Unique Properties of Ceria Nanoparticles Supported on Metals: Novel Inverse Ceria/Copper Catalysts for CO Oxidation and the Water-Gas Shift Reaction

This manuscript describes the properties of inverse catalysts composed of CeOx nanoparticles supported on Cu(111) or CuOx/Cu(111) as determined through the methods described above. Ceria is an important material for redox chemistry because of its interchangeable oxidation states (Ce4+ and Ce3+). Cu(111), meanwhile, is a standard catalyst for reactions such as CO oxidation and the water-gas shift (WGS). This metal serves as an ideal replacement for other noble metals that are neither abundant nor cost effective. To prepare the inverse system we deposited nanoparticles (220 nm) of cerium oxide onto the Cu(111) surface. During this process, the Cu(111) surface grows an oxide layer that is characteristic of Cu2O (Cu1+). This oxide can influence the growth of ceria nanoparticles. Evidence suggests triangular-shaped CeO2(111) grows on Cu2O(111) surfaces while rectangular CeO2(100) grows on Cu4O3(111) surfaces. We used the CeOx/Cu2O/Cu(111) inverse system to study two catalytic processes: the WGS (CO + H2O → CO2 + H2) and CO oxidation (2CO + O2 → 2CO2). We discovered that the addition of small amounts of ceria nanoparticles can activate the Cu(111) surface and achieve remarkable enhancement of catalytic activity in the investigated reactions. In the case of the WGS, the CeOx nanoparticle facilitated this process by acting at the interface with Cu to dissociate water. In the CO oxidation case, an enhancement in the dissociation of O2 by the nanoparticles was a key factor. The strong interaction between CeOx nanoparticles and Cu(111) when preoxidized and reduced in CO resulted in a massive surface reconstruction of the copper substrate with the introduction of microterraces that covered 2535% of the surface. This constitutes a new mechanism for surface reconstruction not observed before. These microterraces helped to facilitate a further enhancement of activity towards the WGS by opening an additional channel for the dissociation of water. In summary, inverse catalysts of CeOx/Cu(111) and CeO2/Cu2O/Cu(111) demonstrate the versatility of a model system to obtain insightful knowledge of catalytic processes. These systems will continue to offer a unique opportunity to probe key catalytic components and elucidate the relationship between structure and reactivity of novel materials and reactions in the future.

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Ref: Senanayake, S.D., Stacchiola, D., and Rodriguez, J.A. Accounts of Chemical Research, 2013. 46(8): p. 1702-1711.  DOI: 10.1021/ar300231p

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