| Since the morphology of a material has a great influence on its catalytic performance, controlled preparation of nanomaterials with regular morphologies and their applications have been the hot topic in catalysis. Due to the high surface area and porous structure, porous metal oxides have been widely used in catalysis. The porous structure not only decreases the mass-transfer resistance of reactants and products, but also benefits the dispersion of active phases on the surface, thus greatly enhancing the catalytic performance of porous materials. In addition, the loading of noble metal nanoparticles on the surface of a metal oxide support with a regular morphology and/or a porous structure can further improve the catalytic activity. In this dissertation,Mn O2 and La Fe O3 nanocatalysts with regular morphologies, mesoporous Co3O4,Mn O2, and Cr2O3, three-dimensionally ordered macroporous La0.6Sr0.4Co O3, Pr6O11,and Tb4O7 were prepared using different methods. With the above-mentioned materials as support, the supported nobel metal nanocatalysts were generated via the polyvinyl alcohol-protected Na BH4 reduction route. A number of techniques were used to characterize physicochemical properties of the as-obtained catalysts, and their catalytic performance were evaluated for the oxidation of CO or toluene. The main results obtained in the investigations are as follows:(1) MnO2 nanorods, MnO2 nanotubes, and MnO2 nanowires were fabricated using the the hydrothermal method under different conditions(e.g., manganese precursor,hydrothermal temperature, and hydrothermal time). The optimal hydrothermal temperature was 140, 120, and 240 ℃, and the optimal hydrothermal time was 12 h, 12 h, and 24 h, respectively. Surface areas of Mn O2 nanorods, Mn O2 nanotubes,and Mn O2 nanowires were 48.4, 64.3, and 114 m2/g, respectively. The Au/Mn O2 nanomaterials with various morphologies were prepared via the polyvinyl alcohol-protected Na BH4 reduction route. Under the conditions of CO/O2 molar ratio = 1/20 and space velocity(SV) = 20,000 m L/(g h) or toluene concentration =1000 ppm, toluene/O2 molar ratio = 1/400, and SV = 20,000 m L/(g h), Mn O2 nanowires and Mn O2 nanotubes outperformed Mn O2 nanorods for the total oxidation of CO or toluene. The loading of Au nanoparticles on the surface of Mn O2 could greatly enhance catalytic performance of the sample. The 4 wt%Au/Mn O2 nanorods exhibited the highest catalytic activity. It is concluded that the good catalytic performance of the supported gold nanocatalysts was associated with better low-temperature reducibility and stronger interaction between gold nanoparticles and Mn O2 support.(2) One-dimensional single crystalline La(OH)3 and Fe2O3 with nanowire-, nanorod-,and nanotube-like morphologies were fabricated using the hydrothermal method.The Fe2O3 with nanowire-, nanorod-, and nanotube-like morphologies possessed a rhombohedral crystal structure. The La(OH)3 nanorods were hexagonal in crystal structure. With the as-prepared La(OH)3 and/or Fe2O3 nanomaterials as template,single crystalline La Fe O3 nanomaterials were synthesized. It is found that an increase in calcination temperature favored formation of the La Fe O3 perovskite phase.(3) With KIT-6 as hard template, mesoporous Co3O4, Mn O2, and Cr2O3 were fabricated via the nanocasting route. The mesoporous Co3O4-, Mn O2-, and Cr2O3-supported Au nanocatalysts were obtained using the polyvinyl alcohol-protected Na BH4 reduction method. The KIT-6 hard template could inhibit the aggregation and growth of the metal oxide crystallites. The mesoporous Co3O4 and Mn O2 displayed high surface areas of 139–161 m2/g. Among the Au/meso-MOx catalysts under the conditions of CO/O2 molar ratio = 1/20 and SV= 20,000 m L/(g h), the 8 wt% Au/meso-MOx catalysts showed the best catalytic performance for CO oxidation. The temperatures(T50% and T90%) required for achieving 50 and 90% conversions of CO over 8 wt% Au/meso-Co3O4 were 30 and 60 ℃, respectively. It is believed that the good catalytic performance of 8wt% Au/meso-Co3O4 was related to its high surface area and large loading of Au nanoparticles(i.e., a more amount of active sites exposed on the surface).(4) A series of Co3O4 nanomaterials were synthesized via the polyol and liquid deposition routes. With SBA-15 as support, x Co3O4/SBA-15(x = 10–50 wt%)were prepared using the impregnation and in-situ hydrothermal methods. The surface area and pore volume of Co3O4 obtained by the polyol method was 54.4m2/g and 0.093 cm3/g, respectively. The surface area was was 95.4 m2/g for the Co3O4 sample derived by the liquid deposition method, 361 m2/g for the 18 wt%Co3O4/SBA-15 sample derived by the impregnation method, and 521 m2/g for the50 wt% Co3O4/SBA-15 sample derived by the in-situ hydrothermal method. CO could be completely oxidized at room temperature over the Co3O4 nanocatalysts.Although CO conversion over the Co3O4 sample derived by the polyol method decreased after 50 h of on-stream reaction, it could be maintained at 50% even in the presence of a small amount of water in feedstock. The storage time did not influence the catalytic activity of the Co3O4 sample derived by the polyol method.After 7.5 h of on-stream reaction, however, there was a decrease in CO conversion over the Co3O4 sample derived by the liquid deposition method. For the Co3O4/SBA-15 catalysts derived by the impregnation method, there were Co3O4 nanoparticles embedded in the mesopores of SBA-15. After proper pretreatments,the 30 wt% Co3O4/SBA-15 catalyst showed better catalytic activity and CO could be totally oxidized at 100 ℃. In the presence of a small amount of water in feedstock, the T90% increased by 80 ℃. The storage time also did not influence the catalytic activity of the Co3O4/SBA-15 samples derived by the impregnation method. In the Co3O4/SBA-15 samples derived by the in-situ hydrothermal method, n℃o3O4 nanoparticles were loaded on the mesopores of SBA-15. After proper pretreatments, the 50 wt% Co3O4/SBA-15 sample exhibited better catalytic activity and CO could be completely oxidized at 160 ℃.(5) High-surface-area and well-ordered mesoporous x Fe-SBA-15 with x(n Fe/n Fe+Si) =1.0–5.5 mol% and y Fe Ox/SBA-15 with y(n Fe/n Fe+Si) = 1.0–4.0 mol% were prepared using the one-step synthesis and incipient wetness impregnation methods,respectively. The x Fe-SBA-15 and y Fe Ox/SBA-15 samples possessed a rod- or chain-like morphology. At x = 5.5 in x Fe-SBA-15 or y = 4.0 in y Fe Ox/SBA-15, the Fe species were highly dispersed on the skeletons of the former samples or on the surface of the latter samples. Under the conditions of toluene concentration =1000 ppm, toluene/oxygen molar ratio = 1/200, and SV = 20,000 m L/(g h),x Fe-SBA-15 showed better catalytic performance than y Fe Ox/SBA-15 with a similar Fe surface density. The x Fe-SBA-15 sample with x = 5.5 performed the best and toluene could be completely oxidized at 420 ℃. The good activity of this catalyst might be associated with its large surface area, high Fe species dispersion,and good low-temperature reducibility.(6) Rhombohedrally crystallized three-dimensionally ordered macroporous(3DOM)La0.6Sr0.4Co O3 and its supported M(M = Au, Pd) nanocatalysts were prepared using the polymethyl methacrylate-templating and polyvinyl alcohol-protected Na BH4 reduction methods, respectively. 3DOM Pr6O11 and 3DOM Tb4O7 with a cubic crystal structure were generated with PMMA as hard template and F127 and L-lysine as soft template. The as-prepared La0.6Sr0.4Co O3 and Au/3DOM La0.6Sr0.4Co O3 samples displayed a high-quality 3DOM architecture with a macropore size of 60 nm and a wall thickness of 10- 30 nm. The Au/3DOM La0.6Sr0.4Co O3 samples showed better catalytic performance for CO oxidation. CO could be completely oxidized at room temperature over 8 wt% Au/3DOM La0.6Sr0.4Co O3. Unlike in the case of Au/3DOM La0.6Sr0.4Co O3, there was an inhibition in catalytic activity in the case of Pd/3DOM La0.6Sr0.4Co O3 with a larger loading of Pd nanoparticles. The 1 wt% Pd/3DOM La0.6Sr0.4Co O3 catalyst showed better performance for CO oxidation. Under the conditions of CO concentration =1 vol%, CO/O2 molar ratio = 1/20, and SV = 10,000 m L/(g h), CO could be completely oxidized at 400 ℃ over 3DOM Pr6O11, and CO conversion and specific reaction rate increased with a rise in temperature. The catalytic activity and specific reaction rate below 360 ℃ increased in the orders of Pr6O11-Lysine <Pr6O11-F127 and Tb4O7-Lysine < Tb4O7-F127. A further rise in temperature(> 360 ℃), however, resulted in a lower specific reaction rate over the Pr6O11-F127 catalyst than that over the Pr6O11-Lysine catalyst. |
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