrO2 and rutile TiO2 could be formed by the decomposition of ZrTiO4 at higher temperatures. Jung-Chung Wu and co-workers32 also reported the formation of the TiO2 (rutile) phase at higher calcination temperatures. However, no independent lines due to TiO2 (anatase or rutile) or ZrO2 (monoclinic, tetragonal, or cubic) phases are observed in the present study. The ZrTiO4 compound appears to be thermally quite stable even up to 800 °C calcination temperature (Figure 1). The observed higher stability of ZrTiO4 compound and its formation at lower temperatures may presumably be due to a different preparation method adopted and the precursor compounds used for the preparation of this mixed oxide support in the present study.36

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Reddy et al. treatments, in the absence of vanadia on the TiO2-ZrO2 support. (2) Absence of a crystalline V2O5 phase and appearance of ZrV2O7 compound accompanied by the TiO2 rutile phase for calcination temperatures beyond 500 °C. Further, the intensity of the lines due to both ZrV2O7 and TiO2 rutile phase also increase with increase in calcination temperature. The XRD observations described above provide interesting information about the reactivity of vanadia toward the ZrTiO4 compound. As envisaged earlier,36 the vanadia reacts preferably with the ZrO2 portion of ZrTiO4 compound to form the ZrV2O7, thus liberating TiO2. The portion of TiO2 released from the ZrTiO4 compound appears as the crystalline rutile phase as shown in the following equation:

ZrTiO4 + V2O5 f ZrV2O7 + TiO2 (rutile)

Figure 2. X-ray diffraction patterns of the V2O5/TiO2-ZrO2 catalyst calcined at different temperatures; (b) characteristic lines due to ZrTiO4; (2) lines due to ZrV2O7; (O) lines due to TiO2 rutile phase.

The XRD patterns of V2O5/TiO2-ZrO2 catalyst calcined at various temperatures are shown in Figure 2. As can be noted from this figure, the V2O5/TiO2-ZrO2 catalyst is again in an amorphous or a poorly crystalline state when calcined at 500 °C. Most importantly, no lines due to crystalline V2O5 are seen in the spectra. This observation clearly indicates that vanadium oxide is present in a highly dispersed and amorphous state on the support surface. This conclusion is further supported from FTIR and ESCA measurements. The O2 uptake results published earlier40 also revealed that vanadium oxide is in a highly dispersed state on the support surface up to the monolayer capacity. The TiO2-ZrO2 mixed oxide support had a BET surface area of 160 m2 g-1. The quantity of V2O5 required to cover the support surface as a monomolecular layer can be estimated from the area occupied per VO2.5 (10.3 × 104 pm2) unit of the bulk V2O5.41 In fact, the 12 wt % selected in the present investigation corresponds to the monolayer capacity of the support. Hence, no crystalline V2O5 is noted from XRD measurements. Upon increase of calcination temperature from 500 to 600 °C, formation of ZrTiO4, ZrV2O7, and TiO2 rutile phases can be noted (Figure 2). On further raising the calcination temperature from 600 to 800 °C a further improvement in the intensity of the lines due to these phases can be seen. A closer look into Figures 1 and 2 reveals that the heating temperature and presence of V2O5 have two major effects on the TiO2-ZrO2 mixed oxide support: (1) Transformation of amorphous TiO2 and ZrO2 mixed oxide support into a definite crystalline TiO2.ZrO2 or ZrTiO4 compound beyond 500 °C temperature. Crystallinity of this phase increases with increase of calcination temperature. This compound is quite stable in the absence of V2O5. This is evidenced by the absence of crystalline TiO2 (anatase or rutile) and ZrO2 (tetragonal, monoclinic, or cubic) phases, even after high-temperature

It is a widely established fact in the literature7,16,24,41 that highly dispersed vanadia on TiO2 support accelerates the anatase-to-rutile phase transformation by lowering the activation temperature of this phenomenon, which is normally expected to be 550 °C and above in impurity-free TiO2 samples.42 During the transformation of the anatase-to-rutile phase some of the vanadia is normally reduced and gets incorporated into the rutile structure as VxTi(1-x)O2 (rutile solid solution).16,24,41,43 However, in the case of TiO2-ZrO2 mixed oxide, the reactivity of vanadia toward the ZrTiO4 compound appears to be different. Infrared spectroscopy has been widely used to ascertain the nature of the vanadium oxide phase existing on various supports.7,8 The FTIR spectra of the V2O5/TiO2-ZrO2 catalyst calcined at different temperatures are shown in Figure 3, in the range 600-1200 cm-1, where those bands due to VνdO are expected to be recorded. Generally, the IR spectrum of bulk V2O5 shows sharp absorption bands at 1020 and 820 cm-1 due to VdO stretching and V-O-V deformation modes, respectively. The FTIR spectra of the V2O5/TiO2-ZrO2 catalyst calcined at 500 °C indicate that the vanadium oxide is in a highly dispersed state. The spectra show only broad bands at around 980 and 825 cm-1, and further the intensity of these bands decreases with an increase in calcination temperature. The band in the range 990-960 cm-1 has been reported frequently for vanadia-titania catalysts having a vanadium content close to that necessary to cover all the support surface with a single monolayer.43 Incidentally, the V2O5 content selected in the present study also corresponds to the monolayer capacity of the TiO2-ZrO2 support. Nakagawa et al.44 studied the monolayer species in V2O5/TiO2 catalysts by FTIR, and found a shift from 1020 (pure V2O5) to 980 cm-1 for the VdO stretching frequency. They concluded that the vanadium oxide is present as an amorphous VOx at low coverage, and as both amorphous and crystalline V2O5 at high surface coverage. Our recent study23 on the V2O5/TiO2-Al2O3 catalyst also revealed that the amorphous VOx, at less than monolayer coverage, exhibits a VdO stretching frequency in the range 940-980 cm-1. Thus, the FTIR results suggest that the vanadia is present in a highly dispersed state on the carrier, especially when calcined at 500 °C. These observations are in line with XRD results. The Kubelka-Munk-transformed UV-vis spectra of TiO2ZrO2 support and V2O5/TiO2-ZrO2 catalyst calcined at 500 to 800 °C are included in Supporting Information. The band-gap absorption showed a blue shift with increase of calcination temperature, and reached an absorption edge of ca. 3.3 eV for the TiO2-ZrO2 sample calcined at 800 °C. This blue shift, upon high-temperature calcination, can be explained as due to both elimination of carbonaceous impurities and the formation of the ZrTiO4 phase.45 In the case of V2O5/TiO2-ZrO2

Characterization of V2O5/TiO2-ZrO2 Catalysts

J. Phys. Chem. B, Vol. 102, No. 50, 1998 10179 TABLE 1: Binding Energies, XPS Atomic Ratios, and Modified Auger Parameter Values for Ti of TiO2-ZrO2 Support and 12% V2O5/TiO2-ZrO2 Catalyst Calcined at Different Temperatures

Figure 3. FTIR spectra of the V2O5/TiO2-ZrO2 catalyst calcined at different temperatures.

catalysts, the sample calcined at 500 °C showed a pale yellow color typical of the V2O5. At higher calcination temperatures the catalyst color changed from yellow to brown, in agreement with the formation of the highly dispersed vanadia phase together with the formation of new ZrV2O7 phase. The samples of TiO2-ZrO2 and 12% V2O5/TiO2-ZrO2 calcined at different temperatures are investigated by XPS technique. The photoelectron peaks of O 1s, Ti 2p, Zr 3d, and V 2p are depicted in Figures 4, 5, 6, and 7, respectively. For the purpose of better comparison, the XPS photoelectron peaks of O 1s, Ti 2p, and Zr 3d pertaining to the TiO2-ZrO2 carrier and the corresponding peaks of the V2O5/TiO2-ZrO2 catalyst

Ti

temp (°C)

O 1s

2p3/2

RI(Ti) Zr 3d5/2 V 2p3/2 ITi/IZr IV/ITi IV/IZr

500 600 700 800

530.0 529.9 529.8 529.7

458.5 458.5 458.5 458.5

TiO2-ZrO2 Support 873.6 182.5 873.6 182.3 873.6 182.3 873.6 182.2

500 600 700 800

529.9 530.0 530.0 530.0

458.5 458.5 458.5 458.5

V2O5/TiO2-ZrO2 Catalyst 873.4 182.9 516.8 873.5 182.5 517.0 873.5 182.3 517.2 873.6 182.2 517.4

0.80 0.81 0.83 0.92 0.77 0.70 0.67 0.65

0.44 0.47 0.50 0.53

0.33 0.34 0.35 0.35

are presented together in these figures. Binding energies for O 1s, Ti 2p, Zr 3d, and V 2p core levels, and the Auger parameter of Ti 2p3/2 are presented in Table 1. The Ti/Zr, V/Ti, and V/Zr atomic percentage ratios as determined by XPS are shown in Figure 8 and Table 1. All these figures and Table 1 clearly indicate that the XPS bands depend on the calcination temperature and the coverage of vanadium oxide on the carrier, in agreement with the literature reports.29,46-50 In general, the intensity of the O 1s, Ti 2p, Zr 3d, and V 2p peaks increases with an increase in the calcination temperature. This general effect is also accompanied by a diminution of the C 1s peak intensity, which indicates clearly that the calcination treatment is also cleaning the surface of the samples from carbon contamination. The O 1s profile, as shown in Figure 4, is due to the overlapping contribution of oxygen from titania and zirconia in the case of TiO2-ZrO2 support, and to titania, zirconia, and vanadia in the case of the V2O5/TiO2-ZrO2 catalyst, respectively. Figure 5 shows the binding energies of Ti 2p photoelectron peaks at 458.5 and 464.4 eV for Ti 2p3/2 and Ti 2p1/2 lines, respectively, which agree well with the values reported in the literature.29,51 Very interestingly, the intensity of the Ti 2p core level spectra increases with an increase in calcination temperature. This increase is more predominant for pure TiO2-ZrO2 than for the V2O5/TiO2-ZrO2 catalyst, indicating that the intensity of Ti 2p photoelectron signals depend on the calcination temperature as well as on the coverage of V2O5 on the titaniazirconia carrier. The Auger parameter for Ti has been measured and summarized in Table 1, showing very small and insignificant variation for the V2O5/TiO2-ZrO2 catalyst and no change for the TiO2-ZrO2 support. As this parameter does not depend on charge effects, the Ti seems to be in a similar chemical state for all the samples. Therefore, Ti is considered a good reference for binding energy calibrations. The measurement of the Auger parameter is envisaged as a good means to assess the electronic properties of oxides on various supports. In fact, we have recently shown that the BE and RI values of TiO2 deposited on SiO2 change by 0.7 and 2.6 eV, respectively, as the coverage increases.38,52 Figure 6 and Table 1 show that the binding energy of the Zr 3d core levels decrease slightly with an increase in calcination temperature. This decrease in binding energy is more predominant in the case of the V2O5/TiO2-ZrO2 catalyst than in that of the pure support. This decrease in binding energy may presumably be due to the formation of new phases, i.e., crystalline ZrTiO4 and ZrV2O7 compounds, respectively. Another interesting observation to be mentioned here is that the resolution of Zr 3d3/2 core level peak increases slowly with an

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Reddy et al.

Figure 4. XPS of the O 1s binding energy region for the V2O5/TiO2-ZrO2 catalyst and TiO2-ZrO2 support calcined at different temperatures.

Figure 5. Ti 2p XPS spectra of the V2O5/TiO2-ZrO2 catalyst and TiO2-ZrO2 support calcined at different temperatures.

increase in the calcination temperature in the case of V2O5/ TiO2-ZrO2 catalyst, but the same effect is not noticed in the pure titania-zirconia support. Figure 7 shows the V 2p3/2 photoelectron peak of the V2O5/ TiO2-ZrO2 catalyst calcined at various temperatures. At 500 °C calcination the binding energy of V 2p3/2 is 516.8 eV; it probably corresponds to the VIV state. With an increase

calcination temperature from 500 to 800 °C, the binding energy of V 2p3/2 increased from 516.8 to 517.4 eV (Table 1), an indication of an increase in the oxidation state of vanadium which is due primarily to formation of crystalline zirconium bivanadate. Vanadia in the tetravalent state is stabilized on the amorphous TiO2-ZrO2 mixed oxide support when calcined at 500 °C. As the calcination temperature increases, the formation

Characterization of V2O5/TiO2-ZrO2 Catalysts

J. Phys. Chem. B, Vol. 102, No. 50, 1998 10181

Figure 6. Zr 3d XPS spectra of the V2O5/TiO2-ZrO2 catalyst and TiO2-ZrO2 support calcined at different temperatures.

in Table 1 and Figure 8. The intense signal corresponding to the V 2p3/2 level of vanadium was compared with the Ti 2p3/2 and Zr 3d5/2 levels. The V 2p/Ti 2p and V 2p/Zr 3d ratios can be taken as a measure of the relative dispersion of vanadium oxide on the support surface as shown in Figure 8b. In general, the V:Ti and V:Zr intensity ratios are found to increase with an increase in calcination temperature (Figure 8b). The difference between V:Ti and V:Zr intensity ratios also increases with an increase in the calcination temperature. The Ti:Zr atomic ratios for the TiO2-ZrO2 support and for the 12% V2O5/TiO2-ZrO2 catalyst are also measured and shown in Table 1 and Figure 8a. As can be seen from Figure 8a, in the case of titaniazirconia mixed oxide support, the Ti/Zr ratio is increased with an increase of calcination temperature. However, in the case of V2O5/TiO2-ZrO2 catalyst this ratio was found to decrease. These effects can be explained as due to the percolation of Ti on the surface in the case of the titania-zirconia mixed oxide support, and segregation of the ZrV2O7 compound and the phase transformation of titania from anatase to rutile at higher temperatures in the presence of vanadium oxide, in the V2O5/ TiO2-ZrO2 catalyst. Formation of these phases has already been demonstrated from XRD measurements of these samples. Thus, the XPS measurements corroborate the observations made from the XRD study. Figure 7. V 2p XPS spectra of the V2O5/TiO2-ZrO2 catalyst calcined at different temperatures.

Conclusions

of crystalline ZrTiO4, ZrV2O7, and the rutile phase of TiO2 occur, thereby decreasing the proportion of amorphous material. Thus, the newly formed crystalline ZrV2O7 compound stabilizes the pentavalent state of vanadium. From these data it is also clear that the binding energy of the V 2p3/2 peak is more sensitive to the nature of the carrier material.29,47 The relative dispersion of vanadium on the support surface was also estimated from XPS measurements of the V2O5/TiO2ZrO2 catalyst calcined at different temperatures and presented

The following conclusions can be drawn from this study: (1) The titania-zirconia binary oxide is an interesting and promising support material for the dispersion of vanadium oxide. (2) The coprecipitated TiO2-ZrO2 mixed oxide, when calcined at 500 °C, is in an X-ray amorphous state and exhibits reasonably high specific surface area. This amorphous TiO2-ZrO2 gets converted into a crystalline ZrTiO4 beyond the 600 °C calcination temperature, and this compound is thermally quite stable up to 800 °C. (3) The TiO2-ZrO2 mixed oxide also accommodates

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Figure 8. XPS intensity ratio versus calcination temperature for the TiO2-ZrO2 support and V2O5/TiO2-ZrO2 catalyst calcined at different temperatures.

the monolayer equivalent (12 wt %) of V2O5 in a highly dispersed state on its surface. Further, the V2O5/TiO2-ZrO2 catalyst is also thermally stable up to a 600 °C calcination temperature. (4) The V2O5 selectively interacts with the ZrO2 portion of the TiO2-ZrO2 mixed oxide support and readily forms the ZrV2O7 compound with the liberation of TiO2 when subjected to thermal treatments beyond 600 °C. The liberated TiO2 appears in the form of the TiO2 rutile phase. Acknowledgment. E.P.R. thanks the Direccio´n General de Investigacio´n Cientı´fica y Te´cnica (Spain) for financial support. B.C. and I.G. are the recipients of senior research fellowships of the University Grants Commission, New Delhi. Supporting Information Available: UV-vis spectra of the TiO2-ZrO2 support and the 12% V2O5/TiO2-ZrO2 catalyst calcined at different temperatures (2 pages). Ordering information is given on any current masthead page. References and Notes (1) Hucknall, D. J. SelectiVe Oxidation of Hydrocarbons; Academic Press: New York, 1974. (2) Wainwright, M. S.; Foster, N. R. Catal. ReV.sSci. Eng. 1979, 19, 211. (3) Bielanski, A.; Haber, J. Catal. ReV.sSci. Eng. 1979, 19, 1. (4) Bosch, H.; Janssen, F. Catal. Today 1988, 2, 369. (5) Reddy, B. N.; Reddy, B. M.; Subrahmanyam, M. J. Chem. Soc., Faraday Trans. 1991, 81, 1655.

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