Hydrogen recombination reactions

The amount of the hydrogen that is liberated on or near a metal surface, which then enters the metal, varies according to the environment and condition of the metal. The main factor that promotes the entry of hydrogen into a metal is the presence on the metal of a surface poison such as sulfide or other species, which inhibit the hydrogen recombination reaction. 

The sudden expansion of the gases, as they are heated in the arc plasma, causes the formation of a high-speed arc jet so that the atomic hydrogen and the reactive carbon species are transported almost instantly to the deposition surface and the chances of hydrogen recombination and of vapor-phase reactions are minimized. 

However, this formulation yields very little information on the actual course of the reaction, i.e. of which partial processes it consists. This set of partial processes is termed the mechanism of the electrode reaction. In the first case, the electrode reaction at some electrodes involves the formation of an adsorbed hydrogen atom, followed by the recombination reaction ... 

Equations 4.2.31 and 4.2.32 represent radical recombination reactions that might be considered as alternative termination reactions. Since these reactions and reaction 4.2.26 are radical recombination reactions, they will all have very low activation energies. The relative rates of these processes will then be governed by the collision frequencies and thus the concentrations of the reacting species. The relative concentrations of hydrogen and bromine atoms can be determined from the steady-state form of equation 4.2.17. 

We have measured the kinetics of ethylidyne formation from chemisorbed ethylene over Pt(lll) surfaces. The rates of reaction display a first order dependence on the ethylene coverage. There is an isotope effect, since the reaction for CjH is about twice as fast as for CjD. We obtain values for the activation energy of 15.0 and 16.7 Kcal/mole for the normal and deuterated ethylene, respectively. These values are lower than those obtained from TDS experiments, but the differences can be reconciled by taking into account the hydrogen recombination when analyzing the thermal desorption data. 

In the analysis of Iyer et al, tH,ji is not assumed to be very much less than iV, so that iV at steady state equals the sum of the currents representing the Tafel recombination reaction and hydrogen permeation... 

The chain fragments formed by the recombination of free radicals can be reconverted into radicals by a variety of reinitiation processes, some of which are listed in Table 1. Such reactions can occur in the gas phase via electron collision and on the polymer surface by impact of charged particles or photon absorption. Reinitiation may also be induced in both the gas phase and on the polymer surface by hydrogen transfer reactions. These last processes are similar to the chain transfer processes which occur during homogeneous polymerization. Expressions for the rates of reinitiation are given by Eqns. 20 through 23. 

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). 

Following hydrogen recombination the luminosity rises at a rate that is very sensitive to the explosion energy, the envelope mass, and to the opacity in the helium core. The ultimate source of the energy here is the decay of Co to 56Fe, a reaction that has powered the light curve since late March and especially through the peak and tail. Because the amount of 5 Ni synthesized is artificially constrained to be the same (0.07 Me V) in all our models, the peak... 

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