Photocatalytic Redox Properties Of Modified Nanocrystalline Titanium Dioxide

Photocatalysis is an attractive approach for environmental protection, energy transformation and functional group transformation because of its advantage such as low-temperature, nonenergy intensity and non-pollution. Photocatalysts is the key part in photocatalysis, and further improvement on its activity and efficiency by selective surface treatment by modifying the properties of the particles is an essential step to apply this novel technology. In this dissertation, the photocatalytic activity and surface properties of modified nanocrystalline TiO2 have been characterized by XRD, SEM, TEM, XPS. The mechanism of improvement on photoactivity was discussed based not only oxidation but also reduction properties, and the relationship between them were analyzed.Nanocrystalline TiO2 with anatase phase was prepared at room temperature by inflicting ultrasonic irradiation. Quantum size effect appears in photocatalytic oxidation and reduction when particles size is less than 20nm.Doped nanocrytstalline TiO2 with transition metal ions such as Cr,Mn,Fe,Co,Ni,Cu were prepared by sonic-immersion method and were characterized with XPS. The results show that doping ions with variable valance facilitate the trapping of excited electrons and subsequent interfacial electron transfer so that it has advantage over doping ions with single valance referring to the photocatalytic activity in oxidation and reduction. A volcano profiles was found between the activity and Ea/r which represent the ability to trap excited electrons, and the catalyst doped with copper ions which has a moderate value of Ea/r shows the best activity in photocatalytic oxidation and reduction. The activity of adsorption oxygen on the surface of catalyst affect the efficiency of trapping electron so as to affect the performance in oxidation.Doped catalysts with copper ion by different methods were prepared and characterized with XPS, XRD, SEM, TEM. The results show that the distribution and valance of copper element is different among these samples. Because of the high dispersion and higher value of Cu+/Cu2+ on the surface of catalyst prepared by sonic-immersion methods, its photocatalytic activity is higher than any other samples prepared by hydrolysis-precipitation and physical mixing methods.Modified nanocrystalline TiO2 with copper and stannum based on coupled semiconductor were prepared. The effective inhibition of the charge-carrier recombination by coupled effect and the enhancement of interfacial electron transfer by Cu+/Cu2+ lead to the enhancement of the degradation rate of formaldehyde and the products of reduction, i.e., the time to completely mineralize is reduced from 180mins to 60mins and 150mins respectively and the accumulative concentration of all reduction products is increased by 33%-72%.In order to further understand modification mechanism of doping cupper ion, modified nanocrystalline TiO2 with Pd and Sn was obtained by photoreduction method on the base of coupled semiconductor SnO2/TiO2, and it shows higher selectivity for methanol than the modified TiO2 with Cu and Sn in photocatalytic reduction of CO2. The different performance between the samples was attributed to the different effect of different modification methods on transfer process of electron.