Structure-Directing Role of Support on Hg0 Oxidation over V2O5/TiO2 Catalyst Revealed for NOx and Hg0 Simultaneous Control in an SCR Reactor

The crystal structure of TiO₂ strongly influences the physiochemical properties of supported active sites and thus the catalytic performance of the as-synthesized catalyst. Herein, we synthesized TiO₂ with different crystal forms (R = rutile, A = anatase, and B = brookite), which were used as supports to prepare vanadium-based catalysts for Hg⁰ oxidation. The Hg⁰ oxidation efficiency over V₂O₅/TiO₂-B was the best, followed by V₂O₅/TiO₂-A and V₂O₅/TiO₂-R. Further experimental and theoretical results indicate that gaseous Hg⁰ reacts with surface-active chlorine species produced by the adsorbed HCl and the reaction orders of Hg⁰ oxidation over V₂O₅/TiO₂ catalyst with respect to HCl and Hg⁰ concentration were approximately 0 and 1, respectively. The excellent Hg⁰ oxidation efficiency over V₂O₅/TiO₂-B can be attributed to lower redox temperature, larger HCl adsorption capacity, and more oxygen vacancies. This work suggests that to achieve the best simultaneous removal of NO_x and Hg⁰ on state-of-the-art V₂O₅/TiO₂ catalyst, a combination of anatase and brookite TiO₂-supported vanadyl tandem catalysts is supposed to be employed in the SCR reactor, and the brookite-type catalyst should be on the downstream of the anatase-based catalyst due to the inhibition of NH₃ on Hg⁰ oxidation.