Abstract
The surface compositions of K and Cu containing Fe/Mn oxide catalysts for Fischer-Tropsch synthesis were investigated by XPS and ISS. The surface species after calcination are identified as Fe2O3, Mn2O3, MnO2, CuO and most likely KO2, and after in situ reduction at 723 K Fe0, Cu0, Fe2+ and Fe3+ oxides, MnO and KOH. Mn and K are enriched on the surfaces after calcination and reduction; the Cu surface content is approximately equal to the bulk concentration. The K enrichment is especially strong and ISS indicates that potassium is mainly confined to the uppermost layers. The degree of reduction of Fe is strongly dependent on the amount of Cu or K. The change in surface composition during Fischer-Tropsch reaction in the XPS equipment can be correlated to the activity of the catalysts. The pure and Cu containing samples show a constant activity and only a small increase in carbon surface concentration. The K containing catalysts deactivate after a short time and are then totally covered by carbon. On all catalyst surfaces a small amount of carbonate is formed.
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