Cancelled Event - IMX Seminar Series - The Remarkable Influence of Zr-doping on the Chemistry and Reactivity of Ceria

Zirconia substituted ceria is a widely studied material system with immense importance in heterogeneous catalysis. Enhancements to a range of properties in the oxide have been attributed to the presence of zirconium, in particular, oxygen storage capacity, oxygen mobility, and surface reaction rate. All of these features have furthermore been connected with the observation of an increase in the concentration of reduced Ce³⁺ species in the presence of Zr⁴⁺. Using a suite of characterization tools we establish the precise role that zirconium plays in controlling these properties. Our bulk thermogravimetric measurements reveal that in the presence of Zr the oxygen non-stoichiometry (d) of Ce_1-xZr_xO_2-d is indeed enhanced. Using angle-resolved X-ray Absorption Near Edge Spectroscopy (XANES), we quantify under technologically relevant conditions the Ce³⁺ concentration in the surface (2-3 nm), as well as bulk regions, of ceria-zirconia films. We find that the surface of each of the oxides far is more reduced than the bulk. However, the extent to which the surface and bulk regions differ depends strongly on the Zr concentration. Specifically, with increasing Zr, the differential between the two regions diminishes. In parallel, using electrical conductivity relaxation methods, we find that the bulk chemical diffusion of oxygen decreases in the presence of Zr. These observations can be generally explained in terms of the preferred lower coordination of the small Zr⁴⁺ ion relative to the larger Ce⁴⁺ and Ce³⁺ ions and likely trapping of oxygen vacancies in the vicinity of Zr. The substantial difference between surface and bulk properties, where even the trends are reversed, urges caution in the use of bulk-based properties as surrogate descriptors for surface characteristics.
Bio: Sossina M. Haile is the Walter P. Murphy Professor of Materials Science and Engineering at Northwestern University, a position she assumed in 2015 after serving 18 years on the faculty at the California Institute of Technology. She earned her Ph.D. in Materials Science and Engineering from the Massachusetts Institute of Technology in 1992. Haile’s research broadly encompasses materials, especially oxides, for sustainable electrochemical energy technologies. She has established a new class of fuel cells with record performance for clean and efficient electricity generation, and created new thermochemical approaches for harnessing sunlight to meet rising energy demands. Amongst her many awards, in 2008 Haile received an American Competitiveness and Innovation (ACI) Fellowship from the U.S. National Science Foundation in recognition of “her timely and transformative research in the energy field and her dedication to inclusive mentoring, education and outreach across many levels.” She is a fellow of the Materials Research Society, the American Ceramics Society, the African Academy of Sciences, and the Ethiopian Academy of Sciences.