Ni–Nb-Based Mixed Oxides Precursors for the Dry Reforming of Methane

Abstract

Perovskite-related mixed-oxides based on La Ni Nb and La Sr Ni Nb were synthesized by the auto combustion method to use as precursors materials for the catalytic reforming of methane at 700 ºC, atmospheric pressure, CH₄:CO₂ = 1:1. LaNiO₃ and LaNbO₄ were used as reference. XRD analysis show that the synthesis method produce a new series of precursor family formed by a mixture of oxides where Ni crystallized as part of a perovskite and Ruddlesden–Popper structure while Nb formed lanthanum orthoniobate LaNbO₄, a scheelite-type structure alternating with oxide layers, with phase distribution depending on niobium content. For Nb (x ≤ 0.3) Ni crystallizes as LaNiO₃ perovskite-type oxide while for Nb (x ≥ 0.7) it forms mainly the orthoniobate phase LaNbO₄ a scheelite-type structure. At higher calcined temperatures (~1100 °C) La₂Ni_0.8Nb_0.2O₄ was formed with a Ruddlesden–Popper structure consisting of three perovskite type layers along the c-axis alternating with a layer of the rock salt type phase. TEM analysis showed the presence of cubic particles with sizes varying between 5 and 60 nm depending on the extent of substitution of Ni by Nb. Reduction of the perovskite-related precursor oxides produced a series of Ni⁰/La₂O₃–NbOx oxides with high metallic dispersion which favors the activity and stability of the catalysts. Introduction of doping quantities of Sr into LaNi_0.8Nb_0.2O_3±λ structure produced a mixture of oxides with Sr dissolved in the lanthanum orthoniobate LaNbO₄ scheelite-type structure due to the similarity of ionic radii of La and Sr. Under the reaction conditions conversions near the thermodynamic equilibrium were attained which remains for long periods of time assessing the stability of the synthesized catalysts.