Activation Of Persulfate Or Oxygen By Copper Oxide Catalysts For The Degradation Of Typical Phenolic Pollutants: A Mechanistic Study

In recently years,transition metal oxide(M_xO_y)based Fenton-like catalytic oxidation technology has been a hotspot in the water pollution controlling research.Compared with the most used iron oxides catalysts,Fenton-like catalytic oxidation technique based on copper oxide(CuO)have a series of advantages such as high reactivity,high oxidant utilization efficiency and low metal leaching.However,the Fenton-like catalytic mechanisms ofCuO were still controversial and more studies were needed.Therefore,in this dissertation,typical phenolic pollutants were taken as the degradation objects,the activation mechanism of PDS by activated phenol induced by submicronCuO(mCuO)was firstly studied.Subsequently,considering the reaction sites on the surface of mCuO was relatively inefficient and hardly to be regenerated,CuO-CeO₂ composite was thus constructed for the enhanced activation of PDS for p-chlorophenol(4-CP)degradation,the PDS activation mechanism and the synergistic mechanism of CeO₂ was studied.On these bases,it was firstly found that nanoscaleCuO(nCuO)could efficiently activate molecular oxygen(O₂),the most eco-friendly and economic oxidant,and the O₂ activation and 2,4-DCP degradation mechanisms were thereafter systematically studied.Finally,the surface structure ofCuO could be regulated throughCuO particle size regulation and high-temperature calcination,the influence ofCuO surface structure on O₂ activation efficiency and the underlying mechanism were studied.The main results are as follows:(1)In theCuO/PDS/phenol system,phenol could firstly bond to mCuO in the inner-sphere form,then spontaneous electron transfer from phenol to mCuO happened and leading to the generation of resonance stabilized activated intermediate for PDS activation,and this activated intermediate was proposed to be phenoxy radical complex.The catalytic ability of mCuO was highly depended on the content of surface hydroxyl groups,which determined the phenol adsorption amount and PDS activation rate.Phenol could be degraded through the unique“half-radical”pathway in the mCuO/PDS system,on one hand,it could be oxidized degraded by PDS in the form of surface activated intermediate,on the other hand,it could also be degraded by surface bound SO₄^·?.The formation of activated intermediate highly depended on the properties of both organics and metal oxides,it could be formed only if the organics with a Hammett constant?⁺higher than the threshold(between-0.13 to-0.02)and the metal ions have a 3d shell between half-filled and full-filled.(2)The catalytic ability ofCuO could be significantly enhanced by CeO₂ due to the strong chemical interaction between them,the pseudo-first order reaction constant(k_obs)for4-CP degradation was calculated to be 9.6×10^?2 min^?1 in theCuO-CeO₂/PDS system,which was 19.2 times higher than that ofCuO/PDS system.The influence of the critical parameters,i.e.,CuO-CeO₂dosage,PDS and 4-CP concentration,and the underlying mechanisms were fully studied from the kinetics point of view.The activation of PDS still relied on the formation of activated intermediate betweenCuO-CeO₂ and 4-CP.The main reaction sites for 4-CP adsorption and PDS activation were the highly dispersedCuO and the bulkCuO on the surface ofCuO-CeO₂.The superior catalytic performance ofCuO-CeO₂ could be ascribed to three aspects.Firstly,4-CP could complex with Ce³⁺to form relatively stable polycyclic structure,meanwhile,the doping of Cu²⁺into CeO₂ lattice further increase the content of oxygen vacancy,both of which could promote the adsorption of 4-CP and PDS.Secondly,the reactivity of Cu²⁺was greatly enhanced by the strong interaction between Cu and Ce,which further promoted the formation of activated intermediate and the activation of PDS.Thirdly,the redox equilibrium between Ce⁴⁺/Ce³⁺and Cu²⁺/Cu⁺promoted the regeneration of the reaction site?Cu²⁺.(3)nCuO could effectively activate O₂ for 2,4-DCP degradation,the degradation process was almost complete within 30 s.The O₂ activation ability of nCuO was proved to be much higher than that of nano-sized Ni O,Co₃O₄,?-Fe₂O₃,Fe₃O₄and CeO₂ catalysts.The influence of critical parameters,i.e.,nCuO dosage,2,4-DCP concentration,reaction temperature and p H on 2,4-DCP degradation and dechlorination were studied,the results of dechlorination ratios indicated that 2,4-DCP could be degraded through dechlorination and non-dechlorination pathways,and the exact proportion of them mainly depended on nCuO dosage and p H.The oxygen vacancies on the surface of nCuO were the main reactive sites for O₂ activation,while carbonate adsorption on the oxygen vacancies could rapidly deactivate nCuO during the degradation process,and the catalytic reactivity could be totally recovered after 250 ^oC calcination under air atmosphere.The activated O₂ in the nCuO/O₂system was totally originate from the surface bound O₂,and Cu³⁺,O₂^·?and ¹O₂ were produced through in-situ O₂ activation by surface Cu⁺,among them,Cu³⁺and O₂^·?were responsible for 2,4-DCP degradation through oxidation and reduction dechlorination pathway,respectively,and the latter pathway was dominated.(4)CuO size regulation and 600 ^oC calcination could significantly affect the surface structure ofCuO,further determine the catalytic ability for O₂ activation.All the fourCuO catalysts with particle size distribution in the range of 40-600 nm could active O₂ to produce Cu³⁺,¹O₂ and O₂^·?,and the electron paramagnetic resonance(EPR)signal intensities of O₂^·?were well correlated with the degradation ratios of 2,4-DCP(R²=0.993),indicating the O₂activation mechanism remain the same as theCuO particle size changed.Based on the experimental studies and density functional theory(DFT)calculation,CuO size regulation and air calcination on one hand could determine the quantity of surface Cu⁺and absorbed O₂ through affecting the specific surface area and oxygen vacancy proportion,on the other hand,they could also determine the existing form and reactivity of absorbed O₂ through affecting the proportion of oxygen vacancy,further determine the catalytic ability ofCuO for O₂ activation and 2,4-DCP degradation.