Microwave-Assisted Synthesis of Classically Immiscible Ag–Ir Alloy Nanoparticle Catalysts

Abstract

We present the synthesis of Ag–Ir alloys in the form of solid-solution nanoparticles (NPs). Ag and Ir are classically immiscible in the bulk and therefore the physical properties of Ag–Ir alloys are unknown. A convenient microwave-assisted, solution-phase method that employs readily available Ag(NO3) and IrCl3 precursors enables the preparation of small (2.5–5.5 nm) Ag–IrNPs with alloyed structures. AgxIr(100–x)NPs can be obtained by this method between x = 6–31. The Ag–IrNPs resist dealloying upon heating up to 300 °C. Ir-rich Ag–IrNPs dispersed on amorphous silica are significantly more active gas-phase alkene hydrogenation catalysts than pure IrNPs. Density functional theory (DFT) and theoretical modeling studies reveal that the Ag–IrNPs—which are consistently larger than monometallic IrNPs prepared under the same conditions—have comparatively fewer strong H-binding edge sites. This promotes faster H atom transfer to coadsorbed alkenes. Ag–IrNPs supported on amorphous Co3O4 show a linear composition dependence in the selective hydrogenation of C═O versus C═C bonds: more Ag-rich Ag–IrNPs are more selective toward C═O hydrogenation of the α,β-unsaturated aldehyde crotonaldehyde, yielding the industrially desirable crotyl alcohol. Furthermore, deposition of Ag–IrNPs inside Co3O4 mesopores results in an additional ∼56% selectivity enhancement.