Critical Step in the HCl Oxidation Reaction over Single-Crystalline CeO₂₋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 omnipresent HCl waste produced in various industrial processes. Previous density functional theory (DFT) model-calculations1 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) towards 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 re-oxidation of a chlorinated single crystalline Cl(vac)-CeO₂₋ₓ(111)-(√3 × √3)R30° surface structure, we provide unique spectroscopic data (high resolution core level spectroscopy (HRCLS) and X-ray adsorption near edge structure (XANES)) for this oxygen-induced de-chlorination process. Combined with theoretical evidence from DFT calculations, the Cl(vac) → Cl(top) displacement reaction is predicted to be induced by a surface-adsorbed peroxo species (O₂²⁻), 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 re-oxidation of Clvac-CeO₂₋ₓ(111) and is considered essential for understanding the function of ceria in oxidation catalysis.