A facile preparation of highly dispersed ultrasmall CeO2 clusters on g-C3N4 decorated TiO2 for efficient photooxidation of Hg0

•The ultrasmall CeO2 clusters are highly dispersed on the g-C3N4/TiO2 surface.•DFT result showed CeO2 clusters were anchored by the cavity of tri-s-triazine ring.•The composite catalyst has excellent performance in photooxidation of Hg0.•The contribution of ·OH in the photooxidation of Hg0 is more pronounced than ·O2–.•The composite catalyst mechanism of action is a combination of type II and Z-scheme.