Bioinspired catalytic CO2 reduction – CarBioRed

The power of molecular inorganic chemistry or coordination chemistry is to provide tailor made catalysts by changing the nature of the metallic center (CO2 binding site) and the nature of the other molecules (ligands) linked to it to tune the final electronic properties. At the end a fine control on the reactivity of coordination complex is achieved. Electrochemical tools have been largely used in this project to assess the potential of given catalysts and eventually their performances. The analytical tools essential to the characterization and quantification of the products of CO2 reduction have been implemented. Finally mechanistic studies have been performed and relevant catalytic pathways proposed. Improving the performances of the catalyst thus relies on a dynamic feedback loop between synthetic chemistry, analytical chemistry and theoretical chemistry.

In the course of CarBioRed, many coordination complexes have been tested for their electro-assisted catalytic properties toward CO2 reduction, mostly for the production of CO or formic acid. One highlight is the synthesis of pyranopterin-dithiolene ligands which mimick the ligand naturally present in Formate Dehydrogénase. This approach is unprecedented and thus original compared to those commonly described in the literature. A second highlight is the elaboration of cobalt complexes highly selective for the reduction of CO2 into formic acid. Furthermore, a cobalt-substituted polyoxometalate was found to favor the formation of formaldehyde, a reduction product seldom produced.

Promising electrocatalysts have been prepared in the frame of CarBioRed, especially metal-bisdithiolene complexes mimicking formate dehydrogenases and cobalt-diphosphine complexes with pendant amines as proton relays. These will be further investigated. Furthermore, the accumulation of electrons and protons needed for the multi-electron multi-proton reduction of CO2 is rarely addressed and the potential of polyoxometalates as electron/proton shuttles deserve to be tested. In a last step, immobilization of the most efficient catalysts onto photocathodes and integration in a photoelectrochemical cell will be highly valuable.

The results obtained along the CarBioRed project have given 13 publications, 9 of which involving several partners, which is remarkable and underlined the complementarity and collaboration network within the consortium.

The use of carbon dioxide as a feedstock for fuel production is a highly desirable goal, both in the context of finding alternatives to fossil fuels and to limit the greenhouse effect. Although the direct hydrogenation of CO2 to methanol or methane is a possibility, the electro- and photo-catalytic reduction processes remain the focus of much interest. These reactions are difficult to control as they do not only involve the transfer of several electrons, but are often coupled to protonation. Resulting competitive pathways are thus commonly observed, leading to various products. CO or formate are often formed, though with poor energetic efficiency, and are of interest for Syngas or Formic Acid economies. By contrast alcohols (methanol and ethanol) or hydrocarbons are only formed in low amounts.
CarBioRed aims at developing efficient and selective, non-noble metal based molecular electrocatalysts for carbon dioxide reduction, and at gaining some insights into their reaction mechanisms. By contrast with previous studies, a bioinspired approach will be followed for selecting the metals and ligands as part of new catalysts.
Based on the knowledge of the structure and reactivity of metalloenzymes dealing with CO2 (CO Dehydrogenase, Formate Dehydrogenase), synthetic molecular mimics of the active sites will be synthesised: mononuclear complexes based on Mo, W with sulphur rich ligands mimicking the natural pterin-dithiolene cofactor and dinuclear Ni-M (Fe,Ru,Mn) clusters with sulphur rich coordination spheres (Task 1). In parallel, we will also use polyoxometalates (POMs), whose structural analogies with the coordination sites found in some metalloenzymes have been underlined, as new type of multidentate and all-inorganic ligands (Task 2). Transition metal substituted POMs (TMSPs) as inorganic Synzymes could provide multifaceted electrocatalysts for CO2 reduction, at least as electron and proton reservoirs.
Our consortium will implement the analytical and electrochemical methodologies to assess the performances of our complexes in electroassisted CO2 reduction, in particular in terms of overpotential requirement, turnover frequency, stability and selectivity (Task 3). A particular attention will be paid to the adjustment of the distinct electrolysis conditions (solvent, nature of the electrodes, proton sources…) in order to improve their activity. Simultaneously, a particular focus will be set on the elucidation of the catalytic mechanisms, through the investigation of likely intermediates by in-situ solution spectroelectrochemical studies and a back and forth interplay with theoretical studies (Tasks 3 & 4). This should in return help to synthetically tune the properties of the catalyst, in terms of both selectivity and performance, a clear asset of the molecular chemistry approach.
The consortium set up for this project will finally allow to go one step further towards valorisation by the development of modified electrodes and electrolysis cells (Task 5), a step rarely tackled in other molecular studies. Analysis of the performances of these supported systems will benefit from the expertise acquired from homogeneous studies (Task 3).