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

Abstract

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.

Rapid Synthesis of Rhodium–Palladium Alloy Nanocatalysts

“…Alloying two or more metals has been shown to allow for fine and continuous tunability of binding energies between the NP surface and substrates. This type of approach can even lead to a substantial enhancement in overall catalytic activity in cases in which one metal is unable to effect the catalysis on its own (e.g., RhAu or RhAg) …”

“…This enables a unique heating profile that is well matched to NP nucleation and growth, in which it is possible to mimic high‐temperature chemistry in nontoxic solvents through so‐called “hotspot” generation, while avoiding the need for refluxing solvents . Additionally, relative to conventional heating, microwave‐assisted chemistry is more energy efficient and is suitable for continuous‐flow synthesis . As such, we are particularly interested to gain a better understanding of how synthesis time affects the resulting alloy NP structures and their catalytic reactivity, with the ultimate goal of minimizing synthesis time and concomitantly maximizing NP stability and reactivity.…”

Microfluidic Generation of Nanomaterials for Biomedical Applications

“…In addition, NPs can be easily modified by adding new reagents at any demanded time point during the reaction procedures . The single‐phase continuous flow strategy is also highly scalable, and the throughput can be dramatically increased by integrating multiple identical microchannels parallelly on the same lamina . Hydrodynamic focusing (HF) techniques are also categorized as a specific type of continuous flow microfluidics.…”

“…First, although the generation of uniform NMs by microfluidics is experiencing rapid development at the proof‐of‐concept stage, the translation of this technology from academic research to industrial and clinical practice still faces several critical challenges, and one of the greatest hurdles is the mass production. Several investigations on the scale‐up of microfluidic platforms have been reported, and parallelization is one of the most common strategies which can improve the quantity of NMs from microgram to kilogram scale . Another aspect to increase production rate is developing microfluidic devices that can withstand high‐pressure operation.…”

Continuous Flow Routes toward Designer Metal Nanocatalysts

“…Recently, microwave‐mediated radio frequency (RF) heating, instead of thermal addition through an oil bath, has been used to facilitate the chemistry for metal NP formation . Specifically, Rh NPs were synthesized ranging from cubo‐octahedra to branched multipods using a microwave heating unit through manipulating the phase (single‐phase or two‐phase), temperature, and/or residence time . The range of Rh NPs synthesized exhibit different sizes and facets that could lead to differences in catalytic performance.…”