Monometallic mechanism reactivities

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. 

The mechanism shown in (19-10) is valid for bimetallic complexes as well as for monometallic complexes. What is decisive is the stability of the Mt-R bond. An Mt-R bond that is too stable is not made reactive by monomer coordination. An unstable Mt-R bond would, on the other hand, be destroyed under the polymerization conditions. The ligands X must be so chosen that the balanced effect of their electron donor properties gives just the right degree of destabilization. 

At present, the Pt-Ru bimetallic system is recognized as the most promising CO-tolerant anode catalyst for the DMFCs. A large body of hterature exist demonstrating improvement of the CO oxidation on the Pt-Ru alloy and Ru-modified Pt catalysts. The superior CO tolerance of the Pt-Ru bimetallic catalysts compared with the monometallic Pt catalyst is frequently explained with concepts of bifunctional mechanism [17] and ligand effect [22, 23]. The former mechanism proposed by Watanabe and Motoo is widely accepted. They claimed that the Ru has higher reactivity with water than Pt and that formation of Ru-OH at a lower potential promotes the electrooxidation of the chemisorbed CO on the Pt (formulas (4) and (5)). 

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