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Kinetics of Fischer-Tropsch synthesis on supported cobalt: Effect of temperature on CO and H2 partial pressure dependencies

Paper ID Volume ID Publish Year Pages File Format Full-Text
53252 46957 2016 10 PDF Available
Title
Kinetics of Fischer-Tropsch synthesis on supported cobalt: Effect of temperature on CO and H2 partial pressure dependencies
Abstract

•Pco dependency of FT rate decreases from +0.31 to −0.64 as temp increases.•Parallel H-assisted pathways leads to activated denominator term.•Model explains observed presence of atomic carbon on the catalyst surface.•“Lack of fit” P parameter effectively differentiates between kinetic mechanisms.

Kinetic data were measured for Fischer-Tropsch Synthesis (FTS) on a cobalt catalyst supported on silica-modified alumina at various partial pressures of CO and H2 and at four different temperatures (210, 220, 230, and 240 °C). The data were sufficient to determine power law rate expressions at each of the four temperatures which indicate that the dependence of rate on PH2 increases with increasing temperature while the rate coefficient for PCO decreases with temperature going from positive order (+0.3) at 210 °C to negative order (−0.6) at 240 °C. Several mechanisms were explored and Langmuir–Hinshelwood (LH) rate models derived in an attempt to explain the data trends. Traditional FT rate expressions have a denominator term, KCO*PCO which, since the denominator is squared in LH expressions, can allow for an overall negative order PCO dependence. However, since KCO is the equilibrium constant for the adsorption of CO, its value decreases with increasing temperature which causes the overall PCO dependence to become more positive with increasing temperature instead of more negative. In this work we propose a mechanism based on parallel hydrogen-assisted mechanistic pathways that leads to a LH model with the denominator term, k’CO*PCO where k’CO is effectively an activated rate constant instead of an equilibrium constant and therefore increases with increasing temperature instead of decreasing. This model, which fits the data extremely well, also explains the presence of atomic carbon on the surface of the catalyst.

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Keywords
Fischer-Tropsch Synthesis; Cobalt catalyst; Chemical kinetics; Langmuir–Hinshelwood; Power law rate expression; Kinetic rate model
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Publisher
Database: Elsevier - ScienceDirect
Journal: Catalysis Today - Volume 270, 15 July 2016, Pages 9–18
Authors
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Subjects
Physical Sciences and Engineering Chemical Engineering Catalysis
Get Full-Text Now
Don't Miss Today's Special Offer
Price was $35.95
You save - $31
Price after discount Only $4.95
100% Money Back Guarantee
Full-text PDF Download
Online Support
Any Questions? feel free to contact us