New frontiers in heterogeneous Hg0 oxidation by O2 over catalytic single-Fe site on boron-vacancy of h-BN: A density functional theory study

Abstract

The single-atom catalysts (SACs) have shown tremendous potential in heterogeneous catalytic process, because of their high atomic utilization efficiency and controllable characteristic of various active sites. The accurate structure design at atomic scale of SACs brings new opportunities and challenges for the cost-effective Hg0 oxidation and the study in molecularly catalytic mechanism by the oxidant of O2. Herein, a series of single transition metal atom doped (Mn, Fe, Co, Ni, Cu and Zn) on h-BN vacancy (M/V-BN) were well designed and the interaction behaviours were further investigated via Density Functional Theory (DFT) method. The mercury oxidation profiles and mechanism in the existence of O2 had been clarified on the optimized single atom catalytic site. The results show that the six kinds of TM atoms can exist stably on the surface of boron-vacancy (VB-BN) monolayer and have the potential to activate the new two-dimensional material (h-BN) with high surface reactivity. Among them, the most stable configuration Fe/VB-BN exhibited the best affinity to Hg0 with an adsorption energy of -1.697 eV, and the Fe-Hg bond was the shortest of 2.747 Å. The covalent interaction of Fe-Hg bond is demonstrated by the overlaps between Fe d-orbitals and Hg p-orbital. The chemisorption mechanism dominates the adsorption processes of different mercury species (Hg0, HgO, HgCl, HgBr and HgS), with the adsorption energy ranging from -1.2 to -4.2 eV. The Hg0 oxidation over Fe/VB-BN by O2 mainly follows the Eley-Rideal mechanism, and undergoes three key steps, i.e., O2 molecule dissociation, formation of O-Hg-O structure and desorption of oxidized Hg products. O2 was firstly adsorbed on the unsaturated Fe site and strongly activated by the metal center, which contributes to the high Hg0 catalyst oxidation activity. The generation of O-Hg-O structure is the public stage of mercury products desorption. The rate-determining step of Hg0 catalytic oxidation was dependent on Hg2O2 cluster desorption with an energy of 2.91 eV, which was much smaller than the desorption barrier of traditional catalysts. As a whole, the outcomes demonstrate that Fe/VB-BN catalyst is one promising candidate for catalytic oxidation of Hg0 by O2, which enables cleaner coalfired power plant operations.