Operando probing of the surface chemistry during the Haber-Bosch process.

The large-scale conversion of N ₂ and H ₂ into NH ₃ (refs.  ^1,2 ) over Fe and Ru catalysts ³ for fertilizer production occurs through the Haber-Bosch process, which has been considered the most important scientific invention of the twentieth century ⁴ . The active component of the catalyst enabling the conversion was variously considered to be the oxide ⁵ , nitride ² , metallic phase or surface nitride ⁶ , and the rate-limiting step has been associated with N ₂ dissociation ^7-9 , reaction of the adsorbed nitrogen ¹⁰ and also NH ₃ desorption ¹¹ . 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 ¹² . Here we use X-ray photoelectron spectroscopy-capable of revealing the chemical state of catalytic surfaces and recently adapted to operando investigations ¹³ of methanol ¹⁴ and Fischer-Tropsch synthesis ¹⁵ -to determine the surface composition of Fe and Ru catalysts during NH ₃ production at pressures up to 1 bar and temperatures as high as 723 K. We find that, although 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 (NH _x ) coverages on the stepped surfaces. These observations indicate that the rate-limiting step on Ru is always N ₂ dissociation. On Fe catalysts, by contrast and as predicted by theory ¹⁶ , hydrogenation of adsorbed N atoms is less efficient to the extent that the rate-limiting step switches following temperature lowering from N ₂ dissociation to the hydrogenation of surface species.
(© 2024. The Author(s).)