Ultrastable iridium-ceria nanopowders synthesized in one step by solution combustion for catalytic hydrogen production
Résumé
Mesoporous ceria loaded with 0.06-0.93 wt% iridium was synthesized in one step by the ambient air combustion of an aqueous solution of ceric ammonium nitrate, ammonium hexachloroiridate, and glycine fuel. The structural properties of the powders, and the influence of such parameters as Ir loading and thermochemical post-treatments, were investigated combining aberration-corrected HRTEM, SEM, in situ XRD, XPS, DRIFTS, and Raman spectroscopy. The materials, which appeared spongy at the micrometer scale, exhibited ca. 30 nm-sized ceria crystallites with a layered structure at the nanoscale. After reducing treatments, Ir nanoparticles anchored at the surface of ceria grains were identified, and their size (ca. 2 nm) did not evolve upon further heating at up to 900 degrees C. A detailed picture of the Ir-CeO2 interface could be established, with the presence of Irx(+)-O2--Ce3+ entities along with oxygen vacancies. The powders loaded with only 0.1 wt% Ir were successfully employed as catalysts for the production of hydrogen from methane and water in low-steam conditions at 750 degrees C. Due to their higher Ir dispersion and stronger Ir-CeO2 interaction, the combustion-synthesized materials outperformed their conventionally prepared counterparts in terms of activity and stability, making them promising as active catalytic layers for solid-oxide fuel cells integrating the gradual internal reforming concept.