M.José Yacamán

Investigations on magnetic properties and structure for carbon encapsulated nanoparticles of Fe, Co, Ni

Abstract In the present work, experiments aim at the encapsulation of foreign materials within hollow graphitic cage have been carried out for iron group metals (Fe, Co, Ni) using a modified arc-discharge (carbon arc) reactor. HRTEM (high resolution transmission electron miscroscope), and XRD (X-ray diffractometer) studies, for three carbon encapsulated materials, showing nanoparticles of both a metallic phase (α-Fe, γ-Fe; hcp-Co, fcc-Co; fcc-Ni) and also a carbide phase (M3C, M=Fe, Co, Ni) are encapsulated in graphitic carbon. The magnetic measurement for the three as-made nanoparticles, indicating that the values of saturation magnetic moment of three nanoparticle are 37.6, 55.5 and 15.7% of the bulk ferromagnetic elements counterparts, respectively. The different comparison values (Mr/Ms) of remanent magnetization (Mr) and saturation magnetization (Ms) suggest, the encapsulated Fe and Co nanoparticles are shown to be ferromagnetic with a ratio of remnant to saturation magnetization Mr/Ms∼0.3; whereas, the encapsulated Ni nanoparticles exhibits superparamagnetic behavior at room temperature.

The effect of shape and crystal structure of small particles on their catalytic activity

Abstract It is shown by the use of weak beam electron microscopy techniques, that the crystallographic structure of small particles on supported catalysts can be obtained. In the case of Rh, it is found that the particles are mainly icosahedral non-fcc or fcc cubo-octahedral depending on the strength of the interaction with the support. Pentane hydrogenolysis reaction using Rh particles of different structure is studied.

Characterization of small platinum particles supported on graphite by electron microscopy

Abstract Characterization of small platinum particles in the Pt/graphite catalyst is done using high-resolution dark-field electron microscopy and single-particle electron diffraction. Two types of hexagonal-shaped particles were found. Both are shown to correspond to a three-dimensional cubo-octahedral shape. The first type has a (111) face in contact with the (0001) face of the graphite surface. The second type has a (110) face parallel to the (0001) face of the support surface. The faceted Pt particles are bounded by (100) and (111) crystallographic planes, which provide specific sites for a catalytic reaction.

Topographical interpretation of vacuum-deposited fine metallic particles

Abstract Morphological features of vacuum–deposited gold particles are investigated. It is shown that the use of bright–field imaging techniques is inadequate for assessing these features but that unambiguous topographical information can be obtained through the use of a weak–beam dark-field technique. Coalesced gold particles deposited on KC1 substrates are shown to be facetted except when undergoing ooalesoence.

GOLD DECORATION OF PRECIPITATES IN DOPED ALKALI-HALIDE SURFACES.

Sodium‐chloride crystals doped with Ca++, Fe++, and Cd++ exhibit precipitation phenomena of various phases. Gold decoration of cleaved crystal surfaces has been found to be highly sensitive to the formation of these phases. The study extended to other alkali halides and a wide range of other divalent impurity atoms.

Modern Applications of Electron Microscopy to Catalysis

The active sites of ultra-dispersed Pt/γ-Al2O3 catalysts are studied using high-resolution electron microscopy, Z-contrast and dark field. In addition we have calculated using a method based in density functional theory the electrostatic potential and charge distribution of the active sites. It is conclude that the most likely Pt clusters that are formed correspond to Pt13 and Pt with icosahedral and decahedral structure. It is shown that this is consistent with the electron microscopy data.

High resolution tem studies of gold and palladium nano-particles

Abstract In the present work we analyze nano-particles of gold and palladium using a combination of High Resolution Electron Microscopy (HREM) and image processing techniques. The experimental results are compared with calculations of images using dynamical diffraction theory. We focus mainly on the decahedral (D h ) and icosahedral (Ic) Particles. We found that the decahedral particles correspond well with the model for a perfect MPT. The icosahedral particle on the other hand appears to be much more distorted than the MPT model predicts. Images of polycrystalline and amorphous particles are also shown. No significant differences between Au and Pd were found.

Surface roughness studies in small particles using HREM

Abstract High-resolution images, multislice image simulation and image processing are used to study surface roughness in small metallic particles. The images of Pt particles on alumina substrates show displacements of the atomic columns that are shown to be due to local changes in thickness. It is concluded that surface roughness is present in Pt particles on Al 2 O 3 and can be studied with these techniques.

Sublimation of ionic crystals in the presence of an electrical field

A direct observation of the influence of an externally applied electrostatic field on the sublimation steps on (100) surfaces of KCl is reported. Typically, round‐cornered square spirals were found to be distorted into round‐cornered rhombical spirals in the presence of a field. The results are interpreted in terms of charged kinks on the surface.

The back force effect and its influence on sublimation of alkali halide single crystal surfaces

Abstract Sublimation structures at the atomic level were studied on sodium and potassium chloride crystals by the gold decoration technique. Three stages of sublimation were revealed: hole nucleation, spiral-ledge motion, and macroscopic pit formation. The results are in good agreement with recent theories of crystal sublimation by the terrace-ledge-kink model, including those for the back force effect at ledge sources.