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Further insights into methane and higher hydrocarbons formation over cobalt-based catalysts with γ-Al2O3, α-Al2O3 and TiO2 as support materials
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Technology.
KTH, School of Chemical Science and Engineering (CHE), Chemical Engineering and Technology, Chemical Technology.
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2017 (English)In: Journal of Catalysis, ISSN 0021-9517, E-ISSN 1090-2694, Vol. 352, p. 515-531Article in journal (Refereed) Published
Abstract [en]

A range of cobalt-based catalysts varying in Co loading and prepared by incipient wetness impregnation of traditional support materials (γ-Al2O3, α-Al2O3 and TiO2), have been studied in the Fischer-Tropsch reaction at industrially relevant process conditions (483 K, 20 bar, H2/CO = 2.1). A high selectivity to C5+ hydrocarbons (SC5+) is to a great extent connected with a high site activity, but not exclusively. We propose that the ratio of monomer-production rate to C–C coupling rate of a catalyst determines chain-growth probability by means of governing the coverage of the monomer on the cobalt surface. We speculate that this ratio depends on e.g. shape, strain and size of the Co crystallites and, therefore, is highly dependent on the choice of support material. No general relationship between Co particle size and SC5+ is found, but individual correlations exist for each support material. Within each support material, there are indications of negative correlations between the chain-growth probability of the C1 ∗ surface intermediate (αC1) and the higher αCn values. This can be rationalized by assuming that the majority of methane is formed by a different mechanism, separate from chain growth, but connected with chain growth through a common carbon pool. We propose that the monomers and the majority of methane are produced at sites different from the ones involved in chain growth. There is no general correlation between αC1 and SC5+ for catalysts with different support materials, possibly due to small differences in cobalt surface coverage of hydrogen. For the TiO2-supported catalysts, a dramatically increased αC1 value observed for catalysts with Co particles smaller than approx. 15 nm, is probably associated with strong metal-support interactions (SMSI). This phenomenon apparently limits the effect (reduction) on the SC5+ when moving toward smaller Co particles.

Place, publisher, year, edition, pages
Academic Press, 2017. Vol. 352, p. 515-531
Keywords [en]
Alpha values, Alumina, Boron, Chain growth, Cluster, Cobalt, Cobalt loading, Coverage, Fischer-Tropsch, Methane, Monomer, Particle size, Rhenium, Selectivity, Site, Site-time-yield, SMSI, Support material, TEM, Titania, TPR
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-212273DOI: 10.1016/j.jcat.2017.06.003ISI: 000408621100014Scopus ID: 2-s2.0-85021873268OAI: oai:DiVA.org:kth-212273DiVA, id: diva2:1134023
Funder
Swedish Energy Agency
Note

QC 20170817

Available from: 2017-08-17 Created: 2017-08-17 Last updated: 2017-09-26Bibliographically approved

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