| Abstract: | Adsorption of cationic Pt complex, Pt(NH3)42+, in aqueous solution onto carbon was carried out by varying the initial pH of the solution. The Pt uptake showed a maximum as a function of the uptake time and afterward decreased gradually, showing that the adsorbed species is desorbed from the carbon surface. It was found that the degree of desorption was strongly dependent on the solution pH, as well as on the IEP (Isoelectric point) of the carbon support. In the solution pH lower than the IEP of the carbon, the desorption was almost complete. The desorption was found to be caused by a gradual decomposition of the ion-exchanged and/or electrostatically adsorbed Pt amine complex after giving the ligand ammonia to the acidic sites of the carbon surface, most probably, positive sites like protonated carboxyl group. The desorption could be prevented to a considerable extent by using a carbon support treated by nitric acid, because lowering the IEP of the carbon by acid treatment must result in a decrease in the interaction of basic ligand ammonia and positive surface sites, which eventually leads to the desorption of Pt complex. Measurements of the Pt dispersion of the catalysts that have undergone desorption indicate that the desorption of the adsorbed Pt complex occurs on relatively weak adsorption sites like carboxyl group, rather than strong adsorption sites (e.g., phenolic hydroxyl or lactonic groups.). |
