Mechanism of Cobalt-Catalyzed CO Hydrogenation: 1. Methanation

The mechanism of CO hydrogenation to CH4 at 260 degrees C on a cobalt catalyst is investigated using steady-state isotopic transient kinetic analysis (SSITKA) and backward and forward chemical transient kinetic analysis (CTKA). The dependence of CHx residence time is determined by (CO)-C-12/H-2 -> (CO)-C-13/H-2 SSITKA as a function of the CO and H-2 partial pressure and shows that the CH, formation rate is mainly controlled by CH,, hydrogenation rather than CO dissociation. Backward CO/H-2 -> H-2 CTKA emphasizes the importance of H coverage on the slow CH, hydrogenation step. The H coverage strongly depends on the CO coverage, which is directly related to CO partial pressure. Combining SSITKA and backward CTKA allows determining that the amount of additional CH, obtained during CTKA is nearly equal to the amount of CO adsorbed to the cobalt surface. Thus, under the given conditions overall barrier for CO hydrogenation to CH4 under methanation condition is lower than the CO adsorption energy. Forward CTKA measurements reveal that O hydrogenation to H2O is also a relatively slow step compared to CO dissociation. The combined transient kinetic data are used to fit an explicit microkinetic model for the methanation reaction. The mechanism involving direct CO dissociation represents the data better than a mechanism in which H assisted CO dissociation is assumed. Microkinetics simulations based on the fitted parameters confirms that under methanation conditions the overall CO consumption rate is mainly controlled by C hydrogenation and to a smaller degree by O hydrogenation and CO dissociation. These simulations are also used to explore the influence of CO and H, partial pressure on possible rate-controlling steps.