J. Phys. Chem. 1984,88, 2569-2572 obtain photoelectron spectra of short-lived free radicals. Further work is in progress with the multichannel spectrometer to study the n-propyl and isopropyl radicals produced from the reaction of fluorine atoms with n-propane.
2569
sistance, and British Gas and S.E.R.C. for the award of a CASE studentship to A.R.E. The authors gratefully acknowledge the advice of Professor N. Jonathan during the course of this work. Registry No. Ethyl radical, 2025-56-1; ethane, 74-84-0;fluorine atom, 14762-94-8;azoethane, 821-14-7;n-propyl nitrite, 543-67-9.
Acknowledgment. W e thank the S.E.R.C. for financial as-
Photocatalytic Hydrogenation of Alkynes and Alkenes with Water over Ti02. Hydrogenatlon Accompanied by Bond Fission Masakazu Anpo,* Norikazu Aikawa, Sukeya Kodama, and Yutaka Kubokawa Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 591, Japan (Received: October 20, 1983)
Photocatalytic reactions of alkynes and alkenes with water have been investigated over Ti02 powder. The major photoformed products are those formed by hydrogenation accompanied by C=C or C=C bond fission. Oxidation products such as CO and C 0 2 are also formed. From the change in the product yields with the remaining amount of water adsorbed as well as the pressure of water vapor it is concluded that water molecules and not surface OH- groups are responsible for the reaction and also that vacant sites for the alkyne (or alkene) adsorption are necessary for the occurrence of the reaction. The fission of the C=C or C=C bond observed with the hydrogenation is attributed to the interaction of the alkynes (or alkenes) with the 'trapped e- (Ti3+) and h+ (6H) pairs. It is suggested that a close association of photoformed electron and hole pairs plays a significant role in the bond fission.
Introduction
TABLE I: Yields of the Hydrogenated Products Formed in the
As an approach to utilization of solar energy, photodecomposition of water into H2 and O2 over metal oxides has recently received a great deal of attention.' In connection with this problem, Boonstra and Mutsaers have found that photocatalytic hydrogenation of alkenes and alkynes occurs over Ti02 containing adsorbed water, and proposed that H atoms produced according to the reaction Tis-OH + hv Ti,-0 + H are responsible for the hydrogenation.2 As has been shown by Schrauzer and G ~ t h , ~ and Boonstra and Mutsaers: as well as described previously: the observed reaction is not a simple hydrogenation, the reaction products undergoing fission of the C=C or C=C bond. Such a fission of the bond is inexplicable by the concept that the reaction is caused by H atoms.2 Schrauzer and Guth have suggested that the reaction could be related to the photodecomposition of water.3 We proposed that positive holes generated by UV irradiation participate in the reaction4 Recently we have shown that with supported M o o 3 the charge-transfer complex, [Mo5+-O-] *, formed by the reaction of
Photoreaction of Various Unsaturated Hydrocarbons with Water over TiO, at 300 K reactants photohydrogenated products, pmol CH=CH CH4 (1.65), C2H4 (0.112), F Z - C ~(0.378),b H~ C3Hg (0.089) CH,-C=CH" CzH6 (4.86), CH4 (0.451), C3H6 (0.312) C2HS-CzCH C3Hg (4.65), CH4 (l.lO), C4H6 (0.380), C2H6 (0.147), CH3CH=C=CH2 (0.295) CH2=C=CH2 C2H6 (2.27), CH4 (0.023), C4H10 (0.436) CH2=CH2 CH4 (0.301), C&6 (0.045), C3HB (0.01 1 ) c-C~H, C2H6 (0.272), C3Hg (0.090), CH4 (0.029)
-
[Mo6+=02-]
& [Mo5+-0-] * hv'
plays a significant role in the fission of the C=C bond of alkenes5 Accordingly, a study was undertaken to clarify the nature of the bond fission in the photocatalytic hydrogenation over Ti02 Such
(1) A. Mills and G. Porter, J . Chem. SOC.,Faraday Trans. I , 78, 3659 (1982);S. Sat0 and J. M. White, J . Phys. Chem., 85, 592 (1981); T. Sakata, T. Kawai, and K. Hashimoto, Chem. Phys. Left., 88, 50 (1982); C. Leygrof, M. Hendewrk, and G . A. Somorjai, J. Caral., 78,341 (1982);S. C. Tsai, C. C. Kao, and Y. W. Chung, J. Caral., 79, 451 (1983); R. I. Bickley, Spec. Period. Rep.: Caral., 5, 308 (1982), and references therein. (2) A. H. Boonstra and C. A. H. A. Mutsaers, J. Phys. Chem., 79, 2025 (1975). (3) G. N. Schrauzer and T. D. Guth, J . Am. Chem. SOC., 99,7189 (1977). (4) C. Yun, M. Anpo, S. Kodama, and Y. Kubokawa, J. Chem. SOC., Chem. Commun., 609 (1980). (5) M. Anpo and Y. Kubokawa, J . Caral., 75, 204 (1982); M. Anpo, I. Tanahashi, and Y. Kubokawa, J. Chem. SOC.,Faraday Trans. 1 , 78, 2121
(1982).
0022-3654/84/2088-2569$01.50/0
a Small amounts of CH3CH0, C2H5CH0,and CH3COCH3were detected. Others were not measured. Ti02: 0.30 g; reactant: 63.0 pmol. bThis value is not conclusive. The mechanism of its formation is unclear.
information appears to be iiecessary for a more complete understanding of the photocatalytic reactions over TiOz caused by the active species formed from water. Experimental Section Materials. The TiOz was obtained from the Japan Aerosil Co. (P-25, Degussa, BET; surface area: 40 m2/g; average particle size: 0.029 pm; purity: 97%). All the alkynes and alkenes used in the present study as well as H2 were of extrapure grade from the Takachiho Kogyo Co. They were used without further purification. Commercial tank oxygen was purified by low-temperature distillation. Deionized double-distilled water was degassed by alternate freezing and thawing in vacuo. D 2 0 of 99.75% isotopic purity (E. Merck Darmstadt) was used without further purification. Apparatus and Procedure. Two types of quartz cells were used. One was about 10 cm long and 5.0 cm in diameter with a total volume of about 300 cm3, and another was smaller, having a volume of about 30 cm3. The catalyst could be moved between the window section and the furnace section, so that its temperature could be raised from room temperature to about 1200 K. The cell was connected to a conventional vacuum system. Ultimate
0 1984 American Chemical Society
2570 The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 TABLE II: Yields of the Photohydrogenated Products from Various Unsaturated Hydrocarbons over TiOi at 300 K' reactants photohydrogenated products, pmol CH2=CH-CH=CH2 CHSCHsCH2 (0.0385), C3Hs (0.0143), CH4 (traces) (CH3)2-C=CHz C3Hs (0.0203), CH4 (0.0105) CHj-CH=CH, C2H6 (0.0085), CH4 (traces) CH,=CH2 CH4 (0.0017) CH3-CzCH C2H6 (0.035), CH4 (0.0029), C3H6 (0.0016) CH=CH CH4 (0.016), C2H4 (0.0037), n-C4H8(traces)
Anpo et al. TI02 H20
~
i
UV-29
0-
1.3 torr
C H p C H ; 4.0 torr
W
'12 3
OTi02: 0.05 g; reactant: 8.8 pmol. vacua of torr were attainable. The TiOz sample of about 0.3-0.05 g was subjected to oxygen treatment at 720 K, followed by evacuation at the same temperature. The TiOz powder was spread on the quartz window, having a surface area of about 12 cm2. Water vapor was adsorbed under various conditions onto the T i 0 2 catalyst at 300 K. The reactant such as alkynes or alkenes was then introduced onto the catalysts, its pressure being adjusted in the range of 0.01-20 torr. Then, the UV irradiation was carried out at 300 K using Toshiba SHL-100 W high-pressure mercury lamp with a water filter alone or together with color glass filters. The reaction products together with unreacted reactants were collected by means of cold traps. With the noncondensable gases such as CO, CH4, and H2 a Toepler pump was used. Analysis of the reaction products was made by gas chromatography using columns of Porapak Q, silica gel, and molecular sieve 5A and/or a Shimadzu MASPEQ-070 quadrupole mass spectrometer. Isotopic analysis of the deuterated methane was carried out with a Shimadzu MASPEQ using low-voltage electrons in just sufficient amount to produce an ion of the parent mass. To determine the particle size of the catalyst, X-ray diffraction and/or transmission electron microscopy was used. Results
Table I shows the photohydrogenated products and yields obtained with Ti02-A. In this case water vapor was adsorbed, prior to the experiments, on T i 0 2 at room temperature until a pressure of a few torr was reached, after which the evacuation was carried out at room temperature. With alkynes the major photoformed products are alkanes formed by hydrogenation accompanied by fission of the C=C of the reactants, Le., CH4 and CzH6 for CH3-C=CH, CH4 and C3Hs for C2H5-C=CH, and CHI for CH=CH. With alkenes and c-C3H6,in addition to such products undergoing the bond fission, i.e., CHI for C2H4, CH4 and C2H6 for C3H6,and CH4 and C2H6 for c-C3H6,alkanes formed without the C=C bond fission are produced as minor products. Table I1 shows the photoformed products and their yields obtained with Ti02-B, which contains an insufficient amount of water to form a monolayer. It is seen that alkanes, i.e., CHI and C3H6 for C4H6, CH4 and C3Hs for (CH3)$2=CHz, CH4 and C2H6 for CH3CH=CHz and CH3-C=CH, and CH4 for CH2=CH2 and C H e C H , are again formed by hydrogenation accompanied by fission of the C=C or C=C bond, their yields being lower than those show in Table I. If the C-C single bond of the alkenes (or alkynes) is cleaved, elimination of CH3 groups would occur, and C2H4 and C3H6would be formed in the reaction of CH3CH=CH2 and (CH3)2-C=CH2, respectively. The lack of formation of these compounds confirms that C = C or bond fission takes place preferentially over C-C bond fission. It has been found that with a 420- or 390-nm cutoff filter the yield of the products is reduced to 7.7% or 16.9%of its original value (by using a 290-nm cutoff filter), respectively. This suggests that the photoreaction is caused by the electron-hole pair generation by band gap irradiation. As shown in Figure 1, on UV irradiation of T i 0 2 in the presence of the reactant, the photoreaction takes place immediately, its yield increasing with irradiation time. As soon as the irradiation ceases, the reaction stops
Reaction time
,
min
Figure 1. Time course of photohydrogenation reaction of CH3-C=CH with water over TiOz at 300 K.
Ti 02 reactant ; W
'0
373
473
573
Degassing temperatwe, K Figure 2. Effect of the degassing temperature of TiOz on the yields of photoreaction of CH=CH at 300 K. TABLE I11 D Distribution (Percentage) in Methane Formed in the Photohydrogenation over TiO, at 300 K CH,-C=CH CH3-CH=CH2 CH2=CH2 methane D20 DzO D,O CD4 1.5 1.2 0.8 CHD3 51.5 5.8 3.2 CH2D2 27.2 55.3 61.4 CH3D 8.2 18.5 16.5 CH4 11.4 18.9 18.0
+
'TiO,:
+
+
0.30 g; reactant: 38.8 pmol; D20: 10 torr.
immediately. UV irradiation of TiO,, Le., formation of Ti3+ions, was observed by using ESR. It has often been suggested that the reduced titanium ion, Ti3+,would be associated with the splitting of water.6 However, if the photoreduced Ti3+ ions were directly responsible for the photoinduced hydrogenation reaction, such an immediate response for the intermittence of UV irradiation would be unexpected, since the photoformed Ti3+ions should exist even under dark conditions. This lack of reaction in the dark suggests no interaction of the reactants with catalysts having adsorbed water at room temperature, in agreement with the fact that the reactants introduced were completely recovered by desorption up to 323 K. In order to obtain information about how the remaining amount of water adsorbed affects the photoreactions, the following experiments have been carried out: Water vapor was adsorbed on ( 6 ) S. M. Kuznicki and E. M. Eyring, J . Am. Chem. Soc., 100, 6790
(1978).
Photocatalytic Hydrogenation of Alkynes and Alkenes
The Journal of Physical Chemistry, Vol. 88, No. 12, 1984 2571
-0
9 5
-0 0)
E 4
ti 2
*r
g3 Lc
0
ge, 2 Y
I
9 00
9.50
10.0
I P /ev
0
2
4
6
8
Irradiation time , hr Figure 3. Effect of the addition of water on the photohydrogenation of CH,-C=CH over Ti02-A at 300 K (CH3-C=CH, 4.0 torr).
TiOz at room temperature until a pressure of about 10 torr was reached, after which the evacuation was carried out at various temperatures from 300 to 573 K. With each T i 0 2 sample, the yield of photoreaction has been determined. As shown in Figure 2, the yield decreases drastically with increasing degassing temperature, approaching about 9.0% of the original value of 473 K. It is well-known that with the Ti02 outgassed at r w m temperature adsorbed water exists in two forms, Le., molecular water and hydroxyl groups, the former being completely removed by evacuation up to 473 K7v8 This suggests that water molecules and not hydroxyl groups are responsible for the photocatalytic reactions on TiO2. Similar photohydrogenation experiments have been carried out by using the D 2 0 in place of H20. Table I11 shows the D content in the methane formed by the photohydrogenation reactions. The observed D content is in agreement with what is expected from the conclusion that the nondissociated adsorbed water is responsible for the photoinduced hydrogenation. Figure 3 shows the effects of the H 2 0 pressure on the rate of photoreaction of CH3-C=CH over TiO,. The initial rate of the photoreaction is largest in the absence of gaseous water. In this case, however, the rate of photoreaction gradually decreases with UV irradiation time, becoming zero after about 4.0 h. With increasing H 2 0pressure the initial rate decreases, while the extent of decrease in the rate with progress of the reaction is reduced, a constant rate being observed around 0.1 torr of H20.
Discussion Recently Bard et al. have suggested that UV irradiation of semiconductor particles such as Ti02 and ZnO brings about e--h+ pairs which can be represented as a localized e- (Ti3+)and h+ (0or OH).9 Essentially the same conclusion has been obtained by our photoluminescence studies of ZnO and T i 0 2 powders.1° The significance of a close existence of photoformed electrons and holes in the C=C bond fission has been shown in our recent studies of the photoinduced metathesis reaction of propene over Moo3 supported on porous Vycor g l a s s It has been found that the interaction of propene with the charge-transfer excited complex [Mos+-O-*]* brings about formation of the bridge type T (7) (a) G. Munuera, U. R. Arnau, and A. Saucedo, J . Chem. Sor., Faraday Trans. I , 75,736 (1979). (b) A. R. Gonzalez-Elipe, G. Munuera, and J. Soria, J . Chem. SOC.,Faraday Trans. 1 , 15, 748 (1979). (8) M. Egashira, S . Kawakami, S . Kagawa, and T. Seiyama, Bull. Chem. SOC.Jpn., 51, 3144 (1978). (9) M. D. Ward and A. J. Bard, J . Phys. Chem., 86, 3599 (1982). (10) M. Anpo and Y. Kubokawa, to be submitted for publication.
Figure 4. Relationship between log R (rate of cleavage of C=C bond) and ionization potential of alkenes: (1) C2H4,(2) C3H6,(3) i-C4H8,(4)
1,3-C4H6.
complex followed by the back-donation of an electron from the photoreduced Mo ion to the 7c* orbital of the 7-complex, which results in formation of carbene and CH3CH0, the former initiating the metathesis reaction. As a result, the energy produced when the electron and the hole recombine is used for the C=C bond fission. According to the work of Munuera et al., with fully hydrated Ti02such as used in the present studies, Ti4+ions on the surface are completely coordinated by OH- groups and water molecules, no coordination vacancies being present.' On UV irradiation of the T i 0 2 sample, photodesorption of water is expected to take place, since its occurrence has been reported by Munuera et and also confirmed by the present studies. The photodesorption of water is expected to bring about formation of coordination vacancies, which make possible the adsorption of alkynes or alkenes on the Ti4+ions on the surface. Thus, it might be concluded that the trapped e- (Ti3+) and h+ (OH) pairs formed under UV irradiation7,' will interact with the alkynes (or alkenes) adsorbed to form the carbenes and oxygen-containing compounds, in a manner similar to that described previously.5 As shown in Table I, small amounts of oxygen-containing compounds such as C H 3 C H 0 were detected. Although there is no direct evidence supporting the carbene formation, its formation would be expected if the fission of the C=C or C=C bond proceeds via a fourmembered inter~nediate.~ Formation of saturated hydrocarbons has already been proposed by a number of workers as an elementary process in the Fischer-Tropsch synthesis.12 The validity of the above conclusion is confirmed by the results of similar experiments with Pt-loaded T i 0 2 where the separation of e--h+ takes place efficiently, resulting in enhancement of the photohydrogenation reactions without the fission of the C=C or CEC bond. Details will be published in a subsequent paper.14 For some portion of the trapped e- (Ti3+) and h+ (OH) pairs, the following situation would be expected. The electrons and holes are separated from each other, being trapped by H+ and OH- ions to form H and O H radicals, respectively. These radicals attack the carbenes and oxygen-containing compounds. The carbenes will be hydrogenated to form saturated hydrocarbons. The oxidation products such as C O and COz would originate from the reaction of OH radicals with the carbenes and oxygen-containing compounds. As shown in Figure 4, the rate of photohydrogenation of alkenes accompanied by the C=C bond fission increases with decreasing ionization potential of the alkene. Such features are explicable
'
(1 1) H. Van Damme and W. Keith Hall, J . Am. Chem. SOC.,101,4373 (1979). (12) M. A. Vannice, "Catalysis", Vol. 3, J. R. Anderson and M. Boudart, Eds., Springer-Verlag. New York, 1982, p 140.
2572
J. Phys. Chem. 1984,88, 2512-2515
on the basis of the concept that the interaction of alkenes with the trapped e- (Ti3+)and h+ (OH) pairs involves electron transfer from alkenes to the OH moiety, Le., the sites trapping positive holes.3 As described above, water molecules and not surface OHgroups are responsible for the photohydrogenation of alkenes (or alkynes) in the present system. A similar conclusion has been reached by Domen et al.,I3 who investigated the photodecomposition of water on Ni0-SrTiO3 catalyst. According to their views, the oxidation of water proceeds via the attack of the radicals formed from the OH- groups where the equilibrium H20ad, H+ OH- is set up. The OH- groups responsible for the reaction should be distinguished from the stable surfce OH- groups remaining after evacuation at a high temperature. In the present system it appears that the proton exists not as such but in combination with the 012- ions on the surface, Le., the acidic OHgroups as proposed by Munuera et ale7 By losing H+ these OH-
+
(13) K. Domen, S.Naito, T. Onishi, K. Tamaru, and M. Soma, J . Phys. Chem., 86, 3657 (1982). (14) M. Anpo, N. Aikawa, and Y. Kubokawa, J . Phys. Chem., in press.
groups would revert to the 012- ions. By repetition of the cycle, the supply of H+ to the active sites would proceed. Although the nature of the OH- groups associated with the oxidation reactions is unclear, it appears almost certain that the OH- groups remaining after the removal of H+ from HzO is responsible for the oxidation reactions. The significance of the supply of water molecules in the progress of the reaction is obvious from the results shown in Figure 3, where the product yields level off after a certain reaction time where the H 2 0 pressure is very low. Furthermore, it should be noted that the initial rate of the photoreaction is higher the lower the H 2 0 pressure. This suggests that the existence of coordination vacancies, Le., of vacant sites for the adsorption of alkenes (or alkynes), is necessary for the occurrence of the photoreaction, in agreement with the reaction scheme described above. Registry No. HzO, 7732-18-5; TiOz, 13463-67-7; C H z C H , 74-86-2; CH,C=CH, 74-99-7; C2HSCSCH, 107-00-6; CH2=C=CH2, 463-490; CHz=CHz, 74-85-1; c - C ~ H ~75-19-4; , CH4, 74-82-8; n-C4H8, 25 167-67-3; C3H8, 74-98-6; CzHs, 74-84-0; CHjCH=C=CH2, 59019-2; CdH10, 106-97-8; CHz=CHCH=CH2, 106-99-0; (CH,)ZhCHz, 115-11-7; CH$H=CHz, 115-07-1; DzO,7789-20-0.
Quantum Chemical and ‘*O, Tracer Studies of the Activation of Oxygen in Photocatalytic Oxidation Reactions Masakazu Anpo,* Yutaka Kubokawa, Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Sakai, Osaka 591, Japan
Tsuneo Fujii,* and Satoshi Suzuki Department of Engineering Chemistry, Faculty of Engineering, Shinshu University, Wakasato, Nagano, Nagano 380, Japan (Received: September 9, 1983)
Mechanisms for the photocatalytic activation of oxygen have been investigated by EPR techniques and lSO2tracer experiments as well as by the quantum chemical approach for the structure and reactivity of intermediate species. UV irradiation of porous Vycor glass in the presence of a mixture of I8O2and CO leads to the formation of C160180 alone. Addition of CO to the 0,- species leads to the disappearance of their EPR signals and to the formation of COz. These results indicate that the oxygen in the gas phase, and not lattice oxygen, is incorporated into COzmolecules. The C N D 0 / 2 and INDO calculations of the 0,- anion radical indicate that the 0-0 bond of the oxygen is weakened through the interaction of it with the 0hole center and also the total energy of the 03-species shows two minima at the LOO0 bond angle (e) of 60 and 120’. From these results it is concluded that oxygen molecules undergoes photoactivation, Le., weakening of the 0-0 bond, which results in the formation of active oxygen species responsible for photooxidation reactions on surfaces.
Introduction Photocatalytic oxidation is one of the most important fields in photocatalysis on metal oxides.] The nature of the active oxygen species responsible for photocatalytic oxidation has been discussed by a number of workers.’” Pichat et al. have suggested that 0,(1) (a) K. Tanaka and G. Blyholder, J . Phys. Chem., 76, 1807, 3184 (1972); (b) M. Formenti and S.J. Teichner, “Catalysis”, Vol. 2, C. Kemball, Ed., The Chemical Society, London, 1978, p 87; (c) Y. Kubokawa and M. Anpo, Shokubai, 22, 189 (1981); (d) R. I. Bickley, “Catalysis”, Vol. 5 , G . C. Bond and G. Webb, Ed., The Royal Society of Chemistry, London, 1982, p 308. (2) K. M. Sancier and S. R. Morrison, Surf. Sci., 83, 29 (1979). (3) J. Cunningham, B. Doyle, and E. M. Leahy, J . Chem. SOC.,Faraday Trans. 1 , 75, 2000 (1979). (4) S.Yoshida, Y. Matsumura, S.Noda, and T. Funabiki, J . Chem. Soc., Faraday Trans. 1,17, 2237 (1981). (5) H. Courbon, M. Formenti, and P. Pichat, J. Phys. Chem., 81, 550 (1977). (6) Y. Kubokawa, M. Anpo, and C. Yun, Proc. In?. Congr. Catal. 7th, 1170 (1981).
species are intermediates for the hydrocarbon photooxidation as well as the photoinduced 1602-1802 isotopic exchange reaction over Ti02.s Recently, we have investigated the photooxidation of alkenes over porous Vycor glass (PVG) by EPR measurements of the photoformed oxygen species as well as by analysis of the reaction products. We showed that the 0- hole center, formed at low-coordination sites by a charge-transfer process, i.e.
reacts easily with oxygen to form an unstable 03-species, which is closely associated with the photooxidation of alkenes as well isotopic exchange reaction over as the photoinduced 1602-1802 PVG.6 It seems important to clarify the mechanism by which the 03species brings about the oxidation of hydrocarbons. For this purpose, the present work was undertaken to investigate the photooxidation of a simple molecule such as CO using l8OZtracer. Furthermore, quantum chemical studies of the structure and
0022-3654/84/2088-2512$01.50/00 1984 American Chemical Society
