The effect of powder characteristics on washcoat quality. Part I: Alumina washcoats

The adhesion of γ-alumina washcoats on cordierite honeycombs was investigated with the aim to correlate the adhesion properties to the characteristics of the deposited powders, and, determine the conditions that can ensure integrity and endurance of the substrate/washcoat system under the severe operating conditions of an automobile catalytic system. It is shown that adhesion depends primarily on the particle size of the deposited powder. Depending on the production method, some powders have the form of large agglomerates and, if deposited as such upon the honeycombs, exhibit very poor adhesion properties. Reduction of the agglomeratesʹ size down to the order of a few (2–5) microns is necessary in order to ensure adhesion comparable to that of commercial catalysts. Particles of colloidal dimensions exhibit excellent adhesion to the support, provided that reduction of the agglomeratesʹ size that they form during calcination, takes place before deposition.