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J. Phys. Chem. 1980, 84, 3440-3443
(6) 6.B. Wayland, J. V. Minkiewicz, and M. E. Abd-Elmageed, J. Am. Chem. Soc., 96, 2795 (1974). (7) H. A. 0. Hill, P. J. Salder, and R. J. P. Wllllams, J. Chem. Soc., Dalton Trans., 1663 (1973). (6) F. A. Walker, J. Am. Chem. Soc., 92, 4235 (1970). (9) H. A. 0. Hill, P. J. Sadler, R. J. P. Williams, and C. D. Barry, Ann. N . Y . Acad. Scl., 208, 247 (1973). (10) F. A. Walker, J. M g n . Reson., 15, 201 (1974). (1 1) B. E. Wayiand and M. E. Abd-Elmageed, J . Am. Chem. SOC.,96, 4809 (1974). (12) L. C.Dicklnson and J. C.W. Chlen, Inofg. Chem., 15, 1111 (1976). (13) S. Konishi, M. Hoshino, K. Yamamoto, and M. Imamura, Chem. Phys. Lett., 72, 459 (1980).
(14) B. M. Hoffman, D. L. Diemente, and F. Basolo, J. Am. Chem. Soc., 92, 61 (1970). (15) D. Getz, E. Melamud, B. L. Silver, and 2. Dori, J. Am. Chem. Soc., 97, 3846 (1975). (16) B. S. Tovrog, D. J., Kitko, and R. S. Drago, J . Am. Chem. SOC., 98, 5144 (1976). (17) R. S. Drago, Inorg. Chem., 18, 1408 (1979). (18) T. Shlda and W. H. Hamill, J. Am. Chem. Soc., 88, 3689 (1966). (19) W. H. Hamill In “Radical Ions”, E. T. Kaiser and L. Kevan, Eds., Interscience, New York, 1968, pp 321-32. (20) M. Nakamura and S. Fujiwara, J. Coord. Chem., 1, 221 (1971). (21) B. R. McGarvey, Can. J. Chem., 53, 2498 (1975). (22) W. C. Lin, Inorg. Chem., 15, 1114 (1976).
Photoluminescence and Photoreduction of V205 Supported on Porous Vycor Glass Masakazu Anpo, * Ichiro Tanahashi, and Yutaka Kubokawa” Depertment of ApplW Chemistry, College of Englneerlng, University of Osaka Prefecture, Sakal, Osaka 59 1, Japan (Recelved: June 24, 1980)
The studies of the fluorescence and the phosphorescence of V206supported on porous Vycor glass (PVG) have led to determination of the energies of the charge-transferexcited singlet and triplet states on it. The added CO quenches the phosphorescence alone, suggesting the interaction of CO with the excited triplet states. The decay of the phosphorescence in the presence of CO is characterized by superposition of two time constants (one of which is longer than the time constant in the absence of CO). From the comparison of the excitation spectra, it is concluded that the photoformation of CO,, as well as the photouptake of CO observed with Vz05/PVG,is closely associated with the excited triplet states. The vibrational structure of the phosphorescence suggests that the nuclear distance between vanadium and oxygen ions will become longer in the excited states, in agreement with the ease of the photoreduction of VzOs,Le., its photoremoval of lattice oxygen. The quantum yield for photoformation of COz has been determined as -0.04.
Introduction The studies of photocatalysis on metal oxides have received considerable attention in connection with the utilization of solar energy.’ However, there seem to be few studies concerned with the primary process in photocatalysis, e.g., photoformation of hole and electron pairs, the charge separation, and the capture of hole and electrons.2 In view of the significant contribution of the studies of phosphorescence, as well as fluorescence, to the development of photochemistry in the gas phase and in solution? studies of the photoluminescence of metaI oxides4 relating to the photoreaction on them appear to be very useful for clarifying the mechanism of the photocatalysis. A paper about this has already been p ~ b l i s h e d . ~ Although Kazansky et al. have investigated the excited states formed on metal oxides supported on silica gel by spectroscopic technique and found that they exhibit a high reactivity toward the hydrogen abstraction reactions: information on the structure and reactivity of the excited states is still unsufficient. In the present work, by means of emission measurements, we have investigated the photoluminescence of VZO5 supported on porous Vycor glass (PVG) and its photoreduction with CO, since the use of PVG makes it possible to determine the quantum yields of photoreactions on oxides because of its high transparency and large surface area. There have been few studies made of such a determination on oxide surfaces. Experimental Section Materials. The V205/PVG(0.003-0.187 vanadium w t %) was prepared by impregnation of PVG (Corning No. 746685-7930; 160 m2/g, 9.0 X 30 X 1.0 mm) with an 0022-3654/80/2084-3440$01 .OO/O
aqueous solution of NH4V03. The V205/PVG was dried at 350 K and calcined at 850 K in air. The vanadium content was determined by atomic absorption spectrometry. CO (Takachiho Kogyo Co.) was of extrapure grade and used without further purification. Apparatus and Procedures. Details of the apparatus were described previ0usly.~7~ The reactant was introduced to a quartz cell at 298 K on the VZO5/PVGcatalysts which had been evacuated at 623 K (standard treatment) for 2 h. Then UV irradiation was carried out at 298 K by using a high-pressure mercury lamp with a water filter. The dependence of excitation wavelength on the yields of the reactions was measured by using a monochromator equipped with a 500-W Xe lamp. The absorption spectra of the V205/PVGwere determined by measuring transmission through the sample with a Hitachi EPS 3T spectrophotometer at 300 K. A pure PVG specimen pretreated under the same condition was used as a reference. The amount of photoformed COz and the photoinduced uptake of CO were measured with a Pirani vacuum gauge and a Shimazu quadrupole mass spectrometer. The photoluminescence spectra were measured by using a Shimazu RF-501 spectrofluorophotometer with filters to eliminate scattered light in the temperature range of 300-77 K. Details were described previ~usly.~ In the case of the decay curves of photoluminescence, the V205/PVG catalyst was excited with a N2 laser with a nanosecond pulse width at 300 K. Its excitation wavelength of 331.7 nm agreed with the peak of the absorption band of the catalyst. The number of incident photons to the cell was determined by means of potassium ferrioxalate actinometry. The difference between the number of photons passing through the V205/PVG and the corresponding 0 1980 American Chemical Soclety
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The Journal of Physical Chemistry, Vol. 84, No. 25, 1980 3441
Photoluminescence and Photoreduction of V205 eV
?-A
'
..B 8a, *-
E
!i
L
-
350
4
u
u
450
-
,
550
\
0
4
250
350
450
1
Wavelength, nm
IO
II ,
&
-ij a,
a:
6.
I
\
\
\
4. -if
2L-LI
Flgure 1. Phoioluminescence spectra of V205/PVG at 300 K: (a) photoemission excited sit 290 nm (slit width for excitation, 18 nm; slit width for emission, 5.0 nm); (b) excitation spectrum of 500-nm luminescence (slit width for excitation, 3.0 nm; slit width for emission, 7.0 nm).
phosphorescence
VI
fluorescence
77
'
173
223
Temperature,
273 "K
Figure 3. Effect of temperature upon the photoluminescenceIntensity excited at 300 f 3.5 nm. eV 2.48
r3'
phosphorescence
7.07
3-3 bond
\
0-2
0-4 band
L
673
723
A 773
Degassing temperature , "K
Flgure 2. Effect of degassing temperature upon the photoluminescence intensity at 300 I( excited at 300 f 3.5 nm.
number with the pure PVG gives the. number of photons absorbed by the V2O5 catalyst. Thus, the quantum yield for the photoprocesses was determiined. Neither photoformation of COz nor photoadsorption of CO occurred on the pure PVG.
Results and Discussion Excited States of V205/PVG. Figure 1 shows the photoluminescencespectra of V205/PVG together with the corresponding excitation spectrum, Although similar photoluminescence [spectrahave been reported for V2O6 supported on silica gel by Kazansky et d.,8it is to be noted that in the present work the fluorescence extending in the range of 300-400 nul has been observed. The excitation spectrum is essentially the same as that obtained by Kazansky et al., who attributed it to the charge-transfer band due to electron transfer from oxygein to vanadium ions. The intensity of both phosphorescence and fluorescence decreases with increasing degassing temperature of the catalyst (Figure 2). Such behavior confiim the conclusion that the photoluminescence of V2O5 is closely associated with the V=O bond, since it is well known that, because of the weakness of the V=O bond, its oxygen is easily removed on degassing of V206at relaltively low temperatures such as 600-7010 K. As seen in Figure 3, with decreasing temperature from 300 to 77 K, the phos~phorescenceintensity increases while the fluorescence scarcely changes in intensity. Such behavior is expected sirice the lifetime of the excited triplet states is affected moire markedly than that of the singlet
810
I . . . , , 4.00
L 10
600
Wavelength, nm
Flgure 4. Flne structure of phosphorescence of V205/PVG at 77 K (excitation, 300 f 8.5 nm; silt width for emission, 3.0 nm).
states by the temperature-dependent radiationless proce~ses.~ Both fluorescence and phosphorescence spectra at 77 K have a well-resolved vibrational structure. Figure 4 shows the fine structure of the phosphorescence spectrum. The energy separation of the vibrational bands in the spectrum is in agreement with the energy of the vibration of the double bond in the surface vanady groups? e.g., the separation between the 0 0 and 0 1 transitions corresponds to the energy of 1040 cm-'. The absorption of light corresponding to the charge-transfer transition band brings about electron transfer from oxygen to vanadium ions, i.e. V5+-02- + hv V4+-0-
- -
resulting in formation of pairs of hole centers (0-) and trapped electrons (V4+). The photon energy absorbed is mainly localized oin the V 4 surface double bonds. Thus, the energy diagram of Figure 5 can be used for V205/PVG. The energies of the excited singlet and triplet states are determined from the fluorescence and phosphorescence spectrum, respectively.
3442
The Journal of Physical Chemistty, Vol. 84, No. 25, 7980
Anpo et al.
80 K c o l b l e
I
30-
ground state
Flgure 5. Energy diagram of V,OS/PVG.
c
I
eV
3.54
310
276
I
I
1
t
350
400
450
500
600
550
Wavelength, nm
Figure 8, Effect of the addition of carbon monoxide upon the photoluminescence of V,05/PVG at 300 K (excitation, 290 f 7.5 nm; slit width for emission, 5.0 nm). Wavelength, nm
I
Figure 9. Effect of the excitation wavelength upon the yields of photoformatbn of COPand photoadsorption of CO on V,OBIPVG at 298 K (initial GO pressure, 0.25 torr; slit width for excltation, 18 nm).
This suggests that the adsorption mechanism is important; i.e., CO molecules interact with the excited triplet states (V4+-0-)* to form a weak adsorption complex, which leads to destruction of radiative pathways associated with the emission: 1 2 Time
3
4
1
2
3
4
5
co + (v4+-0-)* 2 co (v4+-0-)* 2
6
Time
Figure 7. Effect of the addition of carbon monoxide upon the decay phosphorescence of V205/PVGat 300 K (a) CO, 0 torr; (b) CO, 0.21 torr.
If such a reversible process takes place, the lifetimes of the excited triplet states will become longer in a manner similar to that in the delayed fluorescence (Sl + T1, S1 + hv' Thus, the presence of two decay constants (Figure Interaction of CO Molecules with the Excited States. 7) seems to be explicable, although other explanations Figure 6 shows that the phosphorescence is quenched while might be possible. the fluorescence is unaffected by added C O , suggesting Photoreduction of V205/PVG. U V irradiation of that CO molecules interact with the excited triplet states. V205/PVG in the presence of CO molecules was found to As seen in Figure 7, in the absence of CO the phosinduce formation of C02 (Figure 8). Simultaneously, the phorescence decays exponentially with a time constant of photouptake of CO occurred. The initial rate of phato218 f 5 ps, while in the presence of CO (0.21 torr) its decay formation of COzwas 1.25 X lo-*mol/s at 259 K, the ratio is characterized by superposition of two time constants of of C02formed/CO uptake being 0.52. The ratio changed 218 f 5 and 522 f 5 ps. As for the mechanism of with the experimental conditions such as the degassing quenching the photoemission from oxide surfaces by added temperature of the catalyst, etc. As seen in Figure 9, the gases, two mechanisms appear to be operating: (1)colliof the excitation wavelength upon the yields ~ sional quenching and (2) quenching due to a d s o r p t i ~ n . ~ , ~ dependence for photoformation of C02is almost the same as that with The phosphorescence was found to be quenched by oxygen the photoadsorption of CO, being in agreement with the in a manner similar to that with CO, but unaffected by Xe, excitation band of the photoemission. These results towhich is an effective quencher of excited triplet state^.^
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The Journal of Physlcal Chemistry, Vol. 84, No. 25, 7980 3443
Photoluminescence and Photoreduction of V,05
TABLE I : Q u a n t u m Y i e l d s for P h o t o f o r m a t i o n of CO, a n d P h o t o a d s o r p t i o n of CO on V,O,/PVG a t 300 K“ CO p h o t o a d s o r b e d 0.115 CO, p h o t o f o r m e d
0.04,
“ E x c i t a t i o n , 330 rim. gether with those of the GO quenching experiments (Figure 6) suggest that both1 reactions are closely associated with the charge-transfer excited triplet stabs. The rates of both reactions increased with decreasing reaction temperature in the range of 259-:321 K, the apparent activation energy being -2.2 kcal/mol for photoadsorlption of CO and -1.7 kcal/mol for photoformation of GOz. Such features can be attributed to longer lifetimes of the excited triplet states at lower temperatures. Understanding of the mechanism of photouptake of CO appears to be insufficient, although some fraction of the uptake is explicable in terms of the concept that it occurs on the sites formed by the photoreduction with CO molecules, as suggested by Kazansky et al.ll After the photoreaction of GO, thle color of the caltalyst became blueblack and the ESR signal due to V4+ ions12appeared, thus confirming the occurrence of the photoreduction of the catalyst. The charge-transfer excited state (V4+-O-) are expected to be active in oxidation reactions, since it is well known that 0- anion radicals are active intermediates in the oxidation of GO on oxides13 and since 0-hole centers in V4+-O- complexes exhibit the reactivity similar to that The vibrational structure of the of 0- anion ra.dical~.~J~ phosphorescence spectrum (Figure 4) shows that the 0 3 transition is the strongest. It is therefore concluded from the Franck-Condon principle3 that the nuclear distance of the V5+=02- complex will become longer in the excited states. Since photoformation of GOz is accompanied by the removal of oxygen from metal oxides, such an elongation of the nucleair distance in the! excited states may be associated with tlhe fact that with V205/PVG photoformation of C!Oz proceeds very easily. Although it seems too early to establish such a correlation, it should be noted that in the case ZnO, where photoformation of C02,is considered hardly feasible at room temperature, the intensity of the 0 1 transition is the strongest, suggesting that no change in the nuclear distance Zn-0 occurred. The
-
-
details will be reported in the near future. The quantum yields shown in Table I are regarded as the lowest valueis in view of the loss of the light intensity due to scattering by the catalyst, although its contribution to the quantum yields appears to be small. It is to be noted that the magnitude of the yields is similar to that observed with the organic photoreactions via the excited triplet statesa3Further work is necessary to obtain the general characteristics of the quantum yields for such systems.
Acknowledgment. We thank Dr. K. Murata of Seikei University for the measurements of the decay curves.
References andl Notes G. N. Schrauzor and T. D. Guth, J. Am. Chem. Soc., 99, 7189 (1977); J. C. Heimminger, R. Can, and G. A. Somorjai, Chem. phys. Lett., 57, 100 (1978); H. Relche and A. J. Bard, J. Am. Chem. Soc., 101, 3127 (197’9); T. Kawai and T. Sakata, Nature(London),282, 283 (1979). Y. Kubokawa and M. Anpo, Hyomen, 16, 463 (1978). J. 0. Cabeft and J. N. Pilts, Jr., “Photochemisby”, Wiley, New York, 1966; N. J. Turro, “Modern Molecular Photochemistry”, Benjamln/Cumiings i4blishing CO.,Menlo Park, Ca, 1978; D. M. Hercules, “Fluorescence (andPhosphorescence Analysis”, Wiley, New York, 1966. A. J. Tench and G. T. Pott, Chem. Phys. Lett., 26, 590 (1974); S. Coluccla and A. J. Tench, Roc. Int. Congr. Catal., 7ih, 7980,535 (1980). M. Anpo, C. Yun, and Y. Kubokawa, J. Chem. Soc., Faraday Trans. 1,76, 1014 (1980); C. Yun, M. Anpo, and Y. Kubokawa, J. Chem. SOC., Chem. Commun., 665 (1977). S. A. Sourin, Bl, N. Shelimov, and V. 8. Kazansky, Khim. Vys. Energ., 6, 120 (1972); A. P. Shuklov, S. A. Sourin, B. N. Shelimov, and V. 8. Kazainsky, Kinet. Katal., 16, 468 (1975). M. Anpo, C. Yun, and Y. Kubokawa, J. Catal., 61, 267 (1980); Y. Kubokawa, M. Anpo, and C. Yun, Proc. Int. Congr. Catal. 7th, 7980, 8-36 (1980). A. M. Ortscov, V. A. Shvet, and V. 8. Kazansky, Chem. phys. Lett., 35, 511 (1975); V. B. Kazansky, Proc. Int. Cow.Cats/., 6ih, 7976, 50 (1977). K. Tarama, S. ‘reranishi, S. Yoshida, and N. Taniura, Proc. Int. Congr. Catal. 5kd, 7964, 282 (1968). S.Coluccla, A. hl. Deane, and A. J. Tench, J. Chem. Soc.,Faraday Trans. I,74, 2913 (1978); S. Coluccla, A. M. Deane, and A. J. Tench, Proc. Int. Congr. Catal., Bth, 7976,171 (1977); A. Zecchina, M. G. Lotthouse, and F. S. Stone, J. Chem. Soc., Faraday Trans. I , 71, 1476 (1975). V. B. Kazansky, ,A. N. Pershin, and B. N. Shelimov, Roc. Int. Cow. Catal., 7th, 7980, 8-39 (1980). L. L. Reijen and 1). Cossee, Discuss. Faraday Soc., 41, 277 (1966). J. H. Lunsford, Catal. Rev., 8, 135 (1973), and references therein. V. B. Kazansky, Kinet. Katal., 18, 43 (1977); S. C. Kaiiaguine, B. N. Shellmov, and V. B. Kazansky, J. Catal., 55, 384 (1978).
