PREPARATION OF CARBON-SUPPORTED PLATINUM CATALYSTS - ADSORPTION MECHANISM OF ANIONIC PLATINUM PRECURSOR ONTO CARBON SUPPORT

Adsorption of platinum precursor onto carbon was carried out using solutions of PtCl6(2-) anions at various initial solution PH values. It was found that the adsorption mechanism of the platinum precursor was greatly affected by the pH of the solution, and this was attributed to ammonium ions that were added in order to adjust the initial solution pH. Three types of Pt uptake can be distinguished, depending on the initial solution pH: fast and strong adsorption of Pt anions on protonated sites (OH2+) at low solution pH (type A), weak adsorption on preadsorbed ammonium sites at medium pH (type B), and the deposition of neutral Pt compound formed by the precipitation of (NH4)2PtCl6 at high pH (type C). It was also found that the dispersion of Pt after a reduction treatment was greatly affected by the uptake type as well as by oxygen groups on the carbon surface. The results suggest that each adsorption site and/or adsorbed species has a different sintering resistance.