Intermediate oxygen partial pressure

Figure 8.4 shows the steady-state effect of po2 and imposed catalyst potential Uwr on the rate of C2H4 oxidation and compares the results with the open-circuit kinetics. The sharp rate decline for high po2 values is due to the formation of surface Rh oxide.13 Increasing UWr causes a significant increase in the oxygen partial pressure, po2, where oxide forms and thus causes a dramatic increase in r for intermediate (1 to 2.5 kPa) Po2 values. For low P02 values (reduced surface) the effect of Uwr is moderate with p values up to 2. For highp02 values (po2>Po2 > oxidized surface) Uwr has practically no effect on the rate. 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

Figure 2.33. Ni-Co-O phase diagram (isothermal section at 1600 K). log p02 (oxygen partial pressure) is plotted against the molar fraction in the metallic alloy. The metallic alloy, (Ni, Co) solid solution is stable in (1) and the mixed oxide (Ni, Co)0 solid solution in (3). In the intermediate region (2) we have coexistence of alloy and oxide. For the value of the partial oxygen pressure corresponding to y, within the two-phase field, we will have the alloy of composition xt in equilibrium with an oxide containing the two metals in the ratio x2-...

Selective oxidation of methyl ethyl ketone to diacetyl has been studied by passing a mixture of the ketone in artificial air over vanadium phosphorus oxide catalysts in the temperature range 200-350 C. Products observed included diacetyl, methyl vinyl ketone, acetaldehyde, acetic acid and carbon dioxide. C4 products were favoured at low temperatures and at low or zero oxygen partial pressures. These results are rationalised in terms of two pathways for C2 products, namely oxidation of the double bond in the enol form of methyl ethyl ketone to yield acetic acid and acetaldehyde, and acid catalysed hydration of the keto form to yield acetaldehyde only. The C4 products are envisaged to go through a common intermediate, namely, CH3COCHOHCH3, formed by interaction between methyl ethyl ketone and lattice oxygen. 

From Rado s study, it can be concluded that the nature of SiC surface changes dramatically at a temperature T which depends on the oxygen partial pressure in the furnace (in Rado s experiments T % 1050°C). At lower temperatures, the SiC surface is oxidised while at higher temperatures it is graphitized. The transition is continuous around T i.e., without an intermediate state of a non-contaminated,... 

Long-term successful operation of the SOFCs requires that the electrolyte possess adequate chemical and structural stability over a wide range of oxygen partial pressures, from air or oxygen to humidified hydrogen or hydrocarbons. The requirements for the electrolyte used in the intermediate-temperature SOFCs (IT SOFCs) include ... 

Reaction of a metal and high valence oxide to form an oxide of intermediate valence (e.g., in 3Nb + Nb20s -> 5NbO), which must be carried out under carefully controlled oxygen partial pressures product formation is assisted by heating all constituents above their melting points (e.g., in an arc melter). 

Tables 1-5 present the effects of different reaction conditions and catalyst composition on the variation of TOC in solution, the percentage of initial carbon converted into polymers (quantity recovered on the catalyst divided by the quantity of carbon in the initial solution) and the percentage of initial carbon converted into CO2 (quantity of carbon reacted from the initid solution minus the quantity of carbon recovered on the catalyst, divided by the quantity of carbon in the initial solution), and the percentage of converted carbon recover as polymers on the catalyst (quantity of carbon recovered on the catalyst divided by the quantity of carbon removed from the solution). Experiments were performed varying the pH (with NaOH or HCIO4) of the solution. One important objective of these measurements was to see if acidity favours polymerization. Effects of the oxygen partial pressure, the reaction temperature, the nature of the oxides, and hydroquinone, maleic acid and acetic acid, which have been proposed as intermediates in the mechanism of the reaction, were tested.

Region 2. In this intermediate range of the oxygen partial pressure, there are no dominating reactions and, therefore, the concentrations of metal and oxygen vacancies are comparable. As a consequence, the composition of the oxide phase is nearly stoichiometric. This allows the electroneutrality condition to be approximated by using two altemative expressions ... 

In the intermediate region when A, >> A,-, Eq. (6.31) holds and Vo = and is thus independent of the oxygen partial pressure. Substituting this result in Eq. (6.23) yields... 

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