Mechanism of Cobalt-Catalyzed CO Hydrogenation: 1. Methanation

Abstract

The mechanism of CO hydrogenation to CH4 at 260 °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 12CO/H2 → 13CO/H2 SSITKA as a function of the CO and H2 partial pressure and shows that the CH4 formation rate is mainly controlled by CHx hydrogenation rather than CO dissociation. Backward CO/H2 → H2 CTKA emphasizes the importance of H coverage on the slow CHx 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 CH4 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 H2 partial pressure on possible rate-controlling steps.

Cobalt@Silica Core‐Shell Catalysts for Hydrogenation of CO/CO2 Mixtures to Methane

“…In literature, two deactivation reasons are most frequently considered -coking and sintering. According to the mechanistic study of Chen et al [18] on a Co/SiO 2 catalyst, the CO methanation proceeds by the direct dissociation of CO to atomic C and O, which is subsequently hydrogenated to CH 4 and H 2 O. Carbon deposition induced by the Boudouard reaction (Equation 4), as frequently reported for similar systems, [19,47,48] is underlined by the significant CO 2 formation.…”

“…We assume a change in the underlying mechanism induced by the presence of both CO and CO 2 , whose identification is still a matter of debate, [18,19] since only little is known for CO x hydrogenation on cobalt as active material thus far beyond Fischer-Tropsch synthesis. For an increasing CO 2 share in the feed, CO 2 is converted to methane at a significant rate, while for smaller CO 2 fractions the CO methanation is the predominant reaction and CO 2 is produced via the WGS.…”

“…[17,18] Miao et al [19] reviewed possible mechanisms during CO and CO 2 methanation focusing mainly on Ni and Ru catalysts. [17,18] Miao et al [19] reviewed possible mechanisms during CO and CO 2 methanation focusing mainly on Ni and Ru catalysts.…”

“…The reaction mechanisms of CO and CO 2 hydrogenation are still under debate in literature. [17,18] Miao et al [19] reviewed possible mechanisms during CO and CO 2 methanation focusing mainly on Ni and Ru catalysts. In general, two different routes are claimed: On the one hand the associative mechanism meaning H atoms assisted CÀ O bond breaking and on the other hand the dissociative mechanism, where the CÀ O bond breaking takes place before the hydrogenation.…”

Influence of Carbon Deposits on the Cobalt-Catalyzed Fischer–Tropsch Reaction: Evidence of a Two-Site Reaction Model

“…Procedures to determine the residence time and coverages of CO and CH x (intermediates of CH 4 ) are provided in our earlier work. 38 …”