The effect of the addition of Cl to Ru/γ-AlO catalysts for the Fischer-Tropsch synthesis has been investigated. A series of four catalysts containing 3 wt.% of Ru supported on γ-AlO 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/AlO catalysts has been studied by means of DRIFT concluding that Cl addition leads to a higher fraction of CO species adsorbed on Ru atoms with low coordination numbers. In addition, the stability of such CO species, characterized by high frequency bands in the IR, increases with the increasing content of Cl in the catalysts. The reactivity of the CO 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.

J.M. González-Carballo acknowledges Ministerio de Ciencia e Innovación and the Formación de Profesorado Universitario program (FPU) for the PhD grant.

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