LAUSR

Supported catalysts based on metal nanoparticles are a class of widely used heterogeneous catalysts in industry. The catalytic performances of supported metal catalysts are highly determined by many parameters of metal nanoparticles such as their particle size, composition, surface structure and also interfacial interaction with supports [1]. Supported metal catalysts are commonly prepared by depositing metal precursors on high-surface area supports through various methods such as impregnation (IMP), co-precipitation (CP), and depositionprecipitation (DP), followed by thermal or chemical reduction of the metal precursors deposited thereon to form supported metal nanoparticles. The difficulty for conventional methods to prepare high-quality supported metal nanoparticles with uniform particle size and composition mainly lies in the following two aspects: 1) The interaction between metal precursors and supports is not strong enough to prevent their sintering during the post thermal or chemical reduction processes; 2) The deposition of metal precursors is not homogenous so that the phase separation often occurs after the reducing treatment. By addressing these two issues, the DP method has been outstanding among other conventional methods in preparing supported small metal nanoparticles [2]. However, the DP method has met great difficulties in preparing high-quality metal nanoparticles catalysts on many acidic supports (such as silica, WOx, zeolites) due to their relatively low point of zero charge (PZC) [3]. During the past decade, there has thus been an emergence of research efforts directed toward the use of pre-made uniform nanoparticles as precursors to prepare well-controlled metal nanocatalysts for investigating complicated interfacial catalytic mechanism [4–6]. In these colloidal methods, the influence of organic capping agents is often concerned. As compared to supported monometallic nanoparticles, the preparation of high-quality supported alloyed nanoparticles is even more challenging. Alloying has been well-documented as an effective strategy to tailor the catalytic performance of supported metal catalysts. On one hand, alloying provides an avenue to reduce the usage of expensive scare metal without compromising the overall catalytic activity. On the other hand, the electronic density and/or dispersion of catalytically active metal can be manipulated by the introduction of another metal species, thus tailoring the reaction pathway and often optimizing the overall catalytic properties. In order to obtain high-quality supported alloyed catalysts with uniform ultrasmall size and uniform composition, it is essential to have the metal precursors uniformly deposited onto the surface of supports before being converted into alloyed nanoparticles. Recently, Regalbuto and colleagues have developed a general and simple method to prepare ultrasmall alloyed metal nanoparticles based on both noble (i.e., Pt, Pd) and base metals (i.e., Ni, Co, Cu) [7]. The homogeneously alloyed metal nanoparticles have a narrow size distribution around 1 nm. In the developed method, positively charged metal-amine complex precursors were deposited onto negatively charged surface of silica support through strong electrostatic adsorption (SEA) [8,9], and then reduced by H2 at an elevated temperature to give supported uniform ultrasmall alloyed nanoparticles (Fig. 1a). The key of this work is to use positively charged metal-amine complexes as metal precursors. Since the surface point of zero charge of silica (Aerosil 300 silica) is around 3.6, the surface charge of silica in water is negative in a wide range of pH value (4–13) before silica is dissolved. The strong electrostatic interaction can thus promote the homogenous deposition, making the method outstanding conventional preparation methods for the production of ultrasmall alloyed nanoparticles with different combina-