Amorphous TiO₂ And Its Supported Plasma-activated Nano-Au Photocatalysts

Photocatalysis is very promising to remove air pollutant such as formaldehyde and CO.TiO2 is the most widely used photocatalyst,for its great properties such as high photocatalytic activity,chemical stableness and nontoxicity.Anatase is commonly believed to have the highest photocatalytic activity,while amorphous TiO2 has little photocatalytic activity.In this thesis,high active hydro-oxygenated amorphous TiO2(AmT)UV photocatalyst was prepared by atmospheric-presure and low-temperature chemical vapor deposition(CVD)via the gas-phase hydrolysis of titanium tetraisopropoxide(TTIP).Mechanism and apparent kinetics of photocatalytic oxidation of formaldehyde over AmT film was investigated.Furthermore,high active AmT-supported nano-Au visible-light(VL)photocatalysts were achieved via a novelly low-temperature activation of plasma.Mechanisms of plasma activation and VL photocatalytic oxidation of CO for Au/AmT were further discussed.The main work and results are summarized as follows:1.AmT films,which have abundant hydroxyl groups with good uniformity,were deposited by atmospheric CVD via gas-phase hydrolysis of TTIP at low temperature.It intensely absorbs UV light with a band gap of 3.4 eV.Films deposited on glass and polyethylene terephthalate(PET)are both highly active for formaldehyde photocatalytic oxidation under 254 nm irradiation,and their activities are almost the same.Photoluminescence(PL)spectra of films calcined at different temperatures indicate that hydroxyl groups of AmT effectively inhibit the recombination of charge carriers at surface defects,and significantly prolong the lifetime of photogenerated charge carriers.Therefore,hydroxyl groups greatly improve the photocatalytic activity of AmT films.2.Adsorption and photocatalytic oxidation(PCO)of formaldehyde over hydroxyl-oxygenated AmT film were investigated by in-situ FTIR characterizations.Results show that formaldehyde converts to dioxymethylene(DOM)when it adsorbs on AmT.Under UVC 254 nm irradiation,DOM is oxidized to formate,which is further oxidized to carbonate,followed by the release of CO2 from carbonate decomposition.The important role of water vapor was demonstrated by comparing the dry and humid cases.The existence of vapor significantly accelerates the oxidiations of DOM to formate and formate to carbonate and the decomposition of carbonate.3.Apparent kinetics of formaldehyde PCO over hydroxyl-oxygenated AmT film was investigated,and a 0.3 order kinetics was obtained,which means reaction rate is little influenced by reactant concentration.There are two regimes of humidity effect on the photocatalytic reaction.Reaction rate constant keeps maximum at water concentration below 0.4%,and when water concentration increases and exceeds 0.4%,the rate constant linearly decreases.In the whole experimental range,reaction rate constant is proportional with light intensity and the apparent quantum yield reaches about 60%.Apparent kinetic equation was further obtained by fitting rate constants with different light intensity and humidity.At typical formaldehyde concentrations in indoor air(0.1?1mg·m-3),reaction rate of formaldehyde PCO over AmT film is tens to hundreds times higher than that over anatase film.4.Visible-light(VL)active Au/AmT photocatalyst was prepared by impregnation with ammonia washing(IAW)and activated by plasma.Residual ammonia was detected on the photocatalyst because of the hydroxyl groups,and its variation in plasma activation and influences on photocatalytic activity were examined.By hydrogen plasma activation,Au0 content,VL absorbance and photogenerated charge carrier lifetime of Au/AmT increase significantly,meanwhile,the content of residual ammonia decreases,thus the sample possesses a relatively high VL photocatalytic activity of CO oxidation with a CO turnover frequency(TOF)about 0.3 mmol·mg-1·h-1.Oxygen plasma activation leads to reduction in Au0 content and oxidation of partial residual ammonia to nitrogen oxides.The generation of surface oxygen on Au/AmT by oxygen plasma makes the activated sample also VL active,but the activity is quite low and the TOF is an order of magnitude lower than that of hydrogen plasma activated sample.As to nitrogen plasma,it cannot activate Au/AmT effectively,and the sample shows no VL photocatalytic activity.5.In order to avoid the influence of residual ammonia,Au/AmT VL photocatalyst without residual ammonia was prepared by deposition-precipitation with NaOH as precipitant(DPN).By hydrogen plasma activation,TOF of(Au/AmT)HPDPN sample reaches 3.8 mmol·mg-1·h-1,which is an order of magnitude higher than that of hydrogen plasma activated Au/AmT prepared by IAW.Although oxygen plasma activation endows Au/AmT higher CO adsorption capacity than that of(AU/AMT)HPDPN,it dose not improve the VL photocatalytic activity of the sample compared with(Au/AMT)HPDPN Properties of TiO2 supports have great influence on plasma activation.For the Au/P25 sample prepared by the same method,hydrogen plasma consumes its surface oxygen species and reduces electron-deficient CO adsorption sites,leading to a dramatic decrease of CO adsorption capacity.This makes a negative effect of hydrogen plasma activation on VL photocatalytic activity of Au/P25.