Study On The Preparation Of Nickel-based Catalysts From Hydrotalcite And Its Catalytic Reaction Of Methane Decomposition

The more stringent requirements of environmental protection pose severe challenges to the traditional fossil energy system,forcing the countries around the world to accelerate improving their energy structure.As clean energy,hydrogen will play an important role in the future energy structure.The rapid growth of hydrogen demand poses a challenge to produce hydrogen in a more environmental-friendly and efficient way.The catalytic decomposition of methane（CDM）process not only has the advantages of low energy consumption and simple process flow but also can greatly reduce greenhouse gas emissions during the hydrogen production process,providing a practical technical route for clean and efficient hydrogen production.However,the formation of carbon deposits in this reaction is inevitable,leading to the rapid deactivation of the catalyst.Using a circulating fluidized bed reactor to separate the methane cracking reaction from the carbon deposition catalyst regeneration process is a feasible route to realize the continuous reaction of methane cracking.The development of high efficient catalyst is the key to this process.In this research,nickel-based catalysts with different physical structures and chemical properties were obtained by adjusting the preparation method with hydrotalcite as the precursor.The synthesis of nickel-based catalysts with high activity and good stability was guided by establishing the dependence of nickel particle size on reaction activity and the type of carbon deposits.The regeneration behaviors of catalysts were also investigated in different regenerative media.To improve the poor stability of the catalyst during the reaction-regeneration cycle,a Ni Mo alloy catalyst was developed.This research provides theoretical and technical support for the systematic development of the CDM process.First,the tartaric acid intercalated Ni Mg Al-TA-LDH hydrotalcite was prepared by the ion exchange method.When calcining in a high-purity nitrogen atmosphere,the reduced nickel particles could be formed through the self-reduction reaction that takes advantage of the carbonization product of intercalated tartrate to deprive oxygen atoms in Ni（Mg Al）O_X nearby.By altering the calcination temperature,a series of catalysts with different nickel particle sizes（2.6～41.0 nm）were prepared.In the evaluation of the intrinsic reaction activity of methane cracking,it was found that there was an obvious dependence between the hydrogen generation rate and the size of nickel particles,the catalyst with an average nickel particle size of 2.6 nm has the highest hydrogen formation rate of 20.6 mmol/(g _Ni·min).When the nickel particle size was less than 15 nm,the hydrogen generation rate increased rapidly as the nickel particle size decreased.Based on the relationship between the proportions of different crystal surfaces and nickel particle size,it is speculated that the stepped surface（represented by the crystal surface of Ni（211））is the dominant crystal surface for methane cracking and has the highest methane cracking activity.It was also found that as the nickel particle size decreased,the strength of methane and hydrogen adsorption increased.Subsequently,a series of Ni Mg Al hydrotalcite with different Mg/Al molar ratios were prepared by the co-precipitation method,a series of catalysts with different nickel particle sizes（13.2～25.4 nm）were obtained after calcination and reduction.The increase of the Mg/Al molar ratio is beneficial to the formation of smaller nickel particles.In the methane cracking reaction,the catalyst with larger nickel particles maintained a stable activity during the 30min reaction.As the size of the nickel particles decreased,the rate of catalyst deactivation increased.By multiple characterizations,we found that carbon nanotubes that could keep catalyst activity were more prone to form on large nickel particles,while encapsulated carbon species that led to deactivation were inclined to deposit on small particles.Supported by DFT calculations,we proposed the insufficient supply of carbon atoms and rapid nucleation of carbon precursors caused by the lesser terrace/step ratio on smaller nickel particles inhibit the formation of carbon nanotube,leading to the formation of encapsulated carbon species.By establishing the relationship between nickel particle size and hydrogen generation rate as well as carbon deposition type,it can be concluded that catalysts with small nickel particles not only had higher methane cracking activity,but also the amorphous carbon generated could be easily eliminated by oxidation.Under the guidance of this conclusion,an 8Ni Mg Al-800catalyst with a nickel particle size of 7.7 nm was synthesized by using hydrotalcite as a precursor.The catalyst performed a high initial hydrogen formation rate(52 mmol/(g _Ni·min))in CDM reaction but deactivated rapidly within 30 minutes of evaluation.After regenerating the deactivated catalyst for 15 min at 650 ^oC using air as the regeneration medium,the carbon deposits can be eliminated.At the same time,nickel species which were difficult to be reduced were produced during regeneration,which led to the decrease of catalytic activity after regeneration.When using CO₂ as the regeneration medium,the carbon deposition on the catalyst cannot be eliminated completely after 15 min.However,the nickel species on the catalyst remained reduced,and the reactivity of the catalyst was completely restored after regeneration.In the process of reaction-regeneration cycles,the catalysts regenerated in different media both show a trend of gradual decline in activity.In order to improve the poor stability of small-sized nickel particles during the reaction-regeneration cycles,an efficient methane cracking Ni Mo alloy catalyst was designed and prepared.Ni Mg Al Mo-LDH hydrotalcite was prepared by taking advantage of the memory effect of hydrotalcite.After calcination and reduction,a Ni Mg Al Mo catalyst with high activity and stability was obtained.At the reaction temperature of 700 ^oC,the hydrogen formation rate of Ni Mg Al Mo-800 catalyst was 60.4 mmol/(g _Ni·min),which was better than most catalysts reported in recent literature.The Ni Mg Al Mo-800 catalyst showed good stability during10 times methane cracking-air regeneration cycles.By characterizations,it was found that Ni Mo alloy particles with an average particle size of 7.4 nm were formed on Ni Mg Al Mo-800 catalysts after reduction.Due to the high melting point of Mo,the stability of the alloy particles was improved.The introduction of Mo could not only optimize the catalyst structure to form small-sized Ni Mo alloy particles but also promoted the adsorption of reactant methane and the desorption of product hydrogen through the transfer of electrons from Mo atoms to Ni,thereby improving the catalyst activity.