Stabilizer-Free CuIr Alloy Nanoparticle Catalysts

We present the direct synthesis of stabilizer-free Cu–Ir alloy nanoparticles (NPs) with tunable compositions. Cu and Ir are classically immiscible in the bulk. Therefore, the physical and catalytic properties of Cu–Ir alloys are largely unknown. A convenient microwave-assisted method utilizes readily available Cu2+ salts and IrCl3 in a modified polyol reaction; NaOH facilitates rapid solvent-assisted coreduction to yield CuxIr(100–x)NPs, where x can be varied within the approximate range 10–50. The as-synthesized NPs form stable dispersions of small (∼2 nm) and near-monodisperse cuboctahedra without the requirement of organic stabilizing (capping) agents, resulting in nanostructures that are directly amenable to heterogeneous catalysis. CuxIr(100–x)NPs supported on mesoporous Co3O4 show high selectivity toward the selective partial hydrogenation of crotonaldehyde to yield >40% crotyl alcohol in the vapor phase. The selectivity toward C═O versus C═C bond hydrogenation is largely insensitive to the Cu:Ir ratio but more Ir rich compositions are also more highly active. Density functional theory (DFT) studies explain this behavior in two ways: first the Co3O4 support enhances selective C═O bond activation of the crotonaldehyde feedstock via favorable support-reagent adsorption; second, increasingly Ir-rich CuIrNP surfaces provide more Ir- sites, resulting in higher hydrogenation activity.