Morphology Changes of Co Catalyst Nanoparticles at the Onset of Fischer–Tropsch Synthesis

Abstract

Cobalt nanoparticles play an important role as catalysts for the Fischer− Tropsch synthesis, which is an attractive route for production of synthetic fuels. It is of particular interest to understand the varying conversion rate during the first hours after introducing synthesis gas (H 2 and CO) to the system. To this end, several in situ characterization studies have previously been done on both idealized model systems and commercially relevant catalyst nanoparticles, using bulk techniques, such as X-ray powder diffraction and X-ray absorption spectroscopy. Since catalysis takes place at the surface of the cobalt particles, it is important to develop methods to gain surface-specific structural information under realistic processing conditions. We addressed this challenge using small-angle X-ray scattering (SAXS), a technique exploiting the penetrating nature of Xrays to provide information about particle morphology during in situ experiments. Simultaneous wide-angle X-ray scattering was used for monitoring the reduction from oxide to catalytically active metal cobalt, and anomalous SAXS was used for distinguishing the cobalt particles from the other phases present. After introducing the synthesis gas, we found that the slope of the scattered intensity in the Porod region increased significantly, while the scattering invariant remained essentially constant, indicating a change in the shape or surface structure of the particles. Shape-and surface change models are discussed in light of the experimental results, leading to an improved understanding of catalytic nanoparticles. ■ INTRODUCTIONThe Fischer−Tropsch synthesis (FTS) is a set of chemical reactions that forms hydrocarbon chains from a mixture of CO and H 2 . The product can be upgraded to petroleum substitutes, for example synthetic diesel.1 Typical commercial FTS catalysts consist of cobalt nanoparticles of diameter ∼20 nm dispersed on a porous support material, 2 such as γ-alumina. Optimal particle size, temperature, and pressure are required for obtaining high activity and high selectivity to long-chain hydrocarbons. The reaction output is dependent on the temperature and the pressure in the reactor cell; the standard industrial process operates at T ≈ 220°C and pressure of 25− 45 bar, conditions favorable for producing waxes.1 At ambient pressure the CO conversion is still high, but the products are predominantly short molecules, thus tending to remain in the gas phase.The FTS shows an initial stage lasting a few hours where the conversion rate increases to a high level, followed by a much slower decrease of the reaction rate that continues on a time scale of days and months.3 Understanding the mechanisms behind this behavior is of high commercial and academic interest. Tsakoumis et al.3 combined in situ X-ray absorption spectroscopy (XAS) and X-ray powder diffraction to investigate the cobalt catalyst nanoparticles during the FTS synthesis. The deactivation was detected by mass spectrometry, but no apparent changes in the X-ray signal could be...

X-ray spectroscopic and scattering methods applied to the characterisation of cobalt-based Fischer–Tropsch synthesis catalysts

“…[50] It is therefore necessary to use a broader range of techniques to fully understand these effects. The use of nanospectroscopy and TXM has proved effective in observing the spatial elemental composition under reaction conditions including the behaviour of the surface.…”

“…WAXS was performed simultaneously to study the reduction of the cobalt oxide, the results of which were consistent with previous studies. ASAXS (Anomalous Small Angle X-ray Scattering), a technique in which the X-ray energy is deliberately tuned near to a absorption edge to provide some element specificity in the features being probed, was performed independently on one sample to distinguish between Co particles and pores in the -alumina, resulting in a more meaningful discussion of the SAXS data [50].…”

Simultaneous investigation of the structure and surface of a Co/alumina catalyst during Fischer–Tropsch synthesis: discrimination of various phenomena with beneficial or disadvantageous impact on activity

“…24,25 However, the first four expressed possible causes should lead to catalyst deactivation, which is clearly not the case during the first period when the activity increases to a maximum. 24,25 However, the first four expressed possible causes should lead to catalyst deactivation, which is clearly not the case during the first period when the activity increases to a maximum.…”

CO oxidation over supported gold nanoparticles as revealed by operando grazing incidence X-ray scattering analysis

“…The signal also increases at larger q ('Porod' region) for the in-plane proles taken under the reaction conditions that could indicate a more abrupt interface between catalyst and gas atmosphere. 27 In order to extract changes in NPs' shape and phase under reaction conditions the 2D experimental patterns were tted using the model with bimodal spheres arranged into the planar hexagonal superlattice (Fig. S2 †).…”