Small-Angle Scattering of Heterogeneous Catalysts

Benedetti, Alvise

doi:10.1107/s0021889897001003

Cited by 13 publications

(7 citation statements)

References 35 publications

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“…Supported metal catalysts − are systems mostly composed of a support and an active phase (metal) responsible for the principal chemical reaction. Generally, the metal is highly dispersed on the support by a chemical procedure in order to have aggregates so small that a lot of the atoms are on the surface.…”

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confidence: 99%

Au/C Catalyst: Experimental Evidence of the Coexistence of Nanoclusters and Larger Au Particles

1998

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In this study, using X-ray diffraction Rietveld analysis, we have determined that a great quantity of very fine particles (<2 nm) can be present in a supported metal catalyst, but that they are almost undetectable if larger particles (even a very small fraction) are contained in the sample. The large amount of hidden fine particles found in the discussed catalyst convinced us that, probably, such a nanostructure could be characteristic of catalysts containing a low percentage of metal. Since heterogeneous catalysis is a surface phenomenon, it is clear that losing great fractions of nanoparticles biases the microstructural study, and hence, the understanding of the catalytic working mechanism.

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confidence: 99%

Au/C Catalyst: Experimental Evidence of the Coexistence of Nanoclusters and Larger Au Particles

1998

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“…4, inset) using the GNOM package [19]. This profile was obtained as usual [20] by subtracting the ZSM5 normalized SAXS spectrum to that of the In/ZSM5 catalysts to get rid of the scattering contribution coming from the porous ZSM5 support. Figure 4 shows that there is a broad distribution of In 2 O 3 particle sizes in the catalysts and there is a maximum of D V (R) at R=50Å with a long tail to larger radii.…”

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confidence: 99%

Structure of Extremely Nanosized and Confined In-O Species in Ordered Porous Materials

et al. 2003

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Perturbed-angular correlation, x-ray absorption, and small-angle x-ray scattering spectroscopies were suitably combined to elucidate the local structure of highly diluted and dispersed InOx species confined in porous of ZSM5 zeolite. This novel approach allow us to determined the structure of extremely nanosized In-O species exchanged inside the 10-atom-ring channel of the zeolite, and to quantify the amount of In2O3 crystallites deposited onto the external zeolite surface.The study of structural, magnetic and electronic properties of nanostructured, subnanostructured and, in the other extreme, highly diluted monoionic species in solids deserves increasing attention not only from a fundamental point of view but also for their technological applications [1]. In one extreme, highly-dispersed metallic exchanged-atoms in an infinite variety of compounds present, e.g., very important catalytic properties [2], which are not clearly correlated with the active species responsible for them since their physical entities are often unknown. The design of new catalysts with improved activity, selectivity and stability requires the complete knowledge of the local environment of the active centers and its correlation with the desired reaction. The complete characterization of this kind of structures is at present a challenging problem not only in catalysis but also in many fields of physics.An additional difficulty is the confinement of the diluted species inside porous materials. In general, experimental techniques based on energetic probes that strongly interact with the materials, may destroy them. On the other side, low energetic probes are unuseful since they have to pass through the walls around the "hidden" locations of the confined clusters or atoms and their kinetic energy is attenuated.The importance of the extended X-ray absorption fine structure (EXAFS) spectroscopy to study structural properties in crystalline solids [2], nanoclusters [3] and highly dispersed ionic species in catalysis [4] has been long acknowledged. EXAFS constitutes a powerful "atom selective" technique to extract direct information about type, number and distances of neighbors of the absorber atom [5,6]. However, in EXAFS analysis, if more than one species has the same type of bonds (same element and similar bond-lengths), the information from each one would be almost impossible to extract directly, unless some additional information is known.The perturbed-angular-correlation (PAC) technique requires a suitable probe-atom (native or foreign) to be in the system under study, being in this sense also "atom selective". PAC enables the precise determination -at the probe site -of the electric-field gradient tensor (EFG), which is extremely sensitive to the anisotropy of the electronic density near the nucleus, which in turn reflects the probe-neighboring coordination. PAC has been intensively applied to many fields in science during the past two decades with success [7] and, very recently, to the development of accurate ab initio calculations of th...

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“…max denotes the h value of the maximum [30]. A more careful determination of the volume weighted average size was obtained by fitting the calculated scattering intensities to the experimental data using a Schultz distribution of spheres [31]. The small angle intensity scattered by a dilute system of a continuous normalised distribution P(r) of particles with radius r can be represented by the monotonically decreasing curve given by…”

Section: Saxsmentioning

confidence: 99%

Properties of Nanocrystalline Nickel Particles in Ni-Si0 2 Composites

Ennas

Falquri

Piccaluga

et al. 2000

Zeitschrift Für Naturforschung A

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Nickel-Silica nanocomposites with a nickel content equal to 10, 15, 20 wt% have been prepared by a sol-gel method starting from ethanolic solutions of tetraethoxysilane and nickel nitrate hexahydrate. After selation the samples were reduced in H-> flow at selected temperature (450 °C < T < 600 °C).The morphological, structural and magnetic properties were investigated by transmission electron microscopy (TEM), wide and small angle X-ray scattering (WAXS, SAXS), magnetic susceptibility in zero field cooled and field cooled mode (ZFC and FC), and magnetic hysteresis loop.Nanometric nickel particles are observed in all the investigated samples. TEM, WAXS and SAXS techniques indicate that the average nickel particle size grows slightly but almost regularly with the nickel concentration. TEM results moreover indicate that also the width of the particle size distribution, which can be simulated by log-normal functions, follows this trend.All the sample treated in hydrogen show superparamagnetic behaviour. The magnetisation falls to reach saturation up to highest measuring field of 70 kOe even at 3 K, while the observed coercivity H c is much higher than the theoretical bulk one. Some uncertainty in the complete interpretation of the sequence of magnetic measurements is attributed to a progressive oxidation of the samples when these are air exposed.

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Small-Angle Scattering of Heterogeneous Catalysts

Cited by 13 publications

References 35 publications

Au/C Catalyst: Experimental Evidence of the Coexistence of Nanoclusters and Larger Au Particles

Au/C Catalyst: Experimental Evidence of the Coexistence of Nanoclusters and Larger Au Particles

Structure of Extremely Nanosized and Confined In-O Species in Ordered Porous Materials

Properties of Nanocrystalline Nickel Particles in Ni-Si0 2 Composites

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