Langmuir 1999, 15, 3521-3528
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Synthesis, Characterization, and Activity Studies of Vanadia Supported on Zirconia and Phosphorus-Modified Zirconia L. Jhansi Lakshmi, Zhang Ju, and Elmer C. Alyea* Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario N1G 2W1, Canada Received August 25, 1998. In Final Form: January 26, 1999 A series of catalysts with vanadia contents varying between 1 and 14 wt % were prepared by the wet impregnation technique on zirconia and phosphorus-modified zirconia. The catalysts were characterized by employing X-ray diffraction (XRD), electron spin resonance (ESR) spectroscopy, 51V, 31P, and 1H solidstate magic angle spinning nuclear magnetic resonance (MAS NMR), Fourier transform Raman spectroscopy (FT-Raman), diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), and BrunauerEmmett-Teller (BET) surface area measurements. The activities of the catalysts were determined by an ethanol partial oxidation reaction. 51V solid-state NMR studies of both series of catalysts indicated the presence of tetrahedral vanadate species at lower loadings, and octahedrally coordinated vanadium species at higher concentrations. DRIFTS studies of the catalysts were in conformity with NMR studies as the vibrations corresponding to tetrahedral vanadate species were observed at low V2O5 loadings and at higher vanadia contents the vibrations corresponding to polymeric decavanadate species were seen. All of the XRD, ESR, 51V solid-state NMR, FT-Raman, and DRIFTS results indicated weaker interaction of vanadia with zirconia support upon phosphate modification. Activity studies were in agreement with spectroscopic data, as reflected in higher ethanol partial oxidation activities of the V2O5/ZrO2 catalysts.
Introduction Zirconia is used as a support and as a catalyst because of its high thermal stability and acid-base properties.1 It is known to catalyze reactions such as dehydration, hydrogenation, and alkane isomerization.2,3 The acidic and basic properties of the zirconia can be modified by the addition of cations or anions. Sulfate ion addition to zirconia results in an increase in the acidic properties, producing a solid superacid. Recently, much attention is being focused on the use of metal-promoted sulfated zirconia for the skeletal isomerization of alkanes, disproportionation reactions, Fischer-Tropsch synthesis, etc.4-8 Cr2O3 supported on ZrO2 was employed for hydrogenation of propene and oxidation of carbon monoxide.9,10 CuO/ZrO2 catalysts were employed in methanol synthesis from syngas.11 MoO3/ZrO2 catalysts were found to be highly active and selective in catalyzing the oxidation reactions of lower olefins and alcohols.12-14 Vanadia supported on * Author to whom correspondence should be addressed. Fax: (519) 766-1499. E-mail:
[email protected]. (1) Yamaguchi, T. Catal. Today 1994, 20, 199. (2) Nakano, Y.; Iizuka, T.; Hattori, H.; Tanabe, K. J. Catal. 1979, 59, 1. (3) Kono, J.; Domen, K.; Maryya, K.-I. J. Chem. Soc., Faraday. Trans. 1990, 86, 3021. (4) Song, X.; Sayari, A. Catal. Rev. Sci. Eng. 1996, 38, 329. (5) Fogash, K. B.; Larson, R. B.; Dumesic, J. A. J. Catal. 1996, 163, 138. (6) Tabora, J. E.; Davis, R. J. J. Catal. 1996, 162, 125. (7) Rezgui, S.; Gates, B. C. Catal. Lett. 1996, 40, 167. (8) Srinivasan, R.; Sparks, D. E.; Davis, B. H. Catal. Lett. 1996, 40, 167. (9) Wu, P.; Kershaw, R. P.; Dwight, K.; Wold, A. J. Mater. Sci. Lett. 1987, 6, 753. (10) Indovina, V. J. Mol. Catal. 1992, 75, 305. (11) Amenomiya, Y. Appl. Catal. 1987, 30, 57. (12) Wachs, I. E.; Hu, H. J. Phys. Chem. 1995, 99, 10911. (13) Miyata, H.; Tokuda, S.; Ono, T. J. Chem. Soc., Faraday. Trans. 1990, 86, 2291. (14) Miyata, H.; Tokuda, S.; Ono, T. J. Chem. Soc., Faraday. Trans. 1990, 86, 3659.
zirconia was employed for catalyzing reactions such as partial oxidation,15-17 oxidative dehydrogenation of propane,18 ammoxidation,19 and selective catalytic reduction of NOx.20,21 Different characterization techniques such as X-ray diffraction (XRD),22,23 electron spin resonance (ESR),23 51V solid-state nuclear magnetic reasonance (NMR),22,24 and laser Raman spectroscopy25,26 were employed earlier for the characterization of V2O5/ZrO2 catalysts. Much research has been focused in recent years on the characterization of supported vanadia catalysts by 51V solid-state nuclear magnetic resonance spectroscopy.22,27,28 The vanadium nucleus has the attributes of high natural abundance (99.8%), a large nuclear magnetic moment, and a short spin-lattice relaxation time (T1). Different experimental techniques such as static, magic angle spinning (MAS), and spin-echo pulse sequences were (15) Miyata, H.; Kohno, M.; Ono, T.; Ohno, T.; Hatayama, F. J. Chem. Soc., Faraday Trans. 1989, 85, 3663. (16) Sohn, J. R.; Pae, I.; Jo, S. G. React. Kinet. Catal. Lett. 1995, 55, 325. (17) Deo, G.; Wachs, I. E. J. Catal. 1994, 146, 323. (18) Khodokov, A.; Yang, J.; Bell, A. T. J. Catal. 1998, 177, 343. (19) Sanati, M.; Andersson, A.; Wallenberg, L. R. Appl. Catal. 1993, 106, 51. (20) Szakacs, S.; Altena, G. J.; Fransen, T. Catal. Today 1993, 16, 237. (21) Ohno, T.; Hatayama, F.; Toshino, K. Appl. Catal. 1994, 5, 89. (22) Sohn, J. R.; Cho, S. G.; Pae, Y. I.; Hayashi, S. J. Catal. 1996, 159, 170. (23) Chary, K.V. R.; Rao, B. R.; Subrahmanyam, V. S. Appl. Catal. 1991, 74, 1. (24) Lapina, O. B.; Mastikhin, V. M.; Shubin, A. A.; Krasilnikov, V. N.; Zamaraev, K. I. Prog. NMR Spectrosc., 1992, 24, 457. (25) Roozeboon, F.; Hazeleger, M. C.; Moulijn, J. A.; de Beer, V. H. J.; Gellings, P. J. J. Phys. Chem. 1980, 84, 2791. (26) Jehng, J.-M.; Deo, G.; Weckhuysen, B. M.; Wachs, I. E. J. Mol. Catal. 1996, 110, 41. (27) Eckert, H.; Wachs, I. E. J. Phys. Chem. 1989, 93, 6796. (28) Das, N.; Eckerdt, H.; Hu, H.; Wachs, I. E.; Walzer, J. F.; Feher, F. J. J. Phys. Chem. 1993, 97, 8240.
10.1021/la981103m CCC: $18.00 © 1999 American Chemical Society Published on Web 04/24/1999
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applied earlier to determine the various kinds of vanadia species, chemical shift anisotropy, and quadrupolar interactions in the supported vanadia catalysts. The nature of vanadia species on the supports was found to depend on the type of precursor used, the amount of V2O5, the chemical nature of the support material, the preparation technique, and more importantly the nature of pretreatment prior to recording the resonance spectra. At ambient conditions, the support surface will be in a hydrated state and vanadia species cannot be observed in the dispersed form but rather in coagulated hydrated amorphous V2O5like clusters. Eckert and Wachs,27 Lapina et al.,24 and Das and co-workers28 in their 51V NMR studies on supported V2O5 catalysts observed broadening of the peaks in the dehydrated samples because of removal of water molecules from the coordination sphere of vanadium. Soria and co-workers29 reported formation of surface vanadyl phosphates in V2O5/TiO2 catalysts upon phosphoric acid addition. Van Hengstum et al.30 and Zhu and Andersson31 in their studies on the influence of phosphorus additives on V2O5/TiO2 catalysts observed a decrease in activity and selectivity in the toluene oxidation reaction to benzoic acid. Deo and Wachs32 observed a slight decrease in the catalytic activity toward partial oxidation of methanol upon the addition of phosphorus to V2O5/TiO2 catalysts. They identified the formation of vanadyl phosphates by XRD when the P/V ratio in the catalysts is greater than 1.25. The decrease in catalytic activity was attributed to the poisoning of surface vanadium oxide sites because of the formation of V-O-P bonds with the surface phosphorus oxide (POx) phases. In our earlier investigation33 on vanadia catalysts supported on phosphate-modified titania for the partial oxidation of ethanol, we also found a decrease in the activity of the catalysts in comparison to V2O5/TiO2 catalysts. In the present investigation, we report the synthesis, characterization, and ethanol partial oxidation studies of V2O5 supported on ZrO2 and phosphorus-modified ZrO2. The characterization techniques employed are X-ray diffraction, electron spin resonance, 51V, 31P, and 1H solid-state NMR, and Fourier transform Raman (FT-Raman) and diffuse reflectance Fourier transform infrared (FT-IR) spectroscopies. The influence of phosphorus addition on the activity and selectivity of the V2O5/ZrO2 catalysts was tested for the partial oxidation of ethanol and correlated with the characterization data. Experimental Section A series of catalysts with vanadia contents varying between 1 and 14 wt % were prepared by impregnating zirconia support with an oxalic acid solution of V2O5 (Fisher Scientific). Commercial ZrO2 (Aldrich) of surface area 30 m2/g was used as a support. The support was calcined at 500 °C prior to its use. Phosphorus-modified zirconia (PZr; 1.6 wt %) was prepared by the addition of 2 mL of orthophosphoric acid in deionized water to 50 g of the support. The excess solution was evaporated slowly to dryness in a rotary evaporator. The resulting material was dried at 110 °C overnight followed by calcination at 500 °C 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 (29) Soria, J.; Conesa, J. C.; Lopez Granados, M.; Mariscal, R.; Fierro, J. L. G.; Garcia De La Banda, J. F.; Heinnemann, H. J. Catal. 1989, 120, 457. (30) van Hengstum, A. J.; Pranger, J.; van Ommen, J. G.; Gellings, P. J. Appl. Catal. 1984, 11, 317. (31) Zhu, J.; Andersson, S. L. T. J. Chem. Soc., Faraday Trans. 1989, 85, 3629. (32) Deo, G.; Wachs, I. E. J. Catal. 1994, 146, 335. (33) Alyea, E. C.; Lakshmi, J. L.; Ju, Z. Langmuir 1997, 13, 5621.
Lakshmi et al. 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 catalysts V/Zr, vanadia supported on zirconia, and V/PZr, vanadia-supported on phosphorusmodified zirconia, were dried at 110 °C overnight followed by calcination at 500 °C. The vanadia contents in the catalysts 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. ESR spectra were recorded at ambient temperature on a Varian 1000 Century series with 100 kHz modulation. The microwave frequency was 9.5 GHz; g values were referenced to diphenylpicrylhydrazyl. The calcined 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. 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 delay; typically 1500 scans were acquired for the samples. Chemical shifts were referenced to external VOCl3. 51V, 31P, and 1H MAS NMR experiments were carried out on a Bruker Avance DPX 300 multinuclear FT-NMR instrument. A standard bore Bruker MAS/ CPMAS probe with 4 mm zirconia rotors was used. The samples were dehydrated at 350 °C for 30 min in a flow of He prior to recording the 51V and 1H MAS NMR spectra. 51V static and MAS spectra were obtained at 78.9 MHz with a pulse length of 1 µs and relaxation delays of 1 s over a spectral window of 149 kHz. Chemical shifts were referenced to external VOCl3. MAS spectra were recorded by variable spinning speeds ranging from 6 to 10 kHz. 1H MAS NMR spectra were recorded at 300 MHz with a 30° pulse; the pulse length was 3 µs with a 1 s delay between the pulses over a spectral window of 120 kHz. The chemical shifts in ppm were referenced to external tetramethylsilane (TMS) using neat p-dioxane as a secondary reference. The samples were spun at 10 kHz, and 124 free induction decays (FIDs) were accumulated for each sample. 31P MAS NMR spectra were recorded at 121.5 MHz over a spectral window of 48.6 kHz, a pulse length of 1.33 µs, and a delay of 1 s. The samples were spun at 10 kHz, and typically 256 FIDs were collected for each sample. The chemical shifts were referenced to external 85% H3PO4 (δ ) 0 ppm). FT-Raman spectra were recorded on a Bruker FRA 106 FTRaman module interfaced to a Bruker IFS-66 FTIR bench. All Raman spectra were recorded at room temperature under ambient conditions 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 cooled with liquid nitrogen for optimum sensitivity. The samples were dehydrated in flowing dry air at 400 °C and then sealed in a vacuum prior to recording the spectra in the case of dehydrated samples. The number of scans for the lower concentrations of vanadia was 1000, and for higher loadings, the spectra were recorded after 100 scans. DRIFT spectra were acquired using a spectra tech DRIFT accessory “the collector” in an ATI Mattson Research Series FT-IR spectrometer equipped with a KBr beam splitter, a deuterated triglycerol sulfate (DTGS) detector, a spectral range of 6000400 cm-1, and a standard high-intensity source. The diffuse reflectance FT-IR spectra were recorded after evacuating the samples at 350 °C and under reduced pressures in a controlled environmental chamber, which was designed to be used with the collector DRIFT accessory. 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 of the catalysts for the partial oxidation of ethanol were carried out on an on-line microcatalytic fixed-bed reactor interfaced to the gas chromatograph with a six-way gas sampling valve. The activity data were obtained in the temperature range 150-250 °C by taking 200 mg of the catalyst packed in a fixed-bed tubular glass reactor of 6 mm i.d. Air was introduced into the reactor at a flow rate of 60 mL/min and saturated with ethanol (by passing through a saturator maintained at 25 °C). After a steady-state period of 30 min, the products were analyzed 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 product stream
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Table 1. Vanadia Composition and Surface Areas of V/Zr Catalysts catalyst V2O5/ZrO2
catalyst code
wt % V2O5a
surface area (BET), m2/g
V, atoms/nm2
1 2 3 4 5 6
Zr V/Zr-1 V/Zr-2 V/Zr-3 V/Zr-4 V/Zr-5 V/Zr-6
0.9 2.7 5.4 8.0 10.7 13.4
30 29 28 26 25 23 22
2.0 5.9 11.8 17.7 23.6 29.6
a
V2O5 contents of the catalysts determined from ICP analysis.
Table 2. Vanadia Composition and Surface Areas of V/PZr Catalysts catalyst V2O5/PZrO2
catalyst code
wt % V2O5a
surface area (BET), m2/g
V, atoms/nm2
1 2 3 4 5 6
PZr V/PZr-1 V/PZr-2 V/PZr-3 V/PZr-4 V/PZr-5 V/PZr-6
0.9 2.8 5.7 8.4 11.2 14.2
25 23 21 18 18 17 17
2.4 7.4 15.0 22.3 29.7 37.6
a
V2O5 contents of the catalysts determined from ICP analysis.
Figure 2. ESR spectra of V/PZr samples recorded at ambient conditions.
Figure 1. Pore size distributions of Zr and PZr supports. comprised mainly acetaldehyde with traces of ether, ethylene, ethyl acetate, diethoxyethane, acetic acid, CO, and CO2.
Results and Discussion The vanadia contents and BET surface areas of V/Zr and V/PZr catalysts are shown in Tables 1 and 2. Zr and PZr supports exhibited pore volumes of 0.14 and 0.13 cm3/ g, respectively. The pore size distributions of Zr and PZr are shown in Figure 1. It can be seen from the figure that with phosphorus addition to zirconia there is a decrease in the surface area which is expected because of the blockage of the pores on the Zr support surface. Addition of phosphorus to zirconia resulted in a shift in the pore size distribution to higher values. There is a further expected decrease in the surface area of the PZr support upon addition of vanadia, which is due to the addition of the second component. Wachs34 has shown from Raman spectroscopy that the monolayer coverage of V2O5 on various supports is between 6 and 8 atoms/nm2 except in (34) Wachs, I. E. Catal. Today 1996, 27, 437.
the case of SiO2-supported V2O5 catalysts. Therefore, the samples V/Zr-1,2 and V/PZr-1,2, with V2O5 loadings of 2.0, 5.9 and 2.4, 7.4 atoms/nm2, respectively, are below the monolayer. The XRD diffraction studies indicated the presence of only a monoclinic zirconia phase for both unmodified and phosphorus-modified zirconia. Small peaks corresponding to V2O5 microcrystallites were observed in the higher loaded samples V/Zr-6 and V/PZr-6. There is no evidence in the X-ray diffractograms of the V/PZr catalysts for the presence of vanadyl phosphate. However, the presence of microcrystallites of vanadia or vanadyl phosphate of less than 40 Å in size in the catalysts cannot be ruled out. ESR is a convenient technique to detect the presence of V4+ in the supported vanadia catalysts. Eight resolved hyperfine lines may result from interaction between the 3d1 electron (S ) 1/2) and the nuclear magnetic moment (I ) 7/2) of the vanadium atom. Vanadia supported on various carriers such as Al2O3, SiO2, TiO2, MgO, and ZrO2 has been characterized by employing the ESR technique by various investigators.23,35,36 The observance of clear hyperfine splitting is difficult in calcined samples if the spectra are recorded at ambient temperature, because of the predominance of spin-spin interactions or because of the presence of vanadium in an undistorted environment of oxygen atoms. When the recording temperature was lowered or when the sample was reduced in a hydrogen atmosphere, hyperfine splitting was observed. The ESR spectra of the V/PZr samples recorded at ambient temperature are shown in Figure 2. Well-resolved spectra (35) Sharma, V. K.; Wokaun, A.; Baiker, A. J. Phys. Chem. 1986, 90, 2715. (36) Davidson, A.; Che, M. J. Phys. Chem. 1992, 96, 9909.
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Figure 3. ESR spectra of V/Zr samples recorded at ambient conditions.
with clear hyperfine splitting could be seen, indicating the predominance of hyperfine interaction over the spinspin interaction. The presence of hyperfine splitting might be due to the presence of V4+ species in a distorted environment of oxygen atoms. V/Zr samples did not exhibit any hyperfine splitting under ambient temperature; however, when samples were evacuated at 250 °C, hyperfine splitting was observed, and the ESR spectra of these samples are shown in Figure 3. The absence of hyperfine structure at ambient temperature may be due to spin-spin interaction between closely spaced V4+ ions or may be due to the presence of vanadium in an undistorted environment of oxygen atoms. However, in the lower loaded sample V/Zr-1, hyperfine splitting was observed even without the outgassing treatment. Evacuating the samples under vacuum at 250 °C might have resulted in the removal of water molecules from the coordination sphere of V4+, causing a tetragonal distortion in symmetry and enabling the observance of hyperfine splitting at ambient temperature. The hyperfine structure became diffuse gradually at higher vanadia loadings, which may be due to increased spin-spin interaction between more closely spaced V4+ ions. The axially symmetric spin Hamiltonian parameters, B values calculated from the Hamiltonian parameters, and parallel and perpendicular g values calculated from the spectra for V/Zr and V/PZr catalysts are shown in Tables 3 and 4 as a function of catalyst composition. The values |g| ) 1.968 and |A| ) 118 are characteristic of vanadium in a distorted octahedral/square-pyramidal environment.36 Our average g values and hyperfine coupling constants (|g| and |A|) match with the values obtained by Chary et al.23 for reduced samples of V2O5 supported on zirconia. The higher values of B and hyperfine coupling constants A|| and A⊥
Figure 4. 52.6 MHz solid-state wide-line 51V NMR spectra of V/Zr samples under ambient conditions: (a) V/Zr-1, (b) V/Zr-2, (c) V/Zr-4, (d) V/Zr-6. Table 3. Spin Hamiltonian Parameters of V V/Zr Catalysts
4+
in the
catalyst
g|
g⊥
|g|
A|
A⊥
|A|
B
V/Zr-1 V/Zr-2 V/Zr-3 V/Zr-4 V/Zr-5
1.935 1.936 1.938 1.933 1.934
1.979 1.979 1.978 1.978 1.975
1.946 1.964 1.964 1.963 1.961
184 183 182 183 182
88 87 87 87 87
119 119 117 118 118
2.9 2.8 2.6 2.9 2.5
Table 4. Spin Hamiltonian Parameters of V4+ in the V/PZr Catalysts catalyst
g|
g⊥
|g|
A|
A⊥
|A|
B
V/PZr-1 V/PZr-2 V/PZr-3 V/PZr-4 V/PZr-5
1.938 1.925 1.924 1.926 1.924
1.992 1.988 1.987 1.987 1.987
1.974 1.968 1.969 1.967 1.969
193 201 202 201 201
78 76 76 77 77
111 119 118 118 119
6.1 5.4 5.1 5.1 5.1
for the V/PZr samples indicate weaker interaction of V2O5 with the PZr support in comparison to ZrO2. Thus, the strength of the VdO bond is higher in the case of V/PZr samples than in the case of V/Zr samples. Figures 4 and 5 show the wide-line 51V solid-state NMR spectra of the V/Zr and V/PZr calcined catalysts. It can be seen from the figures that at a lower loading of 0.9 wt % in both series of samples there is a broad peak centered at -500 ppm corresponding to the presence of vanadium in a distorted tetrahedral environment. With an increase in the V2O5 content, there is an increase in the intensity
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Langmuir, Vol. 15, No. 10, 1999 3525
Figure 6. 51V MAS NMR spectra of dehydrated V/Zr samples (at 10 kHz spinning rate). The central resonances are indicated by the symbol *: (a) V/Zr-1, (b) V/Zr-2, (c) V/Zr-3.
Figure 5. 52.6 MHz solid-state wide-line 51V NMR spectra of V/PZr samples under ambient conditions: (a) V/PZr-1, (b) V/PZr2, (c) V/PZr-4, (d) V/PZr-6.
of the peak at -310 ppm corresponding to octahedral vanadium species in the V/Zr and V/PZr series of catalysts. The presence of POx on the surface appears to favor the formation of octahedral vanadia sites. The static spectra of the V/Zr and V/PZr catalysts dehydrated at 350 °C exhibited a peak corresponding to octahedrally coordinated vanadium at -310 ppm, and a broad feature at -550 ppm was noticed even in the samples up to 12 wt % V2O5 loading. Figures 6 and 7 show the 51V MAS NMR spectra of the V/Zr and V/PZr samples dehydrated at 350 °C. The isotropic chemical shift was observed at -635 ppm in all of the samples except in the case of V/PZr-2, where a peak at -794 ppm corresponding to a tetrahedral vanadate species was noticed. 1H MAS NMR spectra of the V/Zr and V/PZr catalysts dehydrated at 350 °C are shown in Figures 8 and 9. The peaks were observed at 0.6 and 4.8 ppm in zirconia and at 3.2 and 6.6 ppm in phosphorus-modified zirconia. The peaks at 0.6, 3.2, and 4.8 ppm can be attributed to hydroxyl groups of zirconia, and the downfield peak at 6.6 ppm corresponds to acidic P-OH groups. Similar observations were made by Riemer et al.;37 they found peaks at 1.6 (37) Riemer, T.; Spielbauer, D.; Hunger, M.; Mekhemer, G. A. H.; Knozinger, H. J. Chem. Soc., Chem. Commun. 1994, 10, 1181.
Figure 7. 51V MAS NMR spectra of dehydrated V/PZr samples (at 10 kHz spinning rate). The central resonances are indicated by the symbol *: (a) V/PZr-1, (b) V/PZr-2, (c) V/PZr-3.
and 3.9 ppm for zirconia and at 5.9 ppm for sulfated zirconia. The downfield shift of about 2 ppm was attributed to an increase in the acidic nature of the zirconia upon sulfate addition. Kraus and Prins38 in their 1H MAS NMR investigations on Mo-P/Al2O3 catalysts reported peaks at -0.3 and 3.0 ppm. The first peak was assigned to basic Al-OH groups and the 3.0 ppm peak to P-OH groups. They observed that molybdate ions preferentially react with basic hydroxyl groups at lower loadings and with acidic hydroxyls groups at higher MoO3 loadings. DeCanio et al.,39 in their studies on phosphorus-modified Al2O3, attributed the signal at 1.6 ppm to the -OH groups of (38) Kraus, H.; Prins, R. J. Catal. 1996, 164, 260. (39) DeCanio, E. C.; Edwards, J. C.; Bruno, J. W. J. Catal. 1994, 148, 76.
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Figure 8. 1H MAS NMR spectra of dehydrated Zr and V/Zr samples (at 10 kHz spinning rate): (a) Zr, (b) V/Zr-1, (c) V/Zr-3, (d) V/Zr-4, (e) V/Zr-5. Figure 10. FT-Raman spectra of the V/Zr and V/PZr catalysts dehydrated at 400 °C in the presence of air (in the region 1300700 cm-1).
Figure 9. 1H MAS NMR spectra of dehydrated PZr and V/PZr samples (at 10 kHz spinning rate): (a) PZr, (b) V/PZr-1, (c) V/PZr-3, (d) V/PZr-4, (e) V/PZr-6.
monophosphate and the signal at 3.2 ppm to those of polyphosphate. Mastikhin et al.,40 in their studies on SiO2 surface modified with PCl3, observed four types of hydroxyl groups. The signal at 1.6 ppm was assigned to Si-OH groups; three signals at 2.2, 4.4, and 5.1 ppm were assigned to P-OH groups. 31P MAS NMR spectra of the PZr support and V/PZr catalysts exhibited a resonance at -23 ppm with a shoulder at -10 ppm. Mastikhin and Zamaraev,41 in their 31 P MAS NMR studies on H3PO4 interacted with SiO2, have shown that the resonance at -10 ppm corresponds to terminal phosphate groups on silica and the resonances at -30 and -45 ppm were attributed to bridged phosphate groups. On the basis of these observations, the resonance (40) Mastikhin, V. M.; Nosov, A. V.; Filimonova, S. V.; Terskikh, V. V.; Kotsarenko, N. S.; Shmachkova, V. P.; Kim, V. I. J. Mol. Catal. 1995, 101, 81. (41) Mastikhin, V. M.; Zamaraev, K. I. Appl. Magn. Reson. 1990, 1, 295.
at -23 ppm observed in the present PZr samples can be assigned to bridged phosphate groups and the shoulder peak at -10 ppm could be attributed to terminal phosphate groups. With the addition of vanadia to the PZr support, the latter peak decreased in intensity, which shows the interaction of V2O5 with the phosphate groups on zirconia. Laser Raman spectroscopy is increasingly being employed25,26,34,42 for the characterization of supported oxide catalysts because it facilitates the determination of useful structural information about the metal oxide species present on the support surface. The metal oxide species exhibit characteristic Raman shifts depending on their coordination environments. Isolated vanadia species corresponding to tetrahedral vanadium exhibit a characteristic Raman frequency near 1030 cm-1, indicating a very strong VdO bond. With an increase in the vanadia concentration in the supported catalyst, new Raman bands appear in the region of 800-1000 cm-1 that are attributable to the terminal VdO bonds of polymeric vanadia species. In the Raman spectra of the V/Zr and V/PZr catalysts, there is a gradual decrease in the intensity of the support peaks with an increase in the vanadia loading, which may be due to coverage of the support. Except for the Raman shift at ∼1059 cm-1, Zr and PZr supports exhibited common peaks. The peak at 1059 cm-1 observed in PZr may be due to PdO bonds in surface POx species formed on the modified support. The intensity of this peak decreased in the samples V/PZr-1 and V/PZr-2, indicating that vanadia is interacting with the phosphorus-modified sites on zirconia. Raman spectra of the samples V/Zr-1,2 and V/PZr-1,2 dehydrated at 400 °C are shown in Figure 10 expanded between 1200 and 700 cm-1. It can be seen from the figure that there are two broad peaks at 1049 and 995 cm-1 in the V/Zr-1 catalyst, indicating the presence of both monomeric and microcrystalline vanadia species. It should be mentioned here that the intensities of Raman bands of V2O5 microcrystallites are 5-10 times higher than those of the surface vanadia species.42 The observance of Raman peaks corresponding to V2O5 clusters at lower loadings may also be due to the low surface of the (42) Roozeboom, F.; Mittelmeijer-Hazeleger, M. C.; Moulijn, J. A.; Medema, J.; de Beer, V. H. J.; Gellings, P. J. J. Phys. Chem. 1980, 84, 2783.
Studies of Vanadia Supported on Zirconia
ZrO2 support. In the sample V/Zr-2, an increase in the intensity of the peak at 995 cm-1 and the appearance of a new peak near 700 cm-1 was observed, indicating the formation of V2O5 microcrystallites. In the V/PZr-1 sample there is a broad peak at 1050 cm-1, which may be due to phosphorus coordination as mentioned earlier, while the absence of a peak at 995 cm-1 indicates the presence of only monomeric tetrahedrally coordinated vanadyl species. The intensity of the 1050 cm-1 peak decreased in the V/PZr-2 sample, and the peaks at 995 and 700 cm-1 indicate the formation of microcrystallites of vanadia. FTRaman studies of the catalysts indicate formation of V2O5 clusters in both V/Zr and V/PZr catalysts with an increase in the vanadia content except in the lower loaded samples V/Zr-1 and V/PZr-1. Diffuse reflectance FT-IR studies of the PZr support indicated a peak at 1071 cm-1 corresponding to asymmetric stretching vibrations of PO43- tetrahedra.43 In the hydroxyl region, peaks were observed at 3455 and 3655 cm-1, which can be assigned to hydroxyl groups of zirconia. The zirconia support is reported to exhibit the vibrations corresponding to terminal and bridged hydroxyl groups at 3780 and 3680 cm-1. The DRIFT spectra of the V/Zr and V/PZr samples evacuated at 200 °C exhibited IR bands at 1021 and 867 cm-1 in the V/Zr samples corresponding to vibrations of VdO bonds. The absence of vibrations at lower wavenumbers, i.e., between 900 and 1000 cm-1, indicates the selective presence of polyvanadate species at all loadings studied. In the V/PZr samples the peaks at 1020 and 1077 cm-1 were observed to correspond to the stretching vibrations of VdO groups and phosphate species. With an increase in the V2O5 content, the intensity of the 1020 cm-1 vibration increased while the 1077 cm-1 peak decreased, which may be due to the interaction of the active component, i.e., vanadia with the phosphate groups. In the hydroxyl region the vibrations corresponding to Zr-OH groups were seen at 3687, 3505, and 3300 cm-1. DRIFT spectra of the V/Zr and V/PZr samples outgassed at 350 °C in the range 1300-700 cm-1 are shown in Figure 11. The vibrations corresponding to lower coordinated vanadia species which were not detectable in the 200 °C evacuated samples clearly became evident in these spectra in both series of catalysts. The peak at 923 cm-1 corresponds to tetrahedral VO43- species, and this peak is noticeable only in V/Zr-1 and V/PZr-1, 2 samples. In the samples V/Zr-3 and V/Zr-5, the vibrations at 924, 949, 984, and 1022 cm-1 correspond to decavanadate species.44 Oyama and Somorjai45 studied ethanol oxidation activities of supported vanadia catalysts. Ethanol oxidation was found be a structure-insensitive reaction by these authors because the activities and selectivities of the V2O5/ SiO2 catalysts were found to remain invariant even with an increase in the vanadia loading. At low temperatures total selectivity to acetaldehyde, at intermediate temperatures total selectivity to acetaldehyde and acetic acid, and at high temperatures total selectivity to COx and ethylene were observed. In the present investigation partial oxidation activities of the V/Zr and V/PZr catalysts were tested in ethanol oxidation as a model reaction. The supports ZrO2 (Zr) and phosphorus-modified ZrO2 (PZr) exhibited very low conversions at 150 and 175 °C (less than 4%). However, with an increase in the temperature to 200-250 °C, the activities of the supports increased up to 20%, with PZr being more active than ZrO2. Zirconia support exhibited total selectivity to acetaldehyde up to (43) Andersson, S. L. T. Appl. Catal. A 1994, 112, 209. (44) Day, V. W.; Klemperer, W. G.; Maltbie, D. J. J. Am. Chem. Soc. 1987, 109, 2991. (45) Oyama, S. T.; Somorjai, G. A. J. Phys. Chem. 1990, 94, 5022.
Langmuir, Vol. 15, No. 10, 1999 3527
Figure 11. Diffuse reflectance FT-IR spectra of the V/Zr and V/PZr catalysts evacuated at 350 °C (in the region 1300-700 cm-1).
200 °C, with traces of ether and diethoxyethane at 250 °C. PZr exhibited 90% selectivity to ether with traces of acetaldehyde, showing that acid-base properties of the ZrO2 support were considerably influenced by phosphoric acid impregnation; i.e., increased acidity of the support resulted in the formation of dehydrated product diethyl ether. V2O5 itself also exhibited total selectivity to acetaldehyde at low reaction temperatures of 150 and 175 °C; at and beyond 200 °C the selectivity to acetic acid increased, reaching 20% at 250 °C. The ethanol partial oxidation rates of catalysts V/Zr and V/PZr, at a reaction temperature of 150 °C, are shown in Figure 12. Both series of catalysts exhibited total
3528 Langmuir, Vol. 15, No. 10, 1999
Figure 12. Ethanol partial oxidation rates of the V/Zr and V/PZr catalysts at a reaction temperature of 150 °C (reaction conditions were given in the Experimental Section).
selectivity to acetaldehyde at 150 °C. With an increase in the temperature, the selectivity to ether increased in the case of V/PZr-1 whereas in V/Zr-1 the selectivity to acetic acid increased. The V/Zr-2 sample showed maximum conversion, which may be due to the well-dispersed monolayer of vanadia. There is a decrease in activity followed by leveling off beyond 6 wt % vanadia loading, which indicates the formation of agglomerates of V2O5. The influence of reaction temperature on the ethanol partial oxidation activity and selectivity to various products for the catalysts V/Zr and V/PZr are shown in Figure 13. The selectivity to ether of the V/PZr-1 catalyst indicates the presence of the exposed support sites, which are selective for the dehydration reaction. Its selectivity increased up to 225 °C, beyond which acetic acid selectivity was significant. The V/Zr-1 sample behaved just like bulk V2O5; i.e., with an increase in the temperature, the selectivity to acetic acid increased. The ZrO2 support by itself did not exhibit any selectivity to acetic acid. This confirms that the nature of active vanadia sites is different in the V/Zr and V/PZr series. The active sites for ethanol oxidation reaction may probably be V-O-Zr sites rather than V-O-PZr sites. The intensity of the peak corresponding to phosphorus sites on zirconia decreased upon vanadia addition to PZr, indicating vanadium-phosphorus interaction. With a gradual increase in the vanadia
Lakshmi et al.
Figure 13. Percent conversions and percent selectivities to various products in ethanol oxidation reaction for the V/Zr (s) and V/PZr (‚‚‚) catalysts.
content, the number of V-O-Zr sites might have increased (which is expected), resulting in higher activity. Deo and Wachs32 made similar observations for V2O5/TiO2 catalysts upon P2O5 addition and also attributed the decrease in activity to the formation of V-O-P bonds. Conclusions The 51V NMR and DRIFTS studies indicated the presence of isolated tetrahedrally coordinated vanadia species, in the V/Zr and V/PZr samples at submonolayer loadings. 1H MAS NMR studies indicated the presence of acidic hydroxyl P-OH groups in the phosphate-modified zirconia. The 1H NMR signal intensity decreased with an increase in the V2O5 loading in both the V/Zr and V/PZr series of catalysts. 31P MAS NMR studies indicated terminal and bridged phosphate groups in the PZr support; upon vanadia addition the signal corresponding to terminal phosphates decreased in intensity, indicating the formation of V-O-P bonds. The decrease in the activity of the V/PZr series of catalysts can be explained on the basis of the observation from spectroscopic studies of the formation of catalytically inactive V-O-P sites. LA981103M
