Chemisorption of nitrogen

V.I. Tsivenko, Investigation of Chemisorption of Nitrogen Atoms on Semiconductor Oxides of Metals, Doctorate thesis (Chemistry), Moscow, 1971. 

The character of the chemisorption of nitrogen can be also judged from the results of studies of ammonia synthesis kinetics at the reversible poisoning of the catalyst with water vapor (102,103). If a gas mixture contains water vapor, an adsorption-chemical equilibrium of adsorbed oxygen, hydrogen gas, and water vapor sets in on the iron catalyst. 

The chemisorption of nitrogen on an iron ammonia catalyst was also studied by Emmett and Kummer 313), who found that the surface behaved as if it were of a homogeneous character. 

Fig. 36. Activation energy of the chemisorption of nitrogen on an iron catalyst (360).

Fig. 38. Heats of chemisorption of nitrogen on an iron film as a function of 0 (S63).

Figure 38 gives the heats of chemisorption of nitrogen on iron films, recently published by Bagg and Tompkins (363), and we may compare the decrease of this curve with the increase shown by the curve of Fig. 36. 

The activation energy for the chemisorption of nitrogen on an iron catalyst, as measured by Zwietering and Roukens (360) (hence the curve of Fig. 36 just mentioned), can be represented by the expression... 

Numerous studies of the kinetics of ammonia synthesis and decomposition have been made. With a few exceptions, work has tended to show that the slow step in the synthesis of ammonia is the chemisorption of nitrogen and the slow step for the decomposition is the desorption of nitrogen. Furthermore, it turns out that the decomposition and synthesis of ammonia usually involve in the rate expression a term where y/x is close to 1.5. In 1940, Temkin and Pyzhev derived an equation consistent with both of these observations [M. I. Temkin and V. Pyzhev, Acta Physiochim. U.R.S.S. 12, 327 (1940)]. It has formed the basis for most of the kinetic treatments of ammonia synthesis and decomposition in recent years. 

The alkali metals have a role to play in ammonia synthesis. The K20 promoter in the conventional NH3 synthesis catalyst enhances the chemisorption of nitrogen and causes a hydrogen-promoted dissociation of the N2... 

Temkin and Pyzhev 49) successfully derived a kinetic expression for the ammonia s3mthesis assuming that the chemisorption of nitrogen is... 

Ozaki et al. (33) compared the rate of ammonia synthesis on a doubly promoted iron catalyst with that of deuteroammonia, and found that deuterium reacts markedly faster than hydrogen imder the same reaction condition. From the kinetic data, as well as the isotope effect, they reached the conclusion that the rate-determining step of the overall reaction is the chemisorption of nitrogen on a surface mainly covered with NH radicals, and that the isotope effect is due to the fact that NH is adsorbed more strongly than ND. 

Scholten, J. J. F., Chemisorption of nitrogen on iron catalysts in connection with ammonia synthesis. Thesis, Delft, 1959. 

The basis of this equation is the rate being determined by two consecutive steps, the first being chemisorption of nitrogen and the second combination of a chemisorbed nitrogen molecule with hydrogen to give a chemisorbed radical. This equation converts at conditions close to equilibrium into equation (3.156) and at very low ammonia concentration relative to the equilibrium value to equation (3.157). 

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