(583al) New Insight Into the Mechanism of Ammonia Formation in the Haber-Bosch Process

Ammonia (NH₃) is produced large quantities in the chemical industry, where it is mainly used in the production of fertilizer. Since the early 20^th century, NH₃ has been primarily synthesized via the Haber-Bosch process, in which gaseous nitrogen (N₂) and hydrogen (H₂) are passed over a Ru or Fe-based catalyst at a temperature of approximately 400 ^oC and a pressure of 150 bar according to N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g).

Much effort has been focused on optimizing this process and determining the reaction mechanism. Pioneering work by Ertl and coworkers [1] focused on determining whether the reaction proceeded by initial dissociation of adsorbed N₂ and subsequent hydrogenation (dissociative mechanism), or whether the adsorbed N₂ molecule is first hydrogenated and then dissociates (associative mechanism). By using surface science experimental techniques such as AES, LEED and work function measurements, it was concluded that the dissociative mechanism was responsible for synthesis of NH₃ in the Haber-Bosch process. Density functional theory (DFT) calculations of the dissociative mechanism of NH₃ formation have been utilized to predict the rate of NH₃ formation on Ru nanoparticles to within a factor of 3 to 20 of the experimentally observed rate [2].

In the current study, we use DFT to further investigate the mechanism of NH₃ formation under Haber-Bosch conditions on a stepped Ru surface. Stable intermediates were identified and minimum energy paths between states were calculated to identify saddle points and determine activation energy barriers in both mechanisms. The rate determining step (RDS) of the dissociative mechanism was found to be the dissociation of the adsorbed N₂ molecule and for the associative mechanism the RDS was found to be the addition of the first hydrogen atom to the N₂ molecule. This latter step involves a large loss in entropy in adsorption of gaseous H₂ (as well as N₂) to the surface, and thus would be expected to have a prohibitively high free energy barrier. However, a surprising low-energy intermediate was identified in the associative mechanism with a correspondingly low-energy saddle point. Free energy calculations were performed and it was found that at room temperature, this associative mechanism is favored. Even for the high-temperature Haber-Bosch conditions, the calculated results indicate that both the associative and the dissociative mechanisms can be active.

References:

[1] G. Ertl, Catal. Rev.-Sci. Eng.,21, 201 (1980).

[2] K. Honkala, A. Hellman, I. N. Remediakis, A. Logadottir, A. Carlsson, S. Dahl, C. H. Christense, J. K. Nørskov, Science, 307, 555 (2005).