Preparation Of Multilamellar Ni-Al Catalysts For CO Methanation

Compared with oil and natural gas in China,coal will still be the main and long-lasting energetic material.The burning of coal emits harmful gases and dirty the air.Thus,it is urgent to develop a clean and effective way to use the coal.The development of coal-based syngas to methane can not only adjust the energy demands to increase the amount of methane,but also give a clean utilization of coal resources and make a great contribution to environmental protection.In CO methanation,besides main reaction,the side reaction,such as,CO Boudouard and methane decomposition,can easily lead to the carbon deposit.This strong exothermic reaction may also cause the sintering of catalyst.Therefore,design and preparation a catalyst with high low-temperature activity and stability is the vital importance to CO methanation.In this dissertation,we propose a MOF-template strategy to synthesize multilamellar Ni/Al₂O₃ and Ni/NiAl₂O₄ catalysts by a facile calcination method using nickel nitrate in DMF solution as Ni source and Al-based MOF as morphological template and Al source.Two kinds of multilamellar catalysts?Ni/Al₂O₃ and Ni/NiAl₂O₄?were made upon calcinating the impregnated samples at 600 and 900 ? in air.Multilamellar Ni-Al catalysts show three-dimensional architecture with mesoporous multilayers and the surface area of multilamellar NiO/Al₂O₃ and NiO/NiAl₂O₄ are 206 and 185 m2·g-1,respectively.In particular,Ni nanoparticles?NPs?are directly contacted and confined by two layers,which subsequently provides more interfacial sites and builds strong metal–support interaction.For multilamellar catalysts,more interfacial sites facilitate the spillover of hydrogen,and improved interaction prevents Ni NPs from sintering,both contributing to high activity and stability in CO methanation.Multilamellar Ni/Al₂O₃ and Ni/NiAl₂O₄ catalysts are then developed and used for coal-based CO/H₂ mathanation.CO and H₂ with the ratio of 1:2 are used and originated from coal gasification without further water gas shift reaction to increase the amount of H2.For both multilamellar catalysts,CO conversion increases with temperature at first and maintains at a stable value above 400? because of the thermodynamic equilibrium.Multilamellar Ni/Ni Al₂O₄ presents better low temperature activity than multilamellar Ni/Al₂O₃.Clearly,the highest CO conversion reaches 99.8%?with a CH₄ selectivity of 53.8%?at 350? for the former in comparison with 99.6%?with a CH₄ selectivity of 54.7%?at 400? for the latter.The catalytic activity is hardly changed with the variation of the mass of catalyst or the diameter,indicating that the reaction is in the kinetic region.The effect of mass transport limitations and heat transfer can also be removed for this process by checking the Weisz-Prater criterion and the Mears criterion,respectively.TOF values for multilamellar Ni/Al₂O₃ and Ni/NiAl₂O₄ are 2.79 and 3.26 molCO?molNisurf-1?s-1 at 400? with a WHSV of 240 L?gcat-1?h-1,respectively.CO dissociation has been also considered as one of steps for methanation.The carbon formed on the catalyst surface from CO dissociation can be a chemical intermediate for methanation?by hydrogenation of the formed carbon?.In this case,the controlling step is normally the hydrogenation of adsorbed carbon on Ni surface.A fast CO dissociation with a slow hydrogenation of adsorbed carbon would lead to a carbon deposition,which would finally cause a catalyst deactivation.A good methanation catalyst could be made by reducing the catalyst activity for CO dissociation to make a better balance with the activity for hydrogenation.CO dissociates on Ni catalyst firstly to generate active carbon,and spillover hydrogen on lamellar catalysts benefit for the rapid removal of the deposited carbon by hydrogenation on the metal-support interfaces.On the other hand,Ni nanoparticles are confined in the multilayers of the multilamellar catalysts.These layers immobilize Ni NPs and restrain the migration and Ostwald ripening of Ni NPs at high temperatures.Also,this unique structure of Ni nanoreactor facilitates to thermal transmission and diffusion,minimizing the heat effect on the particle size during the strong exothermic reaction.