Promoter action of rare earth oxides in rhodium/silica catalysts for the conversion of syngas to ethanol

Abstract

The promoter action of rare earth oxides (REO) (La 2O 3, CeO 2, Pr 6O 11, Nd 2O 3, and Sm 2O 3) in Rh/SiO 2 catalysts for syngas conversion to ethanol and the characterization of these catalysts by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) have been investigated. The addition of REO, especially CeO 2 and Pr 6O 11, increases markedly the selectivity for production of ethanol. High-temperature reduction of the catalyst favours selectivity for ethanol in all REO promoted rhodium catalysts. XPS measurements revealed that CeO 2 in Rh-CeO 2/SiO 2 mostly exists as Ce 2O 3 after reduction, in contrast to that in rhodium-free CeO 2/SiO 2. It is concluded that rhodium assists in the reduction of CeO 2. Correlation was found between the selectivity for ethanol and the reducibility of REO. The surface mean diameters of rhodium particles in Rh-REO/SiO 2, as observed by TEM, lie in the range 4 to 6 nm and do not change after high-temperature reduction. The nature of rhodium-promoter interactions and a synergistic mechanism of rhodium coupled with REO for catalytic conversion of carbon monoxide are discussed.