Abstract
The early reported pseudo-first-order reaction kinetics of the polymer-supported metallic nanocatalysts for the model reaction of p-nitrophenol (p-NP)/NaBH4 were probably oversimplified. Here a detailed study of p-NP reduction by NaBH4 in the presence of the raspberry-like poly(allylamine hydrochloride)-modified polymer poly(glycidyl methacrylate) composite sub-microspheres with tunable gold nanoparticles (PGMA@PAH@AuNPs) was presented. Effects of polyelectrolyte concentration, the ratio of polymer spheres to gold nanoparticles, and the solution pH value for composite synthesis on the induction period, reaction time, average reaction rate and average turnover frequency were systematically investigated. Experimental results in all cases of our study revealed an n(th) order (n > 1) of the p-NP/NaBH4 catalytic reaction by the prepared polymer composite particles. The apparent order of reaction, n, is dependent on the total surface area of the coated gold nanoparticles on the polymer spheres, which can be closely correlated with the tunable gold nanoparticle surface coverage. The mechanism of the observed catalytic activity enhancement was proposed based on active epoxy groups of the polymer spheres and a large adsorption of p-nitrophenolate anions onto the positively-charged spheres.
Highlights
Catalytic reduction of p-nitrophenol by borohydride ions (BH4À) in the presence of a metal catalyst has become one of the model reactions for evaluating the catalytic activity of noble metallic nanoparticles.[1,2,3,4,5,6,7,8,9,10,11,12] the reaction is thermodynamically feasible, it is kinetically restricted in the absence of a catalyst because the kinetic barrier between the mutually repelling negative ions p-NP and BH4À is very high.[13]
In the kinetic studies of the as-prepared composite catalyst, we found that the catalytic reduction of p-nitrophenol by metallic nanoparticles coated on poly(glycidyl methacrylate) (PGMA) spheres does not obey the rst-order reaction law, in which the reaction rate does not depend linearly on the concentration of p-NP
The catalytic reduction of 0.125 mmol p-nitrophenol to p-aminophenol (p-AP) in the presence of an excess amount of NaBH4 was chosen to investigate the catalytic activity of PGMA@poly(allylamine hydrochloride) (PAH)@AuNP composites with mild stirring at ambient conditions
Summary
Metal nanoparticles are important catalysts for oil re ning and petrochemical industries, energy conversion and vehicle exhaust gas removal, since they can catalyze many important transformations including oxidation of hydrocarbons, C–C cross coupling, electron-transfer reaction, and hydrogenation– dehydrogenation.[17,18,19,20,21] Research into noble metal nanoparticles as the catalyst has remained an active topic Their high surface energy due to the large surface-to-volume ratio and high collision frequency associated with their greater mobility cause serious stability problems, such as a tendency to aggregate, changes in shape and damage to their surface states during catalytic reactions, and eventual loss of their initial activity and selectivity. The apparent reaction order, n, was correlated with the surface coverage of gold nanoparticles coated on the PGMA spheres
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