Evidence for the catalytic oxidation of La2NiO4+δ

Abstract

The redox behaviour of lanthanum nickelate La₂NiO₄₊δ (LNO) has been studied by in situhigh temperature X-ray diffraction. Higher order Ruddlesden–Popper phases, La₃Ni₂O_7−δ and La₄Ni₃O_10−δ, rapidly form on heating LNO in contact with platinum. These oxidation products were not formed when an alumina barrier layer was placed between LNO and platinum over the same heating regime. Platinum catalysis of the oxidation of LNO occurs at 800 °C and above, with significant formation of the higher order Ruddlesden–Popper phases at temperatures greater than 800 °C. It is proposed that the mechanism of formation of the higher oxidation state phases occurs by reduction of molecular oxygen in the gas phase catalysed by platinum to form O²⁻ ions, which are then incorporated into bulk LNO. The incorporation of these additional O²⁻ ions is compensated for by an increase in the nickel oxidation state, leading to formation of the higher order Ruddlesden–Popper phases.