Oxygen defect, electron transfer and photocatalytic activity of Ag/CeO₂/SBA-15 hybrid catalysts

Ag/CeO₂/SBA-15 hybrid photocatalysts were synthesized and their catalytic activities were investigated in the photodegradation of Congo red (CR) under different conditions. The results of X-ray diffraction (XRD) analysis, Rietveld refinement, X-ray photoelectron spectroscopy (XPS) analysis, and DRS-UV–vis characterization confirmed that oxygen defects were created in CeO₂ layer and its concentration decreased on increasing of ceria loading of the catalysts. Ag⁰/Ag⁺ and Ce³⁺/Ce⁴⁺ couples coexisted in the Ag/CeO₂/SBA-15 catalysts, resulting from the electrons transferring from Ag⁰ nanoparticles (AgNPs) to Ce⁴⁺ around the oxygen defects in CeO₂. The values of surface Ag⁰/Ag⁺ and Ce³⁺/Ce⁴⁺ ratios were correlated well with the variation of oxygen defect concentration and the photocatalytic activity. The catalysts with the lower CeO₂ content, i.e. 5–10 wt%, exhibited the better photocatalytic activity due to the formation of a greater number of oxygen defects and more electrons transferring between AgNPs and CeO₂. CR could be photodegraded with a higher conversion under acidic than basic condition, following the first order kinetics. A four-stage reaction mechanism consisting of the primary, secondary, aromatic ring opening, and mineralization steps was proposed. This work also confirms that the combination of computer simulation technique for the crystalline structure refinement with the catalytic experiments can gain new insights into the reaction mechanism and help to better understand the photocatalytic behaviors.