Preparation of Nanosized Noble Metals on Well-defined Oxide Particles Via Selective Precipitation

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T. Sugimoto, S. Kawasaki, A. Muramatsu
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Particle Formation
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In the IFPRI Annual Report of (1998), we described a novel method for selective deposition of nanometer-size metallic Au particles (ca. 1 nm) onto monodispersed well-defined metal oxide particles simply by heating a HAuCl, solution around pH 6 at 100 “C, without addition of any specific reducing agent. This was found to be a kind of catalytic reaction of the metal oxides. Similarly, we tried to extend this technique to the selective deposition of noble metal nanoparticles of the platinum group (Ru, Rh, Pd, Ir, and Pt) onto well-defined metal oxide particles, such as a-Fe,O, a-FeOOH, B-FeOOH, TiO, and ZrO. However, in contrast to the gold system, the noble metals in the platinum group were found to deposit onto the supports in the form of oxide or hydroxide by the aging at 100 “C, without being reduced to metallic particles. Metallic particles (l-4 nm) were finally obtained by reduction of the hydrous(oxide) precursor particles on the support powders with H, gas at 250 “C. The support particles in this case were found to play a decisive role in the acceleration of the selective deposition of the precursor particles and in the final formation of dense and well-dispersed metallic nanoparticles of the noble metals. In general, the specific surface area and surface roughness of the supports and their affinity to metal particles were found to be the decisive factors for obtaining well-dispersed metal particles by inhibiting the aggregation and/or sintering during the reduction process, Above all, the combination of Pt and monodispersed ellipsoidal TiO, particles (anatase) with a large specific surface area and a rough surface yielded well-dispersed nanoparticles of Pt as small as 1.3 5 0.5 nm. When it was used as a catalyst for hydrogenation of 1-octene to octane, an outstanding catalytic activity was shown over the other combinations. The Pt/TiO, catalyst prepared by the precursor deposition method in this study was superior to other Pt/TiO, catalysts prepared even by the best conventional methods, such as the ion-exchange method or the impregnation method.

We are now planning to study the correlation between the structural factors of heterogeneous catalysts and their catalytic performance, mainly as optical catalysts using new Pt/TiO, composite particles with nanosized titania supports precisely controlled in size, crystal habit, and crystal structure.