Hexarhodium clusters on lanthana: Synthesis, characterization, and catalysis of ethene hydrogenation

Rhodium carbonyl clusters were prepared on the surface of La2O3 powder (calcined at 673 K) by a surface-mediated synthesis from La2O3-supported Rh(CO)(2)(acac) in the presence of CO at 1 atm and 373 K. The cluster preparation and subsequent decarbonylation by treatment in He were characterized by infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. Treatment in He at 573 K removed the carbonyl ligands, giving site-isolated La2O3-supported clusters that are well approximated as Rh-6 octahedra, being characterized by a first-shell Rh-Rh coordination number of 3.9 +/- 0.4 at a distance of 2.64 +/- 0.02 Angstrom. The supported clusters were characterized by IR and EXAFS spectroscopies in the presence of ethene and H-2 reacting catalytically to give ethane. The EXAFS first-shell Rh-Rh coordination number was found to be about 4, consistent with the presence of Rh-6 Octahedra, which are inferred to be the catalytically active species. IR spectra show that both hydrocarbons and hydride ligands were present on the working cluster catalyst, including pi-bonded ethene and others, inferred to be ethyl, ethylidyne, and di-sigma-bonded ethene. The concentration of hydride on Rh-6 increased during the initial induction period in a flow reactor as the catalytic activity increased almost proportionately; hydrides are inferred to be reactive intermediates. H-1 NMR spectroscopy showed that hydride remained on the clusters following catalysis. The results suggest that the hydrogenation of ethene on Rh-6/La2O3 proceeds by insertion of pi-bonded ethene into a Rh-H bond to form ethyl, which is subsequently hydrogenated to give ethane. Rh-6/La2O3 is about 50 times more active for ethene hydrogenation catalysis than Rh-6/gamma-Al2O3, and we suggest that the difference is related to the electron-donor properties of the supports.