Development of Pd_n/g-C₃N₄ adsorbent for Hg⁰ removal – DFT study of influences of the support and Pd cluster size

Elemental mercury in the flue gas is highly hazardous to the ecosystem. However, its removal from gas phase is of challenges as it is highly volatile, chemically very stable and insoluble in water. It is therefore imperative to develop novel adsorbents that are efficient in the removal of elemental mercury from gas mixtures. In this study, density functional theory (DFT) was adopted to assist the development of novel adsorbents for mercury removal based on the in-depth understanding of the adsorption of Hg⁰ on g-C₃N₄, single Pd atoms and Pd_n(n=2–4) clusters as well as the influences of the support and the size of Pd clusters on Hg⁰ adsorption. It is found that Hg⁰ atoms are physically adsorbed on the pristine g-C₃N₄ and are chemisorbed on the pure Pd_n clusters and the Pd_n/g-C₃N₄. The strongest adsorption happens on the Pd₂ cluster and the Pd₃/g-C₃N₄, while single Pd atoms doped on the g-C₃N₄ do not adsorb Hg⁰ atoms effectively. For a Pd₄ cluster, there are at least four adsorption sites for the adsorption of Hg⁰, while these sites become more active in the presence of the support and the number of active sites for Hg⁰ adsorption on a Pd₄ cluster doped on the g-C₃N₄ becomes seven due to the enhanced charge transfer from Hg atoms to the Pd cluster and the g-C₃N₄ surface. The most charge transfers are found to take place in the case of Pd₂, Pd₂/g-C₃N₄ and Pd₃/g-C₃N₄, indicating the most intensive interactions between Hg⁰ and these configurations. Moreover, the shortened Pd-Hg bonds in the Pd₃/g-C₃N₄ and the Pd₄/g-C₃N₄ also shows the enhanced mercury adsorption, while opposite phenomena were observed in the other two structures, i.e., the Pd/g-C₃N₄ and the Pd₂/g-C₃N₄.