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 dissociation7–9, 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 (NHx) 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.

Barium-doped iron nanoparticles supported on MgO as an efficient catalyst for ammonia synthesis under mild reaction conditions

“…At lower temperatures, H(a) may be strongly adsorbed on the surface, while at higher temperatures NH x (a) replaces H(a) as discussed in ref. 33. The apparent activation energies which are equivalent to the slope of the curves were 57.3, 74.5, 111.5, and 85.5 kJ mol −1 for Fe/Ba(1)/MgO-700red, Fe/Ba(1)/MgO-500red, Fe/MgO-700red, and the commercial catalyst at temperatures lower than 300 °C, respectively.…”

Atomically Resolved Transition Pathways of Iron Redox

“…The dynamic study on the complete reduction of iron oxides has been a long-standing challenge for ETEM techniques due to the requirement of an extremely pure gas environment. − In principle, even a trace amount of oxidants can impact the equilibrium state of the redox of iron . To optimize the gas environment of the gas cell, we revised the configuration for the entire gas supply system by incorporating leakage-preventing alternatives, such as those constructed from stainless steel or poly­(ether–ether–ketone)–silica.…”

Fischer–Tropsch Synthesis for the Production of Sustainable Aviation Fuel: Formation of Tertiary Amines from Ammonia Contaminants

“…It needs however to be noted that providing a complete mechanism involving dissociation and chain growth, which also accounts for the formation of byproducts like oxygenates and amines, warrants substantial follow-up work involving both experimental and modeling studies. Potential candidates are DFT, , (isotopic) transient kinetics, and operando XPS, which was recently shown to be able to probe the N speciation over Ru and Fe catalysts during the Haber–Bosch reaction . The observed similarity between the chain growth probability of amines with oxygenates and alkanes also warrants further attention in light of the long debate on the chain growth mechanism in Fischer–Tropsch synthesis involving either CO or CH x insertion .…”