Lattice structure nickel sulfide

The most important undesired metallic impurities are nickel and vanadium, present in porphyrinic structures that originate from plants and are predominantly found in the heavy residues. In addition, iron may be present due to corrosion in storage tanks. These metals deposit on catalysts and give rise to enhanced carbon deposition (nickel in particular). Vanadium has a deleterious effect on the lattice structure of zeolites used in fluid catalytic cracking. A host of other elements may also be present. Hydrodemetallization is strictly speaking not a catalytic process, because the metallic elements remain in the form of sulfides on the catalyst. Decomposition of the porphyrinic structures is a relatively rapid reaction and as a result it occurs mainly in the front end of the catalyst bed, and at the outside of the catalyst particles. 

The nickel arsenide (NiAs) structure is also based on an hep array in this case, the cations form the backbone lattice, and the larger anions occupy both octahedral sites. This structure, also associated with metal sulfides, selenides, tellurides, and antimonides, is only adopted for weakly ionic compounds. Since the octahedral sites are extremely close to one another, purely ionic compounds would be much too unstable due to strong anion-anion repulsions. 

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