Nitrogen-doped 2H-phase Graphite Nanoplatelets Catalyze PMS Degradation Of Azo Dyes

In recent years,peroxymonosulfate advanced oxidation technology has received much attention due to its advantages such as low cost,high efficiency,and thorough oxidation.This technology typically uses transition metal catalysts to activate peroxymonosulfate,but there is a problem of metal leaching during the reaction,which can cause secondary pollution.Therefore,this study aimed to address the shortcomings of traditional catalysts by preparing a carbon-based catalyst and investigating its effectiveness and mechanism for activating peroxymonosulfate to degrade Reactive Yellow X-RG.This study selected the typical high-concentration azo dye（Reactive Yellow X-RG）in the environment as the target pollutant,and nitrogen-doped carbon-based catalysts were successfully synthesized using 2H-graphite nanoplatelets and urea as carbon and nitrogen sources.The catalyst can promote the generation rate of peroxymonosulfate（PMS）radicals,thereby achieving rapid degradation of Reactive Yellow X-RG.At the same time,we studied the catalytic oxidation performance of the optimal catalyst and explored the catalytic oxidation mechanism of the optimal catalyst/PMS system.The research results provide theoretical support for treating high-concentration azo dye waste-water using peroxymonosulfate advanced oxidation technology.The main research contents are as follows:（1）From the SEM images,it can be observed that the 2H-graphite nanoplatelets before and after nitrogen doping both have a sheet-like structure,and the surface morphology of the catalyst did not undergo significant changes.TEM images showed some wrinkles on the surface of the2H-graphite nanoplatelets after nitrogen doping,which provided a larger surface area for the catalyst.The peak position of XRD was blue-shifted due to the successful incorporation of nitrogen into the 2H-graphite nanoplatelets.Scherrer’s formula calculates that the average particle size of N-GNP_S0.5-600 is 19.5 nm.XPS analysis showed four types of nitrogen configurations in2H-graphite nanoplatelets doped with different nitrogen ratios,namely,graphite N,pyridinic N,pyrrolic N,pyridinic N,pyrrolic N,and nitrogen oxide.Through XPS depth profiling,it was found that graphite N,pyridinic N,and pyrrolic N were distributed on the surface of the N-GNP_S0.5-600catalyst,while nitrogen oxide was located at the edge of the catalyst.（2）Under conditions of 20℃,the activation of PMS by nitrogen-doped 2H-graphite nanoplatelets（N-GNP_S）for the degradation kinetics of Reactive Yellow X-RG was investigated.The fitting results of first-order kinetics indicated that the catalytic oxidation rate of N-GNP_S0.5-600,synthesized at 600℃,was the fastest,with a k value of 0.2735 h^-1.Meanwhile,the catalytic oxidation rate of N-GNP_S0.5-800,synthesized at 800℃,was the fastest among all the catalysts,with a k value of 0.1133 h^-1.The catalytic oxidation rate of N-GNP_S0.5-600 was 2.4 times faster than that of N-GNP_S0.5-800.The catalyst synthesized at 600℃exhibited significantly better catalytic activity than that synthesized at 800℃.N-GNP_S0.5-600 showed the best catalytic oxidation performance among all the catalysts,with a removal rate of 94%for Reactive Yellow X-RG in 24 hours and a first-order kinetics fitting rate of k=0.2735 h^-1.Compared with PMS alone,GNP_S,and GNP_S-600,N-GNP_S0.5-600 increased the degradation rate by about 72%,64%,and 67%,respectively.The catalytic oxidation rate of the N-GNP_S0.5-600 reaction system(k=0.2735 h^-1)was 25 times that of the PMS reaction system(k=0.0109 h^-1),19 times that of the GNPS reaction system(k=0.0141 h^-1),and 20 times that of the GNPs-600 reaction system(k=0.0135 h^-1).（3）An increase in temperature can accelerate the catalytic oxidation rate of the N-GNP_S0.5-600/PMS system.At 20℃,30℃,and 40℃,the catalytic oxidation rates of N-GNP_S0.5-600 were 0.2735 h^-1,0.3528 h^-1,and 0.5095 h^-1,respectively.^-had a slight inhibitory effect on the catalytic oxidation of the PMS system,reducing the reaction rate from0.2735 h^-1to 0.2115 h^-1.CO₃^2-,PO₄^3-had a greater inhibitory effect on the catalytic oxidation of the N-GNP_S0.5-600/PMS system,reducing the catalytic oxidation rate from 0.2735 h^-1to 0.0104 h^-1and 0.0042 h^-1,respectively.NO₃^-had no significant effect on the N-GNP_S0.5-600/PMS system.Both an increase in the amount of catalyst and PMS added can accelerate the catalytic oxidation rate,but the catalytic oxidation rate does not exhibit a trend of exponential growth.The initial pH had little effect on the catalytic degradation of Reactive Yellow X-RG by the N-GNP_S0.5-600/PMS reaction system;therefore,the initial pH value conditions in this experiment do not need to be adjusted.（4）N-GNP_S0.5-600 exhibits excellent cycling performance,with a degradation rate of 90%for Reactive yellow X-RG,even after five consecutive cycles.Full-band scanning was performed for the degradation process of low-concentration Reactive yellow X-RG（50 mg/L）,and no characteristic absorption peaks or intermediate product peaks were observed after 40 minutes.TOC analysis showed that the TOC and TN contents after 40 minutes of reaction were 3.29 mg/L and 0.821 mg/L,respectively,demonstrating that Reactive yellow X-RG was completely degraded into water and carbon dioxide,respectively,and the intermediate products were also wholly degraded.（5）The main degradation pathway of the N-GNP_S0.5-600/PMS system is through non-radical processes,with the main active species being singlet oxygen（¹O₂）,₄^·-and^·catalytic oxidation rate being accelerated as a radical process.Electron paramagnetic resonance（EPR）spectroscopy further confirms the existence of three active oxidative species.The results of the energy band and density of states before and after nitrogen doping show that the conduction band portion formed by the 2PZ orbitals of nitrogen atoms partially intersects with the Fermi level.The2PZ orbitals of carbon atoms also cross the Fermi level,showing electron transitions rising to the conduction band,resulting in a significant increase in the electronic activity of the system.