Co2+/Co3+ redox reaction in Co-doped Bi4Ti3O12 for enhancing photocatalytic NO removal: Interplay of superoxide radicals and in-situ DRIFTS study

Removing low-concentration NO in the air by means of photocatalytic technology is a promising approach. Herein, our work pins down the mechanism of cobalt metal doping within the lattice of Bi4Ti3O12 (BTO) for the photocatalytic NO conversion. As a result, the Co-BTO exhibits a NO conversion efficiency of 58.3 %, which is substantially higher than that of BTO (30 %). More interestingly, the Co-BTO also shows a lower production of intermediate NO2 than that of the BTO. These findings demonstrate that within Co-BTO, both Co2+ and Co3 + coexist. The redox cycle between these two cobalt species engenders active sites possessing strong reducing capabilities for O2 which can be adsorbed and activated at the Co sites, thereby generating superoxide radicals (center dot O2-). Furthermore, the doping of a certain portion of cobalt broadened light absorption and enhanced the separation efficiency of photogenerated carriers, further facilitating the activation of O2. Lastly, electron spin resonance (ESR) and in-situ diffuse reflective infrared Fourier transform spectroscopy (in-situ DRIFTS) were respectively utilized to further clarify the active species involved in the photocatalytic oxidation of NO and its adsorption and conversion pathways from a mechanistic standpoint. Overall, this work represents a novel endeavor for the advancement of visible-light-driven photocatalysts usage for environmental remediation against NO pollution.