Direct CO Oxidation by Lattice Oxygen on Zr-Doped Ceria Surfaces
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Direct CO Oxidation by Lattice Oxygen on Zr-Doped Ceria Surfaces Zongxian Yang • Zhaoming Fu • Yanning Zhang Ruqian Wu
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Received: 2 March 2010 / Accepted: 31 August 2010 / Published online: 21 September 2010 Ó The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract Systematic density-functional calculations have been performed to address an important issue for CO oxidation on redox ceria: the role of lattice oxygen. One major findings is that CO easily grasps one lattice oxygen atom to form CO2- and CO2 on CeO2 (111) and Ce0.75Zr0.25O2 (111) with small activation energies. Zr dopants facilitate the reduction of Ce?4 to Ce?3 and hence weaken the Ce–O bonds, which benefit the direct formation and release of CO2. Keywords Density-functional theory (DFT) CO oxidation Zr-doped ceria Vienna ab-initio simulation package (VASP)
1 Introduction Rational design of robust nanocatalysts for CO oxidation and NOx reduction is crucial for many important applications such as pollution control and energy production. Owing to its particular redox properties, ceria has been extensively utilized in catalytic converters [1, 2]. Z. Yang (&) College of Physics and Information Engineering, Henan Normal University, Xinxiang, Henan, 453007, People’s Republic of China e-mail: [email protected] Z. Fu Department of Physics, Fudan University, Shanghai 200433, People’s Republic of China Y. Zhang R. Wu (&) Department of Physics and Astronomy, University of California, Irvine, CA 92697-4575, USA e-mail: [email protected]
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Significantly, its redox performance can be further enhanced by the presence of various dopants, such as Zr4?, Ca2? and some noble metal [3–6]. This offers a great opportunity to control the activity and selectivity of oxide catalysts with the aid of innovative nanofabrication techniques. One of the most important tasks for fundamental research is to attain a clear understanding of mechanisms: (1) how lattice oxygen atoms contribute toward chemical reactions and, (2) how dopants modify surface activity. It is widely believed that CO directly combines with lattice oxygen atom on ceria, through the Mars–van Krevelen mechanism [7]. However, the relevant reaction dynamics has not been satisfactorily unravelled. Shapovalov and Metiu [8] recently investigated the effects of noble metal dopants (Au, Ag and Cu) on the energetics of CO oxidation on CeO2 (111). Their density functional calculations indicate that bonds between lattice oxygen and metal atoms in oxide are significantly weakened by the presence of dopants and hence the doped CeO2 (111) surfaces become much more active in both releasing CO2 and healing vacancies. Chen et al. [9] also found that Au adsorption on Ce-vacancies activates O atoms nearby. In particular, releasing and restocking oxygen through the redox processes on ceria involve the Ce4?/Ce3? transformation and therefore it is crucial to study the effect of irreducible dopants (e.g., Zr4? and Ca2? dopants) on the charge state of Ce. The present paper reports results of sy
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