Effect Of A-Site Substitution Of LaNiO₃/SiO₂ On Catalytic Performance Of Methanation Reaction

The main problem of the Ni-based catalyst in the methanation of carbon monoxide is carbon deposition and sintering.The core problem to be solved is to increase the resistance to carbon deposition and sintering of the catalyst while maintaining a high activity.LaNiO₃belongs to perovskite-type composite oxide?PTO?.In the PTO lattice,the site of A?La?and B?Ni?ions are highly dispersed at the atomic level and are confined in the PTO grains.The obtained nanoparticles of the A-position La oxide and the B-site Ni metal are in sufficient contact to facilitate the interaction,and the Ni particles can be highly dispersed,and the strontium carbonate species which can be formed are easy to remove the carbon,thereby simultaneously preventing the sintering of Ni particles and anti-carbon deposition.The partial substitution of the A site of LaNiO₃ can change the oxidation state and oxygen vacancy concentration of the B site cation,and the A,A'and B formed after the reduction are in close contact with each other,which is expected to improve the catalytic activity.In this paper,LaNiO₃/SiO₂ catalyst was prepared by citric acid complex method for carbon monoxide methanation.It was characterized by XRD,H₂-TPR,XPS,TG,BET,etc.,and the were analyzed.The influence of doping of different elements?the doping amount of similar elements and the carrier structure on the structure of the catalyst?the reduction state of the active component and the catalytic performance were analyzed;and explored the relationship between them.The catalytic performance of La_0.75A_0.25NiO₃/SiO₂?A=Sr,Ce,Sm,Ca?catalysts,especially the medium temperature activity is significantly higher than that of LaNiO_3/SiO₂catalyst,and the performance of La_0.75Ca_0.25NiO₃/SiO₂ catalyst is optimal and has similar properties with La_0.75Ce_0.25NiO₃/SiO₂ catalyst which not formed the PTO structure.Doping the ionic(Ca²⁺,Sr²⁺and Sm²⁺)which radius is close to the La³⁺,they can enter the LaNiO₃lattice and partially replace the A-site element La.After the substitution,the interaction between the active material and the carrier is enhanced,and the reduced Ni⁰ is beneficial to increase the mesophilic activity of the methanation reaction;while ionic radius of Ce⁴⁺is large difference with ionic radius of La³⁺,the doping element enters the La₂O₃ lattice to cause lattice shrinkage to form NiO and La-Ce-O solid solution,which is easier to reduced,it can provide more active sites for methanation reaction after reduction,which helps to improve its catalytic performance.The stability test results of 30h show that all the catalysts have good stability.The catalysts prepared by doping Sr,Ca and Sm produced La₂O₂CO₃ after the reaction,which played an important role in eliminating carbon deposition.At the same time,the amount of carbon accumulated in the catalyst prepared by doping Sm elements was the least,which was close to that of pure LaNiO₃/SiO₂ catalysts.This is related to the degree of substitution of doping elements in LaNiO₃.The doping amount influences the structure of the preparation catalyst,0.1%?Wt%?Ce doping amount of La_0.9Ce_0.1NiO₃/SiO₂ forms a perovskite structure,is favorable for the reduction of the PTO,inhibits the micro-crystal growth,is beneficial to the distribution of the catalyst on the carrier,and has better high-temperature activity.The doping of the high Ce content??0.25?prevents the formation of the perovskite structure and produces the separation phase of the La₂O₃,NiO and the rare earth oxide,which is not conducive to the dispersion of the active components,It is easy to aggregate at high temperatures to reduce its catalytic performance.The support structure affects the catalytic performance.The activation temperature of La_0.9Ce_0.1NiO₃/SiO₂-M catalyst with mesoporous SiO₂ as support is significantly reduced.The conversion of CO is above 90%at 350°C,and it is nearly complete conversion at 400°C.Mainly because of the high specific surface area and confinement effect of the mesoporous structure of the support,which can enhance the dispersibility of the active component and prevented the sintering by undergoing migration and aggregation during the reaction.In addition,the Ce element doped on the support successfully replaces the La of LaNiO₃,and generates CeO₂ and NiO particles.The interaction between the NiO particles and the carrier is moderate,and it is easy to be reduced and not easy to be sintered.So the medium-low temperature activity is well maintained,and the high-temperature reduction peak of LaNiO₃is superposed on each other to form a broad peak,that is,as the reaction temperature increases,Ni particles are continuously reduced,and the active site can be continuously provided for the reaction.