Chemistry - Oxygen Vacancies and Catalysis on Ceria Surfaces
Chemistry occurring at the surface of metal oxides is critical in a variety of industrial applications including catalysis and photocatalysis, optical display technology, solar energy devices and corrosion prevention. Defects have long been recognized to be the most reactive sites on the surfaces of many oxide materials. The most common types of defects present on the surfaces of metal oxides are oxygen vacancies and step edges. The nature of surface oxygen vacancies, and their number, distribution and diffusion across the surface of oxides, are thus issues of major scientific importance. One of the most interesting oxides in this respect is CeO2, since oxygen vacancies play the key role in giving this material it's industrially important ''oxygen-storage capacity''. This capacity makes modern automotive exhaust treatment catalysts containing CeO2 much more effective than their predecessors without CeO2. Ceria is also well known as a support which enhances the performance of transition metal catalysts, relative to other oxide supports, in a variety of other reactions including water-gas shift, steam reforming of oxygenates and PROX 1-7, all of which hold promise for enabling a hydrogen economy 1. Related to ceria's facile redox capacity (ability to rapidly form and eliminate oxygen vacancy defects) is the poorly understood observation that some less reducible oxides, such as zirconia (ZrO2), are used as additives that actually enhance this ''oxygen storage'' property of CeO2. In this issue, Esch and coworkers in Trieste, Italy report an exciting study that for the first time clearly elucidates the structure, distribution and formation of oxygen vacancies on a cerium oxide surface 8. They have elegantly combined beautiful, atomic-resolution imaging using scanning-tunneling microscopy (STM) on a ceria surface with state-of-the-art quantum mechanical calculations using Density Functional Theory (DFT) to raise our understanding of CeO2 surfaces to a much higher level. They show that surface oxygen vacancies on CeO2(111) are immobile at room temperature, but at higher temperatures linear clusters of these vacancies are formed which expose exclusively Ce3+ ions to gas-phase reactants. The resulting exposed Ce3+ ions are thus grouped into rather large ensembles, with the sites immediately adjacent to these vacancy clusters remaining as pure Ce4+ ions. The authors further show that one subsurface oxygen vacancy is required to nucleate each such vacancy cluster. Guided by this knowledge, they performed DFT calculations that suggest an exciting new explanation for the role of Zr promoters in ceria-based catalysts: to enable growth of these linear vacancy chains without the need for a subsurface vacancy, which is energetically more costly. It should be noted that Namai et al. 9,10 also recently reported such linear vacancy clusters on CeO2(111), for which Esch et al. now provide much needed atomic-level structural detail.
- Research Organization:
- Pacific Northwest National Laboratory (PNNL), Richland, WA (US), Environmental Molecular Sciences Laboratory (EMSL)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- AC05-76RL01830
- OSTI ID:
- 15017428
- Report Number(s):
- PNNL-SA-45523; 8215; KC0302010; TRN: US200517%%455
- Journal Information:
- Science, Vol. 309, Issue 5735
- Country of Publication:
- United States
- Language:
- English
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