Understanding synergetic effect of TiO2-supported silver nanoparticle as a sorbent for Hg0 removal

Abstract

The adsorption of elemental mercury (Hg0) on various sorbent models such as Agn (n=2, 4, and 8) clusters, TiO2 (110) surface, and Agn/TiO2 composites have been carried out using periodic density functional theory (DFT) calculations to understand the synergetic effect of metal-support toward Hg0 adsorption. The calculated d-band center predicts that the reactivity of the Agn/TiO2 composites were higher than both the bare Agn cluster and the TiO2 (110) surface, which is confirmed by the calculated Hg0 adsorption energies. The Hg0 binds moderately to the Agn clusters and the TiO2 (110) surface, but it is adsorbed strongly on the Agn/TiO2 composites. Mulliken charge analysis and charge density differences show that after Hg0 adsorption on the Agn/TiO2 composites, electrons are transferred from the Hg0 to the deposited Agn and are then distributed to the entire TiO2 support via the Ag–O interface-bonds. The partial density of state (PDOS) plots demonstrates the strong hybridization among orbitals of the adsorbed Hg0 and Agn/TiO2 composites. Overall, the key finding is that the synergy between deposited Agn and TiO2 support promotes electron transfer from adsorbed Hg to Agn/TiO2. This amount of electron transfer is an important descriptor for the Hg0 adsorption efficiency of the sorbents due to it being directly proportional to the Hg0 binding strength. The DFT calculation results explain the experimental observations well, showing that the Hg0 adsorption efficiency of the sorbents is crucially enhanced when TiO2 and Ag nanoparticle cooperate. The obtained results are useful for further design of high efficient adsorbents for removing Hg0 as well as other hazardous elements.