While the current state of understanding of electrocatalytic CO2 reduction with molecular transition-metal-based catalysts has progressed significantly in recent years, many unanswered questions remain and critical challenges lie ahead. Acquiring an advanced mechanistic understanding of metastable reaction intermediates in the CO2 reduction cycle will allow us to manipulate these species under non-equilibrium conditions to promote a desired outcome and develop new processes and materials to secure a clean energy future. To this end, we are making use of a new technique that we recently developed, namely pulse radiolysis combined with nanosecond time-resolved infrared detection (PR-TRIR). Pulse radiolysis is a powerful method for rapidly oxidizing or reducing dissolved species, such as catalysts. Taking advantage of the structural specificity of TRIR spectroscopy, we have significantly improved the characterization of transient species produced by pulse radiolysis, and gained an understanding of energy-related processes that would have been difficult without TRIR detection. Here, we present our recent work in this area, focusing on mechanistic studies of new and existing Mn- and Re-based CO2 reduction electrocatalysts. For example, we show that we can turn on the previously elusive “protonation-first” CO2 reduction pathway at reduced overpotential by the incorporation of Brønsted-basic methoxy groups into the second coordination sphere of a new Mn catalyst, [fac-Mn({(MeO)2Ph}2bpy)(CO)3(CH3CN)](OTf). We have used PR-TRIR to monitor the reactivity of this and related catalysts with bulky bipyridyl ligands upon one-electron reduction, discovering a mechanistic pathway to the two-electron reduced active catalytic species. We are also characterizing other intermediates deeper into the catalytic cycle of CO2 reduction through the use of starting materials that can be used to enter the cycle at different points. The work at BNL and use of the facilities of the BNL Accelerator Center for Energy Research were supported by the US-DOE Office of Science, Division of Chemical Sciences, Geosciences and Biosciences under contract DE-SC0012704.

Mechanistic details of CO2 reduction processes revealed by pulse radiolysis with time-resolved infrared detection

253rd National Meeting (2017)

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