The impact of sequential H2-CO-H2 activation treatment on the structure and performance of cobalt based catalysts for the Fischer-Tropsch synthesis

Abstract

Different activation protocols were applied to a promoted Re/Co/γ-Al2O3 catalyst for Fischer-Tropsch synthesis. The activation included treatment in either pure H2 or a sequence of Reduction (H2) -Carburization (CO) -Reduction (H2) in order to vary the microstructure of Co nanoparticles and investigate their performance. The alternative activation protocol applied under specific conditions (carburization at 230 ⁰C and final reduction at 350 ⁰C) outperform the conventional activation both in terms of catalyst activity and C5+ selectivity. The catalyst was characterized in all the stages of the activation process by synchrotron based X-ray diffraction (XRD) and X-ray absorption near edge structure spectroscopy (XANES). The effect of air exposure at room temperature of the carburized sample was investigated and it was found that air exposure leads to minor phase changes that significantly affect catalyst performance. Carburization temperature and temperature used for the decomposition of the carbide on the final reduction step both had a severe effect on catalyst performance, but neither of them influenced considerably the microstructure of Co nanoparticles that was primarily hexagonal (hcp). All carburized samples contained a significant amount of carbon as observed by Thermogravimetric analysis (TGA) and Raman spectroscopy. The final reduction step could remove part of the excess carbon produced during CO disproportionation. It appears that the carbon removal\minimization is a key factor for the exploitation of the benefits of hcp configuration of Co nanoparticles catalysing Fischer-Tropsch synthesis.

In-depth characterisation of metal-support compounds in spent Co/SiO2 Fischer-Tropsch model catalysts

“…The main product of the FT synthesis is H2O, which has been linked to deactivation of cobalt-based FT catalysts [10,18,[20][21][22][23][24][25][26]. Three me-chanisms have been proposed in literature: hydrothermal sintering of cobalt particles leading to a loss of specific surface area [23,27], oxi-dation of metallic cobalt to FT inactive cobalt oxides [24][25][26], and the formation of MSCs [18,19], which are also not active for the FT synthesis [20].…”

Structure–Performance Relationships on Co-Based Fischer–Tropsch Synthesis Catalysts: The More Defect-Free, the Better

“…For operation in diffusion limitation free regime, the catalyst samples were sieved to a particle diameter of 53−90 μm. More details on catalyst synthesis can be found in a different source 29 .…”

“…The reactor was first pressurized with Ar to 14 bar. 29 Then CO was introduced by replacing the Ar flow gradually, in order to keep the temperature increase in the catalyst bed below 2 ⁰C during the carburization. Carburization lasted 3 hours with the exception of the 350 B catalyst that was exposed at 14 bar CO for 6 hours.…”

“…This alternative multistep activation was first studied by Hofer and Peebles 14 and reinvestigated, leading to uncover of phase sensitivity in FTS by Ducreux et al 25 . The differences in rate and selectivity between NPs of fcc-Co and hcp-Co structures, although challenging to interpret, appear clear [26][27][28][29] .…”

“…As mentioned above, predominately hcp-Co NPs can be prepared when a reduction -carburization -reduction (RCR) activation protocol is followed. However, to achieve the best catalytic properties, carbon produced during the carburization (CO disproportionation) step has to be minimized 29 .…”