| Due to low carbon content and small amount of greenhouse gas CO2 generated during the combustion process, methane, the main component of natural gas, has become into an ideal fuel. Combustion (oxidation) of natural gas at high temperatures, however, can produce harmful NOx gases. The catalytic combustion of natural gas is one of effective pathways for lowing NOx emmisions, in which the catalysts used are the key factor in carrying out the low-temperature combustion processe. It is known that ceria-zirconia solid solutions possess oxygen storage/release ability and good thermal stability that rend their important application in the catalytic conversion of automotive exhaust. The doping of a small amount of yttria into ceria-zirconia can generate a material Ce0.6Zr0.4-xYxO2 (CZY, x = 0.1, 0.05) that possesses more amount of oxygen vacancies, stronger lattice oxygen mobility, and better oxygen storage/release (redox) ability, and such a material is suitable for the use as a catalyst support for natural gas combustion. The active components can be cheaper Pd, Ag, and Au. In this thesis, we have fabricated a series of CZY nano-/macro-particles, and adopted them as a support to generate MOy/CZY (M = Pd, Ag, Au) catalysts. The XRD, HRSEM, HRTEM/SAED, H2-TPR, TGA/DSC, and XPS techniques were used to determine the physicochemical properties these materials. We also evaluated the catalytic performance of CZY and MOy/CZY for the oxidation of methane. The results obtained are followed below:(1) Single-phase fluorite-type cubic Ce0.6Zr0.4-xYxO2 (x = 0.05, 0.1) solid solution nanoparticles with high surface areas were synthesized using a modified co-precipitation strategy with hexadecyltrimethylammonium bromide (CTAB) as template, aqueous ammonia solution as precitation agent, nitrates of cerium, zirconium, and yttrium as metal precursors. The as-fabricated particles were of cauliflower-like morphology, with the diameters of the particle clusters and their single particle of 20~80 nm and 2~7 nm, respectively.(i) The y% PdO/CZY0.05-CTAB and y% PdO/CZY0.1-CTAB (y% represents the Pd weight percentage, y = 4, 6, 8, 10) catalysts showed good catalytic performance for the addressed reaction. Under the conditions of CH4/O2 molar ratio = 1/4, space velocity = 50,000 h?1 and reaction temperature = 310℃, total methane conversion could be achieved. Such a better catalytic performance might be associated with the presence of highly dispersed active phase PdO on catalyst surfaces and good oxygen storage/release ability of CZY0.05-CTAB and CZY0.1-CTAB. (ii) High-surface-area wormhole-like mesoporous Ce0.6Zr0.35Y0.05O2 (CZY0.05-CTAB) solid solution nanoparticles with a cubic crystal structure were synthesized using a modified co-precipitation strategy with the CTAB as template, and the CZY0.05-CTAB-loaded 0.2~4.0 wt% Ag2O catalysts were prepared using the incipient wetness impregnation method. The catalytic activities of these materials were measured for the total oxidation of methane. It is found that among the x% Ag2O/CZY0.05-CTAB catalysts, the one at x = 2.0 showed the best catalytic performance: under the conditions of CH4/O2 molar ratio = 1/4, space velocity = 50,000 h?1 and reaction temperature = 600℃, methane could be oxidized completely to carbon dioxide and water; furthermore, the hysteresis phenomenon of methane conversion versus temperature over this catalyst during the processes of temperature rise and drop was basically eliminated. Based on the above results, we concluded that the good catalytic performance of Ag2O/CZY0.05-CTAB is associated with the formation of three-dimensional wormhole-like mesoporous structure, larger surface area, stronger oxygen storage/release ability, and the stabilization of active phase Ag2O by CZY0.05-CTAB.(iii) The y% AuOx/CZY0.1-CTAB (y% represents the Au weight percentage, y = 0.2~10.0) were prepared via an in situ reduction pathway with HAuCl4 and NaBH4 as starting materials, and their catalytic activities were examined for the combustion of methane. It is shown that the gold catalysts supported by CZY0.1-CTAB outperformed the ones supported by other cariers reported in literature, especially the 0.2% AuOx/CZY0.1-CTAB catalyst over which methane could be totally oxidized at 660oC. This might be due to the result of the presence of highly active AuOx domains on the catalyst surfaces at lower gold loadings. The"catalytic activity"hysteresis phenomenon caused by the decomposition of active pahse AuOx to Au0 was undermined significantly.(2) Rod-like ceria-zirconia-yttria (CZY0.1-CTAB) solid solution nano-/micro-particles with 10 nm in diameter and 30 nm~2μm in length were fabricated using the CTAB-assisted hydrothermal method with CO(NH2)2 as precipitation agent. Ceria-zirconia-yttria (CZY0.1-P123) solid solutions with morphologies of microspheres, microbowknots, and microoctahedra were synthesized using the P123-assisted sol-gel method. It is shown that these specifically morphological ceria-zirconia-yttria solid solutions possessed fluorite-type cubic polycrystalline structures, strong oxygen storage/release ability, and good thermal stability.
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