Role of Transport Phenomena in the Impregnation of Porous Media by Active Catalytic Materials

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The effect of transport phenomena (including electrokinetic effects) during the preparation of supported metal catalysis by wet impregnation is examined theoretically. A model for the transient diffusion - adsorption process coupled with the Poisson- Boltzmann equation for the electrical potential (which arises because of the presence of pH - dependent charged sites on the support surface) is developed for a cylindrical pore. The influence of process variables, such as the pH and ionic strength of the impregnating solution and of the addition of a competing species on the catalyst weight loading and distribution profile, is considered. The question of the values of the process variables for given catalyst design conditions is also addressed. Detailed calculations are carried out for Pt on alumina catalyst. The relevance of the proposal resides in the fact that the appropriate conditions (pH, ionic strength, nature of the substrate and concentration of the impregnant) for the preparation of the desired catalytic materials can be identified. The novelty of the approach consists in the inclusion of the electrokinetic phenomena in the description of the impregnation process. In this manner, the charge of the substrate and the influence of the pH and ionic strength on the impregnation process are taken into account.