A Study On The Construction Of WO₃-based Photoanodes And Their Photoelectrocatalytic Degradation Of Refractory Organic Pollutants

With the rapid development of chemical industry,a variety of refractory organic pollutants with trace concentration have been detected in almost all environmental matrices,including surface water,groundwater,as well as drinking water.These pollutants have high biological toxicity,which endanger the ecological environment and sustainable development of human beings.Therefore,achieving high degradation and mineralization efficiencies of the refractory organic pollutants have become a crucial task in water environment protection.Photoelectrocatalytic（PEC）technology enables the complete oxidation of organic pollutants through the reactive oxygen species（ROS）generated during the process,which reduces the risk of secondary pollution and shows great potential in the refractory organic pollutant treatments.However,many semiconductors utilized in the PEC technology possess the shortcomings such as the limited light absorption properties and high electron-hole recombination rates,which restricts their PEC efficiencies.Hence,it is urgent to develop efficient photoanodes for improving the separation and transfer efficiency of charge carriers,and thus achieving the high degradation and mineralization efficiencies of refractory organic pollutants.Herein,a series of WO₃-based photoanodes have been rational designed for PEC degradation of these refractory organic pollutants.Besides,the enhanced PEC performance of the catalysts as well as the mechanism of pollutant degradation during PEC process have been deeply investigated.The main research contents are summarized as follows:（1）The WO₃-based photoanode with hexagonal and monoclinic phases grown on Tungsten（W）mesh（hm-m-WO₃）is prepared by a two-step hydrothermal method,which is utilized for PEC degradation of bisphenol A（BPA）.The effects of monoclinic and hexagonal phase configuration on the PEC performance of WO₃-based photoelectrodes have been systemically investigated,and results indicate the optimized photoelectrode with a mixed phase structure of WO₃ on the W mesh and both hexagonal and monoclinic WO₃ exposed on the surface of the photoelectrode exhibits the highest PEC performance.It shows the highest photocurrent density of ca.5.6 m A cm^-2 at 1.2V_RHE,achieving a complete degradation（99.9%）and nearly total mineralization（60.3%）of BPA within 150 min,reaching an apparent reaction rate constant of 5.7×10^-2 min^-1,which is two orders of magnitude higher than ever reported.According to the results of band structures and charge carrier dynamics,the obtained hm-m-WO₃/W mesh photoelectrode processes a Schottky junction in the WO₃/W interface together with a rationally constructed heterophase junction structure,which greatly promotes the charge separation of the catalyst.Computational fluid dynamics（CFD）simulations indicate that the network structure of the photoelectrode is in favor of the diffusion and mass transfer of the pollutants around the surface of catalyst and the whole reactor.Based on the above reasons,BPA can be degraded and mineralized efficiently.（2）It is still a serious challenge to decompose the fluorinated organic pollutants.Aiming to solve the problem,a functional bilayer WO₃ photoelectrode（Double-WO₃）is constructed on W mesh with WO₃ nanoflake as the inner layer and hexagonal-monoclinic WO₃ phase junction as the outer layer,which is implemented in the PEC degradation of various fluorinated organic pollutants including bisphenol AF（BPAF）,4-fluorophenol（4-FP）and pentafluorophenol（PFP）.Double-WO₃ shows the superior PEC performances,achieving the highest photocurrent density of ca.6.3 m A cm^-2 at1.6 V_RHE,and the complete degradation（99.9%）of BPAF,4-FP and PFP and nearly total mineralization（99.9%）of PFP are achieved within 150 min.Combining the results of band structures and charge carrier dynamics,it is found that the inner layer of Double-WO₃ is constructed the monoclinic WO₃ nanoflake acting as an electron transport layer（ETL）for facilitating the charge transfer from catalyst to substrate of the photoelectrode,while the outer layer is composed of the hexagonal-monoclinic WO₃phase junction which provides the driven force for charge separation.Transient photocurrent（TPC）measurements demonstrate that the construction of the bilayer photoelectrode can not only achieve fast electron extraction kinetic,but can accelerate surface reaction kinetic by suppressing the surface charge recombination,therefore improving the PEC performance of Double-WO₃.At the same time,large amounts of ROS including?OH,?O₂^-and h⁺are generated for decomposing and mineralizing the fluorinated organic pollutants.（3）In order to improve the PEC activity and stability of WO₃-based photoelectrodes with single phase,the WO₃ photoelectrode loaded with needle-like Ti O₂ nanoparticles（Ti O₂/WO₃）is synthesized by the hydrothermal-liquid phase deposition methods,and is employed for the PEC degradation of BPAF.At the optimized condition of liquid phase deposition,Ti O₂ nanoparticles have been uniformly dispersed onto the WO₃ nanoplates,and a type II heterojunction is formed in the interface between Ti O₂ and WO₃ which provides the driven force for photogenerated charge separation.The Ti O₂/WO₃ exhibits a photocurrent density of ca.3.4 m A cm^-2 at1.4 V_RHE,which is ca.1.5 times than that obtained for the pristine WO₃(2.2 m A cm^-2),and it shows high PEC degradation activities with 99.9%of BPAF and 55.8%of TOC removal efficiencies achieved.When the actual medical wastewater is selected as the target pollutant,the TOC removal efficiency can reach to 40.6%after 10 h PEC degradation process,indicating the Ti O₂/WO₃/PEC system can be applied to the treatment of the complex actual wastewater.In comparison with WO₃,the Ti O₂/WO₃shows improved surface reaction kinetics in PEC degradation of BPAF.It is found that?OH,?O₂^-and h⁺all play important roles in the decomposition of BPAF while?OH is the most crucial ROS.A flow reaction system is established which shows enhanced mass transfer of the pollutant during the PEC degradation process.At the operation conditions where a flow rate is set as 4.5 m L min^-1 and cell voltage is set as 2.5 V,98.2%of BPAF and 60.5%of TOC are removed within 2.5 h,and 81.2%of TOC is eliminated with reaction time prolonging to 5 h.CFD simulations indicate the liquid accumulates in the front of Ti O₂/WO₃ photoelectrode,which improves the contact between the pollutant and catalysts.