Mn promotion effects in Co/TiO2 Fischer-Tropsch catalysts as investigated by XPS and STEM-EELS

The addition of small amounts of Mn to a Co/TiO2 catalyst affects the catalytic performance in the Fischer-Tropsch synthesis (FTS) by increasing the activity and suppressing the CH4 yield. These variations in the catalyst selectivity are due to Mn promotion effects that influence the final catalyst active site distribution, playing a role under reaction conditions. The use of STEM-EELS and XPS has provided more insight into the location of Mn as a function of the catalyst surface composition observed after the main preparation steps, that is, calcination and reduction. XPS shows that after calcination the catalysts contain mainly a Co3O4 phase, and that after 4 h of reduction in H2 flow at 350 °C, a fair fraction of the Co3O4 is fully reduced to Co0. The STEM-EELS measurements reveal the existence of a clear Co-Mn association in the calcined catalyst, in which the Mn apparently forms a mixed oxidic phase with the Co particles and thus causes a decrease in the Co reducibility with respect to a Mn-free Co/TiO2 catalyst. The Mn tends to segregate over the TiO2 support after reduction, as indicated by the increase in the Mn/Co and Mn/Ti atomic ratios obtained from XPS. Hence, the Mn compounds migrate from the Co particles and are enriched at the surface of the TiO2 support during Co3O4 reduction to Co0. However, the STEM-EELS images obtained for the reduced catalyst reveal a remaining Co-Mn interaction, which presumably causes the promotion effect in FTS.