Solution combustion synthesis of noble metal-loaded ceria catalysts and application to hydrogen production and purification for fuel cells
Résumé
Mesoporous ceria powders doped with up to 2 wt% platinum-group metals (Pt, Pd, Ir, Rh, Ru) were synthesized in one step by the ambient air combustion of an aqueous solution of ceric ammonium nitrate, chloride or nitrate metal precursor, and glycine or oxalyl dihydrazide used as fuels. The structural properties of the powders, and the influence of such parameters as metal loading and thermochemical post-treatments, were investigated combining aberration-corrected HRTEM, SEM, in situ XRD, XPS, DRIFTS, and Raman spectroscopy. The materials, whose texture appeared spongy at the micrometer scale and depended on the fuel nature, exhibited ca. 30 nm-sized ceria crystallites with a layered structure at the nanoscale. Comparisons with pure ceria showed that the presence of the metal inhibited ceria grain coarsening. The powders were successfully employed as catalysts for the production of hydrogen from the steam reforming of methane (SRM) in water-deficient conditions, and for the purification of hydrogen through the preferential oxidation of CO (PROX). For SRM, 0.1 wt% Ir-CeO2 exhibited the best performances. Due to its higher Ir dispersion and stronger Ir-CeO2 interaction, the combustion-synthesized material was more active and stable than its conventionally prepared counterpart. Moreover, it was not permanently deactivated by the introduction of H2S in the reactant feed. After reducing treatments, Ir nanoparticles anchored at the surface of ceria grains were imaged, and their size (ca. 2 nm) and morphology did not evolve upon further heating at up to 900 °C. A complete picture of the Ir-CeO2 interface could be established, with the presence of Irx+-O2--Ce3+ entities along with oxygen vacancies. For CO oxidation and PROX, systematic comparisons between the samples, which exhibited similar metal nanoparticle sizes, allowed us to rank the Pt-group metals. Rh-CeO2 appeared as the most active system in H2-free CO oxidation. The presence of H2 boosted the CO oxidation activity of all catalysts, except that of Rh-CeO2, which promoted the decomposition of CO and the subsequent formation of methane. Pt-CeO2, which was the most active and selective PROX catalyst, was further investigated by changing the nature of the fuel and the metal precursor. Although the catalyst activities were influenced by such parameters, the selectivities were strikingly unaffected.