A DFT Study Of Propane Oxidative Dehydrogenation Reaction On NiOOH

Propylene is one of the important chemical raw materials in the industrial society,its selective conversion is significant in the context that petroleum is still the main energy source nowadays.However,traditional propylene production processes,such as petroleum catalytic cracking and propane direct dehydrogenation,have their own shortcomings and disadvantages,making propane oxidative dehydrogenation（ODH）reaction an attractive alternative to propylene producton.Understanding the mechanism of propane ODH and exploring the relationship between propane activation capacity and propylene selectivity are of great significance for improving propane conversion and propylene selectivity.Herein,based on density functional theory（DFT）,（001）and（010）surfaces of nickel oxyhydroxide（Ni OOH）are modeled to investigate propane ODH reaction mechanism and propylene further oxidation.In order to study the mechanism of propane ODH,we first calculated adsorption structures and energies of propane on Ni OOH（001）and（010）surafaces.The results indicate that a physisorption nature of propane on both surfaces,and the dispersive interaction on Ni OOH（001）is slightly stronger than（010）.Then using DFT calculations,two pathways of propane ODH were studied,including n-propyl（CH₃CH₂CH₂^*）formation pathway and i-propyl（（CH₃）₂CH^*）formation pathway.The relatively low activation barriers on catalyst surfaces make Ni OOH potentially a good catalyst for propane ODH.The activation barriers on（001）surface are lower than（010）,indicating that（001）has better catalytic performance.Finally,the catalyst cycle process was calculated,the process of gas-phase H₂O fromation is endothermic by1.82 e V and 2.19 e V respectively.The subsequent catalyst reoxidation process is facile in the presence of oxygen and not likely relevant with kinetics in reaction.The activation barriers of propylene further oxidation were calculated,the activation barrier for forming allyl radical is higher than that for propane dehydrogenation.The delocalization of allyl radical unpaired electron weakens the interaction between radical and surface,which makes transition state structure unstable and increases the activation barrier.Subsequently,the effects of radical-surface van der Waals interaction and steric force at transition states of propane and propylene dehydrogenation process were quantitatively investigated.The results show that van der Waals interaction of propylene dehydrogenation transition state is weaker than that of propane,and distortions caused by steric force contribute to the activation barrier of the propylene transition state are higher than that of propane,leading to incrasing propylene selectivity on Ni OOH surface.This study provided a theoretical basis for understanding the mechanism of propane ODH reaction.Ni OOH was applied for a thermal catalyst for the first time and showed good activity and selectivity,providing new ideas for experimental workers in designing the high efficient propane ODH catalysts.