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

Abstract

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 TiO2 was used as support to obtain efficient recyclable PGM@TiO2 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@TiO2 and Rh@TiO2, the NPs were well dispersed on the support in contrast to the large agglomerates of Pd@TiO2. The PGM@TiO2 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@TiO2 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@TiO2 synthesized from the real solution showed a similar catalytic activity (70% conversion at pH 14) as that obtained from model solution.

Studies on the Application of PGM Nanocatalysts from Spent Automotive Converters for Degradation of Ibuprofen in Aqueous Solutions

“…As shown in our previous studies [ 19 , 20 ], the use of a TiO 2 support facilitated the straightforward separation of the obtained Pt–NPs from the reaction mixture. The synthesized materials underwent preliminary testing in a model catalytic reduction reaction of 4-NPh to 4-APh.…”

“…An amount of 1% of Pd@TiO 2 or Pt@TiO 2 formed from model solutions was chosen as the nanocatalyst. These nanomaterials were characterized elsewhere (TEM, AFM images, and the distribution of particle size in Supplementary Materials of [ 20 ]), and it was confirmed using TEM images that the size of PGM–NPs was less than 10 nm.…”

“…The metal ion concentration was measured in the aqueous solutions before and after precipitation, and the mass balance indicated complete precipitation of the target metals. Additionally, our previous studies have demonstrated that the catalytic material PGMs@TiO 2 is reusable for at least 7 cycles in a model reaction, reducing 4-NPh to 4-APh, without a decrease in catalytic activity [ 20 ].…”

Platinum Group Metals: Green Recovery from Spent Auto-Catalysts and Reuse in New Catalysts—A Review

“…Moreover, as far as we know, only a few examples are found in the literature about the synthesis of PGM NPs or PGM-based catalysts directly obtained starting from the leach liquors of SACs. A recent work on the leaching of a Pt-Rh SAC [62] demonstrated for the first time that after pH correction strongly acidic solutions (pH < 0.5) obtained after hydro-metallurgical treatment of the SAC may be a valuable secondary source of PGMs nanoparticles to be employed in TiO 2 -based catalysts. According to this study, and as predictable, the difficulty in NPs formation from the leach solution lies in the high acidity of this solutions (pH < 0.5) and the presence of accompanying/contaminating ions of non-precious metals, as shown in Table 5 where the composition of the leach solution is shown.…”

“…Metal ion concentrations after leaching with HCl/H 2 O 2 /H 2 SO 4 (S/L = 1/100 g/cm 3 , 3 h, 70 • C, 300 rpm)[62].…”