Surface Structure Dependence by Monofaceted CeO2 Nanoparticles: Catalytic Oxidation Reactions

LI, MEIJUN; WU, ZILI; CALAZA, FLORENCIA C.; MULLINS, DAVID R.; OVERBURY, STEVEN H.

Nashville, TN

Simposio; AVS 58th Annual International Symposium and Exhibition; 2011

American Vacuum Society

Reducibility of pure and doped CeO2 is of interest in emission control catalysts because of the ability of the CeO2 to store and supply oxygen during oxidation catalysis. But, it is not known how the structure or crystallographic termination of the CeO2 affects the catalytic reaction rates and selectivity. Using CeO2 nanoparticles with controlled shapes including cubes, octahedra and rods that are terminated on (100), (111) and (110) surfaces respectively, we have investigated this structure dependence. Temperature programmed desorption, temperature programmed reaction, flow reactor rates, and in situ DRIFTS were used to probe adsorption states, desorption, reaction, oxidation rates and product selectivity for CO and ethanol oxidation. Results show pronounced differences between the three different morphologies. All morphologies show evidence of surface ethoxide species at room T, but during subsequent TPD, the DRIFTS exhibits variation in surface species between the different surfaces with evidence for formation of adsorbed acetaldehyde and acetate. Temperature induced changes in the C-H stretching regions, different also for each polymorph, suggest competing dehydrogenation and dehydration of surface species. Desorption temperatures and product distributions also vary. The ratio of H2/H2O, and the H2 peak desorption temperature is highest for the octahedra, consistent with its highest vacancy formation energy and therefore least available oxygen. This ratio is lowest for high surface area multi-faceted nanoparticles, and its variability has important implications for tailoring and understanding CeO2 catalysts or supports for production of H2 in ethanol fuel cells. Product profiles during TPR of ethanol in O2 were also dependent upon the surface structure. Octahedra show the highest selectivity to acetaldehyde and an onset of H2 evolution above 400 °C while the cubes and rods showed lower temperatures for the onset of H2, indicating that the hydrogen is evolved by two different pathways on different shaped ceria. Similarly, in a steady state flow reactor, the ratio of selective oxidation product (acetaldehyde) to the total oxidation product (CO2) followed the order (111) > (100) > (110). Such results provide a basis for fundamental understanding of how surface coordination, bonding, decomposition and reaction are affected by the atomic structure of an oxide surface, especially important for reducible oxides.