Topochemical insertion

A simple topochemical model for the growth of NiO islands on the Ni surface during the reaction of oxygen with a Ni(lll) crystal is clearly described by Holloway and Hudson [112]. They considered three cases in which the rate-determining step is, respectively (a) oxygen sorption from the gas phase (surface diffusion is fast), (b) surface diffusion of oxygen, and (c) oxygen insertion over the island boundary. 

Two types of transformations can be very broadly distinguished. The first is the formation of a solid solution, in which solute atoms are inserted into vacancies (lattice sites or interstitial sites) or substitute for a solvent atom on a particular sublattice. Many types of synthetic processes can result in this type of transformation, including ion-exchange reactions, intercalation reactions, alloy solidification processes, and the high-temperature ceramic method. Of these, ion exchange, intercalation, and other so-called soft chemical (chimie douce) reactions produce no stmctural changes except, perhaps, an expansion or contraction of the lattice to accommodate the new species. They are said to be under topotactic, or topochemical, control. 

The reduction of the high-temperature superconductor YBa2Cu307 to YBa2Cu30g is also a topochemical reaction. Similarly, many of the reactions involving insertion of atomic species into host oxides are topochemical. 

Insertion and intercalation electrode materials are electroactive and have composition, chemical and physical properties that change with oxidation state. The materials have a common characteristic that they serve as a host solid into which guest species are inserted from an electrolyte, and the insertion process is reversible. In particular, the intercalation process is described as the reversible insertion of guest species into a host lattice without structural modification of the host. The term topotactic or topochemical is also used to describe the insertion reaction. 

In case the reaction product bears a close structural relationship to the pristine electrode (unreacted) material (H), this type of reaction is classified as topochemical [2]. A special case of the topochemical process is the intercalation or insertion reaction which proceeds without any breakage of bonds in the host structure. This reaction (Eq. 13.1) is generally reversible. 

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