Chemisorption of nitrogen on iron

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. 

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

Surface Science of Ammonia Synthesis Structure Sensitivity of Ammonia Synthesis Kinetics of Dissociative Nitrogen Adsorption Effects of Aluminum Oxide in Restructuring Iron Single-Crystal Surfaces for Ammonia Synthesis Characterization of the Restructured Surfaces Effect of Potassium on the Dissociative Chemisorption of Nitrogen on Iron Single-Crystal Surfaces in UHV... 

J.J. Scholten, P. Zwietering, J.A. Konvalinka, and J.H. de Boer. Chemisorption of Nitrogen on Iron Catalysts in Connection with Ammonia Synthesis. Part 1. The Kinetics of the Adsorption and Desorption of Nitrogen. Trans. Faradav Soc. 55 2166 (1959). 

In this section a large amount of work which has been performed outside the Lawrence Berkeley Laboratory (except for Section 4.4.4) is summarized. The structure and bonding strength of potassium will be discussed along with the effect of potassium on the dissociative chemisorption of nitrogen on iron single-crystal surfaces. 

The Effect of Potassium on the Dissociative Chemisorption of Nitrogen on Iron Single-Crystal Surfaces in UHV... 

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).

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... 

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. 

The study on the pattern of nitrogen chemisorption is very important for the understanding of reaction mechanism of ammonia synthesis. But the chemisorption of nitrogen on the surface of transient metals is very complex. Taking chemisorption on iron catalyst for ammonia synthesis as an example, it was found that the formation rate of ammonia depends on pre-adsorption temperature if the adsorption of nitrogen was preceded first on the surface of activated catalyst, and then hydrogen was passed on surface of pre-adsorbed nitrogen. It is considered that there exist two adsorption patterns The first one is of L-type which appeared at 200°C of pre-adsorption temperature, in which the formation rate of ammonia was proportional to second one is of H-type which appeared at 400°C 40°C of... 

The heat of chemisorption of nitrogen on Ru-K is estimated from the adsorption constant in the above equation to be 167 kJ/mol. It is to be noted, however, that both isotopic equilibration and N2 chemisorption are enhanced by hydrogen on potash-promoted iron catalysts [26]. No enhancement by hydrogen was found on pure iron [181, 182]. Recently, N2 isotopic equilibration was found to be enhanced by hydrogen on rare earth-promoted Ru catalysts [122]. The hydrogen effect on Ru seems to depend on the kind of support and promoter. The guiding principle of this phenomena has not been solved, yet, the study is quite important to construct an effective Ru catalyst under high pressure. 

Fig. XVIII-13. Activation energies of adsorption and desorption and heat of chemisorption for nitrogen on a single promoted, intensively reduced iron catalyst Q is calculated from Q = Edes - ads- (From Ref. 130.)...

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 heat of chemisorption of hydrogen, adsorbed on iron that has previously been covered with nitrogen up to 0 = 0.18, is, indeed, lower than the heat of chemisorption of hydrogen adsorbed on a clean surface 395). The heat of chemisorption of CO on an iron film partly covered with nitrogen is also lower than on a clean film, but Bagg and Tompkins 395) found that hydrogen when adsorbed on a film partly covered with CO shows a higher heat of adsorption than when adsorbed on a clean film. 

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