Efficient and selective molecular catalyst for the CO ₂ -to-CO electrochemical conversion in water

Abstract

Substitution of the four paraphenyl hydrogens of iron tetraphenylporphyrin by trimethylammonio groups provides a watersoluble molecule able to catalyze the electrochemical conversion of carbon dioxide into carbon monoxide. The reaction, performed in pH-neutral water, forms quasi-exclusively carbon monoxide with very little production of hydrogen, despite partial equilibration of CO 2 with carbonic acid-a low pK a acid. This selective molecular catalyst is endowed with a good stability and a high turnover frequency. On this basis, prescribed composition of CO-H 2 mixtures can be obtained by adjusting the pH of the solution, optionally adding an electroinactive buffer. The development of these strategies will be greatly facilitated by the fact that one operates in water. The same applies for the association of the cathode compartment with a proton-producing anode by means of a suitable separator.CO 2 -to-CO conversion | contemporary energy challenges | electrochemistry | catalysis | solar fuels O ne of the most important issues of contemporary energy and environmental challenges consists of reducing carbon dioxide into fuels by means of sunlight (1-3). One route toward this ultimate goal is to first convert solar energy into electricity, which will then be used to reduce CO 2 electrochemically. Direct electrochemical injection of an electron into the CO 2 molecule, forming the corresponding anion radical CO 2 .− requires a very high energy [the standard potential of the CO 2 / CO 2 .− couple is indeed −1.97 V vs. normal hydrogen electrode (NHE) in N,N′dimethylformamide (DMF)] (4, 5). Electrochemical conversion of CO 2 to any reaction product thus requires catalytic schemes that preferably avoid this intermediate. Carbon monoxide may be an interesting step en route to the desired fuels because it can be used as feedstock for the synthesis of alkanes through the classic Fischer-Tropsch process. A number of molecular catalysts for the homogeneous electrochemical CO 2 -to-CO conversion have been proposed. They mainly derive from transition metal complexes by electrochemical generation of an appropriately reduced state, which is restored by the catalytic reaction. So far, nonaqueous aprotic solvents (mostly DMF and acetonitrile) have been used for this purpose (5-16). Brönsted acids have been shown to boost catalysis. However, they should not be too strong, at the risk of leading to H 2 formation at the expense of the CO. Trifluoroethanol and water (possibly in large amounts) have typically played the role of a weak acid in the purpose of boosting catalysis while avoiding hydrogen evolution.One of the most thoroughly investigated families of transitionmetal complex catalysts of CO 2 -to-CO conversion is that of iron porphyrins brought electrochemically to the oxidation degree 0. The importance of coupling electron transfer and introduction of CO 2 into the coordination sphere of iron with proton transfers required by the formation of CO, CO 2 + 2e − + 2AH ↔ CO + H 2 O + 2A − , appeared from the very beginning of these ...

Recent Advances in Breaking Scaling Relations for Effective Electrochemical Conversion of CO2

“…Furthermore, suitable functional groups can be introduced to the porphyrin rings to adjust the redox potential and pKa. [73] It is important to note that Shen et al also found the production of small amounts of CH 4 and CH 3 OH originating from CO further reduction [69] and they claimed that the CO further reduction conformed to coupled proton-electron transfer. [72] …”

A Hybrid Co Quaterpyridine Complex/Carbon Nanotube Catalytic Material for CO2 Reduction in Water

“…[4,5] These catalysts are mainly active in aprotic solvent with reaction products being mostly CO and more rarely HCOOH. [9,10] In that case,C O production was achieved with over 90 %selectivity at neutral pH and am oderate 450 mV overpotential. Ni cyclam was first reported as as elective and efficient catalyst of the CO 2 -to-CO conversion in water at am ercury surface electrode,with favorable specific interactions of the catalytic adsorbed species with the surface, [6,7] while the use of acarbon electrode leads to much less efficiencyi nt erms of rate (maximal turnover frequency of 90 s À1 )a nd turnover (4 cycles).…”

NiII Complexes of C‐Substituted Cyclam as Efficient Catalysts for Reduction of CO2 to CO

“…Hence, CO 2 reduction/activation by 3 d metal‐based catalysts has received intense interest. Among noteworthy examples of homogeneous catalysts are Fe‐tetraphenylporphyrin and derivatives by Savéant and co‐workers, selective reduction of CO 2 to formate using [Fe 4 N(CO) 12 ] – by Berben and co‐workers, and reduction of CO 2 to methanol by borane with Ni and Fe pincer complexes by Guan and co‐workers . Li and co‐workers reported photo‐reduction of CO 2 to CO catalyzed by molecular Co complexes immobilized on solid support .…”