Critical Step in the HCl Oxidation Reaction over Single-Crystalline CeO_2-x(111): Peroxo-Induced Site Change of Strongly Adsorbed Surface Chlorine

The catalytic oxidation of HCl by molecular oxygen (Deacon process) over ceria allows the recovery of molecular chlorine from the omnipresent HCl waste produced in various industrial processes. In previous density functional theory (DFT) model-calculations by Amrute et al. [ J. Catal. 2012, 286, 287−297.], it was proposed that the most critical reaction step in this process is the displacement of tightly bound chlorine at a vacant oxygen position on the CeO₂(111) surface (Cl_vac) toward a less strongly bound cerium on-top (Cl_top) position. This step is highly endothermic by more than 2 eV. On the basis of a dedicated model study, namely the reoxidation of a chlorinated single-crystalline Cl_vac-CeO_2-x(111)-(Formula Presented × Formula Presented)R30° surface structure, we provide in situ synchrotron-based spectroscopic data (high resolution core level spectroscopy (HRCLS) and X-ray adsorption near edge structure (XANES)) for this oxygen-induced dechlorination process. Combined with theoretical evidence from DFT calculations, the Cl_vac → Cl_top displacement reaction is predicted to be induced by an adsorbed peroxo species (O₂^2-), making the displacement step concerted and exothermic by 0.6 eV with an activation barrier of only 1.04 eV. The peroxo species is shown to be important for the reoxidation of Cl_vac-CeO_2-x(111) and is considered essential for understanding the function of ceria in oxidation catalysis.