Activation, of hydrogen

Xs are surface fractions (or active centers), free and covered by chemisorbed species of hydrogen, carbon monoxide, and methanol. H, C, and M are activities of hydrogen, carbon monoxide, and methanol. Primes indicate equilibrium values. 

Griess has observed crevice corrosion of titanium in hot concentrated solutions of Cl , SOj I ions, and considers that the formation of acid within the crevice is the major factor in the mechanism. He points out that at room temperature Ti(OH)3 precipitates at pH 3, and Ti(OH)4 at pH 0-7, and that at elevated temperatures and at the high concentrations of Cl ions that prevail within a crevice the activity of hydrogen ions could be even greater than that indicated by the equilibrium pH values at ambient temperatures. Alloys that remain passive in acid solutions of the same pH as that developed within a crevice should be more immune to crevice attack than pure titanium, and this appears to be the case with alloys containing 0-2% Pd, 2% Mo or 2[Pg.169]

Electrochemically, the system metal/molten salt is somewhat similar to the system metal/aqueous solution, although there are important differences, arising largely from differences in temperature and in electrical conductivity. Most fused salts are predominantly ionic, but contain a proportion of molecular constituents, while pure water is predominantly molecular, containing very low activities of hydrogen and hydroxyl ions. Since the aqueous system has been extensively studied, it may be instructive to point out some analogues in fused-salt systems. 

Hydrogen Electrode an electrode at which the equilibrium (aq.) + jHj, is established. By definition, at unit activity of hydrogen ions and unit fugacity of hydrogen gas the potential of the standard hydrogen electrode h+/y//2 =... 

Nevertheless, Ta5+ and Nb5+ interact with aqueous media containing fluorine ions, such as solutions of hydrofluoric acid. On the other hand, as was clearly shown by Majima et al. [448 - 450], the increased hydrogen ion activity can also significantly enhance the dissolution rate of oxides. The activity of hydrogen ions can be increased by the addition of mineral salts or mineral acids to the solution. 

The use of molybdenum catalysts in combination with hydrogen peroxide is not so common. Nevertheless, there are a number of systems in which molybdates have been employed for the activation of hydrogen peroxide. A catalytic amount of sodium molybdate in combination with monodentate ligands (e.g., hexaalkyl phosphorus triamides or pyridine-N-oxides), and sulfuric acid allowed the epoxidation of simple linear or cyclic olefins [46]. The selectivity obtained by this method was quite low, and significant amounts of diol were formed, even though highly concentrated hydrogen peroxide (>70%) was employed. 

Begue and coworkers recently achieved an improvement in this method by performing the epoxidation reaction in hexafluoro-2-propanol [120]. They found that the activity of hydrogen peroxide was significantly increased in this fluorous alcohol, in relation to trifluoroethanol, which allowed for the use of 30% aqueous H202. Interestingly, the nature of the substrate and the choice of additive turned out to have important consequences for the lifetime of the catalyst. Cyclic dis-ubstituted olefins were efficiently epoxidized with 0.1 mol% of MTO and 10 mol%... 

The dependence of the equilibrium potential on the activities of hydrogen and sulfuric acid is given by the corresponding Nernst equation ... 

Recently, other authors when studying the activation of hydrogen by nickel and nickel-copper catalysts in the hydrogen-deuterium exchange reaction concentrated for example only on the role of nickel in these alloys (56) or on a correlation between the true nickel concentration in the surface layer of an alloy, as stated by the Auger electron spectroscopy, and the catalytic activity (57). 

J. J. Chessick and A. C. Zettlemoyer The Catalytic Activation of Hydrogen in Homogeneous, Heterogeneous, and Biological Systems J. Halpern... 

Heterolytic activation of hydrogen by transition metal complexes. P. J. Brothers, Prog. Inorg. Chem., 1981,28,1-61 (168). 

Value calculated from jo assuming a = 0.5 and the given acidity, activity of hydrogen set to 1. The authors report a considerable scattering of the obtained data. 

Activators of Hydrogen Peroxide, Functional Catalase-Peroxidase Replicas 494... 

B. Activation of Hydrogen. Activation of hydrogen is clearly as important for the catalytic processes under consideration as the activation of CO. For Co and Mn the hydrogen activation mechanism is the equilibrium ... 

Another merit of the AhupL mutant is that its activity of hydrogen reabsorption is very small even at high cell densities (Masukawa et al., unpublished observation). These results indicate that deletion of the hupL gene could be employed as a source for further improvement of H2 production in a nitrogenase-based photobiological H2 production system. 

Compounds and complexes of the early transition metals are oxophilic because the low d-electron count invites the stabilization of metal-oxo bonds by 7T-bond formation. To a substantial extent, their reactivity is typical of complexes of metals other than rhenium. That is particularly the case insofar as activation of hydrogen peroxide is concerned. Catalysis by d° metals - not only Revn, but also CrVI, WVI, MoVI, Vv, ZrIV and HfIV - has been noted. The parent forms of these compounds have at least one oxo group. Again the issue is the coordination of the oxygen donating substrate, HOOH, to the metal, usually by condensation ... 

Direct heterolytic activation of hydrogen by RhCl(CO)(PPh3)2 has been suggested, but likely involves an intermediate dihydride D. Evans, J.A. Osborn, G. Wilkinson, /. Chem. Soc. A 1968, 3133. 

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