Adsorption And Oxidation Of Co On The Highly Active Catalyst Cu-ceo₂ (111): First Principles Study

In terms of its low-cost and high activity, CuO-CeO₂ mixed-metal oxide catalysts have important applications as electrolytes in fuel cells and efficient catalysts for various reactions such as the water-gas shift reaction, selective CO oxidation at low temperature and de-SOx. Although some progress has been made in the field, a systematic first-principle study of the Cu-CeO₂ (111) systems and the mechanisms of how the supported copper species participates in oxidation reactions are still lacking. We use quantum-mechanical calculations to explore, among other things, the charge state of Cu and the structure around it for two limiting model systems, Cu/CeO₂(111) and CuxCe1-xO2(111), i. e. for isolated Cu atoms adsorbed on CeO₂(111) and for Cu incorporated into the CeO₂ lattice, replacing one Ce4+ ion close to the (111) surface.With the use of the DFT+U method, the properties of Cu adsorbed on the stoichiometric CeO₂(111) surface, Cu doped CeO₂(111) (denoted as Cu_0.08Ce_0.92O₂) surface,and CO oxidation on the stoichiometric Cu_0.08Ce_0.92O₂(111) surface are studied systematically. It is found that (i) Cu is stable both as an adsorbed atom on the surface and as dopant in the surface region. Cu adsorbed at the surface is in the state of Cu(+I), while Cu as a dopant atom is a Cu(+II). (ii) The Cu dopant facilitates O vacancy formation considerably, while Cu adsorption on the stoichiometric CeO₂ (111) surface may suppress oxygen vacancy formation. (iii) Physisorbed CO, physisorbed CO2, as well as chemisorbed CO (carbonate) species are observed on the Cu-doped CeO₂(111) surface, in contrast, on the clean ceria(111) surface, only physisorbed CO was previously observed. These results may lead to a better understanding for the CuxCe1-xO2 catalysts and give clues for solving the problem of coking when Cu/CeO₂ materials used as electrolytes in fuel cells.