Immobilization of gaseous elemental mercury by Ag nanoparticles: A combined DFT and experimental study

Abstract

Silver nanoparticles (AgNPs) have attracted considerable attention as promising industrial adsorbents of gaseous mercury. The underlying mechanism of immobilization of Hg0 on different AgNP surfaces was investigated in theoretical and experimental approaches. The crystallographic properties of AgNPs obtained by X-ray diffraction and selected area electron diffraction were incorporated into comparative DFT calculations to evaluate the adsorption energy on AgNP surfaces such as low-index and edge planes. In addition, the energy pathway of oxidation of adsorbed Hg to HgO on AgNP surfaces was analysed by an ab-initio method, which suggested chemisorption rather than catalytic oxidation to be the main mechanism of mercury removal by AgNPs in the low-to-middle temperature range (≤326 °C). Mercury adsorption and desorption tests were conducted in a fixed-bed reactor with Ag nanoparticles and microparticles as adsorbent, where AgNPs were more efficient than microparticles in mercury removal, and the removal efficiency remained stable after four rounds of regeneration. Furthermore, temperature-programmed desorption analysis suggested a second-order desorption of Hg from AgNPs with an activation energy of 0.84 eV. X-ray photoelectron spectroscopy showed that the chemical characteristics of AgNPs remained stable while adsorbing Hg0 and that the adsorbed Hg was not in an oxidation state, validating the DFT results. By providing an integrated understanding of mercury adsorption on AgNPs, these computational and experimental results will help to guide the design and optimisation of Ag-based mercury adsorbents for industrial applications.