Clusters and catalysis: on the requirement for multinuclear centers to catalyze the hydrogenation of carbon monoxide

Abstract

Efficient homogeneous catalysis by mononuclear complexes of certain reactions, such as methanation and Fischer-Tropsch synthesis has not been achieved. It has recently been demonstrated that temperature programmed decomposition (TPDE) of carbonyls adsorbed on silica or alumina leads to the formation of novel subcarbonyl species which are highly dispersed and which can be stable to 7250/sup 0/C, a temperature sufficient for catalysis of methanation over supported metals. A detailed study was undertaken of the dependence of CH/sub 4/ formation (during TPDE) on the nuclearity of the alumina supported complexes. The results suggest a novel reaction sequence for catalytic methanation: M + CO ..-->.. M(CO)ads (8); 4sigma - OH + M(CO)ads ..-->.. MO/sub 3/ + 2sigma - O/sup -/ + CH/sub 4/ (9); 3H/sub 2/ + MO/sub 3/ ..-->.. M + 3H/sub 2/O (10); 2H/sub 2/O + 2sigma - O/sup -/ ..-->.. 4sigma - OH (11); CO + 3H/sub 2/ ..-->.. CH/sub 4/ + H/sub 2/O (12). Reactions 8, 10, and 11 are well known. Reaction 9 simply expresses the known stoichiometry of CH/sub 4/ formation during the TPDE of carbonyls in flowing He. Reaction 9 presumedly represents a sequence of steps which would cycle with reactions equivalent to 10 and 11more » so that M only gets slightly oxidized during reaction. A unique feature of this scheme is that CH/sub 4/ is formed by reaction with sigma-OH, the role of the H/sub 2/ being to rereduce the metal. High reactivity for catalytic methanation of Mo(CO)/sub 6/ W(CO)/sub 6/ supported anAl/sub 2/O/sub 3/ has also been discovered, the activities being roughly 100-fold higher than for analogous traditional heterogeneous catalysts. 4 figures, 2 tables. (DP)« less