An Infrared Study of Supported Ruthenium Catalysts Prepared from Ru₃(CO)₁₂

Abstract

The structure and the decomposition process of R₃(CO)₁₂ dispersed on various oxidei were investigated by use of infrared spectroscopy. Whereas interaction of Ru₃(CO)₁₂, with SiO₂ and ZnO was found to be relatively weak, Ru₃(CO)₁₂ strongly interacted with the surface γ-Al₂O₃ to give a very stable species, which was assumed to be a mononuclear ruthenium carbonyl. The infrared bands due to this Ru carbonyl on γ-Al₂O₃ were located at 2O63 and 199Ocm^-1 and the bands were little affected by evacuation at temperatures up to 25O°C as shown in Fig.4.
Thermal decomposition of the Ru carbonyl on γ-Al₂O₃ followed by H₂ reduction at 45O°C resulted in the formation of Ru catalysts with high dispersion (<9O%) as determined by H₂ chemisorption. The dispersion of Ru/Al₂O₂ catalysts prepared by conventional impregnation using RuCl₃ was low (<5O%) as shown in Fig.5.
Infrared spectra and adsorption measurement of CO and H₂ indicated that CO was adsorbed in linear- and twin-types on Ru/Al₂O₃ derived from Ru₃(CO)₁₂ and that multiple- and twintypesof CO were predominant of Ru/Al₂O₃ prepared from RuCl₃. With increasing dispersion of Ru, the relative amount and the strength of adsorption of twin-type CO increased. The twin-type species was little reactive to hydrogen (Fig.8), indicating that these species rather act as a poison for the hydrogenation of CO over Ru catalyst. The characteristic trend of Ru catalysts that specific activity (turnover frequency) for CO conversion increased with the decreasing metal dispersion in the hydrogenation of CO can be explained by this retardation effect of multiple- and twin-type CO. Infrared bands of adsorbed CO shifted to lower frequencies when Ru was supported on MgO or K₂CO₃-doped Al₂O₃ (Fig.9, 1O), probably due to electron transfer from the support to Ru.