Kinetic\nEvaluation of Dendrimer-Encapsulated Palladium Nanoparticles in the 4‑Nitrophenol\nReduction Reaction

Abstract

The\nsynthesis of dendrimer-encapsulated palladium nanoparticles\nwith ratios of 13 and 55 metal atoms to templating dendrimer, (Pd₁₃- and Pd₅₅-DENs) was successfully demonstrated\nwith the use of hydroxyl-terminated generation 4 and 5 (G4 and G5)\npoly­(amidoamine) (PAMAM) dendrimers as both templating and stabilizing\nagents. These Pd-DENs catalysts were fully characterized using spectroscopic\ntechniques. High resolution transmission electron microscopy (HRTEM)\nwas used for the determination of particle size. The average particle\nsizes were found to be 1.33 ± 0.15 and 1.66 ± 0.20 nm in\ndiameters for Pd₁₃ and Pd₅₅-DENs, respectively.\nThese catalysts were evaluated using the widely utilized model reaction,\n4-nitrophenol (NP) reduction by sodium borohydride (NaBH₄). The experimentally determined kinetic data was modeled using the\nLangmuir–Hinshelwood equation which relates the apparent rate <i>k</i>_app, NP and BH₄^– adsorption constants, <i>K</i>_NP and <i>K</i>_BH₄^– respectively, the surface rate constant <i>k</i>, and the surface area, <i>S</i>. The behavior\nof the adsorption constants with increasing temperature was also investigated\nby varying the reaction temperature between 298 and 318 K. The Pd₁₃-DENs showed greater adsorption of NP and BH₄^– when compared to Pd₅₅-DENs. The overall\nresults showed that the Langmuir–Hinshelwood model can be successfully\nused for full kinetic analysis of NP reduction by BH₄^– in the presence of Pd_<i>n</i>-DENs catalysts.