| Summary: | 博士 === 國立臺灣大學 === 環境工程學研究所 === 103 === The continuous increase in concentration of CO2 in the atmosphere, as well as the depletion of fossil fuels, has become a public concern in recent years. The use of solar energy (i.e., unlimited energy) to convert CO2 as fuels, such as formic acid and methanol, could address those concerns. The reactions for the generation of these fuels are based on the premise that dissolved CO2 can be reduced by accepting protons and electrons. Promoting the reduction reaction requires catalysts with high efficiency under favorable operation conditions. Two kinds of graphene-loaded TiO2, which were prepared from pure graphite and waste graphite, were used in this research to convert CO2 into fuels. This dissertation also focused on the mechanism of the reactions that are related to the characteristics of the catalysts, the selectivity of the final products, and the radicals involved in the reaction.
In this research, the components of the catalysts were characterized via elemental analysis (EA), X-ray fluorescence (XRF), and energy-dispersive spectroscopy (EDS). The surface area was determined using an N2 adsorption/desorption isotherm analyzer (BET). X-ray diffraction (XRD) results confirmed that TiO2 had a mixed crystal phase of anatase and rutile. Functional groups that could affect the surface potential and polarity of the catalyst were determined via Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The presence of single- and multi-layered graphene was determined via morphological studies, specifically transmission electron microscopy (TEM). The optical characteristics and charge transfer ability of the catalysts were tested via UV-visible (UV-Vis) spectroscopy and photoelectrochemical analysis.
To obtain the highest conversion efficiency, parameters such as graphene loading, catalyst loading, pH, and recycle times were analyzed. The maximum yield of the final products was obtained with 40% graphene loading and 0.4 g L-1 catalytic loading at neutral condition under visible light irradiation. Based on the results, the penetration efficiency of light was related to catalytic loading, which can inhibit the efficiency when the catalysts were loaded in excess. Furthermore, the efficiency of the reaction was also affected by graphene loading because of the charge separation ability of the different catalysts. Moreover, the relationships between the surface potential of the catalysts and the carbon species in the solution at varying pH were also found to be critical factors that affected CO2 reduction. The recycled catalysts exhibited stable reduction efficiency after two recycle times, thereby indicating the possibility for reuse.
The final products and the radicals generated in the intermediate reaction step were identified to determine the plausible mechanism of the reaction. In this research, Gas Chromatography Mass Spectrometry (GC-MS) results showed that the final products were formic acid and methanol. Electron paramagnetic resonance (ESR), which can be used to analyze unpaired electrons, was utilized to determine and identify the radicals involved in the reaction. The ESR results indicate that carbon monoxide radicals were present, and these radicals can react with hydrogen ions and electrons to generate CH3OH. Combining the results of ESR and GC-MS, the possible reduction paths can be summarized as CO2→ HCOOH → CH2O → CH3OH and CO2 →∙CO2−→∙CO- → CH3OH.
Two kinetic models were then developed based on the result of mechanism studies. First, the kinetic model for formic acid and methanol can be assumed to be a pseudo-first order model. Second, based on the possible pathway of CO2 reduction, the pseudo-steady-state hypothesis (PSSH) model was also utilized. This model was suit for the system with several intermediates of unknown concentration and was then utilized to investigate the process of CO2 reduction.
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