Dynamic Response of Oxygen Vacancies in the Deacon Reaction over Reduced Single Crystalline CeO2- x(111) Surfaces

Abstract

The heterogeneously catalyzed HCl oxidation reaction (Deacon reaction) that produces chlorine and the byproduct water leads to a reduction and surface chlorination of the ceria (CeO2) catalyst under typical reaction conditions. The interaction of HCl with reduced ceria can be modeled with a reduced single crystalline CeO2-x(111) model surface that is able to stabilize various ordered surface structures, e.g., (√7 × √7)R19.1°, (3 × 3), or (4 × 4), depending on the concentration of oxygen vacancies (VO). Saturating these phases with HCl at room temperature, followed by annealing to the Deacon process temperature of 700 K, results in all cases in a uniformly covering (√3 × √3)R30°-Clvac overlayer structure with identical adsorption geometry and Cl coverage. Low energy electron diffraction (LEED) fingerprinting, density functional theory (DFT) calculations, and X-ray photoelectron spectroscopy (XPS) indicate that Cl adsorbs in the surface oxygen vacancies (Clvac) with a high adsorption energy (>2 eV). From thermal desorption spectroscopy (TDS) and XPS of Cl 2p, it is found that both the adsorption energy of Clvac and the water formation ability depend on the degree of reduction x of CeO2-x(111). TDS spectra show that chlorine desorption shifts from 1175 to 1320 K when the degree of reduction x is increased from CeO1.8(111) (x = 0.2) to CeO1.6(111) (x = 0.4). In order to rationalize why the formation of the (√3 × √3)R30°-Clvac structure on CeO2-x(111) is independent of the original degree of reduction x of CeO2-x(111), efficient diffusion of surface and bulk oxygen vacancies is required.