Operando probing of the surface chemistry during the Haber–Bosch process

Christopher M. Goodwin (), Patrick Lömker, David Degerman, Bernadette Davies, Mikhail Shipilin, Fernando Garcia-Martinez, Sergey Koroidov, Jette Katja Mathiesen, Raffael Rameshan, Gabriel L. S. Rodrigues, Christoph Schlueter, Peter Amann and Anders Nilsson ()
Additional contact information
Christopher M. Goodwin: Stockholm University, AlbaNova University Center
Patrick Lömker: Stockholm University, AlbaNova University Center
David Degerman: Stockholm University, AlbaNova University Center
Bernadette Davies: Stockholm University
Mikhail Shipilin: Stockholm University, AlbaNova University Center
Fernando Garcia-Martinez: Deutsches Elektronen-Synchrotron DESY
Sergey Koroidov: Stockholm University, AlbaNova University Center
Jette Katja Mathiesen: Stockholm University, AlbaNova University Center
Raffael Rameshan: Montan University Leoben
Gabriel L. S. Rodrigues: Stockholm University, AlbaNova University Center
Christoph Schlueter: Deutsches Elektronen-Synchrotron DESY
Peter Amann: Stockholm University, AlbaNova University Center
Anders Nilsson: Stockholm University, AlbaNova University Center

Nature, 2024, vol. 625, issue 7994, 282-286

Abstract: 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.

Date: 2024
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DOI: 10.1038/s41586-023-06844-5

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