Kinetics of deactivation by carbon of a cobalt Fischer–Tropsch catalyst: Effects of CO and H2 partial pressures

Keyvanloo, Kamyar; Fisher, McCallin J.; Hecker, William C.; Lancee, Remco J.; Jacobs, Gary; Bartholomew, Calvin H.

doi:10.1016/j.jcat.2015.01.022

Cited by 55 publications

(44 citation statements)

References 40 publications

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“…This difference can be explained by the larger amount of graphitic carbon deposited at higher temperature and lower H 2 /CO ratio. 54 Accordingly, we propose that the initial decrease in the CO consumption rate can be assigned to carbon deposition. Notably, there are also differences in the reactivity of the deposited graphitic carbon.…”

Section: Resultsmentioning

confidence: 99%

“…15,32 At a low H 2 /CO ratio, a fraction of these C atoms will be converted to graphitic carbon, as we observed in the present work, causing deactivation. 54 Similarly, O migrating to terraces will be converted to CO 2 due to the high CO coverage. Graphitic carbon on terrace sites will suppress CH 4 and CO 2 formation, in line with our experimental observations.…”

Section: Discussionmentioning

confidence: 99%

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

et al. 2018

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One of the well-known observations in the Fischer–Tropsch (FT) reaction is that the CH4 selectivity for cobalt catalysts is always higher than the value expected on the basis of the Anderson–Schulz–Flory (ASF) distribution. Depositing graphitic carbon on a cobalt catalyst strongly suppresses this non-ASF CH4, while the formation of higher hydrocarbons is much less affected. Carbon was laid down on the cobalt catalyst via the Boudouard reaction. We provide evidence that the amorphous carbon does not influence the FT reaction, as it can be easily hydrogenated under reaction conditions. Graphitic carbon is rapidly formed and cannot be removed. This unreactive form of carbon is located on terrace sites and mainly decreases the CO conversion by limiting CH4 formation. Despite nearly unchanged higher hydrocarbon yield, the presence of graphitic carbon enhances the chain-growth probability and strongly suppresses olefin hydrogenation. We demonstrate that graphitic carbon will slowly deposit on the cobalt catalysts during CO hydrogenation, thereby influencing CO conversion and the FT product distribution in a way similar to that for predeposited graphitic carbon. We also demonstrate that the buildup of graphitic carbon by 13CO increases the rate of C–C coupling during the 12C3H6 hydrogenation reaction, whose products follow an ASF-type product distribution of the FT reaction. We explain these results by a two-site model on the basis of insights into structure sensitivity of the underlying reaction steps in the FT mechanism: carbon formed on step-edge sites is involved in chain growth or can migrate to terrace sites, where it is rapidly hydrogenated to CH4. The primary olefinic FT products are predominantly hydrogenated on terrace sites. Covering the terraces by graphitic carbon increases the residence time of CHx intermediates, in line with decreased CH4 selectivity and increased chain-growth rate.

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Section: Resultsmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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

et al. 2018

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“…This trend confirmed that concluded from TPR reduction profiles. It should be mentioned that some authors [51] have found that the degree of reduction may be underestimated for large Co crystallites (10-12 nm) under present pulse oxidation conditions. Titration is then proposed to be conducted at higher O 2 partial pressure (1-2 atm), temperature (450˝C instead of 400˝C), and exposure times (longer than 1 h) to completely oxidize Co 0 to Co 3 O 4 .…”

Section: Cobalt Oxide Reducibilitymentioning

confidence: 77%

Influence of Cobalt Precursor on Efficient Production of Commercial Fuels over FTS Co/SiC Catalyst

et al. 2016

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β-SiC-supported cobalt catalysts have been prepared from nitrate, acetate, chloride and citrate salts to study the dependence of Fischer-Tropsch synthesis (FTS) on the type of precursor. Com/SiC catalysts were synthetized by vacuum-assisted impregnation while N 2 adsorption/desorption, XRD, TEM, TPR, O 2 pulses and acid/base titrations were used as characterization techniques. FTS catalytic performance was carried out at 220˝C and 250˝C while keeping constant the pressure (20 bar), space velocity (6000 Ncm 3 /g¨h) and syngas composition (H 2 /CO:2). The nature of cobalt precursor was found to influence basic behavior, extent of reduction and metallic particle size. For β-SiC-supported catalysts, the use of cobalt nitrate resulted in big Co crystallites, an enhanced degree of reduction and higher basicity compared to acetate, chloride and citrate-based catalysts. Consequently, cobalt nitrate provided a better activity and selectivity to C 5 + (less than 10% methane was formed), which was centered in kerosene-diesel fraction (α = 0.90).On the contrary, catalyst from cobalt citrate, characterized by the highest viscosity and acidity values, presented a highly dispersed distribution of Co nanoparticles leading to a lower reducibility. Therefore, a lower FTS activity was obtained and chain growth probability was shortened as observed from methane and gasoline-kerosene (α = 0.76) production when using cobalt citrate.

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“…Extracted catalyst was treated mildly with 1%O 2 /N 2 at 300 °C for 4 h to remove the wax product formed from FTS, prior to characterization of temperature programmed reduction (TPR) measurements. In a very recent study, Keyvanloo et al [30] also followed similar procedure for the wax extraction.…”

Section: Catalyst Characterizationmentioning

confidence: 99%

Fischer–Tropsch synthesis: Effect of ammonia on supported cobalt catalysts

Pendyala

Jacobs

Bertaux

et al. 2016

Journal of Catalysis

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The effect of ammonia in syngas on the performance of various supported cobalt catalysts (i.e., Al 2 O 3 , TiO 2 and SiO 2) was investigated during Fischer-Tropsch synthesis (FTS) using a continuously stirred tank reactor (CSTR). The addition of ammonia (10 ppmv NH 3) caused a significant deactivation for all supported cobalt catalysts, but the rate of deactivation was higher for the silica-supported catalysts relative to the alumina and titania-supported catalysts used in this work. Ammonia addition had a positive effect on product selectivity (i.e., lower light gas products and higher C 5 +) for alumina and titania-supported catalysts compared to ammonia free conditions, whereas, the addition of ammonia increased lighter hydrocarbon (C 1-C 4) products and decreased higher hydrocarbon (C 5 +) selectivity compared to ammonia-free synthesis conditions for the silica-supported catalyst. For alumina and titania-supported catalysts, the activity almost recovered with mild in-situ hydrogen treatment of the ammonia exposed catalysts. For the silica-supported catalyst, the loss of activity is somewhat irreversible (i.e., cannot be regained after the mild hydrogen treatment). Addition of ammonia led to a significant loss in BET surface area and changes in pore diameter (consistent with pore collapse of a fraction of pores into the microporous range as described in the literature), as well as formation of catalytically inactive cobalt support compounds for the silica-supported catalyst. On the other hand, the pore characteristics of alumina and titania-supported catalysts were not significantly

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Kinetics of deactivation by carbon of a cobalt Fischer–Tropsch catalyst: Effects of CO and H2 partial pressures

Cited by 55 publications

References 40 publications

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

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

Influence of Cobalt Precursor on Efficient Production of Commercial Fuels over FTS Co/SiC Catalyst

Fischer–Tropsch synthesis: Effect of ammonia on supported cobalt catalysts

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