Poisonous Species in Complete Ethanol Oxidation Reaction on Palladium Catalysts

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 CO₂ 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 CO₂ 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 CO₂ 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.