Diffuse reflectance infrared spectroscopy study of Co2(CO)8 supported on alumina

Surface species formed by the deposition of Co-2(CO)(8) on partially dehydroxylated aluminas were identified by diffuse reflectance IR spectroscopy (DRIFT). Co-2(CO)(8) decomposes when it interacts with the surface of alumina, giving rise to smaller surface species, such as Co(CO)(4)(-) and Co2+(CO)(x) in which cobalt is supposed to be in octahedral coordination, as well as to larger Co-6 species. The behavior of these species and the formation of new ones, a divalent cobalt carbonyl, in which cobalt is in tetrahedral coordination, and one unidentified, were studied by heating the sample gradually until nu(CO) due to carbonyl species had disappeared. Co2+(CO)(x) with cobalt in octahedral coordination is the first surface species to disappear. Tetrahedrally coordinated divalent cobalt carbonyl, in turn, was found to be the most stable of the cobalt carbonyl surface species. As side reactions, bicarbonates, carbonates, and formates have been formed. A bicarbonate species has been proposed to be formed simultaneously with the disproportionation of Co-2(CO)(8). Liberating CO reacts with O- sites, forming monodentate carbonate, which turns to bidentate carbonate species during heating. At higher temperatures; hydrogen releases and reacts with CO forming formate species. We proposed the disproportionation of Co-2(CO)(8) for the main initial surface reaction on alumina 200. Besides the disproportionation, other initial reactions, such as a formation of Co-6 species, exist on aluminas with more Lewis acid/basic sites. Formation of Co-6 species and bicarbonates was found to be competitive reactions, most probably they adsorb at similar sites.