J . Phys. Chem. 1984,88, 5210-5214
5210
Effect of Platinum Deposits on Oxygen Adsorptiorl and Oxygen Isotope Exchange over Variously Pretreated, Ultraviolet-Illuminated Powder TiO, Henri Courbon, Jean-Marie Herrmann, and Pierre Pichat * Institut de Recherches sur la Catalyse, CNRS, 69626 Villeurbanne CZdex, France (Received: May 21, 1984)
The amounts of oxygen photoadsorbed at room temperature on neat Ti02 and on Pt/TiOz catalysts containing 0.5, 5, or 10 wt % Pt (-2-nm-diameter Pt particles) have been determined after a 423 K evacuation and a 295 K equilibration in O2of the samples in the dark. These amounts decreased with increasing Pt contents, which is interpreted by electron transfer from Ti02 to Pt in line with photoconductancemeasurements. The resulting electron enrichment of the Pt particles is estimated. Oxygen isotope exchange (OIE) has been studied at room temperature over the same solids. For preoxidized samples, increasing Pt contents decreased the OIE rate. For oxide surfaces with a slightly reduced stoichiometry, this rate was markedly enhanced over 0.5 wt % Pt/Ti02, whereas higher Pt contents progressively canceled this effect. In addition, the OIE revealed that the homogeneity of the surface oxygen species was decreased by the Pt deposits for these latter samples, whereas it was not affected for the preoxidized surfaces. The OIE results can also be explained by the Ti02 to Pt electron transfer which occurs only with the less stoichiometric samples.
Introduction Catalysts consisting of metal deposits on powder semiconductors, particularly Pt/Ti02, have recently drawn attention because (i) they extend the possibilities of photosensitive semiconductors to uphill reactions1-' and (ii) under certain conditions the metal catalytic properties are deeply modified.*-I' In previous studies, we have shown that the photoconductance of powder TiOz under vacuum at room tcmperature is decreased by Pt12or Ni4deposits, and we have attributed this phenomenon to electron transfer from the titania to the metal. In addition, oxygen is photoadsorbed on TiOZand some other n-type semiconductors by electron capture,I3J4 and band-gap illumination also promotes oxygen isotope exchange (OIE) over the same solids at room t e m p e r a t ~ r e . ' ~ - ' ~ The purpose of this investigation is to assess the effect of Pt deposits (with some reference to Ni deposits) upon Ti02 and especially upon its free electron density by studying the adsorption and the isotopic exchange of O2over illuminated samples in two different states (preoxidized or not). Experimental Section 1. Apparatus. A thin layer of catalyst (100 mg) was deposited from an aqueous suspension onto the fused silica optical window consituting the lower end of the cylindrical cell (1 cm high, 5-cm diameter). This cell was glass blown to a vacuum system equipped with an Edwards oil diffusion pump (residual pressure 10-5-106 Pa), Granville-Phillips metallic valves, an ionization gauge, a Datametrics-Dresser barocel pressure sensor, and a Riber QMM 17 quadrupole analyzer. The catalyst was illuminated by a Philips HPK 125-W high-pressure mercury lamp through a water-circulating cuvette and a 300-410-nm filter (Corning 7.60). The radiant flux received by the sample (-3.75 m W cm-z) was measured with a power meter (United Detector Technology, Model 21 A) calibrated against a microcalorimeter. For pretreatments, the cell was surrounded by a furnace. The power meter was also used to control the uniformity of the catalyst layer by placing it at various locations behind this layer; any nonuniform deposit after this criterion was not employed. 2. Materials. The three Pt/Ti02 samples (Pt contents: -0.5, 5, and 10 wt %) were prepared by impregnation (H2PtC16solution) of T i 0 2 (Degussa P-25, nonporous anatase, 50 mz g-l) and by reduction in flowing Hz as describedS2The T i 0 2 sample, used as a reference, was similarly treated, except that H2PtC16 was replaced by HCl. The Ni/Ti02 samples, which were examined in much less detail, were obtained, one by impregnating the Degussa P-25 TiO, with hexaminenickel nitrate solution as described4 and the other by similarly impregnating an anatase
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*Present address: Ecole Centrale de Lyon, Bp 163, 69131 Ecully C€dex, France.
0022-3654/84/2088-5210$01.50/0
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specimen (9 mz g-', particle diameter 170 nm) prepared from T i c 4 in a flame reactor.20 The Ni contents were 4.83 and 4.53 wt %, respectively. Transmission electron micrographs2v3showed that the Pt particle size distribution was narrow with a surface-weighted mean diameter of ca. 2 nm, the largest and much less numerous particles having a diameter of 4 nmS3Measurements of the Pt dispersion by H2and O2chemisorptions and H 2 a 2titrations confirmed these results.z' Unlike many other cases of metal-supported catalysts, the particle size distribution was almost unaffected by the Pt l ~ a d i n g provided ,~ the preparation method was thoroughly followed. By contrast, the Ni particles were much bigger and their size depended on the Ni content: an average diameter of 13.5 nm was determined by magnetic measurements for the 4.83 wt % Ni/TiOz ample.^ The micrograph of Figure 1 shows that the diameter of the metal particle was also of this order of magnitude in the case of the 4.53 wt % N i / T i 0 2 sample, for which the greatest size of the TiOz grains allows one to display the Ni particles contrary to the other Ni/TiOz sample. Through the text the catalysts will be designated as i-Pt (Ni), where i is the metal content in wt %. I8O2from the Yeda Research and Development Co. (Israel) contained 94.6 atom % l*O. (1) See ref 2-7 and references therein. (2) Pichat, P.; Herrmann, J.-M.; Disdier, J.; Courbon, H.; Mozzanegga, M.-N. Nouo. J. Chim. 1981, 5, 627. (3) Pichat, P.; Mozzanega, M. N.; Disdier, J.; Herrmann, J.-M. N o w . J . Chim. 1982,6, 559. (4) Prahov, L. T.; Disdier, J.; Herrmann, J.-M.; Pichat, P. Znt. J . Hydrogen Fnpruv 1984 9, _ 797 ,,---., _._ ( 5 ) Bard, A. J. J . Phys. Chem. 1982, 86, 172. (6) Mills, A.; Porter, G. J. Chem. SOC.,Faraday Trans. 1 1982, 78, 3659. (7) Bornarello. E.; Pelizzetti. E. Chim. Znd. (Milan) 1983, 65, 474. (8) See-ref 9-1 1 and references therein. (9) Tauster, S. J.; Fung, S. C.; Baker, R. T. K.; Horsley, J. A. Science 1981, 211, 1121. (10) Herrmann, J.-M.; Pichat, P. J . Catal. 1982, 78, 425. (l!) Imelik, B. et al., Eds.; "Studies in Surface Science and Catalysis"; Elsevier: Amsterdam, 1982; vol. 11. (12) Disdier, J.; Herrmann, J.-M.; Pichat, P. J . Chem. SOC.,Faraday Trans. 1 1983, 79, 651. (13) Herrmann, J.-M.; Disdier, J.; Pichat, P. J . Chem. SOC.,Faraday Trans. 1 1981, 77, 2815. (14) Morrison, S . R. In "The Chemical Physics of Surfaces"; Plenum: New York. 1977: ChaDter 9. Vol'kenstein, F. F. Rum. J. Phys. Chem. ( E n d . Transl.) 1974, 48, 1
