Sinter-Resistant Catalysts: Supported Iridium Nanoclusters with Intrinsically Limited Sizes

Abstract

Results from a wide selection of literature sources reviewed here show that treatments of iridium complexes on various supports under harsh reductive conditions (e.g., 873 K in H2) lead to the formation of only uniform iridium clusters limited to a critical diameter of ~1 nm. The observations have been explained by the results of calculations at the level of density functional theory ory showing that cubic structures of this size are resistant to aggregation because coalescence of two such clusters would require energetically unfavorable rearrangements of their surface atoms, and this point has been reinforced by scanning transmission electron microscopy images demonstrating the non-coalescence behavior of iridium clusters of the critical size—which instead bounce off each other. Here we consider supported iridium catalysts in light of the literature, aiming to (1) demonstrate the generality of the sinter-resistant property of iridium nanoclusters (in contrast to those of other noble metals) and (2) summarize information regarding sample synthesis and preparation methods that lead to supported iridium catalyst with sinter-resistant properties.