Leaching, redistribution, and dynamic heterogeneity in supported catalysts

Supported metal catalysts are often regarded as purely heterogeneous systems, yet under liquid-phase reaction conditions many undergo elemental leaching, leading to the loss of active species, catalyst deactivation and product contamination. Beyond deactivation, leached metal and non-metal species can affect turnover numbers and modify surface active sites. In some cases, a transition from solid catalysts to active species in the liquid phase is observed, or coupled “release and capture” mechanisms operate. These effects blur the boundaries between heterogeneous and homogeneous catalysis and complicate structure-activity relationships. Herein we summarize the most common elemental leaching pathways in supported metal catalysts and critically evaluate the diagnostic tools used to determine the true nature of the active species. Diagnostic approaches are organized into three categories: reaction progress-based diagnostics (e.g., hot filtration test, poisoning experiments, three-phase test, kinetic signatures), spectroscopy-based diagnostics (e.g., ICP-OES/MS, UV-Vis, NMR, IR/Raman, and in-situ / operando X-ray absorption spectroscopy), and theory-based (indicative) diagnostics (e.g., density functional theory, Pourbaix-type diagrams). We highlight key benefits and limitations, and emphasize the need for converging evidence from complementary diagnostic techniques. Finally, we discuss strategies to mitigate leaching and propose a structured workflow to reduce ambiguity regarding catalyst heterogeneity and guide the design of leaching-resistant supported catalysts.