Topochemical Reaction

a chemical reaction that occurs at the boundary of solid phases. Examples of topochemical reactions are the dehydration of crystal hydrates, the reduction of oxides, and the thermal decomposition of azides of heavy metals.

Topochemical reactions are characterized by two specific features. First, they begin at individual sites of a solid that are the most reactive (active sites), rather than throughout the entire solid. Second, having arisen at an active site, the reaction continues in neighboring regions of the crystal (autocatalysis). The origin of active sites in topochemical reactions is usually related to the presence of crystal defects and the low mobility of the ions, atoms, or molecules forming the crystal lattice. Autocatalysis results from the catalytic effect of the solid or gaseous reaction product, and from the crystallochemical features of the propagation of the reaction in the crystal. The phase boundary on which topochemical reactions are localized arises as a result of the formation and growth of reaction nuclei; the rate of the process is usually proportional to the magnitude of the surface at any given moment. Thus, kinetic analysis of topochemical reactions takes into account the propagation of a process not only over time but also through space.

Crystal defects have a significant effect on the rate of topochemical reactions. This effect is evident in the change in both the number of potential reaction centers on the surface and the conditions for transport phenomena in solids. The significant role of defects in the propagation of topochemical reactions is also associated with the well-known effect of a preparation’s prior history on its reactivity and with the variety of factors affecting rates. The nature of the effect of crystal defects on the rate of topochemical reactions in each specific case depends both on the type and concentration of defects and on the mechanism of the elementary steps in the reaction.

Topochemical reactions are commonly used in practice. The most important topochemical reactions include the roasting, reduction, and chlorination of the ores of many metals; the case-hardening of steel; the production of ceramics, refractories, and ferrites; some steps in the photographic process; and the gaseous corrosion of metals and alloys. In many cases, the processes of synthesis and purification of semiconductor materials and the decomposition of explosives upon heating are also related to topochemical reactions.