Mixed metals ensemble effect

The effect of precursor-support interactions on the surface composition of supported bimetallic clusters has been studied. In contrast to Pt-Ru bimetallic clusters, silica-supported Ru-Rh and Ru-Ir bimetallic clusters showed no surface enrichment in either metal. Metal particle nucleation in the case of the Pt-Ru bimetallic clusters is suggested to occtir by a mechanism in which the relatively mobile Pt phase is deposited atop a Ru core during reduction. On the other hand, Ru and Rh, which exhibit rather similar precursor support interactions, have similar surface mobilities and do not, therefore, nucleate preferentially in a cherry model configuration. The existence of true bimetallic clusters having mixed metal surface sites is verified using the formation of methane as a catalytic probe. An ensemble requirement of four adjacent Ru surface sites is suggested. 

In order to verify the presence of bimetallic particles having mixed metal surface sites (i.e., true bimetallic clusters), the methanation reaction was used as a surface probe. Because Ru is an excellent methanation catalyst in comparison to Pt, Ir or Rh, the incorporation of mixed metal surface sites into the structure of a supported Ru catalyst should have the effect of drastically reducing the methanation activity. This observation has been attributed to an ensemble effect and has been previously reported for a series of silica-supported Pt-Ru bimetallic clusters ( ). 

This so called ensemble or geometric effect, however, is not the only reason for the distinct properties of mixed metal catalysts. The diluent metal can... 

Due to their high surface-to-volume ratio and size-dependent electronic properties nanostructured materials like NPs are good as catalysts. NPs of different sizes and structures can show significantly different catalytic activities and thus provides an opportunity to understand the structure-function relationship. NPs prepared usually in ensembles of NPs immobilized on an electrode. Thus the electrocatalytic property result of the average properties of the ensemble. Optimization of the catalyst requires increasing the number of sites available for the reaction to occur, shape and size effect of NP and composition of particles (in case of mixed metal... 

By alloying a metal A is dispersed (more or less, it depends on the type of alloys) in a metal B. If a certain reaction requires a big ensemble of contiguous atoms A in the surface of alloys, this reaction will be suppressed strongly by alloying. This may lead to selectivity changes, if other potential reactions in the system can occur on smaller ensembles or even individual atoms. This is true for systems when B is much less active than A. If both components are active, one has to consider also the possibility that a big ensemble required can be formed by a mixture of A and B. In some cases (Pt/Ir, Pd/Ni, Pt/Re,. . . ) the mixed ensembles may even be suspected to be more active than the one-component ensembles. In the literature, this kind of effect is called an ensemble size effect (1-5). 

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