Enhancing Electrocatalytic Hydrodechlorination through Interfacial Microenvironment Modulation

Pd1-O-Ti dual sites for efficient electrochemical active hydrogen generation and bromate reduction

Atomic hydrogen (H*) plays a crucial role in electrochemical reduction technology towards various environmental and energy applications, but suffers from low utilization efficiency arisen from the undesirable H-H dimerization and the competitive adsorption between water molecule with reactants on the traditional adjacent catalytic sites. Herein, we anchored Pd single atoms on the naturally formed titanium oxide of titanium foam to construct Pd1-O-Ti dual-site electrocatalyst with spatially isolated water dissociation and H* utilization site, which synchronously inhibits the H-H dimerization and the competitive adsorption of water molecule and targeted reactants. Experiments and theoretical calculations revealed that the Ti-O sites could synergistically dissociate water to H*, which overflowed to nearby Pd single-atom sites for designed reduction reactions and utilization benefiting from the hydrogen spillover ability of titanium oxide substrate. These Pd1-O-Ti dual sites delivered almost 100 % bromate reduction efficiency with a rate constant of 1.57 h-1, far superior to those of Pdn-O-Ti with adjacent Pd sites (0.52 h-1), Pd1-N-C with single sites (0.04 h-1) and commercial Pd/C (0.18 h-1), respectively. This study sheds light on the importance of integrating synergistic active sites for complicated electrochemical reactions, and provide new insights in improving H* utilization for environmental remediation.

Combining a Pd Cluster and a Built-in Electric Field as a Biomimic for Stable C-Cl Bond Polarization

Adopting the essence of enzyme catalysis, the strong binding of substrates into the active site pocket for their selective activation through multiple noncovalent interactions in the reactive site design can effectively enhance the electrocatalysis process. However, mimicking the enzyme catalytic process, particularly the introduction of reactant activation mechanisms, remains a significant challenge. Herein, we present a Pd cluster inside the Fe2N-Fe3O4-based built-in electric field (BEF), denoted as Pd/Fe2N-Fe3O4, to serve as an enzyme mimic to activate stable C-Cl bonds. Theoretical calculations and in situ Raman indicate that the probe molecule 2,4-dichlorophenol (2,4-DCP) adsorbs onto the Pd site and rotates inside the BEF with the C4-Cl bond being selectively activated and elongated from 1.73 to 1.82 Å. This makes Pd/Fe2N-Fe3O4 an excellent electrocatalytic hydrodechlorination catalyst, with Pd usage down to 2.5 μg cm-2, which is 32.7-360 times less than that of conventional catalysts like Pd/C, and achieving a Faradaic efficiency exceeding 20%. We reveal that besides H*-mediated electrochemical reduction, Pd/Fe2N-Fe3O4 also hydrodechlorinates activated 2,4-DCP via the proton-electron coupled transfer pathway. This understanding of the role of BEF in reactant activation, along with the strategy of integrating BEF and noble metals to mimic enzymes, provides a direction for the design of advanced electrocatalysts.

Electronic structure modulation on PdCo bimetal for enhanced atomic hydrogen mediated electro-dehalogention of 2,4,6-trichlorophenol

Atomic hydrogen (H*)-mediated indirect electrochemical reduction has regarded as a highly effective strategy for halogenated organic pollutants (HOPs) removal in wastewater. The efficient generation and prompt utilization of H* are critical factors which directly determine the degradation performance. This study presented a new strategy for the electro-reduction of a typical HOP-2,4,6-trichlorophenol (2,4,6-TCP), achieved by modulating the electronic structure of a PdCo bimetal electrode (Pd1Co1/CP). Through characterizations and density functional theory (DFT) analysis, ZIF-67 as the Co source could effectively improve the uniform distribution of Pd and Co, which exposed more active sites for H* on-demand generation. Meanwhile, the introduction of Co atoms and the optimum of Pd/Co ratios modulated the electronic structure of Pd sites. In 120 min, 92 % 2,4,6-TCP (initial concentration 10 mg/L) was degradated with a reaction rate constant (kobs) of 0.103 min-1, which was 2.1 times higher than that of Pd/CP electrode. Lower Gibbs free energy for hydrogen adsorption (-0.477 eV) was found in Pd1Co1/CP which benefited the surface-adsorbed H* stability for enhanced utilization. The degradation pathway of 2,4,6-TCP was further studied. These findings highlight the critical role of modulating the electronic structure of the electrode to optimize the utilization of surface-adsorbed active intermediates, presenting a novel and promising approach for advancing wastewater treatment technologies.

Electrochemical Characterization of Pyrogenic Carbons: Implications for Degradation of Groundwater Contaminants with Carbon-Based Amendments

Pyrogenic carbons (PCs) are important mediators of biogeochemical redox reactions and contribute to contaminant degradation in some remediation methods. Contaminant degradation by electron transfer from PC is influenced by its electron conductivity and storage capacity, and atom transfer reactions involving atomic hydrogen, but resolving their contributions is difficult. In this study, we developed a method to modify working electrodes with agarose-stabilized thin films of PCs and used them to characterize PC reactivity using 2,4,6-trinitrotoluene (TNT) as a model contaminant and several electrochemical methods. Chronoamperometry showed that TNT reduction rates increase with increasing PC temperature, and electrochemical impedance spectroscopy showed that this is because the heterogeneous electron transfer rate constants (k0) are inversely correlated to resistivity (Rp) of the PC films. As a result, PCs like biochars, which have higher charge transfer resistance (Rct), give lower TNT reduction rates, whereas lower Rct PCs like activated carbons give faster TNT reduction. Current from reduction of H2O/H+ to H2 (HER) was significant only for low Rp PC-films on Pt working electrodes. The reactive intermediates involved in HER react preferentially with quinonoid moieties on the PC, thereby competing with and limiting TNT reduction at low overpotentials ( Collapse

Key Words

• capacitance
• hydrogen evolution reaction
• pyrogenic carbon
• resistance
• rotating disk electrode
• thin film

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MESH Headings

• Carbon/chemistry
• Groundwater/chemistry
• Trinitrotoluene/chemistry
• Water Pollutants, Chemical/chemistry
• Electrochemical Techniques
• Oxidation-Reduction
• Charcoal/chemistry

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Grants Collapse

Affiliation(s)

Jeffrey M Hudson OHSU/PSU School of Public Health, Oregon Health & Science University, Portland, Oregon 97239, United States
Paul G Tratnyek OHSU/PSU School of Public Health, Oregon Health & Science University, Portland, Oregon 97239, United States

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Collaborators Collapse

Electrochemical reduction for chlorinated hydrocarbons contaminated groundwater remediation: Mechanisms, challenges, and perspectives

Electrochemical reduction technology is a promising method for addressing the persistent contamination of groundwater by chlorinated hydrocarbons. Current research shows that electrochemical reductive dechlorination primarily relies on direct electron transfer (DET) and active hydrogen (H⁎) mediated indirect electron transfer processes, thereby achieving efficient dechlorination and detoxification. This paper explores the influence of the molecular charge structure of chlorinated hydrocarbons, including chlorolefin, chloroalkanes, chlorinated aromatic hydrocarbons, and chloro-carboxylic acid, on reductive dechlorination from the perspective of molecular electrostatic potential and local electron affinity. It reveals the affinity characteristics of chlorinated hydrocarbon pollutants, the active dechlorination sites, and the roles of substituent groups. It also comprehensively discusses the current progress on electrochemical reductive dechlorination using metal, carbon-based, and 3D electrode catalysts, with an emphasis on the design and optimization of electrode materials and the impact of catalyst microstructure regulation on dechlorination performance. It delves into the current application status of coupling electrochemical reduction technology with biodegradation and electrochemical circulating well technology for the remediation of groundwater contaminated by chlorinated hydrocarbons. The paper discusses practical application challenges such as electron transfer, electrode corrosion, water chemistry environment, and aquifer heterogeneity. Finally, considerations are presented from the perspectives of environmental impact and sustainable application, along with a summary and analysis of potential future research directions and technological prospects.

B-Modified Pd Cathodes for the Efficient Detoxification of Halogenated Antibiotics: Enhancing C-F Bond Breakage beyond Hydrodefluorination

Halogenated antibiotics pose a great threat to aqueous environments because of their persistent biotoxicity from carbon-halogen bonds. Electrochemical reduction (ER) is an efficient technology for dehalogenation, but it still suffers from limited efficiencies in breaking C-F bonds. Herein, we present a strategy to enhance C-F cleavage and promote detoxification by loading benchmark palladium cathodes onto boron-doped carbon. This improves the florfenicol (FLO) degradation rate constant and defluorination efficiency by 1.24 and 1.05 times, respectively, and improves the defluorination of various fluorinated compounds. The cathode with optimal B content shows superior mass activity for FLO degradation (1.11 mmol g-1 Pd min-1), which is 5.9 times that of commercial Pd/C and is among the best-reported cathodes. Notably, the exclusive formation of the direct defluorination product (i.e., FLO-F) on Pd/B-C implies a higher intrinsic C-F cleavage ability endowed by B doping. As revealed by experiments and theoretical calculations, boron modification enhances palladium binding and induces stronger strain effects and higher electron density for surface palladium atoms, which boosts H* generation and reduces the energy barrier for C-F cleavage. This study provides an effective cathode design strategy to enhance C-F activation, which may broadly benefit the destruction and detoxification of fluorinated organics that are limited by sluggish C-F cleavage kinetics.

Co-Activating Lattice Oxygen of TiO2-NT and SnO2 Nanoparticles on Superhydrophilic Graphite Felt for Boosting Electrocatalytic Oxidation of Glyphosate

Glyphosate (GH) wastewater potentially poses hazards to human health and the aquatic environment, due to its persistence and toxicity. A highly superhydrophilic and stable graphite felt (GF)/polydopamine (PDA)/titanium dioxide nanotubes (TiO2-NT)/SnO2/Ru anode was fabricated and characterized for the degradation of glyphosate wastewater. Compared to control anodes, the GF/PDA/TiO2-NT/SnO2/Ru anode exhibited the highest removal efficiency (near to 100%) and a yield of phosphate ions of 76.51%, with the lowest energy consumption (0.088 Wh/L) for degrading 0.59 mM glyphosate (GH) at 7 mA/cm2 in 30 min. The exceptional activity of the anode may be attributed to the co-activation of lattice oxygen in TiO2-NT and SnO2 by coupled Ru, resulting in a significant amount of •O2- and oxygen vacancies as active sites for glyphosate degradation. After electrolysis, small molecular acids and inorganic ions were obtained, with hydroxylation and dephosphorization as the main degradation pathways. Eight cycles of experiments confirmed that Ru doping prominently enhanced the stability of the GF/PDA/TiO2-NT/SnO2/Ru anode due to its high oxygenophilicity and electron-rich ability, which promoted the generation and utilization efficiency of active free radicals and defects-associated oxygen. Therefore, this study introduces an effective strategy for efficiently co-activating lattice oxygen in SnO2 and TiO2-NT on graphite felt to eliminate persistent organophosphorus pesticides.

Surfactant Directionally Assembled at the Electrode-Electrolyte Interface for Facilitating Electrocatalytic Aldehyde Hydrogenation

Electrocatalytic hydrogenation of unsaturated aldehydes to unsaturated alcohols is a promising alternative to conventional thermal processes. Both the catalyst and electrolyte deeply impact the performance. Designing the electrode-electrolyte interface remains challenging due to its compositional and structural complexity. Here, we employ the electrocatalytic hydrogenation of 5-hydroxymethylfurfural (HMF) as a reaction model. The typical cationic surfactant, cetyltrimethylammonium bromide (CTAB), and its analogs are employed as electrolyte additives to tune the interfacial microenvironment, delivering high-efficiency hydrogenation of HMF and inhibition of the hydrogen evolution reaction (HER). The surfactants experience a conformational transformation from stochastic distribution to directional assembly under applied potential. This oriented arrangement hampers the transfer of water molecules to the interface and promotes the enrichment of reactants. In addition, near 100 % 2,5-bis(hydroxymethyl)furan (BHMF) selectivity is achieved, and the faradaic efficiency (FE) of the BHMF is improved from 61 % to 74 % at -100 mA cm-2. Notably, the microenvironmental modulation strategy applies to a range of electrocatalytic hydrogenation reactions involving aldehyde substrates. This work paves the way for engineering advanced electrode-electrolyte interfaces and boosting unsaturated alcohol electrosynthesis efficiency.

Utilizing an Integrated Flow Cathode-Membrane Filtration System for Effective and Continuous Electrochemical Hydrodechlorination

Pd-based electrodes are recognized to facilitate effective electrochemical hydrodechlorination (EHDC) as a result of their superior capacity for atomic hydrogen (H*) generation. However, challenges such as electrode stability, feasibility of treating complex matrices, and high cost associated with electrode synthesis hinder the application of Pd-based electrodes for EHDC. In this work, we investigated the feasibility of degrading 2,4-dichlorophenol (2,4-DCP) by EHDC employing Pd-loaded activated carbon particles, prepared via a simple wet-impregnation method, as a flow cathode (FC) suspension. Compared to other Pd-based EHDC studies, a much lower Pd loading (0.02-0.08 mg cm-2) was used. Because of the excellent mass transfer in the FC system, almost 100% 2,4-DCP was hydrodechlorinated to phenol within 1 h. The FC system also showed excellent performance in treating complex water matrices (including hardness ion-containing wastewater and various other chlorinated organics such as 2,4-dichlorobenzoic acid and trichloroacetic acid) with a relatively low energy consumption (0.26-1.56 kW h m-3 mg-1 of 2,4-DCP compared to 0.32-7.61 kW h m-3 mg-1 of 2,4-DCP reported by other studies). The FC synthesized here was stable over 36 h of continuous operation, indicating its potential suitability for real-world applications. Employing experimental investigations and mathematical modeling, we further show that hydrodechlorination of 2,4-DCP occurs via interaction with H*, with no role of direct electron transfer and/or HO•-mediated processes in the removal of 2,4-DCP.

Enhanced Mineralization of Organic Pollutants through Atomic Hydrogen-Mediated Alternative Transformation Pathways

Electrocatalytic hydrogen atom-hydroxyl radical (H*-·OH) redox system is a promising approach for contaminant removal and mineralization. However, its working mechanism, especially the effect of H*, remains unclear, hindering its practical application. Herein, we constructed an electrochemical reactor equipped with our self-made Pd-loaded Ti/TiO2 nanotube cathode and a commercial boron-doped diamond anode. After fulfilling the electrode characterization and free radical detection, we employed coumarin and 7-azido-4-methylcoumarin as probes to confirm the participation of H* in the transformation of organic compounds. A comprehensive study on the degradation kinetics, reaction, and mineralization mechanisms using benzoic acid (BA) and 4-chlorophenol (4-CP) as model compounds was further conducted. The rate constants and total organic carbon removal of BA and 4-CP in the redox system increased compared with those of the individual oxidation and reduction processes. Theoretical calculations demonstrate that H* opens up alternative pathways for BA and 4-CP ring cleavage, forming quinones as reactive intermediates. Furthermore, H* facilitates the mineralization of the typical intermediates, maleic acid and fumaric acid, through C=C bond addition and H-abstraction from the 1,1-diol structure. The presence of H* provides alternative pathways for pollutant transformation, consequently reducing the treatment duration.

Nanostructured electrocatalysts for organic synthetic transformations

Organic chemists have made and are still making enormous efforts toward the development of novel green catalytic synthesis. The necessity arises from the imperative of safeguarding human health and the environment, while ensuring efficient and sustainable chemical production. Within this context, electrocatalysis provides a framework for the design of new organic reactions under mild conditions. Undoubtedly, nanostructured materials are under the spotlight as the most popular and in most cases efficient platforms for advanced organic electrosynthesis. This Minireview focuses on the recent developments in the use of nanostructured electrocatalysts, highlighting the correlation between their chemical structures and resulting catalytic abilities, and pointing to future perspectives for their application in cutting-edge areas.

Hydrogen Nanobubbles Generated In Situ from Nanoscale Zerovalent Iron with Water to Further Enhance Selenite Sequestration

Gas nanobubbles used for water treatment and recovery give rise to great concern for their unique advantages of less byproducts, higher efficiency, and environmental friendliness. Nanoscale zerovalent iron (nZVI), which has also been widely explored in the field of environmental remediation, can generate gas hydrogen by direct reaction with water. Whether nanoscale hydrogen bubbles can be produced to enhance the pollution removal of the nZVI system is one significant concern involved. Herein, we report direct observations of in situ generation of hydrogen nanobubbles (HNBs) from nZVI in water. More importantly, the formed HNBs can enhance indeed the reduction of Se(IV) beyond the chemical reduction ascribed to Fe(0), especially in the anaerobic environment. The possible mechanism is that HNBs enhance the reducibility of the system and promote electron transport in the solution. This study demonstrates a unique function of HNBs combined with nZVI for the pollutant removal and a new approach for in situ HNB generation for potential applications in the fields of in situ remediation agriculture, biotechnology, medical treatment, health, etc.

Critical Review of Pd-Catalyzed Reduction Process for Treatment of Waterborne Pollutants

Catalyzed reduction processes have been recognized as important and supplementary technologies for water treatment, with the specific aims of resource recovery, enhancement of bio/chemical-treatability of persistent organic pollutants, and safe handling of oxygenate ions. Palladium (Pd) has been widely used as a catalyst/electrocatalyst in these reduction processes. However, due to the limited reserves and high cost of Pd, it is essential to gain a better understanding of the Pd-catalyzed decontamination process to design affordable and sustainable Pd catalysts. This review provides a systematic summary of recent advances in understanding Pd-catalyzed reductive decontamination processes and designing Pd-based nanocatalysts for the reductive treatment of water-borne pollutants, with special focus on the interactions and transformation mechanisms of pollutant molecules on Pd catalysts at the atomic scale. The discussion begins by examining the adsorption of pollutants onto Pd sites from a thermodynamic viewpoint. This is followed by an explanation of the molecular-level reaction mechanism, demonstrating how electron-donors participate in the reductive transformation of pollutants. Next, the influence of the Pd reactive site structure on catalytic performance is explored. Additionally, the process of Pd-catalyzed reduction in facilitating the oxidation of pollutants is briefly discussed. The longevity of Pd catalysts, a crucial factor in determining their practicality, is also examined. Finally, we argue for increased attention to mechanism study, as well as precise construction of Pd sites under batch synthesis conditions, and the use of Pd-based catalysts/electrocatalysts in the treatment of concentrated pollutants to facilitate resource recovery.

Metal-organic framework derived bio-anode enhances chlorobenzene removal and electricity generation: Special Ru4+/Ru3+-bridged intracellular electron transfer

Efficient removal of chlorinated organic contaminants using the microbial fuel cell (MFC) provides a promising strategy to alleviate water pollution and energy crisis. However, bio-degradation is challenged by poor biofilm formation and sluggish intracellular electron transfer, causing unsatisfactory electricity generation. To address those problems, a metal-organic framework derivative, Ru-porous TiO2 (Ru-PT) bio-anode has been artfully designed herein for chlorobenzene removal. The Ru-PT bio-anode not only formed a compact anodic biofilm due to the large specific surface area of PT, but more importantly, it introduced special pseudocapacitance-enhanced intracellular electron transfer by slowly implanting Ru4+/Ru3+ redox pair into bacteria. Such a Ru4+/Ru3+ implantation was then found to directionally induce the enrichment of a dual-functional genus (degrader & exoelectrogen), Pseudomonas, thereby enhancing the conversion of bio-refractory chlorophenols towards biodegradable carboxylic acids. These features allowed our MFC to have a resilient chlorobenzene removal and accompanied satisfactory electricity generation, with power density, coulombic efficiency, and turnover frequency reaching 662 mW m-2, 8.7%, and 386,622 s-1, which outcompeted those of other MFCs reported. Further, benefiting from the reversible pseudocapacitance, the Ru-PT bio-anode intriguingly functioned as an internal capacitor for electricity storage. This work provided important insights into cost-effective bio-anode development and offered an avenue for engineering MFC.

Insights into Electrochemical Dehalogenation by Non-Noble Metal Single-Atom Cobalt with High Efficiency and Low Energy Consumption

It is critical to discover a non-noble metal catalyst with high catalytic activity capable of replacing palladium in electrochemical reduction. In this work, a highly efficient single-atom Co-N/C catalyst was synthesized with metal-organic frameworks (MOFs) as a precursor for electrochemical dehalogenation. X-ray absorption spectroscopy (XAS) revealed that Co-N/C exhibited a Co-N4 configuration, which had more active sites and a faster charge-transfer rate and thus enabled the efficient removal of florfenicol (FLO) at a wide pH, achieving a rate constant 3.5 and 2.1 times that of N/C and commercial Pd/C, respectively. The defluorination and dechlorination efficiencies were 67.6 and 95.6%, respectively, with extremely low Co leaching (6 μg L-1), low energy consumption (22.7 kWh kg-1), and high turnover frequency (TOF) (0.0350 min-1), demonstrating excellent dehalogenation performance. Spiking experiments and density functional theory (DFT) verified that Co-N4 was the active site and had the lowest energy barrier for forming atomic hydrogen (H*) (ΔGH*). Capture experiments, electron paramagnetic resonance (EPR), electrochemical tests, and in situ Fourier transform infrared (FTIR) proved that H* and direct electron transfer were responsible for dehalogenation. Toxicity assessment indicated that FLO toxicity decreased significantly after dehalogenation. This work develops a non-noble metal catalyst with broad application prospects in electrocatalytic dehalogenation.