Manganese Carbonyl Complexes as Selective Electrocatalysts for CO2 Reduction in Water and Organic Solvents

[Image: see text] The electrochemical reduction of CO(2) provides a way to sustainably generate carbon-based fuels and feedstocks. Molecular CO(2) reduction electrocatalysts provide tunable reaction centers offering an approach to control the selectivity of catalysis. Manganese carbonyl complexes, based on [Mn(bpy)(CO)(3)Br] and its derivatives (bpy = 2,2′-bipyridine), are particularly interesting due to their ease of synthesis and the use of a first-row earth-abundant transition metal. [Mn(bpy)(CO)(3)Br] was first shown to be an active and selective catalyst for reducing CO(2) to CO in organic solvents in 2011. Since then, manganese carbonyl catalysts have been widely studied with numerous reports of their use as electrocatalysts and photocatalysts and studies of their mechanism. This class of Mn catalysts only shows CO(2) reduction activity with the addition of weak Brønsted acids. Perhaps surprisingly, early reports showed increased turnover frequencies as the acid strength is increased without a loss in selectivity toward CO evolution. It may have been expected that the competing hydrogen evolution reaction could have led to lower selectivity. Inspired by these works we began to explore if the catalyst would work in protic solvents, namely, water, and to explore the pH range over which it can operate. Here we describe the early studies from our laboratory that first demonstrated the use of manganese carbonyl complexes in water and then go on to discuss wider developments on the use of these catalysts in water, highlighting their potential as catalysts for use in aqueous CO(2) electrolyzers. Key to the excellent selectivity of these catalysts in the presence of Brønsted acids is a proton-assisted CO(2) binding mechanism, where for the acids widely studied, lower pK(a) values actually favor CO(2) binding over Mn–H formation, a precursor to H(2) evolution. Here we discuss the wider literature before focusing on our own contributions in validating this previously proposed mechanism through the use of vibrational sum frequency generation (VSFG) spectroelectrochemistry. This allowed us to study [Mn(bpy)(CO)(3)Br] while it is at, or near, the electrode surface, which provided a way to identify new catalytic intermediates and also confirm that proton-assisted CO(2) binding operates in both the “dimer” and primary (via [Mn(bpy)(CO)(3)](−)) pathways. Understanding the mechanism of how these highly selective catalysts operate is important as we propose that the Mn complexes will be valuable models to guide the development of new proton/acid tolerant CO(2) reduction catalysts

Interactive biocatalysis achieved by driving enzyme cascades inside a porous conducting material

An emerging concept and platform, the electrochemical Leaf (e-Leaf), offers a radical change in the way tandem (multi-step) catalysis by enzyme cascades is studied and exploited. The various enzymes are loaded into an electronically conducting porous material composed of metallic oxide nanoparticles, where they achieve high concentration and crowding – in the latter respect the environment resembles that found in living cells. By exploiting efficient electron tunneling between the nanoparticles and one of the enzymes, the e-Leaf enables the user to interact directly with complex networks, rendering simultaneous the abilities to energise, control and observe catalysis. Because dispersion of intermediates is physically suppressed, the output of the cascade – the rate of flow of chemical steps and information – is delivered in real time as electrical current. Myriad enzymes of all major classes now become effectively electroactive in a technology that offers scalability between micro-(analytical, multiplex) and macro-(synthesis) levels. This Perspective describes how the e-Leaf was discovered, the steps in its development so far, and the outlook for future research and applications

A Manganese Complex on a Gas Diffusion Electrode for Selective CO2 to CO Reduction

Manganese carbonyl complexes have been studied extensively in solution as low cost, selective electrocatalysts with a low overpotential for CO2 reduction but experiments are typically at low current densities. In...</jats:p

Zero-Gap Bipolar Membrane Electrolyzer for Carbon DioxideReduction Using Acid-Tolerant Molecular Electrocatalysts

[Image: see text] The scaling-up of electrochemical CO(2) reduction requires circumventing the CO(2) loss as carbonates under alkaline conditions. Zero-gap cell configurations with a reverse-bias bipolar membrane (BPM) represent a possible solution, but the catalyst layer in direct contact with the acidic environment of a BPM usually leads to H(2) evolution dominating. Here we show that using acid-tolerant Ni molecular electrocatalysts selective (>60%) CO(2) reduction can be achieved in a zero-gap BPM device using a pure water and CO(2) feed. At a higher current density (100 mA cm(–2)), CO selectivity decreases, but was still >30%, due to reversible product inhibition. This study demonstrates the importance of developing acid-tolerant catalysts for use in large-scale CO(2) reduction devices

Potential Dependent Reorientation Controlling Activity of a Molecular Electrocatalyst.

The activity of molecular electrocatalysts depends on the interplay of electrolyte composition near the electrode surface, the composition and morphology of the electrode surface, and the electric field at the electrode-electrolyte interface. This interplay is challenging to study and often overlooked when assessing molecular catalyst activity. Here, we use surface specific vibrational sum frequency generation (VSFG) spectroscopy to study the solvent and potential dependent activation of Mo(bpy)(CO)4, a CO2 reduction catalyst, at a polycrystalline Au electrode. We find that the parent complex undergoes potential dependent reorientation at the electrode surface when a small amount of N-methyl-2-pyrrolidone (NMP) is present. This preactivates the complex, resulting in greater yields at less negative potentials, of the active electrocatalyst for CO2 reduction

Design principles for a nanoconfined enzyme cascade electrode via reaction-diffusion modelling

The study of enzymes by direct electrochemistry has been extended to enzyme cascades, with a key development being the 'electrochemical leaf': an electroactive enzyme is immobilized within a porous electrode, providing in situ cofactor (NADP(H)) regeneration for a co-immobilized downstream enzyme. This system has been further developed to include multiple downstream enzymes, and it has become an important tool in biocatalysis, however, the local environment within the porous electrode has not been investigated in detail. Here, we constructed a 1D reaction-diffusion model, comprising the porous electrode with 2 kinds of enzymes immobilized, and an enzyme-free electrolyte diffusion layer. The modelling results show that the rate of the downstream enzyme is a key parameter, and that substrate transport within the porous electrode is not a main limiting factor. The insights obtained from this model can guide future rational design and improvement of these electrodes and immobilized enzyme cascade systems

Enzyme-material composites for solar-driven reactions

Using sunlight to drive chemical reactions has long been one of the goals in developing sustainable processes. Previous research has focused on solar fuel production in the form of H2, but this thesis demonstrates that solar-to-chemicals processes can be constructed to produce more complex compounds, using hybrid systems composed of enzymes and inorganic materials. Tetrachloroethene reductive dehalogenase (PceA), an enzyme that catalyzes the conversion of tetrachloroethene (PCE) to trichloroethene (TCE) and subsequently to cis-dichloroethene (cDCE), was shown to accept electrons from both graphite and TiO2 electrodes. Irradiation by UV light onto PceA-adsorbed TiO2 particles led to the selective production of TCE and cDCE, which was not possible without PceA as a catalyst. Ferredoxin-NADP+ reductase (FNR) is a key enzyme in photosynthesis, as it receives energetic electrons from Photosystem I and produces NADPH as an energy carrier for downstream 'Dark' reactions involving CO2 assimilation. This thesis presents the discovery of FNR activity on indium tin oxide (ITO) electrodes which led to direct electrochemical investigation of the properties of FNR, both in the absence and presence of its substrate, NADP+. The FNR-adsorbed electrode, termed 'the electrochemical leaf', rapidly interconverts NADP+/NADPH, and this was coupled to a downstream NADPH-dependent enzyme, thus demonstrating a new approach to cofactor regeneration for enzyme-catalyzed organic synthesis. The NADP+ reduction by FNR was also driven by light using a photoanode made of visible-light responsive semiconductors.</p

Generalizability and limitations of machine learning for yield prediction of oxidative coupling of methane

Product yields of catalytic reaction networks are dependent on many factors, encompassing both catalyst properties and reaction conditions. The oxidative coupling of methane (OCM) is a complex heterogeneous-homogeneous process, and the yield of the desired C2 products is non-linear with respect to reaction conditions. Herein, using two published datasets of OCM catalytic experimental results, I show that various machine learning (ML) algorithms can predict C2 yields from reaction conditions with a mean absolute error (MAE) of 0.5 – 1.0 percentage points in the best case. However, complications arising from real-world applications should be anticipated, therefore I investigated the effects of training set size, added noise, and out-of-sample partitions on the performance of ML algorithms. These results provide insights into the generalizability of the algorithms as well as caveats into the applicability of ML to reaction yield predictio

Improving the stability, selectivity, and cell voltage of a bipolar membrane zero-gap electrolyzer for low-loss CO2 reduction

Abstract Electrolyzers for CO2 reduction containing bipolar membranes (BPM) are promising due to low loss of CO2 as carbonates and low product crossover, but improvements in product selectivity, stability, and cell voltage are required. In particular, direct contact with the acidic cation exchange layer leads to high levels of H2 evolution with many common cathode catalysts. Here, Co phthalocyanine (CoPc) is reported as a suitable catalyst for a zero‐gap BPM device, reaching 53% Faradaic efficiency to CO at 100 mA cm−2 using only pure water and CO2 as the input feeds. It is also shown that the cell voltage can be lowered by constructing a customized BPM using TiO2 water dissociation catalyst, however this is at the cost of decreased selectivity. Switching the pure‐water anolyte to KOH improved both the cell voltage and CO selectivity (62% at 200 mA cm−2), but cation crossover could cause complications. The results demonstrate viable strategies for improving a BPM CO2 electrolyzer toward practical‐scale CO2‐to‐chemicals conversion