Microscopic metals bimetallic particles

Figure 3.3.13 (A) Pt core-shell nanoparticle ORR electrocatalysts are prepared by (electro) chemical selective removal of a less noble metal, M, from a Pt-M nanoparticle alloy. The near-surface region is Pt enriched, while the particle core remains bimetallic. (B) Transmission electron microscopic elemental map of dealloyed Pt-Cu core-shell nanoparticle ORR electrocatalysts.

In relation to platinum highly dispersed as particles with sizes of the order of 10 A, Sinfeld makes the points (i) that X-ray diffraction is of little use for such cases and (ii) that semi-empirical analysis of the observed platinum EXAFS indicates the average coordination number of the platinum atoms in such catalysts to be lower than for bulk platinum metal [96a]. Furthermore, Sinfeld et al. have prepared bimetallic Ru/Cu clusters of very small size dispersed on oxide supports of very large surface area. The resultant high proportion of surface relative to bulk metal atoms represented favourable conditions in which surface metal atoms and their environment would dominate EXAFS. Semi-empirical analysis of the EXAFS led Sinfeld et al. to conclude that Cu atoms in surface layer(s) of the small bimetallic Cu/Ru clusters had (on average and in contrast to microscopic non-miscibility) several near-neighbour Ru atoms [96b]. 

The high surface area of the charcoal used in this work prevented us from using gas chemisorption to determine metallic dispersion of our catalysts. We used an electron microscope (Jeol JEM 100 CX) to investigate the geometric appearance of the particles. At first sight, the micrographs show that in the pure platinum catalyst the metal crystallites form two populations whereas in the bimetallic preparations the size is much more uniform. 

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