Modeling wet impregnation of nickel on γ-Alumina

Abstract

A transport model is presented for the coimpregnation of a metal salt and an acid into a porous oxide where equilibrium adsorption is controlled by hydrolysis of surface hydroxyl groups and ion binding on these groups. The transport and adsorption equations for the metal species and the acid are solved simultaneously to determine metal profiles for the wet impregnation of NiCl 2 and HCl in a γ-Al 2O 3 support with a background electrolyte of NaCl. For acid pretreatment, the transport and adsorption equations of HCl may be solved alone to establish the pellet pH profile, which is then used as the initial condition for subsequent metal impregnation. An important feature of the equilibrium adsorption model is the generation of protons upon nickel adsorption. As nickel diffuses in and adsorbs, a local excess of H + ions is generated, producing a pH profile inside the pellet and reducing nickel uptake. This blocking phenomenon, which cannot be described by Langmuir isotherms, produces unique, concave-convex metal profiles in the pellets. New experimental nickel profiles on γ-Al 2O 3 are determined by electron microprobe analysis over a 0.001 to 0.1 M nickel concentration range in 0.1 M NaCl aqueous solutions ranging in pH from 5.2 to 7.85. By controlling the initial pH profile through acid pretreatment, the final nickel profile may be modified to produce egg-shell, egg-white, egg-yolk, and uniform catalysts. Comparison of the proposed transport theory and the experimental metal profiles shows good agreement, establishing a new tool for designing oxide support catalysts.