Transient mechanistic study of the gas-phase HCl oxidation to Cl _2 on bulk and supported RuO_2 catalysts

摘要

The oxidation of HCl to Cl_2 (Deacon process) was studied over RuO_2-based catalysts in bulk and supported forms. RuO_2 (7 wt.%) was incorporated onto different TiO_2 carriers (rutile, anatase, and P25) by deposition-precipitation of Ru(OH)_2 followed by calcination. RuO_2/TiO_2-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 RuO_2 and the influence of the TiO_2-rutile carrier were gained by studies in the Temporal Analysis of Products (TAP) reactor at 623 K. HCl multipulsing in the absence of O_2 leads to chlorination of the catalysts with no detectable gas-phase Cl_2 production. The chlorine uptake of RuO _2 amounts to 0.16 mmol Cl gcat-1, which enables to estimate that ca. 75% of the total surface Ru atoms (coordinatively unsaturated and bridge sites) are chlorinated, forming a RuO_(2-x)Cl_x oxychloride. RuO_2 is also chlorinated by Cl_2, but to a lesser extent compared to HCl. The chlorine uptake of fresh RuO _2/TiO_2-rutile per gram of sample was four times higher than that of fresh RuO_2, 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 O_2 and HCl over the catalysts revealed that both gas-phase Cl_2 evolution and H_2O desorption are impeded processes. Pump-probe experiments of O_2 and HCl evidenced the tight dependence of the net Cl_2 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 Cl _2 at very low coverages. The equilibration procedure (ambient or TAP conditions) influences the reaction mechanism and the net Cl_2 production, giving account of the reaction complexity due to the highly dynamic state of the catalyst surface. The Deacon process on RuO_(2-)_xCl_x-based catalysts can be mostly described by a Langmuir-Hinshelwood scheme with a minor 2D Mars-van Krevelen contribution.