Evidence for the catalytic oxidation of La2NiO4+delta

The redox behaviour of lanthanum nickelate La2NiO4+delta (LNO) has been studied by in situ high temperature X-ray diffraction. Higher order Ruddlesden-Popper phases, La3Ni2O7-delta and La(4)Ni(3)O10(-delta), 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 degrees C and above, with significant formation of the higher order Ruddlesden-Popper phases at temperatures greater than 800 degrees 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.