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.
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Arras et al. (2010) studied this question.