The formation of gold nanoparticles in the first 2–20 ms of the reaction was studiedin situwith XAS using microfluidics.

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 ≥ Au x Pd y /TiO2 ≥ Pd/TiO2. Hence, the physical and chemical properties of these catalysts can be fine-tuned.

Metal oxide supported bimetallic AuPd nanoparticles (NPs) are known to exhibit significantly enhanced activity and selectivity in numerous reactions compared to their monometallic counterparts. An atomic-level understanding of the nature of AuPd nanoalloys is among the most important and challenging topics in catalysis and nanoscience. Here, colloidal monometallic Au and Pd as well as bimetallic AuxPdy NPs (~1 nm) with different Au:Pd ratios were synthesized in a continuous microfluidic reactor and then deposited on TiO2. The structural, electronic and reactive properties of AuxPdy/TiO2 were first investigated by a multi-technique approach including scanning transmission electron microscopy, energy dispersive X-ray spectroscopy mapping, in situ X-ray absorption spectroscopy, FTIR spectroscopy, and X-ray photoelectron spectroscopy. Temperature-dependent IR spectroscopy using CO as a probe molecule provided deeper and solid evidence for the presence of a variety of active sites on the surface of monometallic Au and Pd NPs and AuxPdy nanoalloys. The results demonstrated consistently strong electronic interactions between Au and Pd upon alloying, leading to an interatomic charge transfer and electronic modifications in the d bands of Au and Pd. The AuPd/TiO2 sample with an Au:Pd ratio of 3:7 exhibited the highest catalytic activity in CO oxidation compared to the other alloys. This was attributed to a synergistic effect where the activation of dioxygen is facilitated at the Pd-enriched sites while both bimetallic Au and Pd sites chemisorb CO. Hence, the combination of microfluidic synthesis and advanced characterization including FTIR allowed deeper insights into the nature of AuPd nanoalloys for catalytic applications.

Monometallic Au and Pd nanoparticles (NPs) and homogeneous AuPd nanoalloy particles were synthesized in a continuous flow of reactants (HAuCl4, K2PdCl4, NaBH4 and polyvinylpyrrolidone (PVP)) using a microfluidic reactor with efficient micromixers. The obtained ultrasmall NPs were subsequently deposited onto SnO2 supports with different surface area (32.7 and 3.6 m 2 g-1). Samples with 1.0 and 0.1 wt.% metal loading were prepared. After calcination at 380 °C for 1 h the supported NPs aggregated to some extent. SnO2 supported AuPd nanoalloys with low (0.1 wt.%) metal loadings showed the smallest NP diameters (~ 5-7 nm) and the narrowest size distribution among the samples. The gas sensing performance of the materials was investigated at 300 °C in four different gas atmospheres containing either CO, CH4, ethanol or toluene using dry and humid conditions. They exhibited a distinct variation in the response patterns and selectivity toward the test gases depending on composition and metal loading: Au increased the sensor signals compared to pristine SnO2 in all cases and decreased the interference of water vapor; the supported Pd NPs showed a weak response to toluene, strong sensitivity in CO sensing and slightly better response in ethanol sensing in humid air compared to dry air. However, they showed a high selectivity toward CH4 when used in dry air; AuPd alloy particles provided lower sensor signals compared to pristine SnO2 and no remarkable CH4 selectivity, in contrast to the Pd system. Operando diffuse reflectance infrared Fourier-transformed spectroscopy (DRIFTS) indicates a strong band bending in the case of Pd and AuPd NPs, whereas in the case of Au no band bending occured, indicating a strong electronic interaction between the support and Pd-containing NPs (Fermi-level control mechanism), and a weak electronic interaction between SnO2 and Au NPs (spill-over mechanism).

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