(A) Proposed reaction scheme for the photocatalytic oxidation of formaldehyde on the pure TiO₂: superoxide radicals as the oxidant; hydroxyl radicals as the oxidant [58]; (B) schematic diagram of the proposed pathway for photocatalytic oxidation of formaldehyde over Au/TiO₂ under visible light [59]; (C) schematic illustration for the binary reaction pathways for HCHO degradation [60]; (D) calculated activation energies and reaction energies for the primary and secondary reaction pathways of HCHO degradation, positive and negative depict energy absorption and release respectively [60]; (E) schematic for photocatalytic oxidation of HCHO through selective breakage of C-H in intermediate species (HCOOH) on the surface of Sn self-doped CaSn(OH)₆ [61]; the charge difference distribution after HCHO adsorption and the corresponding plot of in situ DRIFTS tests for HCHO interaction in the dark on the HBPO (F) and MBOP (G) surface [62].