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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The performance of transition metal catalysts for ethanol oxidation reaction (EOR) in direct ethanol fuel cells (DEFCs) may be greatly affected by their oxidation. However, the specific effect and catalytic mechanism for EOR of transition metal oxides are still unclear and deserve in-depth exploitation. Copper as a potential anode catalyst can be easily oxidized in air. Thus, in this study, we investigated C-C and C-H bond cleavage reactions of CHCO (x = 1, 2, 3) species in EOR on CuO(111) using PBE+U calculations, as well as the specific effect of +U correction on the process of adsorption and reaction on CuO(111). It was revealed that the catalytic performance of CuO(111) for EOR was restrained compared with that of Cu(100). Except for the C-H cleavage of CHCO, all the reaction barriers for C-C and C-H cleavage were higher than those on Cu(100). The most probable pathway for CHCO to CHCO on CuO(111) was the continuous dehydrogenation reaction. Besides, the barrier for C-C bond cleavage increased due to the loss of H atoms in the intermediate. Moreover, by the comparison of the traditional GGA/PBE method and the PBE+U method, it could be concluded that C-C cleavage barriers would be underestimated without +U correction, while C-H cleavage barriers would be overestimated. +U correction was proved to be necessary, and the reaction barriers and the values of the Hubbard U parameter had a proper linear relationship.

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Economically advantageous Cu-based catalysts have been widely used for a great number of reactions related to ethanol. However, serious obstacles still remain, such as the high reaction energy barrier and low selectivity for the first step of the dehydrogenation of ethanol. In this study, O-H and α-C-H bond cleavages in ethanol on a Cu3X(111) surface (X= Zr, In, Ag, Au) were carried out using DFT to explore the effect of alloying on the selective and effective dehydrogenation of ethanol. Cu3Zr(111) was found to have superior catalytic performances for dehydrogenation with significantly low reaction barriers for both O-H bond cleavage (0.13 eV) and α-C-H bond cleavage (0.73 eV), which are much lower than the results on Cu(111). Thus this work indicates that alloying Zr can selectively break the O-H bond of ethanol, which cannot be accomplished using Pt, Pd, or Cu catalysts. Meanwhile, through PDOS analysis, Mülliken charge analysis, and d-band center analysis, there are two key fac-tors that contribute to the great improvements on the dehydrogenation catalytic activities of Cu3X(111). Firstly, the specific inherent properties of the second alloyed metal X, including the d-band center, are crucial to the adsorption and activation of ethanol on surfaces. Sec-ondly, the electronic distribution on the surfaces resulting from the difference of electronega-tivity between the metals Cu and X is associated with the dehydrogenation reaction barrier. More electron density around the Cu atoms on these surfaces is more beneficial for dehydro-genation reactions, especially when H atoms were adsorbed stably on the Cu sites.