Brennan and Fletcher

Fig. 9. The dependence on pressure of the rate of atomisation of hydrogen over tungsten at about 1200 K. The differently styled points denote separate experiments made over a period of a few days. (Courtesy Brennan and Fletcher [6].)...

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. 

Fig. 10. The dependence on temperature of rates of atomisation, uncorrected for end cooling. The lines are the least squares fit to the experimental points, (a) System 02 Pt (Brennan and Fletcher [7]) the parameters in the text are corrected for end cooling, (b) System N2—W (Gregory and Hayward [25]) the first two low temperature points have been excluded.

We have seen in Sect. 2.2.3(b) that, when the rate of adsorption of molecules becomes comparable with the rate of desorption of atoms, we expect failure of the half-order rate law and, eventually, first-order behaviour will be observed. All the metals investigated by Brennan and Fletcher [5, 7] exhibited this transition. Figure 11(a) illustrates the behaviour using the example of the H2— Pt system at 1750 K. According to eqn. (51), the limiting probability of atomisation is the molecular sticking coefficient, s2, values of which are listed in Table 5. 

Fig. 11. The transition from half-order to first-order kinetics, (a) H2—Pt at 1750 K (Brennan and Fletcher [7]) the points represent experimental data and the curve has been drawn using eqn. (20). (b) 02 — Pt (courtesy Weber et al. [47]). X, 1370 K 1480 K , 1570 K , 1640 K.

The assumption of mechanism (I) requires that the activation energy in the region of half-order kinetics should be — E = 51.5 kcal mole [Eq. (31)]. This is in very good agreement with the result of Brennan and Fletcher (Table II) and the theoretical expectation that the energy of activation Ei should be + i, being less than... 

The value of the pre-exponential factor is in good agreement with that obtained by Brennan and Fletcher (Table II), especiaUy when it is recalled that they used a roughness factor of 1.4, whereas Eq. (44) im-... 

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