Structure-Controlled Sulfur Poisoning and Hydrogen-Induced Regeneration in Single Pd Nanoparticles Probed by Nanospectroscopy

Abstract

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 structure on deactivation by comparing sulfuric acid (H2SO4) poisoning on polycrystalline and defect-rich Pd NPs versus facetted Pd NPs, thus identifying the influence of structure on the density and stability of sulfur-based poisoners. Electron microscopy and diffraction analyses show that annealing transforms the initially rough, aggregated nanoparticles, composed of small crystallites, into spherical nanoparticles consisting of a few ordered crystalline domains with well-defined grain boundaries. Nanoscale Atomic Force Microscopy Infrared (AFM-IR) analysis show that pristine NPs rapidly decompose SOx upon H2 exposure, and SOx is mainly located on the rim of the NP. Annealed NPs maintain stable, localized SOx signatures across multiple N2-H2 cycles, and only after the second H2 exposure cycle the SOx is removed from the central part of the NP. Contact potential difference analysis show the strong affinity of SOx 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 SOx reduction and desorption along with on-surface diffusion of SOx species from their centers to the rim. Annealed, crystalline NPs stabilize SOx more strongly and exhibit delayed desorption. This direct link between structural order, SOx 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.