CeO 2 is a promising catalyst for the HCl oxidation (Deacon process) in order to recover Cl 2 . Employing shape-controlled CeO 2 nanoparticles (cubes, octahedrons, rods) with facets of preferential orientations ((100), ( 111), ( 110)), we studied the activity and stability under two reaction conditions (harsh: Ar:HCl:O 2 = 6:2:2 and mild: Ar:HCl:O 2 = 7:1:2). It turns out that both activity and stability are structuresensitive. In terms of space time yield (STY), the rods are the most active particles, followed by the cubes and finally the octahedrons. This very same trend is reconciled with the complete oxygen storage capacity (OSCc), indicating a correlation between the observed activity STY and the OSCc. The apparent activation energies are about 50 kJ/mol for cubes and rods, while the octahedrons reveal an apparent activation energy of 65 kJ/mol. The reaction order in O 2 is positive (0.26−0.32). Under mild reaction conditions, all three morphologies are stable, consistent with corresponding studies of CeO 2 powders and CeO 2 nanofibers. Under harsh reaction conditions, however, cubes and octahedrons are both instable, forming hydrated CeCl 3 , while rods are still stable. The present stability and activity experiments in the catalytic HCl oxidation reaction over shape-controlled CeO 2 nanoparticles may serve as benchmarks for future ab initio studies of the catalyzed HCl oxidation reaction over well-defined CeO 2 surfaces.

Transient spectroscopic surface-chemistry experiments in combination with spatially resolved kinetic Monte Carlo (KMC) simulations offer great potential to gain a wealth of molecular information on the kinetics of catalytic surface reactions as exemplified by the CO oxidation reaction over RuO2(110). This approach surpasses the common problem that in the steady-state reactions, the prevailing species detectable by in operando surface-sensitive spectroscopy are frequently spectator species, thereby obscuring the reactive surface species. Our experiment is sensitive to the relative activity of different oxygen species by saturating the surface with loosely bound oxygen, leaving only single vacancies where CO can adsorb and recombine with oxygen. With in situ reflection–absorption infrared spectroscopy (RAIRS) in combination with ab initio based KMC simulations, we follow the time evolution toward steady state (transient experiment). In this way, we are able to resolve a long-standing controversy about the active oxygen species in the CO oxidation over RuO2(110), evidencing that both surface O species (Obr and Oot) are equally active, although their adsorption energies differ by more than 150 kJ/mol.

Ultrathin crystalline CeO 2 (111) films were grown on Ru(0001) in order to study the interaction of HCl with this surface as a first step of the Deacon reaction with experimental techniques, including low energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS) in combination with density functional theory calculations (DFT+U). HCl molecules adsorb dissociatively on CeO 2 (111), forming a well-ordered °R ( 3 x 3 ) 30 overlayer structure with one Cl and one H per surface unit cell. DFT calculations indicate that HCl adsorption is exothermic by 1.15 eV and proceeds via an acid−base reaction. The mixed overlayer structure is stabilized by Lewis acid−base pairing (∼0.4 eV). Stoichiometric CeO 2 (111) films are likely to be not very active in the Deacon process since at 800 K the recombination of adsorbed H* and Cl* to form HCl is far more preferred over Cl* + Cl* recombination to form the desired product Cl 2 .

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 (CeO 2 ) catalyst under typical reaction conditions. The interaction of HCl with reduced ceria can be modeled with a reduced single crystalline CeO 2−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 (V O ). 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°-Cl vac 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 (Cl vac ) 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 Cl vac and the water formation ability depend on the degree of reduction x of CeO 2−x (111). TDS spectra show that chlorine desorption shifts from 1175 to 1320 K when the degree of reduction x is increased from CeO 1.8 (111) (x = 0.2) to CeO 1.6 (111) (x = 0.4). In order to rationalize why the formation of the (√3 × √3)R30°-Cl vac structure on CeO 2−x (111) is independent of the original degree of reduction x of CeO 2−x (111), efficient diffusion of surface and bulk oxygen vacancies is required.

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