Preparation Of Rare Earth Elements Doped Manganese Oxide Electrode And Electrocatalytic Oxidation Removal Anmonia-nitrogen

With the ammonia pollution becoming more and more serious,it has been put great attentions on the removal of ammonia-nitrogen by electrochemical oxidation.In this paper,the manganese oxide electrode with a matrix of titanium metal was prepared by impregnation pyrolysis and used in electrocatalytic denitrification.At the same time,since the lower of oxygen evolution overpotential and poor stability,the manganese oxide electrodes were modified by doping with rare earth elements such as Er,Ce and Nd.First,manganese oxide electrodes were prepared by thermal decomposition method and characterized by X-ray powder diffraction?XRD?,Transmission electron microscope?TEM?,Raman spectroscopy?Raman?and X-ray photoelectron spectroscopy?XPS?.The results showed that there were two kinds of crystal forms of MnO₂ and Mn₂O₃ in the MnO_x electrode materials,and the main component was MnO₂.The average grain size was about 27.3 nm.The optimal preparation conditions of the electrode were as follows:temperature of crystallization was 450°C,thermal time was 2 h and the coating quality was 0.83±0.17 mg/cm².The experimental results suggested that the removal ratio reached 77.1%after 10 hours of reaction,when the ammonia-nitrogen concentration was 100 mg/L,the cell voltage was 2.0 V,pH was 9.5.By studying the effect of initial ammonia-nitrogen concentration,it was found that the electrolysis reaction accorded with apparent zero-order reaction kinetic characteristics at the initial stage,but the average ammonia-removal ratio of the electrode decreased with the prolonging of reaction time,which might be due to the prevention of ammonia-nitrogen oxidation at the electrode surface by the intermediate products.The analysis of the reactants and products during the denitrification process revealed that most of the ammonia-nitrogen was oxidized to nitrogen and produced0.74%of nitrate nitrogen and 0.15%of nitrite nitrogen.The reaction mechanism might be:ammonia was adsorbed on the surface of the electrodes firstly,than oxidized by the active component Mn?IV?;after that the generated Mn?III?was transformed into Mn?IV?again through anodic oxidation.Ammonia-nitrogen was removed during this cycle.Second,Er-doped manganese oxide electrodes were prepared by thermal decomposition method and characterized by XRD,TEM,Raman and XPS.It was found that the main crystal form was MnO₂ and its grain size was about 9.1 nm,which much smaller than the grain size of MnO₂ in MnO_x.The experimental results showed that the removal ratio reached 87.1%after 10 hours of reaction,when the cell voltage was 2.0 V,pH was 9.5.By studying the effect of initial ammonia-nitrogen concentrations,it was found that the electrolysis reaction conformed to zero-order reaction kinetic characteristics at the beginning,but the average removal ratio of the electrode decreased with the increasing of reaction time,which might be due to the prevention of ammonia-nitrogen oxidation at the electrode surface by the intermediate products.After 6 runs of ammonia removal test,the removal efficiency remained80.2%,meaning the exaltation of electrode stability.It was revealed that most of the ammonia-nitrogen was oxided to nitrogen,only 0.85%was converted to nitrate,and0.23%was converted to nitrite,by detecting the reactants and products during the ammonia-removal process.The reaction mechanism might be:ammonia-nitrogen was adsorbed on the surface of the electrodes firstly,than oxidized by the active component Mn?IV?;after that the generated Mn?IV?was transformed into Mn???through anodic oxidation.The doping of Er suppressed the formation of Mn₂O₃ and increased the content of Mn?IV?in the manganese oxide,thus improving the electrocatalytic activity.Third,Ce-doped manganese oxide electrods were prepared by thermal decomposition and characterized by XRD,TEM,Raman and XPS.The results showed that the main crystal form was MnO₂ and the grain size was about 11.8 nm,which much smaller than the grain size of MnO₂ in MnO_x.The experimental results revealed that the removal efficiency reached 97.9%using 8 hours of reaction,when the cell voltage was 2.0 V,pH was 9.5.By studying the effect of initial ammonia-nitrogen concentrations on the denitrification performance of the electrode,it was found that the electrolysis reaction agreed with the zero-order reaction kinetic characteristics at the beginning,but the average denitrification rate decreased with with the increasing of reaction time which might because the intermediate products prevented further oxidized of ammonia.After 6 runs of ammonia removal test,the removal efficiency remained at 90.3%.It was revealed that most of the ammonia nitrogen was oxided to nitrogen,only 0.62%was converted to nitrate,and 0.12%was converted to nitrite,by detecting the reactants and products during the denitrification process.The reaction mechanism might be:ammonia was adsorbed on the surface of the electrodes firstly,than oxidized by the active component Mn?IV?;after that the generated Mn?IV?was transformed into Mn?III?through anodic oxidation.The grain refinement reached the nanometer level,and the contact area between the electrodes and ammonia-nitrogen was increased,enhancing the ability of electrocatalytic oxidation of ammonia-nitrogen.Fourth,Nd-doped manganese oxide electrodes were prepared by thermal decomposition method and characterized by XRD,TEM,Raman and XPS.It was found that Mn₂O₃ was the dominant electrodes material,meaning that Mn?III?was the main component in the manganese oxide electrodes.The experimental results showed that the removal retio reached 93.9%after 10 hours of reaction,when the cell voltage was 2.5 V,pH=9.5.By studying the effect of initial ammonia-nitrogen concentrations,it was found that the electrolysis reaction agreed with zero-order reaction kinetic characteristics at the initial stage,but the average ammonia-removal ratio of the electrode decreased with the prolonging of reaction time,which might be due to the prevention of ammonia-nitrogen oxidation at the electrode surface by the intermediate products.After 6 times runs of denitrification tests,the removal ratio still remained86.2%.It was revealed that most of the ammonia-nitrogen was oxided to nitrogen,only 0.97%was converted to nitrate nitrogen,and 0.31%was converted to nitrite nitrogen,by detecting the reactants and products during the denitrification process.The mechanism of action might be:Mn?III?was the main component,after the doping of Nd.Mn?III?was oxidize to Mn?IV?under a higher anode potential,increasing the oxygen evolution overpotential of the electrodes and saving energy consumption.