Preparation Of Pd/MnO₂-Ni Foam Electrode And Its Electrocatalytic Hydrodechlorination Performance

Chlorinated organic compounds represent one important chemical raw materials,which are widely used in medicine,pesticide,rubber production and other industries.The large-scale use of them has led to the increase of their exposure to the ecological environment.These chlorinated organic compounds are stable in chemical structure,and have teratogenicity,carcinogenicity,which pose a huge threat to the health of animals and plants.Compared with the traditional technologies,electrocatalytic hydrochlorination（EHDC）has a wide application prospect due to its high efficiency,green feature,mild reaction condition and low risk of secondary pollution.Palladium（Pd）is an important EHDC catalyst because it can produce atomic state（H^*）from aqueous solution in a wide pH range.However,Pd is a precious metal with low earth reserves,which limited the large-scale application.Loading Pd onto three-dimensional porous conductive materials（such as foam metal）is an important strategy to improve the performance of Pd,which can effectively increase the dispersion of Pd atoms and enhance the mass transfer of pollutants,promoting EHDC reaction.In this paper,Pd/MnO₂ array-nickel foam（Pd/MnO₂-Ni foam）3D porous electrode was prepared 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.In EHDC of 2,4-DCP in water,the Pd/MnO₂-Ni foam electrode delivered a removal efficiency of 98.7%in 120 min.The mass activity of Pd was determined to be 0.883 min^-1 mmol_Pd^-1,much higher than that reported in the literature.During the reaction,about 50%of 2,4-DCP is converted to phenol（P）,while the rest is further reduced.The stability test show that the electrode could maintain the high performance in the long-term run（the activity remained at 95%after 5 cycles）.The biotoxicy of the treated solution by the Pd/MnO₂-Ni foam electrode were tested on by a MTT assay.The results showed that the cell survival rate in the treated water increased to about 99%in comparison to 87%in the original 2,4-DCP solution.For the EHDC reaction,various factors in the reaction environment affect the efficiency.Firstly,we found the EHDC efficiency displayed a volcano dependence on the cathode potential.A too positive cathodic potential slowed down the generation of atomic H*,while too negative cathodic potential accelerated the hydrogen evolution reaction,which suppressed the EHDC reaction.The best EHDC rate was obtained at the cathode potential of-0.8V vs.Ag/AgCl.The pH of the solution also affected the reaction and the optimal pH was 2.0.Effects of the coexisting ions on the EHDC efficiency was studied.The presences of the reducing sulfides(S^2-、SO₃^2-)and nitrite were detrimental to the EHDC reaction.The effect of S^2-was the most significant,and 1mM of them could totally deactivate the electrode.In the presences of 1 mM SO₃^2-and2.5 mM NO₂^-,the EHDC efficiency dropped from 100%to 48.3%and 49.6%,respectively.Nitrate and chloride ion had no effect on EHDC efficiency.A continuous flow system was established to test the EHDC performance of the Pd/MnO₂-Ni foam electrode.The effects of the cathode potential and hydraulic retention time on EHDC efficiency were investigated.A volcano-like dependence of EHDC efficiency on the cathode potential were firstly found,and a peak efficiency of75.3%was achieved at-0.8 V.The hydraulic retention time was also determinant to EHDC efficiency.With the retention time decreasing from 66.6 to 10.5 min,the removal efficiency decreased from 97.8%to 71.4%.The mechanism of enhancing the EHDC activity of Pd/Ni foam electrode by MnO₂introduction was explored.It was found that depositing Pd NPs on MnO₂ nanosheet array minimized the particle size and improved their dispersion on electrode,which raised the proportion of accessible Pd atoms in each Pd NP,leading to the higher EHDC efficiency and mass activity.On the other hand,the MnO₂ enabled to accommodate the H^*that was produced continuously on Pd surface,and spread them over the whole electrode for reaction.The unique feature extended the reaction area beyond Pd,and alleviated the pressure of access H^*on the small Pd surface.Overall,the research results will provide theoretical support for the subsequent development of new and efficient palladium-based cathode materials as well as the practical application of EHDC in practical environmental remediation.