J. Phys. Chem. 1992,96,7076-7078
7076
Characterization of Vanadium Oxide Catalysts Supported on Ti02-Zr02 by SolibState 51V and 'H NMR Spectroscopy B. Mahipal Reddy,* E. Padmanabha Reddy, S. T. Srinivas, Catalysis Section, Indian Institute of Chemical Technology. Hyderabad 500 007, India
V. M. Mastikhin, A. V. NOSOV,and 0. B. Lapina Institute of Catalysis, Novosibirsk 630 090, Russia (Received: January 23, 1992; In Final Form: April 8, 1992)
A series of Ti02-Zr02 mixed oxide supported vanadia catalysts with various V2O5 loadings ranging from 1 to 16 wt W were prepared by a wet impregnation method and were characterized by means of solid-state slV and 'H NMR spectroscopic techniques. The solid-state 51VNMR spectra of V205/TiO2-ZrOZcatalysts reveal the existence of two types of dispersed surface vanadium oxide complexes in a tetrahedral oxygen environment at lower vanadium loadings and a tbird thrediensional crystalline V2O5 in distorted octahedral environment at higher vanadium contents. The proton NMR results provide evidence for the existence of metal oxide support interaction through the support surface hydroxyl groups.
deionized water to remove residual chloride ions, dried at 120 OC Introduction for 16 h, and calcined at 500 OC for 6 h in an open-air furnace. Vanadium oxide catalysts are well-known for selective oxidation The TiOz-Zr02 supported vanadia catalysts with various V20s of various hydrocarbons.'.2 However, to achieve good activity and loadings ranging from 2 to 16 wt% were prepared by the standard selectivity levels, V205should be dispersed on a suitable s ~ p p o r t . ~ ? ~ wet impregnation technique with stoichiometricaqueous solutions Titania, in the form of anatase,4' is considered to be the more of ammonium metavanade (Fluka, AR grade). The impregnated successful support for the phthalic anhydride production from samples were oven dried at 120 "Cfor 12 h and calcined at 500 o-xylene, as well as for the reduction of N O by NH3. Similarly, OC for 6 h in dry air atmosphere. highest activity in methanol oxidation has been observed using X-ray Diffraction. X-ray diffraction analysis were made on Z r 0 2 as the support.6 Thus, the supporting metal oxide plays a a Philips PW 1051 diffractometer with nickel filtered Cu Kru major role in determining the dispersion and activity of the VzO5 radiation. when supported. The combination of both support oxides (TiNuclear Magnetic Resonance Studies. 51VNMR spectra were 02-Zr02) form another class of interesting supports, whose adrecorded on a Bruker MSL-400 NMR spectrometer at a frequency vantages are yet to be explored for various industrial reactions. of 105.2 MHz (magnetic field 9.4 T) in the frequency range of The effect of support material on the structure and dispersion 250 kHz with a radio frequency pulse duration of 1 ps and a pulse of vanadium oxide has been investigated using several spectroscopic repetition rate of 0.1 Hz. The accumulation number of free technique^,^*^-'^ such as UV-vi~ible,~J' Raman,'-" ESR,8,'2 induction decay accounted ranged from lo3to lo5. Chemical shifts XPS,9,'3-'5and EXAFS,l6 etc. The technique of solid-state 51V were measured relative to V0Cl3 as an external reference. NMR represent a novel and promising approach to study these 'H MAS NMR spectra were obtained on a Bruker CXP-300 Since only the local environmentof the nucleus under spectrometer at a frequency of 300 MHz. The frequency range study is probed by NMR, this method is well suited for the was 50 kHz, (*/2) pulse duration was 5 ecs, and the pulse repetition structural analysis of disordered systems such as the two-difrequency was 0.2 Hz. Prior to 'HNMR measurements, the mensional surface vanadium oxide phase to be present on the samples were placed in a specially made NMR tubes and were support surfaces. Owing to a large magnetic moment, high natural evacuated Torr) at 250 OC for 12 h and sealed under vacabundance (99.76%),and favorable relaxation characteristic, the uum. The samples thus prepared were placed in quartz rotors slV nucleus is very amenable to solid-state NMR investigafor MAS. The rotation frequency of the rotor was 3.0-3.5 kHz. tions.'8-20 Similarly, the recent development of the magic angle The chemical shifts were measured relative to tetramethylsilane spinning (MAS) technique has made 'H MAS NMR spectroscopy (TMS) as an external reference. Total number of surface OH a powerful tool for characterizing Briinsted acidity of zeolites and groups were quantitatively estimated by measuring the area under related catalyst^.^^-^^ The main reason for its success is that the peak with reference to a known standard Si02 (1.1 X 1020 different types of OH groups can be determined from their difOH groups g-' sample) samplez4with an accuracy of f2. ferent resonance positions in the 'H MAS NMR spectra of the samples. Apart from the structural information provided NMR Result.# and Macussion methods, the direct proportionality of the signal intensity to the The Ti02-Zr02 mixed oxide support prepared according to the number of contributing nuclei makes NMR a useful technique procedure described in the Experimental Section had a N 2 BET for quantitative studies. In the present investigation the techniques surface area of 160 m2 g-I. This mixed oxide support was also of 51V and ' H MAS NMR have been utilized to characterize a found to be quite stable and uniform throughout the bulk.% When series of V205/Ti02-Zr02catalysts of various vanadia loadings. the mixed oxide support was calcined at various temperatures Experimental Section ranging from 500 to 800 OC, no X-ray diffraction lines due to Ti02 (anatase or rutile) and ZrO2 individual phases were observed. Catalyst Preparation. The Ti02-ZrOz ( 1 :1 molar ratio) mixed However, a definite Ti02-ZrOz compound crystalline phase oxide support was prepared by coprecipitation method by adopting (analogous to TiZr04) was noted from 600 OC and above temthe following p r o c e d ~ r e . ~An ~ .aqueous ~~ mixture solution conperatures and whose crystallinity was also increased with increase taining the required quantities of ZrOC12(Fluka, AR grade), TiC14 in calcination temperature. A similar observation was also made (Fluka, AR grade) and urea (Loba Chemie, GR grade) was heated earlier by Daly et aLZ5from XPS measurements that this particular to 95 OC with constant stimng. Precipitation was complete after method of preparation generally yields a uniform Ti02-Zr02 (1:l 2-3 h, at which time the pH of the solution was approximately molar ratio) mixed oxide support. The XRD patterns of Ti7. The precipitate was then filtered off, washed several times with 02-Zr02 mixed oxide support calcined at 500 and 800 OC temperatures with unsupported V,05 and V205/Ti02-Zr02catalysts To whom correspondence should be addressed. 0022-365419212096-7076503.0010 0 1992 American Chemical Society , I
,
The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 7077
Vanadium Oxide Catalysts
C
TiZrOL
A
B
I
'
0 "205
c
I I
I
'
T j
I I
I
f
1
! I [ I
1
I
- 1 1 1 4 , 1 - 1 1
I
I
I
I
40
30
20
6
2
1
o a
0
I 1t
--e Figure 1. X-ray diffraction patterns of Ti02-Zr02 mixed oxide support calcined at 500 (a) and 800 OC (b) and V2O5/TiO2-ZrO2catalysts of various V205 loadings.
of various V205loadings are shown in Figure 1. As can be noted from Figure 1, the Ti02-Zr02 binary oxide support calcined a t 500 OC is in amorphous state. However, upon heating of this sample a t 800 OC,the formation of TiZrO, compound can be noted. The V205/Ti02-Zr02 catalysts with a vanadia loading of 12 wt % and above show broad reflections due to crystalline V205phase. This observation suggests that vanadium oxide below 12 wt % is in a highly dispersed state or the crystallites formed are less than 4 nm in size, i.e., beyond the detection capability of the XRD technique. In fact, the monolayer capacity of Ti02-Zr02 support for vanadia appears to be between 10 and 12 wt % of V205,which is in agreement with the results of Bond et aLZ7 They found a monolayer capacity of 0.07 wt % V20S/m2 of T i 0 2 support. As expected from this calculation the XRD results also reveal the appearance of three-dimensional V2O5 structures on the Ti02-Zr02 support (SA 160 m2g-') beyond 11.2 wt 96 (monolayer coverage) of V205. The solid-state 51VNMR spectra of V20S/TiOz-Zr02catalysts obtained without preevacuation of the samples are shown in Figure 2. As can be noted from this figure, there are at least three types of distinct signals in the spectra of catalysts with varying intensities depending on V2OScontent on the support surface. Line A with a peak maximum at -540 ppm is the main signal in the spectrum of the sample containing 2 wt % V205. The increase in V205 content resulted appearance of additional signals B with a peak in the range from -640 to -670 ppm and C with 6, = -300 ppm and 6, = -1260 ppm respectively. The intensity of the latter signal increases with increase in V2O5 loading. A remarkable change in spectral positions and intensities can be noted upon evacuation Torr) of the samples at 250 OC for 2 h (Figure 3). Here again three types of distinct signals can be noted in the spectra, but two of them have their peak positions changed compared to the nonevacuated samples. Signal B' has a peak maximum in the range -660 to -680 ppm, while the signal A' is at -450 ppm, different from A, and the signal C' has parameters (6, = -310 ppm and 6,,= -1265 ppm) slightly different from those of signal C. Here again, species A' and B' dominate in the spectra of samples at lower VZOS loadings, while species C' appears in the spectra of samples containing large contents of VZO5. Different peak positions normally indicate the differences of the spectral parameters and are observed due to different local environments of vanadium nuclei.~8-z0*zs~zg Thus species A, A' and
-500
I
I
-1000
-1500
6,ppm
Figure 2. Solid-state slV NMR spectra of V20S/Ti02-Zr02catalysts.
Spectra were obtained without evacuation of the samples.
I
I
1
1
I
0
-500
,
-1000
1
-1500
6.ppm
Figure 3. Solid-state "V NMR spectra of V205/Ti02-Zr02catalysts obtained after evacuation of the samples at 250 OC for 2 h.
B, B' can be attributed to two types of tetrahedral vanadium complexes with different oxygen environments, while species C, C' are due to V atoms in distorted octahedral environment. This observation is in agreement with the structural models proposed by Bond and Tahir3 for V205/Ti02and related monolayer catalysts more recently. The latter lines (C and C') can unambiguously be attributed to hydrated and unhydrated forms of V2O5 on the support surface. A comparison of the spectra with the spectra of unsupported V2O5 and also from known vanadium compounds shows that this species is similar to that of V205.19920929 Of course, the presence of crystalline V205 a t higher vanadia contents is also noted from the XRD results. Further, the spectral parameters of the signals B and B' are very close to each other. Therefore, these lines can be attributed to surface tetrahedral complexes of vandium which do not contain OH groups or adsorbed water molecules in their first coordination sphere, since the evacuation treatment does not change their peak positions. On the contrary, the signals A and A' can be attributed to the surface vanadium complexes containing one or more OH groups
Reddy et al.
7078 The Journal of Physical Chemistry, Vol. 96, No. 17, 1992 3 0 1
A
stantial loss of Si-OH groups were observed when Si02 support was impregnated with v20532or Mo03.33 A small extent of decrease in NMR signal intensity due to Ti02-Zr02 surface OH groups for V20S/Ti02-Zr02catalysts may be due to the nonavailability of all surface OH groups to the interacting vanadia phase. Most probably, the hydroxyls in Ti02-Zr02 bindary oxide support are not at the exposed surface, but rather in the interior of the structure. Similar results were also observed when V 2 0 5 was impregnated on SnOz support.20 Thus, the present solid-state 51Vand IH NMR results show, at lower V2OSloadings, two types of surface tetrahedral vanadium complexes with different oxygen environments, and one of them is loosely bound to the support surface. At higher vanadia loadings in excess of monolayer coverage the vanadia species exists preferably as three-dimensional V2O5.
Acknowledgment. We thank Prof. K. I. Zamaraev, Director, Institute of Catalysis, Novosibirsk, Russia, for providing us the NMR facilities. E.P.R. and S.T.S. are the recipients of UGC and CSIR research fellowships respectively. Constructive suggestions by the referees are gratefully acknowledged. References and Notes
6.p p m
Figure 4. Solid-state 'HNMR spectra of hydroxyl groups of Ti02-Zr02 and V205/Ti02-Zr02samples.
or water molecules in their coordination sphere. This is evidenced by a change in their peak position upon evacuation. The decrease in the signal intensity of species A' on evacuation compared to that of A before evacuation clearly indicates that the evacuation treatment at 250 OC for 2 h resulted the transformation of loosely bound tetrahedral vanadium-oxygen complexes to a microcrystalline V2Os phase.21 The 'H MAS NMR spectra of Ti02-Zr02 support and V20S/Ti02-Zr02catalysts with various V205 loadings are shown in Figure 4. The IH NMR spectrum of Ti02-Zr02 support shows the presence of at least three overlapping signals with chemical shifts 6 = 3.0, 4.8, and 7.1 ppm respectively. The lines at 6 = 3.0 and 4.8 ppm can be attributed to two types of Zr-OH groups (ZrOz support is known to contain two types of distinct OH groups with chemical shifts 6 = 2.4 and 4.8 ppm, respecti~ely~~). Some part of the intensity of the signal at 6 = 3.0 ppm and a shoulder at 6 = 7.1 ppm are believed to belong to Ti-OH groups. (On Ti02 support two types of surface OH groups with 6 = 2.4 and 6.8 ppm, respectively, were f ~ u n d . ~The ~ * additional ~~) narrow line in the region 0.9-1.4 ppm is due to the traces of water molecules on the outer surface of the rotor and the sample t ~ b e s . As ~ ~can , ~ be ~ noted from Figure 4, vanadia appears to interact more strongly with Zr-OH groups (6 = 4.8 ppm), whose relative concentration is deminished upon increase in V205loading. Some part of vanadia also appears to interact with Ti-OH groups since the signal intensity at 6 = 7.1 ppm also vanished at small V205 contents. However, the peak in the high field (6 = 3.0 ppm) rest intact even at the highest vanadia loading. The total number of surface OH groups as a function of vanadia content is also shown in Figure 4. No substantial loss of Ti02-Zr02 support surface OH groups upon impregnation with V2O5 can be noted. In contrast, a sub-
(1) Hucknall, D. J. Selective Oxidation of Hydrocarbons; Academic Press: New York, 1974. (2) Gellinas. P. J. Caralvsis: Roval Societv of Chemistrv: London., 1985., Vol. 7, p 165: (3) Bond, G. C.; Tahir, S. F. Appl. C a r d 1991, 71, 1 and references therein. (4) Wachs, I. E.; Saleh, R. Y.; Chan, S. S.; Chersich, C. Chemrech 1985, 756. (5) Wong, W. C.; Nobe, K. Ind. Eng. Chem. Prod. Res. Dev, 1984, 23, 564. (6) Roozeboom, F.; Cordingley, P. D.; Gellings, P. J. J. Card 1981, 68, 464. (7) Hausinger, G.; Schmelz, H.; Knijzinger, H. Appl. Caral. 1988,39,267. (8) Busca, G.; Centi, G.; Marchetti, L.; Trifiro, F. Langmuir 1986,2,568. (9) Nieto, J. M. L.; Kremenic, G.; Fierro, J. L. G. Appl. Caral. 1990,61, 235. (10) Wachs, I. W. Chem. Eng. Sci. 1990,45, 2561. (11) Oyama, S.T.; Went, G. T.; Lewis, K. B.; Bell, A. T.; Somorjai, G. A. J. Phys. Chem. 1989, 93,6786. (12) Chary, K. V. R.; Reddy, B. M.; Nag, N. K.; Sunandana, C. S.; Subrahmanyam, V. S. J . Phys. Chem. 1984,88, 2622. (13) Anderson, S.L. T. J. Chem. Soc., Faraday Trans. 1 1979,75,1357. (14) Bond, G. C.; Zurita, J. P.; Flamerz, S.Appl. Caral. 1986, 27, 353. (15) Nag, N. K.; Massoth, F. E. J . Card. 1990, 124, 127. (16) Kozlowski, R.; Pettifer, R. F.; Thomas, J. M. J . Phys. Chem. 1983, 87, 5176. (17) Zamaraev, K. I.; Mastikhin, V. M. Colloids Surf. 1984, 12, 401. (18) Eckert, H.; Wachs, I. E. J . Phys. Chem. 1989,93,6796. Eckert H.; Wachs, I. E. Mater. Res. Sot. Symp. Proc. 1988, 1 1 1 , 455. (19) Le Cbtumer, L. R.; Taouk, B.; Le Meur, M.; Payen, E.; Guelton, M.; Grimblot, J. J . Phys. Chem. 1988, 92, 1230. (20) Narsimha, K.; Reddy, B. M.; Rao, P. K.; Mastikhin, V. M. J . Phys. Chem. 1990, 94, 7336. (21) Mastikhin, V. M.; Lapina, 0. B.; Shubin, A. A.; Krasilnikov, V. N.; Zamaraev. K. I. Caral. Rev. Sci. E m . . submitted. (22) Clague, A. D. H. Catalysis; koyal Society of Chemistry: London, 1985; Vol. 7, p 61. (23) Freudo, D.; Hunger, M.; Pfeifer, H. 2.Phys. Chem. (Munich) 1987, IC1
1-1
I J L , 111.
(24) Reddy, B. M.; Mastikhin, V. M. In Proceedings ofrhe 9rh International Congress on Caralysis; Phillips, M. J., Ternan, M., Eds.; Chemical Institute of Canada: Ottawa, 1988; Vol. 1, p 82. (25) Daly, F. P.; Ando, H.; Schmitt, J. L.; Strum, E. A. J. Caral. 1987, 108, 401. (26) Reddy, B. M.; Manohar, B.; Mchdi, S. J . Solid State Chem. 1992, 97, 233. (27) Bond, G. C.; Zurita, J. P.; Flamerz, S.;Gellings, P. J.; Basch, H.; Van Ommen, J. G.; Kip, B. J. Appl. Cural. 1986, 22, 361. (281 Chary, K.V. R.; Rao, V. V.; Mastikhin, V. M. J. Chem. Sot., Chem. Commun. 1989, 202. (29) Sobalik. Z.: LaDina. 0. B.: Novaorodova. 0. N.: Mastikhin. V. M. Appl. Card. 1990, 63, -191. (30) Mastikhin, V. M.; Zamaraev, K. I. Appl. Magn. Reson. 1990, I , 295. (31) Mastikhin, V. M.; Nosov, A. V., unpublished results. (32) Reddy, B. M.;Narsimha, K.; Rao, P. K.; Mastikhin, V. M. J . Caral. 1989, 118, 22. (33) Reddy, B. M.; Rao, K. S.P.; Mastikhin, V. M. J . Coral. 1988, 113, 556.
