J. Phys. Chem. 1988, 92, 2516-2520
2516
I
AEleVI
..........
adsorbed water for cathodic applied potentials. These results support the recent hypothesis put forth concerning the reactivity of the H 2 0 and H atom ensemble in the potential range of H atom electroadsorption/electrodesorption on Pt electrodes with either (100)- or (1 11)-type preferred crystallographic orientation in acid electrolytes.*
P I 2 l t HZ0
-
0.0 e v
Figure 9. Energy changes of each step involved in the decomposition
reaction of water over a Pt(100) surface at different applied potentials. The height of the energy barrier decreases for cathodic potentials due to bonding stabilization of products: (-) uncharged surface; (---) cathodic potentials; .) anodic potentials. (e
products can produce the lowering of the activation barrier when going toward cathodic potentials (Table IV). Therefore, in the framework of the EHM calculations the present results can be explained through two main effects, mainly the delocalization of the orbital of the adsorbed species on the metal cluster, which lowers the energy of the adsorbed system for each step of the reaction in the anodic direction, and chargetransfer effects, which influence the bonding stability of the adsorbed products in the opposite direction. The last question is the most important in explaining the favorable decomposition of
Conclusions The semiempirical extended Hiickel method appears to be a useful tool to discuss the electronic properties of water molecules, HO radicals, and H atoms interacting with different Pt surfaces that are simulated by means of the cluster approximation. In this way an explanation of the different reactivities of an adsorbed water molecule over either charged or uncharged Pt( 100) and Pt( 1 11) metal surfaces can be advanced. Transition-state geometries are similarly proposed on both surfaces although they comprise a different way that products coordinate those metal structures. The smaller activation energy found over Pt( 100) justifies the different reactivity of this substrate. Negative charging of the surface (cathodic direction) lowers the activation energy as a result of the bonding stabilization of the reaction products through charge-transfer effects from the metal to the adsorbed species. Therefore, by means of the simple molecular orbital theory the initial stages of complicated electrochemical processes can be qualitatively approached from a molecular standpoint, and in this way theoretical methods become a valuable complement for experimental electrochemical research. Acknowledgment. This work was financially supported by the Consejo Nacional de Investigaciones Cientificas y Tccnicas and the Comisidn de Investigaciones Cientificas de la Provincia de Buenos Aires. Registry No. Pt, 7440-06-4; H 2 0 , 7732-18-5.
Quantum Chemical Study of Metal Oxide Catalysts. Structures of Vanadium Oxide and Niobium Oxide Clusters Supported on Silica and Alumina Hisayoshi Kobayashi,* Masaru Yamaguchi, Faculty of Living Science, Kyoto Prefectural University, Sakyo- ku, Kyoto, Japan
Tsunehiro Tanaka, Yasuo Nishimura, Hiroshi Kawakami, and Satohiro Yoshida Department of Hydrocarbon Chemistry and Division of Molecular Engineering, Faculty of Engineering, Kyoto University, Sakyo- ku, Kyoto, Japan (Received: May 13, 1987)
Geometrical structures of vanadium oxide and niobium oxide clusters supported on silica and alumina are investigated by the ab initio molecular orbital method. Optimized geometries are searched by using the energy gradient technique. For the bare cluster and the clusters supported on silica, the monoxo form is more stable than the dioxo form independent of the transition-metal atoms, whereas the possibility of greater stability for the dioxo form is suggested on alumina. Stabilization of the dioxo form is ascribed to the coordinative bonding from the oxygen atom to the three-coordinated aluminum atom within the cluster and the resulting four-membered ring formation. The calculated bond lengths for the V 4 , V-O, Nb==O, and Nb-O bonds are compared with those estimated from EXAFS/XANES measurements, and the agreement is better for the silica-supported clusters than the alumina-supported clusters.
Introduction Metal oxides of transition elements catalyze various types of reactions, especially oxidation. These catalysts are usually employed in the dispersed form on the supports. The atomic configuration around the transition-metal atom is expected to be different from that in the oxide crystal itself. It has been difficult to obtain this type of information from experiments. Only a few quantum chemical calculations have been reported so far, and one 0022-3654/88/2092-25 16$01.50/0
of the reasons for this is ascribed to a lack of information. Recently, new analyzing techniques using X-ray, such as extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure ( X A N B ) , came into wide use.' =hey Provide certain information that is necessary to build up the calculational (1) E.g.: Lee, P. A,; Citrin, P. H.; Eisenberger, P.;Kincaid, B. M. Rec. ~ o d~ . h y s1981, 53, 769.
0 1988 American Chemical Society
Vanadium and Niobium Oxide Clusters
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2517
0;
-2M ‘ 0
p
H H Model I
I
2
\
H Model II
Figure 1. Model clusters without support. Models I and I1 represent the monoxo and dioxo forms, respectively. Atom symbol M means V or Nb,
M o d e l 111
M o d e l 111’
and some atoms are labeled to specify that atom in the cluster.
models and to directly compare calculated results. On the other hand, remarkable progress in computers and programs has enabled large-scale calculations including several heavy atoms. Ab initio molecular orbital (MO) methods with the energy gradient technique are a powerful tool for systems where the geometrical structure is not exactly known by experimental measurements alone. Vanadium oxide on oxide supports is present as a mixture of various vanadate forms, i.e., V 0 4 tetrahedra, V 0 5 square pyramids, V 0 6 octahedra, and aggregated forms of these vanadate unitsS2 However, the isolated V 0 4 tetrahedron has been suggested as the most probable species on silica by many studies based on ESR3-8 and UV-vis.”* This species is also identified as an active species. Geometrical parameters for the V - 0 bond length in the V 0 4 unit have been estimated by EXAFS.I3 The existence of the V 0 4 tetrahedron on alumina has also been pointed out by the EXAFSIXANES ana lyse^'^^'^ and by the Raman spectra mea~urements.’~J’This V 0 4 unit has been thought to possess the V=O double bond, which is the active center of the catalyst, similarly to the oxide crystal.I8 We have already adopted the V 0 4 unit model in our quantum chemical calculation on the photocatalysis of vanadium oxide.I9 For niobium oxide, still fewer works have been reported. It seems reasonable to think that the oxides of the SA elements in the periodic table possess one M=O (M means V or Nb) double bond (monoxo form) independent of support materials. However, some workers suggested that even the oxides of the 5A elements have a structure with two M=O double bonds (dioxo form) like the oxides of the 6A element^.'^,^^
(2),Gellings, P.J. In Cafalysis; Bond, G. C., Webb, G., Eds.; Specialist Periodical Reports; Royal Society of Chemistry: London, 1985; Vol. 7, p 104. See references therein. (3) Yoshida, S.; Iguchi, T.; Ishida, S.; Tarama, K. Bull. Chem. SOC.Jpn. 1972, 45, 376. (4) van Reijen, L. L.; Cossee, P. Discuss. Faraday SOC.1966, 41, 277. (5) Che, M.; Canosa, B.; Gonzalez-Elipe, A. R. J . Phys. Chem. 1986.90, 618. (6) Narayana, M.; Narasimhan, C.; Kevan, L. J. Catal. 1983, 79, 237. (7) Shvets, V. A.; Sarichev, M. E.; Kazansky, V. B. J. Cafal.1968,11,378. (8) Kazansky, V. B.; Shvets, V. A.; Kon, M.; Ya, N.; Kisha, V. V.; Hselimov, B. N. Cafal.,Proc. Int. Congr., 5th 1973, 2, 1423. (9) Praliaud, H.; Mathieu, M. V. J . Chim. Phys. Phys.-Chim. Biol. 1976, 73, 689. (10) van Hanke, W.; Heise, K.; Jerschkewitz, H.-G.; Lischke, G.; Ohlmann, G.; Parlitz, B. Z. Anorg. Allg. Chem. 1978, 438, 176. (1 1) Lischke, G.; Hanke, W.; Jerschkewitz, H.-G.; Ohlmann, G. J . C a r d 1985, 91, 54. (12) Gritscov, A. M.; Shvets, V. A.; Kazansky, V. B. Chem. Phys. Letf. 1975, 35, 511. Kinet. Katal. 1974, 15, 1257. (13) Tanaka, T.; Nishimura, Y.; Kawasaki, S.; Funabiki, T.; Yoshida, S. J. Chem. Soc., Chem. Commun., in press. (14) Kozlowski, R.; Pettifer, R. F.; Thomas, J. M. J . Phys. Chem. 1983, 87, 5176. (15) Tanaka, T.; Yamashita, H.; Tsuchitani, R.; Funabiki, T.; Yoshida, S. J. Phys. Chem., in press. (16) 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. (17) Rwzeboom, F.; Medema, J.; Gellings, P. J. Z . Phys. Chem. (Munich) 1978. 111. 215. (18) Iwamoto, M.; Furukawa, H.; Matsukami, K.; Takenaka, T.; Kagawa, S.J . Am. Chem. SOC.1983, 105, 3719. (19) Kobayashi, H.; Yamaguchi, M.; Tanaka, T.; Yoshida, S . J. Chem. Soc., Faraday Trans. 1 1985, 81, 1513. (20) Haber, J.; Kozlowska, A,; Kozlowski, R. J . Catal. 1986, 102, 52.
\
H Model h,
Model
N’
H Model V
M o d e l V’
Figure 2. Six model clusters for the catalyst-support systems. Models 111-V and models 111’-V’ are used for the catalyst supported on silica and alumina, respectively. Models I11 and 111’ and models IV, IV’, V, and V’ represent the monoxo and dioxo forms, respectively. See the caption of Figure 1.
Si (OH )4
A I ( O H I 3 + HO ,
Figure 3. Model clusters for the silica and alumina units. The latter includes a water molecule to reproduce the environment in the alumina crystal partly. These are referred to as models 0 and 0’, respectively. Optimized bond lengths and bond angles are also shown.
In this article, the structures of vanadium oxide and niobium oxide catalysts supported on silica and alumina as well as the unsupported catalysts are investigated by using the ab initio MO method. Model clusters used in the calculation are comprised of the metal oxide and/or the support. Structures of the catalysts and the electronic interactions between the catalyst and support are discussed in terms of the geometrical parameters, electron density, and energy for the optimized clusters. Models The model clusters used in the calculation are shown in Figures 1-3. Terminal H represents Si or Al atoms adjacent to the cluster. The monoxo and dioxo forms are most simply represented by models I and 11, respectively, where all the silica and alumina moieties connected to the M 0 4 unit are replaced by H atoms. Calculations including the support moieties are indispensable in examining the influence of the silica and alumina supports on the
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The Journal of Physical Chemistry, Vol. 92, No. 9, 1988
TABLE I: Geometrical Parameters and Relative Energies ( E ) for Models 1 and 11“ V
Kobayashi et al. TABLE 11: Geometrical Parameters and Relative Energies ( E ) for Models 111-V and 111”V’“
Nb
V/Si
Model I M=Ol, M-02, A 02-H, A f 0 , - M - 0 2 , deg f M-02-H, deg E , kcal/mol
1.56 1.75 0.97 109 144 0.0
Model I1 1.59 1.77 2.00 0.96 04-H, A 0.98 1 I6 io,-M-0,, deg iO,-M-O,, deg 104 iM-0,-H, deg 151 iM-O,-H, deg 123 LH-O,-M-O,,~ deg 69 E , kcal/mol +17.3 M=O,, A M-03, A M-04, A 03-H. A
1.72 1.89 0.97 109 148’ 0.0 1.74 1.91 2.17 0.97 0.98 117 105 161 124 68 +32.6
“ A t o m symbol M means V or Nb. QNb-0-H is fixed since optimization leads to a linear Nb-O-H form. eLH-03-M-0, is a dihedral angle.
transition-metal oxide catalysts. The extended model clusters are built up by substituting the silica and alumina units for the H atoms in the clusters of models I and 11. At present it is very difficult to deal with the fully substituted clusters. An alternative method adopted in the present work is to employ one support moiety and two H atoms. These model clusters are shown in Figure 2. One type of cluster, that is, model I11 or 111’, is enough to be examined for the monoxo form since the three M-0 bonds in model I are all equivalent. For the dioxo form, however, the two M-0 bonds in I1 are not equivalent, and two types of cluster are required. They are models IV and V or IV’ and V’. Models 111-V are used for the silica support, and models 111’-V’ for the alumina support. Models 111’-V’ are produced from models 111-V, respectively, by replacing the Si atom with the A1 atom and by removing one OH group. The number of component atoms is taken to be equal among the three models. The distinction between models IV and V and between models IV’ and V’ is caused by limitation in the cluster size. However, the position of the three-coordinated aluminum atom has an important meaning in the case of alumina, and this point will be discussed. The structures of silica and alumina moieties alone are optimized by using additional models, model 0 and O’,respectively, in advance of the calculation for models 111-V and 111’-V’. They are shown in Figure 3. The silica model is Si(OH)4 The alumina model consists of Al(OH)3 and a coordinated water molecule, which partly reproduces the environment in the alumina crystal. The 0-AI-0 angle is fixed to the tetrahedral angle (109.47O).
M=Ol, A M-02, A M-O,, 8,
oz-x, A
f01-M-02, deg i 0 , - M - 0 3 , deg E , kcal/mol M=O,, %I M-O,, A M-O,, A
o,-x, A
LO,-M-O,,A iM-0,-X, deg LO,-M-O,, deg iX-0,-M-O,,‘ deg E, kcal/mol M=O,, A M-O,, 8,
M-O,, A LOl-M-03, deg LO,-M-0,, deg f M - 0 , - X , deg LH-O,-M-O,,C deg E, kcal/mol
V/Al
Nb/AI
Model 111, 1.56 1.72 1.77 1.90 1.75 1.89 1.64 1.64 113 112 109 109 0.0 0.0
NbiSi
1.56 1.74 1.77 1.73 111 1IO 0.0
1.72 1.89 1.90 I71 112 1 IO 0.0
Model IV, IV’ 1.59 1.74 1.77 1.92 2.00 2.17 1.62 1.63 117 118 165 171 102 106 70 69 +16.8 +32.3
1.60 1.76 2.03 1.70 117 161 107 71 12.4
+
1.75 1.91 2.19 170 118 162 108 71 + 3 1.8
Model V, V’ 1.59 1.74 1.76 1.91 2.02 2.19 1 I8 118 99 101 126 126 70 68 +24.7 +41.9
1.58 1.94 1.93 115 78 99 67 -59.2
1.73 2.10 2.10 I I8 73 100 70 -39.5
“Atom symbol M means V or Nb, and X means Si or AI. * i M 0,-X is fixed and taken to be equal to LM-O,-H in models I and 11. ‘LX-O,-M-O, and LH-O,-M-O, are dihedral angles.
silicon, oxygen, and hydrogen atoms, respectively.26
Results and Discussion Structures of Models I and II. The optimized structural parameters for models I and I1 are shown in Table I along with the total energies. The M=O double-bond length is shorter than the M - 0 single-bond length by 0.17-0.19 A in both models. (The M-O4 bond length in model I1 will be discussed.) The angle Ol-M-02 indicates that the MO, unit of Model I is shaped almost tetrahedrally. In model 11, the two M-O bonds are not equivalent: one has a length nearly equal to that in model I, whereas the other has a longer length (2.00 and 2.17 A). This longer length suggests that the bonding is relatively weak and has coordinative character. Among models I and 11, the Nb=O and Nb-0 bond lengths are longer than the V=O and V-0 bond lengths, respectively, by 0.14-0.17 A. There are no other remarkable changes in the whole structure dependent upon the metal atoms. Therefore the calculated difference in the structure could be explained in terms of the atomic radii of the metal atoms. (The atomic radii for V and N b atoms are 1.35 and 1.45 A, respectively.) Method of Calculation The cited total energies are relative values where the energies for model I are set to zero as a standard. It is found that the The calculations are carried out at the Hartree-Fwk level with monoxo form is more stable than the dioxo form for both vanadium the GAUSSIAN 80/82 programs.2’ All the basis sets are split and niobium oxides, and further the relative destabilization for valence type. For the vanadium atom, the (12s6p4d) set reported the dioxo form is larger in the case of niobium. by Roos et a1.22was contracted to [5s2pld], and a d orbital with Structures of Models 0 and 0’. The optimized structural the exponent 0.0885 was added to the set.23 In order to represent parameters for models 0 and 0’ are shown in Figure 3. The bond the 4p atomic orbitals (AOs), a p orbital with the exponent 0.22 length of the four Si-0 bonds is 1.64 A. The 0-Si-0 bond angles was added. For the niobium atom, the (12~8p7d)/[4~3p2d] set are in the range 106-109’. For model 0’, the optimized bond reported by Friedlander et al.24was reinforced by the Ss, Ss’, and 5p AOs with the exponents 0.130,0.043, and 0.124, respecti~ely.~~ length of the A1-0 bond in the A1(OH)3 moiety is 1.71 A, and that between the water and the AI(OH)3 moiety is 1.84 A. The The 33-216, 3-21G, and 21G sets are used for the aluminum and Al-0 bond length is a little longer than the Si-0 bond. This trend is general throughout the calculation and is shown again in the (21) Binkley, J. S.;Whiteside, R. A,; Krishnan, R.; Seeger, R.; DeFrees, results for models 111-V and 111’-V’. D.J.; Schlegel, H. B.; Topiol, S.; Kahn, L. R.; Pople, J. A. QCPE 1981, 13, Structures of Models 111-Vand III‘-V’. Table I1 shows the 406. (22) Roos, B.; Veillard, A.; Vinot, G. Theor. Chim. Acra 1971, 20, 1 . optimized structural parameters for models 111-V and 111’-V’. (23) Hay, P. J. J . Chem. Phys. 1977, 66, 4377. (24) Friedlander, M. E.; Howell, J. M.; Snyder, G. J . Chem. Phys. 1982, 77, 1921. (25) Taylor, T. E.; Hall, M. B. J . Am. Chem. Soc. 1984, 106, 1576.
(26) Binkley, J. S.; Pople, J. A.; Hehre, W . J. J . Am. Chem. Sot. 1980, 102, 939.
Vanadium and Niobium Oxide Clusters Except for the M-O bonds in model V’, the M=O and M-O bond lengths agree with the corresponding bond lengths in models I or I1 to within 0.04 A. The two V-0 bonds in model V’ are characteristic compared with those in the other models and have a middle value between the longer and shorter single bond lengths. To specify the individual catalyst-support systems simply, we use the abbreviations V/Si, Nb/Si, V/Al, and Nb/Al for vanadium and niobium oxides supported on silica and alumina, as shown in Table 11. Within models I11 and 111’, the corresponding bond angles are nearly independent of the V or N b atom or silica or alumina. Among models IV and IV’, the bond angles change to some degree. The largest changes are 9” and 10” and occur between the Nb-03-Si and Nb-O,-Al angles and between Nb0,-Si and V-0,-A1 angles. Other changes are equal to or less than 6 ” . The bond-angle changes are much larger (more than 25”) between models V and V’. These changes do not occur between the V and N b atoms with the same support but occur between the silica and alumina supports independent of the transition-metal atom, where drastic deformation of the cluster structure is clear, as shown in Figure 2. Relative Stability among Model Clusters. Table I1 also shows the relative total energy of each model cluster where the energies of models 111 and 111’ are taken to be zero. Similar to the result of models I and 11, both the vanadium and niobium oxides are more stable in the monoxo form on silica, and the N b atom destabilizes the dioxo form more than does the V atom. The energy values, however, clearly indicate a remarkable difference between models V and V’. Both the oxides prefer the dioxo form on the alumina support. For the case of silica, the energy differences between models IV and V are 8 and 10 kcal/mol, and they are an artifact caused by the smallness of the cluster size. However, we could draw further information on the relative stability for the fully silicasubstituted clusters by comparing these energies with the energies for models I and 11. Model I1 is more unstable by 17 and 33 kcal/mol for the V and N b clusters, respectively, than is model I. These values are very close to the energies for models IV, Le., 17 and 32 kcal/mol. Substituting the silica unit for the OZ-H hydrogen atom in model I and the 0,-H hydrogen atom in model I1 brings about almost equal effects. Substituting for the 04-H hydrogen atom in model 11, however, causes some relative destabilization (8-10 kcal/mol). Therefore we could expect that for the fully substituted clusters the energy difference between the monoxo and dioxo forms remains intact or becomes larger, and the superiority of the monoxo form is valid. For alumina, the mode of interactions is very different between models V and V’, and similar discussion is not given. However, since the energies for model IV’ are close to those for model IV, the effects of the substitution may be estimated to the same degree as for the case of silica. When the remaining H atoms are substituted by the alumina units in model V’, the relative stability of the dioxo form will decrease by at most 10 kcal/mol, but the superiority of the dioxo form will not be changed. Figure 2 clearly shows the bond formation between the 0, atom and the aluminum atom in model V’. This point will be discussed in detail from the viewpoint of the electronic structure in the next section. It is required that the aluminum atom located in this position must be a three-coordinated one. This condition is thought to be neither too severe nor unrealistic because there may exist only one three-coordinated aluminum atom near the M 0 4 unit on the alumina surface, and other aluminum atoms may be fouror six-coordinated. Electronic Structures of Model Clusters. Mulliken’s atomic population and atomic bond population are used to estimate the charge of atoms and the nature of bondings, respectively, and their values for models 111-V and 111’-V’ are shown in Table 111. The charges on M and 0 atoms indicate that the ionic character or polarization of bonds is stronger for the Nb=O bond than for the V=O bond. The Nb=O bond also has the larger value of bond population. This result is reasonable when we consider that the distribution of valence electrons spreads further away from the core region in the N b atom than in the V atom and contributes
The Journal of Physical Chemistry, Vol. 92, No. 9, 1988 2519 TABLE HI: Charge of Atoms (Q) and Bond Population (P)” V/Si
Nb/Si
V/A1
Nb/AI
Model +1.40 -0.36 -0.92 -0.72 f0.51 0.42 0.26 0.29 0.23
111, 111’ +1.54 -0.50 -0.95 -0.76 +0.53 0.45 0.26 0.29 0.22
+1.35 -0.36 -0.84 -0.72 +0.53 0.41 0.29 0.29 0.20
+1.54 -0.50 -0.89 -0.76 +0.56 0.44 0.28 0.29 0.21
Model +1.25 -0.48 -0.99 +0.52 +0.19 0.43 0.25 0.13 0.24
IV, IV’ +1.33 -0.54 -0.98 +0.52 +0.21 0.44 0.25 0.14 0.23
+1.19 -0.50 -0.94 +0.57 +O. 19 0.43 0.28 0.12 0.22
+1.28 -0.56 -0.94 +0.56 +0.21 0.45 0.27 0.13 0.22
Model V, V’ +1.20 +1.28 -0.49 -0.55 -0.78 -0.78 -0.95 -0.93 +0.14 +0.20 0.42 0.44 0.27 0.28 0.1 1 0.13 0.03 0.01 0.17 0.16
+1.29 -0.43 -0.94 -0.93 +0.06 0.42 0.16 0.18 0.1 1 0.1 1
+1.39 -0.50 -0.9 1 -0.91 +0.06 0.44 0.17 0.19 0.10 0.1 1
aAtomsymbol M means V or Nb, and X means Si or AI. b n equals 4 if X means Si or equals 3 if X means AI.
to the (overlap) bond population more largely. The strength of the V=O and Nb=O bonds is, however, not compared simply by their bond population values. The negative charge on the 0 atom for the single bond is higher than that for the double bond. These trends are consistent throughout the models shown in Table 111. The electron density for the M=O and M-0 bonds is higher for models IV, IV’, V, and V’ than for models I11 and 111’. This accumulation of charge is explained as follows: For example, from model I11 to IV, one M-0 single bond with a more negative 0 atom is replaced by an M=O double bond with a less negative 0 atom. The excess electron density is redistributed among the M=O and M-0 bonds, which relatively increases the electron density around both the M=O and M-O bonds. The OHz moiety, where the oxygen atom is labeled as O4in models IV and IV’, possesses a positive charge. A long bond length and a small bond population for the M a 4 bond compared with those for the M-0, bond indicate that the M-O4 bond has a coordinative character. The electronic structures for models V and V‘ are very different from each other as expected from the remarkable difference in the geometrical structures. In model V, the M-O4 bond is weak compared with the M-0, bond, again as estimated from the bond population, and the moiety possesses a positive charge similar to the OHz moiety in model IV. In model V’, in contrast with model V, the difference between the M-O4 and M-0, bonds disappears, and the positive charge in the Al(OH), moiety becomes very small compared with that in the Si(OH)4 moiety. Further the negative charge on the O3 atom increases. These results suggest the accumulation of electrons to the O3and aluminum atoms, which is favorable for the bond formation in terms of the electron donation from the O3atom to the aluminum atom. The bond population for the 03-X bond also shows the bond formation for model V’ and not for model V. The values of bond population for the M-O,, M-04, 0,-X, and 03-X bonds show that in model V’ the M-0, and 04-Al bonds are further weakened and contrarily the M-04 bond as well
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The Journal of Physical Chemistry, Vol. 92, No. 9, 1988
as the 03-Al bond is strengthened. Thus, the formation of the 0,-A1 bond affects the whole electronic structure of the 03-M04-A1-03 four-membered ring. Comparison with Experimental Data. Calculated results are compared with adequate experimental data so far available. Recently Tanaka et al. reported 1.61-1.62 and 1.77-1.78 8, as the V=O and V-0 bond lengths, respectively, in the V 0 4 unit for the silica-supported vanadium oxide catalysts by an EXAFS/XANES study.13 These values are compared with 1.56 and 1.75-1.77 8, for the V/Si system in model 111, respectively. The difference for the V=O bond length is 0.05 A, and excellent agreement was given for the V-O bond. They also reported 1.67 and 1.77 A for V=O and V-0 bond lengths for the aluminasupported ~ata1ysts.l~ The calculated bond lengths are 1.58 and 1.93-1.94 8, for the V/Al system in model V’. The differences go up to ca. 0.1 A, and the agreement is not good. For the niobium oxide, Iwasawa et al. have reported the result of an EXAFS study for sam les dispersed on various supports.*’ They report 1.76 and 1.92 for the Nb=O and Nb-0 bonds in the N b 0 4 catalyst supported on silica. These are in fair agreement with 1.72 and 1.89-1.90 A, respectively, for the Nb/Si system in model 111. For the catalyst supported on alumina, the experimentally estimated bond lengths are 1.66 and 2.04 A, and the calculated values are 1.73 and 2.10 8, for the Nb/Al system in model V’. The conformity is somewhat poor, and the discrepancy is 0.07 and 0.06 A, respectively. Comparison of the calculated bond lengths with those estimated from the EXAFS/XANES studies for the four systems shows that the agreement is better for the silica-support catalysts. This result is not very surprising. The electronic structure of the silica unit is well understood in terms of the usual single-bond concept, although it includes the electronegative oxygen atoms. In the case of alumina, the unoccupied orbital localized on the aluminum atom has subtle influence on the whole structure of catalysts. In fact, the atomic configuration of actual alumina surfaces is much more complicated than that of silica surfaces. The worst agreement in the bond lengths is found for the V/A1 system. We think that the strain by the four-membered ring formation results in the extraordinarily long V-0 bond lengths. This means that some artificial effects occur in the results of model V‘ to gain the largest stabilization energy. However, the agreement is not so bad for the Nb/A1 system of model V’, which may be a fortunate consequence for the relatively long Nb-0 distance. When the relative stabilization energy is disregarded, the V=O and V-0 bond lengths for models 111’ and IV’ agree better with the values estimated from the EXAFS/XANES study.I5
R
(2!) Kuroda, H.; Iwasawa, Y.; Asakura, K.; Kosugi, N.; Yokyama, T.; Nishimura, M. Photon Factory Activity Report; National Laboratory for High Energy Physics, Japan, 1985, 1984/1985 VI-83.
Kobayashi et al. The present calculation should not be entirely blamed for the discrepancy with the experimental data referred here, and the accuracy of individual EXAFS/XANES measurements and of their data analysis should be also discussed carefully. So that the discrepancy between the experimental and calculated bond lengths for the alumina-supported catalysts can be explained, however, further calculations with different models are desired, which may include more than two aluminum atoms or the octahedrally coordinated aluminum atom. Either the unrestricted Hartree-Fock calculations or those including the electron correlations may be necessary to describe accurate pictures of alumina surfaces and catalysts on alumina.
Conclusion The geometrical and electronic structures of the vanadium and niobium oxide catalysts on the silica and alumina supports are investigated by the ab initio MO methods. The following information is drawn from the present calculations: (1) Both catalysts on silica are more stable in the monoxo form, which is seen to be the normal configuration for the metal oxide of the 5A elements. (2) In the case of alumina, both catalysts are more stable in the dioxo form through the bonding interactions between the oxygen and aluminum atoms within the catalystsupport system. (3) The calculated M=O and M-O bond lengths are generally in good agreement with the experimental values exce t for the V/Al system, where the discrepancy is up to ca 0.1 and an improvement in calculations and models is necessary. (4) The charge on atoms and the bonding nature estimated by the population analysis show that the M-0 oxygen atom is more negatively charged than the M=O oxygen. The charges on the M and 0 atoms in the M=O bonds are larger for N b than for V. (5) For the dioxo form, one of the two M - 0 bonds is the relatively weak coordination type if the four-membered ring is not formed. The formation of the four-membered ring drastically affects the electron density of the whole ring. Thus, the present calculation drew some useful information that is difficult to know from experiments alone and shed light on the fundamental problems in surface science and heterogeneous catalysis.
1,
Acknowledgment. We are grateful to Dr. Kanji Kajiwara and Dr. Yuzuru Hiragi for their helpful discussion. We thank the Data Processing Center of Kyoto University for generous use of the FACOM M380/780 computer and the Computer Center of the Institute for Molecular Science for permission to use the HITAC M680H computer. Registry No. Vanadium oxide, 11099-1 1-9; niobium oxide, 1262700-8,
