Study On Alkaline Earth Metal And Lanthanum Oxide Modified Nickel Based Catalysts For Dry Reforming Of Methane

Methane carbon dioxide reforming（DRM）could convert these two greenhouse gases into syngas,which had excellent environmental benefits and good economic efficiency.The nickel-based catalyst was the most common in the field of methane reforming due to its high activity and low price.It was still a thorny problem about sintering and carbon deposition over the nickel-based catalyst for the challenges of large-scale commercialization and it was significant to develop efficient and stable nickel-based catalysts.This work aimed to explore nickel-based catalysts with high catalytic performance,combing the strong alkalinity of lanthanum oxide and alkaline earth metal oxide and the texture characteristics of silica.The effects of alkaline earth metal modified lanthanum oxide supported nickel-based catalyst and lanthanum doped silica-supported nickel-based catalyst on the catalytic performance of methane dry reforming were investigated.The specific research contents were as follows:（1）A series of lanthanum oxide supported nickel-based catalysts doped with alkaline earth metal oxides were synthesized in one step by complexing citric acid with lanthanum precursors,excess ammonia with nickel precursors,and doping with alkaline earth metals（Mg,Ca,Sr,Ba）.The effects of alkaline earth metal oxides on the physicochemical properties,catalytic performance,and anti-carbon deposition ability were investigated.Although Ni-Ca-La₂O₃,Ni-Sr-La₂O₃,and Ni-Ba-La₂O₃catalysts exhibited higher catalytic activity than Ni-La₂O₃catalysts due to the stronger metal-support interaction,they showed obvious carbon deposition.However,the appearance of carbon deposition in all spent catalysts increases the specific surface area.And the Ni-Sr-La₂O₃,catalyst possessed the highest catalytic activity with the most carbon deposition.The amount of carbon deposition was not the main factor affecting the activity and stability of the catalysts.The Ni-Mg-La₂O₃ catalyst showed good stability and anti-carbon deposition ability.The addition of Mg increased the low coordination of O^2-species on the catalyst surface significantly,improved the adsorption of CO₂,and was conducive to the removal of carbon.The addition of Mg promoted the reaction between La₂O₂CO₃ and carbon.It could significantly improve the activity and carbon deposition resistance with the doping amount of Mg of a certain amount（2 wt%）in the Ni-x Mg-La₂O₃ catalyst.Further increasing the doping amount of Mg could not significantly improve the catalyst performance.（2）It could not expose more metal nickel active sites on the lanthanum oxide support due to the small specific surface area of lanthanum oxide,resulting in relatively low catalytic activity.A series of lanthanum modified silica-supported nickel-based catalysts were prepared by ammonia evaporation to explore the influence of rare earth metal Lanthanum on the catalytic performance.The Ni-SiO₂ catalyst prepared by ammonia evaporation showed better catalytic performance than the Ni-SiO₂-N catalyst prepared without ammonia.The addition of ammonia in the preparation process could obtain a catalyst with a higher specific surface area,and was conducive to the formation of nickel-phyllosilicate species with strong metal-support interaction,improving the catalyst performance.The La doping with a suitable amount（1.5wt.%）could induce a large amount of 1:1 and Tran-2:1 nickel-phyllosilicate having robust interaction with SiO₂ support,remarkably enhanced the catalyst reducibility to 95.3%and consequently produced a larger number of small reduced Ni⁰ on the catalyst surface.Ni-SiO₂catalyst with weak resistance to sintering resulted in its lower stability than the lanthanum-modified catalysts.The suppression of carbon formation and improvement of catalyst stability depended strongly on basic sites.The appropriate medium-strength basic site ensured the quick reaction between carbon and adsorbed CO₂,hence resulting in effective carbon removal.