Role of Ni in Bimetallic PdNi Catalysts for Ethanol Oxidation Reaction

Bimetallic PdNi catalysts have garnered great interest in the study of ethanol oxidation reactions (EORs), though mechanistic insights into their catalytic performances are lacking, which hinders further improvement and rational design of the next generation of PdNi catalysts. As such, density functional theory (DFT) calculations were performed for six key elementary reactions using four model catalysts, one with pure Pd and three for PdNi. DFT results indicate that the reduced catalytic activities observed experimentally when Ni atoms were placed under Pd layers are the result of an increase in the reaction barrier for CH3COOH formation. Further analysis illustrated that this is largely owing to the charge transfer from the Ni to the Pd atoms. On the other hand, the enhanced activities of the PdNi catalysts with respect to pure Pd catalysts in EORs when Ni atoms are exposed at the catalyst surfaces are due to the lowering of the reaction barrier toward C–C bond cleavage and increasing of that toward C–O bond coupling. Therefore, surface Ni atoms are responsible for the superior activity of the PdNi catalysts in EORs. Further analysis of DFT results suggests that the reaction barriers of the C–C bond cleavage and the C–O bond coupling approach similar values when the composition of surface Ni atoms in a PdNi catalyst reaches about 44%. To achieve a complete EOR, the estimated surface Ni atoms should be as high as 77%. However, stability may become a concern for catalysts with such a high exposure of Ni atoms at the catalyst surface.