A.L. Cabrera

Kinetics of hydrogen desorption from palladium and ruthenium-palladium foils

The absorption of hydrogen and carbon monoxide at room temperature by palladium and 5% ruthenium-palladium foils was studied using thermal desorption spectroscopy. It was found that hydrogen readily diffused in the palladium and desorbed as one broad peak at about 650 K. Plots of the In (rate) versus inverse absolute temperature indicate that the desorption order is n = 1.25 and the activation energy is about 8.5 Kcal/mol. Carbon monoxide is adsorbed, as two different states, on the surface of the foil and complete coverage is quickly reached below 100 L. Hydrogen also diffuses in 5% ruthenium-palladium foil but to a lesser degree. Two hydrogen desorption peaks are observed in the Ru-Pd alloy. The desorption traces can be fitted with two peaks and the desorption orders are n = 2 for the first peak and n = 1.25 for the second peak. Activation energies of 10.7 and 5.6 Kcal/mol are obtained for the first and second hydrogen peaks, respectively. The first hydrogen desorption peak is regarded as hydrogen desorbing from the surface sites while the second peak is regarded as hydrogen diffusing from below the surface. Activation energies for bulk diffusion were obtained from hydrogen uptake measurements using a sensitive microbalance. These energies corresponded to 4.4 Kcal/mol for Pd foil and 4.9 Kcal/mol for the Ru-Pd alloy. Discussion about the relation between these results with prior studies of hydrogen adsorption on Pd single crystal is included. The appearance of a fractional order for hydrogen desorption is also discussed.

Changes in crystallographic orientation of thin foils of palladium and palladium alloys after the absorption of hydrogen

The adsorption/absorption of hydrogen at room temperature by palladium, 16% silver-palladium and 5% ruthenium-palladium foils was studied using thermal desorption spectroscopy. Hydrogen readily diffused in the palladium and desorbed as one broad peak at about 650 K. Hydrogen also diffuses in the 16% silver-palladium foil and in the 5% ruthenium-palladium foil but with a smaller diffusion constant. Two hydrogen desorption peaks are observed for the Ru-Pd and Ag-Pd alloys, at 440 and around 650 K. The first hydrogen desorption peak is regarded as hydrogen desorbing from the surface sites while the second peak is regarded as hydrogen diffusing from the subsurface sites. The desorption order for surface hydrogen corresponds ton = 2 while the diffused hydrogen desorbs with a fractional order ofn = 1.25. The crystallographic orientation of the foils determined by X-ray diffraction shows a preferential (1,1,0) orientation along the direction of rolling of the foils before hydrogen absorption. This preferential orientation is destroyed after hydrogen adsorption for Pd and Pd-Ag but unaltered for the Pd-Ru alloy. This preferential orientation of the foils might have significant implications in membrane fabrication, since the absorption of hydrogen by Pd is very dependent on surface orientation.

Structural changes induced by hydrogen absorption in palladium and palladium–ruthenium alloys

The structural changes in Pd and Pd–Ru alloys induced by the repeated absorption–desorption of hydrogen have been studied. It is found that absorption–desorption cycles produce structural changes in Pd whereas the addition of small amounts of Ru inhibits these hydrogen‐induced changes. The experimental results show that bulk hydrogen absorption occurs in Pd, while hydrogen surface adsorption becomes dominant over bulk absorption, in the Pd–Ru alloy.

Magnetooptic properties of Fe/Pd and Co/Pd bilayers under hydrogen absorption

The magnetooptical (MO) properties of hydrogenated Fe∕Pd and Co∕Pd bilayers were studied as a function of the H2 pressure. For samples with a Pd overlayer thickness 5.3nm and Fe film thickness in the range of 4.0–11.0nm, the fractional change in MO response under hydrogen loading, ΔθK∕θK0, increases approximately 12%, independent of the Fe film thickness. For H2 pressures less than PH2=25Torr, the enhancement obeys Sievert’s law (ΔθK∕θK0∝PH21∕2). Thicker Pd overlayers increase the MO enhancement, with remarkably large enhancement of 50% for a Fe(4.1nm)∕Pd(10.0nm) sample. Because ΔθK∕θK0 is independent of the Fe thickness, this effect results from a change in the optical properties of the Pd overlayer. This was confirmed by vibrating sample magnetometry. In contrast, no effect is observed in the Co∕Pd bilayers, presumably due to a small amount Co interdiffusion that prevents the Pd from absorbing H2.

Studies of carbon monoxide and hydrogen adsorption on nickel and cobalt foils aimed at gaining a better insight into the mechanism of hydrocarbon formation

Systematic studies of adsorption of hydrogen and carbon monoxide on polycrystalline surfaces of nickel and cobalt have been carried out. The aim of these studies was to gain a better insight into the catalyzed formation of hydrocarbons from H2-CO mixtures. We have studied the adsorption of these gases by means of thermal desorption spectroscopy (TDS) on nickel foils and powders. More recently, we have obtained desorption spectra of hydrogen adsorbed on cobalt foils and powders. In this work we described desorption spectra of carbon monoxide on cobalt foils. Carbon monoxide desorption from cobalt foils was studied in a similar way as prior work, using a mass spectrometric method in an ultra high vacuum system. Two carbon monoxide desorption peaks were observed. These two states correspond to molecular CO and presumably dissociated CO, as it is observed in the case of stepped surfaces of Ni and Co single crystals. An activation energy of around 4.0 kcal/mol is obtained for the molecular state while for the dissociated state the energy is coverage-dependent with a value between 8.0 and 15.0 kcal/mol. The carbon monoxide desorption peaks were fitted with near Gaussian curves which facilitates the analysis of the data to obtain activation energies for desorption. Kinetic parameters for carbon monoxide and hydrogen desorption from nickel and cobalt foils are provided and compared with already published data involving single crystals.

Influence of Crystallographic Phase Transitions in Small Ferroelectric Particles on Carbon Dioxide Adsorption

The aim of this work is to understand surface properties of ferroelectric crystals towards gas adsorption. Various ferroelectric crystals involved in these studies readily adsorb carbon dioxide, thus our studies were centered on adsorption studies of this molecule. It has been claimed by other authors that a dipole moment is induced on carbon dioxide molecules that are near an oxide surface. Thus, our experiments explored the possibility of a dipole-dipole interaction between the gas molecule and the ferroelectric oxide surface in order to explain its adsorption. We describe Raman studies of small particles of BaTiO 3 and KNbO 3 in order to determine the ferroelectric nature of the particles as well as to study the temperature dependent phase transitions. We were able to correlate desorption of CO 2 with the occurrence of the Curie transition in BaTiO 3 and with the orthorhombic to tetragonal transition in KNbO 3 in particles of 50 w m size.

Kinetics of subsurface hydrogen adsorbed on niobium: Thermal desorption studies

The adsorption/absorption of hydrogen and the adsorption of carbon monoxide by niobium foils, at room temperature, was studied using thermal desorption spectroscopy. Two hydrogen desorption peaks were observed with a maximum at 404 and 471 K. The first hydrogen desorption peak is regarded as hydrogen desorbing from surface sites while the second peak, which represents desorption from surface sites stronger bound to the surface, also has a component-due to its tailing to higher temperatures-of hydrogen diffusing from subsurface sites. Carbon monoxide adsorption was used to determine the number of surface sites, since it does not penetrate below the surface. Two carbon monoxide desorption peaks are observed in these experiments: at 425 and 608 K. The first peak is regarded as the adsorption of molecular carbon monoxide, and the second, as carbon monoxide dissociated on the niobium surface. The crystallographic orientation of the foils was determined by x-ray diffraction and showed a preferential (110) orientation of the untreated foil due to the effect of cold rolling. This preferential orientation decreased after hydrogen/heat treatment, appearing strong also in the (200) and (211) orientations. This change in texture of the foils is mainly due to the effect of heat treatment and not to hydrogen adsorption/desorption cycling. The kinetics of hydrogen and CO desorption is compared with that of Pd and Pd alloys.

Ferroelectric Properties of Flash Evaporated Barium Titanate Thin Films

Thin films of barium titanate were fabricated by flash evaporation on Pt-coated silicon at 630°C in the thicknesses range from 30 to 300 nm. X-ray diffraction shows that the films are polycrystalline, in the tetragonal phase. Raman Spectroscopy shows the cubic phase in the 30 nm films and the tetragonal phase in the 300 nm films. The values of the refractive index obtained by spectroscopic ellipsometry indicate that the films contain, also, an amorphous phase. We think that the amorphous phase contributes to high resistivity, I–V measurements present typical resistivity values in the range of 1011 to 1012 Ω-cm, and that the crystalline phase is responsible of the ferroelectric behavior. The latter was observed in 300 nm thick films, showing unsaturated hysteresis loops. However, 30 nm films present rounded hysteresis loops, due to current leakage. Both cases were demonstrated by a numerical simulation of the ferroelectric capacitor, which fits the experimental measurements. The dielectric constant and loss factor are affected, also, by the amorphous character of the films.