Electrochemical Mechanisms in Thermochemical Catalysis

Abstract

Electrochemistry and thermochemical catalysis have historically been viewed as disparate fields with distinct mechanistic paradigms. This divide, along with an inadequate experimental and methodological electrochemical toolkit, has impeded understanding of electrochemical mechanisms in thermochemical catalysis. This dissertation applies both legacy and newly-developed electrochemical approaches to assess the role of coupled half-reactions in three different thermochemical catalytic reactions. We first examine the hydrogenation of nitrate on bimetallic PdCu/C catalysts (Chapter 2) and determine that the reaction proceeds via the coupling of electrochemical hydrogen oxidation on Pd and nitrate reduction on Cu. This understanding is employed to optimize catalyst formulation, determine new bimetallic catalyst pairings with nitrate hydrogenation activity, and improve activity in the state-of-the-art catalyst. In Chapter 3, this mechanistic framework is extended to the Pd-catalyzed vinyl acetate synthesis. We use a combination of liquid-phase and gas-phase electrochemical measurements, with the latter being a new tool developed in our laboratory, to investigate the reaction. This study reveals the bifunctionality of homogeneous Pd(II) species and heterogeneous Pd/C in facilitating the overall oxidation, revising the legacy mechanism for this industrial catalytic reaction. Building on the mechanistic analysis and electrochemical probes developed in Chapters 2 and 3, we investigate the synthesis of ammonia (Chapter 4). Our study challenges the historical mechanism for the reaction, revealing the essential role of interfacial charge transfer in activating and reducing dinitrogen with driving force derived from the coupling of electrochemical half-reactions. Taken together, these studies unravel the role of galvanic coupling in redox thermochemical catalysis, and serve as a call to action for the application of electrochemical methods in mechanistic investigations of catalysis.