Activation Of Peroxymonosulfate By N Doped And Co,N Co-modified Porous Carbons For The Degradation Of Organic Pollutants

The sulfate radical(SO4·-)based advanced oxidation process,in which the SO4·-is generated through peroxymonosulfate(PMS)activation,shows great promise for the degradation of organic pollutants due to the strong oxidative capability,long radical lifetime,low pH dependence and high mineralization efficiency.Traditional transition metal or metal oxide-based catalysts showed high-efficiency in PMS activation,but they suffered from inevitable metal leaching.Hence,it is highly desirable to employ cost-effective and environmentally friendly carbon-based catalyst as the alternative to metal-based catalysts for PMS activation.However,the catalytic efficiency and stability of current carbon materials still need further improvement.Moreover,the PMS catalytic mechanism and active sites on carbon remain unclear,which limits the development of high-efficiency carbon-based PMS catalysts.To address the above problems,metal organic frameworks(MOFs)were employed as the precursors to construct the N doped porous carbon,Co and N codoped porous carbon and the Co@N doped porous carbon for enhanced PMS activation.Benefiting from the diverse species and tunable structure of MOFs,the relationship between the catalyst structure,dopants content/species and the catalytic performance could be well studied,which provides the scientific basis and theoretical guidance for preparing the high-effciency,stable and cost-effective carbon-based PMS catalysts.The main content and results are as follows:(1)Different nitrogen-rich MOFs(ZIF-8,IRMOF-3,NH2-MIL-53 and MOF-5)and carbonization temperatures(800?,900?,1000? and 1100?)were chosen to prepare nitrogen doped porous carbons(NPCs)with different nitrogen content and species.Various refractory organic pollutants such as phenol,bisphenol A,rhodomine B and methyl orange were chosen as the model pollutants to evaluate the PMS catalytic performance of NPCs,and the relationship between nitrogen content or species and catalytic performance was studied.The results showed all the NPCs exhibited enhanced PMS activation for phenol degradation compared with the nitrogen-free porous carbon,and was even superior to homogeneous Co2+(the best PMS activator reported before).The catalytic activity of NPC was enhanced with the increase of graphitic N content.The NPC derived from ZIF-8 and carbonized at 1000 ?,which contained the most graphtic N content,performed best with the kinetic constant for phenol degradation 4.4 times higher than that on porous carbon.Both radical process(OH and SO4·-)and nonradical process were present in the NPC/PMS system,and the radical process played a dominant role(60%).The graphitic N played a key role in PMS activation,which significantly redistributed the charge density on neighbouring carbon atoms and activated them,promoting the adsorption and dissociation of PMS on carbon.(2)Co substituted ZIF-8 was used as the precursor to construct the Co and N codoped porous carbon(Co-N-PC).Various refractory organic pollutants such as phenol,bisphenol A,atrazine and sulfamethoxazole were used as the model pollutants to evaluate the PMS catalytic efficiency of Co-N-PCs,and the synergetic mechanism originating from Co and N codoping was explored.The results showed that codoping of Co and N into carbon generated the synergistic effect to enhance the PMS activation for pollutants degradation.The kinetic constant of phenol degradation on Co-N-PC(0.30 min-1)was 48.4 or 17.6 times higher than that on porous carbon with only N(0.0062 min-1)or Co(0.017 min-1)doping,and the rate constant of SO4·-generation for Co-N-PC(0.72 min-1)was 4.8 or 12.9 times higher than that for Co(0.15 min-1)or N doped porous carbon(0.056 min-1).Moreover,the catalytic efficiency of Co-N-PC was greatly enhanced with the increase of Co-N coordination(Co-Nx).The kinetig constants of the best Co-N-PC for the degradation of phenol,bisphenol A,atrazine and sulfamethoxazole were 1.6-3.4 times higher than that of homogeneous Co2+/PMS system.Compared with the previously reported PMS activators,the catalytic activity(measured by reaction rate constant per catalyst mass)of Co-N-PC for pollutant degradation was.about 1.9-77.6 times higher than those of N doped graphene,N doped CNT,S and N codoped graphene and a-MnO2 et al.,respectively.The Co-Nx not only played a key role in PMS activation,but also inhibited the Co leaching(the maximum Co leaching was 0.12 mg·L-1)due to its higher binding energy(781.8 eV)than the Co-Co bond of Co metal(778.2 eV).DFT calculation revealed that the Co and N codoping not only synergistically activated carbon atoms by redistributing their spin and charge population respectively,but also provided Co active sites for PMS activation.The effluent COD of the secondary effluent of refinery oil wastewater(feed COD=92 mg·L-1)after tertiary treatment by Co-N-PC/PMS system was less than 50 mg·L-1,which satisfied the National Standard for Wastewater Discharge,and the COD and TOC removal increased gradually with the increase of catalyst dosage.(3)The Co@N doped porous carbon core-shell structure(Co@NPC)was prepared from ZIF-67 through two-steps heating treatment,and the carbonization temperature was varied to control the structure and composition of carbon shell.The phenol,bisphenol A and rhodamine B were used to evaluate the PMS catalytic efficiency of Co@NPC,and the relationship between the structure of carbon shell and catalytic performance was studied.The results indicated that the carbon thickness as well as the mesopore volume increased gradually with the increase of carbonization temperature,and the catalytic performance of Co@NPC was significantly enhanced with the increase of the ratio of mesopore volume to carbon thickness.The kinetic constant of the best Co@NPC for phenol degradation(0.41 min-1)was 4.6 times and 110.8 times higher than that for homogeneous Co2+ and heterogeneous Co3O4,respectively,and was higher than that of the Co and N codoped porous carbon(0.30 min-1)of my second work.When the number of carbon layers was larger than 3,the Co@NPC displayed good reusability and stability,which showed almost no performance decline after 5 repeated use.The maximum Co leaching percentage reached 2%,which was lower than that of Co and N codoped porous carbon(3%)reported in my second work.The mesopore volume and the thickness of carbon shell played significant role for catalytic performance.More mesopores are favorable for reactant transfer,while the moderate thickness of graphitic carbon shell could not only inhibit the Co leaching,but also promote the interaction between Co core and carbon shell.The catalytic performance of Co@NPC was also influenced by Co-N coordination and graphitic N content.