Paper SS2-WeM9
Surface Structure Dependence by Monofaceted CeO₂ Nanoparticles: Catalytic Oxidation Reactions

Wednesday, November 2, 2011, 10:40 am, Room 109

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.

Research sponsored by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, US DOE.