Kinetics of iridium reduction by hydrogen in hydrochloric acid solution

Abstract

The effects of stirring speed, initial iridium ion concentration, hydrogen pressure, hydrochloric acid, chloride concentrations, seeding, and temperature on the reduction rate of iridium ion by hydrogen have been investigated. The whole process of reduction occurs in a complicated heterogeneous stage. The reduced metallic Ir serves itself as a catalyst for the reduction. The rate in the earlier stage of the reduction (including the nucleation period) is controlled by chemical reaction; the apparent activation energy obtained in the temperature range of 363 to 423 K was 76.1 kJ/mol. The order of the reaction with respect to the iridium ion concentration and the hydrogen partial pressure was found to be one, and the reaction rate increased with the increase of HC1 and Cl concentrations. The rate in the later stage was controlled by diffusion, and the apparent activation energy was reduced to 25.5 kJ/mol. The reaction mechanism has been given by the analysis of experimental results.