Effect of Bi2O3 content on characteristics of Bi2O3–GDC systems for direct methane oxidation

Ceramic systems of Bi₂O₃ and gadolinia-doped ceria (GDC) solid mixture were prepared as catalysts for direct methane oxidation. These systems were characterized by temperature-programmed reduction using hydrogen and carbon monoxide, temperature-programmed reaction of methane, fixed-temperature direct methane oxidation, and X-ray diffraction analysis. Adding Bi₂O₃ to GDC promotes both hydrogen and CO oxidation activities, because of the presence of surface Bi₂O₃ and the high content of mobile oxygen in Bi₂O₃. The reactivity of CO with surface lattice oxygen is enhanced to a higher extent than that of H₂, and this enhanced extent shows a maximum in Bi₂O₃ content. Such a maximum also exists for the catalytic activity of direct methane oxidation. A synergistic effect occurs due to a combination of the high methane reactivity of GDC and the high content of mobile oxygen in Bi₂O₃. The CO₂ selectivity of direct methane oxidation can be modulated by varying the Bi₂O₃ content. The mixing of Bi₂O₃ with GDC also increases the self-de-coking capability of the catalyst during direct methane oxidation, which stabilizes the activity.