In-situ study of the promotional effect of chlorine on the Fischer–Tropsch synthesis with Ru/Al2O3

• Cl addition to Ru/Al2O3 increases TOF for the FTS.
• Cl addition leads to higher number of low-coordinated Ru sites.
• Steady-state FTS conversions for Cl-free and Cl-promoted catalysts are identical.
• Cl removal and/or coke deposition are responsible for lower reaction rates.

The effect of the addition of Cl to Ru/γ-Al2O3 catalysts for the Fischer–Tropsch synthesis has been investigated. A series of four catalysts containing 3 wt.% of Ru supported on γ-Al2O3 and promoted with different amounts of chlorine (Cl/Ru atomic ratios of 0, 1, 5 and 10) has been prepared. The catalysts were tested in the Fischer Tropsch synthesis in a fixed bed reactor. The initial rates for the CO hydrogenation increase with the increasing amount of Cl in the catalyst. However, at the steady-state all catalysts exhibit similar Fischer Tropsch yields, irrespectively of their initial amount of Cl. The nature of the CO adsorption sites in the Cl-promoted Ru/Al2O3 catalysts has been studied by means of DRIFT concluding that Cl addition leads to a higher fraction of COad species adsorbed on Ru atoms with low coordination numbers. In addition, the stability of such COad species, characterized by high frequency bands in the IR, increases with the increasing content of Cl in the catalysts. The reactivity of the COad species in hydrogen atmospheres and under FTS environments has been studied by in situ MS-DRIFT. We have observed that low-coordinated Ru atoms are more reactive for CO dissociation than those in a higher coordination environment. However, the former sites are prone to deactivation during FTS catalysis, at the steady-state, and only CO adsorbed on highly coordinated Ru atoms is observed in the IR spectra. Consequently, all catalysts exhibit the same reaction rates at the steady-state.

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