Density Functional Theory Studies on Selective Hydrogenation of Acetylene to Ethylene Over Ni-based Intermetallic Catalysts
Author:Rao De Ming
Semi-hydrogenation of acetylene to ethylene is the most commonly used method in ethylene polymerization industry to remove the acetylene impurity in the ethylene feedstock.Today,the mainly used Pd-based catalysts for acetylene hydrogenation still suffer from the drawbacks of scanty resource and high cost.Therefore,design and research of highly selective and active non-noble metal catalysts to substitute noble metal catalysts is a subject of great theoretical and practical application value.Recently,Ni-based intermetallic compounds such as Ni-Ga and Ni-Sn used as catalysts have exhibited high reaction activity and selectivity in semi-hydrogenation of acetylene,but the mechanism of hydrogenation and the origin of the selectivity are not clear.This restricts a further development of Ni-based intermetallic catalysts.Against this problem,based on density functional theory(DFT),in this dissertation,the mechanism of hydrogenation of acetylene on the surface of Ni-based intermetallic compounds was studied,including desorption barrier,hydrogenation barrier of ethylene and geometric effects,which clarifies the origins of ethylene selectivity.This provides theoretical information and useful instruction for Ni-based intermetallic compounds on selective hydrogenation of acetylene.The main research contents and conclusions of this work are as follows:(1)The hydrogenation reaction of acetylene on the surface of pure Ni(111)and Ni-Ga intermetallic compounds with different Ni/Ga ratios,Ni3Ga(111)and NiGa(111),were calculated,including the adsorption of the intermediates,the charge transfer mechanism and the reaction mechanism.The results indicate that the adsorption energy.of acetylene enhances along with the increase of Ni/Ga ratio.Bader charge analysis shows that electrons transfer from Ga atoms to Ni atoms and further deliver to acetylene when acetylene adsorbs on the surfaces.The effective barrier(Eaeff)and the barrier difference(△Ea)between ethylene hydrogenation and ethylene desorption were also calculated to describe the reaction activity and selectivity to ethylene,respectively.The ethylene pathway of acetylene hydrogenation is facile for its low effective barrier.NiGa(111)is more active and selective than the other surfaces due to its lower Eaeff and higher △Ea Therefore,NiGa(111)surface turns out to be the most favorable reaction facet for semi-hydrogenation of acetylene among the calculated surfaces.(2)DFT studies have been carried out to investigate the mechanism of acetylene hydrogenation(including the adsorption energies and hydrogenation barriers)on six surfaces of Ni-Sn IMCs(Ni3Sn(111),Ni3Sn(001),Ni3Sn2(101),Ni3Sn2(001),Ni3Sn2(101)-2 and Ni3Sn2(001)-2 surfaces),and the geometric effects towards ethylene selectivity was investigated.Two key parameters(adsorption energy(Ead)and hydrogenation barrier of ethylene(Ea,hydr)),which determine the ethylene selectivity,were studied quantitatively to reveal the origin of ethylene selectivity.It was found that.the adsorption strength significantly depends on site ensemble:The better the Ni active site is isolated by the inert Sn,the easier it is to promote the desorption of ethylene.However,the surface roughness mainly dominates the hydrogenation barrier of ethylene.Either a low or high roughness decreases the interactions between H and C2H4(Eint),resulting in an enhanced energy barrier for over-hydrogenation of C2H4(Ea,hydr).From this point of view,according to the selectivity denoted as △Ea = Ea,hydr-E,des,Ni3Sn(001)and Ni3Sn2(101)surfaces with well-isolated Ni ensembles and low(or high)surface roughness exhibit decreased Ea,des and increased Ea,hydr,giving rise to excellent selectivity to ethylene.Electronic effect studies show that the presence of subsurface C atoms weakens the adsorption of C2 species on the surface of Ni3Sn(111),and also promotes the over-hydrogenation of C2H4,which leads to a decrease of ethylene selectivity.(3)DFT calculations on the hydrogenation reaction of acetylene on the surfaces of Ni3In(111)and Niln(110)intermetallic compounds have been carried out.Compared with the ethylene pathway,it was found that the effective,energy barrier(Eaeff)over Niln(110)surface is lower than that over Ni3In(111)surface;while the difference between hydrogenation energy barrier and desorption energy barrier of ethylene(△Ea)over Niln(110)surface is lower than that over Ni3In(111)surface,indicating that the Niln(110)surface has better activity and selectivity than Ni3In(111)surface at the same time.Comparing the hydrogenation process on Ni3In(111)surface with those on Ni3Ga(111)and Ni3Sn(111)surfaces in the previous studies,it was found that both the activity and selectivity curves of Ni3M(111)(M = Ga,Sn,In)are linearly related to the adsorption energy of acetylene,and Ni3In(111)surface possess high activity and selectivity,simultaneously.These conclusions are consistent with the experimental report well.This work provides theoretical understandings for the mechanism of acetylene hydrogenation reaction,reactivity,and the origin of ethylene selectivity over intermetallic compounds,which would give useful instruction for the design and preparation of high-performance intermetallic catalysts for substitution of noble-metal catalysts.