Stability of formates species on CeO2(111): Influence of the oxidizing or reducing environment

Abstract

Trabajo presentado en el VI Encuentro de Física y Química de Superficies, celebrado en Río Cuarto (Argentina) del 08 al 10 de septiembre de 2014.Ceria-based materials have a wide variety of applications including ceramics, gas sensors, solid state electrolytes, automotive exhaust catalysts, among others [1]. Particularly, ceria is used in the field heterogeneous catalysis as support or promoter to improve the activity, selectivity and/or stability of the catalysts. One of the crucial properties of a ceria-based material is its oxygen storage capacity by means of redox shift between Ce3+ and Ce4+ under oxidizing and reducing environments, which reduces the carbon formation and/or facilitate the oxidation of pollutants in three way catalysts (exhaust catalysts). Since then, other industrial reactions have been studied on such materials, for instance the water-gas-shift reaction (WGSR, CO + H2O → CO 2 + H2) and the steam reforming of methanol (SRM, CH3OH + H2O → CO2 + 3 H2). Numerous reaction mechanisms have been proposed to determine the limiting step of each reaction [2], where the possible reaction pathways involve formate species (HCOO −) formation as either spectator or intermediate [2]. The formates species can be coordinated differently on oxide surfaces, classifying the species according to the number of bondings with the Ce atom (Monodentate, Bidentate, Bridge). IR measurements performed under various experimental conditions have shown three types of formate species for the WGS and SRM reactions. From the theory, various efforts were made to assign the IR peaks to different formates species but in any case the experimental conditions consistently considered [3]. The aim of this work is to study the influence of oxidizing/reducing environment on the stability of various formate species. For this we include in our calculations H, OH, O2 and H2O co-adsorbed in the vicinity of formate, being calculated the Gibbs free energy from data obtained by the DFT + U and the results are summarized in phase diagrams depending on the hydrogen and oxygen chemical potentialsPeer Reviewe