Design, Synthesis, and Characterization of Nanostructured Catalysts for Clean Energy and Environmental Applications

This thesis contains three parts: 1) electrocatalytic reduction of CO 2, 2) synthesis of useful compounds from CO2 by Fischer-Tropch synthesis, and 3) doped octahedral molecular sieve manganese oxides with enhanced catalytic activity. Heterogeneous (electro)catalytic conversion of carbon dioxide is one of the viable solutions to reduce greenhouse gas emission and simultaneous production of useful compounds. In the first part, a proof-of-concept experiment has demonstrated that CO2, water, and renewable electricity could be used for the production of transportation fuels. Nano-sized platinum has been deposited on calcia-stabilized zirconia by metal organic chemical vapor deposition. The activation of CO2 was realized by polarizing metal/metal oxide interfaces with a DC voltage or current at 600-900°C. Cryptomelane-type octahedral molecule sieve manganese oxides (K-OMS-2) exhibited excellent conductivity and some activity at a low temperature of 350°C. Real time electrochemical impedance spectra (EIS) have shown the effects of polarization. The products were analyzed by gas chromatograph, nuclear magnetic resonance (NMR) spectroscopy, and liquid chromatography–mass spectrometry (LC-MS). The high selectivity to paraformaldehyde was achieved as high as 100% and CO2 conversion was 8-33%. Ethylene and methanol were also produced using other low cost ZnO, Fe, SiC, and Co catalysts. ^ In the second part, Fe and Co catalysts supported on K-OMS-2 have proven to be efficient Fischer-Tropsch catalysts for CO2 hydrogenation with 45% CO2 conversion and for CO hydrogenation with 87% CO conversion. Valuable light olefins, carboxylic acids, jet fuel, and α-hydroxylic acids have been selectively produced under different conditions. The synergistic effects of metal carbides, potassium, and manganese oxides contribute to a high activity and selectivity. A new mechanism has been developed based on experimental data. ^ In the third part, Cu2+, CO3+, and Ce 4+ doped porous K-OMS-2 have been synthesized using one-step hydrothermal methods. Doped K-OMS-2 has shown higher catalytic activities in phenol oxidation, oxidation of tetralol, and microbial fuel cells compared with undoped K-OMS-2. This study demonstrated that increase in defects (edge dislocations and oxygen vacancies) results in enhanced catalytic reactions. The controllable edge dislocations in doped K-OMS-2 nanofibers were shown in high-resolution transmission electron microscopy (HRTEM) images.