Rhodium/titania catalysts preparation

The pH of the solution can be critical in the impregnation process. In the preparation of Rh/Ti02 catalysts by impregnating titania with aqueous solutions of rhodium chloride it was found that at low pH the interaction between the salt and the support surface takes place in several steps. Initially the oxide particles become more positively charged and, thus, adsorb the anionic species such as RhCl4 . After adsorption, these anions are more firmly anchored to the support by the displacement of one or more chloro ligands by OH groups on the titania... 

Supported mixed metal catalysts are also prepared by other means such as the deposition of bimetallic colloids onto a support O and the decomposition of supported bimetallic cluster compounds.208 The photocatalytic codeposition of metals onto titania was also attempted with mixed results.209 with a mixture of chloroplatinic acid and rhodium chloride, very little rhodium was deposited on the titania. With aqueous solutions of silver nitrate and rhodium chloride, more rhodium was deposited but deposition was not complete. In aqueous ammonia, though, deposition of both silver and rhodium was complete but the titania surface was covered with small rhodium crystallites and larger silver particles containing some rhodium. With a mixture of chloroplatinic acid and palladium nitrate both metals were deposited but, while most of the resulting crystallites were bimetallic, the composition varied from particle to particle.209... 

This new single-step synthesis unites the simplicity of preparation and lower production costs, with the outstanding properties of the final catalysts. By the single-step procedure proposed here, deposition of dispersed nanoparticles of noble metals on ceramic supports with customised textural properties and shape was achieved. Noble metals including platinum, palladium, rhodium, ruthenium, iridium, etc. and metal oxides including copper, iron, nickel, chromimn, cerium oxides, etc on sepiolite or its mixtures with alumina, titania, zirconia or other refractory oxides have been also studied. 

Rh dispersion appears to be a key factor in the activity of zeolite-supported systems. In studies of the effect of Si/Al ratio in Rh/NaX (faujasite-type zeolite), prepared by cation exchange of NaX with [Rh(NH3)5Cl](OH)2, maximum carbonylation activity (ca. 8 mol/(gRh-h) at 200°C) correlates with maximum Rh dispersion in the prepared catalyst (48). Similarly, cation exchange of NaX with [Rh(NH3)5Cl]Cl2 has been reported to give carbonylation rates of ca. 1 mol/(gRh h) at 150°C (49), and NaY exchanged with rhodium salts (50) gives carbonylation activity of 0.4 mol/(gRh-h) at 170°C. These rates are claimed to be higher than those for Rh impregnated alumina, silica-alumina, silica, or titania (49,50). Optimum Rh dispersion corresponded to approximately two Rh atoms per vmit cell (50). 

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