The effect of water on the Fischer-Tropsch performances of a supported cobalt catalyst has been studied in a fixed bed reactor by running co-feeding experiments for more than 1000 h at industrially relevant process conditions. The adopted γ-Al 2 O 3 support (D p =300 µm) has been modified with superficial cobalt-aluminates so to prevent the formation of these species during the reaction as a consequence of the strong interaction between the active phase and the support. In spite of this precaution, the addition of water results in a slow deactivation of the catalyst, possibly caused by the oxidation of the reduced cobalt metal centers. More complex is the effect of water on the product distribution: in this case water has both irreversible effects, which become stronger upon increasing the concentration of co-fed water, and reversible effects, which are clearly visible even with small concentrations of co-fed water. The two effects bring, in the presence of co-fed water, to decreased CH 4 and alcohols selectivities, and increased C 25+ hydrocarbons, olefins and CO 2 selectivities. These results can be explained by assuming that water suppresses hydrogenation reactions.
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