Gold nano particles catalyzed oxidation of hydrazine by a metallo-superoxide complex: experimental evidences for surface activity of gold nano particles

Abstract

Gelatin-capped gold nano particles (GNPs) of diameter 23, 28 and 36 nm were prepared and characterized as almost monodispersed, near-spherical solids. In acidic media, these GNPs at their very low concentration level (∼10⁻¹³ M) catalyze the oxidation of hydrazine by the metallo-superoxide, [(NH₃)₄Co^III(μ-NH₂,μ-O₂)Co^III(NH₃)₄](NO₃)₄ (1). In the presence of a large excess of hydrazine over [1], the catalyzed oxidation is first-order in [1], [GNPs] and media alkalinity. The pure first-order dependence implies that the size as well as the nature of the catalyst remained unchanged during the reaction. The catalytic efficacies increased with increased total surface area of the GNPs. Increasing T_Hydrazine (T_Hydrazine is the analytical concentration of hydrazine) tends to saturate the first-order rate constant (k_o) for hydrazine oxidation and a plot of 1/k_oversus T_Hydrazine was found to be linear at a particular [GNPs], indicating the GNPs assisted deprotonation of N₂H₅⁺ to N₂H₄. The rate constants show a non-linear behavior with temperature studied in the range 288–308 K. At a lower temperature interval, viz. 288–298 K, k_o increases with increasing temperature whereas at temperature interval, viz. 303–308 K, k_o decreases with temperature. Such a variation indicates the important process of absorption and desorption of the reactants on and from the surface. A plausible mechanism for the GNPs catalyzed oxidation of hydrazine is suggested.