Surface reactivity of LaCoO3 and Ru/LaCoO3 towards CO, CO2 and C3H8:Effect of H2 and O2 pretreatments

摘要

The differences in surface reactivity of LaCoO3 and Ru/LaCoO3 solids after pre-treatment in a hydrogen or oxygen gas atmosphere towards oxidative steam reforming (OSR) of propane was probed by performing temperature-programmed desorption (TPD) of CO and CO2, temperature-programmed surface reaction (TPSR) of C3H8, transient isothermal oxidation of "carbon" formed after TPSR of C3H8, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) studies. The TPD of CO and CO2 studies revealed a greater adsorption of these molecular product species of the OSR of propane on the surface of LaCoO3 than Ru/LaCoO3 solid, while XPS studies performed after reduction in hydrogen revealed a higher concentration of Co~(2+) in the LaCoO3 than Ru/LaCoO3 solid catalyst composition. The Ru/LaCoO3 solid was found to exhibit after an induction period enhanced reactivity towards H2 production for the OSR of propane reaction compared to LaCoO3 after hydrogen reduction at 750 °C These results led to the conclusion that the introduction of a small amount of Ru (0.8 wt%) on lanthanum cobaltite surface leads to a higher concentration of metallic cobalt (Co~0) after hydrogen reduction is performed, thus favouring the enhancement of necessary active catalytic sites on the Co, LaCoO3 and La2O3 surfaces formed. TPSR of propane used as model compound of diesel revealed the participation of surface lattice oxygen of both LaCoO3 and Ru/LaCoO3 pre-oxidized solids as well as the formation of hydrogen by reaction of propane on the Co~0 (metallic state) surface once the latter is formed. A greater reactivity of propane during TPSR was observed over the Ru/LaCoO3 solid surface for either oxidative or reductive pre-treatments. Also, a lower amount (2.13mmol/g) of "carbon" deposition was found on Ru/LaCoO3 compared to LaCoO3 (3.65 mmol/g) at the end of the TPSR of propane experiment. These results provide important fundamental information on the role of Ru in the Ru/LaCoO3 system towards catalytic oxidative steam reforming of propane.