Density Functional Theory Study Of Mercury Adsorption On Pd And Pd/γ-Al₂O₃

Mercury is a harmful heavy metal due to its volatility, migration,bioaccumulation and neurological toxicity. Mercury can participate in theatmospheric circulation and long-distance migration, and deposit in variousforms, and finally concentrate in the food chain, leading to human healthproblems and environmental pollution. In the society which promotes harmonybetween human and nature, and environmental protection, it is necessary tocontrol and reduce the release of mercury. The study of mercury adsorptionmechanism can provide theoretical basis for the design of a new and efficientmercury adsorbents.Noble metal palladium (Pd) show high mercury adsorption capacity at hightemperatures and it can be recycled easily. However, the addition of secondmetal in Pd sorbents should be considered due to its high costs. Our studyselected noble metal palladium as research objects. The adsorption of mercuryon Pd, Pd/γ-Al2O3and PdM/γ-Al2O3were investigated with the aims ofexploring the effects of support and second metal on the Pd sorbents.Pd(111) surfaces models were constructed including step surface,vacancy-defective surface and the second metal-doped (M-substituted) surfaces,in order to exploring the effects of defects and the addition of second metal onthe mercury adsorption capacity. The Pd/γ-Al2O3and PdM/γ-Al2O3(M=Au, Ag, Cu) surfaces were investigated to understand the impact of the support onthe adsorption performance. The Hgn(n=1~3) clusters adsorption were studiedto elucidate how the mercury coverage affect mercury removal capacity. Mainconclusions are listed as follows:(1) Step and vacancy-defective Pd(111) surfaces showed better Hgadsorption capacity than the perfect surface, indicating that step and vacancydefects were the main mercury adsorption sites.(2) Hg adsorption capacity of M-substituted Pd(111) surfaces was notaffected by the addition of second metal at low Hg coverage. M-substitutedPd(111) surfaces can contribute to the stepwise adsorption of Hg atoms on thesurfaces at high Hg coverage.(3) Mercury coverages have effects on mercury adsorption capacity ofsupported Pd-based sorbents. For a single Hg atom adsorption, mercuryadsorption capacity of PdM/γ-Al2O3sorbents was affected bymetal-support-interaction, and the support has no impact on that of Pd/γ-Al2O3.However, for high mercury coverage, the properties of metal clusters PdM wasthe main factor for mercury adsorption capacity of the supported bimetallicsorbents, while the mercury adsorption capacity was weaken by the support forPd/γ-Al2O3sorbents.(4) Pd was the main active composition for Hg adsorption in spite of theaddition of the second metal. The γ-Al2O3support can reduce the activity ofpure metal Pd2while enhance the activity of bimetallic PdM for Hg adsorption,especially the PdCu. Therefore, Cu addition was a comparatively good candidatetaking Hg adsorption capacity and economic costs into account.