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• MnO 2 is induced to Pd/Ni foam for enhanced Volmer step and atomic H* generation. • Electrode reactivity was correlated to atomic H* generation and cathode potential. • The optimal cathode potential was determined to be −0.85 V vs Ag/AgCl. • The effects of various influence factors on the ECH reactivity were investigated. • Efficient utilization of atomic H* (>70%) was achieved at weak acidic conditions. Atomic H* has been regarded as the key active species for the electrocatalytic hydrodechlorination (ECH), and the electrode design and operation conditions are related to the atomic H* generation. However, there is limited understanding of how the operation parameters affect the atomic H* generation and utilization. Due to the fact that Pd was inefficient in water dissociation step, MnO 2 was combined with Pd to construct a novel Pd/MnO 2 /Ni foam cathode. Cyclic voltammetry confirmed that Pd/MnO 2 /Ni foam cathode with MnO 2 load of 0.51 mg cm−2 possessed the highest atomic H* generation and ECH activity. Excessive Pd load (>0.44 mg cm−2) was detrimental to ECH because of the side hydrogen evolution reaction (HER). Operation conditions (e.g. applied current and electrolyte concentration) would directly affect the cathode potential, which was an important indicator of the atomic H* generation. The optimal cathode potential was determined to be −0.85 V when the applied current was 10 mA in this study. The electrolyte concentration of 10–15 mM was able to provide sufficient atomic H* for ECH, while excessive input of electrolyte would favor HER but inhibit ECH. ECH at acidic and neutral/alkaline solution underwent different pathways for atomic H* generation and further affect the utilization of atomic H*. After 120 min reaction, >70% of atomic H* could be selectively used for ECH at initial pH of 4.0 and 5.0, while only ∼37% was used for ECH at initial pH of 7.0. This work correlated the reactivity of electrode with the atomic H* and cathode potential, which would provide strategies for ECH catalyst design and operation optimization. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • TiC doped palladium/nickel foam cathode was synthesized via electroless deposition. • Pd/TiC/Ni foam show superior activity for 2,4-DCBA dechlorination to Pd/Ni foam. • Optimized conditions on 2,4-DCBA dechlorination were investigated. • Possible promotion mechanism by doped TiC over Pd/Ni foam cathode was proposed. Abstract Titanium carbide (TiC) with excellent electrical conductivity, chemical and thermal stabilities has been recognized as one of the most promising electrocatalysts. A novel cathode, titanium carbide doped palladium/nickel foam (TiC-Pd/Ni foam), was synthesized via electroless deposition to improve the performance of Pd/Ni foam in electrocatlytic hydrodechlorination (ECH). TiC can be co-precipitated onto the surface of cathode during galvanic replacement reaction between Pd(II) solution and Ni foam. Both constant potential and constant current tests proved that TiC-Pd/Ni foam cathode performed remarkably higher activity for 2,4-dichlorobenzoic acid (2,4-DCBA) than Pd/Ni foam cathode, owing to the excellent conductivity of TiC and enhanced water dissociation over TiC-Pd/Ni foam cathode. Under the optimized reaction conditions of –0.85 V (vs Ag/AgCl), electrolyte of 10 mM and initial pH of 4, 99.8% of aqueous 2,4-DCBA (0.2 mM) was removed within 90 min. The removal process of the aqueous 2,4-DCBA obeyed first-order decay kinetic model. Over 86.3% of 2,4-DCBA can still be removed by TiC-Pd/Ni foam cathode in the fifth consecutive run within 120 min, which was much higher than that of Pd/Ni foam cathode (37.5%). Consequently, TiC-Pd/Ni foam cathode was a promising design for enhanced ECH activity and reduced operation cost. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Hierarchical Pd/MnO 2 /Ni foam electrode was fabricated for efficient ECH process. • Dosage of precious metal Pd was drastically reduced by the introduction of MnO 2. • MnO 2 promoted the water dissociation and provide more atomic H∗ for dechlorination. • Pd/MnO 2 /Ni foam electrode exhibited good reusability and stability. Abstract A Pd/MnO 2 /Ni foam electrode with hierarchical structure was synthesized via electrodeposition for efficient electrocatalytic hydrodechlorination. Compared with the ordinary Pd/Ni foam electrode, the introduction of MnO 2 greatly enhanced the catalytic reactivity and reduced the dose of precious metal Pd. Only a quarter of Pd was required for the Pd/MnO 2 /Ni foam compared to the Pd/Ni electrode to achieve complete dechlorination of 2,4-dichlorobenzoic acid (2,4-DCBA) within 120 min. Various characterizations suggested that MnO 2 covered the surface of the Ni foam and increased the specific surface area of the electrode, while Pd nanoparticles were subsequently deposited on MnO 2. The atomic H∗-based indirect dechlorination was the dominant pathway while only approximately 13% of 2,4-DCBA was removed by direct electron transfer. Atomic H∗ adsorbed on Pd acted as the key active species for the dechlorination of 2,4-DCBA by the Pd/MnO 2 /Ni foam electrode in this study. The introduction of MnO 2 would promote the water dissociation and the hydrogen evolution reaction, and provide Pd with more atomic H∗. Pd/MnO 2 /Ni foam exhibited good stability and reusability according to the XPS spectra and consecutive electrocatalytic experiments, which suggested its long-term potential for efficient removal of chlorinated contaminants. This work demonstrated a new strategy to design efficient electrocatalysts with less precious metal. [ABSTRACT FROM AUTHOR]

To simultaneously enhance the catalytic activity of Pd and lower its dosage, we report a Pd/Co-MOF nanosheets/Ni foam electrode (Pd/Co-MNSs/Ni foam) with conductive 2D MOF nanosheets as interlayer for electrocatalytic hydrodechlorination (ECH). Such electrode delivers high ECH activity for nearly complete dechlorination (~97%) of chloramphenicol (CAP) with an apparent rate constant of 0.11 min−1, outcompeting the well-reported and the commercial electrode materials. Pd/Co-MNSs/Ni foam maintains its activity after 9 consecutive runs and 5-day exposure to air, and shows rapid degradation kinetics for other priority contaminants. The formation of Pd/Co-MNSs heterojunction interface increases the electron density of metallic Pd and brings the d states closer to Fermi level than Pd alone, thus optimizing the binding of ECH intermediates. This support construction-based strategy tunes the interface of catalytic electrode to expose active sites and modulate the electronic states of catalyst to promote intrinsic activity, boosting electroreductive remediation of organic halides. [Display omitted] • Co-MNSs mitigates Pd agglomeration and provides abundant active sites for ECH. • Pd/Co-MNSs/Ni foam rapidly catalyzes the reduction of priority contaminants. • Pd/Co-MNSs/Ni foam maintains its ECH activity after runs and long exposure to air. • The Pd/Co-MNSs heterojunction interface enriches electron on Pd. • Stronger H* and CAP binding, and weakened product adsorption promote ECH. [ABSTRACT FROM AUTHOR]

[Display omitted] • Pd-catalyzed electrochemical HDC is a key process for the conversion of CAPs. • High concentration (0.05 ∼ 1 M) CAPs were converted to value-added chemicals. • CAPs (4 categories) have high yield (>95 %) and chemo-selectivity (>97 %). • Pd mass activity and area activity of cathode have been increased by 16 ∼ 685 times. Pd-catalyzed electrochemical hydrodechlorination (HDC), as a key process before discharge and biological treatment, is an effective strategy for the in-situ conversion of low concentration chlorinated aromatic pollutants (CAPs) in wastewater or polluted water. However, its application has been greatly hindered by the low Pd utilization and operation efficiency. Herein, we developed an electrocatalytic HDC method using Pd nanoparticles modified nickel foam (Pd NPs/Ni) as cathode and alkaline aqueous solution as catholyte, and applied it to convert high concentration CAPs to value-added chemicals. The results showed that the reaction efficiency and selectivity of the developed HDC method mainly depend on the cathode material, pH and cathode potential. The HDC method can convert CAPs (4 categories) with various high concentrations (0.05 ∼ 1 M) into a single value-added chemical (picolinic acid, phenoxyacetic acid, benzoic acid or phenol) with very high yield (>95 %) and chemo-selectivity (>97 %). Compared with the conventional Pd-catalyzed HDC methods, the new patterned one can greatly improve Pd utilization and operation efficiency (The Pd mass activity and the area activity of cathode increased by 16 ∼ 685 times). In addition, the underlying reaction pathway and catalytic mechanism of the developed HDC system has also been studied using 3,6-dichloropicolinic acid (3,6-D) as the model CAPs. The HDC of 3,6-D on the Pd NPs/Ni in a stepwise fashion with 3-chloropicolinic acid (3-ClPA) as the main intermediate product is suggested, and it follows the indirect HDC mechanism with electrochemically adsorbed H as the reductant. The end product (picolinic acid) of the HDC could be further reduced to pipecolinic acid at very negative potentials following direct hydrogenation mechanism, which would significantly lower the selectivity of HDC. [ABSTRACT FROM AUTHOR]