Further insights into methane and higher hydrocarbons formation over cobalt-based catalysts with γ-Al2O3, α-Al2O3 and TiO2 as support materials

• 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.

Subject headings

TEKNIK OCH TEKNOLOGIER  -- Kemiteknik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Chemical Engineering (hsv//eng)

Keyword

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

Publication and Content Type

ref (subject category)

art (subject category)