Distinct reduction pathways of PdO/Al₂O₃ in the aqueous phase: formate vs. borohydride

Catalyst reduction is a key step in heterogeneously catalysed liquid-phase reactions for fine chemicals production and biomass upgrade. However, the liquid-phase reduction of supported PdO remains poorly understood at the molecular level, despite its widespread use in synthetic protocols. Here, we investigate the reduction of PdO/Al₂O₃, containing PdO nanoparticles with a diameter of ca. 2 nm, using two widely employed aqueous reducing agents: sodium formate (HCOONa) and sodium borohydride (NaBH₄). The formation of hydrides from the initial PdO phase was monitored in situ by X-ray total scattering and X-ray absorption spectroscopy. Phase fractions and dynamic structural features were evaluated using principal component analysis and multivariate curve resolution, Rietveld refinement, pair distribution function analysis and multiphase extended X-ray absorption fine structure analysis. The two reducing agents promote distinct reduction pathways: HCOONa induces the rapid and concurrent formation of Pd and PdH_x, whereas NaBH₄ promotes the development of metallic Pd prior to hydride formation. The final hydride phase exhibits a different degree of lattice expansion with respect to metallic Pd. The lattice expansion observed for the hydride formed with HCOONa is larger, pointing to more effective hydrogen intercalation. These insights provide a molecular-level understanding of how reductants shape structural reorganization in supported Pd catalysts, with implications for optimizing hydrogenation performance.