Unsupported Bimetallic Particles

Nanometer-size materials have attracted remarkable academic and industrial research interest due to their fundamental properties and their potential apphca-tion, ranging from fundamental studies to catalysis [125-130]. Precise control of size and chemical behavior (stability and reactivity) by means of the synthesis itself is a major aim due to the direct correlation of intriguing new properties with particle size, bridging the gap between molecules and bulk materials. 

There is currently considerable interest in controlling the stracture of a material at the microscopic level as this influences its physical and chemical properties and 

The preparation of unsupported metal nanoparticles was therefore achieved following an organometallic approach since this does not employ drastic reaction conditions and avoids surface contamination. In addition, the size, the shape and the surface state of the particles can be controlled using various stabilizing agents. Besides catalysis, such work can find applications in different areas such as chemical sensors [141, 142] or magnetic properties [143]. 

Metal clusters may also find applications in catalysis. Here the smaller the particle size, the higher the number of their surface atoms [144,145]. This makes the particles interesting candidates for catalytic applications. Crooks used palladium clusters for hydrogenation and showed an elevated reactivity with the particles, depending on the particle size [146]. Nanoparticles are also used as the sensitive layer in selective gas detectors [147]. 

The radiation method was described by Rogninski and Schalnikoff for the first time and is based on condensation of the metal atoms after collision [154]. Reetz et al. prepared nanoparticles via electrochemical synthesis [155]. Salt reduction was developed by Bonnemann to obtain mono- and bi-metallic nanoparticles in solution [156]. Salt reduction is the most widely practised method for the synthesis of colloidal metal suspensions. Faraday synthesised gold particles by the reduction of HAuCb [157]. 

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For single crystal surfaces, a reaction is deemed insensitive if its rate is about the same on all low Miller index planes, but since these differ from small metal particles in not having atoms of very low co-ordination number, the term face sensitivity should be used in this case. Two further approaches to the general problem have been tried (1) systematic variation of particle in supported metal catalysts, and (2) alteration of the composition of the surface of bimetallic catalysts, either supported or unsupported (Section 5.7). These lead respectively to particle size sensitivity and ensemble size sensitivity, but the three types are not necessarily exactly the same. 

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