Theoretical Study On The Promoters Modified Cu-Al Spinel Surfaces

Cu-based catalysts have been extensively studied in the methanol steam reforming hydrogen production(MSR)system.Among them,the spinel catalyst system has excellent performance in related catalytic reactions,especially its stability is better than traditional supported catalysts.Among the reported copper-based spinel catalysts,CuAl2O4 spinel catalysts have higher catalytic activity.In this paper,the method of density functional theory(DFT)calculation is used to study the mechanism of the promoters to the spinel catalyst surface structure.The DFT method was used to calculate the adsorption of Nin(n=1-4)on the perfect and oxygen-defective CuAl2O4 spinel surface.The results show that one single Ni atom adsorption on the surface of perfect CuAl2O4(100)is exothermic by-5.30 eV.Nin(n=1-4)adsorbed on the surface of oxygen-defective CuAl2O4(100)is less stable that on the perfect surface,but the adsorption energy on the surface of oxygen-defective CuAl2O4(110)is close to that on the perfect surface.Bader charge and partial density of states(PDOS)analysis shows that the adsorption of Ni on the surface of CuAl2O4 is accompanied by a charge transfer from the metal to the support.The growth and aggregations capabilities of Nin clusters follow the order:gas phase>?-Al2O3(110)>CuAl2O4(110)>CuAl2O4(100).This result can give a reasonable explanation for the experimental phenomenon that Ni supported on CuAl2O4 spinel has better stability than ?-Al2O3.The transition metal(Fe,Co and Ni)substitution of CuAl2O4 spinel A1 and Cu sites are thermodynamically feasible.It has been found that Ni is more likely to be doped on octahedral sites in spinel surface as well as in bulk,while Fe and Co prefer to locate in the tetrahedral sites of the spinel in the surface layers,vs.in the octahedral sites in spinel bulk.Bader charge and partial density of states(PDOS)analysis reveal that Fe,Co and Ni substitution for Cu and Al site of CuAl2O4 spinel surface is accompanied by charge transfer.The order of charge transfer from Fe,Co and Ni to the spinel surface capability is Fe>Co>Ni.In addition,the(100)surface of CuAl2O4 spinel has stronger ability than the(110)surface to get electrons from Fe,Co and Ni.As indicated by PDOS,new peaks emerged above the Fermi level due to the empty orbitals of the positively charged Fe,Co and Ni resulting that the doped CuAl2O4 has narrower band gap than that of CuAl2O4 spinel surface.After M(Fe,Co and Ni)substitution of CuAl2O4 spinel,the by-product of CO adsorption on the he active site of Cu,M and O.The most stable adsorption of CO on the surface of M-substituted CuAl2O4 spinel(110)is bonding with lattice oxygen to form a similar CO3 structure,and a higher degree of oxidation on the surface of metal-substituted oxygen defects(110).The most stable adsorption site for CO adsorption on the complete(100)surface substituted by Fe is the two-coordinated Cu site,but the most stable adsorption of CO on the complete(100)surface substituted by Co and Ni is the formation of a similar CO2 structure,which promote CO oxidation.The adsorption of CO on the oxygen defects(100)is the two-coordinated Cu sites on the surface,which is not conducive to CO oxidation.