Surface-supported metal cluster carbonyls. Chemisorption, decomposition and reactivity of Os3(CO)12, H2Os3(CO)10 and Os6(CO)18 supported on silica and alumina and the investigation of the fischer-tropsch catalysis with these systems

Abstract

Abstract On silica and alumina, Os 3 (CO) 12 , H 2 Os 3 (CO) 10 and Os 6 (CO) 18 are physisorbed (or weakly adsorbed) at room temperature. When the physisorbed cluster Os 3 (CO) 12 is thermally decomposed at ca. 150°C, there is an oxidative addition of a surface MOH group (M  Al???, Si???) to the OsOs bond of Os 3 (CO) 12 with formation of the surface species Os 3 (H)(CO) 10 (OM) (M  Al???, Si???) in which the cluster is covalently bonded to the surface by MOOs 3 bonds.Such a grafted cluster was also obtained was also obtained by bringing Os 3 (CO) 10 (CH 3 CN) 2 or H 2 Os 3 (CO) 10 into reaction with the surface of silica and alumina. On silica the grafted cluster, when treated with CO + H 2 O, can regenerate the starting Os 3 (CO) 12 cluster. The structure of the covalently bonded cluster was also confirmed by the synthesis of the model compound Os 3 (H)(CO) 10 (OSi(Ph) 3 ). Such covalent attachment of a cluster to a surface can be regarded as a model for the metal—support interaction which is frequently involved in heterogeneous catalysis. When the physisorbed clusters Os 3 (CO) 12 , H 2 Os 3 (CO) 10 , Os 6 (CO) 18 or the chemisorbed cluster Os 3 (H)(CO) 10 (OM), (M  Al???, Si???), are heated at about 200°C, there is a breakdown of the cluster cage with simultaneous oxidation of the osmium to two osmium(II) carbonyl species by the surface proton with simultaneous release of hydrogen. The [Os II (CO) 3 X 2 ] 2 and [Os II (CO) 2 X 2 ] n surface species in which X is a surface oxygen atom can be interconverted by a reversible carbonylation-decarbonylation process at 200°C. These two species can be also obtained by decomposition of [Os(CO) 3 X 2 ] 2 , (X  Cl, Br) onto silica or alumina surface or by CO reduction at 200°C of OsX 3 adsorbed on silica or alumina. The structure of one these surface compounds was confirmed by synthesis of the model compound [Os(CO) 3 (OSiPh 3 ) 2 ] m . These surface osmium carbonyl species exhibit a rather high thermal and chemical stability. They appear to be reduced by H 2 to metallic osmium only at 400°C. The thermal decomposition of the supported clusters is followed by a stoichiometric water gas shift reaction as well as a stoichiometric formation of methane. Under CO + H 2 , a Fischer-Tropsch catalyst, which exhibits a high selectivity for methane, is obtained. From the range of temperatures over which those stoichiometric and catalytic reactions are observed it seems reasonable to assume that they involve the Os II carbonyl species rather than metallic osmium.