| Mercury is one kind of highly toxic heavy metals,which is volatile,persistent and bioaccumulation,and is harmful to human and environment.According to "global mercury assessment report" which released by UNEP in 2013,mercury emissions in China accounted for 30% of global emissions,and mercury emissions from coal-fired flue gas account for 47% of total mercury emissions in China.Therefore,how to effectively reduce mercury emissions from coal combustion is imminent.Elemental mercury is hard to be removed because of low activity in chemical reactions,poor solubility in water and volatile,so the removal of elemental mercury becomes the focus of researches.Noble metal palladium（Pd）has received broad attention recently.It shows high adsorption capacity of mercury in relatively high temperature and can be regenerated by increasing temperature.However,the hydrogen sulfide in the coal gas has strong toxic effect on palladium adsorbent.The study on the mechanism of the mercury adsorption on palladium in the atmosphere containing hydrogen sulfide（H2S）plays an important role in the design of new and efficient mercury adsorbents.In this paper,the density functional theory has been used to study the adsorption mechanism of mercury on noble metal palladium in the reduction atmosphere with hydrogen sulfide.The Pd and Pd/γ-Al2O3 are investigated in order to exploring the effects of hydrogen sulfide coverage,defects and support on the hydrogen sulfide adsorption capacity.At the same time,the co-adsorption of hydrogen sulfide and mercury is investigated,which provide theoretical basis and technical support for the influence mechanism of hydrogen sulfide to the removal of mercury on noble metal palladium.Main conclusions are listed as follows:（1）Vacancy and step defect on the Pd（111）surfaces can enhance the interaction between H2S,HS,S and surfaces,and improve the mercury adsorption capacity of Pd sorbents.For low H2S adsorption,Pd/γ-Al2O3 and Pd surfaces show similar H2S adsorption activities.For higher H2S adsorption,the deposition of single Pd atom on the γ-Al2O3 surface can enhance the activity of the Pd atom for H2S adsorption.（2）For one and two H2S molecules adsorption,H2S prefers dissociative adsorption on different substrates.For three H2S molecules adsorption,both molecular and dissociative adsorption co-exists on the Pd（111）prefect and vacancy surfaces,while the dissociative adsorption is most stable adsorption mode on step Pd（111）,γ-Al2O3（110）and Pd/γ-Al2O3（110）surfaces.（3）The adsorption energies of Hg on the Pd（111）surfaces are less than that of H2S,and HgS is likely to dissociatively adsorb on the Pd（111）surfaces.Hg atoms will be repelled strongly by S atom on Pd（111）surfaces,so elemental mercury cannot be effectively removed.（4）Pdn clusters show weaker Hg adsorption capacity than the Pd（111）surfaces,and γ-Al2O3 support can enhance the mercury adsorption capacity of Pdn clusters.The adsorption process of Hg can enhance the interaction between the small size Pdn clusters（n=15）and γ-Al2O3 support,but it will decrease the interaction between the large size Pdn clusters（n=68）and γ-Al2O3 support.（5）Pd is the active component for H2S and Hg adsorption on the supporting adsorbents.The adsorption of H2S on Pdn/γ-Al2O3（110）is much stronger than that of Hg,indicating that the presence of H2S can inhibit the adsorption of Hg on the supporting adsorbents. |
