Heat And Mass Transfer And Conversion In Synergistic Surface Plasmonic Photothermal Catalytic Reduction Of P-Nitrophenol

With rapid development of this society,water pollution becomes more severe,which has been regarded as one of critical issues facing this society.Among various pollutants,p-nitrophenol（4-NP）,discharged by chemical,pharmaceutical and pesticide industires,is carcinogenic and hard to degrade.Therefore,it is necessary to develop an efficient and green method to treat 4-NP contained wastewater.The use of sodium borohydride as a reducing agent represents a promising catalytic hydrogen transfer method to convert 4-NP to p-aminophenol（4-AP）,which offers the advantages of free secondary pollution and low risk and son.Moreover,4-AP is an important raw material in industires.As a result,using this method cannot only effecitively degrade 4-NP but also realize the wastewater resource recovery,so that it has become one of hot topics.At present,past efforts on the use of sodium borohydride as a reducing agent to convert 4-NP to 4-AP have devoted to the development of highly active and selective noble metal catalysts.Among them,silver nanoparticles（Ag NPs）have received extensive attention because they have relatively low cost and surface plasmonic effect that can boost catalytic reactions.However,Ag catalysts still face the issues of low precursor utilization and catalyst loading.Moreover,the underlying mechanism of how the illumination affects the catalytic conversion of 4-NP to 4-AP remains unclear.Therefore,this thesis aims at efficient catalytic conversion of 4-NP to 4-AP over the Ag catalysts in the presence of sodium borohydride and a comprehensive study is performed.First of all,because the polydopamine（PDA）has good adhesion and reduction properties,it was used to prepare a high-performance Ag-PDA catalytic layer on the inner wall of a reactor,with which the effects of the precursor type,p H value and precursor reduction time on the nanoparticle size and loading were investigated.Then,a reactor with synergistic surface plasmonic photothermal conversion was developed to study the effect of catalytic layer on photothermal response and conversion under illumination.To reveal contributions of the illumination and temperature to the reaction,kinetics of the 4-NP to 4-AP conversion over Ag-PDA was explored under dark and light conditions.Based on the transition state theory,the underlying mechanism was revealed.To enhance the photothermal conversion,a reactor with highly efficient photothermal conversion was developed by embedding carbon powders into PDMS and constructing microcube cavities or porors foam structure.The photothermal conversion properties of these substrates and their effect on the catalytic conversion of 4-NP were studied.Finally,a theoretical model descrbing the fluid flow,heat and mass transport,photothermal conversion and catalytic reaction was developed,by which the effects of the illumination intensity,liquid flow rate and inlet 4-NP concentration on the reactant concentration and temperature distribution as well as the conversion were studied.The main outcomes of this thesis are summaried as below.1 Ag nanoparticles were synthesized on the inner surface of the PTFE tubes using the adhesion and reduction properties of PDA and silver ammonia solution as a precursor.As compared to the traditionally used Ag NO₃as a precursor with low p H value,due to the zwitterionic property of the PDA surface groups,more negative potential of PDA could be achieved when using silver ammonia solution with higher p H value,which enhanced the adsorption of silver ions and thereby the precursor utilization.It was found that Ag nanoparticles prepared by using silver ammonia solution as a precursor had smaller particle size and higher loading,thereby yielding higher conversion of 4-NP to 4-AP and better stability.The size and loading of Ag nanoparticles could be controlled by the reduction time of the silver ammonia solution.Increasing the reduction time of silver ammonia solution led to the increase of the catalyst loading and the promotion of the catalytic conversion of 4-NP to 4-AP.2 To fully utilize the merits of the surface plasmonic effect and photothermal conversion of Ag and PDA as well as firm adhension of PDA on various substrates,a catalytic layer of Ag-PDA with excellent photothermal conversion was prepared on the inert PDMS substrate.Upon illumination,the reaction temperature was increased because of the PDA bottom layer and catalys layer possessing photothermal conversion,which enhanced the converson of 4-NP to 4-AP.In particular,the increment by synergistic surface plasmonic photothermal catalytic reduction of 4-NP was higher than simple combination of increment by sole illumination without temperature increase and increment by sole temperature increase without illumination.When the liquid flow rate was 0.5 m L/min,the conversion increment was 1.7%and 7.15%by sole illumination without temperature increase and sole temperature increase without illumination,respectively,while it was 17.4%for synergistic surface plasmonic photothermal catalytic reduction.This fact confirms that the synergy of surface plasmonic effect and photothermal conversion can boost the conversion of 4-NP to 4-AP.3 Ag nanoparticles were prepared by in-situ reduction on the surface of PDA microspheres to study the kinetics of Ag-PDA catalysts under light and dark conditions.It was found that the intrinsic reaction rate could be increased with increasing the reaction temperature regardless of dark or light conditions,which enhanced the conversion of 4-NP to 4-AP.Based on the transition state theory,the underlying mechanism leading to the improvement of the catalytic reaction rate under illumination was analyzed.It is demonstrated that the high-energy electrons generated by light can improve the initial energy state of the reactants adsorbed on the catalyst surface,reducing the activation energy required for the reaction from 47.2 k J/mol to 32.6 k J/mol and thereby enhancing the catalytic conversion of 4-NP to 4-AP.4 A substrate with the carbon powders embedded in PDMS was developed,which enhanced the photothermal conversion.Besides,the microcube-cavity structure and porous foam structure were constructed on the carbon powders embedded PDMS substrate,by which the reflection was enhanced by pore structure and the photon capture was also enhanced.Moreover,because the microcube-cavity structure and porous foam structure increased the specific surface area,the loading of Ag NPs was also increased.As compared with similar substrates without embedding carbon powders,the reactors with flat plate,microcube cavity and porous foam structures yielded better photothermal conversion under illumination,promoting the conversion of 4-NP.The reactor with the porous foam structure had the largest catalyst loading and the best photothermal conversion efficiency,the highest conversion was achieved.5 A 3D steady-state thermal model describing the fluid flow,heat and mass transfer,photothermal conversion and catalytic reaction was developed,by which the heat and mass transfer and conversion characteristics occurring in the synergistic surface plasmonic photothermal catalytic reactor were studied.The simulation results are in good agreement with the experimental data.It was also found that increasing the illumination intensity could generate more heat via photothermal conversion,leading to the increased reaction temperature and thereby the conversion.The increase of the liquid flow rate reduced the residence time and enhanced the heat transfer between the catalytic layer and flow stream,which lowered the reaction temperature.As a result,the conversion was decreased.Because Ag nanoparticles had exceleent activity,the reaction could be approximated to be first-order reaction in the ranges of liquid flow rate and reactant concentration used in this study.The reaction rate mainly depended on the reactant concentration.Therefore,under the same reaction concentration,the effect of the liquid flow rate on the conversion can be ignorable.