Study Of Photo-Electro-Catalytic Activation Of Molecular Oxygen And Peroxymonosulfate By Single Atom Metal

Hydroxyl radical（HO·）is a strong oxidant with a high oxidation potential(E⁰=2.8 V_NHE).HO·,which can non-selectivity and high efficiently degrade most persistent organic pollutants and even mineralize to H₂O and CO₂.However,low steady-state concentration(7.5×10^-12–3×10^-11 M)and ultrashort half-life(t_1/2≤1μs)of HO·results in a low apparent rate constant(4×10⁹ M^-1s^-1)of pollutant degradation.Therefore,how to improve the production rate of HO·is the key issue of advanced oxidation process for water pollution control.Photo-electro-catalytic（PEC）oxidation technology is a new advanced oxidation technology based on the synergistic effect of photocatalysis and electrocatalysis.PEC oxidation technology owns the characteristics of both photocatalytic and electrocatalytic process.Electrocatalysis（EC）can improve the separation efficiency of photo-generated carriers during photocatalytic（PC）process and promote the rapid transfer of photo-generated electron(e_CB^-),and then to enhance PEC performances.Molecular oxygen（O₂）is an environmentally friendly and earth abundant oxidant.Most organics can’t be directly oxidized by O₂ under moderate conditions due to spin forbidden reactions.However,PEC activation of O₂ can produce various reactive oxygen species（ROS）,such as·O₂^-,H₂O₂,HO·.Besides,O₂ can be converted to ¹O₂through energy transfer process.All these ROS can be applied for wastewater treatment.Moreover,PEC activation of peroxymonosulfate（PMS）can also selectively produce ROS since the asymmetric structure and O-O bond in PMS.Single atom catalysts（SACs）have attracted wide attention of researchers due to advantages of dramatically increasing surface free energy and maximum utilization of metal atoms.Compared to nanoparticles（NPs）,SACs own unsaturated coordination environment between metal and substrate.New chemical bonds formed between metal and substrate provide additional channel for electron transfer.Titanium dioxide（TiO₂）,a mature semiconductor material,has been used in various fields due to the advantages of nontoxicity,high chemical stability and low cost.In addition,TiO₂ as substrate for the anchoring of SACs can show excellent catalytic activity and stability.Based on the above observations,this work focus on the promotion of photoelectrocatalytic performance of TiO₂ with various single atom catalysts for activating O₂ and PMS to produce HO·during water purification.The obtained main results were exhibited as follows:（1）The promotion of enhanced electron transfer on photo-electro-catalytic activation of molecular oxygen for fast HO·productionBased on fact that low steady-state concentration of HO·resulted in low apparent rate constant of pollutant degradation,a novel strategy of increasing HO·production rate by adjusting the electron transfer pathway during O₂ reduction reaction in PEC was proposed.The HO·production rate was determined by the rate of production of H₂O₂ via 2e reduction pathway and 1e reduction of H₂O₂.Single atom Pd on the surface of F-TiO₂favored H₂O₂ formation with 99%selectivity in EC process.Simultaneously,HO-O···Pd-F-TiO₂ chemical bond promoted the transfer of photo-generated electron from conduction band to single atom Pd,and then reduce Pd···O-OH to HO·.Optimal production rate of HO·was up to 9.18μmol L^-1 min^-1,which was 2.6-52.5 times higher than that of conventional advanced oxidation processes.In wastewater treatment,the kinetic rate constants of bisphenol A and acetaminophen were 0.324 and 0.175 min^-1,respectively.At the same time,93.2%TOC removal and 99.3%acute toxicity removal were obtained.Water quality and pH values had less influence on the degradation efficiency.There is no metal leaching during degradation process.This research provided some theoretical guidance and technical support for wastewater purification based on HO·.（2）The photo-electro-catalytic activation of molecular oxygen for the production of HO·under alkaline conditionsTraditional Fenton reaction suffers from the limitation of acidic conditions（pH=2.5-3.5）,which hampered the practical application in environmental remediation.This work proposed a strategy of efficient HO·production under alkaline conditions（pH=7-11）in sustainable PEC activation of O₂.Single atom Cu was innovatively anchored on titanium dioxide with oxygen vacancy(Cu-SA/TiO_2-x).At pH=9,optimal accumulated concentration of HO·during PEC activation of O₂ was 276.23μmol L^-1.The removal efficiency of sulfamethizole（SMX）was up to 96.3%within 10min and the apparent kinetic constant(k_obs)was 0.311 min^-1.In addition,high COD removal（69.1-93.9%）and TN removal（25.1-87.7%）were maintained for five actual water samples and hardly affected by water quality.This work not only proposed a strategy of high HO·production under alkaline conditions,but also expanded the application of PEC process for oxygen activation.（3）The photo-electro-catalytic activation of peroxymonosulfate for efficient production of HO·PEC activation of PMS showed high removal efficiency of organic pollutants in a wide pH range.This work constructed a novel Mn-SA/TiO₂ photoelectrode using modified molten-salt method（MSM）for the activation of PMS to HO·.This process was further applied for the removal of fluoroquinolone antibacterial norfloxacin（NOR）.HAADF-STEM result indicated the existence of Mn on the surface of TiO₂ is isolated atom.Mn-O-Ti chemical bond formed on photoelectrode surface identified by XPS characterization enhanced the separation of photo-generated carriers and provided an additional channel for fast electron transfer,promoting PEC activation of PMS to produce HO·.The degradation efficiency of NOR achieved 98.5%within 15min.（4）The selective removal of phenolic pollutants by photocatalytic oxidation processPhenolic pollutants as highly toxic and hazardous organics are widely generated from industrial and domestic process.Phenolic pollutants with different hydroxyl position（catechol,resorcinol,hydroquinone and phenol）were preferentially and efficiently oxidized by Boron doped commercial TiO₂（B-TiO₂）.Interfacial Ti³⁺-O interaction weakened the competitive adsorption of H₂O in aqueous solution and favored the formation of hydrogen bond on B-TiO₂ surface,leading to enhance the adsorption of phenolic pollutants.The degradation rate constant of B-TiO₂(k _B-TiO2)was regardless of the corresponding oxidation potential of phenolic pollutants.The k _B-TiO2 for catechol was as high as 3.46 min^-1,which was 18.2 and 1.6 times higher than that of biodegradation and ozonation methods,respectively.The result of in-situ attenuated total reflectance ATR-IR indicated the nine-membered ring formed between Ti（Ⅲ）on B-TiO₂ surface and phenolic pollutants enhanced adsorption of phenolic pollutants and weakened the competitive adsorption of H₂O.The results were helpful for developing preferential oxidation technology based on HO·for selectively removing pollutants with low concentration but high toxicity.