O2 exchange

If O2 exchange at the gas-exposed surface is rate limiting (Figure 24a), the relatively fast transport of oxygen ions in the film will cause the film to act as a uniformly concentrated and electrically neutral reservoir for oxygen vacancies and associated electrons/ holes. Conservation of vacancies in the film in this case is therefore given by... 

It is generally concluded that 0 species are the most reactive in CO oxidation and in homomolecular O2 exchange under illumination. The latter reaction proceeds via... 

Homomolecular O2 exchange is very slow on ZnO at 298 K in the dark. However, cooling to 77 K produces a sudden increase in rate. There is debate over the active intermediate for this strange, non-photocatalytic reaction. Tanaka et al. and Hirota et al. favour O4 and 4 species, respectively, whereas Russian workers conclude that these cannot be involved. Tanaka et al. published e.s.r. studies using " 02 in support of neutral O4 species, but Gundrizer et al. state that such species in liquid O2 are inactive in exchange and, therefore, that it is more likely for the intermediate on ZnO to be a non-radical , molecularly adsorbed species. It is not clear in their paper what exactly is meant by this nor how such species can function as intermediates. 

It should be mentioned that the French chemist L.], Thenard was the first to obtain hydrogen peroxide (in 1818) by reacting BaOj with dilute mineral acid. Today Na Oj is used in breathing apparatus (for example by fire brigades) and in submarines lor CO2/O2 exchange according to equation (3). 

The exchange of lattice oxide anions in nanocrystals of MgO has been observed when H2018 is passed over MgO particles at various temperatures (Li and Klabunbe, 1992). At 300°C, die surface hydroxyls underwent exchange however, at higher temperatures surface lattice O2- exchanged, while at 700°C, inner lattice oxide anions exchanged. 

Role of Various Factors in Placental O2 Transfer. These experiments characterize the dependence of 02 transfer and umbilical venous po2 on maternal arterial p02> fetal placental flow rate, and fetal inflowing po2 on O2 exchange in a single cotyledon of the sheep placenta and on fetal placental flow in the rabbit placenta. Each factor was studied individually while the fetal placental circulation was isolated and perfused in situ. The present findings do not apply for an intact fetus whose blood recirculates between peripheral tissues and the placenta because compensations would tend to maintain 02 transfer equal to fetal 02 consumption in this latter instance. The present data take account of changes in only a single variable. 

The catalysts were reduced in flowing H2 (200 cc/min) at 300°C for 2 hr. The prereduced catalysts were oxidized prior to measuiments related to CH4 oxidation, isotopic O2 exchange, CO chemisorption, and EXAFS spectroscopy. 

The calculated barrier for the reaction, even including configuration interaction, was 40 kcal/mole, which is inconsistent with the observance that H2/O2 exchange occurred at 78K. They therefore suggested that the reaction path was probably asymmetrical in nature, although examination of this possibility was impractical with computational resources then available. 

Net O2 exchanges were determined polarographically using a Clark-type O2 electrode. Unidirectional O2 and CO2 fluxes were measured simultaneously with a mass spectrometry technique as previously described (7). For O2 and CO2 exchange determinations, 2 ml of O2 and 0.15 ml of CO2 (99% 0 and 99 % C) were bubbled in... 

Photosynthetic O2 exchange was measured at 25C in a Rank O2 electrode using cells suspended in C02-free MOPS-NaOH (20 mM pH 7.0) (3). labelling of photosynthetic products was done as described in (2) with. these changes. Cells were equilibrated in a Rank O2 electrode in the dark, bicarbonate added, (100 /iM or 2.5 mM initial concentration) and photosynthesis initiated by turning on the light (500 /iE min" ). Soluble cell extracts were fractionated on a cation exchange column of Bio-Rad AG 50W-8X, H+. 

Table A smnmary of the photosynthetic and respiratory O2 exchange of leaves grown at different canopy positions. Data points were en from a combination of four separate hght curves per canopy level. Light saturation points were estimated visually, and maximum net photosynthesis values are Wed on rates at 1800 /iEi m s PAR. Apparent quantum yields, hght compensation points, and dark respiration rates were determined by a linear equation fitted to hght intensity points ranging from 0 to 102 /iEi m s (r > 0.9 in ah cases). Absorbed photon yields can be estimated by dividing apparent quantum yields by the average leaf absorbancy.

The results of many studies carried out with zeolites, mostly in the 1970s and reviewed in ref [52], did not meet these expectations. ZeoUtes cannot compete with conventional catalytic systems both in the selective and complete oxidations. The main reason is that the transition metals introduced into a zeoUte matrix lose their ability to activate dioxygen, as was evidenced by isotopic O2 exchange [53, 54]. The... 

The H2 production rate depends directiy on the rate of O2 removal from the H2O decomposition zone. This also depends on the O2 permeability of the membrane, which is a function of the electron and 02-ion conductivities, surface O2 exchange kinetics of the membrane, and oxygen partial pressure (PO2) gradient across the membrane (Balachandran et al., 2004 Ma, Balachandran, Chao, Park, Segre, 1997 Maiya et al., 1997). 

A Nafion film decreases the current at the second wave but increases the current at the first one (cf Fig. 2b) and c)). The current decrease at the second wave is made evident by the Levich plots at -200mV (see Fig. 4) and is due to the decrease of the rate constant for the transport of O2 in the fihn. To confirm that, we applied the method of Gough and Leypoldt [6 to estimate Cf l, and the film permeability to O2, Pf 1, which expresses the combined effects on the limiting current of the O2 exchange between the solution and the Nafion film and the rate constant (Kj )i for the transport of O2 in the film. The results obtained are summarized in... 

Fig. 5.6. CO2 and O2 exchange under long- and short-day conditions (Kalanchoe daigremontiana) CO2---------------- O2.(after Marcelle, 1975, by permission)...

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