Atomic hydrogen mechanism

A new, low-pressure, plasma-assisted proeess for synthesising diamonds has been found by Roy et al [83,84]. An intimate mixture of various forms of carbon with one of many metals (e.g., Au, Ag, Fe, Cu, Ni) is exposed to a microwave plasma derived from pure hydrogen at temperatures ranging from 600-1000 °C. Roy et al postulate a mechanism in which a solid solution of atomic hydrogen and the metal. Me, facilitates dissolution of carbon to form molten droplets of Me -Cj,-H. Diamonds nucleate at the surface of the droplets as the temperature is reduced. 

Table III lists the kinetic equations for the reactions studied by Scholten and Konvalinka when the hydride was the catalyst involved. Uncracked samples of the hydride exhibit far greater activation energy than does the a-phase, i.e. 12.5 kcal/mole, in good accord with 11 kcal/mole obtained by Couper and Eley for a wire preexposed to the atomic hydrogen. The exponent of the power at p amounts to 0.64 no matter which one of the reactions was studied and under what conditions of p and T the kinetic experiments were carried out. According to Scholten and Konvalinka this is a unique quantitative factor common to the reactions studied on palladium hydride as catalyst. It constitutes a point of departure for the authors proposal for the mechanism of the para-hydrogen conversion reaction catalyzed by the hydride phase.

The same reaction mechanism operated on the surfaces of both kinds of catalyst on metals and their hydrides as well. The reaction proceeded according to the Rideal-Eley mechanism and was of first order with respect to the atomic hydrogen concentration in the gas phase. 

Values for rs p and T for the three different models considered above are given in Table III. Once again, the equation for the hydrogen atom stripping mechanism was directly integrated to give I to1al, but for the... 

The kinematic theory can now be used to determine the appropriate values of Js, lQtotal, and It (Equations 15, 16, and 17, respectively) for various assumed values of k, the rate constant of Reaction O—a quantity about which nothing is known—and for the various mechanisms. Obviously from the chemistry of the system, Reactions U, V, and W cannot occur by a hydrogen atom transfer mechanism hence, only two cases need be considered—Reactions S, T, U, V, and W, occurring by... 

The deposition is a complex mechanism which is not fully understood at this time. Two conditions seem necessary (a) activation of the carbon species and (b) the action of atomic hydrogen. These factors are reviewed in the following sections. 

Transient computations of methane, ethane, and propane gas-jet diffusion flames in Ig and Oy have been performed using the numerical code developed by Katta [30,46], with a detailed reaction mechanism [47,48] (33 species and 112 elementary steps) for these fuels and a simple radiation heat-loss model [49], for the high fuel-flow condition. The results for methane and ethane can be obtained from earlier studies [44,45]. For propane. Figure 8.1.5 shows the calculated flame structure in Ig and Og. The variables on the right half include, velocity vectors (v), isotherms (T), total heat-release rate ( j), and the local equivalence ratio (( locai) while on the left half the total molar flux vectors of atomic hydrogen (M ), oxygen mole fraction oxygen consumption rate... 

Suggestions that the sulphate catalysed paths may involve a mechanism with a sulphate-bridged activated complex, as opposed to a hydrogen-atom transfer mechanism, have been made. ... 

In the past it had been a popular belief that the electrochemical reduction of any inorganic or organic substance involves the primary electrochemical formation of a special, active form of hydrogen in the nascent state (in statu nascendi) and subsequent chemical reaction of this hydrogen with the substrate. However, for many reduction reactions a mechanism of direct electron transfer from the electrode to the substrate could be demonstrated. It is only in individual cases involving electrodes with superior hydrogen adsorption that the mechanism above with an intermediate formation of adsorbed atomic hydrogen is possible. 

Gerischer H. 1958. Mechanism of electrol5dic discharge of hydrogen and adsorption energy of atomic hydrogen. Bull Soc Chim Belg 67 506. 

Proposed intermediates in the above reaction include atomic hydrogen [27, 28], hydride ions [29, 30], metal hydroxides [31], metaphosphites [32, 33], and excitons [34]. In general, the postulated mechanisms are not supported by direct independent evidence for these intermediates. Some authors [35] maintain that the mechanism is entirely electrochemical (i.e. it is controlled by electron transfer across the metal-electrolyte interface), but others [26] advocate a process involving a surface-catalyzed redox reaction without interfacial electron transfer. 

In this chapter we will list the deep-level centers passivated by atomic hydrogen in the major elemental semiconductor, namely Si, and discuss their thermal stability and the possible passivation mechanisms. As is the case with any aspect of hydrogen in semiconductors, much more work has been performed in Si than any of the other materials. 

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