Theoretical Study On The Active Site And The Reaction Mechanism For The Oxidative Dehydrogenation Of Ethane Over NiO Surfaces

Ethylene is the cornerstone of the petrochemical industry.The catalytic oxidative dehydrogenation（ODH）of ethane is regarded as a promising alternative technology for ethylene production.NiO catalysts exhibit superior low-temperature activity of ethane and high selectivity of ethylene.The aim of this thesis work is to investigate and study the active site and the reaction mechanism of ethane ODH on NiO catalysts by density functional theory.The main research contents and conclusions are summarized as follows:The activation of ethane on（100）and（110）surfaces:The adsorption of ethane on NiO surfaces is a physical adsorption process.Ethane dissociation proceeds via heterolytic mechanism with strong acid-base interactions between ethane and NiO surface.NiO（110）surface exhibits relatively higher activity than NiO（100）surface for ethane activation.On NiO（100）surface,the presence of Ni vacancy will promote ethane activation.But on NiO（110）surface,the existence of O or Ni vacancy will suppress ethane C-H bond breaking.The different impacts of vacancy on ethane activation over NiO（100）and NiO（110）surfaces are derived from the nature of Ni-O pairs,which serve as the active sites for ethane activation.The activation barrier is related to the reactivity of Ni-O pairs.Hydrogen chemisorption（oxygen activity）,C₂H₅ chemisorption（nickel activity）and bond strength（interaction of the oxygen with the nickel）may serve as potential descriptors for the nature of active sites.The strong chemisorption of hydrogen and C₂H₅ combined with the weaker bond strength of Ni-O pair can facilitate C-H bond cleavage and lower the barrier of ethane dissociation.The effect of facet and adsorbed O₂ on the mechanism of ethane dehydrogenation over（100）and（110）surfaces:The oxidative dehydrogenation of ethane on NiO（100）and NiO（110）surfaces is an endothermic reaction,which proceeds via Mars-van Krevelen mechanism.And the final products are ethylene and H₂.NiO（110）surface exhibits higher activity for C-H bond dissociation,but the selectivity of ethylene is higher on NiO（100）surface.O₂ is readily adsorbed on oxygen vacancy containing surface.The adsorbed oxygen can significantly reduce the activation barrier of C-H bond cleavage.The reaction of ethane dehydrogenation is exothermic on O₂adsorbed surfaces.The final products are ethylene and H₂O.The adsorbed oxygen may decrease the selectivity of ethylene on NiO（100）surface,but increase it on NiO（110）surface.Because after the adsorption of O₂,it is easy to form CH₃CH intermediate species on NiO（100）surface,which leads to deep oxidation of ethylene and lower selectivity of ethylene.However,on NiO（110）surface,CH₂CH₂ species are not easily to be oxidized deeply.The selectivity of ethylene is increased.The effect of surface reconstruction on ethane activation on（111）surfaces:Surface reconstruction causes the surface to shrink inward and the interlayer spacing is reduced.Thus,surface is more stable.On ideal NiO（111）surface,ethane activation is a spontaneous process and the active site is O-O pair.On reconstructed NiO（111）surface,activation barrier significantly increases and O-Ni pair serves as the active site.The lattice oxygen atom on ideal surface has a strong ability to adsorb H atom,which is favorable for ethane activation.Surface reconstruction weakens the interaction between lattice oxygen atoms and H atoms,leading to higher activation barrier for ethane dissociation.On reconstructed surface,the activation barrier is lower to form O-H and Ni-C₂H₅ species after C-H bond dissociation.This result is related to the four-centered structure of C₂H₅-Ni-H-O in the transition state and the acid-base interaction in the ethane activation.