Due to high activity of cobalt metal in syngas conversion to higher hydrocarbons and long chain normal paraffins, supported cobalt catalysts have been widely used for Fischer-Tropsch synthesis (FTS) reaction.

We investigated effect of pretreatment procedures on catalytic performance of 15wt% Co/AlO catalyst prepared by incipient wetness impregnation (IWI). Catalyst was characterized by different methods such as BET, XRD, TPR, TGA, H-TPD and TPH/TPO-MS. FTS was performed in a fixed-bed reactor at the following conditions: 593 K, 20 bar, H/CO = 2 and gas space velocities of 3.85 NL/g-cat/h. The catalyst was activated in situ with either hydrogen at 648 K for 12 h, or syngas (CO/H/He) or 10%CO/Ar at 623 K for 12 h.

BET surface areas of calcined catalyst was 154 m2/g. XRD pattern of calcined catalyst showed existence of cubic CoO in addition to -AlO phase. Metallic cobalt (Co fcc) was found after H reduction. Co°(fcc and hcp) and CoxC (x = 2, 3) were found in samples activated in syngas and 10%CO/Ar. Two step reduction of CoO was observed in TPR and TGA experiments. Carbon deposition was observed during TGA using 10%CO/Ar as reducing agent. Degree of reduction was about 83% (H-TPR following H isothermal activation) and dispersion 9.8%. Corrected value of crystallite sizes from combination of chemisorption and H-TPR following hydrogen isothermal activation was 9.8 nm. TPH/TPO-MS results indicate existence of CoxC and different types of deposited carbon including graphitic carbon after syngas-activation.

CO conversion of 10%CO-activated catalyst was very low (7.4%) whereas the H2-activated catalyst was the most active (CO conversion of 58.8%). CH selectivity decreased and C+ selectivity increased, with increasing amount of H in the reducing gas. Moreover, the hydrogen reduced catalyst had the highest 1-olefin selectivity (C-H hydrocarbons). These results show that surface carbide sites (created after 10%CO/Ar and syngas pretreatments) favor methane production and secondary hydrogenation of 1-olefins.

In conclusion, H-activated 15wt% Co/AlO catalyst had the best performance due to high degree of reduction and dispersion.


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