Surface enrichment of alloys

Beck, L., S. Bosonnet, S. Reveillon, D. Eliot, and F. Pilon (2004), Silver surface enrichment of silver-copper alloys A limitation for the analysis of ancient silver coins by surface techniques, Nuclear Instrum. Meth. (B) 226(1-2), 153-162. 

Current views on the surface enrichment of one component over another in alloy systems are, surprisingly, more a consequence of gas titration and Auger electron spectroscopy than XPS and UPS. There is little doubt, however, that looking to the future XPS will provide important clues regarding the mechanism of bimetallic catalysts, the significance of promoters. 

Other workers (165) used X-ray photoelectron spectroscopy (XPS) to examine the influence of ammonia oxidation on the surface composition of alloy gauzes. After several months on stream, the surface was covered by the same types of highly faceted structures noted by others. As illustrated in Fig. 14, XPS analysis provides evidence that the top microns, and in particular the top 100 A of the surface, were enriched in rhodium. This enrichment was attributed to the preferential volatilization of platinum oxide. The rhodium in the surface layers was present in the oxide form. Other probes confirm the enrichment of the surface in rhodium after ammonia oxidation (166). Rhodium enrichment has been noted by others (164, 167), and it has been postulated that in some cases it leads to catalyst deactivation (168). 

Mechanism. The interaction of antimony with supported nickel particles was studied with X-ray diffraction (XRD) (10) These studies suggested that a high level of antimony is present on the surface of Ni-Sb alloys. Further studies of the Ni-Sb alloy, using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES), confirmed the presence of an antimony enriched surface (.11) The surface enrichment of antimony on the Ni-Sb alloy would be expected to significantly alter the catalytic activity of nickel, as indeed occurs when antimony is added to nickel laden cracking catalysts. 

Surface Enrichments.—Alloy catalysts may or may not have the same composition on their surfaces as they do in the bulk. In the case of 22 atom % Pd-Au alloy the surface and bulk compositions, as measured by Maire et al. using AES, are identical for clean surfaces. Now pretreatments in O2 have an effect on catalytic activities for Pd-Au and Pt-Au alloys. In the case of Pd-Au the O2 pretreatment induces surface enrichment of Pd. 

In addition to individual precious metal/support interactions, those between metals themselves must also be considered. Thus, it has been established that Pt and Rh can form alloys, surface enrichment of which, with oxidised Rh species, is adverse to high activity (ref. 52). Thus, preparative methods must target carefully the juxtaposition of all key components for optimum performance and durability. 

In Table 1, the surface free energies of several important oxides are listed. As may be seen in the table, the differences between the various Materials are remarkable. In single and multicomponent. Materials the tendency towards the lowest possible value of surface free energy is the reason for recrystallization effects and for sorption, phase separation, and segregation phenomena. Surface enrichment of the component with the lowest surface-tension value is generally observed. Calculations of the surface-tension and surface-enrichment effects were made for alloys (real... 

According to Eq. 3.51, the metal constituent with the lower heat of sublimation will accumulate at the surface in excess. This relationship, with smalt modifications, has been used to predict surface enrichment at alloy surfaces. Note that, for oxide or organic solid surfaces, no simple 7 A/Z ubi correlation exists like the one found for metal surfaces. 

Pt and Rli atoms may be randomly distributed on Pt-Rh alloy surfaces, but the deptli distribution of Pt and Rli atoms depends strongly on the amiealing temperature. So far, it has been accepted that the higher the amiealing temperature, the higher the surface enrichment of Pt atoms. In fact, layer-by-layer analysis of a Pt-Rli alloy tip amiealed at 700 C ( ca. 1000 K) showed Pt-enricliment on the topmost layer and Pt-depleted (Rli-enriched) 2nd layer [5]. 

STATISTICAL-MECHANICAL THEORY OF SURFACE ENRICHMENT IN ALLOYS, 5.2.2.1 Regular solution model for a two-component alloy. 

While the same basic mechanisms for passivity of pure metals also applies to alloys, the processes involved in the passivation of alloys have an added complexity. In many cases only one component of the alloy has the property of being passive in a particular environment. Alloys such as steiinless steels, which contain highly passive components (chromium in this case), owe their corrosion resistance to the surface enrichment of the passivating component Thus stainless steels resist corrosion in many acidic systems (where iron or carbon steel would be poorly passive or not passive at all) by a passivating oxide film containing Cr predominantly as Cr(III). Surface analytical techniques such as Auger electron and X-ray photoelectron spectroscopies reveal substantial enrichment of chromium in the passivating oxide film on these alloys " . There are only two ways by which this enrichment can... 

The Cr(2p), Co(2p), and Ni(2p) X-ray photoelectron spectra for the samples were also studied, and the oxidation states of Cr, Co, and Ni as well as their relative intensities were obtained. From these data it was found that the Pt-Co/C sample had the lowest overall oxidizing components among the binary- and ternary-alloy electrocatalysts. Surface atomic ratios for Cr Pt, Co Pt, and Ni Pt of the carbon supported electrocatalysts, obtained from their respective X-ray photoelectron spectra, are summarized in Table 10.5. The results indicate some surface enrichment of platinum metal in all the binary-alloy electrocatalysts, namely Pt-Cr/C, Pt-Co/C, and Pt-Ni/ C. However, a surface enrichment of base metals was found in the ternary-alloy electrocatalysts, as can be seen from Table 10.5. The results suggest a higher electrocatalytic activity towards the oxygen... 

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