Failure analysis of three-way catalysts through comprehensive characterization and understanding the role of pretreatment

Currently, most of the studies on waste three-way catalysts (TWCs) have focused on the extraction of platinum group metals (PGMs), with less characterization and analysis of their properties. This can lead to an incomplete understanding of the physicochemical properties and causes of failure of waste TWCs, which in turn affects the recovery of PGMs. This paper conducted various characterization analyses on waste TWCs, including their chemical composition, particle size, surface elements, microscopic morphology, specific surface area, and thermal stability. The PGMs content in the waste TWCs is 1468 g/t, and the total content of rare earth elements is about 1.58%, which has high recovery value. The analysis identified several reasons for TWC failure: the presence of S and P elements caused sulfidation and phosphorylation poisoning of PGMs; PGMs were also partially oxidized to PtO2, PdO2 and Rh2O3; in addition, sintering and agglomeration of waste TWC powder, encapsulation of PGMs in the substrate, reduction of specific surface area and total pore volume, and adsorption of accumulated carbon were also considered as causes of failure. The physicochemical properties of waste TWCs are extremely important for the selection of the PGM extraction process among them. Therefore, the effects of several pretreatment methods on the leaching efficiency of PGM were investigated. The results showed that grinding reduced the leaching rate of PGMs, and HCOOH reduction and NaHSO4 calcination pretreatment could improve the leaching rate of Pt and Rh, respectively; while the Li2CO3 calcination pretreatment could result in 70.87% leaching of Pt directly in HCl. The feasibility of extracting PGMs from waste TWCs using a Li2CO3 calcination pretreatment followed by direct HCl leaching was validated.