Catalytic oxidation over lanthanum-transition metal perovskite materials

The thrust of this work is to follow the defect chemistry of the simple LaCoO₃ system in an attempt to probe if there is a relationship between the defect chemistry and the activity of this perovskite-type material to catalytic methane combustion. A simple flow-through reactor has been used to study the combustion of methane between room temperature and 1100°C. Using a gel-type precipitation method it has been proved possible to synthesise a single phase perovskite material with a bulk La:Co metal atom ratio of 1:1.1. PXRD, Rietveld analysis and density measurements show that the perovskite phase is non-stoichiometric with a deficiency of lanthanum ions in the lattice. This material is rather ineffective as a catalytic material. A more active form can be prepared at a La:Co metal atom ratio of 1:1 when a mixed phase (perovskite/lanthana) is produced. This material exhibits both higher activity to methane combustion and the storage/evolution of oxygen (as measured using DSC and TPD techniques).

The results of activity tests have been rationalised using XPS where large amounts of O⁻ species are seen at the surface. It is proposed that these ions occupy anion vacancies created to compensate for the reduced cation charge in the lattice. This is not possible for the single phase material where vacancies are compensated for by the presence of valency changes of cobalt and/or oxygen.

Further work has been carried by doping of the perovskite with cations of valencies +2, +3 and +4 in an attempt to control the non-stoichiometry. In this way, it has been proved possible to provide simple synthetic routes to active methane combustion catalysts.