Trends in the CO oxidation and PROX performances of the platinum-group metals supported on ceria

PGM-CeO2 (PGM = Pt, Pd, Ir, Rh, Ru) catalysts were prepared by one-step solution combustion synthesis (SCS) and characterized by elemental analysis, N-2 volumetry, aberration-corrected HRTEM, X-ray diffraction, and CO-DRIFTS. The samples, consisting of 2-6 nm metal nanoparticles supported on mesoporous ceria, were tested in CO oxidation in absence and presence of hydrogen (PROX). To our knowledge, this work presents the first comparison of all the platinum-group metals (except Os) for these reactions in the same conditions. The as-prepared SCS catalysts are active in CO oxidation and a reducing treatment has no significant effect on their performances. While the best catalyst in H-2-free CO oxidation is Rh-CeO2, the addition of a high hydrogen excess decreases the Rh catalyst activity but enhances the CO oxidation rate on all other systems, including alumina-supported metals employed as reference catalysts. The resulting PROX turnover frequencies (Pt > Pd > Rh > Ir > Ru) follow the trends predicted by published density-functional-theory calculations considering the COad + O-ad elementary reaction as the rate-determining step. Pt-CeO2 is not only the most active but also the most selective catalyst, reaching near 100% CeO2 at low temperature (ca. 100 degrees C). The alumina-supported catalysts appeared less active than their ceria-supported counterparts in both reactions. The effect of heating/cooling cycles on the reaction kinetics was also investigated. Whereas the Pt, Pd and Ir ceria-supported catalysts were stable throughout PROX cycles, Rh and Ru ones exhibited apparently chaotic behaviors above ca. 200 degrees C, which are proposed to be induced by favorable CO dissociation and methanation pathways and/or variable metal oxidation states. (C) 2015 Elsevier B.V. All rights reserved.