Reactivity of single-vacancy defective graphene (DG) and DG-supported Pd-n and Ag-n (n = 1, 13) for mercury (Hg-0) adsorption has been studied using density functional theory calculation. The results show that Pd-n binds defective site of DG much stronger than the Agn, while metal nanocluster binds DG stronger than single metal atom. Metal clustering affects the adsorption ability of Pd composite while that of Ag is comparatively less. The binding strength of -8.49 eV was found for Pd-13 binding on DG surface, indicating its high stability. Analyses of structure, energy, partial density of states, and d-band center (epsilon(d)) revealed that the adsorbed metal atom or cluster enhances the reactivity of DG toward Hg adsorption. In addition, the Hg adsorption ability of M-n-DG composite is found to be related to the ed of the deposited M-n, in which the closer ed of Mn to the Fermi level correspond to the higher adsorption strength of Hg on Mn-DG composite. The order of Hg adsorption strength on Mn-DG composite are as follows: Pd-13 (-1.68 eV) >> Ag-13 (-0.67 eV) Ag-1 (-0.69 eV) > Pd-1 (-0.62 eV). Pd-13-DG composite is therefore more efficient sorbent for Hg removal in terms of high stability and high adsorption reactivity compared to the Ag-13. Further design of highly efficient carbon based sorbents should be focused on tailoring the ed of deposited metals. (C) 2015 Elsevier B.V. All rights reserved.