Kinetics and mechanistics of reaction between silver (I) and hexachloroiridate(IV) in aqueous acidic media: Evidences of formation of binuclear intermediate complex and Ir(0) nanoparticles with orientation on electron-transfer process

Abstract

Spectrophotometric techniques has been applied for studying the kinetics and mechanistics between Ag+ and [IrCl6]2− reaction in aqueous acidic solutions at a constant ionic strength of 1.0 moldm−3. The naked eyes observation showed that the brownish color of [IrCl6]2− solution was rapidly vanishing with simultaneous appearance of a new dark-blue color on mixing solution of the oxidant with Ag+ ion electrolyte in either neutral or acidic media. The new color was persisted for a few seconds or minutes depending on the reaction conditions. Then, it began to fading out with time elapsing. This result means that the reaction occurs through two distinct stages. The spectral traces indicated the formation of binuclear intermediate complex at the initial fast stage [Ag+…ClIrCl52−] (with the rate constants k1 = 1.15 × 10−2 dm 3 mol−1 s−1; k−1 = 7.35 × 10−5 s−1, formation constant (K) = 1.56 × 102 dm 3 mol−1 and pseudo first- order rate constant (kobs) = 1.31 × 10−4 s−1 at [IrCl6]2− = 2.2 × 10−4, [H+] = 1.0 mol dm−3 and 30 °C. The initial rapid part was followed by a subsequent slow stage corresponding to the decomposition of the formed binuclear intermediate in the rate-determining step to give rise to the final oxidation products. The formation of such binuclear intermediate was found to be of acid-independent; nature; whereas its decomposition was dependent on the acid concentration with inverse fractional first-order in [H+]. This means that the binuclear decomposition is of acid-inhibition nature. In case of presence of a large excess of [Ag+] over that of [IrCl6]2−, Ir(0) nanoparticles was formed as confirmed by the XRD-spectra and TEM morphology. The kinetic parameters for the formation constants and decomposition rate constants of the binuclear complex have been evaluated and a suitable reaction mechanism for the overall redox reaction is suggested and discussed.