J. Phys. Chem. 1992, 96, 3442-3446
3442
A Luminescence Spectroscopy Study on Supported Vanadium and Chromium Oxide Catalysts M. F. Hazedcamp* and G.Blasse Debye Research Institute, University of Utrecht, P.O. Box 80.000, 3508 TA Utrecht, The Netherlands (Received: October 24, 1991; In Final Form: January 2, 1992)
The luminescence properties of dehydrated silica- and alumina-supported vanadium and chromium oxide catalysts at low loadings at 4.2 K are reported and discussed. It is shown that one luminescent oxc-vanadium (-chromium) species is present on silica surfaces and more than one species on alumina surfaces. The Cr/AIz03sample does not show luminescence. The monomeric tetrahedral species show vibrational structure in their emission spectra, and their decay times amount to 10-30 ms. These luminescenceproperties are different from those of similar complexes in the solid state. The discrepancy is ascribed to a different electronic structure of the surface species which localizes the excited state in a short metal-oxygen bond.
Introduction During the past two decades several papers have been published reporting photoluminescence experiments on supported oxotransition metal catalysts. Gritscov et al. reported for the first time the luminescence of a silica-supported oxo-vanadium catalyst.' Later, Anpo et al. published several papers on silica-supported oxo-vanadium, -molybdenum, -tungsten, and -chromium catalysts.z-5 Iwamoto et a1.6and Morys et al.'J also contributed to this field. In all these papers luminescence spectroscopy is mainly used to characterize the catalyst and/or to monitor photocatalytic reactions over these catalysts. In the present work the measurements reported in the literature are extended by spectral and decay measurements down to 4.2 K. It will be shown that the properties of the luminescent complexes in these catalysts are different from those of similar complexes in the solid state. In fact, the observation of efficient luminescence from these catalysts, especially in the case of Cr/SiOz, is rather unexpected. None of the papers mentioned above deals with these more fundamental aspects of the luminescence. In our laboratory a lot of experience with luminescence spectroscopy of related inorganic solid materials is a ~ a i l a b l e . ~ J ~ Therefore, it seemed interesting to perform a closer examination of the luminescence of these catalysts. Moreover, it has only recently become clear that the molecular structures of oxotransition metal complexes at the surface of supporting oxides are strongly different under hydrated and dehydrated conditions. Luminescence spectroscopy can serve as an analytical tool, in addition to other techniques like Raman, solid-state NMR, and X-ray absorption, to reveal the molecular and electronic structures of these complexes. In this paper we focus on supported oxo-vanadium and -chromium catalysts. A subsequent paper will deal with oxomolybdenum, -tungsten, and -niobium systems. Experimental Section All samples were prepared by wet impregnation of the supports. The starting materials used were SiOz (Aerosil 200, Degussa, (1) Gritscov, A. M.; Shvets, V. A.; Kazansky, V. B. Chem. Phys. Lett. 1975, 35, 51 1.
(2) Anpo, M.; Tanahashi, 1; Kubokawa, Y. J . Phys. Chem. 1980,84,3440. (3) Anpo, M.; Sunamoto, M.; Che, M. J . Phys. Chem. 1989, 93, 1187. (4) Anpo, M.; Tanahashi, I.; Kubokawa, Y. J . Phys. Chem. 1982.86, I . (5) Anpo, M.; Kubokawa, Y. Reu. Chem. Intermed. 1987, 5, 105.
surface area -200 m2/g), y-A1203(Engelhard A1-4172P, surface area -200 m2/g), NH,VO, (Merck, p.a.), NH4Cr2O7(Fluka, p.a.). The supports were impregnated with aqueous solutions of the ammonium salts at their natural pH. The samples were dried at 100 "C for 1 h and fired in air at 525 OC for 20 h. The samples were rapidly transferred from the hot furnace into the cryostat to avoid water adsorption as much as possible. In the cryostat the samples are in a helium atmosphere. The loading for all samples was 6 x mol of metal/g of supporting oxide, which corresponds to -5% of a monolayer. Excitation and emission spectra were measured using a Spex Fluorolog-2 spectrofluorometer equipped with a 450-W xenon lamp and an Oxford helium flow cryostat. These spectra were corrected for the output of the xenon lamp and for the sensitivity of the photomultiplier, respectively. Decay measurements in the case of the vanadium samples were performed on a Nd/YAG laser setup described in detail in ref 11. Excitation was at 280 nm. Decay measurements on the chromium samples were performed on a Molectron nitrogen laser setup, described in ref 12. Excitation was at 337 nm in this case.
Results V/SiOp The V/SiOz sample is white in the dehydrated form. When exposed to ambient conditions, the sample turns yellow. The diffuse reflection spectrum of this hydrated sample is presented in Figure 1. Luminescence measurements were performed on the dehydrated sample. At 4.2 K the V/SiOz sample shows a rather intense photoluminescence. The excitation and emission spectra are presented in Figure 1. Two excitation maxima are observed, at 250 and 295 nm. The emission spectrum shows a well-resolved vibrational progression. The average spacing amounts to 990 f 70 cm-'. The band at 522 nm (probably the 0-5 transition) is the one with the highest intensity. No second emission band, denoted by Anpo as fluorescence,2 is observed, in accordance with the papers by Gritscov' and by Iwamoto et aL6 The emission spectrum does not change upon varying the excitation wavelength. The decay curve of the emission at 4.2 K is presented in Figure 2. It shows a rather large deviation from exponential behavior. The tail of the curve could be fitted to an exponential function, resulting in a decay time of 33 ms. Neither the slope nor the shape of the decay curve changes up to 50 K. Above this temperature the decay time starts to decrease. V/A1203. The V/Alz03 sample is white in the hydrated and the dehydrated form. The emission intensity at 4.2 K of the dehydrated V/Alz03 sample is somewhat lower than in the case
(6) Iwamoto, M.; Furukawa, H.; Matsukami, K.; Takenaka, T.;Kagawa, S. J . Am. Chem. SOC.1983, 105, 3719.
(7) Morys, P.; Goerges, U.; Krauss, H.-L. Z.Nafurjorch.1984, 398, 458. (8) Morys, P.; Schmerbeck, S.Z. Narurforch. 1987, 42B. 756. (9) Blasse, G. Struct. Bonding 1980, 42, 1, (10) Blasse, G. Prog. Solid Sfate Chem. 1988, 18, 79.
0022-3654/92/2096-3442$03.00/0
( I 1) Vries, A . J . de; Vliet, J. P. M. van; Blasse, G. Phys. Status Solidi B 1988, 149, 391. ( 1 2 ) Vliet, J . P. M . van; Blasse, G.; Brixner, L. H . J . Electrochem. SOC. 1988, 135, 1574.
0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 8, 1992 3443
Supported Vanadium and Chromium Oxide Catalysts
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Figure 4. Decay curve of the emission of V/A120, at 4.2 K. Excitation is at 280 nm.
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Figure 5. Emission (EM) and excitation (EX) spectra of the luminescence of Cr/Si02 at 4.2 K.
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Figure 2. Decay curve of the emission of V/Si02 at 4.2 K. Excitation is at 280 nm. The solid line presents a fit to an exponential function.
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W a v e l e n g t h (nm) Emission (EM; hLY = 300 nm) and excitation (EX; XEM = 520 nm) spectra of the luminescence of the V/AI20, sample at 4.2 K. of the V/Si02 sample. The emission and the excitation spectra of the luminescence are presented in Figure 3. The excitation and emission maxima are positioned a t 330 and 550 nm, respectively. The width and the position of the maximum of the emission band change upon varying the excitation wavelength. This indicates that there is more than one luminescent species present on the surface.
In first approximation, there is a species with excitation and emission bands at shorter wavelengths and a species with excitation and emission bands at longer wavelengths than the maxima given above. In the emission spectrum of the former species, a weakly resolved vibrational structure is observed, which is indicated by the arrows in Figure 3. The average spacing amounts to 950 & 100 cm-I. The emission of the latter species is structureless. The decay curve of the emission at 4.2 K is presented in Figure 4. The curve is nonexponential. The tail could be fitted to an exponential function resulting in a decay time of 13 ms. Neither the slope nor the shape of the decay curve changes up to 70 K. Cr/Si02. The photoluminescence intensity of the Cr/Si02 sample at 4.2 K is somewhat lower than that of the V/Si02 sample. The emission and excitation spectra at 4.2 K are presented in Figure 5. The excitation spectrum shows maxima at 255,370, and 500 nm. The emission spectrum shows a vibrational progression. The spacing amounts to 955 f 50 cm-'. The vibronic component at 656 nm (the 0-2 transition) is the one with the highest intensity. The width of the vibronic components is similar to that seen in the emission spectrum of the V/Si02 sample. The emission spectrum does not change upon excitation at other wavelengths. The decay curve of this emission at 4.2 K is presented in Figure 6. The decay curve is nonexponential. The tail could be fitted to an exponential function, resulting in a decay time of 30 ms. Neither the slope nor the shape of this curve changes up to 25 K. When the sample is fired at 900 OC, a weak emission is observed in addition to the emission of Figure 5. This emission shows a more complicated vibrational structure, and the position of the 0-0 transition is at a longer wavelength, viz. 610 nm. This emission
3444 The Journal of Physical Chemistry, Vol. 96, No. 8, 1992
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seems to be similar to that reported by Morys et al.' for a Cr/Si02 sample fved at 800 OC. The presence of the oxc-chromium species involved in this emission seems therefore to be related to the higher firing temperature. A Cr/A1203 sample did not show luminescence due to an oxo-chromium(V1) species. Only a weak inhomogeneous broadened line emission around 700 nm was observed, which is ascribed to Cr3+ions incorporated in the amorphous A1203 support.
Discussion V/SiOp All optical transitions are ascribed to charge-transfer transitions in an oxo-vanadium(V) ~ p e c i e s . ~The , ~ absorption is due to singlet-singlet transitions and the emission to a tripletsinglet t r a n s i t i ~ n . ~ *From ' ~ the diffuse reflection spectrum of Figure 1 it appears that species with tetravalent vanadium are absent, since in that case, absorption bands are expected in the red or near-infrared region.14 The emission and excitation spectra in Figure 1 are similar to those published b e f ~ r e . l - ~The * ~ two bands in the excitation spectrum are ascribed to two different one-electron excitations from an occupied MO to an unoccupied M0.9*'3 The independence of the emission spectrum from the excitation wavelength indicates that there is only one kind of luminescent species present. This is confirmed by the fact that the vibrational structure is well-resolved. It is nowadays well-known that several kinds of oxo-vanadium species can be present on the silica s u r f a ~ e . l ~ -Anpo ~ ~ et al. ascribed the emission to monomeric tetrahedral V 0 4 species.2*3 This seems a correct attribution considering the following: (1) Similar spectral positions are observed for the isolated VOd3complex in the solid state.13 (2) The spectral positions of the two excitation maxima are in good agreement with those of the bands in the UV reflectance spectrum of a dehydrated V/Si02 catalyst at very low vanadium 10adings.I~ In ref 19 it is suggested that at very low loadings only monomeric tetrahedral oxo-vanadium species are present. (3) Upon increasing the vanadium loading, which is accompanied by the formation of polymeric species, the luminescence intensity drops sharplye6(4) In addition, it has been shown in the literature, on the basis of Ramanis and X-ray absorptionZomeasurements, that in dehydrated V/Si02 catalysts at ( I 3) Ronde. H.; Blasse, G. J . Inorg. Nucl. Chem. 1978, 40, 2 15. (14) Ballhausen, C . J.; Gray, H . B. Inorg. Chem. 1962, I , 1 1 1 . (IS) Went, G. T.; Oyama, S. T.; Bell, A. T. J . Phys. Chem. 1990, 94,4240. (16) Dco, G.; Wachs, I . E. J . Phys. Chem. 1991, 95, 5889. (17) Roozeboom, F.; Mittelmeijer-Hazeleger, M. C.; Moulijn, J . A.; Medema, J.; Beer, V. H . J . de; Gellings, P. J. J . Phys. Chem. 1980, 84, 2783. (18) Taouk. B.; Guelton, M.; Grimblot, J.; Bonnelle, J. P. J . Phys. Chem. 1988, 92, 6700. (19) Schraml-Marth, M.; Wokaun, A.; Pohl, M.; Krauss, H.-L. J . Chem. SOC.,Faraday Trans. 1991, 87, 2635. (20) Yoshida, S.;Tanaka, T.; Nishimura, Y.; Mizutani, H.; Funabiki, T. Proceedings, 9th InfernafionalCongress on Calalysis, Calgary; Phillips, M . J.; Ternan, M., Eds.; Chemical Institute of Canada: Ottawa, O N , Canada, 1988; Vol. 3, p 1473.
Hazenkamp and Blasse low loadings only monomeric tetrahedral oxo-vanadium species are present. The diffuse reflection spectrum of the hydrated V/Si02 sample in Figure 1 is similar to the absorption spectrum of the decavanadate ion in aqueous solution.2' The presence of decavanadate complexes on hydrated silica surfaces is in accordance with the literature.I6 Since our V/Si02 sample has been exposed for a short time to the ambient temperature and atmosphere during the rapid transfer from the furnace to the cryostat, our sample may also contain a small amount of decavanadate. The absorption band of the decavanadate species shows spectral overlap with the emission band of the sample. The fact that the present emission spectrum peaks at 522 nm, whereas the literagives 500 nm is ascribed to radiative transfer to the decavanadate. The true emission maximum is probably situated at 500 nm. The nonexponentiality of the decay curve is ascribed to energy transfer.22 An important acceptor is of course the decavanadate species. However, adsorbed molecular oxygen, which is known to quench this luminescence,2 and minor amounts of impurities are expected to play a role too. The tail of the decay curve is exponential and corresponds to the radiative decay time of the oxo-vanadium species.1° An analysis shows this to be 33 ms at 4.2 K, a very long time indeed. Let us now compare these results with those of the vo43complex in the bulk. In the solid state, vibrational structure has never been observed and the decay times of the emission are 1 order of magnitude shorter, viz. 1-2 ms.9J3 This discrepancy has never been noted before. In YV04,a representative example of a crystalline vanadate, the VO2- complex is tetrahedral with four equivalent V-O bonds of 1.71 The lowest optical transition is from a nonbonding orbital which has tl symmetry and oxygen 2p character to an antibonding orbital which has e symmetry and metal character.24 According to several authors, the luminescent V04 species at the silica surface has a quite different s t r ~ c t u r e . ~ItJ ~consists of a vanadium ion bonded to the silica surface with three Si-0-V bonds, whereas one very short ( N 1.56 Azs) axial V-O bond sticks out of the surface. The stretching vibration of this short V-O bond is the one that appears in the vibronic progression in the emission spectrum. From Raman measurements, this frequency was determined to be 1042 cm-I,l5 in agreement with the value derived from the emission spectrum. A detailed Franck-Condon analysis of the vibrational structure in the emission spectrum has recently been published.26 From an ESCA study on V/Si02 catalyst^,^' it has been determined that the binding energy of the oxo-vanadium species at the SiOz surface is higher than in reference compounds. This was ascribed to an electron-withdrawing effect of the S i 0 2 supports2' This results in a high ionicity of the S i 4 to V bond; i.e., the Si-O ligands are highly electronegative. This latter property of the bonding in the V 0 4 unit on the surface suggests that the lowest energetic excitation is localized in the short axial V-O bond, since the other three oxygens are not expected to release their electrons easily. This localization is in accordance with the observation of a vibrational progression involving the vibration of the short V-O bond. If this model for the luminescent V 0 4 species at the silica surface is correct, the same luminescence properties are expected if the highly electronegative Si-O ligands are replaced by fluorine ions. In the crystalline compound KVOF,, more or less isolated VOF4- units are present in which the V-O bond is very short, viz. 1.57 In fact, a vibrational progression involving the V-O
-
(21) Praliaud, H.; Mathieu, M.-V. J . Chim. Phys. (Paris) 1976, 73, 689. (22) Inokuti, M.; Hirayama, F. J . Chem. Phys. 1965, 43, 1978. (23) Baglio, J . A.; Gashurov, G. Acta Crystallogr. 1968, B24, 292. (24) Zieglet, T.; Rauk, A,; Baerends, E. J. Chem. Phys. 1976, 16, 209. (25) Hardcastle. F. D.; Wachs. I, E. J . Phys. Chem., in press.
(26) Patterson, H . H.; Cheng, J.; Despres, S.; Sunamotoa, M.; Anpo, M . J . Phys. Chem. 1991, 95, 8813. (27) Horvath, B ; Strutz, J.; Geyer-Lippmann, J.; Horvath, E. G.Z.Anorg. . Allg. Chem. 1981, 483, 181. (28) Rieskamp. H.; Mattes, R. Z . Anorg. Allg. Chem. 1973, 401, 158.
Supported Vanadium and Chromium Oxide Catalysts stretching vibration is observed in the emission spectrum in addition to a decay time of 33.5 ms at 4.2 K.29 It was also shown that the long decay time is due to the low oscillator strength of the singletsinglet transition from which the emissive tripletsinglet transition ‘steals” its intensity. The analogy between the luminescence of the VOF4- ion and the oxo-vanadium species on silica is striking. Presently we are engaged in abinitio calculations on a VOF3 model cluster in order to reveal the background of the low oscillator strength of the singlet-singlet transition. Kobayashi et al. have made efforts to characterize the lowest (luminescent) triplet state of the V 0 4 species on the silica surface using abinitio calculations on a (OH)3V0 model cluster.30 This seems to us an incorrect approach since the electronegativity of the O H ligands is not much higher than that of the axial oxygen. From their calculations it results that the HOMO consists of nonbonding oxygen p-orbitals located on the O H ligands. Therefore, in the approach of Kobayashi et al., the lowest energetic excited state will not be localized in the axial V-0 bond, which is in contradiction with the luminescence measurements (see above). V/A1203. The emission spectrum of V/A1203 resembles the one reported by Iwamoto et al. at 77 K,6 but in the present case, the luminescence intensity is not much weaker than that of a comparable V/Si02 sample as reported in ref 6. The V/Si02 and V/A1203systems have clearly different luminescence properties. The excitation edge of the latter is positioned at longer wavelengths than in the case of the V/Si02 system. In V/SiO, there is one luminescent species whereas in V/A1203there are at least two luminescent species. The existence of different oxo-vanadium species on alumina surfaces compared to silica surfaces has been observed before from Raman meas u r e m e n t ~and ~ ~ ,is~nicely ~ confirmed by the present measurements. Raman studies on dehydrated V/A1203catalysts at low loadings indicate two kinds of species on the surface: (1) monomeric tetrahedral species of a structure similar to that found on the silica surface and (2) oligo- and polymeric vanadate chains with tetrahedrally coordinated vanadium.I5J’ The shorter wavelength emission with the vibrational structure is assigned to the first species and the longer wavelength emission to the second species. The longer wavelength of the excitation edge and the emission maximum supports this latter assignment since it is known that, upon connecting V04 tetrahedra to form polyvanadate chains, the excitation is more or less delocalized over the chain. Delocalization is usually accompanied by a red shift of the absorption edge compared to the m o n ~ m e r . l ~ , ~ ~ The tail of the decay curve is exponential and corresponds to a decay time of 13 ms, which is much longer than that of the emission of the regular vo43tetrahedron. In analogy with V/Si02, this decay time is attributed to the radiative decay time of the monomeric species. The fast part of the curve is nonexponential. This is ascribed to energy transfer from the monomers to acceptors like adsorbed molecular oxygen and impurities. The decay of the emission of the polyvanadate is also located in the fast part of the curve. The decay time for the monomer on alumina is shorter than that for this species on silica (1 3 vs 33 ms). ESCA measurements indicate that the A1-0 to V bond is less ionic than the Si-0 to V bond, since the positive shift in the binding energy with respect to reference compounds is less for V/A1203than for V/Si02.27*33 Theory confirms these ESCA results since the ionicity of the bonding between A1 and 0 is higher than between Si and 0,due to the smaller electronegativity of Al. It is to be expected that the excited state is less well localized in the short V-O bond in the case of V/AI2O3, due to the lower (29) Hazenkamp, M. F.; Strijbosch, A . W. P. M.; Blasse, G. J . Solid State Chem., in press. (30) Kobayashi, H.;Yamaguchi, M.; Tanaka, T.; Yoshida, S. J . Chem. SOC.,Faraday Trans. I 1985, 81, 1513. (31) Vuurman, M . A.; Wachs, I. E., to be published. (32) Berdnikov, S. L.; Zelikin. Y . M . Opr. Spectrosc. 1983, 5 4 , 273. (33) Nag, N . K.; Massoth, F. E. J . Caral. 1990. 124, 127.
The Journal of Physical Chemistry, Vol. 96, No. 8, 1992 3445 ionicity of the AI-0 to V bond. This might be the explanation for the shorter decay time in the case of V/A1203, since it is assumed that this localization is a major requirement for the observation of a long decay time (see above). Cr/Si02. The luminescence of the Cr/Si02 samples is ascribed to charge-transfer transitions involving oxo-chromium(V1) species. The emission is due to one kind of oxo-chromium species,in which the metal is in four-coordination. The first excitation band and the emission band are at lower energy than those of the corresponding oxo-vanadium complex. This is usually observed for oxc-chromium and -vanadium c o m p l e ~ e s ~and q ~ ~can . ~ be ~ related to the values for the fifth and the sixth ionization potential of V and Cr, respectively. Only a limited number of oxc-chromium compounds are known to be l ~ m i n e s c e n t .However, ~~ their spectral and decay characteristics are different from those of the Cr/Si02 sample. For example, the weak emission band of CaCr04 at 4.2 K is broad and structureless; the decay time amounts to 150 ps. K2Cr2O7 shows some vibrational structure in its emission spectrum due to coupling with Cr-O bending vibrations. The decay time of this emission amounts to 800 ps at 4.2 K.34 These decay times are shortened by nonradiative decay. The radiative decay times are estimated to be -9 ms,j4 which is still considerably shorter than the 30 ms in the case of the Cr/Si02 emission. The spectral position of the first absorption band (500 nm) is at longer wavelength than that of the only slightly distorted CrO2tetrahedron in CaCr04 (450 nms4). The spectral position is in better agreement with those of oxo-chromium complexes with less than cubic symmetry like in K2Cr207( 5 0 0 nm34)and Cr02F, (480 n m 9 . This suggests that the luminescent species has a distorted tetrahedral symmetry. The vibrational structure in the emission spectrum and the anomalously long decay time suggest that the excited state is localized in a short Cr-O bond in a way similar to that observed for the V/Si02 system. The vibrational frequency of the progression in the emission spectrum amounts to 955 cm-’, which indicates that the Cr-0 bond in the present luminescent species is shorter than those in the tetrahedral Cr042-ion, for which the frequency of the symmetrical Cr-O stretching vibration amounts to 850 cm-1.36 We propose that the luminescent species is a monomeric four-coordinated oxc-chromium(V1) species with two short Cr-O bonds. The existence of such a species on the silica surface has been suggested before in the l i t e r a t ~ r e . ~ ?Moreover, ~’ following the general trend observed for dehydrated silica-supported oxovanadium, -molybdenum, and -tungsten catalysts at low loadings, only monomeric species are expected to be p r e ~ e n t . ~ ~ ~ ~ ~ J * Another aspect of the luminescence of these oxo-vanadium and -chromium catalysts which has not been discussed yet is the relatively high quantum efficiency. Especially luminescence from oxo-chromium complexes is not frequently observed due to the low-energy position of the a b ~ o r p t i o n .It~ ~has been argued that efficient luminescence of oxo-transition metal complexes is only observed if the expansion of the complex in the excited state is counteracted by a stiff crystal lattice.39 This condition is clearly not fulfilled in the case of the complexes at the surfaces. Coremans40 has suggested a simple model, arguing that more efficient luminescence from oxo-transition metal complexes is to be expected if the complex has a low site symmetry in the ground state. The possibility of coupling with Jahn-Teller active vibrations in the excited state, which can induce nonradiative relaxation, will be lower for lower symmetry.@ This model has been applied to (34) Dalhoeven, G . A. M.; Blasse, G . Chem. Phys. Lerf. 1980, 76, 27. (35) Jasinski, J. P.; Holt, S.L.; Wood, J. H.; Asprey, L. B. J . Chem. Phys. 1975, 63, 751. (36) Gonzalez-Vichez, F.; Griffith, W . P. J . Chem. SOC.,Dalton Trans. 1972, 1416. (37) McDaniel, M. P.; Martin, S. J. J . Phys. Chem. 1991, 95, 3289. (38) Boer, M. de; Dillen, J . van; Koningsberger, D. C.; Vuurman, M . A,; Wachs, I . E.; Geus, J . W . Catal. Lett. 1991, I I , 221. (39) Blasse, G . In Aduances in Nonradiatioe Processes in Solids; Dibartolo, B., Ed.; Plenum: New York, 1991; p 287. (40) Coremans, C . J. M . Ph.D. Thesis, Leiden University, 1989.
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J . Phys. Chem. 1992, 96, 3446-3452
explain the higher quantum efficiency of K2Cr207with respect to CaCr04.40 It can also be applied in the present case since the excitation is localized in a Cr-O or V-O bond. Such a bond has only one nondegenerate vibrational mode.
Conclusions The systems V/Si02, V/A1203, and Cr/Si02 show at low temperature a rather efficient luminescence. This was not expected because an important requirement for efficient luminescence is the presence of stiff surroundings of the luminescent complex which can counteract the expansion in the excited state. A surface clearly does not offer such stiff surroundings. Luminescence spectroscopy indicates one luminescent oxovanadium (-chromium) species on the silica surface and more than one species on the alumina surface. These results confirm the
results from other characterization techniques, such as Raman spectroscopy. The luminescence properties of the tetrahedral monomeric surface species, which are strongly distorted, are different from those of the corresponding regular tetrahedral complexes. This is ascribed to the localization of the excited state in a short metal-oxygen bond. Hence, in addition to information on the molecular structure of these surface complexes, luminescence spectroscopy can also give information on the electronic structure of these complexes.
Acknowledgment. We thank Dr. M. A. Vuurman of the Inorganic Chemistry Department of the University of Amsterdam for a valuable discussion and for sharing some of his results prior to publication.
Catalytic Oxidation of Phosphorus on MOO, As Studied by Infrared Spectroscopy Dilip K. Paul, Ling-Fen Rao, and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: October 31, 1991; In Final Form: December 27, 1991)
The decomposition and oxidation of phosphine has been studied on an MoO3/AI2O3supported catalyst using transmission IR spectroscopy and mass spectroscopy in the temperature range 300-800 K. Phosphine decomposes on Moo3 at 573 K and is oxidized to a surface species containing the H P = O moiety, exhibiting a characteristic H-P mode at 2490 cm-I and a P=O mode at -1 100 cm-l. Further oxidation at 673 K under 0 2 ( g ) produces a surface species (HO),P==O(a) which has probably migrated to the support. The ( H O ) , F O species is characterized by an 0-H stretching mode at 3672 cm-l; it is desorbed from the surface at temperatures near 773 K. The two sequential oxidation steps have been performed for several cycles using additional PH, adsorbate. These observations suggest that the MoO3/AI20, catalyst may be effective for the continuous catalytic oxidation of organophosphorus compounds.
I. Introduction The oxidation of phosphorus on surfaces is of fundamental importance in the development of heterogeneous catalytic methods for environmental protection from phosphorus-containing compounds such as those used in pesticides, herbicides, and chemical warfare agents.l It has been shown that certain metals such as Pt,2 Pd,) and Mo4 are somewhat effective in promoting the catalytic oxidation of an organophosphorus compound, dimethyl methylphosphonate (DMMP). In recent surface science studies on single crystals of Pd3 and M o , ~it has been shown that the key to continued catalytic oxidation activity is the attainment of temperatures where surface phosphorus is readily oxidized away leaving clean metal or metal oxide sites available for continued catalytic chemistry. In model experiments on single crystals in an ultrahigh-vacuum environment, the critical temperatures for phosphorus oxidation were found to be approximately 900 K (M0(110)~)and 1000 K (Pd(111)3). This paper is concerned with the catalytic oxidation of adsorbed phosphorus by an A1203-supportedMOO, catalyst. The fundamental premise of our approach is that the key rate-controlling step in the catalytic destruction of phosphorus-containing compounds will be the elemental phosphorus catalytic oxidation step. Therefore this basic catalytic reaction should first be studied under controlled conditions. Phosphorus is supplied to the MOO, catalyst by phosphine gas, and measurements of partially oxidized phosphorus surface species and their interconversions are made using transmission IR spectroscopy. It has been found that at (1) Ekerdt, J. G.;Klabunde, K. J.; Shapley, J. R.; White, J . M.; Yates, J. T.,Jr. J . Phys. Chem. 1988, 92, 6182. (2) Henderson, M. A.; White, J. M. J . Am. Chem. SOC.1988, 110, 6939. (3) Guo,X.;Ycshinobu, J.; Yates, J. T., Jr. J . Phys. Chem. 1990, 94,6439. ( 4 ) Smentkowski. V . S.; Hagans, P.;Yates, J . T., Jr. J . Phys. Chem. 1988. 92, 6351
1.5 Torr oxygen pressure, on MOO,, the onset of adsorbed phosphorus oxidation may be detected by IR spectroscopy at 673 K. This is a much lower and more practical oxidation temperature than that measured in the single-crystal Mo( 1 10) experiment^.^ This work suggests that a low-temperature catalytic oxidation process for organophosphorus compounds can be designed using MOO,-based catalysis. 11. Experimental Section The stainless steel ultrahigh-vacuum IR cell used in these studies is shown in Figure 1 and has been described in more detail earlier.s The cell is equipped with CaF2 optical windows sealed into 2.75-in.-diameter conflat flanges, allowing transmission IR measurements in the ~ 1 O O O - c mspectral ~' range. In the center of the cell is a tungsten grid, which consists of closely spaced square openings of 0.22-mm width photoetched in tungsten foil of 0.0254-mm thickness. Inside each of the openings in the grid, the high area catalyst is supported in intimate contact with the tungsten foil frames surrounding each grid opening. This permits excellent thermal contact between the powdered catalyst and the tungsten grid, while also permitting high transmission efficiency for infrared radiation. The tungsten grid is rigidly held between Ni clamps which serve both as electrical connections and as heat-transfer connections to a reentrant dewar which supports the Ni clamps on their electrical feedthroughs; these clamps then support the tungsten grid. The grid containing the catalyst can be heated using an electronic controller, and temperatures are measured with a thermocouple welded to the tungsten grid. The upper temperature limit exceeds 1500 K with this cell, and rapid and reproducible ( 5 ) Basu, P.; Ballinger. T.H.; Yates, J. T., Jr. Reu. Sci. Insrrum. 1988, 59, 1321.
0022-365419212096-3446%03.00/0 0 1992 American Chemical Society
