Ethene hydrogenation on zeolite-supported rhodium clusters. A mechanistic study by density functional and microkinetic modeling

Experiments showed small Rh clusters, supported in zeolites, to be catalytically active in the hydrogenation of ethene. We report a computational study on the transformations of ethene over Rh₄ clusters supported in a faujasite zeolite framework, in particular on the influence of the hydrogen loading of the clusters. Our density functional calculations revealed a general trend of decreasing activation energies of the hydrogenation for increasing H coverage of the metal particles. Furthermore, the coordination of the attacking H ligand crucially affects the hydrogenation barriers. At low H coverage, strongly adsorbed, bridge-bonded H ligands are responsible for the calculated higher barriers while lower barriers were calculated for top-coordinated H ligands near the organic reactants. Microkinetic modeling, based on these electronic structure results, suggests that zeolite-supported Rh₄ clusters are active for hydrogenation of ethene regardless the H loading. Ethene, adsorbed in π-coordinated fashion, was determined to be a crucial resting state in this reaction.