J. Phys. Chem. 1991, 95, 8813-8818
8813
Relationship between the Geometry of the Excited State of Vanadium Oxides Anchored onto Si02 and Their Photoreactivity toward CO Molecules Howard H. Patterson,* Jian Cheng, Scott Despres, Department of Chemistry, University of Maine, Orono, Maine 04469
Masatoshi Sunamoto, and Masakazu A n p * Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Sakai. Osaka 591, Japan (Received: November 20, 1990)
The phosphorescence spectrum of vanadium oxide anchored onto S O 2shows well-resolved vibrational fine structure at 77 K. Franck-Condon analysis of the phosphorescence indicates that the internuclear equilibrium distance between oxygen and vanadium ions of V=O vanadyl groups as emitting sites of V/Si02 catalyst anchored onto Si02is larger by 0.12 A in its charge-transfer excited triplet state than in its ground state. UV irradiation of the catalyst in the presence of CO leads to the formation of C 0 2 (photoreduction of the catalyst) and adsorption of CO. In this former photoreaction, a good linear relationship between the yields of photoformed C02and the yields of phosphorescence is observed. Since photoformation of C02is accompanied by the removal of oxygen from the catalyst, both theoretical Franckxondon analysis and experimental results indicate not only that the charge-transfer excited state of the surface vanadyl group of the catalyst plays a significant role in the photoreaction but also that an elongation of the internuclear distance between vanadium and oxygen ions in the excited state of the vanadyl groups may be associated with the easier photoreduction of the catalyst with CO.
Introduction For the complete understanding of photocatalysis on a molecular level, it is important to elucidate the chemical nature and reactivities of photogenerated electrons and holes, as well as the excited states of the catalysts and the primary processes, including their dynamics.’-s With the ‘anchored” catalysts one can design active surface species that span the range from discrete molecules, to aggregated clusters, and finally to extended semiconductors.6 Therefore, with such chemically designed “anchored” catalysts, one can expect not only to obtain much detailed information about the excited states of catalysts and particle size effect and/or size quantization effect on photocatalysis but also to achieve more active and selective photoreaction systems, because when a metal oxide is dispersed on a support surface both its physical and chemical nature are seriously modified, often resulting in high catalytic and photocatalytic activities and selectivities.’” On the other hand, in view of the significant contribution of the studies of photoluminescence to the development of photochemistry, the studies of photoluminescence of catalysts relating to the photoreaction on them also seem to be very useful to clarify the excited-state and primary processes of photocatalysis. Fortunately, many metal oxide catalysts such as vanadium7-15 and molybdenum oxides1626 supported on Si02 exhibit photoluminescence in the visible region as a radiative deactivation from the charge-transfer excited states of the oxides, having higher quantum yields when their dispersion becomes higher. Thus, photoluminescence, which exhibits high sensitivity and nondestruction of the surface active sites, is a convenient technique for the investigation of the structure and properties of the surface active sites on the supported catalysts with much lower metal oxide loading. For supported vanadium oxide catalysts with lower vanadium contents less than 1 .O wt %, the following have been reported by means of photoluminescence measurements, including their dynamics. The vanadium oxide anchored onto Si02by the photochemical vapor deposition method exhibits a much higher quantum yield in the formation of the charge-transfer excited triplet state of the vanadyl group ( V 4 ) of the oxide due to its better dispersion, resulting in much higher photocatalytic reactivity for isomerization of trans-2-butene due to less efficient radiationless deactivation in the excited state than the catalysts prepared by the conventional impregnation m e t h ~ d . ~ * ~ * * ~
* Authors to whom correspondence should be addressed. 0022-3654/91/2095-8813$02SO/O
In this paper, we deal with the Franck-Condon analysis of the well-resolved phosphorescence spectra a t 77 K of vanadium catalysts anchored onto S O 2 with different vanadium concentrations, photoreactivities of these catalysts toward CO molecules, and finally the relationship between these features. The results of the Franck-Condon analysis of the well-resolved photoluminescence spectrum of powdered ZnO and its photoreactivity toward CO are compared with those for vanadium oxides anchored (1) Anpo, M.; Kondo, M.; Coluccia, S.; Louis, C.; Che, M. J . Am. Chem. Soc. 1989,111, 8791. (2) Anw, M.; Kubokawa, Y. Res. Chem. Intermed. 1987.8, 105. (3) A n h , M. Res. Chem. Intermed. 1989, 11, 67. (4) Anpo, M. Hyomen, in press. (5) Anpo, M. Hyomen Kagaku 1990, 11, 39. (6) Anpo, M. In Proceedings of the 8th International Conference on Photochemical Conversion and Storage of Solar Energy (I.PS.4); Kluwer Academic: Dordrecht, 1990, in press. (7) Anpo, M.; Sunamoto, M.; Che, M. J . Phys. Chem. 1989, 93, 1187. (8) Anpo, M.; Tanahashi, 1.; Kubokawa, Y. J. Phys. Chem. 1980,84,3440. (9) Anpo, M.; Tanahashi, 1.; Kubokawa, Y. J. Phys. Chem. 1982,86, 1. (10) Anpo. M.; Suzuki, T.; Yamada, Y.; Che, M. Proc. 9rh Int. Congr. C a r d 1988, 3, 1513. (11) Anpo, M.; Sunamoto, M.; Fujii, T.; Patterson, H.; Che. M. Res. Chem. Intermed. 1989, 11, 245. (12) Anpo, M.; Suzuki, T.; Yamada, Y.; Otsuji, Y.; Giamello, E.; Che, M. In New Developments in Selective Oxidation; Centi, G., Trifiro, F.. Eds.; Elsevier: Amsterdam, 1990; p 683 and references therein. (13) Kazansky, V. B. Proc. 6th Int. Congr. Catal. 1976, 50. (14) Iwamoto, M.; Furukawa, H.; Matsukami, K.; Takenaka, T.; Kagawa, S.J . Am. Chem. Soc. 1983, 105, 3719. (15) Occelli, M. L.; Psaras, D.; Suib, S.L. J . Coral. 1985, 96, 363. (16) Anpo, M.; Tanahashi, I.; Kubokawa, Y. J . Chem. Soc., Faraday Trans. I 1982, 78, 2121. (17) Ono. T.: Anw. M.: Kubokawa. Y. J . Phvs. Chem. 1986.90.4780. (18) Anpb, M.;Skuki, T.;Kubokawa, Y.; Tinaka, F.; Yamashi&, S. J . Phys. Chem. 1984,88, 5778. (19) Anw, M.;Kondo, M.;Kubokawa.. Y.:. Louis.. C.:. Che.. M. J . Chem. Si.,Faradaj Trans. 1 1&38,.84, 2771. (20) Kazansky, V. B.; Pershin, A. N.; Shelimov, B. N. h o c . 7th Inr. Congr. Coral. 1980. 8-1210. (21) Kubokawa, Y.; Anpo, M. In Adsorption and Catalysis on Oxide Surfaces; Che, M., Bond, G. C., Eds.; Elsevier: Amsterdam, 1985; p 127. (22) Shelimov, B. N.; Elev, I. V.; Kazansky, V. B. J . Catal. 1986,98,70. (23) Iwasawa. Y.; Nakano, Y.; Ogasawara, S. J . Chem. Soc., Faraday Trans. 1 1978, 74, 2968. (24) Iwasawa, Y.; Ogasawara, S.J . Chem. Soc., Faraday Trans. 1 1979, 75, 1465. (25) Iwasawa, Y.;Ogasawara, S.Bull. Chem. Soc. Jpn. 1980,53,3709. (26) Iwasawa, Y. Adu. Catal. 1987, 35, 187.
0 1991 American Chemical Society
8814 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 WAVE NUMBER / cm-1 19000
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Figure 1. Phosphorescencesptctra of anchored V/BO2 catalysts at 77 K (excitation wavelength, 280 nm; slit width, 5 nm; concentration of anchored vanadium oxide (in V wt W),1: 1.3, 2: 2.23, 3: 3.07). The spectrum of V/Si02-2catalyst was previously reported in ref 11.
onto SO2. Such studies have never been reported in the literature. These results, therefore, give useful information about the excited state of the catalysts and the mechanisms of the photocatalytic reactions, especially about the primary processes of photocatalysis on a molecular scale.
NUMBER / 30000
cm-1
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nSpe 2. Excitation spectra at 77 K of the phosphorescence of anchored V/Si02 catalysts corresponding to the spectra in Figure 1 (monitoring
the phosphorescence at 500 nm).
deposition m e t h ~ d . ~As. ~described pre~iously,"'~the absorption and phosphorescence are attributed to the following chargetransfer processes on the surface vanadyl groups ( V - 0 ) of tetrahedral V 0 4 unit structure, involving an electron transfer from 01- to V5+ ions and a reverse radiative decay from the charge-transfer excited Experimental Section triplet state. The chemically supported "anchored" vanadium oxides onto As seen in Figure 1, maximum intensity in the phasphorascenoe S O 2 (anchored V/Si02) were prepared by using a facile reaction is observed with the anchored V/Si02-2 catalyst. The lifetimes of VOC13with the surface OH groups of Si02 (Miqsawa Kagaku of the phosphorescence of these catalysts were determined from Co., BET surface area 200 m2/g), which had heen degassed at the decay curves of the phosphorescence. As described in the 423 K for 20 h, in anhydrous conditions at 350 K. Anchored previous the phosphorescence decay curves of the ansamples were dried, evacuated, and then hydrolyzed with H 0, chored vanadium oxide catalyst show a single exponential,except and then finally these samples were calcined in O2at 773 K.S.foJl for the anchored V/Si02-3 with the highest V content. The The contents of anchored vanadium oxide were determined to be lifetimes of the phosphorescence at 77 K of these ancbored V/ 1.30 (anchored V/Si02-l), 2.23 (anchored V/Si02-2), and 3.07 Si02-l, V/Si02-2, and V/Si02-3 are 5.6, 5.6, and 5.0 ms, reV wt % (anchored V/Si02-3), respectively, by plasma emission spectively. From these results, as described previously, in the spectrometry. anchored V/Si02-3 some weak interaction between V 0 4 unit Details of the procedure and apparatus for the photoluminesstructures arises because a higher vanadium loading leads to cence and lifetime measurements were described pre~iously.'*~-*~ additional radiationless pathways, which results in the shorter Prior to the photoreaction, and the photoluminescenceand Raman lifetime of the phosphorescence and some deviation in a singlemeasurements, the anchored V/Si02 catalysts were degassed at exponential decay of the phosphorescence of the catalyst. The 723 K for 4 h, heated in O2 at 743 K for 2 h, and then finally presence of such interaction is also suggested by the red shift in evacuated at 473 K for 1 h (about 5 X lod Torr). The photothe absorption spectrum of anchored V/Si02-3 compared with reaction was carried out at 280 K as follows: CO (Takachiho anchored V/Si02-1 and -2 with lower V contents (Figure 2). Kagaku Kogyo Co.) was introduced onto the catalyst that had 020been pretreated under vacuum at 473 K. The sample was subII I jected to UV irradiation at 280 K using a high-pressure mercury lamp (Toshiba SHL-100UV) with a color filter to cut off wavelengths below 280 nm and a water filter. The.gas phase was analyzed at definite time intervals by using a Pirani vacuum gauge and gas chromatography. The Raman measurements were done Figure 3 shows the second-derivative spectra of the phoswith a Spectra-Physics (Model 2020) Ar ion b r . A prism phorescence shown in Figure 1. The spectrum indicates that the monochromator and interference filter were equipped to eliminate energy separation between the (0 0) and (0 1) transition the plasma lines. Data were recorded by use of a holographic bands is 1050 f 5 cm-',being attributed to the vibronic transition grating double monochromator, a cooled photomultiplier tube, due to the vanadyl V 4 double bond. Figure 4 shows the Raman and a pulse counting photometer system. The monochromator spectrum at 78 K of the anchored V/SiO,-l catalyst. A Raman and photometer were interfaced to a personal Apple IIc computer peak is observed at 1050 cm-I, being in good agreement with the for automated spectra scanning and digitization. results obtained by Beil et aLn on the V/Si02 with 1.4 V wt 5%. They have assigned the peak at 1050 cm-'to a monomeric vanadyl Results and Discussion species bonded to the Si02 support shown in eq I. Busca et al.= PhosphorescenceSpectra of V d u m oxldea Ancholunl onto and Iwamoto et al." have also shown that the Raman and IR SO2. Figure 1 shows the phosphorescence spectra at 77 K of peaks in the region 1035-1049 cm-I are due to the similar anchored vanadium oxide catalysts, i.e., anchored V/Si02-1, structure, i.e., a monoxo ( V 4 ) species on the supported vanaanchQrcdV/Si02-2, and anchored V/Si&-3. Figure 2 shows the dium oxides. Thus, a good accordance between the Raman value excitation spectra of anchored vanadium oxides mesponding to of 1050 cm-' and the energy separation of 1050 cm-' of the (0 the phosphorescence spectra in Figure I . These phosphorescence 0) and the (0 1) transition bands of the phosphorescence and excitation (absorption) spectra are in good agreement with those obtained with supported V/Si02 catalysts prepared by a conventional impregnation method8-I4and with vanadium oxide (27) Went. G. T.;Oyama, S.T.;Bell, A. T.J. Phys. Chem. 1990,944240. catalysts anchored onto Vycor glass by a photochemical vapor (28) Cristiani. C.; Forzatti, P.; B u m , G. J . Card 1989. 116, 586.
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The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8815
Vanadium Oxides Anchored onto Si02 WAVE LENGTH / n m 46 0
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Figure 4. Raman spectrum at 78 K of anchored V/Si02 catalyst (2.23 V wt %). (Figure shows only the region corresponding to the vibration of vanadyl groups.) TABLE I: FranckCondon A and B Values Determined for the Anchored V/SiO,-1. -2. and -3 Catalvsts at 77 K
-
anchored V/Si02- 1 anchored V/Si02-2 anchored V/SiO2-3
0.750
0.1180 0.1 I85 0.1 I75
0.760 0.780
= 0) (u"= 1) to an integral corresponding to (u'= 0) = 0) can be expressed as the f o l l ~ w i n g : ~ ~ * ~ ~ -8001. - 28000
8 o o ~ 3770
24000 20000 WAVE NUMBER / cm-1 WAVE LENGTH / n m 460 550
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+ 1)
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Here D is a parameter proportional to the difference in the internuclear equilibrium separation between the ground and excited electronic states and B is the square root of the frequency of the excited state to the ground electronic state. The ratio of R(n,O) to R(0,O)can be determined from the following relationship for Hermite polynomials: R(n+l,O)/R(n,O) = 2 D / ( ( B 2 + 1) (n 1)'l2) + n ' / * / ( n + 1) (E2 - 1)/(B2 + l)R(n-l,O)/R(n,O) (2)
+
The Franck-Condon overlap integrals are related to the relative intensities for the vibronic emission peaks by intensity of transition for (0 intensity of transition for (0
-- [
n) E(n,O) = 0) .,o,o)]
[ -1
R(n,O)
(3) where E(n,O) refers to the energy of the vibronic peak corresponding to a transition from (0' = 0) to (u" = n). If the internuclear equilibrium distance of the excited electronic state ( r l ) is shifted by the value A from the internuclear equilibrium distance of the ground state ( r c ) , the Franck-Condon principle allows for transitions to a number of excited vibrational levels. The shapes of the harmonic potentials also have an effect on the Franckandon integral. In the present case, the theoretical intensities have been calculated as a function of A and B. The parameters B and A were varied until the theoretical intensities match closest to the experimental intensities. In Figure 5 , the fit of the progression obtained from the experimental photoluminescence spectra for the anchored V/Si02 catalysts and the theoretical Franck-Condon analyses are shown. Table I shows the Franck-Condon results obtained for the anchored V/Si02-1, -2, and -3 samples. Since the average vibrational energy in the progressions is larger than 1OOO cm-', we can assume that most of the vibrational states populate the u' = 0 vibrational level in the excited electronic state, and the emission occurs from this level to the vibrational levels of the ground state
-E 16000 -800 28000
24000 20000 WAVE NUMBER / cm-1
Figure 3. Second-derivativephosphorescence spectrum at 77 K of anchored V/Si02 -1, -2, and -3 catalysts. The spectrum of V/Si02-2 catalyst was previously reported in ref 1I .
obtained in the present work indicates not only that eq I is in good agreement with the model proposed by Bell et ale?' Busca et a1.,2* and Iwamoto et but also that the photon energy absorbed in the catalyst is mainly localized on the vanadyl group, Le., V=O surface bonds of the catalyst. Franckxondon Analysis of the Phosphorescence Spectra. According to the Franck-Condon principle, to evaluate the Franck-Condon overlap integraPN the vibrational wave functions for the ground and excited electronic states have to be expressed in terms of Hermite polynomials including displacement of the origin of the normal coordinates of the ground and excited electronic states. The ratio of an integral corresponding to (u' (29)Herzberg, G.Molecular Spectra and Molecular Structure I. Spectra of Diatomic Molecules; Van Nostrand: New York, 1966. (30)Herzberg, G. Molecular Spectra and Molecular Structure I I . Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand: New York, 1966.
(31) Henderson, J. R.; Muramoto, M.;Willett, R. A. J . Chem. Phys. 1964, 41, 580. (32) Henderson, .I. R.; Willett, R. A.; Muramoto, M.;Richardson, D. R.
Douglass Report SM-45807,Jan 1964.
Patterson et al.
8816 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
EXPERIMENTAL
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>
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80
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40
60
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ieooo
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Figure 6. Time profiles of the photoformation of COz under UV irradiation of anchored V/Si02-I, -2, and -3 catalysts in the presence of CO molecules (4.0 Torr) (1, anchored V/Si02-1; 2, anchored V/Si02-2; 3, anchored V/Si02-3).
values of A, with anchored V/Si02-2 having the highest photoreactivity and the largest value of A. It should be pointed out that there is not anything in the Franck-Condon derivation with harmonic oscillator wave functions that proves the bond is longer and not shorter in the excited state. However, due to anharmonicity, the intensity distribution along the progression is slightly different for r,‘ > r / than for r,‘ < r;’ because a steeper part of the potential is sampled in the latter case, giving rise to a broader maximum in the progression intensity. Comparison of the experimental spectra with the calculated spectra (ignoring anharmonic effects) in Figure 5 indicates the experimental spectra are slightly sharper than the calculated spectra, implying that r,’ > rl’. Assume that the observed structure in the luminescence spectrum is due to a progression for the V = O ground electronic state with the energy of u vibrational quanta given by
ENERGY (CM-1) BVSP
EXPERIMENTAL
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18000
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EXPERIMENTAL E4 C M M T E D I
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ENERGY (CM-1)
Figure 5. Progression of the vibrational fine structure of the phosphorescence spectrum of anchored V/Si02-I, -2, and -3 catalysts at 77 K and fit of the theoretical Franck-Condon analysis (a, anchored V/ SO2-1; b, anchored V/Si02-2; c, anchored V/Si02-3).
in which the Franck-Condon integral is nonzero. The following two facts should be emphasized from the results shown in Table I. First, the excited-state potential is displaced from the ground-state potential by 0.12 A, being in agreement with the assignment of the electronic transition from 02-to Vs+ ions of surface (V-0) vanadyl group^.^-'^ Second, as shown later, the order of the photoreactivity of the anchored V/Si02- 1, -2, and -3 toward CO molecules is in good accordance with that of the
We(U +
!4) - WJe@ + f/2I2
(4)
Since E(u+l) - E(u) = We- 2WJe(u+1), a plot of E(u+l) E(u) versus (u + 1) gives a linear relation with WJe about 6.8 cm-’. This implies that only a small amount of anharmonicity is present in the ground-state vanadyl group vibrational potential, and a harmonic oscillator model can be used to describe the potential surface. It is interesting to compare the present Franck-Condon results with recent ab initio Hartree-Fock molecular orbital results for supported V/Si02 oxide catalysts. Kobayashi and Yoshida et al.33 have calculated that the V=O bond of the oxide is elongated in its lowest charge-transfer excited state by 0.3 A compared with that in the ground state. Their potential energy curves for the ground state versus the lowest excited triplet state have indicated a sizable change in the V=O vibrational energy, being in agreement with the results suggested by the B values in Table I. Thus, it is concluded that the present Franck-Condon calculations of the progressions of the vibrational fine structure of the phosphorescence of anchored V/Si02 are in qualitative agreement with the MO predictions. Photoreduction of Anchored Vanadium Oxides with CO Molecules. As described previously,8 UV irradiation of anchored vanadium oxide catalysts at 280 K in the presence of CO molecules leads to the photoformation of C02 molecules (Le., photoreduction of the catalyst), it accompanying small amounts of photo-uptake of CO molecules. Figure 6 shows the time profile of these photoinduced reactions for the anchored V/Si02-1, -2, and -3 catalysts. After photoreduction of the anchored catalysts and further evacuation of excess CO, O2 was admitted under 4.0 Torr at 280 K for a few minutes and then evacuated. After this treatment the photoreduction with CO proceeded with the same efficiency as before. This result indicates that the original state of the catalyst surface is completely restored at 280 K by contact with (33) Kobayashi, H.; Yamaguchi, M.; Tanaka, T.; Yoshida, S.J . Chem. SOC.,Faraday Trans. I 1985.81, 15 13.
The Journal of Physical Chemistry, Vol. 95, No. 22, 1991 8817
Vanadium Oxides Anchored onto S i 0 2
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Yields of phosphorescence, orb.unit Figure 7. Relationship between the yields of photoformed C02on the anchored V/Si02 catalysts and the relative yields of phosphorescence of the catalysts at 280 K. Fieure 9. Photoluminescence sDectrum of Dowdered ZnO catalvst at 77
KTexcitation wavelength 280 i m ) . I zt
P,
+*:6
0
1 Amount of
2
3
4
5
6
CO, ld*mol/qcm
Figure 8. Experimental plots of Qo/Qversus the amounts of added CO (Stern-Volmer plots) for the intensity ( 0 ) and lifetime (0) of the phosphorescence of V/Si02-1 catalyst at 77 K.
oxygen; Le., the photoreduced surface of the anchored V/Si02 catalysts are easily reoxidid by 02,in agreement with the results obtained with anchored Mo/Si02 catalysts.'J9 As shown in Figure 6, the yield of photoformed C 0 2 is the highest with anchored V/Si02-2 catalyst, and then anchored V/Si02-I and V/Si02-3 catalysts are followed. Figure 7 shows the relationship between the yields of photoformed C 0 2 and the yields of phosphorescence of the catalysts. These results indicate that the chargetransfer excited triplet state of the surface vanadyl groups play a significant role in the photoformation of C 0 2 molecules. In fact, the intensity and lifetime of the phosphorescence are found to decrease in the presence of CO molecules, its extent depending on the amount of CO. And as shown in Figure 8, the Stern-Volmer plots of the phosphorescence quenching in the intensity and lifetime by CO are almost linear functions of the amount of added CO. These results prove that the emitting sites are responsible for the photoreaction of CO. From the Stem-Volmer plots for the V/Si02-l at 77 K, together with the lifetime of the charge-transfer excited triplet state of the surface vanadyl groups, the absolute rate constant of the excited state toward CO molecules was determined to be 4.1 X 1O'O (g of cat. s)/mol at 77 K. This value is larger than those of various alkenes such as C2H4 (3.5 X 1O'O) and CIHB (2.4 X 101o).lo Relation between the Results of the Franck-Condon Analysis and the Photoreduction of the Catalyst with CO Molecules. As reported in a previous paper? the photoreduction of the catalysts with CO molecules is confined to the oxides having metal-oxygen surface doublebond character ( M d ) , such as supported V2Ot MOO,, and CrO3. The photoreducibilityof these oxides decreases in that order, Le., decreasing with the lifetime of the chargetransfer excited triplet state determined from the decay of the phosphorescence of the supported oxides at 295 K. The present
I
0.2
0.4
0.6
0.8 1 1.2 ENERGY (1000 cm-1)
Figure 10. Raman spectrum of powdered ZnO at 300
1.4
1.6
K.
results (Le., parallel relationship between the yield of photoformed C 0 2 and the yield of the phosphorescence, and quenching of the phosphorescence in yield and lifetime with added CO) also indicate the direct participation of the charge-transfer excited state in the photoreduction of the catalyst (photoformation of C02). As mentioned above, the Franck-Condon analysis of the vibronic fine structure at 77 K of the phosphorescence of the anchored V/Si02 catalysts suggested that the equilibrium bond distance V-O of the vanadyl groups in the charge-transfer excited triplet state is elongated by 0.12 A as compared with that in the ground state. Since photoformation of C 0 2 molecules from CO is accompanied by the removal of oxygen from the oxide (i.e., photoreduction of the catalyst), such an elongation of the equilibrium nuclear distance in the excited state is closely associated with the fact that with anchored V/Si02 catalysts photoformation of C 0 2 proceeds very easily. As a result, the 0-hole centers in the charge-transfer (V4+-O-) complexes exhibit the reactivity similar to that of 0-anion radicals. As reported previ~usly,~~ powdered ZnO catalyst (Kadox 25) exhibits photoluminescence with vibrational fine structure at 77 K (Figure 9). Four peaks (a-d) in the photoluminescence spectrum of ZnO are separated from each other by 420,620, and 560 cm-I. Figure 10 shows the Raman spectrum of powdered ZnO at 300 K in the range 200-1600 cm-I. A sharp Raman peak is observed at about 420 cm-I, being in good agreement with the energy separation in the vibronic fine structure of the photoluminescence of ZnO at 77 K. Although it is not clear that the emitting sites of ZnO are located on the surface and/or near surfaces, from these results, peak a in the photoluminescence is assigned as a (0 0) vibrational transition of the ZnO catalysts.
-
(34) Anpo,
M.;Kubokawa, Y.J . Phys. Chem. 1984,88, 5556.
J. Phys. Chem. 1991, 95, 8818-8823
8818
surfaces. Some other part of the photoabsorption of CO occurs The Franck-Condon analysis of the vibronic spectrum of ZnO via the interaction of CO with the excited state, (V4+-O-),3to indicates no change in the equilibrium nuclear distance of Zn-O produce (Vs+_o2-)CO species. The former seems to be associated in the excited state; Le., no displacement for the excited-state with the stronger CO adsorption species and the latter with weaker potential versus the ground-state potential occurred. CO adsorption species which can be removed by the evacuation U V irradiation of powdered ZnO at 298 K in the presence of of the system at 298 K. CO, even after long U V irradiation, shows no evolution of C 0 2 molecules.2w Only photo-uptake of CO molecules was f o ~ n d . ~ J ~ Conclusion Being in good agreement with the prediction of the FranckFranck-Condon analysis of the phosphorescence spectra of Condon analysis, these experimental results indicate that the vanadium oxide anchored onto S i 0 2 indicates that the nuclear removal of oxygen on the ZnO catalyst scarcely proceeds under distance of the V-O bond of the vanadyl groups of the tetraheU V irradiation at 298 K,and it has much stronger Zn-O bond drally coordinated VO, unit structure becomes longer by 0.12 A strength in the excited state of (Zn+-O-) than that of the V-O in its charge-transfer excited triplet state as compared with the bond in the charge-transfer excited (V4+-0-)complex which is ground state. In agreement with the theoretical prediction, U V sufficiently weakened to react with CO to form COP Thus, these irradiation of the anchored V/Si02 catalysts at 280 K in the results clearly indicate that not only the electronic nature of the presence of CO leads to the formation of C02,i.e., reduction of excited state but also the elongation of the M e 4 bond in the catalysts. These results clearly indicate that oxygen atoms which excited state of the oxide, Le., displacement for the excited-state constitute the vanadyl group of the tetrahedral coordinated VO, potential versus the ground-state potential, is one of the most structure are easily incoporated into CO molecules to form COz important factors for the photoreduction of the catalyst with CO under U V irradiation. In addition to this photoformation of C02, at normal temperatures. photoadsorption of CO molecules onto the reduced surface sites Understanding of the complete mechanism of the photeuptake is also observed. of CO molecules appears to be insufficient at the present. It is likely that some fraction of the photo-uptake of CO molecules Acknowledgment. This work has been supported by the U.S. occurs on the sites formed by the photoreduction with CO, as National Science Foundation (Grant ECS-8619520) and by the suggested previously! Some part of the photoadsorption of CO Ministry of Education in Japan for Grant-in-Aid Scientific Remolecules occurs on the sites that were photoreduced to V4+ by search (Grant 01550632) and International Joint Project Research the reaction of CO with the excited state of (V4+-O-)3on the (Grant 02044125). Thanks are also due to Osaka Prefecture for Special Project Research, Functional Dyes and De-NoXingCatalysis (Project Chief: T. Kitao). (35) Volodin, D. M.;Chcrkashin. A. E. Kinet. Katul. 1979, 22, 979. ~
Sorptlon of Water, Methanol, and Ammonia on AIPO,-5 As stwR.d by M d t h u c h r NMR Spectroscopy I. Kustanovich and D.Goklfarb* Department of Chemical Physics, The Weizmann Institute of Science, 76 I O 0 Rehovot, Israel (Received: November 26, 1990)
Multinuclear NMR measurements were performed on several samples of AlPo4-5: samples that are as synthesiztd,calcined and rehydrated, dehydrated and those obtained after adsorption of methanol and ammonia. The dynamics of the adsorbates was followed by 2HNMR, which was correlated with the change the framework undergoes upon adsorption, as manifested b the ,IP and 27AlMAS NMR spectrum. Water adsorption up to 20% of the total AIW4-5 capacity did not affect the *YA1 and 31Pspectra, and the 2H NMR spectrum indicates the presence of rather mobile water molecules. When the samples are completely hydrated, the AI spectrum shows the presence of octahedral Al, the ,'P line is significantly broadened, and the 2H NMR shows the appearance of 'bound" water molecules undergoing a local motion, which could be a r flip about the DOD bisector. These molecules are assigned to water coordinated to the octahedral AI and the flip is about the AI-0 axis. The 2H NMR spectrum of adsorbed ND3 indicates the presence of two types of molecules, just as in the case of water, with the bound molecules undergoing a rotation about the C3axis, along the AI-N axis. In this case three types of AI were detected, tetrahedral, octahedral, and pentacoordinated. While water and ammonia behaved similarly, methanol adsorption did not produce octahedral A1 and the methanol molecules were found to freely reorient within the A1P04-5channels. T2measurements attributed the changes in the 31Pline width to inhomogeneous broadening due to dispersion of chemical shifts. This dispersion was assigned to the changes in the AI neighbors.
IatrodUCtiOn The novel family of aluminophosphate molecular sieves, abbreviated as AIP0,-n, has recently drawn considerable attention due to their potential to act as new large-pore catalysts.' This includes VPI-5, which possesses the largest pore system (18( I ) Flanigcn, E. M.; Lok, B. M.; Patton, R. L.; Wilson, S.T. New Developments in Zeolites Science Technology. In Proceedings of the 7th International Zeolite Conferencr; Murakami, Y..Lijima, A., Wand, J. W. as.; Kodansha: Tokyo, 1986; p 103.
0022-3654/91/2095-8818$02.50/0
member rings) known to date.2 The AlF'04-n materials have a neutral framework and hence no Bransted acidity; however, the possibility of framework substitution with Si, Co, Mg, and other elements' allows the introduction of Bransted acidity and thus enhances their catalytic potential. AlP04-n has a highly symmetrical structure exhibiting only one type of T site. It possesses a onedimensional channel system (2) Davis, M.E.; Saldamaga, C.; Montes, C.; Garces, Y.;Crowder, C. Nature 1988, 331, 698.
0 1991 American Chemical Society
