Using a One-Electron Shuttle for the Multielectron Reduction of CO₂ to Methanol: Kinetic, Mechanistic, and Structural Insights

Abstract

Pyridinium and its substituted derivatives are effective and stable homogeneous electrocatalysts for the aqueous multiple-electron, multiple-proton reduction of carbon dioxide to products such as formic acid, formaldehyde, and methanol. Importantly, high faradaic yields for methanol have been observed in both electrochemical and photoelectrochemical systems at low reaction overpotentials. Herein, we report the detailed mechanism of pyridinium-catalyzed CO(2) reduction to methanol. At metal electrodes, formic acid and formaldehyde were observed to be intermediate products along the pathway to the 6e(-)-reduced product of methanol, with the pyridinium radical playing a role in the reduction of both intermediate products. It has previously been thought that metal-derived multielectron transfer was necessary to achieve highly reduced products such as methanol. Surprisingly, this simple organic molecule is found to be capable of reducing many different chemical species en route to methanol through six sequential electron transfers instead of metal-based multielectron transfer. We show evidence for the mechanism of the reduction proceeding through various coordinative interactions between the pyridinium radical and carbon dioxide, formaldehyde, and related species. This suggests an inner-sphere-type electron transfer from the pyridinium radical to the substrate for various mechanistic steps where the pyridinium radical covalently binds to intermediates and radical species. These mechanistic insights should aid the development of more efficient and selective catalysts for the reduction of carbon dioxide to the desired products.

Organic reactions for the electrochemical and photochemical production of chemical fuels from CO2 – The reduction chemistry of carboxylic acids and derivatives as bent CO2 surrogates

“…Such organic-mediated reactivity with regards to the conversion of CO 2 to chemical fuels has been reviewed by Hu and coworkers [5]. The report describes the use of tetraalkylammonium salts [6], ionic liquids [7], and pyridinium derivatives [8] that have been shown to act as mediators in the selective (photo-) electrochemical reduction of CO 2 to products such as CO and methanol.…”

Organic, Organometallic and Bioorganic Catalysts for Electrochemical Reduction of CO2

“…In this study56 they concluded that pyridinium is reduced on a Pt electrode, including a pyridinium‐bound proton to form a surface hydride which was further supported by the work of Batista 57. Indeed, the study from Bocarsly and co‐workers55 created more controversy in the field and drove several studies 58, 59, 60, 61. Another study was conducted by Savéant and co‐workers arguing the plausibility of this catalytic process 62.…”

“…They also clarified that formic acid and formaldehyde are the intermediate or side products formed during the electrochemical reduction of CO 2 to methanol. That reaction requires a proton‐assisted six‐electron reduction with a formal potential of −380 mV (vs. NHE) 55. According to their findings the mechanism involves multiple single‐electron transfers.…”

“…Later in 2010, Bocarsly and his team gave mechanistic insights into how the electrochemical reduction of CO 2 to methanol proceeds 55. They also clarified that formic acid and formaldehyde are the intermediate or side products formed during the electrochemical reduction of CO 2 to methanol.…”

“…The authors argue that the process apparently works only on Pt and/or Pd, metals that are known to reduce hydrated protons 63. Their counterargument further continues in two main aspects: challenging the fact that the faradaic efficiency was found to be 20 %,53, 55 while 100 % of the electrochemical characteristics from cyclic voltammetry were attributed to the process. Second, the authors find confusing the lack of a typical catalytic current enhancement for which the replacement of the peak by a plateau is accompanied by a large increase.…”