Photocatalysis Recovery of Precious Metals from Spent Three-Way Catalysts: The Solvent Effects of Halide Salts

Abstract

Photocatalytic leaching has emerged as a promising platform for the recovery of platinum group metals (PGMs) from spent three-way catalysts (TWCs). However, the complex leaching dynamics in halide-rich media and the fundamental influence of the ionic environment on leaching performance remain to be fully elucidated. In this work, a BiOCl/TiO2/diatomite heterostructure was developed as a high-performance photocatalyst for PGM extraction within a halide salt solution. Under optimized conditions, recovery efficiencies of 97.74, 93.63, and 74.77% were achieved for Pd, Pt, and Rh, respectively, in a 1.4 mol/L NaCl solution. Mechanistic studies reveal that while leaching efficacy is primarily governed by the coordination strength between halide anions and PGMs, it is kinetically constrained by the rapid formation of a PdI2 passivation layer, resulting in a performance hierarchy of NaBr > NaI > NaCl. Furthermore, we demonstrate that cations with larger ionic radii and lower valence states (such as K+) promote photocatalytic activity by modulating the Stern layer occupation and elevating the zeta potential. Radical trapping experiments identify photogenerated holes (h+) as the dominant reactive species, complemented by the synergistic contributions of 1O2, ·O2–, ·Cl, and ·OH in enhancing the leaching kinetics. This study provides a sustainable and economical strategy for industrial PGM recovery and delivers deep insights into the ionic effects governing photocatalytic dissolution.