Study On Anti- Sintering And Coking Of Functional Ni-based Catalysts For Partial Oxidation Of Methane

Taking into account reducing reserves of oil and coal on the earth and increasing pressure from environmental protection, the utilization of natural gas resource will play a more and more important role in energy and chemical industry in future. As a mildly exothermic reaction, the partial oxidation of methane（POM） has been research hotspot in consideration of the merits of less energy consumption, affordable high space velocity, small reactor and suitable H₂/CO ratio for downstream product. However, the most widely used Ni-based catalysts results in catalyst deactivation due to the issue of sintering and carbon deposition. In order to solve the previous problem, prolong the life of catalysts and improve the catalytic performance, the Ni@SiO₂, Ni/Zr O₂@SiO₂ core-shell catalysts and Ni/SiO₂, Ni-Yb/Al₂O₃ supported catalysts have been prepared in the text for POM reaction and tested for physicochemical property. The effects of size of Ni nanoparticles, preparation method, surface valence state, coated silica shell and modification of promoter on sintering and carbon deposition of catalysts have been investigated. In addition, the reaction mechanisms of POM at surface of different catalysts have been explored. The main contents are as follows.1. A novel preparation method employing the polyhydric alcohols as solvents was proposed. The polyhydric alcohols delivered the nickel salt precursor into the pore channel of silica. The confinement of pore prevented the sintering of active component. The results are as follows.（1） During process of preparation, the confinement of carbon templates derived from ethylene glycol and glycerol effectively prevents aggregation and growth up of Ni particles, thus producing the homogeneous and small particles. The Ni particles were confined into the pore channel.（2） POM test showed Ni/SiO₂-EG å’Œ Ni/SiO₂-GLcatalysts possessed higher catalytic activity and stability; while Ni/SiO₂-H₂ O å’ŒNi/SiO₂-Et catalysts deactivated quickly. The reason may be attributed to the fact that Ni/SiO₂-EG and Ni/SiO₂-GL possess small and highly dispersed Ni nanoparticles. For the Ni/SiO₂-H₂ O and Ni/SiO₂-Et catalysts, most of Ni particles were located at surface of support and aggregated easily, thus deactivating fast. The 70 h life test of Ni/SiO₂-GL catalyst did not show reduced activity, due to the confinement of pore as well as more active sites contributed by smaller, highly dispersed Ni particles. The catalytic performance of catalysts prepared using glycerol as solvents was superior to the catalyst using water.（3） Catalytic activity of POM reaction decreased with increasing space velocity, GHSV = 5×104 m L·h-1·g-1 was considered as the optimal space velocity. CH4 conversion increased with the rise of reaction temperature.2. The Ni@SiO₂ core-shell catalysts have been successfully synthesized to solve the issues of sintering. The effects of calcination temperature on size of particles as well as morphology, pore structure on surface valence state have been discussed. The dependence of POM catalytic performance, sintering and coking on above-mentioned factors have also been illustrated. The results are as follows.（1） The size of particles increased with the increasing calcinations temperatures. POM test showed the Ni-350@SiO₂ catalyst with the smallest metal core showed higher CH4 conversion and H₂/CO ratio. However, the carbon deposition on the catalysts was severe, which resulted in the low CO selectivity. The carbon deposition was related with the transform to Ni O. There existed a dynamic equilibrium between Ni and Ni O states, whose ratio was determined by size of Ni core and pore structure in silica shell. The ratio of Ni/Ni O also affected the yield of synthesis gas. Taking into consideration of CH4 conversion and H₂, CO selectivity, 500℃ was the optimal calcination temperature.（2） Compared with supported catalysts, core-shell catalysts using the entire surface showed superior catalytic activity and stability, the coated silica shell can effectively prevent the sintering of active component.3. Ni/Zr O₂@SiO₂, Ni@SiO₂ core-shell catalysts and Ni/Zr O₂ supported catalysts was prepared to investigate POM reaction. The effect of silica shell and Zr O₂ promoter on reducibility, structural characteristics and inhibition of carbon deposition was investigated. The results are as follows.（1） The structure of core-shell catalyst was stable and there was not obvious sintering and carbon deposition even suffering from high temperature reaction. The tube carbon and shell carbon occurred at used Ni/Zr O₂ catalyst after reaction.（2） Catalytic performance of POM reaction indicated Ni/Zr O₂@SiO₂ catalyst showed higher activity and stability compared with Ni/Zr O₂ supported catalyst and Ni@SiO₂ catalyst. The reason was silica shell and addition of Zr O₂ effectively inhibited the carbon deposition. Ni/Zr O₂@SiO₂ catalyst（Ni/Zr=1） was proved to possess highest activity. The result indicated that silica shell played an important role in anti-coking by coating the edge and corner atom, preventing the growth up of carbon fiber. The addition of Zr O₂ effectively improved reducibility and oxygen storage capacity of catalyst, which helped to reduce carbon deposition.4. Ni/Al₂O₃ catalyst showed highly catalytic activity for POM reaction. However, the plenty of acid sites at the surface of catalyst resulted in severe carbon deposition and catalyst deactivation. The effect of Yb-doped on physicochemical property of catalyst was investigated. More important, the effect of modification on catalytic performance and carbon deposition was discussed. The results are as follows.（1） The addition of Yb improved the interaction between Ni species and support, enhancing the dispersibility of Ni species, reducing the size of Ni particles. The obtained small and dispersive Ni nanoparticles were favorable to reduce coking.（2） TG and TEM indicated the Ni/Al₂O₃ catalyst suffered from the serious coking compared with Ni-Yb/Al₂O₃ catalyst. The kind of carbon deposition included the fibrous carbon and shell carbon. NH3-TPD and CO₂-TPD test indicated that the addition of Yb reduced the Lewis acid sites of support and improved the resistance to carbon deposition.（3） POM test showed the addition of Yb improved the activity of catalyst, among the catalysts with different Ni/Yb ratio, Ni-Yb/Al₂O₃（Ni/Yb = 1/1） was proved to be the most active. In addition, the addition of Yb promoted the methane combustion and CO adsorption, improving the CH4 conversion and reducing carbon deposition.