Oxidation mechanism of elemental mercury by HCl over MnO2 catalyst: Insights from first principles

Catalytic oxidation of elemental mercury from flue gas is a promising way in the field of mercury pollution control. Among catalysts for Hg oxidation, MnO2-based materials showed high catalytic activity. The oxidation mechanism of Hg on the MnO2(1 1 0) surface was studied by periodic density functional theory calculations. The thermodynamic stability analysis suggests that the stoichiometric MnO2(1 1 0) is the most stable surface. Hydroxylation and chlorination MnO2 surfaces can exist under the SCR conditions. The reaction energy profiles of two possible routes (Hg -> HgCl -> HgCl2 and Hg -> HgCl2) and the corresponding configurations were examined. The results show that HCl can undergo dissociative chemisorption to form a surface hydroxyl and manganese-chlorine complex. Hg oxidation reaction occurs through Langmuir-Hinshelwood mechanism in which adsorbed Hg reacts with adsorbed Cl from HCl dissociation. The HgCl2 direct formation on MnO2 (Hg -> HgCl2) is hindered by high energy barrier of 98.25 or 101.97 kJ/mol. Hg can react with adsorbed Cl with a substantially lower barrier (40.12 or 43.59 kJ/mol) to form HgCl, and HgCl can react with adsorbed Cl with a low barrier (57.72 or 66.27 kJ/mol) to form HgCl2. The calculated results suggest Hg oxidation by HCl over MnO2 surface prefers the Hg -> HgCl -> HgCl2 pathway rather than a pathway Hg -> HgCl2. The HgCl -> HgCl2 process is the rate-determining step for the overall oxidation reaction due to its high energy barrier. (C) 2015 Elsevier B.V. All rights reserved.