Periodic Density Functional Theory Study Of Hydrogenation Mechanism On Co₃O₄ And Graphene Catalysts

Recently,non-noble materials are attracting increasing interests as hydrogenation catalysts due to the energy crisis.Metal oxides and carbon materials are potential hydrogenation catalysts because they are low-cost,earth-abundant,safety and stability.Herein,the hydrogenation mechanism of ethylene on metal oxide Co₃O₄ and graphene were deeply studied using periodic density functional theory.What's more,the critical factors for hydrogenation performance of these two catalysts were discussed.Firstly,the hydrogenation of ethylene on perfect and oxygen defective Co₃O₄?1 1 1?surfaces were studied.The results demonstrated that?i?H₂ dissociation on Co₃O₄ is a complicated two-step process through a heterolytic cleavage,followed by the migration of H atom and finally yields the homolytic product on both perfect and oxygen defective Co₃O₄?1 1 1?surfaces easily.?ii?After introducing the surface oxygen vacancy,the stepwise hydrogenation of ethylene by atomic hydrogen is much easier than that on perfect surface due to the weaker bond strength of O-H group.The strength of O-H bond is a crucial factor for the hydrogenation reaction which involves the breakage of O-H bond.The formation of oxygen vacancy increases the electronic charges at the adjacent surface O,which reduces its capability of further gaining electrons from adsorbed atomic hydrogen and then weakens the strength of O-H bond.These results emphasize the importance of the oxygen vacancies for hydrogenation on metal oxides.Secondly,the hydrogenation of ethylene on perfect and oxygen defective Co₃O₄?1 1 0?surfaces were studied.The results demonstrated that?i?H₂ dissociation is in heterolytic cleavage way on both perfect and oxygen defective Co₃O₄?1 1 0?surfaces easily,and H atom could attack ethylene without migration.?ii?On perfect Co₃O₄?1 1 0?surface,the barriers of the two step hydrogenation are significantly reduced compared with perfect Co₃O₄?1 1 1?surface.What's more,the formation of oxygen vacancy further improved the hydrogenation performance of Co₃O₄?1 1 0?surface.These results show that surface morphology has great effect on Co₃O₄ hydrogenation activity.Co₃O₄ is an excellent hydrogenation catalyst due to the cooperation between surface morphology and oxygen vacancy.Finally,the hydrogenation mechanism on modified graphene was studied.The results demonstrated that?i?pristine graphene couldn't be used as hydrogenation catalyst due the high barrier of H₂ dissociation.?ii?After introducing the surface carbon vacancy,the dissociation of H₂ is much easier.However,H atom is strongly bound to carbon,which restrains the subsequent hydrogenation of ethylene because it requires C-H bond breaking.Heterogeneous elements doping?N,P,S?could reduce the barrier of H₂ dissociation to a certain extent,but not obvious.There is still a high barrier for H₂ dissociation.?iii?The synergy of carbon defects with heterogeneous atoms doping not only significantly reduces the energy barrier of H₂ dissociation,but also weakens the binding energy of H atoms and surface.However,on trimerized pyridine-type graphene,the binding energy between H atoms and surface is still high,which is not good for subsequent hydrogenation.On carbon defect with phosphorus-doped graphene,C₂H₅ strongly adsorbes on P site and the second hydrogenation of ethylene is difficult to occur.