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Fischer-Tropsch Synthesis: An In-Situ TPR-EXAFS/XANES Investigation of the Influence of Group I Alkali Promoters on the Local Atomic and Electronic Structure of Carburized Iron/Silica Catalysts Mauro C. Ribeiro,† Gary Jacobs,† Burtron H. Davis,*,† Donald C. Cronauer,‡ A. Jeremy Kropf,‡ and Christopher L. Marshall‡ UniVersity of Kentucky Center for Applied Energy Research, 2540 Research Park DriVe, Lexington, Kentucky 40511, and Chemical Sciences and Engineering DiVision, Argonne National Laboratory, 9700 South Cass AVenue, CSE/205, Argonne, Illinois 60439-4837 ReceiVed: December 15, 2009; ReVised Manuscript ReceiVed: March 2, 2010

The promoting impact of alkali metals (i.e., Li, Na, K, Rb, Cs) on the carburization rate of Fe in Fe/Si catalysts was investigated by X-ray absorption spectroscopy. A multisample holder was used, allowing nearly simultaneous examination of the catalysts during activation in a CO/He mixture. With the white line intensity and shape as a fingerprint for oxidation state, TPR/XANES analysis enabled us to measure the relative composition of the different compounds as a function of the carburization time, temperature, and atomic number of the group 1 promoter. At the same time, TPR/EXAFS provided information on the changes in local atomic structure that accompanied the oxidation state changes. The rate of carburization increased in the following order: unpromoted < Li < Na < K ) Rb ) Cs. After 10 h of treatment the samples containing K, Rb, and Cs were completely carburized, and residual quantities of iron oxides were detected in both unpromoted and Li-promoted samples. The EXAFS spectra after carburization could be fitted well by considering a model containing Ha¨gg carbide and Fe3O4. After 10 h of CO/He treatment at 290 °C, the main component observed was Ha¨gg carbide. A model containing Ha¨gg and ε-carbides, and Fe3O4, was also investigated. However, the r-factor was not significantly impacted by including ε-carbide in the fitting, and the resulting contribution of ε-carbide in each catalyst from the model was virtually negligible. Selectivity differences are thus not likely due to changes in the carbide distribution. Rather, the alkali promoter increases the CO dissociative adsorption rate, resulting in an increase in the surface coverage of dissociated CO and an inhibition in the olefin readsorption rate. This in turn results in higher olefin selectivities, in agreement with previous catalytic tests. 1. Introduction Although Co, Fe, and Ru are active catalysts for the Fischer-Tropsch (FT) synthesis reaction, Co and Fe are most commonly employed. Co catalysts are currently used in the FT step during commercial gas-to-liquids (GTL) production. On the other hand, Fe catalysts are perhaps better suited for converting lower H2/CO ratio syngas (e.g., coal or biomassderived), due to their intrinsic water-gas shift (WGS) activity. Fe catalysts also tend to produce more oxygenate and olefin products relative to Co.1 Iron catalysts often contain structural promoters (e.g., SiO2), along with chemical promoters (e.g., Cu, Mn, and alkali elements). The structural modifiers promote the surface area of the catalyst under reaction conditions but also influence chemical properties.2 As for the chemical promoters, Cu increases the active surface area by facilitating the iron oxide reduction. Mn has been proposed to enhance olefin selectivities3 and is currently under study. The presence of the alkali metal increases both the activity and the selectivity for high molecular weight and olefinic products. Explanations for this enhanced activity/ selectivity are often given in terms of the basicity of the alkali promoter,4 which is suggested to cause an increase in the heat * Author to which correspondence should be addressed. † University of Kentucky Center for Applied Energy Research. ‡ Chemical Sciences and Engineering Division, Argonne National Laboratory.

of CO adsorption on the catalytic surface,5 thereby increasing the surface coverage of carbon species due to increased CO bond scission rates. This, in turn, is proposed to lead to higher conversion rates, as well as higher selectivities toward higher molecular weight hydrocarbon products. From this point of view, an expected order of promotion may be based on the relative basicity of the promoter (i.e., in the order Cs > Rb > K > Na > Li > unpromoted). Regarding the promoting effect of the group I elements on the CO conversion rate, it is important to consider that conversion also depends on the availability of H2; when low H2/CO ratio syngas is used, the availability of H2 relies on the WGS reaction rate. Ngantsoue-Hoc et al.6 observed that different alkali promoters added to Fe:Si displayed varying impacts on catalytic behavior, depending on the CO conversion rate. K was the best promoter, because it positively impacted the CO conversion rate at all conversion levels relative to the unpromoted catalyst. Using the unpromoted catalyst as a reference, addition of lighter alkali dopants (e.g., Li and Na) improved the CO conversion rate in the low to moderate conversion range; however, in the high conversion range, they were ineffective or even inhibited the CO conversion rate. CO conversion rates were lower relative to the unpromoted catalyst when heavier alkali dopants (e.g., Rb and Cs) were added over the entire range of conversion. In terms of product selectivities, the alkali promoter type greatly impacted the olefin-to-paraffin ratios, in which the order

10.1021/jp911856q  2010 American Chemical Society Published on Web 04/01/2010

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of effectiveness followed the basicity order Cs > Rb > K > Na > Li . unpromoted. Furthermore, the CO2 selectivity (i.e., indicative of the WGS rate) was impacted, such that higher WGS rates were observed in the cases of the K and Na promoters. Finally, while the unpromoted catalyst produced the highest light gas (i.e., C2-C4) selectivities and lowest gasoline + diesel range selectivities, the alkali-doped catalysts achieved higher gasoline + diesel selectivities and lower light gas selectivities. Among the alkali-doped catalysts, Na had the highest selectivity toward light gases (i.e., it was the worst), and K had the lowest light gas selectivity and the highest C19+ selectivity. These results illustrate the changes that the presence of alkali promoters can cause in both activity and selectivity of iron catalysts. One may expect that these changes are due to modifications in the surface basicity of the catalyst. On the other hand, one cannot discard the possibility that the alkali promoter may also influence the distribution of compounds in the active catalyst. The major compounds present in the carburized working catalyst are almost exclusively iron carbides and iron oxides, although the presence of γ-Fe has been observed in some cases for catalysts pretreated with synthesis gas;7 this is probably a minority bulk phase, if present, since it is not thermodynamically stable under reaction conditions. Iron oxide, present in the carburized working catalyst mainly as magnetite (Fe3O4), is believed to be located in the core of the catalyst particles and readily converts to iron carbides close to the surface of the particle.8 The active sites for FT are likely iron carbides, represented by different types.9 The most stable carbides are Ha¨gg (χ-Fe2.5C) and cementite (θ-Fe3C), which are formed above 190 and 300 °C in flowing CO, respectively, from pure Fe2O3 catalysts prereduced with H2.10 Metastable phases, such as η-Fe2C and ε-Fe2.2C are also present in the form of small particles (i.e., containing a high concentration of defects), when the activation with CO or synthesis gas is made at mild temperatures.10,11 There is evidence that not only the temperature but also the reaction time and the presence of promoters tend to change the relative composition of carbides. For example, as shown by Li et al.,8 the presence of either Cu or K was suggested to promote the formation of a larger quantity of small nuclei of Fe3O4 and iron carbides, thus contributing to the formation of smaller crystallites and, in turn, a higher density of active sites for FT. While χ-Fe2.5C tends to be the major carbide phase in larger particles, smaller particles are composed to a large extent of ε-Fe2.2C.12 In some reports, the deactivation of Fe catalysts is correlated with the conversion of metastable ε-Fe2.2C to χ-Fe2.5C;12,13 others suggest that deactivation is connected with oxidation and carbon layer formation, regardless of the composition of the carbides present.8 The carburized working catalyst should therefore be comprised of iron oxides and iron carbides, the latter component being a mixture of at least two carbides (χ-Fe2.5C and ε-Fe2.2C). There is not a consensus regarding whether the alkali promoter acts solely by changing the surface chemistry of the catalyst or by changing the relative composition of iron carbides and oxides. For this reason, many studies have been carried out attempting to correlate changes in the catalyst phase composition to changes in the nature of the promoter. Mo¨ssbauer spectroscopy has been largely used to identify Fe-containing compounds being formed in situ, although there are limitations with measurements at high temperatures.7 Alternatively, the structure of Fe-based catalysts under working conditions has been studied in situ using X-ray absorption spectroscopy (XAS).7,8,14,15 More recently, de Smit et al.7 followed the evolution of the catalyst composition after

Ribeiro et al. pretreating the catalyst by different activation procedures (i.e., using either H2 or CO/H2 activation) and also by carrying out FT; furthermore, different promoters were studied. Using Fe2O3-Cu-K-Si as the untreated starting catalyst, following temperature-programmed activation in syngas (i.e., CO/H2 to 450 at 2 °C min-1), the authors identified γ-Fe, along with χ-Fe2.5C. EXAFS data were qualitatively analyzed based on comparison with theoretically generated χ(k) functions. In a continuation of a study based on catalytic tests that have been carried out previously by our group,6 the objective of the present work was to investigate the promoting effect of group 1 alkali metals (Li, Na, K, Rb, Cs) on the Fe carburization rate and relative composition of the different Fe-containing phases present in the activated catalyst. In doing so, the carburization (activation under CO/He flow) of 100 Fe:4.6 Si:1.5 M catalyst samples (where M is Li, Na, K, Rb, or Cs, atomic basis) was followed by in situ TPR-EXAFS/XANES. Cu-free samples were selected, as we aimed to decouple the effect of the alkali promoter from that of Cu during activation. 2. Experimental Section The Fe:Si catalyst base was prepared by continuous precipitation of amorphous ferric oxyhydroxide from aqueous iron(III) nitrate nonahydrate (1.17 M) and concentrated ammonium hydroxide (15.6 M) in a continuously stirred tank reactor at pH 9.5. Hydrolyzed tetraethyl orthosilicate was added to the iron(III) nitrate solution to give the desired level of silicon (100 Fe:4.6 Si). The catalyst slurry was filtered continuously with one or more 6 in. rotary drum vacuum filters. The resulting catalyst cake was washed and filtered twice with a volume of distilled deionized water equal to the volume of the previous filtrate. The slurry was then oven-dried at 393 K and finally calcined in flowing air at 623 K and crushed to ∼300 mesh. Alkali metals (nitrate salts) were added by incipient wetness impregnation. Therefore, the final atomic ratios were 100 Fe:4.6 Si:1.5 M (where M is Li, Na, K, Rb, or Cs). The carburization of Fe/Si catalysts promoted with group I alkali metals under CO/He flow was investigated by XAS near the Fe K-edge. The temperature was increased for 2 h from room temperature to 290 °C and then held constant for 10 h. Successive X-ray absorption spectra were recorded during this time. The spectra (in energy space) were background subtracted and normalized with a Victoreen function and further normalized using a two-polynomial method with degree 1 for both the preand postedge regions. XANES spectra were then obtained by trimming the entire baseline corrected spectra from 7.07 to 7.202 keV (i.e., 90 eV above the Fe absorption edge). Changes in the phase composition of the catalyst samples during the carburization process were analyzed by least-squares linear combination (LC) fitting of the XANES region of the spectra over the range 7.1-7.162 keV using the WinXAS16 software. LC fitting was carried out following conversion of Fe2O3 to Fe3O4 (two reference spectra used) and Fe3O4 to iron carbides (again, two reference spectra used). The standards used for the LC method were: (1) Fe3O4, the spectrum recorded for each catalyst at the temperature in which the white line absorption achieved a minimum before the sample started to become carburized; (2) Fe3O(4-x), the spectrum recorded for each catalyst at the temperature where the white line absorption reached a maximum; and (3) FeCx, the spectrum of the most carburized catalyst (i.e., the Cs promoted one following carburization), which exhibited the lowest white line intensity and was nearly completely carburized. Though not shown for the sake of

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TABLE 1: Atomic Positions Using Cartesian Coordinates and Corresponding Wyckoff Letters, Structures Shown in Figure 9 carbide compound

atom

Fe 4c Fe′ 8d C 4c Ha¨gg carbide Fe 8f Fe′ 8f Fe″ 4e C 8f ε-carbide Fe 6g C 2c C′ 2d η-carbide Fe 4g C 2a magnetite Fe 8a Fe′ 16d O 32e

color (see Figure 9)

cementite

white light gray dark gray black white black gray

white gray black

TABLE 2: Path Parameters Generated by FEFF (Single Scattering) for the Carbides (Structures Shown in Figure 9) Ha¨gg carbide atom interaction no. of degeneracies distance (Å)

Cartesian coordinates (x1, y1, z1) ) (0.036, 0.250, 0.852) (x2, y2, z2) ) (0.186, 0.063, 0.328) (x3, y3, z3) ) (0.890, 0.250, 0.450) (x1, y1, z1) ) (0.092, 0.091, 0.421) (x2, y2, z2) ) (0.207, 0.577, 0.302) (x3, y3, z3) ) (0.000, 0.566, 0.250) (x4, y4, z4) ) (0.109, 0.300, 0.082) (x1, y1, z1) ) (0.333, 0.000, 0.000) (x2, y2, z2) ) (0.333, 0.667, 0.250) (x3, y3, z3) ) (0.333, 0.667, 0.750) (x1, y1, z1) ) (0.671, 0.250, 0.000) (x2, y2, z2) ) (0.000, 0.000, 0.000) (x1, y1, z1) ) (0.125, 0.125, 0.125) (x2, y2, z2) ) (0.500, 0.500, 0.500) (x3, y3, z3) ) (0.255, 0.255, 0.255)

brevity, overlays of all the catalyst spectra (i.e., unpromoted, Li, Na, etc.) corresponding to the maximum fraction of Fe3O4 showed virtually no differences; the same was true for the catalyst spectra corresponding to the maximum fraction of Fe3O(4-x). EXAFS spectra were treated using the WinXAS16 software. After background removal and normalization (previously described), the spectra were converted to k-space and background subtracted in k-space using a cubic spline fit. The spectra were then truncated within the range of 2.5-12 Å-1. To obtain the radial distribution function, the Fourier transform was carried out on the spectra in k-space making use of a Bessel window; a k-weighting of 1 was employed, to emphasize scattering by the light atoms (i.e., C and O).17 The first coordination shells were fitted in the k- and r-ranges of 2.5-10 Å-1 and 1.0-3.0 Å, respectively. The experimental EXAFS data (k1χ(k)) were fitted with theoretically generated spectra derived from structural models, assuming the presence of χ-Fe2.5C (Ha¨gg carbide), ε-Fe2.2C (epsilon carbide) and Fe3O4 (magnetite). These phases have been detected in active carburized catalysts by Mo¨ssbauer spectroscopy studies9 and are reasonable considering XRD investigations of the carburization process.11 With the use of the ATOMS software,18 structural information of each one of the phases (Table 1) was transformed into spatial coordinates, which were then employed by the software FEFF version 6.0119 to calculate the scattering paths (Tables 2 and 3). The scattering paths specified in Tables 2 and 3 were used as inputs for the software FEFFIT20 to generate theoretical χ(k), which were compared to their experimental counterparts. Two models were considered: one comprising the mixing of carbide phases (Ha¨gg, epsilon) and Fe3O4, and the other model considering a mixture of Ha¨gg carbide and Fe3O4. For the structural fitting, scattering paths up to 3.5 Å were considered. Structural fitting parameters used in the model included: two “R” global lattice expansion coefficients, one for both Fe-C and Fe-Fe in the carbides and the other for both Fe-O and Fe-Fe in the Fe3O4 phase; ∆e0, an overall energy shift applied to each path; σ21 and σ22, the Debye-Waller factors for the carbides (Ha¨gg and ε) and for Fe3O4, respectively; and the amplitude coefficients that account for the fraction of each phase (Ha¨gg, epsilon, and Fe3O4) and changes in the amplitude due to particle size (i.e., surface atoms have lower degrees of coordination). The E0 value was assumed to be the first peak in the first derivative after the pre-edge and corresponded to a value of ∼7.122 eV for the final carburized catalysts; the ordinate

ε-carbide

Fe2 Fe1 Fe1 Fe2 Fe3 Fe3 Fe2 Fe3 Fe2 Fe1 Fe1 Fe1 Fe3 Fe1 Fe2 Fe3 Fe1 Fe3 Fe2 Fe2 Fe1 Fe1 Fe2 Fe1 Fe2 Fe1 Fe2 Fe1 Fe3 Fe1 Fe2 Fe1 Fe2 Fe1 Fe1 Fe2 Fe3 Fe2 Fe1 Fe1 Fe1 Fe1

Fe-C Fe-C Fe-C Fe-C Fe-C Fe-C Fe-C Fe-Fe Fe-Fe Fe-C Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-Fe Fe-C Fe-C Fe-Fe Fe-Fe Fe-C Fe-C Fe-Fe Fe-Fe Fe-C

1.0 1.0 1.0 1.0 2.0 2.0 1.0 2.0 1.0 1.0 1.0 1.0 2.0 1.0 2.0 2.0 1.0 2.0 2.0 1.0 2.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 2.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 2.0 1.0 4.0 6.0 6.0 4.0

1.9309 1.9433 1.9766 1.9921 2.0022 2.0388 2.3475 2.3616 2.3616 2.4964 2.4958 2.4965 2.4965 2.5415 2.5502 2.5762 2.5901 2.5901 2.6010 2.6170 2.6378 2.6428 2.6428 2.6463 2.6463 2.6737 2.6737 2.6937 2.6937 2.6988 2.6988 2.7788 2.7788 2.8702 3.4258 3.4642 3.4642 3.4887 1.9259 2.6951 2.7520 3.3594

TABLE 3: Path Parameters Generated by FEFF (Single Scattering) for Magnetite (Structure Shown in Figure 9) magnetite

atom

interaction

no of degeneracies

distance (Å)

Fe1 Fe2 Fe2 Fe1 Fe2 Fe1

Fe-O Fe-O Fe-Fe Fe-Fe Fe-Fe Fe-O

1.0 1.0 1.0 1.0 2.0 2.0

1.8861 2.0602 2.9684 3.4807 3.4807 3.4933

corresponding to this abscissa was ∼0.7. In order to constrain the fits to achieve more physically meaningful results, the data sets were fitted simultaneously. Due to limitations in the fitting program, only five of the data sets could be fitted simultaneously. Considering that Rb and Cs exhibited virtually identical reduction behavior in TPR-XANES, five data sets chosen included the unpromoted, and catalysts promoted with Li, Na, K, and Rb. To further constrain the model, global parameters (i.e., over all samples) were used for the isotropic lattice expansion (R) and Debye-Waller factor (σ2) parameters for carbide (i.e., combined epsilon and Ha¨gg) and oxide (i.e., Fe3O4) fractions. Local parameters (i.e., for each sample) were used for the energy shift (e0) and amplitude function multipliers of each component.

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Figure 2. (a) Dependence of the XANES white line intensities with the carburization temperature and on the alkali type in promoted Fe catalysts. (b) Temperatures of conversion of Fe3O4 to Fe3O4-x as a function of the alkali promoter.

Figure 1. XANES spectra of the temperature-programmed carburization of the alkali-promoted catalysts. Temperatures ranged from 25 to 290 °C in 2 h.

3. Results 3.1. XANES. Figure 1 shows the XANES spectra of the samples promoted with the different alkali metals. There are some common features observed during the evolution of the XANES spectra for each catalyst as the carburization temperature was increased from room temperature to 290 °C. For all the spectra at temperatures below 170 °C, a sharp pre-edge peak located at 7114 eV is observed. At higher temperatures, the preedge peak disappears, giving rise to a pre-edge shoulder, located at higher energies. Hematite (R-Fe2O3) exhibits a corundum structure in which the O2- ions are hexagonal close packed and the Fe3+ ions occupy octahedral sites.21 In γ-Fe2O3 (hematite), the O2- is cubic and the Fe3+ randomly occupies both octahedral and tetrahedral sites.21 As the pre-edge absorption feature in Fe K-edge XANES spectra originates from a 1s to 3d transition in the 3d metallic ions located in tetrahedral coordination symmetries,22 the presence of a pre-edge peak in the sample at temperatures below 170 °C indicates that the calcined sample was at least partially composed of γ-Fe2O3. In Fe3O4 the O2ions are distributed in a structure with cubic symmetry, in which Fe2+ ions occupy the octahedral interstices, whereas half the Fe3+ ions occupy octahedral sites, and the other half occupy tetrahedral sites. Therefore, the decrease in the intensity of the pre-edge feature at temperatures above 170 °C suggests that Fe2+ ions (which tend to occupy octahedral sites in the Fe3O4 structure) are formed from the reduction of Fe3+, during the reduction from Fe2O3 to Fe3O4. Figure 2 shows the evolution of the white line at temperatures between 100 and 290 °C, the temperature range where the largest changes in the white line peak intensity are observed. The absorptivity reaches a minimum at approximately 200 °C

and then a maximum at around 230 °C before an abrupt decrease at temperatures above 250 °C. The stepwise reduction of Fe2O3, involves the formation of Fe3O4 (magnetite) and an oxygendeficient iron oxide, denoted here as Fe3O(4-x). At low temperatures, as we have already seen, the predominant phase is Fe2O3. At temperatures above 150 °C, Fe2O3 undergoes reduction to Fe3O4, which presents a lower white line intensity as the Fe(II)/ Fe(III) ratio increases. At temperatures higher than 200 °C, an increase in the white line absorptivity occurs due to the formation of Fe3O(4-x). After the white line achieves a maximum at around 230 °C, it then decreases, because of the formation of iron carbides, as the Fe-C bond has much more covalent character than the Fe-O bond.23 Comparison of the curve for the unpromoted sample with those of the alkali metal promoted samples (Figure 2) reveals some differences. All the promoted catalysts present well-defined white line absorptivity minima (at around 200 °C), whereas this is not observed in the case of the unpromoted sample. As shown in Figure 2b, the temperature in which the maximum is observed decreases according to the order: unpromoted > Li > Na > K > Rb ∼ Cs. The presence of the promoter, even in small quantities, influences the reduction process of the catalyst. As for the maxima observed, it is clear that the order in ease of reduction from Fe3O4 to Fe3O(4-x) improves with increasing promoter basicity. Taking the temperatures in which the white line absorption is maximum and minimum, respectively, as the Fe3O4 and Fe3O(4-x) standards, we performed LC fittings of the XANES spectra in order to calculate the phase composition as a function of the reduction/carburization temperature. Some differences were previously noted between the bulk (powder) and catalyst Fe2O3 and Fe3O4 electronic and local structures;24 these were significant enough such that we did not use the bulk references in LC fittings of catalysts. The bulk compounds exhibit longer range order, and moreover, the catalysts are influenced by the structural promoter, Si. Both of these factors led us to use internal spectra (i.e., those most

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Figure 5. (top) Molecular orbital bonding of CO to a metal site according to the Blyholder model. Charge is donated from carbon to the metal by the 5σ bond and stabilized by back-donation from the metal d-orbitals to the 2π* antibonding molecular orbitals, weakening the CO bond.29 (bottom) Alkali addition has been proposed to strengthen the M-C bond and weaken the CO bond by increased back-donation.

Figure 3. Comparison between the bulk and internal Fe2O3 and Fe3O4 XANES and EXAFS transform magnitude spectra. Bulk spectra were taken from Fe2O3 and Fe3O4 powders. Internal Fe2O3 and Fe3O4 spectra were taken from the Fe:Si catalyst samples, respectively, before carburization and at a carburization extent in which the white line achieved a minimum. The Na-promoted sample is shown (Fe3O4 recorded at 195.9 °C and Fe3O(4-x) at 250.9 °C). Figure 6. Concentration profiles obtained from least-squares linear combination fitting of alkali-promoted Fe/Si catalysts with (filled symbols) Fe3O(4-x) and (open symbols) FeCx reference compounds during carburization at 290 °C.

precedent for following this procedure. In carrying out LC fittings of Co/Al2O3 catalysts,25 the white line of CoO along the TPR trajectory exhibited a higher intensity than the bulk CoO reference. This necessitated the use of the catalyst CoO spectrum obtained from the TPR experiment as a reference for LC fitting. TABLE 4: Temperatures Where the Internal Standards for Fe3O4 and Fe3O(4-x) Were Recorded

Figure 4. Concentration profiles obtained from least-squares linear combination fitting of alkali promoted Fe/Si catalysts with (filled symbols) Fe3O(4-x) and (unfilled symbols) FeCx reference compounds as a function of temperature.

resembling bulk Fe2O3 and Fe3O4, as can be seen in Figure 3), along the trajectories as references for LC fittings. There is

catalyst

T (°C) at which highest fraction of Fe3O4 was recorded

T (°C) at which highest fraction of Fe3O(4-x) was recorded

unpromoted Li Na K Rb Cs

192.7 194.8 195.9 190.5 189.5 187.2

257.3 259.4 250.9 234.2 232.0 230.9

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Figure 7. (a) Fourier transform magnitudes of the k1-weighted χ(k) recorded during temperature-programmed carburization. (b) Fourier transform magnitudes of the k1-weighted χ(k) recorded during the 10 h temperature hold at 290 °C.

The nature of the alkali promoter did not significantly influence the transition from Fe2O3 to Fe3O4. Also, as shown in Table 4, the reduction temperature range in which the maximum fraction of Fe3O4 was detected was narrow. However, it is clear that the nature of the promoter directly influences the reduction of Fe3O4 to Fe3O(4-x), as can be seen in Figure 4. The reduction to Fe3O(4-x) takes place at lower temperatures in the presence of K, Rb, or Cs, whereas the composition profiles are quite the same when the Na, Li, and the unpromoted catalyst are compared. In agreement with this, the temperature in which the maximum amount of O-deficient Fe3O4 is formed decreases with increasing promoter basicity (Table 4). Finally, during carburization (Fe3O(4-x) to FeCx), the third and most important step from the standpoint of catalyst activation, the order of promotion is Cs > Rb > K > Na > Li ) unpromoted, which correlates closely with the basicity of the alkali metal promoter. The olefin selectivity also changes in the same manner as the carburization rate as a function of the type of alkali promoter, with the most active promoters being those that are capable of forming more basic salts and oxides.6 According to the Blyholder model,26 nonbonding 5σ levels of the CO molecule interact with empty d levels of atoms located at the metallic surface. There is also an interaction between populated d levels of the metal and 2π* (antibonding) empty levels of the CO molecule, as illustrated in Figure 5. While the former interaction causes an increase in the absorption rates of CO on certain metallic surfaces, the latter

interaction contributes to the C-O bond scission of the adsorbed CO. It has been shown that the alkali promoter increases CO adsorption heats on Fe catalysts.4,5 At the same time backdonation by the alkali decreases the C-O bond frequency, and therefore the C-O bond strength,27-29 in certain metals. Therefore, it has been argued that the presence of alkali leads to a promotion by charge transfer from the promoter to at least its immediate vicinity. An increase in the coverage of dissociatively adsorbed CO from the decreased C-O bond strength could explain the higher carburization rates observed with alkali addition and, more specifically, as a function of increasing alkali basicity. Such a higher CO surface coverage would likely decrease the olefin hydrogenation rate, due to inhibition of olefin readsorption or a decrease in the surface hydrogen pool. Figure 6 shows the evolution of the composition of the samples throughout the 290 °C hold for 10 h. Although the promoted catalysts begin this period with different relative concentrations of Fe3O(4-x) and Fe-carbides, all the catalysts tend to reach comparable extents of carburization at the end of the 10 h treatment. The Li and unpromoted catalyst samples, though, still possess the highest concentration (though small) of iron oxides following activation. 3.2. EXAFS. In order to investigate the effect of the presence of different promoters on the structure of the different phases formed after long-term carburization treatment, the EXAFS spectra were also analyzed.

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Figure 8. Temperature range of specific iron carbides observed in XRD studies.11

Figure 7a shows the k1 weighted Fourier transform magnitude spectra of samples promoted with different alkali elements at different temperatures (Figure 7a) during the ramp, and also during the 10 h temperature hold at 290 °C (Figure 7b). It can be seen that at lower temperatures, there are two major peaks located between 1 and 2 Å and between 2 and 3 Å, respectively. The first peak corresponds to the Fe-O distance in the first Fe-O coordination shell around Fe (d ) 1.9 - 2.1 Å). The second peak corresponds to scattering by Fe atoms located in the first Fe-Fe coordination shell (2.9-3.5 Å). As the temperature increases, both peaks decrease, and at temperatures close to 290 °C, the peaks transform into a less intense, wider peak, located between 1 and 3 Å. This represents the stepwise reduction of Fe2O3 to Fe3O4, and the reduction/carburization of Fe3O4 to form a mixture of iron carbides and iron oxide. Reduction/carburization takes place at lower temperatures for the K-, Rb-, and Cs-promoted catalyst samples relative to the unpromoted catalyst and the catalysts doped with lighter alkali elements (i.e., Li and Na). Though the catalysts with heavier alkali promoters carburized more rapidly (Figure 7b), all the EXAFS Fourier transform magnitude spectra look qualitatively similar after 10 h of carburization. In order to determine the phase composition in the carburized samples, EXAFS fitting of the spectra of samples following 10 h carburization at 290 °C were made. It is certainly true that higher temperatures induce larger deviations of atoms from their equilibrium positions (i.e., higher Debye-Waller factors); nevertheless, fittings were carried out to ascertain whether a theoretical model based on a mixture of iron carbide and Fe3O4 could satisfactorily fit the experimental results of carburized catalysts. Earlier Mo¨ssbauer results indicated that carburized Fe-based Fischer-Tropsch synthesis catalysts were mixtures of Ha¨gg and ε-carbides, and magnetite as well. With XRD studies as a guide,11 while ε-carbide is typically formed at lower temperatures ( Rb > K > Na > Li ) unpromoted. 2. Reasonable fittings of EXAFS experimental spectra were obtained using theoretical spectra derived from a model that

included Ha¨gg carbide and Fe3O4. According to our fittings, the main phase present in each catalyst following carburization was the Ha¨gg-carbide (χ-Fe2.5C), with only a minor contribution from Fe3O4 (i.e., 8-13% contribution to the amplitude function). Adding ε-carbide to the fitting did not significantly alter the fit, and the contribution from ε-carbide was virtually negligible for this case (carburization in CO/ He at 290 °C for 10 h). 3. As has been previously demonstrated, olefin selectivity is likely correlated to the basicity of the promoter in the order Cs > Rb > K > Na > Li > unpromoted. In the present work we have shown by XANES that the carburization rate increases in the same order. The results suggest that the presence of the alkali

Fischer-Tropsch Synthesis promoter leads to an increase in the CO dissociative adsorption rate. This in turn results in an increase in the adsorbed CO surface coverage and likely inhibits the olefin readsorption rate, resulting in higher olefin selectivities. Since the Ha¨gg carbide was the most prevalent, differences in product selectivity do not apparently reflect a difference in the carbide distribution. Acknowledgment. The work carried out at the CAER was supported in part by funding from a seed grant from the Kentucky Governor’s Office of Energy Policy (Solicitation 08GOEP-02), as well as the Commonwealth of Kentucky. Argonne’s research was supported in part by the U.S. Department of Energy (DOE), Office of Fossil Energy, National Energy Technology Laboratory (NETL). The use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. MRCAT operations are supported by the Department of Energy and the MRCAT member institutions. References and Notes (1) Dry, M. E. Handbook of Heterogeneous Catalysis; Ertl, G., Knozinger, H., Schuth, F., Weitkamp, J., Eds.; Wiley: Chichester, 2008; Vol. 6, p 2965. (2) Sirimanothan, N.; Hamdeh, H. H.; Zhang, Y.; Davis, B. H. Catal. Lett. 2002, 82, 181. (3) Hindermann, J. P.; Hutchings, G. J.; Kiennemann, A. Catal. ReV.Sci. Eng. 1993, 35 (1), 1. (4) Dry, M. E.; Oosthuizen, G. J. J. Catal. 1968, 11, 18. (5) Dry, M. E.; Shingles, T.; Boshoff, L. J.; Oosthuizen, G. J. J. Catal. 1969, 15, 190. (6) Ngantsoue-Hoc, W.; Zhang, Y.; O’Brien, R. J.; Luo, M.; Davis, B. H. Appl. Catal., A 2002, 236, 77. (7) de Smit, E.; Beale, A. M.; Nikitenko, S.; Weckhuysen, B. M. J. Catal. 2009, 262, 244. (8) Li, S.; Ding, W.; Meitzner, G.; Iglesia, E. J. Phys. Chem. B 2002, 106, 85. (9) Niemantsverdriet, J. W.; van der Kraan, A. M.; van Dijk, W. L.; van der Baan, H. S. J. Phys. Chem. 1980, 84, 3363.

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