Oxygen partial pressure limits

FIGURE 11.24 Oxygen partial pressure limits The partial pressure of oxygen in air at sea level is 0.21 atm. If this pressure drops by 50%, fatal hypoxia can result. High oxygen levels can also be harmful, but only if the partial pressure of oxygen increases by a factor of 7 or more. 

Among the oxygen ion conductors, CaO or Y2C>3 stabilized Zr02 (CSZ and YSZ) [67, 68], and Y2C>3 or La203 stabilized Th02 [69] are frequently used. CSZ and YSZ are limited to oxygen partial pressures in the range from 10-13 to 1010 Pa at 1273 K [68], Lower partial pressures are allowed with the thoria-based... 

Eor illustration purposes, we consider here a simple scenario of this interplay. We evaluate the effectiveness factor at a fixed cell voltage and thus at a fixed rig. We can express the corresponding current density as a two-variable function, jg =f f, Sqi), where the reaction penetration depth, CL/ depends on rjg. This function can be used to determine the effectiveness factor, rcL- In the case of severely limited oxygen diffusion, the following relations for local oxygen partial pressure and current density can be obtained ... 

Figure 6.7 shows the average correlation g(PQ) as a function of oxygen partial pressure. At the low-pressure limit (determined by the pair correlation only) there is no clear-cut monotonic dependence on temperature. At the high-pressure limit (as determined by the quadruplet correlations, see Section 5.8), we see that the... 

Equation 18 can be used to write the ORR rate in eq 13 or 15 in terms of the limiting current density, an experimentally measurable quantity, rather than the oxygen partial pressure. 

An oxidative environment is also an essential element in maintaining catalytic activity. Air is used as the copper(l) reoxidant for safety reasons. Oxygen partial pressure must be held between 2 volume % and 6 volume % during the redox cycle. If the oxygen partial pressure falls below 2 volume %, monoatomic palladium(O) does not reoxidize to palladium(Il) at a sufficient rate, and some catalytic activity is lost due to polymeric palladium metal formation. Under typical oxycarbonylation conditions, copper(ll) cannot reoxidize polymeric palladium metal. An oxygen partial pressure greater than 6 volume % affords a potentially explosive gas mixture with carbon monoxide. Oxygen partial pressure control within these limits was easily achieved in the oxidative-carbonylation pilot plant reactor. 

At 25 IK, the observed bimolecular rate constant reaches its limiting value, ki 6 x lO cnAnolecule V1, at an oxygen partial pressure of 100 Torr. At this point, adduct formation becomes the rate limiting step and further increases in oxygen partial pressure produce no increase in the observed rate constant. Applying a steady state analysis to the reaction sequence (la,-la,2) gives the expression... 

These models consider the mechanisms of formation of oscillations a mechanism involving the phase transition of planes Pt(100) (hex) (lxl) and a mechanism with the formation of surface oxides Pd(l 10). The models demonstrate the oscillations of the rate of C02 formation and the concentrations of adsorbed reactants. These oscillations are accompanied by various wave processes on the lattice that models single crystalline surfaces. The effects of the size of the model lattice and the intensity of COads diffusion on the synchronization and the form of oscillations and surface waves are studied. It was shown that it is possible to obtain a wide spectrum of chemical waves (cellular and turbulent structures and spiral and ellipsoid waves) using the lattice models developed [283], Also, the influence of the internal parameters on the shapes of surface concentration waves obtained in simulations under the limited surface diffusion intensity conditions has been studied [284], The hysteresis in oscillatory behavior has been found under step-by-step variation of oxygen partial pressure. Two different oscillatory regimes could exist at one and the same parameters of the reaction. The parameters of oscillations (amplitude, period, and the... 

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