Metal nanoparticles with high catalytic activity in degradation of methyl orange: An electron relay effect

Gold, silver and platinum nanoparticles have been synthesized following a green approach by reducing the corresponding salt using tannic acid as reducing agent at room temperature in aqueous medium. The reaction is instantaneous and the average diameter of the particles formed is around 10 nm in all the three cases as measured by TEM. These nanoparticles have been used as a catalyst for the degradation of methyl orange in the presence of sodium borohydride (NaBH4). Silver nanoparticles have a drastic catalytic effect as compared to gold or platinum nanoparticles on the degradation of methyl orange in the presence of sodium borohydride. From the kinetic data it is concluded that the rate constant follows the order: k(Ag) (nanoparticles) >> k(Au nanoparticles) > k(pt nanoparticles) >> k(uncatalyzed reaction). The high catalytic effect of silver nanoparticles has been attributed to its low value of work function as compared to Au and Pt. The uncatalyzed reaction does not show any decrease in the absorbance value within the given experimental time due to the large kinetic barrier, i.e. high activation energy. Decrease in absorbance value for uncatalyzed reaction is observed after nearly 48 h that too at a very high concentration of reducing agent, thereby indicating that reaction is extremely slow and reduction of methyl orange is thermodynamically feasible. (C) 2010 Elsevier B.V. All rights reserved.