Atomisation kinetics

Atomisation kinetics when molecular adsorption is activated... 

In the atomisation reaction, we have a flux of molecules impinging on the surface, generally at a temperature (T ) very much lower than the temperature (T) of the surface, and a flux of atoms and molecules departing from the surface. Before we consider the kinetics for these practical conditions, we will examine the situation for an equilibrium system. 

Kinetics of atomisation under stationary conditions when molecular adsorption is unactivated... 

It is evident that two distinct types of behaviour can occur depending on the conditions. When the rate of atomisation is slow relative to the collision rate, is proportional to (Fj)-172 (half-order kinetics) and when the rate of atomisation is comparable to the collision rate, approaches a limiting upper value (first-order kinetics). The circumstances in which these two kinds of behaviour are to be expected will now be examined. 

Fig. 3. The transition from half-order to first-order kinetics calculated by means of eqn. (43) for the atomisation of hydrogen at 1200 K and 1300 K (courtesy Brennan [6]).

The physical interpretation of the conditions defined by eqn. (44) is that there will be many molecules adsorbed and desorbed for every atom which leaves the surface. This means that the stationary concentration of the adsorbed layer is negligibly different from the equilibrium value. In the region of half-order kinetics, the mechanism of atomisation is... 

So that we will be able to consider the controversy concerning the atomisation of hydrogen by gold [111], we now develop relations for the kinetics of atomisation when s2 is not constant, but depends on temperature. The discussion will also be relevant to our later development of the kinetics of recombination. We assume that the adsorption of a molecule requires the presence of two adjacent vacant surface sites and that the probability that adsorption occurs when a molecule strikes such a pair of sites is k2,1 so we write... 

Tret yakov [111] did not find half-order kinetics for the atomisation of hydrogen by gold under these conditions. On the contrary, they reported first-order kinetics. They attributed the observation of half-order kinetics by Brennan and Fletcher for this system to the presence of surface impurities capable of rapidly dissociating molecular hydrogen, whereas molecular adsorption on the clean surface, being activated, was considered by them to be rate-determining. We will return to the H2—Au system in Sect. 3.2.1(d). Nomes and Donaldson [8] have demonstrated half-order behaviour for the N2—W system, but over a narrower pressure range. 

Sticking coefficients for the adsorption of molecules, derived from the probability of atomisation in the region of first-order kinetics... 

Figure 13(a) shows a typical set of plots for the rate of desorption of atoms at different temperatures and pressures. Although straight lines have been drawn through the points, it should not be concluded that Fj depends linearly on the pressure in these reactions. On the contrary, as we have seen, Fj will depend on the rate of desorption of atoms as compared with the rate of adsorption of molecules. For all the conditions represented in Fig. 13(a), the rate of atomisation is only an extremely small fraction of the rate of adsorption of molecules, so the kinetics will be half-order with respect to P2. However, the range of pressure over which the measurements have been made is insufficient to reveal the /P dependence. The variation of d1 with temperature is shown in Fig. 13(b) and the equation of the least squares straight line gives the relation... 

Cas temperature This depends on the kinetic energy of the atoms and ions in the plasma. It can be determined from the Doppler peak broadening. This is however not completely straightforward, as the contributions of Doppler and temperature line broadening have to be separated by mathematical deconvolution. Together with the rotational temperature it is an indicator for the vaporisation and atomisation capability of a plasma. 

---