| Ethylene is the basic raw material for industrial production of polymers and fine chemicals;it becomes an important mark to evaluate the production level of petrochemical industry in a country.The thermal cracking of petroleum is the main method to obtain ethylene in the industry,however,it inevitably involves the formation of a small amount of acetylene(0.1~1%)that causes an irreversible damage of the catalysts used in the process of ethylene polymerization.Thus,the content of acetylene must be reduced to below 1 ppm to meet the requirements of ethylene polymerization.Acetylene selective hydrogenation to ethylene has been widely used as an effective method to solve this problem on the precious metal Pd-based catalysts,which results in the double effects of acetylene removal and a higher ethylene production.However,precious metal Pd-based catalysts have low selectivity and high costs;thus,developing a non-precious metal catalyst with high-performance has become a research priority.Although non-precious Cu catalysts present poor selectivity and activity at low temperature and only be added to the precious metal catalysts as an assistant,it can act as an active components in the process of acetylene selective hydrogenation at high temperature.In the current work,taking the acetylene selective hydrogenation on Cu catalysts as the research project,the effects of Cu catalyst valence state,surface structures and surface alloying on the catalytic performance have been systematically investigated by means of density functional theory calculations.For Cu catalysts with different valence state,using Cu,Cu2 O and CuO represents Cu(0),Cu(I)and Cu(II)catalysts.For the surface structures of Cu catalysts,using different low-index surfaces of Cu,Cu2 O and CuO,(111),(110)and(100)represents different surface structures,in which the corresponding perfect and defective surfaces have been considered;For metal-doped Cu catalysts,surface alloying of four metal(Ni,Pd,Pt and Au)is considered.Firstly,the adsorption and dissociation of H2 and H2 O over different Cu O(111)surfaces,including the perfect,defective,pre-covered oxygen surfaces,have been explored to illuminate the effect of surface structures on the first key step of acetylene selective hydrogenation;then,acetylene selective hydrogenation on three different valence state of Cu(0),Cu(I)and Cu(II)catalysts with different surface structures(perfect,defective)have been systematically explored to clarify the effect of Cu catalysts valence state and surface structures on reaction;further,acetylene selective hydrogenation on Cu(0)/Cu(I)bi-component catalyst have been investigated to illustrate the effect of Cu component on reaction;Finally,Ni,Pd,Pt,Au alloyed Cu(0)and Cu(I)catalysts have been considered to illuminate the effect of surface alloying on reaction.Based on the above research,we further obtain the effect of Cu valence state,surface structures,component and surface alloying on catalytic performance of acetylene selective hydrogenation to ethylene and provide an important theoretical foundation for designing a novel Cu catalyst with high performance.The main conclusions are as follows:(1)The surface structures of CuO(111)surface present structure-sensitivity to the adsorption and dissociation of H2 and H2 O.a)H2 is the dissociative adsorption on the perfect surface to form a H2 O molecule with surface lattice oxygen,which ultimately contributes to the formation of oxygen-vacancy surface.Due to the decrease of surface oxygen as the active center,H2 mainly exists in the form of molecular adsorption on the oxygen-vacancy surface.H2 dominantly exists in the form of the dissociative adsorption on the pre-covered oxygen surface.b)For the initial dissociation of H2 O molecule(H2O?OH+H),the oxygen-vacancy surface exhibits a stronger catalytic activity than the perfect surface,the pre-covered oxygen surface is the most favorable both thermodynamically and kinetically than other two surfaces;OH species is difficult to be dissociated into H and O on the perfect surface,for the oxygen-vacancy and pre-covered surfaces,the presence of H atom(produced from H2 O dissociation)can promote OH dissociation.OH species is the dominant product of H2 O dissociation on different Cu O(111)surfaces.(2)Cu valence state and surface structures can affect the selectivity and activity towards ethylene formation in acetylene selective hydrogenation.a)Both the selectivity and activity of ethylene formation on Cu catalysts are completely different along with Cu valence state change of Cu(II)?Cu(I)?Cu(0),Cu(II),Cu(I)and Cu(0)catalysts all present better selectivity and activity towards ethylene formation,in which Cu(I)catalysts have the highest ethylene selectivity while Cu(0)catalysts present the highest activity.b)For Cu(II)catalysts,the perfect CuO surfaces present a poor ethylene selectivity;however,the defective CuO(111)surface has both high ethylene selectivity and activity.c)For Cu(I)catalysts,the perfect Cu2 O surfaces have a poor selectivity towards ethylene formation,whereas the defective Cu2O(111)and Cu2O(110)surfaces present a higher selectivity and activity,both surfaces have almost the same selectivity while the defective Cu2O(110)surface exhibits a relatively high catalytic activity.d)For Cu(0)catalysts,Cu(111)and Cu(211)surfaces possess a higher selectivity and activity towards ethylene formation,and the most stable Cu(111)surface presents a better selectivity and activity compared to Cu(211)surface.(3)The active component of Cu catalysts can affect the selectivity and activity towards ethylene formation in acetylene selective hydrogenation.Cu(0)/Cu(I)bi-component catalyst exhibits a very low catalytic activity towards ethylene formation compared to the single-component Cu(0)and Cu(I)catalysts,thus,Cu(0)/Cu(I)bi-component catalyst cannot be selected as an ideal catalyst applied to the reaction of acetylene selective hydrogenation,Cu catalysts should have only one valence state in the system of acetylene selective hydrogenation.(4)The surface alloying of Ni,Pd,Pt and Au on Cu catalysts can affect the selectivity and activity towards ethylene formation in acetylene selective hydrogenation.a)For different Ni,Pd,Pt,and Au-doped Cu(111)surfaces,the selectivity towards ethylene formation are in the following order: PdCu(111)>Cu(111)>PtCu(111)>NiCu(111)>Au Cu(111)>>Pd(111);the order of activity is PdCu(111)>PtCu(111)>>Pd(111)>Cu(111)>NiCu(111)>AuCu(111);PdCu(111)presents the highest selectivity and activity towards ethylene formation,which is also superior to the pure Cu(111)and Pd(111)surfaces,while AuCu(111)surface has the lowest catalytic activity.b)The alloying of Ni,Pd,Pt and Au metals on Cu(111)surface contributes to the change of surface atom d-band center,further affects the ethylene selectivity and activity in acetylene selective hydrogenation.According to two roughly volcano type of curves formed by selectivity and activity as a function of surface atom d-band center on different metal-doped Cu(111),pure Cu(111)and Pd(111)surfaces,the following conclusions can be obtained:Both AuCu(111)and Pd(111)surfaces present the lowest selectivity due to the lowest and highest d-band centers among different surfaces,respectively;PdCu(111)surface with the surface d-band center at medium site exhibits the highest selectivity and activity towards ethylene formation compared to other alloyed Cu(111)surfaces,as well as Cu(111)and Pd(111)surfaces.c)For Pd-doped the defective Cu2O(111)and Cu2O(110)surfaces,since Pd atoms are located at the defective sites,the catalytic performance of the alloyed Cu(I)surfaces are similar to that on the corresponding perfect surfaces,namely,Pd-doped Cu(I)catalysts present poor selectivity and activity towards ethylene formation.(5)The four main structural factors of Cu catalysts,including valence state,surface structures,component and surface alloying,have effects on the catalytic performance of acetylene selective hydrogenation to ethylene,acetylene can be removed completely with high selectivity and activity by regulating these factors,and further adjust their catalytic performance.The current theoretical work can provide the basic theoretical clues and methods for the modification and design of novel efficient Cu-based catalysts. |
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