Potassium dispersion on silica-supported ruthenium catalysts

Abstract

Alkali modifiers are known to be quite effective at improving catalyst activity or selectivity for several metal-catalyzed reactions of industrial importance. Yet it is still difficult to address the location and distribution of alkali species in most catalysts. This paper reports on an investigation of the potassium dispersion in a series of 3 wt% Ru/SiO 2 catalysts sequentially doped with potassium nitrate up to (K/Ru)awm = 0.2 followed by rereduction. This series was evaluated extensively using gas volumetric hydrogen chemisorption and the structure-sensitive ethane hydrogenolysis reaction. Hydrogen chemisorption results indicate that the alkali was apparently atomically dispersed on the ruthenium surface. The added potassium species interfered with hydrogen chemisorption on a one-to-one atomic basis. Potassium addition resulted in a decrease in the apparent activation energy and an increase in the apparent hydrogen reaction order for ethane hydrogenolysis. Using the statistical poisoning model of Martin ( Catal. Reu.-Sci. Eng. 30, 519 (1988)) which assumes that the metal surface is uniform for adatom adsorption, the apparent ensemble required for the reaction was estimated to be made up of 12 ± 3 adjacent exposed surface ruthenium atoms. Using an extension of Martin's model, this structure-sensitive reaction also revealed that at the higher potassium levels the alkali dispersion became nonuniform. This nonuniform dispersion is suggested to be due to a preference of the dopant for certain metal sites. Because of this nonuniform dispersion, the “true” reaction ensemble size is suggested to be less than 12.