The large-scale conversion of N2 and H2 into NH3 over Fe and Ru catalysts for fertilizer production occurs through the Haber-Bosch process, which has been labelled the most important scientific invention of the 20th century. The active component of the catalyst enabling the conversion was variously considered to be the oxide, nitride, metallic phase or surface nitride; and the rate-limiting step has been associated with N2 dissociation, reaction of the adsorbed nitrogen and also NH3 desorption. This range of views reflects that the Haber-Bosch process operates at high temperatures and pressures, whereas surface-sensitive techniques that might differentiate between different mechanistic proposals require vacuum conditions. Mechanistic studies have accordingly long been limited to theoretical calculations. Here, we use X-ray photoelectron spectroscopy—capable of revealing the chemical state of catalytic surfaces and recently adapted to operando investigations of methanol and Fischer-Tropsch synthesis— to determine the surface composition of Fe and Ru catalysts during NH3 production at pressures up to 1 bar and temperatures as high as 723 K. We find that while flat and stepped Fe surfaces and Ru single crystal surfaces all remain metallic, the latter are almost adsorbate free whereas Fe catalysts retain a small amount of adsorbed N and develop at lower temperatures high amine (NHx) coverages on the stepped surfaces. These observations indicate that the rate-limiting step on Ru is always N2 dissociation. On Fe catalysts, in contrast and as predicted by theory, hydrogenation of adsorbed N atoms is less efficient to the extent that the rate-limiting step switches upon temperature lowering from N2 dissociation to the hydrogenation of surface species.
By using the Bar-XPS, we have been able to probe a reaction that was previously thought impossible; Nobel laurette Gerhard Ertl wrote, “… spectroscopic measurements may never be performed under pressure conditions as applied for the [Haber-Bosch] reaction.” For almost 40 years, this was true. The most valuable surface spectroscopy could not study reaction at pressure over several 10s of mbar. Being able to research ammonia synthesis at relevant pressures, we have shown that the flat Fe(110) surface is covered in long-lived surface nitrogen species, while the stepped surface Fe(210) has a much higher coverage. We also have proven that even the stepped Ru(10 3) surface has a far lower coverage than any other surface studied. Through this work, we have proven that nitrogen fixation is the rate-limiting step at high temperatures. Yet, at low temperatures, where future catalysts aim to operate, the reaction is limited by hydrogenation of the surface nitrogen. An improved iron-based catalyst may operate by decreasing the thermal energy needed for hydrogenation. Finally, though we verified that Ruthenium is the most active catalyst, we also showed it has the lowest coverage. It may be possible to improve the Ruthenium catalyst by increasing the surface nitrogen coverage.
Image 2. XPS spectra of N1s at various conditions, 200, 500, and 1000 mbar, 523 and 673K, and Fe(110), Fe(210), and Ru(10 3). Please note that the scale of the Ru data is far smaller than that of the other samples.