Kinetics of electrochemical dechlorination of 2-chlorobiphenyl on a palladium-modified nickel foam cathode in a basic medium: From batch to continuous reactor operation

Electrocatalytic hydro-dehalogenation of halogenated organic pollutants from wastewater: A critical review

Halogenated organic pollutants are often found in wastewater effluent although it has been usually treated by advanced oxidation processes. Atomic hydrogen (H*)-mediated electrocatalytic dehalogenation, with an outperformed performance for breaking the strong carbon-halogen bonds, is of increasing significance for the efficient removal of halogenated organic compounds from water and wastewater. This review consolidates the recent advances in the electrocatalytic hydro-dehalogenation of toxic halogenated organic pollutants from contaminated water. The effect of the molecular structure (e.g., the number and type of halogens, electron-donating or electron-withdrawing groups) on dehalogenation reactivity is firstly predicted, revealing the nucleophilic properties of the existing halogenated organic pollutants. The specific contribution of the direct electron transfer and atomic hydrogen (H*)-mediated indirect electron transfer to dehalogenation efficiency has been established, aiming to better understand the dehalogenation mechanisms. The analyses of entropy and enthalpy illustrate that low pH has a lower energy barrier than that of high pH, facilitating the transformation from proton to H*. Furthermore, the quantitative relationship between dehalogenation efficiency and energy consumption shows an exponential increase of energy consumption for dehalogenation efficiency increasing from 90% to 100%. Lastly, challenges and perspectives are discussed for efficient dehalogenation and practical applications.

TiO2@PDA inorganic-organic core-shell skeleton supported Pd nanodots for enhanced electrocatalytic hydrodechlorination

The development of catalysts with high atom utilization and activity is the biggest challenge for electrocatalytic hydrodechlorination (EHDC) technology. Herein, a design strategy of TiO₂@PDA inorganic-organic core-shell skeleton for loading lower dosage of noble palladium (Pd) with robust activity is reported. The self-supported TiO₂@PDA nanorod arrays provides exposed surface area for anchoring Pd and PDA as interlayer controls the Pd nucleation to form nanodots with high dispersion, realizing high atom utilization. Moreover, the strong interaction between PDA and Pd realizes the coexistence of electron-rich and deficient Pd species with suitable proportion, which facilitate the H* formation and the C-Cl bond activation, respectively, resulting in the promoted activity. The optimal TiO₂@PDA/Pd electrode exhibits a low dosage of Pd (0.093 mg cm^-2) and excellent activity for 4-chlorophenol reduction with a mass activity (MA) of 23.96 min^-1g^-1, which is 3.31 times as high as that of TiO₂/Pd. The design scheme with inorganic-organic core-shell skeleton as support is benefit for developing highly efficient and lower price elctrocatalysts for EHDC.

Degradation Kinetics and Mechanism of Polychloromethanes Reduction at Co-MoS2/Graphite Felt Electrode

In this study, the electrochemical dechlorination of different polychloromethanes (CCl4, CHCl3, and CH2Cl2) on a Co-MoS2 graphite felt cathode was investigated. The Co-MoS2 electrocatalyst was prepared hydrothermally on a graphite felt support. The prepared catalyst’s characterization revealed the formation of hybridized CoSx and MoS2 nanosheets deposited on the pore structures of graphite. The influencing factor for the electro-dechlorination parameters such as applied current density, pH, and sample concentration on the dechlorination rate was optimized. A significant capacitive reduction current density peak of approximately 1 mA/cm2 was noted for CCl4 at a potential of −0.3 V (vs. AgCl). The dechlorination mechanism was attributed to the stepwise hydrogenolysis mechanism that involves the organochlorides bond cleavage by H* insertion. It was noted that the Co-MoS2 graphite felt electrode exhibited excellent catalytic activity toward the reduction of each of the chlorinated compounds with high selectivity toward the higher-order organochloride. Moreover, the dechlorination rates for each of the compounds were suited to the first-order kinetic model, and the estimated apparent rate constants showed the dechlorination in the following sequence CH2Cl2 (k3 = 9.1 × 10−5 s−1) < CHCl3 (k2 = 1.5 × 10−3 s−1) < CCl4 (k1 = 2.8 × 10−3 s−1).

Identifying the rate-determining step of the electrocatalytic hydrodechlorination reaction on palladium nanoparticles

Identifying the rate-determining step over the catalysts and clarifying the underlying mechanisms are crucial for maximizing the electrocatalytic hydrodechlorination (EHDC) efficiency for detoxification of the chlorophenol pollutants in water. Here, monodisperse palladium nanoparticles (Pd NPs) separately supported on carbon (C) and titanium nitride (TiN) were synthesized as two model catalysts. The support effects on EHDC efficiency, kinetics and current efficiency towards 2,4-dichlorophenol (2,4-DCP), and the electronic structure of Pd and its binding strengths with 2,4-DCP, phenol and Cl- (the primary EHDC product) were investigated by experimental and density functional theory (DFT) analyses. The low current efficiency (<30%) of both catalysts and the good description of EHDC kinetics by the Langmuir-Hinshelwood model suggest that the 2,4-DCP coverage on Pd, rather than the well-known adsorbed hydrogen generation, determines EHDC efficiency. Furthermore, the superior EHDC efficiency on TiN-Pd (96.4% vs. 80.9% on C-Pd), coupled with the weakened adsorption of 2,4-DCP and phenol on TiN-supported Pd, demonstrates that the 2,4-DCP coverage is largely influenced by phenol due to its poisoning effect by blocking active sites, and phenol desorption is the rate-determining step of EHDC on the catalyst. The support TiN enables alleviation of the phenol poisoning by modulating the electronic structure of Pd. The d band center of Pd can serve as a potential descriptor of EHDC efficiency, and its optimization for balancing 2,4-DCP and phenol adsorption should be an effective strategy to enhance EHDC.

Efficient degradation of tetrabromobisphenol A via electrochemical sequential reduction-oxidation: Degradation efficiency, intermediates, and pathway

Tetrabromobisphenol A (TBBPA), a toxic persistent pollutant, should be effectively removed from the environment. In this study, an electrochemical sequential reduction-oxidation system was proposed by controlling reaction atmosphere with Pd-Fe nanoparticles modified Ni foam (Pd-Fe/Ni) electrode as cathode for TBBPA degradation. To obtain an efficient Pd-Fe/Ni electrode for TBBPA degradation, various factors, like Pd loading, Fe²⁺ adding amounts, were examined. The Pd-Fe/Ni electrode exhibited higher TBBPA conversion and debromination than the counterparts, due to the synergism of Fe⁰ and electrochemical reduction. Similar TBBPA conversions and debromination ratios were observed for the cases of sparging N₂ only and sparging N₂ followed by air, which were higher than those of aeration. Reductive debromination occurred while first bubbling N₂, forming tri-BBPA, di-BBPA, mono-BBPA and BPA; and these intermediates were likely to be further oxidized by OH generated from H₂O₂ together with Pd-Fe/Ni electrode under aeration. Reductive and oxidative intermediates (including aromatic ring-opened product) were identified by HPLC and UPLC-QTOF-MS. Based on the intermediates, the possible TBBPA degradation mechanism and pathway were proposed. This study demonstrates that sequential reduction-oxidation process tuned by N₂ and air bubbling was favored for TBBPA degradation, thus, it should be a promising process for HOCs degradation.

Cathodic photoelectrochemical detection of PCB101 in environmental samples with high sensitivity and selectivity

A novel cathodic photoelectrochemical (PEC) sensing method was developed for fast and convenient detection of PCB101 taking advantages of the excellent PEC reducibility of Pd quantum dots (QDs) modified molecularly imprinted TiO₂ nanorods (NRs). Attributed to the efficient PEC reduction of PCB101 on the cathode surface, sensitive cathodic photocurrent would be produced with increasing PCB101 concentration under a negative bias potential, giving a low PCB101 detection limit of 5×10^-14molL^-1. Meanwhile, molecular imprinting (MI) technique was integrated by in situ introduction of MI sites on the surface of TiO₂ NRs, so that highly specific adsorption and reduction of PCB101 congener could be obtained. The results indicated that the PEC sensor presented excellent selectivity toward PCB101 with the coexistance of 100-fold excess of other pollutants including the PCBs congeners, other aromatic pollutants, and heavy metal ions. The cathodic PEC sensor was sucessfully applied in determination of PCB101 in real water and soil samples, and the results had good consistency with that obtained by the traditional GC-MS. This work provides a new concept and research basis for fabricating cathodic PEC sensor for the environmental pollutants with specific structures that were easily reduced.

Identification of Active Hydrogen Species on Palladium Nanoparticles for an Enhanced Electrocatalytic Hydrodechlorination of 2,4-Dichlorophenol in Water

Clarifying hydrogen evolution and identifying the active hydrogen species are crucial to the understanding of the electrocatalytic hydrodechlorination (EHDC) mechanism. Here, monodisperse palladium nanoparticles (Pd NPs) are used as a model catalyst to demonstrate the potential-dependent evolutions of three hydrogen species, including adsorbed atomic hydrogen (H*_ads), absorbed atomic hydrogen (H*_abs), and molecular hydrogen (H₂) on Pd NPs, and then their effect on EHDC of 2,4-dichlorophenol (2,4-DCP). Our results show that H*_ads, H*_abs, and H₂ all emerge at -0.65 V (vs Ag/AgCl) and have increased amounts at more negative potentials, except for H*_ads that exhibits a reversed trend with the potential varying from -0.85 to -0.95 V. Overall, the concentrations of these three species evolve in an order of H*_abs < H*_ads < H₂ in the potential range of -0.65 to -0.85 V, H*_ads < H*_abs < H₂ in -0.85 to -1.00 V, and H*_ads < H₂ < H*_abs in -1.00 to -1.10 V. By correlating the evolution of each hydrogen species with 2,4-DCP EHDC kinetics and efficiency, we find that H*_ads is the active species, H*_abs is inert, while H₂ bubbles are detrimental to the EHDC reaction. Accordingly, for an efficient EHDC reaction, a moderate potential is desired to yield sufficient H*_ads and limit H₂ negative effect. Our work presents a systematic investigation on the reaction mechanism of EHDC on Pd catalysts, which should advance the application of EHDC technology in practical environmental remediation.

The influence of cathode material on electrochemical degradation of trichloroethylene in aqueous solution

In this study, different cathode materials were evaluated for electrochemical degradation of aqueous phase trichloroethylene (TCE). A cathode followed by an anode electrode sequence was used to support reduction of TCE at the cathode via hydrodechlorination (HDC). The performance of iron (Fe), copper (Cu), nickel (Ni), aluminum (Al) and carbon (C) foam cathodes was evaluated. We tested commercially available foam materials, which provide large electrode surface area and important properties for field application of the technology. Ni foam cathode produced the highest TCE removal (68.4%) due to its high electrocatalytic activity for hydrogen generation and promotion of HDC. Different performances of the cathode materials originate from differences in the bond strength between atomic hydrogen and the material. With a higher electrocatalytic activity than Ni, Pd catalyst (used as cathode coating) increased TCE removal from 43.5% to 99.8% for Fe, from 56.2% to 79.6% for Cu, from 68.4% to 78.4% for Ni, from 42.0% to 63.6% for Al and from 64.9% to 86.2% for C cathode. The performance of the palladized Fe foam cathode was tested for degradation of TCE in the presence of nitrates, as another commonly found groundwater species. TCE removal decreased from 99% to 41.2% in presence of 100 mg L(-1) of nitrates due to the competition with TCE for HDC at the cathode. The results indicate that the cathode material affects TCE removal rate while the Pd catalyst significantly enhances cathode activity to degrade TCE via HDC.

Electrochemical transformation of thichloroethylene in groundwater by Ni-containing cathodes

In this study, we evaluate the use of different stainless steel (SS) materials as cost-effective cathode materials for electrochemical transformation of trichloroethylene (TCE) in contaminated groundwater. Ni, which is present in certain SS, has low hydrogen overpotential that promotes fast formation of atomic hydrogen and, therefore, its content can enhance hydrodechlorination (HDC). We a flow-through electrochemical reactor with a SS cathode followed by an anode. The performance of Ni containing foam cathodes (Fe/Ni and Ni foam) was also evaluated for electrochemical transformation of TCE in groundwater. SS type 316 (12% Ni) removed 61.7% of TCE compared to 52.6% removed by SS 304 (9.25% Ni) and 37.5% removed by SS 430 (0.75% Ni). Ni foam cathode produced the highest TCE removal rate (68.4%) compared with other cathodes. The slightly lower performance of SS type 316 mesh is balanced by the reduction in treatment costs for larger-scale systems. The results prove that Ni content in SS highly influences TCE removal rate.

Electrodeposition of palladium and reduced graphene oxide nanocomposites on foam-nickel electrode for electrocatalytic hydrodechlorination of 4-chlorophenol

A high-performance palladium (Pd) and reduced graphene oxide (RGO) composite electrode was prepared on foam-nickel (foam-Ni) via two-step electrodeposition processes. The scanning electron microscopic (SEM) observation showed that the obtained Pd/RGO/foam-Ni composite electrode displayed a uniform and compact morphology. The X-ray diffraction (XRD) and X-ray photoelectron spectroscopic (XPS) analysis confirmed the successful deposition of Pd and RGO on nickel substrate. The cyclic voltammetric (CV) measurements indicated that the presence of RGO greatly enhanced the active surface area of Pd particles deposited on foam-Ni. The as-deposited Pd/RGO/foam-Ni electrode was applied to electrocatalytic hydrodechlorination (ECH) of 4-chlorophenol (4-CP). Various factors influencing the dechlorination of 4-CP such as dechlorination current, initial concentration of 4-CP, Na2SO4 concentration and initial pH were systematically investigated. The thermodynamic analysis showed that the dechlorination reaction of 4-CP at different temperatures followed the first-order kinetics and the activation energy for 4-CP dechlorination on Pd/RGO/foam-Ni electrode was calculated to be 51.96 kJ mol(-1). Under the optimum conditions, the dechlorination efficiency of 4-CP could reach 100% after 60-min ECH treatment. Moreover, the prepared Pd/RGO/foam-Ni composite electrode showed good stability for recycling utilization in ECH of 4-CP.