الفهرس 
cover eOnly 14 pages are availabe for public view
 |  | from | 123 |  |  |
| | | from | 123 | | |
Abstract
The oxidation of methanol to formaldehyde is one of the most important industrial reactions nowadays. The conversion of methanol to formaldehyde typically takes place on the surface of a heterogeneous catalyst and the reaction can be catalyzed by the thermochemical or photochemical driving force. Titaniabased photocatalysts have attracted immense attention in the scientific community for being a low-cost and robust catalyst material, widely employed for many applications those related to photocatalytic oxidation/reduction processes and light energy conversion. However, titania is characterized by a wide optical band gap (~ 3.2 eV for anatase and 3.0 eV for rutile) which limits its light-harvesting properties to the ultraviolet (UV) region of the electromagnetic spectrum which in turn constitute a small fraction of only about 5% of the solar spectrum energy. Therefore, the physicochemical and optical properties of titania must be modified in order to enhance its absorption characteristics toward a light in the visible range and hence increase its energy conversion and photocatalytic efficiencies. This can enable the utilization of abundant visible light from the natural solar energy spectrum for sustainable energy conversion. Consequently, the improved photocatalytic activity will be reflected on the efficacy of the conversion of the light energy into chemical energy and can lead to enhanced oxidation rates, for example, enhanced photooxidation rates during the photocatalytic oxidation of methanol to formaldehyde. In order to achieve this requisite feature, huge efforts have been dedicated to developing approaches for modification of titania and strategies to design a titania-based catalyst system with high performance. The primary focus of this master thesis is to develop visible-light sensitive photocatalysts with improved performance for photocatalytic oxidation of methanol (CH3OH) to formaldehyde (HCHO) over heterojunctions of nitrogen-doped titania (n-TiO2) and selected transition metal oxides, namely hematite (α-Fe2O3) and tungsten trioxide (WO3). The underlying hypothesis was that nitrogen-doped titania can demonstrate enhanced light-harvesting capability compared to non-doped titania and that hematite and tungsten oxide can absorb light in the visible range in addition to their charge acceptance properties. Thus, the heterojunctions of n-TiO2/α-Fe2O3 and nTiO2/ WO3 could lower the recombination of the photogenerated charges in the composite photocatalysts resulting in efficient photocatalytic oxidation of methanol to formaldehyde under visible light illumination. In this study, a simple hydrothermal approach for the preparation of hematite (α-Fe2O3) nanocubes and tungsten trioxide (WO3) micro rods and for the fabrication of the n-TiO2@α-Fe2O3 and n-TiO2@ WO3 composite photocatalysts have been investigated. The promoting effect of the nitrogen-doped titania on the photocatalytic properties of the prepared materials was studied by varying the ratio of n-TiO2 in the prepared composite photocatalysts. In order to gain insights into the structure of the prepared photocatalysts and understand the structure-properties relationships, the prepared materials were fully characterized using standard physiochemical techniques including transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive X-ray (EDX), X-ray photoelectrons spectroscopy (XPS), physisorption (BET), Fourier-transform infrared (FTIR), Raman, diffuse reflectance spectroscopy (DRS) and photoluminescence (PL) spectroscopy. Upon the employment of the differently prepared photocatalysts for the photocatalytic oxidation of methanol to formaldehyde under visible-light illumination, it was possible to reveal the promoting effect of nitrogen doping and the combination of the semiconductor which were found to enhance the optical and photocatalytic properties of the material toward methanol oxidation in an aqueous medium. The obtained results demonstrate the superior performance of the nitrogendoped titania over non-doped titania for methanol photooxidation. The n-TiO2 expressed higher photoconversion efficiency and enhanced formation of formaldehyde of 24.5 mmol L-1 compared to 6.4 mmol L-1 of that of TiO2 under the same experimental conditions of visible-light illumination with a light intensity of 150 mW/cm2 for 3 h using the same photocatalyst loading of 1 mg/ml and under mechanical stirring. Further enhancement of the photooxidation rates of methanol associated with hematite and tungsten trioxide was achieved upon their modification with combination with nitrogen-doped titania anchored onto their surfaces. The highest photocatalytic conversion of methanol to formaldehyde of 48.4 mmol L-1 was recorded for the 2.3 wt.% n-TiO2/WO3 composite photocatalyst after visible-light illumination (150 mW/cm2) under continues mechanical stirring for 3 h. It is important to mention that, besides the improved performance of the prepared materials as photocatalysts photooxidation of methanol and its conversion into formaldehyde, the prepared photocatalysts could serve as efficient photocatalysts for light-assisted oxidation of other small organic molecules such as organic dye molecules or other alcohols.