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

Abstract The addition of small amounts of Mn to a Co/TiO 2 catalyst affects the catalytic performance in the Fischer–Tropsch synthesis (FTS) by increasing the activity and suppressing the CH 4 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 Co 3 O 4 phase, and that after 4 h of reduction in H 2 flow at 350 °C, a fair fraction of the Co 3 O 4 is fully reduced to Co 0 . 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/TiO 2 catalyst. The Mn tends to segregate over the TiO 2 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 TiO 2 support during Co 3 O 4 reduction to Co 0 . However, the STEM-EELS images obtained for the reduced catalyst reveal a remaining Co–Mn interaction, which presumably causes the promotion effect in FTS.