Effects of hydroxylation of a silica surface on the metal dispersion in supported platinum catalysts

The influence of the surface hydroxyl groups of porous silica on the degree of platinum dispersion was studied using silica supports with the same surface area but with an increasing amount of these functional groups. The supports were prepared by immersing a calcined silica in water at 70°C for various periods of time. Platinum was deposited on the supports by wet impregnation with aqueous solutions of platinum tetraamine dichloride (PTD) and chloroplatinic acid (CPA). For catalysts with PTD reduced at 400°C, the degree of platinum dispersion indicated a maximum for a support whose surface was moderately hydroxylated. Such an effect was not observed for the catalysts reduced at the lower temperature of 330°C and for those with CPA. The surface hydroxyl groups affected the reduction of PTD and certain types of surface hydroxyl groups may be responsible for the effects observed.