Copper catalysts EXAFS data

Figure 2. Normalized EXAFS data (copper K absorption edge) at 100°K, with associated Fourier transforms and inverse transforms, for silica supported copper and ruthenium-copper catalysts. Reproduced with permission from Ref. 8. Copyright 1980, American Institute of Physics.

Figure 4. Contributions of nearest neighbor copper and osmium backscattering atoms (circles in fields B and C, respectively) to the EXAFS (solid line) associated with the osmium Ltjj absorption edge of a silica supported osmium-copper catalyst, me circles in field A represent the combined contributions resulting from the data analysis. Reproduced with permission from Ref. 12. Copyright 1981, American Institute of Physics.

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... 

Figure 4.9 Interaction between ruthenium and copper dispersed on silica, as illustrated by the marked difference in the EXAFS envelope functions derived from EXAFS data associated with the K-absorption edge of copper in silica-supported copper and ruthenium-copper catalysts (31). (Reprinted with permission from the American Institute of Physics.)...

In contrast, the magnitude of the same peak in the transform of the ruthenium EXAFS data for the ruthenium-copper catalyst does not decrease when the data are obtained with oxygen present. This result suggests that the presence of copper tends to shield the ruthenium from the oxygen, as might be expected if the copper concentrated at the surface in the ruthenium-copper clusters (2,10-12). 

In the analysis of EXAFS data on osmium-copper catalysts, we consider two XiM functions, one for the osmium EXAFS and the other for the copper EXAFS. Each of these functions consists of two terms, one due to osmium atom neighbors and the other to copper atom neighbors about the absorber atom (32). Thus for the osmium EXAFS we can write the expression ... 

For the osmium EXAFS, the first term in Eq. 4.10 represents the contribution of osmium backscattering atoms. In this term, the quantity /V, represents the number of nearest neighbor osmium atoms about an osmium absorber atom and R, represents the distance between the osmium atoms. The phase shift function 28, (/0 is that for an OsOs atomic pair. The quantity f,(/0 exp(—2/C 2tr,2) differs from the analogous quantity for pure metallic osmium by a factor exp(—2X 2Ao-,2), where Ao-,2 is the difference between the value of o-,2 for the OsOs pair in the osmium-copper catalyst and the value for the same pair in the pure metallic osmium. Note that the quantity F,(K) exp( —2/C2cr,2) for the pure metallic osmium is known from the analysis of EXAFS data on it, as indicated earlier. 

To continue the analysis of EXAFS data on osmium-copper catalysts, it is necessary to have phase shifts for OsCu and CuOs. The following procedure is used to obtain the phase shifts. From the paper of Teo and Lee (34),... 

The approach adopted amounts to a trial and error procedure in which a series of values is chosen for OsCu and CuOs subject to the constraint of Eq. 4.12. For each set of trial phase shift functions, Eqs. 4.10 and 4.11 for the function Xi(XT, incorporating expressions of the form of Eq. 4.9 for the various x/MO terms, are fit to the corresponding functions derived from the osmium and copper EXAFS data on the osmium-copper catalyst. The fitting exercise yields values of various structural parameters, including the distance between an osmium atom and a copper atom (nearest neighbor atoms). For a given set of phase shift functions for OsCu and CuOs, limited only by the constraint of Eq. 4.12, this distance as derived from the osmium EXAFS will not in general be equal to the distance derived from the copper EXAFS. 

The size of the osmium-copper clusters of interest in the catalyst considered here is such that the number of metal atoms which could be present in a full surface layer is significantly higher than the number that would be located in the interior core. For a stoichiometry of one copper atom per osmium atom, there are, then, too few copper atoms to form a complete surface layer around the osmium. It should be realized that parameters derived from the EXAFS data on the osmium-copper clusters are average values, since there is very likely a distribution of cluster sizes (9) and compositions in a silica-supported osmium-copper catalyst. 

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