The use of in situ approaches to study mechanisms of experimentally demanding and complex reactions under conditions of practical relevance opens doorways to discover new or improved heterogeneous catalysts. A representative example within this category of reactions is the gas-phase oxidation of HCl to Cl2 (Deacon process). Studies in the temporal analysis of products (TAP) reactor complemented by flow experiments at ambient pressure evidenced pronounced mechanistic differences between copper catalysts (CuO, CuCl, and CuCl2, that is, Deacon-like catalysts) and RuO2 (the basis of the recent Sumitomo’s catalyst for large-scale Cl2 production). RuO2 is 1 order of magnitude more active than the copper-based materials. HCl oxidation on RuO2 obeys a Langmuir-Hinshelwood mechanism in the presence of adsorbed species over a partially chlorinated surface. HCl oxidation on the copper catalysts is more complex: all the samples experience bulk changes, namely, chlorination, resulting in multiphase materials. Besides, lattice species also participate in the reaction. In particular, fresh CuO follows a Mars-van Krevelen mechanism, in which lattice oxygen is active for Cl2 and H2O production, leading to bulk chlorination, whereas for copper chlorides activated upon exposure to the Deacon mixture and used CuO, a combination of Mars-van Krevelen and Langmuir-Hinshelwood mechanisms seems to be a better description. Investigations over the copper-based phases and characterization of the fresh and used samples by X-ray diffraction indicate that a copper (hydr)oxychloride is the main active phase. Our mechanistic study suggests that a more active and particularly stable copper catalyst can be achieved by controlling the degree of surface chlorination. This is more or less a self-regulated process on the ruthenium-based catalyst.
