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Article

MnO2-Supported Pd Nanocatalyst for Efficient Electrochemical Reduction of 2,4-Dichlorobenzoic Acid

by
Yaxuan Peng
and
Meiyan Wang
*
School of Energy and Environmental Engineering, Hebei University of Technology, Tianjin 300400, China
*
Author to whom correspondence should be addressed.
Clean Technol. 2025, 7(4), 102; https://doi.org/10.3390/cleantechnol7040102
Submission received: 18 August 2025 / Revised: 17 September 2025 / Accepted: 27 October 2025 / Published: 11 November 2025
(This article belongs to the Collection Water and Wastewater Treatment Technologies)
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Abstract

Chlorobenzoic acids (CBAs) are a group of chlorinated persistent environmental pollutants with hard biodegradability, high water solubility, and well-documented carcinogenic and endocrine-disrupting properties. Electrocatalytic hydrodechlorination (ECH) is a highly efficient method under mild conditions without harmful by-products, but the ECH process commonly requires adding precious metal catalysts such as palladium (Pd). To address the economic constraints and more effective utilization of Pd, a palladium/manganese dioxide (Pd/MnO2) composite catalyst was developed in this study by chemical deposition. This method utilized the excellent electrochemical activity of MnO2 as a carrier as well as the hydrogen storage and activation capacity of Pd. The test showed the optimal Pd loading was 7.5%, and the removal percent of 2,4-dichlorobenzoic acid (2,4-DCBA), a typical CBA, reached 97.3% using 0.5 g/L of Pd/MnO2 after 120 min of electrochemical reaction. Under these conditions, the dechlorination percent can also be as high as 89.6%. A higher current density enhanced the dechlorination efficiency but showed the lower current utilization efficiency. In practical applications, current density should be minimized on the premise of compliance with the water treatment requirement. Mechanistic studies showed that MnO2 synergistically promoted hydrolysis dissociation and hydrogen spillover and facilitated Pd-mediated adsorption of atomic hydrogen (H*) for dehydrogenation of 2,4-DCBA. The presence of MnO2 can effectively disperse the loaded Pd and reduce the amount of Pd via the above process. The catalyst exhibited excellent stability over multiple cycles, and the 2,4-DCBA removal could still reach more than 80% after the five cycles. This work establishes electrocatalytic strategies for effectively reducing Pd usage and maintaining high removal of typical CBAs to support CBA-related water treatment.

1. Introduction

Chlorobenzoic acids (CBAs) represent a class of environmental organic acidic pollutants characterized by aromatic rings substituted with chlorine atoms. Owing to the strong electronegativity of chlorine, these compounds form electron-withdrawing benzene rings that are resistant to oxidation and create stable conjugated systems [1]. Consequently, they exhibit relatively stable chemical properties, making them recalcitrant to degradation and persistent in the environment. Among those, 2,4-dichlorobenzoic acid (2,4-DCBA) is a typical CBA, exhibiting high environmental persistence, broad distribution, and significant biological toxicity [2]. Furthermore, 2,4-dichlorobenzoic acid is widely identified as a principal terminal derivative in the anaerobic breakdown of polychlorinated biphenyls (PCBs), as well as a transient metabolic intermediate in the transformation of several herbicide classes [3,4,5]. 2,4-DCBA is primarily used in industries such as agrochemicals (e.g., herbicide intermediates), pharmaceuticals (active ingredient synthesis), and dye manufacturing (chlorinated aromatic precursors). These substances are commonly detected in multiple environmental compartments, such as soils, rivers, and aquifers, with concentration levels spanning from micrograms per liter to milligrams per liter in regions experiencing significant pollution. The concentration of 2,4-DCBA in agricultural chemical manufacturing wastewater can reach 50–200 mg/L due to incomplete herbicide synthesis or equipment cleaning [6]. The leachate from garbage affected by the decomposition of chlorinated waste typically contains 0.5–10 mg/L of 2,4-DCBA. Some municipal and pharmaceutical wastewater typically exhibit much lower 2,4-DCBA concentrations (0.01–0.5 mg/L attributed to metabolic excretion or drug degradation [7]. They are potential carcinogens and endocrine disruptors, capable of causing microcephaly, cancer, and mutations [8,9,10]. Due to the water solubility, 2,4-DCBA can readily enter aquatic environments, where it can contribute to sustained pollution and biomagnification effects [11].
Various techniques have been applied to remove 2,4-DCBA and other types of chlorobenzoic acids (CBAs) from the environment over the past few decades. However, conventional biological treatment and oxidation processes exhibit inherent limitations in the remediation of 2,4-DCBA, due to the recalcitrant nature of chlorinated organic compounds. Biological treatment of chlorinated organic compounds is limited by the narrow substrate specificity of microorganisms and the stereospecificity of catabolic enzymes, resulting in the low performance of dechlorination [11]. Physical methods, while simple and relatively rapid, merely transfer pollutants between media without dechlorination, thus failing to reduce toxicity or resolve pollution fundamentally [12]. Although the advanced oxidation method has the advantages of good deep decomposition effect of pollutants, the main reasons that limit its widespread application so far are its high energy consumption and poor mineralization of pollutants [13]. And even after degradation, smaller molecules of chlorinated organic compounds remain toxic, while high mineralization rates require a large amount of energy and catalytic material investment. Therefore, the main source of toxicity for CBAs comes from chlorine, and reductive dechlorination can convert organic chlorine into inorganic chloride ions, essentially solving the above problems.
Electrochemical reduction is a technology that utilizes reducing free radicals or functional groups generated by electrodes to reduce organic pollutants [14]. It has the advantages of high reaction activity, mild conditions, convenient operation, and high selectivity, and is an effective method for removing CBA compounds. The dechlorination mechanisms in electrochemical systems primarily involve two distinct pathways: direct electron transfer to the target pollutant and indirect reduction mediated by surface-adsorbed hydrogen atoms (H*) [15]. Among them, electrocatalytic hydrodechlorination (ECH) mainly comes from indirect reduction, as this pathway enhances the electron utilization efficiency and current efficiency by reducing the production of by-product H2 [16]. Due to the main use of H*, it can be considered as an environmentally friendly clean reduction technology. The electrochemical reduction process suffers from rapid H* recombination via the electrocatalytic hydrogen evolution (HER) reaction and weak H* adsorption on non-catalytic surfaces, necessitating supported metal catalysts for efficient dechlorination [17]. Common metal catalysts, such as Pd, Ag, and other precious metal nanoparticles, have the characteristics of wide application and excellent performance due to their ability to generate and store H* at low overpotentials [18]. The indirect reduction through H* (E0 = −2.106 V) has been proven to be the main pathway for hydrodechlorination. Although Pd is considered a good catalyst for adsorbing H*, its efficiency is relatively low in the hydrolysis dissociation step. In addition, the system containing solely Pd nanoparticles tends to form aggregates due to the interparticle forces, and their high dosage leads to cost concerns. According to the reported studies, the addition of metal oxides can enable Pd-based metals to more effectively cleave HO-H bonds and generate H*, allowing H* to be adsorbed by uniformly distributed Pd-based catalysts to enhance reaction rate [19].
The common metal oxide MnO2 has great potential as a carrier electrode material due to its outstanding capacitive behavior and catalytic performance. Considering the multiple valence states of Mn, it can be easily switched between each valence state through electron capture/release. Research has found that the Mn element can capture H* flowing out from the Pd surface and help it diffuse to the entire catalyst surface for subsequent hydrogenation reactions [20]. In this case, more H* tends to undergo the ECH reaction rather than evolving into H2, which facilitates improved reaction kinetics and higher current efficiency and is advantageous for the process. For example, Huang et al. [21] used a two-step electrodeposition method to prepare MnO2 and Pd-modified foam nickel electrode. MnO2 deposited on the foam nickel framework forms an irregular connection structure with the nano sheet, which enhances the deposition of Pd and promotes the dissociation of H2O. And due to the high specific capacitance of MnO2, the catalyst can have good catalytic activity and electrochemical performance, which can have greater advantages in the manufacturing of electrodes using trace precious metals instead of large amounts of precious metals. Previous studies mainly focus on loading MnO2 and Pd mixture onto the electrode surface [22]. This approach not only results in insufficient exposure of MnO2 and Pd particles to the solution, but also reduces the loading effect of MnO2 and Pd particles due to CBA substances easily sticking to the electrode surface [23]. Therefore, this study employs MnO2 and Pd composite particles, which were added as independent materials (without electrode immobilization) to water for the degradation of 2,4-DCBA. Furthermore, we focus on elucidating the mechanism of MnO2 as a carrier for the active component Pd in the catalyst to enhance the production of active hydrogen in the system, to compensate for the insufficient electrochemical reduction efficiency of conventional metal cathode systems, and to develop a type of catalyst added to the cathode chamber for strong reduction and dechlorination of 2,4-DCBA. On the basis of improving the reaction process, it further enhances the lifespan and recyclability of the catalyst. The environmental benefit of the presented electrochemical methodology could be further amplified by coupling it with renewable energy sources, such as solar or wind power. Utilizing green electricity to drive the hydrodechlorination process is imperative for the generation of clean hydrogen and the achievement of a net-zero carbon footprint, ultimately advancing this technology as a sustainable solution for water purification [24,25].
In this study, a Pd and MnO2 composite catalyst was synthesized for the efficient ECH of 2,4-DCBA. The influence of key process parameters on catalytic hydrodechlorination was comprehensively examined. The stability of the catalyst was assessed through repeated recycling experiments. The mechanism of the catalyst was explored using multiple characterization methods. Electrochemical techniques, including linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV), were employed in conjunction with quenching experiments to elucidate the enhancement mechanism of the Pd/MnO2 catalyst in electrochemical reduction for 2,4-DCBA degradation. Special emphasis was placed on analyzing the role of MnO2 in this process, providing the theoretical support for the electrochemical reduction of CBA-type pollutants.

2. Materials and Methods

2.1. Chemicals

Reagent-grade regents, including PdCl2, MnO2, C6H5Na3O7, NaBH4, Na2SO4, NaHCO3, and NaNO3, together with 2,4-DCBA (high-performance liquid chromatography (HPLC) grade), were purchased from Aladdin (Shanghai, China). Methanol for the HPLC measurement was purchased from McLean Company (Shanghai, China). All experimental solutions were formulated using high-purity deionized water (DI) (below 18 MΩ·cm−1). All other regents were analyzed at a pure level.

2.2. Preparation of Pd/MnO2 Catalyst

The Pd/MnO2 catalyst was synthesized through a chemical deposition method, wherein an aqueous mixture of PdCl2 and sodium citrate was utilized. In this process, sodium citrate served as a complexing ligand to stabilize Pd ions in solution, thereby inhibiting their early precipitation and promoting homogeneous distribution onto the MnO2 support. Subsequently, 2.0 g of MnO2 was introduced into the solution and subjected to ultrasonic treatment for 30 min. Subsequently, an aqueous solution of NaBH4, which serves as a strong reducing agent, was added dropwise into the mixture under constant stirring to reduce the Pd2+ ions to metallic Pd0 nanoparticles and deposit them onto the MnO2 support. Following 4 h of stirring, the solid catalyst was collected by filtration, washed with deionized water, and vacuum-dried at 60 °C for subsequent applications. It is noteworthy that during catalyst synthesis, the absolute mass of the support material (MnO2) was maintained constant at 2.0 g, while the amount of the Pd precursor was varied to achieve different Pd loadings.

2.3. Electrochemical Experiments

Electrochemical measurements were performed in a two-chamber electrolytic cell (250 mL per compartment), employing a copper foam cathode and a platinum wire anode, powered by a CHI 760e potentiostat (Shanghai Chenhua Co., Ltd., Shanghai, China). The reaction chambers are separated by a cation exchange membrane to prevent the generated Cl− from spreading to the anode surface to be oxidized to Cl2. In the anode chamber, 200 mL of 0.5 mM Na2SO4 aqueous solution was added, and in the cathode chamber, 200 mL of 2,4-DCBA solution and catalyst were added. A magnetic stirrer was employed to enhance reactant transport throughout the reaction process. At designated time intervals, 1.0 mL aliquots were withdrawn from the cathode compartment for subsequent detection and analytical procedures. The results represent the average of three experimental repeats.

2.4. Analysis Methods

Concentration of 2,4-DCBA was analyzed using a high-performance liquid chromatography (Agilent 1260, Agilent Technologies, Santa Clara, CA, USA) equipped with an Agilent C18 column (2.1 mm × 100 mm). The HPLC separation utilized a mobile phase of methanol and acidified water (0.2% H3PO4) at a volume ratio of 55:45, delivered at 1.0 mL·min−1. Detection was performed at 234 nm, and each sample injection was 20 μL.The dechlorination extent was determined by comparing the measured chloride ion concentration following the reaction with the theoretical maximum chlorine content present in the initial substrate [26].
The observed pseudo-first-order rate constant (kobs, min−1) was employed to quantify the electrocatalytic hydrodechlorination kinetics. For all experimental conditions, the temporal concentration data of 2,4-DCBA were fitted to the integrated rate equation:
ln(C0/Ct) = kobs* t
where C0 and Ct represent the initial concentration and the concentration at time t (min), respectively. The values of kobs were derived from linear regression analysis using OriginPro 2022, with all fittings yielding regression coefficients (R2) greater than 0.97.
The surface morphologies of the Pd/MnO2 catalyst were studied using high-resolution transmission electron microscopy (JEM-2100 plus, JEOL Ltd., Tokyo, Japan) at a 13 eV pass energy and a resolution of 0.05 eV. The surface element identification and distribution of the catalyst were analyzed using energy dispersive X-ray spectroscopy (EDS) analysis and mapping. X-ray photoelectron spectroscopy (XPS) analysis of the catalyst was carried out with an Escalabxi spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), equipped with a monochromatic Al Kα X-ray source operated at 14.4 kV and 13.6 mA.
The actual metal loadings of palladium and manganese in the Pd/MnO2 catalyst were determined using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES, Thermo Scientific iCAP 7400, Thermo Fisher Scientific, Waltham, MA, USA ). Specifically, 10 mg of catalyst was completely digested in 10 mL of aqua regia (HCl:HNO3 = 3:1) by heating at 120 °C for 2 h. The obtained solution was then cooled to room temperature, filtered through a 0.22 μm membrane, and diluted to 50 mL with deionized water for ICP-OES measurement. The measured Pd loading was 7.05 wt%, demonstrating the high precision of the preparation method. The Mn content was determined to be 45.2 wt%, which is consistent with the expected composition of the MnO2 support.
Linear voltammetry (LSV) was used to qualitatively compare the reaction rates of hydrogen evolution before and after the addition of the catalyst. The resistance of charge transfer and the electron transfer rate in the electrode were determined by electrochemical impedance spectroscopy (EIS). Electrode capacitances were qualitatively compared using cyclic voltammetry CV curves, and active sites were compared by the number of reduction peaks.

2.5. Theoretical Calculation Methods

All density-functional theory (DFT) computations were conducted within the Vienna Ab Initio Simulation Package (VASP) [27,28], employing the projector augmented-wave (PAW) approach [29]. The exchange correlation interactions were described using the Perdew–Burke–Ernzerhof (PBE) formulation of the generalized gradient approximation (GGA) [30]. Spin-polarized computations were incorporated. The plane-wave basis set was defined with a kinetic energy cutoff of 450 eV. Ionic relaxations were performed via the conjugate gradient (CG) algorithm until convergence criteria were met: total energy changes within 10−5 eV and atomic forces below 0.05 eV/Å. To mitigate spurious periodic interactions, a vacuum spacing of 15 Å was introduced normal to the surface. The lattice constants of the isolated Pd substrate were adopted from standard references: Pd/MnO2 is 5.50 Å × 5.50 Å × 20.00 Å and 8.89 Å × 13.65 Å × 17.86 Å, respectively. The Brillouin zone was sampled using 6 × 6 × 1 and 4 × 3 × 1 Monkhorst-Pack k-point grids, respectively.

3. Results

3.1. Optimization and Characterization of Electrochemical Catalysts

3.1.1. Comparison of Different Catalyst Materials

The pre-inquiry of the performance of Pd/MnO2 in enhancing the electrochemical reduction of 2,4-DCBA is shown in Figure 1. Al2O3 is a commonly used Pd-supported substrate; thus, the performance of Pd/Al2O3 was also explored for comparison. As shown in Figure 1a, without the addition of Pd/Al2O3 and Pd/MnO2 catalysts, the electrified Cu foam cathode has little degradation effect on 2,4-DCBA. It can be seen that the electrocatalytic effect of Pd/MnO2 was significantly better than that of the Pd/Al2O3 catalyst. It might be because Mn possessed less electronegativity and a stronger attraction to H+ than Al, which was conducive to the formation of Hads [31]. Studies have found that Mn can capture H* flowing from the Pd surface and help them to diffuse to the whole catalyst surface so as to carry out the subsequent hydrogenation reaction [32]. Therefore, MnO2 in the Pd/MnO2 catalyst probably played the role of a medium [33] to transfer H* from Pd to 2,4-DCBA, enhancing the reducible degradation of 2,4-DCBA.
The influence of the Pd-loading ratio on the catalytic dechlorination performance is shown in Figure 1b and Figure S1. The degradation efficiency of 2,4-DCBA exhibited an initial rise, followed by a decline, as the Pd-loading ratio increased, with the optimal loading amount identified as 7.5% in this investigation. The dechlorination percentage and 2,4-DCBA degradation trends were consistent, with 89.62% dechlorination of 2,4-DCBA at the end of the reaction at 7.5% of Pd loading (Figure S1). In general, a higher Pd-loading ratio means that more atomic H* can be stored, thus speeding up the catalytic effectiveness of the ECH reaction. However, with the increase in the Pd load, Pd particles would agglomerate and thus increase in size, which accelerated the HER reaction. The loaded Pd with a smaller particle size reacts easily with the carrier [33], while Pd with a larger particle size is conducive to adsorb H2 and generate Pd-H [34]. The findings suggest that an overproduction of H2 bubbles could impede the accessibility of 2,4-DCBA to the catalytic cathode surface, thus constraining the overall reaction efficiency. Obviously, the presence of MnO2 can decrease the required amount of Pd. Adding extra Pd will also increase costs; therefore, the Pd/MnO2 with a 7.5% Pd loading ratio was selected as the best material for the subsequent experiments.
To contextualize the performance of the synthesized Pd/MnO2 catalyst, a comparative analysis with other recently reported Pd-based electrocatalysts for the dechlorination of chlorobenzoic acids is summarized in Table S1. As clearly demonstrated, the Pd/MnO2 catalyst developed in this study exhibits a remarkable removal efficiency of 97.3% for 2,4-DCBA, substantially outperforming other systems such as Pd/Al2O3 (42.3%) and TiC-Pd/Ni foam (37.5–86.3%) under comparable conditions. This superior activity highlights the crucial role of the MnO2 support in enhancing the electrocatalytic dechlorination performance.

3.1.2. Morphological Characterization of Pd/MnO2 Catalyst

The structural features and spatial element composition of the Pd/MnO2 catalyst were analyzed by high-resolution transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy. The results showed the presence of Mn, O, and Pd elements (Figure 2a,b) and confirmed the successful deposition of Pd nanoparticles on the MnO2 support (Figure 2c–f). Specifically, it can be seen that there were small, irregular, clustered particles distributed on the surface of the matrix particles. EDS results showed that the surface-clustered particles were Pd, while the matrix material was MnO2. The high-resolution transmission electron microscopy (TEM) micrograph exhibits well-defined lattice fringes measuring 0.417 nm in interplanar spacing [35], which are consistent with the (1 1 1) crystallographic plane of metallic palladium. It indicated that the Pd nanoparticles were well-dispersed and integrated into the MnO2 layer, forming an active catalytic structure. An X-Ray diffraction (XRD) analysis of the Pd/MnO2 catalyst also confirms this (Figure S3).

3.2. Influencing Factors of Electrochemical Reduction

3.2.1. Effects of Current Density

The effects of applied current density were investigated, since atomic H* generation was potentially dependent on it. As shown in Figure 3a, the removal of 2,4-DCBA increased from 78.55% to 95.36% after a 120 min reaction when the current density increased from 4 mA·cm−2 to 16 mA·cm−2. It indicated that a higher current density was conducive to the removal of 2,4-DCBA. In general, a higher current density means that more electrons are transferred to H2O in a given time, generating more H*, an important reducing agent for the dechlorination of 2,4-DCBA, thus enhancing the dechlorination efficiency. However, current efficiency (CE), which indicates energy efficiency, should also be considered to determine the effectiveness of the charge used during the dechlorination of 2,4-DCBA. According to Faraday’s law, the formula for calculating the current efficiency in this work can be provided in Text S1. The CE model was fitted using data collected after the initial 30 min, owing to the presence of two distinct forms of atomic H* within the solid solution phase (Hs) and inert H* in the palladium hydride phase (HPd). Under equilibrium conditions between the active and inert phases, an adequate concentration of active sites remains accessible to participate in the reductive dechlorination of 2,4-DCBA, thereby facilitating the reaction while maintaining the overall equilibrium. [36]. Additionally, the internal ohmic resistance resulting from ion migration across the electrodes required mitigation during the operation.
As shown in Figure 3b, at a lower current density (4 mA·cm−2), CE gradually increased with the potentially continuous generation of H* at the beginning, and then decreased, probably due to the limitation of mass transfer. At elevated current densities, however, the two atomic H* phases rapidly attain equilibrium, resulting in a monotonically declining profile devoid of inflection points [37]. Notably, while the 2,4-DCBA removal efficiency exhibited a positive correlation with applied current, the Coulombic efficiency (CE) demonstrated an inverse relationship with increasing current density, indicating that the accelerated production of H* on the electrode enhanced the dechlorination, but also accelerated the side reaction of electrocatalytic dehydrogenation, leading to a decrease in the CE. In practical applications, current density should be minimized subject to compliance with the water treatment requirement.

3.2.2. Solution Influencing Factors of the Electrochemical System

As depicted in Figure 4a, as the concentration of 2,4-DCBA continuously increased, both the removal percentage and the reaction apparent rate constant (kobs) showed a decreasing trend. Using a starting concentration of 1 mg/L, the removal of 2,4-DCBA was up to 99.6%, with kobs being 0.024 min−1. At a concentration of 20 mg/L 2,4-DCBA, the removal percentage slightly dropped to 97.3%. However, when it escalated above 30 mg/L, the removal significantly decreased to below 53.2%. The reduction was attributed to incomplete dechlorination. Throughout the electrocatalytic reduction, intermediate dechlorination products and residual 2,4-DCBA may poison catalytic sites on the electrode surface, leading to diminished degradation efficiency. To verify the hypothesis of active site deactivation by adsorbed dechlorination products, a post-reaction characterization of the used Pd/MnO2 electrode was performed. The HPLC analysis of the concentrate confirmed the presence of dechlorination products (2-CBA and benzoic acid) on the surface, with an estimated total adsorption of 11.5 ± 1.3 μmol g−1, consistent with reported values for aromatic compounds on metal oxides [38,39]. Another conceivable reason was that higher substrate concentrations necessitated greater charge transfer in electrocatalytic reduction reactions [40]. In this study, the charge under constant current (200 mA) conditions might not be adequate to satisfy the escalating demand associated with higher substrate concentrations. The influence of catalyst dosage on the reaction is shown in Figure 4b. It is evident that the optimal catalytic performance was achieved at a catalyst dosage of 0.5 g/L, with a slight decline observed when the dosage was increased to 0.7 g/L. When the catalyst dosage was excessive, the catalytic material might form the agglomeration, thereby reducing the exposure of active surfaces and leading to the reduced catalytic efficiency [41].
In the investigation of the pH influence, the effect of electrocatalytic dechlorination decreased gradually with the increase in pH (Figure 4c,d). When the initial pH increased from 4.0 to 8.0, the kobs of 2,4-DCBA decreased from 0.00694 min−1 to 0.01117 min−1, mainly since the lower pH environment could enhance the generation of atomic H*. In general, atomic H* is mainly produced during the reduction of H3O+ and the dissociation of H2O in acidic and neutral/basic solutions [42]. The generated H* species may subsequently engage in both the HER and the hydrodechlorination process, thus increasing the dechlorination of 2,4-DCBA. Secondly, the surface charge of the MnO2 support (point of zero charge ~4–5) becomes increasingly positive at lower pH values, which may enhance the adsorption of anionic chlorobenzoate species through electrostatic interactions, thereby promoting their accessibility to Pd-active sites. Conversely, at a near-neutral pH (8.0), the diminished H3O+ concentration reduces H* generation kinetics, while the increasingly negative catalyst surface could electrostatically repel the deprotonated 2,4-DCBA anions (pKa ≈ 2.9), limiting their adsorption and subsequent dechlorination. Moreover, the competitive HER is markedly enhanced in acidic media, which could consume a portion of the applied current but simultaneously maintain a high H* flux. As the reaction proceeded, OH− ions generated from the HER accumulated in the cathode chamber due to the cation exchange membrane, causing the pH to rise and eventually plateau. This dynamic pH change explains the observed stabilization of the removal efficiency over time, as the system approached a new steady-state condition under the operating pH.

3.3. Mechanisms of Electrocatalytic Reduction by Pd/MnO2 Catalyst

3.3.1. Electrochemical Characterization Analysis

To explore the electrocatalytic reduction mechanisms of 2,4-DCBA by Pd/MnO2, C4H10O was added for the quenching experiment of active H* removal. As can be seen from Figure 5a, the reaction was partially inhibited with 5 mM C4H10O and almost completely inhibited with 10 mM C4H10O, indicating that the reduction of 2,4-DCBA was mainly dependent on active H*. Electrochemical characterization of the catalyst was also carried out to illustrate the role of MnO2 in electrocatalytic reduction dechlorination. Electrochemical impedance spectroscopy (EIS) was employed to determine the charge-transfer resistance at the electrode interface. The diameter of the high-frequency semicircle in the alternating current (AC) impedance spectrum corresponds to the electron transfer kinetics at the electrode interface. As shown in Figure 5b, compared with Pd/Al2O3, the semicircle diameter of Pd/MnO2 catalyst was smaller, indicating a smaller electrode resistance and lower electron transport impedance. The introduction of MnO2 enhanced the electron transfer [43,44], and such higher electron transfer rates can lead to the higher catalytic efficiency [45].
To elucidate the evolutionary behavior of H* on the Pd/MnO2 catalyst, CV was performed within a potential window of −1.2 to 0.4 V. From Figure 5c, it can be seen that, compared to Pd/Al2O3, the CV curve area of Pd/MnO2 catalyst was larger, indicating a higher capacitance. It also suggested that Pd/MnO2 catalyst possessed a superior electrochemical active surface area, positively changing the chemical environment of Pd nanoparticles. Expanding the electron-active region beyond Pd and distributing it across the entire electrode enables potential reaction pathways. The addition of metal oxides can effectively accelerate water splitting [46], and Al2O3 is also a common metal oxide used as a carrier, which can partially enhance the electrocatalytic hydrogenation. Notably, the CV curve in Figure 5c did not show a significant reduction peak in the Pd/Al2O3 cyclic voltammetry curve when scanning from the negative direction. However, a reduction peak at around 0.08 V could be observed in the Pd/MnO2 catalyst system, indicating the production of H*. The excellent hydrogen adsorption capacity and catalytic reduction performance provided abundant active sites for H* generation, thereby enhancing the reduction reaction. The intrinsic HER activity was quantitatively compared through the calculated overpotentials. The Pd/MnO2 catalyst exhibited a significantly lower overpotential of 390 mV at 10 mA cm−2, compared to 740 mV for the Pd/Al2O3 catalyst. This remarkable reduction of approximately 350 mV unequivocally demonstrates the superior proton reduction capability of the MnO2-supported catalyst. The enhanced HER activity directly contributes to its higher efficiency in generating reactive H* atoms, thereby explaining its superior performance in the electrocatalytic hydrodechlorination of 2,4-DCBA.
In the HER reaction, the overpotential is an extremely important parameter. Typically, a smaller hydrogen evolution overpotential indicates superior catalytic activity. As shown in Figure 5d, Pd/MnO2 showed a smaller hydrogen evolution overpotential compared to the Pd/Al2O3 catalyst, and the HER intensity on the electrode was greater. Pd was the major catalyst for storing atomic H*, which was the main reducing agent in this system. The presence of MnO2 made it easier for Pd to generate H*, enhancing the reduction of 2,4-DCBA. Based on the comprehensive characterization of LSV and EIS, due to the fact that Pd itself only has the function of catalyzing the activation of H2 and cannot accelerate water splitting [21], the introduction of MnO2 might mainly serve as an intermediate for electron transfer and storage, facilitating further dissociation of water molecules for H* generation.

3.3.2. Electrocatalytic Mechanisms of 2,4-DCBA Reduction

Combining references and the above analysis, it can be inferred that the dechlorination in this study was mainly conducted through indirect electrochemical reduction, and the specific process is as follows [47]:
2H2O + 2e− → H2 + 2OH−
H2 + M → 2(H*)adsPd
R − Cl + 2(H*)adsPd → 2Pd + R − H +H+ + Cl−
(H*)adsPd + H2O + e− → Pd +OH− +H2
(H*)adsPd + (H*)adsPd → 2Pd + H2
This indicated that the reduction of 2,4-DCBA mainly relied on active H*. MnO2 as an intermediate enhanced the electron transfer and the ability of Pd to capture hydrogen, and Pd/MnO2 as a carrier could effectively enhance the hydrogen evolution ability of electrode, further accelerating the catalytic generation of H* from the electrode. The binding energy (Eads) of H* on Pd clusters and Pd/MnO2 clusters are compared in Figure 6a and Figure 6b, respectively. The Eads of H* on Pd clusters were smaller than that on Pd/MnO2 clusters (–2.06 eV vs. –3.29 eV). A higher absolute value of Eads suggested a stronger adsorption tendency of H* on Pd-supported MnO2 compared to unsupported Pd, which contributed to the improved capacity of the Pd/MnO2 composite in sustaining elevated surface concentrations of atomic hydrogen. The adsorption behavior of H* was further confirmed by projected density of states (PDOS) analysis. The observed PDOS overlap revealed the establishment of three distinct Pd–H bonds (Pd1–H, Pd2–H, and Pd3–H), primarily attributed to interactions between H 1s and Pd 4d orbitals (Figure 6c,d). Crucially, a comparative analysis of the Pd-H and Pd/MnO2-H systems revealed a notable electronic effect induced by the MnO2 support. The d-band center of Pd in the Pd/MnO2 system was shifted upwards compared to bare Pd, indicating an enhanced ability to adsorb reactive intermediates like H* [48]. This shift resulted in stronger orbital hybridization, as evidenced by the higher hybrid orbital energies in the Pd/MnO2-H system compared to the Pd-H system. The intensified electronic interaction suggests that more electrons from the Pd 4d orbitals participate in forming robust Pd-H bonds with adsorbed H atoms. The PDOS results were consistent with Eads analysis, further demonstrating that Pd/MnO2 was superior to bare Pd in adsorbing H*. The atomic H* anchored on the surface of the Pd/MnO2 system thus enhanced the dechlorination of 2,4-DCBA through an indirect mechanism.
The proposed reaction mechanism for the Pd/MnO2 catalyst is schematically illustrated in Figure 7. The H2 generated by H2O electrolysis would exert a certain hydrogen pressure on the electrode. The H2 that entered the active center Pd of the catalyst for the first time was able to generate a solution phase [H] [49]. Subsequently, some of the solid-solution phase would reversibly react with H2 to form the metal hydride Pd-H. When the two were in equilibrium, the remaining active-solution phase [H] would participate in the reduction and dechlorination process. At the same time, the newly generated [H] in the solution phase might also generate H2 again through Heyrovsky and Tafel side reactions (Equations (4) and (5)). The function of MnO2 was to enhance the cleavage of HO-H bonds in water molecules. When Pd electrolyzed H2O to produce H2, it catalyzed the formation of H* atoms adsorbed on the catalyst surface. The 2,4-DCBA adsorbed on the catalyst surface might then be reduced by (H*)adsPd (hydrogen radical adsorbed on Pd surface), via removing one chlorine atom. Depending on the position of the substituent, 2,4-DCBA, which was further reduced by H*, probably dechlorinated to form benzoic acid (BA), thereby reducing toxicity [50]. In addition to accelerating water splitting, MnO2 was able to serve as an intermediate for electron transfer or storage, participating in the process of hydrogen overflow [51]. The spillover effect involved the Hads diffusing from Pd particles to the surface of MnO2. MnO2 acted as a medium to transfer H* from Pd to 2,4-DCBA, altering the chemical environment of Pd nanoparticles and diffusing H* to the entire system for reaction. Such a process was able to expand the region of action beyond Pd and reduce the pressure of adsorbed H* on the Pd surface. Even at high yields, more H* tended to undergo hydrogenation for 2,4-DCBA dechlorination rather than evolving into H2, which helped to improve the reaction efficiency.

3.4. Stability of the Pd/MnO2 Particle Catalyst

XPS spectra of Mn 2p and Pd 3d before and after dechlorination of 2,4-DCBA are shown in Figure S2. In the Mn 2p spectrum of the synthesized Pd/MnO2 catalyst, the peaks observed at 641.5 eV and 653.5 eV correspond to Mn 2p3/2 and Mn 2p1/2, respectively, characteristic of Mn(IV) in the MnO2 structure [52]. The intensity of these tow peaks maintained well before and after the reaction, and no other valence Mn peaks were found, suggesting that MnO2 was still well preserved, which further illustrated the stability of the catalyst. The dominant peaks observed at 334.3 eV and 339.7 eV are attributed to Pd0 3d5/2 and Pd0 3d3/2, respectively, whereas the satellite peaks located at 336.4 eV and 342.6 eV correspond to the 3d5/2 and 3d3/2 states of Pd(II). The presence of Pd2+ species in the catalyst may result from either incomplete reduction of Pd2+ by oxygen vacancies during synthesis or partial oxidation of Pd0 upon air exposure, resulting in the co-existence of both Pd0 and Pd2+ oxidation states on the electrode [53]. Overall, the Pd element mainly existed in the form of Pd0, which was considered as the active part in the reaction.
Five consecutive cycle experiments of 2,4-DCBA dechlorination were conducted to further verify the reusability of Pd/MnO2 catalyst. As can be seen from Figure 8, the removal of 2,4-DCBA decreased slightly with the progress of the experiment, but could still reach more than 80% after the five cycles. The decrease in catalytic performance was partly related to the fact that the granular catalyst was always in a hydrogen-rich state [54], and there was certain morphological changes in Pd due to metal expansion and other reasons, leading to partial blockage of surface active sites [55]. The decrease in adhesion influenced the efficiency of reduction reaction.

4. Conclusions

A Pd/MnO2 catalyst for 2,4-DCBA dechlorination was developed by chemical deposition. Pd nanoparticles have excellent hydrogen storage and activation properties, while MnO2 is an effective carrier with good electrochemical properties. By exploiting the synergistic effect between MnO2 and Pd, the activation and spillover of atomic H* were enhanced, thereby promoting the effective removal and degradation of 2,4-DCBA. The optimized system achieved near-complete contaminant removal and high dechlorination efficiency under mild conditions, showing its potential for practical water treatment applications. In addition, the catalyst exhibited robust stability over multiple cycles, ensuring its long-term availability. DFT calculations revealed that Pd/MnO2 outperformed bare Pd in adsorption of H*. Moreover, the atomic H* enhanced the dechlorination of 2,4-DCBA through an indirect mechanism. The findings provide a feasible electrocatalytic strategy to minimize the amount of precious metals without compromising the treatment efficiency for typical CBA-related pollutants.

Supplementary Materials

The supporting information can be downloaded at https://www.mdpi.com/article/10.3390/cleantechnol7040102/s1. References [10,56] are cited in in the supplementary materials.

Author Contributions

Y.P.: writing—original draft, conceptualization, methodology, software; M.W.: formal analysis, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data presented in this study are available on request from the corresponding author due to data are not publicly available due to privacy.

Conflicts of Interest

The authors declare no conflict of interest.

Abbreviations

The following abbreviations are used in this manuscript:
CBAsChlorobenzoic acids
ECHElectrocatalytic hydrodechlorination
2,4-DCBA2,4-dichlorobenzoic acid
H*Atomic hydrogen

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Figure 1. Degradation efficiency of 2,4-DCBA in the electrochemical system by different catalyst supports with Pd (12.5%) (a) and effects of different Pd-loading ratios (0%, 1%, 2.5%, 5%, 7.5%, 10%, and 12.5%) for 2,4-DCBA removal (b).
Figure 1. Degradation efficiency of 2,4-DCBA in the electrochemical system by different catalyst supports with Pd (12.5%) (a) and effects of different Pd-loading ratios (0%, 1%, 2.5%, 5%, 7.5%, 10%, and 12.5%) for 2,4-DCBA removal (b).
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Figure 2. HRTEM (a), corresponding overall element distribution diagram (b), O element distribution (c), Mn element distribution (d), and Pd element distribution (e). EDS-mapping results (f) of Pd/MnO2.
Figure 2. HRTEM (a), corresponding overall element distribution diagram (b), O element distribution (c), Mn element distribution (d), and Pd element distribution (e). EDS-mapping results (f) of Pd/MnO2.
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Figure 3. Reductive efficiency of 2,4-DCBA (a) and CE results (b) under different current densities.
Figure 3. Reductive efficiency of 2,4-DCBA (a) and CE results (b) under different current densities.
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Figure 4. Effects of initial concentration of 2,4-DCBA (a), catalyst dosage (b), as well as initial pH (c) on the reduction efficiency of 2,4-DCBA and kobs for initial pH experiment (d).
Figure 4. Effects of initial concentration of 2,4-DCBA (a), catalyst dosage (b), as well as initial pH (c) on the reduction efficiency of 2,4-DCBA and kobs for initial pH experiment (d).
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Figure 5. Effects of C4H10O on the reductive dechlorination of 2,4-DCBA (a), EIS results (b), CV profiles (c), and LSV analysis (d) of Pd/Al2O3 and Pd/ MnO2.
Figure 5. Effects of C4H10O on the reductive dechlorination of 2,4-DCBA (a), EIS results (b), CV profiles (c), and LSV analysis (d) of Pd/Al2O3 and Pd/ MnO2.
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Figure 6. Optimized structures of an H atom adsorbed on a Pd (a) and an MnO2-supported Pd (b). PDOS analysis for H, Pd1, Pd2, and Pd3 atoms after adsorption for Pd-H system (c) and Pd/MnO2 system (d).
Figure 6. Optimized structures of an H atom adsorbed on a Pd (a) and an MnO2-supported Pd (b). PDOS analysis for H, Pd1, Pd2, and Pd3 atoms after adsorption for Pd-H system (c) and Pd/MnO2 system (d).
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Figure 7. Mechanisms of enhanced dechlorination of 2,4-DCBA-based on Pd/MnO2 catalyst.
Figure 7. Mechanisms of enhanced dechlorination of 2,4-DCBA-based on Pd/MnO2 catalyst.
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Figure 8. Catalytic reusability of Pd/MnO2 in the hydrodechlorination of 2,4-DCBA.
Figure 8. Catalytic reusability of Pd/MnO2 in the hydrodechlorination of 2,4-DCBA.
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Peng, Y.; Wang, M. MnO2-Supported Pd Nanocatalyst for Efficient Electrochemical Reduction of 2,4-Dichlorobenzoic Acid. Clean Technol. 2025, 7, 102. https://doi.org/10.3390/cleantechnol7040102

AMA Style

Peng Y, Wang M. MnO2-Supported Pd Nanocatalyst for Efficient Electrochemical Reduction of 2,4-Dichlorobenzoic Acid. Clean Technologies. 2025; 7(4):102. https://doi.org/10.3390/cleantechnol7040102

Chicago/Turabian Style

Peng, Yaxuan, and Meiyan Wang. 2025. "MnO2-Supported Pd Nanocatalyst for Efficient Electrochemical Reduction of 2,4-Dichlorobenzoic Acid" Clean Technologies 7, no. 4: 102. https://doi.org/10.3390/cleantechnol7040102

APA Style

Peng, Y., & Wang, M. (2025). MnO2-Supported Pd Nanocatalyst for Efficient Electrochemical Reduction of 2,4-Dichlorobenzoic Acid. Clean Technologies, 7(4), 102. https://doi.org/10.3390/cleantechnol7040102

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