Taneki Tokuda

Oxygen species adsorbed on ultraviolet-irradiated magnesium oxide

The adsorption of oxygen, under u.v. irradiation, on the surface of MgO outgassed at 1123 K has been studied at 77 K by temperature-programmed desorption and e.s.r. spectroscopy. During adsorption ozonide and superoxide ions were formed in equal amounts. The formation of these oxygen species is explained by the reaction of oxygen molecules with a short-lived exciton produced by the absorption of a photon at a surface O2– species in a very low coordination state. Thermal decomposition of the oxygen species adsorbed at 77 K has also been studied under a dynamic vacuum. It proceeds in four steps: (i) O–3 is transformed into O– and O2 between 77 and 300 K; (ii) O– is changed into O2–2 between 77 and 473 K; (iii) O–2 is also transformed into O2–2, with the evolution of O2 between 300 and 673 K; (iv) finally O2–2 is changed into O2– and O2 between 673 and 1123 K. This mechanism is also consistent with energy considerations. The formation and thermal decomposition of superoxide ions proceeds similarly both on surfaces subjected to u.v. irradiation and on surfaces containing thermally preadsorbed hydrogen.

O–2 formation on magnesium oxide powders containing preadsorbed hydrogen

Temperature-programmed desorption (t.p.d.) and e.s.r. spectroscopy have been used to study the mechanism of O–2 formation following the admission of O2 onto the surface of MgO powders which had previously been outgassed at high temperatures (mainly at 1123 K) and then exposed to hydrogen at 308 K. Four types of preadsorbed hydrogen have been previously identified and oxygen has been found to interact with all of them. For a given amount of preadsorbed hydrogen twice as much as oxygen can be adsorbed (as O–2). The presence of at least four varieties of O–2 has been identified by e.s.r. spectra and five desorption peaks of O2 have been found in the t.p.d. spectra following O–2 formation. However, no specific relation has been found between the types of preadsorbed hydrogen and the O–2 species resulting from subsequent oxygen adsorption.O–2 formation can be explained on the basis of the presence of heterolytically dissociated hydrogen; a first O–2 species is formed by electron transfer between H– and O2 and a resultant H atom forms a second O–2. This mechanism is also consistent with energy considerations.Thermal decomposition of O–2in vacuo proceeds in three steps: (i) O–2 is transformed into O2–2 with the simultaneous desorption of O2, forming four t.p.d. peaks in the temperature range 350–790 K; (ii) by further heating O2–2 is decomposed into O2– and O2, forming a fifth t.p.d. peak at 790–1000 K; (iii) preadsorbed hydrogen is desorbed as H2O above 900 K, with the O atom thought to originate from O–2 from material-balance considerations.

Mechanism of diffusion and sorption of carbon dioxide in poly(vinyl acetate) above and below the glass transition temperature

Abstract The pressure dependence below 1 atm of the apparent diffusion and permeation coefficients were observed by using the permeation time lag method for carbon dioxide in poly(vinyl acetate), which has a glass transition near room temperature, at temperatures ranging from 8 to 50°C. Above the glass transition temperature, pressure dependence of the diffusion and permeation coefficient has not been observed; hence, Ficks law with a concentration independent diffusion coefficient applies. On the other hand, in the glassy state, the apparent diffusion coefficient shows pressure dependence. Moreover, the behavior of the pressure dependence does not show a clear curve in the ranges between 30°C to 17°C. Above 17°C, the apparent diffusion coefficients show discontinuities, but below 17°C increase with pressure is regular. Using the theoretical prediction of Paul, a computer was used in the numerical calculation to determine the true diffusion coefficient and other dual sorption parameters. p]The compensated diffusion coefficients controlled only by Henrys law dissolution was described by three straight lines with two intersection in the form of Arrhenius plots, which give good agreement with both our results for He and Ar and those of Meares. It is assumed that beside the dual sorption mechanism, another effect, for instance some relaxation effect may also contribute to the diffusion for carbon dioxide in poly(vinyl acetate) near the glass transition temperature region.

Isotopic study of hydrogen adsorption on magnesium oxide powders

By measuring temperature-programmed desorption (t.p.d.) spectra of hydrogen adsorbed on MgO powders outgassed at 1123 K, eight different states of adsorbed species have been found. Species W1 is reversibly adsorbed at 77 K and other irreversible species desorb with maximum rates at 190 (W2), 229 (W3), 293 (W4), 327 (W5), 460 (W6), ca. 500 (W7) and 608 K (W8). All the species are hydrogen chemisorbed on different active sites. The numbers of active sites are > 1.7 (W1), 32±3 (W2+W3), 23±3 (W4+W5), 1.9±0.5 (W6+W7) and 2.3±0.5 (W8)× 1015 site m–2 and the total number of active sites except for type W1 amounts to ca. 60 × 1015 site m–2.A dominant species W5 has been studied in detail. T.p.d. spectra, observed by using a mass-filter, of HD molecules have shown that molecular identity is conserved in the desorption process. This implies that the adsorbed species is immobile and localized on the surface and that pseudo-first-order kinetics controls the desorption process. The activation energy for H2 desorption at the peak maximum is 99.4±0.6 kJ mol–1, less than that for D2 desorption by only 1.5 kJ mol–1. The heat of adsorption is roughly estimated to be 90 kJ mol–1. The values of isotope effects (H/D) observed for species W5 on the sample having nearly half-coverage at 308 K are 2.2±0.2 and 0.71±0.02 for the adsorption rate and the equilibrium, respectively. It is revealed, on the basis of these isotope effects and available i.r. spectra on adsorption, that the formation of species W5 is consistent with a heterolytic dissociation of hydrogen on a surface ion pair O2–LC—Mg2+LC with very low coordination numbers. O—H and Mg—H bonds formed have no hydrogen bonding with neighbouring ions and behave as ‘free’ groups. Their frequencies, including bending modes, have been assigned.Species W2(and/or W3), dominant in the lower-temperature region, also seems to be formed via the heterolytic dissociation of hydrogen. Its adsorption process requires a higher activation energy than that of species W5, while the desorption process, controlled by first-order kinetics, requires a lower activation energy than that of species W5.

Hydrogen adsorption on magnesium oxide powders

Hydrogen adsorption on MgO powders has been studied using t.p.d. spectra, adsorption isotherms and e.s.r. spectra. When H2 is adsorbed at 308 K, MgO outgassed at 773 K shows only one t.p.d. peak (M2) at 385 K. On the other hand, MgO outgassed at 1123 K shows three new peaks at 480 (H1), 550 (H2) and 680 K (H4) as well as peak M2. Both samples have another adsorbed species (M1) which is reversible at 308 K. These five adsorbed species come from different adsorption sites and the surface concentrations of the sites are given.A model is proposed such that every adsorption site consists of a pair of O2–cus and Mg2+cus, in highly coordinative unsaturation, formed by high-temperature activation; the model further proposes that H2 is dissociatively adsorbed on the sites to form H+ and H–. The coordination number (3 or 4) of O2–cus seems to be the main factor in determining the adsorption energy. This type of H2 adsorption may be widespread in other oxide systems and useful as a probe for some coordinatively unsaturated surface ions.

Thermal Diffusion of Silicon Tetrafluoride and Silane

Separation of silicon isotopes by thermal diffusion method was studied. When the hot-wire-type thermal diffusion column 150 cm long was used, the separation factors were 529/28 = 1.017 and 330/28 = 1.087 after 6 hours operation using silicon tetrafluoride as a diffusion material. This experimental results were compared with the theoretical molecular models based on the inverse power repulsion and the Lennard-Jones (12—6) potential. In both models, the calculated optimum pressure was in good agreement with the observed one, but the value of maximum separation factor was in poor agreement. The inverse power model gave a better reproduction of the observed data than the Lennard-Jones (12—6) potential model. Then these two models can be used to estimate the optimum pressure. Silane, having the greatest relative mass difference, was also used as a diffusion gas. The maximum separation factors obtained were 329/28 = 1.15 and 330/28 = 1-23 after 648 hours by using silane and the column of 390 cm long. From the viewpoint of thermal stability, silicon tetrafluoride is suitable as a diffusion gas for the enrichment of silicon isotopes by thermal diffusion.

Sintering of the Magnesium Oxide Particles in the Water Vapor and the Properties of the Products

In order to study the influence of the atmosphere on the sintering of magnesium oxide particles, the particles prepared by rapid decpmposition of magnesium oxalate in vacmo were sintered in water vapor of 20 mmHg at the temperatures from 500 to 900eC for various times ranging from one to ten hours. The features of this sintering system were described by measurements of the crystallite size, specific surface area, and lattice constant of the products, Eaeh crystallite of the sintered magnesium oxide particles showed a cubic form (Fig. 1), aqd the degree of secondary agglomeration between them was relatively srilight(Fig. 6). The decrease of the specific surface area observed with the increase of the sintering time was an immediate consequence of the crystallite growth, andboth the phenomena could be treated in one kinetic equation (Equation (5)). The values of the apparent activation energy for the sintering were found to be 33.3.v36.lkcal/mol. It seems likely that the sintering in water vapor is acceleratedby increase of anion mobility due to repetition of the adsorption-desorption cycle of water molecules.Aging of the sintered magnesium oxide particles upon exposure to the air was observed for 8 minutes to 2 years (Fig. 8, 9, and Table 1). lt was found that both lattice constants, and crystallite sizes were kept constant over the whole period, but the decrease qf the spe-cific suMrface area was already observed within 3 hours after exposure to the air. Through these measurements on aging, it is suggested that degrees of interaction of magnesium exide particles with water vapor vary during the lbng period of exposure, but the lattice constants of magnesium oxide particles are not affeeted by the degrees of interaction.