Direct Ethanol Fuel Cells: Cleavage of the C-C Bond on Different Pt-M Catalysts Using Reactivity Descriptors through Density Functional Theory

Direct ethanol fuel cells (DEFCs) are devices to convert directly chemical energy into electrical energy efficiently and producing low CO₂ emissions. The main application of the DEFCs are electronic portable devices, portable auxiliary chargers and transport. To date a lot of effort has been made to improve the Pt-based catalysts to reach the C-C bond cleavage in this molecule, avoiding the poisoning of the Pt based catalysts and improving the rate of the ethanol oxidation reaction (EOR) [1-3].

The use of ethanol in this fuel cell offers advantages such as its non-toxicity, renewability, high energy density (8 kWh/kg, compared with 33 kWh/kg for hydrogen and about of 11 kWh/kg for gasoline), and its easy production in large quantities [4-6], despite this advantage of ethanol as fuel, the research to achieve the C-C bond cleavage in this molecule on several Pt-Metal (Pt-M) alloys is still in development.

Complete oxidation of ethanol implies the cleavage of the C-C bond, whose mechanism is currently not completely understood. The most accepted mechanism proposes that ethanol oxidation on Pt-based catalysts occurs through an initial adsorption step and then a series of dehydrogenation reactions occurs to produce adsorbed acetaldehyde CH₃CHO and acetyl CH₃CO. Acetyl could react with OH molecules, formed by H₂O oxidation to OH^-, producing acetic acid CH₃COOH, considered a final product in ethanol oxidation because of the difficulty of further oxidation. This partial oxidation of ethanol is considered the most favorable route on actual catalysts, but it only releases 4 electrons [5, 6].

To achieve the complete oxidation to CO₂ and to avoid the partial oxidation of ethanol is necessary to gain understanding of the C-C bond cleavage after of the first dehydrogenation steps, so in this work the C-C cleavage step of the ethanol oxidation is described on Pt and different Pt-M catalytic alloys (Pt-Sn, Pt-Re) using reactivity descriptors (representative molecules showing destabilization on catalytic alloys, for instance CHCO) [7].

The reactivity descriptors are characterized calculating their binding energies, electron density, density of states, and electron localization function. With this data and analyzing the reactivity surface of the descriptors on each catalytic alloy, a comparison of the behavior of the C-C bond cleavage on the different metallic alloys is presented based on the chemical bonding theory. Also available experimental data are compared with the theoretical calculations carried out with density functional theory.

References:

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