PREPARATION AND CHARACTERIZATION OF MULTIPLE ION-EXCHANGED PT/TIO2 CATALYSTS

The amount of platinum introduced by ion exchange on TiO2 anatase (63 M2 g-1) can be increased by repeating the exchange procedure after reduction of the catalyst under hydrogen. From IR spectroscopy studies, two kinds of Lewis acid sites on the TiO2 SUPPort (alpha and beta sites) have been found to participate in the ion-exchange mechanism. Only the strongest Lewis acid sites (alpha sites) are involved in anion exchange, whereas both kinds of sites participate in cation exchange. These adsorption sites are partly liberated by reduction under hydrogen allowing further ion-exchange procedures and thus increasing the amount of platinum. By modelling the quantity of platinum introduced by each exchange, a limit for the amount of platinum is expected to be reached after several exchange-reduction cycles: 1.3 wt.% for anion exchange and 3.1 wt.% for cation exchange.