Corrugated Ionic Liquid Surfaces with Embedded Polymer Stabilized Platinum Nanoparticles Richard Knapp, † Sonja A. Wyrzgol, † Manuela Reichelt, ‡ Tobias Hammer, ‡ Harald Morgner,* ,‡ Thomas E. Mu ¨ ller,* ,†,§ and Johannes A. Lercher* ,† Lehrstuhl II fu ¨r Technische Chemie, Department Chemie, Technische UniVersita ¨t Mu ¨nchen, Lichtenbergstrasse 4, 85747 Garching, Germany and Wilhelm-Ostwald-Institute for Physical and Theoretical Chemistry, UniVersita ¨t Leipzig, Linne ´strasse 2, 04103 Leipzig, Germany ReceiVed: April 12, 2010; ReVised Manuscript ReceiVed: June 29, 2010 Ionic liquids such as 1-butyl-2,3-dimethyl-imidazolium trifluoromethane sulfonate stabilize complex structures of polyvinylpyrrolidone protected nanoparticles on their surface. The polyvinylpyrrolidone shell around the Pt nanoparticles remains intact, but the ionic liquid partly penetrates the protective layer and interacts with the metal. This leads to a pronounced increase of the viscosity of the ionic liquid, as observed also with metal complexes. The protected particles form small islands of aggregated moieties that are connected by interacting polymer strands, a process enhanced by diluting the sample in methanol during preparation. Some of these ensembles segregate at the surface protruding significantly out from the bulk ionic liquid. The outermost layer of the surface, however, consists of the ionic liquid with the cation being on top and the SO 3 group of the anion underneath being oriented toward the bulk. Introduction The immobilization of organometallic complexes and metal nanoparticles in thin films of supported ionic liquid has been introduced as a new concept to combine the high activity and selectivity of homogeneous catalysts with the facile separation of solid catalysts. 1-4 Supported ionic liquid catalysts are prepared by impregnation of a porous support with an ionic liquid containing the catalytically active components. Such materials are an improved alternative to supported aqueous phase and supported liquid phase catalysts, 5 as the catalysts are particularly stable for gas phase reactions due to the low vapor pressure of the ionic liquid. 6 Catalysts with immobilized ionic liquids have successfully been used in different reactions such as hydrogena- tion, 7 hydroformylation of olefins, 1 hydroamination, 2,3 Heck- reaction, 8 and hydrogenation. 4,9 The concept allows combining different functionalities in a single material as cocatalysts and stabilizing agents can be added to the ionic liquid phase. 3 The properties of the catalyst can be fine-tuned by choosing an appropriate ionic liquid. Particularly, the solubility of different reactants and, thus, the selectivity of the catalyst toward these reactants can be enhanced by adjusting the polarity of the ionic liquid. Ordering phenomena and domain formation of the ionic liquid may be induced by the interaction of the ionic liquid with the support as well as with the active components. 4,10,11 Especially strong interactions were observed with metal nano- particles immobilized in ionic liquids. 4,12,13 These metal nano- particles can be either attached to the support or dispersed in the ionic liquid. Usually, the metal particles are formed by reduction of a salt or complex precursor in situ in the ionic liquid. 12 The synthesis of Pt nanoparticles in ionic liquids via such decomposition of organometallic complexes, for example, was explored in detail by Dupont et al. 14,15 A new method is presented here, whereby polymer stabilized metal nanoparticles 16 were prepared prior to the immobilization in the ionic liquid and then dispersed in it. An advantage of this method is that the particle size of the nanoparticles can be tuned by changing the conditions during their preparation. Here, we used an alcohol as reducing agent, but also hydrogen can be applied. 16 Depending on the alcohol, the reduction kinetics varies. Low molecular weight alcohols provide a higher reduc- tion rate leading to smaller metal particles. 16 The present contribution describes a new surface corrugation of the ionic liquid 1-butyl-2,3-dimethyl-imidazolium trifluo- romethane sulfonate (BDiMIm, see Figure 1) with embedded polyvinylpyrroldone (PVP) stabilized Pt nanoparticles. The alignment of the ionic liquid and the nanoparticles at the surface of the ionic liquid was analyzed in detail by neutral impact collision ion scattering spectroscopy (NICISS). Special focus was laid on modeling the local arrangement between the metal particles, the polymer and the ionic liquid. BDiMIm was chosen for this study, as it is weakly coordinating to metal surfaces, has a blocked C2 position on the imidazolium ring to avoid carbene formation, is stable to relatively high temperatures and * To whom correspondence should be addressed. (H.M.) Tel: +49 341 9736500. Fax: +49 341 9736399. E-mail: hmorgner@rz.uni-leipzig.de. (T.E.M.) E-mail: Thomas.Mueller@catalyticcenter.rwth-aachen.de. (J.A.L.) E-mail: johannes.lercher@ch.tum.de. Tel. +49 89 28913540. Fax +49 89 28913544. † Technische Universita ¨t Mu ¨nchen. ‡ Universita ¨t Leipzig. § Current address: CAT Catalytic Center, ITMC, RWTH Aachen University, Worringerweg 1, 52074 Aachen, Germany. Tel. +49 241 8028594, Fax +49 241 8022593. Figure 1. 1-Butyl-2,3-dimethyl-imidazolium trifluoromethane sulfonate (BDiMIm) and poly(n-vinyl-2-pyrrolidone) (PVP) (* indicates the end group of the polymer). J. Phys. Chem. C 2010, 114, 13722–13729 13722 10.1021/jp103250k  2010 American Chemical Society Published on Web 07/26/2010