Recombination of Hydrogen Atoms

The temperature behavior of the alloy catalysts in the heterogeneous recombination of hydrogen atoms was different for rich in nickel alloys from one side and for rich in copper from the other. For the three alloy catalyst films, i.e. Ni97Cu3, Ni77Cu23, and Ni57Cu43 (numbers represent... 

On the basis of the related experimental evidence and its discussion one can regard the poisoning effect of the hydride hydrogen in nickel and its alloys with copper as normally accompanying the heterogeneous recombination of hydrogen atoms on these catalysts at lower temperatures. 

Further investigations of the above discussed effects show that, at fixed temperature of the oxide film (catalyst), the jump in the electric conductivity first increases in amplitude, as the portion of alcohol vapor admitted into the vessel increases. On further increase of the admitted portion of alcohol, the jump amplitude reduces (starting with the pressure of 3.6-10 2 Torr). At the pressure of 3.2-10 Torr, the jump in the electric conductivity of the zinc oxide film is less pronounced. Finally, at still higher pressures, it disappears (Fig.4.9). This effect is not unexpected. On our mind, it is associated with the fact that, as the concentration of alcohol vapor increases, the sum of the rate of interaction of the vapor with adsorbed hydrogen atoms and the rate of surface recombination of hydrogen atoms at the time instant of production becomes higher than the chemisorption rate of these atoms. The latter is responsible for the increase of the electric conductivity of the semiconductor oxide film via the reaction... 

Such a possibility has been pointed out for the first time in [13], where processes of diffusion and recombination of hydrogen atoms (protium and deuterium) have been studied in a water layer frozen on a semiconductor zinc oxide layer. 

However, due to the difficulties in calculating ion yields in SIMS, quantitation of the data is not very reliable, and their work was not conclusive. We have determined here that the reaction of chemisorbed ethylene to form ethylidyne is first order in ethylene coverage. A noticeable isotope effect was observed, with activation energies of 15.0 and 16.7 Kcal/mole for C H and 02 respectively. These values are smaller than those calculated from TDS, but the differences can be reconciled by including the recombination of hydrogen atoms on the surface in the interpretation of the thermal desorption experiments. 

Depending on the pressure, recombination reactions (the reverse of the dissociation reactions) may be bimolecular or termolecular reactions. Recombination of hydrogen atoms is typically at its low-pressure limit and proceeds as a termolecular reaction,... 

The alkyl radical may also dissociate thermally to form an alkene and a smaller alkyl radical. The mechanism that is initiated by these reactions is chain propagating rather than chain branching and for this reason the overall oxidation rate of the fuel decreases. Also there is a change from OH to HO2 as the main chain carrier, and as we have seen, the HO2 radical is much less reactive than OH. The HO2 radical is formed both from alkyl + O2 hydrogen abstraction reactions such as (R69) and from recombination of hydrogen atoms with O2, H + O2 + M HO2 + M (R5). Under lean conditions any hydrogen atoms formed will primarily react with oxygen. At intermediate temperatures the reaction H + O2 O + OH (Rl) is still too slow to compete with (R5). 

Hardy and Linnett (26) have compared the recombination of hydrogen atoms on Pd-Ag and Ni-Cu alloys under conditions where the surface composition is nearly equal to that of the bulk. In both cases they observe a maximum in activity, as shown in Fig. 14. The maximum is at 40% Ag in the Ag-Pd alloy, but at 60% Cu in the Ni-Cu alloy. Since the final decrease in activity is probably the result of a decrease in rate of adsorption, the rapid decline in the Pd-Ag alloys compared with that in the Ni-Cu alloys could be due to the loss of holes at 60% Ag. Another contribution to the lower rate can be the solution of hydrogen or deuterium in the Pd-Ag alloy. 

The qualitative evidence that the delay occurs at the stage of recombination of hydrogen atoms thus appears fairly good. Though he did not specifically claim a general relationship between catalytic power and overpotential, Tafel introduced the idea that the rate of recombination is important implicitly into his equations in 1905,6 It is interesting that... 

Methane formation at very high light intensities is probably due to the recombination of hydrogen atoms and methyl radicals , viz. 

S. R. Federman and L. Frommhold. Recombination of hydrogen atoms via free-to-bound Raman transitions. Phys. Rev. A, 25 2012-2016 (1982). 

Three slices of single-crystal nickel, 10 mm. in diameter and 2 mm. in thickness, respectively, parallel to the (110), (100), and (111) planes were prepared by S. Kaya and denoted, respectively, a, b, and c. They were studied by Matsuda (unpublished) about ten years ago with particular reference to the rates of recombination of hydrogen atoms and to the activation energies for the chemisorption of hydrogen. The latter had been theoretically estimated by Horiuti et ah, as mentioned elsewhere, as functions of the different arrangement of the nickel atoms on the three main crystal planes mentioned above. The relative rates of recombination on the electropolished surfaces of these specimens observed by Matsuda at room temperature followed the qualitative rule ... 

Glarborg, P., Ostberg, M., AIzueta, M.U., Dam-Johansen, K., and Miller, J.A. (1998) The Recombination of Hydrogen Atoms with Nitric Oxide at High Temperatures. 11 Symp. (Int.) on Combust., TTie Combustion Institute, Pittsburgh, PA, USA, 219-226. 

In principle, the decomposition may be a one-step process. It is more likely, however, that two hydrogen atoms are formed in consecutive steps followed by recombination of hydrogen atoms to Ha... 

Bond Type in the Adsorbed Film Production of Hydrogen Atoms by Hot Tungsten Recombination of Hydrogen Atoms The Parahydrogen Conversion. ... 

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