| Summary: | 博士 === 國立交通大學 === 環境工程系所 === 104 === This study intends to propose and study a technology which combines CO2 capture and utilization (CCU) by using ethanolamine (MEA) as the CO2 absorbent and reductant in the solution. To explore the feasibility of ethanolamine as the CO2 absorbent and photocatalytic reducing agent, the CO2 reduction efficiency was compared with commonly used reducing agents in the literature. Photocatalysts with high specific surface area were then prepared and tested under different operating parameters including the light wavelenths from UV to visible light. The CO2 reduction efficiency, photo-reduction quantum efficiency (PQE), and the possible reaction mechanisms were proposed in this study.
The innovative results of this study include the prove of MEA to be the best absorbent/reductant as compared to the NaOH and H2O solution. Therefore, MEA solution is employed in this study for studying the photocatalytic reduction of CO2 to form valuable energy source of methane (CH4) and the total combustible organic compounds (TCOCs) at different light spectra of UV light sources (254, 365 nm) and solar concentrator as well. The results showed that methane yields of the modified photocatalysts of Ti-MCM-41(X) and Mo-TNTs were better than the pure TiO2 when irradiated under both UV and visible light sources. The 8 hours test results showed that the best metal photocatalyst at 254nm was Ti-MCM-41(50), which has the methane yield of 62.42μmol/g and the carbon monoxide yield of 27.65μmol/g under 32μW/cm2 light intensity. And the best photocatalyst was Mo-T-500 under 365nm UV light, which material was prepared under pH=3. Its methane production rate was 0.52μmol/g with the light intensity of 63μW/cm2. And the other products of carbon monoxide and TCOCs yields were 10.41μmol/g and 13.53μmol/g, respectively. In addition, the Mo-T-500 was also tested for its long-term (24hrs) stability under visible light condition (fluorescent lamp, 840nm, 8W). The product yield of TCOCs was up to 10.29 µmol/g after 24 hrs. The long-term stability test for photocatalytic reduction of CO2 under visible light proved the feasibility of Mo-TNTs to work in MEA solution.
From the analysis of chemical and physical properties of Mo-TNTs, it revealed that the structure of Mo-TNTs was changed with the increase of calcination temperature. For Mo-TNTs calcined at 500 °C, the partial corruption of titanate nanotubes into anatase particles caused the reduction of Mo species from Mo6+ to Mo5+ and produced oxygen vacancies, which resulted in the highest CO2 reduction ability. It was found that the molybdenum structure and oxygen vacancies could be the key factors controlling the photocatalytic reduction efficiency of CO2. Possible structure transformation of Mo-TNTs at different calcination temperatures was inferred. And reaction mechanism for photocatalytic CO2 reduction with oxygen vacancy sites of Mo-TNTs was proposed.
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