In situ scanning tunneling microscopy of the poisoning of a Co(0 0 0 1) Fischer–Tropsch model catalyst by sulfur

• In situ STM data show sulfur poisoning of the Fischer–Tropsch reaction on the atomic scale.
• Investigations were performed in the mbar regime at 493 K with a Co(0 0 0 1) model catalyst.
• In the presence of sulfur, the cobalt surface restructures into many small terraces.
• In addition, ordered superstructures were observed.
• XPS taken after the reaction shows sulfur and carbon.

The poisoning of a Co(0 0 0 1) model catalyst for the Fischer–Tropsch reaction by sulfur was investigated by in situ scanning tunneling microscopy (STM). Experiments were performed under syngas at pressures around 10 mbar and at sample temperatures around 500 K under methanation conditions. After the reaction experiments, the surface was analyzed by X-ray photoelectron spectroscopy. Mass spectrometry showed that the sample was poisoned when hydrogen with <0.5 ppm of sulfur was used in the syngas. STM images recorded under these conditions showed a strong restructuring of the surface into small terraces. The terraces were fully covered by a stable surface phase, displaying small ordered areas with (2 × 2) and (√19 × √19)R23.4° structures. The observations contrast with the flat terraces covered by a mobile adsorption layer when sulfur-free hydrogen was used. The poisoned surface state is interpreted as a mixed sulfur/carbon layer that additionally involves a reconstruction of the Co(0 0 0 1) surface.

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