Morphology-controlled synthesis of gold nanoparticles with chitosan for catalytic reduction of nitrophenol

Gold nanoparticles (AuNPs) were synthesized using chitosan as both reducing agent and stabilizer and charac-terized by UV-vis spectra and transmission electron microscope (TEM). Synthesized AuNPs were used in the reduction of nitrophenol to aminophenol in the presence of NaBH4. The experimental parameters such as the dosage of catalyst, NaBH4/4-nitrophenol molar ratio, temperature, AuNPs obtained under different synthetic conditions were investigated in detail. The reaction rate constants were calculated through the pseudo-first-order kinetic equation. The results indicated that the morphology of AuNPs was controllable just by adjusting the concentration, molecular weight (MW) and degree of deacetylation (DD) of chitosan. The catalytic activities of as-prepared AuNPs was dependent on their morphology and the nature of the reducing agent and stabilizer. Triangular AuNPs synthesized by 0.1% (w/v) chitosan with low DD (63.1%) showed the highest reaction rate constant 0.7050 min-1, which was followed by spherical and small AuNPs (around 10 nm) synthesized by 0.01% (w/v) chitosan and irregular and large AuNPs with wide size distribution synthesized by 0.3% (w/v) chitosan. The values of thermodynamic parameters including activation energy, enthalpy change, entropy change and Gibbs free energy change for the activation were calculated and revealed that the activation of catalytic process was non-spontaneous and endothermic. A comparative study for the catalytic reduction of three kinds of nitrophenols showed that the catalytic activity order of AuNPs was 3-nitrophenol > 2-nitrophenol > 4-nitrophenol.