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J. Phys. Chem. 1986, 90, 2715-2718 reactions which poison the catalytic sites. Additives which keep the surface cleaner, such as potassium and silicon, enhance the rate of benzene formation. Sulfur and chlorine appear to enhance the polymerization or decomposition reactions which poison the catalytically active sites. The low-pressure stoichiometric reaction that yields benzene from acetylene is inhibited by site blocking for electron-donating additives and enhanced due to electronic interactions for electron-accepting additives. From these results it is evident that the effect of the additives on the acetylene cyclotrimerization varies considerably between low pressure and atmospheric pressure. They exhibit both site
blocking and electronic interactions that markedly change the benzene yield.
Acknowledgment. This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division of the U S . Department of Energy, under Contract DE-AC03-76SF00098. T.M.G. thanks Dow Corning Corp. for a graduate fellowship. Registry NO,H-H, 74-86-2; HZC=CHz, 74-85-1;C6Hs,71-43-2; Pd(lll), 7440-05-3; Si, 7440-21-3; C12, 7782-50-5; P, 7723-14-0; K, 7440-09-7.
ESR Characterization of V205 Monolayers and Double Layers Supported on Various Carriers V. K. Sharma,? A. Wokam,$ and A. Baiker*+ Department of Industrial and Engineering Chemistry and Physical Chemistry Laboratory, Swiss Federal Institute of Technology (ETH), CH 8092 Zurich, Switzerland (Received: October 23, 1985)
Monolayers and double layers of V2O5 supported on A1203,SO2,and MgO have been investigated by electron spin resonance spectroscopy. The g and hyperfine tensors of V4+determined from the spectra are used to estimate the influence of the carriers on the vanadyl bond strength and the delocalization of the 3d electron on the coordinatively bound oxygen ligands. Spectra of V2OSsupported on A1203and S O 2 exhibit a large g tensor anisotropy, corresponding to a relatively short V-0 bond. Reduction is found to have a different influence on the V=O bond strength in these two systems: On A120, the bond is weakened, whereas on Si02the bond strength is increased. On MgO the spectra indicate a weak vanadyl bond which does not change significantly upon reduction. The delocalization of the V4+ electron into ligand orbitals is largest for the MgO-supported systems. Reduction results in an increased delocalization on A1203and S O 2 carriers, whereas with MgO no significant change of this parameter is observed.
The deposition of monolayers or double layers of an active phase
on a carrier is an attractive technique for the tailoring of catalytic properties. It offers the advantage of a uniform distribution of active sites and allows one to influence the properties of the active phase by its interaction with the support. This approach has recently been applied for the immobilization of V 2 0 5on A1203, S O 2 , MgO, and Ti02.' The preparation starting with vanadyl triisobutoxide, the textural properties of the resulting catalysts, and the carrier influence on the reducibility of the V5+ions have been investigated.l Of particular interest was the bifunctional activity of these catalysts for dehydration and selective oxidation of alcohols to the corresponding aldehydes. The oxidative activity could be tailored by the acidity of the carrier and the deposition of V205 species. Catalytic activity has been tested for methanol and 1-heptanol oxidation. The vanadyl bond strength, as qualitatively derived from temperature-programmed reduction (TPR), was found to parallel the selectivity of methanol oxidation to formaldehyde. Spectroscopic information on the coordination geometry of the supported V205 species is desired as a more quantitative measure of the V=O bond strength. ESR spectroscopy has been used to provide this information from an analysis of the g and hyperfine tensors of s1V4+ions. ESR measurements have been performed on a conventional Varian Century series X-band spectrometer with 100-kHz modulation at room temperature. The microwave frequency was 9.51 1 GHz. The signals due to slV with I = were observed from all the catalysts. Samples were analyzed before and after reduction (TPR) in hydrogen. The spectra could be attributed to V4+ ions for all catalyst except one. Some of the observed spectra are presented in Figures 1 and 2. t Department of Industrial and Engineering Chemistry. *Physical Chemistry Laboratory.
TABLE I: Parameters Characterizing the ESR Spectra of Monolayer and Double Layer Catalysts
A1203-V4+ after TPR A1203-2V4+ after TPR
1.942 1.947 1.938 1.946
1.974 2.1 1.950 1.1 1.979 2.8 1.953 1.1
191 179 194 181
63 73 66 78
0.75 0.63 0.75 0.61
0.53 0.53 0.55 0.56
Si02-V4+ after TPR Si02-2V4+ after TPR
1.934 1.930 1.932 1.930
1.974 1.979 1.971 1.975
2.4 3.1 2.3 2.7
193 194 195 196
73 71 75 70
0.69 0.70 0.69 0.72
0.57 0.57 0.58 0.57
MgO-V4+ after TPR Mg0-2V4+ after TPR, V2+ after TPR, V4+
1.954 1.953 1.953 1.981 1.946
1.965 1.964 1.966 1.981
1.3 1.3 1.4 1.0
174 175 172
75 77 77
0.59 0.55 0.58 0.55 0.56 0.55
80
80
176
In the following section the relationships between ESR parameters, coordination geometry, and electron delocalization are summarized. Subsequently, the experimental results are presented and discussed in terms of changes in the V - 0 bond strength and in electron delocalization over the ligand orbitals. Analysis of the Spectra The powder spectra have been reduced by computer simulations including second-order corrections. The envelopes exhibit shoulders at magnetic field positions corresponding to gil,with a multiplet splitting due to the hyperfine coupling tensor component A , , . The magnetic field positions corresponding to g,, with an (1) Kijenski, J.; Baiker, A.; Glinski, M.; Dollenmeier, P.; Wokaun, A. J .
Carol. in press.
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The Journal of Physical Chemistry, Vol. 90, No. 12, 1986
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Sharma et al. (b)
(a)
+&.&&SP" g?-3q-& y 4 .;Y
pa/
-
d,> -
tp. a
d,,
>v
y2
-
,+,.e
4. d,,
dx,
3
2Ex, y ,
9
I' b2
AIZO,-V" after TPR
h
calculated
'+7
I
Figure 3. Geometry and energy level scheme of the surface vanadyl species. (a) The average symmetry corresponds to C,, where the oxygen ligands 1-4 form a square parallel to the surface Some of the orbitals used in eq 3 are indicated. (b) The level degeneracy present in 0, symmetry is lifted by tetragonal distortion. The splitting AExr is a measure of distortion, whereas shows no dependence on distortion.
Al203- 2V"
AIZ03-2V"after TPR
2800
3wo
3400
3800
4200
B/G Figure 1. Typical ESR spectra of VzOs supported on A1203. [AI2O3-V4+ and AIzO3-2V4+correspond to ESR signals of V4+ in V20, monolayers and double layers, respectively.] A simulated spectrum is included (second trace), where a Gaussian line width of 50 G was used.
MgO -2V"
I
features is included in Figure 1. The parameters determined from the computer simulations are given in Table I. The (g,,, A,,) multiplet is well-resolved, with line positions deviating only slightly from those expected for first-order hyperfine splitting. The accuracy is estimated as f0.005and f 3 G for glland A,,,respectively. The perpendicular transitions are generally less well resolved in the spectra. Consequently, the corresponding error margins are larger, i.e. f0.01 for g , and =k5 G for A , . Molecular orbital theory is used to relate the measured principal components of g and A tensors to changes in the geometry and in the electronic structure of the vanadyl species. The splitting of the d orbitals of transition-metal ions in octahedral and distorted ligands field has been considered by numerous authors (e.g., ref 2-7). Our experimental observation (cf. Table I) that gIl < g, in the electronic ground state indicates a tetragonal d i s t ~ r t i o n ~ , ~ . For simplicity we assume that the geometry of a vanadyl complex on the surface of a carrier corresponds to approximate C, symmetry, shown in Figure 3. The surface normal defines the distinguished axis ( z ) . The bond to oxygen atom 5, which corresponds to the V=02+ double bond, is considerably shorter than the bond to oxygen atoms 1-4, which form a square parallel to the plane of the surface. The complex is anchored to the support by a relatively long bond. We briefly review the extraction of information on the tetragonal distortion and electron delocalization from the ESR parameters. For this purpose the notation' of the molecular orbitals is defined. Each ligand contributes a filled a-type orbital directed along the bond toward the vanadium ion, and two filled p orbitals perpendicular to this direction [cf. Figure 3b]. On the central vanadium ion 3d and 4s orbitals are considered. The bonding molecular orbitals are mainly centered on the ligands and are filled by the ligand electrons. The antibonding linear combinations are predominantly centered on the vanadium ion. Three orbitals' relevant for the present discussion are listed in order of increasing energy, referring to the level scheme of Figure 3b:
The excitation energy from +*(b2) to +*(b,), denoted by A E x ~ - ~ , is independent of the tetragonal distortion and is determined by 2em
3ooo
34w
3800
4200
B/G Figure 2. Typical ESR spectra of V20s supported on MgO. Note that in the bottom spectrum (Mg0-2V4+ after TPR) the predominant eight-line feature is due to V2+ ions.
(2) Ballhausen, C. J.; Gray, H. B. Inorg. Chem. 1962, I, 1 1 1 . (3) Kivelson, D.; Lee, S. K. J . Phys. Chem. 1964, 42, 1896. (4) Abragam, A,; Bleaney, 8. Electron Paramagnetic Resonance of Transition Ions;Clarendon Press: Oxford, 1970. (5) Griffith, J. S. The Theory of Transition Metal Ions; Cambridge
eight-line multiplet pattern due to A , , give rise to divergences in the spectrum. A calculated stick spectrum indicating these
University Press: London, 196 1. (6) McMillan, J. A.; Halpern, T. J . Chem. Phys. 1971, 55, 3 3 . (7) Kohin, R. P. Magn. Reson. Rev. 1979, 5 , 7 5 .
The Journal of Physical Chemistry, Vol. 90,No. 12, 1986 2717
ESR Characterization of V2OS the splitting parameter lODq of an average octahedral environm e ~ ~In t . contrast, ~ the excitation energy from $*(b2) to $*(e,), PE,,, is zero for octahedral symmetry and becomes increasingly positive with shortening of the V=O bond relative to the bonds in the basal plane. Expressions for the principal components of the g tensor are given next. Neglecting small terms one finds3vsfrom second-order perturbation theory
Agll = gll - ge = -8w1*2P2*2/AEx2-9 Agl = g , - g, = -2Xe,*2/32*2/AExr
(2)
where g, is the free electron g value, and X is the spin-orbit coupling constant of the free ion. The ratio
(3) is a sensitive indicator of tetragonal distortion. With a shortening of the V = O bond, or with increasing distance of the four oxygen ligands in the basal plane, AE,, and thereby also E are increasing. [Note that in the limit of undistorted octahedral symmetry, where B = 1 and AE,, = 0, the perturbation expression eq 2 loses its validity.] The principal components of the hyperfine tensor are given by the approximate relationships’
(4) where Kerfrepresents the Fermi contact term for the vanadium ion, and P is proportional to the expectation value of f 3 in the 3d orbitals of a free V4+ ion. A value of P = 184.5 G6 has been used in the calculations. The squared M O coefficient /3z*2 can be determined from a suitable linear combination of the g and A tensor principal axis values by use of eq 4,
While the spectra yield only the magnitudes of All and A,, a negative sign of the hyperfine tensor components is required to obtain positive values for j32*2 from eq 5 . Results are listed in Table I. The quantity C = 1 - p2*2, which corresponds to the fraction of unpaired d electrons that is delocalized over ligand orbitals, is used below to discuss the catalyst properties. The Fermi contact parameter K,ff is obtained from eq 4 by inserting eq 5 and is listed in Table I. The results for Agll/Agl, j32*2,and K& observed on the various camers will now be discussed in sequence.
Results and Discussion The powder spectra of all investigated V20, monolayer and double layer catalysts are characterized by broad lines, as is seen in Figures 1 and 2. In the computer simulations, where a Gaussian line shape was used to represent the broadening, line widths on the order of 50 G were required to obtain a fit between the calculated and experimental spectra on A1203and S O 2carriers; on MgO the line widths are 25-30 G. These widths are quite broad as compared to the spectra from impurity doped vanadium ions in crystals. W e interpret the observed widths as an inhomogeneous broadening due to static disorder. For vanadyl species immobilized on a carrier surface one expects a relatively wide distribution of coordination geometries. Deviations from C4, symmetry are likely to occur for the individual ion, which would result in nonaxially symmetric g and A tensors (g, # gv and A, # A,,). The fact that the observed spectra are well reproduced by an axially symmetric spin Hamiltonian shows that the average over geometries occurring on the surface corresponds to C4, symmetry. In this sense the subsequent discussion always refers
to this average geometry. However, the trends and differences in the spectra observed for various carriers remain valid in the presence of static disorder. The ESR parameters obtained from our analysis are listed in Table I. They permit the estimation of changes in the catalytically important geometry and delocalization upon immobilization on different carriers, and upon reduction. The quantity B = Agll/Agl reflects changes in the tetragonal distortion. As indicated in the previous section, an increase of B indicates a shortening of the V = O bond, or a lengthening of the distance to the oxygen ligands in the basal plane. An increasing distance between the four electron-rich ligands and the central vanadium ion will in turn result in a stronger Coulomb attraction of the vanadyl oxygen [atom 5 in Figure 3a]. Therefore both geometry changes that are consistent with an increase in B can be expected to have similar consequences for the catalytic properties, as they both corresponded to a strengthening of the V=O bond. On A1203,an increase in bond strength is observed upon second layer impregnation. Reduction weakens the V=O bond to the same extent on both monolayer and double layer catalysts on A1203 and increases the electron delocalization (1 - /32*2)onto the ligand orbitals. In contrast, the Si02-supported monolayer of V205 exhibits a slightly weaker V=O bond than the corresponding double layer catalyst. Reduction strengthens the bond considerably in both Si02-supportedsystems. In the electron delocalization no marked change is observed upon increase in the V205 load, and upon reduction. V205 supported on MgO exhibits the largest electron delocalization. The tetragonal distortion observed on MgO is the smallest among all carriers, indicating a weak V=O bond. Reduction has no influence on the V=O bond strength for the monolayer catalyst (MgO-VS+). After reduction, a weak contribution (5-10%) of a spectrum corresponding to V2+ions is seen superimposed on the V4+ signals. During reduction of the Mg0-2VS+ catalyst a symmetric spectrum, shown in the bottom trace of Figure 2, is observed, which is due to V2+ ions in octahedral symmetry. With higher gain the outermost hyperfine components of a residual V4+ minority species are visible in the spectrum, from which the parameters glland All given in Table I have been estimated. It is interesting to compare our results, which show appreciable electron delocalization, with the work of Takahashi et a1.8 who have studied V205 on alundum and Neobead carriers. On the latter support material, the authors found no delocalization of the V4+electron into ligand orbitals at surface coverages corresponding to one quarter of a monolayer.s On alundum, onto which about eight V2OSlayers had been deposited, ESR spectra resembled those of pure V205. Table I shows that the Fermi contact term, Kea is essentially constant for all samples investigated. Keffis found to be independent of electron delocalization ( 1 - /32*2), and varies only slightly between the three carriers. It is worthwhile to comment on the origin of the Fermi contact term for the 3d ion V4+. An electron in the +*(b2) orbital, defined by eq 1, does not give rise to a Fermi contact interaction directly, as the wave function vanishes at the nucleus. However, the unpaired electron density polarizes the core s electrons3 which results in an indirect hyperfine interaction. Note that the term -PKeff in eq 4 represents the dominant negative contribution to both Alland A,. McMillan and Halperd have discussed the influence of electron delocalization on the Fermi contact term. Following their notation Keffis split into two contributions Ken = 82*2K1 + (1 - P2*2)K2
(6)
Here /32*2K1represents the core polarization due to the electron density in the 3d, orbital; this is the only contribution considered by Kivelson and Lee.3 The second term in eq 6, (1 - @2*2)K2, (E) Takahashi, H.; Shiotani, M.; Kobayashi, H.; Sohma, J. J . Catal. 1969, 14, 134.
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J. Phys. Chem. 1986, 90, 2718-2723
is the vanadium core polarization due to electron density delocalized into other orbitals. Our data do not provide information on the mechanisms that are contributing to K2. One possible interpretation is the existence of static disorder, i.e. an inhomogeneous distribution of local deviations from the average C,, geometry assumed in the analysis. If the local distortions are of low symmetry, e.g. C2or C 1 ,an admixture of vanadium (3d,z + 4s) orbitals becomes symmetry allowed. In this case the term (1 - @2*2)K2 would contain a direct Fermi contact contribution from the 4s orbital. Experimentally we observe (Table I) that Ken is practically independent of @2*2, which implies that K1 and Kz must be of equal magnitude. Conclusions Electron spin resonance proved to be a powerful method for the characterization of monolayers and double layers of V 2 0 s supported on A1203,SO2,and MgO. From the spectra estimates of the V=O bond strength, and the delocalization of the V4+ unpaired electron onto the coordinatively bound oxygen ligands, have been obtained. The analysis of the spectra was based on a tetragonally (C,) distorted average coordination geometry. Increasing tetragonal distortion, which is due either to shortening of the V=O bond or to an increased ligand distance in the basal plane, corresponds in both cases to a higher bond strength.
Therefore carrier-induced changes of the g tensor anisotropy could be related to alterations of the catalytically important V=O bond strength. An interesting correlation of this parameter was found with previous investigations of methanol oxidation on these samples. A relatively weak vanadyl bond strength, as observed on MgO, resulted in low selectivity to formaldehyde, whereas the stronger V=O bond found on Si02 led to higher selectivity. The Al,O,-supported V 2 0 s system exhibited a more complicated behavior, since the product distribution is also influenced by the acidic sites present on this carrier. Electron delocalization was estimated from the hyperfine tensor principal values. Delocalization was found to increase upon reduction on A1203and S O 2 carriers. For V2Os supported on MgO the delocalization was largest and was unchanged by reduction. The Fermi contact parameter Kerfwas found to be independent of the support material and was unchanged by electron delocalization.
Acknowledgment. Thanks are due to A. Schweiger (Physical Chemistry Laboratory) for the use of the ESR facilities. Financial support by the Swiss National Science Foundation and by ETH (Schweizerischer Schulrat) is gratefully acknowledged. Registry No. V,05, 1314-62-1; MgO, 1309-48-4.
I n Situ Photoluminescence Studies of H y p e Gallium Phosphide Semiconductor Electrodes. Intermediates and Mechanism of Photoanodic Electron-Transfer Reactions Yoshihiro Nakato,* Kiyoyuki Morita, and Hiroshi Tsubomura* Laboratory f o r Chemical Conversion of Solar Energy and Department of Chemistry, Faculty of Engineering Science, Osaka University, Toyonaka, Osaka, 560 Japan (Received: October 24, 1985; In Final Form: February 3, 1986)
Photoluminescence spectra and photocurrents in n-type gallium phosphide (n-Gap) electrodes were studied in relation with electrode potential, solution pH, excitation wavelength, and intensity of the excitation light. It has been confirmed from these studies that the luminescence band peak at 800 nm arises from a surface species, which we called “surface-trapped hole”. It has been concluded that this surface-trapped hole is formed at the interface between the GaP crystal and an oxide (or hydroxide) layer covering it. Experimental results on the quenching of the 800-nm band by reductants in the solution strongly support the conclusion that the surface-trapped hole acts as the precursor of photoanodic electron-transfer reactions at the n-GaP electrode.
Introduction Photoelectrochemical properties of semiconductor and powder photo catalyst^^ have been studied extensively in view of solar energy conversion. The understanding of the molecular mechanism of surface reactions is very important for the achievement of efficient and stable conversion. Many workers have proposed the existence of surface states acting as electronhole recombination centers or as mediators for interfacial electron but the chemical structures of these states remain obscure. (1) Williams, F.; Nozik, A. J. Nature 1984, 312, 21-27. (2) Parkinson, B. A. J . Chem. Educ. 1983,60, 338-340. (3) Gratzel, M., Ed. Energy Resources Through Photochemistry and Catalysis; Academic: New York, 1983. (4) Gerischer, H. Faraday Discuss. Chem. SOC.1980, No. 70, 137-151. ( 5 ) Kelly, J. J.; Memming, R. J . Electrochem. SOC.1982, 129, 730-738. (6) Parkinson, B. A.; Heller, A.; Miller, B. J. Electrochem. Soc. 1979, 126, 954-960. (7) Gutierrez, C.; Salvador, P. J. Electroanal. Chem. 1982, 138, 457-463. (8) Wilson, R. H. J . Electrochem. SOC.1980, 127, 228-234. (9) Siripala, W.; Tomkiewicz, M. J . Electrochem. SOC.1983, 130, 1062-1 067. (10) Kobayashi, K.; Aikawa, Y.; Sukigara, M. Chem. Lett. 1981,679-680. (1 1) Allongue, P.; Cachet, H. J. Electroanal. Chem. 1984, 176, 369-375.
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We have investigated the electrochemical and luminescent properties of various n-type semiconductor electrodes such as n-GaP,I2-l7 n-CdS,16 and n-Ti02,16118and assigned some of the luminescence bands as being caused by electronic transitions from the conduction band to the vacant states acting as intermediates of photoanodic reactions. These studies have proved that luminescence measurement is a technique effective for the in situ investigation of reaction intermediates at semiconductor electrodes. We have reported that the luminescence band in the n-GaP electrode peaked at 800 nm arises from the precursor of photoanodic reactions, called “surface-trapped hole”.l4 The existence (12) Nakato, Y.; Tsumura, A,; Tsubomura, H. J. Electrochem. SOC.1980, 127, 1502-1506. (13) Nakato, Y.; Tsumura, A.; Tsubomura, H. J . Electrochem. SOC.1981, 128, 1300-1304. (14) Nakato, Y.; Tsumura, A,; Tsubomura, H. Chem. Lett. 1981, 127-130. (15) Nakato, Y.; Tsumura, A,; Tsubomura, H. Chem. Lett. 1981,383-386. (16) Nakato, Y.; Tsumura, A,; Tsubomura, H. Chem. Phys. Lett. 1982, 85, 387-390. (17) Nakato, Y.; Tsumura, A.; Tsubomura, H. Bull. Chem. SOC.Jpn. 1982, 55, 3390-3393. (18) Nakato, Y.; Tsumura, A,; Tsubomura, H. J . Phys. Chem. 1983,87, 2402-2405.
0 1986 American Chemical Society
