Reactivity of bimetallic clusters

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. 

In the interval 0.2 to 2 s we also observe slower decay than for t 0.2 s which was assigned, as previously for silver, to the reverse electron transfer from sub- 

The MV+ formed can transfer an electron back to supercritical clusters (Eqs. 38, 41, and 42) so that it acts, as for silver, as an electron relay from subcritical to supercritical clusters3 °  

After 2 s, the absorbance at 700 nm increases drastically (Fig. 12). This absorbance is no longer that of MV+, already consumed, but of an increasing red component of large clusters, which were developed from a small concentration of nuclei.  

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