Aspectos generales:

The preparation of nanoalloy particles is a rapidly expanding field, which has recently been reviewed.85 Materials scientists commonly approach this task in a very different manner to catalytic scientists but, typically, the materials produced by the materials science community

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are not tested as catalysts. Nanoparticles can be prepared by many different methods, from those where only a few particles are synthesised through to those used in larger scale material preparations. Many of these methods are potentially suited to catalyst preparation but have not been exploited, thereby highlighting an interesting possibility where the fields of catalysis and materials synthesis can be united to mutual benefit. An illustration of relevant processes is given below. Ligands have been used in many reactions to help control the reactivity of metals. Suitable ligands (e.g. 1,2-dimyristoyl-snglycero-3-[phospho-rac-(1-glycerol)](sodium salt) (DMPG) or hexamethylene-1,6-bis (dodecyldimethylammonium bromide) (12-6-12) are often used to control particle growth in liquid phase syntheses of nanoparticles. Schaak et al. studied the reduction of copper acetate and tetrachloroauric acid, either by thermolysis in tetraethylene glycol86 or by borohydride reduction87,88 in the presence of polyvinylpyrrolidone (PVP) stabiliser, to produce gold–copper colloids of various compositions. XRD was again used to demonstrate alloy formation; it also showed that Cu2O was also observed in some

cases due to re-oxidation. Selected area electron diffraction (SAED) showed that the particles were bimetallic, in agreement with bulk XRD data. The particles synthesised by thermolysis at 310 oC were ordered alloys, whilst those prepared at lower temperatures were disordered. CuAu particles with an unusual structure have been prepared by seed growth88 using DMPG as stabiliser. Gold seed particles were prepared by borohydride reduction of a tetrachloroauric acid–sodium citrate solution, and these were used to grow particles from a tetrachloroauric acid–copper sulfate–DMPG solution. The amount of copper added to the CuAu growth solution affected the structure of the gold particles and, at higher copper concentrations, decoration of the gold particles with smaller copper particles was observed. These are similar to the species which are believed to form, following reduction and high temperature calcination,79,80 and so could possess similar catalytic properties. TEM showed that in the presence of DMPG, there was a pseudo-core–shell arrangement. Interestingly, when the

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stabiliser was changed to hexamethylene-1,6-bis (dodecyldimethylammonium bromide), the effect of the copper on the structure of the gold particles was small, although some elongated particles were observed. In this system, higher copper content led to sintering of the gold particles, in contrast to the particles produced by borohydride reduction and dodecanethiol capping (described above), where the particles with the highest copper content sintered least on calcination. The stabiliser, sodium bis (2-ethylhexyl) sulfosuccinate (AOT), was used to prepare bimetallic CuAu particles using anionic microemulsion.90 Cu(AOT)2 was added to

AOT in isooctane–water to form oil-in-water microemulsions. The copper was reduced by the addition of hydrazine and then, after 2 h, HAuCl4 was added. The resulting particles were

stabilised by the addition of 1-dodecanethiol to prevent further agglomeration. Characterisation of the nanoparticle solution by TEM and UV/visible spectroscopy demonstrated the formation of bimetallic particles. The nanoparticles produced were dispersed in polymers for sensor applications. However, a gold-containing polymer composite, without copper added, was a more sensitive detector for both ammonium hydroxide and hydrogen peroxide. Copper–gold particles in a silica matrix have been prepared by a sol–gel reaction of copper nitrate and tetrachloroauric acid with tetraethyl orthosilicate.91 A high-temperature hydrogen reduction (500–900 oC for 5 h) was used to produce the alloys after calcination at 250 oC. The alloy structure was confirmed by electron diffraction, TEM and XRD. Electrodeposition from suitable copper–gold solutions has also been used to synthesise AuCu alloy particles.92 A cyanoalkaline solution (made up of 6.4–8.0 g KCN, 6.4–8.8 g KCu(CN)2, and 1.0–3.0 g KAu(CN)2) in de-ionized water was used to

electroplate a 10–20 mm thick AuCu foil onto a titanium substrate. The as-deposited material required a vacuum annealing treatment to form a crystalline CuAu alloy, as demonstrated by XRD. The stability of the nanocrystalline grain size of the alloy coating was of interest, as it is strongly linked to mechanical properties such as hardness and strength. Physical methods

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have also been used to prepare CuAu particles. Implantation of Cu+ and Au+ ions onto a silica slide, followed by annealing in hydrogen at 900 oC 93-95 and sputtering of atoms from gold and copper metals onto a suitable support, both gave bimetallic particles.96-98 A high temperature reduction step was found to be crucial to the alloy formation in systems where the particles are produced directly from ions or atoms. Mattei et al. 99 showed, using visible spectroscopy, that a reduction treatment for their sputtered CuAu/SiO2 materials was required

to give alloying. For the as-prepared samples, only a gold plasmon band was observed, whilst, for the reduced bimetallic systems, the plasmon band position changed monotonically with the copper content, showing that the Cu was alloyed with the gold particles. There are many synthetic methods which can be applied to the synthesis of bimetallic copper–gold particles, from straightforward procedures, such as co-impregnation, through to methods which need highly complex and expensive instrumentation. One common theme is that a high-temperature reduction step is necessary for the formation of alloyed particles.