Fischer-Tropsch (FT) synthesis is an important process to manufacture hydrocarbons and oxygenated hydrocarbons from mixtures of carbon monoxide and hydrogen (syngas). The catalysis process occurs on, for example, cobalt metal surfaces at elevated temperatures and pressures. A fundamental understanding of the reduction pathway of supported cobalt oxides, and the intermediate species present during the activation, can assist in developing improved industrial supported cobalt catalysts. Hard synchrotron X-rays have the unique ability to probe atomic processes both in terms of phases present as well as the crystallographic and local structure (using the pair distribution function approach) under realistic conditions. In this manuscript we present results from measurements during in situ hydrogen activation of a model Co/alumina catalyst using in situ synchrotron X-ray powder diffraction and pair-distribution function (PDF) analysis on beam line ID31 at the ESRF in Grenoble, France. The PDF analysis showed a substantially improved understanding of the reduction of cobalt oxides, as for the first time all cobalt could be accounted for by using total scattering analysis.

SynopsisThe atomic configuration of the pseudo-monoclinic structure of Hägg carbide (χ-

The crystal structure of χ-Fe5C2 Hägg carbide was re-determined using the simulated annealing capability of Topas Academic (Coelho, 2000). The unit cell is triclinic (space group P-1) with a = 11.5695(5) Å, b = 4.5713(2) Å, c = 5.0588(2) Å, α = 89.884(4)°, β = 97.765(3)° and γ = 89.964(5)°. The crystal structure was fitted by Rietveld refinement, resulting in R wp = 8.19% and R Bragg = 2.54%. The triclinic space group of χ-Fe5C2 Hägg carbide was determined using selected area electron diffraction (SAED). A TEM study was done to determine exposed crystallographic faces on coarse χ-Fe5C2 Hägg carbide particles. Energy calculations using Castep (Cambridge Sequential Total Energy Package, Payne, et al., 1992 and Milman et al., 2000) indicated the triclinic unit cell of χ-Fe5C2 Hägg carbide to have the same total energy as the monoclinic unit cell. Thermogravimetric analysis was used to characterize the carbon species in the sample.

Supported cobalt Fischer-Tropsch catalysts are characteristically nanoparticulate and the reduced SiC supported catalyst was found to contain both HCP and FCC polymorphs. This is reflected in the powder XRD patterns and generally there is a poor fit between the experimental and calculated diffractograms. This was ascribed to small crystallite sizes and the occurrence of disorder, manifested as peak broadening and peak shifts. Selected area electron diffraction data of suitably oriented cobalt catalyst grains on silicon carbide supports show non-periodic disorder in the zone axis orientations that contain the common (001) (HCP) and (111) (FCC) reciprocal lattice planes. Both FCC and HCP polymorphs are present in the same grains and these show disorder mainly in the HCP component. The disorder is further examined using high angle annular dark field (HAADF) scanning transmission electron microscopy at atomic resolution and the stacking sequences elucidated. Random sequences of mainly FCC are interrupted by HCP sequences and twin surfaces with reverse stacking sequences are also present. This study highlights the presence of significant disorder in cobalt catalyst grains confirmed by HAADF microscopy.

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