A Study Of The Effects Of Transition Metals On Catalytic Combustion Of Methane

Conventional combustion of natural gas (i.e. methane) not only causes serious air pollution, but also has low utilization efficiency. Catalytic combustion of methane is becoming attractive in recent years due to the high combustion efficiency and less pollutant emission and therefore is more environmentally friendly. In this thesis, the effects of trasition metals on the catalytic combustion of methane have been studied systematically with respect to both the catalyst support and the catalytic reactivity of Ce_1-xM_xO_y (M=Fe,Cu,Ni) of the composite metal oxides by adding trasition metals, i.e., Fe,Cu,Ni into cerium oxides.The author analyzed the properties of coal fly ash including size distribution, magnetic separation, chemical composition, crystal structure and microstructure and morphology by means of X-ray Diffraction (XRD), Scanning Electronic Microscope (SEM) and Laser Particle Size Analyser, etc. The catalytic reactivity of coal ash during methane combustion was assessed. The results indicated that coal ash, due to its porous structure and some active components, may catalyse the comubstion of methane under the present expeimental conditions. A series of surpported catalysts were prepared using coal ash as the support with transition metal, i.e., ferric oxide, as the main active component and the effects of coal ash particle size, magnetic separation and the loading level of active catalytic components were carefully studied. The results demonstrated that catalysts supported on coal ash with smaller particle size or on magnetic fraction of the coal ash with a higher loading level of active components showed better catalytic reactivity during methane combustion.By using co-precipitation in combination with microwave drying and calcination, composite metal oxides Ce_1-xM_xO_y (M=Fe,Cu,Ni) were prepared by introducing transition metals such as Fe,Cu and Ni into ceria oxide and the precursors of composite oxides were analyzed in details by using a thermogravimetry with differential thermal analysis (TGA-DTA). The effect of substitution ratio ,x, of trasition metals on the catalysts reactivity during methane combustion was investigated.The results showed that the catalytic reactivity of methane combustion increased dramatically through the use of Ce_1-xFe_xO_y and Ce_1-xNi_xO_y composite oxides which were formed from above mentioned method. The substitution ratio ,x, was an important factor influencing the catalyst reactivity during methane combustion. Under the present conditions, the optimum substitution ratio is 0.3, i.e. the compostion of Ce0.7Fe0.3Oy has the highest catalytic reactivity. However, for Ni, the optimum value of x was 0.5, i.e. Ce0.5Ni0.5Oy showed better catalytic reactivity. The addition of Cu seemed not significantly influence the catalytic reactivity of the cerium oxide. The effect of doping La on the catalytic reactivity of the composite oxide Ce_1-xM_xO_y (M=Fe,Cu,Ni) was also carefully studied. The results showed that La may be effective to improve the performance of the composite oxide Ce_1-xM_xO_y (M=Fe,Cu) when the loading level is low, further increase in the doping level will lead to the decrease of the catalytic reactivity of the catalyst.However,doping Ce_1-xNi_xO_y composite oxides with La decreases their catalytic reactivity dramatically.