Reactions of colloidal platinum in aqueous solutions containing methyl viologen, its cation radical, and hydrogen, studied by pulse radiolysis

The reactions of methyl viologen ions, MV²⁺ and MV⁺, and molecular hydrogen in the presence of colloidal Pt were investigated by pulse radiolysis. In the pH range 1.5-4 all the reducing species in the system were converted into molecular hydrogen. At pH 10, practically all the reducing species produced MV⁺, while in the intermediate pH range (6-8) the predominant species were Pt particles loaded with hydrogen in the form of hydride ions. The kinetics of reactions of the Pt colloidal particles with MV⁺ and H₂ were investigated. The rate of reaction of MV⁺ with the Pt was first order in MV⁺ and second order in Pt. When the reaction of MV²⁺ with loaded Pt was also important, the overall order with respect to Pt was less than 2. A loading mechanism involving particle-particle collisions is examined in relation to the effects of pH, [Pt], and repetitive pulses.