Mechanistic Investigation Of Nitrogen Vacancy On Graphitic Carbon Nitride Based Photocatalytic Reaction

Graphitic carbon nitride(g-C₃N₄,denoted CN)driven photocatalytic system has widely used for the removal of emerging organic contaminants in wastewater.However,limited active site,high charge recombination rate and narrow visible-light absorption of CN greatly reduced its oxidation capacity.It has been demonstrated that oxygen vacancy could enhance the photocatalytic performance of material via the modulation of different interfacial reactions.Given that nitrogen vacancy and oxygen vacancy had similar structure and were both anionic vacancy defects,it was inferred that nitrogen vacancy also had the potential to modulate the electronic structure of CN,thus improving the above issues of CN.Nevertheless,the photocatalytic reaction was a relatively complex multiphase catalytic process,and the enhancement of reaction rate was not only related to the structural properties of catalyst,but also the interaction between micro-interfaces in photocatalytic system.Although nitrogen vacancy might have an impact on the interface of photocatalytic system,the detailed mechanism has not been deeply elucidated so far.Therefore,in this article,the impact of nitrogen vacancy on CN-based photocatalytic reaction was investigated in depth for different interfaces.The details were as follows:(1)The nitrogen vacancy was introduced into CN by the reduction of sodium borohydride,and Z-scheme Ni Co₂O₄/CN-V_ncatalyst with nitrogen vacancy was directional constructed to investigate the impact of nitrogen vacancy on the charge transport path at solid/solid interface.Theoretical calculations and experimental results demonstrated that nitrogen vacancy reduced the potential difference between valence band of Ni Co₂O₄and conduction band of CN-V_nto promote the charge transport of Ni Co₂O₄/CN-V_nin a Z-scheme route,achieving efficient charge separation,and accelerating Ni(II)and Co(II)to activate PMS into active species for the rapid oxidation of contaminants.Compared to the degradation rate constant of tetracycline hydrochloride(TCH)by Ni Co₂O₄/CN(0.0724 min^-1),the rate constant for Ni Co₂O₄/CN-V_nwas increased to 0.1169 min^-1.In addition,Ni Co₂O₄/CN-V_nexhibited excellent degradation of carbamazepine(CBZ),4-chlorophenol(4-CP),atrazine(ATZ)and p-nitrophenol(PNP),with efficiencies of 95.5%,94.2%,98.0%and 91.4%within 30 min,respectively.(2)The nitrogen vacancy was introduced into CN to precisely construct nitrogen vacancy-rich Co CO₃/CN-V_ncatalyst and to investigate the mechanism between the nitrogen vacancy and exogenous oxidant peroxymonosulfate(PMS)at the solid/liquid interface.Theoretical calculations and experimental results demonstrated that the nitrogen vacancy lowered the activation energy barrier of Co(II)-PMS complex by promoting the adsorption between Co(II)and PMS and then accelerated the production of Co(IV)=O.In addition,the nitrogen vacancy enhanced the reactivity of Co(IV)=O by redistributing the electrons in Co 3d orbital,ultimately achieving efficient degradation of contaminants.Compared to the TCH removal efficiency of10CCH/CN(92.1%),the removal efficiency of 10CCH/CN-V_nwas increased to94.1%.In addition,it was found that 10CCH/CN-V_ncould selectively remove contaminants with low ionization potential.(3)The nitrogen vacancy was introduced into CN to precisely construct a nitrogen vacancy-rich FIS/CN-V_ncatalyst and to investigate the mechanism between the nitrogen vacancy and the endogenous oxidant hydrogen peroxide(H₂O₂)at the solid/liquid interface.Theoretical calculations and experimental results demonstrated that nitrogen vacancy not only provided abundant unpaired electrons to accelerate the reduction of oxygen to H₂O₂and lowered the energy barrier for*OOH formation,but also facilitated the conversion of H₂O₂to ¹O₂,thus achieving the rapid degradation of contaminant.The TCH degradation efficiency of FIS/CN-V_nwas increased to 88.5%compared to that of FIS/CN(81.6%).In addition,FIS/CN-V_nexhibited unique selectivity for contaminants with low ionization potential.(4)The nitrogen vacancy was introduced into CN to precisely construct nitrogen vacancy-rich Fe-POM/CN-V_ncatalyst and to investigate the mechanism of nitrogen vacancy on the production and conversion pathways of active species at the solid/liquid interface.The results of radical trapping experiments and electron spin resonance spectra demonstrated that nitrogen vacancy not only enhanced the light absorption and photogenerated charge separation and transfer,accelerated the Fe(III)/Fe(II)conversion to produce·OH and ¹O₂,but also regulated the electronic structure and directly promoted·O₂^-generation,thus achieving efficient degradation of contaminants.The TCH degradation rate constant of Fe-POM/CN-V_nwas increased to 0.1520 min^-1compared to that of Fe-POM/CN(0.0950 min^-1).In addition,45Fe-POM/CN-V_ndegraded atrazine(ATZ),atrazine(ALA),4-chlorophenol(4-CP)and methyl orange(MO)with efficiencies up to 81.6%,91.1%,95.0%and?100.0%within 18 min,respectively.(5)The nitrogen vacancy was introduced into CN to precisely construct nitrogen-rich Fe Co O_x/CN-V_ncatalysts and to investigate the mechanism of between nitrogen vacancy and contaminants at the solid/liquid interface.Theoretical calculations and experimental results demonstrated that nitrogen vacancy promoted Fe and Co cycle and induced the direct extraction of electrons from the TCH to positively charged Co(III),accelerating the regeneration of Co(II).This unique dual electron transfer pathway rapidly enhanced the activation efficiency of oxidant,ultimately achieving the rapid degradation of contaminant.Compared with the TCH degradation rate constant of Fe Co O_x/CN(0.2479 min^-1),the rate constant of Fe Co O_x/CN-V_nwas enhanced to 0.6674 min^-1.