Molecular tuning of CO2-to-ethylene conversion.

The electrocatalytic reduction of carbon dioxide, powered by renewable electricity, to produce valuable fuels and feedstocks provides a sustainable and carbon-neutral approach to the storage of energy produced by intermittent renewable sources1. However, the highly selective generation of economically desirable products such as ethylene from the carbon dioxide reduction reaction (CO2RR) remains a challenge2. Tuning the stabilities of intermediates to favour a desired reaction pathway can improve selectivity3-5, and this has recently been explored for the reaction on copper by controlling morphology6, grain boundaries7, facets8, oxidation state9 and dopants10. Unfortunately, the Faradaic efficiency for ethylene is still low in neutral media (60 per cent at a partial current density of 7 milliamperes per square centimetre in the best catalyst reported so far9), resulting in a low energy efficiency. Here we present a molecular tuning strategy-the functionalization of the surface of electrocatalysts with organic molecules-that stabilizes intermediates for more selective CO2RR to ethylene. Using electrochemical, operando/in situ spectroscopic and computational studies, we investigate the influence of a library of molecules, derived by electro-dimerization of arylpyridiniums11, adsorbed on copper. We find that the adhered molecules improve the stabilization of an 'atop-bound' CO intermediate (that is, an intermediate bound to a single copper atom), thereby favouring further reduction to ethylene. As a result of this strategy, we report the CO2RR to ethylene with a Faradaic efficiency of 72 per cent at a partial current density of 230 milliamperes per square centimetre in a liquid-electrolyte flow cell in a neutral medium. We report stable ethylene electrosynthesis for 190 hours in a system based on a membrane-electrode assembly that provides a full-cell energy efficiency of 20 per cent. We anticipate that this may be generalized to enable molecular strategies to complement heterogeneous catalysts by stabilizing intermediates through local molecular tuning.

NHC-CDI Ligands Boost Multicarbon Production in Electrocatalytic CO2 Reduction by Increasing Accumulated Charged Intermediates and Promoting *CO Dimerization on Cu.

Copper-based materials exhibit significant potential as catalysts for electrochemical CO2 reduction, owing to their capacity to generate multicarbon hydrocarbons. The molecular functionalization of Cu electrodes represents a simple yet powerful strategy for improving the intrinsic activity of these materials by favoring specific reaction pathways through the creation of tailored microenvironments around the surface active sites. However, despite its success, comprehensive mechanistic insights derived from experimental techniques are often limited, leaving the active role of surface modifiers inconclusive. In this work, we show that N-heterocyclic carbene-carbodiimide-functionalized Cu catalysts display a remarkable activity for multicarbon product formation, surpassing bare Cu electrodes by more than an order of magnitude. These hybrid catalysts operate efficiently using an electrolyzer equipped with a gas diffusion electrode, achieving a multicarbon product selectivity of 58% with a partial current density of ca. -80 mA cm-2. We found that the activity for multicarbon product formation is closely linked to the surface charge that accumulates during electrocatalysis, stemming from surface intermediate buildup. Through X-ray photoelectron spectroscopy, we elucidated the role of the molecular additives in altering the electronic structure of the Cu electrodes, promoting the stabilization of surface CO. Additionally, in situ Raman measurements established the identity of the reaction intermediates that accumulate during electrocatalysis, indicating preferential CO binding on Cu step sites, known for facilitating C-C coupling. This study underscores the significant potential of molecular surface modifications in developing efficient electrocatalysts for CO2 reduction, highlighting surface charge as a pivotal descriptor of multicarbon product activity.

Selective CO2 Electrochemical Reduction Enabled by a Tricomponent Copolymer Modifier on a Copper Surface.

Electrochemical CO2 reduction over Cu could provide value-added multicarbon hydrocarbons and alcohols. Despite recent breakthroughs, it remains a significant challenge to design a catalytic system with high product selectivity. Here we demonstrate that a high selectivity of ethylene (55%) and C2+ products (77%) could be achieved by a highly modular tricomponent copolymer modified Cu electrode, rivaling the best performance using other modified polycrystalline Cu foil catalysts. Such a copolymer can be conveniently prepared by a ring-opening metathesis polymerization, thereby offering a new degree of freedom for tuning the selectivity. Control experiments indicate all three components are essential for the selectivity enhancement. A surface characterization showed that the incorporation of a phenylpyridinium component increased the film robustness against delamination. It was also shown that its superior performance is not due to a morphology change of the Cu underneath. Molecular dynamics (MD) simulations indicate that a combination of increased local CO2 concentration, increased porosity for gas diffusion, and the local electric field effect together contribute to the increased ethylene and C2+ product selectivity.

Molecular enhancement of heterogeneous CO2 reduction.

The electrocatalytic carbon dioxide reduction reaction (CO2RR) addresses the need for storage of renewable energy in valuable carbon-based fuels and feedstocks, yet challenges remain in the improvement of electrosynthesis pathways for highly selective hydrocarbon production. To improve catalysis further, it is of increasing interest to lever synergies between heterogeneous and homogeneous approaches. Organic molecules or metal complexes adjacent to heterogeneous active sites provide additional binding interactions that may tune the stability of intermediates, improving catalytic performance by increasing Faradaic efficiency (product selectivity), as well as decreasing overpotential. We offer a forward-looking perspective on molecularly enhanced heterogeneous catalysis for CO2RR. We discuss four categories of molecularly enhanced strategies: molecular-additive-modified heterogeneous catalysts, immobilized organometallic complex catalysts, reticular catalysts and metal-free polymer catalysts. We introduce present-day challenges in molecular strategies and describe a vision for CO2RR electrocatalysis towards multi-carbon products. These strategies provide potential avenues to address the challenges of catalyst activity, selectivity and stability in the further development of CO2RR.

In-Situ Nanostructuring and Stabilization of Polycrystalline Copper by an Organic Salt Additive Promotes Electrocatalytic CO2 Reduction to Ethylene.

Bridging homogeneous molecular systems with heterogeneous catalysts is a promising approach for the development of new electrodes, combining the advantages of both approaches. In the context of CO2 electroreduction, molecular enhancement of planar copper electrodes has enabled promising advancement towards high Faradaic efficiencies for multicarbon products. Besides, nanostructured copper electrodes have also demonstrated enhanced performance at comparatively low overpotentials. Herein, we report a novel and convenient method for nanostructuring copper electrodes using N,N'-ethylene-phenanthrolinium dibromide as molecular additive. Selectivities up to 70 % for C≥2 products are observed for more than 40 h without significant change in the surface morphology. Mechanistic studies reveal several roles for the organic additive, including: the formation of cube-like nanostructures by corrosion of the copper surface, the stabilization of these nanostructures during electrocatalysis by formation of a protective organic layer, and the promotion of C≥2 products.

Progress on all ends for carbon-carbon bond formation through photoredox catalysis.

Dual role for catalysts: Novel routes for the generation of asymmetric stereocenters using photoredox catalysis were recently developed. Different chiral catalytic systems allowed new CC bonds to form in good yields and enantioselectivities using a mild methodology in which light is used as the energy source.

Efficient palladium-catalyzed electrocarboxylation enables late-stage carbon isotope labelling.

Carbon isotope labelling of bioactive molecules is essential for accessing the pharmacokinetic and pharmacodynamic properties of new drug entities. Aryl carboxylic acids represent an important class of structural motifs ubiquitous in pharmaceutically active molecules and are ideal targets for the installation of a radioactive tag employing isotopically labelled CO2. However, direct isotope incorporation via the reported catalytic reductive carboxylation (CRC) of aryl electrophiles relies on excess CO2, which is incompatible with carbon-14 isotope incorporation. Furthermore, the application of some CRC reactions for late-stage carboxylation is limited because of the low tolerance of molecular complexity by the catalysts. Herein, we report the development of a practical and affordable Pd-catalysed electrocarboxylation setup. This approach enables the use of near-stoichiometric 14CO2 generated from the primary carbon-14 source Ba14CO3, facilitating late-stage and single-step carbon-14 labelling of pharmaceuticals and representative precursors. The proposed isotope-labelling protocol holds significant promise for immediate impact on drug development programmes.

Recent advances in the application of chiral phosphine ligands in Pd-catalysed asymmetric allylic alkylation.

One of the most powerful approaches for the formation of simple and complex chiral molecules is the metal-catalysed asymmetric allylic alkylation. This reaction has been broadly studied with a great variety of substrates and nucleophiles under different reaction conditions and it has promoted the synthesis of new chiral ligands to be evaluated as asymmetric inductors. Although the mechanism as well as the active species equilibria are known, the performance of the catalytic system depends on the fine tuning of factors such as type of substrate, nucleophile nature, reaction medium, catalytic precursor and type of ligand used. Particularly interesting are chiral phosphines which have proved to be effective asymmetric inductors in several such reactions. The present review covers the application of phosphine-donor ligands in Pd-catalysed asymmetric allylic alkylation in the last decade.

Breaking Scaling Relationships in CO2 Reduction on Copper Alloys with Organic Additives.

Boundary conditions for catalyst performance in the conversion of common precursors such as N2, O2, H2O, and CO2 are governed by linear free energy and scaling relationships. Knowledge of these limits offers an impetus for designing strategies to alter reaction mechanisms to improve performance. Typically, experimental demonstrations of linear trends and deviations from them are composed of a small number of data points constrained by inherent experimental limitations. Herein, high-throughput experimentation on 14 bulk copper bimetallic alloys allowed for data-driven identification of a scaling relationship between the partial current densities of methane and C2+ products. This strict dependence represents an intrinsic limit to the Faradaic efficiency for C-C coupling. We have furthermore demonstrated that coating the electrodes with a molecular film breaks the scaling relationship to promote C2+ product formation.

Photo- and Electrochemical Valorization of Carbon Dioxide Using Earth-Abundant Molecular Catalysts.

The dramatic increase in anthropogenic carbon dioxide emissions in recent decades has forced us to look for alternative carbon-neutral processes for the production of energy vectors and commodity chemicals. Photo- and electrochemical reduction of CO2 are appealing strategies for the storage of sustainable and intermittent energies in the form of chemical bonds of synthetic fuels and value-added molecules. In these approaches, carbon dioxide is converted to products such as CO, HCOOH and MeOH through proton-coupled electron transfer reactions. The use of earth-abundant elements as components of the catalytic materials is crucial for the large-scale applicability of this technology. This review summarizes the most recent advances related to this issue, with particular focus on studies where molecular metal complexes are used as catalysts. In addition, with the aim of aiding in the design of more robust and efficient non-noble metal-based catalysts, we discuss the lessons learned from the corresponding mechanistic studies.

Highly active and selective photochemical reduction of CO2 to CO using molecular-defined cyclopentadienone iron complexes.

Herein, we report highly active (cyclopentadienone)iron-tricarbonyl complexes for CO2 photoreduction using visible light with an Ir complex as photosensitizer and TEOA as electron/proton donor. Turnover numbers (TON) of ca. 600 (1 h) with initial turnover frequencies (TOF) up to 22.2 min(-1) were observed. Operando FTIR measurements allowed for the proposal of a plausible mechanism for catalyst activation.