Activity of alloys

A very convenient method of investigating electronic effects in catalysis is to study the activities within a series of metal alloys. Changes in alloy composition will alter the energy density of electron levels at the Fermi surface. Moreover, in the particular case of transition metals, it has been seen that alloying with a metal of Group 16 decreases the number of holes in the d-band. These effects should profoundly alter the catalytic activities of the alloys. It is important to keep in mind that within such 

The catalyst was used in the form of a wire sealed down the axis of a cylindrical reaction vessel and care was taken to prevent its contamination, by baking it out and using a liquid air trap. Instead of the usual 

The effect of temperature on the first-order rate constant of the ortho-para hydrogen conversion, given by the Arrhenius equation k — was determined for the series of Pd-Au alloys from 0 to 

Dowden and Reynolds (49,50) in further experimental work on the hydrogenation of benzene and styrene with nickel-copper alloys as catalysts, found a similar dependence. The specific activities of the nickel-copper alloy catalysts decreased with increasing copper content to a negligible value at 60% copper and 30-40% copper for benzene and styrene, respectively. Low-temperature specific heat data indicated a sharp fall (1) in the energy density of electron levels N(E) at the Fermi surface, where the d-band of nickel becomes filled at 60 % copper, and (2) from nickel to the binary alloy 80 nickel -)- 20 iron. Further work by these authors (50) on styrene hydrogenation with nickel-iron alloy 

The rise in activity from 80 nickel-20 iron to 100% nickel is attributed to the large increase in N E), and the fall from nickel to nickel-copper is attributed to both the decrease in the number of d-holes and to the large fall in N(E). 

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In these particular experiments it proved impossible to investigate the effect of copper concentration on the catalytic activity of alloys free of the hydride phase. Figure 10 69, 64a, 65) illustrates the changing values of the recombination coefficient on nickel-copper alloys related to the composition of the alloy at room temperature. The small amount of copper introduced into the nickel already distinctly decreased the catalytic ac-... 

A general problem existing with all multicomponent catalysts is the fact that their catalytic activity depends not on the component ratio in the bulk of the electrode but on that in the surface layer, which owing to the preferential dissolution of certain components, may vary in time or as a result of certain electrode pretreatments. The same holds for the phase composition of the surface layer, which may well be different from that in the bulk alloy. It is for this reason that numerous attempts at correlating the catalytic activities of alloys and other binary systems with their bulk properties proved futile. 

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... 

The LDOS-based frontier orbital model is different from the simpler collective-electron model, in which all local information is averaged out. In the context of alloys. Ponec and Bond (23. p. 451). stated that it must be clear to the reader that (the collective electron model of catalytic activity of alloys] has now been consigned to the trash can of science. . . [because of] the discovery that, more in tune with chemists intuition, the atoms in an alloy retain their identity more or less completely. Such local properties disappear from the collective-electron model in which it was supposed that the available electrons were equally shared by aU the atoms present, and that. .. the density of states at the Fermi surface or some related... 

Figure 3. Comparison of the activity of alloyed and mixed Rh/Pt/C catalysts in the crotonic acid test.

A second method to determine partial molar Gibbs energies and activities of alloy components in an alloy is the measurement of the cell voltage of the cell shown in Figure 3.7. The alloy can be formed by metallurgical alloying or, as is shown in Figure 3.7, be deposited on a substrate either electrochemically or by vapor deposition methods. 

The irradiation of steels in a neutron flux results in activation of alloying elements and impurities and the packaging of steels must take account of the inventory of activation products. The key issue for OCR steel is heat production and radiation dose due to the activation products cobalt-60, iron-55 and nickel-63. Generally for normal steels the dominant activation product for heat and dose is Co-60 but it is important to understand the chemical composition of steels used inside a reactor as special grades or alloys may introduce unusual radionuclides which could be important contributors to heat, dose and other aspects of package performance such as post-closure risk. 

Fig. 71. Activities of alloys in the samarium-gadolinium system at 900°C. The straight diagonal lines represent ideal behavior as indicated by Raoult s law.

A promising technique for the study of the catalytic activity of alloys is the study of chemisorption on iron overlayers on single crystals of other metals such as Fe/Ru [375], Fe/Re [376], and Fe/W [377]. 

Analyses of alloys or ores for hafnium by plasma emission atomic absorption spectroscopy, optical emission spectroscopy (qv), mass spectrometry (qv), x-ray spectroscopy (see X-ray technology), and neutron activation are possible without prior separation of hafnium (19). Alternatively, the combined hafnium and zirconium content can be separated from the sample by fusing the sample with sodium hydroxide, separating silica if present, and precipitating with mandelic acid from a dilute hydrochloric acid solution (20). The precipitate is ignited to oxide which is analy2ed by x-ray or emission spectroscopy to determine the relative proportion of each oxide. 

Lime is added to the reaction to increase the activity of MnO and reduce the activity of Si02. This allows greater extraction of manganese in equilibrium with less than 2% Si in the alloy. 

Toxic heavy metals and ions, eg, Pb, Hg, Bi, Sn, Zn, Cd, Cu, and Fe, may form alloys with catalytic metals (24). Materials such as metallic lead, ziac, and arsenic react irreversibly with precious metals and make the surface unavailable for catalytic reactions. Poisoning by heavy metals ordinarily destroys the activity of a precious-metal catalyst (8). 

FIG. 28-10 Six possible types of behavior for an active/passive alloy in a corrosive environment. 

We developed a sensor for determination of content of phosphorars in metallurgical melts. In quality of ion conductor used orthophosphate of calcium which pressed in tablets 010 mm. Tablets (mass 1-2 g) annealed at a temperature 400°C during 7-10 h. Tablets melts then in a quartz tube and placed the alloy of iron containing 1 mass % P. Control of sensor lead on Fe - P melts. Information on activities (effective concentration) of phosphorars in Fe - P melts was received. It is set that the isotherm of activity of phosphorars shows negative deviations from the Raouls law. Comparison them with reliable literary inforiuation showed that they agree between itself. Thus, reliable data on activities (effective concentration) of phosphorars in metallic melts it is possible to received by created electrochemical sensor for express determination. 

There are no films or protective surface films on active metals, e.g., mild steel in acid or saline solutions. Passive metals are protected by dense, less readily soluble surface films (see Section 2.3.1.2). These include, for example, high-alloy Cr steels and NiCr alloys as well as A1 and Ti in neutral solutions. Selective corrosion of alloys is largely a result of local concentration differences of alloying elements which are important for corrosion resistance e.g., Cr [4],... 

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. 

Pure aluminum cannot be used as an anode material on account of its easy passivatability. For galvanic anodes, aluminum alloys are employed that contain activating alloying elements that hinder or prevent the formation of surface films. These are usually up to 8% Zn and/or 5% Mg. In addition, metals such as Cd, Ga, In, Hg and T1 are added as so-called lattice expanders, these maintain the longterm activity of the anode. Activation naturally also encourages self-corrosion of the anode. In order to optimize the current yield, so-called lattice contractors are added that include Mn, Si and Ti. 

The addition of cathodically active elements to pure lead was the main objective of investigations to improve its corrosion resistance to H2SO4 [42,44]. Best known is copper-lead with 0.04 to 0.08% Cu. By adding combinations of alloying elements, it was possible to produce lead alloys that not only had much better corrosion resistance, but also had greater high-temperature strength. Lead alloy with 0.1% Sn, 0.1% Cu and 0.1% Pd is an example [45]. 

The development of new polymer alloys has caused a lot of excitement in recent years but in fact the concept has been around for a long time. Indeed one of the major commercial successes of today, ABS, is in fact an alloy of acrylonitrile, butadiene and styrene. The principle of alloying plastics is similar to that of alloying metals - to achieve in one material the advantages possessed by several others. The recent increased interest and activity in the field of polymer alloys has occurred as a result of several new factors. One is the development of more sophisticated techniques for combining plastics which were previously considered to be incompatible. Another is the keen competition for a share of new market areas such as automobile bumpers, body panels etc. These applications call for combinations of properties not previously available in a single plastic and it has been found that it is less expensive to combine existing plastics than to develop a new monomer on which to base the new plastic. 

Two different sets of experimental conditions have been used. Buu-Hoi et al. and Hansen have employed the method introduced by Papa et using Raney nickel alloy directly for the desulfurization in an alkaline medium. Under these conditions most functional groups are removed and this method is most convenient for the preparation of aliphatic acids. The other method uses Raney nickel catalysts of different reactivity in various solvents such as aqueous ammonia, alcohol, ether, or acetone. The solvent and activity of the catalyst can have an appreciable influence on yields and types of compounds formed, but have not yet been investigated in detail. In acetic anhydride, for instance, desulfurization of thiophenes does not occur and these reaction conditions have been employed for reductive acetylation of nitrothiophenes. Even under the mildest conditions, all double bonds are hydrogenated and all halogens removed. Nitro and oxime groups are reduced to amines. 

With special techniques for the activation of the metal—e.g. for removal of the oxide layer, and the preparation of finely dispersed metal—the scope of the Refor-matsky reaction has been broadened, and yields have been markedly improved." The attempted activation of zinc by treatment with iodine or dibromomethane, or washing with dilute hydrochloric acid prior to use, often is only moderately successful. Much more effective is the use of special alloys—e.g. zinc-copper couple, or the reduction of zinc halides using potassium (the so-called Rieke procedure ) or potassium graphite. The application of ultrasound has also been reported. ... 

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