Electro-oxidation of propylene by palladium functionalized titanium hollow fibre electrodes

• Palladium functionalized Titanium Hollow Fibre Anodes can be used to convert propylene to various partial oxidation products in acidic conditions (pH 1 and pH 4). • Metallic Pd induces the conversion of propylene and water to form allyl alcohol and acroleine. • At pH 1, significant dissolution of Pd 2+ occurs in the potential range of 0.9 to 1.1 V vs RHE, resulting in the conversion of propylene to acetone. • At pH 1 and 1.2 V vs RHE, dissolution is less significant and the electrode is composed of PdO 2 , inducing formation of propylene glycol. • At pH 4 dissolution of Pd and formation of acetone is less significant and PdO is formed, inducing formation of allyl alcohol and acroleine in the potential range 0.7-1.0 V, and propylene glycol at 1.1 to 1.2 V. • Mass transfer limitations are absent and propylene conversion is kinetically limited. The performance of Pd functionalized titanium hollow fibre electrodes (Pd@Ti-HFEs) in the electro-oxidation of propylene was investigated at pH 1 and pH 4 and at applied potentials ranging from 0.7 to 1.2 V vs RHE. Larger quantities of products are obtained at pH 1 in comparison to pH 4, mainly due to extensive dissolution of Pd to Pd 2+ in the range of 0.9-1.1 V, initiating a homogeneous chemical reaction of propylene and water to form acetone and Pd nanoparticles. At the lower end of the applied potentials (0.7-0.9 V), allyl alcohol and acrolein are formed, likely heterogeneously by the Pd surface. At the most oxidative potential investigated (1.2 V), acetone production decreases, while propylene glycol and CO 2 emerge as reaction products. PdO 2 likely forms at these potentials, limiting dissolution and inducing the shift in product selectivity. At pH 4, allyl alcohol and acrolein were detected in the electrolyte up to 1.1 V, while quantities of acetone were significantly smaller than observed at pH 1 at this potential. SEM images show extensive dissolution of Pd does not occur at pH 4, and formation of PdO is favored, as confirmed by post-analysis by XPS. At 1.2 V, quantities of propylene glycol again increase. Varying the flow rate of propylene through the HFE did not affect the product distribution, indicating that mass transfer limitations are likely absent. By taking results of cyclic voltammetry, and SEM and XPS of used electrodes into consideration, we provide an illustration of the correlation between electrode morphology and composition, and the product selectivity.