Abstract
The most important model catalytic reaction to test the catalytic activity of metal nanoparticles is the reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride as it can be precisely monitored by UV–vis spectroscopy with high accuracy. This work presents the catalytic reduction of 4-nitrophenol (4-Nip) to 4-aminophenol (4-Amp) in the presence of Pd nanoparticles and sodium borohydride as reductants in water. We first evaluate the kinetics using classical pseudo first-order kinetics. We report the effects of different initial 4-Nip and NaBH4 concentrations, reaction temperatures, and mass of Pd nanoparticles used for catalytic reduction. The thermodynamic parameters (activation energy, enthalpy, and entropy) were also determined. Results show that the kinetics are highly dependent on the reactant ratio and that pseudo first-order simplification is not always fit to describe the kinetics of the reaction. Assuming that all steps of this reaction proceed only on the surface of Pd nanoparticles, we applied a Langmuir−Hinshelwood model to describe the kinetics of the reaction. Experimental data of the decay rate of 4-nitrophenol were successfully fitted to the theoretical values obtained from the Langmuir–Hinshelwood model and all thermodynamic parameters, the true rate constant k, as well as the adsorption constants of 4-Nip, and BH4− (K4-Nip and KBH4−) were determined for each temperature.
Highlights
The world of nanoscience involving metallic nanoparticles has been the subject of numerous scientific and technological research studies during recent years
The art of synthesizing metallic nanoparticles (mNPs) and nanoalloys in a large range with full control of different shapes and sizes has resulted in widespread applications in engineering, electronics, and catalysis [1]. mNPs such as palladium, platinum, and gold have already shown their potential in catalysis of several specific reactions, such as hydrogenation, water splitting, and cross-coupling reactions [2]
Menumerov showed that the induction period was linked to the level of dissolved oxygen contained in the medium [31]. He determined that the induction period ended once the level of dissolved oxygen fell below a critical value and that this level was dependent upon the metal nanoparticles used as a catalyst
Summary
The world of nanoscience involving metallic nanoparticles (mNPs) has been the subject of numerous scientific and technological research studies during recent years. 4-Nitrophenol is a significant side product obtained from pharmaceutical industries, synthetic dyes, herbicides, and pesticides. The reduction of 4-nitrophenol by sodium borohydride is a model reaction to evaluate the catalytic activity of mNPs and to study the reaction kinetics. The presence of isosbestic points in the UV−vis absorption spectra at different times provides clear proof that 4-nitrophenol is completely reduced to 4-aminophenol with no side products generated. This reaction proceeds under mild conditions (at room temperature in water as a solvent). The apparent rate constant kapp is determined in terms of pseudo first-order kinetics
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