A series of twelve second coordination sphere (SCS) functionalized manganese tricarbonyl bipyridyl complexes are investigated for their electrocatalytic CO2reduction properties in acetonitrile. A qualitative and quantitative assessment of the SCS functional groups is discussed with respect to the catalysts’ thermodynamic and kinetic efficiencies, and their product selectivities. In probing a broad scope of functional groups, it is clear that only the aprotic ortho‐arylester SCS is capable of promoting the highly desired low‐overpotential proton‐transfer electron‐transfer (PT‐ET) pathway for selective CO production. The ortho‐phenolic analogues cause an increase in overpotential with a product selectivity favoring H2evolution, consistent with a high‐overpotential pathway via the anionic [Mn−H]−intermediate. Alternative aprotic Lewis base functional groups such as trifluoromethyl, morpholine and acetamide are shown to also be capable of intermediate manganese hydride generation. The tertiary amine substituent, 2‐morpholinophenyl, exhibits a desirable product distribution characteristic of syn‐gas (CO : H2=30 : 48) with an impressive turnover frequency, while the secondary amine group, 2‐acetamidophenyl, induces a notable shift in selectivity with a faradaic yield of 55 % for the formate (HCO2−) product. In addition to their catalytic properties, cyclic voltammetry and infrared spectroelectrochemistry (IR‐SEC) studies are presented to probe pre‐catalyst electronic properties and the two‐electron reduction activation pathway.
A critical challenge in electrocatalytic CO2reduction to renewable fuels is product selectivity. Desirable products of CO2reduction require proton equivalents, but key catalytic intermediates 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 CO2reduction to formate generally proceed through a metal hydride intermediate. We apply thermodynamic relationships that describe the reactivity of metal hydrides with H+and CO2to generate a thermodynamic product diagram, which outlines the free energy of product formation as a function of proton activity and hydricity (∆GH−), or hydride donor strength. The diagram outlines a region of metal hydricity and proton activity in which CO2reduction is favorable and H+reduction is suppressed. We apply our diagram to inform our selection of [Pt(dmpe)2](PF6)2as a potential catalyst, because the corresponding hydride [HPt(dmpe)2]+has the correct hydricity to access the region where selective CO2reduction is possible. We validate our choice experimentally; [Pt(dmpe)2](PF6)2is a highly selective electrocatalyst for CO2reduction to formate (>90% Faradaic efficiency) at an overpotential of less than 100 mV in acetonitrile with no evidence of catalyst degradation after electrolysis. Our report of a selective catalyst for CO2reduction illustrates how our thermodynamic diagrams can guide selective and efficient catalyst discovery.
more » « less- NSF-PAR ID:
- 10080025
- Publisher / Repository:
- Proceedings of the National Academy of Sciences
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 115
- Issue:
- 50
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- p. 12686-12691
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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