H2 production from ethanol over Rh-Pt/CeO2 catalysts:: The role of Rh for the efficient dissociation of the carbon-carbon bond

The reaction of ethanol over a Rh-Pt/CeO2 catalyst has been investigated by temperature-programmed desorption (TPD) and infrared spectroscopy (FTIR) and in steady state conditions. On the unreduced surface, ethoxides (the main species observed upon ethanol adsorption) are directly dehydrogenated to adsorbed acetaldehyde (eta(1) mode at 1705 cm(-1), IR) that desorbs at 390 and 450 K (TPD). Comparison with TPD of ethanol over Pt/CeO2 and Rh/CeO2 separately shows that acetaldehyde desorption at 390 K is characteristic of the former and that at 450 K is characteristic of the latter. In contrast to this, on an H-2-reduced Rh-Pt/CeO2 surface ethoxide is directly decomposed to adsorbed CO (2032 cm(-1), IR): most likely via an oxametallacycle intermediate on Rh, with the formation of only very small amounts of acetaldehyde. This results in a considerable shift of the primary reaction product from acetaldehyde to CO and methane (TPD). Steady state reactions show that large amounts of H-2 can be formed depending on the ethanol-to-O-2 ratio and reaction temperature. At an O-2 : ethanol ratio = 2 and a reaction temperature above 600 K, total conversion of ethanol with an H-2 yield approaching 25 mol%, is seen. Dehydration to ethylene was not observed under any of the reaction conditions investigated. Moreover, large amounts of CO were converted to CO2, as evidenced by high CO2-to-CO ratios (between 7 and 10). It appears that both Rh and Pt are required for efficient low-temperature production of H-2 from ethanol. (C) 2002 Elsevier Science (USA).