Recovery of platinum group metals from spent automotive converters and their conversion into efficient recyclable nanocatalysts

The study reported in this article has shown for the first time that strongly acidic solutions (pH < 0.5) obtained after hydrometallurgical treatment of spent automotive converters (SAC) may be valuable secondary sources of platinum group metal (PGM) nanoparticles (NPs). The PGM precipitation strongly depended on the solution pH; the yield of the precipitated PGM NPs increased considerably from 40% to almost 100% when the pH was adjusted to 7–8. To improve the NPs stability, commercial TiO(2) was used as support to obtain efficient recyclable PGM@TiO(2) catalysts. The size of the PGM NPs was smaller than 5 nm, while the diameter of the supported particles varied from 10 to 50 nm. The size and dispersion of PGM NPs on the support strongly depended on the pH of the medium: at pH < 0.5, the Pt and Pd NPs were significantly smaller than the NPs obtained at pH 7–8. Also, in the case of Pt@TiO(2) and Rh@TiO(2), the NPs were well dispersed on the support in contrast to the large agglomerates of Pd@TiO(2). The PGM@TiO(2) showed catalytic properties in the reduction of 4-nitrophenol to 4-aminophenol, particularly, at pH above 11. The highest conversion of 98% was obtained with 1% Pd@TiO(2) at pH 14 after only 15 min. The catalyst was easily separated from the reaction mixture and reused in 7 consecutive cycles without significant loss of activity. The PGM@TiO(2) synthesized from the real solution showed a similar catalytic activity (70% conversion at pH 14) as that obtained from model solution. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s11356-022-24593-2.