Atomic-Scale Understanding of the HCl Oxidation Over RuO2, A Novel Deacon Process

Heterogeneous catalysis of the HCl oxidation by oxygen (Deacon process) over Ruo(2) is a green chemistry route to recover high purity Cl-2 from HCl waste in an almost energy neutral way on a large industrial scale. The outstanding properties of RuO2-based catalysts are long-term stability under such harsh reaction conditions and high catalytic activity, allowing for lower reaction-temperatures and hence for higher Cl-2 conversions at equilibrium. In this Feature Article, I will be reviewing the atomic-scale insights into this Deacon process gained on single-crystalline RuO2(110) model catalysts. The extraordinary stability of RuO2(110) is traced to the selective and self-limited replacement of bridging surface oxygen by chlorine, thereby transforming active surface sites with basic Bronsted character into inactive sites and thereby suppressing the bulk chlorination of Ruo(2). The reaction mechanism has been clarified by utilizing experimental surface science techniques together with density functional theory (DFT) calculations. Oxygen adsorption proceeds dissociatively across two neighboring undercoordinated Ru sites, thereby forming two undercoordinated surface on-top O (O-ot) atoms (homolytic cleavage). These O-ot species are able to accept H from dissociative adsorption of HCl to form on-top Cl (Cl-ot) and on-top hydroxyl groups (OotH). The heterolytic cleavage of HCl requires both acidic (undercoordinated Ru) and basic surface centers (undercoordinated O). Another H-transfer to the hydroxyl groups produces the byproduct water which desorbs at 420 K. The recombination of adjacent adsorbed Cl-ot produces finally the desired product Cl-2, an elementary reaction step with the highest activation barrier of 228 kJ/mol. Yet, oxygen adsorption constitutes the rate-determining step in the Deacon process over RuO2(110) under typical reaction conditions since strongly adsorbed Cl-ot blocks dissociative oxygen adsorption. The overall reaction mechanism is governed by a delicate interplay of surface kinetics and thermodynamics, i.e., the adsorption energies of reactants, reaction intermediates and products.