Role of WO3 in the Hg Oxidation across the V2O5–WO3–TiO2 SCR Catalyst: A DFT Study

Selective catalytic reduction (SCR) units can be exploited to reduce Hg emissions from coal-based power plants; hereby, Hg0 is oxidized into Hg2+, which has a higher solubility than the former and can therefore be scrubbed before leaving the stacks. With the purpose of examining the effect of the surface composition and surface coverage on the reactivity of a commercial SCR catalyst (V2O5–WO3–TiO2) toward Hg oxidation, two models were used to represent TiO2-supported systems with low and high loading of the two active phases (i.e., V2O5 and WO3). The reactivities of these systems were compared through the analysis of the adsorption energies of Hg, Cl•, HgCl, and HCl, which are likely involved in the Hg oxidation mechanism. These adsorption energies were complemented by results from both the Bader charge and projected density of states (PDOS) analyses, thus providing an increased understanding of the effects of surface coverage and composition on the electronic structure of these materials. An enhanced reactivity of the SCR catalyst is observed with increasing loadings of both V2O5 and WO3 phases. Binary systems (i.e., V2O5–TiO2 or WO3–TiO2) were compared against ternary systems (V2O5–WO3–TiO2 with different V2O5/WO3 ratios) and indicate a higher reactivity of the latter.