Radiolytically induced one-electron reduction of methylviologen in aqueous solution. Platinum-catalyzed formation of dihydrogen

The reaction of the methylviologen dication (MV2+) with radiolytically generated (CH3)2COH radicals in deaerated aqueous solution is rapid and quantitative, producing the methylviologen cation radical (MV+). At pH 1, MV+ decays via [MV2+]-dependent second-order kinetics in the course of minutes according to an H+-assisted disproportionation reaction that yields hydrogenated methylviologen as a final product; at pH 4.2, MV+ is more stable but does decay in the course of a day. Dihydrogen is a product of the radiolysis with G(H2) equaling the background yield of H2 from the primary radiolytic act and the scavenging of H atoms by 2-propanol. In the presence of poly(vinyl alcohol)-stabilized Pt sols (total [Pt] = 50 μM), MV+ decays in less than 1 s. G(H2) is at or near zero during the inital phases of the reaction when the disporportionation. Continued exposure causes G(H2) to rise above background level and reach a plateau; the yield of hydrogenation product decreases concomitantly and reaches a minimum level that is not negligible. The maximum efficiency of H2 production above background, relative to that of the same system in the absence of MV+, is approx. 79% at pH 1 and approx. 43% at pH 4.2; the minimum efficiency of hydrogenation, relative to the zero-dose limit, is approx. 10% at pH 1 and approx. 25% at pH 4.2. The implications of these results to the photochemical generation of H2 in the presence of Ru(bpy)32+, MV2+, and Pt are discussed