Activity Enhancement Of Titania-based Photocatalysts By Oxygen Plasma Activation And Hydroxylation

Titania is a typical photocatalyst. One of the critical problems for practical application is how to enhance the photocatalytic activity in the visible and ultraviolet (UV) regions. The enhancement on visible-light (VL) photocatalytic activity of Au/TiO2 by O2 plasma activation and U V photocatalytic activity of amorphous titania by hydroxylation, was studied in this thesis.Due to the surface plasmonic resonance effect of Au nanoparticles (Au NPs), supported Au/TiO2 as a VL photocatalyst has been attracted great attention. To obtain high VL activity Au/TiO2, plasma activation of Au/TiO2 using atmospheric-pressure dielectric barrier discharge (DBD) was studied in this thesis. The effects of discharge gas composition, flow rate, input power and discharge time were investigated. Comparing the performance of Au/TiO2 activated by calcination, Ar plasma and O2 plasma, it was found that Au/TiO2 activated by O2 plasma has the highest VL photocatalytic activity. Meanwhile, the mechanism of the activity enhancement of Au/TiO2 activated by O2 plasma was elucidated. Then, the kinetics of the formaldehyde (HCHO) photocatalytic removal on the Au/TiO2 activated by O2 plasma under VL irradiation was studied. Finally, the hydroxylated amorphous titania was deposited at atmosphere pressure and room temperature by chemical vapor deposition (CVD) method and its calcinated samples at different temperatures were obtained. The structure, optical properties, chemical composition, morphology, specific surface area and UV photocatalytic activity of the hydroxylated amorphous titania and anatase samples were compared. The main results are summarized as follows:(1) Comparing with the VL activity of Au/TiO2 photocatalysts activated by plasmas of Ar, N2 and synthesis air in CO oxidation, the Au/TiO2 activated by O2 plasma has the best activity. Using the mixture of Ar and O2 as the discharge gas, the VL activity of Au/TiO2 rose rapidly at first and then kept constant as O2 content increased. When H2O content in O2 plasma activation increased, the VL activity of Au/TiO2 activated by O2 plasma decreased slightly. In the investigated range of this study, the VL activity of Au/TiO2 activated by O2 plasma increased first and then decreased as the O2 flow rate increased, monotonically increased as the input power rose, and increased first and then kept unchanged as the discharge time extended.(2) Among the three samples activated by calcination, Ar and O2 plasmas, the samples activated by O2 and Ar plasmas exhibited the highest and lowest activity in CO oxidation under the VL irradiation, respectively. According to XPS analysis and temperature programmed reduction by CO, Au/TiO2 activated by O2 plasma had the highest content of surface oxygen which was advantageous to the formation of active oxygen species in the photocatalytic reaction. In addition, the Au/TiO2 activated by O2 plasma had the lowest content of metallic Au based upon XPS analysis, but its cationic Au could be rapidly reduced during the induction period. The results accorded with those of UV-vis spectra, CO pulse chemisorption and in-situ DRIFTs of CO adsorption. Au/TiO2 activated by O2 plasma has large amounts of surface oxygen and low-coordinated Au species, which can promote its VL photocatalytic activity.(3) In VL photocatalytic oxidation for formaldehyde removal, the Au/TiO2 activated by O2 plasma showed higher activity and shorter induction period than the sample activated by calcination. The induction period of Au/TiO2 activated by O2 plasma shortened rapidly and then reduced slowly when light intensity or humidity increased. The in-situ DRIFTs revealed that, compared with dry gas, the moisture could accelerate the oxidation rate of formate species, shorten the induction period and prohibit the accumulation of monodentate carbonate. The oxidation of formate as the rate-controlling step was accelerated over Au/TiO2 activated by O2 plasma. As a result, the activity of Au/TiO2 activated by O2 plasma was higher than that by calcination. Subsequently, according to the VL photocatalytic kinetics study of the Au/TiO2 activated by O2 plasma, the equation of VL intensity-and humidity-dependent rate constant was built, and the inherent rate constant and humidity impact factor were deduced. The humidity impact factor quantitatively revealed the strongly positive, strongly negative and slightly negative effects of humidity on reaction rate in the low, medium and high humidity regimes, respectively.(4) The as-prepared sample (AM-RT) hydroxylated amorphous titania at atmosphere pressure and room temperature and the samples calcined at different temperatures were compared. The AM-RT had the largest content of hydroxyl group, surface area and HCHO adsorption capacity, therefore it exhibited the highest apparent quantum efficiency (AQE) of 60.4% under UV irradiation of 254 nm. AN-550 had the highest photocatalytic activity among anatase samples, but its AQE only reached 49.5%. The specific surface area and HCHO adsorption capacity of AN-350 and AN-450 were very close, while the AQE of AN-350 was higher than that of AN-450 under the irradiation of 254 nm. The order of their AQE was the same as that of hydroxyl group content measured by XPS and FTIR, which supported the contribution of hydroxylation to the enhancement on photocatalytic activity.