A complete catalytic reaction scheme for Hg⁰ oxidation by HCl over RuO₂/TiO₂ catalyst

RuO₂-based catalysts have attracted great attention in mercury emission control region due to their outstanding catalytic activity and long-term stability. Quantum chemistry calculation was performed to uncover the atomic-scale reaction mechanism of Hg⁰ oxidation by HCl over RuO₂/TiO₂ catalyst. The results indicate that Hg⁰ adsorption on RuO₂/TiO₂(110) surface is controlled by a weak chemisorption mechanism. The 5-fold coordinated surface Ru atom is identified as the active center for Hg⁰ adsorption. HgCl molecule serves as an intermediate connecting reactant state to product state. The weak interaction between HgCl₂ and catalyst surface is favorable for product desorption. HCl activation is an O-assisted surface reaction process in which HCl is oxidized into active Cl atom for Hg⁰ oxidation. The heterolytic cleavage of HCl molecule occurs without noticeable activation energy barrier. Hg⁰ oxidation by HCl over RuO₂/TiO₂ catalyst proceeds through two independent reaction channels. The dominant reaction channel of Hg⁰ oxidation is identified as a four-step process. Finally, a complete catalytic cycle that can produce the correct stoichiometry was proposed to understand the heterogeneous reaction mechanism of Hg⁰ oxidation over RuO₂/TiO₂ catalyst. The catalytic cycle consists of HCl activation, mercury oxidation and surface reoxidation. Mercury oxidation is the rate-determining step of the catalytic cycle.