Spin-polarized density functional theory calculations were carried out to investigate the adsorption of Hgn (n = 1-3) on the perfect, step and vacancy-defective Pd(111) surfaces as well as the Pd/γ-Al2O3(110) surface. It is found that Hg atoms prefer to adsorb on the hollow sites on Pd(111) surfaces. The adsorption of Hg on the step and vacancy-defective Pd(111) surfaces is stronger than on the perfect Pd(111) surface, which indicates that the existence of vacancy and step defects can enhance the mercury adsorption activity of Pd adsorbents. As indicated by the calculated adsorption energies, the mercury adsorption on γ-Al2O3 is weak. The γ-Al2O3 supported single Pd atom shows as good Hg adsorption activity as the perfect Pd(111) surface at low Hg coverage, while more coordination unsaturated active Pd atoms is needed to achieve high Hg adsorption capacity. In addition, it was also found that the Hg adsorption on Pd/γ-Al2O3 weakens the binding of Pd to the γ-Al2O3 surface.