Trivalent nickel ions

Nickel containing scales exhibit higher conductivity because of the presence of trivalent nickel ions, which introduce vacancies in the lattice of the scale. Therefore, nickel-based coating can lead to superior conductivity and good protection provided that it is alloyed properly with corrosion resistant elements. Cobalt has a lower solubility in molten carbonate and electroless Co has been successfully used for a variety of corrosion-resistant applications. Electroless plating of Ni-Co gives rise to deposition of uniform layers of nanostructured material, which would result in better protection of the substrate. 

For the positive electrode, the reaction actually is more complicated. Nickel hydroxide is not an exactly defined chemical compound, and not only trivalent nickel ions are present in the charged material, but it can be a mixture between Ni, Ni ", and Ni" + (cf. Fig. 1.6). Thus 2 NiOOH as the charged state more precisely should be written... 

R indicates the organic portion of the resin and SO3 is the immobile portion of the ion active group. Two resin sites are needed for nickel ions with a plus 2 valence (Ni ). Trivalent ferric ions would require three resin sites. As shown, the ion exchange reaction is reversible. The degree the reaction proceeds to the right will depend on the resins preference, or selectivity, for nickel ions compared with its preference for hydrogen ions. The selectivity of a resin for a given ion is measured by the selectivity coefficient. K. which in its simplest form for the reaction... 

Cobaltites with spinel stractnre have compositions MC02O4, where M is a metal forming divalent cations, snch as zinc, cadminm, magnesinm, nickel, manganese, and divalent cobalt. In contrast to the perovskites, the cobaltites have a rather high catalytic activity already at room temperatnre. Experiments show that the activity increases with increasing spinel structure content (i.e., increasing number of Co ions) of the catalyst snrface. The trivalent cobalt ions promote the withdrawal of... 

The permanent moments of the cations in the octahedral and tetrahedral sites are antiparallel. Thus, the moments of the trivalent cations (Fe +) cancel out, and the net moment is due to the divalent cations. If the individual divalent cation moments are known, then it is possible to calculate the net moment per formula unit (see Cooperative Learning Exercise 6.7). In a formula unit of magnetite, there is one divalent iron ion that has a total moment of 4/tb resulting from four unpaired electrons, which is very close to the observed value. Similarly, in NiFc204, the moment per formula unit is 2/j.b identical to that of the divalent nickel ion. 

Another process that has benefited from procedures such as that outlined above is the removal of cobalt from leach liquors derived from the extraction of nickel-bearing concentrates or mattes.406 Traditionally, the removal of cobalt from such acidic sulfate solutions has been based on the affinity of the trivalent metal ions (such as those of iron and cobalt) for hydroxide, which... 

Nickel spinel, NiAl204, was used for making synthesis gas by C02 reforming of methane. The spinel structure is based on a cubic close-packed array of oxide ions. Typically, the crystallographic unit cell contains 32 oxide ions one-eighth of the tetrahedral holes (of which there are two per anion) are occupied by the divalent metal ion (N +), and one-half of the octahedral holes (of which there is one per anion) are occupied by the trivalent metal ion (AP+). These spinels are usually very stable and have been used for high temperature catalytic reactions [12,13]. 

Fig. 7. Schematic representation of the reaction mechanism for the transformation of bivalent nickel ions to trivalent nickel within the oxide scale.

The idea that covalent bonds may exist between adjacent metal ions is not new, but so far no good method has been available for defining the strength of such bonds. A possible definition of intercation covalehcy is to be found in the diminution of magnetic moment from that theoretically probable. For instance, suppose that trivalent nickel is found to have a moment of 2.7 instead of the theoretical 3.8. The degree of covalent bond formation may be expressed as the percentage 100 X (3.8 - 2.7)/3.8 = 29%. 

The electrical properties of NiO, doped with monovalent ions (Li+) and trivalent ions (Ga +, Cr +) have been studied quite extensively. The study of Cr-doped NiO is of particular interest for understanding the oxidation behavior of Ni—Cr alloys. When trivalent Cr ions are introduced into a NiO crystal, the electrical compensation could occur by formation of doubly ionized nickel vacancies, according to... 

The opposite happens when nickel oxide is doped with a trivalent metal ion such as chromium. The defect formation reaction on dissolving Cr203 in NiO is... 

A possible explanation of these phenomena involves the presence in heavily lithiated nickel oxide samples of two different phases or, more simply, of two different intercalation mechanisms for lithium ions. Assuming that pristine nickel oxide films are defective p-type materials due to the presence of trivalent nickel [26], it may be supposed that the electrons initially injected to balance the insertion of the first lithium ions (Qinteract with Ni " in the host matrix. However, and as described for proton intercalation in... 

Stable the nickel ion at a B-site exists not in its usual bivalent state but rather in a trivalent state. Thus, perovskite-type oxides are highly flexible materials that are able to incorporate various metal ions into their stable structures, and can give rise to an abnormal valency as well as a vacancy by the substitution of constituent elements. 

The crystallinity of the material is also dependent on the natiure of the trivalent metal ion present in the network. C0-AI-CO3-HT are more oystalline in comparison with Fe and Cr containing samples. In the case of C0- -CO3-HT, preparation by sequential precipitation yielded amorphous material whereas preparation under low supersaturation resulted in a better oystalline material. In our Co-Fe containing samples, it is not completely possible to exclude the presence of hexagonal Co(OH)2 prepared under sequential precipitation. These results indicated that presence of Arfavours the formation of crystalline HTlc phase which is in accordance with the results observed by Clause et al ftn nickel containing hydrotalcites [7]. TEM results showed spherical to hexagonal platelets of thin and wide nature characteristic of these materials [3]. 

Allmann found that when suitable trivalent ions were introduced during the precipitation of the hydroxides of Mg, Zn, Mn, Fe, Co, and Ni, these were incorporated in the lattice and the structure changed from the brucite (Mg(OH)2) to the pyroaurite ([Mg6Fe2(OH)l6] [CO,- 4H20]) type of structure [68], One of the nickel materials he prepared was an Ni/Al hydroxide. Axmann et al. [69-71] have given the nickel compounds the general formula... 

For metallic iron and nickel electrodes, the transpassive dissolution causes no change in the valence of metal ions during anodic transfer of metal ions across the film/solution interface (non-oxidative dissolution). However, there are some metals in which transpassive dissolution proceeds by an oxidative mode of film dissolution (Sefer to Sec. 9.2.). For example, in the case of chromium electrodes, on whidi the passive film is trivalent chromium oxide (CrgOj), the transpassive dissolution proceeds via soluble hexavalent chromate ions. This process can be... 

There are ammoniates of PtCl2, of halides of other platinum metals and of cobalt and nickel, too, some of which have been mentioned before in, Section 50. The cobalt complexes clearly show the importance of the completed d shells for the stability of the complex. Non complex compounds of trivalent cobalt are very unstable. Solutions of divalent cobalt in ammonia, however, are readily oxidized by air, because the NH3 complex of trivalent cobalt Co(NH3)6 3+ClT has eighteen electrons used in bond formation, whereas the ion Co(NH3) + would have nineteen electrons. 

With its 3d84,r2 electron configuration, nickel forms Ni2 ions. Having a nearly complete 3d subshell, nickel does not yield a 3d electron as readily as iron and cobalt, and trivalent and tetravalent forms are known only in the hydrated oxides. Ni203 and Ni()2. and a few complexes. 

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