The study investigates the structural and compositional changes in core-shell Rh$_{0.5}$Pd$_{0.5}$ and Pt$_{0.5}$Pd$_{0.5}$ nanoparticles during reactions involving NO, O$_2$, CO, and H$_2$. Using X-ray photoelectron spectroscopy (XPS) at ambient pressures, the researchers found that Rh$_{0.5}$Pd$_{0.5}$ nanoparticles exhibit significant and reversible changes in composition and chemical state under oxidizing or reducing conditions. Rh atoms segregate to the shell in oxidizing conditions, while Pd atoms diffuse to the shell in reducing atmospheres. In contrast, Pt$_{0.5}$Pd$_{0.5}$ nanoparticles show no significant segregation of Pd or Pt atoms. These findings highlight the flexibility and tunability of bimetallic nanoparticle catalysts during catalytic reactions, suggesting potential for developing "smart" catalysts with enhanced activity and selectivity. The study also demonstrates the use of ambient pressure XPS to study the structural and chemical changes in nanoparticles under various reaction conditions.The study investigates the structural and compositional changes in core-shell Rh$_{0.5}$Pd$_{0.5}$ and Pt$_{0.5}$Pd$_{0.5}$ nanoparticles during reactions involving NO, O$_2$, CO, and H$_2$. Using X-ray photoelectron spectroscopy (XPS) at ambient pressures, the researchers found that Rh$_{0.5}$Pd$_{0.5}$ nanoparticles exhibit significant and reversible changes in composition and chemical state under oxidizing or reducing conditions. Rh atoms segregate to the shell in oxidizing conditions, while Pd atoms diffuse to the shell in reducing atmospheres. In contrast, Pt$_{0.5}$Pd$_{0.5}$ nanoparticles show no significant segregation of Pd or Pt atoms. These findings highlight the flexibility and tunability of bimetallic nanoparticle catalysts during catalytic reactions, suggesting potential for developing "smart" catalysts with enhanced activity and selectivity. The study also demonstrates the use of ambient pressure XPS to study the structural and chemical changes in nanoparticles under various reaction conditions.