Real-Time Scattering-Contrast Imaging of a Supported Cobalt-Based Catalyst Body during Activation and Fischer-Tropsch Synthesis Revealing Spatial Dependence of Particle Size and Phase on Catalytic Properties

Pierre Senecal (1, 2) , Simon D. M. Jacques (1, 3, 4) , Marco Di Michiel (5) , Simon A. J. Kimber (5) , Antonis Vamvakeros (1, 2) , Yaroslav Odarchenko (1, 2) , Ines Lezcano-Gonzalez (1, 2) , James Paterson (6) , Ewen Ferguson (6) , Andrew M. Beale (1, 2)

1 Rutherford Appleton Lab, Res Complex Harwell, Harwell OX11 0FA, Berks, England
2 UCL, Dept Chem, 20 Gordon St, London WC1H 0AJ, England
3 Univ Manchester South, I13, Didcot OX11 0DE, Oxon, England
4 Univ Manchester, Sch Mat, Manchester M13 9PL, Lancs, England
5 ESRF - European Synchrotron Radiation Facility
6 BP Chem, Convers Technol Ctr, HRTC DL10 Saltend, Kingston Upon Hull HU12 8DS, N Humberside, England

A combination of time-resolved synchrotron mu-X-ray diffraction computed tomography (mu-XRD-CT) and mu-pair distribution function computed tomography (mu-PDF-CT) has been applied for the study of an individual Co/gamma-Al2O3 catalyst pellet during reduction and the early stages of the Fischer-Tropsch synthesis (FTS) reaction, revealing insight into the solid-state changes occurring from within such crystalline materials. Both sets of data were of sufficient quality so as to be able to follow the spatial dependency of Co speciation evolution from Co3O4 to CoO to face-centered cubic (fcc) Co metal nanoparticles. These data revealed the samples to be highly heterogeneous and contain two types of Co species: small (<= 6.5 nm) nanoparticles that interact strongly with the gamma-Al2O3 support which are difficult to reduce (remaining as CoO) and nanoparticles that agglomerate and have little interaction or else are weakly interacting with the support but readily reduce in H-2. The Co phase evolution under FTS conditions shows a strong dependence on the Co nanoparticle location; the complementarity between the observations made using mu-XRD-CT vs mu-PDF-CT allowed us to conclude that at the sample periphery a significant amount of agglomerated, weakly interacting with the support, small fcc Co metal nanoparticles (<= 7.5 nm) oxidize to CoO/Co3O4 during FTS, most likely due to the presence of water vapor produced during the reaction. Catalytic tests demonstrated that this oxidation coincided with a decrease in CH4 selectivity and increased water-gas shift (WGS) activity. This oxidation of fcc Co nanoparticles (i.e., the removal of the contribution to the XRD signal) also explains the observation of sintering previously reported for such catalysts in the early stages of the FTS reaction

Mots clés: Fischer-Tropsch; PDF-CT; XRD-CT; alumina; cobalt; particle size; pellet

Domaines: Chimie

Pierre Senecal, Simon D. M. Jacques, Marco Di Michiel, Simon A. J. Kimber, Antonis Vamvakeros, et al.. Real-Time Scattering-Contrast Imaging of a Supported Cobalt-Based Catalyst Body during Activation and Fischer-Tropsch Synthesis Revealing Spatial Dependence of Particle Size and Phase on Catalytic Properties. ACS Catalysis, 2017, 7 (4), pp.2284-2293. ⟨10.1021/acscatal.6b03145⟩. ⟨hal-01691820⟩