Uncovering Cooperative Redox Enhancement Effects in Bimetallic Catalysis

ConspectusElectro- and thermo-catalysis are frequently considered as disparate fields of research. The former is critical for the future electrified green chemical industry where renewable electrical energy will power production in place of fossil-derived sources; however, the necessary scale-up from the laboratory to commodity chemical production is in its infancy. In contrast, thermo-catalysis is at the heart of the modern chemical industry with the associated capital investments in infrastructure and technology. It is, however, far more energy intensive and typically requires the use of elevated temperatures and pressures to obtain economically viable productivity. Our recent discovery of a new approach to catalyst design, termed Cooperative Redox Enhancement (CORE), bridges the gap between these traditionally distinct fields. In this Account, we outline how CORE can facilitate a unifying approach to these fields and describe how electrochemical methods can provide detailed thermochemical mechanistic information.Industrial heterogeneous catalysts often comprise supported precious metal nanoparticles with alloys frequently providing superior performance over monometallic counterparts. However, we have found that spatial separation of the two metals on an electronically conductive support leads to substantial enhancement in activity through electrochemical coupling. This CORE effect demonstrates that thermochemical redox reactions can, and often do, operate like nanoscale electrochemical fuel cells. The electrochemical coupling in systems containing at least two discrete active sites accelerates both half reactions in a mechanism analogous to galvanic coupling in corrosion science. CORE demonstrates that leveraging electrocatalytic approaches is a key tool for the development of the next generation of thermochemical catalysts and vice versa.Here, our primary aim is to provide a critical overview of CORE effects that are exhibited in thermocatalytic redox reactions over bimetallic catalysts. We will provide a chronological timeline of the research in this area that led to this discovery. This will include comparing CORE to other effects which are commonly exhibited by bimetallic catalysts, e.g., the synergistic electronic and geometric effects observed through the formation of nanoalloys. We will provide a detailed overview of CORE, how it can be studied, and how thermochemical enhancements can be predicted by utilizing electrochemical methods. Specifically, we will discuss the importance of using linear sweep voltammetry, Tafel analysis, and mixed potential theory to acquire a host of new electrochemical terms that we have defined, such as <i>E</i>^CORE (operating mixed potential for bimetallic catalysts) and <i>j</i>^CORE (operating current density for bimetallic catalysts), which underpin the electrochemical study of CORE. Primarily, the discussion will be centered on the CORE effects observed in coupled systems that involve dehydrogenation and oxygen reduction, as this is the primary model system we have studied to date. However, we also include examples of how CORE has relevance in other redox reactions, demonstrating the generality of the effect. Finally, we provide a short perspective on the future directions of this field and the impact that can be expected on catalysis over the coming decade.