Direct ethanol fuel cell technology suffers from a lack of effective anode catalysts for complete ethanol oxidation reaction (EOR). Pd and Pd-based catalysts showed some promise, but only a trace amount of CO 2 was detected as the product. The difficulty of C−C bond cleavage and the formation of acetic acid are commonly believed to be great obstacles toward complete EOR. The limited formation of CO 2 also suggests that acetic acid may not be the only dead-end product that prevents complete EOR. A careful study on the reaction pathway leading to complete EOR is needed to better understand and design effective EOR catalysts. As such, we studied 17 key elementary reactions on Pd surfaces using density functional theory (DFT) and designed experiments to confirm some of the DFT findings. The results show that, in addition to the acetic acid formation, other poisonous species, C, CH, CCO, or dimerization of acetaldehyde, are also largely responsible for the limited formation of CO 2 on Pd catalysts due to their strong adsorptions to the catalysts which block the active sites. The ethanol oxidation shows totally different reaction pathways in neutral and alkaline media. The DFT calculation result provided important insights into the catalysis of complete ethanol oxidation. The experiment result showed that EOR on PdCu alloy nanoparticle catalyst has higher catalytic activity than that on Pd nanoparticle catalyst, suggesting fast kinetics of initial dehydrogenation on the alloy catalyst.

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.

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