5752
J . Phys. Chem. 1986,90, 5752-5755
Dynamical Change In the Crystal FleM around the V(1V) Ion on 110, (Rutile) Surface Accompanled by the Interaction with Adsorbed Oxygen Molecules Yoshiya Kera* and Yoshihiro Matsukaze Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, Higashiosaka, Osaka 577, Japan (Received: August 16, 1985; In Final Form: May 21, 1986)
A broad ESR spectrum with an unresolved hyperfine splitting (hfs) was observed in Ti02 (rutile) deposited with vanadium oxide (V(1V)-TiO,) at 77-298 K under vacuum. Upon the introduction of oxygen gas, a very sharp spectrum with eight hf limes (slV nucleus, I = 7/2) appeared at 77 K. The sharp spectrum, however, broadened out with an increase in the temperature. The hfs disappeared completely at 145 K and at 298 K it changed to a broad spectrum. The change was reversible. A computer simulation for the broad and sharp spectra was satisfactorily carried out by using the following parameters: g, = 1.983, gll = 1.950, A , = 5 5 G, All = 173.1 G, and the line width (d)= 35 G; and g, = 1.931, 1.922, gll = 1.972, A, = 46, 47 G, All= 152.2 G, and d = 7 G, respectively. The broad spectrum was assigned to the V(1V) ion that replaced the Ti ion site on the surface, the crystal field symmetry of which was ascribed to a distorted square pyramid, and the sharp spectrum was assigned to the V(1V) ion placed in a field of octahedral symmetry. Based on these results, it is suggested that the crystal field around the V(1V) ion changes dynamically from the distorted square pyramid to the octahedral symmetry accompanied by oxygen molecule adsorption. Part of an electron on the V(1V) ion evidently transferred to the oxygen molecule through some interaction. Similar ESR changes were also found in the cases of N 2 0 and NO gas adsorptions on V(1V)-TiO,.
Introduction The deposited state of supported metal oxide catalysts and the adsorbed state of reactant gases on them have been effectively investigated by using ESR spectrometry.L*2 Many authors have studied the vanadium oxide catalyst, using the slV nucleus ( I = ' / J as a probe for the study of the vanadium(1V) i~n.~-'OWe have previously found that the g value was greatly influenced by the matrix into which the vanadium(1V) ion was placed. It decreased in the sequence V205 V307 V6OI3 V02,11and in fact, the crystal field symmetry around the V(1V) ion was shown to change from a distorted square pyramid in V205to an octahedron in VOz. Considering the vanadium(1V) ion doped into the single crystal of titania (rutile and anatase types), the g tensors have been extensively a n a l y ~ e d . ' ~ JThese ~ results suggest that the field around the V(1V) ion belongs to an octahedral symmetry in the rutile and is considerably distorted toward tetrahedral symmetry in the anatase, as expected, from the original crystalline lattice of titania.I4 Recently, we prepared vanadium oxide catalysts supported on various carriers by the impregnation of the oxyvanadium ion under a controlled pH.I5 In order to examine the deposited state of vanadium oxide, ESR spectrometry was utilized. Based on gtensor analysis, the V(1V) ion appeared into the metal ion site on the carrier surface in almost all the catalysts in which the deposition was limited to less than a monolayer.I6 With the
- - -
(1) Lunsford, J. H. Adv. Catal. 1972, 22, 265;Catal. Rev. 1973,8 , 135. (2) Van Reijen, L. L.; Cossee, P. Discuss. Faraday Soc. 1966, 41, 277. (3) Shvets, V. A.; Vorotyntsev, V. M.; Kazansky, V. B. Kinet. Karal. 1969, 10, 356. Shvets, V. A.; Kazansky, V. B. J . Coral. 1972, 25, 123. Kazansky, V. B.;Pershin, A. N.; Shelimov, B. N. Proc. Int. Congr. Catal., 7th, Tokyo, 1980; 1980, B-39. (4) Yoshida, S.; Iguchi, T.; Ishida, S.; Tarama, K. Bull. Chem. Sac. Jpn. 1972, 45, 376. (5) Takahashi, H.; Shiotani, M.; Kobayashi, H.; Sohoma, J. J. Catal. 1969, 14, 134. (6)Khube, K. C.; Mann, R. S.; Manoogian, A. J . Chem. Phys. 1974,60, 4810. (7)Chien, J. C.W. J. Am. Chem. Sac. 1971, 93,4675. (8) Akimoto, M.; Usami, M.; Echigoya, E. Bull. Chem. Sac. Jpn. 1978, 51, 2195. (9) Bond, G. C.; Sarkany, A. J.; Parfitt, G. D. J . Catal. 1976, 57, 476. (10) Inomata, M.; Mori, K.; Miyamoto, A.; Ui, T.; Murakami, Y. J . Phys. Chem. 1983, 87, 754. (11) Kera, Y.; Kuwata, K. Bull. Chem. Sac. Jpn. 1979, 52, 1268. (12)Gerritsen, H.J.; Lewis, H.R. Phys. Rev. 1960, 119, 1010. (13) Grunin, V. S.;Ioffe, V. A.; Patrina, I. V.; Davityan, G. D. Sou. Phys. Solid State 1976, 17, 2012. (14)Huckel, W.Structural Chemistry of Inorganic Compounds; Elsevier: New York, 1951;Vol. 11, pp 675-678. (15) Roozcboom, F.; Fansen, T.; Mars, P.; Gellings, P. J. Z.Anorg. Allg. Chem. 1979, 449, 25.
vanadium oxide catalyst supported on T i 0 2 (rutile) we found a broad spectrum with unresolved hfs which changed to a new, sharp spectrum with eight clear hf lines at 77 K in the presence of oxygen gas. We could never observe such sharp spectrum at 298 K. This change was reversible when oxygen was introduced into or evacuated from the system. Similar ESR changes were also seen during N 2 0 and N O gas adsorptions at -77 K. In this paper, we will describe in detail the ESR changes during adsorption of the gas molecule and/or molecular ions. On the basis of a computer simulation and a g-tensor analysis, we will show that these changes are caused by a modulation of the surface crystal field around the V(1V) ion with a dynamical interaction of these adsorbed gas molecules.
Experimental Section I . Materials and Procedures. TiOz(rutile), as a carrier, was prepared by heating T i 0 2 (anatase type, Kanto Chem. Co. Ltd., guaranteed grade) at 1100 O C for 3 h. Eight grams of the TiO, (rutile) powder, which was pulverized and sieved between 100 and 170 mesh sizes, was packed into a glass tube (inner diameter: 15 mm). NH4V03(Kanto Chem. Co. Ltd., guaranteed grade) was dissolved in water (ca. 0.5 wt %) and the pH was controlled at 2.0 by H N 0 3 . In order to make the vanadium ion adsorb fully, the NH4V03solution was poured into a column packed with the T i 0 2 until both the pH and vanadium ion concentration of the effluent solution had returned to their original values. After the adsorption, the Ti02 was dried at 110 OC for 8 h and then calcined at 400 O C for 2 h in order to prepare the vandium oxide catalyst supported on Ti02. The catalyst was placed into a quartz ESR measurement tube having an inner diameter of 4 mm. The tube was evacuated below lo-" Torr at 470 OC for 1 h, and sealed with or without gases such as 02,NO, NzO, and NOz. 2. Measurements and Simulation. Surface area was measured by the usual BET method of N2 gas adsorption at 77 K. The concentration of oxyvanadium ion was determined by the Mohr salt method (iron(I1) ammonium sulfate titration; J I S G1221). The characterization of the carrier was done by X-ray powder diffraction analysis. The diffractometer was a Rigaku Denki Model GF-Rad-yA, using Cu Kar radiation and a Ni filter. ESR spectrum was recorded at 77-298 K in a magnetic field of 2500-4200 G (v = 9.3 GHz, 100-kHz modulation). The spectrometer was a Nippon Denshi Model JES-PE (X-band). Computer simulation for ESR spectrum was done using an IBM 4381 system, described in more detail later in this report. (16)Kera, Y.;Matsukaze, Y.; Inoue, T. Bull. Chem. Sac. Jpn., submitted for publication.
0022-3654/86/2090-5752$01.50/00 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5753
Crystal Field around V(1V) Ion on TiOz
-
V( IV) Ti09
V(IV). TI02
at 7 T K
a
Evac. at R. T.
A I
298 K
' I
.
.
.
,
2500
I
.
3x0
.
9.
.
,
l
.
3500
,
.
He /G
,
I
.
.
4000
Figure 1. Temperaturedependence of the ESR spectrum for V(1V)-Ti02
evacuated previously at 470 OC. The arrow points out the position of free spin; go = 2.0023. 05 :V(IV)
- Ti02 208 K
245 K
I
b
S
I
.
I
.
.
3500
3000
.
.
.
)
.
4000
k / G
Figure 3. ESR spectrum of V(1V)-Ti02 containing various gases at 77 K: (a) after an evacuation at room temperature; (b) in 5 Torr of NO2, added after an evacuation at 400 OC;(c) in 5 Torr of N20;and (d) in 5 Torr of NO.
C
TABLE I: The Best Fitting Parameters for the ESR Spectra of V(IV)-Ti02 and 02/V(IV)-Ti02 g tensor A tensor/G
sample
Rl 1.983
02/V(IV)-Ti02
1.931, 1.922
RII
A,
A 11
1.950 1.972
55 46, 47
173.1 152.2
a broad spectrum similar to that in the evacuated sample as seen in Figure 2a-f. When V(IV)-TiO2 was evacuated at room temperature, a mixed spectrum of broad and sharp peaks appeared at 77 K as shown in Figure 3a. A similar spectrum was also observed at 77 K when N20 and NO gases were introduced into V(1V)-TiOz, which had been previously evacuated at 400 O C for 1 h. They are shown in Figure 3c,d. Upon the addition of NOz gas only the broad spectrum was observed (Figure 3b).
f
I
V( IV)-Ti02
. _ . .I . . . .
2500
3000
3500
4ooo H. /G
Figure 2. Temperature dependence of the ESR spectrum for 0 2 / V (IV)-Ti02 under O2 gas (7 Torr).
The T i 0 2 was identified as rutile by X-ray powder diffraction. The BET surface area of the T i 0 2 (rutile) was 2.0 m2/g. The amount of the vanadium oxide (as V205)deposited was estimated to be 0.73 mg/g of carrier. The surface coverage was 0 = 0.66. Ti02 deposited with the vanadium oxide (V(1V)-TiO,) gave a broad ESR spectrum upon evacuation below 10-4 Torr at 470 O C for 1 h, although the T i 0 2 itself did not show any ESR signal. The spectrum did not change when the measurement temperature was raised from 77 to 298 K except for the intensity as shown in Figure la-c. Upon the addition of oxygen gas at 7 Torr, a sharp spectrum with 8 hf lines was obtained a t 77 K as seen in Figure 2f. However, the sharp spectrum with the hfs varied greatly when the temperature was raised. The hfs broadened out and completely disappeared at 145 K. At 298 K, it changed to
Discussion 1 . Computer Simulation. The broad spectrum appears to contain the eight hf lines of the 51Vnucleus. In order clarify this point a computer simulation for the line shape due to the V(1V) ions randomly oriented on the Ti02 powder surface was tried. The first derivative curve was calculated according to the usual simulation m e t h ~ d , ' ~at , ' ~intervals of 10 G in the magnetic field of 2500-4200 G. The intensity of the microwave power absorbed at each resonance field was obtained by a numerical integration at intervals of 1.5O to 0 and 9 for all directions under the assumption of a Gaussian shape function and 35 G of the half-width. Upon the application of the ESR parameters, gL = 1.983, gll = 1.950, A , = 55 G, and All = 173.1 G, given in Table I, we found (17)Sands, R.H.Phys. Reu. 1955,99,1222. Kneubuhl, F. K. J . Chem. Phys. 1960,33, 1074. Lefebvre, R.; Maruani, J. J . Chem. Phys. 1965,42, 1480. (18) Bleaney, J. Philos. Mag. 1951,42,441.Rockenbauer, A.;Simon, P. J . Mag. Reson. 1973,J J , 217. Iwasaki, M. J . Magn. Reson. 1974,16,417. Ishida, S.;Magatani, Y.; Kobayashi, Y.; Yoshida, S.Annu. Meeting Card. SOC.Jpn., (Fukuoka, 1979) 1979,4R01.
5754 The Journal of Physical Chemistry, Vol. 90, No. 22, 19186 v(Iv)at
TI 02 K
rr
- Ob*.
n
... Slmul.
- - A d b........ .. __.... .. ..__....'
,e.,
I
.
.
.
- . . ,9- ;
: ',i
~
.
I
__.. ..,.... - .. ._......._ .... ..-
.
.
.
.
I 4Ooo
3500
3000
H./G
Figure 4. The best-fit spectrum simulated for V(IV)-Ti02:
- - -,simu-
lated and -, observed. 02 : V ( I V l - T I 0 2 af 7 7 1
- 005
..-.... SIYUL
I
.
.
3000
. 3500
4000 H./G
Figure 5. The best-fit spectrum simulated for 02/V(IV)-Ti02:
- - -,
simulated and -, observed. the best fit to the observed spectrum as shown in Figure 4. The base line was slightly corrected with a sinelike curve near the center in this simulation. With respect to the sharp spectrum, the simulation was most satisfactorily carrier out under the assumptions of a Lorentzian-type shape function19and 7 G of the half-width. The best set of ESR parameters was gi = 1.931, 1.922, gll= 1.972, A , = 46, 47 G, and All = 152.2 G, as also given in Table I. The spectrum was simulated a t intervals of 5 G in the field of 2800-4000 G. It is shown with the observed spectrum in Figure 5. The positions of the sharp lines in the simulated spectrum compare closely with those in the observed one. However, the consistency in the intensity was not as good especially a t the stronger peaks. Also the base line of the observed spectrum deviated from the simulated spectrum over the wide range. We will not discuss these differences further. 2. Crystal Field around V(II/)Ion Deposited on T i 4 (Rutile). We have previously found a clear ESR spectrum due to V(IV) ion in a V2O5 single crystal which was prepared from a purified fully V205 melt. The g value was determined as g, = 1.978 and gll= 1.932." On the other hand, for V(IV) ion contained in a V 0 2 single crystal (rutile type), g, = 1.895, 1.930, and gll = 1.925 have been reported.20 We have previously suggested that in a change of the crystal field of the V(IV) ion from distorted square pyramidal (V2O5 matrix) to octahedral (rutile-type matrix), the g , tends to greatly decrease, while the glldecreases only slightly." With V(1V) ion doped into a T i 0 2 (rutile) single crystal, g, = 1.915, 1.912, and gll= 1.956 have been reported.', In fact, this value corresponds well to that for the V 0 2 single crystal (rutile type) rather than the V205.crystal. In order to estimate the crystal field splitting of the d-orbitals corresponding to these g value, we (19) Carrington, A.; McLachlan, A. D. Introduction to Magnetic Resonance with Applications to Chemistry and Chemical Physics; Harper and Row: New York, 1967; Chapter 11. (20) Grunin, V. S.;Ioffe, V. A.; Patrina, I. 8. Phys. Status Solidi B 1974, 63, 629.
Kera and Matsukaze applied the following familiar equations: g,, = go - 4X/A, g,, = go- X/61, and gyy= go - h/S2,where h is the spin-orbit coupling constant.2'*22 If X is reasonably assumed to be 250 cm-1,22the 6- and A-values for the V20s crystal (g, = 1.978 and gl,= 1.932) are estimated to be 1.03 and 1.42 pm-', respectively. Similarly, the J1, S2, and A for the TiO, crystal (g, = 1.915, 1.912, and gll = 1.956) are estimated to be 2.8, 2.7, and 20.3 cm-', respectively. These splittings compare with those given by S h i m i z ~who , ~ ~have also given an exact energy diagram for the d-orbitals of V(IV) ion incorporated into the rutile-type crystal such as Sn0224and Ge02.25 The coordination models and the d-orbital splittings for the V(IV) ion replaced into the positive ion site of TiO,(rutile) and V2O5 crystalline lattice are summarized as parts a and b of Figures 6, respectively. Many authors have investigated the deposited state of vanadium oxide supported on T i 0 2 powder in connection with its catalytic properties.8-'0.26 Bond et al.9 reported the g values as g , = 1.980 and gll = 1.940, which correspond well to those for the present V(IV)-Ti02 (g, = 1.983 and gll= 1.950). The deposited state, therefore, seems to be analogous to that of our catalyst. Akimoto et aL8have reported the values g , = 1.922 and gll= 1.983. These correspond to our values for the V(1V) ion supported on TiO, (anatase), g, = 1.912 and gll= 1.981,16rather than the present V(IV)-Ti02, since their carrier (TiO,) belongs to the anatase type. Inomata et aLIO observed an ESR spectrum with unresolved hfs for vanadium oxide supported on rutile and anatase. The g value was the same in both cases, ( g ) = 1.971. The value is close to that in the V2O5 matrix rather than the TiO,. Thus, the surface of their catalysts appears to be covered by a multilayer of vanadium oxide. Rusiecka et al.27prepared the V205-rutile catalysts with the different vanadia content of 0.8-20%. They have reported for the samples an ESR spectrum composed of an assembly of narrow lines (gi = 2.00 and gll = 1.96) superimposed on a broad signal ( ( g ) = 1.975). They assigned the broad and narrow components to V4+ ion in the vanadium oxide deposited and vanadyl groups (V=O)2+ dispersed on rutile, respectively. The broad component seems to correspond to the spectrum of Inomata et al.IO 3. Crystal Field Change with O2 Adsorption. The g values for the present V(IV)-Ti02 corresponds to those for the V,O, crystal (gl = 1.978 and gll = 1.932)" rather than the T i 0 2 crystal.12 On the other hand, the gvalues for V(IV)-Ti02 in the presence of adsorbed O2correspond to those for the TiO, crystal (g, = 1.915, 1.922, and gl, = 1.956)12 rather than the V205 crystal." The estimated crystal field splittings and the proposed coordination models are shown in Figure 6c,d. This change in the energy diagram, Figure 6, part c to part d, corresponds well to the change of Figure 6 part b to part a. The dYzand d,, levels decrease and the dxylevel increases. Therefore, we assume that, accompanied by oxygen adsorption on the V(IV) ion, the crystal field symmetry changes from a distorted square pyramid to an octahedron, as shown by the coordination models in Figure 6. Accompanied by the oxygen adsorption, the hf-coupling constant A. (= I13(Ag+ 2A,)), decreases from 94.3 G (for the broad spectrum) to 81.7 G (for the sharp one), as evaluated from Table I. Therefore, a part of an electron in the V(1V) ion should be transferred to the oxygen molecule, although only to a small degree. The sharp spectrum with hfs observed at 77 K in the presence of oxygen gas was replaced at 298 K with a broad spectrum, which was always observed in the evacuated sample, as previously mentioned. During a rise in temperature the hfs was completely broadened out at 145 K as shown in Figure 2e. Thus, a so-called adsorption life time for the interaction of the (21) Pryce, M. H. L. Proc. Phys. SOC.1950, A63, 25. ( 2 2 ) Pake, G. C. Paramagnetic Resonance, W. A. Benjamin: New York, 1962. (23) Shimizu, T.J. Phys. SOC.Jpn. 1967, 23, 848. (24) Kikuchi, C.; Chen, I.; From, W. H.; Dorain, P. B. J . Chem. Phys. 1965, 42, 181.
( 2 5 ) Siegel, I. Phys. Reu. 1963, 134, A193. (26) Wachs, I. E.; Saleh, R. Y.; Chan, S.S.; Chersich, C . C. Appl. Catal. 1985, 15, 339. (27) Rusiecka, M.; Grzybowska, B.; Gasior, M. Appl. Curul. 1984, IO, 101.
The Journal of Physical Chemistry, Vol. 90, No. 22, 1986 5155
Crystal Field around V(1V) Ion on TiOz
(C
(0)
1.915, 1.912
g,
Qn
(d 1
1
8
1.956
P ,
I I
.-‘
Q
A
b
---
---= I
- - _ - --
I
33
3.s
3.1
Figure 6. Coordination model and crystal field splitting for the surface V(IV) ion in V(IV)-Ti02 and 02/Ti02; those for the V(IV) ion in Ti02(rutile)12 and V2OS1’single crystals are given for comparison in (a) and (b), respectively.
oxygen molecule with the surface V(1V) ion can be postulated to become shorter than the time scale of the measurement (microwave frequency used: 9.3 GHz) at temperatures higher than 145 K.I9 A spectrum composed of both the sharp and broad spectra was observed at 77 K in the sample evacuated at room temperature, as shown in Figure 3a. The sharp spectrum is thought to be caused by air remaining on the sample’s surface because of improper evacuation. In other words, we could observe the broad spectrum with the sharp one because the remaining oxygen’s concentration was too low to simultaneously occupy all the open V(IV) ion sites on the surface. A similar composite spectrum appeared at 77 K for the NzO and NO additions while for NOz, only the broad spectrum appeared, as shown in Figure 3b,d. The intensity ratio of the sharp spectrum to the broad one seems to be large in the case of the NzO and NO additions and different in the case of the evacuation at room temperature. The vapor pressure of NO, 1 X lo-’, and 2 X N20, and NOz is ca. 5 X Torr, respectively, at 77 K.” From these data we can understand the reason why the sharp spectrum has appeared in the NO addition but not in NO2 addition. However, we do not expect the appearance of the sharp spectrum for the N 2 0 addition because of its quite low vapor pressure. Probably, the sharp signal would
be caused by oxygen produced by the decomposition of N20 on the surface, since it can be easily decomposed on such a catalyst.z9 The hf-coupling constants for the sharp spectrum are almost identical regardless of the type of gas utilized. The reason for this is not known at this time. Since a part of the electron is certainly transferred from the V(IV) ion to the adsorbed gas molecules, some spectrum from such adsorbed species should be o b s e r ~ e d . ~ ’ ~However, ~ we cannot detect them in the present spectra. It may be due to the small surface area of the catalyst, partial electron transfer, and/or some relaxation problem.
Acknowledgment. We express our appreciation to Dr. J. Asakura for his help on ESR measurement, Dr. S. Ishida for his help on the computer simulation programming, and Professor K. Kuwata for his stimulating discussions. Registry NO. 02,7782-44-7;NO, 10102-43-9;NZO, 10024-97-2;V02, 12036-21-4; Ti02, 13463-67-7. (29) Vorotyntsev, V. M.; Shvets, V. A.; Kazanskii, V. B. Kiner. Kazal. 1971, 12, 618.
(30) Cornaz, P.F.;Van Hooff, J. H. C.; Pluijim, F. I.; Schuit, G. C. A.
Discuss. Faraday SOC.1966,41, 290.
(31) Iyengar, R.D.;Codell, M.; Karra, J. S.;Turkevich, J. J. Am. Chem. SOC.1966,88, 5055.
(32) Naccache, C.; Meriaudeau, P.; Che, M.; Tench, A. J. Tram.Faraday (28) Chemical Society of Japan, Ed. Kagaku Binran I& Maruzen: Tokyo, 1984; p 113.
SOC.1971, 67, 506.
(33) Shiotani, M.; Moro, G.; Freed,J. H. J. Chem. Phys. 1981,74,2616. Tatsumi, K.; Shiotani, M.; Freed, J. H. J . Phys. Chem. 1983, 87, 3425.
