Influence of synthesis route on the properties of doped lanthanum cobaltite and its performance as an electrochemical reactor for the partial oxidation of natural gas

Abstract

La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) perovskite powders have been synthesised by solid-state reaction, co-precipitation, drip pyrolysis and citrate gel routes, and characterised using XRF, XRD, SEM and BET. Co-precipitation using oxalic acid or aqueous ammonia as precipitant failed to achieve the correct chemical composition. Perovskite structures were achieved in all other cases. Surface areas ranging from 0.6 to 17.4 m² g⁻¹ were obtained, which was reflected in the different microstructures observed. The citrate gel product exhibited a convoluted network morphology resulting in its large surface area. Thin-walled (∼200 µm) tubular membranes have been manufactured from the LSCF powders using viscous plastic processing. The tubes have been characterised using a custom-built gas analysis rig with on-line mass spectrometry. Porosity levels of the membranes were found to be very low (<0.1%). The spontaneous oxygen flux across the tubular membranes was determined as a function of temperature. Oxygen permeation rates ranged from 0.1 to 0.3 µmol cm⁻² s⁻¹ at 1273 K. The catalytic behaviour of the LSCF tubes towards methane oxidation has been studied using temperature programmed reaction and conventional catalytic measurements. The tubes favoured combustion reactions, with smaller amounts of partial oxidation and oxidative coupling products observed. Powder coatings have been incorporated to establish the effect of increasing surface area.