Bimetallic alloy clusters from

Formation of bimetallic alloy clusters from M, Na-Y (M=Pt, Ir, Rh, Ru) after partial ion exchange with CUSO4 solution, oxygen treatment and subsequent reduction in flowing hydrogen was evidenced by the shift of the band position of adsorbed CO from 2090 cm" by about 50 cm" to lower wavenumbers [632]. Similar experiments were carried out with silicalite-1 or ZSM-5 as supports. 

The reactivity of short-lived bimetallic clusters has also been studied by the kinetics method. Under conditions when a transient alloyed cluster of Ag-Au was formed,reactivity with the electron donor MV+ was probed and compared with that of monometallic Ag clusters previously observed. Just after the pulse a mixed solution of Ag and Au cyanides is partially reduced into atoms Ag and Au , while is partially reduced to the redox probe MV+. It is observed that in the first 20 ms the kinetics, at 400 nm, of cluster growth are the same as in the absence of the probe. Thus the coalescence of atoms to form an alloyed small cluster is, at first, not affected. The mechanism should be the same as in Eqs. (20)-(23). After this period, however, the decay of MV" at 700 nm starts in correlation with the increase of the cluster absorbance which results from electron transfer (Fig. 12). When the bimetallic cluster formed reaches the critical size where its potential becomes slightly higher than °(MV +/MV+ ), it acts as a nucleus that initiates a catalyzed growth fed alternately by electron transfer from the donor and the adsorption of excess Ag or Au ions. For i +J > ny. 

As an aside, we should mention that the same principles apply to the formation of bimetallic clusters on a support. In the case of Pt-Re on AI2O3 it has been shown that hydroxylation of the surface favors the ability of Re ions to migrate toward the Pt nuclei and thus the formation of alloy particles, whereas fixing the Re ions onto a dehydroxylated alumina surface creates mainly separated Re particles. As catalytic activity and selectivity of the bimetallic particles differ vastly from those of a physical mixture of monometallic particles, the catalytic performance of the reduced catalyst depends significantly on the protocol used during its formation. The bimetallic Pt-Re catalysts have been identified by comparison with preparations in which gaseous Re carbonyl was decomposed on conventionally prepared Pt/Al203 catalysts. ... 

Supported bimetallic clusters (with well-defined metal frames) are commonly prepared from organometallic precnrsors, where the bimetallic clnster frame is already present in the strnctnre with reactive ligands that can be removed under specific treatments [57, 58]. Snpported bimetallic particles and alloys, which are used in naphtha reforming (Re-Pt, Sn-Pt, Ir-Pt) and antomobile exhaust conversion (Rh-Pt) will not be reviewed here. Recent reviews on this topic can be found elsewhere [59]. 

Alloyed multi-metallic clusters have been synthesized in the same way [3,6]. However, characterization ofthe alloyed structure is still more difficult than for bimetallic clusters. The alloyed character is inferred rather from their catalytic properties, for example, which are enhanced when they are produced at high compared to low dose rate. 

X-Ray Diffraction Studies. The dependence of the lattice parameter of bulk platinum-iridium alloys on composition is shown in Figure 4.22 (4,45). Lattice parameters are commonly obtained from X-ray diffraction measurements. For platinum-iridium catalysts, X-ray diffraction measurements provide a way of demonstrating the presence of bimetallic clusters of platinum and iridium, if the metal dispersion is not too high. 

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