Laboratoire de chimie des processus biologiques

The future of energy supply depends on innovative breakthroughs regarding the design of cheap, sustainable, and efficient systems for the conversion and storage of renewable energy sources, such as solar energy. The production of hydrogen, a fuel with remarkable properties, through sunlight-driven water splitting appears to be a promising and appealing solution. While the active sites of enzymes involved in the overall water-splitting process in natural systems, namely hydrogenases and photosystem II, use iron, nickel, and manganese ions, cobalt has emerged in the past five years as the most versatile non-noble metal for the development of synthetic H(2)- and O(2)-evolving catalysts. Such catalysts can be further coupled with photosensitizers to generate photocatalytic systems for light-induced hydrogen evolution from water.

Herein we discuss band gap modification of MIL-125, a TiO2/1,4-benzenedicarboxylate (bdc) metal-organic framework (MOF). Through a combination of synthesis and computation, we elucidated the electronic structure of MIL-125 with aminated linkers. The band gap decrease observed when the monoaminated bdc-NH2 linker was used arises from donation of the N 2p electrons to the aromatic linking unit, resulting in a red-shifted band above the valence-band edge of MIL-125. We further explored in silico MIL-125 with the diaminated linker bdc-(NH2)2 and other functional groups (-OH, -CH3, -Cl) as alternative substitutions to control the optical response. The bdc-(NH2)2 linking unit was predicted to lower the band gap of MIL-125 to 1.28 eV, and this was confirmed through the targeted synthesis of the bdc-(NH2)2-based MIL-125. This study illustrates the possibility of tuning the optical response of MOFs through rational functionalization of the linking unit, and the strength of combined synthetic/computational approaches for targeting functionalized hybrid materials.

Abstract It is generally believed that CO 2 electroreduction to multi‐carbon products such as ethanol or ethylene may be catalyzed with significant yield only on metallic copper surfaces, implying large ensembles of copper atoms. Here, we report on an inexpensive Cu‐N‐C material prepared via a simple pyrolytic route that exclusively feature single copper atoms with a CuN 4 coordination environment, atomically dispersed in a nitrogen‐doped conductive carbon matrix. This material achieves aqueous CO 2 electroreduction to ethanol at a Faradaic yield of 55 % under optimized conditions (electrolyte: 0.1 m CsHCO 3 , potential: −1.2 V vs. RHE and gas‐phase recycling set up), as well as CO electroreduction to C 2 ‐products (ethanol and ethylene) with a Faradaic yield of 80 %. During electrolysis the isolated sites transiently convert into metallic copper nanoparticles, as shown by operando XAS analysis, which are likely to be the catalytically active species. Remarkably, this process is reversible and the initial material is recovered intact after electrolysis.

In the recent years, significant progress has been made toward designing active and selective catalysts for electrochemical CO2 reduction, with particular interest focused on the two major C2 products—ethylene and ethanol. Numerous efforts have been made to enhance the understanding of the heterogeneous copper-based CO2 reduction mechanisms by computational studies. Here we provide a critical assessment of various proposed scenarios of the initial and post C–C coupling steps that result in either ethylene or ethanol. In silico rationalization of the parameters controlling the product selectivity, such as the catalyst structure and composition (Cu facets, the presence of defective sites and/or subsurface oxygen atoms, or the interplay with a second metal) and the reaction conditions (pH, applied potential, and electrolyte), is provided. A comprehensive scheme combining the proposed pathways is derived, and the issues that are still under debate and require further investigations are highlighted.

Polypyridyl transition metal complexes represent one of the more thoroughly studied classes of molecular catalysts towards CO₂ reduction to date.

Catalysis is a key enabling technology for solar fuel generation. A number of catalytic systems, either molecular/homogeneous or solid/heterogeneous, have been developed during the last few decades for both the reductive and oxidative multi-electron reactions required for fuel production from water or CO(2) as renewable raw materials. While allowing for a fine tuning of the catalytic properties through ligand design, molecular approaches are frequently criticized because of the inherent fragility of the resulting catalysts, when exposed to extreme redox potentials. In a number of cases, it has been clearly established that the true catalytic species is heterogeneous in nature, arising from the transformation of the initial molecular species, which should rather be considered as a pre-catalyst. Whether such a situation is general or not is a matter of debate in the community. In this review, covering water oxidation and reduction catalysts, involving noble and non-noble metal ions, we limit our discussion to the cases in which this issue has been directly and properly addressed as well as those requiring more confirmation. The methodologies proposed for discriminating homogeneous and heterogeneous catalysis are inspired in part by those previously discussed by Finke in the case of homogeneous hydrogenation reaction in organometallic chemistry [J. A. Widegren and R. G. Finke, J. Mol. Catal. A, 2003, 198, 317-341].

Selective electrochemical reduction of CO2 into energy-dense organic compounds is a promising strategy for using CO2 as a carbon source. Herein, we investigate a series of iron-based catalysts synthesized by pyrolysis of Fe-, N-, and C-containing precursors for the electroreduction of CO2 to CO under aqueous conditions and demonstrate that the selectivity of these materials for CO2 reduction over proton reduction is governed by the ratio of isolated FeN4 sites vs Fe-based nanoparticles. This ratio can be synthetically tuned to generate electrocatalysts producing controlled CO/H2 ratios. It notably allows preparing materials containing only FeN4 sites, which are able to selectively reduce CO2 to CO in aqueous solution with Faradaic yields of over 90% and at low overpotential.

We report here on a new series of CO2-reducing molecular catalysts based on Earth-abundant elements that are very selective for the production of formic acid in dimethylformamide (DMF)/water mixtures (Faradaic efficiency of 90 ± 10%) at moderate overpotentials (500–700 mV in DMF measured at the middle of the catalytic wave). The [CpCo(PR2NR′2)I]+ compounds contain diphosphine ligands, PR2NR′2, with two pendant amine residues that act as proton relays during CO2-reduction catalysis and tune their activity. Four different PR2NR′2 ligands with cyclohexyl or phenyl substituents on phosphorus and benzyl or phenyl substituents on nitrogen were employed, and the compound with the most electron-donating phosphine ligand and the most basic amine functions performs best among the series, with turnover frequency >1000 s–1. State-of-the-art benchmarking of catalytic performances ranks this new class of cobalt-based complexes among the most promising CO2-to-formic acid reducing catalysts developed to date; addressing the stability issues would allow further improvement. Mechanistic studies and density functional theory simulations confirmed the role of amine groups for stabilizing key intermediates through hydrogen bonding with water molecules during hydride transfer from the Co center to the CO2 molecule.

Abstract Proton conductive materials are of significant importance and highly desired for clean energy-related applications. Discovery of practical metal-organic frameworks (MOFs) with high proton conduction remains a challenge due to the use of toxic chemicals, inconvenient ligand preparation and complication of production at scale for the state-of-the-art candidates. Herein, we report a zirconium-MOF, MIP-202(Zr), constructed from natural α-amino acid showing a high and steady proton conductivity of 0.011 S cm −1 at 363 K and under 95% relative humidity. This MOF features a cost-effective, green and scalable preparation with a very high space-time yield above 7000 kg m −3 day −1 . It exhibits a good chemical stability under various conditions, including solutions of wide pH range and boiling water. Finally, a comprehensive molecular simulation was carried out to shed light on the proton conduction mechanism. All together these features make MIP-202(Zr) one of the most promising candidates to approach the commercial benchmark Nafion.

The sandwich-type polyoxometalate (POM) [(PW9O34)2Co4(H2O)2]10– was immobilized in the hexagonal channels of the Zr(IV) porphyrinic MOF-545 hybrid framework. The resulting composite was fully characterized by a panel of physicochemical techniques. Calculations allowed identifying the localization of the POM in the vicinity of the Zr6 clusters and porphyrin linkers constituting the MOF. The material exhibits a high photocatalytic activity and good stability for visible-light-driven water oxidation. It thus represents a rare example of an all-in-one fully noble metal-free supramolecular heterogeneous photocatalytic system, with the catalyst and the photosensitizer within the same porous solid material.

Many industrial catalysts contain isolated metal sites on the surface of oxide supports. Although such catalysts have been used in a broad range of processes for more than 40 years, there is often a very limited understanding about the structure of the catalytically active sites. This Review discusses how surface organometallic chemistry (SOMC) engineers surface sites with well-defined structures and provides insight into the nature of the active sites of industrial catalysts; the Review focuses in particular on olefin production and conversion processes.

The last few years have seen huge growing interest in the heterogenisation of molecular catalysts since it allows combining the advantages of homogeneous and heterogeneous catalyses. Besides bringing recyclability, the immobilisation of the catalyst may increase its stability while allowing tuning its selectivity. In this respect, Metal-Organic Frameworks (MOFs) attract evergrowing interest as a platform for their confinement within their pores or channels. In this review, Cat@MOF composites wherein molecular catalysts (Cats) are immobilised into MOFs through non-covalent interactions with their host are reviewed thoroughly. Polyoxometalates (POMs) and other metal-based complexes as immobilised molecular species are covered. In the first part, the different synthetic methods and analytical tools are described. A critical analysis of the various physico-chemical methods available to characterise the Cat@MOF composites is provided - particular attention being paid toward their pertinence to the investigation of the content, the position and the stability of the catalyst within the MOF. Besides, the focus is on non-conventional techniques such as the Pair Distribution Function (PDF) method and a section is dedicated to the contribution of DFT calculations. In the second part, the applications of these materials in the fields of catalysis, including oxidation and reduction reactions, acid-base catalysis, and photo- and electrocatalysis, are detailed.

intermediate catalytic centers revealed by electron spin resonance (ESR) measurements and recent knowledge of lifetime excited states in MIL-125-type of solids.

Abstract To use water as the source of electrons for proton or CO 2 reduction within electrocatalytic devices, catalysts are required for facilitating the proton‐coupled multi‐electron oxygen evolution reaction (OER, 2 H 2 O→O 2 +4 H + +4 e − ). These catalysts, ideally based on cheap and earth abundant metals, have to display high activity at low overpotential and good stability and selectivity. While numerous examples of Co, Mn, and Ni catalysts were recently reported for water oxidation, only few examples were reported using copper, despite promising efficiencies. A rationally designed nanostructured copper/copper oxide electrocatalyst for OER is presented. This material derives from conductive copper foam passivated by a copper oxide layer and further nanostructured by electrodeposition of CuO nanoparticles. The generated electrodes are highly efficient for catalyzing selective water oxidation to dioxygen with an overpotential of 290 mV at 10 mA cm −2 in 1 m NaOH solution.

The Keggin-type polyoxometalate (POM) PW12O403– and the catalytic complex Cp*Rh(bpydc)Cl2 (bpydc = 2,2′-bipyridine-5,5′-dicarboxylic acid) were coimmobilized in the Zr(IV) based metal organic framework UiO-67. The POM is encapsulated within the cavities of the MOF by in situ synthesis, and then, the Rh catalytic complex is introduced by postsynthetic linker exchange. Infrared and Raman spectroscopies, 31P and 13C MAS NMR, N2 adsorption isotherms, and X-ray diffraction indicate the structural integrity of all components (POM, Rh-complex and MOF) within the composite of interest (PW12,Cp*Rh)@UiO-67. DFT calculations identified two possible locations of the POM in the octahedral cavities of the MOF: one at the center of a UiO-67 pore with the Cp*Rh complex pointing toward an empty pore and one off-centered with the Cp*Rh pointing toward the POM. 31P–1H heteronuclear (HETCOR) experiments ascertained the two environments of the POM, equally distributed, with the POM in interaction either with the Cp* fragment or with the organic linker. In addition, Pair Distribution Function (PDF) data were collected on the POM@MOF composite and provided key evidence of the structural integrity of the POM once immobilized into the MOF. The photocatalytic activity of the (PW12,Cp*Rh)@UiO-67 composite for CO2 reduction into formate and hydrogen were evaluated. The formate production was doubled when compared with that observed with the POM-free Cp*Rh@UiO-67 catalyst and reached TONs as high as 175 when prepared as thin films, showing the beneficial influence of the POM. Finally, the stability of the composite was assessed by means of recyclability tests. The combination of XRD, IR, ICP, and PDF experiments was essential in confirming the integrity of the POM, the catalyst, and the MOF after catalysis.

Electrochemical CO2 reduction presents a sustainable route to storage of intermittent renewable energy. Ethanol is an important target product, which is used as a fuel additive and as a chemical feedstock. However, electrochemical ethanol production is challenging, as it involves the transfer of multiple electrons and protons alongside C–C bond formation. To date, the most commonly employed and effective catalysts are copper-based materials. This Review presents and categorizes the most efficient and selective Cu-based electrocatalysts, which are divided into three main groups: oxide-derived copper, bimetallics, and copper- and nitrogen-doped carbon materials. Only a few other specific examples fall outside this classification. The catalytic performance of these materials for ethanol production in aqueous conditions is discussed in terms of current density, overpotential, and faradaic efficiency. A critical evaluation of the factors that contribute to high performance is provided to aid the design of more efficient catalysts for selective ethanol formation.

The first photosensitization of a rhodium-based catalytic system for CO2 reduction is reported, with formate as the sole carbon-containing product. Formate has wide industrial applications and is seen as valuable within fuel cell technologies as well as an interesting H2 -storage compound. Heterogenization of molecular rhodium catalysts is accomplished via the synthesis, post-synthetic linker exchange, and characterization of a new metal-organic framework (MOF) Cp*Rh@UiO-67. While the catalytic activities of the homogeneous and heterogeneous systems are found to be comparable, the MOF-based system is more stable and selective. Furthermore it can be recycled without loss of activity. For formate production, an optimal catalyst loading of ∼10 % molar Rh incorporation is determined. Increased incorporation of rhodium catalyst favors thermal decomposition of formate into H2 . There is no precedent for a MOF catalyzing the latter reaction so far.

Homoleptic terpyridine complexes of first row transition metals are evaluated as catalysts for the electrocatalytic reduction of CO2. Ni and Co-based catalytic systems are shown to reduce CO2 to CO under the conditions tested. The Ni complex was found to exhibit selectivity for CO2 over proton reduction while the Co-based system generates mixtures of CO and H2 with CO : H2 ratios being tuneable through variation of the applied potential.

Iron-sulfur (Fe-S) clusters are essential protein cofactors whose biosynthetic defects lead to severe diseases among which is Friedreich's ataxia caused by impaired expression of frataxin (FXN). Fe-S clusters are biosynthesized on the scaffold protein ISCU, with cysteine desulfurase NFS1 providing sulfur as persulfide and ferredoxin FDX2 supplying electrons, in a process stimulated by FXN but not clearly understood. Here, we report the breakdown of this process, made possible by removing a zinc ion in ISCU that hinders iron insertion and promotes non-physiological Fe-S cluster synthesis from free sulfide in vitro. By binding zinc-free ISCU, iron drives persulfide uptake from NFS1 and allows persulfide reduction into sulfide by FDX2, thereby coordinating sulfide production with its availability to generate Fe-S clusters. FXN stimulates the whole process by accelerating persulfide transfer. We propose that this reconstitution recapitulates physiological conditions which provides a model for Fe-S cluster biosynthesis, clarifies the roles of FDX2 and FXN and may help develop Friedreich's ataxia therapies.

Understanding the activity and selectivity of molecular catalysts for CO₂ reduction to fuels is an important scientific endeavour in addressing the growing global energy demand.