Microwave chloride leaching of valuable elements from spent automotive catalysts: Understanding the role of hydrogen peroxide

This study reports on the microwave-assisted leaching behavior of platinum group metals (PGMs) and light rare earth elements (LREEs) from two representative end-of-life automotive ceramic catalyst materials in 6 M HCl at 150 degrees C with and without the addition of 10 v/v% H2O2 solution. It was shown that the in situ generated headspace gas in the pressure-tight reactor and the elemental speciation in the spent catalysts influence the leachability of PGMs and LREEs. The formation of Cl-2 in the headspace provided the 6 M HCl system with a suitable oxidizing environment to leach PGMs as soluble chloro-complexes. Indeed, spent catalyst containing prevalently oxidized PGM species leached best in 6 M HCl (93.9 +/- 0.7% Pd, 98 +/- 3% Pt, 70.7 +/- 0.4% Rh). Whereas, H2O2 addition moderately decreased the PGM leaching efficiency due to surface passivation by an oxide layer. However, spent catalyst containing other oxidizable substrates (e.g. Ce3+, zerovalent PGMs) that give rise to H-2 evolution, compensated partially the oxidation potential of the HCl system. In such case, H2O2 addition slightly improved the PGM leachability (Rh 91.8 +/- 0.1% Pd, 96 +/- 4% Pt, 89.9 +/- 0.2% Rh). Among the studied LREEs, Ce leaching was mainly affected by the passivation of Ce3+ through oxidation, thus in the absence of H2O2 and at lower initial Ce3+ concentration Ce leached best (87-94%). The effect of hydrogen peroxide was negligible for La and Nd, and moderate towards Y leaching. The leaching of these elements was constrained by their association with Al and Zr oxides, respectively.

Automated summary

This study investigated the microwave-assisted leaching behavior of platinum group metals (PGMs) and light rare earth elements (LREEs) from end-of-life automotive ceramic catalysts. The in situ generated headspace gas, elemental speciation, and the addition of H2O2 solution all influenced the leachability. The presence of oxidized species and other oxidizable substrates played a role in the leaching efficiency, with H2O2 addition affecting the surface passivation and overall leachability differently for different catalyst compositions.