Surface composition of alloys

Discuss the role of the surface free-energy in phenomena such as alloy segregation, surface reconstruction, faceting and sintering of small particles. How does a gas environment (air, vacuum) affect the surface composition of alloys ... 

Sachtler WMH. 1973. Surface composition of alloys in equilibrium, Le Vide 164 67. 

Considerable progress achieved in the quantum theory of alloys and in the theory predicting the surface composition of alloys (see, e.g., 5 for a review). 

Development in the field of measurements on metals without carrier can now be considered as satisfying and there is still some progress going on. However, the problem of a reliable determination of the surface composition of alloys on carriers is still too far from being solved. In particular, problems like the detection of small amounts of unalloyed active metals on carrier and the question of homogeneity in distribution of the active metals in an inactive matrix have not been solved yet, and just such problems are most likely responsible for some controversies in the results on alloys. More work has to be done in the future in this field. 

The ascertaining by classical and modern methods that the surface composition of alloys can strongly deviate from the composition of the bulk. These methods have opened up the possibility of correlating catalytic phenomena with catalyst surface composition, thus removing a major drawback of older studies with alloy catalysts. 

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). 

A summary of some results for various anode electrocatalyst materials is given in Table III in relation to the d-electron configuration (357). It must be remarked that the linear dependence of several quantities on d-band vacancy, found by Arikado et al. in their CI2 evolution studies (357,434), will arise, in part, from the (probably doubtful) assumption that the d-band vacancy for alloys varies in linear proportion to the values for the elemental components, weighted by the composition ratio. A further factor is that the surface composition of alloys is rarely identical with that of the bulk (cf. Refs. 43 , 432). 

A model P4-18-SPM scanning tunneling microscope (NT-MDT, Russia) was employed to investigate the structure, in atmosphere, of nanometer-scale thin film materials and also to measure the thickness of the film. A setup [28] combining electrochemical studies and X-ray photoelectron spectroscopic (XPS) analysis served to characterize the surface composition of alloy films. 

Surface Composition of Alloys from Model Calculations... 

In a staged multi-scale approach, the energetics and reaction rates obtained from these calculations can be used to develop coarse-grained models for simulating kinetics and thermodynamics of complex multi-step reactions on electrodes (for example see [25, 26, 27, 28, 29, 30]). Varying levels of complexity can be simulated on electrodes to introduce defects on electrode surfaces, composition of alloy electrodes, distribution of alloy electrode surfaces, particulate electrodes, etc. Monte Carlo methods can also be coupled with continuum transport/reaction models to correctly describe surfaces effects and provide accurate boundary conditions (for e.g. see Ref. [31]). In what follows, we briefly describe density functional theory calculations and kinetic Monte Carlo simulations to understand CO electro oxidation on Pt-based electrodes. 

TABLE III Surface Composition of Alloys Experimental Results and Predictions of the Regular Solution Model... 

B. Heinrichs, F. Noville, J.-P. Schoebrechts, and J.-P. Pirard, PaUadinm-Silver Sol-Gel Catalysts for Selective Hydrodechlorination of 1,2-Dichloroethane into Ethylene II. Surface Composition of Alloy Particles, J. Catal., 192, pp. 108-18, 2000. 

Heinrichs B., Noville F., Schoebrechts J.P., Pirard J.P. Palladium-silver sol-gel catalysts for selective hydrodechlorination of 1,2-dichloroethane into ethylene 2 surface composition of alloy particles. J. Catal. 2000 192 108-118... 

Change of the surface composition of alloy by selective dissolution of the less noble element... 

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