Chemical Nature of Microfluidically Synthesized AuPd Nanoalloys Supported on TiO₂

Abstract

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 multitechnique 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.