Transient mechanistic study of the gas-phase HCl oxidation to Cl2 on bulk and supported RuO2 catalysts

The oxidation of HCl to Cl₂ (Deacon process) was studied over RuO₂-based catalysts in bulk and supported forms. RuO₂ (7 wt.%) was incorporated onto different TiO₂ carriers (rutile, anatase, and P25) by deposition-precipitation of Ru(OH)₂ followed by calcination. RuO₂/TiO₂-rutile exhibits the highest activity at ambient pressure in a continuous flow fixed-bed reactor. Insights into the mechanism of this experimentally demanding reaction on RuO₂ and the influence of the TiO₂-rutile carrier were gained by studies in the Temporal Analysis of Products (TAP) reactor at 623 K. HCl multipulsing in the absence of O₂ leads to chlorination of the catalysts with no detectable gas-phase Cl₂ production. The chlorine uptake of RuO₂ amounts to 0.16 mmol Cl , which enables to estimate that ca. 75% of the total surface Ru atoms (coordinatively unsaturated and bridge sites) are chlorinated, forming a RuO₂–_xCl_x oxychloride. RuO₂ is also chlorinated by Cl₂, but to a lesser extent compared to HCl. The chlorine uptake of fresh RuO₂/TiO₂-rutile per gram of sample was four times higher than that of fresh RuO₂, namely due to chlorination of the high surface area carrier. Upon equilibration at ambient pressure under Deacon conditions, the TAP-derived chlorine uptake in the supported catalyst experienced a 6-fold decrease with respect to the fresh sample due to permanent chlorination. Pulsing of a mixture of O₂ and HCl over the catalysts revealed that both gas-phase Cl₂ evolution and H₂O desorption are impeded processes. Pump-probe experiments of O₂ and HCl evidenced the tight dependence of the net Cl₂ production on both the oxygen and chlorine coverage. In addition, a limited contribution of lattice surface species could be detected by the production of small amounts of Cl₂ at very low coverages. The equilibration procedure (ambient or TAP conditions) influences the reaction mechanism and the net Cl₂ production, giving account of the reaction complexity due to the highly dynamic state of the catalyst surface. The Deacon process on RuO₂–_xCl_x-based catalysts can be mostly described by a Langmuir-Hinshelwood scheme with a minor 2D Mars-van Krevelen contribution.