Structure analysis of multiphase systems by anomalous small-angle X-ray scattering

Tatchev, Dragomir

doi:10.1080/14786430802279760

Cited by 23 publications

(26 citation statements)

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“…the number of phases in the sample is increased to three, the complexity increases dramatically, drastically lowering the fields of application [180]. Some existing examples are studies on the extraction of hydrocarbons from coal [28], absorption studies on carbon fibres [79] and determination of closed versus open pores in geopolymers [112] 1 .…”

Section: Scattering To Small Anglesmentioning

confidence: 99%

“…Small-angle x-ray scattering can be applied to a large variety of samples, with the majority consisting of two-phase systems [180]. In multiphase systems where the electron density of one phase is drastically higher than that of the remaining phases a two-phase approximation can be made [100].…”

Section: Scattering To Small Anglesmentioning

confidence: 99%

“…For multiphase systems straightforward SAXS is rarely attempted, though some groundwork for such applications has recently been laid [181]. Instead, element-specific techniques such as anomalous SAXS (ASAXS) [192,180] or combinations between SAXS and SANS [129] are used to extract element-specific information.…”

Section: Scattering To Small Anglesmentioning

confidence: 99%

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Everything SAXS: small-angle scattering pattern collection and correction

Pauw

2013

J. Phys.: Condens. Matter

169

136

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Section: Scattering To Small Anglesmentioning

confidence: 99%

Section: Scattering To Small Anglesmentioning

confidence: 99%

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Everything SAXS: small-angle scattering pattern collection and correction

Pauw

2013

J. Phys.: Condens. Matter

169

136

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“…In anomalous SAXS (ASAXS) experiments, the pronounced element-specific dispersion occurring near absorption edges are exploited for increased contrast. The scattered intensity from a two-phase system can be understood as a sum of three terms, 40,41…”

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confidence: 99%

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

et al. 2014

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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...

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“…Anomalous small-angle X-ray scattering experiments [45][46][47] were carried out at the PTB beamline at BESSY in Berlin. The X-ray energy range used was between 8000 and 8331 eV just below the K absorption edge of Ni (8333 eV).…”

Section: Anomalous Small-angle X-ray Scattering Characterizationmentioning

confidence: 99%