Chemisorption and catalysis by metal clusters: characterisation and chemisorption. Properties of Ru3(CO)12 and H4Ru4(CO)12 supported on alumina, silica and titania

Abstract

Ru₃(CO)₁₂ and H₄Ru₄(CO)₁₂ have been impregnated on to silica, alumina and titania and characterised in the freshly impregnated state and in states achieved by subjecting the freshly impregnated material to washing and to heating to 523 K. The original clusters interact with the support surfaces and are converted to a species of empirical formula Ru_n(CO)_xnC_ynZ_zn, where Z represents adsorption sites, n≈ 6, 0 ⩽x 2.8, 0.2 ⩽y 1.5 and 0.2 ⩽z 1.6. Retention of metal–metal bonding in this species is demonstrated by ultraviolet/visible reflectance spectra while the infrared spectra contain bands indicative of the presence of carbonyl ligands bonded to ruthenium atoms in formal zero, partial negative and partial positive oxidation states. Carbon monoxide adsorption isotherms have a conventional appearance, but most are composed of a primary and a secondary region. Material adsorbed in the secondary region is removed by evacuation at room temperature whereas that adsorbed in the primary region is removed by evacuation at elevated temperatures. From the experimental evidence it is deduced that carbon monoxide and oxygen each absorb molecularly. In the primary region carbon monoxide adsorption occurs at ruthenium sites on the cluster framework, whereas in the secondary region it occurs at ligand-carbon bonded to ruthenium. For oxygen, adsorption is again at ruthenium sites on the cluster framework.