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 °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 Cl2 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 H2 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.