Behaviour of advanced materials in environmental aquatic media – dissolution kinetics and dispersion stability of perovskite automotive catalysts

Abstract

Tighter regulations on exhaust emission limits require more effective catalysts and here advanced materials (AdMa) play an increasingly important role. Perovskite-based catalysts are among the promising candidates. However, like other automotive catalysts, they contain metal elements of potential concern like nickel, cobalt and noble metals; hence, their likelihood to be released and their fate under environmentally relevant conditions must be assessed at the early stages of material development, so as to align with the goals of the EU Chemical Strategy for Sustainability. The aim of this study is to provide insights into the dissolution and agglomeration behaviour of perovskites in aqueous media with different ionic strengths and salt contents, as well as the influence of the presence of natural organic matter (NOM). The current OECD guidance document and testing guidelines (GD 318 and TG 318, respectively) for nanomaterial testing were applied to three different perovskite AdMa with a lanthanum–cobalt–nickel (LaCoNi) structure with and without doping with palladium or platinum. These tests resulted in a range of practical insights into the feasibility of this methodological cross-over as well as evidence on transferability and applicability to other case studies. Our findings rank the dissolution kinetics of these perovskites to lie between the two reference nanomaterials ZnO and BaSO₄. Dissolution rates were found to be, respectively, for ZnO NM110, LaCoNi, and BaSO₄ NM220: 0.13, 0.041, and 0.013 μg m⁻² s. The ionic strength of the media used in this study did not seem to impact the overall leachable amount of metals (% w/w); however, we found that metal release was mostly incongruent and metal specific i.e., a lower lanthanum ratio with respect to either cobalt or nickel. The presence of NOM increased the dissolution of both benchmark materials; however, no strong influence on dissolution was observed for the perovskite materials. The dispersion stability of perovskites in solution was substantially increased by the presence of NOM and decreased by increased hardness in the test media. Finally, this study provides methodological insights and practical recommendations for testing the dissolution and dispersion stability in different media relevant for ecotoxicological testing and environmental risk assessment.