Highly Active And Durable Carbon-based Catalyst For Electrochemical Nitrate Reduction

The considerable consumption of agricultural fertilizers and the random discharge of improperly treated industrial wastewater have resulted in the gradually increased concentration of nitrate in the surface water and groundwater,seriously threatening the sustainability of the ecosystem and the safety of human society.In order to cope with the severe nitrate pollution situations,various treatment technologies have been proposed in the field of denitrification.Owing to the advantages of high efficiency,easy and controllable operation,low cost,wide p H working range,low risk of secondary contamination,and being driven by renewable energy sources?such as wind energy,solar energy,and tidal energy?,the electrochemical denitrification technology has received much attention from academia and industry.As the core component of the electrochemical denitrification technology,the cathode catalyst directly determines the overall denitrification efficiency of the electrochemical system.The current electrocatalysts generally have the problems of low activity and poor stability,because of the direct contact between the catalytically active site and the electrolyte,which leads to the continuous leaching of catalytically active components.This not only affects the feasibility of long-term operation,but also brings serious secondary pollution,eventually limiting the development and application of the technology.In order to solve the above-mentioned problems,we design and synthesize a new class of carbon electrocatalysts having the unique core-shell structure:N-doped graphitic carbon-encapsulated metal nanoparticles,which achieved outstanding activity and durability.Its promising performance for electrochemical reduction of nitrate available in the real wastewater was also verified.The main experimental results are as follows:?1?The first effort was made to prepare N-doped graphitic carbon-encapsulated iron nanoparticles via the simple high-temperature pyrolytic method and explore its catalytic performance for nitrate removal.The resulting Fe?20%?@N-C achieves a better nitrate removal proportion of 83.0%?attained in the first running cycle?compared to the efficiencies of other reference catalysts,including those with lower entrapped Fe content.The nitrogen selectivity is 25.0%in the absence of Cl^-and increases to 100%when supplemented with 1.0 g L^-1 Na Cl.More importantly,there is no statistically significant difference?at a 95%confidence interval?regarding the removal percentage recorded over 20 cycles for the Fe?20%?@N-C cathode.The density functional theory?DFT?calculations indicate that the iron nanoparticles could attenuate the work function on the neighboring carbon atoms,which are the reactive sites for NRR,and that the graphitic shells hinder the access of the electrolyte,thus protecting the iron particles from dissolution and oxidation.Testing with the real industrial wastewater further demonstrates the superiority of Fe?20%?@N-C cathode towards NRR,as evidenced by efficient removal of nitrate available in the biological effluent from a local coking wastewater treatment plant.?2?In order to develop and boost the performance of the core-shell catalysts,it is greatly significant to understand the structure-activity relationship of this class of catalysts from the atomic scale.The second attempt was made to investigate the effects of the different encapsulated metal nanoparticles on the catalytic activity and explore the origin of the activity difference.Experimental and theoretical results show that the strong interaction between inner metal particles and the doped-N atom on the surface of carbon shell is responsible for the improved catalytic performance.Under the same reaction condition as shown in the first part,the optimized catalyst?Co@NCNT?achieves nearly complete nitrate removal and itscatalytic activity is well retained after 40-cycle operation.In addition,it is found that the dissolved oxygen and natural organic matter have little influences on the nitrate removal,indicating the excellent anti-interference ability of the catalyst.The results of electron spin resonance?ESR?characterizations and electrochemical scavenging experiments confirm the involvement of H radical-mediated nitrate removal mechanism in the Co@NCNT electrochemical system.On the whole,we have successfully demonstrated that metal-encapsulating N-doped carbon catalysts show high efficiency and durability for electrochemical nitrate reduction.Benefiting from the synergistic effects between inner metal particles and doped-N on the surface of carbon shell,continuous metal release can be avoided and the originally inert carbon atom on the surface can be activated by the inner metal particles and doped-N atom,thus leading to the high catalytic activity and stability.These findings might provide insights and understanding for practical application of the electrochemical nitrate reduction in the future,especially for the complicated and severe wastewater situations?i.e.,high salinity and pollutant concentration,extreme acid or alkaline solution,and high concentration of organic matters?.