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Anthracene hydrogenation in aqueous micellar solutions is catalyzed at room temperature by ionic-surfactantprotected gold and silver nanoparticles (NPs) with well-controlled particle sizes. When sodium borohydride was used as the hydrogen source in the presence of these NPs, the only product obtained in the reaction is 9,10-dihydroanthracene. During the course of the reaction, a blue shift in the surface plasmon resonance band of silver NPs was more prominent and lasted longer than that of gold NPs. The difference in the optical property of metal NPs was related to the charging of their surfaces, indicating that the metal NPs play a role as the nanoelectrode storing electrons from hydrides. At the same time, a remarkable size-dependence of catalytic activity was found for both gold and silver NPs. Mechanism about the electron transfer-relaying effects of metal NPs is proposed for the hydrogenation reaction.

Size-sensitive catalysis of chemical reactions by metallic nanoparticles (NPs) is an important research direction attracting a lot of attention. [1] In most of the investigations, the NPs are supported on a surface, usually a metal oxide, to allow formation of highly dispersed NPs.[2] For heterogeneous catalysts involving gold NPs deposited on metal oxides, size-sensitive catalysis by gold NPs is reported for the CO oxidation, [3a] the propylene epoxidation [3b] and the selective hydrogenation. [4][5][6][7][8][9][10] However, there often exists a strong interaction between the NP and the support which makes the size effect in catalysis difficult to understand. In a second approach colloidal particles in solution are used for catalysis in the liquid state. [11] Compared to supported catalysts, the size and the shape of colloidal particles can easily be controlled. Also the strong interaction with the support is essentially eliminated. In contrast to supported gold catalysis, a clear size-sensitive catalysis effect has not been reported in liquid-phase catalysis by gold colloids. Herein, we report the hydrogenation of anthracene catalyzed by size-controlled gold colloids which are protected by ionic surfactants in aqueous solution at room temperature. The sizes of gold NPs in solution may be controlled by the properties of the surfactant and its concentration used for the preparation (cationic surfactant: hexadecyltrimethylammonium bromide, CTAB, and anionic surfactant: sodium dodecyl sulfate, SDS). Then, the less volatile reactant such as anthracene was solubilized in surfactant-protected gold colloids to study its hydrogenation catalyzed by gold NPs. A remarkable size-dependent catalytic activity is found. Compared with the reported hydrogenation of anthracene catalyzed by TiCl 4 , [12] our Au colloidal particles exhibit higher catalytic activity and higher selectivity at lower reaction temperatures.For the preparation of gold NPs in the SDS system, 0.02 mL of an aqueous 25 mm HAuCl 4 ·3 H 2 O solution was added to 5 mL of an aqueous solution of 20 mm SDS. Then, 0.06 mL of an aqueous NaBH 4 (11 mm) solution was added (see left side of Scheme 1). After the addition of NaBH 4 , the solution turned pink and an UV/Vis absorption spectrum, shown in Figure 1 a (solid line), was measured. The surface plasmon resonance (SPR) band at 520 nm indicates the formation of gold NPs. [13] The average diameter of gold NPs (denoted as Au@SDS) is[a] Dr.

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