Challenges in Product Selectivity for Electrocatalytic Reduction of Amine-Captured CO2: Implications for Reactive Carbon Capture.

Abstract

CO₂ is a potential feedstock for carbon-based fuels or materials, but is only available in dilute streams. Integrated processes for CO₂ capture and conversion directly valorize the CO₂ captured by sorbent materials, skipping the energetically expensive sorbent regeneration step. Amines are the most heavily studied liquid-phase sorbent materials for CO₂ capture from dilute streams. Amines react with CO₂ in a 2:1 ratio to form the corresponding ammonium carbamate. Ammonium carbamate [NH₄]-[H₂NCO₂] was tested as the substrate using the highly selective and robust CO₂-to-formate reduction electrocatalyst [(^tBuPOCOP)-Ir-(H)-(NCCH₃)₂], where (^tBuPOCOP) is the tridentate pincer ligand 2,6-bis-(ditert-butyl-phosphonito). When ammonium carbamate was used as the substrate instead of CO₂, only hydrogen was produced. An equivalent electrolysis with ammonium hexafluorophosphate with CO₂ also resulted in primarily hydrogen. Methyl carbamate and urea were also tested as substrates as proxies for carbamate that do not contain an equivalent of ammonium, and there was also negligible reduction to carbon-based products. These results indicate that the loss of selectivity observed for amine-captured CO₂, or ammonium carbamate, is likely due to the generation of the acidic ammonium equivalent as well as the greater challenge of reducing carbamate compared to CO₂. This study illustrates that catalysts with high selectivity for concentrated CO₂ can favor hydrogen evolution and loss of carbon-based products when amine-captured CO₂ is used instead.