Activated chemisorption of hydrogen on supported ruthenium: II. Effects of crystallite size and adsorbed chlorine on accurate surface area measurements

Abstract

A crystallite size effect has been observed during the activated adsorption of hydrogen on supported ruthenium catalysts. This effect does not occur in the absence of adsorbed chlorine and is increasingly more pronounced as metal dispersion is increased. Activation energies for adsorption, at constant hydrogen coverage, may be as high as 16 kcal/mole and have been measured utilizing (i) Al 2O 3 and SiO 2 support materials, (ii) crystallites which range in size from 2.6 to 22.0 nm, and (iii) surface chlorine coverages which vary from 0.01 to 0.35 of the number of surface ruthenium atoms. Results of these studies suggest that preferential adsorption of chlorine atoms at high coordination sites reduces electron density at adjacent low coordination sites, thereby increasing the activation energy barrier for electron donation to, and dissociative chemisorption of, incoming hydrogen molecules. While this effect may be viewed as primarily electronic in origin, a structure-sensitive (i.e., crystallite size-dependent) mechanism has been proposed to account for the observed adsorption behavior based on the number of high coordination-chlorine adsorption sites in close proximity to low coordination-hydrogen adsorption sites. Activated adsorption sites, attributed to the above-noted mechanism, can be responsible for a greater than two-fold underestimation of the number of surface ruthenium atoms measured by irreversible hydrogen adsorption at 298 K. Significant differences in adsorption behavior between silica- and alumina-supported crystallites of equal dispersion also suggest that deliberate addition of chlorine adatoms may provide a sensitive probe for discriminating differences in crystallite shape and surface texture.