CH₄/CO₂ Reforming Over Pt/promoter Modified Ni Catalyst:Reaction Mechanism And Characteristics Study

The comprehensive utilization of CH₄ and CO₂,can not only make rational usage of natural gas and carbon dioxide resources,but also alleviate the greenhouse effect caused by CH₄,CO₂ and other gas emissions,which plays a key role in abating environmental deterioration and global warming.In addition,in the academic sense,CH₄ and CO₂ belong to the very stable structure of small molecules,the study of the activation processes and their catalytic reactions is important for understanding the catalytic process of small molecules,energy conversion and efficient usage.Ni based catalysts show good activity.Considering the natural reserves and price factors,Ni catalyst is the most suitable for large-scale application in methane dry reforming process.However,the stability of Ni-based catalyst is the main obstacle for its industrialization.In this work,surface modification of Ni based catalysts is an effective approach to improve the catalytic performance by adding a certain amount of second metal Pt or oxide promoter.Through experimental study and molecular simulation,a systematical DFT study of methane activation by O*and OH*assisted activation as well as direct deprotonation and CH_x oxidation on Ni@Pt is performed and compared with Ni（111）and Pt（111）surface.The effect of surface alloy Ni-Pt catalysts on the activation mechanism of CH₄ and CO₂ and carbon deposition resistance are investigated.The role of oxide promoters（CeO₂、ZrO₂、ZnO）in the Ni based catalysis of CH₄/CO₂ reforming is studied.In addition,the effects of Pt addition into Ni catalyst on the CH₄dehydrogenation process and surface carbon formation are discussed.The main innovative achievements obtained in this paper are shown as follows:（1）The active sites of Ni-core/Pt-shell catalyst（Ni@Pt）in CH₄/H₂O and CH₄/CO₂reforming processes and their resistance to carbon deposition were revealed to keep the catalytic activity in the reaction and reduce the formation of carbon at the same time.The direct methane activation is energetically favorable reaction pathway on Ni,while the OH*assisted methane activation is the favorable pathway on Pt and Ni@Pt catalysts.The compensation between the entropy change and the effective activation energy makes Ni@Pt without significant scarifying activity compared to Ni.In addition,the mechanism study based on DFT calculation shows that whether the activation of H₂O/CO₂ on the support will determine the optimal path of the reaction,and the activation of H₂O on the support is in good agreement with the experimental results.The coking rate for Ni catalyst is around 0.044 mg_coke/(mg_cat·h),while for Ni@Pt it is decreasing to 0.026 mg_coke/(mg_cat·h),thus Ni@Pt catalyst shows the better anti-carbon formation ability compared to monometallic Ni.（2）The effects of bimetallic Ni-Pt catalyst on the inhibition of carbon deposition,ratio of H₂/CO formation and catalytic activity were revealed.The addition of 0.5¹.0wt%Pt can significantly enhance the resistance to carbon deposition of the catalyst during the reaction,and meanwhile maintain a higher reaction activity.The results of TGA-MS and TEM measurements showed that the amount of carbon deposited on the surface of bimetallic Ni-Pt catalyst is lower than that of the single Ni and Pt catalysts,and the filamentous carbon is dominant,which has little effect on the activity of bimetallic Ni-Pt catalyst.The apparent activation energy measurements showed that1.0Pt-12Ni not only reduces the activation energy for CH₄ dissociation and enhances the catalytic activity of the catalyst,but also reduces the energy barrier for CO₂ activation and promotes the formation of surface O*by CO₂ adsorptive dissociation.It is beneficial to enhance the resistance to carbon deposition of the catalyst and prolong its service life in the reaction process.In addition,through DFT calculations,compared with Ni（111）,CH is more favorable to be oxidized instead of cracking into surface carbon on the Ni-Pt surface.Even if a small amount of carbon deposited on the Ni-Pt surface,its oxidation process requires lower activation barrier.The Ni-Pt catalyst has the best ability to resist carbon deposition compared with monometallic samples.（3）The effects of different oxide additives（CeO₂,ZrO₂,ZnO）on the inhibition of carbon deposition,the ratio of H₂/CO formation and the catalytic activity were obtained.We found that both the increased basicity of oxide and the decreased electronegativity of metal element help to promote the CO₂ activation and enhance the catalyst’s stability.Based on the kinetic experiments,the reaction order and activation energy,and compared with the results of DFT calculations and G free potential energy curve,to evaluate the action mechanism of different oxide additives and the effect on the active sites for CH₄ and CO₂ activation during the reaction.The lower deactivation function,indicating a lower deactivation rate and better catalyst stability,was obtained at a lower electronegativity.The enhanced CO₂ activation at lower electronegativity promoted the generation of O*.Through diffusion of O*into Ni surfaces,it enhanced the surface reaction steps of CH_x with O*and thus reduced the coke formation and stabilized the catalysts.We propose that CeO₂-Ni has best catalytic performance in the reaction.（4）With the addition of Pt,the activation of CH₄ is slightly suppressed,but the inhibitory effect on CH*cracking into C*is obviously strengthened.The increased barrier inhibits the CH cracking reaction and reduces the possibility of carbon formation.The adsorption ability and sites for different adsorbents on Ni and Ni-Pt surfaces were analyzed and compared.By comparing the activation energy required for the CH₄dehydrogenation on different catalysts’surfaces,the optimum ratio of Ni/Pt was obtained,and the ability of carbon resistance was improved significantly.In general,the first and last step of dehydrogenation of CH₄ are the key steps in the whole dehydrogenation process,which could directly affect the activation rate of CH₄ and the formation rate of surface carbon.