Metal segregation in supported bimetallic catalysts: γ-Al 2O 3-supported CO hydrogenation catalysts prepared from RhOs 3, Rh 4, and FeOs 3 clusters

Abstract

Al 2O 3-supported metals were prepared from [H 2RhOs 3(CO) 10(acetylacetonate)], [Rh 4(CO) 12], and [H 2FeOs 3(CO) 13]. The samples were characterized by infrared spectroscopy after reaction with CO + H 2 and tested as catalysts for conversion of CO + H 2 in a flow reactor at 200 and 270 °C and 10 atm. Used catalysts were characterized by transmission electron microscopy, X-ray photoelectron spectroscopy, infrared spectroscopy, and elemental analysis. The catalyst lost Os during operation, presumably as a result of formation of volatile carbonyls. The catalytic reaction products were a nearly Schulz-Flory-Anderson distribution of hydrocarbons with small yields of dimethyl ether (formed from methanol). The performance of the catalyst prepared from the RhOs 3 cluster was closely similar to that of the catalyst prepared from the Rh 4 cluster. Characterization of the samples after treatment in CO + H 2 and after catalysis demonstrated that the RhOs 3 clusters broke apart, first giving triosmium clusters and mononuclear Rh complexes and then, at higher temperatures, giving Rh crystallites and mononuclear Os complexes. The catalytic activity for hydrocarbon synthesis is attributed to the Rh metal; the activity for methanol synthesis is tentatively associated with ionic Rh complexes. The FeOs 3 catalyst was two orders of magnitude less active than the Rh Os 3 catalyst, apparently consisting of small iron oxide particles and mononuclear Os complexes. The selectivity of this catalyst for dimethyl ether formation increased markedly with time onstream in the flow reactor; after 55 h, 36 mol% of the organic product was ether.