CATALYTIC SITE REQUIREMENTS FOR ELEMENTARY STEPS IN SYNGAS CONVERSION TO OXYGENATES OVER PROMOTED RHODIUM

Article abstract of DOI:10.1021/j100411a009

Following P.H.Emmett's pioneering work, much reserach was focused on the mechanism of Fischer-Tropsch catalysis, and it has been discovered that certain base metal ions promote the activity and selectivity of Rh metal to catalyze the conversions of CO + H2 to aldehydes and alcohols.Recent IR, Moesbauer, and EXAFS evidence in conjunction with catalytic tests suggests that the highly oxophilic ions Mn, Ti, Zr, and Nb, when located at the Rh surface, enhance CO dissociation, possibly through direct interaction with the oxygen atom of tilted adsorbed CO.A different mechanism emerges for Fe and Zn ions.These seem to block sites, thus inhibiting the formation of bridging CO and its dissociation to C + O.These steps are characterized by large ensemble requirements.Remarkably, the rate of CO insertion on Rh appears enhanced but the rate of hydrogenation is markedly suppressed by Zn.Consequently, the hydroformylation of olefins and the synthesis of methanol are efficiently catalyzed.This result suggests that the latter reactions require only very small Rh ensembles; possibly they are catalyzed by single Rh atoms.