Microfluidic Synthesis of Ultrasmall AuPd Nanoparticles with a Homogeneously Mixed Alloy Structure in Fast Continuous Flow for Catalytic Applications

Abstract

Ultrasmall Au, Pd, and AuPd nanoparticles (NPs) stabilized by PVP were prepared in a microfluidic reactor with cyclone micromixers for rapid mixing of reactants. In this system, pulsation-free flow of reactants was achieved at a total flow rate of 2.6 L h–1. A rapid homogeneous mixing within 2 ms was obtained with three cyclone micromixers. Controlled NP nucleation and growth occur in a following meandering microchannel. The resulting colloidal NPs were characterized thoroughly by various complementary techniques, e.g., high-resolution transmission electron microscopy (HRTEM), energy-dispersive X-ray spectroscopy (EDX), and ultraviolet–visible spectroscopy. The average NP diameter was about 1 nm with a narrow size distribution, and electron microscopy showed homogeneously alloyed NPs. Moreover, the particles were supported on TiO2 for catalytic tests and further structural characterization. Electron microscopy showed a uniform distribution of NPs on the support with some aggregation. X-ray absorption spectroscopy (XAS) confirmed the formation of well-mixed AuPd alloys in NP cores with Pd-rich surfaces. Finally, 1 wt % metal-loaded supports showed catalytic activities in CO oxidation in the following order: Au/TiO2 ≥ AuxPdy/TiO2 ≥ Pd/TiO2. Hence, the physical and chemical properties of these catalysts can be fine-tuned.