Density Functional Study On Selective Bond Breaking Of Ethane Catalyzed By Pd Single Atom In CO₂ Atmosphere

Ethane is the most abundant alkane in natural gas,coal-bed methane,or shale gas besides methane,but it is not efficiently used,and CO₂ is a greenhouse gas that causes global warming.Using CO₂ as raw material to convert alkanes into more valuable chemicals through catalytic action is a research hotspot in the field of green catalysis.Ethane/CO₂ oxidation dehydrogenation and reforming reaction can achieve efficient conversion of carbon-based energy and reduce the content of CO₂.However,the reaction mechanism of these two pathways is still unclear.Therefore,it is of great academic and engineering significance to explore the micro reaction mechanism of ethane/CO₂ catalytic reaction at the molecular scale.In this paper,combining density functional theory（DFT）and transition state theory,using B3LYP method,mixed base group（LANL2DZ for Pd atom,6-311G for C,H,and O atoms）,different catalytic reaction paths of ethane and CO₂ were studied.The main conclusions are as follows:（1）Clarfied the main reaction path of oxidative dehydrogenation of ethane to ethylene and obtained a method to improve the selectivity of ethylene.When CO₂ as the oxidant,the optimal path of ethane dehydrogenation to ethylene is CH₃CH₃→CH₃CH₂→CH₂CH₂.The elementary reactions are all exothermic processes,which is more thermodynamically advantageous.The reaction rate constant of each elemental reaction to produce ethylene is much larger than the consumption path of ethylene,which helps to improve the selectivity of ethylene.When H atoms exist,on the one hand,the dissociation energy barrier of CH₂CH₂ increases,which inhibits the deep dehydrogenation of CH₂CH₂;on the other hand,it promotes the desorption of the generated CH₂CH₂ into ethylene molecules.Therefore,an appropriate amount of H₂ can be added in the reaction to improve the selectivity of CH₂CH₂.（2）Revealed the reaction mechanism and rate-determing step of ethane/CO₂oxidative dehydrogenation and dry reforming,and clarified the tendency of Pd atom catalyst for different reaction paths.When CO₂and ethane undergo oxidative dehydrogenation at 1:1,the decomposition steps is CH₃CH₃+CO₂→CH₃CH₂+H+CO₂→CH₃CH₂+COOH→CH₃CH₂+CO+OH→CH₂CH₂+H+OH+CO→CH₂CH₂+H₂O+CO,which follows the coupling reaction mechanism,and the hydrogenation activation of CO₂is rate-derterming step,requiring activation energy of 165.31 k J/mol.When CO₂ and ethane undergo reforming reaction at 2:1,the C-C bond breaking in the intermediate CH₃CHO is the optimal path,the direct decomposition of CO₂ is a rate-determing step,requiring activation energy of 209.95 k J/mol.The first step in both oxidative dehydrogenation and reforming reactions is the first C-H bond break in CH₃CH₃.Pd atom as a catalyst is more prone to oxidative dehydrogenation of ethane/CO₂.（3）The main path for C-C bond breaking in the reforming reaction and the optimal path for generating CO are obtained.The activation energy of C-C bond breaking in C₂H_xO is obviously less than that of C₂H_x,which is the main way of C-C bond breaking.The CO in syngas is partly from the direct decomposition of CO₂ and partly from ethane.The path CH₃CH₃→CH₃CH₂→CH₃CH→CH₃CHO→CH₃+CHO→CH₃+CO needs to overcome a lower energy barrier and a smaller activation energy,which is the optimal path to generate CO.Among them,the activation energy required for C-C bond breaking in CH₃CHO is 132.5 k J/mol,which is the rate-determing step of this path.The H₂ in syngas is generated by the reaction of H atoms generated during the C-H bond breaking.