Hierarchical Pd/MnO₂ nanosheet array supported on Ni foam: An advanced electrode for electrocatalytic hydrodechlorination reaction

Highlights

• An advanced hierarchical Pd/MnO₂-Ni foam electrode was synthesized for EHDC.

• The electrode has 3D foam structure, porous skeleton surface and high Pd dispersion.

• The Pd/MnO₂-Ni foam electrode delivered an unprecedented high mass activity.

• MnO₂ in the electrode served as a mediator to transfer the H* from Pd to 2,4-DCP.

• The electrode displayed high efficiency and durability in continuous-flow system.

Abstract

Electrocatalytic hydrodechlorination (EHDC) is deemed as one promising approach for efficient and safe detoxification of the trace halogenated organic pollutants in water. Here we prepared one advanced Pd/MnO₂-Ni foam electrode via the construction of oxygen-deficient MnO₂ nanosheet arrays on Ni foam skeleton, which then served as the support and electron donator to capture and reduce Pd precursor to nanoparticles. With the three-dimensional porous structure, hierarchical skeleton surfaces and improved Pd dispersion, the Pd/MnO₂-Ni foam electrode delivered an unprecedented large mass activity (k_obs) of 0.883 min⁻¹ mmol_Pd⁻¹ for EHDC of 2,4-dichlorophenol (2,4-DCP), in comparison to 0.081 min⁻¹ mmol_Pd⁻¹ of the Pd/Ni foam electrode and those reported in literatures. The Pd/MnO₂-Ni foam electrode also displayed high durability during the repeated batch EHDC experiments without the efficiency decay and the leaching of Mn/Ni/Pd, unless some reduced sulfur compounds and nitrite were included. The mechanism study revealed the MnO₂ in electrode served as a mediator to transfer H* from Pd to 2,4-DCP, which extended the reactive area beyond Pd and hindered the molecular hydrogen evolution, leading to the enhanced reactions between H* and 2,4-DCP. The Pd/MnO₂-Ni foam electrode was also tested in a continuous-flow EHDC system, and displayed the potential and retention time-dependent performances.

Introduction

Halogenated organic compounds, including halogenated chain and aromatic hydrocarbons, are an important class of chemical and industrial feedstock with wide applications in pharmaceutical, agricultural and polymer industries. The large-scale use, however, increases their exposure and impact in ecotope [1]. As the leading member of persistent organic pollutants, these halogenated compounds are stable in chemical structure, and highly resistant to natural degradation. They can also be easily accumulated in living bodies via food chain, exerting long-term harms to organs and immune systems [2]. In this case, the technologies that can remove them in an effective and green manner are highly desired [3], [4], [5], [6], [7], [8]. Electrocatalytic hydrodechlorination (EHDC) represents one promising alternative by its high efficiency, mild condition, green feature and low secondary pollution risk [9], [10]. In EHDC, numerous atomic hydrogen (H*) were in situ produced from aqueous solution at cathode via electrolysis of water, which served as the reductive agent to attack and cleave C-Cl bond, converting halogenated organics to their nonhalogenated analogues and chloride ions [11], [12], [13].

The metallic palladium (Pd) was one priority cathode catalyst due to its high efficiency and durability in producing H* from aqueous solution at a wide pH range [14], [15]. Additionally, it showed strong power in adsorption and activation of the halogenated pollutants for sequent hydrodechlorination reaction [16]. However, Pd is one precious metal, and its low earth-abundance forces us to maximize its mass activity and reduce consumption. Engineering the particle into a nanoscale is one efficient strategy, which enabled to raise the exposure of Pd atoms at particle surface, making them accessible for the desired reactions [17], [18], [19]. To further improve the performance, these NPs were supported on the metallic Ti, Cu or Ni foam substrate that owned a self-supported three-dimensional (3D) porous structure facilitating the pollutant mass diffusion [20], [21], [22]. Cheng reported the first Pd NP/Ti mesh electrode for removal of 2,4-dichlorophenol [23]. Since then, various foam electrodes, such as the Pd/Ni foam and Pd/Cu foam electrode, were developed [24], [25], [26]. However, as the Pd NPs were grown on the foam via a simple spontaneous galvanic reaction between the foam metal and a Pd salt, their dispersion and size were usually not well-controlled, making their overall mass activity still unsatisfactory. On the other hand, the intrinsic activity of Pd in these electrodes was actually not improved, due to the little synergy between Pd and the support in EHDC.

In recent years, the researchers found that decoration of the foam electrode with some other active species can significantly promote the mass activity of Pd. He ever deposited an Ag or Cu layer between the Ni foam and Pd NPs, and found that the presence of Ag improved the dispersion of Pd NPs and contributed to the adsorption of pollutants on electrode [27], [28]. Instead, Mao decorated the Cu foam with N-doped graphene (N-GR) before the loading of Pd, and identified that N-GR contributed to an enhanced H* generation [29]. Sun introduced the conductive polymer in electrodes, which was proved to promote NP dispersion and H* generation [30]. Xu ever modified the Pd/Ni foam electrode with TiN or TiC NPs as both of these NPs could contribute a promotional synergy for H* generation [31], [32]. The oxides with the metal component of diverse valences (such as MnO₂ and TiO₂) represented another important class of active additives. Lou ever deposited the Pd NPs on Ni foam with its skeleton covered by layers of MnO₂. Their experimental results confirmed that the introduction of MnO₂ could reduce the Pd NP size, and enhance H* generation at the Pd-MnO₂ interfaces, leading to a significant enhancement in mass activity [33], [34]. In addition, the hydrophilic features of the oxide benefited the mass diffusion of reactants around electrode.

In this work, we developed another more efficient Pd/MnO₂-Ni foam electrode for EHDC of 2,4-dichlorophenol (2,4-DCP, one typical halogenated organic pollutant). In contrast to that in Lou’s work with a compact layer structure, the MnO₂ in our work displayed a uniform nanosheet array structure with much larger surface areas. Notably, sequent Pd depositing was conducted by pre-constructing oxygen vacancies on MnO₂ sheet by a reductive current, which served as the active sites to catch and reduce Pd²⁺ to Pd. By our binder-free approach, the formed Pd NPs are small in size (around 3.5 nm), well dispersed on MnO₂ nanosheet array and form strong interactions with MnO₂. As expected, the Pd/MnO₂-Ni foam electrode displayed an unprecedented high EHDC performance and mass activity in batch experiments, in comparison to the Pd/Ni foam and that reported in known literatures. The cathode potential and coexisting anions effect on EHDC performance of Pd/MnO₂-Ni foam electrode were then investigated. Given the robust EHDC performance, the electrode was applied into a continuous flow EHDC system to assess its feasibility in practical applications. Finally, the real role of MnO₂ played during the EHDC was explored.