Yasunori Yanagisawa

Thermal and photo-stimulated desorption of chemisorbed oxygen molecules from titanium dioxide surfaces

Abstract Thermal desorption (TD) and photo-stimulated desorption (PSD) studies of 18 O 2 pre-adsorbed TiO 2 powders have been carried out under high-temperature UHV treatment. At peak temperatures ( T p ) 180 and 470 K with a shoulder at about 520 K, 18 O 2 TDs are observed. TD of 16 O 18 O with a small amount of 16 O 2 is observed at 530 K, indicating that oxygen isotope exchange occurs between 18 O 2 gas and defective TiO 2 surfaces. By UV illumination with the threshold energy of 3.0 eV, 18 O 2 PSD takes place at room temperature, which results in the decrease of an 18 O 2 TD peak. PSD rate decays as t − n with n about 0.5, where t is the illumination time. A plausible PSD mechanism is discussed based on the substrate-dependent process induced by photons with band-gap energy.

Interaction of Oxygen Molecules with Surface Centers of UV-Irradiated MgO

Oxygen-surface interaction has been studied in an ultra high vacuum by means of thermal desorption mass spectrometry. ESR, emission of after glow and thermoluminescence. During the O 2 exposure of less than 10 4 L to UV-irradiated MgO powders, F s + centers were found to decay following two monomolecular stages and two types of O 2 - centers were found to be formed. The activation energy of the frst decay was found to be 0.04 eV. The luminescence was also found to be quenched by the adsorption of O 2 . The chemisorbed O 2 - molecules desorbed thermally with activation energies of 0.83 and 1.10 eV. Through the estimation of the order of kinetics and activation energies, the interaction of O 2 with F s + is discussed as a diffusion-limited process causing charge transfer on surfaces of MgO.

Oxygen isotope exchange between O2 adsorbate and UV-irradiated MgO surfaces

Abstract By using thermal desorption (TD) gas analysis, oxygen-isotope exchange is found between 18 O 2 gas and UV-irradiated MgO powders baked in high vacuum. At a peak temperature ( T p ) 125 K, only 18 O 2 TD takes place. At 200 K, TDs of 18 O 2 and 16 O 18 O with a small amount of 18 O 2 are observed. In the temperature range 550–800 K, a group of three TD peaks composed of 18 O 2 , 16 O 18 O and 16 O 2 is obtained. The exchange ratios, R ( 16 O 18 O) and R ( 16 O 2 ) defined respectively as the ratios of 16 O 18 O and 16 2 O TD intensities to the sum of three TD intensities at the same peak, are: R ( 16 O 18 O) = 0.45 and R ( 16 O 2 ) = 0.05 for T p = 200 K, and R ( 16 O 18 O) = 0.20 and R ( 16 O 2 ) = 0.20 for T p = 690−750 K. A plausible exchange mechanism is discussed in terms of interactions of 18 O 2 adsorbates with low-coordinated 16 O − ions and 16 O 2 2- peroxy defects on MgO surfaces.

NO interaction with thermally activated CaO and SrO surfaces

By using temperature-programmed desorption gas analysis, thermal desorption (TD) of N 16 O with a small amount of N 18 O was observed in the temperature range 400-1000 K after N 18 O adsorption on both CaO and SrO powders at room temperature. TDs of N 2 and N 2 16 O with a small amount of N 2 18 O were also observed in the temperature range 400-600 K. Based on IR measurements, NO, (NO) 2 and tentative NO 2 (or NO 3 ) species were found after N 16 O adsorption on CaO powders. This suggests that oxygen exchange of N 18 O with CaO and SrO may occur through NO 2 - or NO 3 - type intermediates during migrating on the surfaces involving of a lattice oxygen. N 2 O desorption may be due to the decomposition of (NO) 2 , while exchange process is not clear.

Possibility of hydrogen-deuterium exchange at surface irregularities of MgO: an ab initio MO study

Abstract Two models of irregular sites of MgO surfaces as candidates for the H2-D2 exchange reaction are investigated by the use of the ab initio method. After the heterolytic H2-chemisorbed states are analyzed, it is suggested that the exchange may take place at step sites on (001) surfaces. This comes from the steric condition that the (Mg-D Mg) configuration is readily converted to the (Mg D-Mg) one.

Enhanced Photon Emission during Hydrogen Adsorption on UV-Excited MgO Surfaces

Phosphorescence enhancement induced by hydrogen adsorption is observed on UV-excited MgO powders in vacuo. The main peak of the spectrum is at 3.2 eV with a sub peak at 2.3 eV. The enhancement depends on the gas pressure and sample temperatures and the phosphorescence is quenched by the light illumination at 2.5 eV after UV excitation. The role of surface centers in the luminescence is discussed.

Oxygen exchange between CO2 and metal (Zn and Ti) oxide powders

Using temperature-programmed desorption gas analysis, thermal desorption of C 18 O 2 , C 16 O 18 O and C 16 O 2 was observed in the temperature range 350-700 K after C 18 O 2 adsorption on vacuum-annealed ZnO and TiO 2 powders at room temperature. This indicates that oxygen isotope exchange takes place between C 18 O 2 gas and oxide surfaces. After 16 O 2 exposure of the C 16 O 18 O-preadsorbed powders, the amounts of C 16 O 2 and C 16 O 18 O desorption increased, suggesting that isotope mixing may occur through a CO 2 O 2 intermediate complex

Exchange of strong carbon dioxide O C bonds on an MgO surface

Thermal desorption (TD) of 16OC18O and 16OC16O gases after 18OC18O adsorption on MgO powders has been observed in the temperature range 350–1000 K. [graphic omitted]. These two exchange reactions were investigated by ab initio calculations with various cluster models including a coordinatively unsaturated oxygen site. The single exchange is found to take place via a bidentate CO3 intermediate, and the double one proceeds with CO3 migration on the surface.

Oxygen exchange between adsorbed NO and MgO surfaces

Abstract By using temperature-programmed desorption gas analysis, thermal desorption of N16O and N18O with a small amount of N216O gases was observed in the temperature range 350–1000 K after N18O exposure at room temperature on MgO powders. This suggests that oxygen echange between N18O and Mg16O may occur through an NO2-type intermediate involving lattice oxygen during migration on the surfaces.

An ab initio MO study of hydrogen adsorption on LiF surfaces

Abstract The adsorption energies ( E ads ) of a hydrogen molecule and an atom onto (001) and (110) surfaces are calculated by an ab initio MO with the STO-3G basis set. The surfaces are approximated by cluster models, (LiF) 9 for (001) and (LiF) 8 for (110). E ads is -0.18 eV for the H atom on the Li ion of both surfaces. E ads for H 2 is -0.02 eV and its dissociation does not take place on the surfaces. The state density after hydrogen adsorption has a new peak at the bottom of the valence band. The difference of E ads between the ionic surface and the lithium surface for H adsorption is also examined.