Topotactic transformation and molten salt electrodeposition: Upcycling spent catalysts into platinum nanospheres.

Spent catalysts are vital platinum resources yet hazardous wastes, threatening irreversible environmental harm if mishandled. This study introduces a strategy for upcycling platinum from spent catalysts into nanospheres. Demonstrated using a simulated system and validated with a real spent catalyst, the designed Pt→Li2Pt→Li2PtO3 pathway enables the topotactic conversion of Li2Pt to Li2PtO3 via oxygen intercalation, thereby reconstructing the electronic structure of platinum. Zintl-phase Li2Pt intermediate enhanced the oxidation kinetics. Li2PtO3 stabilized platinum in the +4 state and facilitated its dissolution in the molten salt. Preferential Li+ dissolution triggered lattice destabilization and the release of Pt4+. Pt4+ coordination evolved from O2- to Cl--dominated soluble complexes through mixed-coordination intermediates like [PtOCly](y-2)-. Electrochemical analysis confirmed a two-step reduction mechanism, which suppressed explosive nucleation and dendritic growth. Electrodeposition at 1.0 V produced uniform platinum nanospheres. This study develops a clean and efficient upcycling strategy for spent catalysts, which provides a viable solution for the sustainable utilization of strategic metal resources.