Selective Electrochemical Oxidation Research Articles

A Green and Efficient Electrocatalytic Route for the Highly-Selective Oxidation of C-H Bonds in Aromatics over 1D Co3O4-Based Nanoarrays.

Electrocatalytic oxidation of C-H bonds in hydrocarbons represents an efficient and sustainable strategy for the synthesis of value-added chemicals. Herein, a highly selective and continuous-flow electrochemical oxidation process of toluene to various oxygenated products (benzyl alcohol, benzaldehyde, and benzyl acetate) is developed with the electrocatalytic membrane electrodes (ECMEs). The selectivity of target products can be manipulated via surface and interface engineering of Co3O4-based electrocatalysts. We achieved a high benzaldehyde selectivity of 90% at a toluene conversion of 47.6% using 1D-Co3O4 nanoneedles (NNs) loaded on a microfiltration (MF) titanium (Ti) membrane, i.e, Co3O4 NNs/Ti. In contrast, the main product shifted to benzyl alcohol with a selectivity of 90.1% at conversion of 32.1% after modifying MnO2 nanosheets (NSs) on Co3O4 NNs/Ti (Co3O4@MnO2/Ti) catalyst. Moreover, benzyl acetate product can be obtained withselectivity of 92% at a conversion of 58.5% at high current density (> 1.5mA cm-2), demonstrating that the pathway of toluene oxidation is readily maneuvered. DFT results reveal that modifying MnO2 on Co3O4 optimizes the electron structure of Co3O4@MnO2/Ti and modulates the adsorption behavior of intermediate species. This work demonstrates a sustainable,and continuous-flow process for precise control over production selectivity of value-added oxygenated derivatives in electrochemical oxidation of aromatic hydrocarbons.

Recovery of niobium and titanium from low-grade niobium ores and the evaluation of Fe5Si3 alloy by-product as oxygen evolution reaction electrocatalysts

A low-grade ore containing 4 % niobium and 7 % titanium has been extracted from Bayan Obo tailing, but its potential has not been fully utilized. This study aims to recover the niobium and titanium elements from this low-grade ores in the form of their carbides. The process involves reducing niobium and titanium to (Nb,Ti)C through a carbothermal process. During this process, iron and silicon are also reduced to Fe-Si intermetallic compounds, allowing (Nb,Ti)C to diffuse into them. Subsequently, selective electrochemical oxidation is used to separate (Nb,Ti)C from the Fe-Si intermetallic compounds in an aqueous ferrous sulfate solution. The Fe-Si intermetallic compounds are first oxidized to the Fe3Si phase, and then to the Fe5Si3 phase in the solution with pH 0.35. The (Nb,Ti)C/matrix interfaces are identified as the preferred site of electrochemical pitting corrosion. By exploiting the electrochemical oxidation reaction of iron in the matrix, (Nb,Ti)C powders are separated along these interfaces. The by-products of this process are FeSi alloys, which are evaluated for their electrocatalytic properties in oxygen evolution reactions. It is found that the Fe5Si3 alloy by-product exhibits good electrocatalytic activity and kinetics in the oxygen evolution reaction, demonstrating its potential for practical applications.

Structural Modification Effect of Se-doped Porous Carbon for Hydrogen Evolution Coupled Selective Electrooxidation of Ethylene Glycol to Value-added Glycolic Acid.

The ethylene glycol oxidation reaction (EGOR) has attracted attention because ethylene glycol (EG), which exhibits large-scale production and a low market price, can be reformed into valuable glycolic acid (GCA) with the cogeneration of high-purity hydrogen gas during the reaction. In this study, a noble catalyst material of Pt nanoparticles supported on Se-doped porous carbon (Pt/SePC) is prepared and investigated for the selective electrochemical oxidation of EG to GCA. Pt/SePC achieved a maximum EG conversion of 94.6% and GCA selectivity of 84.4% and maintained this high performance with negligible degradation during durability tests. Furthermore, the EGOR required lower overpotential rather than the oxygen evolution reaction, thus the EGOR coupled with the hydrogen evolution reaction can reduce the cell overpotential to 0.60 V, which is much lower than that of water electrolysis (1.58 V). The effect of Se doping is investigated through experimental analyses and density functional theory (DFT) calculations, and they shows that Se modified the binding energy of Pt nanoparticles and the adsorption energy of reactants by lattice deformation and charge density modification. This study provides scientific insights and strategies for electrocatalyst design for the selective oxidation of polyols to value-added chemicals via the cogeneration of hydrogen gas.

High-Dimensional Nb2O5 with NbO6 Octahedra for Efficient Electrocatalytic Upgrading of Methanol to Formate.

Facilitating the selective electrochemical oxidation of methanol into value-added formate is essential for electrochemical refining. Here we propose a high-dimensional Nb2O5 on Ni foam (Nb2O5-HD@NF) composite as anode for methanol oxidation reaction (MOR) for efficient production of formate. In an electrolyte containing 3 M methanol aqueous solution, the Nb2O5-HD@NF anode requires only 240 mV overpotential to deliver an industrial-level current density of 100 mA cm-2 with a formate Faraday efficiency of 100%. In situ Raman and electrochemical kinetic analyses reveal that the origin of the excellent activity in 3 M methanol electrolyte can be ascribed to the NbO6 octahedra as active sites and the Lewis acid sites on the surface of Nb2O5-HD. This work may pave a way for the design of non-noble metal electrocatalysts with surface acidity engineering for the effective electrocatalytic upgrading of biomass molecules.

A Highly Efficient Electrosynthesis of Formaldehyde Using a TEMPO-Based Polymer Electrocatalyst.

In the chemical industry, formaldehyde is an important bulk chemical. The traditional synthesis of formaldehyde involves an energy intensive oxidation of methanol over a metal oxide catalyst. The selective electrochemical oxidation of methanol is challenging. Herein, we report a catalytic system with an immobilized TEMPO electrode that selectively oxidizes methanol to formaldehyde with high turnover numbers. Upon the addition of various organic and inorganic bases, the activity of the catalyst could be tuned. The highest Faradaic efficiency that was achieved was 97.5 %, the highest turnover number was 17100. Additionally, we found that the rate determining step changed from the step in which the carbonyl specie is created from the methanol-TEMPO adduct to the oxidative regeneration of the TEMPO+ species. Finally, we showed that the system could be applied to the oxidation of other aliphatic alcohols.

Selective electrochemical oxidation of organic compounds in a mass transfer-enhanced electrochemical flow reactor (e[sbnd]NETmix)

eNETmix stands as an electrochemical flow reactor engineered to enhance mass transfer. This study aimed at assessing the performance of the eNETmix reactor in the realm of organic electrosynthesis. Specifically, the research focused on the selective electrochemical oxidation of 4-methoxybenzyl alcohol (4-MBA) to p-anisaldehyde (PAA) using a bare fluorine-doped tin oxide (FTO) anode. The efficiency of the process was assessed for distinct current densities (j), Reynolds numbers (Re), supporting electrolyte contents, and substrate initial contents. The eNETmix reactor was extensively compared to a commercial electrochemical flow reactor (MicroFlowCell from ElectroCell, Denmark). eNETmix facilitated the use of a broader range of j (0.8–2.0 mA cm−2versus 0.8 mA cm−2) together with smaller Re (≥190 versus >1750), supporting electrolyte contents (≥1 mM versus ≥30 mM), and substrate initial contents (≥2.0 mM versus ≥3.0 mM) with no loss of PAA production or energy consumption. These findings underscore a remarkable suitability of eNETmix as a reactor for organic electrosynthesis.

Highly Selective Electrochemical Baeyer-Villiger Oxidation through Oxygen Atom Transfer from Water.

The Baeyer-Villiger oxidation of ketones is a crucial oxygen atom transfer (OAT) process used for ester production. Traditionally, Baeyer-Villiger oxidation is accomplished by thermally oxidizing the OAT from stoichiometric peroxides, which are often difficult to handle. Electrochemical methods hold promise for breaking the limitation of using water as the oxygen atom source. Nevertheless, existing demonstrations of electrochemical Baeyer-Villiger oxidation face the challenges of low selectivity. We report in this study a strategy to overcome this challenge. By employing a well-known water oxidation catalyst, Fe2O3, we achieved nearly perfect selectivity for the electrochemical Baeyer-Villiger oxidation of cyclohexanone. Mechanistic studies suggest that it is essential to produce surface hydroperoxo intermediates (M-OOH, where M represents a metal center) that promote the nucleophilic attack on ketone substrates. By confining the reactions to the catalyst surfaces, competing reactions (e.g., dehydrogenation, carboxylic acid cation rearrangements, and hydroxylation) are greatly limited, thereby offering high selectivity. The surface-initiated nature of the reaction is confirmed by kinetic studies and spectroelectrochemical characterizations. This discovery adds nucleophilic oxidation to the toolbox of electrochemical organic synthesis.

Rational coupling of selective electrochemical oxidation and reduction reactions for in-situ value-added chemical generation

In this review, the recent developments achieved in the electrochemical production of Bi-value-added chemicals by replacing the conventional (oxygen evolution reaction) OER and (hydrogen evolution reaction) HER with thermodynamically favorable reactions are summarized. Beyond conventional electrolysis used for the generation of a single value-added product at the anode or cathode, hybrid electrolysis opens a new avenue for the simultaneous production of value-added concurrently at the anode and cathode. This new strategy can not only increase the efficiency of value-added products, but also abolish the generation of explosive gaseous products by eliminating conventional OER and HER. To construct a hybrid electrolyzer, various anodic oxidation reactions including the oxidation of glucose, glycerol, CH4, 1,2-propanediol, 1-phenylethanol, isopropanol, benzyl alcohol, biomass-derived HMF, syringaldehyde, and o-phenylenediamine are used to replace the anodic OER, whereas CO2RR is mostly used to replace the cathodic HER. Furthermore, the collective efforts of the electrocatalyst, device architecture, and fabrication method toward selective product formation are explained in detail.

Selective Electrochemical Oxidation of Benzylic C-H to Benzylic Alcohols with the Aid of Imidazolium Radical Mediators.

Selective oxidation of benzylic C-H to benzylic alcohols is a well-known challenge in the chemical community since benzylic C-H is more prone to be overoxidized to benzylic ketones. In this work, we report the highly selective electro-oxidation of benzylic C-H to benzylic alcohols in an undivided cell in ionic liquid-based solution. As an example, the selectivity toward xanthydrol could be as high as 95.7% at complete conversion of xanthene, a typical benzylic C-H compound, on gram-scale in imidazolium bromide/H2O/DMF. Mechanism investigation reveals that the imidazolium radical generated in situ participants in a proton-coupled electron transfer process and low-barrier hydrogen bonds stabilize the reaction intermediates, together steering the redox equilibrium, favoring benzylic alcohols over benzylic ketones.

Factors affecting selective electrochemical glycerol oxidation to three-carbon products over cuprous oxide microcubes

BackgroundIn spite of the promising potential of selective electrochemical glycerol oxidation reaction (EGOR) as a sustainable method for converting glycerol into valuable products, the factors governing selective EGOR to desired products remain inadequately understood. MethodIn this study, we investigated the key factors governing selective EGOR using Cu2O as an electrocatalyst. By combining electrochemical studies paired with high-performance liquid chromatography, we gained a detailed understanding of the reaction pathway. Significant findingsThe results demonstrate that Cu2O is an efficient and stable electrocatalyst, producing valuable three-carbon products, including dihydroxyacetone (DHA), glyceraldehyde (GLYD), and glyceric acid (GLAC), with a selectivity of 90%. Notably, DHA was produced with selectivity as high as 45%. Our investigations reveal that while the applied potential and glycerol concentration primarily govern the reaction rates, temperature is the most significant factor for tuning the product distribution. However, it should be noted that raising the electrolyte temperature to 50 °C can lead to the self-degradation of intermediates to a certain content, which can impact product selectivity. Our work contributes to a better understanding of the factors that control EGOR and provides a foundation for future research in this field.

Selective Electrochemical Oxidation of p-Xylylene Glycol Coupled with the Hydrogen Evolution Reaction via a Continuous Flow Strategy

Selective Electrochemical Oxidation of <i>p</i>-Xylylene Glycol Coupled with the Hydrogen Evolution Reaction via a Continuous Flow Strategy

Self-supporting CuM LDH (M = Ni, Fe, Co) carbon cloth electrodes for selective electrochemical ammonia oxidation to nitrogen

Electrochemical ammonia oxidation reaction (eAOR) has important applications in ammonia fuel cells and wastewater treatment due to its environmental friendliness and convenient operation. In this study, hydrothermal synthesis was used to create self-supported CuM (M = Ni, Fe, Co) carbon cloth electrodes. After three hours of electrolysis at 1.65 V vs. RHE, a CuCo/CC electrode with the ideal Cu/Co ratio (1:2) achieved the maximum dinitrogen selectivity of 90.0% and Faraday efficiency of 77.4%. The reaction mechanism was investigated using Raman and in-situ IR spectroscopy. Cu serves as NH3 adsorption sites and Co catalyzes the oxidation, and together achieve high efficiency.

Pt-Based Electrocatalyst Modified by CsH2PO4/SiP2O7 for Electrochemical Oxidation of NH3 to H2 in Solid Acid Electrolysis Cell

Ammonia (NH3) has received much attention as a hydrogen carrier because it can be easily liquefied with a high hydrogen storage density and emits no greenhouse gas during the dihydrogen evolution process. The ammonia oxidation reaction (AOR) in an electrochemical system has an important merit in which a very high-purity dihydrogen gas can be obtained without an additional separation process that is typically needed for thermochemical decomposition processes. Herein, the electrochemical AOR was carried out in a solid acid electrolysis cell (SAEC) at an intermediate temperature around 250 °C, in which a solid composite of CsH2PO4 mixed with SiP2O7 was used as an electrolyte and Pt/C-based electrocatalysts were employed as the electrode materials of both anode and cathode. The Pt/C electrode material was modified with the CsH2PO4/SiP2O7 electrolyte in order to enhance the electrocatalytic activity for the AOR with an improved H2 production rate. Over the SAEC system reported here, a high AOR performance was obtained with a current density of 67.1 mA/cm2 and Faradaic efficiency (FE) of 98.2%. This study can suggest the significant potential of SAEC for the carbon-free H2 production from the selective electrochemical oxidation of NH3.

Efficient and highly selective direct electrochemical oxidation of ammonia to dinitrogen facilitated by NiCu diatomic site catalysts

Direct electrochemical oxidation has recently received increasing attention in ammonia-nitrogen wastewater treatment. However, how to catalyze the oxidation of ammonia to N2 with high selectivity retains a tremendous challenge. In this work, NiCu3-N-C diatomic site catalyst (DAC) anchored in nitrogen-doped carbon was synthesized via solid-phase pyrolysis. HAADF-STEM and XAFS characterizations demonstrated the high dispersion of adjacent Ni/Cu diatomic sites and the micro-environment of Ni-N4/Cu-N4 groups. The experiment results exhibit that NiCu3-N-C DAC has an eminent intrinsic activity towards AOR with a TOF of 3.64 s−1 at 1.50 V vs. RHE. Consequently, in a real wastewater with 616 mg/L NH4+-N, 99.52% of removal efficiency and 86.60% of faradaic efficiency (FE) after a 6-hour degradation experiment were acquired with a high N2 selectivity of 97.87%, confirming its excellent catalytic performance. This work furnishes a good example of rationally designing functionally coordinated diatomic site catalysts (DACs) for the efficient treatment of ammonia-nitrogen wastewater.

Selective indirect electrochemical oxidation of sulfides and thiols using DDQ as an efficient and cost-effective electrocatalyst

The efficient oxidation of sulfides to sulfoxides and the oxidative coupling of thiols to disulfides was performed electrochemically in the presence of DDQ as an electrocatalyst. The electrochemical reactions were conducted in a mixture of water and acetonitrile as solvent under constant potential (0.6 V vs Ag/AgCl) for regeneration of DDQ at room temperature and the products were selectively produced in good to high yields. The electrocatalytic properties of the applied electrocatalyst for the oxidation of substrates and the proposed electrocatalytic mechanism were investigated by the cyclic voltammetry (CV) technique. The proposed electrochemical regeneration of DDQ can be developed for the synthesis of different organic compounds in a single operation under mild conditions

Advances in Selective Electrochemical Oxidation of 5-Hydroxymethylfurfural to Produce High-Value Chemicals.

The conversion of biomass is a favorable alternative to the fossil energy route to solve the energy crisis and environmental pollution. As one of the most versatile platform compounds, 5-hydroxymethylfural (HMF) can be transformed to various value-added chemicals via electrolysis combining with renewable energy. Here, the recent advances in electrochemical oxidation of HMF, from reaction mechanism to reactor design are reviewed. First, the reaction mechanism and pathway are summarized systematically. Second, the parameters easy to be ignored are emphasized and discussed. Then, the electrocatalysts are reviewed comprehensively for different products and the reactors are introduced. Finally, future efforts on exploring reaction mechanism, electrocatalysts, and reactor are prospected. This review provides a deeper understanding of mechanism for electrochemical oxidation of HMF, the design of electrocatalyst and reactor, which is expected to promote the economical and efficient electrochemical conversion of biomass for industrial applications.

Highly selective electrochemical oxidation of 2-nitro-4-methylsulfonyltoluene to 2-nitro-4-methylsulfonylbenzoic acid enabled by molybdate

Highly selective electrochemical oxidation of 2-nitro-4-methylsulfonyltoluene to 2-nitro-4-methylsulfonylbenzoic acid enabled by molybdate

Proton conduction and electrochemical glucose sensing property of a newly constructed Cu(II) coordination polymer

The assembly between 5-sulfo-1, 2, 4-benzoic acid (H4SBTB) and Cu2+ in the presence of 4, 4´-bipyridine (Bipy) results in a novel ionic coordination polymer with two crystallographically independent hexa-coordinated Cu1 and Cu2 ions, namely, [Cu3(HSBTB)2(Bipy)3(H2O)8]n (1), which consists of an anionic Cu12(C1O1O2)2-Bipy-bridged double-chain and a cationic Cu2-Bipy-bridged mono-chain. With the aid of H-bonds, the anionic Cu12(C1O1O2)2-Bipy-bridged double-chains extend into a 2D supramolecular anionic double-layer, which is further linked by the cationic Cu2-Bipy-bridged mono-chain, resulting in a 3D supramolecular network. The H4SBTB acid exhibits a new coordination mode, i.e. one carboxyl group still remains protonated in 1. The analysis of the solid-state diffuse reflectance UV-Vis-NIR spectrum reveals that the indirect bandgap exists in 1, which was further illustrated by the theoretical calculation on the basis of Density Functional Theory (DFT) using the CASTEP program. The electrochemical studies indicate that the composite membrane of 1 with nafion shows interesting temperature-insensitive proton conduction behavior at pH = 3. Cyclic voltammograms indicate that 1 exhibits electrocatalysis to glucose oxidation in 0.1 M NaOH. The current-time (i-t) response curves reveal that 1 demonstrates sensitive and selective electrochemical oxidation sensing of glucose with an exponential detection range of 0-30 mM. The limit of detection (LOD) was calculated to be 38.74 μM.

Highly Sensitive and Selective Detection of Formaldehyde via Bio-Electrocatalysis over Aldehyde Dehydrogenase.

Formaldehyde (HCHO), as one of the prominent indoor pollutants, causes many health-related problems. Although the detection of HCHO is a widespread concern and a variety of detection methods have been continuously developed, the volatile organic chemical (VOC) interference remains to be solved. Here, we report a highly sensitive and selective method for HCHO detection, relying on the selective electrochemical oxidation of formaldehyde catalyzed by aldehyde dehydrogenases (ALDHs) on a Cu electrode. The detection signal exhibits a standard power law relationship against the analytes with a broad detection range of 10-5-10-15 M and a limit of detection (LOD) of 1.46 × 10-15 M, far below the indoor safe exposure limit (about 10-9 M) for formaldehyde. In comparison to the standard spectrophotometry method, the ALDH-based electrochemical method shows a much high specificity to formaldehyde among common VOCs, such as benzene, toluene, and xylene. This simple yet effective detection technique opens up a new path for developing advanced formaldehyde sensors with high sensitivity and selectivity.

C, N co-doped mesoporous Co-based phosphates via glucose-mediated regulation for the selective electrochemical water oxidation in alkaline realistic seawater

The development of highly efficient and stable anode electrocatalysts is highly demanding and challenging in seawater electrocatalysis for industrial application. Herein, we have constructed C, N co-doped mesoporous Co-based hybrid phosphates via a facile glucose-mediated regulation. The optimal C4-2-0.5Glu-550 by calcination at a mild temperature exhibits superior OER performance in 1.0 M KOH seawater with an overpotential of 296 mV at 10 mA cm−2 and a Tafel slope of 109 mV dec−1 as well as long-term stability (for at least 19 h). Combined with structural characterization, its excellent electrocatalytic performance should be attributed that the proper heteroatom doping regulates its electronic structure, correspondingly, adjusts the adsorption strength of the intermediates, in addition, its open porous structure increases exposed active sites and facilitates the electrons/mass transfer and the adsorption of reactants, above all, the carbon layer formed by the introduction of glucose significantly improves its corrosion resistance, further enhancing its performance in alkaline realistic seawater.