H2O2 Electrocatalytic Synthesis and the Enhancement of Generation and Utilization Efficiency of ·OH

Author:Zhou Wei

Supervisor:qin yu zuo gao ji hui

Database:Doctor

Degree Year:2019

Download:90

Pages:171

Size:6326K

Keyword:

Pollutants control technologies and high-efficiency utilization technologies for heavy carbon call for green and efficient oxidation technology.Hydrogen peroxide(H2O2),an environmentally-friendly oxidant,could decompose into highly oxidative hydroxyl radicals(·OH)and thus be an important choice for these two technologies.H2O2 is conventionally produced by the anthraquinone process which is energy-intensive and polluted.Moreover,the concentrated H2O2 is also difficult to transport.Electrochemical methods for H2O2 production,has characteristics such as green and on-site,have received wide attention recently.However,the low efficiency makes the scaling-up still a challenge.The efficient catalytic decomposition of H2O2 to highly oxidative·OH and the enhancement methods of the reaction between·OH and target chemicals is another important question in H2O2-based oxidation systems.H2O2 generation via O2 electroreduction is the most promising technology for H2O2 electrochemical production,where the low efficiency hinders its real application.This work explores the efficiency improvement methods from multi-scale of“reaction-transportation-flow”.(1)To enhance the O2 electroreduction and H2O2 generation from the scale of reaction,this work investigated H2O2 electrogeneration by activated carbon-based cathode,H2O2electrogeneration and enhancement by graphitic carbon;(2)To enhance the diffusion of O2 and H2O2 from the scale of species transportation,this work studied H2O2 electrogeneration enhanced by the control of diffusion process of O2 and H2O2;(3)To achieve the continuous H2O2 production from the scale of flow conditions regulation,this work developed the flow-through reactor for continuous H2O2production.Moreover,aimed at improving the efficiency of·OH,this work studied the enhanced regeneration of Fe2+ to improve the ·OH production,and the control of ·OH by adjusting the addition methods of reactants.Conventional cathode materials are expensive and the fabrication are relatively complex.To solve this obstacle,this work proposed an activated carbon/stainless steel mesh(ACSS)hybrid electrode,which achieved simultaneous H2O2 generation,H2O2 activation,and organic pollutants adsorption.Results of characterization,H2O2generation and activation show that both electrolyte pH and current influence the H2O2 production.The initial concentration of H2O2,the mass of AC,electrolyte pH,and stirring could impact the activation of H2O2.Coupled with adsorption process,the ACSS-enabled electro-Fenton process is more effective.Moreover,this work found that although AC adsorbed organics,the“self-cleaning”mechanism can regenerate the ACSS electrode over several operational cycles.Conventional graphitic carbon has a low activity towards H2O2 production.This work presented the oxidative modification of graphitic carbon cathodes by electrode polarity reversal(PR) technique,and verify its applicability on graphite felt(GF) and reticulated vitreous carbon foam(RVC foam).Different with previous reports,this work achieved the in situ O doping in acid-free,low-conductivity electrolyte.Results show that the PR technique can significantly increase the O/C ratio and improve the hydrophilicity of electrodes.After modification,the catalytic activity of electrodes towards O2 reduction increased and thus the H2O2 production increased ca.2 times.Neutral pH and low current could support effective H2O2production.Moreover,the electro-Fenton(EF) process enabled by the modified electrode could generate more·OH,and thus resulted in higher removal efficiency compared with the original electrode.To enhance the diffusion of O2 to porous cathode,this work presented the“floating cathode”structure,and studied the effects of key parameters on H2O2 production.Results show that compared with submerged cathode,the“floating cathode”can improve the H2O2 production by 4.3 times.Solution pH,current,and stirring can have significant effect on H2O2 production.Additionally,to study the disproportion pathway,anodic oxidation,and cathodic reduction of H2O2,this work studied the contribution of these pathways in a decoupling way,and proposed that pulsed current could enhance the H2O2 production.Results show that H2O2 disproportion pathway is significantly affected by the initial concentration of H2O2,and the concentration of electrolyte,the H2O2 anodic oxidation pathway is affected by current,and the H2O2 cathodic reduction has relation with both current and the porosity of cathode.This work found“2sON+2sOFF”was the best parameters and increased the H2O2 yield 61.3%.To solve the problem that O2 and acidic conditions are required in conventional process,this work fabricated a 3-electrode flow-through reactor where reactants O2 and H+ can be self-supported.Compared with 3-electrode static batch reactor,the following conclusions were drawn:in static batch reactor,Ti/mixed metal oxides(Ti/MMO) anode could supply O2 to cathode via oxygen evolution reaction.The mass transfer of dissolved O2 to cathode vicinity and the acidic conditions determine the H2O2 production.In flow-through reactor,a localized pH of 2.5-3.5 could be generated after the Ti/MMO anode.The current and flow rate significantly impact the concentration of H2O2 and accumulative production of H2O2 in column reactor.Experiments also show that the accumulation of O2 bubbles underneath the cathode could potentially deteriorate the O2 mass transfer.How to effectively transform the electrogenerated H2O2 into·OH and increase its utilization efficiency is another key focus of this work.This work proposed that the enhancement of·OH production can be facilitated by the accelerated Fe3+ reduction,and the control of·OH competitive reactions can be regulated by the addition approaches of reactants.Hydroxylamine,hydroquinone(HQ),1,4-benzoquinone(BQ),and sodium sulfate were selected as model additives to reveal the different regeneration mechanism of Fe2+.Results show that hydroxylamine facilitate Fe2+ regeneration by transforming itself into other products,HQ and BQ could build quinone cycle which could continuously regenerate Fe2+.The higher the molar ratio of initial concentration of HQ and BQ to Fe2+,the longer time that Fe2+concentration could retain.However,the stability of quinone cycles decreased due to the consumption of HQ and BQ by highly oxidative ·OH.Results of different addition approaches of Fe2+ and H2O2 show that the micro-scale ·OH competitive reactions could be effectively regulated by the macro-scale addition approaches and the diffusion processes of reactants.The essence of the regulation of OH competitive reactions are that the useless consumption of ·OH was regulated by Fe2+ and H2O2.