Directing the Reactivity of Metal Hydrides for Selective CO2 Reduction

2018-05-01T19:35:35Z (GMT) by Bianca M. Ceballos Jenny Yang
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 (ΔGH-), 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.