Kinetic Models of N2 Chemisorption

The numerical models of the kinetics of NH3 synthesis will be discussed in Sect. 2.7.3 All contain a model of the kinetics of N2 adsorption 

N2 is weakly adsorbed on the surface of Fe. One model [396] calculates that the equilibrium coverages of N2 under 101 kPa of N2 is 0.27 for Fe and 0.30 for K/Fe at 500 K. The coverage is even smaller at higher temperatures [396]. These calculations are made based on the assumption that N2 does not dissociate to form N. N is much more strongly adsorbed than N2. Under 101 kPa of N2, the equilibrium coverage by N is close to unity at 500-1000 K [396]. 

Estimates of the sticking probability based on transition state theory are not particularly successful. This may be caused by incomplete equilibration of the transition state [396], by an underestimate of non-adiabatic quantum effects, or because the one-dimensional potential energy surface is an oversimplification of the problem. 

If a Fe surface is given a brief but large exposure to N2 at low temperature, the surface will be covered by N2 [396]. The dissociation to N is thermodynamically favorable, but does not proceed as virtually no free sites are left [396]. At a higher temperature the coverage by N2 will be less complete and there will be a sudden transition to complete coverage by N [396]. At even higher temperatures the coverage by N will be less than unity [396]. 

The thermal desorption of N2 has been treated by Bowker, Parker and Waugh and by Stoltze and Norskov. In their first paper [535], Bowker, Parker and Waugh used a conventional prefactor for the rate of N recombination. 

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