Thermodynamic Considerations for Optimizing Selective CO2 Reduction by Molecular Catalysts

Energetically efficient electrocatalysts with high product selectivity are desirable targets for sustainable chemical fuel generation using renewable electricity. Recycling CO2 by reduction to more energy dense products would support a carbon-neutral cycle that mitigates the intermittency of renewable energy sources. Conversion of CO2 to more saturated products typically requires proton equivalents. Complications with product selectivity stem from competitive reactions between H+ or CO2 at shared intermediates. We describe generalized catalytic cycles for H-2, CO, and HCO2 formation that are commonly proposed in inorganic molecular catalysts. Thermodynamic considerations and trends for the reactions of H+ or CO2 at key intermediates are outlined. A quantitative understanding of intermediate catalytic steps is key to designing systems that display high selectivity while promoting energetically efficient catalysis by minimizing the overall energy landscape. For CO2 reduction to CO, we describe how an enzymatic active site motif facilitates efficient and selective catalysis and highlight relevant examples from synthetic systems.