CO2 HYDROGENATION AND FISCHER-TROPSCH TO OLEFINS CATALYST

Abstract

The invention relates to nanocatalysts composed of iron oxide nanoparticles supported on porous interconnected carbon nanosheets (CNS) fabricated from the carbonization of potassium citrate, that are remarkably active for CO.sub.2 hydrogenation and Fischer-Tropsch to Olefins (FTO) synthesis, as well as a method for directly converting CO.sub.2 and H.sub.2 to C.sub.2-C.sub.4 olefins and direct FTO synthesis.

Claims

1. A nanocatalyst, comprising: a support structure, comprising: a plurality of porous interconnected carbon nanosheets, and a potassium promoter embedded in the carbon nanosheets; and a plurality of iron oxide nanoparticles supported on the support structure.

2. The nanocatalyst of claim 1, wherein the iron oxide nanoparticles comprise Fe.sub.3O.sub.4.

3. A method of forming a nanocatalyst, comprising: preparing a support structure, comprising: interconnecting a plurality of porous carbon nanosheets; and embedding a potassium promoter in the carbon nanosheets; depositing a plurality of iron oxide nanoparticles on the support structure; reducing the plurality of iron oxide nanoparticles to metallic iron; and transforming the metallic iron into iron carbide.

4. The method of claim 3, wherein the depositing step comprises an iron precursor.

5. The method of claim 4, wherein the iron precursor comprises ammonium iron citrate.

6. The method of claim 3, wherein the preparing step comprises carbonization of potassium citrate.

7. The method of claim 3, wherein the reducing and transforming steps comprise reducing Fe.sub.3O.sub.4 nanoparticles to metallic iron nanoparticles and transforming to active Fe.sub.5C.sub.2, respectively.

8. The method of claim 7, wherein the reducing step comprises H.sub.2 activation.

9. The method of claim 7, wherein the transforming step comprises exposing the metallic iron nanoparticles to syngas.

10. The nanocatalyst of claim 1, wherein said nanocatalyst is used in CO.sub.2 hydrogenation and Fischer-Tropsch to Olefins synthesis.

11. The nanocatalyst of claim 10, wherein said nanocatalyst is reusable repeatedly without degradation in catalytic performance for at least 500 hours of cumulative TOS.

12. A method of preparing C.sub.2-C.sub.4 olefins, comprising: fabricating a nanocatalyst, comprising: preparing a support structure, comprising: obtaining a plurality of carbon nanosheets; interconnecting the plurality of carbon nanosheets; carbonizing a potassium precursor; and dispersing potassium promoter throughout the plurality of carbon nanosheets; and depositing a plurality of iron oxide nanoparticles on the support structure; initiating H.sub.2 activation for reducing the plurality of iron oxide nanoparticles to metallic iron nanoparticles; and exposing the metallic iron nanoparticles to carburization for transforming the metallic iron into active iron carbide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] A full understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings.

[0023] FIG. 1 includes views (a), (b), (c) and (d), wherein 1(a) is a scanning electron microscopy (SEM); 1(b) is transmission electron microscopy (TEM) images of the Fe.sub.xO.sub.y/CNS catalyst, (inset) size distribution of Fe.sub.xO.sub.y nanoparticles from analysis of >800 nanoparticles; 1(c) is a XRD pattern of Fe.sub.xO.sub.y/CNS catalyst compared with the standard reference of Fe.sub.3O.sub.4 (PDF 01-076-1849); and 1(d) is a high resolution TEM (HRTEM) image of Fe.sub.xO.sub.y nanoparticles showing lattice fringes consistent with the Fe.sub.3O.sub.4 phase;

[0024] FIG. 2 includes views (a) and (b), wherein 2(a) is a plot of the catalytic performance of Fe.sub.xO.sub.y/CNS with iron time yield (FTY) (circles, right y axis), CO conversion (%, triangles, lefty axis); and 2(b) is a plot of light olefins selectivity (wt. %, squares, left y axis) and olefin to paraffin ratio, O/P, (diamonds, right y axis) as a function of time on stream (TOS) under reaction condition of 350° C., 20 bar, H.sub.2/CO=1 and WHSV=30,000 cm.sup.3(STP)/(g.sub.cat.Math.h);

[0025] FIG. 3 includes views (a) and (b), wherein 3(a) is a TEM image of fresh Fe.sub.xO.sub.y/carbon nanotube (CNT) catalyst, (inset) particle size distribution; and 3(b) is a plot of FTY (circles, left y-axis) and O/P ratio (diamonds, right y-axis) of Fe.sub.xO.sub.y/CNT for FTO as a function of time on stream;

[0026] FIG. 4 includes views (a), (b), (c) and (d), wherein 4(a) is a TEM image of spent Fe.sub.xO.sub.y/CNS catalysts, (inset) particle size distribution of spent iron nanoparticles based on >500 nanoparticles; 4(b) is a HRTEM image of an isolated iron carbide/iron oxide core/shell nanoparticle with lattice spacing in the core consistent with that of Fe.sub.5C.sub.2; 4(c) shows .sup.57Fe Mössbauer spectra (hashed lines) of the fresh Fe.sub.xO.sub.y/CNS catalyst with the overall spectral simulation (solid line) and a magnetic subcomponent simulation representing the ferrous sites of Fe.sub.3O.sub.4 (solid line); and 4(d) shows the spent Fe.sub.xO.sub.y/CNS catalyst with the overall spectral simulation (solid line) as well as the spectral simulation representing x-Fe.sub.5C.sub.2 (solid line) and an additional iron carbide phase Fe.sub.xC (solid line); the arrows indicate the spectral component similar to the iron oxide component in the fresh catalyst;

[0027] FIG. 5 includes views (a) and (b) including Fourier transformed Fe K-edge extended x-ray absorption fine structure (EXAFS) data of fresh and reacted 5(a) Fe.sub.xO.sub.y/CNS and 5(b) Fe.sub.xO.sub.y/CNT samples at TOS=0 h (fresh), 0.5 h, 1 h, 2 h, 3 h, 4 h, 7 h and 10 h in H.sub.2/CO (1:1) syngas at 20 bar and 350° C.; the inset shows the corresponding EXAFS spectra in k-space; the solid lines represent experimental data, and the circles are fitted spectra; the vertical dashed lines indicate the feature of Fe—Fe coordination from Fe.sub.5C.sub.2;

[0028] FIG. 6 includes views (a) and (b), wherein 6(a) is the comparison of the evolution of the coordination number (CN) of Fe—Fe scattering from Fe.sub.5C.sub.2 composition in the Fe.sub.xO.sub.y/CNS (circles) and Fe.sub.xO.sub.y/CNT (squares) catalysts as a function of TOS; and 6(b) is Fourier transformed Fe K-edge EXAFS spectra of H.sub.2 reduced Fe.sub.xO.sub.y/CNS and H.sub.2 reduced Fe.sub.xO.sub.y/CNT catalysts; the most significant distinction between the two spectra is the two additional peaks at 2.2 Å and 4.4 Å observed in reduced Fe.sub.xO.sub.y/CNS, which correspond to the Fe—Fe bonds from metallic iron; in contrast, the reduced Fe.sub.xO.sub.y/CNT catalyst is mostly comprised of oxidized Fe species;

[0029] FIG. S1 is x-ray diffraction (XRD) pattern of carbon nanosheets;

[0030] FIG. S2 includes views (a) and (b) wherein S2(a) and S2(b) show additional high resolution transmission electron microscopy (HRTEM) images of fresh Fe.sub.xO.sub.y/CNS catalysts;

[0031] FIG. S3 includes views (a) and (b) wherein S3(a) is Raman spectrum and S3(b) is x-ray photoelectron spectroscopy (XPS) C is spectrum and fitting analysis of the fresh Fe.sub.xO.sub.y/CNS catalyst;

[0032] FIG. S4 includes views (a), (b), (c) and (d) wherein is S4(a) is scanning transmission electron microscopy (STEM) image and energy dispersive x-ray analysis (EDX) mapping of fresh Fe.sub.xO.sub.y/CNS catalysts showing the distribution of Fe, O and K elements, i.e., S4(b), S4(c) and S4(d), respectively, on the CNS support (scale bars, 100 nm);

[0033] FIG. S5 is in situ XRD patterns of fresh Fe.sub.xO.sub.y/CNS catalyst under 4% H.sub.2/Ar (20 SCCM) reduction while heating from 25° C. to 400° C.; temperature ramp rate was 5° C./min; the temperatures were held for 10 min to reach the stable state; the 10 scans at 400° C. were collected with 30 min interval; the scans were taken with a step size of 0.017° and scan rate of 200 s/step; Al.sub.2O.sub.3 signal was from the sample holder;

[0034] FIG. S6 is XRD pattern of Fe.sub.xO.sub.y/CNT catalyst and blank CNT with standard reference of Fe.sub.3O.sub.4 (PDF 01-076-1849);

[0035] FIG. S7 includes views (a) and (b) wherein S7(a) is Raman spectrum and S7(b) is C is XPS spectrum of Fe.sub.xO.sub.y/CNT catalyst;

[0036] FIG. S8 includes views (a) and (b) wherein S8(a) is CO conversion and S8(b) is O/P ratio of Fe.sub.xO.sub.y/CNS and Fe.sub.xO.sub.y/CNT catalysts for FTO as a function of time on stream;

[0037] FIG. S9 includes views (a), (b), (c) and (d) wherein S9(a) is a high-angle annular dark field-STEM image of an iron-based nanoparticle in the spent Fe.sub.xO.sub.y/CNS catalyst; EDX elemental mapping images of iron in S9(b), carbon in S9(c) (dashed line shows the outline of the Fe.sub.5C.sub.2 core) and oxygen elements in S9(d), because of the carbon support used for the catalysts, additional C signal is seen in areas outside of the nanoparticle;

[0038] FIG. S10 is laboratory based Fe 2p XPS spectra of spent Fe.sub.xO.sub.y/CNS catalyst as a function of sputtering time; the Fe.sub.5C.sub.2 feature at 708.0 eV becomes increasingly pronounced as the surface oxide layer is removed by sputtering; the features at 711.3 eV and 710.4 eV correspond to Fe.sup.3+ and Fe.sup.2+ species, respectively; the very weak Fe features in the first spectrum at 0s are likely due to the surface coating of catalysts with contaminants such as carbon; as sputtering removed the surface coating, the spectral features become clearer;

[0039] FIG. S11 includes views (a), (b), (c) and (d) showing Fe K edge x-ray absorption near edge structure (XANES) profiles of Fe.sub.xO.sub.y/CNS (in S11(a)) and Fe.sub.xO.sub.y/CNT (in S11(b)) fresh and reacted samples at different TOS as indicated by the legend; iron oxides reference standards including Fe.sub.3O.sub.4, Fe.sub.2O.sub.3 and FeO are also shown in S11(a); S11(c) and S11(d) show the zoom-in and overlaid spectra in S11(a) and S11(b), respectively, illustrating the spectral changes in pre-edge and white line as a function of TOS; the arrow at pre-edge shows the appearance and evolution of features associated with iron carbide phase, and the arrow at line points to the intensity decrease of oxide phase with TOS;

[0040] FIG. S12 is Fourier transformed Fe K-edge EXAFS data of fresh Fe.sub.xO.sub.y/CNS samples; the inset shows the corresponding EXAFS spectra of these samples in k-space; the lines represent experimental data, and circles are fitted spectra; a single model magnetite Fe.sub.3O.sub.4 is applied to fit the spectra of the fresh catalysts (Table S2);

[0041] FIG. S13 includes views (a) and (b) showing relative amplitudes of iron oxide and iron carbide phases indicated by EXAFS coordination numbers (CN) of neighboring atoms in reacted Fe.sub.xO.sub.y/CNS (in S13(a)) and Fe.sub.xO.sub.y/CNT (in S13(b)) samples with TOS from 0.5 h to 10 h; contributions from neighboring Fe-O coordination shell from Fe.sub.3O.sub.4, and Fe—C and Fe—Fe neighboring shells from Fe.sub.5C.sub.2 crystal structures are presented;

[0042] FIG. S14 includes views (a), (b), (c) and (d) showing high-angle annular dark field-STEM image of S14(a) a Fe-based nanoparticle in reduced Fe.sub.xO.sub.y/CNS catalyst showing a core-shell structure; EDX mapping images of Fe, O and C in S14(b), S14(c), S14(d), respectively;

[0043] FIG. S15 is H.sub.2-temperature programmed reduction (TPR) profiles of Fe.sub.xO.sub.y/CNS and Fe.sub.xO.sub.y/CNT catalysts; the inverse peak at 700° C. of Fe.sub.xO.sub.y/CNS profile is due to the methanation of CNS support that typically occurred in the carbon supported iron catalysts; a lower onset reduction temperature˜200° C. and a larger reduction peak below 400° C. appear in the Fe.sub.xO.sub.y/CNS catalyst; the reduction of Fe.sub.xO.sub.y particles is less efficient on CNTs;

[0044] FIG. S16 is synchrotron XPS K 2p core-level spectra of fresh Fe.sub.xO.sub.y/CNS catalysts;

[0045] FIG. X is a bar graph that shows CO.sub.2 hydrogenation catalytic performance for a Fe.sub.xO.sub.y/CNS catalyst, in accordance with certain embodiments of the invention;

[0046] FIG. Y is a bar graph that shows C.sub.1-C.sub.5 hydrocarbon product distribution from CO.sub.2 hydrogenation, in accordance with certain embodiments of the invention; and

[0047] FIG. Z is a plot that shows CO.sub.2 hydrogenation activity of the Fe.sub.xO.sub.y/CNS catalyst as a function of time on stream (TOS), in accordance with certain embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0048] The invention relates to nanocatalysts composed of iron oxide nanoparticles supported on carbon nanosheet. In certain embodiments, the carbon nanosheet includes a plurality of porous interconnected carbon nanosheets (CNS) fabricated from the carbonization of potassium citrate, that are remarkably active for CO.sub.2 hydrogenation and Fischer-Tropsch to Olefins (FTO) synthesis. The inventors have found that FTO catalysts according to the invention have a very high iron time yield, e.g., 1882 μmol.sub.CO/g.sub.Fe.Math.s, light olefins selectivity, e.g. 41%, and extended stability, e.g., over 100 hours of testing.

[0049] Detailed characterization of the inventive nanocatalysts has shown that the CNS support facilitates iron oxide, e.g., Fe.sub.3O.sub.4, reduction to metallic iron, leading to efficient transformation to an active iron carbide phase during FTO reaction. For example, the iron oxide catalyst is transformed to metallic iron nanoparticles during H.sub.2 activation step and then to active iron carbide upon syngas exposure. The K-promoted CNS is effective to stabilize the metallic iron particles during H.sub.2 reduction, which enhances formation of iron carbide under FTO reaction conditions, and the efficient carburization of the iron oxide/CNS catalyst results in high catalytic activity, selectivity and stability.

[0050] In certain embodiments, prior to FTO reaction, the Fe.sub.3O.sub.4/CNS catalyst is reduced in H.sub.2 to form FeO and α-Fe phases, and further completely transformed to α-Fe metal. Under FTO conditions, the metal α-Fe is readily carburized and forms the active iron carbide species Fe.sub.5C.sub.2.

[0051] The invention includes the novel carbon nanosheet support material with embedded potassium (K) promoter. The K-promoted carbon nanosheets (CNS) are used as support for iron-based FTO catalysts which allows the formation of Fe.sub.3O.sub.4 nanoparticles on the surface. The K-promoted CNS catalyst effectively provides for the direct conversion of CO.sub.2 and H.sub.2 to light olefins, and also for direct FTO synthesis with extremely high activity.

[0052] The CNS supports are fabricated from the carbonization of potassium citrate as a K precursor that serves as an inexpensive carbon source with the added benefit of dispersing K promoter throughout the support. The catalyst demonstrates high activity and stability towards C.sub.2-C.sub.4 light olefins, and exhibits very high iron time yield (FTY) values, e.g., 1790-1990 μmol.sub.CO/g.sub.Fe's for ˜100 hour time on stream (TOS). Furthermore, the catalyst can be repeatedly used while maintaining high activity for an extended period, e.g., at least 500 hours of cumulative TOS. Various characterization results have shown that the as-deposited iron oxide catalyst nanoparticles on K-promoted CNS are more readily reduced and stabilized as metallic iron after the initial H.sub.2 activation compared with a control catalyst sample supported on carbon nanotubes (CNTs). In certain embodiments, ammonium iron citrate is used as a Fe precursor for depositing iron oxide nanoparticle catalysts on the CNS support. The more robust formation of metallic iron on CNS results in more efficient conversion in the subsequent FTO reaction to form highly active iron carbide. Also, the K embedded in CNS can enhance catalyst activity and selectivity. Moreover, K is a common promoter for a broad range of reactions and therefore, the inventive process for fabricating K-promoted CNS catalyst supports has broad utility.

[0053] In certain embodiments, the catalyst according to the invention can be effective to catalyze direct CO.sub.2 hydrogenation to produce light olefins with up to 37% CO.sub.2 conversion and 65% light olefins in the hydrocarbon distribution produced, as well as providing an extremely high iron time yield (FTY) of 1882 μmol.sub.CO/g.sub.Fe.Math.s with 41% selectivity for light olefins and excellent stability (approximately 100 hours on stream) for FTO processes. The FTY value is 50 to 1300 times higher as compared to catalysts exhibiting similar light olefin selectivity known in the art. The catalyst according to the invention is highly active for 100 hours continuous time on stream and demonstrates very low degradation after repeated catalytic reaction cycles totaling 550 hours.

[0054] Since the novel carbon nanosheet (CNS) support material advantageously allows the formation of Fe.sub.3O.sub.4 phase of the iron oxide nanoparticles on the surface of the catalyst support, the transformation to highly active iron carbide Fe.sub.5C.sub.2 upon exposure to CO for FTO synthesis is facilitated. In contrast, similar synthesis parameters result in the Fe.sub.2O.sub.3 phase of iron oxide on other carbon support materials in the art. The advantages of Fe.sub.3O.sub.4 (as compared to Fe.sub.2O.sub.3) include (i) it is a highly active reverse water gas shift (RWGS) catalyst (CO.sub.2+H.sub.2=CO+H.sub.2O), and (ii) it is more readily reduced to metallic iron compared to Fe.sub.2O.sub.3 which is essential to the subsequent conversion to form highly active Fe.sub.5C.sub.2 in the presence of CO for FTO synthesis (2nH.sub.2+nCO=C.sub.nH.sub.2n+nH.sub.2O). Moreover, the novel CNS support offers advantages of 2D carbon-based catalyst support materials, and also contains a promoter, K, embedded uniformly in the CNS structure. The CNS support material has multiple functional groups such as carboxyl, carbonyl and hydroxyl groups. Without being bound by any particular theory, it is believed that these functional groups contribute to catalyst performance by anchoring and stabilizing supported iron oxide nanoparticles. Further, the ubiquitous and even distribution of K throughout the support contributes to the superior activity and performance of the nanocatalysts. It has been shown that using other carbon-based catalyst support, such as carbon nanotubes, or intentionally adding promoter K to other carbon support materials, will not achieve the high activity that is demonstrated by the CNS support material according to the invention.

[0055] Pursuant to the invention, the Fe.sub.3O.sub.4/CNS catalyst selectively converts CO.sub.2 to C.sub.2-C.sub.4 olefins. Further, selectivity to the C.sub.2-C.sub.4 olefins is tuned by optimizing reaction parameters such as feed gas composition and space velocity. According to the invention, unwanted CH.sub.4 production is suppressed by increasing H.sub.2/CO.sub.2 ratio of the feed gas or by lowering the reaction temperature.

[0056] It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed and the following examples conducted, but it is intended to cover modifications that are within the spirit and scope of the invention.

EXAMPLES

Experimental Details

[0057] The following experimental details apply to the below Examples that were conducted.

[0058] Brunauer-Emmett-Teller (BET) surface area measurements of fresh catalysts were conducted in a Quantachrome Autosorb 1-C analyzer. Prior to N.sub.2 isotherms at −196° C., the catalysts (approximately 40 mg) were degassed at 110° C. under vacuum overnight.

[0059] Raman spectra were obtained using a Horiba (LabRam HR-Evolution) spectrometer with a 633 nm laser excitation source. The laser excitation power was 67 mW and the filter size was 10%.

[0060] X-ray photoelectron spectroscopy (XPS) experiments were conducted with a PHI 5600ci spectrometer equipped with a hemispherical electron analyzer and A1 Kα (1486.6 eV) radiation source. The powder samples were mounted on double-sided carbon tapes for analysis. The core-shell structure of spent catalyst samples was analyzed with sputtering experiments conducted by Ar ion bombardment. The XPS data were collected with sputtering times of 30, 60, 90, 120, 180, and 300 seconds. Both C 1s and Fe 2p spectra were recorded. All binding energies were calibrated to the C is peak located at 284.6 eV.

[0061] Synchrotron-based XPS spectra of fresh, H.sub.2 reduced, and spent FTO catalysts were collected at the beamline 23-ID-2 (IOS) at NSLS-II. The powder samples were pressed on an indium foil and the spectra were collected in UHV at room temperature using a SPECS Phoibos 150 NAP analyzer.

[0062] CO conversion, product selectivity, and FTY were determined/calculated as follows:

[00001]$\mathrm{CO}\mathrm{conversion}\left(\%\right)=\frac{\mathrm{moles}\mathrm{of}\mathrm{CO},\mathrm{in}-\mathrm{moles}\mathrm{of}\mathrm{CO},\mathrm{out}}{\mathrm{moles}\mathrm{of}\mathrm{CO},\mathrm{in}}\times 100$$\mathrm{Selectivity}\mathrm{of}C\mathrm{product}i\left(\mathrm{wt}\%,\mathrm{excluding}\mathrm{CO}2\right)=\frac{\left[\left(\mathrm{molar}\mathrm{flow}\mathrm{rate}\right)\left(\mathrm{MW}\right)\right]i}{.Math.\left[\left(\mathrm{molar}\mathrm{flow}\mathrm{rate}\right)\left(\mathrm{MW}\right)\right]i}\times 100$$\mathrm{FTY}\left(\mathrm{.Math.molCO}\text{/}g\mathrm{Fe}.Math.s\right)=\frac{\mathrm{molar}\mathrm{flow}\mathrm{rate}\mathrm{of}\mathrm{CO}\mathrm{converted}\mathrm{to}\mathrm{HC}\mathrm{products}}{\left(\mathrm{catalyst}\mathrm{weight}\right)\left(\%\mathrm{Fe}\mathrm{loading}/100\right)}$

[0063] The D and G bands (at ca. 1322 and 1580 cm-1) were clearly observed, corresponding to the structural disorder of the defects of carbon materials, and the vibration of the sp.sup.2-hybridized carbon atoms, respectively. The D/G band ratio is 1.13, indicating surface defect or structural disorder on the CNS support. Surface disorder and functional groups on the carbon catalyst support have been reported to be important factors affecting the FTO catalyst performance.

[0064] The C is spectra were deconvoluted into four peaks for Fe.sub.xO.sub.y/CNS catalyst. Nonoxygenated C ring (284.4 eV), C in C—O (285.8 eV), carbonyl C (C═O, 287.2 eV) and carboxyl C (O═C—O, 289.4 eV) were applied to fit the spectra, with occupations of 68.1%, 20.7%, 6.1% and 5.1%, respectively. The C—O peak could be either C—OH or C—O—C functional groups, and cannot be distinguished from each other. The Fe.sub.xO.sub.y/CNS sample clearly contained significant amounts of oxygen functional groups, which is beneficial to the homogeneous distribution of the supported iron nanoparticles.

[0065] The D and G bands at 1323 and 1590 cm.sup.−1 exhibited a D/G band ratio of 1.38, indicating a slightly higher defective surface or structural disorder compared with the CNS support, which was likely responsible for the stronger interaction between CNT and Fe.sub.xO.sub.y nanoparticles.

[0066] The C 1s spectra were deconvoluted into five peaks for Fe.sub.xO.sub.y/CNT catalyst. Nonoxygenated C ring (284.4 eV), C—O (285.6 eV), carbonyl C (C═O, 286.9 eV), carboxyl C (O═C—O, 289.4 eV) and π-π* satellite at 291.2 eV were applied to fit the spectra, with occupations of 64.3%, 16.8%, 8.8%, 4.5% and 5.6% respectively. The Fe.sub.xO.sub.y/CNT sample exhibited similar amounts of oxygen functional groups, which was also beneficial to the homogeneous distribution of the supported iron nanoparticles.

TABLE-US-00001 TABLE S1 Mössbauer simulation parameters of the fresh and spent Fe.sub.xO.sub.y/CNS catalysts. δ ΔEq B.sub.hf Γ Sample Phase (mm/s) (mm/s) (T) (mm/s) % Comments Fresh Fe.sub.xO.sub.y-I 0.46 0 52.1 0.5 35 Ferric sites, Fe.sub.xO.sub.y/ Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4 CNS Fe.sub.xO.sub.y-II 0.43 0 49.8 0.5 37 Ferric sites, Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4 Fe.sub.xO.sub.y-III 0.83 −1.14 46.3 0.5 16 Ferric sites, Fe.sub.3O.sub.4 Spent Fe.sub.xO.sub.y-I 0.46 0 52.1 0.5 6 Ferric sites, Fe.sub.xO.sub.y/ Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4 CNS Fe.sub.xO.sub.y-II 0.43 0 49.8 0.5 6 Ferric sites, Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4 Fe.sub.xO.sub.y-III 0.45 0 46.5 0.5 5 Ferric sites, Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4 χ-Fe.sub.5C.sub.2-I 0.39 0.09 25.5 0.4 30 χ-Fe.sub.5C.sub.2-II 0.33 0 21.8 0.55 27 χ-Fe.sub.5C.sub.2-III 0.33 0.05 10.6 0.5 15 Fe.sub.xC 0.36 0 18.3 0.3 10

[0067] The Mössbauer spectrum of fresh Fe.sub.xO.sub.y/CNS catalyst (FIG. 4c) can be simulated with three components (Table S1), two of them have .sup.57Fe hyperfine field value of ˜50 T and isomer shift value of ˜0.45 mm/s, which represent ferric sites in either Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4. The third component has a .sup.57Fe hyperfine field value of 46.3 T, an isomer shift value of 0.83 mm/s, and represents typical ferrous sites in Fe.sub.3O.sub.4. The spent catalyst (FIG. 4d) has two components from ferric sites in either Fe.sub.2O.sub.3 or Fe.sub.3O.sub.4. These features accounted for 12% of the total Fe signal in the spent sample (the spectral features indicated by the arrows in FIG. 4d) and are also seen in the fresh, unactivated, catalysts. A third iron oxide feature was observed with a hyperfine field value of 46.5 T, an isomer shift of 0.45 mm/s, which accounted for 5% of the Fe signal; this feature was not seen in the fresh catalyst and likely arises from exposing the active catalyst to ambient after the reaction. The Mössbauer results also indicated that 72% of the Fe signal in the spent catalyst is composed of 3 Ψ—Fe.sub.5C.sub.2 features with hyperfine field values ranging from 10 T to 25 T and isomer shift values of ˜0.35 mm/s. There is an additional feature associated with an Fe.sub.xC phase (possibly Fe.sub.2C or Fe.sub.2.2C) that accounted for 10% of the Fe signal with a hyperfine field value of 18 T and an isomer shift of 0.36 mm/s.

[0068] The Fe K-edge XANES data indicated that the fresh Fe.sub.xO.sub.y/CNS and Fe.sub.xO.sub.y/CNT catalysts closely resemble the Fe.sub.3O.sub.4 phase rather than Fe.sub.2O.sub.3 and FeO phases (FIG. S11a,b), which is consistent with the TEM, XRD and Mössbauer results. The XANES spectra of samples after FTO reaction showed an absorption feature at 7111.2 eV similar to the iron carbide phase reported in the art and this feature continued to grow as a function of TOS (FIG. S11a,c). In addition to the distinct iron carbide feature, Fe.sub.3O.sub.4 phase was still present in the spent catalysts, reflected by the line at 7131.2 eV (FIG. S11c). The line intensity reduces and the spectral feature broadens as the reaction further proceeds. The line correlates with 1s to 4p transition and its intensity is an indicator of the degree of reduction and carburization of catalysts. The smaller line intensity is associated with higher degree of carburization and higher iron carbide content. A shift of edge rising position from 7126.4 eV of fresh catalyst to 7123.0 eV of spent catalyst after 10 h TOS implies a reduction process of iron species during FTO reaction. Consequently, the spent Fe.sub.xO.sub.y/CNS catalysts appeared to be a mixture of iron carbide and oxide phases, which is consistent with the HRTEM, XPS and Mössbauer results. Fe.sub.xO.sub.y/CNT catalysts showed similar conversion from iron oxide to iron carbide phase during FTO reaction (FIG. S11b,d). However, unlike the Fe.sub.xO.sub.y/CNS catalysts, the changes in the XANES spectra were much less pronounced for the reacted Fe.sub.xO.sub.y/CNT catalyst (FIG. S11b,d).

TABLE-US-00002 TABLE S2 Results of EXAFS data fitting analysis of fresh Fe.sub.xO.sub.y/CNS sample. Fe.sub.3O.sub.4 (magnetite) model was used for fresh Fe.sub.xO.sub.y/CNS fitting analysis. Note that S.sub.0.sup.2 is fixed at 0.77. Samples Model Atoms R(Å) CN σ.sup.2(Å.sup.2) ΔE (eV) Fresh Tetrahedral Fe Fe—O 1.96 ± 0.00 2.8 ± 0.2 0.002 ± 0.001 3.38 ± 0.29 Fe.sub.xO.sub.y/CNS Fe—Fe 3.38 ± 0.04 7.7 ± 1.4 0.022 ± 0.006 Fe—O 3.54 ± 0.02 9.6 ± 1.9 0.022 ± 0.005 Fe—Fe 3.70 ± 0.01 4.6 ± 1.2 0.009 ± 0.002 Octahedral Fe Fe—O 2.11 ± 0.01 3.8 ± 0.4 0.018 ± 0.002 3.38 ± 0.29 Fe—Fe 3.00 ± 0.00 2.3 ± 0.6 0.005 ± 0.002 Fe—Fe 3.50 ± 0.00 6.2 ± 0.8 0.004 ± 0.001 Fe—O 3.76 ± 0.02 4.9 ± 2.4 0.016 ± 0.012 Tetrahedral Fe Fe—O 1.86 4 Fe.sub.3O.sub.4 Fe—Fe 3.45 12  (theoretical) Fe—O 3.46 12  Fe—Fe 3.6  4 Octahedral Fe Fe—O 2.05 6 Fe—Fe 2.94 6 Fe—Fe 3.45 6 Fe—O 3.62 6

TABLE-US-00003 TABLE S3 Results of EXAFS data fitting analysis of fresh Fe.sub.xO.sub.y/CNT sample. Fe.sub.3O.sub.4 (magnetite) model is used for fresh Fe.sub.xO.sub.y/CNT fitting analysis. Note that S.sub.0.sup.2 is fixed at 0.77. Samples Model Atoms R(Å) CN σ.sup.2(Å.sup.2) ΔE(eV) Fresh Tetrahedral Fe Fe—O 1.89 ± 0.00 2.02 ± 0.10 0.003 ± 0.001 1.24 ± 0.23 Fe.sub.xO.sub.y/CNT Fe—Fe 3.47 ± 0.01 2.45 ± 0.70 0.005 ± 0.003 Fe—O 3.45 ± 0.04 2.15 ± 0.61 0.012 ± 0.007 Fe—Fe 3.97 ± 0.03 1.19 ± 0.62 0.010 ± 0.007 Octahedral Fe Fe—O 2.04 ± 0.01 1.99 ± 0.11 0.003 ± 0.001 1.24 ± 0.23 Fe—Fe 2.97 ± 0.02 3.35 ± 0.30 0.011 ± 0.002 Fe—Fe 3.14 ± 0.06 1.05 ± 0.65 0.021 ± 0.007 Fe—O 3.46 ± 0.13 1.52 ± 0.46 0.013 ± 0.009

[0069] In order to determine the relative quantities of iron oxides and iron carbides in these reacted catalysts, both Fe.sub.3O.sub.4 and Hägg carbide χ-Fe.sub.5C.sub.2 were employed as models for the EXAFS fitting analysis. χ-Fe.sub.5C.sub.2 was used instead of other carbides such as s-Fe.sub.2.2C and O-Fe.sub.3C because (a) HRTEM and Mössbauer spectroscopy analysis suggest Fe.sub.5C.sub.2 as the major carbide species in the spent catalysts, and (b) Hägg carbide x-Fe.sub.5C.sub.2 is a typical product formed between 250 and 350° C. under FT conditions, and the FTO reaction was carried out at 350° C. The Fe.sub.5C.sub.2 EXAFS spectrum showed two major peaks below 3 Å, corresponding to Fe—C (1.9 Å) and Fe—Fe (2.2 Å) scattering. Although the Fe—C peak overlaps with the Fe—O peak in iron oxides, it can be distinguished by EXAFS fitting analysis. FIG. S13a and Table S4 display the fitting results of the reacted Fe.sub.xO.sub.y/CNS catalysts. The build-up of iron carbide phase as a function of TOS is illustrated by the coordination number (CN). The first Fe—C coordination shell grows gradually from 1.3 to 1.8 as TOS increases from 0.5 h to 10 h, due to the increasing contribution of Fe—C scattering from x-Fe.sub.5C.sub.2, whereas the coordination number of Fe—O from Fe.sub.3O.sub.4 slowly decreases from 3.2 to 2.0. There is a remarkable increase from 4.7 to 8.2 associated with the Fe—Fe coordination from x-Fe.sub.5C.sub.2 from TOS=0.5 h to 10 h. This is clear evidence of the carburization process of iron catalysts during FTO reaction. Debye-Waller factors (σ.sup.2) evaluated the crystal disorder and deviations from standard references. The rise of Debye-Waller factor of Fe—Fe shell from Fe.sub.3O.sub.4 from 0.5 h to 10 h originated from the gradual structural transformation from Fe.sub.3O.sub.4 to Fe.sub.5C.sub.2 (see details in Table S4).

[0070] The models of Fe.sub.3O.sub.4 and Fe.sub.5C.sub.2 and fitting procedure similar to those used for analyzing Fe.sub.xO.sub.y/CNS spectra were applied for Fe.sub.xO.sub.y/CNT. The detailed fitting results are in Table S3 and S5. Coordination number is a good indicator for phase composition change as a function of TOS (FIG. S13b). From 0.5 h to 10 h, the CN of Fe—O bond remained almost constant around 3.0, suggesting an insignificant change of Fe.sub.3O.sub.4 phase in the CNT supported nanoparticles. The lack of pronounced variation in reacted catalysts was also observed in the Fe—C and Fe—Fe coordination from the Fe.sub.5C.sub.2 phase (FIG. S13b). These results are in marked contrast to the significantly more pronounced conversion of Fe.sub.3O.sub.4 to Fe.sub.5C.sub.2 phase seen in CNS supported catalysts (FIG. S13a).

TABLE-US-00004 TABLE S4 Results of EXAFS data fitting analysis of reacted Fe.sub.xO.sub.y/CNS catalysts at different TOS. Hägg carbide and Fe.sub.3O.sub.4 (magnetite) are applied as the fitting models. Fitting intervals for k and R space are 2.5~10 Å.sup.−1, 1~3 Å, respectively. Global parameters of S.sub.0.sup.2 0.77 and energy shift ΔE 2.78 eV are employed for the Fe—C and Fe—Fe (Fe.sub.5C.sub.2) paths from Hägg carbide and Fe—O and Fe—Fe (Fe.sub.3O.sub.4) paths from magnetite in all samples analysis. TOS Atoms R(Å) CN σ.sup.2(Å.sup.2) 0.5 h.sup.  Fe—O 1.98 ± 0.01 3.2 ± 0.6 0.010 ± 0.001 Fe—C 1.98 ± 0.02 1.3 ± 0.3 0.003 ± 0.002 Fe—Fe (Fe.sub.5C.sub.2) 2.69 ± 0.01 4.7 ± 0.5 0.014 ± 0.001 Fe—Fe (Fe.sub.3O.sub.4) 2.99 ± 0.01 7.5 ± 1.0 0.022 ± 0.002 1 h Fe—O 1.98 ± 0.01 3.4 ± 0.3 0.011 ± 0.001 Fe—C 1.97 ± 0.02 1.3 ± 0.3 0.003 ± 0.002 Fe—Fe (Fe.sub.5C.sub.2) 2.69 ± 0.01 4.7 ± 0.5 0.014 ± 0.001 Fe—Fe (Fe.sub.3C.sub.4) 2.98 ± 0.01 6.7 ± 0.9 0.022 ± 0.002 2 h Fe—O 1.98 ± 0.01 3.0 ± 0.2 0.010 ± 0.001 Fe—C 1.97 ± 0.02 1.5 ± 0.2 0.003 ± 0.002 Fe—Fe (Fe.sub.5C.sub.2) 2.69 ± 0.01 5.2 ± 0.7 0.015 ± 0.001 Fe—Fe (Fe.sub.3C.sub.4) 2.98 ± 0.01 6.5 ± 1.1 0.024 ± 0.001 3 h Fe—O 1.98 ± 0.02 2.8 ± 0.3 0.010 ± 0.002 Fe—C 1.97 ± 0.01 1.5 ± 0.3 0.003 ± 0.002 Fe—Fe (Fe.sub.5C.sub.2) 2.69 ± 0.01 6.0 ± 0.9 0.016 ± 0.001 Fe—Fe (Fe.sub.3O.sub.4) 2.98 ± 0.02 6.1 ± 1.2 0.024 ± 0.001 4 h Fe—O 1.98 ± 0.01 2.4 ± 0.2 0.010 ± 0.001 Fe—C 1.97 ± 0.01 1.6 ± 0.2 0.003 ± 0.002 Fe—Fe (Fe.sub.5C.sub.2) 2.69 ± 0.01 6.5 ± 0.7 0.016 ± 0.001 Fe—Fe (Fe.sub.3O.sub.4) 2.97 ± 0.02 6.0 ± 1.1 0.028 ± 0.003 7 h Fe—O 1.98 ± 0.02 2.0 ± 0.3 0.007 ± 0.005 Fe—C 1.95 ± 0.05 1.8 ± 0.2 0.002 ± 0.001 Fe—Fe (Fe.sub.5C.sub.2) 2.67 ± 0.02 7.9 ± 0.9 0.016 ± 0.003 Fe—Fe (Fe.sub.3O.sub.4) 2.96 ± 0.04 5.9 ± 1.6 0.029 ± 0.005 10 h  Fe—O 2.00 ± 0.02 2.0 ± 0.3 0.008 ± 0.003 Fe—C 1.95 ± 0.02 1.8 ± 0.2 0.002 ± 0.001 Fe—Fe (Fe.sub.5C.sub.2) 2.69 ± 0.02 8.2 ± 0.9 0.018 ± 0.001 Fe—Fe (Fe.sub.3O.sub.4) 2.94 ± 0.02 5.5 ± 1.2 0.030 ± 0.004

TABLE-US-00005 TABLE S5 Results of EXAFS data fitting analysis of reacted Fe.sub.xO.sub.y/CNT samples at different TOS. Hägg carbide and Fe.sub.3O.sub.4 (magnetite) are applied as the fitting models. Fitting intervals for k and R space are 2.5~10 Å.sup.−1, 1~3 Å, respectively. Global parameters of S.sub.0.sup.2 0.77 and energy shift ΔE 3.26 eV are employed for the Fe—C and Fe—Fe (Fe.sub.5C.sub.2) paths from Hägg carbide and Fe—O and Fe—Fe (Fe.sub.3O.sub.4) paths from magnetite in all samples analysis. TOS Atoms R(Å) CN σ.sup.2(Å.sup.2) 0.5 h.sup.  Fe—O 1.99 ± 0.01 3.1 ± 0.2 0.009 ± 0.001 Fe—C 1.93 ± 0.02 1.0 ± 0.1 0.002 ± 0.001 Fe—Fe (Fe.sub.5C.sub.2) 2.61 ± 0.01 2.9 ± 0.3 0.013 ± 0.001 Fe—Fe (Fe.sub.3O.sub.4) 2.95 ± 0.02 5.5 ± 1.4 0.030 ± 0.004 1 h Fe—O 1.99 ± 0.01 3.1 ± 0.2 0.009 ± 0.001 Fe—C 1.94 ± 0.01 1.0 ± 0.2 0.002 ± 0.001 Fe—Fe (Fe.sub.5C.sub.2) 2.61 ± 0.01 3.0 ± 0.4 0.013 ± 0.001 Fe—Fe (Fe.sub.3O.sub.4) 2.94 ± 0.03 5.4 ± 1.1 0.032 ± 0.004 2 h Fe—O 1.99 ± 0.01 3.2 ± 0.3 0.009 ± 0.005 Fe—C 1.93 ± 0.03 1.1 ± 0.2 0.002 ± 0.001 Fe—Fe (Fe.sub.5C.sub.2) 2.61 ± 0.01 3.1 ± 0.4 0.013 ± 0.001 Fe—Fe (Fe.sub.3O.sub.4) 2.94 ± 0.02 5.3 ± 1.3 0.031 ± 0.006 3 h Fe—O 1.99 ± 0.00 3.2 ± 0.2 0.009 ± 0.005 Fe—C 1.93 ± 0.01 1.1 ± 0.2 0.002 ± 0.001 Fe—Fe (Fe.sub.5C.sub.2) 2.61 ± 0.01 3.2 ± 0.3 0.013 ± 0.001 Fe—Fe (Fe.sub.3O.sub.4) 2.93 ± 0.03 5.0 ± 1.2 0.032 ± 0.004 4 h Fe—O 1.96 ± 0.01 3.1 ± 0.4 0.011 ± 0.001 Fe—C 2.01 ± 0.02 1.1 ± 0.2 0.008 ± 0.004 Fe—Fe (Fe.sub.5C.sub.2) 2.61 ± 0.01 3.2 ± 0.3 0.013 ± 0.001 Fe—Fe (Fe.sub.3O.sub.4) 2.93 ± 0.02 5.2 ± 1.1 0.032 ± 0.004 7 h Fe—O 1.96 ± 0.04 3.4 ± 0.2 0.011 ± 0.001 Fe—C 1.97 ± 0.02 1.1 ± 0.2 0.006 ± 0.003 Fe—Fe (Fe.sub.5C.sub.2) 2.61 ± 0.01 3.2 ± 0.3 0.015 ± 0.001 Fe—Fe (Fe.sub.3O.sub.4) 2.94 ± 0.02 5.6 ± 1.2 0.032 ± 0.004 10 h  Fe—O 1.96 ± 0.01 3.1 ± 0.3 0.011 ± 0.001 Fe—C 1.96 ± 0.03 1.0 ± 0.3 0.006 ± 0.003 Fe—Fe (Fe.sub.5C.sub.2) 2.63 ± 0.01 3.8 ± 0.4 0.015 ± 0.001 Fe—Fe (Fe.sub.3O.sub.4) 2.93 ± 0.02 5.2 ± 1.3 0.032 ± 0.005

[0071] The electronic structure of fresh Fe.sub.xO.sub.y/CNS catalysts was evaluated by means of synchrotron XPS technique. The K 2p spectra indicate the presence of potassium promoter in range of 292.5-298.5 eV. The significant asymmetry of K 2p region in fresh Fe.sub.xO.sub.y/CNS material confirmed potassium species of partially oxidized potassium states. No plasmon features at higher binding energies for metallic K were found in the spectra.

Catalytic Performance for FTO Synthesis with Fe.sub.xO.sub.y/CNS Catalysts

[0072] The as-synthesized iron oxide on interconnected carbon nanosheets (Fe.sub.xO.sub.y/CNS) catalyst consisted of 10.0 nm±4.8 nm Fe.sub.xO.sub.y nanoparticles well dispersed on CNS with a rose-like structure (FIG. 1a,b). XRD indicated the main crystal phase of Fe.sub.xO.sub.y particles was Fe.sub.3O.sub.4 (FIG. 1c). The calculated size of Fe.sub.3O.sub.4 nanoparticles was 11 nm from the Scherer formula using the peak (311) at 35.4°, consistent with the TEM particle size analysis. The broad peak at 230 arises from the CNS (FIG. S1). The HRTEM images of the iron oxide nanoparticles in FIG. 1d and S2 show lattice fringes of 4.9 Å and 2.6 Å corresponding to the d spacing of (111) and (311) planes in Fe.sub.3O.sub.4, respectively, which further confirmed the XRD results. Good reducibility of iron oxides in iron-based FTO catalysts is well known to be essential to achieving high catalytic activity. Fe.sub.3O.sub.4 was more readily reducible compared to Fe.sub.2O.sub.3 reported in the art, and can be more efficiently transformed into Fe metal during the H.sub.2 activation step and subsequently the active iron carbide phase upon syngas exposure under typical FTO reaction conditions.

[0073] Raman and X-ray photoemission spectroscopy (XPS) results illustrated that the CNS had multiple functional groups such as carboxyl, carbonyl and hydroxyl groups (FIG. S3). These functional groups have been proposed to contribute to catalyst performance by anchoring and stabilizing supported iron oxide nanoparticles. EDX mapping confirmed the presence of very evenly distributed K that was introduced during CNS synthesis by the potassium citrate precursor (FIG. S4). This ubiquitous and even distribution of K throughout the support plays a key role in the activity and performance of this catalyst system.

[0074] Prior to FTO reaction, Fe.sub.xO.sub.y/CNS catalysts were reduced in H.sub.2 for 3 hours at 400° C. to obtain metallic iron. In situ XRD confirmed the excellent reducibility of the Fe.sub.xO.sub.y/CNS system, which formed FeO and α-Fe phases in 4% H.sub.2 at 300° C., and further completely transformed to α-Fe metal at 400° C. (FIG. S5). Under FTO conditions, the metallic α-Fe was then readily carburized and formed the active species Fe.sub.5C.sub.2 (see below).

[0075] The CO conversion, iron time yield (FTY, the number of CO molecules converted to hydrocarbons per gram of Fe per second), light olefins selectivity and olefin to paraffin ratio (O/P) are illustrated in FIG. 2. In the first 10 h, the CO conversion quickly increased from 50% to 70%. This induction period corresponded to the carburization process that transformed metallic iron to catalytically active iron carbide phases. After 10 h of TOS, CO conversion slowly increased and subsequently reached a steady state value around ˜70%. The Fe.sub.xO.sub.y/CNS catalysts demonstrated exceedingly high FTY values between 1790-1990 μmol.sub.CO/g.sub.Fe.Math.s that were far superior to high performance known in the art, Fe-based carbon supported FTO or FT catalysts evaluated under similar reaction conditions. For example, FTY value of 29.8 μmol.sub.CO/g.sub.Fe*s was reported for Fe.sub.2O.sub.3 on carbon nanofibers,.sup.3 27.9 μmol.sub.CO/g.sub.Fe.Math.s for Fe supported on N-doped carbon nanotubes, while commercial Ruhrchemie catalysts produced FTYs of 22.5 μmol.sub.CO/g.sub.Fe.Math.s. The inventive FTYs are among the highest values achieved for iron based FTO and FT catalysts. Similarly impressive FTYs were recently reported in the art for Mg and K-promoted Fe on reduced graphene oxide catalyst, but in this study the activity decreased to 800-900 μmol.sub.CO/g.sub.Fe.Math.s after˜90 h of TOS. Additionally, FIG. 2b shows that the Fe.sub.xO.sub.y/CNS catalyst exhibited good and stable selectivity towards C.sub.2-C.sub.4 olefins with a steady-state olefin to paraffin ratio (O/P) above 3. The reaction conditions were specifically chosen to favor short chain hydrocarbon production and essentially all of the products were C.sub.1-C.sub.5 molecules, with only trace amounts of hydrocarbons of C.sub.6 and beyond.

[0076] To evaluate the role of carbon support material, Fe.sub.xO.sub.y/CNT catalyst was prepared and tested for FTO as a control sample. The as-received CNTs had an outer diameter of ˜10 nm and length of 3-20 μm. The average size of the CNT supported Fe.sub.xO.sub.y nanoparticles was ˜ 7.3 nm (FIG. 3a) and XRD indicates the oxide was in the Fe.sub.3O.sub.4 phase (FIG. S6), which was the same starting phase as the Fe.sub.xO.sub.y/CNS samples. Despite similar characteristics between the CNT and CNS supported samples, the Fe.sub.xO.sub.y/CNS catalyst outperformed the Fe.sub.xO.sub.y/CNT catalyst in all aspects of FTO synthesis (FIG. 3b, S8, Table 1). Although the CO conversion was relatively stable (˜45%) for Fe.sub.xO.sub.y/CNT, it was significantly lower than that of Fe.sub.xO.sub.y/CNS (˜70%), and the FTY slowly decreased from 1000 to 860 μmol.sub.CO/g.sub.Fe.Math.s over 90 h TOS. Compared with the stable O/P ratio of 3.4 for the Fe.sub.xO.sub.y/CNS catalyst, the Fe.sub.xO.sub.y/CNT exhibited a substantially lower O/P ratio which also changed over time from 0.3 to 1.2 in 90 h (FIG. 3b and S8).

[0077] The catalytic performance of several Fe based catalysts is summarized in Table 1. To make a more direct comparison with the K-promoted CNS system, the activity of Fe.sub.xO.sub.y/CNT promoted with 1% K (1K-Fe.sub.xO.sub.y/CNT) was evaluated. While the 1K-Fe.sub.xO.sub.y/CNT sample had better olefin selectivity (54.4%) than the unpromoted Fe.sub.xO.sub.y/CNT (33.1%), the activity and stability of the 1K-Fe.sub.xO.sub.y/CNT catalyst were drastically reduced, with a CO conversion of only 4.1% at 10 h TOS and almost complete deactivation after ˜18 h TOS. This finding validated that high FTO performance cannot be simply achieved by adding K onto CNTs and highlights the advantage of using CNS as a support. A standard non-supported sample, bulk Fe—Cu—K—SiO.sub.2 catalyst promoted by Cu and K, achieved 52.3% CO conversion, but demonstrated poor stability after ˜18 h. Finally, compared with one of the best FTO catalysts known in the art, Fe.sub.2O.sub.3 supported on CNF, the 1882 μmol.sub.CO/g.sub.Fe.Math.s FTY value for the inventive Fe.sub.xO.sub.y/CNS catalysts was ˜60 times higher and this activity was maintained with high CO conversion (72.6%) and high olefin selectivity (41.2%). The data and comparisons indicate that the K-promoted Fe.sub.xO.sub.y/CNS system is one of the highest known performing Fe based FTO catalysts.

TABLE-US-00006 TABLE 1 Catalytic performance of Fe.sub.xO.sub.y/CNS compared with Fe.sub.xO.sub.y/CNT, 1 wt. % K promoted Fe.sub.xO.sub.y/CNT and other reference catalysts as measured by CO conversion, FTY and product selectivity under the same FTO reaction conditions (350° C., 20 bar, H.sub.2/CO = 1) except Fe.sub.2O.sub.3/carbon nanofiber (CNF) (340° C., 20 bar, H.sub.2/CO = 1). Selectivity (% wt.) Sample TOS (h) CO (%) FTY CH.sub.4 C.sub.2-C.sub.4 C.sub.2.sup.═-C.sub.4.sup.═ C.sub.5.sup.+ O/P Fe.sub.xO.sub.y/CNS 90 72.6 1882 29.9 53.5 41.2 16.6 3.35 Fe.sub.xO.sub.y/CNT 90 42.1 861 29.7 61.0 33.1 9.0 1.19 1K—Fe.sub.xO.sub.y/CNT 10 4.1 89.2 26.3 64.5 54.4 9.1 5.36 Fe—Cu—K—SiO.sub.2 18 52.3 161 47.1 46.5 26.0 6.4 1.27 Fe.sub.2O.sub.3/CNF.sup.3 64 88 29.8 13 (% C) 64 (% C) 52 (% C) 18 (% C) 6.5 .sup.3FTY unit is μmol.sub.CO/g.sub.Fe•s. Note that the selectivity to C.sub.2-C.sub.4 includes both paraffins and olefins, whereas selectivity to C.sub.2.sup.═-C.sub.4.sup.═ is specifically for olefins.

Structure-Activity Correlation

[0078] Detailed characterization of the FTO catalysts has been carried out to understand the structure-activity correlation. The Fe.sub.xO.sub.y/CNS catalysts undergo significant structural and phase changes under FTO reaction conditions (FIG. 4a, b). The post-reaction iron-based nanoparticles slightly increased to an average particle size of 12.1 nm (FIG. 4a inset), compared to 10.0 nm in fresh catalysts (FIG. 1b inset). Some larger nanoparticles (>30 nm) were also observed (FIG. 4a), indicative of some agglomeration.

[0079] Despite the occurrence of some sintering, both the catalytic activity and product selectivity were stable throughout the entire reaction testing process (FIG. 2). Analysis of the HRTEM images indicated post-reaction catalyst nanoparticles were composed of a Fe.sub.5C.sub.2 core with a thin amorphous iron oxide shell. The observed lattice fringes in the core were consistent with the d spacing of (510), (200), and (11-2) planes of Fe.sub.5C.sub.2 phase. In addition, the 700 angle also agreed with the expected angle between (021) and (−11-1) planes in Fe.sub.5C.sub.2 structure (FIG. 4b).

[0080] EDX mapping clearly illustrates the Fe.sub.5C.sub.2/amorphous iron oxide core/shell structure in the spent catalyst (FIG. S9): Fe was present in both the core and shell with C in the core and O in the shell. The same conclusion is further supported by XPS depth-profiling studies (FIG. S10). As the oxide shell was gradually removed by sputtering, the embedded Fe.sub.5C.sub.2 core became increasingly exposed as evidenced by the growth of the peak associated with Fe.sub.5C.sub.2 in the XPS spectra. The amorphous iron oxide shell may have resulted from exposing the post-reaction catalyst to air, or it may have formed in situ due to H.sub.2O generation during FTO reaction.

[0081] Mössbauer spectroscopy had been utilized to study the evolution of Fe.sub.xO.sub.y nanoparticles during the FTO process. The Mössbauer results also quantified the types of Fe species present in the catalyst before and after reaction (Table S1). The Mössbauer spectrum of the fresh Fe.sub.xO.sub.y/CNS catalyst showed a sextet splitting pattern typically associated with magnetic iron species (FIG. 4c). Analysis of the spectrum (Table S1) suggests that Fe.sub.3O.sub.4 was the predominate phase in the fresh, unactivated, Fe.sub.xO.sub.y/CNS catalyst, consistent with the XRD and HRTEM analysis.

[0082] In comparison, the Mössbauer spectrum of the spent catalyst showed significant differences (FIG. 4d). Simulation of the spectrum of the spent Fe.sub.xO.sub.y/CNS catalyst (Table S1) showed that it was mainly composed of x-Fe.sub.5C.sub.2 (72% of the Fe species), with contributions from a minor Fe.sub.xC phase and a mixed oxide phase. The high x-Fe.sub.5C.sub.2 content in the spent catalyst correlated well with its high activity and selectivity.

[0083] X-ray absorption spectroscopy (XAS) provides additional details on the transformation of iron oxide nanoparticles during FTO reactions. The evolution of the X-ray absorption near edge structure (XANES) spectra for both Fe.sub.xO.sub.y/CNS and Fe.sub.xO.sub.y/CNT catalysts at different reaction times demonstrated the conversion of iron oxide to iron carbide during FTO reaction (FIG. S11), consistent with TEM, XRD and Mössbauer results shown above. Despite the similar conversion processes for the two catalyst systems, the degree of carburization of Fe.sub.xO.sub.y/CNS catalysts appeared to be much more complete than that of Fe.sub.xO.sub.y/CNT catalysts. This difference is more clearly illustrated by Fourier transformed extended X-ray absorption fine structure (EXAFS) in FIG. 5. For the fresh catalysts, the first peak located between 1 and 2 Å is associated with the first Fe—O shell around the Fe absorbing core at 1.9-2.1 Å. The second peak between 2 and 3.5 Å originates from the Fe—Fe scattering of Fe.sub.3O.sub.4 at 3.0-3.5 Å (see fitting results in Table S2, S3). In the reacted catalysts, the Fe—Fe scattering from Fe.sub.3O.sub.4 in the fresh catalyst diminished as a function of TOS. Instead, a new peak at ˜ 2.0 Å appears (denoted by the vertical dashed lines in FIG. 5), which is ascribed to the Fe—Fe coordination from iron carbide composition (Table S4, S5). The growth of this Fe.sub.5C.sub.2 peak is much more pronounced for Fe.sub.xO.sub.y/CNS catalysts (FIG. 5a) than Fe.sub.xO.sub.y/CNT catalysts (FIG. 5b). These results suggest more effective carburization of iron oxide supported on CNS, leading to much improved catalytic performance for Fe.sub.xO.sub.y/CNS catalysts compared with Fe.sub.xO.sub.y/CNT catalysts (Table 1).

Effect of Catalyst Support

[0084] As discussed in the previous section, the difference in catalyst performance for CNS and CNT supported catalysts was a result of the more complete transformation from iron oxide to iron carbide for CNS supported catalysts. For quantitative comparison, the evolution of Fe—Fe EXAFS coordination number from Fe.sub.5C.sub.2 for Fe.sub.xO.sub.y/CNS and Fe.sub.xO.sub.y/CNT catalysts as a function of TOS is illustrated in FIG. 6a and FIG. S13. To understand the difference in carburization on different catalyst supports, the EXAFS spectrum of the H.sub.2 reduced Fe.sub.xO.sub.y/CNS catalyst (FIG. 6b) offers the direct evidence of the presence of metallic iron, whereas the H.sub.2 reduced Fe.sub.xO.sub.y/CNT catalyst contained mostly oxidized Fe species (FIG. 6b). Furthermore, EDX mapping of reduced Fe.sub.xO.sub.y/CNS was consistent with a core/shell structure with a metallic Fe core and an amorphous iron oxide shell presumably due to exposure to air (FIG. S14). The improved reducibility of Fe.sub.xO.sub.y/CNS catalyst is also suggested by the H.sub.2-TPR profiles (FIG. S15).

[0085] These results suggest more robust formation of metallic Fe nanoparticles on the CNS support than on the CNT. The stabilization of metallic Fe nanoparticles by CNS subsequently leads to more effective carburization upon introduction of CO under FTO conditions. In contrast, for the reduced CNT supported catalyst, the less effective formation and/or stabilization of metallic iron entails that the conversion to iron carbides was hindered due to the extra barrier to push out oxygen in iron oxides. These different effects of the two support materials can explain the improved catalytic activity and selectivity for CNS supported catalysts.

[0086] A potential reason that CNS is a superior support material may be that CNS can offer an optimal interaction with the catalyst particles leading to enhanced reduction/carburization. The average particle size formed on CNSs is 10±5 nm but decreased to 7±3 nm on CNTs despite using identical synthesis procedures. This suggests a stronger interaction between the iron oxide nanoparticles and the CNT support resulting in the stabilization of smaller sized particles. This stronger interaction with the CNT support was consistent with the observation that the Fe.sub.xO.sub.y/CNT catalyst was more difficult to reduce and carburize.

[0087] Another factor contributing to the catalytic performance of Fe.sub.xO.sub.y/CNS was the potassium contained in the CNS support. Potassium is widely used as a promoter for improving olefin selectivity and activity by facilitating the formation of Hägg carbide, improving the surface CO/H.sub.2 ratio, and stabilizing active iron facets. The CNS support was specifically chosen because it derives from carbonization of potassium citrate with residual K distributed evenly through the entire support (FIG. S4). The Fe.sub.xO.sub.y/CNS catalyst contained 1.8 wt. % K in a partially oxidized state (FIG. S16) and offers a promoter effect. The K effect in Fe.sub.xO.sub.y/CNT catalysts was evaluated by adding ˜ 0.1-1 wt. % K (1K-Fe.sub.xO.sub.y/CNT, Table 1); however, these 1K-Fe.sub.xO.sub.y/CNT catalysts had a much lower activity and reduced stability, despite an enhancement in light olefins selectivity. This result suggested the synergistic effect of the CNS support and the inherently embedded potassium evenly distributed on the CNS is more effective than simply adding K to CNT support.

[0088] K promoters are commonly used in various catalytic applications, such as ammonia synthesis. These K-promoted CNS supports should therefore have utility for a wide variety of catalyst applications beyond the current demonstration for FTO reactions.

CO.SUB.2 .Hydrogenation

[0089] CO.sub.2 hydrogenation was examined with the K-promoted CNS supported Fe.sub.xO.sub.y catalysts (FIG. X). Catalytic performance of the Fe.sub.xO.sub.y/CNS catalyst for CO.sub.2 hydrogenation was investigated using the following range of reaction conditions: T=350-400° C., P=20 bar, H.sub.2/CO.sub.2=1-4, WHSV=18,000-30,000 cm.sup.3 (STP)/(g.sub.cat-h). CO.sub.2 conversion increased with reaction temperature and H.sub.2/CO.sub.2 molar ratio of the feed gas. High CO.sub.2 conversion up to 37% could be achieved at 400° C. and H.sub.2/CO.sub.2=3. CO was the main C-containing product under these reaction conditions, but small quantities of C.sub.2-C.sub.4 products were observed.

[0090] The C.sub.1-C.sub.5 hydrocarbon product distribution from CO.sub.2 hydrogenation was characterized in more detail (FIG. Y). CO was excluded in this product distribution to better demonstrate that the selectivity to C.sub.2-C.sub.4 olefins can be tuned by optimizing key reaction parameters such as feed gas composition and space velocity. The Fe.sub.xO.sub.y/CNS catalyst selectively converted CO.sub.2 to C.sub.2-C.sub.4 olefins. Unwanted CH.sub.4 production could be suppressed by increasing the H.sub.2/CO.sub.2 ratio of the feed gas or by lowering the reaction temperature.

[0091] The stability of the CO.sub.2 hydrogenation activity of the K-promoted CNS supported Fe.sub.xO.sub.y catalyst was also evaluated. FIG. Z shows catalytic performance as a function of time on stream (TOS) under the conditions of 400° C., 20 bar, H.sub.2/CO.sub.2=3 and WHSV=30,000 cm.sup.3 (STP)/(g.sub.cat-h). The catalyst exhibited stable performance for at least 80 h with no real evidence of degradation of performance. Additionally, the carbon balance remained close to 100% throughout the experiment illustrating the sample was not coking and/or that the reaction products were being accounted for, completely.

CONCLUSIONS

[0092] New K-promoted CNS supported Fe.sub.xO.sub.y catalysts for CO.sub.2 hydrogenation and FTO synthesis have been developed. The catalyst demonstrated superior catalytic activity and stability, with good olefins selectivity. Its FTY in the range of 1790-1990 μmol.sub.CO/g.sub.Fe.Math.s appeared to be the highest values of reported Fe based FTO or FT catalysts in the art. The catalyst was robust over ˜100 h of TOS. Moreover, the catalyst could be reused repeatedly without degradation in catalytic performance for at least 500 h cumulative TOS. The effect of the CNS support had been evaluated and compared with other carbon support materials such as CNT. EXAFS studies indicated that K-promoted CNS could stabilize the metallic iron nanoparticles during H.sub.2 reduction, which enhanced the formation of iron carbide under FTO reaction conditions. The efficient and complete carburization of Fe.sub.xO.sub.y/CNS catalyst resulted in its high catalytic activity, selectivity and stability. In contrast, the CNT supported catalyst nanoparticles exhibited smaller average sizes and were more difficult to reduce, leading to less efficient transformation to catalytically active iron carbide. These observations suggest that the K-promoted CNS support offers a relatively weak but balanced interaction with the catalyst nanoparticles that enables the improved catalyst reduction and carburization while maintaining the structural integrity under reaction conditions.

Methods

Catalyst Preparation

[0093] Carbon nanosheets (CNS) were prepared by carbonization of potassium citrate, which was heated in an alumina ceramic tube under N.sub.2 to 850° C. with a ramp rate of 1° C./min and was held at this temperature for 1 h. This fabrication formulation and method were chosen to not only form interconnected CNS, but also to efficiently incorporate the K promoter into the catalyst support. The product was then cleaned with 10% HCl and subsequently washed with copious amounts of water until the solution pH was neutral. The carbon nanosheets were then dried at 70° C. for 2 h and further dried under vacuum for 12 h.

[0094] Ammonium iron citrate was used as the Fe precursor for depositing iron oxide nanoparticle catalysts on the CNS support. The nominal Fe content of all catalysts prepared in this study was fixed at 5 wt. %. Ammonium iron citrate solution (1.4 M, 333 μL) was diluted by 5 mL of water and added slowly to 500 mg of CNS until the powder was fully wet. The mixture was then allowed to dry slowly at 50° C. for several hours and further dried under vacuum overnight. Subsequently, the mixture was calcined at 500° C. for 2 h with a ramp rate of 5° C./min under N.sub.2 to form the Fe.sub.xO.sub.y/CNS catalyst.

[0095] For comparison, iron-based nanoparticles supported on carbon nanotubes (CNTs) were also prepared. Ammonium iron citrate solution was mixed with multiwalled carbon nanotubes (Sigma-Aldrich) with 6-13 nm in outer diameter and 2.5-20 μm in length and the resulting mixture was processed in the same manner as described above to form Fe.sub.xO.sub.y/CNT catalyst. For K-promoted Fe.sub.xO.sub.y/CNT catalyst, K.sub.2CO.sub.3 was added to ammonium iron citrate solution for the iron oxide deposition step. Elemental analysis of the blank CNT indicated that there is a trace amount (0.01 wt. %) of Fe in the as-received CNT.

[0096] The Fe.sub.xO.sub.y/CNS catalyst were determined to contain 3.6 wt. % Fe and 1.8 wt. % K using ICP-MS. The unpromoted Fe.sub.xO.sub.y/CNT contained 4.2 wt. % Fe and no K (below the ICP-MS detection limit). The nominal 1 wt. % K-promoted Fe.sub.xO.sub.y/CNT contained 3.9 wt. % Fe and 1.2 wt. % K.

Catalyst Performance Evaluation

[0097] CO.sub.2 hydrogenation and FTO tests were conducted in a fixed-bed reactor system (Process Integral Development Engineered & Tech.). The prepared catalysts were evaluated at 350-400° C. and 20 bar, which favored the production of short chain hydrocarbons. The total gas flow rate was 100 cm.sup.3(STP)/min. For CO.sub.2 hydrogenation, the weight hourly space velocity (WHSV) range was between 18,000 and 30,000 cm.sup.3(STP)/(g.sub.cat.Math.h). The CO.sub.2 flow rate was fixed at 13 cm.sup.3(STP)/min while the H.sub.2/CO.sub.2 molar ratio was varied between 1 and 4. N.sub.2 was used as a diluent and an internal standard. For FTO, the feed gas composition was CO/H.sub.2/N.sub.2=45/45/10 and the WHSV was 30,000 cm.sup.3(STP)/(g.sub.cat.Math.h). Prior to reaction, catalyst samples (200 mg) were activated in situ in flowing H.sub.2 (50 cm.sup.3(STP)/min) at 400° C. and 1 bar for 3 h. The feed and product streams were analyzed online using an Agilent GC7890A equipped with flame ionization and thermal conductivity detectors (FID/TCD) as well as a methanizer. Separation of the compounds was performed using Ar as a carrier gas and 2 columns: molecular sieve 13× (6 ft×⅛ in. SS, 60/80 mesh) for light gases (H.sub.2, N.sub.2, CH.sub.4, and CO) and Hayesep Q (10 ft×⅛ in. SS, 80/100 mesh) for CO.sub.2 and C.sub.2-C.sub.6 hydrocarbons.

Catalyst Characterization

[0098] Scanning transmission electron microscopy (STEM) images were taken with a Hitachi HD-2300A dedicated scanning transmission electron microscope with a field-emission gun (FEG) and an optimal resolution of 0.204 nm. The catalyst sample was crushed in an agate mortar and pestle and it was suspended in ethanol. Approximately 1-2 drops of the suspension were spread onto a Cu grid coated with a holey carbon film (HC—Cu grid). The grid was then dried in air. The bright-field imaging (BF), high-angle annular dark-filed (HAADF, or atomic number contrast, Z-contrast) imaging, and secondary electron imaging (SE) were carried out with a 200-kV electron probe. A Thermo Scientific Noran System SIX (NSS) energy dispersive X-ray spectroscopy (EDX) system was used to collect elemental chemistry and X-ray maps.

[0099] The atomic-resolution transmission electron microscopy (TEM) images of the catalysts were collected using an FEI Talos F200X instrument operated at 200 keV. With the aid of the ultra-bright field emitter, this instrument can image at near diffraction limit in annular dark-field STEM (ADF-STEM) mode and routinely achieve 1.4-1.5 angstrom resolution.

[0100] X-ray diffraction (XRD) measurements were carried out using a PANalytical X'pert pro X-ray diffractometer with Cu Kα radiation (X=1.5418 Å) with a step size of 0.017° and 200 s/step in the 2θ range from 10° to 80°. The XRD was operated at 45 kV and 40 mA. The crystallite size of Fe.sub.3O.sub.4 was calculated by the Scherer equation.

[0101] .sup.57Fe Mössbauer spectra were collected using a .sup.57Co radiation source mounted on a velocity transducer operating under a constant acceleration mode. Velocity was calibrated with α-Fe metal. During the measurements, the samples were kept at 4.2 K in a SuperVaritemp dewar designed by Janis Research (Wilmington, Mass.). Mössbauer spectral simulations were performed by using the WMOSS software package (SEE Co., Edina, MN). Isomer shifts were quoted relative to α-Fe metal at 25° C.

[0102] X-ray absorption fine structure (XAFS) experiments were carried out at the 8-ID ISS beamline at Brookhaven National Laboratory's National Synchrotron Light Source II (NSLS-II) and the XAS beamline at Louisiana State University's Center for Advanced Microstructures and Devices (CAMD). Samples were prepared by mixing with boron nitride (BN) and were pressed into a pellet of ˜1 mm in thickness. Reference samples such as Fe.sub.2O.sub.3, Fe.sub.3O.sub.4 and FeO were mixed with BN with 5 wt. % Fe in BN. The pellets for Fe.sub.xO.sub.y/CNS and Fe.sub.xO.sub.y/CNT catalyst samples were prepared at a mass ratio of approximately 1:2 of catalyst:BN due to their low Fe concentration. For X-ray spectroscopy experiments, fresh catalysts, catalysts reduced under H.sub.2 at 400° C. for 1 h, and catalysts undergoing FTO catalytic reaction conditions for different periods of time were prepared using a fixed-bed reactor under the same FTO conditions as the catalyst performance studies. All X-ray absorption measurements were conducted ex situ under ambient conditions. Fe K-edge XAFS data were collected in transmission mode for reference samples and in fluorescence mode for Fe.sub.xO.sub.y/CNS and Fe.sub.xO.sub.y/CNT samples. The IFEFFIT software package was used to analyze the XANES and EXAFS data to obtain the local structural information of iron. FEFF6 was applied to calculate single scattering paths modeled the χ(R).

[0103] Temperature-programmed reduction (TPR) was conducted in a Micromeritics Autochem 2950 HP chemisorption analyzer. Initially, samples (approximately 50 mg) were heated in flowing He at 300° C. for 30 min to remove moisture followed by cooling to room temperature. Subsequently, H.sub.2-TPR was performed by heating the samples in flowing 10% H.sub.2/Ar (50 cm.sup.3(STP)/min) from room temperature to 1100° C. with a ramp rate of 10° C./min. H.sub.2 consumption during TPR was monitored by a thermal conductivity detector (TCD). A bath containing isopropyl alcohol (IPA) and liquid nitrogen was utilized during these measurements to trap water generated during H.sub.2-TPR.