Effects of dispersion on the activity and selectivity of alumina-supported ruthenium catalysts for carbon monoxide hydrogenation

Abstract

A study was performed to determine the extent to which metal dispersion affects the activity and selectivity of Ru Al 2O 3 catalysts used for CO hydrogenation. For dispersions below 0.7, the specific activity for synthesis of methane and C 2+ products decreases with increasing dispersion, but neither the probability for chain growth nor the olefin-to-paraffin ratio is affected. The decrease in activity over this range is ascribed to a decrease in the fraction of sites present on planar surfaces. For dispersions above 0.7, the specific activity for synthesis of all products decreases dramatically, and is accompanied by a slight decrease in the probability of chain growth and a rapid decrease in the olefin-to-paraffin ratio. These changes are attributed to changes in the electronic properties of the Ru microcrystallites with size and the occurrence of metal/support interactions. In situ infrared spectra reveal that only those sites that adsorb one linearly bound CO molecule per Ru atom are active for CO hydrogenation. Adsorption of two CO molecules per Ru site is also observed but these site are catalytically inactive.