title: On the origin of the cobalt particle size effects in Fischer-Tropsch catalysis.
abstract: The effects of metal particle size in catalysis are of prime scientific and industrial importance and call for a better understanding. In this paper the origin of the cobalt particle size effects in Fischer-Tropsch (FT) catalysis was studied. Steady-State Isotopic Transient Kinetic Analysis (SSITKA) was applied to provide surface residence times and coverages of reaction intermediates as a function of Co particle size (2.6-16 nm). For carbon nanofiber supported cobalt catalysts at 210 degrees C and H(2)/CO = 10 v/v, it appeared that the surface residence times of reversibly bonded CH(x) and OH(x) intermediates increased, whereas that of CO decreased for small (<6 nm) Co particles. A higher coverage of irreversibly bonded CO was found for small Co particles that was ascribed to a larger fraction of low-coordinated surface sites. The coverages and residence times obtained from SSITKA were used to describe the surface-specific activity (TOF) quantitatively and the CH(4) selectivity qualitatively as a function of Co particle size for the FT reaction (220 degrees C, H(2)/CO = 2). The lower TOF of Co particles <6 nm is caused by both blocking of edge/corner sites and a lower intrinsic activity at the small terraces. The higher methane selectivity of small Co particles is mainly brought about by their higher hydrogen coverages.
authors.affiliation: Inorganic Chemistry and Catalysis, Debye Institute for NanoMaterials Science, Utrecht University, NL-3508 TB Utrecht, P.O. Box 80003, The Netherlands.
authors.forename: J P
authors.initials: JP
authors.lastname: den Breejen
authors.forename: P B
authors.initials: PB
authors.lastname: Radstake
authors.forename: G L
authors.initials: GL
authors.lastname: Bezemer
authors.forename: J H
authors.initials: JH
authors.lastname: Bitter
authors.forename: V
authors.initials: V
authors.lastname: Frøseth
authors.forename: A
authors.initials: A
authors.lastname: Holmen
authors.forename: K P
authors.initials: KP
authors.lastname: de Jong