Desorption oxygen

With regard to the liqiiid-phase mass-transfer coefficient, Whitney and Vivian found that the effect of temperature upon coiild be explained entirely by variations in the liquid-phase viscosity and diffusion coefficient with temperature. Similarly, the oxygen-desorption data of Sherwood and Holloway [Trans. Am. Jnst. Chem. Eng., 36, 39 (1940)] show that the influence of temperature upon Hl can be explained by the effects of temperature upon the liquid-phase viscosity and diffusion coefficients. 

Figure 5.25. Redhead plot for oxygen desorption from a Pt film deposited on YSZ for various catalyst film potentials vs Au reference electrode. The slope of each line is equal to Ed/R.7 Reprinted with permission from Academic Press.

Figure 5.26. Effect of catalyst potential on the oxygen desorption activation energy, Ed, calculated from the modified Redhead analysis for Pt, Ag and Au electrodes deposited on YSZ.44,46 Reprinted from ref. 44 with permission from the Institute for Ionics.

Oxygen adsorption that occurs at platinum at potentials more positive than 0.9 to 1.0 V is irreversible, in contrast to hydrogen adsorption. Oxygen can be removed from the surface by cathodic current, but the curves obtained in the anodic and cathodic scan do not coincide cathodic oxygen desorption occurs within a narrower region of potentials, and these potentials are more negative than the region where the... 

Avery NR. 1983. An EELS and TDS study of molecular-oxygen desorption and decomposition on Pt(lll). Chem Phys Lett 96 371-373. 

It follows from the formula that the experimentally evaluated value of / activation energy on the ZnO surface may be related to the activation energy of oxygen desorption from the zinc oxide surface. This value well agrees with the desorption activation energy measured with the aid of semiconductor detectors in work [109]. 

According to the results obtained above, the following mechanism seems to be reasonable for this reaction. This mechanism, in which oxygen desorption is the slowest step, has been suggested by many investigators. 

This (3-oxygen desorption is promoted by Cu substitution as indieated by the results of O2-TPD experiments in accordanee with the conclusion that Cu incorporation enhanced the... 

Temperature-Programmed Surface Reaction (TPSR) Experiments at 800 Torr. Pretreated and preoxidized silver exhibited no reactivity toward an ethylene/argon mixture at reaction temperatures (443 - 543 K) and atmospheric pressures (750-800 torr). The desorption spectrum of a pretreated sample showed no evidence of oxygen desorption when the sample was heated in vacuo to 673 K. These... 

Asymmetry of the response curve to the point of the exposition end reflects the different nature of the exposition and relaxation output signals. A transition from an exposition into relaxation phase corresponds to a return of gas-sensitive matter contact with the initial atmosphere. A variety of processes take place simultaneously in that phase. They may include oxidation of adsorbed molecules by the air oxygen, desorption of the previously adsorbed molecules, competitive adsorption of the ambient atmosphere components. These circumstances cause a complicated shape of the relaxation curve. In general, its course reflects the dynamics of the surface concentration of conductivity clusters. Almost all relaxation curves are characterized by presence of a maximum. It is often more prominent that the corresponding exposition maximum. The origin of this phenomenon is determined by higher conductivity of clusters formed by the oxidized molecules of compounds adsorbed during the exposition phase. 

The most important properties used as a measure of the bonding strength are the heat of formation of the metal oxides, the heat of oxygen desorption, the reducibility of the metal oxide and the activation energy for isotope exchange between l802 in the gas phase and oxygen in the catalyst. 

Fig. 30. Apparent reaction order plot for oxygen desorption from Ir(110) as function of coverage (124).

From the critical comparison of all observations included in the determination of the compensation trends in Table V, A-C, and also remembering the scatter of data and the interrelationship between errors in B and e, we conclude that the kinetic results for all three rate processes discussed can probably be regarded as a single compensation trend. Allowing for standard error, there is overlap of the values of e at 0.049. This common pattern of kinetic behavior is consistent with the conclusion reached by Winter (263, 264) that the three rate processes involve the same rate-limiting step, oxygen desorption. No mechanistic interpretations of these examples of compensation beha vior have been provided and it is not known which of the theoretical models described in Section II, A gives the most satisfactory explanation of the systematic variation of log A with E. 

It should be noted that while UPS could not detect an adsorbed hydroxide species with surface coverage less than 10% of a monolayer, such a species could be chemically active. The same hole-acceptor could be present at both the gas-solid and liquid-solid interface, but the rate of its formation may be inadequate to compete with oxygen diffusion into the bulk from the gas-solid interface. MUnuera (AO) has found that measurable rates of restoration of a hydroxyl species linked to the photoactivity of Ti02 powders for oxygen desorption require treatments harsher than immersion in liquid water. 

Figure 7.2 shows a supposed mechanism of a-site formation [83, 111], starting from an initial binudear iron complex (A). The existence of such complexes was documented by many authors [81, 109, 110, 114—116]. Upon oxygen desorption at elevated temperature (step 1), complex A may transform into the reduced complex B. If the oxygen desorption is reversible, a-site formation does not occur and, upon cooling, the system returns to its initial state, restoring complex A. Such reversibility... 

Oxygen Desorption and Conductivity Change of Palladium-Doped Tin(IV) Oxide Gas Sensor... 

Relation between Oxygen Desorption and Conductivity Change... 

Electron hole pairs generated by photon absorption enable oxygen to desorb from the surface (bottom of Fig. 3). The oxygen desorption annihilates some of the holes, thereby decreasing the surface, so that electrons are now able to move from one ZnO grain to another. Thus, photoconductivity of the layer is produced. In the dark period which follows, the photoconductivity of the layer is preserved for some time due to the large number of shallow electron traps. 

Reaction 13 represents the production of a paramagnetic defect by capture of a photogenerated hole. The paramagnetic centres may be anihilated by the reverse of reaction 12 by capture either of photoelectrons from the conduction band or of electrons liberated during oxygen desorption. 

When the catalyst is illuminated, the ZnO bonds are excited and loosened. In the linear combination of the ZnO bonds (Eq. 25) the nonbonding form increases at the expense of the bonding forms. This means that there is a shift of the bond electrons from the oxygen to the zinc atom. The electron density in the orbitals of the Zn atoms is increased, while the electron density in the orbitals of the oxygen atoms is decreased as compared with that of the ground state of the bond. A similar state is generated by the oxygen desorption of the surface where electron-enriched zinc atoms remain. ZnO, as an n-conductor, even in the... 

Effects of System Physical Properties on kG and kL When designing packed towers for nonreacting gas-absorption systems for which no experimental data are available, it is necessary to make corrections for differences in composition between the existing test data and the system in question. The ammonia-water test data (see Table 5-24-B) can be used to estimate HG, and the oxygen desorption data (see Table 5-24-A) can be used to estimate HL. The method for doing this is illustrated in Table 5-24-E. There is some conflict on whether the value of the exponent for the Schmidt number is 0.5 or 2/3 [Yadav and Sharma, Chem. Eng. Sci. 34, 1423 (1979)]. Despite this disagreement, this method is extremely useful, especially for absorption and stripping systems. 

E] Studied oxygen desorption from water into N2. Packing 0.22-mm-diameter stainless-steel mesh. 

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