Role of Local Coordination and pH on Electrochemical Oxidation of Alkene in Aqueous Solutions

Electro-oxidation of alkenes powered by renewable sources offers a promising pathway for producing target chemicals. However, to compete with well-established, energy-intensive processes that utilize high temperatures and/or high pressures, selectivity and yield must be perfectly controlled. This can only be achieved through a deep understanding of reaction pathways and reaction intermediates. Recent studies on propylene oxidation have shown that various chemicals, such as propylene glycol, propylene oxide, acetone, and acrolein, can be produced depending on the conditions, including applied potential and pH of the solution. However, few studies have been conducted at high anodic potentials, where a significant amount of oxygen evolution reaction can occur. In this research, we first investigated the mechanism of propylene oxidation to propylene glycol on Pd-based catalysts within a high-potential range in acid media aqueous solutions, exploring the effects of oxidation state, substitution and local coordination on the yield, selectivity and kinetics of the reaction. We found that the faradaic efficiency (FE) peaks at 1.4 V (RHE) for metallic Pd, while it increases to 1.6 V (RHE) for other materials, including PdO and Pt-doped PdO, before higher anodic potentials, where the OER becomes dominant. Interestingly, although cyclic voltammetry (CV) and X-ray absorption spectroscopy revealed that the Pd surface is oxidized at potentials where the FE is at its maximum, Pd shows a much lower FE (23% at 1.4 V) compared to PdO (56% at 1.6 V). This suggests that the structural differences between PdO and the oxide layer formed on Pd play a crucial role in electrocatalysis, guiding us through the design of novel Pd-based oxide catalysts with enhanced activity, including Pt-doped PdO that shows FE of 67% at 1.6 V vs. RHE. Operando attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy was then employed to elucidate the reaction mechanism and intermediates, demonstrating the crucial role played by oxo-intermediates in the reaction. These results gathered in acidic conditions were then compared to those collected in neutral conditions, where propylene oxide is found as major product but in very limited amount (maximum FE of 13% was measured for Pt-doped PdO catalyst). This major discrepancy when compared to previous reports using Pd-based catalysts in neutral conditions shines the light on the extreme sensitivity of this reaction to experimental conditions. Our study provides insights into the propylene oxidation behavior in different media and emphasizes the significant impact of crystallographic coordination of active sites on electrolysis efficiency.