How a Supported Metal Is Influenced by an Ionic Liquid: In-Depth Characterization of SCILL-Type Palladium Catalysts and Their Hydrogen Adsorption

A novel approach to tremendously influence the gas adsorption behavior of heterogeneous metal catalysts consists of adding a small quantity of an ionic liquid during the catalyst synthesis yielding a supported catalyst with an ionic liquid layer (SCILL). Herein, SCILL-type catalysts derived from silica-supported palladium were characterized by means of ICP-OES, TPR, X-ray absorption spectroscopy (XANES/EXAFS), X-ray photoelectron spectroscopy (XPS), hydrogen pulse chemisorption, and differential scanning hydrogen adsorption calorimetry. EXAFS analysis of SCILL-type palladium catalysts did not indicate significant changes in the bulk properties of Pd compared to untreated Pd/SiO₂, the results are in good agreement with metallic Pd. XPS analysis revealed a more oxidized Pd surface if Pd/SiO₂ was treated with ionic liquids. In the case of [BMIM][N(CN)₂], Pd(II) species were present in part indicating a complexation of palladium by [N(CN)₂]⁻. Hydrogen uptakes determined by pulse chemisorption varied between 38 and 609 μmol gPd⁻¹ depending on the chosen IL, IL content, and temperature and were decreased in comparison to Pd/SiO₂ without IL. The initial heats of hydrogen adsorption were also affected in the presence of ionic liquids and were lowered compared to those of the untreated Pd/SiO₂. Mechanisms showing how the ionic liquids interact with the active palladium site are proposed and might be in summary interpreted as a ligand effect by the ionic liquid similar to the influence of second metals in bimetallic catalysts