Structure-controlled sulfur poisoning and hydrogen-induced regeneration in single Pd nanoparticles probed by nanospectroscopy

Structural order and surface crystallinity of palladium nanoparticles (Pd NPs) significantly affect their chemical and mechanical stability, and impact their resistance to poisoning and efficiency in hydrogen (H₂) (de)sorption. This study investigates the impact of sulfuric acid (H₂SO₄) poisoning on pristine and annealed Pd NPs to identify the influence of structure on the density and stability of poisoners. Electron microscopy and diffraction analyses show that annealing transforms initially amorphous, rough, and aggregated particles into spherical and crystalline NPs with better-defined grain boundaries. Nanoscale AFM-IR analysis shows that pristine NPs rapidly decompose SO_x upon H₂ exposure, and SO_x is mainly located on the rim of the NP. Annealed NPs maintain stable, localized SO_x signatures across multiple N₂–H₂ cycles, and only after the second H₂ exposure cycle the SO_x is removed from the central part of the NP. Contact potential difference analysis shows the strong affinity of SO_x to the rim of the NP. Together, these results reveal that crystallinity and defect density dictate both the spatial distribution and chemical stability of sulfur species on Pd surfaces. Pristine, defect-rich NPs promote rapid H₂-induced SO_x reduction and selective desorption from their centers, whereas annealed, crystalline NPs stabilize SO_x more strongly and exhibit delayed desorption. This direct link between structural order, SO_x binding strength, and hydrogen (de)sorption affinity underscores the critical role of nanoscale morphology in controlling poisoning dynamics and highlights nanospectroscopy as a powerful tool for correlating local structure with chemical functionality.