The scaling-up of electrochemical CO2 reduction requires circumventing the CO2 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 H2 evolution dominating. Here we show that using acid-tolerant Ni molecular electrocatalysts selective (>60%) CO2 reduction can be achieved in a zero-gap BPM device using a pure water and CO2 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 CO2 reduction devices.
The electrochemical reduction of CO
provides a way to sustainably generate carbon-based fuels and feedstocks. Molecular CO
reduction electrocatalysts provide tunable reaction centers offering an ...approach to control the selectivity of catalysis. Manganese carbonyl complexes, based on Mn(bpy)(CO)
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)
Br was first shown to be an active and selective catalyst for reducing CO
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
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
electrolyzers.Key to the excellent selectivity of these catalysts in the presence of Brønsted acids is a proton-assisted CO
binding mechanism, where for the acids widely studied, lower p
values actually favor CO
binding over Mn-H formation, a precursor to H
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)
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
binding operates in both the "dimer" and primary (via Mn(bpy)(CO)
) 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
reduction catalysts.
Strontium niobium oxynitride (SrNbO2N) particles were coated on fluorine-doped tin oxide (FTO) glass and examined as a photoelectrode for water splitting under visible light in a neutral aqueous ...solution (Na2SO4, pH ≈ 6). SrNbO2N, which has a band gap of ca. 1.8 eV, acted as an n-type semiconductor and generated an anodic photocurrent assignable to water oxidation upon irradiation with visible-light photons with wavelengths of up to 700 nm, even without an externally applied potential. Under visible light (λ > 420 nm) with an applied potential of +1.0–1.55 V vs RHE, nearly stoichiometric H2 and O2 evolution was achieved using a SrNbO2N/FTO electrode modified with colloidal iridium oxide (IrO2) as a water oxidation promoter. This study presents the first example of photoelectrochemical water splitting involving an n-type semiconductor with a band gap smaller than 2.0 eV that does not require an externally applied potential.
Photocatalytic activities of perovskite‐type niobium oxynitrides (CaNbO2N, SrNbO2N, BaNbO2N, and LaNbON2) were examined for hydrogen and oxygen evolution from water under visible‐light irradiation. ...These niobium oxynitrides were prepared by heating the corresponding oxide precursors, which were synthesized using the polymerized complex method, for 15 h under a flow of ammonia. They possess visible‐light absorption bands between 600–750 nm, depending on the A‐site cations in the structures. The oxynitride CaNbO2N, was found to be active for hydrogen and oxygen evolution from methanol and aqueous AgNO3, respectively, even under irradiation by light at long wavelengths (λ<560 nm). The nitridation temperature dependence of CaNbO2N was investigated and 1023 K was found to be the optimal temperature. At lower temperatures, the oxynitride phase is not adequately produced, whereas higher temperatures produce more reduced niobium species (e. g., Nb3+ and Nb4+), which can act as electron‐hole recombination centers, resulting in a decrease in activity.
The preparation conditions and characterization of a perovskite niobium oxynitride, CaNbO2N, are described. This photocatalyst, containing a band gap of 2.0 eV, produces hydrogen and oxygen from water containing an electron donor and acceptor, respectively, under irradiation with wavelengths up to 560 nm.
Protein film electrochemistry (PFE) has given unrivalled insight into the properties of redox proteins and many electron-transferring enzymes, allowing investigations of otherwise ill-defined or ...intractable topics such as unstable Fe–S centers and the catalytic bias of enzymes. Many enzymes have been established to be reversible electrocatalysts when attached to an electrode, and further investigations have revealed how unusual dependences of catalytic rates on electrode potential have stark similarities with electronics. A special case, the reversible electrochemistry of a photosynthetic enzyme, ferredoxin-NADP+ reductase (FNR), loaded at very high concentrations in the 3D nanopores of a conducting metal oxide layer, is leading to a new technology that brings PFE to myriad enzymes of other classes, the activities of which become controlled by the primary electron exchange. This extension is possible because FNR-based recycling of NADP(H) can be coupled to a dehydrogenase, and thence to other enzymes linked in tandem by the tight channelling of cofactors and intermediates within the nanopores of the material. The earlier interpretations of catalytic wave-shapes and various analogies with electronics are thus extended to initiate a field perhaps aptly named “cascade-tronics”, in which the flow of reactions along an enzyme cascade is monitored and controlled through an electrochemical analyzer. Unlike in photosynthesis where FNR transduces electron transfer and hydride transfer through the unidirectional recycling of NADPH, the “electrochemical leaf” (e-Leaf) can be used to drive reactions in both oxidizing and reducing directions. The e-Leaf offers a natural way to study how enzymes are affected by nanoconfinement and crowding, mimicking the physical conditions under which enzyme cascades operate in living cells. The reactions of the trapped enzymes, often at very high local concentration, are thus studied electrochemically, exploiting the potential domain to control rates and direction and the current–rate analogy to derive kinetic data. Localized NADP(H) recycling is very efficient, resulting in very high cofactor turnover numbers and new opportunities for controlling and exploiting biocatalysis.
Reduction of CO2 and its direct entry into organic chemistry is achieved efficiently and in a highly visible way using a metal oxide electrode in which two enzyme catalysts, one for electrochemically ...regenerating reduced nicotinamide adenine dinucleotide phosphate and the other for assimilating CO2 and converting pyruvate (C3) to malate (C4), are entrapped within its nanopores. The resulting reversible electrocatalysis is exploited to construct a solar CO2 reduction/water-splitting device producing O2 and C4 with high faradaic efficiency.
Pulsed electrolysis can significantly improve carbon dioxide reduction on metal electrodes, but the effect of short (millisecond to seconds) voltage steps on molecular electrocatalysts is largely ...unstudied. In this work, we investigate the effect pulse electrolysis has on the selectivity and stability of the homogeneous electrocatalyst Ni(cyclam)2+ at a carbon electrode. By tuning the potential and pulse duration, we achieve a significant improvement in CO Faradaic efficiencies (85%) after 3 h, double that of the system under potentiostatic conditions. The improved activity is due to in situ catalyst regeneration from an intermediate that occurs as part of the catalyst’s degradation pathway. This study demonstrates the wider opportunity to apply pulsed electrolysis to molecular electrocatalysts to control activity and improve selectivity.
This study simulates a high-temperature reaction in a plug-flow reactor (PFR) for the aromatization of methane via oxidative coupling of methane (OCM) using a state-of-the-art gas-phase chemical ...kinetic mechanism. Benzene is formed from a methane–oxygen (CH4–O2) feed via formation of ethylene through OCM followed by homogeneous gas-phase aromatization of C2H4 after O2 depletion. Because both OCM and C2H4 aromatization are exothermic reactions, the process is advantageous over an endothermic nonoxidative methane aromatization reaction. For the OCM reaction, the previously reported mechanism in which the catalyst achieves the quasi-equilibrated formation of OH• from an H2O–O2 mixture is included in the gas-phase combustion chemistry reaction network. It is evident that OH• formation increases benzene yield as a consequence of enhanced C2H4 yield from the OCM. The influence of temperature, CH4/O2 ratio, and contact time on benzene yield is elucidated, and reaction pathways leading to aromatic formation are analyzed. The maximum benzene yield on a carbon basis at a total pressure of 1 atm reaches 10% at CH4/O2 ratios from 3 to 6 and temperatures of 800–900 °C (isothermal). Our analysis on the differential rates of production suggests that benzene is formed from the benzyl radical via toluene and from the reaction between allyl and propargyl radicals. Simulations show that using the exothermicity of the process enables adiabatic reactor operation, which is beneficial for reducing the external heat supply (i.e., inlet temperature) by utilizing the exothermic reactions.
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