Activity and selectivity of heterogeneous catalysts are often inversely correlated with one another. One reason for this phenomenon is that the reaction energetics of the activity- and selectivity-determining steps of catalytic processes often scale differently with surface reactivity. The semi-hydrogenation of acetylene to ethylene is a simple yet ubiquitous reaction, for its role in purifying polymer grade ethylene streams, suitable as a template reaction for the study of this phenomenon. Earth abundant Cu-based catalysts exhibit superior ethylene selectivity over conventional Pd catalysts due to their near thermoneutral ethylene adsorption energy. Unfortunately, Cu-based catalysts also exhibit low acetylene hydrogenation activity due to their sluggish H2 activation kinetics. However, this tradeoff can be circumvented by performing acetylene semi-hydrogenation in an electrochemical hydrogen pump. Electrochemical hydrogen pumps are an emerging technology for H2 purification that operate by oxidizing H2 to H+ over the anode and transporting the H+ through a proton-exchange membrane to the cathode, where they are reduced back into H2. These devices can be utilized to perform acetylene semi-hydrogenation by feeding the acetylene stream directly into the cathode chamber. This configuration enables acetylene semi-hydrogenation to be performed over Cu-based catalysts without the kinetic burden of H2 activation, significantly enhancing intrinsic activity and breaking the activity-selectivity tradeoff. We demonstrate this concept by performing acetylene semi-hydrogenation over Pt and Cu in a reactor setup that enables acetylene semi-hydrogenation to be performed both thermocatalytically and thermo-electrocatalytically at 80 °C. We demonstrate that the mode of operation does not impact the intrinsic activity of metals that can rapidly activate H2, such as Pt. However, the acetylene semi-hydrogenation activity of Cu is enhanced by nearly three orders of magnitude when performed thermo-electrocatalytically. H2/D2 scrambling experiments confirm that the kinetic limitations typically exhibited by Cu arise due to sluggish H2 activation and are further supported by density functional theory calculations and microkinetic modelling. Thus, this presentation demonstrates that hybrid thermo-electrocatalytic reactors exhibit significant advantages over either individual approach.
