(197b) Direct Propylene Epoxidation By Water Oxidation on PdPtOx Electrocatalysts

Direct electrochemical propylene epoxidation shows promise in displacing incumbent routes of epoxidation involving hazardous and costly oxidants (peroxides, chlorine) while using renewable electricity. Equimolar Pd and Pt maximize partial current densities and faradaic efficiencies (FE) of propylene oxide formation, greatly exceeding the performance of monometallic PdO_x or PtO_x. Increasing oxidation temperatures of Pd₁Pt₁ increases epoxidation FE non-monotonically, maximized at 500°C. This temperature enriches an oxidic tetragonal phase relative to a metallic face-center cubic phase, consistent with XAS showing a maximized Pt–O coordination. However, monometallic Pd and Pt catalysts treated 500°C exist primarily as PdO or metallic Pt, respectively. This conclusion agrees with XPS data showing that>85% of surface Pt exists in a 2⁺ oxidation state in PdPtO_x. Thus, combining Pd and Pt stabilizes PtO-like structures, which are likely the most active sites for water oxidation and O-atom transfer to propylene.

Epoxidation rates increase with H₂O concentration with a 2^nd-order dependence, suggesting two water molecules participate in the kinetically relevant step (KRS), consistent with (H₂O/D₂O) kinetic isotope effects. Epoxidation rates increase sublinearly with C₃H₆ pressure (n=0.4-0.7), consistent with C₃H₆* species saturating the surface. Rates and FEs of epoxidation increase exponentially with potential in aqueous electrolytes with Tafel slopes of 102 mV dec^-1, consistent with one-electron transfer to the most abundant reactive intermediate in the KRS. These findings agree with a mechanism involving the adsorption of H₂O and C₃H₆ to the surface of PtO active sites, which oxidize by sequential proton-coupled electron transfer steps (OH*→O*→OOH*→O₂). The KRS, however, likely involves the oxidation of O* to OOH* by two H₂O molecules, in which the fate of OOH* determines the selectivity of forming C₃H₆O or O₂ by distinct charge transfer steps. Overall, we elucidate the activate phase of PdPtO_x and determine the reaction mechanisms of epoxidation, leading to record epoxidation rates and FEs.