Langmuir 1997, 13, 5621-5626
5621
Spectroscopic and Activity Studies on Vanadia Supported on Titania and Phosphorus-Modified Titania Elmer C. Alyea,* L. Jhansi Lakshmi, and Zhang Ju Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario N1G 2W1, Canada Received February 27, 1997. In Final Form: August 12, 1997X A series of catalysts with vanadia contents varying between 1 and 14 wt % were prepared on titania (Degussa) and phosphorus-modified titania. The catalysts were characterized by employing BET surface area, X-ray diffraction (XRD), electron spin resonance (ESR), FT- Raman (FT-Raman), and 51V solid state nuclear magnetic resonance (51V NMR) spectroscopy. The activities of the catalysts were tested in the ethanol partial oxidation reaction. XRD studies indicated the formation of V2O5 microcrystallites only at the highest loading of 14 wt % in both series of catalysts. FT-Raman studies indicated the formation of microcrystalline V2O5 species beyond 5.4 wt % loading (V-Ti 3) in the TiO2-supported catalysts and 2.8 wt % (V-PTi 2) in the phosphorus-modified samples. 51V NMR spectra of the V2O5/TiO2 catalysts showed the presence of both octahedral (a peak at -310 ppm) and tetrahedral vanadia species (a shoulder peak at -500 ppm) in the samples up to 5.4 wt % V2O5; beyond this loading only the peak corresponding to octahedrally coordinated vanadia species was observed. V2O5 supported on phosphorus-modified TiO2 exhibited peaks corresponding to only octahedrally coordinated vanadia at all loadings studied. The ESR spectra of the V-Ti catalysts exhibited hyperfine lines corresponding to VO6c2+ or VO5c2+ species. The ESR spectra of V-PTi samples were found to be more complex with two kinds of signals with both broad and sharp hyperfine lines with different peak to peak line widths, which may correspond to the presence of VO6c2+ or VO5c2+ and V6c4+ species. Ethanol partial oxidation activities of the catalysts decreased upon phosphorus addition to titania, turnover frequency for these catalysts being 3 orders of magnitude less than that for the unmodified samples.
Introduction Titania-supported vanadia catalysts have been subjected to investigation by various researchers as they find extensive application in synthesis of phthalic anhydride from o-xylene,1,2 selective catalytic reduction (SCR) of NOx,3,4 partial oxidation,5,6 and ammoxidation7,8 reactions. In comparison with carriers like Al2O3, SiO2, and ZrO2, TiO2-supported vanadia catalysts were found to exhibit higher activities and selectivities, which was attributed to the strong oxide support interaction (SOSI) that results in the formation of a well-dispersed monolayer of surface vanadia species.9 Vejux and Courtine10 noticed the selective exposure of the 010 plane of V2O5 on TiO2 support due to the crystallographic fit between vanadia and titania. The spectroscopic techniques such as laser Raman spectroscopy,11,12 infrared spectroscopy,13,14 diffuse reflectance spectroscopy,15 51V NMR,12,16-20 and electron spin reso* Fax: (519) 766-1499. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, September 15, 1997. (1) Wainwright, M. S.; Foster, N. R. Catal. Rev.-Sci. Eng. 1979, 1, 19. (2) Nikolov, V.; Klissurski, D.; Anastasov, A. Catal. Rev.-Sci. Eng. 1991, 33, 319. (3) Bosch, H.; Janssen, F. Catal. Today 1988, 2, 369. (4) Wachs, I. E.; Deo, G.; Amiridis, M. D. J. Catal. 1996, 161, 211. (5) Grabowski, R.; Grzybowska, B.; Samson, K.; Sloczynski, J.; Stoch, J.; Wcislo, K. Appl. Catal. 1995, 125, 129. (6) Miki, J.; Osada, Y.; Shikada, T. Catal. Lett. 1995, 30, 263. (7) Sanati, M.; Ansersson, A. Ind. Eng. Chem. Res. 1991, 30, 312. (8) Cavalli, P.; Cavani, F.; Manenti, I.; Trifiro, F. Catal. Today 1987, 1, 471. (9) Bond, G. C.; Tahir, S. F. Appl. Catal. 1991, 71, 1. (10) Vejux, A.; Courtine, P. J. Solid State Chem. 1978, 23, 93. (11) (a) Went, G. T.; Leu, L.-J.; Bell, A. T. J. Catal. 1992, 134, 479. (b) Went, G. T.; Oyama, S. T.; Bell, A. T. J. Phys. Chem. 1990, 94, 4240. (12) Deo, G.; Turek, A. M.; Wachs, I. E.; Machej, T.; Haber, J.; Das, N.; Eckerdt, H.; Hirt, A. M. Appl. Catal. 1992, 91, 27. (13) Topsoe, N.-Y. J. Catal. 1991, 128, 499. (14) Busca, G. Langmuir 1986, 2, 577. (15) del Arco, M.; Holgado, M.; Martin, C.; Rives,V. Langmuir 1990, 6, 801. (16) Eckerdt, H.; Wachs, I. E. J. Phys. Chem. 1989, 93, 6796.
S0743-7463(97)00219-9 CCC: $14.00
nance21-25 were employed earlier to determine the structural information on V2O5/TiO2 catalysts. Went and coworkers11 in their in situ Raman and temperatureprogrammed reduction studies on V2O5 supported on anatase TiO2 identified a peak at 1030 cm-1 corresponding to isolated vanadyl species and at 920-945 cm-1 due to the formation of polyvanadate chains. They noticed preferential elimination of oxygen atoms from terminal VdO groups of monomeric and polymeric vanadia species to V-O-V bridging oxygens upon reduction of the catalysts. Deo and co-workers12 employed Raman and 51V NMR spectroscopic techniques on vanadia supported on different phases of titania, and it was observed that similar vanadia species exist on all supports irrespective of the crystal structure of the bulk titania phase. The methanol partial oxidation studies also showed essentially similar activity and selectivity on various catalysts indicating that the type of titania support is not critical but the presence of impurities in the titania was found to influence the nature of vanadia coordination. Various promoters like alkali metals, phosphorus, and other oxides were added to the V2O5 /TiO2 catalysts either before or after vanadia impregnation on the support. Soria et al.25 noticed the formation of vanadyl phosphate species upon phosphoric acid addition to titania. Deo and Wachs26 (17) Blasco, J.; Nieto, J. M. L. Colloid Surf. 1996, 115, 187. (18) Lapina,O. B.; Mastikhin,V. M.; Shukri, A. A.; Krasilnikov, V. N.; Zamaraev, K. I. Prog. NMR. Spectrosc. 1992, 24, 457. (19) Pinaeva, L. G.; Lapina, O. B.; Mastikhin,V. M.; Nosov, A. V.; Balzhinimaev, B. S. J. Mol. Catal. 1994, 88, 311. (20) Eckerdt, H.; Deo, G.; Wachs, I. E.; Hirt, A. M. Colloids Surf. 1990, 45, 347. (21) Davidson, A.; Che, M. J. Phys. Chem. 1992, 96, 9909. (22) Inomata, M.; Mori, K.; Miyamoto, A.; Murakami, Y. J. Phys. Chem. 1983, 87 ,761. (23) Miller, J. B.; DeCanio, S. J.; Michel, J. M.; Dybowski, C. J. Phys. Chem. 1985, 89, 2592. (24) Busca, G.; Centi, G.; Marchetti, L.; Trifiro, F. Langmuir 1986, 2, 568. (25) Soria, J.; Conesa, J. C.; Granados, M. L. J. Catal. 1989, 120, 457. (26) Deo, G.; Wachs, I. E. J. Catal. 1994, 146, 335.
© 1997 American Chemical Society
5622 Langmuir, Vol. 13, No. 21, 1997
Alyea et al.
Table 1 (a) % Composition and Surface Areas of V-Ti Catalysts catalyst Ti V-Ti 1 V-Ti 2 V-Ti 3 V-Ti 4 V-Ti 5 V-Ti 6
wt % V2O5
surface area (BET), m2/g
V atoms/nm2
0.89 2.67 5.35 8.02 10.70 13.4
48 46 44 43 42 42 40
1.2 3.7 7.4 11.1 14.8 18.5
a
(b) % Composition and Surface Areas of V-PTi Catalysts catalyst PTi V-PTi 1 V-PTi 2 V-PTi 3 V-PTi 4 V-PTi 5 V-PTi 6
wt % V2O5
surface area (BET), m2/g
V atoms/nm2
0.89 2.8 5.6 8.4 11.2 14.2
37 36 35 35 35 34 31
1.6 5.0 10.0 15.0 20.0 25.4
observed vanadyl phosphates by X-ray diffraction (XRD) only when the P/V ratio in the catalysts is greater than 1.25. The decrease in activity upon the addition of phosphorus to the support was attributed to the formation of vanadium-oxygen-phosphorus bonds with the surface phosphorus oxide (POx) species. Van Hengstum et al.27 and Zhu and Andersson28 also observed a lowering in the activity and selectivity of the catalysts in the toluene oxidation reaction to benzoic acid when phosphorus was added to the V2O5/TiO2 catalysts. In the present study the influence of phosphorus addition on the structural properties of V2O5/TiO2 samples was investigated by applying the spectroscopic techniques such as X-ray diffraction, electron spin resonance, 51V NMR, and BET surface area. These characterization studies were done at ambient conditions on the calcined samples. The activities of both series of catalysts were investigated by ethanol partial oxidation as a model reaction.
b
Experimental Section A series of catalysts with vanadia content varying between 1 and 14 wt % was prepared by impregnating titania and phosphorus-modified titania support with an oxalic acid solution of V2O5 (Fisher Scientific). Commercial TiO2 (Degussa) of surface area 48 m2/g was used as a support. The support was calcined at 773 K prior to its use. Phosphorus-modified titania (1.3 wt %) was prepared by the addition of 2 mL of orthophosphoric acid in deionized water to 50 g of titania support. The excess solution was evaporated slowly in a rotary evaporator. The resulting material was dried at 383 K overnight followed by calcination at 773 K for 5 h. The amount of phosphorus was determined by inductively coupled plasma (ICP) analysis on a Varian Liberty 100-OES spectrometer after calibrating the instrument with NIST traceable standards. A weighed sample was digested in an acid mixture of nitric acid, sulfuric acid, and perchloric acid until the dissolution was complete and then the solution was diluted to a specific volume prior to analysis. The samples were dried at 283 K overnight followed by calcination at 773 K. The vanadia contents of the catalysts, VTi-vanadia supported on titania and VPTi-vanadia supported on phosphorus-modified titania were determined by ICP analysis following the similar procedure mentioned above for phosphorus estimation. The X-ray diffractograms were recorded on a Rigaku Geigerflex DMAX II diffractometer using Co KR radiation. Electron spin resonance (ESR) spectra were recorded at ambient temperature on a Varian E-line Century series with 100 kHz modulation. The microwave frequency was 9.51 GHz. g values were referenced (27) Van Hengstum, A. J.; Pranger, J.; Van Ommen, J. G.; Gellings, P. J. Appl. Catal. 1984, 11, 317. (28) Zhu, J.; Andersson, S. L. T. J. Chem. Soc., Faraday. Trans. 1 1989, 85, 3629.
Figure 1. (a) ESR spectra of V-Ti samples recorded at 298 K. (b) ESR spectra of V-PTi samples recorded at 298 K. to DPPH. The catalysts were sealed in quartz tubes of 4 mm diameter prior to the exposure of the samples to ambient conditions for recording the ESR spectra. Similarly for FTRaman and 51V solid state NMR measurements, the calcined samples were sealed in glass ampules. FT-Raman spectra were recorded on a Bruker FRA 106 FT-Raman module interfaced to a Bruker IFS-66 FT-IR bench. All Raman spectra were recorded at room temperature under ambient condition using a 80 mW power setting for the incident radiation of 943.4 nm from a Nd: YAG laser. The high sensitivity Raman detector (D 418-S) was
Vanadia Supported on Titania
Langmuir, Vol. 13, No. 21, 1997 5623 Table 2
a
(a) Spin Hamiltonian Parameters of V4+ in the V-Ti Catalysts catalyst V-Ti 1 V-Ti 2 V-Ti 3 V-Ti 4 V-Ti 5 V-Ti 6
g|
g⊥
|g|
A|
A⊥
|A|
B
1.9355 1.9382 1.9266 1.9364 1.9350 1.9386
1.9604 1.9600 1.9588 1.9626 1.9590 1.9560
1.9521 1.9527 1.9481 1.9538 1.9520 1.9502
193 201 202 201 201 198
189 188 187 187 189 188
108 107 106 107 107 106
1.59 1.52 1.74 1.65 1.50 1.47
(b) Spin Hamiltonian Parameters of V4+ in V-PTi Catalysts catalyst V-PTi 1 V-PTi 2 V-PTi 3 V-PTi 4 V-PTi 5 V-PTi 6
b
g|
g⊥
|g|
A|
A⊥
|A|
B
1.922 1.929 1.926 1.924 1.925 1.931
1.991 1.991 1.991 1.991 1.991 1.991
1.968 1.970 1.969 1.969 1.969 1.971
199 199 200 200 200 197
76 75 76 76 76 76
117 116 117 117 117 116
6.6 6.6 6.7 6.9 6.9 6.2
for higher loadings the spectra were recorded after 100 scans. Wide-line 51V solid state NMR spectra were obtained on a Bruker ASX 200 MHz spectrometer, operating at 52.6 MHz for vanadium, equipped with a wide-line probe and a 10 mm insert. A 2 µs pulse was applied following a 2 s relaxation decay; typically 1500 scans were acquired for the samples. Chemical shifts were referenced to external VOCl3. BET surface areas of the supports and catalysts were determined by the multipoint adsorption method using N2 on a Quantachrome Autosorb-1 apparatus. Activity studies for partial oxidation of ethanol were carried out in the temperature range 373-498 K taking 200 mg of the catalyst packed in a fixed bed tubular glass reactor of 10 mm i.d. Purified air at a flow rate of 60 mL/min saturated with ethanol (by passing through a saturator maintained at 298 K) was introduced into the reactor. After a steady state period of 30 min the products were analyzed on-line using a Varian 3400 gas chromatograph employing Porapaq N (stainless steel column, 80/100 mesh, 6 ft × 0.125 in. diameter) and a thermal conductivity (TC) detector. The catalysts exhibited total selectivity to acetaldehyde at lower reaction temperatures, whereas at higher temperatures of 473 and 498 K significant amounts of acetic acid and trace amounts of ether, ethyl acetate, diethoxyethane, and CO and CO2 were noticed. Turnover frequencies (TOFs, s-1) for the monolayer and submonolayer catalysts were calculated from moles of ethanol converted per mole vanadium atom per second assuming that all the vanadium atoms present act as the active sites participating in the reaction.29
Results and Discussion
Figure 2. (a) 51V wide-line solid state NMR spectra of the V-Ti samples. (b) 51V wide-line solid state NMR spectra of the V-PTi samples. cooled with liquid nitrogen for optimum sensitivity. The number of scans for the lower concentrations of vanadia was 1000, and
The vanadia contents and BET surface areas of V-Ti and V-PTi samples are shown in Table 1. There is a decrease in surface area of titania upon the addition of phosphorus which is due to blockage of pores on the support surface. There is a further decrease in surface area upon addition of vanadia to phosphate-modified titania. The X-ray diffractograms indicated the presence of both anatase and rutile forms of titania in the support. Small peaks corresponding to the presence of V2O5 microcrystallites were observed in the samples V-Ti 6 and V-PTi 6 with ∼14 wt % vanadia loading. Deo and Wachs29 observed the formation of two- dimensional overlayers on the TiO2 support (Degussa S.A. 50 m2/g) at loadings of 7.9 vanadium atoms/nm2 (6 wt %). Thus the present V-Ti sample with a vanadia loading of 7.4 vanadium atoms/ nm2 is close to the monolayer whereas the V-PTi sample with a vanadium loading of 5.4 atoms/nm2 has the monolayer loading (considering the S.A. of P-TiO2 37 m2/ g). Electron spin resonance spectra of V-Ti and V-PTi catalysts recorded at room temperature are shown in Figure 1. Surface V4+ ions can be detected in the calcined samples as observed earlier,30 even without reduction. (29) Deo, G.; Wachs, I. E. J. Catal. 1994, 146, 323. (30) Cavani, F.; Busca, G. Mater. Chem. Phys. 1990, 25, 475.
5624 Langmuir, Vol. 13, No. 21, 1997
a
b
Figure 3. FT-Raman spectra of (a) V-Ti and (b) V-PTi series of catalysts.
Alyea et al.
Davidson and Che21 noticed two kinds of signals for the V2O5/TiO2 (rutile) sample made by the impregnation method. Signal 1 with broad peaks was assigned to VO6c/5c2+ vanadyl ions. The second signal with sharper peaks was assigned to V6c4+ ions inside the TiO2 matrix. The hyperfine splitting pattern in the V-PTi catalysts with both broad and sharp peak to peak line widths is complicated with overlapping of hyperfine lines, unlike the V-Ti samples, indicating the presence of different kinds of vanadium species. There may be the presence of V6c4+ and VO6c/5c2+ vanadyl ions in the phosphorusmodified sample as assigned by Davidson and Che. Further studies on the simulation of the ESR signals and influence of different pretreatments such as evacuation and reduction at different temperatures on these samples are in progress. The axially symmetric g values g| and g⊥ and hyperfine coupling constants A| and A⊥ of the V-Ti and V-PTi series of catalysts are given in Table 2. According to Davidson and Che21 any value of Aiso larger than 100 G is a characteristic of either octahedral or square pyramidal vanadyl ions. In both series of catalysts Aiso is larger than 100 indicating, on the basis of this criterion, octahedrally coordinated vanadyl species. The B values, which represent the strength of the VdO bond, are higher in the case of V-PTi catalysts, indicating weaker interaction between vanadia and phosphate-modified titania. Solid state wide-line 51V NMR spectra of selected samples in the V-Ti and V-PTi series of catalysts are shown in Figure 2. 51V NMR spectroscopy is effectively employed for the characterization of supported vanadia catalysts as this technique provides valuable information on the various coordination environments of vanadia nuclei. Eckerdt and Wachs16 in their 51V NMR studies on V2O5/TiO2 catalysts observed the peaks corresponding to tetrahedral vanadia species at low vanadia contents while at medium loadings both four- and six-coordinated vanadia species were present; at higher concentrations of vanadia they observed only peaks corresponding to octahedrally coordinated vanadia species. The V-Ti series of samples (Figure 2a) show a peak at about -330 ppm with a broad shoulder at -550 ppm in the samples V-Ti 1 and V-Ti 2 indicating the presence of both tetrahedrally and octahedrally coordinated vanadia species. However at higher loadings the absence of the broad shoulder at -550 ppm indicates only octahedrally coordinated vanadia in these samples. In contrast to this V-Ti series of samples, V-PTi samples (Figure 2b) show a peak at -330 ppm, revealing that in these samples vanadium coordination is almost exclusively octahedral. This observation is in accordance with the 51V NMR studies of Blasco et al.17 They noticed that more acidic supports result in the formation of polymeric octahedrally coordinated vanadia species due to decrease in interaction between vanadia and the support resulting in aggregates of V2O5. Lapina et al.18 in their 51V NMR studies on V2O5/TiO2 catalysts observed the formation of distorted octahedrally coordinated vanadia species upon the interaction of tetrahedrally coordinated vanadia species with water. The presence of impurities on the titania support were found to influence the vanadia coordination on vanadia-titania catalysts.12 Pinaeva et al.19 also noticed broadening of the peaks upon the removal of water molecules from vanadia-titania catalysts, indicating an increased number of tetrahedral sites. Eckerdt and co-workers20 concluded that under ambient conditions the surface vanadium oxide on anatase and rutile supports exists predominantly in distorted octahedral coordination. Figure 3 shows the Raman spectra of the V-Ti and V-PTi series of catalysts. Raman spectra of the supports are also included in the figure. Figure 4 shows the same spectra expanded between the Raman shifts 650 and 1200
Vanadia Supported on Titania
Langmuir, Vol. 13, No. 21, 1997 5625
a
Figure 5. Ethanol partial oxidation rates of the V-Ti and V-PTi series of catalysts at a reaction temperature of 398 K.
b
Figure 4. FT-Raman spectra expanded in the region 6501200 cm-1: (a) V-Ti samples; (b) V-PTi.
cm-1. The major Raman shifts of titania characteristic of anatase at 144, 399, 520, and 640 cm-1 and rutile at 450, 610, and 830 cm-1 could be seen in both the series. In the PTi support there is a broad peak centered around 1050 cm-1, which indicates surface POx species. Hadjiivanov and co-workers31 in their IR studies on phosphate-modified TiO2 reported a peak at about 1030-1010 cm-1 with a shoulder at about 1120 cm-1. These bands were assigned to phosphorus-oxygen stretching modes in the phosphate (31) Hadjiivanov, K. I.; Klissurski, D. G.; Davydov, A. A. J. Catal. 1989, 116, 498.
anion. There is a decrease in the intensity of this peak with vanadia addition indicating the reaction between vanadia and surface POx species. The Raman peak at 1050 cm-1 is clearly resolved in the V-Ti catalysts (Figure 4a) indicating the presence of isolated vanadyl species in the catalysts at lower vanadia concentrations. 51V NMR data are also consistent with Raman results confirming the presence of both tetrahedrally and octahedrally coordinated vanadia species in the V-Ti series of catalysts at and below 5.4 wt % vanadia contents and the presence of only octahedrally coordinated vanadia species in the V-PTi series of samples even at 1 wt % vanadia loading. With an increase in vanadia concentration in both series of catalysts, there is an appearance of peaks at 995 and 700 cm-1 indicating the presence of V2O5 microcrystallites. V2O5 crystallites are known to be considerably more Raman active than the surface species. However the persistence of a broad peak at about 1050 cm-1 even in the higher concentrations of vanadia in V-Ti and V-PTi series of catalysts indicates that isolated tetrahedral vanadia species are still present. Eckerdt and Wachs16 observed Raman shifts corresponding to surface vanadate species even up to 7 wt % loading on V2O5/TiO2 catalysts. Considering V-V bond distances they calculated the amount of vanadia for monolayer formation as 3.3 wt %, which shows that two monolayers of vanadia are formed before crystallization to V2O5. Ethanol partial oxidation was employed earlier as a model reaction to determine the activities of supported and unsupported vanadia catalysts.32,33 Oyama and Somorjai32 in their studies on silica-supported vanadia catalysts showed that ethanol oxidation is a structure insensitive reaction as they obtained similar activities even with an increase in vanadia concentration. At low temperatures acetaldehyde was the predominant product; with an increase in temperature they noticed acetic acid and at still higher temperatures COx and ethylene were observed as the major products. Quaranta et al.33 obtained higher activity and acetaldehyde selectivity in the ethanol oxidation reaction upon TiO2 addition to SiO2 before vanadia addition. The supports TiO2 (Ti) and phosphorus-modified TiO2 (PTi) exhibited very low conversions at reaction temperatures of 423-473 K. However with an increase in temperature to 498-523 K the activities of the supports (32) Oyama, S. T.; Somorjai, G. A. J. Phys. Chem. 1990, 94, 5022. (33) Quaranta, N. E.; Corberan, V. C.; Fierro, J. L. G. Stud. Surf. Sci. Catal. 1992, 72, 147.
5626 Langmuir, Vol. 13, No. 21, 1997
Alyea et al. Table 3. TOF for the Ethanol Partial Oxidation on V-Ti and V-PTi Catalysts at a Reaction Temperature of 398 K catalyst
TOFa (s-1)
catalyst
TOFa (s-1)
V-Ti 1 V-Ti 2 V-Ti 3
6.9 4.5 3.4
V-PTi 1 V-PTi 2 V-PTi 3
2.1 1.3 1.0
a
Figure 6. Percent conversions and percent selectivities to various products in ethanol oxidation reaction for the catalysts: (a) V-Ti 1; (b) V-PTi 1.
increased with PTi being more active than Ti. Titania support exhibited total selectivity to acetaldehyde at low conversions while at the higher reaction temperatures of 523 K traces of diethoxyethane, ethyl acetate, and acetic acid were observed. PTi exhibited about 25% selectivity to ether, at the reaction temperatures of 498 and 523 K, indicating an increase in acidity of the titania upon the addition of phosphoric acid. V2O5 itself also exhibited total selectivity to acetaldehyde at low reaction temperatures of 423 and 448 K; at and beyond 473 K the selectivity to acetic acid increased, reaching 20% at 523 K. Figure 5 shows ethanol partial oxidation rates of the V-Ti and V-PTi series of catalysts at a temperature of 398 K. Both series of catalysts exhibited total selectivity to acetaldehyde at this temperature. The reaction rates are higher in the V-Ti catalysts in comparison to V-PTi catalysts. The V-Ti 3 sample showed the highest
Based on amount of vanadia loaded.
conversion, which may be due to the well-dispersed monolayer of vanadia at this composition. There is a decrease in activity followed by a leveling off beyond 5.4 wt % vanadia loading which can be attributed to the presence of microcrystallites of V2O5. The influence of reaction temperature on the ethanol oxidation activity and selectivity to various products for the catalysts V-Ti 1 and V-PTi 1 is shown in Figure 6. With increase in the temperature, the selectivity to ether increased in the case of V-PTi 1 indicating the presence of exposed support sites which are selective for ether formation, whereas in V-Ti 1 the selectivity to acetic acid increased at higher reaction temperatures. The selectivity to acetic acid is much higher in the case of the V-Ti series of samples in comparison to the V-PTi series, at all the vanadia loadings studied. The turnover frequencies (TOFs) of the catalysts calculated from the amount of the vanadia loaded in the V-Ti and V-PTi samples are given in Table 3. The TOFs of the samples above the monolayer loadings are not calculated since microcrystallites of V2O5 were identified by FT-Raman spectroscopy. The turnover frequencies of the V-Ti catalysts are higher by 3 times compared to the V-PTi samples, indicating a decrease in the activity per site in the latter series of catalysts. According to Deo and Wachs23 the reactivities of the vanadia catalysts are dependent on the specific oxide support and thereby the V-O-support bond. TiO2- and ZrO2-supported vanadia catalysts were found to exhibit higher TOFs in comparison to other supports such as Al2O3, SiO2, and Nb2O5. Deo and Wachs23 also observed a decrease in the methanol partial oxidation activities of their phosphorus-modified titania supported vanadia catalysts, which was attributed to the formation of V-O-P bonds. Conclusions 51
V solid state NMR and FT-Raman studies indicate the presence of both tetrahedral and octahedral vanadia species in the V-Ti series of catalysts. Phosphate addition to the titania support prior to vanadia addition resulted in the formation of only octahedrally coordinated vanadia species (V-PTi series). ESR results are in conformity with the NMR and Raman results indicating a decrease in interaction between vanadia and titania upon phosphorus addition. Turnover frequencies of the V-PTi catalysts were found to be 3 times less than those for the V-Ti catalysts. LA9702190
