Directing the Reactivity of Metal Hydrides for Selective CO2 Reduction

A critical challenge in electrocatalytic CO₂ reduction to renewable fuels is product selectivity. Desirable CO₂ reduction products require proton equivalents, but key catalytic intermediates in CO₂ reduction can also be competent for direct proton reduction to H₂. Understanding how to manage divergent reaction pathways at these shared intermediates is essential to achieving high selectivity. Both proton reduction to hydrogen and CO₂ reduction to formate generally proceed through a metal hydride intermediate. We apply thermodynamic relationships that describe the reactivity of metal hydrides with H+ and CO₂ to generate a modified Pourbaix diagram which outlines product favorability as a function of pro-ton activity and hydricity (ΔG_H-), or hydride donor strength. The diagram outlines a region of metal hydricity and proton activity in which CO2 reduction is favorable and H+ reduction is suppressed. We apply our diagram to inform our selection of [Pt(dmpe)₂](PF₆)₂ as a potential catalyst because the corresponding hydride [HPt(dmpe)₂]+ has the correct hydricity to access the region where selective CO2 reduction is possible. We validate our choice experimentally; [Pt(dmpe)₂](PF6)₂ is a highly selective electrocatalyst for CO₂ reduction to formate (>90 % Faradaic efficiency) at an overpotential of less than 100 mV with no evidence of catalyst degradation after electrolysis. Our report of a new selective catalyst for CO₂ reduction illustrates how our modified Pourbaix diagrams can guide selective and efficient catalyst discovery.