Characterization of precursors and reactivity of LaNi1-xCoxO3 for the partial oxidation of methane

Perovskite type oxides LaNi1-xCoxO3 were prepared by thermal decomposition of amorphous citrate precursors followed by annealing at 1073 K in air atmosphere. These systems were characterized by X-ray diffraction (XRD), temperature-programmed reduction, temperature programmed desorption, specific surface area measurements and photoelectron spectroscopy. All the samples exhibited a single perovskite phase as revealed by XRD and rather low specific surface areas (0.8-2.2 m(2)/g). The reduction behaviour and the extent of oxygen desorption were found to depend on the substitution degree (x). They were tested in the partial oxidation of methane (POM) using both continuous flow and pulse experiments. Both CH4 conversion and CO and H-2 selectivities were found to increase with the reaction temperature. However, selectivities strongly depended on x. Particularly, for LaCoO3 (x = 1) the catalyst became inactive for the POM reaction. As revealed by cycling experiments, thermal effects on the catalyst bed appeared also important in CH4 pulse experiments over the LaNi1-xCoxO3 samples indicated that both surface and lattice oxygen species are involved in the oxidation reaction producing CO2 and CO besides H2O and H-2. Pulsing results point out to a mechanism in which CH4 reacts first with lattice oxygen forming H2O and CO2, which subsequently react with excess CH4 to yield CO and H-2. (c) 2005 Elsevier B.V. All rights reserved.