Platinum-nickel alloy films

Typical of such studies is that of Nava et al. [224] on the interaction of an Ni-Pt alloy with crystalline silicon. The system is a particularly favorable one since both nickel and platinum are much heavier than silicon, and platinum is in turn much heavier than nickel. Alloy films about... 

The corrosion behavior of non-ferrous alloys such as those based on nickel, cobalt, copper, zirconium, and titanium has been reviewed in detail in this chapter. Besides exotic materials such as tantalum and platinum, nickel-based alloys are the most resistant to corrosion by mineral acids, and they are especially resistant to localized corrosion in chloride-containing environments, which troubles stainless steels. Nickel-based alloys can broadly be divided into alloys, e.g. Ni-Mo (B-2, B-3) and Ni-Cu (alloy 400), that do not contain chromium, and are not, therefore, passivated under oxidizing conditions, and alloys, e.g. Ni-Cr-Mo (C-22, C-2000,59,686, etc.) and Ni-Cr-Fe (G-30, 825, etc.), that form a chromium oxide passive film under oxidizing conditions. Ni-Mo alloys such as B-3 have excellent corrosion resistance in hot reducing acids such as hydrochloric and sulfuric. Ni-Mo alloys cannot withstand oxidizing conditions such as nitric acid and hydrochloric acid contaminated with ferric ions. Ni-Cr-Mo alloys such as C-2000 alloy are multipurpose alloys that can be used both in reducing and oxidizing conditions. 

In alloys with 0-23% Cu the activation energy of the total conversion of n-hexane is only marginally influenced and the observed effects are consequently connected with the preexponential factors. Since the selectivity of nickel diluted with copper is near the value found by Anderson et al. (113) for highly dispersed films, considering a common cause is suggested (60). Anderson assumes that with a large fraction of surface atoms in very small crystals the isolated corner atoms favor the formation of carbocyclic intermediates of isomerization, whereas hydrogenolysis requires two or more adjacent platinum atoms in a crystal plane. 

Other metals, such as copper, nickel, or silver, have been used as electrode materials in connection with specific applications, such as the detection of amino acids or carbohydrates in alkaline media (copper and nickel) and cyanide or sulfur compounds (silver). Unlike platinum or gold electrodes, these electrodes offer a stable response for carbohydrates at constant potentials, through the formation of high-valence oxyhydroxide species formed in situ on the surface and believed to act as redox mediators (40,41). Bismuth film electrodes (preplated or in situ plated ones) have been shown to be an attractive alternative to mercury films used for stripping voltammetry of trace metals (42,43). Alloy electrodes (e.g., platinum-ruthenium, nickel-titanium) are also being used for addressing adsorption or corrosion effects of one of their components. The bifunctional catalytic mechanism of alloy electrodes (such as Pt-Ru or Pt-Sn ones) has been particularly useful for fuel cell applications (44). 

For anodic processes the choice of materials for the electrode is much more limited than for cathodic ones, as the anode could bo easily attacked by the products of the electrolysis (chlorine, oxygon etc.), or electrochemioally dissolved. In alkaline solutions the selection will be restricted to the application of platinum (or alloys of platinum with irridium or rhodium), palladium, carbon (or rather graphite) iron and nickel, while for acid solutions only metals of the platinum group and graphite will be suitable in a special case of the electrolysis in sulphuric acid solutions lead has found wide use, it getting coated with a conductive film of lead dioxide. 

The alloy catalysts used in these early studies were low surface area materials, commonly metal powders or films. The surface areas, for example, were two orders of magnitude lower than that of platinum in a commercial reforming catalyst. Hence these alloys were not of interest as practical catalysts. The systems emphasized in these studies were combinations of metallic elements that formed continuous series of solid solutions, such as nickel-copper and palladium-gold. The use of such systems presumably made it possible to vary the electronic structure of a metal crystal in a known and convenient manner, and thereby to determine its influence on catalytic activity. Bimetallic combinations of elements exhibiting limited miscibility in the bulk were not of interest. Aspects of bimetallic catalysts other than questions related to the influence of bulk electronic structure received little attention in these studies. 

---