EXAFS morphologies

The Cu/ZnO system is very dynamic. The morphology of the Cu particles responds immediately to a change in reduction potential of the gas mixture above it. EXAFS studies suggest that the change in morphology is associated vith the extent that the metal particles vet the underlying support [B.S. Clausen, J. Schiotz, L. Gr4-bffik, C.V. Ovesen, K.W. Jacobsen, J.K. Norskov and H. Topsoe, Top. Catal. 1 (1994)... 

Chen et al. [123] examined the amount-dependent change in morphology for a series of Au/Pt bimetallic nanoparticles. The EXAFS results confirmed the formation of a core/shell structure and inter-diffusion between Au and Pt atoms. The composition of the shell layer was found to be Pt-enriched AuPt alloy. They also characterized bimetallic Ag-core/Au-shell nanoparticles by the EXAFS [124]. 

The microscopy results characterizing the Cu/ZnO catalyst are in accord with EXAFS data representing the dynamic morphology changes (39—41), and they also provide an important additional insight On the basis of the lattice-resolved images, the nature of the exposed facets of the projected copper nanoclusters and the epitaxial relationship between the copper and ZnO can be identified. The majority of the copper nanocrystals appear to be in contact with the ZnO support with their (111) facets, as was also observed for copper particles prepared by vapor... 

The DFT optimised cluster FSl is found to have the most energetically favourable free-space structure compared to other clusters investigated and is closest in morphology to silica-bound cluster Bl. Cluster B1 is also found to be the best representative silica-bound cluster with respect to the experimental data found thus far and is compared favourably with the experimental EXAFS data for both coordinations and bond-lengths in ref. 54. Conversely cluster Bl is not found to be the lowest energy cluster structure compared with other possible clusters bound to a silica four-ring in the same manner. 

However, EXAFS data show that in reality the average Pd particle size is larger in Pd/NaY than in Pd/HY. This observation is clearly related to the interaction of small Pd particles with zeolite protons, which will further be discussed in Section VIII. A change of Pt particle morphology in zeolite L has also been reported due to H2 (26). This effect has tentatively been attributed to the support, though its chemical nature remains to be clarified. 

An interesting example of application of the Wulff rule is given by Ovesen et al. [61]. They have analyzed the kinetics of methanol synthesis on nm Cu particles supported by ZnO. The generalized surface tension for the particle-substrate interface was assumed to be dependent on the reduction potential of the gas phase. The latter resulted in the dependence of the areas of the (111), (100), and (110) facets on the gas-phase concentrations (such changes were observed by using EXAFS). The total reaction rate, represented as a sum of the reaction rates on different facets, was found to be affected by the changes in particle morphology. 

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