Effects of inter/intralayer adsorption and direct/indirect reaction on photo-removal of pollutants by layered g-C₃N₄ and BiOBr

Abstract

Photocatalysis is an effective process to utilize solar energy for contaminants degradation, and the catalytic performance depends on the intrinsic structural properties of catalysts and organic molecules as well as their interactions. This work innovatively reveals that layer-structured graphitic carbon nitride (GCN) and BiOBr exhibit distinctly different adsorption and photocatalytic behaviors and that they show respective advantages in catalytic removals of Congo red (CR) and ibuprofen (IBP). The kinetic rate constant of CR removal by GCN was about 1.3 times higher than that by BiOBr, while BiOBr has a superior photocatalytic degradation performance towards IBP with a 25-fold-higher kinetic rate constant than that of GCN. Experiments including temperature-controlled adsorption and degradation as well as mechanism discussion confirmed that IBP and CR could be strongly adsorbed in the surface intralayer of BiOBr and GCN, through unique coordination interaction, while CR adsorption on BiOBr was dominated by interlayer adsorption. Thermodynamically, we examine that these layered catalysts can induce different reactive species according to their unique valence band and conduction band locations and then trigger different oxidation pathways for degradation reaction, as confirmed by the quenching experiments. Depending on the dissociated superoxide (O2•−) radical induced indirect oxidation pathway, GCN has higher kinetics for CR degradation than BiOBr. In contrast, BiOBr offers better photocatalytic performance for IBP degradation through the direct oxidation pathway of photogenerated holes (h+). BiOCl, with an analogous structure to BiOBr, also exhibits a similar behavior to BiOBr. This work provides a new perspective for understanding the different behaviors of layer-structured materials in selective oxidation of pollutants.