Gold nanoparticles (Au NPs) have attracted broad interests for their activity towards many reactions of industrial and environmental importance. An example is supported Au NPs catalyzing low temperature CO oxidation. Their activity relies heavily on the NP size and is usually limited by the rate of O2 dissociation. Bimetallic PdAu catalysts are promising for CO oxidation since Pd is known to dissociate O2 and the activation mechanism have been studied in single crystal and single atom alloy prototypes. However, the study of PdAu NPs on high surface area supports under reaction conditions usually falls short of elucidating the role of Pd and Au, because of the limitations of most experimental techniques caused by heterogeneities, sintering of Au NPs and dynamic alloying effect of Pd and Au. Here we report the results of in-situ, temperature-dependent IR spectroscopy for tracking the CO chemisorption on Pd and Au in temperature range from 77 K to 673 K under various CO and O2 partial pressure. The samples were dilute Pd in Au NPs of ca. 6 nm in diameter (with narrow size and compositional distribution, as ascertained by STEM) supported on raspberry colloid-templated (RCT) silica. The local structure of the Pd environment in as-prepared catalysts was characterized by X-ray absorption spectroscopy (XAS) showing higly dispersed Pd atoms within Au NPs. Results of the in-situ IR measurements revealed that characteristic bands of CO species on Pd and Au shift under various reaction conditions, indicating CO-induced Pd migration between quasi-random alloy and surface-segregated locations. The IR frequencies as well as the chemisorption/desorption process were supported by DFT calculations. The study provides understandings of the role of Pd and Au in NPs and elucidates how adsorption- or reaction- induced segregation or restructuring of Pd in Pd-Au NPs affect their catalytic properties.

Tracking CO-induced migration of highly dispersed Pd in Au nanoparticles supported on raspberry colloid-templated silica

2019 Fall National Meeting

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