Continuous Flow Synthesis of Rh and RhAg Alloy Nanoparticle Catalysts Enables Scalable Production and Improved Morphological Control

The pursuit of scalable methods for the preparation of well-defined metallic nanoparticles (MNPs) is addressed in this work via a novel microwave-assisted continuous flow synthesis technique. It is shown that single- and two-phase flow synthesis methods provide access to morphologically well-defined and near-monodisperse RhNPs. The RhNPs can be prepared in shorter reaction times and at lower temperatures than are commonly required in conventional batch reactions. Under single-phase flow conditions, in which Rh(III) is reduced in ethylene glycol, near-monodisperse cuboctahedral RhNPs are obtained; the average NP size can be controlled as a function of the residence time of the reactant stream within the microwave cavity. In contrast, a two-phase microfluidic droplet flow method leads to the highly selective formation of Rh multipods. When compared to cuboctahedral RhNPs of comparable size, the Rh multipods are found to exhibit significantly higher catalytic activity in the vapor-phase hydrogenation of cyclohexene. The potential versatility of this new two-phase flow method coupled with microwave-assisted heating is further demonstrated in the synthesis of well-defined isotropic NPs comprised of classically immiscible RhAg random alloys.