Electrocatalytic Oxygen Reduction For Selective H₂O₂ Synthesis And In-Situ Heterogeneous Electro-Fenton

Fenton reaction has been regarded as one of advanced oxidation technologies for wastewater treatment.Fenton reaction proceeds by catalytic decomposition of H₂O₂ to·OH for pollutant degrdation,which shows the advantages of high reaction rate and high mineralization rate.However,Fenton technology suffers from the issues of acidic p H,massive discharge of iron sludge,etc.Heterogeneous electro-Fenton technology is cabaple of coupling electrocatalytic oxygen reductive H₂O₂ synthesis in situ and activation of H₂O₂ to·OH,showing the advantages of wide p H,environmental friendliness,safe and controllable operation.However,the kinetic rate of pollutant degradation of heterogeneous electro-Fenton is 1～2 orders of magnitude lower than that of homogeneous Fenton reaction.Firstly,electrocatalytic oxygen reductive H₂O₂synthesis suffers from low reaction selectivity,activity and product concentration caused by the competitive electrocatalysis of O₂ to H₂O.Moreover,in situ electrochemical activation of H₂O₂to·OH shows poor activity,resulting in low·OH productivity.To solve these problems,this thesis focuses on the design and preparation of carbon-based metal catalysts to achieve high-performance H₂O₂ electrosynthesis and in-situ heterogeneous electron-Fenton catalysis,which are successfully manipulated by the effects of hydrophobic surface,heteroatom doping,and single-atom catalysis.Futhermore,the mechanism behinds enhanced performance is clarified.The major research contents and results are as follows:（1）In order to enhance H₂O₂productivity from electrocatalytic oxygen reduction,a highly hydrophobic and superaerophilic carbon electrode consisting of densely distributed N-doped carbon（NPC）nanopolyhedra derived from ZIF-8 was prepared for accelertating gaseous O₂trapping.The effects of surface hydrophilicity or hydrophobicity and N doping configuration on current density and current efficiency of electrosynthesis H₂O₂ were estimated.Using aerophilic NPC electrode,it could delivery markedly high current of 50～250 m A cm^-2 with83～99%current efficiency,achieving high-concentration H₂O₂ manufacture（0.66～5.38 wt%）with 8735.3 mmol h^-1 g^-1_cat of H₂O₂ production rate that was higher than those values reported.Density functional theory（DFT）calculations revealed that pyrrolic N site showed the excellent ability for O₂ adsorption and suitable adsorption energy for*OOH,which was favorable to enhance the selectivity of electrocatalysis of O₂ to H₂O₂.NPC electrode showed the striking durability for 200-hours electrocatalytic H₂O₂ synthesis at 100 m A cm^-2 with 85～99%current efficiency.（2）Aiming at co-enhancing the activity and selectivity of electrocatalysis of O₂ to H₂O₂,it is proposed to optimize the binding energy of*OOH(ΔG_*OOH)by regulating metal centers and coordination atoms of single metal site catalysts.DFT calculations showed that Co-N₄ site coordinated with O atom had the optimalΔG_*OOH value that situated at the peak of volcano-tpye diagram describing the performance of electrocatalysis of O₂ to H₂O₂,theoretically suggesting the optimal activity and selectivity.Single cobalt site catalyst with O₁-Co-N₄coordination structure was prepared using cobalt phthalocyanine and oxygen modified carbon nanotubes as precursor（denoted as Co Pc-OCNT）.The H₂O₂ selectivity of Co Pc-OCNT almost reached 100%in the potential range of 0～0.6 V vs RHE under 0.01 mol L^-1 KOH solution.The TOF value of Co Pc-OCNT catalyst for H₂O₂ electrosynthesis was 603 min^-1at 100 m A cm^-2,which was 1.3 or 14.7 times higher than that of Cu Pc-OCNT and Fe Pc-OCNT,and even 2～10times higher than that of homogeneous oxygen reductive H₂O₂ synthesis based on highly active Co Pc catalysts（with similar overpotential）.（3）Based on highly selective H₂O₂ electrosynthesis,in-situ electrochemically activating H₂O₂ to·OH for enhanced heterogeneous electron-Fenton can be driven by a bifunctional catalyst of Cu,N co-doped porous carbon supported iron that is prepared by using iron-based metal-organic-framework（Fe-MOF）as precursor.And the effects of nitrogen and Cu doping on H₂O₂ selectivity and electrochemical activation of H₂O₂ to·OH were estimated.The electro-Fenton performance of optimized bifunctional catalyst in pollutants degradation was investigated.At-0.1 V vs SCE,the Cu doped catalyst exhibited the higher activity in electrochemical activation of H₂O₂ to·OH compared with the undoped catalyst,contributing to four-folds enhanced kinetic rate of phenol degradation.Under the conditions of-0.6 V vs SCE and p H 4～10,the kinetic rate constant of phenol degradation by electro-Fenton process was4.02～2.52 h^-1,suggesting the effective pollutants degradation in a wide p H range.（4）In the process of heterogenenous electro-Fenton,the electrochemical activation of H₂O₂into·OH is slow due to limited catalytically active sites and poor activation ability for H₂O₂.Porous carbon supported Fe₃O₄ nanocrystalline catalyst（HD-Fe NP/PC）was prepared using modified iron-based MOF as precursor.The roles of nanoscale effect and dominant crystal face exposure of Fe₃O₄ in improving the ability for H₂O₂activation were investigated and the mechanisms behinds enhanced performance of electrochemical activation of H₂O₂ to·OH were explored.Compred with the microscale catalyst,surface iron atom exposure on nanoscale HD-Fe NP/PC catalyst was raised by 3-folds,which contributed to 2.3-folds enhanced·OH production rate and 1-fold enhanced kinetics in SMX degradation.DFT calculations revealed that H₂O₂ showed a lower adsorption energy barrier at active Fe site via end-on mode than that of side-on,and the Fe site on dominant Fe₃O₄（311）face had a higher free energy for H₂O₂decomposition,which greatly contributed to electron-Fenton performance.（5）Focusing on the low·OH productivity and efficiency in pollutants degradation of heterogeneous electro-Fenton as compared to homogeneous Fenton reaction,the atomically dispersed iron settled by defective three-dimensional porous carbon was prepared,and the effect of iron single atom on enhanced electrocatalysis of O₂ to·OH was investigated.Under a near-neutral condition（p H 6）,the·OH productivity of iron single atom catalyst from electrocatalysis of O₂process is 262 mmol L^-1 h^-1 g _Fe^-1,which was 77 times higher as compared to that by iron oxide nanocatalyst.In the range of p H 3 to 8,the kinetic rate constant in SMX degradation of heterogeneous electro-Fenton is 401～708 min^-1g _Fe^-1,which was comparable to Fenton reaction(p H 3～4,535～848 min^-1g _Fe^-1).DFT calculations revealed that H₂O₂ activation to·OH on single iron sites showed an increased reaction free energy,which greatly improved the heterogeneous electro-Fenton efficiency.In summary,the performance of H₂O₂ electrosynthesis and heterogeneous electro-Fenton for organic pollutants degradation have been effectively enhanced by regulating catalyst surface hydrophobicity,introducing heteroatom doping and metal single atom,which provides a new insight on the design and preparation of efficient,stable and multifunctional electro-Fenton catalysts.