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

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.

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. 60-200 kJ/mol (exptl) vs 145-245 kJ/mol (DFT) Few reliable data on polycrystalline Fe; effects of coverage, dispersion, promoters are not defined

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.

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.

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

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

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

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.

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

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

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.

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

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

TPD Fit for 99% Fe, %D = 0.07

Effects of Adsorption Temperature

Effects of Adsorption Temperature

Effects of Adsorption Temperature

Rates vs T

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).

DOE BYU catalysis group: Acknowledgements

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

Effects of Promoter (Adsorption at 25oC)

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

Nonlinear Least Squares Regression