Superoxide generated by blue light photocatalysis of g-C₃N₄/TiO₂ for selective conversion of amines

Reactive oxygen species (ROS) are remarkably reactive chemical oxygen-containing molecules that not only occupy critical positions in cell signaling and homeostasis for regulating aerobic living organism's growth and development but also broadly participate in the environmental management as extraordinary oxidizing agents. Inspired by the behaviors of ROS, we designed an artful visible light photocatalytic system for the selective conversion of amines due to the activation of oxygen (O₂) to superoxide (O₂^-) over g-C₃N₄/TiO₂. Here, blue light was manipulated as a light source to circumvent the initiation of the strong nonselective hydroxyl radical (OH) that is often generated by valence band holes (h_vb⁺) of TiO₂. Aerial O₂ was employed to achieve the long-lived, exclusive ROS, O₂^-, while acetonitrile, an aprotic solvent, was utilized to prolong the lifetime of O₂^-. Importantly, the g-C₃N₄/TiO₂ possesses an exceptional capability for the generation of O₂^-. Based on the synergistic effect of two ingredients of the g-C₃N₄/TiO₂ photocatalyst, the highly selective conversion of amines was achieved with superior conversions in comparison with the pristine TiO₂ and g-C₃N₄. Furthermore, a mechanism dominated by O₂^- was proposed according to the kinetic studies, electron paramagnetic resonance (EPR), and ROS quenching experiments. This work highlights the importance of ROS in defining the desirable outcomes over semiconductor photocatalysts.