Mechanistic investigation of elemental mercury adsorption over silver-modified vanadium silicate: A DFT study

Ag-modified vanadium silicate (EVS-Ag) has been regarded as a superior sorbent for elemental mercury (Hg⁰) capture from coal-fired flue gas. However, the atomic-level reaction mechanism which determines Hg⁰ adsorption capacity of EVS-Ag sorbent remains elusive. Reaction mechanism and active sites of Hg⁰ adsorption over EVS-Ag sorbent were studied using density functional theory (DFT) calculations systematically. DFT calculation results indicate that silver exchange shows little effects on the geometric structure of EVS-10 sorbent. Hg⁰ adsorption on EVS-10 and EVS-Ag surfaces is controlled by the physisorption and chemisorption mechanisms, respectively. Ag₂ cluster is determined to be the most active site of Hg⁰ adsorption over Ag-modified EVS sorbent. The adsorption energy of Hg⁰ on Ag₂ cluster is -51.93 kJ/mol. The orbital hybridization and electron sharing between Ag and Hg atoms are responsible for the strong interaction between EVS-Ag surface and Hg⁰. HgO prefers to adsorb on Ag₂ cluster of EVS-Ag sorbent, and yields an energy release of 306.21 kJ/mol. HgO desorption from EVS-Ag sorbent surface needs a higher external energy, and occurs at the relatively higher temperatures. O₂ molecule promotes Hg⁰ adsorption over EVS-Ag sorbent. HgO species can be easily formed during Hg⁰ adsorption over EVS-Ag sorbent in the presence of O₂.