Enhancing Stability of Surface Au under Oxidizing Conditions through Reduced Bulk Au Content

Contrary to the common assumption that a higher bulk content of precious metals facilitates the preservation of more surface noble metal by serving as a reservoir for surface enrichment, we demonstrate that a lower bulk content of Au results in a more stable arrangement of Au atoms at the surface of Cu–Au nanoparticles when exposed to an O₂ atmosphere. Using ambient pressure X-ray photoelectron spectroscopy, we investigate the surface segregation and oxidation behavior of Cu–Au nanoparticles across various compositions. Our results reveal that in Au-rich nanoparticles exposed to an H₂ atmosphere, surface segregation prompts the formation of a continuous Au-enriched shell, which subsequently oxidizes into a complete CuO_x shell upon transitioning to an O₂ atmosphere. Conversely, in Au-poor nanoparticles during H₂ treatment, segregation results in the emergence of Au clusters embedded within the surface layer, persisting upon exposure to O₂. This unexpected phenomenon shows that reducing the bulk content of precious metals can enhance the surface stability of noble atoms under oxidizing conditions, as further demonstrated by comparing the catalytic performance of Cu–Au nanoparticles with varying Au bulk contents in CO oxidation.