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| Journal Article | PUBDB-2024-01323 |
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2024
Nature Publ. Group
London [u.a.]
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Please use a persistent id in citations: doi:10.1038/s41586-023-06844-5 doi:10.3204/PUBDB-2024-01323
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 nitride6, and the rate-limiting step has been associated with N$_2$ dissociation$^{7,8,9}$, 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 investigations13 of methanol14 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.
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