Vliv metody přípravy ZnO na fotokatalytický rozklad hydroxidu amonného

Abstract

This thesis examines the influence of the preparation method of zinc oxide on the photocatalytic decomposition of ammonia, where as ammonium hydroxide was used as a model reaction mixture. Hydrogen and nitrogen were generated by photocatalytic decomposition of ammonia. The photocatalytic decomposition of ammonia was carried out in a batch annular stirred reactor with suspended photocatalyst. In the axis of symmetry of the reactor the UV lamp providing a radiation having a wavelength of 254 nm was placed. Zinc oxide photocatalyst was prepared by three different ways, namely: thermal annealing of zinc acetate (ZnO(1)), precipitation reaction of zinc acetate and sodium hydroxide followed by thermal annealing (ZnO(2)) and a precipitation reaction of zinc acetate with sodium hydroxide under the UV radiation and followed by thermal annealing (ZnO(3)). Prepared nanoparticles were characterized by thermogravimetry, X-ray diffraction analysis, UV-Vis spectroscopy, photoluminescence spectroscopy, nitrogen physisorption and transmission electron microscopy. The hydrogen production was observed in both, photocatalytic and photochemical decomposition of model reaction mixture. The hydrogen production in the presence of a ZnO photocatalysts was compared to the production of H2 in the presence of a commercially manufactured photocatalyst TiO2 Evonik P25. The activity of tested photocatalysts was decreasing in this order: ZnO(1) < ZnO(3) = Evonik P25 < ZnO(2). Experimental data of the photocatalytic decomposition of ammonia were evaluated by integral method, kinetic constants were calculated and the assumption the reaction can be described by the equation of 1st order was confirmed. Based on the correlation between the photocatalytic activity and physico-chemical characteristics of prepared photocatalysts. It was found out the key parameters influencing photocatalytic decomposition of ammonia are different defects in the crystal lattice of the ZnO oxygen vacancies (VO) or interstitial oxygen (Oi) and the oxygen in the place of Zn (OZn) which originate from different preparation way of photocatalysts. The most active photocatalyst ZnO(1), which was prepared by thermal annealing, had the lowest specific surface area, but also the lowest amount of oxygen vacancies. The lowest activity of ZnO(2) photocatalyst, prepared by precipitation reaction, can be attributed to the highest amount of oxygen vacancies. These vacancies capture excited electrons, which arenot available for the photocatalytic reaction itself.