Understanding trends in the mercury oxidation activity of single-atom catalysts

Abstract

Mercury pollutants emitted from coal-fired power plants are recognized as a global environmental problem. Rapid and sustainable catalytic oxidation of elemental mercury (Hg⁰) to oxidized mercury (Hg²⁺) is an essential step to remove mercury from coal-fired power plants. Very recent proof-of-concept experiments firstly found that single-atom catalysts (SACs) have an outstanding performance for Hg⁰ oxidation. However, looking for an effective catalyst for this reaction is a remaining big challenge. In this work, ten 3d transition metal (TM) SACs with nitrogen-doped carbon substrates (TM₁–N₄–C, where TM is from Ti to Zn) were designed and analyzed as the catalysts to oxidize Hg⁰ using O₂ as the oxidant. The reaction kinetics and thermodynamics were analyzed based on spin-polarized density functional theory calculations with van der Waals corrections (DFT-D3). We found that Fe₁–N₄–C has the highest catalytic activity with the lowest energy barrier in the rate-determining step. By analyzing the relationships between reaction thermodynamics and kinetics, the adsorption energy of atomic O was found as an effective descriptor that can predict the rate-determining step barriers of 72 catalysts. The predicted values of the energy barriers were then successfully verified by subsequent DFT-D3 calculations. To further understand the trends in Hg⁰ oxidation, a volcano-shaped microkinetic model was derived based on the linear scaling relations of the reaction kinetics and thermodynamics, as a function of O adsorption energy. The catalytic activities of 28 transition metal SACs were then predicted by the volcano map, showing that Fe₁–N₄–C has the highest catalytic activity among the analyzed 3d, 4d, and 5d SACs. Furthermore, correlations between activity and electronic structure were discussed through the analyses with Bader charge, d-band center, and system electronegativity. Most importantly, this study provides an understanding of the activity trends of Hg⁰ oxidation and a descriptor-based design guideline for this environmentally important reaction.