XAFS Investigation of Monometallic Catalysts (Au, Pt, Pd, Ru, Cu) on Ionic Liquid-Functionalized SBA-15: Structural Characterization and Catalytic Hydrogenation of p -Nitrophenol

A series of monometallic nanoparticles (Au, Pt, Pd, Ru, Cu) were synthesized via the immobilization of corresponding metal chlorides onto imidazolium ionic liquid (IL)-functionalized SBA-15. The structure transformation of the immobilized metal chlorides (MCl x _IL-SBA-15) into metal nanoparticles (MNP_IL-SBA-15) was systematically investigated using XAFS analysis. Results revealed that the immobilized metal chlorides formed anionic complexes, electrostatically stabilized by imidazolium cations. This stabilization maintains spatial separation among the complexes, effectively preventing nanoparticle agglomeration upon reduction with NaBH 4 . The resulting metal nanoparticles exhibit element-dependent phase compositions: Au is fully metallic, Pd is predominantly metallic with a minor oxidized fraction, whereas Pt, Ru, and Cu display heterogeneous mixtures of metallic and oxidized species. The activity and stability of the nanoparticle catalysts were evaluated in the hydrogenation of p -nitrophenol (PNP) to p -aminophenol (PAP), with Pd exhibiting the highest activity, followed by Au, Pt, Ru, and Cu. Based on the EXAFS-derived phase compositions and the catalytic trends, the catalytically relevant surface of Pd is partially oxidized, while Pt, Ru, and Cu are dominated by oxidized species. These findings highlight the significant roles of particle size and oxidation state in governing catalytic behavior and provide key insights into the structural evolution of metal species within IL-functionalized SBA-15 derived from metal chloride precursors.