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 Bi 4 Ti 3 O 12 (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 NO 2 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 (•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.