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preparation of metal nanoclusters stabilized by EG and
Source:    Published:2011/6/9 16:44:29

   size in colloidal solutions of high metal concentration is derived from the difficulty in controlling the processes of core formation and its further growth in the previous

   methods. A strategy to solve this problem is to separate the core formation process from the reduction of metalions in the cores as shown in Scheme l, and use solvent (EG) and simple ions (OH etc.) as the stabilizers [11]. In the first step of this process, metal salts hydrolyzed in the alkaline solution of EG to give rise to metal hydroxide or oxide colloids, which were then reduced by EG at elevated temperature to produce colloidal metal nanoclusters in the

   y:Wang and XWang (1) NaOH, pH >12 in glycol HZPtCI6 Pthydroxidecolloid hydrolysis/condensation(2) 1600C, Nz flow "unprotected" Pt nanoclusterreduction colloidal solution Scheme l. Procedure of alkaline EG method for the chemical

  

   preparation of metal nanoclusters stabilized by EG and sampleions. second step. TEM measurements showed the formation of the metal hydroxide or oxide colloidal particles before the

   reduction process in the preparation [11]. Liu and coworkers monitored the reaction course using UV-vis spectroscopy and investigated the evolution of metal species in the synthesis process by XPS [15]. Their results also support the two-step formation mechanism of the unprotected metal nanoclusters.

  

   2.2. Preparation of Nanocomposite Metal nanoclusters are promising building blocks in the preparation of heterogeneous catalysts, providing new possibilities of universal significance for designing and constructing structure-controllable catalysts. The support-

   entrapment method provides a realistic strategy, which builds a framework of inorganic supports via stacking metal oxide nanoparticles around preformed metal nano clusters protected with organic ligand or polymer. In this method, to obtain a close contact of the metal nanoparticles with inorganic supports, organic stabilizers originally adsorbed on the metal nanoclusters usually have to be removed by extraction [26,27] or pyrolysis [28-31] at

  

   the end of the catalyst preparation, which may cause the aggregation of metal nanoclusters in some cases, especially at high metal loading. However, these processes are not necessary in the preparation of heterogeneous catalysts using unprotected metal nanoclusters as building blocks. The solvent molecules and simple ions absorbed on the surface of metal nanoclusters can be easily removed during the immobilizing process.Recently, a new strategy for synthesizing the nanocomposites of metal nanoclusters and inorganic semiconductor nanoparticles was proposed [18,19]. As illustrated in Scheme 2, this strategy includes capturing the unprotected metal nanoclusters on the colloidal particles of metal oxides via electrostatic interaction and gelating the complex sol by adjusting its pH value or heating. This

  

   assembling process, using two kinds of nanoparticles sta bilized with solvent and simple ions as building blocks, is different from those based on an in situ hydration process

   of M(OR)n compounds in the presence of ligandprotected metal nanoclusters as reported previously

   [26,29l. The catalysts prepared by this strategy are more regulable in structure, i.e. it is convenient to control both the size and composition of the metal and support nanoparticles in the formed catalysts, and to modulate the catalytic properties by the interaction between the differ-

   ent nanoparticles as well as the external environment surrounding the catalytic sites.

  

CAS NO.71-23-8
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