Insights into the mechanism of heterogeneous mercury oxidation by HCl over V2O5/TiO2 catalyst: Periodic density functional theory study

The selective catalytic reduction (SCR) for the reduction of NOx can enhance the oxidation of elemental mercury, which is regarded as a low-cost option for mercury control in coal-fired power plants. First-principles calculations based on the density functional theory and the periodic slab models were used to gain a fundamental understanding of mercury oxidation mechanism across V2O5/TiO2 SCR catalyst. The adsorption of Hg0, HCl, HgCl, and HgCl2 on V2O5/TiO2(0 0 1) surface were studied. The energy profile of the oxidation reaction and the structures of related transition states and intermediates were examined. The results show that Hg0 is mainly physically absorbed on vanadyl-oxygen sites of the V2O5/TiO2(0 0 1) surface with an adsorption energy of −27.93 kJ/mol. HCl is chemisorbed on vanadyl-oxygen sites of V2O5/TiO2(0 0 1) surface, and can undergoes dissociation process with an energy barrier of 101.53 kJ/mol to form the vanadium oxy-chloride complex which is essential in Hg0 oxidation reaction on V2O5/TiO2(0 0 1) surface. The mercury oxidation reaction occurs through an Eley-Rideal mechanism in which Hg reacts with HCl that has previously been adsorbed and dissociated on V2O5/TiO2(0 0 1) surface to form surface HgCl, and then surface HgCl reacts with HCl to form HgCl2, finally HgCl2 desorbs from the V2O5/TiO2(0 0 1) surface. In the whole Hg oxidation reaction, the formation of HgCl2 is the rate-determining step based on its high energy barrier.