1. Microkinetic Study of CO Adsorption and Dissociation on Fe Catalysts
Calvin H. Bartholomew, Uchenna P. Paul, Hu Zou, Denny Frost, and William C. Hecker
BYU Catalysis Laboratory
Department of Chemical Engineering
Brigham Young University

2. Mechanism of FTS
Pichler in his 1950 review notes that the publication of Fischer & Tropsch caused a great stir; scientists of the time had not expect such complexity with such a simple molecule as CO.

3. CO Adsorption on Fe CO* ↔ CO(g) + * (1) Initial step in FT synthesis
Energetics of SCs well studied: DHad varies with coverage, surface structure, and coadsorbates
CO may adsorb on 3F, 2F, or 1F (topside) sites; binding preference varies with crystal plane and coverage.
-DHad ↓ from 245 to 142 as qCO ↑ from 0.25 to 1.0 [Fe(100)]
-DHad ↑ with ↑ surface roughness (some exceptions)
i.e. Fe(110) < Fe(100) < Fe(211)
Experimental values of -DHad generally much smaller than theoretical.
kJ/mol (exptl) vs kJ/mol (DFT)
Few reliable data on polycrystalline Fe; effects of coverage, dispersion, promoters are not defined

4. CO Dissociation on Fe CO* ↔ C* + O* (2)
Important kinetically relevant mechanistic steps in FT synthesis. Rate controlling or not?
SCs studied experimentally and theoretically; recent surface science and DFT studies provide new data on site requirements and energetics
Rates of CO dissociation are not available for polycrystalline or supported Fe.
Rate depends upon reaction conditions, coverage, metal/metal carbide surface structure, and interaction of sites with promoters and supports.

5. Eact of 60, 85, and 105 kJ/mol reported for molecular desorption of CO from a1, a2, and a3 states.
Eact of 105 kJ/mol for CO dissociation (b-peak).
[Benzinger and Madix, 1980]
Potassium reported to (1) increase binding energy of CO and (2) increase rate of CO dissociation.

6. CO Dissociation on Fe(110) [Courtesy of Mavrikakis et al.]

7. CO Dissociation: Data for SC Fe surfaces
CO dissociation is facile on stepped SC iron. Need for data on PC Fe surfaces

8. Objective
Develop and validate a microkinetic model of CO desorption and dissociation on PC and supported Fe
Approach
Use TPD to obtain rate parameters for CO desorption and dissociation

9. CO-TPD Procedure [Reduction] 10%H2/He He He He 450ºC 12 h 800ºC
[Purge]
[Adsorption]
[Desorption]
10%CO
/He
10%H2/He He He He
450ºC 12 h
800ºC
1ºC/min
430ºC 30 min
15-30ºC/min
RT 2 h
or 50ºC
or 100ºC RT RT
60 min
Experimental guidelines recommended by Demmin and Gorte [1984] and Kanervo et al. [2006] were followed.

10. Effects of Fe Dispersion
6 peaks
Effects of Fe Dispersion
Reflects increasing heterogeneity of adsorption sites
4 peaks
2 peaks

12. Effects of Adsorption Temperature
99% Fe, D = 0.07%
Effects of Adsorption Temperature

13. Parameter Fit Determining Kinetic Parameters for Elementary Steps
Set up differential equations for each elementary step—including CO2 formation.
Assume plug flow and divide reactor into nodes.
Use ODE solver and optimization code to minimize errors in the kinetic parameters.

14. TPD Fit for 99% Fe, %D = 0.07 CO Adsorbed at 25oC

15. TPD Fit for 99% Fe, %D = 0.07;CO Adsorbed at 25oC

16. TPD Fit for 99% Fe, %D = 0.07

17. Effects of Adsorption Temperature

18. Effects of Adsorption Temperature

19. Effects of Adsorption Temperature

20. Rates vs T

21. Conclusions
Kinetic parameters can be obtained quantitatively from TPD data. It’s critical to (a) follow published experimental guidelines and (b) model the reactor as plug flow; CSTR doesn’t work.
Complexity of TPD spectrum increases with increasing dispersion, reflecting increasing heterogeneity, e.g. 3F, 2F, and 1F sites at higher %D, only 3F at low %D.
Consistent trends in enthalpies, activation energies, and coverages are found for data obtained after adsorption at 25, 50, 100, and 150oC. The most significant changes are observed between 100 and 150oC: (a) slight increase in DHads; (b) moderate increase in Edes; and (c) a 1000-fold increase in the rate of CO dissociation.
Promoter additives greatly influence CO desorption and dissociation rates; Pt facilitates CO dissociation, while K increases the binding energy of CO and the rate of CO dissociation (on sites in close association with Fe).

22. DOE
BYU catalysis group:
Acknowledgements

23. Unsupported Fe FTS Catalysts
Samples (Co-precipitation)
Composition
Treatment
SBET
(m2/g)
H2 Uptake
(mmol/g)
99FeAl
1 wt% Al2O3-99 wt% Fe
Calc. 300ºC 6 h 59
Red.
500ºC 12 h 12 97
99FeAl-Pt
1 wt% Pt-1 wt% Al2O3-98 wt% Fe 51 10
99FeAl-K
1 wt% K-1 wt% Al2O3-98 wt% Fe 56 14

24. Effects of Promoter
(Adsorption at 25oC)

25. 2 adsorption peaks 3 dissociation peaks Effects of Potassium
(Adsorption at 25oC)

26. Nonlinear Least Squares Regression