Organic molecules as mediators and catalysts for photocatalytic and electrocatalytic CO₂reduction

Abstract

Reduction of CO 2 by electrochemical and photoelectrochemical methods to produce carbon-rich fuels is a heavily pursued research theme. Most of the current efforts are focused on the development of transition-metal-based catalysts. In this tutorial review, we present an overview of the development of organic molecules as mediators and catalysts for CO 2 reduction. Four classes of organic molecules are discussed: tetraalkylammonium salts, aromatic esters and nitriles, ionic liquids, and pyridinium derivatives. It is shown that reactions mediated or catalyzed by these organic molecules can be competitive compared to their metal-catalyzed counterparts, both in terms of product selectivity and energy efficiency.

Isotopic Probe Illuminates the Role of the Electrode Surface in Proton Coupled Hydride Transfer Electrochemical Reduction of Pyridinium on Pt(111)

“…This process has been documented on platinum disk microelectrodes, 34,35 suggesting that proton reduction from hydronium is the likely process in either direct hydronium or PyrH + reduction cases. Using the relationship in Equation 4 and approximating hydronium and PyrH + reduction as reversible over the scan rates chosen so that the formal potential, E H A/H 2 0 ∼ = E 1/2 , the E 1/2 values calculated for hydronium and PyrH + protons are estimated to be −0.42 V vs. SCE and −0.55 V vs. SCE, close to that observed experimentally.…”

“…In order to provide lower energy pathways to CO 2 reduction and pursue mechanistic understanding, solution based catalysts have been employed. 2,4,5 Pyridinium (PyrH + ) catalyzes the electrochemical reduction of CO 2 to methanol and formic acid at a platinum (Pt) electrode at low overpotentials (ca. −600 mV vs. SCE).…”

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

“…Organic molecules regenerated at electrode surfaces have been shown to perform energy-relevant redox chemical transformations [14,3,5]. Such molecules acting as either ligands or additives have also been involved in metal/organic fragment cooperativity in electrocatalytic dehydrogenation reactions [15,16].…”

“…These transformations are reminiscent of reactions in nature, where organic cofactors and coenzymes often mediate biological pathways [4]. 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.…”

Visible-Light Photoredox Catalysis: Selective Reduction of Carbon Dioxide to Carbon Monoxide by a Nickel N-Heterocyclic Carbene–Isoquinoline Complex

“…With regard to selectivity, it is critical to minimize the competitive reduction of water to hydrogen that is typically kinetically favored over CO 2 reduction, as well as selectively convert CO 2 to one carbon product. 22,23 Another primary consideration is the use of visible-light excitation, which more effectively harvests the solar spectrum and avoids deleterious high-energy photochemical pathways. Semiconductors such as TiO 2 and SiC have been widely employed as heterogeneous catalysts for photochemical and photoelectrochemical conversion of CO 2 to a variety of carbon products such as carbon monoxide, methanol, and methane.…”