Active Ni and Fe species on catalysts Ni/Al₂O₃ and NiFe/Al₂O₃ for the oxidative dehydrogenation (ODH) of ethane to ethylene assisted by CO₂

Ni/Al₂O₃ and NiFe/Al₂O₃ catalysts were synthesized by incipient wetness impregnation and tested for oxidative dehydrogenation (ODH) of ethane to ethylene. The effect of Ni loading and Fe as a promoter for bimetallic catalysts were characterized by X-ray diffraction (XRD), Raman spectroscopy, ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), scanning electron microscopy (SEM), and temperature-programmed reduction (H₂-TPR). The catalyst with 10 wt% Ni and 6.6 wt% Fe (NiFe32) displayed the highest ODH activity. XRD results showed smaller crystalline NiO (˂ 7 nm) for bimetallic catalysts than the monometallic ones. Raman results depicted NiO_x species in the monometallic catalysts. The bimetallic catalysts exhibited spinel-type structures as NiFe₂O₄ and FeAl₂O₄ related to more active species in ODH of ethane. DRS results revealed that the higher ODH activity for bimetallic catalysts is related to a predominance of Ni and Fe in tetrahedral symmetry (T_d) than octahedral symmetry (O_h). These suggest that tetrahedral symmetry species were more effective than octahedral symmetry. TPR profiles showed that the iron addition increased the nickel species with strong interaction with the support. This rearrangement of Ni and Fe species in octahedral and tetrahedral sites of the NiO and alumina lattice could explain the shift reduction temperatures for Ni species and the formation of the spinel-type structure in the bimetallic catalysts. A relation between the metal-support interaction and the selectivity toward ethylene was found. A higher strong metal-support interaction favors a better catalytic activity.