Principle on Selecting the Coordination Ligands of Palladium Precursors Encapsulated by Zeolite for an Efficient Purification of Formaldehyde at Ambient Temperature

High-performance zeolite-supported noble metal catalysts with low loading and high dispersion of active components are the most promising materials for achieving the complete oxidation of formaldehyde (HCHO) at room temperature. In this work, palladium nanoparticles (Pd NPs) with different sizes were successfully encapsulated inside the silicalite-1 (S-1) zeolite framework by using diverse stabling ligands via the one-pot method. Thereafter, the rule on selecting the coordinative ligands for palladium was clarified: more N atoms, a short carbon chain, a smaller branch chain, and bidentate coordination are characteristics of an ideal ligand. Accordingly, the best-performing 0.2Pd@S-1(Ethylenediamine) catalyst exhibited outstanding performance for HCHO oxidation, achieving 100% conversion even at room temperature. High-resolution high-angle annular dark-field scanning transmission electron microscopy (HR HAADF-STEM) and density functional theory (DFT) calculations indicate that the chelate is formed by complexation of Pd²⁺ ions with ethylenediamine, displaying the smallest spatial site resistance simultaneously with the zeolite synthesis, resulting in Pd located mostly within the 5-membered ring (5-MR) channels of S-1 after calcination, thus limiting the growth of Pd clusters and promoting their dispersion.