Excited-State Dynamics of 5,10,15,20-Tetraphenyl- 21H,23H-porphine

Immobilization of Tetraphenylporphyrin Manganese (III) Chloride in HMS Modified by Zr, Cu, and Zn Oxides and Their Catalytic Activity. Wei-jie Zhang ...
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J. Phys. Chem. B 2001, 105, 8513-8518

8513

Excited-State Dynamics of 5,10,15,20-Tetraphenyl- 21H,23H-porphine Manganese(III) Chloride Encapsulated in TiMCM-41 and MCM-41; Proved by fs-Diffuse Reflectance Laser Photolysis Yanghee Kim, Jun Rye Choi, and Minjoong Yoon* Department of Chemistry, Chungnam National UniVersity, Taejon, 305-764, Korea

Akihiro Furube, Tsuyoshi Asahi, and Hiroshi Masuhara Department of Applied Physics, Osaka UniVersity, Suita, Osaka, 565-0871, Japan ReceiVed: May 3, 2001; In Final Form: July 2, 2001

5,10,15,20-Tetraphenyl-21H,23H-porphine manganese(III) chloride (MnIIITPP(Cl)) was encapsulated into MCM-41 and TiMCM-41, and its photoinduced electron transfer had been studied by using femtosecond diffuse reflectance photolysis. Two different transient species (c.a. 10 ps and c.a. 80 ps) were observed. The shorter-lived species should be originated from relaxation of a “tripmultiplet” state and the longer-lived species should be attributed to the spin-forbidden relaxation (slower than the spin-allowed decay of triptquintet) via a quintet CT state. After irradiation, MnTPPCl+• radicals are detected in MCM-41 or TiMCM-41, indicating that the mesoporous silicate framework plays a good electron acceptor. Furthermore, it has been found that the formation MnTPPCl+• is easier in TiMCM-41 than in MCM-41, indicating that framework modification by incorporating the Ti4+ into the MCM-41 enhances the electron-accepting ability of the MCM-41 framework.

Introduction In the last several decades, interfacial photoinduced electron transfer and photochemical reaction on the surface of semiconductor particles have been interesting topics because it has a relation to solar energy conversion into chemical energy and the environmental cleaning.1-6 From this point of view, a large number of research works have been reported.7-20 Using several semiconductors, the common output of these works is the following. (i) TiO2 is the most suitable material with relatively favorable band gap energy, offering the highest light energy conversion efficiency; (ii) the efficiency of photoinduced electron transfer is generally limited by the occurrence of deactivating back electron transfer, which competes with other useful reactive pathways of the generated radical ion pairs; (iii) the medium and large pore molecular sieves can provide an appropriate microenvironment for retarding dramatically the back electron transfer and lengthening enormously the lifetime of the photogenerated ion pairs. However, in most photocatalytic systems the photon-to-electron conversion efficiency has remained still low and it has not been appropriate for a direct application to the bulk systems. So, nowadays, many research groups are making efforts to improve the photon-to-electron conversion efficiency through modification of the TiO2 surface by adsorption of photosensitive dyes and/or heterogeneous guests.12,13,21-31 Furthermore, it is also reported that the large pore zeolitic aluminosilicates are very promising hosts to perform fruitful photoinduced electron-transfer reaction.18-20 However, overall modified TiO2 systems still have a problem in application because it just responds to the UV lights which corresponds to only 5% of the whole sunlight. Therefore, it is worthwhile to attempt to make a new photocatalytic system effectively operated by whole sunlight. * Author to whom correspondence should be addressed. Tel: 82-42821-6546. Fax: 82-42-823-7008. E-mail: [email protected].

In this point of view, to modify the TiO2 particle surface, titanosilicate TiMCM-41 molecular sieves have been synthesized, and used as heterogeneous hosts for long-lived photoinduced electron transfer state of metallo-porphyrin, particularly 5,10,15,20-tetraphenyl-21H,23H-porphine manganese(III) chloride (MnIIITPP(Cl)). As well-known, metallo-porphyrin molecules and their derivatives have been extensively used as sensitizers in photocatalytic systems exhibiting photoinduced energy and electron transfer because of their structure and functional similarities to chlorophylls and their absorption of visible lights.32-34 Especially, MnIIITPP(Cl) has been continuously interesting in the last several decades because of its versatile characteristic behaviors in solution: feasible formation of dioxygen complexes,35,36 photolytic redoxreactions,37-39 dependency of redox potential on a coordinated monoanion ligand,40 etc. Therefore, it could be a nice representative molecular model as a photocatalyst. Furthermore, it has been reported that titanosilicate TiMCM-41 and mesoporous silicate have promising utility for photoreactions of bulky molecules in addition to catalysis and separation of large organic/inorganic molecules with their unique properties, i.e. they show a regular hexagonal array of uniform silica tubes with diameters from 15 to 100 Å, a high surface area of about 1000 m2/g, and a pore size distribution nearly as sharp as that of zeolites.18,41,42 Thus, if the above two systems were combined, it could be effectively applied to photocatalysis, responding to whole sunlight. In this point of view, several research groups have made an effort to combine the porphyrin and titanium incorporated and/or mesoporous materials.18,43 Nevertheless, their photoinduced electron and/or charge transfer mechanism has not been investigated by time-resolved spectroscopy despite its importance related to design of the advanced photocatalytic system. This is probably because it is a very fast and complicated process to investigate spectroscopically in the combined

10.1021/jp0116757 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/17/2001

8514 J. Phys. Chem. B, Vol. 105, No. 36, 2001 photocatalytic systems. Furthermore, the study of excited dynamic for metallo-porphyrin in MCM-41 has not been reported because the optical condition is not appropriate to conventional femtosecond and picosecond transient absorption spectroscopy. In this work, we have attempted to investigate the fast electron or charge-transfer processes between metallo-porphyrin and mesoporous silica by using the ultrafast time-resolved diffuse reflectance spectroscopy.

Kim et al. molecules in the host matrix was 5.0 × 10-4 M. The slurry of MCM-41, TiMCM-41 in MnIIITPP(Cl) solution was stirred and allowed to equilibrate for 24 h, and the solvent was then removed under vacuum. Spectroscopic Measurements. X-ray diffractograms were recorded by a MO3X-HF diffractometer (Model-1031, Mac Science Co.). The diffuse reflectance UV-Vis spectra were recorded by using a Shimadzu UV-3101PC spectrometer equipped with an integrating sphere. Absorption spectra of the ground state are evaluated by the Kubelka-Munk function:

Experimental Section Materials. 5,10,15,20-Tetraphenyl-21H,23H-porphine manganese(III) chloride (MnIIITPP(Cl)), Ludox HS-40 colloidal silica (40 wt % suspension in water), hexadecyltrimethylammonium chloride (25 wt % solution in water, HTACl), ammonium hydroxide (28% NH3 in water), hydrogen peroxide (35 wt % solution in water), tetrapropylammonium bromide (TPABr), dodecyltrimethylammonium bromide (DTABr), and titanium(IV) ethoxide (titanium 20% in ethanol) were purchased from Alrdrich Chemical Co. and used as received. All solvents (Merck Chemical Co.) were spectroscopic grade and used without further purification. MCM-41 Synthesis Procedure. A pure MCM-41 has been synthesized following a procedure.18,42 A clear sodium silicate (Na/Si: 0.5) solution was prepared by combining 46.9 g of aqueous NaOH (1.00 M) solution with 14.3 g of a colloidal silica (Ludox HS-40) and heating the resulting gel mixture with stirring for 2 h at 353 K. The sodium silicate solution was added dropwise to a Teflon bottle containing a mixture of 0.29 g aqueous NH3 solution and 20.0 g of HTACl solution, and the resulting solution was vigorously mixed by a magnetic stirrer at room temperature. The molar composition of the resulting gel mixture in the bottle was 6 SiO2/1 HTACl/1.5 Na2O/0.15 (NH4)2O/250 H2O. After stirring for 1 h more, the gel mixture was heated to 370 K for 1 day. The HTA-silicate mixture was then cooled to room temperature. Subsequently, the pH of the reaction mixture was adjusted to 10.2 by dropwise addition of 30 wt % acetic acid with vigorous stirring. The pH adjustment of the reaction mixture to 10.2 and subsequent heating for 1 day were repeated twice more. The precipitated product, MCM41 with HTA template, was filtered, washed with deionized water, and dried in an oven at 370 K. The product was calcined in air under static conditions using a tube furnace, and the calcination temperature was increased from room temperature to 823 K over 10 h and maintained at 823 K for 24 h. TiMCM-41 Synthesis Procedure. Titanosilicate TiMCM41 was synthesized in same manner used for MCM-41 except that titanium source and surfactant source (DTABr (1.2 g) and TPABr (1.04 g)) were added. To prepare the titanium silicate (Ti/Si:0.008) solution, a certain amount of titanium(IV) ethoxide solution which was prepared by dissolving in a mixture of ethanol and hydrogen peroxide, was added slowly into surfactant and sodium silicate gel mixture. The gel mixture was stirred for 4 h at room temperature. The resulting surfactant-titanium silicate gel mixture had molar composition of 6 SiO2/0.05 TiO2/1 HTACl/0.25 DTABr/0.25 TPABr/0.15 (NH4)2O/1.5 Na2O/300 H2O. Encapsulation of MnIIITPP(Cl) in MCM-41 and TiMCM41 and Photocatalysis. All MCM-41 materials were dried at 150 °C for 15 h to remove the physisorbed water before introducing MnIIITPP(Cl). To adsorb MnIIITPP(Cl) into the channel of MCM-41, a certain amount of MCM-41 or TiMCM41 (1 g) was immersed into 10 mL of methylene chloride containing 5.0 µmol of MnIIITPP(Cl). The concentration of

K (1 - r) ) S 2r

2

(1)

where K and S are absorption and scattering coefficients, respectively, and r is diffuse reflectance. The details of a femtosecond diffuse reflectance spectroscopic system have been reported elsewhere.44,45 Briefly, a light source consists of a cw self-mode-locked Ti:sapphire laser(Mira 900) Basic, Coherent) pumped by an Ar+ laser (Innova 310, Coherent) and a Ti:sapphire regenerative amplifier system (TR 70, Continuum) with a Q-switched Nd:YAG laser (Surelight I Continuum). The fundamental output from the regenerative amplifier (780 nm, 3-4 mJ/pulse, 170 fs fwhm, 10 Hz) was frequency doubled (390 nm) and used as an excitation light pulse. The energy of the excitation pulse measured with a Joule meter (P25, Scientech) was several tens of microjoules and its spot size on the sample was nearly 2 mm. The shot-to-shot fluctuation of the energy was less than (10%. The residual of the fundamental output was focused into a quartz cell (1 cm path length) containing H2O to generate a white-light continuum as a probe pulse. Transient absorption intensity was displayed as percentage absorption:44

% absorption ) 100 × (1 - R/Ro)

(2)

where R and Ro represent intensity of the diffuse reflected light of the probe pulse with and without excitation, respectively. The time resolution of system is less than 1 ps for the powder with a large absorption coefficient at the excitation wavelength. Results and Discussion The X-ray diffraction patterns of MCM-41 and TiMCM-41 are shown in Figure 1, which have been assigned to a hexagonal lattice as reported previously,18,42 indicating that the framework of the synthesized MCM-41 and TiMCM-41 is well established. No change in the XRD patterns was observed when MnIIITPP(Cl) was encapsulated in MCM-41 and TiMCM-41,46 meaning that the framework of the MCM-41 is not broken upon encapsulation as shown in Figure 2. Furthermore, we could not observe any spectral features (XRD pattern, ground-state absorption, etc.), originating from the self-aggregation of porphyrin molecules. Figure 3 shows the ground-state absorption spectra of MnIIITPP(Cl) in MCM-41 and TiMCM-41 as well as in benzene. All absorption spectrum consists of the two Soret bands (380 and 480 nm) and three Q-bands (520, 580, and 610 nm). Upon encapsulation, the absorption band was dramatically changed, i.e., (i) the Q-band was blue-shifted, (ii) the ratio of Soret to Q-bands was reduced, and (iii) the Soret bands became broader. It has been reported that adsorption of organic molecules on oxide supports produces strong perturbations to the absorption maxima and the molar absorption coefficients.47,48 The electronic spectroscopy and surface photochemistry of organic molecules adsorbed on silica gel have been also reported,18,47-49 and the

Excited-State Dynamics of MnIIITPP(Cl)

J. Phys. Chem. B, Vol. 105, No. 36, 2001 8515

Figure 3. Absorption spectra of MnIIITPP(Cl) encapsulated in MCM41, TiMCM-41, and in benzene.

Figure 1. XRD patterns of MCM-41 (s) and MnIIITPP(Cl) encapsulated MCM-41 (- - -) (a), TiIMCM-41 (Si/Ti ) 120) (s) and MnIIITPP(Cl) encapsulated TiMCM-41 (- - -) (b).

Figure 2. Schematic representation of the MnIIITPP(Cl) encapsulated in MCM-41 (or TiMCM-41).

adsorption generally results in spectral blue shifts because the excited state of the molecules has a decreased permanent dipole. The broadening of absorption bands of aromatic molecules adsorbed on metal oxide is originated from the π-electrons interacting with the surface hydroxyl groups. Thus, these absorption spectral changes indicate that MnIIITPP(Cl) molecules are adsorbed onto MCM-41 and TiMCM-41, that the porphyrin π-electrons interact with the surface hydroxyl group of MCM41 and TiMCM-41.18,47-49 To understand the microenvironmental effects on the photophysical dynamics of MnIIITPP(Cl), the femtosecond and picosecond 390 nm laser photolysis was conducted in MCM41 or TiMCM-41 as well as in benzene. Figure 4 shows the transient absorption spectra of MnIIITPP(Cl) in benzene at delay times of 1 ps with an excitation of the Soret band at 390 nm. In benzene, the characteristic spectral feature of these absorption spectra shows a weak flat absorption in the entire probe wavelength region with a dip at 470 nm due to the groundstate bleaching of the Soret band. In addition, another weak dip is observed in the low energy region (650-800 nm) at 630 nm due to the ground-state bleaching of the Q-band. In earlier picosecond studies for MnIIITPP(Cl) in methylene chloride (CH2H2),50 the authors attributed the observed decay at 450500 nm region with a decay time of approximately 17 ps,

Figure 4. (a) Transient absorption spectra of MnIIITPP(Cl) in benzene at delay times of 1 ps with respect to the 390 nm excitation. (b) Decay profile of the transient absorption at 490 nm for MnIIITPP(Cl) in benzene following 390 nm excitation.

to relaxation of a “tripmultiplet” state. As well-known, MnIIITPP(Cl) has a d4 ground-state electron configuration (S ) 2). Therefore, the “(π, π*)” states of the complex are not the normal singlets and triplets because of coupling of the unpaired metal electrons with the ring π electrons. The ground state is a quintet (5So), and a quintet excited state (5S1) is derived from the lowested excited ring (π, π*) singlet; a “tripmultiplet” manifold (3T1, 5T1, 7T1) is derived from the lowest ring (π, π*) triplet.50 The essentially nonluminescent behavior of manganese(III) prophyrin complexes is generally attributed to relaxation from normally emissive (π, π*) excited states to CT or (d,d) states at lower energy. Irvine et al.51 based this assignment largely on the observation that many porphyrin (π, π*) states, such as the triplet (π, π*) of ZnIITPP,51 show strong absorption near 500 nm (i.e., to the red Soret band): such absorption is seen in manganese(III) prophyrin transient spectra at short time delays. In agreement with the general conclusion by Irvine et al.51 and Holten et al.52,53 we believe that the most transient is a tripmultiplet state and more specially it can be assigned that

8516 J. Phys. Chem. B, Vol. 105, No. 36, 2001

Figure 5. Transient absorption spectra (a) and the temporal profiles (b) of MnIIITPP(Cl) encapsulated in MCM-41.

the tripquintet, 5T1 (π, π*), which is apparently formed rapidly via intersystem crossing from the lowest singquintet, 5S1 (π, π*) decaying very rapidly (sub ps). In relation to that in benzene solution we observed a very short-lived component at 450500 nm region, which has approximately 2-3 ps lifetime corresponding to the singlet state as shown in Figure 4. In MCM-41 or TiMCM-41, however, the transient spectra were greatly changed (Figures 5 and 6). The transient absorption around 450-530 nm is observed more broadly compared to that observed in benzene, and the low energy transient is observed around 650-800 nm. The temporal profiles of transient absorption in Figures 5b and 6b indicates that a new decay component, which has ca. 60 ps lifetime, is observed in MCM41 and TiMCM-41. A state energy diagram was shown in Figure 7 with the aid of an energy-level diagram that is generally accepted in the photophysics for manganese(III) prophyrins.50-52 As suggested by Holten et al.,52,53 the low energy region transient absorption originated from a spin-forbidden relaxation (slower than the spin-allowed decay of triptquintet) via a quintet CT state. Otherwise, the 5T1 (π, π*) decay gives rise to the longer lived 80-100 ps component, which is assigned to 7T1 (π, π*). The rapid 5-20 ps decay can be viewed in part as the time for equilibration of the tripmultiplets and the longer lifetime of 7T1 (π, π*) to that of 5T1 (π, π*) can be rationalized on the basis of spin selection. First, direct relaxation of the tripseptet to the quintet ground state is spin-forbidden. Second, no low-energy septet CT or (d,d) excited state to which 7T1 (π, π*) could decay rapidly are expected. However, no septet (d,d) excited states are existent because no metal T ring CT transition involving the half-filled d orbitals can lead to the septet excited state. Therefore, the only possible septet CT states are 7(π, dx2- y2), and these will be very high in energy and unlikely to participate in decay of 7T1 (π, π*). The encapsulating with MCM-41 and TiMCM-41 could immobilize the guest molecules and decrease the vibrational

Kim et al.

Figure 6. Transient absorption spectra (a) and the temporal profiles (b) of MnIIITPP(Cl) encapsulated in TiMCM-41.

Figure 7. Proposed energy-level diagram for MnIIITPP(Cl) in MCM41 and TiMCM-41.

relaxation with medium. Thus, enormous transient absorption in the low energy region could be explained by this phenomenon. It should be noted that the longer-lived transient of metalloporphyrins is similarly observed in the ligating solvents, such as pyridine.54 Likewise the enhancement of low-energy region transient absorption in MCM-41 or TiMCM-41 could be also attributed to the framework of MCM-41 and the central metal of porphyrin. The longer-lived component in TiMCM41 is even more enhanced than in MCM-41, indicating that the

Excited-State Dynamics of MnIIITPP(Cl)

J. Phys. Chem. B, Vol. 105, No. 36, 2001 8517 41 was significantly blue shifted and, the resulting spectra were well overlapped with MnIIITPP(Cl) encapsulated in TiMCM41. The results indicate that the medium properties, such as electron density and/or polarity should be changed. However, no spectral changes were observed in MnIIITPP(Cl) encapsulated in TiMCM-41. The differences between these spectral changes could be explained with different framework compositions. Ti4+ sites in TiMCM-41 effectively works electron-accepting site and can be easily reduced to Ti3+. On the other hand MCM-41 does not have a special reducing site. Therefore, transferred electron(s) should be accumulated on the oxygen of surface of MCM-41 and changed surface electron density and/or composition, and then generated reduction site. Therefore, those photogenerated electrons and MnIIITPP(Cl) cation radical in this system could be applied to the photocatalytic reaction. Conclusions

Figure 8. Absorption spectra of MnIIITPP(Cl) encapsulated in MCM41 and TiMCM-41 as a function of various irradiation times.

framework of TiMCM-41 could be more easily interacted and/ or ligated with MnIIITPP(Cl) compared with one of MCM-41, and represents that the spin-forbidden relaxation (slower than the spin-allowed decay of triptquintet) via a quintet CT state were favorable in the order TiMCM-41 > MCM-41. As mentioned above, a broad transient absorption of MnIIITPP(Cl) encapsulated in MCM-41 or TiMCM-41 was observed around 500 nm, indicating that it was not originated from a single component. In this work, it was observed that the spectra of really long-lived and/or permanent components were widely observed (500-800 nm). However, their lifetime could not be discussed because it was out of our experimental boundary. Those components did not decay and were more strongly observed for MnIIITPP(Cl) encapsulated in TiMCM41 than in MCM-41. As in the previous reports,18,41 the MCM41 framework has a role as an electron acceptor, and porphyrin π cation radical in MCM-41 were generated by photoirradiation (g350 nm). Furthermore, this radical cation observed at not only 77 K but also at room temperature did not decay. Therefore, no decaying transient absorption at 500-800 nm could be originated from the porphyrin π cation radical, which was generated by the irradiation. The spectral broadening and enhancement at 500-530 nm may indicate that other transient absorption is overlapped in the lower energy region. Moreover, this porphyrin π cation radical is more easily generated and detected in TiMCM-41 than in MCM-41. The generation of porphyrin π cation radical could suggest that the photoinduced electron transfer occurs from porphyrin to MCM-41 framework and the transferred electron can be accumulated in MCM-41 framework. This possibility is also supported by the ground-state absorption spectral changes of MnIIITPP(Cl) encapsulated in MCM-41 upon irradiation (g400 nm). As shown in Figure 8, upon irradiation (g400 nm) the ground-state absorption of MnIIITPP(Cl) encapsulated in MCM-

Mesoporous MCM-41 and TiMCM-41 molecular sieves are found to be promising hosts for long-lived photoinduced charge separation of adsorbed MnIIITPP(Cl). Longer-lived transient species should be originated by the spin-forbidden relaxation (slower than the spin-allowed decay of triptquintet) via a quintet CT state. After irradiation, MnTPPCl+• radicals are detected in MCM-41 or TiMCM-41, indicating that the mesoporous silicate framework plays good electron acceptor. The MnIIITPP(Cl)+• generation increases in the order MCM-41 < TiMCM-41, indicating that framework modification by incorporating the Ti4+ into the MCM-41 enhances the electron-accepting ability of the MCM-41 framework. Acknowledgment. This work was supported in part by the Korea Research Foundation (Grant 1998-001-D01141). References and Notes (1) Moser, J.; Gra¨tzel, M. J. Am. Chem. Soc. 1983, 105, 6547-6555. (2) Navio, J. A.; Marchena, F. J. J. Photochem. Photobiol. A: Chem. 1991, 55, 319-322. (3) Wang, Y.; Wan, C. J. Photochem. Photobiol. A: Chem. 1994, 84, 195-202. (4) Duonghong, D.; Ramsden, J.; Gra¨tzel, M. J. Am. Chem. Soc. 1982, 104, 2977-2985. (5) Muzyka, J. L.; Fox, M. A. J. Photochem. Photobiol. A: Chem. 1991, 57, 27-39. (6) Pichat, P.; Mozzanega, M.; Courbon, H. J. Chem. Soc., Faraday Trans. 1 1987, 83, 697-704. (7) Hagfeldt, A.; Gra¨tzel, M. Chem. ReV. 1995, 95, 49-68. (8) Bach, U.; Lupo, D.; Comte, P.; Mpser, J. E.; Weisso¨rtel, F.; Salbeck, J.; Spreitzer, H.; Gra¨zel, M. Nature 1998, 395, 583-585. (9) Moser, J.; Gra¨tzel, M. J. Am. Chem. Soc. 1984, 106, 6557-6564. (10) Kalyanasundaram, K.; Vlachopoulos, N.; Krishnan, V.; Monnier, A.; Gra¨tzel, M. J. Phys. Chem. 1987, 91, 2342-2347. (11) Vlachopoulos, N.; Liska, P.; Augustynski, J.; Gra¨tzel, M. J. Am. Chem. Soc. 1988, 110, 1216-1220. (12) Fan, F. F.; Bard, A. J. J. Am. Chem. Soc. 1979, 101, 6139-6140. (13) Giraudeau, A.; Fan, F. F.; Bard, A. J. J. Am. Chem. Soc. 1980, 102, 5138-5142. (14) Tennakone, K.; Tilakaratune, C. T. K.; Kottegoda, I. R. M. J. Photochem. Photobiol. A: Chem. 1995, 87, 177-179. (15) Cermenati, L.; Pichat, P.; Guillard, C.; Albini, A. J. Phys. Chem. B 1997, 101, 2650-2658. (16) Hidaka, H.; Horikoshi, S.; Ajisaka, K.; Zhao, J.; Serpone, N. J. Photochem. Photobiol. A: Chem. 1997, 108, 197-205. (17) Mills, A.; Hunte, S. L. J. Photochem. Photobiol. A: Chem. 1997, 108, 1-35. (18) Sung-Suh, H. M.; Luan, Z.; Kevan, L. J. Phys. Chem. B 1997, 101, 10455-10463. (19) Borja, M.; Dutta, P. K. Nature 1993, 362, 43. (20) Ledney, M.; Dutta, P. K. J. Am. Chem. Soc. 1995, 117, 7687. (21) Xu, Y.; Chen, X. Chem. Ind. (London) 1990, 6, 497. (22) Mattews, R. W. J. Phys. Chem. 1988, 92, 6853. (23) Sahate, J.; Anderson, M. A.; Kikkawa, H.; Edwards, M.; Hill, G. G. J. Catal. 1991, 127, 167.

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