Structural and electronic properties of Pd-decorated graphene oxides and their effects on the adsorption of nitrogen oxides: insights from density functional calculations

Abstract

The structural and electronic properties of Pd-decorated reduced graphene oxides (rGOs) and their effects on the adsorption of nitrogen oxides NO_x (x = 1, 2, 3) were studied by density functional theory calculations. Our results indicate that the Pd atom and Pd₄ and Pd₆ clusters can be strongly bound to the rGOs surface through the Pd⋯O coordination bond or Pd–O covalent bond between Pd and the active sites provided by the hydroxyl and epoxy functional groups. Generally, the Pd clusters exhibit stronger binding to rGO than single metal atoms. The strong binding of the Pd clusters to the substrate effectively modifies the position of the d-band center accompanied by larger charge transfer from the cluster to the support. The decorated Pd on rGO improves the adsorption of NO_x with larger binding energy, compared to the pristine graphene and rGO nanomaterials. The adsorption strength of the gas molecule is well correlated with the d-band center of the Pd cluster. The electronic structure calculations show that the strong hybridization of the frontier orbitals of free NO₂ and NO₃ with the d electronic states of decorated Pd around the Fermi level is responsible for the large charge transfers from molecules to the Pd–GO complex, giving rise to the acceptor doping by these molecules on this complex, whereas the adsorbed NO shows both donor and acceptor doping character, depending on the adsorption sites of NO.