Kinetic aspects and deactivation behaviour of chromia-based catalysts in hydrogen chloride oxidation

Abstract

The gas-phase HCl oxidation was studied over bulk and supported Cr₂O₃-based catalysts by means of kinetic experiments in a fixed-bed reactor at ambient pressure and variable temperature, inlet O₂/HCl ratio, and contact time. Cr₂O₃ exhibited high activity for Cl₂ production. X-ray diffraction of the used catalysts detected no phase change. Temperature-programmed reduction with hydrogen and X-ray photoelectron spectroscopy showed that the surface of the fresh catalyst contains chromium species in oxidation state (III) as well as higher oxidation states (V and VI) while that of the used sample features the reduction of Cr⁵⁺/Cr⁶⁺ and comprises a little amount of chlorine. Coupling the catalytic and characterisation data over Cr₂O₃ with activity tests over CrO₃, a redox cycle is proposed in which chromium species shift between Cr³⁺ and Cr⁵⁺ + Cr⁶⁺. The positive dependence of the HCl conversion on the inlet O₂ concentration suggests that catalyst re-oxidation is rate limiting. SiO₂ was identified as a better carrier for Cr₂O₃ than Al₂O₃ and TiO₂-anatase, as it favours the formation of Cr₂O₃ nanoparticles rather than (unstable) isolated chromate species. However, all the supported catalysts suffered from severe deactivation due to substantial chromium loss. The deactivation mechanism is assigned to the in situ formation of the highly volatile CrO₂Cl₂ from Cr⁶⁺ species and CrO₂(OH)₂ from both Cr³⁺ and Cr⁶⁺ species. The deactivation rate can be reduced, though not suppressed, by applying a high O₂ excess in the feed mixture, thus indicating that the deactivation route via the oxychloride might be predominant. The features observed represent critical reasons justifying the restricted industrial implementation of chromium-based catalysts for HCl oxidation.