Electron Microscopy Study of γ-Al2O3 Supported Cobalt Fischer–Tropsch Synthesis Catalysts

Abstract

Three supported catalysts containing 20 wt% cobalt and 0.5 wt% rhenium were subjected to electron microscopy studies in their calcined state. The catalysts were prepared by incipient wetness impregnation of c-Al 2 O 3 supports of different pore characteristics with aqueous solutions of cobalt nitrate hexahydrate and perrhenic acid. The influence of the support on the Co 3 O 4 crystallite size and distribution was studied by X-ray diffraction and electron microscopy. There was a positive correlation between the pore diameter of the support and the post calcination Co 3 O 4 crystallite size. On all three c-Al 2 O 3 supports, Co 3 O 4 was present as aggregates of many crystallites (20-270 nm in size). Cobalt oxide did not crystallise as independent crystallites, but as an interconnected network, with a roughly common crystallographic orientation, within the matrix pore structure. The internal variations in crystallite size between the catalysts were maintained after reduction. Fischer-Tropsch synthesis was carried out in a fixed-bed reactor at industrial conditions (T = 483 K, P = 20 bar, H 2 /CO = 2.1). Although the cobalt-time yields varied significantly (4.6-6.7 9 10 -3 mol CO/mol Co s), the site-time yields were constant (63-68 9 10 -3 s -1 ) for the three samples. The C 5? selectivity could not be correlated to the cobalt oxide aggregate size and is more likely related to the cobalt particle size and chemical properties of the c-Al 2 O 3 support.

Active phase distribution changes within a catalyst particle during Fischer–Tropsch synthesis as revealed by multi-scale microscopy

“…The formation of nanoparticle aggregates has previously been shown for Co/Al 2 O 3 and Co/SiO 2 FTS catalysts. 38,40,41 We demonstrate here that aggregates of Co nanoparticles are also formed using TiO 2 as support. The formation of aggregates of nanoparticles is most probably related to the drying temperature of the catalyst.…”

In-Situ Reduction of Promoted Cobalt Oxide Supported on Alumina by Environmental Transmission Electron Microscopy

“…The nanoparticles interact more readily with the support [21] making them more difficult to reduce. Preliminary reduction experiments performed on c-Al 2 O 3 catalysts in the ETEM, confirmed that they were more difficult to reduce and that the degree of reduction was not so easy to establish due to the more complicated microstructure.…”

Speciation of Ruthenium as a Reduction Promoter of Silica-Supported Co Catalysts: A Time-Resolved in Situ XAS Investigation

“…This is in line with earlier interpretation by Iglesia et al 1 and with experimental results obtained with rhenium and platinum as reduction promoters. 16,18,19 A recent report has rather concluded on the existence of bimetallic Co-containing Ru nanoparticles, on the basis of XAS recorded at the Ru K edge. 84 In that case, catalysts were prepared by sequential impregnation on a formerly calcined Co catalyst, which may explain the differences in terms of interactions between Co and Ru.…”