Highly active Pt ₃ Rh/C nanoparticles towards ethanol electrooxidation. Influence of the catalyst structure

Abstract

The electrochemical oxidation of ethanol results in the formation of strongly adsorbed intermediates. Pt–Rh catalysts are proposed as alternatives since they easy the C[sbnd]C bond breaking. However, the effect of the Pt–Rh structure on the catalytic activity and selectivity to CO ₂ is not well understood. Here, we synthesised Pt/C and two different Pt–Rh/C catalyst architectures, an alloy (Pt ₃ Rh/C)and a bimetallic mixture (Pt ₃ –Rh/C)to study the effect of catalyst structure on its catalytic activity and on the products formed during the ethanol oxidation in acid media. The nanoparticles were prepared by a modified polyol reduction method using ethylene glycol as a co-reducing agent and Pb as a material of sacrifice, to obtain very small and well-dispersed nanoparticles on the carbon support. Fourier transform infrared spectroscopy and derivative voltammetry was used to give insights about the ethanol oxidation mechanism occurring at the developed catalysts. The samples characterised by X-ray diffraction analysis showed distortions in the Pt lattice parameters for the Pt-Rh alloy structure due to the presence of Rh in the catalyst's composition. Transmission electron microscopy analyses indicate that nanoparticles were well-dispersed on a carbon support, with spherical shapes and small particle sizes (2–3 nm). in situ X-ray absorption spectroscopy data evidence that Pt–Rh interactions produce changes in the Pt 5d band vacancy. The electronic effect is maximized when Pt forms an alloy with Rh, resulting in the highest d-band vacancy of the Pt ₃ Rh/C. The Pt ₃ Rh/C catalyst showed the highest activity towards ethanol oxidation, presenting current densities in a quasi-steady-state condition (measured at 600 mV)around 5.2 times higher than the commercial Pt/C (Alfa Aesar). Moreover, the onset potential for ethanol oxidation shifts to more negative potentials (110 mV lower taken at 1 mA cm ^–2 )was also observed. In situ FTIR data revealed that Pt/C catalyst favours the formation of acetic acid. The synergistic effect between Rh and the alloy structure results in an easier C[sbnd]C bond breaking for Pt ₃ Rh/C, in comparison to Pt ₃ –Rh mixture, thus favouring CO ₂ formation at lower potentials.

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