Keio Toi

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.

Sorption and diffusion of water vapor in poly(ethylene terephthalate) film

The sorption and diffusion of water vapor in poly(ethylene terephthalate) (PET) film were measured by applying a thermogravimetric analyzer (TG-DTA), which customarily has been used to detect the weight loss of a sample with the increase of temperature under a given atmosphere. In this case, we detected the weight gain of PET film by sorption of water vapor under a given humidity at a constant temperature. Sorption-rate curves were successfully obtained in spite of the low solubility of PET film and the presence of Fickian-type curves. The solubility was better described according to the dual-mode sorption model. The diffusion coefficients were determined in their initial slopes by the short-time method. We found that the diffusion coefficient depended on vapor pressure.

Reaction of preadsorbed methane with oxygen over magnesium oxide at low temperatures

Partial oxidation by O2 of methane, which had previously been adsorbed at room temperature on MgO pretreated at 1123 K in vacuo, has been examined near room temperature by t.p.d., i.r. and e.s.r. spectroscopies and compared with a methanol adsorption system. The oxidation reaction of methane preadsorbed either in the dark or under u.v.-irradiation proceeds according to the same reaction mechanism, where low-coordination surface ions of MgO play an important role. Methane is adsorbed in a heterolytically dissociated form (CH–3+ H+), and then oxidized to a methoxide species, OCH–3(2), on the admission of O2 at room temperature. On heating under evacuation, OCH–3(2) is either decomposed into H2 and CO in the temperature range 495–535 K or further oxidized by coexisting O–2 into HCO–2 below 473 K. The HCO–2 formed gives either CO or bidentate CO2–3 at ca. 600 K. Another more stable methoxide species, OCH–3(1), is also sometimes formed eithere directly on room-temperature oxidation or through a transformation of the less stable species, OCH–3(2), on heating at ca. 500 K. Part of the OCH–3(1) present then decomposes into H2 and CO at 760 K and the rest changes into bidentate CO2–3. The CO2–3 species thus formed ultimately are desorbed as CO2 above 600–700 K.

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.

Partial oxidation of methane with oxygen over magnesium oxide at low temperatures

Partial oxidation of CH4 by O2 on MgO pretreated at 673 and 1123 K has been investigated near room temperature by temperature-programmed desorption (TPD), infrared (IR) and electron paramagnetic resonance (EPR) spectroscopy. The UV-irradiation effect on this reaction was also studied. In the dark, this oxidation reaction proceeded only on MgO pretreated at 1123 K and is initiated by the heterolytic dissociation of CH4 into CH–3 and H+ on a low-coordinated surface ion pair, Mg2+LC–O2–LC. CH–3 thus formed reacts with O2 and then an oxide ion O2– to give OCH2–3. Upon subsequent heating in vacuo, part of the OCH–3 decomposes into CO and H2, and the rest is further oxidized into CO2–3. Under UV-irradiation, the oxidation reaction proceeded on MgO irrespective of its pretreatment temperature. This reaction seems to be initiated by an active oxygen species, O–, which can be formed on MgO in the presence of O2 under UV-irradiation. The initial event in O– formation is photon absorption at OH– and Mg2+LC–O2–LC sites on MgO pretreated at 673 and 1123 K, respectively. Such a site reacts with O2 to produce O–. The O– so formed can abstract an H atom from CH4 to give a CH3 radical. This CH3 radical reacts with a surface oxide ion O2– to produce OCH–3, which is mainly further oxidized into HCO–2 by adsorbed oxygen species during UV-irradiation. A part of the HCO–2 decomposes to give CO during subsequent heating in vacuo, and the remainder is further oxidized into CO2–3.

Reaction of methane with nitrous oxide over magnesium oxide at low temperatures

Partial oxidation of CH4 by N2O on MgO below 573 K (mainly at room temperature) was investigated using TPD, IR and ESR spectroscopies. The effect of UV-irradiation was also studied. In the dark, the partial oxidation reaction did not proceed at room temperature but did when the reaction temperature was raised above 423 K. This temperature coincides well with that of decomposition of N2O, and O–2 seems to be the active oxygen species. However, most of the partial oxidation products, OCH–3, are easily oxidized above this temperature to give carbonates. Under UV-irradiation, the partial oxidation reaction of CH4 by N2O proceeded at 323 K and gave HCOO–, OCH–3 and C2–C7 hydrocarbons. Photodecomposition of N2O also easily proceeded at 323 K, and O– was formed at the initial stage. O– thus formed is an active oxygen species. Hence, it is demonstrated that decomposition of N2O is a key factor in the reaction of CH4 with N2O at low temperatures, either in the dark or under UV-irradiation. Low-coordinated surface ions play no important role in the partial oxidation with N2O as the oxidant.

Nature of CO2 adsorbed on MgO surface at low temperatures

Among five types of species formed by CO 2 adsorption on MgO, dynamic behavior of unidentate-type carbonate (species-1) upon desorption was investigated. Surface diffusion, O-exchange, and molecular substitution appreciably take place prior its desorption even below the room temperature. Since these dynamic processes necessitate bond rupture in species, MgO may be useful in CO 2 conversion at low temperatures.

Adsorption of methane on magnesium oxide surfaces under ultraviolet irradiation

CH4 adsorption on MgO under the influence of UV-irradiation near room temperature was studied, mainly using temperature-programmed desorption. In addition to an adsorbed species (type M1) observed in the dark adsorption, an adsorbed species (type IM), which was desorbed at a higher temperature range of 350–800 K than M1 in a TPD run, was found. The active site IM was formed by pretreatment of MgO above 973 K in vacuo, and the number of sites increased remarkably with increase in pretreatment temperature, although the maximum number of sites observed was still very small (1015 site m–2). Many similarities were found between M1 and IM adsorptions and both types of active site are believed to involve Mg2+LC and O2–LC ions, where LC (low coordination) refers mainly to 3C and partly 4C (three-coordinated and four-coordinated sites), on which CH4 is heterolytically dissociated into CH–3 and H+. However, the locations of both the LC ions are different for the two types of site; Mg2+LC and O2–LC are separately located in IM while they constitute the nearest lattice pair in M1. Therefore, the IM site can adsorb CH4 only under more severe adsorption condition; UV-irradiation causes activation of the site to produce O–LC from O2–LC prior to the adsorption process.

Analysis of the permeation of CO2 through glassy polymers

Abstract Dual-mode sorption and mobility parameters for the transport of CO 2 in glassy poly (ethylene terephthalate) were determined from the pressure dependence of the apparent diffusion coefficients, D a , below 1 atm at various temperatures. Experimental values of these diffusion coefficients were obtained from permeation time-lag measurements. Values of D a calculated by means of the above parameters are in excellent agreement with the experimental data. Moreover, the permeation time-lag curves calculated by the finite-difference technique in conjunction with the dualmode sorption parameters are consistent with the experimental time-lag curves. These results appear to support the validity of the dual-mode sorption model of gas transport in glassy polymers. However, equally good agreement between the calculated and experimental curves was obtained by using the apparent solubility and diffusion coefficient without the dual-mode sorption parameters. Similar results were obtained with poly(vinyl acetate)/CO 2 system at temperatures both above and below the glass transition temperature.

Relationship between thermal properties and diffusion coefficients of gases for polyimide films

The relationships between the chemical structure, packing density (1/V F ), and cohesive energy density (CED) and the thermal properties of polyimides were investigated. Particularly, the correlation of tan δ measured by stress-strain/thermal mechanical analysis with 1/V F and CED was found for eight polyimides. We measured the relationship between the apparent diffusion coefficient (D a ) and 1/V F and CED, respectively, as described in previous articles. From these experiments, we found that the thermal properties, especially tan δ, were correlated with the apparent diffusion coefficient of gas. These results are well explained by use of micro-Brownian motion.