Dissociative adsorption of nitrogen

For a second-order desorption process, both the desorption-peak maximum temperature Tp and the half-width change with increasing initial coverage. An example of this type of behavior is the desorption of N2 from W(IOO) (Figure 4.17), clearly indicating the dissociative adsorption of nitrogen before desorption under the conditions of the experiment. 

Dissociative adsorption of nitrogen proceeds through molec-ularly adsorbed intermediate states, where the behavior of the Fe(l 11) surface is of particular interest Exposure to N2 at low temperature (< 80K) causes the formation of an adsorbed species (a-state) with an adsorption energy of 24kJ/mol and a N-N stretch frequency of 2100 cm (for N2), which is very close to the value of the free N2 molecule. Together with the analysis of data from photoelectron spectroscopy [14,15], this led to the conclusion that this is a physisorbed species with its molecular axis perpendicular to the surface. 

FIGURE 6.4. Schematic potential diagram for the dissociative adsorption of nitrogen on a Fe(l 11) surface. 

It was early recognised that the rate limiting step in the ammonia synthesis is the dissociative adsorption of nitrogen (23) and that hydrogenation proceeds at a much faster rate (24). Temkin and Pyzhev (25) proposed a rate expression. 

The role of potassium as an electronic promoter is in increasing the sticking coefficient for dissociative adsorption of nitrogen by stabilizing both the a-state and the transition state. In the presence of adsorbed K alone, the sticking coefficient was found to be increased up to 4 x 10 , whereby the enhancement was stronger for Fe(llO) and Fe(lOO) than for Fe(lll) and so the differences between the clean surfaces became almost eliminated. 

Napolskikh also considered that the dissociation adsorption of nitrogen was RDS. He found that the following reactions proceeded very slowly ... 

For ruthenium catalyst, same as iron, it was in general considered that dissociated adsorption of nitrogen was also rate determining step, but the reaction... 

The potential energy diagram was therefore modified following suggestions by Ertl, on the basis that, under the conditions of temperature and pressure required for industrial ammonia synthesis, the dissociative adsorption of nitrogen becomes an activated process. The potential energy diagram is reported in Fig. 4.13. While there is no direct experimental evidence for this postulate, it is noteworthy that the same basic assumption was taken by Temkin for the development of his kinetic equation. ... 

Dissociative adsorption of nitrogen as the rate-determining step under all reaction conditions. 

Assuming the rate-determining step to be dissociative adsorption of nitrogen, the rate of synthesis becomes... 

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