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

Abstract

The large-scale conversion of N₂ and H₂ into NH₃ (refs. ^1,2) over Fe and Ru catalysts³ for fertilizer production occurs through the Haber-Bosch process, which has been considered the most important scientific invention of the twentieth century⁴. The active component of the catalyst enabling the conversion was variously considered to be the oxide⁵, nitride², metallic phase or surface nitride⁶, and the rate-limiting step has been associated with N₂ dissociation^7-9, reaction of the adsorbed nitrogen¹⁰ and also NH₃ desorption¹¹. 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 calculations¹². Here we use X-ray photoelectron spectroscopy-capable of revealing the chemical state of catalytic surfaces and recently adapted to operando investigations¹³ of methanol¹⁴ and Fischer-Tropsch synthesis¹⁵-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, 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 N₂ dissociation. On Fe catalysts, by contrast and as predicted by theory¹⁶, hydrogenation of adsorbed N atoms is less efficient to the extent that the rate-limiting step switches following temperature lowering from N₂ dissociation to the hydrogenation of surface species.