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

Abstract

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/SiO 2 , the results are in good agreement with metallic Pd. XPS analysis revealed a more oxidized Pd surface if Pd/SiO 2 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 g Pd -1 depending on the chosen IL, IL content, and temperature and were decreased in comparison to Pd/SiO 2 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/SiO 2 . 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.

Ligand Effects in SCILL Model Systems: Site‐Specific Interactions with Pt and Pd Nanoparticles

“…Enhanced selectivity has been demonstrated for SCILL systems in hydrogenation catalysis, [ 6 , 7 ] but the microscopic origins of such effects are still unclear. [ 8 ] On the one hand, the concentration of reactants available at the catalytic center may be tuned via the reactant solubility in the IL. [ 1 ] On the other hand, the IL may act as a ligand directly interacting with the catalytically active nanoparticle.…”

Surface Science and Model Catalysis with Ionic Liquid‐Modified Materials

“…[ 19 ] However, the origins of such selectivity effects are yet unclear. [ 21 ] On the one hand, the IL fi lm may act as a differential reactant collector, which depending on the solubility of individual reactants and products may either enhance or suppress specifi c reaction channels. On the other hand, the IL may directly interact with the active catalytic sites, similar to what a ligand would do.…”

Ruthenium‐Catalyzed Selective Hydrogenation of Benzene to Cyclohexene in the Presence of an Ionic Liquid

“…Ionic liquids have numerous important applications, such as solvents in organic synthesis, as stabilizers for nanoparticles, [9] and as constituents of SILP (supported ionic liquid phase) catalysts [10] and SCILL (solid catalyst with an ionic liquid layer) systems. [11] As shown in Table 1, the ILs first used in the screening affect both the activity of the catalyst Ru/Al 2 O 3 -41 (mean Ru particle size d Ru = 4.1 nm) and the cyclohexene selectivity. In the reference test without IL (entry 1) only cyclohexane was produced in the entire range of conversion, as expected.…”