Platinum Group Metal Recovery from End-of-Life Ammonia Oxidation Catalysts in Oxidative Chloride Media

Abstract

Platinum group metals (PGMs) are critical raw materials, and their sustainable recovery from secondary sources supports supply security and a circular economy. End-of-life ammonia oxidation catalysts are considered attractive feedstocks due to their high PGM content. A systematic evaluation of oxidising agents was conducted to assess the dissolution of palladium, platinum and rhodium in chloride media under identical conditions (80°C, 8 h). Near-complete and non-selective dissolution (~100% for palladium, platinum and rhodium) was achieved with aqua regia. Selective palladium dissolution was observed with hydrochloric acid (HCl) combined with oxidants: 92.9% (hydrogen peroxide (H 2 O 2 )), 95.4% (sodium chlorate (NaClO 3 )) and 87.9% (sodium hypochlorite (NaClO)), while platinum and rhodium remained <70% in single-stage runs. The HCl–NaClO system exhibited the highest palladium selectivity (S Pd:Pt 1.8; S Pd:Rh 2.0). A combined HCl–H 2 O 2 –NaClO system increased overall recoveries to palladium 93.3%, platinum 81.4%, rhodium 75.5%, but reduced selectivity. A two-stage dissolution strategy reconciled selectivity and recovery, achieving >99.5% overall dissolution while concentrating 85–92% of palladium in the first stage (S Pd:Rh up to 8.9). Thermodynamic analysis (Eh-pH (Pourbaix) diagrams, standard ΔG°, ΔS° and ΔH°) confirmed that redox control governs selectivity. This work provides a quantitative, mechanistically grounded framework for designing selective hydrometallurgical routes to recover PGM from high-grade catalysts.