Nickel oxide point defects

An effect which is frequently encountered in oxide catalysts is that of promoters on the activity. An example of this is the small addition of lidrium oxide, Li20 which promotes, or increases, the catalytic activity of dre alkaline earth oxide BaO. Although little is known about the exact role of lithium on the surface structure of BaO, it would seem plausible that this effect is due to the introduction of more oxygen vacancies on the surface. This effect is well known in the chemistry of solid oxides. For example, the addition of lithium oxide to nickel oxide, in which a solid solution is formed, causes an increase in the concentration of dre major point defect which is the Ni + ion. Since the valency of dre cation in dre alkaline earth oxides can only take the value two the incorporation of lithium oxide in solid solution can only lead to oxygen vacaircy formation. Schematic equations for the two processes are... 

Figure 1.13 Point defects in nickel oxide, NiO (schematic) Ni2+ vacancy Ni2+ interstitial Li+ on a Ni2+ site Mg2+ on a Ni2+ site Fe3+ on a Ni2+ site O2- vacancy N3- on an O2-site F on an O2- site free electron free hole.

Nickel oxide, NiO, which adopts the sodium chloride structure (Fig. 1.14), can readily be made slightly oxygen rich, and, because the solid then contains more oxygen than nickel, the crystal must also contain a population of point defects. This situation can formally be considered as a reaction of oxygen gas with stoichiometric NiO, and the simplest assumption is to suppose that the extra oxygen extends the crystal by adding extra oxygen sites. Atoms are added as neutral atoms, and... 

An excellent example of the attribution of a chemisorption to a specific atomic point defect is afforded by the Harwell work on magnesium oxide and nickel oxide (37, 38). The work was undertaken to discover whether the specific electronic nature of an oxide was the determining factor in chemisorption, or whether more general structural features were important, and the choice of these oxides, isomorphous but different electronically, was dictated by this intention. Likewise, neutron bombardment was chosen in order to emphasize structural defects and determine whether vacancies and interstitials, which would be similar in the two oxides, would lead to similar changes in adsorption, or whether the electronic differences in the host lattices would impose differences in adsorptive behavior. 

Some of the transition metal-oxide systems have become a subject of intensive research in the last two decades. The relation between the parabolic oxidation kinetics and the predominating point defect in the oxide was verified. To discuss the high-temperature oxidation mechanism of non-noble metals it is appropriate to start with a brief survey of some of the literature on the point defect dependent properties of, for example, nickel oxide. 

The value of the current in passive region is very important parameter for the prediction and modeling of the metallic corrosion. In Fig. 103 the current in the passive region is plotted as a function of temperature. It is noted that the passive current passes through a maximum at a temperature of aroimd 300°C. The passive current is expected to increase with temperature, because it is partly determined by the transport of ions through the oxide film, a process that is temperature activated. However, the passive current is also determined by the rate of dissolution of the passive oxide film, which is sensitive to the chemical and physical properties of the environment. The Point Defect Model for the growth and breakdown of passive films on metal surfaces gives the steady state current on nickel in terms of... 

We treat the ease of the selective formation of nickel oxide starting from the nickel-platinum alloys [LAL 73] (nickel and platinum are miscible in all proportions). We choose mole fractions of nickel vacancies in the alloy as variables of compositiom If we assume non-ionized point defects of nickel oxide (cationic vacancies and associated electron holes), the various oxidation steps are as follows ... 

The relaxation of La2Ni04 to La2Ni04,i8 illustrates a couple of important points. Firstly, the defect and electronic modes of relaxation necessarily work together since the change in oxidation state of NP+ is directly related to the amount of interstitial present. This simultaneous relaxation of both the stretched and the compressed layers is a feature found in many, if not all, of the observed mechanisms for relaxing lattice-induced strain. Secondly, the lattice-induced strain is directly responsible for the crystallization of a stable compound with a fixed, but irrational, composition, involving a fixed, but nonintegral, oxidation state for nickel. 

Braid surfaces at the failure point and a spot away from that area were analyzed. The external and internal surfaces of the braid at the failure area were mildly discolored because of oxidization, similar to the effect of overheating a metallic object. Optical microscopy and scanning electron microscopy showed no disturbance to the weave pattern, no deformation of metal wires, and no surface defects, based on the comparison of defective and normal areas. Energy dispersive x-ray (EDX) showed similar concentrations of iron (Fe), nickel (Ni), chromium (Cr), and molybdenum (Mo) in both areas, indicating stainless steel. A small amount of silicon (Si) was detected that was attributable to environmental contamination such as dust and dirt. 

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