Alloy reaction path

A number of metals have the ability to absorb hydrogen, which may be taken into solid solution or form a metallic hydride, and this absorption can provide an alternative reaction path to the desorption of H,. as gas. In the case of iron and iron alloys, both hydrogen adsorption and absorption occur simultaneously, and the latter thus gives rise to another equilibrium involving the transfer of H,<,s across the interface to form interstitial H atoms just beneath the surface ... 

Figure 7-6. a) Schematic A-B-O phase diagram of the second kind (/i0//VB) and possible oxidation reaction paths involving (A, B) alloy and oxide compounds, b) Corresponding reaction paths (NB( )) in real space at time t. 

The basic parameters which determine the kinetics of internal oxidation processes are 1) alloy composition (in terms of the mole fraction = (1 NA)), 2) the number and type of compounds or solid solutions (structure, phase field width) which exist in the ternary A-B-0 system, 3) the Gibbs energies of formation and the component chemical potentials of the phases involved, and last but not least, 4) the individual mobilities of the components in both the metal alloy and the product determine the (quasi-steady state) reaction path and thus the kinetics. A complete set of the parameters necessary for the quantitative treatment of internal oxidation kinetics is normally not at hand. Nevertheless, a predictive phenomenological theory will be outlined. 

The comparison of the catalytic performances of metals and their alloys is sometimes hampered by the different degree of deactivation by carbonaceous residues (107, 67). Therefore, it seems appropriate to start with a discussion of the exchange reactions of the hydrogen isotopes protium and deuterium on platinum and Pt-Au films (31). A comparison of this reaction on platinum and its alloy shows that of the two reaction paths possible on platinum in the temperature region studied, one remains unchanged on the alloy but the other, which prevails on platinum except at very low temperatures, seems... 

The H/D exchange between D2 and benzene was found to have a rate exceeding that of benzene deuteration by several orders of magnitude. This result shows that exchange and hydrogenation reactions follow different reaction paths. The exchange parameters were also found to be independent of the overall alloy composition. 

In the case of the co-deposited catalyst (non-alloyed), the number of electron from the methanol stripping experiment is close to 2, indicating that almost only CO is formed at the electrode surface. The number of electrons for the oxidation of bulk methanol is close to 6 (6.6), which indicates that only a small amount of formic acid is formed, and that the main reaction path leads to the formation of CO2, according to the following steps ... 

The trend in hydrogenation practice appears to be toward the use of higher pressures, where the apparatus is smaller. Under such conditions, the reaction velocity is increased the equilibrium positions are made more favorable the reaction path is better defined, with fewer side reactions and heating and cooling and heat interchange are facilitated. The design and construction of equipment for high-pressure work are, however, somewhat more complicated than for low-pressure operations. Alloy steels are by far the most common materials of construction. 

Structure promoters can act in various ways. In the aromatization of alkanes on Pt catalysts, nonselective dissociative reaction paths that lead to gas and coke formation can be suppressed by alloying with tin. This is attributed to the ensemble effect, which is also responsible for the action of alkali and alkaline earth metal hydroxides on Rh catalysts in the synthesis of methanol from CO/H2 and the hydroformylation of ethylene. It was found that by means of the ensemble effect the promoters block active sites and thus suppress the dissociation of CO. Both reactions require small surface ensembles. As a result, methanol production and insertion of CO into the al-kene are both positively influenced. 

Sometimes the adsorption of a molecule A—X with resolution into the radicals A and X depends upon the correct interatomic spacings on the catalyst, and this opens the way to studies of the relation of catalytic power and crystal structure. The formation of covalencies with adsorbed atoms of one kind and another is a function of the electron orbitals of metalKc catalysts, and a considerable field of investigation exists in the relation between the occupation of electron levels in metals and alloys and their catalytic properties. Electron distributions in soM carbon may also play a significant part in its catalytic reactions. Metallic impurities modify these distributions and so change activation energies directly, without opening qualitatively new reaction paths. These matters demand specialized study and we shall not enter further into them here. 

As noted before, metal dusting is to be expected if metallic materials are carburised at carbon activities ac > 1, i.e. under a strong driving force for graphite formation. The carbon from the gas molecules should react to graphite (and, in fact, that is the overall reaction which occurs in metal dusting) and destroy the materials. As yet, two different reaction paths have been observed. For iron and Fe-based alloys, the reaction sequence is as follows (see Fig. 1.7) ... 

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