The
mechanism of CO and HCOOH electrooxidation in an acidic solution
on carbon-supported Au–Pd core–shell nanoparticles was
investigated by differential electrochemical mass spectrometry and
in situ Fourier transform infrared (FTIR) spectroscopy. Analysis performed
in nanostructures with 1.3 ± 0.1 nm (CS1) and 9.9 ± 1.1
nm (CS10) Pd shells provides compelling evidence that the mechanism
of adsorbed CO (COads) oxidation is affected by structural
and electronic effects introduced by the Au cores. In the case of
CS10, a band associated with adsorbed OH species (OHads) is observed in the potential range of CO oxidation. This feature
is not detected in the case of CS1, suggesting that the reaction follows
an alternative mechanism involving COOHads species. The
faradaic charge associated with COads oxidation as well
as the Stark slope measured from FTIR indicates that the overall affinity
and orbital coupling of CO to Pd are weaker for CS1 shells. FTIR spectroscopy
also revealed the presence of HCOOads intermediate species
only in the case of CS1. This observation allowed us to conclude that
the higher activity of CS10 toward this reaction is due to a fast
HCOOads oxidation step, probably involving OHads, to generate CO2. Density functional theory calculations
are used to estimate the contributions of the so-called ligand and
strain effects on the local density of states of the Pd d-band. The
calculations strongly suggest that the key parameter contributing
to the change in mechanism is the effective lattice strain.
To achieve complete oxidation of ethanol (EOR) to CO2, higher operating temperatures (often called intermediate-T, 150-200 °C) and appropriate catalysts are required. We examine here titanium oxycarbide (hereafter TiOxCy) as a possible alternative to standard carbon-based supports to enhance the stability of the catalyst/support assembly at intermediate-T. To test this material as electrocatalyst support, a systematic study of its behavior under electrochemical conditions was carried out. To have a clear description of the chemical changes of TiOxCy induced by electrochemical polarization of the material, a special setup that allows the combination of X-ray photoelectron spectroscopy and electrochemical measurements was used. Subsequently, an electrochemical study was carried out on TiOxCy powders, both at room temperature and at 150 °C. The present study has revealed that TiOxCy is a sufficiently conductive material whose surface is passivated by a TiO2 film under working conditions, which prevents the full oxidation of the TiOxCy and can thus be considered a stable electrode material for EOR working conditions. This result has also been confirmed through density functional theory (DFT) calculations on a simplified model system. Furthermore, it has been experimentally observed that ethanol molecules adsorb on the TiOxCy surface, inhibiting its oxidation. This result has been confirmed by using in situ Fourier transform infrared spectroscopy (FTIRS). The adsorption of ethanol is expected to favor the EOR in the presence of suitable catalyst nanoparticles supported on TiOxCy.
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