Using Enzymes to Understand and Control the Local Environment of Catalysis

Abstract

Local environments within porous electrodes are an inherent, but often neglected component of catalysis as the local conversion of reactants to products means catalysis occurs in a very different environment to bulk solution. By understanding and modifying these local environments using a combination of experimental and computational techniques, we show how to improve the performance of electrocatalytic reactions to address the climate crisis by efficiently converting renewable energy to chemical fuels. The selectivity and activity of enzymes means they are ideal model catalysts that can guide the design of synthetic systems. However, they must be in an environment that is close to their optimal to operate efficiently, with small changes in properties such as pH drastically affecting their activity. By optimising their local environment, the rates of fuel formation can be drastically (>18×) increased.[1] We also demonstrate the crucial role of CO2 hydration kinetics on the local pH and CO2 concentration using the enzyme Carbonic Anhydrase co-immobilised with Formate Dehydrogenase.[2] Carbonic Anhydrase catalyses CO2 hydration, causing CO2 to act as a better buffer to mitigate changes in the local pH environment allowing the system to operate closer to its optimal and how this contrasts with heterogeneous CO2 reduction. (fig. 1a)We extend this approach to low CO2 concentrations, taking inspiration from the natural carboxysome to develop a system where Formate Dehydrogenase and Carbonic Anhydrase are co-immobilised in a nanoconfined structure to improve low CO2 concentration utilisation. (fig. 1b).[3] The electrolysis of dilute CO2 streams suffers from low concentrations of dissolved substrate and its rapid depletion at the electrolyte-electrocatalyst interface. These limitations require first energy-intensive CO2 capture and concentration, before electrolyzers can achieve acceptable performances. For direct electrocatalytic CO2 reduction from low-concentration sources, we introduce a strategy that mimics the carboxysome in cyanobacteria by utilizing microcompartments with nanoconfined enzymes in a porous electrode. Carbonic Anhydrase accelerates CO2 hydration kinetics and minimizes substrate depletion by making all dissolved carbon available for utilization, while a highly efficient formate dehydrogenase reduces CO2 cleanly to formate; down to even atmospheric concentrations of CO2. This bio-inspired concept demonstrates that the carboxysome provides a viable blueprint for the reduction of low-concentration CO2 streams to chemicals by using all forms of dissolved carbon.