The nature of colloidal platinum in aqueous solution: features of catalytic reactions

The specific features of the catalytic reduction of methylviologen by dihydrogen in water in the presence of platinum colloids synthesized by various methods are studied. The colloids prepared by the radiation-chemical reduction of PtCl42- in the presence of polyacrylate or polyphosphate as stabilizers and colloids prepared by the reduction with dihydrogen efficiently catalyze the reaction. The "citrate" colloids synthesized by the reduction of PtCl62- with citric acid are characterized by a prolonged induction period after which these colloids also gain the catalytic activity. We assume that the "citrate" platinum colloids are giant clusters with the close-packed metal core containing the "magic" number of platinum atoms (Pt-561), which are coordinated by the ligand molecules blocking the surface of the metal particle. In the presence of H-2, the ligand molecules are decomposed or removed from the surface, which is accompanied by the appearance of the catalytic activity.