Partially reduced $Co/Al_2O_3$ catalysts have been studied to investigate their adsorption and catalytic properties in CO hydrogenation. $H_2$ and CO chemisorption, reaction tests in $H_2$/CO stream, TPD of CO and TPSR of deposited carbon were made with 10 wt.% $Co/Al_2O_3$ catalysts.
Catalyst was manufactured by incipient wetness method with $Co(NO_3)_2.6H_2O$ solution. To obtain partially reduced catalysts, temperature and period of reduction were varied between 275℃ and 525℃ and between 0.25 and 18 hr, respectively. The degree of reduction of the catalyst was varied from 20% to 70% by changing the reduction conditions.
Typical changes in adsorption properties with decreased catalyst reduction are suppression of hydrogen adsoption and increase in the ratio of irreversibly adsorbed $(CO/H)_{irr}$.
CO hydrogenation with poorly reduced catalyst has a few distinct trends in activity, selectivity and kinetic parameters. As the catalyst is poorly reduced, the overall rate decreased, but the specific activity calculated from hydrogen adsorption was greatly enhanced. The poorly reduced catalyst has a high olefin selectivity.
The activation energy of methane formation increased with increasing degree of reduction, and that of ethylene formation and ethane formation decreased with increasing degree of reduction. The partial pressure dependency of $Co/Al_2O_3$ catalyst also varied with degree of catalyst reduction.
Production pattern of hydrocarbons with respect to reactant mole ratio of $H_2$/CO was the same regardless of the extent of catalyst reduction, but the poorly reduced catalyst produced more olefins than paraffins up to higher value of $H_2/CO$ ratio than well reduced one.
The results suggest that unreduced cobalt oxide on the catalyst surface is not simply inert, but it interacts with the reduced cobalt metal to modify the overall catalytic properties. It has been concluded that the extent of catalyst reduction is an important variable in determining the kinetic behavior of supported cobalt catalysts.
