Physical Models for Impregnation

Basically, impregnation has been rationaKzed following two physical models. For the sake of clarity, their presentation starts with the simplest one (wet/diffusional impregnation) that will be further completed to describe dry impregnation. 

The distribution of the precursor along the pellet depends on the balance between diffusion and adsorption when the precursor adsorbs on the support surface, its concentration in solution decreases and as a consequence, diffusion decreases too. Diffusion can be enhanced by imposing a high concentration 

An important parameter from the introduction of Darcy s law is the solution viscosity g. In the case of aqueous solutions and in the common range of concentrations used for impregnation, viscosity increases almost proportionally with concentration [18]. It also increases with the presence of organic ligands attached to the metal ions [33]. Viscosity and concentration behave contrariwise as far as the precursor diffusion is concerned a high concentration tends to favor the diffusion of the solute toward the center 

Temperature of impregnation has seldom been investigated as a parameter determining the profile in a pellet. It may act on the adsorption equilibrium of the precursor [30] but has been considered a minor factor as far as the precursor distribution is concerned [29]. 

The step after impregnation is the elimination of the solvent. Typically, the impregnated system, spread in fine layers, is heated in an oven up to the boiling point of the solvent or at a lower temperature for gentle drying, in static conditions or under a flow of gas. The elimination of water from the pores leads to an increase of the precursor concentration up to saturation and crystallization, preferably on seeds resulting from the interactions with the support, if one wants to obtain a highly dispersed phase. However, hydrated salts like nitrates melt at moderate temperature [35, 36], which may cause the coalescence of their initially dispersed particles, or even their exclusion from the pores. 

---