A comparison of ceria and Sm-doped ceria for hydrocarbon oxidation reactions

The oxidation of methane, ethane, propane, and n-butane has been studied over CeO2 and Samaria-doped ceria (SDC) catalysts. The rates for methane and ethane were found to be indistinguishable over the two catalysts, while the rates for propane and n-butane were much higher on ceria compared to SDC. The difference between n-butane oxidation over ceria and SDC is shown to result from a low-temperature rate process on ceria that is not present on SDC. Measurements using CD4 and C4D10 show that both low- and high-temperature rates exhibit a similar kinetic-isotope effect; however, the low-temperature process is half-order in O-2, While the high-temperature process is zero-order. Pulse studies demonstrate that oxygen from the bulk becomes accessible for reaction at approximately the same temperature as that at which the high-temperature rate process becomes important. The implications of these results for understanding the effect of doping on reactions over ceria, and for characterization of oxides using standard test reactions, are discussed. (C) 2004 Elsevier B.V. All rights reserved.