Surface Modification And Kinetic Study Of Ni-based Mesoporous Catalyst For CH₄/CO₂ Reforming

Carbon dioxide reforming of methane attracted more widespread attention in recent years,which is an especially promising route to generate high-value-added syngas with an H₂/CO ratio close to 1 and remove CH₄ and CO₂ and an important link between carbon resources and liquid fuels or high-value chemicals.Among them,Nickel-based catalysts have been widely used in the CH₄-CO₂ reforming due to their excellent catalytic activity and low cost,but easily carbon deposition.So,the key to the catalyst not only lies in solving the catalytic activity of catalysts but also in solving the carbon deposition.The molecular sieve MCM-41 with high specific surface area and high stability is employed as the support,which not only improve the dispersion of active nickel metal,but also maintain the thermal stability of the microporous structure at high temperature.On this basis,alcohol was introduced as a surface modification substance to control the size of metal active within a certain range and enhance metal-support interaction.In addition,we designed the different preparation method of the catalyst by directly regulating the way of the introduced alcohol to alter the form between the support and Nickel.Aiming to the serious sintering of the metallic phase at high temperatures,the effect of the plasma treatment ways on the coke resistance was systematically discussed to realize the controlled preparation.Firstly,Ni-based catalysts were modified by methanol,ethanol,n-propanol,n-butanol,and ethylene glycol.Presumably,the main influencing factor for the catalytic performance and coke resistance of the alcohol-promoted catalysts is hydroxyl group numbers rather than the carbon chain length of the introduced alcohol.Especially,the NM-EG catalyst has the highest catalytic performance and the best coke resistance in all of the alcohol-promoted catalysts.The conversions of CH₄ and CO₂over the NM-EG-C catalyst finally reaches 71.76%and 82.34%after 20 h CH₄-CO₂ reforming reaction,increased by 29.43%and 36.50%.The characterization results confirm that the introduction of alcohol enhances the metal-support interaction,and so lead to the formation of small sizes and high dispersion metal particle,which will inhibit the coke formation.On the other hand,more Ni²⁺species are taken into the channels of the MCM-41 during impregnation due to the complex formed between alcohol and Ni（NO₃）₂or alcohol-induced capillary action,and a high ratio of surface adsorbed oxygen species will increase the carbon elimination rate.Besides,the influence of the number of hydroxyl group numbers on the Ni-based catalyst was analyzed by kinetics.The activation energy of the Ni-based catalyst prepared by surface modification of ethanol and ethylene glycol was 53.19 kJ/mol and 56.53 kJ/mol,respectively.With the increase of hydroxyl group numbers,the activation energy of the catalyst is further reduced,thereby enhancing the CH₄ activation and increasing the rate of reaction.Secondly,based on our previous works,ethanol with insufficient alcoholic hydroxyl groups and carbon source could not effectively improve performance activity and coke resistance.However,ethanol（EA）is more accessible,easier to perform,and less expensive than ethylene glycol（EG）.we propose a newly ethanol and cold plasma coupling-actived synthesized method to improve performance activity and coke resistance,and illuminate effect of ethanol and H₂-plasma on structural and catalytic properties of ethanol and cold H₂-plasma coupling-activated Ni/MCM-41 for CH₄/CO₂ reforming.In this work,H₂-plasma in-situ reduces Ni²⁺to Ni⁰,and forms a Ni⁰/MCM-41 catalyst.The plasma-enhanced catalyst has improved CH₄/CO₂ reforming activity,but the high energy density of the generated plasma environment will destroy well-ordered hexagonal mesoporous structure of MCM-41.However,a substantially larger number of Ni are confined in the mesopores of MCM-41,and Ni²⁺can be highly dispersed in MCM-41 channels.The addition of ethanol will lead to partial inhibition of the molecular sieve direct decomposed by energetic electron bombardment under high energy status.The ordered mesoporous materials are regularly arranged throughout the crystal domain to form short-range order.NM-EA-P not only improves low-temperature reducibility of the catalyst while maintaining strong metal-support interaction,but also decrease the particle size of Ni⁰.Correspondingly,NM-EA-P shows the highest initial activity of CH₄and CO₂,about 76.40%and 76.36%,and relatively stable among all catalysts.The carbon deposition is significantly suppressed on the catalyst NM-EA-P.Besides,the effects of different plasma treatment methods on the performance of the catalyst are explored,and whether plasma can replace H₂ reduction and calcination is discussed.Without H₂ reduction,the catalyst NM-EA-P prepared by direct plasma treatment can effectively improve the initial activity and stability of the catalyst.After 20 h reaction,the conversions of CH₄and CO₂ only decreased by 2.62%and 4.91%,and the carbon deposited over the catalyst is only 3.56%,therefore,H₂-plasma can replace reduction under certain conditions.The characterization results confirm that the catalyst precursor rapidly decomposes into Ni O at a moderate temperature,and even reduces to the active phase metal Ni⁰ under H₂ atmosphere,which prevents the agglomeration of the active metal under high-temperature calcination,thereby improving the coke resistance of the catalyst.The effect of the coupling activation of ethanol and plasma on the Ni-based catalyst is analyzed by kinetics.The activation energy of the direct plasma activated catalyst is 95.55 kJ/mol.The catalyst needs more energy during the reaction to realize the effective conversion of CH₄ and CO₂,this is because direct plasma activation destroys the molecular sieve structure,and part of the Ni species is blocked inside the pores of the molecular sieve.The reaction activation energy of the Ni-based catalyst activated by the coupling of ethanol and H₂ radio frequency plasma is 65.48 kJ/mol,which proves that the surface modification of ethanol enables Ni²⁺to be highly dispersed in the pores of MCM-41,thereby inhibiting the molecular sieve direct decomposed by energetic electron bombardment under high energy status.Thirdly,based on our previous works,we designed the regulation of ethylene glycol（EG）during preparation of catalysts to alter the form between reducing carbon species and Nickel.By directly impregnating with ethylene glycol（EG）solution of Ni（NO₃）₂ to prepare the catalyst NM-EG-C,reducing carbon species partially in-situ reduces Ni O to Ni⁰ under N₂atmosphere calcination.The catalyst NM-EG-C is well-dispersed without agglomeration phenomenon between the particles,and Ni⁰ with adjacent ones is atomically dispersed at each loading to form the structure of carbon-coated Ni⁰.This special structure plays an active role in the formation of well-dispersed and small particle sizes of Ni⁰.As a result,the catalyst shows the highest initial activity of CH₄ and CO₂ at 700°C,about 77.01%and 83.63%,and relatively stable.Interestingly,due to the presence of Ni⁰,even without H₂ reducing conditions,the NM-EG-C catalyst shows the activity of CH₄ and CO₂reach 73.83%and80.26%after 20 h reaction.However,for the EG-Sup-C-Ni-C catalyst,the ordered mesoporous support is well wrapped by the carbon layer to form C-support and then Ni supports on the surface of the C-support.Ni O cannot be in-situ reduced into Ni⁰ during the second N₂ calcination process and form the structure of carbon-stacked Ni.A large amount of active metal Ni O form larger agglomeration and consequently decrease active surface area.This structure is less effective for enhancing the stability of CH₄-CO₂ reforming.Through kinetic analysis,it was found that the carbon-coated structure formed during the preparation process of NM-EG-C effectively reduced the catalyst activation energy（51.81kJ/mol）,which improves the rate of reaction.However,for the catalyst EG-Sup-C-Ni-C,there is a strong C-O-Ni bond between the Ni species and the composite support（C-MCM-41）,leading to an increase in the activation energy of the catalyst EG-Sup-C-Ni-C,eventually reaches 62.10 kJ/mol.