Kinetics and Mechanisms of the Reaction of PH2O2− with O2, Photocatalyzed by [Ru(bpy)3]2+* (bpy = 2,2′-bipyridine), Forming Peroxodiphosphate Ions in an Aqueous Solution

Abstract

The formation reaction of peroxodiphosphate (denoted by PDP and meaning P2O84−, HP2O83− etc.) by the reaction of molecular oxygen (O2) with the phosphinate ion (PH2O2−) was extremely accelerated by irradiation with visible light of aqueous solutions containing the tris(2,2′-bipyridine)ruthenium(II) ion ([Ru(bpy)3]2+). The [Ru(bpy)3]2+ acted as a photocatalyst during the reaction. The reaction mechanism consists of a chain reaction being accompanied by the O2-quenching of a photoexcited ruthenium(II) complex ([Ru(bpy)3]2+*) to make the superoxide (O2−•) and [Ru(bpy)3]3+, followed by the reduction of [Ru(bpy)3]3+ by PH2O2− to make [Ru(bpy)3]2+ and PH2O2•. However, such PDP once formed in the presence of O2 began to decrease with a rate law of -d[PDP]/dt = kobsd[PDP] after the solution was saturated with N2 gas, and when the solution was again saturated with air, the PDP began to increase again with light irradiation and remained constant in the dark. Such behaviors are discussed while presenting the reaction mechanisms.