Anion adsorption on alumina

Abstract A series of inert metal complex anions with net charges of −1 to −3 have been used to examine the anion adsorption process on alumina. We have found that this process involves the simultaneous coadsorption of both hydrogen ion (from added acid) and complex anion. Adsorption is fast and depends only slightly upon changes in solution ionic strength and complex anion concentration. The adsorption process is also reversible with the addition of base. These findings are interpreted on a molecular level in the following way. A surface-bound hydroxyl group is protonated, thereby creating a positively charged surface group which can bind a complex anion by electrostatic attraction. The addition of base removes the positively charged surface group by reaction with the bound proton and releases the complex anion. This work implies that one may vary a catalyst metal loading simply by controlling the amount of acid present in a preparation where an inert metal complex anion is employed.