Operando Probing of the Surface Chemistry During the Haber-Bosch Process

The Haber-Bosch process produces NH3 from N2 and H21,2, typically with Fe and Ru3.  HB has been proposed as the most important scientific invention in the 20th century4. The chemical state during reaction has been proposed as oxides5, nitrides2, metallic, or surface nitride6. The proposed rate-limiting step has been the dissociation of  N27–9, reaction of adsorbed nitrogen10, or desorption of NH311. Due to the vacuum requirement for surface-sensitive techniques, studies at reaction conditions are limited to theory computations12–14. We determined the surface composition, during NH3 production, at pressures up to 1 bar and temperatures as high as 723 K on flat, stepped Fe, and stepped Ru single crystal surfaces using operando X-ray Photoelectron Spectroscopy15. We found that all surfaces remain metallic. On Fe only a small amount of adsorbed N remains, yet Ru’s surface is almost adsorbate free. At 523 K, high amines (NHx) coverages appear on the stepped Fe surface. The results show that the rate-limiting step on Ru is always N2 dissociation. Still, on Fe the hydrogenation step involving adsorbed N atoms is essential for the total rate, as predicted by theory13. If the temperature is lowered on Fe, the rate-limiting steps switch and become surface species’ hydrogenation.