Preparation Of Catalysts For CO₂ Methanation Reaction And Insight Into The Reaction Mechanism

CO₂ is the main component of greenhouse gases,but it is also a kind of abundant and inexpensive carbon resource.CO₂ transformation by renewable energy has attracted much interest and become one of the most promising strategies for sustainable development,where the CO₂ converted into high value-added chemicals is expected to curb the anthropogenic CO₂emissions and alleviate the impending energy crisis and environmental deterioration caused by the excessive use of fossil energy.For now,Ni-based catalysts have attracted wide attention due to their low price and high catalytic activity and selectivity for CO₂ hydrogenation.However,the high catalytic temperature of traditional Ni-based catalysts?250??450??makes high energy consumption and complicated equipment.At the same time,the catalyst is easy to accumulate carbon and agglomerate during the long-term reaction,which in turn leads to catalyst deactivation and restricts its industrial application.Therefore,it is a popular research aspect for researchers to find a method or catalyst that can oriented convert CO₂ under mild conditions.In this paper,RNi₅?R=La,Ce,Pr,Nd,Sm,etc.?is used as a precursor for catalyst preparation.On the one hand,it aims to obtain methanation with high catalytic activity and high selectivity under room temperature under ball-milling conditions.On the other hand,it is hoped that the room temperature hydriding characteristics of La Ni₅ can be utilized to develop a novel way for the value-added utilization of industrial hydrogen waste.Firstly,the CO₂ methanation reaction of RNi₅-[H₂+CO₂+Ar]system under ball-milling condition was investigated.Under the catalysis of RNi₅,the experiment applied mechanochemical force instead of thermal energy to achieve room temperature reduction of CO₂.The effects of rotation speed,reaction gas pressure,hydrogen sources,cycle times,and hydrogen storage alloy type on catalytic performance were analyzed.The CO₂ conversion rate and methane production rate of La Ni₅-[H₂+CO₂+Ar]system reached 100%and 83.8%at500rpm and 0.3MPa pressure.The mechanism of hydrogen storage alloy in CO₂ hydrogenation reaction was explored via the tracking test.It was concluded that La Ni₅absorbed hydrogen to obtain La Ni₅H₆ firstly,then the hydride interacted with CO₂ and accompanied by the generation of Ni/La₂O₃ catalyst and active H atoms.Half-in-situ FTIR analysis was adopted to analyze the methanation pathway of carbon dioxide,it was found that the CO₂ in the system was converted to methane through the formate route.Then,we performed the carbonate reduction reaction in the system[Me CO₃?or Me HCO₃?+La Ni₅H₆]under ball-milling condition at room temperature for the first time,where carbonate served as CO₂ gas source,La Ni₅H₆ adopted as a hydrogen source and catalyst precursor.For comparison,the effects of H₂ source,the ratio of H₂ to carbonate,types of carbonates,and kids of hydrogen storage alloy hydrides on the reactivity experimented.When the carbonate was Na₂CO₃,the hydrogen storage alloy hydride was La Ni₅H₆,and H₂:La₂O₂CO₃=12:1,the yield of CH₄ could reach 91.9%,and that of Na₂CO₃ and Ca CO₃ were30.3%and 30.8%,respectively.Meanwhile,the experiment compared the effects of active H atoms in hydride and H₂in the gas phase when acted as hydrogen source on methanation reactions.Finally,In order to further improve the activity and stability of the catalyst.The similar homogeneous catalyst chemical environment catalysts?atomic catalyst?were synthesized through the rational design of supports and structures.The author employed the dielectric barrier discharge plasma-assisted ball milling?DBDP-BM?technology to prepare defective graphene supported Ru catalyst?Ru/DG?with higher activity and stability.Owing to the etching function of DBDP over graphene,it generated rich defects on the surface of graphene,and the defects subsequently anchored the Ru atoms to form nano-Ru or even single-atom Ru catalysts.By controlling the plasma type and gas pressure,the Ru/DG and N-doped Ru/DG catalysts were synthesized.The Ru/NG catalyst prepared under the condition of 0.5 atm N₂ reached a CH₄yield of 62.3%at 250?and maintains high stability during a long cycle reaction.