Ethanol promotion by the addition of cerium to rhodium–silica catalysts

Abstract

The selectivity of rhodium-based catalysts is very dependent on the choice of support or the promotors. Cerium oxide is very effective for ethanol promotion when added to Rh/SiO₂ catalysts or used as support (70–80% ethanol selectivity). The role of the cerium oxide has been studied by various techniques, namely chemical trapping, Fourier-transform infrared spectroscopy and the use of probe molecules. The results show that when cerium oxide is added to rhodium catalysts, a new absorption band appears at 1725 cm^–1 in the infrared spectra which may be attributed to a CO molecule bonded through both the carbon and oxygen atoms. Such an interaction could be responsible for the formation and stabilization of the formyl species which, from chemical trapping experiments, is proposed as a key intermediate. These results and those obtained with probe molecules would be in accordance with a reactional pathway where the formyl species is a building block of the C₂-oxygenates and the acetaldehyde a primary product of the synthesis. In addition to its role in the activation of carbon monoxide towards hydrogenation, it is also shown that cerium oxide is responsible for the reduction of acetaldehyde to ethanol.