Structure and Catalytic Properties of Pt-Modified Hyper-Cross-Linked Polystyrene Exhibiting Hierarchical Porosity Lyudmila M. Bronstein,* ,² Gu 1 nter Goerigk, ‡ Maxim Kostylev, ² Maren Pink, ² Irina A. Khotina, § Peter M. Valetsky, § Valentina G. Matveeva, | Esther M. Sulman, | Michael G. Sulman, | Alexei V. Bykov, | Nataliya V. Lakina, | and Richard J. Spontak ⊥ Department of Chemistry, Indiana UniVersity, Bloomington, Indiana 47405, Institut fu ¨r Festko ¨rperforschung, Forschungszentrum Ju ¨lich, Postfach 1913, D-52425 Ju ¨lich, Germany, A. N. NesmeyanoV Institute of Organoelement Compounds, Moscow 117813, Russia, TVer Technical UniVersity, TVer 170026, Russia, and Departments of Chemical Engineering and Materials Science & Engineering, North Carolina State UniVersity, Raleigh, North Carolina 27695 ReceiVed: August 6, 2004 The structural transformation and catalytic properties of metal/polymer nanocomposites derived from hyper- cross-linked polystyrene (HPS) exhibiting both microporosity and macroporosity, and filled with Pt nanoparticles, are investigated in the direct oxidation of L-sorbose to 2-keto-L-gulonic acid. Transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, anomalous small-angle X-ray scattering, and catalytic studies suggest that the catalytically active species, nanoparticles of mixed composition with a mean diameter of 1.6 nm, develop after the initial induction period. At the highest selectivity (96.8%) at 100% L-sorbose conversion, the catalytic activity is measured to be 2.5 × 10 -3 mol/mol Pt-s, which corresponds to a 4.6-fold increase in activity relative to the Pt-modified microporous HPS previously reported. This substantial increase in catalytic activity is attributed to the presence of macropores, which facilitate mass transport and, consequently, accessibility of the nanoparticle surface for reactants. Introduction Metalated polymer nanocomposites continue to receive considerable attention since their bulk properties can be greatly, and desirably, altered relative to those of pure polymers. 1-4 Nanoparticles incorporated into a polymer system may impart magnetic, semiconducting, or catalytic properties, depending on the nanoparticle species and its characteristics. In recent years, catalysis employing metal nanoparticles has become a subject of intense interest due, in major part, to the enhanced activity and selectivity of nanostructured catalysts. 5-7 When catalytic nanoparticles are formed on the surface of an inorganic or carbon substrate, however, the nanoparticle size and morphology cannot be precisely regulated. 8,9 One reliable means by which to overcome this challenge is to grow the nanoparticles in nanostructured polymers exhibiting well-defined buried inter- faces. 10-12 Such interfaces can be conveniently generated via (i) microphase separation in solution or bulk 13-23 or (ii) formation of nanopores, or nanocavities, within a polymer matrix. 24 Nanopores can be designed into polymers by frustrat- ing the chain packing of complex macromolecules. 25 Conversely, nanoporosity can be induced in polymers by special treatment, such as exposure to a supercritical fluid under a select set of conditions. 26 When nanoporous polymers are used as matrixes in which to grow nanoparticles, the size of the nanoparticles may be physically restricted by the pore size of the polymer. This approach is widespread in the production of mesoporous solids wherein metal compounds are incorporated inside a porous inorganic medium so that nanoparticles can subsequently be grown within the pores. 27,28 This general strategy is not typically employed for use with polymeric media, since most polymers are dense and do not consist of regular interpenetrating cavities. The development of nanoporous polymers has been greatly stimulated by the need for materials with a low dielectric constant for next-generation microelectronics. 29 Production of such materials with closed nanopores can be achieved, for instance, by templating over block copolymers possessing a body- or face-centered cubic spherical morphology and a thermally labile block. If the pores are openly connected through the use of other morphologies, the resultant polymer membrane can be surface-decorated by metal deposition, resulting in metal-polymer nanocomposites. 24,30 Alternatively, delocalized solvent crazing can yield nanoporous polymers possessing interpenetrating pores that can serve as nanoreactors in the formation and stabilization of highly dispersed metal nanopar- ticles. Amorphous porous poly(ethylene terephthalate) prepared in this fashion has been successfully used as a matrix in which to grow metallic Ni nanoparticles via reduction of nickel perchlorate by sodium borohydride. 31 At the reaction conditions examined, pore dimensions influence the state of the final product. Subtle control over nanoparticle size and size distribu- tion, which are vital to ultimate property development, cannot be realized, however, with the ill-defined pores formed in a crazed polymer matrix. Nanoporous polymer microspheres containing continuous channels lined with poly(acrylic acid) have been reported by Lu et al. 32 These microspheres are prepared by UV-cross-linking of poly(tert-butyl acrylate-b-2- * To whom correspondence should be addressed. E-mail: lybronst@ indiana.edu. ² Indiana University. ‡ Institut fu ¨r Festko ¨rperforschung. § A.N. Nesmeyanov Institute of Organoelement Compounds. | Tver Technical University. ⊥ North Carolina State University. 18234 J. Phys. Chem. B 2004, 108, 18234-18242 10.1021/jp046459n CCC: $27.50 © 2004 American Chemical Society Published on Web 11/02/2004