Kinetic evaluation of dendrimer-encapsulated palladium nanoparticles in the 4-nitrophenol reduction reaction

The synthesis of dendrimer-encapsulated palladium nanoparticles with ratios of 13 and 55 metal atoms to templating dendrimer, (Pd₁₃- and Pd ₅₅-DENs) was successfully demonstrated with the use of hydroxyl-terminated generation 4 and 5 (G4 and G5) poly(amidoamine) (PAMAM) dendrimers as both templating and stabilizing agents. These Pd-DENs catalysts were fully characterized using spectroscopic techniques. High resolution transmission electron microscopy (HRTEM) was used for the determination of particle size. The average particle sizes were found to be 1.33 ± 0.15 and 1.66 ± 0.20 nm in diameters for Pd₁₃ and Pd ₅₅-DENs, respectively. These catalysts were evaluated using the widely utilized model reaction, 4-nitrophenol (NP) reduction by sodium borohydride (NaBH₄). The experimentally determined kinetic data was modeled using the Langmuir-Hinshelwood equation which relates the apparent rate k_app, NP and BH₄^- adsorption constants, K _NP and K_BH4- respectively, the surface rate constant k, and the surface area, S. The behavior of the adsorption constants with increasing temperature was also investigated by 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 results showed that the Langmuir-Hinshelwood model can be successfully used for full kinetic analysis of NP reduction by BH₄^- in the presence of Pd_n-DENs catalysts.