Catalysis on alloy films

R. Van Hardeveld and F. Hartog Adsorption and Catalysis on Evaporated Alloy Films... 

It is particularly helpful that we can take the Cu-Ni system as an example of the use of successive deposition for preparing alloy films where a miscibility gap exists, and one component can diffuse readily, because this alloy system is also historically important in discussing catalysis by metals. The rate of migration of the copper atoms is much higher than that of the nickel atoms (there is a pronounced Kirkendall effect) and, with polycrystalline specimens, surface diffusion of copper over the nickel crystallites requires a lower activation energy than diffusion into the bulk of the crystallites. Hence, the following model was proposed for the location of the phases in Cu-Ni films (S3), prepared by annealing successively deposited layers at 200°C in vacuum, which was consistent with the experimental data on the work function. 

Gold forms a continuous series of solid solutions with palladium, and there is no evidence for the existence of a miscibility gap. Also, the catalytic properties of the component metals are very different, and for these reasons the Pd-Au alloys have been popular in studies of the electronic factor in catalysis. The well-known paper by Couper and Eley (127) remains the most clearly defined example of a correlation between catalytic activity and the filling of d-band vacancies. The apparent activation energy for the ortho-parahydrogen conversion over Pd-Au wires wras constant on Pd and the Pd-rich alloys, but increased abruptly at 60% Au, at which composition d-band vacancies were considered to be just filled. Subsequently, Eley, with various collaborators, has studied a number of other reactions over the same alloy wires, e.g., formic acid decomposition 128), CO oxidation 129), and N20 decomposition ISO). These results, and the extent to which they support the d-band theory, have been reviewed by Eley (1). We shall confine our attention here to the chemisorption of oxygen and the decomposition of formic acid, winch have been studied on Pd-Au alloy films. 

The Pt-Au films were not used in catalysis, but the chemisorption of CO was studied. The work function of Pt was only raised by 0.03 eV and there was no change with the alloys after short exposures to CO. It was therefore not possible to titrate the Pt content of the surface with CO in the same way as hydrogen was used with Cu-Ni alloy films (2). Long-term exposure of the films to 10 5-10-4 Torr CO at 20°C for periods up to four days caused the work function of the alloys to increase slowly Fig. 31. After 16 hr this increase was more evident in the Pt-rich region, but the effect was observed on the Au-rich regions after longer exposures. The effect was accelerated if the films were maintained at 100°C. These results were cited as direct evidence for the enrichment of the surface with platinum... 

The subject of Adsorption and Catalysis on Evaporated Alloy Films is reviewed and Moss and Whalley conclude that phase separation caused a variety of complications which makes it difficult to define the nature of catalytic activity. 

Influence of Metal Particle Size in Nickel-on-Aerosil Catalysts on Surface Site Distribution, Catalytic Activity, and Selectivity R. Van Hardeveld and F. Hartog Adsorption and Catalysis on Evaporated Alloy Films... 

In comparison to most other methods in surface science, STM offers two important advantages (1) it provides local information on the atomic scale and (2) it does so in situ [50]. As STM operates best on flat surfaces, applications of the technique in catalysis relate to models for catalysts, with the emphasis on metal single crystals. Several reviews have provided excellent overviews of the possibilities [51-54], and many studies of particles on model supports have been reported, such as graphite-supported Pt [55] and Pd [56] model catalysts. In the latter case, Humbert et al. [56] were able to recognize surface facets with (111) structure on palladium particles of 1.5 nm diameter, on an STM image taken in air. The use of ultra-thin oxide films, such as AI2O3 on a NiAl alloy, has enabled STM studies of oxide-supported metal particles to be performed, as reviewed by Freund [57]. 

---