| The main use of abundant natural gas is based on combustion.Unburned methane as the second largest greenhouse gas will cause serious environmental problems.Catalytic combustion as an efficient and environmentally friendly combustion technology,which can oxidize low-concentration methane at the low temperature.In addition,it can avoid the production of pollutant gases such as NOx and CO during high-temperature combustion,which is of great significance for improving combustion efficiency and processing low-concentration methane in mines and purification of industrial exhaust gases.However,most catalysts are not resistant to high temperatures,easily sintered,and easily poisoned and deactivated by water vapor and sulfides.This further limit the practical application of catalytic combustion technology.In this thesis,NiCo2O4 spinel catalyst synthesized by hydrothermal 60h has a porous nanosheet structure,and its activity is better than that of NiCo2O4 synthesized by coprecipitation.At a space velocity of 24000 ml h-1 g-1,its T50(temperature at which methane conversion is 50%)is about 280?.Moreover,H2O has little effect on the activity of the NiCo2O4 catalyst synthesized by hydrothermal method,especially at high space velocity.This is because the catalyst undergoes long-term hydrothermal synthesis conditions,and the surface has less adsorbed oxygen and less crystal defects,so the adsorption capacity of H2O on its surface is relatively weak.Then,the hydrothermal synthesis time was tubed to control the amount of surface high-valent ion(Ni3++Co3+),and it was found that the amount of surface high-valent ion(Ni3++ Co3+)has a good correlation with catalytic activity.Therefore,we experimentally confirmed that both Ni3+and Co3+can be used as methane activation sites on the spinel surface.In order to further improve the low temperature activity of the catalyst,ultrafine PdOx nanoparticles(about 1 nm)were grown in situ on NiCo2O4 support to obtain a tight Pd-NiCo2O4 interface.Compared with the catalyst prepared by the traditional method,the sample prepared by the galvanic deposition method not only has good catalytic performance,but also has relatively good water vapor resistance.The reason is that the catalyst obtained by replacement method has more high-valence Pd and more oxygen vacancies are generated on the support.High-valence Pd activates methane,and a large number of oxygen vacancies on the support are conducive to the activation of gas phase oxygen.Synergy effect between the noble metal Pd and the support to increase the catalytic reaction rate.Therefore,the catalyst shows excellent catalytic activity(T90=260?).Sintering and sulfur poisoning are the causes of deactivation of most catalysts.The Pd clusters were encapsulated into the all-silica zeolite(S-1)by one-step hydrothermal synthesis to obtain a core-shell structure(noted as Pd@S-1)with good catalytic activity and stability.After aging at 800? for 10 hours,10%water vapor aging for 100 hours,due to the limitation of the S-1 shell,it can effectively prevent the migration and growth of Pd particles.In addition,the introduction of Ni into the catalyst can not only enhance the catalytic activity,but also improve its thermal stability.Because the cohesive energy of Ni is greater than that of Pd,and Ni can interact with the substrate,the Pd0.8Ni0.2@S-1 catalyst will not sinter severely even after aging at 900? for 5 h.The methane was completely converted at 450?.However,the doping of Ni inhibited the water resistance and sulfur resistance of the catalyst,which exacerbated the deactivation of the catalyst.Finally,the sulfur resistance of Pd@S-1 samples was investigated.A series of characterization methods show that the S-1 shell has a certain blocking effect on SO2.We speculate that due to the confined effect of S-1 micropores,the formation of large-sized PdSO4 clusters is prevented,leading to the easy desorption of SO2 on the surface of Pd@S-1,which significantly reduced the regeneration temperature of Pd@S-1 catalyst. |
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