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 (Hg) capture from coal-fired flue gas. However, the atomic-level reaction mechanism which determines Hg adsorption capacity of EVS-Ag sorbent remains elusive. Reaction mechanism and active sites of Hg 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. Hg adsorption on EVS-10 and EVS-Ag surfaces is controlled by the physisorption and chemisorption mechanisms, respectively. Ag cluster is determined to be the most active site of Hg adsorption over Ag-modified EVS sorbent. The adsorption energy of Hg on Ag 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 Hg. HgO prefers to adsorb on Ag 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. O molecule promotes Hg adsorption over EVS-Ag sorbent. HgO species can be easily formed during Hg adsorption over EVS-Ag sorbent in the presence of O.