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

Abstract

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 Rh6 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 H2 reacting catalytically to give ethane. The EXAFS first-shell Rh−Rh coordination number was found to be about 4, consistent with the presence of Rh6 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 Rh6 increased during the initial induction period in a flow reactor as the catalytic activity increased almost proportionately; hydrides are inferred to be reactive intermediates. 1H NMR spectroscopy showed that hydride remained on the clusters following catalysis. The results suggest that the hydrogenation of ethene on Rh6/La2O3 proceeds by insertion of π-bonded ethene into a Rh−H bond to form ethyl, which is subsequently hydrogenated to give ethane. Rh6/La2O3 is about 50 times more active for ethene hydrogenation catalysis than Rh6/γ-Al2O3, and we suggest that the difference is related to the electron-donor properties of the supports.