Metal Particle Size Distribution Research Articles

Study of electrode products emitted by vacuum arcs in form of molten metal particles

The electrode products emitted by vacuum arcs in the form of molten metal particles were studied for Au, Pd, and Mg in the current range 2−6 A. The study includes the size distribution of these molten metal particles, the volume carried by these particles versus the volume carried by the metal vapor, and the velocity distribution of these particles. It was found that the ratio of volumes carried by the particles and the vapor is greatly dependent on the cathode material.

Small angle X-ray scattering investigation of platinum metal dispersions on alumina catalysts

The metal particle size diameters and particle size distributions of platinum supported on alumina catalysts have been investigated using small angle X-ray scattering techniques. The interfering background scattering from the micropores of the alumina support was eliminated by adsorption of small concentrations of CH2I2 and C4H9I, organic liquids of electron density similar to that of the alumina. Although CH2I2 is twice as effective as C4H9I in reducing the pore scattering from the η-alumina support, care must be exercised to prevent an excess of organic material which will cause unwanted scattering. The primary parameters derived from the scattering of X-rays by the platinum crystallites are RG, the Guinier radius, and RP, the Porod radius. These radii are expressed as the ratio of moments of a distribution which is related to σ2 and μ, the variance and geometric mean, respectively, of a log-normal particle size distribution function. Using this simple procedure, we find that both the mean particle diameter and the particle size distribution of an aged 0.62 wt % Pt on alumina catalyst have shifted to larger sizes. In the experimental intensity curves of a 0.35 wt % Pt catalyst and that of the fresh 0.62 wt % Pt catalyst, shoulders are observed that are more or less pronounced. This behavior indicates interparticle scattering effects, suggesting the presence of small clusters of platinum particles approximately 200 Å in diameter before any agglomeration has ocurred.

Surface reactivity of supported gold: I. Oxygen transfer between CO and CO 2

The rate of redistribution of isotopic carbon between CO and CO 2 has been studied on Au supported on MgO in the temperature range 300 to 400 °C, P co 2 P co ratios 0.1 to 1.2 and total pressure of 50 Torr. A few experiments were also carried out on supported Ru and Pt. The effect of Au concentration, temperature, and catalyst preparation method have been selected for investigation. In addition, determinations of the particle size of Au have been carried out by X-ray to illustrate the effect of the temperature of reduction and decomposition of the Au salt upon the particle size of the metal in the supported catalyst. Chemical reduction of the Au salt at low temperature (< 100 °C) produced Au particles with diameters ≤ 150 Å, while particles with diameters 20 times larger were obtained by thermal decomposition (≤ 350 °C) of the Au salt. Since particle growth may occur by direct addition of the decomposing salt and/or by sintering among metal particles, it is suggested that the latter process cannot readily occur at temperatures ⩽ 0.3 Tm. The implications of these findings for separate control of the degree of dispersion and of the support coverage by the metal are pointed out. Kinetic observations have been employed to study the thermodynamic and kinetic factors contributing to the activity of Au surfaces in the oxygen transfer step between gas and surface phases. Au activity was found to decrease with increasing P co 2 P co ratio, indicating that reduced surface species (metal atoms) play a dominant role in the reactivity of the surface. A similar trend was found for Ru and Pt at low ratios P co 2 P co . For Pt at higher P co 2 P co ratios, a reactivity inversion was found. Under similar conditions of gas composition, temperature and support, the affinity of the Au surface for oxygen increased with decreasing particle size. The degree of dispersion of Au was found to influence the rate of the catalytic reaction. The effect has been interpreted in terms of a relation between metal particle size and gas mean free path. The usefulness of these studies for developing criteria for control of oxidation depth and selectivity behavior in catalytic oxidations through optimization of size, size distribution of metal particles, and their morphological connection with the supporting agent is emphasized.

The influence of crystallite size on the adsorption of molecular nitrogen on nickel, palladium and platinum: An infrared and electron-microscopic study

A specified study was made of the phenomenon of infrared-active nitrogen adsorption on nickel, the existence of which was first shown by Eischens and Jacknow. The study combines an infrared spectroscopic investigation of the adsorbed nitrogen with an electron-microscopic determination of the metal particle size distribution for the various nickel-on-silica samples. It is shown that this form of nitrogen adsorption is not limited to nickel but also exists on platinum and palladium (and, doubtless, also on other metals). However, this type of adsorption is measurable only if the diameter, d, of the metal crystallites is within the range ~ 15 Å < d < ~ 70 A. This is because crystals within this range possess a large number of special surface sites which are absent, or much scarcer, on larger or smaller crystals. This has appeared from a study of their surface structure in which use was made of marble models and of mathematical computations. The infrared activity of the nitrogen adsorbed on these special sites is due to the induction of a transition-moment by the strong polarizing field of the special sites. The observed absorption band is, in fact, the nitrogen Raman band (2331 cm −1) which has shifted under the influence of the Stark effect; in nickel, palladium and platinum it is observed at 2202, 2260 and 2230 cm −1 resp. Measurement of the heat and entropy of adsorption revealed that the initial heat of adsorption is about 12 kcal/mole and that the adsorption of the molecules is immobile at low coverage. Although the high heat of adsorption might suggest chemisorption, it is made plausible, i.a. in view of the non-specificity of the phenomenon to a particular metal, that this type of nitrogen adsorption is, in fact, physisorption.

Support structure and particle size influences on the effect of adsorbed hydrogen on the magnetization of catalytically active nickel

Saturation magnetizations have been obtained for small particles of catalytically active nickel supported on silica gel and on gamma-alumina. The measurements were made in fields up to 17 koe, and in the temperature range 2 ° to 300 °K. The saturation magnetization of nickel under these conditions appears to be independent of particle size over the radius range 13 to 58 Å as determined from low and high field magnetizations. There is no obvious effect on the saturation magnetization of changing the support from silica to alumina, although the absolute accuracy is poor owing to problems found in the chemical reducibility of the samples on alumina. The effect of adsorbed hydrogen in lowering the saturation moment of the nickel was determined for all samples. All samples on silica gave about the same effect, regardless of particle size. A possible influence of the alumina was shown in a diminished effect of hydrogen on the magnetization of alumina-supported samples. This is attributed to the change of metal particle size, shape, and size-distribution when the carrier is changed.