X-ray Absorption Spectroscopic Studies at the Cobalt K-Edge on a Reduced Al₂O₃-Supported Rhenium-Promoted Cobalt Fischer−Tropsch Catalyst

Abstract

In situ XAFS spectroscopic studies have been carried out at 450 °C on the hydrogen reduction of a rhenium-promoted Co 3 O 4 /Al 2 O 3 catalyst. The results show that reduction at this temperature yields a highly dispersed system with small cobalt metal particles (<40 Å). In addition, a lower fraction of the cobalt content is randomly dispersed over the tetrahedral vacancies of the alumina support. This dispersion occurs during reduction and not calcination. The cobalt in these sites cannot be reduced at 450 °C, a temperature that is too low to permit formation of the spinel CoAl 2 O 4 . A particularly significant feature of the spectrum of the reduced material is that the energy of the K-edge is increased by 3-4 eV. This is attributed to the small size of the metal particles and consequently a deviation from bulk properties. The effect is a manifestation of changes in shielding of the 1s electrons by the valence orbitals relative to the situation pertaining to the bulk metal. The actual crystal form of the metal particles could not be determined, but the recently reported metastable non-close-packed body-centered cubic form cannot be excluded, especially since graphite was present and carbon is suggested to catalyze transformation to this structure.

1H and 13C solution- and solid-state NMR investigation into wax products from the Fischer–Tropsch process

“…The FT process is therefore a key part of processes to convert non-oil feedstocks such as gas, coal or biomass into liquid transportation fuels. The catalysts of interest in the current work are cobaltbased, supported on alumina; either cobalt-only or containing approximately 1% of a promoter, rhenium (Re), [7][8][9][10] or cerium (Ce). 11 Promoters are added to the catalyst to improve performance in different ways.…”

Soft‐Chemical Exfoliation Route to Layered Cobalt Oxide Monolayers and Its Application for Film Deposition and Nanoparticle Synthesis

“…Even though it is not allowed by the electronic dipolar selection rule (Dl = AE 1), the pre-edge peak is detectable because of the mixing of 4p and 3d states. [33] Therefore, this feature obtains significant spectral weight in the case of non-centrosymmetric tetrahedral geometry of an absorbing atom through the enhancement of d-p orbital mixing. In fact, the rocksalt-structured CoO with octahedral cobalt ions displays only a weak pre-edge peak P. In contrast, a rather intense feature P appears for the reference Co 3 O 4 with the spinel structure, in which one third of cobalt ions exist in tetrahedral sites and the rest are located in octahedral sites.…”

Capturing metal-support interactions in situ during the reduction of a Re promoted Co/γ-Al2O3 catalyst

“…Rutherford backscattering spectrometry (RBS) and X-ray Absorption Near Edge Structure (XANES) studies also Furthermore detailed TPR and XANES studies pointed on the formation of such a compound during catalyst reduction 7,8,19 . Although (XANES) has been regularly applied for the detection of Co-support mixed compounds 14,20,21 , due to its sensitivity towards oxidation state and local geometry, direct observation on the exact timing of this interaction has not been reported.…”

“…The loss of active metal due to strong interaction with the γ-Al2O3 support has been a topic of scientific interest for many years. It has been proposed that the formation of mixed cobaltsupport oxide phases takes place during calcination [16][17][18] , reduction 7,8,33 and at reaction condition 14,34,35 . Although the high temperature chemistry of this solid-state reaction resulting in the crystalline spinel CoAl2O4 rich in Co 2+ Td is reasonably known, the formation of the non-stoichiometric surface compound that lacks long range order is difficult to be detected.…”