Steering Selectivity in Electrocatalytic Furfural Reduction via Electrode-Electrolyte Interface Modification.

Electrocatalytic reduction of biomass-derived furfural (FF) represents a sustainable route to produce furfuryl alcohol (FA) and 2-methylfuran (MF) as a value-added chemical and a biofuel, respectively. However, achieving high selectivity for MF as well as tuning the selectivity between FA and MF within one reaction system remain challenging. Herein, we have reported an electrode-electrolyte interface modification strategy, enabling FA and MF selectivity steering under the same reaction conditions. Specifically, by modifying copper (Cu) electrocatalysts with butyl trimethylammonium bromide (BTAB), we achieved a dramatic shift in selectivity from producing FA (selectivity: 83.8%; Faradaic efficiency, FE: 68.9%) to MF (selectivity: 80.1%; FE: 74.8%). We demonstrated that BTAB adsorption over Cu modulates the electrical double layer (EDL) structure, which repels interfacial water and weakens the hydrogen-bond (H-bond) network for proton transfer, thus impeding FF-to-FA conversion by suppression of the hydrogen atom transfer (HAT) process. On the contrary, FF-to-MF conversion was less affected. This work shows the potential of engineering of the electrode-electrolyte interface for selectivity control in electrocatalysis.

Single-Atom Titanium on Mesoporous Nitrogen, Oxygen-Doped Carbon for Efficient Photo-thermal Catalytic CO2 Cycloaddition by a Radical Mechanism.

Developing efficient and earth-abundant catalysts for CO2 fixation to high value-added chemicals is meaningful but challenging. Styrene carbonate has great market value, but the cycloaddition of CO2 to styrene oxide is difficult due to the high steric hindrance and weak electron-withdrawing ability of the phenyl group. To utilize clean energy (such as optical energy) directly and effectively for CO2 value-added process, we introduce earth-abundant Ti single-atom into the mesoporous nitrogen, oxygen-doped carbon nanosheets (Ti-CNO) by a two-step method. The Ti-CNO exhibits excellent photothermal catalytic activities and stability for cycloaddition of CO2 and styrene oxide to styrene carbonate. Under light irradiation and ambient pressure, an optimal Ti-CNO produces styrene carbonate with a yield of 98.3 %, much higher than CN (27.1 %). In addition, it shows remarkable stability during 10 consecutive cycles. Its enhanced catalytic performance stems from the enhanced photothermal effect and improved Lewis acidic/basic sites exposed by the abundant mesopores. The experiments and theoretical simulations demonstrate the styrene oxide⋅+ and CO2⋅- radicals generated at the Lewis acidic (Tiδ+) and basic sites of Ti-CNO under light irradiation, respectively. This work furnishes a strategy for synthesizing advanced single-atom catalysts for photo-thermal synergistic CO2 fixation to high value products via a cycloaddition pathway.

One-Step Electrochemical Ethylene-to-Ethylene Glycol Conversion over a Multitasking Molecular Catalyst.

Ethylene glycol is an essential commodity chemical with high demand, which is conventionally produced via thermocatalytic oxidation of ethylene with huge fossil fuel consumption and CO2 emission. The one-step electrochemical approach offers a sustainable route but suffers from reliance on noble metal catalysts, low activity, and mediocre selectivity. Herein, we report a one-step electrochemical oxidation of ethylene to ethylene glycol over an earth-abundant metal-based molecular catalyst, a cobalt phthalocyanine supported on a carbon nanotube (CoPc/CNT). The catalyst delivers ethylene glycol with 100% selectivity and 1.78 min-1 turnover frequency at room temperature and ambient pressure, more competitive than those obtained over palladium catalysts. Experimental data demonstrate that the catalyst orchestrates multiple tasks in sequence, involving electrochemical water activation to generate high-valence Co-oxo species, ethylene epoxidation to afford an ethylene oxide intermediate via oxygen transfer, and eventually ring-opening of ethylene oxide to ethylene glycol facilitated by in situ formed Lewis acid site. This work offers a great opportunity for commodity chemicals synthesis based on a one-step, earth-abundant metal-catalyzed, and renewable electricity-driven route.

Understanding the Activity Trends in Electrocatalytic Nitrate Reduction to Ammonia on Cu Catalysts.

Cu-based catalysts possess great potential in the electrocatalytic nitrate (NO3-) reduction reaction for ammonia (NH3) synthesis. However, the low atomic economy limits their further application. Here we report a Cu single-atom (SA) incorporated in nitrogen-doped carbon (Cu SA/NC) with high atomic economy, which exhibits superior NH3 Faradaic efficiency (FE) of 100% along with an impressive NH3 yield rate of 7480 µg h-1 mgcat.-1. As counterparts, Cus+n/NC, with mixed SA and nanoparticles (NPs), shows decreasing NH3 FE with decreasing SA content, but the production of N2 and N2O increases gradually, which reaches the maximum on pure Cu NPs. In situ characterizations and theoretical calculations reveal that a higher NH3 FE of Cu SA/NC is ascribed to a lower free energy of the rate-limiting step (HNO* → N*) and effective inhibition for the N-N coupled process. This work provides the intuitive activity trends of Cu-based catalysts, opening an avenue for subsequent catalysts design.

Electrochemical carbon-carbon coupling with enhanced activity and racemate stereoselectivity by microenvironment regulation.

Enzymes are characteristic of catalytic efficiency and specificity by maneuvering multiple components in concert at a confined nanoscale space. However, achieving such a configuration in artificial catalysts remains challenging. Herein, we report a microenvironment regulation strategy by modifying carbon paper with hexadecyltrimethylammonium cations, delivering electrochemical carbon-carbon coupling of benzaldehyde with enhanced activity and racemate stereoselectivity. The modified electrode-electrolyte interface creates an optimal microenvironment for electrocatalysis-it engenders dipolar interaction with the reaction intermediate, giving a 2.2-fold higher reaction rate (from 0.13 to 0.28 mmol h-1 cm-2); Moreover, it repels interfacial water and modulates the conformational specificity of reaction intermediate by facilitating intermolecular hydrogen bonding, affording 2.5-fold higher diastereomeric ratio of racemate to mesomer (from 0.73 to 1.82). We expect that the microenvironment regulation strategy will lead to the advanced design of electrode-electrolyte interface for enhanced activity and (stereo)selectivity that mimics enzymes.

Unraveling the potential-dependent structure evolution in CuO for electrocatalytic biomass valorization.

Electrocatalytic oxidation of renewable biomass (such as glucose) into high-value-added chemicals provides an effective approach to achieving carbon neutrality. CuO-derived materials are among the most promising electrocatalysts for biomass electrooxidation, but the identification of their active sites under electrochemical conditions remains elusive. Herein, we report a potential-dependent structure evolution over CuO in the glucose oxidation reaction (GOR). Through systematic electrochemical and spectroscopic characterizations, we unveil that CuO undergoes Cu2+/Cu+ and Cu3+/Cu2+ redox processes at increased potentials with successive generation of Cu(OH)2 and CuOOH as the active phases. In addition, these two structures have distinct activities in the GOR, with Cu(OH)2 being favorable for aldehyde oxidation, and CuOOH showed faster kinetics in carbon-carbon cleavage and alcohol/aldehyde oxidation. This work deepens our understanding of the dynamic reconstruction of Cu-based catalysts under electrochemical conditions and may guide rational material design for biomass valorization.

Scalable electrosynthesis of commodity chemicals from biomass by suppressing non-Faradaic transformations.

Electrooxidation of biomass platforms provides a sustainable route to produce valuable oxygenates, but the practical implementation is hampered by the severe carbon loss stemming from inherent instability of substrates and/or intermediates in alkaline electrolyte, especially under high concentration. Herein, based on the understanding of non-Faradaic degradation, we develop a single-pass continuous flow reactor (SPCFR) system with high ratio of electrode-area/electrolyte-volume, short duration time of substrates in the reactor, and separate feeding of substrate and alkaline solution, thus largely suppressing non-Faradaic degradation. By constructing a nine-stacked-modules SPCFR system, we achieve electrooxidation of glucose-to-formate and 5-hydroxymethylfurfural (HMF)-to-2,5-furandicarboxylic acid (FDCA) with high single-pass conversion efficiency (SPCE; 81.8% and 95.8%, respectively) and high selectivity (formate: 76.5%, FDCA: 96.9%) at high concentrations (formate: 562.8 mM, FDCA: 556.9 mM). Furthermore, we demonstrate continuous and kilogram-scale electrosynthesis of potassium diformate (0.7 kg) from wood and soybean oil, and FDCA (1.17 kg) from HMF. This work highlights the importance of understanding and suppressing non-Faradaic degradation, providing opportunities for scalable biomass upgrading using electrochemical technology.

Rechargeable Biomass Battery for Electricity Storage/generation and Concurrent Valuable Chemicals Production.

The development of a rechargeable battery that can produce valuable chemicals in both electricity storage and generation processes holds great promise for increasing the electron economy and economic value. However, this battery has yet to be explored. Herein, we report a biomass flow battery that generates electricity while producing furoic acid, and store electricity while yielding furfuryl alcohol. The battery is composed of a rhodium-copper (Rh1 Cu) single-atom alloy as anode, a cobalt-doped nickel hydroxide (Co0.2 Ni0.8 (OH)2 ) as cathode, and furfural-containing anolyte. In a full battery evaluation, this battery displays an open circuit voltage (OCV) of 1.29 V and a peak power density up to 107 mW cm-2 , surpassing most catalysis-battery hybrid systems. As a proof-of-concept, we demonstrate that this battery produces 1 kg furoic acid with 0.78 kWh electricity output, and yields 0.62 kg furfuryl alcohol when 1 kWh electricity is stored. This work may shed light on the design of rechargeable batteries with value-added functionality such as chemicals production.

Protocol for photoelectrocatalytic synthesis of aromatic azo compounds from aromatic amines.

Photoelectrocatalytic (PEC) strategy has emerged as a promising approach to drive organic reactions under mild conditions. Here, we present a protocol for PEC oxidative coupling of aromatic amines to produce aromatic azo compounds over a porous BiVO4 nanoarray (BiVO4-NA) photoanode. We describe the fabrication of BiVO4-NA photoanode and the detailed steps for the PEC oxidative coupling reaction, including key performance data of the BiVO4-NA photoanode for synthesizing azobenzene from aniline. For complete details on the use and execution of this protocol, please refer to Luo et al. (2022).1.

Electrocatalytic Upcycling of Biomass and Plastic Wastes to Biodegradable Polymer Monomers and Hydrogen Fuel at High Current Densities.

Transformation of biomass and plastic wastes to value-added chemicals and fuels is considered an upcycling process that is beneficial to resource utilization. Electrocatalysis offers a sustainable approach; however, it remains a huge challenge to increase the current density and deliver market-demanded chemicals with high selectivity. Herein, we demonstrate an electrocatalytic strategy for upcycling glycerol (from biodiesel byproduct) to lactic acid and ethylene glycol (from polyethylene terephthalate waste) to glycolic acid, with both products being as valuable monomers for biodegradable polymer production. By using a nickel hydroxide-supported gold electrocatalyst (Au/Ni(OH)2), we achieve high selectivities of lactic acid and glycolic acid (77 and 91%, respectively) with high current densities at moderate potentials (317.7 mA/cm2 at 0.95 V vs RHE and 326.2 mA/cm2 at 1.15 V vs RHE, respectively). We reveal that glycerol and ethylene glycol can be enriched at the Au/Ni(OH)2 interface through their adjacent hydroxyl groups, substantially increasing local concentrations and thus high current densities. As a proof of concept, we employed a membrane-free flow electrolyzer for upcycling triglyceride and PET bottles, attaining 11.2 g of lactic acid coupled with 9.3 L of H2 and 13.7 g of glycolic acid coupled with 9.4 L of H2, respectively, revealing the potential of coproduction of valuable chemicals and H2 fuel from wastes in a sustainable fashion.

Identification of Active Sites Formed on Cobalt Oxyhydroxide in Glucose Electrooxidation.

Transition-metal-based oxyhydroxides are efficient catalysts in biomass electrooxidation towards fossil-fuel-free production of valuable chemicals. However, identification of active sites remains elusive. Herein, using cobalt oxyhydroxide (CoOOH) as the archetype and the electrocatalyzed glucose oxidation reaction (GOR) as the model reaction, we track dynamic transformation of the electronic and atomic structure of the catalyst using a suite of operando and ex situ techniques. We reveal that two types of reducible Co3+ -oxo species are afforded for the GOR, including adsorbed hydroxyl on Co3+ ion (µ1 -OH-Co3+ ) and di-Co3+ -bridged lattice oxygen (µ2 -O-Co3+ ). Moreover, theoretical calculations unveil that µ1 -OH-Co3+ is responsible for oxygenation, while µ2 -O-Co3+ mainly contributes to dehydrogenation, both as key oxidative steps in glucose-to-formate transformation. This work provides a framework for mechanistic understanding of the complex near-surface chemistry of metal oxyhydroxides in biomass electrorefining.

Nitrite Electroreduction to Ammonia Promoted by Molecular Carbon Dioxide with Near-unity Faradaic Efficiency.

Electrochemical reduction of nitrite (NO2 - ) offers an energy-efficient route for ammonia (NH3 ) synthesis and reduction of the level of nitrite, which is one of the major pollutants in water. However, the near 100 % Faradaic efficiency (FE) has yet to be achieved due to the complicated reduction route with several intermediates. Here, we report that carbon dioxide (CO2 ) can enhance the nitrite electroreduction to ammonia on copper nanowire (Cu NW) catalysts. In a broad potential range (-0.7∼-1.3 V vs. RHE), the FE of nitrite to ammonia is close to 100 % with a 3.5-fold increase in activity compared to that obtained without CO2. In situ Raman spectroscopy and density functional theory (DFT) calculations indicate that CO2 acts as a catalyst to facilitate the *NO to *N step, which is the rate determining step for ammonia synthesis. The promotion effect of CO2 can be expanded to electroreduction of other nitro-compounds, such as nitrate to ammonia and nitrobenzene to aniline.

Electrocatalytic synthesis of adipic acid coupled with H2 production enhanced by a ligand modification strategy.

Adipic acid is an important building block of polymers, and is commercially produced by thermo-catalytic oxidation of ketone-alcohol oil (a mixture of cyclohexanol and cyclohexanone). However, this process heavily relies on the use of corrosive nitric acid while releases nitrous oxide as a potent greenhouse gas. Herein, we report an electrocatalytic strategy for the oxidation of cyclohexanone to adipic acid coupled with H2 production over a nickel hydroxide (Ni(OH)2) catalyst modified with sodium dodecyl sulfonate (SDS). The intercalated SDS facilitates the enrichment of immiscible cyclohexanone in aqueous medium, thus achieving 3.6-fold greater productivity of adipic acid and higher faradaic efficiency (FE) compared with pure Ni(OH)2 (93% versus 56%). This strategy is demonstrated effective for a variety of immiscible aldehydes and ketones in aqueous solution. Furthermore, we design a realistic two-electrode flow electrolyzer for electrooxidation of cyclohexanone coupling with H2 production, attaining adipic acid productivity of 4.7 mmol coupled with H2 productivity of 8.0 L at 0.8 A (corresponding to 30 mA cm-2) in 24 h.

Electrocatalytic Hydrogenation of 5-Hydroxymethylfurfural Promoted by a Ru1 Cu Single-Atom Alloy Catalyst.

Electrochemical reduction of biomass-derived 5-hydroxymethylfurfural (HMF) represents an elegant route toward sustainable value-added chemicals production that circumvents the use of fossil fuel and hydrogen. However, the reaction efficiency is hampered by the high voltage and low activity of electrodes (Cu, Bi, Pb). Herein, we report a Ru1 Cu single-atom alloy (SAA) catalyst with isolated Ru atoms on Cu nanowires that exhibits an electrochemical reduction of HMF to 2,5-dihydroxymethylfuran (DHMF) with promoted productivity (0.47 vs. 0.08 mmol cm-2 h-1 ) and faradic efficiency (FE) (85.6 vs. 71.3 %) at -0.3 V (vs. RHE) compared with Cu counterpart. More importantly, the FE (87.5 %) is largely retained at high HMF concentration (100 mM). Kinetic studies by using combined electrochemical techniques suggest disparate mechanisms over Ru1 Cu and Cu, revealing that single-atom Ru promotes the dissociation of water to produce H* species that effectively react with HMF via an electrocatalytic hydrogenation (ECH) mechanism.

Selective Photoelectrocatalytic Glycerol Oxidation to Dihydroxyacetone via Enhanced Middle Hydroxyl Adsorption over a Bi2O3-Incorporated Catalyst.

Photoelectrocatalytic (PEC) glycerol oxidation offers a sustainable approach to produce dihydroxyacetone (DHA) as a valuable chemical, which can find use in cosmetic, pharmaceutical industries, etc. However, it still suffers from the low selectivity (≤60%) that substantially limits the application. Here, we report the PEC oxidation of glycerol to DHA with a selectivity of 75.4% over a heterogeneous photoanode of Bi2O3 nanoparticles on TiO2 nanorod arrays (Bi2O3/TiO2). The selectivity of DHA can be maintained at ∼65% under a relatively high conversion of glycerol (∼50%). The existing p-n junction between Bi2O3 and TiO2 promotes charge transfer and thus guarantees high photocurrent density. Experimental combined with theoretical studies reveal that Bi2O3 prefers to interact with the middle hydroxyl of glycerol that facilitates the selective oxidation of glycerol to DHA. Comprehensive reaction mechanism studies suggest that the reaction follows two parallel pathways, including electrophilic OH* (major) and lattice oxygen (minor) oxidations. Finally, we designed a self-powered PEC system, achieving a DHA productivity of 1.04 mg cm-2 h-1 with >70% selectivity and a H2 productivity of 0.32 mL cm-2 h-1. This work may shed light on the potential of PEC strategy for biomass valorization toward value-added products via PEC anode surface engineering.

Selective Electrooxidation of Biomass-Derived Alcohols to Aldehydes in a Neutral Medium: Promoted Water Dissociation over a Nickel-Oxide-Supported Ruthenium Single-Atom Catalyst.

The biomass-derived alcohol oxidation reaction (BDAOR) holds great promise for sustainable production of chemicals. However, selective electrooxidation of alcohols to value-added aldehyde compounds is still challenging. Herein, we report the electrocatalytic BDAORs to selectively produce aldehydes using single-atom ruthenium on nickel oxide (Ru1 -NiO) as a catalyst in the neutral medium. For electrooxidation of 5-hydroxymethylfurfural (HMF), Ru1 -NiO exhibits a low potential of 1.283 V at 10 mA cm-2 , and an optimal 2,5-diformylfuran (DFF) selectivity of 90 %. Experimental studies reveal that the neutral electrolyte plays a critical role in achieving a high aldehyde selectivity, and the single-atom Ru boosts HMF oxidation in the neutral medium by promoting water dissociation to afford OH*. Furthermore, Ru1 -NiO can be extended to selective electrooxidation of a series of biomass-derived alcohols to corresponding aldehydes, which are conventionally difficult to obtain in the alkaline medium.

Electrocatalytic oxidative upgrading of biomass platform chemicals: from the aspect of reaction mechanism.

Oxidation reactions provide a wide range of important chemicals in industry; however, most of these chemicals are produced from fossil feedstocks. As a candidate of oxygen evolution reaction (OER), the electrooxidation of biomass platform chemicals instead of a petroleum source offers a sustainable and atom-economic avenue toward organic oxygenates, with additional benefits when coupled with renewable electricity driven processes. This highlight article describes the representative examples in this nascent area, including oxidative dehydrogenation, coupling, and cleavage. We classify the examples into inner-sphere and outer-sphere electrode reactions based on the classical electrocatalysis concept for better understanding of the reaction mechanism. Moreover, we highlight the recent progress in oxidative biomass electrorefining via inner-sphere anodic reactions, which are strongly dependent on the nature of the electrode material. Particularly, the understanding of the formation of reactive oxygen species, adsorption of substrates, and reconstruction of anode materials is presented. Finally, the existing challenges and perspectives are discussed.

Alcohols electrooxidation coupled with H2 production at high current densities promoted by a cooperative catalyst.

Electrochemical alcohols oxidation offers a promising approach to produce valuable chemicals and facilitate coupled H2 production. However, the corresponding current density is very low at moderate cell potential that substantially limits the overall productivity. Here we report the electrooxidation of benzyl alcohol coupled with H2 production at high current density (540 mA cm-2 at 1.5 V vs. RHE) over a cooperative catalyst of Au nanoparticles supported on cobalt oxyhydroxide nanosheets (Au/CoOOH). The absolute current can further reach 4.8 A at 2.0 V in a more realistic two-electrode membrane-free flow electrolyzer. Experimental combined with theoretical results indicate that the benzyl alcohol can be enriched at Au/CoOOH interface and oxidized by the electrophilic oxygen species (OH*) generated on CoOOH, leading to higher activity than pure Au. Based on the finding that the catalyst can be reversibly oxidized/reduced at anodic potential/open circuit, we design an intermittent potential (IP) strategy for long-term alcohol electrooxidation that achieves high current density (>250 mA cm-2) over 24 h with promoted productivity and decreased energy consumption.

Photoelectrocatalytic C-H halogenation over an oxygen vacancy-rich TiO2 photoanode.

Photoelectrochemical cells are emerging as powerful tools for organic synthesis. However, they have rarely been explored for C-H halogenation to produce organic halides of industrial and medicinal importance. Here we report a photoelectrocatalytic strategy for C-H halogenation using an oxygen-vacancy-rich TiO2 photoanode with NaX (X=Cl-, Br-, I-). Under illumination, the photogenerated holes in TiO2 oxidize the halide ions to corresponding radicals or X2, which then react with the substrates to yield organic halides. The PEC C-H halogenation strategy exhibits broad substrate scope, including arenes, heteroarenes, nonpolar cycloalkanes, and aliphatic hydrocarbons. Experimental and theoretical data reveal that the oxygen vacancy on TiO2 facilitates the photo-induced carriers separation efficiency and more importantly, promotes halide ions adsorption with intermediary strength and hence increases the activity. Moreover, we designed a self-powered PEC system and directly utilised seawater as both the electrolyte and chloride ions source, attaining chlorocyclohexane productivity of 412 µmol h-1 coupled with H2 productivity of 9.2 mL h-1, thus achieving a promising way to use solar for upcycling halogen in ocean resource into valuable organic halides.