CO Adsorption, Dissociation and Hydrogenation on Fe Calvin H. Bartholomew, Hu Zou, and Uchenna P. Paul BYU Catalysis Laboratory Department of Chemical Engineering Brigham Young University

Mechanism of FTS

Van Dijk Microkinetics Model Microkinetics model of van Dijk. Schematic representation of the FT microkinetics model (quantitative model containing all the kinetic parameters for the elementary steps) for FTS on Co/Ru/TiO2 based on 13CO SSITKA at 498 K, 1.2 bar and H2/CO = 1-5 [van Dijk, 2001].

CO Adsorption on Fe Energetics of SCs well studied: DHad varies with coverage, surface structure, and coadsorbates -DHad ↓ from 245 to 142 as qCO ↑ from 0.25 to 1.0 [Fe(100)] -DHad ↑ with ↑ surface roughness, i.e. Fe(110) < Fe(100) < Fe(211) Experimental values of -DHad much smaller than theoretical 100-200 kJ/mol (exptl) vs 145-245 kJ/mol (DFT) Few reliable data on polycrystalline Fe especially over a range of coverages; few data on carbides => Need for experimental data as a function of coverage on stepped SC and polycrystalline Fe and Fe carbides (TPD and calorimetric studies)

CO Dissociation and Carbon Hydrogenation Probably the most important kinetically relevant mechanistic steps in FT synthesis. SCs well studied experimentally and theoretically; recent surface science and DFT studies provide new data on site requirement and energetics Questions remain regarding which step is rate-determining on both Co and Fe catalysts. Rates of both steps depend upon reaction conditions, metal/metal carbide surface structure, and interaction of sites with promoters and supports. => More reliable mechanistic picture and kinetic para-meters are needed for stepped SC, polycrystalline, and supported Fe for development of better kinetic models.

Preferred Adsorption Sites and Binding Energies Courtesy of Professor Manos Mavrikakis, UWM

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

CO Dissociation: Recent Developments New data for smooth SC Fe surfaces New data for Co SC and PC surfaces => CO dissociation is facile on stepped SC cobalt. Need for data on stepped SC and PC Fe surfaces

Carbon Hydrogenation: Recent Developments Activation energies are now available for CO dissociation and carbon hydrogenation to methane on Fe(100) and Fe(110). Potential Energy Surface for CO hydrogenation to CH4 on Fe(110) [Courtesy of Mavrikakis et al] -4 -3 -2 -1 1 Energy (eV) CO* + 4H* C* + O* + 4 H* CH* + O* + 3H* CH2* + 2H* + O* CO(g) + 4H* 1.96 2.20 0.90 0.78 1.52 0.96 1.36 CH4* + O* 0.08 CH3* + H* + O* TS 1.01 0.48 RDS Quantitative rate data are lacking for the early elementary steps on Fe catalysts; energetics are lacking for stepped SC and real PC Fe catalysts.

Our Objectives Approach Present Focus Develop and validate a detailed microkinetic model of the rates of important elementary steps that occur during FTS on PC and supported Fe Approach Use TPD, TPH, and SSITKA to obtain rate parameters for CO adsorption, dissociation, and hydrogenation on PC and supported Fe Present Focus Using TPD and TPH to obtain rate parameters for CO hydrogenation on PC Fe

Unsupported Fe Catalyst 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 Prepared by coprecipitation of Fe and Al nitrates with ammonia Fe dispersion = 1.2%

CO-TPD at Different Adsorption Temperatures on 99FeA Less CO adsorbed molecularly at higher temps due to greater dissociation; complete dissociation at 423 K. Preliminary heat of adsorption for CO/PC-Fe = 100 kJ/mol

Isothermal hydrogenation spectra of 99FeA samples after pretreatment in syngas; data are obtained in a quartz fixed bed containing 0.1-0.2 gcat.

Determining Rate Parameters from Isothermal Hydrogenation Postulate elementary steps Write concentration/site balances (differential equations) for all species Solve stiff differential equations and fit to experimental data to obtain best kinetic parameters

Sequence of Elementary Steps However, adsorbed carbon atoms can be bound to sites of differing coordination (hence surface energy) on the irregular surfaces of small iron crystallites. We postulate two different carbon species, a1 and a2 (C-s species) having different binding energies; this gives us two sets of five or 10 steps altogether.

Differential Equations where Ci and Li have units of mol/m3 and mol/kgcat. Similar equations are written for surface concentrations of H, CH, CH2, and CH3; 5 more equations are written for the Ca2 and CHx2 species; altogether 12 differential equations with 12 constants are obtained,

Parameter Fit

Results of Fitting Data [175°C] Single-site model

Two-site model with irreversible hydrogenation

Two-site model with reversible hydrogenation

Seven site Gaussian fit

Results of Two-site Data Fit (175°C) (rCH4)max = 2.66 x 10-5 mol/m3-s

Conclusions Kinetic parameters can be obtained from isothermal hydrogenation using a microkinetic model but need to be validated with TGA and SSITKA data. Data are consistent with there being a range of atomic carbon reactivities, i.e. a range of binding energies. Rate of carbon hydrogenation is relatively slow on PC Fe compared to CO dissociation; thus carbon hydrogen-ation may be the RDS.

BYU Catalysis Group

Nonlinear Least Squares Regression