Abstract: Spent automotive three-way catalysts (TWCs) are important secondary resources with high recycling value owing to their high concentrations of precious metals, including Pd, Pt, and Rh. Recently, photocatalytic technologies have been explored for the recovery of platinum group metals (PGMs); however, existing studies have predominantly focused on new TWCs or metallic elements as model materials. The composition of new TWCs differs significantly from that of spent TWCs, which contain complex mixtures of carrier materials, oil and carbon deposits, and oxidized precious metals. In this study, TiO₂ was loaded onto the surface of diatomite using typical sol–gel method, followed by a solvothermal process to integrate BiOCl with TiO₂/diatomite, thereby successfully preparing a ternary BiOCl–TiO₂/diatomite composite photocatalyst. Under ultraviolet irradiation, the prepared material exhibited superior PGM leaching efficiency compared with the commercial P25 catalyst. The leaching efficiencies reached 98.58% for Pd, 97.30% for Pt, and 65.37% for Rh. The results demonstrated that the incorporation of diatomite preferentially exposed the BiOCl (001) facets, thereby enhancing photocatalytic activity. Furthermore, a systematic analysis of changes in liquid-phase composition and reactive radical species elucidated the key mechanism responsible for PGM leaching from spent TWCs.