Particle size effects bimetallic systems

Studies of small particles by Sinfelt [29] and his co-workers have shown that when the particles sizes become very small and dispersions tend toward unity (that is, when virtually every atom is at the surface), alloy systems exhibit phase diagrams very different from those that characterize bulk systems. For example, microclusters containing Cu and Ru, Cu and Os, or Au and Ni can be produced in any ratio of the two elements, indicating complete miscibility or solid solution behavior. In the bulk phase these elements are completely immiscible. This very different behavior of the surface phases of bimetallic systems finds important applications in the design of catalysts to carry out selective chemical reactions. Moran-Ldpez and Falicov [30] developed a theory—using pairwise interactions—of alloy surface segregation that explains this effect. Bimetallic systems remain miscible at lower temperatures in two dimensions than in three dimensions. 

Also the activation energy was calculated as 56.3 2 kJ mol. Bimetallic alloy had Pt Ru metal ratio of 1 1. The particle size had a narrow distribution of 3.2 1.4 nm. It was foimd that the Pt-Ru PVP catalyst system had higher catalytic activity than the physically mixed Pt and Ru monometallic catalysts. This may arise from the synergistic effects of using Pt and Ru together or the smaller particle size relative to the monometallic Pt and Ru which were 4.6 nm and 4.2 nm, respectively. However, the bimetallic catalyst system lost 28% of its initial catalytic activity by the end of the fifth run [85]. 

Of the modifiers considered in the previous section, most if not aU were believed to exercise their influence while remaining in a positive oxidation state. With those about to be considered there is good evidence that in the main they are in the zero oxidation state, forming real bimetallic catalysts. It is possible, indeed likely, that in some cases not all of the modifier is metallic " some may remain on the support (e.g. in the Pt-Re system), but even with elements not easily fully reduced it appears that bimetallic particles may be formed (e.g. Pt-Mo, ° Pt-Zr ). In principle however the effects produced are expected to be similar, i.e. a lowering of the mean size of the active ensemble, with the possibility in some cases of electronic modification as well. The main additional feature is the chance that a bimetallic site will be of comparable activity to that composed only of active atoms this chance rises to near-certainty when the two metals are drawn from within Groups 8 to 10. 

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