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

Recent Approaches for Cleaving the C─C Bond During Ethanol Electro-Oxidation Reaction

Direct ethanol fuel cells (DEFCs) play an indispensable role in the cyclic utilization of carbon resources due to its high volumetric energy density, high efficiency, and environmental benign character. However, owing to the chemically stable carbon-carbon (C─C) bond of ethanol, its incomplete electrooxidation at the anode severely inhibits the energy and power density output of DEFCs. The efficiency of C─C bond cleaving on the state-of-the-art Pt or Pd catalysts is reported as low as 7.5%. Recently, tremendous efforts are devoted to this field, and some effective strategies are put forward to facilitate the cleavage of the C─C bond. It is the right time to summarize the major breakthroughs in ethanol electrooxidation reaction. In this review, some optimization strategies including constructing core-shell nanostructure with alloying effect, doping other metal atoms in Pt and Pd catalysts, engineering composite catalyst with interface synergism, introducing cascade catalytic sites, and so on, are systematically summarized. In addition, the catalytic mechanism as well as the correlations between the catalyst structure and catalytic efficiency are further discussed. Finally, the prevailing limitations and feasible improvement directions for ethanol electrooxidation are proposed.

Insight and Activation Energy Surface of the Dehydrogenation of C2HxO Species in Ethanol Oxidation Reaction on Ir(100)

Dehydrogenation of an organic compound is the first and the most fundamental elementary reaction in many organic reactions. In ethanol oxidation reaction (EOR) to form CO 2 , there are a total of 46 pathways in C 2 H x O (x=1-6) species leading to the removal of all six hydrogen atoms in five C-H bonds and one O-H bond. To investigate the degree of dehydrogenation in EOR under operando conditions, we performed density function theory (DFT) calculations to study 28 dehydrogenation steps of C 2 H x O on Ir(100). An activation energy surface was then constructed and compared with that of the C-C bond cleavages to understand the importance of the degree of dehydrogenation in EOR. The results show that there are likely 28 dehydrogenations in EOR under fuel cell temperatures and the last two hydrogens in C 2 H 2 O are less likely cleaved. On the other hand, deep dehydrogenation including 45 dehydrogenations can occur under ethanol steam reforming conditions.

Insights into reaction mechanisms of ethanol electrooxidation at the Pt/Au(111) interfaces using density functional theory

Understanding ethanol electrooxidation reaction kinetics is fundamental to the development of direct ethanol fuel cells.

Potential-Dependent Mechanistic Study of Ethanol Electro-oxidation on Palladium

We herein used the density functional theory (DFT) method and the implicit continuum solvation model to study the potential-dependent mechanism of ethanol oxidation reaction (EOR) on palladium (Pd). Energy evolutions of the EOR on low-index Pd surfaces, including (111), (110), and (100), were obtained as a function of the electrode potential. Moreover, the onset potentials for key intermediates and products were calculated. In addition, the potential range for adsorbed ethanol as the most stable adsorption state for proceeding the EOR was determined to be between 0.15 and 0.78 V via the calculated Pourbaix diagrams when considering hydrogen underpotential deposition and Pd(II) oxide formation as competing reactions. Specifically, the behavior of Pd(111) as the dominating facet decided the overall activity of the EOR with onset potentials to acidic acid/acetate at 0.40 V, to carbon dioxide at 0.71 V, and to oxide formation at 0.78 V. Pd(110) was predicted to exhibit the optimal activity toward the EOR with the lowest onset potentials to both the first dehydrogenation process and carbon dioxide at 0.08 and 0.60 V, respectively. A computational potential-dependent mechanism of the EOR was proposed, which agrees well with the experimental curve of linear sweeping voltammetry on the commercial Pd/C electrocatalyst. Our study suggests that targeted control of products can be tuned with proper overpotential and thus provides a foundation for the future development of EOR electrocatalysts.

Ethanol Dehydrogenation: A Reaction Path Study by Means of Temporal Analysis of Products

Conventional fossil fuels such as gasoline or diesel should be substituted in the future by environmentally-friendly alternatives in order to reduce emissions in the transport sector and thus mitigate global warming. In this regard, iso-butanol is very promising as its chemical and physical properties are very similar to those of gasoline. Therefore, ongoing research deals with the development of catalytically-supported synthesis routes to iso-butanol, starting from renewably-generated methanol. This research has already revealed that the dehydrogenation of ethanol plays an important role in the reaction sequence from methanol to iso-butanol. To improve the fundamental understanding of the ethanol dehydrogenation step, the Temporal Analysis of Products (TAP) methodology was applied to illuminate that the catalysts used, Pt/C, Ir/C and Cu/C, are very active in ethanol adsorption. H2 and acetaldehyde are formed on the catalyst surfaces, with the latter quickly decomposing into CO and CH4 under the given reaction conditions. Based on the TAP results, this paper proposes a reaction scheme for ethanol dehydrogenation and acetaldehyde decomposition on the respective catalysts. The samples are characterized by means of N2 sorption and Scanning Transmission Electron Microscopy (STEM).

Conversion of Ethanol via C-C Splitting on Noble Metal Surfaces in Room-Temperature Liquid-Phase

Rh-catalyzed decomposition of ethanol into CO₂ and CH₄ via C-C bond splitting is reported in room-temperature liquid phase under atmospheric pressure. Mechanistic investigations show that C-C bond splitting of ethanol on the noble metal surface is rapid, and CO₂ forms through the oxidation of α-CH _xO and β-CH _x fragments after C-C bond splitting, while CH₄ forms through the hydrogenation of β-CH _x utilizing H atoms from -OH, β-CH _x, and α-CH _xOH fragments.

Hydrogen adsorption-mediated synthesis of concave Pt nanocubes and their enhanced electrocatalytic activity

Concave nanocubes are enclosed by high-index facets and have negative curvature; they are expected to have enhanced reactivity, as compared to nanocubes with flat surfaces. Herein, we propose and demonstrate a new strategy for the synthesis of concave Pt nanocubes with {hk0} high-index facets, by using a hydrogen adsorption-mediated electrochemical square-wave potential method. It was found that Pt atoms prefer to deposit on edge sites rather than terrace sites on Pt surfaces with intensive hydrogen adsorption, resulting in the formation of concave structures. The as-prepared concave Pt nanocubes exhibit enhanced catalytic activity and stability towards oxidation of ethanol and formic acid in acidic solutions, compared to commercial Pt/C catalysts.

Elucidation of the surface structure–selectivity relationship in ethanol electro-oxidation over platinum by density functional theory

We have successfully built a general framework to comprehend the structure–selectivity relationship in ethanol electrooxidation on platinum by density functional theory calculations.

Methanol electro-oxidation on platinum modified tungsten carbides in direct methanol fuel cells: a DFT study

Bilayer Pt-modified WC catalysts exhibit up to 2.4 times higher MOR reactivity compared to that of pure Pt.