Sossina M. Haile, Northwestern University
Untangling the Surface Chemistry and Reactivity of Ceria
It is well-established that the surface of ceria (doped or undoped) is substantially more reduced than the bulk, a feature that has been identified as critical to the catalytic activity of these materials. In parallel, strong evidence has emerged that the activity of different facets of ceria can differ substantially, which has in turn, led to an explosion in efforts to control catalyst particle morphology and thus exploit the facet-dependent activity. Surprisingly, however, direct measurements of the extent of reduction at the ceria surface are lacking. We address this gap using angle-resolved X-ray Absorption Near Edge Spectroscopy (XANES) to quantify, under technologically relevant conditions, the Ce3+ concentation in the surface (2-3 nm) and bulk regions of ceria-zirconia single-crystal films. In all circumstances, we observe substantial Ce3+ enrichment at the surface. For example, in CeO2-d, at 1100 °C and » 3 ´ 10-6 atm oxygen partial pressure, at which bulk reduction is negligible, the surface Ce3+ concentration is an astonishing 62%. Even more remarkable are the trends with facet and Zr concentration. In stark contrast to computational investigations which suggest, on the basis of oxygen vacancy formation energies, that vacancy concentrations increase in the order (111) < (001) < (110), we find a negligible dependence on facet. Furthermore, in contrast to bulk behavior, which shows an Ce reduction with increasing Zr concentration, the fraction of surface Ce ions that are reduced is unchanged with cation composition. In the absence of other charge compensating defects, this implies a decrease in the surface oxygen vacancy concentration as the Zr content increases. In turn, this trend suggests that high Zr concentration, which is known to enhance thermal stability and bulk oxygen storage capacity, may be detrimental in terms of area-specific catalytic reaction rates. Preliminary kinetic data, obtained using conductivity relaxation methods, support this conclusion.