Nonuniform catalyst distributions in porous supports are obtained primarily by multicomponcnt impregnation techniques. In general, an intermediate level of interaction between catalyst precursor and support is required, so that the precursor can attach to the support, but can also desorb if another competing adsorbing species is present. Depending upon the interplay between competitive adsorption and diffusion of the various species in the porous support, a variety of nonuniform catalyst distributions can be obtained. The above physicochemical processes are also encountered in chromatographic separations (Ruthven, 1984). This chapter is divided in two parts. The first deals with adsorption on powders, while the second is focused on simultaneous diffusion and adsorption phenomena.

Although diffusion–adsorption methods are dominant for the preparation of nonuniform catalyst pellets, other procedures have also been employed. One such technique is deposition precipitation in preshaped carriers (l)e Jong, 1991). It involves deposition inside pellets of insoluble compounds, such as hydroxides which are formed by a precipitation reaction. The latter can be induced by a change of solution pH. Immediately after imbibition, a pH profile develops inside the pellets, which depends on the initial solution pH and the isoelectric point of the carrier. Since precipitation reactions depend on pH, the insoluble compound distribution reflects the pH gradient. Hence, by appropriate choice of the impregnation conditions, precipitation can occur in either the inner (egg-yolk distribution) or the outer (eggshell distribution) region of the pellet. However, preparation of eggshell catalysts leads to the problem of precipitation outside the pellets.