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 O^2- to Cl^--dominated soluble complexes through mixed-coordination intermediates like [PtOCl_y]^(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.