Abstract
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.