Low oxygen partial pressure

The amorphous structure favours internal oxidation unless a protective oxide film is formed as, for example, under low oxygen partial pressures . 

Metal dusting usually occurs in high carbon activity environments combined with a low oxygen partial pressure where carburisation and graphi-tisation occur. Usually pits develop which contain a mixture of carbon, carbides, oxide and metal (Fig. 7.52). Hochmann" proposed that dusting occurs as the result of metastable carbide formation in the high carbon activity gas mixture which subsequently breaks down into metal plus free carbon. The dependence of the corrosion resistance of these nickel alloys on the protective oxide him has been described accelerated or internal oxidation occurs only under conditions that either prevent the formation, or lead to the disruption, of this him. In many petrochemical applications the pO is too low to permit chromia formation (ethylene furnaces for example) so that additions of silicon" or aluminium are commonly made to alloys to improve carburisation resistance (Fig. 7.53). 

At low oxygen partial pressures, that is, reducing conditions, assume that oxygen is lost to the surrounding atmosphere to create oxygen vacancies to add to those present due to the Frenkel defects. The charged oxide ion is extracted as a neutral atom, leaving behind two electrons ... 

Peck DH, Miller M, and Hilpert K. Phase diagram study in the Ca0-Cr203-La203 system in air and under low oxygen partial pressure. Solid State Ionics 1999 123,47-57. 

The next two chapters discuss two supporting components of the fuel cell stack —specifically, interconnects and sealants. The interconnect conducts the electrical current between the two electrodes through the external circuit and is thus simultaneously exposed to both high oxygen partial pressure (air) and low oxygen partial pressure (fuel), which places stringent requirements on the materials stability. Ceramic interconnects have been used, but metallic interconnects offer promise... 

A key factor in the possible applications of oxide ion conductors is that, for use as an electrolyte, their electronic transport number should be as low as possible. While the stabilised zirconias have an oxide ion transport number of unity in a wide range of atmospheres and oxygen partial pressures, the BijOj-based materials are easily reduced at low oxygen partial pressures. This leads to the generation of electrons, from the reaction 20 Oj + 4e, and hence to a significant electronic transport number. Thus, although BijOj-based materials are the best oxide ion conductors, they cannot be used as the solid electrolyte in, for example, fuel cell or sensor applications. Similar, but less marked, effects occur with ceria-based materials, due to the tendency of Ce ions to become reduced to Ce +. 

De Groot H, Noll T. 1989. Halomethane hepatotoxicity Induction of lipid peroxidation and inactivation of cytochrome P-450 in rat liver mierosomes under low oxygen partial pressures. Toxieol Appl Pharmaeol 97 530-537. 

At low oxygen partial pressures an equilibrium is established between lattice oxygen, gas phase oxygen, oxygen ion vacancies and free electrons and conduction is n-type. 

NiOi + y could be measured precisely by use of the solid electrolyte, without chemical analysis. This result is shown in Fig. 1.51, as log Po. versus T curves, in which the dotted lines indicate the iso-y values. This figure shows that in order to get stoichiometric NiO, the sample has to be prepared at low temperature and low oxygen partial pressure in the NiO phase region. 

The second feature of our results at variance with the literature networks is the observed relationship between oxygen partial pressure and product distribution low oxygen partial pressures favoured DA and MVK formation over C2 products (see figs. 3 and 4). These data taken with the ability of MEK to reduce the VPO catalyst, and in so doing generate DA, MVK and AcH (AcOH was not analysed for in these experiments), imply that lattice oxygen must play a role in the reaction. 

Most kinetic studies (e.g., 29) and a thermodynamic study (30) of nuclear waste host dissolution focus on heterogeneous reactions with major concern for temperature, pH, and complexation. We believe that strongly reducing conditions are necessary to inhibit the undesirable U(IV)-U(VI) oxidation in nuclear waste matrices. Therefore, nuclear waste matrices should incorporate reductants or oxidation-reduction Eh buffers to maintain very low oxygen partial pressure, and leach studies should be conducted under oxidation-reduction conditions that nearly match repository conditions. 

In addition to a proper membrane, CMRs also need a good catalyst. Due to the specific conditions under which catalysts are placed in CMRs, conventional active phases could behave differently from when under classical conditions. For example, in dehydrogenation reactions, due to the removal of H2, the hydrogen hydrocarbon ratio is smaller in CMRs when compared to other reactors, which will probably affect the stability of the catalyst. The low oxygen partial pressure used in CMRs for selective oxidation (Section A9.3.3.2) could also lead to some changes in catalyst behavior. These aspects could necessitate the specific design of catalysts for CMRs. 

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