Structural Changes in Gas-Solid Reactions

Broadly speaking, one could envision two types of change, one leading to larger pores, or a more open structure, and the other causing densification, finer pores, and hence a lower effective diffusion coefficient. 

The structural changes that may accompany gas-solid reactions can be quite complex but may be classified somewhat arbitrarily into the following categories  

Structures developed for NpJpp 1 frequently have a large number of very small pores and as a consequence the solid has a large specific surface. This effect is exploited in the manufacture of high surface area heterogeneous catalysts. The development of such microporosity is a complex phenomenon 

Sintering is a complex phenomenon and, in spite of much careful investigation, it is still not possible to predict the rate at which the various structural characteristics (e.g., porosity, surface area) change under a given set of circumstances. 

A photograph of a partially reduced and sectioned nickel oxide pellet, showing surface layers of dense sintered nickel. Diagonal striations were caused by the tool used to section the pellet. 

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