Papers, communications and reviews… our recent published work is here.


Advances in Molecular Electrochemical Activation of Dinitrogen

Lydia Merakeb, Marc Robert

Curr. Opin. Electrochem. 29, 100834, 2021

Nitrogen fixation is an important and challenging transformation in chemistry. It is currently achieved by the Haber–Bosch process that reduces N2 to NH3 on large scale but at the cost of a huge environmental footprint. In the search for greener processes, electrochemically driven N2 reduction might become a sustainable answer. The development of molecular homogeneous catalysis has gained momentum over the last decades, and we focus in this review on the emerging electrochemical systems based on molecular catalysts for N2 fixation.
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Electrocatalytic and Photocatalytic Reduction of Carbon Dioxide by Earth-abundant Bimetallic Molecular Catalysts

Huan Chen, Lingjing Chen, Gui Chen, Marc Robert, Tai-Chu Lau

ChemPhysChem 22 (18), 1835-43, 2021

Converting CO2 into useful resources by electrocatalysis and photocatalysis is a promising strategy for recycling of the gas and electrification of the industry. Numerous studies have shown that multinuclear metal catalysts have higher selectivity and catalytic activity than monometallic catalysts due to the synergistic effects between the metal sites. In this review, we summarize some of the recent progress on the electrocatalytic and photocatalytic reduction of CO2 by earth-abundant bimetallic molecular catalysts.
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Taming Electron Transfers: From Breaking Bonds to Creating Molecules

Niklas von Wolff, Marc Robert

The Chemical Record 21 (9), 2095-2106, 2021

The electron is the ultimate redox reagent to build and reshape molecular structures. Understanding and controlling the parameters underlying dissociative electron transfer (DET) reactivity and its coupling with proton transfer is crucial for combining selectivity, kinetics and energy efficiency in molecular chemistry. Reactivity understanding and mechanistic elements in DET processes are traced back and key examples of current research efforts are presented, demonstrating a large variety of applications. The involvement of DET pathways indeed encompasses a broad range of processes such as photoredox catalysis, CO2 reduction and alcohol oxidation. Interplay between these experimental examples and fundamental mechanistic study provides a powerful path to the understanding of driving force-rate relationships, which is crucial for the development of future generations of energy efficient catalytic schemes in redox organic chemistry.
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Highlights and Challenges in the Selective Reduction of Carbon Dioxide to Methanol

Sara Navarro-Jaén, Mirella Virginie, Julien Bonin, Marc Robert, Robert Wojcieszak and Andrei Y. Khodakov

Nat. Rev. Chem. 5, 564-579, 2021

Carbon dioxide (CO2) is the iconic greenhouse gas and the major factor driving present global climate change, incentivizing its capture and recycling into valuable products and fuels. The 6H+/6e− reduction of CO2 affords CH3OH, a key compound that is a fuel and a platform molecule. In this Review, we compare different routes for CO2 reduction to CH3OH, namely, heterogeneous and homogeneous catalytic hydrogenation, as well as enzymatic catalysis, photocatalysis and electrocatalysis. We describe the leading catalysts and the conditions under which they operate, and then consider their advantages and drawbacks in terms of selectivity, productivity, stability, operating conditions, cost and technical readiness. At present, heterogeneous hydrogenation catalysis and electrocatalysis have the greatest promise for large-scale CO2 reduction to CH3OH. The availability and price of sustainable electricity appear to be essential prerequisites for efficient CH3OH synthesis.
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Molecule/Semiconductor Hybrid Materials for Visible-Light CO2 Reduction: Design Principles and Interfacial Engineering

Akinobu Nakada, Hiromu Kumagai, Marc Robert, Osamu Ishitani and Kazuhiko Maeda

Acc. Mater. Res. 2 (6), 458–470, 2021

Because of increasing concerns over the depletion of energy sources and the concomitant increase in CO2 emissions, much attention has been devoted to carbon capture and utilization technologies. Among the various methods and schemes proposed, visible-light-driven CO2 reduction in combination with water oxidation, one of the representative models of artificial photosynthesis, is an attractive solution because it enables abundant water and inexhaustible solar energy to be used to produce value-added chemicals. Molecular metal complexes and semiconductors are promising candidates for photocatalysts that can reduce CO2 to CO, formate, formaldehyde, or other hydrocarbons. Although both molecular metal complexes and semiconductors have strengths and weaknesses, their weaknesses (low oxidation ability and low selectivity for reduction reactions) can be overcome via the construction of a suitable molecule/semiconductor hybrid material. However, facilitating electron transfer at the molecule/semiconductor junction while suppressing unfavorable back electron transfer events is challenging. Consequently, the number of molecule/semiconductor hybrid systems that show a reasonable level of visible-light photocatalytic activity is limited, despite the development of a large number of visible-light-driven semiconductors and molecular photocatalysts (or catalysts). In this Account, we describe our approaches to developing hybrid photocatalysts and photoelectrodes for CO2 reduction. We have been developing both molecular (photo)catalysts and semiconductor photocatalysts individually, the latter of which are also designed for visible-light water splitting. For example, supramolecular photocatalysts that possess both photosensitizer and catalyst units in a single molecule can reduce CO2 to formate or CO in a homogeneous system, with high selectivity toward the desired product and high quantum yields of several tens of percent. However, nonoxide semiconductors such as C/N-based polymers and mixed-anion compounds exhibit a strong photooxidation ability under visible light. Carefully designed molecule/semiconductor hybrid materials achieve CO2 reduction under visible light with high product selectivity and stability even in an aqueous environment, where the concentration of CO2 is low but that of protons is high. Visible-light CO2 reduction combined with H2O oxidation is possible via the construction of a photoelectrochemical cell that comprises a molecular photocathode and an n-type semiconductor photoanode. Although our photosystems can be regarded as model systems for artificial photosynthesis, their light-energy conversion efficiencies are still unsatisfactory. To improve the efficiency, materials design, including interfacial engineering at the molecule/semiconductor junction, is important and is the general theme of the results highlighted in this Account.
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Hybridization of Molecular and Graphene Materials for CO2 Photocatalytic Reduction with Selectivity Control

Bing Ma, Matías Blanco, Laura Calvillo, Lingjing Chen, Gui Chen, Tai-Chu Lau, Goran Dražić, Julien Bonin, Marc Robert, Gaetano Granozzi

J. Am. Chem. Soc. 143 (22), 8414-8425, 2021

In the quest for designing efficient and stable photocatalytic materials for CO2 reduction, hybridizing a selective noble-metal-free molecular catalyst and carbon-based light-absorbing materials has recently emerged as a fruitful approach. In this work, we report about Co quaterpyridine complexes covalently linked to graphene surfaces functionalized by carboxylic acid groups. The nanostructured materials were characterized by X-ray photoemission spectroscopy, X-ray absorption spectroscopy, IR and Raman spectroscopies, high-resolution transmission electron microscopy and proved to be highly active in the visible-light-driven CO2 catalytic conversion in acetonitrile solutions. Exceptional stabilities (over 200 h of irradiation) were obtained without compromising the selective conversion of CO2 to products (>97%). Most importantly, complete selectivity control could be obtained upon adjusting the experimental conditions: production of CO as the only product was achieved when using a weak acid (phenol or trifluoroethanol) as a co-substrate, while formate was exclusively obtained in solutions of mixed acetonitrile and triethanolamine.
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Light-Driven Catalytic Conversion of CO2 with Heterogenized Molecular Catalysts Based on Fourth Period Transition Metals

Alessandro Perazio, Grace Lowe, Roberto Gobetto, Julien Bonin, Marc Robert

Coord. Chem. Rev. 443, article number 214018

This review examines recent advances in photocatalytic CO2 reduction using heterogenized molecular catalysts. The main part of the discussion is focused on the chemistry used to attach catalysts to different supports to produce hybrid materials, and how this effects photocatalytic performance. Examples of hybrid materials used for colloidal dispersions and solid suspensions are presented, including those based on carbon nitride, chalcogenide and perovskite quantum dots, and metal oxides. Some key examples in which this chemistry has been employed to make electrodes and photoelectrodes for photoelectrochemical CO2 reduction are also presented. In addition, the incorporation of molecular catalysts into ordered, porous frameworks (MOFs and COFs) is discussed because it offers many new and unique chemical pathways for heterogenization. Lastly, an outlook for this field and the potential future impact of these systems on solar fuels research is given.
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CO2 Reduction to Methanol with a Molecular Cobalt Catalyst Loaded Porous Carbon Electrode Assisted by a CIGS Photovoltaic Cell

Ruwen Wang, Etienne Boutin, Nicolas Barreau, Fabrice Odobel, Julien Bonin, Marc Robert

ChemPhotoChem 5, 705-710, 2021

Conversion of CO2 into valuable compounds, including fuels, with renewable energy source and sustainable compounds is a challenge addressed by artificial photosynthesis research. In particular, solar assisted electrochemical (EC) processes, in which electrons are furnished by a photovoltaic (PV) cell is a promising approach. A PV‐EC system is described, consisting in a CIGS PV unit linked to a carbon electrode loaded with cobalt phthalocyanine as molecular catalyst, able to achieve the CO2 reduction to CO and then to methanol in aqueous media with limited bias voltage. Using CO as starting material, a partial current density of ca. 0.6 mA cm‐2 for methanol is obtained at a bias voltage corresponding to a low 240 mV overpotential. Remarkably, the liquid fuel production can be sustained for at least 7h. Under ideal conditions, the CO2‐to‐CH3OH reaction shows a global Faradaic efficiency of 28%.
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A Pioneering Career in Electrochemistry: Jean-Michel Savéant

Cyrille Costentin, Benoît Limoges, Marc Robert, Cédric Tard

ACS Catal. 11, 3224-3238, 2021

Prof. Jean-Michel Savéant sadly passed on August 16, 2020. We would like to honor his memory, his tremendous contribution to electrochemistry, and its use for a general understanding of the laws of physical chemistry. In this review, we highlight his decisive role in the foundation of molecular electrochemistry. We also present his major achievements in the field of molecular and biomolecular catalysis. Finally, we review his unique contribution to dissociative electron transfers and to the electrochemical approach of proton-coupled electron transfers. This shows how various concepts rigorously established and experimentally validated assemble to each other to enlighten complex systems.
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Molecular Electrochemical Reduction of CO2 beyond Two Electrons

E. Boutin, M. Robert

Trends Chem. 3 (5), 359-372, 2021

CO2 molecular electrochemical reduction has recently led to remarkable advancements. High performing catalysts have been prepared that mainly produce carbon monoxide (CO) and formic acid (HCOOH). Studies reporting highly reduced products (formed with more than two electrons) remain scarce, but the field is now quickly emerging. Driving such multi-electron, multi-proton catalytic reactions with molecular catalysts would offer unlimited possibilities for tuning and controlling catalytic active site configuration/environment. In this perspective, we discuss known examples and draw a first unified reactivity scheme. Starting with reports on C1 products such as formaldehyde, methanol, and methane, we then address the formation of C–C bonds and associated intricate pathways. Eventually, we provide critical insights to sharpen the interpretation and analysis of existing and upcoming reports.
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