Electron spin resonance studies on the mechanism of the formation of p-benzosemiquinone anion over manganese dioxide

The formation of p-benzosemiquinone anion radical in a hydroquinone aqueous solution over manganese dioxide has been studied by e.s.r. spectroscopy. The rate of the formation of the radical strictly obeyed equation (i) d[SA]/dt = k(b[HQ]₀ - [SA])(c[MnO₂]₀ - [SA]) (i) where k, b, and c are constants. [MnO₂]₀ is the MnO₂ weight: liquid volume ratio, and [HQ]₀ and [SA] are the initial concentrations of hydroquinone and semiquinone anion radical, respectively. The following reaction mechanism is proposed. The surface species MnO ₂* abstracts a hydrogen atom from hydroquinone to form a neutral semiquinone radical which desorbs into the liquid phase and is converted into semiquinone anion radical, while the surface species MnO ₂* is deactivated and reduced to Mn(OH)₂*. The number of surface species, MnO₂*, is determined as 7.7 x 10¹⁸ m^-2.