Development of Pdn/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 Hg-0 on g-C3N4, single Pd atoms and Pdn(n=2-4) clusters as well as the influences of the support and the size of Pd clusters on Hg-0 adsorption. It is found that Hg-0 atoms are physically adsorbed on the pristine g-C3N4 and are chemisorbed on the pure Pdn clusters and the Pd-n/g-C3N4. The strongest adsorption happens on the Pd-2 cluster and the Pd-3/g-C3N4, while single Pd atoms doped on the g-C3N4 do not adsorb Hg-0 atoms effectively. For a Pd-4 cluster, there are at least four adsorption sites for the adsorption of Hg-0, while these sites become more active in the presence of the support and the number of active sites for Hg-0 adsorption on a Pd-4 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 Pd-2, Pd-2/g-C3N4 and Pd-3/g-C3N4, indicating the most intensive interactions between Hg-0 and these configurations. Moreover, the shortened Pd-Hg bonds in the Pd-3/g-C3N4 and the Pd-4/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 Pd-2/g-C3N4.