Catalytic activity of palladium and gold dendrimer-encapsulated nanoparticles for methylene blue reduction: A kinetic analysis

A kinetic analysis of the catalytic reduction of methylene blue (MB) by sodium borohydride (BH₄) using colloidal dendrimer-encapsulated palladium (Pd) and gold (Au) nanoparticles (PdDENs and AuDENs) is presented. The nanoparticles were prepared by the template method using generation 4 (G4) and generation 5 (G5) hydroxyl-terminated poly (amido)amine (PAMAM-OH) dendrimers for PdDENs and amine-terminated poly (amido)amine (PAMAM-NH₂) for AuDENs in aqueous solution. The catalysts were characterized by UV/vis spectrophotometry and transmission electron microscopy (TEM). The sizes of the particles obtained ranged from 1.3 to 2.3 nm with volume normalized surface areas between 0.02 and 0.06 m² L^-1. The reduction of MB by BH₄^- was monitored by UV/vis spectrophotometry using the stopped-flow technique and the kinetic data obtained was modeled to the Langmuir-Hinshelwood mechanism. The apparent rate constant for the reduction is related to the surface area S of the nanoparticles, the adsorption constants of MB (K_MB) and BH₄^- (KB H4) as well as to the kinetic constant k which is related to the rate-determining step of the reaction. The kinetic constant k was largest for PdDENs with the largest volume normalized surface area of 0.06 m² L^-1. K_MB values were 10 times larger than those of KB^H4- in the case of PdDENs. The activation energy was lowest for the most catalytic active PdDENs at 34.5 ± 1.5 kJ mol^-1 as compared to 71.3 ± 1.6 kJ mol^-1, for the least active AuDEN catalyst.