Transforming carbon dioxide (CO₂) into higher value-added chemicals, such as fuels, is a major challenge in green chemistry. The CO molecule2 being very stable, this transformation requires energy and the development of efficient and inexpensive catalysts. Several electrochemical processes have thus been developed in recent years, which use copper-based electrodes, which serve as a catalyst, or carbon-based electrodes covered with molecular catalysts*. Until now, the latter have mainly succeeded in producing simple compounds such as carbon monoxide (CO) or formate (HCOO–). Obtaining more complex products involving carbon-carbon bonds still remains difficult.
If copper-based catalysts succeed, they often generate a mixture of molecules with two or more carbons, typically ethanol but also ethylene and acetate, with little control over selectivity. From the perspective of a circular carbon economy, it would obviously be preferable to produce ethanol directly, without having to add purification and product separation steps. An international team of scientists recently developed a new catalytic system that overcomes these obstacles using an organometallic iron complex, iron tetraphenylporphyrin or Fe-TPP, deposited on a nickel support.
Dutch, American, Canadian and French researchers (at the Paris Institute of Molecular Chemistry – CNRS/Sorbonne University) used an innovative approach which consists of hanging the molecular catalyst, and iron complex, on a nickel foam. This strategy, which abandons traditional carbonaceous supports to use metallic electrodes despite their potential for competing electrochemical reactions, allowed them to obtain an almost total conversion of CO2 in ethanol. Remarkably, the nickel-Fe-TPP electrode provides high ethanol yields at low potentials without producing unwanted byproducts like acetate or methane, outperforming all non-copper-based systems. And this efficiency persists over many hours of use.
This advance, published in Nature Catalysischallenges long-held assumptions about molecular catalysts. Above all, it offers new electrochemical routes to convert CO₂ and CO into alcohols such as ethanol, but also methanol or propanol. The ethanol produced can easily be stored and used as a renewable fuel, providing a sustainable alternative to fossil resources and reducing dependence on biomass and water-intensive bioethanol production. And if the loop was soon closed for the CO2 ?
*Unlike traditional solid or metal catalysts, molecular catalysts are often organometallic complexes, where a central metal atom is surrounded by organic ligands that modify its chemical properties.
Editor: AVR
