Chemisorption and catalysis by metal clusters: I. Characterisation of materials obtained by impregnation of Os 3(CO) 12 and Os 6(CO) 18 onto silica, alumina, and titania

Abstract

Os 3(CO) 12 and Os 6(CO) 18 were impregnated onto silica, alumina, and titania and characterised in the freshly impregnated state and in states achieved by subjecting the freshly impregnated material to (i) washing, (ii) heating to 523K (temperature-programmed decomposition), and (iii) storage at room temperature. The original clusters interact with the support surfaces and are converted to a family of species A of empirical formula Os n (CO) xn C yn , where the most likely value of n is 12, 2.0 ≤ x ≤ 3.0, and 0.0 ≤ y ≤ 0.4. Retention of osmium-osmium bonding in species A is demonstrated by ultraviolet/visible reflectance spectroscopy and the upper limit of n is suggested by electron microscopy. Infrared spectra of species A contain three bands and indicate the presence of carbonyl ligands bonded to osmium atoms in formal zero, partial negative, and partial positive oxidation states. Species A chemisorbs carbon monoxide and oxygen at 293K, the extent of oxygen chemisorption being the same as that of strong CO chemisorption. A bridged structure for adsorbed-CO is proposed. [ 18O]CO adsorbed onto species A does not equilibrate, even at high temperatures, with linearly bonded [ 16O]CO-ligands, confirming that adsorbed-CO and ligand-CO are different states of bound CO. CO 2 is formed, probably by a Boudouard reaction, during temperature-programmed decomposition of all freshly impregnated materials, and hence species A prepared in this way may contain ligand-C. Speculations as to likely cluster structures for species A are presented. Chemisorption and catalytic properties will be described in later papers.