This study investigates the structural and chemical changes in Rh-Pd and Pt-Pd core-shell nanoparticles during oxidizing, reducing, and catalytic reactions involving NO, O₂, CO, and H₂, using in situ X-ray photoelectron spectroscopy (XPS) at ambient pressures. The Rh-Pd nanoparticles undergo significant and reversible changes in composition and chemical state depending on the reaction conditions. Under oxidizing conditions, Rh atoms segregate to the shell, while under reducing conditions, Pd atoms diffuse to the shell. In contrast, Pt-Pd nanoparticles show no significant segregation of Pd or Pt atoms. These differences highlight the structural flexibility and tunability of bimetallic nanoparticle catalysts during catalytic reactions. The research demonstrates that bimetallic nanoparticle catalysts can undergo profound structural and chemical changes in response to reactive environments at ambient pressures. This was achieved using an ambient pressure XPS (APXPS) apparatus, which allows for X-ray photoelectron spectra to be obtained under relatively high gas pressures. The study analyzed the structure and composition of Rh-Pd and Pt-Pd nanoparticles during catalytic reactions in different gas environments, revealing the unique structural flexibility of nanoparticles and the interplay of structure and reactivity. Rh-Pd and Pt-Pd nanoparticles with diameters of 15 ± 2 nm were synthesized using colloidal chemistry methods and characterized by TEM and XRD. The nanoparticles were deposited on silicon wafers to form model catalysts for XPS studies. The mean free paths of photoelectrons excited at different X-ray energies were approximately 0.7, 1.0, and 1.6 nm. The structure of Pt-Pd nanoparticles was studied at various photon energies, with similar mean free paths for the generated photoelectrons. The study found that the atomic fractions of Rh and Pd in Rh-Pd nanoparticles change significantly in response to changes in reactant gas composition. In contrast, Pt-Pd nanoparticles showed a more stable structure. The observed changes in atomic distribution and chemical state are reversible and depend on the composition of the surrounding reactive gases. The restructuring behavior of Rh-Pd nanoparticles can be explained by the surface energy differences between Rh and Pd, with Pd having lower surface energy and being more reactive than Rh. The opposite segregation behavior of Rh and Pd under oxidizing and reducing conditions is attributed to their different surface energies and oxidation stabilities. The findings suggest that the combination of tunable colloid chemistry-based synthesis and controlled structural engineering using reactive gases offers a new approach for designing new catalysts and shaping catalytic properties of nanomaterials. The study highlights the potential for "smart" catalysts whose structures change advantageously depending on the reaction environment.This study investigates the structural and chemical changes in Rh-Pd and Pt-Pd core-shell nanoparticles during oxidizing, reducing, and catalytic reactions involving NO, O₂, CO, and H₂, using in situ X-ray photoelectron spectroscopy (XPS) at ambient pressures. The Rh-Pd nanoparticles undergo significant and reversible changes in composition and chemical state depending on the reaction conditions. Under oxidizing conditions, Rh atoms segregate to the shell, while under reducing conditions, Pd atoms diffuse to the shell. In contrast, Pt-Pd nanoparticles show no significant segregation of Pd or Pt atoms. These differences highlight the structural flexibility and tunability of bimetallic nanoparticle catalysts during catalytic reactions. The research demonstrates that bimetallic nanoparticle catalysts can undergo profound structural and chemical changes in response to reactive environments at ambient pressures. This was achieved using an ambient pressure XPS (APXPS) apparatus, which allows for X-ray photoelectron spectra to be obtained under relatively high gas pressures. The study analyzed the structure and composition of Rh-Pd and Pt-Pd nanoparticles during catalytic reactions in different gas environments, revealing the unique structural flexibility of nanoparticles and the interplay of structure and reactivity. Rh-Pd and Pt-Pd nanoparticles with diameters of 15 ± 2 nm were synthesized using colloidal chemistry methods and characterized by TEM and XRD. The nanoparticles were deposited on silicon wafers to form model catalysts for XPS studies. The mean free paths of photoelectrons excited at different X-ray energies were approximately 0.7, 1.0, and 1.6 nm. The structure of Pt-Pd nanoparticles was studied at various photon energies, with similar mean free paths for the generated photoelectrons. The study found that the atomic fractions of Rh and Pd in Rh-Pd nanoparticles change significantly in response to changes in reactant gas composition. In contrast, Pt-Pd nanoparticles showed a more stable structure. The observed changes in atomic distribution and chemical state are reversible and depend on the composition of the surrounding reactive gases. The restructuring behavior of Rh-Pd nanoparticles can be explained by the surface energy differences between Rh and Pd, with Pd having lower surface energy and being more reactive than Rh. The opposite segregation behavior of Rh and Pd under oxidizing and reducing conditions is attributed to their different surface energies and oxidation stabilities. The findings suggest that the combination of tunable colloid chemistry-based synthesis and controlled structural engineering using reactive gases offers a new approach for designing new catalysts and shaping catalytic properties of nanomaterials. The study highlights the potential for "smart" catalysts whose structures change advantageously depending on the reaction environment.