(669c) First Principles Study of Low Temperature Electrocatalytic Propane Dehydrogenation to Propylene

Growing demand for propylene, coupled with rising availability of propane due to an increase in shale gas production, has motivated numerous investigations of the propane dehydrogenation (PDH) reaction in recent years. PDH is traditionally performed using thermal catalysis on platinum and platinum-based alloys at temperatures of 550-650 °C, requiring significant input of energy. Electrocatalytic production of propylene in voltage-driven reactors, in contrast, could be performed near room temperature with significantly less energy input. However, electrocatalytic processes are not well understood at the molecular level, and there is a compelling need for fundamental studies to provide this crucial information.

In this study, we use periodic Density Functional Theory (DFT) calculations to elucidate the mechanism of the PDH reaction in an aqueous electrochemical environment. Inspired by thermal PDH catalysts, we begin our analysis on a platinum, which is known to be stable under voltages relevant for electrochemical PDH (around ~0.3 V vs. SHE). We analyze a mechanism proceeding via standard, homolytic C-H bond breaking on the catalyst surface, followed by electrochemical desorption of the adsorbed atomic hydrogen via proton-coupled electron transfer, and ultimately leading to propylene or deeper dehydrogenation products. The energetic parameters associated with the corresponding elementary steps are obtained from DFT, and the voltage dependence of the proton-electron formation steps is determined using Nernstian thermodynamics. Initial C—H bond activation is found to be the likely rate-limiting step (Figure 1). Microkinetic analysis of the energetics reveals that adsorbed hydrogen competes with propane adsorption on the catalyst below 0.3 V, resulting in low PDH rates. As the hydrogen coverage decreases at ~0.3V, however, the PDH rate increases and plateaus, as observed by our experimental collaborators. The results suggest that PDH can be realized on platinum catalysts in electrochemical environments and form a basis for future studies on Pt alloy catalysts.