Core intermetallic compounds

M. Campagna and F.U. Hillebrecht, f-electron hybridization and dynamical screening of core holes in intermetallic compounds 75... 

Studies of Dislocation Cores in Metals and Intermetallic Compounds, Ultramicroscopy 56, 79 (1994). 

Bi- and multicomponent metallic catalysts supported on metal oxides are complex objects for preparation and characterization. It is difficult to achieve a uniform particle composition within the catalyst [1] and exclude strong metal/support interactions [2]. Moreover, the question whether the supported particles of the two or more metals are coexisting as elements (next to each other, separated or as core/shell particles), or forming alloys (randomly distributed elements on the crystallographic sites) or intermetallic compounds... 

Campagna, M., and F.U. Hillebrecht, 1987, f-Electron hybridization and dynamical screening of core holes in intermetallic compounds, in Handbook on the Physics and Chemistry of Rare Earths, Vol. 10, eds K.A. Gschneidner Jr, L. Eyring and S. Hufner (North-Holland, Amsterdam) ch. 63, p. 75. 

For example, the addition of a small content of Au to Pd, e.g., a Pd9Auj alloy core achieves remarkable enhancement of the stability while retaining the ORR activity as high as that on PtMu/Pd/C [16]. MEA fuel-cell tests showed that PtML/Pd9Au/C remains active even after 200,000 potential cycles. In addition to alloys, highly stable, inexpensive intermetallic compounds can also be attractive candidates as the supports for PtMu- PtniL electrocatalyst with intermetallic Pd-Pb core exhibits an ORR activity superior to Pt/C [17]. 

Also for Yb (de Boer et al. 1979) a most satisfactory account for the valence state of different intermetallic compounds was obtained, de Boer et aL (1979) also extended their treatment to discuss the possible existence of tetravalent Ce intermetallic systems. However, in this case, using the same scheme as for Eu and Yb, it was not possible to reproduce the separation between y- and a-like Ce compounds as a transition from trivalent (f ) to tetravalent (f°) Ce. This demonstrated, in full agreement with many other later observations, that the 4f electron cannot always be treated as completely core-like, i.e. the Ce 4f electron is not always chemically inert. As a further example of valence investigations, a simple extension of eq. (27) has been used to determine the stabilities of various valence states for lanthanide- and actinide-hahdes and oxides (Johansson 1978b). Again a good agreement with the experimental data was obtained. 

Fig. 13 Core level photoelectron spectra of AB intermetallic compounds and their hydrides. The concentration ratio A B as measured by the integrated peak height is almost the same for the inter-metallics and the hydrides, indicating that there is no significant H-induced surface segregation (Schlapbach et al., 1984).

Adsorption and absorption of H could induce a surface reconstruction or surface segregation. Single crystal adsorption studies were not yet performed. XPS core level analysis (Fig. 13) of hydrides of intermetallic compounds never gave a significant surface enrichment of one of the components as compared to the pure intermetallic compound (Schlapbach, 1982 Schlapbach et al., 1984), Whenever a surface segregation was observed, oxygen as impurity was also present. 

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