Preparation And Application Of High Performance Nano-photocatalyst For Chemical Oxygen Demand Determination

Water environment is one of the fundamental elements of human society on which the human survival and development.With the rapid social economic development,soaring population and the improving of living standards,the pollution to environment is more and more serious,especially the water environment pollution, which has become one of the main environmental problem and a serious threat to the survival and development of human.Whether the sewage can reach the discharge standard is determined by the pollution index of the sewage,among which the most important is the COD value of the sewage.Chemical oxygen demand(COD),which displays great significance on the evaluation of water environment,has preferentially been used as a parameter to represent the degree of organic pollution of wastewater. The conventional method for COD determination is manual and is based on titration of the samples with a toxic reagent such as potassium permanganate or potassium dichromate.This type of standard manual method is time consuming,involves complicated procedure,has secondary pollution and its accuracy depends on operators' skill.Therefore,an alternative method for COD determination,if it is convenient,time-saving,accurate,environment friendly and highly sensitive,is most desirable.Since the discovery of photocatalytic properties of TiO₂,the Photocatalytic Materials has been researched extensively,especially nano-TiO₂.Under the excitation of light or electricity,nano-TiO₂ can produce strong oxidizing agents such as hydroxide radicals,which can achieve the complete degradation and mineralization of organic compounds and.can directly transform the light energy to chemical energy to motivate chemical reaction,by reacting with hydroxyl and/or water on their surface. However,TiO₂ exists some disadvantages,such as the high recombination ratio of photogenerated electron/hole pairs,low photocatalytic efficiency,showing photochemical activity only in ultraviolet region because of the high band gap up to 3.2 eV(the utilizing efficiency of the sunlight is less than 10%).All of these have restricted its application in environment analysis.To broaden the light absorption spectrum of TiO₂,lower the recombination ratio of photogenerated electron/hole pairs and make its photocatalysis come into practical application,various modifications have been performed on nano-TiO₂ by researchers.In this dissertation,some high performance nano-TiO₂ photocatalysts were prepared and characterized,their mechanisms and behaviors in photoelectrocatalyzing organic substances were discussed and a new method for COD determination based on the mechanism of photocatalysis of nano-TiO₂ was described. Chapter 1 OverviewIn this part,a detail outline and reviews mainly on the mechanisms,preparation, and development of novel nano-TiO₂ photocatalysts and the current situation and development of the COD determination were described.Chapter 2 Preparation of novel Au/TiO₂ nanotube and its application in the detection of chemical oxygen demandA new method for fabrication of Au/TiO₂ nanotube using photoelectrocatalysis was described.The Au/TiO₂ nanotube promoted the transfer of interfacial electrons and facilitated the separation of electron-hole(e^--h⁺) pair,then showed higher photocatalytic efficiency compared to the single TiO₂ nanotube.It was showed that Au nanoparticles were successfully encapsulated into highly ordered TiO₂ nanotube and thus formed the honeycomb-like structure.This material can be used for chemical oxygen demand(COD) determination.Under the optimizing conditions,the sensor responded.Chapter 3 Preparation and photoelectroehemical properties of a hybrid electrode composed of polypyrrole encapsulated in highly ordered titanium dioxide nanotube array.We demonstrated in this paper that polypyrrole encapsulated in highly ordered TiO₂ nanotubes array(PPy/TiO₂ NTs) electrode could be obtained via two steps:(1) electrosynthesis of TiO₂ nanotube array in HF solution by anodization method;(2) followed by electropolymerization of pyrrole encapsulated in the TiO₂ nanotubes array.The photoelectrochemical properties were studied in a three-electrode cell with 0.5 M Na₂SO₄ solution as supporting electrolyte.The photocurrent was dramatically enhanced on the PPy/TiO₂ NTs electrode,comparing with that on bare TiO₂ NTs electrode. Chapter 4 Sonophotoelectrocatalytic degradation of azo dye on TiO₂ nanotube electrodeThe degradation of azo dye,methylorange(MeO) in aqueous solution with sonophotoelectrocatalytic process was investigated.The TiO₂ nanotubes were used as electrode in photoelectrocatalytic(PEC),sonophotoelectrocatalytic(SPEC)processes or as photocatalyst in photocatalytic(PC),sonophotocatalytic(SPC)processes,respectively. Experimental results showed that the hybrid processes could effiently enhance the degradation eciency of MeO,and followed pseudo-frst-order kinetics.At the optimized experimental conditions,the rate constants of decolorization of MeO were 0.0073 min^-1 for SPC process and 0.0035 min^-1 for PC process.The rate constants obviously indicated that there existed synergistic effect in the ultrasonic, electro-assisted and photocatalytic processes.Chapter 5 Preparation of ZrO₂/TiO₂ composite film Using sol-gel method for chemical oxygen demand determinationA new composite film using the sol-gel technology was described.The nano-ZrO₂/TiO₂ film promoted the separate efficiency of the charge and extended the range of spectrum,then showed a higher photocatalytic efficiency than the single nano-TiO₂ film.This method was originated from the direct determination of the Cr (â…¥) concentration change resulting from photocatalytic oxidation of organic compounds on the nano-ZrO₂/TiO₂ composite film.Under the optimal operation conditions,the detection limit is 3mg/L,COD values with the linear range of 10.0～800.0 mg/L were achieved.The results were in good agreement with those from the conventional COD methods.