THE ROLE OF SURFACE-STRUCTURE AND DISPERSION IN CO HYDROGENATION ON COBALT

The effects of surface structure on the CO hydrogenation reaction have been investigated by comparing the steady-state activity and selectivity of submonolayer cobalt deposited on W(110) and W(100) with those of carbonyl-derived Co/alumina catalysts of varying dispersion and extent of reduction. The Co/W surfaces have highly strained and different geometries (1) but have similar activity. The activity matches that of the highly active, highly reduced Co/alumina catalysts, suggesting that the steady-state activity of cobalt surfaces is independent of surface structure. AES spectra show the after-reaction Co/W surfaces to have high coverages of both carbon and oxygen, with carbon lineshapes characteristic of carbidic carbon. Carbonyl-derived Co/dehydroxylated alumina catalysts have high extents of reduction, high dispersions, and good activity stability. Increasing the dehydroxylation temperature of the alumina support increases metal dispersion while decreasing CO2 and olefin selectivities. Specific CO hydrogenation activity is constant over the range of dispersion of 5-37% for highly reduced 3 and 5% Co/alumina catalysts and over the entire range of dispersion (0-100%) if polycrystalline Co and Co/W surfaces are included. The specific activity of carbonyl-derived catalysts appears to be more closely related to the extent of reduction and the support dehydroxylation temperature than to the dispersion. Thus, the chemical nature of the support surface appears to be the controlling factor in determining the specific activity of supported cobalt catalysts. © 1991.