J. Phys. Chem. 1988, 92, 467-470 of O2 over Ti02 it would seem reasonable to speculate that O2 may act as a recombination center at the surface under these conditions. In conclusion, we believe that this extraordinary photochromic effect in Ti02,reported here in brief, is of fundamental importance in photocatalysis and warrants thorough investigation. Possibilities that immediately spring to mind are studies of sensitization, Le., charge injection into T i 0 2 by visible-light-absorbing surface compounds or dopant ions; the effect of surface-deposited metals; ~~
(17) Pichat, P. In Heterogeneous and Homogeneous Photocatalysis; Pelizzetti, E., Serpone, N., Eds.; Reidel: Dordrecht, in press.
467
and more fundamental characterization of the trapping states and the role of the chemical state, and physical texture, of the TiOz surface in controlling electron-hole recombination. Acknowledgment. We thank Dr. C. Dimitropoulos and Professor w. Benoit in the Dgpartement de Physique for the use and adaption of the single-beam PAS spectrometer, and Dr P. P. Infelta for his generous assistance in the development of the software programming for data acquisition and manipulation. This work was supported by a grant from the Gas Research Institute, Chicago (subcontract with S.E.R.I., Golden, CO) and by the Swiss Office of Energy. Registry No. Ti02, 13463-67-7; H20, 7732-18-5; methylviologen, 1910-42-5.
Room Temperature Photoassisted Formation of Hydrogen-Molybdenum Bronzes with an Alcohol as a Hydrogen Source Pierre Pichat, * Marie-Noelle Mozzanega, Ecole Centrale de Lyon, Equipe CNRS Photocatalyse, BP 163, 69131 Ecully Cedex, France
and Can Hoang-Van Universitd Claude Bernard Lyon I, UA 231 du CNRS de Catalyse AppliquPe et Cindtique H?tProg?ne, 69622 Villeurbanne Cedex, France (Received: May 18, 1987)
The possibility of using the photosensitive properties of a highly divided MOO, (30 m2 8-I) to prepare hydrogen-molybdenum bronzes has been explored. Ultraviolet illumination of MOO, suspensions in an alcohol (methanol, 2-propanol) as a source Addition of Ti02 allows the formation of H0,93M003, which indicates interparticle of hydrogen atoms produces Ho,~&~OO~. hydrogen atom transfer. The presence of Pt metal, either directly deposited on MOO, or by adding Pt/TiO,, suppresses any hydrogen-molybdenum bronze formation. The origin of the detrimental role of Pt, which contrasts with the necessary presence of a group VI11 (groups 8-10) metal for the preparation of bronzes by hydrogen spillover, is discussed.
Introduction Molybdenum trioxide is capable of incorporating hydrogen, thus forming four phases of bronze H,MoO, in the range 0 < x 6 2l which, inter alia, are catalysts for h y d r ~ g e n a t i o n ~and - ~ isomerization5 reactions. These bronzes are usually prepared from single crystals or from powders of low surface area at temperatures below 373 K by using atomic hydrogen which can be generated by various ways6 cathodic reduction, reaction of HCl on Zn, hydrogen plasma, mercury photosensitization,' and dissociative chemisorption on an appropriate metal supported on MOO, (at least during the activation step'). At 473-488 K, the action of gaseous methanol on MOO, of high surface area also results in the formation of bronze^.^^'^ On the other hand, molybdenum trioxide is a photosensitive n-type semiconductor with a band gap of ca. 3.15 eV. Our intention was to determine whether this photosensitivity can be used to produce a bronze. Indeed, hydrogen is abstracted from gaseous or liquid alcohols by means of photocatalysts formed from group VI11 (groups 8-1 0)29metals deposited on semiconductor oxides, (1) Birtill, J. J.; Dickens, P. G. Muter. Res. Bull. 1978, 13, 31 1. (2) Sermon, P. A.; Bond, G. C. J . Chem. Soc., Faraday Trans. 1 1980, 76, 889. (3) Marcq, J. P.; Wispenninckx, X.;Poncelet, G.; Fripiat, J. J. J . Catal. 1982, 7 3 , 309. (4) Benali, R.; Hoang-Van, C.; Vergnon, P. Bull. SOC.Chim. Fr. 1985, 417. (5) Hoang-Van, C.; Benali, R.; Vergnon, P., unpublished work. (6) Fripiat, J. J. Surface Properties and Catalysis by Non-Metals; Bonnelle, J. P., et al., Eds.; Reidel, Dordrecht, 1983; p 477. (7) Fleisch, T. H.; Mains, G. J. J. Chem. Phys. 1982, 7 6 , 780. (8) Erre, R.; Van Damme, H.; Fripiat, J. J. Surf. Sci. 1983, 127, 48. (9) Vergnon, P.; Tatibouet, J. M. Bull. SOC.Chim. Fr. 1980, 1-45. (10) Guidot, J.; Germain, J. E. React. Kinet. Catal. Lett. 1980, 15, 389.
0022-3654/88/2092-0467$01.50/0
such as titanium dioxide."*12 Over UV-illuminated TiO, without deposited metal, an evolution of H2 is also observed, but it is not a catalytic phenomenon. Accordingly, the purpose of this study was to find out whether MOO, suspended in a liquid alcohol, and activated by photons of suitable energy, can yield hydrogen bronzes at room temperature.
Experimental Section Materials. MOO, was prepared by decomposing Mo02C12 vapors in an oxygen stream using a flame r e a ~ t 0 r . l ~After calcination at 673 K in O2 for 15 h, the powder obtained had a surface area of ca. 30 m2 g-' and was nonporous, which enabled a complete exposure to illumination. Its composition and structure were verified by its Raman spectrum and X-ray diffractogram which revealed the orthorhombic phase. Ti02 was nonporous Degussa P.25 (50 m2 g-l; mainly anatase). Procedures. Bronze formation was carried out in a cylindrical static slurry photoreactor (volume ca. 100 cm3) with an optical Pyrex window at its base, transmitting wavelengths >300 nm. Illumination was provided by a Philips HPK 125 W mercury lamp through a 2.5-cm water-circulating cuvette. The radiant power at the entrance of the photoreactor was in the range 45-50 mW as measured with a UDT 21A radiometer calibrated against a microcalorimeter. Unless otherwise indicated, 120 mg of MOO, was dispersed in 10 cm3 of methanol. The suspension was deaerated by using a rotatory pump before illuminating and was (1 1) Pichat, P. ACS Symp. Ser. 1985, 278, 21 and references cited therein. (12) Pichat, P.; Herrmann, J.-M.; Disdier, J.; Courbon, H.; Mozzanega, M.-N. Nouu. J . Chim. 1981, 5, 627. (13) Vergnon, P.; Bianchi, D.; Benali, R.; Coudurier, G. J . Chim. Phys. 1980, 7 7 , 1043.
0 1988 American Chemical Society
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Pichat et al.
The Journal of Physical Chemistry, Vol. 92, No. 2, 1988
I -1
dark
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2
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I
1
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17
time/h
Figure 2. Variations of the logarithm of the photoconductance MOO, sample as a function of time.
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Figure 1. Reflectance spectrum of the M o o 3 sample scanned using a Perkin-Elmer Lambda 9 spectrophotometer equipped with an integrating sphere (reference: MgSO,).
stirred magnetically during the experiment. The gas phase was analyzed by on-line gas chromatography (N2as carrier gas for H2determinations). The solid was recovered by centrifugation. X-ray diffraction was performed with a Siemens diffractometer (Kristalloflex D500) using Cu K, radiation filtered through nickel. Photoconductance measurements were carried out in the cell described in ref 14 with the same lamp and fused silica windows; the radiant power reaching the sample was 14 mW cm-’.
Results 1. Characterization of MOO,. Reflectance Spectrum. Figure 1 shows that the reflectance of MOO, begins to change slightly at ca. 500 nm. The absorbance of MOO, is quite important at 365 nm, the wavelength that corresponds to one of the more intense lines of mercury. In other words, a mercury lamp used with Pyrex filters (A > 300 nm) is appropriate for exciting MOO, and thereby for trying to reach the aim of this study. The absorption range of our sample is extended toward the visible, since forbidden band gapvalues of 2.9 eV (-427 nm)I5 and 3.1-3.15 eV (-396 nm)I6 have been reported for single crystals. This is usual for samples of high surface area. A slowly rising absorbance above 450 nm has been mentioned for a MOO, catalyst whose surface area was not indicated.” Photoconductance. Figure 2 shows the effect of illumination under vacuum. Whereas the conductance in the dark of the MOO, (14) Herrmann, J.-M.; Disdier, J.; Pichat, P. J . Chem. SOC.,Faraday Trans. I 1981, 77, 2815. (15) Krylov, 0.Catalysis by Non-Metals;Academic: New York, London, 1980. (16) Erre, R.; Legay, M. H.; Fripiat, J. J. Surf. Sci. 1983, 127, 69. (17) Liu, Y . C.; Griffin, G. L.; Chan, S. S.; Wachs, 1. E. J . Catal. 1985, 94, 108.
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of the
pellet cannot be measured because of its too small value (
