CO hydrogenation with alumina-supported catalysts prepared from [Os(CO) 5]

γ-Al 2O 3-supported osmium was prepared from [Os(CO) 5] and characterized by infrared spectros-copy and transmission electron microscopy before and after use in catalytic hydrogenation of CO at 1 or 10 bar and 523–598 K. Mononuclear osmium carbonyl complexes were formed initially on the Al 2O 3 surface, and when the samples were brought in contact with H 2, the osmium was reduced, forming small metal aggregates. Samples were reduced either severely (in 10 bar of H 2 for 10 h), giving large aggregates, or mildly (in 1 bar of H 2 for 1 h), giving smaller aggregates. The properties of the catalysts for CO hydrogenation were compared with those of samples prepared from [H 2OsCl 6] and [Os 3(CO) 12], which consisted of mononuclear osmium complexes and ensembles consisting of three or fewer osmium ions. The activities of the latter catalysts for hydrocarbon formation and the chain growth probabilities were significantly less than those of the catalysts containing Os aggregates. The chain growth probability and the olefin: paraffin ratios were greater for the larger aggregates. Surface intermediates are suggested involving bifunctional interactions of CO with Os metal (or Os ion) sites and neighboring Al 3+ sites.