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 CO<sub>2</sub> reduction to renewable fuels is product selectivity. Desirable CO<sub>2</sub> reduction products require proton equivalents, but key catalytic intermediates in CO<sub>2</sub> reduction can also be competent for direct proton reduction to H<sub>2</sub>. Understanding how to manage divergent reaction pathways at these shared intermediates is essential to achieving high selectivity. Both proton reduction to hydrogen and CO<sub>2</sub> reduction to formate generally proceed through a metal hydride intermediate. We apply thermodynamic relationships that describe the reactivity of metal hydrides with H+ and CO<sub>2</sub> to generate a modified Pourbaix diagram which outlines product favorability as a function of pro-ton activity and hydricity (ΔG<sub>H-</sub>), 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)<sub>2</sub>](PF<sub>6</sub>)<sub>2</sub> as a potential catalyst because the corresponding hydride [HPt(dmpe)<sub>2</sub>]+ has the correct hydricity to access the region where selective CO2 reduction is possible. We validate our choice experimentally; [Pt(dmpe)<sub>2</sub>](PF6)<sub>2</sub> is a highly selective electrocatalyst for CO<sub>2</sub> 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 CO<sub>2</sub> reduction illustrates how our modified Pourbaix diagrams can guide selective and efficient catalyst discovery.