Nickel-copper alloys surface composition

Nickel-copper alloys provide a good example of a bimetallic catalyst system in which the variation of catalytic activity with composition depends markedly on the type of reaction, thus leading to substantial selectivity effects. The catalysts to be considered here are alloy powders with a surface area of approximately 1 m2/g (6). Approximately one atom out of a thousand is a surface atom in such catalysts. 

Figure 2.5 Lattice constants for nickel-copper alloys as a function of composition (6). [Circles are data for low surface area ( — 1 m2/g) catalysts squares are data of Coles (38) on metallurgical specimens.] (Reprinted with permission from Academic Press, Inc.)...

Evidence for a marked difference between the surface and bulk compositions of dilute copper-nickel alloys has been reported recently by a number of investigators (82, 87-90). Much of the experimental evidence comes from hydrogen adsorption data (74, 82, 87, 90). The conclusions of van der Plank and Sachtler were based on the premise that nickel chemisorbs hydrogen while copper does not (82, 87). The total adsorption of hydrogen at room temperature was taken as a measure of the amount of nickel in the surface. However, in hydrogen adsorption studies on the catalysts used to obtain the catalytic results in Fig. 6, the amount of adsorption on the copper catalyst, while small compared to the adsorption on nickel, is not negligible (74) However, the amount of strongly adsorbed... 

Spectra for a series of Cu-Ni alloys have been obtained (91) and these are reproduced in Fig. 11. Because of overlapping of peaks from the component metals, separate indications of each element are only obtained from the 925 eV Cu peak and the 718 eV Ni peak. The results have only qualitative significance because the quoted nickel concentrations are bulk values. Nevertheless, they do suggest that for these particular samples of Cu-Ni alloys, the surface composition varies smoothly from pure copper to pure nickel. Auger spectroscopy has subsequently shown that the surface composition of the (110) face of a 55% Cu-Ni crystal was identical with the bulk composition (95a). Ono et al. (95b) have used the technique to study cleaning procedures argon ion bombardment caused nickel enrichment of... 

On the alloy surface the reaction proceeded both via the anhydride and formate intermediates (117). As the copper concentration was increased, the formate species dominated the reaction, until at 63% copper the CO/COj ratio was less than 0.1. This change was due to the decrease in the amount of anhydride formed with increasing copper and the corresponding increase in formate. Since only the anhydride decomposition produced CO, the relative amount of anhydride formed could be determined as a function of surface composition. This relationship is shown in Fig. 21 the anhydride concentration fell as the fourth power of the nickel concentration, suggesting the requirement of four nickel atoms for its stabilization. This value agreed with the earlier determination for the saturation density of anhydride intermediates on Ni(llO) (99). 

It should be noted that, in the interpretation of activity patterns of alloy catalysts, extreme care is needed to ensure that the surface composition is known. It has been shown [321,322] with copper—nickel alloys, which show two phases in the composition range 2—80% copper, that, within this miscibility gap, the surface composition remains constant at 80% Cu—20% Ni, independent of the nominal bulk composition. Furthermore, the surface composition may vary depending upon the catalyst pretreatment [322], No miscibility gap occurs with palladium—gold or palladium-silver alloys [323]. 

If the alloy exists in two phases, that phase with the lower sublimation energy will form on the outer surface. If the nickel-copper system is equilibrated at temperatures near 200°C, it exists as two phases of constant composition in equilibrium. 35 The relative amounts of the phases depends on the overall... 

A comparison of catalytic and corrosion properties is possible, of course, only if the alloy happens to corrode without change of surface composition. For alloys, one component of which is relatively much more noble compared with other components, enrichment of the noble metal or its intermetallic compounds may occur at the surface. This makes it difficult to predict the corrosion behavior of Hume-Rothery alloys based on their catalytic behavior as described, for example, by Schwab 26). Copper-nickel alloys and transition metal alloys involving Fe, Cr, Ni, etc., corrode apparently without evidence of surface composition change of this kind. 

An environmentally friendly sensor was developed by fabricating a nickel-copper (NiCu) alloy electrode to determine the chemical oxygen demand. The NiCu alloy film was applied to modify the surface of a glassy carbon electrode which led to a very stable detecting element. The surface morphology of NiCu alloy was investigated by atomic force microscopy which confirmed its continuity and uniform thickness over the entire electrode. The chemical composition of the developed NiCu film was evaluated by energy-dispersive X-ray spectrometry which revealed 69 % presence of Ni in the alloy. 

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