Theoretical Study On Ethylene Conversion In Pd-phosphorene Composites

Olefin oligomerization is widely used for production of linear alpha olefins for synthesis of copolymerization monomers,surfactants,synthetic lubricants,etc.It was showed that Pd complexes are efficient catalysts for ethylene oligomerization into linear alpha olefins as well as many chemical conversions involving C-C coupling reactions.In these complexes,Pd centers are stabilized by O,N,P,etc.containg electron donor ligands to exhibit superior catalytic performance.One major obstacle for application these homogeneous catalysts is the separation of products,making the reuse of catalysts impossible.It is of great significance to design novel heterogeneous catalysts for efficient conversion of ethylene with low cost.Phosphorene is a 2D allotrope of phosphorous.Due to its unique structure,the binding of transition metal atoms to phosphorene is quite flexible and is highly desired as support for catalysts.In this work,the electronic structure and stability of the composites formed by Pd and phosphorene were investigated by extensive first principles based calculations.Based on the results,the mechanisms and other potential chemical processes for ethylene oligomerization catalyzed by Pd phosphorene composites were also investigated,the major contents and findings are as the follows:(1)The existence and thermostability of Pd clusters deposited on phosphorene were investigated with extensive first principles based calculations.Pd clusters would be deformed by interfacial Pd-P interactions into plausible atomic strips aligned parallel to the zigzag direction of phosphorene.The barriers for Pd atomic diffusion along zigzag direction are small(<0.3 e V)and are less dependent on the length of Pd atomic strips,but those along the armchair directions are too high.Stable Pd species would be formed in CO or ethylene atmosphere,with decreased reaction barriers,making atomic dispersion of Pd possible on phosphorene.(2)The mechanisms of Pd species catalyzed ethylene oligomerization were also investigated by extensive first principles based calculations.It is showed that the reactions may proceed through Cossee-Arlman mechanism that can be divided into activation of Pd species,chain growth and chain transfer,etc.Activation of Pd species involves ethylene adsorption and insersion,β-H and desorption of propene,and the corresponding reaction barriers are 0.62 and 0.53 e V.Chain growth involves ethylene adsorption and insersion at barriers of ～0.43 e V.Chain transfer involves β-H elimination and olefin desorption and the reaction barriers are ～0.80 e V.Competing with the chain growth and transfer,isomerization of olefins may also take place.The calculated barriers for isomerization of 1-butene to cis-2-butene and trans-2-butene are 0.75 and 0.80 e V,respectively.As the barrier for chain growth is smaller than that for chain transfer and isomerization,linear olefins would be formed as primary products.Dehydrogenation and disproportionation of adsorbed olefins would not take place according to the high reaction barriers.The findings highlight the activity of Pd species on phosphorene for ethylene oligomerizations.