The Fundamental Research On Catalytic Combustion Of Methane

The natural gas used as fuels possesses various advantages, such as high calorificvalue and utility efficiency, low pollutant emissions, poor ratio of power to gas.Nevertheless, the conventional flame combustion usually leads to the production oflarge amounts of nitrogen oxides and the decrease of energy utilization due to theoptical radiation. Therefore, the development of the techniques of natural gas catalyticcombustion with higher efficiency and lower pollutant emissions becomes quitesignificant.Three series of catalysts for methane combustion, including the perovskites,hexaaluminates and supported noble metals, were prepared. The activities of thecatalysts for the catalytic combustion of methane were examined. The results wereobtained and an activity data base of catalysts for methane catalytic combustion wasestablished, which will afford a practical data base for novel catalyst design.Noble-metal based catalysts are well-known to be active for methane catalyticcombustion at low temperatures. However, they are not stable under operatingconditions due to sulfur poisoning. In addition, the evaluation of catalyst lifetimeusually needs to characterize the catalysts for more than8000h, so there is no suitableevaluation method under laboratory conditions. In this thesis, the1.0wt%Pd/γ-Al₂O₃,1.0wt%Pd/10wt%CeO₂/γ-Al₂O₃and1.0wt%Pd/10wt%Ce_0.6Zr_0.4O₂/γ-Al₂O₃catalysts were prepared by the impregnation method and used for the catalyticcombustion of methane in the presence or absence of SO₂. The physico-chemicalperformance of these catalysts were characterized by means of techniques, such asnitrogen adsorption-desorption (BET), X-ray diffraction(XRD), X-ray photoelectronspectroscopy (XPS), temperature programmed desorption (TPD), sulfur contentmeasurements, and thermogravimetric analysis (TG). It was found that the presence ofSO₂in the feed gas had a negative effect on the activities of the Pd-based catalysts.Pretreatment of the catalysts in SO₂also led to a decrease in catalytic activity to someextent. The sulfur poisoning or the sulfate formation was the main reason for thedeactivation of catalysts. It was the PdO species that contributed to the formation ofsulfates by oxidizing SO₂to SO3species. The sulfur poisoning showed no obviouseffects on the particle morphologies and sizes of the catalysts. The introduction of CeO₂or Ce_0.6Zr_0.4O₂decreased the decomposition temperature of sulfates by50-100°C, indicating the improvement of the sulfur resistance. The sulfur concentration onthe surface of each catalyst accumulated to a constant after the treatment in5%SO₂for more than18h. No observable changes in catalytic activity were observed for thecatalysts after200h of reaction, meanwhile, the Pd contents were also unchangeable.A catalyst evaluation method can be establishted according to the extrapolation of thesulfur and Pd contents. It becomes possible to evaluate the catalyst lifetime underlaboratory conditions.To study the oxidation state effect of Pd during the methane catalytic combustion,the catalysts prepared directly from Pd nanoparticles (NPs) were investigated. Withcetyltrimethylammonium bromide (CTAB) as surfactant, a series of Pd NPs withvarious morphologies were successfully synthesized by controlling the concentrationsof CTAB and NaBH₄in an aqueous solution. The morphologies of palladium NPscould be tuned from nanospheres to nanowires and networks by controlling theconcentrations of CTAB and NaBH₄. At higher CTAB concentrations, the shapes ofthe obtained NPs altered more obviously with a change in NaBH₄concentration. ThePd NPs with uniform particle sizes were encapsulated inside mesoporous silica (SiO₂),affording core-shell NPs (Pd@SiO₂), which were loaded on γ-Al₂O₃to give thesupported catalysts, Pd@SiO₂/γ-Al₂O₃. For comparison, the Pd NPs supportedcatalysts (Pd/Al₂O₃) were also fabricated by the same way. Both kinds of catalystswere calcinated at500,700,800,900, and1050oC, respectively, in air for3h beforeuse. Techniques such as BET, XRD, XPS, temperature programmed oxidation (TPO),temperature programmed reduction (H2-TPR), and temperature programmed surfacereaction (CH₄/O₂-TPSR) were used to characterize the fresh and used catalysts. Thecatalytic activities for methane combustion over the supported catalysts prepareddirectly from NPs, Pd@SiO₂/γ-Al₂O₃and Pd/γ-Al₂O₃, were evaluated. The resultsshowed that the three kinds of surface Pd species (Pd, active PdOxand bulk PdO)coexisted in the catalysts. The Pd@SiO₂/Al₂O₃catalyst, both the fresh one and agedone in air, exhibited higher activity for CH₄oxidation than the correspondingPd/Al₂O₃catalyst. The surface PdOx dispersed on Pd was the active species for CH₄catalytic combustion.Premixed catalytic combustion of natural gas was studied over the self-designedexperimental platform. The influence of the Pd content, cell density of the carrier,air/fuel ratio, height of catalytic bed, and power of the catalytic combustor on catalytic combustion of natural gas was investigated. In addition, the radial and axialtemperatures on the catalytic bed were also examined. The results showed that stablecombustion of natural gas with lower emissions could be achieved on theself-designed experimental platform, which is helpful for the indutrial catalyticcombustor designing.