Hexarhodium Clusters on Lanthana: Synthesis, Characterization, and Catalysis of Ethene Hydrogenation

Rhodium carbonyl clusters were prepared on the surface of La 2 O 3 powder (calcined at 673 K) by a surface-mediated synthesis from La 2 O 3 -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 La 2 O 3 -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 Å. 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 π-bonded ethene and others, inferred to be ethyl, ethylidyne, and di-σ-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. 1 H NMR spectroscopy showed that hydride remained on the clusters following catalysis. The results suggest that the hydrogenation of ethene on Rh 6 /La 2 O 3 proceeds by insertion of π-bonded ethene into a Rh−H bond to form ethyl, which is subsequently hydrogenated to give ethane. Rh 6 /La 2 O 3 is about 50 times more active for ethene hydrogenation catalysis than Rh 6 /γ-Al 2 O 3, and we suggest that the difference is related to the electron-donor properties of the supports.