Surface Modification Enhanced TiO₂ Photocatalytic Production Of Hydrogen Peroxide And Its Strengthening Mechanism

Hydrogen peroxide?H₂O₂?is an important green oxidant,and has been widely used in diverse fields likepollution control,medical care,textile.However,the storage and transportation of H₂O₂ are subject to explosion risks.Photocatalytic production of H₂O₂ is a reaction to use oxygen and water as raw materials and sunlight as energy source,and it is expected to realize on-site production of H₂O₂ at low-cost and high-efficiency.TiO₂ has the advantages including high catalytic activity,low price,high stability,safety and non-toxicity.However,there are four limitations in the production of H₂O₂ based on TiO₂ photocatalysis:?1?TiO₂ can only absorb ultraviolet light below 385 nm,resulting in a low efficiency in solar energy utilization;?2?TiO₂photocatalytic oxygen reduction reaction?ORR?is mainly a single electron process,and the selectivity and efficiency in H₂O₂ production are low;?3?H₂O₂ can adsorb on the surface of TiO₂ and form?Ti-OOH,which can be photocatalyzed and decomposed;?4?Since the ORR is a single-electron pathway,the production of H₂O₂ is limited to take place under acidic conditions.This thesis targets the above problems,investigates the surface modification of TiO₂ and the additive electron donors,explores approaches to fabricate a highly active system for photocatalytic H₂O₂ production with a high selectivity in a two-electron ORR pathway,and reveals the corresponding reaction mechanism.The main contents and conclusions are as follows.?1?Study on the production of H₂O₂ on humic acid?HA?photosensitized TiO₂.Humic acid is a natural organic matters that widely presents in surface water.It can sensitize TiO₂ to produce H₂O₂ under visible light irradiation.The effect of phosphate and perfluorosulfonic acid?Nafion?modification on the photocatalytic performance for HA sensitized TiO₂ was studied.After Nafion modification,the yield of H₂O₂ produced on HA-sensitized TiO₂ increases from 93?M to 99?M after 120 min visible light illumination,while H₂O₂ was not detected on phosphate modified TiO₂.According to the results from impedance spectroscopy,Mott-Schottky curve,scavenging tests and electron spin resonance spectra,we found that Nafion modification can promote the migration of sensitized electrons across the interface between HA and TiO₂,and improve the efficiency of ORR by photogenerated electrons.The production of H₂O₂ is a continuous two-step single-electron reaction.Phosphate modification inhibits the migration of photogenerated electrons from excited HA to the conduction band of TiO₂.?2?Study on the production of H₂O₂ on sulfur,nitrogen co-doped graphene quantum dots modified TiO₂:The sulfur-nitrogen co-doped graphene quantum dots?SNG?were synthesized by using citric acid and thiourea as the raw materials,and SNG/TiO₂ was prepared by an impregnation method.Under visible light,there is negligible H₂O₂ production on the pristine TiO₂,however,the yield increases to 82.8?M on SNG modified TiO₂,which is 5.3 and 3.1 times of that for graphene quantum dots?GQD?and nitrogen-doped graphene quantum dots?N-GQD?modified TiO₂,respectively.Fluorescence and UV-visible diffuse reflectance analyses show that SNG has stronger fluorescence characteristics than GQD and N-GQD.The edge of absorption wavelength of SNG/TiO₂ can be extended to 620 nm.H₂O₂ is not detected on pure SNG.The visible light activity of SNG/TiO₂ can be attributed to photosensitization of SNG.SNG can cover partial hydroxyl groups on the surface of TiO₂,and accordingly inhibit the photocatalytic decomposition of H₂O₂.Under simulated sunlight,N-GQD and SNG modification improves the photocatalytic production of H₂O₂,and produced 339 and 451?M H₂O₂ after 60 min illumination,which are 2.4 and 3.2 times of that on pure TiO₂,respectively.The effect of pH indicates that SNG modification can extend the applicable pH range of H₂O₂production.The mechanism is revealed:the incorporation of sulfur and nitrogen changes the charge distribution of graphene quantum dots;the carbon and sulfur atoms with higher positive charge densities can increase molecular oxygen adsorption and be served as the active sites in ORR;and carbon and nitrogen atoms with the higher negative charge densities can act as proton relays to promote proton-coupled electron transfer reactions in a multi-electron pathway.Therefore,photocatalytic H₂O₂production on SNG/TiO₂ exhibits a high selectivity in two-electron ORR.?3?Nafion enhanced sulfur,nitrogen codoped graphene quantum dots modified TiO₂ production of H₂O₂:The Nafion layer is introduced to SNG/TiO₂ by the immersion deposition method.Nafion modification improves H₂O₂ production on SNG/TiO₂,which is 70%and 65%enhancement under visible light and simulated sunlight,respectively.The H₂O₂ production reaches 745.5?M after 120 min simulated sunlight irradiation.The yield of H₂O₂ does not slow down at the prolong irradiation,suggesting that Nafion modification can effectively inhibit the decomposition of H₂O₂.At the same time,Nafion modification makes the zeta potentials of the catalyst becoming strongly negative,which facilitates the transport,enrichment and activity of protons on the catalyst surface,and provides more protons for ORR-produced H₂O₂.The perfluorinated groups?-?CF₂CF₂?_n CFCF₂-,n=6-10?of Nafion makes the catalyst surface hydrophobic,which is beneficial to the transport of O₂ and increases the O₂ concentration on catalyst surface.Nafion modification reduces the interfacial impedance and results in the negative shift of fermi level,which can promote the migration and separation of photogenerated charges.Above effects of Nafion modification can accelerate photocatalytic oxygen reduction and promote H₂O₂production.?4?Alcohol-based organic substrates enhanced TiO₂ photocatalytic production of H₂O₂:The effect of alcohols?furfuryl alcohol,phemethylol,methanol,ethanol,isopropanol and tert-butanol?on photocatalytic H₂O₂ production on TiO₂ is studied.At the conditions that the concentration of furfuryl alcohol is 650 mM,pH is 3,and the concentration of TiO₂ is 1 g/L,the yield of H₂O₂ is the highest?1.44 mM?after 120 min simulated sunlight irradiation.The surface complexation between furfuryl alcohol and TiO₂ is proved by the control test of fluorine modification.The DFT calculation shows that H₂O₂ production depends on the electron donating ability and the dehydrogen ability alcohols.At the same time,furfuryl alcohol is more conducive to consume photogenerated holes and provide protons.The ORR tests prove there is a high selectivity two-electron ORR pathway in the furfuryl alcohol system.Summary,the thesis investigates that the ORR on TiO₂ can be enhanced by surface modification of Nafion and SNG,and the proper organic electron donor.The enabling two-electron ORR pathway greatly improves H₂O₂ production on TiO₂.The research results provides theoretical support and technical reference for the studies and applications of environmental photocatalysis,H₂O₂ production and oxygen reduction.