DFT study on the mechanisms of mercury removal from natural gas over Se-modified activated carbon

Deeper target reservoir of natural gas leads to higher mercury (Hg) concentration in the product stream, which is a severe threat to both equipment safety and human health. The stringent Hg concentration standards in natural gas flow calls for the highly efficient adsorbent. The extreme high binding affinity between selenium (Se) and Hg implies the potential for Se-modified materials in Hg removal. For the first time, the adsorption, oxidation and desorption behaviors of Hg over Se-modified activated carbon (AC) in natural gas were investigated in this study through density functional theory (DFT) method. For comparison, the Hg removal process over the commonly used adsorbent, S-modified AC, was also calculated. The results indicate that Hg is easier to be absorbed on Se-modified AC surface. Besides p and d orbitals, s orbital of mercury also contributes to the hybridization with carbon on Se-modified AC. CH4 is unfavorable for Hg adsorption but can enhance Hg oxidation without participating in the chemical reaction directly. Lower activation energy is required for the formation of HgSe compared with the formation of HgS and the Hg-Se bond is stronger. HgSe can be regarded as the stable adsorbed species due to the high desorption energies. This work reveals the mechanisms of the excellent performance in heterogeneous Hg removal of Se-modified AC, which has large potential in Hg mitigation from natural gas.