Significance of β-dehydrogenation in ethanol electro-oxidation on platinum doped with Ru, Rh, Pd, Os and Ir

Abstract

In the exploration of highly efficient direct ethanol fuel cells (DEFCs), how to promote the CO2 selectivity is a key issue which remains to be solved. Some advances have been made, for example, using bimetallic electrocatalysts, Rh has been found to be an efficient additive to platinum to obtain high CO2 selectivity experimentally. In this work, the mechanism of ethanol electrooxidation is investigated using the first principles method. It is found that CH3CHOH* is the key intermediate during ethanol electrooxidation and the activity of β-dehydrogenation is the rate determining factor that affects the completeness of ethanol oxidation. In addition, a series of transition metals (Ru, Rh, Pd, Os and Ir) are alloyed on the top layer of Pt(111) in order to analyze their effects. The elementary steps, α-, β-C-H bond and C-C bond dissociations, are calculated on these bimetallic M/Pt(111) surfaces and the formation potential of OH* from water dissociation is also calculated. We find that the active metals increase the activity of β-dehydrogenation but lower the OH* formation potential resulting in the active site being blocked. By considering both β-dehydrogenation and OH* formation, Ru, Os and Ir are identified to be unsuitable for the promotion of CO2 selectivity and only Rh is able to increase the selectivity of CO2 in DEFCs.

Recent Advances on Electro-Oxidation of Ethanol on Pt- and Pd-Based Catalysts: From Reaction Mechanisms to Catalytic Materials

“…A detailed understanding of the reaction mechanism and in particular of the rate-limiting step(s) in EOR under continuous reaction conditions is of critical importance for the design of highly active catalysts [11,12]. Although numerous experimental studies using Fourier transform infrared spectroscopy (FTIR) [13][14][15][16][17][18][19][20][21][22][23][24][25][26] or differential electrochemical mass spectrometry (DEMS) [27][28][29][30][31][32][33][34][35][36][37], as well as theoretical studies [38][39][40][41][42][43][44][45] have been conducted to understand the EOR process, a detailed mechanism of EOR remains unclear or even contradictory. Nevertheless, a so-called dual-pathway (C1 and C2) mechanism has been largely agreed upon: the C1 pathway proceeds via adsorbed carbon monoxide (COads) intermediate to form CO2 (or carbonate in alkaline solutions) by delivering 12 electrons, and the C2 pathway mainly leads to the formation of acetic acid (or acetate in alkaline solutions) by delivering four electrons and/or acetaldehyde by delivering two electrons.…”

Reaction Energy for an Electrode Surface Atom Inserting into an R-H Bond and Its Dependence on Electrode Potential: Application to Pt(111)

“…The calculated 0.84 eV to 1.06 eV activation energy barriers for C-H dissociations on Pt(111) presented in Ref. 3 suggest that these reactions may be prohibited at low temperatures. In the case of water oxidation to OH ads , the reversible potential is around 0.14 V greater than 0.35 V, which means the O-H dissociation on the surface will probably have a high activation energy barrier and the ∼0.1 eV activation energies 20 calculated for direct oxidation into solution make this the likely mechanism.…”

“…It is noted that at the vacuum-surface interface the activation energies calculated in Ref. 3 for the surface dissociation of C-H bonds of three fragments derived from ethanol range from 0.81 to 1.06 eV. These energies seem to be too high for allowing significant reaction at standard temperature.…”

“…There are two paths for oxidizing an R-H bond in R-H ads on an electrode surface. The first path considered is the R-H ads oxidation into solution reaction with simultaneous electron transfer to the electrode and discharge of a hydrated hydronium ion, H 3 …”

“…Most theoretical model calculations have considered only the surface dissociation step without regard to the subsequent oxidation to H + aq . In such work the vacuum-surface interface is employed as a model for the liquid electrolyte-surface interface as, for example, in the work of Shen et al 3 Reference 3 provides an entry into more of this literature. Knowing the reaction energies for the dissociation reactions as functions of electrode potential might help the analysis of the reaction mechanism and activity of an electrocatalyst.…”