PNNL

Abstract

The interaction between an active oxide and an oxide support plays a critical role in controlling the reactivity of oxide-on-oxide catalysts. In the present study, the reactivity of a small ceria cluster (Ce2O4) supported on the reducible monoclinic zirconia and the irreducible ?-alumina was investigated using first-principles density functional theory (DFT+U) method. Our results showed that the binding energies of the Ce2O4 cluster on the supporting ZrO2(111) and ?-Al2O3(100) substrates are -5.32 and -4.06 eV, respectively, indicating a very strong interaction. Based on these oxide-on-oxide model catalysts, the effects of supports on the reactivity of Ce2O4 cluster were probed by the adsorption of CO2 and CO. The acidic CO2 molecule chemisorbs at the O sites of the cluster, forming a carbonate-like (CO32-) species through an acid-base interaction. Neither ZrO2(111) nor ?-Al2O3(100) exhibits significant effect on CO2 adsorption over the supported Ce2O4 cluster. In contrast, the reactive adsorption of CO on the supported Ce2O4 cluster shows a strong dependence on the supporting oxides: The reactive adsorption energy for CO on the ?-Al2O3(100) supported Ce2O4 is -4.33 eV whereas that on the ZrO2(111) supported cluster is only -0.55 eV. This reactive adsorption was accompanied by the reduction of Ce4+ to Ce3+ in the Ce2O4 clusters, leading to the formation of (Ce2O2)2+CO32- that can be considered as an intermediate for CO oxidation to CO2. The very different stability of the (Ce2O2)2+CO32- intermediate on the two oxide supports were analyzed in the context of CO oxidation catalyzed by ceria. The ZrO2(111) supported Ce2O4 cluster is expected to be highly active for CO oxidation whereas the turnover from CO to CO2 on the ?-Al2O3(100) supported and the unsupported Ce2O4 clusters is hindered by the desorption of CO2 from the (Ce2O2)2+CO32- intermediates. Pacific Northwest National Laboratory is operated by Battelle for the US Department of Energy.