13 C NMR Characterization of the Organic Constituents in Ligand-Modified Hexagonal Mesoporous Silicas: Media for the Synthesis of Small, Uniform-Size Gold Nanoparticles Edward W. Hagaman,* Haoguo Zhu, Steven H. Overbury, and Sheng Dai Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received November 14, 2003. In Final Form: July 12, 2004 This paper reports the 13 C NMR characterization of functionalized MCM-41’s and describes the chemistry that occurs in the pores of these materials in the process of forming gold nanoparticles. Nanoparticles formed on hexagonal mesoporous silica (MCM-41) by hydrogen reduction of chloroauric acid have little affinity for pure silica surfaces. The gold can be removed from the support with very mild treatment, for example, solvent extraction. The loss of gold from the substrate can be prevented using a pore functionalization methodology that entails synthesis of the silica containing polydentate amine functionality chemically bound in the mesopores. The synthetic scheme introduces solvents and templating reagents (surfactants) into the mesopores that are chemically reactive under the conditions required for gold particle formation. Extensive base-catalyzed elimination and nucleophilic substitution reactions involving the tetraalkylammonium surfactant occur during the reduction of chloroauric acid to gold. Introduction Highly dispersed gold nanoparticles have been dem- onstrated to be very active catalysts for a number of important chemical reactions ranging from oxidation 1-6 to hydrogenation, 7 hydrogen production, 8 and hydrochlo- rination. 9 Catalytic activities of gold strongly depend on its particle size. It is necessary to have homogeneous distributions of small gold nanoparticles with diameters between 2 and 5 nm for excellent catalytic activities. To achieve the controlled synthesis of dispersed gold nano- particles in the narrow size range, several methods have been developed. These methodologies include coprecipi- tation from an aqueous solution of HAuCl 4 , 1,2 deposition- precipitation using precipitation agents, 7,8,10,11 cosputter- ing of gold and metal oxide on a substrate, 12 and chemical vapor deposition of gold nanoparticles on porous matrixes. 3 The key drawback associated with these synthetic pro- cesses is the difficulty controlling both location and size of the gold nanoparticles in oxide matrixes. Gold nano- particles are normally formed either on external surfaces of the oxide particles or embedded in microporous oxide matrixes. Gold nanoparticles on external surfaces are susceptible to aggregation because of the decreased melting point of nanoparticles 13 and the lack of space confinement, while gold nanoparticles embedded in the microporous matrixes are inaccessible to reactants for effective catalytic reactions. Alternative methodologies for synthesis of gold nanoparticles inside the pores of mesoporous materials are summarized in our previous work. 14 We have recently developed a cosynthesis methodology for the preparation of gold-containing mesoporous silica materials. The essence of this sol-gel cosynthesis method is to combine surfactant template synthesis of mesoporous silica materials with the introduction of metal ions via bifunctional amino-silane ligands, so that the formation of mesostructures and metal-ion doping occur simulta- neously. 14 The purpose of the amine functionality is to complex and stabilize the gold(III) precursors and the gold nanoparticles. 15-17 This strategy does work and allows the preparation of small uniform (2-5 nm) gold particles on silica. Fumed silica 18 and silica gels 19,20 have been studied extensively by solid-state NMR methods, as have ligand- modified silicas. 21-23 Amine 24 and polydentate amine (1) Avgouropoulos, G.; Ioannides, T.; Papadopoulou, Ch.; Batista, J.; Hocevar, S.; Matralis, H. K. Catal. Today 2002, 75, 157-167. (2) Harut, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301-309. (3) Okumura, M.; Tsubota, S.; Iwamoto, M.; Haruta, M. Chem. Lett. 1998, 315-316. 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