Electrochemical hydrogenation of cis,cis-muconic acid: Effects of surface structure on reactivity and selectivity

cis,cis-Muconic acid (ccMA) has been identified as a candidate platform intermediate for the production of sustainably-sourced commodity and specialty monomers. Readily obtained by fermentation of sugars and/or lignin, ccMA can be completely (to adipic acid, AA) or partially (to monounsaturated hexenedioic acids, HDA) hydrogenated by varying the catalyst or electrochemical environment. Electrochemical pathways to these species are particularly attractive due to their ability to curb emissions associated with current fossil carbon-derived routes to adipic acid. Our recent work has demonstrated appreciable production of AA via electrochemical hydrogenation (ECH) on supported Pd/C nanoparticles.

In this presentation, we will share insights obtained from density functional theory (DFT) calculations into the detailed role of the local surface environment in controlling activity and selectivity of ccMA ECH. By performing calculations on terrace and stepped surface models of Pd and Pt, our results suggest a dual mechanism by which early hydrogenation steps occur primarily as outer-sphere processes, whereas later steps are surface-mediated. We further show that the presence of subsurface hydrogen, which may form readily on Pd surfaces, plays a crucial and structure-dependent role in modifying activation energies. Our computational results are coupled with detailed electrochemical experiments to identify that an intermediate binding strength on Pd terraces is a critical factor influencing the activation energies of hydrogenation processes, suggesting structure-sensitive design strategies maximizing exposure of terrace atoms for further improvement of ccMA ECH.