In situ measurements of ammonia synthesis were long considered impossible even Gerhard Ertl wrote, “… spectroscopic measurements may never be performed under pressure conditions as applied for the [Haber-Bosch] reaction.” Ambient pressure XPS has advanced a great deal, and with POLARIS, an XPS capable of measuring in situ at pressures over one bar, it is possible to measure the Haber-Bosch process under reaction conditions. Herein we will present the first in situ XPS data for Haber-Bosch reaction over iron.  We have characterized ammonia synthesis over flat Fe(110) and stepped Fe(210) surfaces from pressures between 200 and 1000 mbar, a wide range of temperatures, and various gas compositions. Since the inception of iron as a Haber-Bosch catalyst, numerous active surface structures have been proposed, including oxides, nitrides, and metallic states. Our research has reconciled the discrepancies between various ex situ studies and thermodynamic calculations that predicted various surface structures. Furthermore, we have ascertained the role of steps in the catalytic process by comparing the differences between flat and stepped surfaces. Finally, we have shown the effects of gas composition, measured the turnover frequency, and determined the surface coverage during the reaction. Our findings indicate that the surface is predominantly metallic with very low coverage of surface nitrogen under the highest turnover conditions.

Iron single crystals catalysts for Haber-Bosch synthesis observed in situ

ACS Spring 2022

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