Ir-Catalysed Nitrous Oxide (N2O) Decomposition:Effect of Ir Particle Size and Metal–Support Interactions

The effect of the morphology of Ir particles supported on γ-Al2O3, 8 mol%Y2O3-stabilized ZrO2 (YSZ), 10 mol%Gd2O3-doped CeO2 (GDC) and 80 wt%Al2O3–10 wt%CeO2–10 wt%ZrO2 (ACZ) on their stability on oxidative conditions, the associated metal–support interactions and activity for catalytic decomposition of N2O has been studied. Supports with intermediate or high oxygen ion lability (GDC and ACZ) effectively stabilized Ir nanoparticles against sintering, in striking contrast to supports offering negligible or low oxygen ion lability (γ-Al2O3 and YSZ). Turnover frequency studies using size-controlled Ir particles showed strong structure sensitivity, de-N2O catalysis being favoured on large catalyst particles. Although metallic Ir showed some de-N2O activity, IrO2 was more active, possibly present as a superficial overlayer on the iridium particles under reaction conditions. Support-induced turnover rate modifications, resulted from an effective double layer [Oδ−–δ+](Ir) on the surface of iridium nanoparticles, via O2− backspillover from the support, were significant in the case of GDC and ACZ

Formation of a Rhodium Surface Oxide Film in Rh n

The structure of small Rhn (n = 1-10) clusters and corresponding Rh-oxide (RhnOm) clusters (n = 1-4; m = 1-9) supported on a stoi¬chiometric CeO2(111) surface has been investigated using density functional theory correct¬ed for on-site Coulombic interactions (DFT+U) with the goal to identify a realistic model for Rh/CeO2-based CO oxidation catalysts. Rhn clusters on ceria prefer to adopt a three-dimensional morphology. The adsorption of oxygen leads to the reconstruction of such clusters into a two-dimensional Rh-oxide film. The stability of RhnOm species is determined by evaluating the reaction energy for the stepwise oxidation of Rhn, which is to be com¬pared with data for the experimental fresh catalysts. It is found that with increasing cluster size the surface oxide phase becomes increasingly stable against the isolated RhO3 form under oxidative conditions. The Helmholtz free energy change for RhnOm clusters with varying m was determined for the reduction by CO and oxidation by O2. In this way, it was found that Rh-oxide species are more stable than the corresponding pure Rh clusters when supported on CeO2(111). This suggests that the active site for CO oxidation is a Rh surface-oxide