Preparation of some supported metallic catalysts from metallic cluster carbonyls

An examination was made of the adsorption of some metallic cluster carbonyls (MCCs), Co2Rh2(CO)12, Co3Rh(CO)12, Co4(CO)12, Ir4(CO)12, Rh6(CO)16, and Ru3(CO)12, from nonaqueous solution onto two typical catalyst supports, ¿-alumina and Aerosil silica. With two MCCs, Co2Rh2(CO)12 and Ir4(CO)12, dispersed metallic catalysts were generated, and a study was made of how the main experimental conditions affected the metallic dispersion. MCC adsorption was more facile on ¿-alumina than on silica and was often assisted by the presence of oxygen. An ir study showed that initial adsorption of Co2Rh2(CO)12 on ¿-alumina occurred with the loss of bridging carbonyls, the remaining carbonyls being progressively lost at temperatures >300 K, while adsorption of Ir4(CO)2 on ¿-alumina resulted in progressive carbonyl loss at 320–620 K. Strong adsorption involves carbonyl loss, probably by ligand exchange with a surface anion, and the effect of oxygen is probably oxidative decarbonylation. Catalysts prepared from Co2Rh2(CO)12 or Ir4(CO)12 were relatively highly dispersed (D ˜ 0.4-1 depending on conditions), and Co2Rh2(CO)12 gave a much higher dispersion than was obtained by conventional impregnation using aqueous salt solutions. MCC adsorption in the presence of oxygen favored higher dispersions.