The structure and activity of supported metal catalysts: VI. Measurement of dispersion in palladium-charcoal catalysts

Abstract

The metal dispersion in Pd-charcoal catalysts can be estimated from X-ray line-broadening and intensity measurements (yielding respectively, mean crystallite sizes and the percentage of Pd detectable, i.e., due to crystallites greater than 50 å size) thereby preserving the high dispersion existing after catalyst manufacture. The theoretical X-ray intensity and intensities from sintered samples relative to an internal standard, MgO, are compared and discussed. Surface cleaning, the normal prerequisite for chemisorption measurements, although very mild in this case (H 2 at 25 °C; evacuation at 100 °C), can induce sintering, particularly at high Pd contents or with nonuniform Pd distributions over the charcoal support. Despite the mild pretreatment, amounts of CO subsequently chemisorbed were consistent with information from X-ray diffraction and electron microscopy. The observed Pd atoms (exposed)/CO molecules (adsorbed) ratios obtained for sintered and high dispersion Pd-charcoal catalysts are discussed.