Activities of Pure Metals

There are few sets of results available for comparing the activities of metals for reactions of alkane greater than C4 by means of Arrhenius parameters based on specific rates or TOFs. Non-specific rates for hydrogenolysis of cyclopentane led27,28 jQ Arrhenius parameters, the compensation plot for which divides the metals examined into three groups, as follows  

Product distributions were recorded, at necessarily very different temperatures (Pd, 573 K Ru, 361 K) the five metals (Os was not studied) all gave some isomerisation, but it was only significant with platinum. With palladium, bond breaking was almost exclusively terminal, and mainly so with rhodium, but with the other three metals it was largely statistical. This behaviom- conforms to that found with the butanes (Chapter 13) and with other alkanes, as we shall see. 

Reflection on some of these results leads to the conclusion that iridium and rhodium sometimes ally themselves with the most active group of metals (e.g. in 

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If correlations do exist for simple metals, predictions are much more difficult for composite materials. On the other hand, cathode activation has two aims (i) to replace active but expensive materials with cheaper ones, and (ii) to enhance the activity of cheaper materials so as to approach or even surpass that of the more expensive catalysts. In the case of pure metals there is little hope to find a new material satisfying the above requirements since in the volcano curve each metal has a fixed position which cannot be changed. Therefore, activation of pure metals can only be achieved by modifying its structure so as to enhance the surface area (which has nothing to do with electrocatalysis in a strict sense), and possibly to influence the mechanism and the energetic state of the intermediate in the wanted direction. This includes the preparation of rough surfaces but also of dispersed catalysts. Examples will be discussed later. 

To obtain good experimental results, it is necessary to know rigorously the ion charge that is responsible for the EMF in cell of type (1). However, the difference in activities of pure metal A and alloy AxB(i-x) can lead to different charges of the A" ion in the vicinity of the electrodes A and AxB(i.x). In this case, even in op>en circuit, a spontaneous transfer of component A to alloys AxB(i-x) is possible and a constant drift of the EMF occurs over time. 

The anodes are generally not of pure metals but of alloys. Certain alloying elements serve to give a fine-grained structure, leading to a relatively uniform metal loss from the surface. Others serve to reduce the self-corrosion and raise the current yield. Finally, alloying elements can prevent or reduce the tendency to surface film formation or passivation. Such activating additions are necessary with aluminum. 

The form of Figure 1.43 is common among many metals in solutions of acidic to neutral pH of non-complexing anions. Some metals such as aluminium and zinc, whose oxides are amphoteric, lose their passivity in alkaline solutions, a feature reflected in the potential/pH diagram. This is likely to arise from the rapid rate at which the oxide is attacked by the solution, rather than from direct attack on the metal, although at low potential, active dissolution is predicted thermodynamically The reader is referred to the classical work of Pourbaix for a full treatment of potential/pH diagrams of pure metals in equilibrium with water. 

In the previous sections we have dealt mainly with the catalytic activity of pure substances such as metallic iron, ruthenium, copper, platinum, etc. Real catalyst, however, are often much more complex materials that have been optimized by adding remote amounts of other elements that may have a profound impact on the overall reactivity or selectivity of the catalyst. Here we shall deal with a few prominent examples of such effects. 

This is explained by a possible higher activity of pure rhodium than supported metal catalysts. However, two other reasons are also taken into account to explain the superior performance of the micro reactor boundary-layer mass transfer limitations, which exist for the laboratory-scale monoliths with larger internal dimensions, are less significant for the micro reactor with order-of-magnitude smaller dimensions, and the use of the thermally highly conductive rhodium as construction material facilitates heat transfer from the oxidation to the reforming zone. 

The activity of a solid The activity of a pure solid in its standard state is unity, so the activity of pure copper or of zinc metal electrodes is one. We write this as a(cU) or a,zn, = 1. 

The mole fraction jc of Fe in pure iron is unity, so the activity of the metallic iron is also unity. The mole fraction x of iron in steel will be less than unity because it is impure. The carbon is evenly distributed throughout the steel, so its mole fraction X(q is constant, itself ensuring that the activity is also constant. Conversely, the sulphur in steel is not evenly distributed, but resides in small (microscopic) pockets . In consequence, the mole fraction of the iron host X(pe) fluctuates, with x being higher where the steel is more pure, and lower in those pockets having a high sulphur content. To summarize, there are differences in the activity of the iron, so a concentration cell forms. 

A metallic electrode consisting of a pure metal in contact with an analyte solution develops an electric potential in response to a redox reaction occurring at its metal surface. Common metal electrodes such as platinum, gold, palladium or carbon are known as inert metal electrodes whose sole function is to transfer electrons to or from species in solution. Metal electrodes corresponding to the first kind are pure metal electrodes such as Ag, Hg and others that respond directly to a change in activity of the metal cation in the solution. For example, for the reaction... 

The metal dusting of pure metals, especially Fe, was studied extensively by Hochman2. The Hochman mechanism for the metal dusting of iron involves three steps. The first step is the formation of metastable iron carbide, FejC, on the surface of iron. This reaction requires carbon activities higher than unity. 

Promoters can, to a certain extent, counterbalance the presence of small amounts of poisons. Thus the oxides of the alkali metals, which in themselves do not improve the activity of pure iron, have an indirect favorable action by binding traces of sulfur. ... 

The problems are mainly with point (3). Some authors point to the fact that in some cases the activity of alloys is higher than that of pure metals, and they conclude that such synergism cannot be explained without... 

Amalgam electrode Metals such as Ca or Na react with water and cannot be used directly as electrodes. They can be used, however, by dissolving them in liquid mercury, forming an amalgam. In the amalgam solution, the activity of the metal is reduced below what it is when pure thus, its reaction with water is suppressed. Mercury does not dissolve in the aqueous solution. 

Figure 5. Schematic arrangement of the surface of a partly crystallized E-L TM amorphous alloy such as Pd-Zr. A matrix of zirconia consisting of the two polymorphs holds particles of the L transition metal (Pd) which are structured in a skin of solid solution with oxygen (white) and a nucleus of pure metal (black). The arrows indicate transport pathways for activated oxygen either through bulk diffusion or via the top surface. An intimate contact with a large metal-to-oxide interface volume with ill-defined defective crystal structures (shaded area) is essential for the good catalytic performance. The figure is compiled from the experimental data in the literature [26, 27].

Beyond human-made Pt- or noble-metal-based ORR electrocatalysts, there exist very active biomimetic carbon-nitrogen-iron ORR electrocatalysts that show great potential for use in PEMFC cathodes, even rivaling the catalytic activity of pure Pt. 

The early conflicting reports on the activity of pure copper metal could not be reconciled without the simultaneous or concurrent measurements of activity, surface area, and surface composition. Moreover, it became evident that it is important to use unsupported copper as the reference material to avoid support-metal interactions that may influence the catalytic properties of the latter. 

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