DIFFUSE-REFLECTANCE INFRARED-SPECTROSCOPY STUDY OF CO-2(CO)(8) SUPPORTED ON ALUMINA

Surface species formed by the deposition of Co-2(CO)(8) on partially d ehydroxylated aluminas were identified by diffuse reflectance IR spect roscopy (DRIFT). Co-2(CO)(8) decomposes when it interacts with the sur face 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 octahedra l coordination, as well as to larger Co-6 species. The behavior of the se 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 carbo nyl species had disappeared. Co2+(CO)(x) with cobalt in octahedral coo rdination is the first surface species to disappear. Tetrahedrally coo rdinated divalent cobalt carbonyl, in turn, was found to be the most s table of the cobalt carbonyl surface species. As side reactions, bicar bonates, carbonates, and formates have been formed. A bicarbonate spec ies has been proposed to be formed simultaneously with the disproporti onation of Co-2(CO)(8). Liberating CO reacts with O- sites, forming mo nodentate 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(C O)(8) for the main initial surface reaction on alumina 200. Besides th e disproportionation, other initial reactions, such as a formation of Co-6 species, exist on aluminas with more Lewis acid/basic sites. Form ation of Co-6 species and bicarbonates was found to be competitive rea ctions, most probably they adsorb at similar sites.