Isotherm oxygen

Dou et al. [95] studied the isothermal oxygen permeation through calda-stabi-... 

Oxygen Adsorption Isotherms. Oxygen adsorption isotherms on the Cab-O-Sil surface at various temperatures up to —47 °C were measured. The isotherms obtained followed Langmuir adsorption at the temperatures examined. Theoretical treatment of the oxygen adsorption isotherm data (27) yields a heat of adsorption of 3.54 kcal/mol (14.8 kj/mol) and the assimilation times of 23.2, 44.1, 99.5, and 238 ps at —47, —64, —80, and —98 °C, respectively. 

Temperature gradients can cause thermal diffusion (Soret effect), which has been measured by Kyser et al. (1998) above the solvus in silicate liquids that are immiscible at lower temperatures. Additional isothermal oxygen diffusion experiments were performed below the solvus in the immiscible liquids and the results from the two kinds of experiments were compared. Although the magnitude and direction of oxygen isotope fractionation was found to be different from that expected, the authors conclude that this process is unlikely to play a significant role in natural processes such as mantle metasomatism. 

Because Dqq oc [V(5o]then if one measured the radio tracer diffusion of cobalt in CoO, the isothermal oxygen pressure dependence should exhibit a one-quarter dependence. This is exactly what Carter and Richardson ( ) did. Their results are shown in Figure 5. The electronic conductivity, o,... 

Figure 16.14 Schematic drawing of the reactor set-up and an incorporated BCFZ hollow fiber for OCM. At both ends the 30 long fiber was coated with gold to obtain a 3 cm long gold-free isothermal oxygen permeation zone. The active surface area for the oxygen permeating BCFZ hollow fiber is 0.78 cm. The OCM catalyst was dipersed between the outer dense alumina tube and the fiber, which was inserted into a porous alumina tube.

For the isothermal oxygen concentration cell, the cell voltage Ueq or E (emf), the Nemst s equation is expressed by... 

Fig. 5.12 (a) Water adsorption isotherms at 20°C on Graphon activated to 24-9 % burn-off, where its active surface was covered to varying extents by oxygen complex. (b) The results of (a) plotted as amount adsorbed per of active surface area (left-hand scale) and also as number of molecules of water per atom of chemisorbed oxygen (right-hand scale). (After Walker.)... 

Isotherms for H2O and / -hexane adsorption at room temperature and for O2 adsorption at Hquid oxygen temperature on 13X (NaX) zeoHte and on the crystalline Si02 molecular sieve siHcaHte are are shown in Figure 8 (43). SiHcaHte adsorbs water very weaMy. Further modification of siHcaHte by fluoride incorporation provides an extremely hydrophobic adsorbent, shown in Figure 9 (44). These examples illustrate the broad range of properties of crystalline molecular sieves. 

Fig. 8. Adsorption isotherms of H2O, O2, and / -hexane on 2eolite NaX (open symbols) and silicalite (filled symbols). Oxygen is at — 183°C and water and...

Analysis of weight loss isotherms displayed in Fig. 8 shows that the first step in the interaction between Nb02F and carbonates of other alkali metals is similar to the interaction described by Equation (11). However, compounds of the M(Nb04F form, where M = Na, K, Rb, Cs, were not found [85]. The instability of such compounds is related to the ionic radii of the alkali metals, which are greater than that of Nb5+, thus the ions are too large to occupy the octahedral cavities formed by the oxygen and fluorine ions. 

Non-isothermal measurements of the temperatures of dehydrations and decompositions of some 25 oxalates in oxygen or in nitrogen atmospheres have been reported by Dollimore and Griffiths [39]. Shkarin et al. [606] conclude, from the similarities they found in the kinetics of dehydration of Ni, Mn, Co, Fe, Mg, Ca and Th hydrated oxalates (first-order reactions and all values of E 100 kJ mole-1), that the mechanisms of reactions of the seven salts are probably identical. We believe, however, that this conclusion is premature when considered with reference to more recent observations for NiC204 2 H20 (see below [129]) where kinetic characteristics are shown to be sensitive to prevailing conditions. The dehydration of MnC204 2 H20 [607] has been found to obey the contracting volume... 

Daw et al. [632] made a microscopic and crystallographic study of talc dehydroxylation. Nucleation, to yield enstatite, occurs inhomogeneously within the particles, perhaps at dislocations. Later, this product is topo-tactically orientated with respect to the reactant lattice, though with extensive faulting on the (010) plane owing to misfit, in addition to the attempt to preserve the oxygen lattice. In an isothermal study (1100— 1160 K) of the same reaction, Ward [633] found first-order obedience and the value of E determined (422 kJ mole-1) is close to that estimated for... 

References to a number of other kinetic studies of the decomposition of Ni(HC02)2 have been given [375]. Erofe evet al. [1026] observed that doping altered the rate of reaction of this solid and, from conductivity data, concluded that the initial step involves electron transfer (HCOO- - HCOO +e-). Fox et al. [118], using particles of homogeneous size, showed that both the reaction rate and the shape of a time curves were sensitive to the mean particle diameter. However, since the reported measurements refer to reactions at different temperatures, it is at least possible that some part of the effects described could be temperature effects. Decomposition of nickel formate in oxygen [60] yielded NiO and C02 only the shapes of the a—time curves were comparable in some respects with those for reaction in vacuum and E = 160 15 kJ mole-1. Criado et al. [1031] used the Prout—Tompkins equation [eqn. (9)] in a non-isothermal kinetic analysis of nickel formate decomposition and obtained E = 100 4 kJ mole-1. 

C.G. Vayenas, C.Georgakis, J. Michaels, and J. Tormo, The role of PtOx in the isothermal rate and oxygen activity oscillations of the Ethylene Oxidation on Pt, J. 

As already noted the strength of chemisorptive bonds can be varied in situ via electrochemical promotion. This is the essence of the NEMCA effect. Following initial studies of oxygen chemisorption on Ag at atmospheric pressure, using isothermal titration, which showed that negative potentials causes up to a six-fold decrease in the rate of 02 desorption,11 temperature programmed desorption (TPD) was first used to investigate NEMCA.29... 

Figure 5.3. Oxygen thermal desorption spectra after electrochemical O2 supply to Pt/YSZ at 673 K (I = +12 pA for 1800 s) followed by isothermal desorption at the same temperature at various times as indicated on each curve.4,7 Reprinted from ref. 7 with permission from Academic Press.

---