Regulation Of Different Space-confined Structures Over Yolk-shell Catalysts And Their Application In CH₄-CO₂ Reforming Reaction

With the depletion of coal and oil resources,natural gas has become a main alternative energy,and methane is the main component of natural gas.It has became an important route for the efficient use of natural gas resources via the conversion of methane into high-value chemicals and liquid fuels.Carbon dioxide reforming of methane?CRM?,has been widely attention due to its significant application prospect for methane and carbon dioxide utilization.Nibased catalysts are considered to be one of the most promising catalysts in CRM owing to its superior activity,low cost and wide availibility.However,catalyst deactivation via severe carbon deposition is the most significant obstacle for their industrialization.In this thesis,we synthesized a highly dispersed Ni@SiO₂ nano-capsule structure catalyst,characterized by its specific geometric structure,Ni nanoparticles were completely isolated from each other without any observable sintering during CRM.By virtue of this,the accumulation and nucleation of carbon materials was sterically hindered due to the very limited space for carbon formation and growth,therefore excellent catalytic performance was achieved.The importance of the space confinement effect in preventing carbon formation was demonstrated by comprehensive characterization and comparison of three different structures of Ni-SiO₂ catalysts?Ni/SiO₂,Ni/SBA-15 and yolk-shell Ni@SiO₂ nano-capsule?,with different metal site external spatial confinement properties?no confinement,partial confinement and total confinement?.The results show that the fully confined nanocapsule catalyst is more effective in avoiding sintering of Ni nano-particles and suppressing carbon formation simultaneously,this paved a new way for the designing and preparation of highly stable catalysts for reforming reactions.The effect of different Ni contents on the structure and properties of the catalysts was investigated by changing the concentration of Ni precursor in xNi@SiO₂ catalyst?x=1.0M,1.5M,2.0M,2.5M?.The results show that the cross-linking phenomenon of SiO₂ shell is getting serious with the increase of Ni precursor concentration.When the Ni precursor concentration is less than 2.5M,the prepared catalysts are basically nanocapsule monomers except for 2.0Ni@SiO₂,who had a tiny cross-linking phenomenon.It is worth noting that the concentration of Ni precursor also has an effect on the pore structure of the catalyst.When x=1.0M,there is almost no shell mesopores at 3-4 nm,so 1.0Ni@SiO₂ exhibited a poor catalytic performance.By characterization and evaluation,the 1.5Ni@SiO₂ catalyst exhibited excellent resistance to metal sintering and carbon deposition at x=1.5 M,and demonstrated a superb longterm CRM reaction stability under rigid reaction condition for more than 310 h.The narrow geometric space of core-shell structure can inevitably lead to the inability of the catalyst system to ensure the rapid diffusion of the reaction gas and subsequent conversion of the carbon intermediates.Herein,we reported the synthesis of a Ni@SiO₂ nanocapsule catalyst with various inner cavities by employing different aging times of a Ni@SiO₂ nanosphere catalyst to demonstrate the critical importance of inner cavity space on both the catalytic activity and stability under operational high space velocity conditions.Compared with Ni@SiO₂ nanosphere catalysts,the availability of sufficient reaction space of these Ni@SiO₂ nanocapsule catalysts can not only provides exposure to more active centers to the incoming reactants,but also enhances the internal reactants adsorption/transportation capacity at high space velocity.Thereafter,the nanocapsule catalysts show excellent catalytic activity and stability.In order to study the universality of the space confinement effect of the core-shell structure,an encapsulation cobalt-based core-shell material is introduced into the N₂O direct catalytic decomposition reaction.The encapsulating shell structure could effectively improve the metal nanocrystallization and dispersion and the porous pores of the shell can also increase the contact interfaces between the active metal oxide and reactants,thus enhancing its catalytic activity in the actual operating conditions of N₂O catalytic decomposition.