PREPARATION OF PLATINUM ON ACTIVATED CARBON

In a study of the preparation of Pt/C catalysts, a set of modified activated carbon carriers has been prepared by nitric acid or air oxidation, or thermal decarboxylation. The carbons obtained by these methods had largely identical pore structures, but differed considerably in surface chemistry. A series of activated carbons with different surface chemistry was impregnated with aqueous hydrochloroplatinic acid solutions. Upon impregnation, the carbon reduced a certain amount of the PtIV complex to PtII, which was strongly chemisorbed at the walls of the carrier's mesopores. Weak physisorption was also observed. The capacity of the carrier for the reductive chemisorption was governed by its surface chemistry, and thus was strongly dependent on the pretreatment of the activated carbon. Aqueous alkaline formaldehyde solutions were then applied as reductor. After a period of slow growth, during which small zero-valent platinum particles were formed, a critical crystallite size was reached. Subsequently, a fast autocatalytic growth was observed until the supply of unreduced platinum became rate limiting. During the reduction process the carrier surface chemistry was (slightly) modified. After the reduction was completed, a catalytic autoredox reaction of excess formaldehyde to methanol and formate was observed. By properly selecting the preparation conditions, platinum-on-carbon catalysts combining a high metal load (5%) and a high metal dispersion (60%) could be obtained. © 1991.