Highly Ordered Mesoporous Fe2O3âZrO2 Bimetal Oxides for an

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Highly ordered mesoporous Fe2O3-ZrO2 bimetal oxides for an enhanced CO hydrogenation activity to hydrocarbons with their structural stability Jae Min Cho, Sae Rom Lee, Jian Sun, Noritatsu Tsubaki, Eun Joo Jang, and Jong Wook Bae ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01989 • Publication Date (Web): 27 Jul 2017 Downloaded from http://pubs.acs.org on July 27, 2017

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Submitted to the ACS Catalysis

Highly ordered mesoporous Fe2O3-ZrO2 bimetal oxides for an enhanced CO hydrogenation activity to hydrocarbons with their structural stability

Jae Min Choa,1, Sae Rom Leea,1, Jian Sunb, Noritatsu Tsubakic, Eun Joo Jangd, Jong Wook Baea,*

a

School of Chemical Engineering, Sungkyunkwan University (SKKU), 2066 Seobu-ro, Jangan-gu, Suwon, Gyeonggi-do, 16419, Republic of Korea

b

Dalian national Laboratory for Clean Energy, Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences, Dalian 116023, China

c

Department of Applied Chemistry, School of Engineering, University of Toyama, Gofuku 3190, Toyama, 930-8555, Japan d

Material Research & Development Center, Samsung Advanced Institute of Technology (SAIT), Suwon, Gyeonggi-do 449-901, Republic of Korea

1

The authors of J.M. Cho and S.R. Lee contributed equally.

---------------------------------------------------------------------------------------------------------------*Corresponding author (J.W. Bae): Tel.: +82-31-290-7347; Fax: +82-31-290-7272; E-mail address: [email protected]

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ABSTRACT Highly ordered mesoporous Fe2O3-ZrO2 mixed bimetal oxides (FeZr) without adding any additional chemical promoters were firstly applied to produce the value-added hydrocarbons by CO hydrogenation through Fischer-Tropsch synthesis (FTS) reaction of syngas. To enhance a catalytic activity and structural stability, an irreducible ZrO2 as a structural promoter was incorporated in the ordered mesoporous Fe2O3 structures with a different Zr/Fe molar ratio from 0 to 1 prepared by using a hard-template of KIT-6. When an optimal amount of zirconia (Zr/Fe molar ratio = 0.25) was incorporated in the ordered mesoporous Fe2O3 frameworks, the catalytic activity was significantly improved almost ten-times higher than the mesoporous monometallic Fe2O3. The highly ordered mesoporous structures were stably preserved even under reductive FTS reaction conditions. The ordered mesoporous FeZr catalysts showed a higher C5+ selectivity even at a higher CO conversion above 80%. This improved catalytic activity and stability on the optimized FeZr catalyst were mainly attributed to the facile formation of active iron carbide species such as the stable χ-Fe5C2 with insignificant structural collapses through a formation of strongly interacted iron nanoparticles with the ZrO2 structural promoter with a suppressed inactive coke deposition in the highly ordered FeZr mesopores.

Key words: Ordered mesoporous Fe2O3-ZrO2 oxides; Fischer-Tropsch synthesis; structural promoter of ZrO2; stable χ-Fe5C2; Structural stability.

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1. Introduction CO hydrogenation to hydrocarbons such as Fischer-Tropsch synthesis (FTS) reaction has been intensively investigated to produce the environmentally-benign and clean liquid fuels and value-added petrochemical intermediates.1 Various heterogeneous transition metalsupported catalysts such as cobalt, iron or ruthenium metals can be used in a fixed-bed or a slurry bubble column reactor for the conversion of syngas, which can be possibly derived from many carbon-containing feedstocks such as coal, biomass or natural (or shale) gases.1-5 A wide range of hydrocarbons from straight-chain hydrocarbons to isomers with light olefins, which can be described by Anderson-Schulz-Flory (ASF) polymerization models,6 can be synthesized at various operating conditions of FTS reaction. The synthesis of specific hydrocarbons by CO hydrogenation has been considered as one of the important transformation routes from syngas, which strongly depends on the types of FTS catalysts and reactors as well.6,7 Among the well-known several active transition metals, iron-based FTS catalysts inevitably containing some chemical promoters such as K, Cu, Al, Si and alkaliearth metals have been largely investigated due to its relatively low catalyst cost, high activities for FTS as well as water-gas shift (WGS) reaction at broad operating conditions even with H2-decifient syngas with a H2/CO ratio below 1.0.8-10 In addition, the iron-based FTS catalysts generally produce a small amount of invaluable CH4 byproduct with a higher light olefin selectivity such as C2-C4 hydrocarbons at a relatively higher temperature.11 However, those chemical promoters on the iron-based FTS catalysts can be easily segregated and transformed to an inactive species on the sintered iron surfaces, which resulted in suppressing the FTS activity with time on stream significantly.7,11 To solve those problems efficiently, some ordered mesoporous metal oxides such as SBA1512 and KIT-613 have been largely used as supporting materials, which can improve the dispersion of active metals and to enhance the catalyst stability by inhibiting the aggregation 3


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of active metals during FTS reaction. These mesoporous materials have been also widely used for other applications such as oxidation reactions or battery applications.14-16 From our previous works,17-19 the cobalt oxide frameworks composed of the highly ordered mesoporous mixed metal oxide structures with the help of the irreducible metal oxides such as Al2O3 or ZrO2 were successfully applied under the reductive FTS reaction conditions with a robust preservation of ordered mesopore structures. These ordered mesoporous metal oxides can be synthesized by using a nanocasting replica method with a hard-template of SBA-15 or KIT-6 after removing the silica templates with NaOH and HF solution.20 The roles of irreducible metal oxides such as Al2O3 and ZrO2 were found to act as efficient structural promoters, which can lessen the structural disintegration by forming a strongly interacted cobalt oxide with structural promoter. The ordered mesopore structures have some advantages of an enhanced mass transport rate of the formed waxy hydrocarbons from the mesopores with less catalyst deactivations. Fortunately, these simple approaches to enhance a structural stability and catalytic activity can be also applied to the iron-based FTS catalysts. In the present investigation, a series of ordered mixed metal oxides consisting of Fe2O3 and ZrO2 without any additional chemical promoters were firstly synthesized at a different molar ratio of Zr/Fe using a nanocasting replica method by using the highly ordered mesoporous KIT-6 as a hard template. The FTS reaction on the ordered mesoporous Fe2O3-ZrO2 were verified in terms of the catalytic activity and structural stability, and the roles of ZrO2 were verified to explain the enhanced activity and structural stability by elucidating the interactions between Fe2O3 and ZrO2 structural promoter and the extent and types of active iron carbide formations.

2. Experimental details 2.1. Synthesis of ordered mesoporous KIT-6 and Fe2O3-ZrO2 with its activity tests 4


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The ordered mesoporous KIT-6 was used as a hard template for synthesizing the ordered mesoporous Fe2O3-ZrO2 structures by well-known nanocasting replica method,19,21-23 For the synthesis of KIT-6 template, an amphiphilic triblock co-polymer of EO20PO70EO20 (Pluronic P123, poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), Aldrich) and tetraethylorthosilicate (TEOS, Alfa Aesar) were used as a structure-directing agent and silica precursor, respectively. For more details, P123 copolymer was firstly dissolved in deionized water and mixed with 2M HCl solution under a vigorous stirring at 35 oC. After mixing for 10 minutes, n-butanol was subsequently added to the above solution and mixed for 1 h again. Subsequently, the silica source of TEOS was poured and stirred for 24 h at 35 oC. The slurry mixtures were transferred to an autoclave and it was kept at 100 oC for 24 h under a static condition. As-synthesized white powder was filtered and washed with deionized water several times, and it was calcined under air flow at 550 oC for 6 h with a heating rate of 1 o

C/min to obtain a final hard template of KIT-6 having a specific surface area of ~850 m2/g. The highly ordered mesoporous Fe2O3-ZrO2 was subsequently prepared by coimpregnation

method using two separate metal precursors with KIT-6 as a hard template. A metal precursor of iron nitrate nonahydrate (Fe(NO3)3·9H2O, Aldrich) and zirconium oxynitrate dehydrate (ZrO(NO3)2·2H2O, Aldrich) were simultaneously dissolved in deionized water, and it was impregnated into exact mesopore volumes of the KIT-6 completely using an incipient wetness method. The slurry was subsequently dried at room temperature overnight, and the KIT-6 filled with metal precursors was calcined in a muffle furnace at 500 oC for 6 h with a heating rate of 1 oC/min. During the calcination steps, two metal precursors were transformed to the mixed metal oxides of the Fe2O3-ZrO2. The hard-template of KIT-6 was removed using an aqueous 2 M NaOH solution at 80 oC for 1 day, and it was washed with deionized water several times again. After removing a hard-template of KIT-6, the mesoporous mixed metal oxide of the Fe2O3-ZrO2 was further dried at 80 oC. The molar ratios of Zr to Fe metal (Zr/Fe) 5


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were varied at a fixed Zr/Fe ratio of 0.125, 0.25, 0.5 and 1. As-synthesized ordered mesoporous Fe2O3-ZrO2 mixed oxides were denoted as FeZr(x), where x represents the molar ratio of Zr/Fe. For comparison, the mesoporous monometallic Fe2O3 was synthesized through similar preparation procedures without Zr promoter addition. Catalytic activity of the mesoporous FeZr catalysts was measured using 0.1 g catalyst mixed with a 1.0 g inert material of α-Al2O3 in a fixed-bed tubular reactor having an outer diameter of 12.7 mm. Prior to the activity tests, the FeZr catalyst was previously reduced under a flow of 5% H2 balanced with N2 at 500 oC for 12 h. After the reduction, the catalytic activity was measured for around 60 h under the reaction conditions of T = 300 oC, P = 2.0 MPa, weight hourly space velocity (WHSV) of 8000 L/(kgcat·h) and a feed gas compositions of H2/CO/N2 = 63.0/31.5/5.5, where N2 was used as an internal standard gas. The effluent gases from the reactor were analyzed by using an on-line gas chromatograph (GC, YoungLin Acme 6500) equipped with a Carboxen-1000 packed column connected with a thermal conductivity detector (TCD) for an in-situ analysis of CO, CO2 and CH4. C1-C6 hydrocarbons were simultaneously analyzed by using a HP-PLOT/Q capillary column connected to a flame ionization detector (FID). The heavier hydrocarbons in a liquid and wax phase were collected in a trap maintained at 60 oC, and they were analyzed by using off-line gas chromatography (GC, Agilent 7890A) equipped with a HP-5 capillary column. CO conversion and product distribution on the FeZr catalysts were calculated based on carbon mole balances, and a chain growth probability (α) was calculated by using Anderson-Schulz-Flory (ASF) distribution model in the range of C12-C30 waxy hydrocarbons formed from FTS reaction.

2.2. Catalyst characterization The surface properties such as a specific surface area, pore volume and average pore diameter of the fresh FeZr catalysts were measured by using a Tristar II 3020 instrument kept 6


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at -196 oC at a vacuum level of 10-6 Pa. Prior to the analysis, as-prepared fresh FeZr catalyst was degassed at 90 oC for 1 h and 350 oC for 4 h to remove any contaminants. The specific surface area, pore volume, average pore diameter and pore size distribution of the fresh FeZr catalysts were calculated from N2 adsorption-desorption isotherms by using BrunauerEmmett-Teller (BET) model and Barrett-Joyner-Halenda (BJH) method from the desorption isotherm, respectively. The morphologies of the fresh and used FeZr catalysts were characterized by Transmittance Electron Microscopy (TEM) using a FEI Tecnai G2-20 STwin instrument operating at 200 kV. A wide-angle powder X-ray diffraction (XRD) analysis of the fresh FeZr catalysts was performed by using a X’Pert PRO Multi-Purpose X-Ray diffractometer operating at 60 kV and 55 mA with a Cu-Kα radiation of 0.15406 nm at a scanning rate of 4o/min from 20 to 80o. The crystallite sizes of the Fe2O3 hematite were further calculated by Debye-Scherrer equation using the values of full width at half maximum (FWHM) from the most intense diffraction peak of the Fe2O3 appeared at 2θ = 33.1o on the fresh FeZr catalysts. The bulk compositions of the fresh FeZr catalysts were confirmed by Xray fluorescence (XRF) analysis using a Bruker S4 instrument, which was operated at 60 kV and 150 mA. The analysis of temperature programmed reduction by H2 (TPR) of the fresh FeZr catalysts was carried out to verify the reduction behaviors of the Fe2O3 crystallites using a BELCAT instrument connected with a TCD analyzer. Prior to the TPR analysis, the FeZr catalyst sample was pretreated under a Ar flow at 350 oC for 1 h to remove any physisorbed water and contaminants. A reducing gas of 10vol% H2/Ar was fed over the sample at a flow rate of 30 ml/min in the temperature range of 100 - 900 oC at a heating rate of 10 oC /min. During the TPR analysis, the effluent gases were passed through a molecular sieve to completely remove any generated water. The degree of reduction (%) of the FeZr catalysts was calculated from a consumed amount of hydrogen below 600 oC from the TPR experiment by dividing it with a 7


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theoretical amount of hydrogen consumption. Binding energies (BE) and electronic states of the surface iron and zirconium species on the fresh and used FeZr catalysts were obtained by X-ray photoelectron spectroscopy (XPS) with a VG Multilab 2000 instrument at a vacuum level of 10-7 Pa using Al Kα monochromatized line of 1486.6 eV. For ex-situ XPS analysis, the fresh and used FeZr catalysts (without removal of any waxy hydrocarbons deposited to minimize the reoxidation under air exposure), the used sample was previously molded to a thin pellet. The BEs of iron and zirconium species were adjusted using a reference BE of C 1s of 284.6 eV. To verify outermost surface concentrations on the fresh and used FeZr, catalysts, the intensity ratios of each Fe 2p3/2 and Zr 3d5/2 peak were calculated by using an integrated each area of the iron and zirconium species. The surface ratios of Zr to Fe are denoted as IZr/IFe and C 1s peak was also analyzed to verify the relative iron carbide formations. X-ray absorption fine structure (XAFS) analyses of the fresh and used FeZr(0) and FeZr(0.3) (without removal of waxy hydrocarbons to minimize reoxidation of the used ones) were performed at Pohang accelerator Laboratory (PAL, 7D XAFS beamline) equipped with a Si(111) channel cut monochromator. The X-ray ring at the PAL has a flux of 2×1011 photons/s at 10 GeV, and the energy range capability was 5 – 30 keV. The extended X-ray absorption fine structure (EXAFS) data were obtained by using a near K-edge of iron metal species. The EXAFS spectra were recorded at room temperature in a cell using average values after several scans. The pre-edge background was subtracted by using power series curves and post-edge background was removed using appropriate splines. The XAFS spectra were further normalized by dividing by a height of absorption edge, and the obtained X-ray absorption near-edge structure (XANES) spectra were compared with reference materials such as Fe foil, FeO, Fe3O4, and Fe2O3 after normalization using an Athena program. Temperature-programmed surface reaction with H2 (TPSR) was carried out with 0.03g of 8


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the used FeZr catalysts to verify the extent of iron carbide formations or coke depositions. Each used FeZr sample was pretreated under a Ar flow at 350 oC for 1 h to remove any physisorbed water and other physisorbed waxy hydrocarbons before the analysis. The hydrogenation gases of 10 vol% H2/Ar was flowed over the sample at a flow rate of 30 ml/min in the temperature range of 100 oC to 800 oC at a heating rate of 10 oC/min. The effluent gases were also passed through a quadruple mass spectroscopy (MS Pfeiffer QMS 200) for analyzing the CH4 species (m/z = 15) formed by hydrogenation of the deposited surface coke precursors or iron carbides formed on the used FeZr surfaces. Raman spectra were further carried out by a SENTERRA Raman Microscope Spectrometer equipped with a laser beam emitting source of 532 nm operating at 20 mW. The used FeZr catalysts were previously ground into a fine powder and then applied for the analysis at an ambient temperature to verify the types of the deposited carbon precursors.

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Fe Mössbauer spectra

(MES) of the used FeZr catalysts were obtained by using a Topologic 500A spectrometer with a proportional counter at room temperature as well. 57Co(Rh) was used as a radioactive source moving in a constant acceleration mode and the data analyses were conducted by assuming a Lorentzian line-shape fitting. The separate iron phases were further identified based on their Mössbauer parameters including isomer shifts, quadruple splitting and magnetic hyperfine field.

3. Results and discussion 3.1. Textural and surface properties of the ordered mesoporous Fe2O3-ZrO2 catalysts Specific surface area (Sg), average pore diameter (Pd) and pore volume (Pv) of the fresh FeZr catalysts are summarized in Table 1. The surface areas of the fresh FeZr catalysts prepared by using a hard template of KIT-6 (TEM image in Figure 1(A-1)) were found to be in the range of 110.8 – 209.5 m2/g by showing the largest surface area of 209.5 m2/g on the FeZr(1) and 9


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smallest one of 110.8 m2/g on the FeZr(0). With an increase of the ZrO2 content on the FeZr catalysts, the specific surface area was increased due to its clear structural promoter effect by generating well-ordered mesopore structures18,19 of the FeZr catalysts. However, the average pore diameter showed no clear correlations with similar sizes of 5.1 - 7.2 nm, where a larger pore diameter was observed on the FeZr(0) with the value of 7.2 nm. The pore volumes were also found to be similar values in the range of 0.2 – 0.4 cm3/g regardless of the amount of the incorporated structural promoter of ZrO2 in the FeZr catalysts. This seems to be attributed to the possible incorporation of relatively larger ZrO2 crystallites into the ordered mesoporous Fe2O3 frameworks by forming relatively thicker Fe2O3-ZrO2 walls except for the FeZr(0) and FeZr(1), which also showed partial structural deformations during the preparation through the segregations of Fe2O3 and ZrO2 crystallites.19 As shown by TEM image in Figure 1(A-2) and by N2 adsorption-desorption isotherms in Figure 1(B) and supplementary Figure S1, the highly ordered mesoporous structures of the FeZr catalysts were clearly observed with a type IV isotherm and H1 hysteresis with a maximum pore size around 3 - 4 nm, which corresponds to a thickness of a hard template of KIT-6 as well. A relatively broad and smaller pore sizes on the FeZr(0) and FeZr(1) revealed the partial structural collapses possibly. The formations of the mesopore structures on all the fresh FeZr catalysts were clearly verified by TEM analysis with all TEM images of the fresh FeZr catalysts in supplementary Figure S2. Even after the incorporation of the structural promoter of ZrO2, the ordered mesoporous structures on all the fresh FeZr catalysts were stably preserved without any significant structural disintegrations during the preparation steps except for the fresh FeZr(0). These observations are well correlated with the pore size distributions of the FeZr catalysts. Interestingly, a relatively stable mesopore structure preservation was observed on the FeZr(0.3) and FeZr(0.1) even after a reductive FTS reaction condition compared with other FeZr catalysts as shown in supplementary Figure S2. 10


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XRD patterns of the fresh FeZr catalysts are displayed in Figure 1(C), and the characteristic diffraction peaks of the alpha-Fe2O3 hematite phases are clearly observed at 2θ = 24.2, 33.2, 35.7, 40.9, 49.5, 54.1, 62.5 and 64.1o which are responsible for the crystalline planes of (012), (104), (110), (113), (024), (116), (214) and (300) of the Fe2O3 hematite, respectively.24 The peak intensities were decreased with an increase of Zr/Fe molar ratio due to a lower content of iron species in the FeZr catalysts. Any incorporated ZrO2 phases in the main frameworks of the mesoporous Fe2O3 structures were not observed on all the FeZr catalysts due to its higher dispersion or intrinsically amorphous characters. The calculated crystallite sizes of the Fe2O3 magnetite24 calculated from the most intense diffraction peak appeared at 2θ = 33.2o are summarized in Table 1. The average crystallite sizes of the Fe2O3 were found to be in the range of 11.8 – 32.4 nm with the largest crystallite size of 32.4 nm on the FeZr(0). The crystallite size of the Fe2O3 was steadily decreased with an increase of Zr/Fe ratio. With the help of the structural promoter of ZrO2 added in the Fe2O3 main frameworks, the smaller Fe2O3 grain sizes can be formed due to well-developed Fe2O3-ZrO2 grain structures. A relatively larger Fe2O3 crystallite size from XRD than that of TEM image can be possibly attributed to its longitudinal distribution of iron oxides in the main frameworks of the FeZr catalysts. Although these highly distributed Fe2O3-ZrO2 crystallites can enhance an initial FTS activity, we believe that the stability of the mesopore structures of the FeZr catalysts can be more strongly affected by their interactions and distributions between the Fe2O3 and ZrO2 under a reductive FTS condition. The reduction behaviors of the fresh FeZr catalysts were characterized by TPR experiments and the reduction patterns are displayed in Figure 1(D), and the degrees of reduction of the FeZr catalysts are summarized in Table 1. Three distinctive reduction peaks were clearly observed on all the FeZr catalysts, which are well-known three reduction steps of the iron oxides from ɑ-Fe2O3 hematite phases to metallic iron species through the transformations of 11


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Fe2O3 → Fe3O4 → FeO → Fe0.25 Comparing with first reduction peak (Fe2O3 → Fe3O4) on the FeZr(0) appearing at 418 oC, all mesoporous FeZr catalysts showed somewhat higher reduction temperature peaks above 490 oC, which can be attributed to the weakly interacted surface iron oxides by the incorporated ZrO2 in the main frameworks of the Fe2O3 structures. However, the temperature shifts of the first maximum reduction peaks on all the FeZr catalysts except for the FeZr(0) were not significantly altered by showing the maximum temperatures in the range of 489 – 511 oC. The gradual decreases of second reduction peak intensity (Fe3O4 → FeO) with an increase of Zr/Fe ratios on the FeZr catalysts can be attributed to a structural instability under a preparation step due to a partial segregation of the Fe2O3 and ZrO2. These two reduction peaks can be assigned to the partial reduction of the Fe2O3 to FeO phases or to the formation of outermost metallic iron surfaces.26 Interestingly, a third reduction peak (FeO → metallic Fe0) above 700 oC showed significantly different behaviors according to the Zr/Fe ratios on the FeZr catalysts. These peak intensities and its maximum reduction temperatures were gradually decreased with an increase of Zr/Fe ratios by showing the highest temperatures on the FeZr(0.1) and FeZr(0.3) in the range of 730 – 774 o

C. However, the observed much higher reduction temperature of the third reduction peak on

the FeZr(0) seems to be attributed to the complete structural collapses of Fe2O3 during the reduction step with a difficult diffusion of H2 through the dense iron surfaces with a large crystallite size formed.26 Therefore, third reduction peaks can be assigned to the full reduction of iron oxides to metallic Fe, and it seems to be directly related with a structural stability of the mesoporous FeZr catalysts. Based on the previous research,27 iron and zirconium metal oxides having similar lattice structures can be interacted strongly between them. The intermediate phases of the FeO formed during the reduction step seems to be hardly reduced,28,29 which corresponds to the characteristic third reduction peak of TPR as well. We believe that these hardly reducible iron phases can be strongly interacted with the irreducible 12


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ZrO2 crystallites resulted in stabilizing the mesopore structures of the FeZr catalysts at an optimal Zr/Fe molar ratio of 0.1 – 0.3. Interestingly, the degree of reduction was increased proportionally due to the incorporation of the ZrO2 into the Fe2O3 structures by forming a small size of iron crystallites with an increase of Zr/Fe ratios as confirmed by XRD analysis. It can be also contributed to the different contents of the Fe2O3 in the FeZr catalysts since the real weight of Fe2O3 was decreased with an increase of Zr/Fe ratio, which can make an easy transport of H2 into the small grains of Fe2O3-ZrO2 by insignificantly forming the dense iron surface phases as well as by weakening the interactions of Fe2O3-ZrO2 structures.26 Therefore, a relatively hard reducibility with a lower reduction degree on the FeZr(0.1) and FeZr(0.3) seems to be responsible for an enhanced structural stability and easy formation of active iron carbides under the FTS reaction conditions. To verify the electronic states and outermost surface concentrations of the Fe and Zr species on the fresh FeZr catalysts, XPS analyses were carried out and the results are displayed in Figure 1(E) and supplementary Figure S3 and summarized in summarized in Table 1. It has been generally accepted that the electrons in an outer core level are sensitive according to the variation of structures, and the weak chemical bonds between Fe2O3 and ZrO2 crystallites can largely alter surface electronic properties. Therefore, the possible electron transfers between Fe 3p3/2 and Zr 3d5/2 orbitals were measured by XPS analysis to verify changes of the interactions between the ZrO2 and Fe2O3 species according to the Zr/Fe molar ratios on the fresh FeZr catalysts. The BEs of Fe 2p3/2 and Zr 3d5/2 peaks revealed the insignificant variations of BEs of Fe 2p3/2 appeared at around 710.3 – 711.1 eV and BEs of Zr 3d5/2 at around 181.6 – 182.4 eV, which corresponds to the phases of Fe2O3 hematite and ZrO2, respectively.30 However, the BEs of Fe 2p3/2 and Zr 3d5/2 were slightly increased at a higher Zr/Fe molar ratios due to intimate bimetal interactions of Fe2O3 and ZrO2 metal oxides, which strongly suggests the relatively stronger interactions between the Fe2O3 and ZrO2 13


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compared to the monometallic FeZr(0). The intensity ratios of the IZr/IFe on the fresh FeZr catalysts were steadily increased with an increase of the concentration of the incorporated ZrO2 from 0.6 on the FeZr(0.1) to 2.2 on the FeZr(1) due to the increased Zr content in the FeZr catalysts as summarized in Table 1. Therefore, the natures of difficult reduction of the FeZr(0.1) and FeZr(0.3) as confirmed by TPR analysis seem to be mainly attributed to the newly formed stronger interactions of the separate phases of the Fe2O3 and ZrO2 metal oxides, which can be beneficial for an enhanced structural stability of the FeZr(0.3) as well.

3.2. Catalytic activity of the ordered mesoporous Fe2O3-ZrO2 catalysts Catalytic activity and product distribution of the FeZr catalysts at a fixed reaction conditions of T = 300 oC, P = 2.0MPa and a relatively higher space velocity of 8000 L/(Kgcat·h) are summarized in Table 2, and their activity variations with time on stream (h) for 60 h are displayed in Figure 2. Even though CO conversions were not stabilized on the FeZr catalysts except for FeZr(0.3) and FeZr(0), the product distributions approached steady-state values after ~30 h on stream as shown in supplementary Figure S4, which can derive proper comparisons of the extraordinary stability of the FeZr(0.3) compared with other FeZr catalysts. The most stable catalytic activity with a higher C5+ and olefin selectivity were observed on the FeZr(0.1) and FeZr(0.3) with CO conversions of 82.1 - 88.3% and C5+ selectivity up to 85.6% (larger diesel-range hydrocarbons of C11+ above 72.3% as well), and vice versa on the FeZr(1) with a lower CO conversion of 44.8%. On the FeZr(0), CO conversion was significantly lower around 5.6% due to a fast initial deactivation by complete structural disintegrations and significant coke depositions (such as dense amorphous heavy hydrocarbons), which was similar with our previous mesoporous Co3O4 catalyst,17-19 Interestingly, the most active FeZr(0.3) without any additional chemical promoter addition showed the smallest formation of CH4 and light C2-C4 hydrocarbons due to the well14


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preserved mesoporous Fe2O3-ZrO2 crystalline phases. It can be originated from an easy formation of the active iron carbides such as the well-known χ-Fe5C2 phases with a less formation of unstable and deformed iron carbide species such as the γ-Fe4C phases (verified by Mössbauer analysis in the next section), where the strong adsorption natures of CO molecules on the edge/corner defect sites are responsible for a higher CH4 formation rate on the various transition metal catalysts.7,31-33 A relatively higher CO conversion to CO2 was found to be proportional to the CO conversion in the range of 38.3 – 46.5% on all the FeZr catalysts due to an enhanced WGS activity, which are similar results with our previous work.34 Especially, the observed higher CO2 formation on the FeZr(0) can be attributed to the deformed iron phases covered with heavy coke layers, which can enhance the activity of WGS reaction.34 With an increase of ZrO2 content such as on the FeZr(0.5) and FeZr(1), the longer induction period to approach a steady-state FTS activity was observed due to the difficult transformations of the metallic iron crystallites to the active iron carbides,35 which can be also prohibited by a significant structure disintegration with a larger heavy hydrocarbon deposition during FTS reaction. Therefore, the enhanced FTS activity and structural stability at an optimal amount of an irreducible ZrO2 promoter on the mesoporous Fe2O3 frameworks, especially on the FeZr(0.1) and FeZr(0.3), seem to be attributed to an increased degree of reduction and stable structural maintenance by forming strongly interacted Fe2O3-ZrO2 mesopore structures and by being easily transformed to the active and stable iron carbide species instead of a waxy hydrocarbon deposition.35 The much larger C11+ selectivity of 72.3% with a lower CH4 selectivity of 4.6% were characterized on the most stable FeZr(0.3) compared to other FeZr catalysts. Based on these superior structural properties on the stable FeZr(0.3) with a less structural deformation of the larger size of ordered mesopores, the much higher olefin selectivity of 53.6% with a high chain growth probability (α) value of 0.928 was observed on the FeZr(0.3) as shown in supplementary 15


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Figure S5 and Table 2. In general, the hydrocarbon distributions from FTS reaction depends on the pore structures as well as its sizes,36 and CH4 selectivity can be significantly increased on the iron-based FTS catalyst having a small pore size. It can be explained that H2/CO ratio can be increased inside of the small pores due to the suppressed diffusion rates of CO molecules compared with those of the H2 reactant through a liquid-like waxy hydrocarbon layers.37,38 The extents of active surface iron carbides on the stable mesoporous Fe2O3-ZrO2 structures seem to be further important to obtain a higher CO conversion and C5+ selectivity.34,35 The formations of the active iron carbide phases (FexC)39 were identified from the XRD analysis on the used FeZr catalysts as shown in supplementary Figure S6, where the FexC species seems to be in the formed of the active cementite carbide (θ-Fe3C), Hagg carbide (χ-Fe5C2) or hexagonal iron carbide (ε-Fe2C) and so on.40 In general, alkali metal components such as Na and SiO2 have been well known as the chemical promoters to improve the catalytic activity and selectivity to olefins on the iron-based FTS catalysts by easily forming active iron carbide phases.41 As summarized in supplementary Table S1, some residual of Na and silica used for a hard template, were not completely removed on the fresh FeZr catalysts even after sodium hydroxide treatment at 80 oC for 1 day. However, the amount of those residuals was similar on all FeZr catalysts around 5wt% as shown in Table S1, which suggest the similar residual effects for activity on the FeZr catalysts. Although the residual SiO2 can also affect the structural stability of FeZr catalysts, the concentrations of Na and Si were found to be similar in the range of 1.1 - 2.4wt% and 2.6 - 4.4wt% on all the FeZr catalysts, respectively. Therefore, we believe the different structural stability and activity on the FeZr catalysts were largely affected by the Zr/Fe ratios by forming different Fe2O3-ZrO2 interactions.

3.3. Roles of ZrO2 promoter for the active carbide formation and structural stability 16


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From the results of XPS analysis on the used FeZr catalysts as shown in supplementary Figure S7 and summarized in Table 1, the BEs of the Fe 2p3/2 peak on the used FeZr catalysts shifted to the much higher BE regions from ~711 eV on the fresh FeZr catalysts to 712.2 – 712.9 eV on the used ones. This observation strongly suggests the possible formations of the active iron carbides30,34,35,40 with the strong interactions of the Fe2O3 with ZrO2 promoter on the used FeZr catalysts. However, BEs of the Zr 3d5/2 appeared at ~ 182 eV on the fresh and used FeZr catalysts were not significantly altered due to its intrinsically irreducible characters. In addition, with an increase of ZrO2 content on the FeZr catalysts, these BEs shifted to the lower values due to the collapses of the ordered mesoporous structures through the complete phase segregations of Fe2O3 and ZrO2 nanoparticles, especially on the used FeZr(0.5) and FeZr(1). Therefore, the facile formations of the active iron carbide species with its strong interactions with ZrO2 promoter were responsible for a higher catalytic activity and structural stability. Furthermore, these stable iron phases seem to be easily formed by the facile transformation of the smaller metallic iron crystallites to the active iron carbide crystallites40,42 by simultaneously maintaining the stable mesopore structures, especially on the stable FeZr(0.3) and FeZr(0.1) compared to the monometallic FeZr(0). The outermost surface concentrations of the iron species were also verified by comparing the IZr/IFe ratios on the used FeZr catalysts and the results are summarized in Table 1. The intensity ratios of the IZr/IFe on the used FeZr catalysts were increased compared to the fresh FeZr catalysts, except for the FeZr(0.3) by showing the similar values of 0.6 – 0.7 before and after FTS reaction. The increased ratios of the IZr/IFe revealed that the ZrO2 nanoparticles can be preferentially moved to the bulk iron oxide surfaces by the disintegration or segregation of the FeZr catalysts. The observed small variations of the IZr/IFe even after FTS reaction on the FeZr(0.3) strongly revealed the enhanced structural stability without any significant structural collapses. It can be originated from the newly formed and 17


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strongly interacted Fe2O3 crystallites with the irreducible ZrO2 promoter by significantly generating a large amount of the active iron carbide surfaces. To further verify the reasons for a less structural disintegration of the FeZr(0.3), XAFS analyses were performed on two selected FrZr(0) and FeZr(0.3) before and after FTS reaction. The XAFS results are displayed in Figure 3(A) and summarized in supplementary Table S2 with the references of Fe foil, FeO, Fe3O4, and the Fe2O3 materials. The normalized XANES spectra on the fresh and used FeZr(0) and FeZr(3) as shown in Figure 3(A-1 and A-2) are similar with the Fe2O3 and metallic Fe with a partial presence of FeO phases, respectively. This observation strongly suggests the formation of the reduced iron metal phases dominantly from the Fe2O3 phases after FTS reaction, which are in line with the results of XRD and XPS analyses. However, the normalized XANES spectra on the used FeZr(0.3) showed a little bit similar with the FeO phases even after FTS reaction due to the strong interactions between the Fe2O3 and ZrO2 nanoparticles as confirmed by TPR analysis. To quantitatively verify the extent of iron phases formed after FTS reaction, XANES fittings were carried out on the fresh and used FeZr(0) and FeZr(0.3), respectively, and the results are summarized in Table S2. On the fresh ones, the main iron phases were found to be Fe2O3 on the fresh FeZr(0) and FeZr(0.3), and the main iron phases were found to be metallic Fe and FeO on the used FeZr(0) and FeZr(0.3), respectively. The XANES fitting results revealed that a large portion of the FeZr(0.3) were preserved in the partially reduced iron phases of FeO (54.4%) mainly, which are well matched with the results of XANES, TPR and XPS analyses. Therefore, the largely enhanced structural stability and a higher catalytic activity on the FeZr(0.3) were mainly attributed to the formation of the strongly interacted Fe2O3 with ZrO2 promoter by preferentially forming active iron carbides during FTS reaction. To further verify the relative amounts of the active iron carbides formed on the FeZr catalysts after FTS reaction, XPS analysis of the C 1s on the used FeZr catalysts was carried 18


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out and the results are displayed in Figure 3(B). The C 1s spectra were deconvoluted in two different peaks according to the types of the deposited carbon types. The BE of C 1s centered at ~ 284.7 eV indicates the sp2 hybridized carbon species (C=C), and the other higher BE at ~ 285.3 eV corresponds to the sp3 carbon (C-C).43 The intensity of the C=C and C-C bonds in the deposited carbons, which are related with the amount of the active iron carbides formed, was increased in the order of FeZr(0.1) > FeZr(0.3) > FeZr(0) > FeZr(0.5) > FeZr(1). The relative peak intensities were proportional to the catalytic activity, except for the FeZr(0) due to an inactive amorphous coke formations17-19 and structural disintegrations as confirmed by TEM in supplementary Figure S2. The relatively higher CO conversion with a lower C5+ selectivity on the FeZr(0.1) than those of the FeZr(0.3), even though the FeZr(0.1) has a larger amounts of iron carbides than that of the FeZr(0.3), can be possibly attributed to the structural instability by forming the segregated ZrO2 nanoparticles on the outermost surfaces of the iron carbides having strong interactions with ZrO2. These phenomena were also supported by the BEs shifts to the much higher ones of 712.9 eV for Fe 2p3/2 and 182.7 eV for Zr 3d5/2 species on the used FeZr(0.1) than those of FeZr(0.3), and these iron carbide sites are more active for a fast reaction of CO hydrogenation to light hydrocarbons such as CH4.7,31-33 In addition, these unstable iron surfaces formed showed a steady increase of CO conversion for 60 h on stream compared with the FeZr(0.3) as shown in Figure 2. In addition, Raman analysis was carried out on the used FeZr catalysts after 60 h reaction and the spectra are displayed in supplementary Figure S8. Two broad peaks appeared at 1320 and 1600 cm-1 were clearly observed and the peak at 1320 cm-1 can be assigned to the breathing mode of A1g symmetry (D band) for the disordered graphitic carbon structures and peak at 1600 cm-1 can be designated to the in-plane bond-stretching motions of carbon sp2 atoms (G band) for the ordered graphite carbonaceous species.44-46 The intensity of Raman spectra on the used FeZr catalysts was significantly altered according to the Zr/Fe ratios, and much smaller peak 19


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intensity was observed on the FeZr(0) due to its less iron carbide formation nature. The intensity ratios of D and G bands (assigned to ID/IG) are also proportional to the number of carbon clusters with the sizes below 2 nm as well.47 The calculated carbon cluster sizes are similar on all the FeZr catalysts with 1.2 nm as summarized in supplementary Table S3, which can be related with the catalytic activity. The somewhat smaller carbon cluster sizes on the FeZr(0.1) and FeZr(0.3) of 1.24 nm compared with those of FeZr(1) and FeZr(0) can be responsible for a higher catalytic activity and stability. In addition, the intensity ratios of ID to IG (ID/IG) as summarized in Table 1 can be also related with an extent of iron carbide formations, and its larger values suggest a facile formation of inactive coke precursors instead of iron carbides, especially on the less active FeZr(0) and FeZr(1) with the larger values of 0.91 – 1.07 compared to that of FeZr(0.1) and FeZr(0,3) with its smaller value of 0.84. The relative quantity of the surface carbonaceous species deposited on the used FeZr catalysts was analyzed by temperature-programmed surface reaction with H2 (TPSR), and the TPSR patterns are shown in Figure 3(C) to measure the evolution of CH4 (m/z = 15). The TPSR spectra can be deconvoluted with three characteristic peaks such as α, ß, γ peaks, where each peaks indicate atomic carbons (carbidic, α), amorphous surface methyl chains (ß) and bulk iron carbides (γ), respectively.48 Each characteristic peaks on the used FeZr catalysts appeared at the temperatures of ~ 340, 480 and 530 oC, and peak temperatures were slightly shifted to lower ones with an increase of the ZrO2 content in the FeZr catalysts as precisely summarized in supplementary Table S2. Bartholomew et al.49,50 have assigned these TPSR peaks as the α, ß, γ carbonaceous species, where the carbide-like ß and γ peaks indicates the possible formation of the active χ-Fe5C2 species as well. The carbide-like ß and γ peak intensities were found to be larger on the FeZr(0.3) and FeZr(0.1), which were most active sites for CO conversion with a higher C5+ selectivity. Interestingly, on the FeZr(0), which showed a larger XPS peak intensity of C 1s, there was no peaks assigned to the active iron 20


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carbides due to the dominant formation of the inactive amorphous hydrocarbons with a lower FTS activity. Among the ordered mesoporous FeZr catalysts, the peaks assigned to carbidelike carbon species were found to be larger on the FeZr(0.3) which showed the stable catalytic activity and structural stability. On the FeZr(0.5) having similar surface properties with the FeZr(0.3), the observed lower activity and structural instability on the FeZr(0.5) containing small amount of iron oxides as confirmed by TEM images in Figure S2 were originated from the unstable iron carbide formations with its small amounts (XPS and TPSR in Figure 3) which can finally disintegrate the mesopore structures of FeZr(0.5) with a lower C5+ selectivity. To verify the types of iron carbides formed on the selected FeZr catalysts, Mössbauer analysis was carried out on the used FeZr(0), FerZr(0.3) and FeZr(1) and the results are summarized in Table 1 with supplementary Table S4 and Figure 3(D) with Figure S9. The Mössbauer spectra were significantly different according to Zr/Fe ratios on the used FeZr catalysts, and the chemical compositions of the iron species were mainly in the forms of FeO, α-Fe2O3, χ-Fe5C2.51-55 For examples as summarized in supplementary Table S4, the iron phases were mainly composed of the χ-Fe5C2 (75.7%) and FeO (24.3%) on the active FeZr(0.3) (Figure 3(D-2)), however, non-permanent-magnetic doublet of the ferrous Fe2+ species was not observed on the FeZr(1) and more complicated iron phases such as Fe2O3 (16.9%), χ-Fe5C2 (53.2%), γ-Fe4C (22.7%) and FeO (7.2%) were observed on the less active FeZr(0) (Figure 3(D-1)). It has been well reported that the χ-Fe5C2 phases are promising active sites for the FTS reaction.12,56-58 As summarized in Table 1, the extent of χ-Fe5C2 formation was found to be much larger on the active FeZr(0.3), which also showed a higher activity with a structural stability due to the stable and smaller iron carbide formation with the help of the strongly interacted FeO-ZrO2 frameworks. On the FeZr(0), the abundant presence of γ-Fe4C phases having less stable tetrahedral carbide structures can be also responsible for an easy structural disintegration during FTS reaction than that of the χ-Fe5C2 21


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phases having a stable trigonal prismatic carbide structures59 on the FeZr(0.3). On the FeZr(1), the observed doublet represents the Fe2+ surface structures with an incomplete carbide formations in the forms of Fe2C species through the inactive coke depositions selectively, which can also be responsible for a lower catalytic activity. In summary, the observed higher catalytic and structural stability of the ordered mesoporous Fe2O3-ZrO2 structures on the used FeZr(0.3) compared with the FZr(0) can be attributed to the strong interactions between Fe2O3 and ZrO2 by easily forming the active iron carbides such as the χ-Fe5C2 species, which was confirmed by TEM images on the used ones in supplementary Figure S2 for all of the fresh and used FeZr catalysts and the magnified TEM images of the FeZr(0) and FeZr(0.3) with the schematic structural formations of the active iron carbides in Figure 4. The structural deformations were clearly observed on the FeZr(0) by forming a larger iron crystallites above 30 nm in size with significant structure collapses. and relatively less structural deformations were observed on the other FeZr catalysts. However, the FeZr catalysts prepared at lower or higher Zr/Fe ratio such as the FeZr(0.1) or FeZr(1) showed a less structural stability during FTS reaction due to a selective deposition of the segregated ZrO2 nanoparticles on the iron carbide surfaces or complete phase segregations of Fe2O3 and ZrO2, respectively. Interestingly, the highly ordered mesoporous structures on the FeZr(0.3) were clearly preserved without any significant structural disintegrations due to the strongly interacted Fe2O3 with ZrO2 at an optimal ZrO2 content (Zr/Fe = 0.25). The facile formations of the stable χ-Fe5C2 species on the structurally stable Fe2O3-ZrO2 mesopores (FeZr(0.3)) under a reductive FTS reaction condition were strongly related with its higher FTS activity and stability.

4. Conclusions A novel highly ordered mesoporous Fe2O3-ZrO2 without any additional chemical promoters 22


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on the iron-based FTS catalysts was applied for CO hydrogenation to valued-added hydrocarbons by varying the concentration of the incorporated ZrO2 promoter in the mesoporous Fe2O3. The significantly enhanced catalytic activity and structural stability were obtained at an optimal molar ratio of Zr/Fe at around 0.25, and the partial segregation of Fe2O3 and ZrO2 phases was observed on the other Zr/Fe ratios. The FTS superior activity and stability on the FeZr(0.3) were mainly attributed to the newly formed strong interactions between Fe2O3 and irreducible ZrO2 crystallites in the ordered mesoporous Fe2O3-ZrO2 main frameworks. In addition, the higher C11+ selectivity at a high CO conversion were also induced from the facile formation of active iron carbide species such as χ-Fe5C2 on the stabilized mesoporous Fe2O3-ZrO2 structures with insignificant structural disintegrations. The highly ordered mesoporous Fe2O3-ZrO2 structures can also enhance a transport rate of the heavy hydrocarbons through the larger stable mesopore structures with less formation of inactive coke precursors on the ordered mesopores of the FeZr catalysts, which were confirmed by coke characterization through XPS, XAFS, TPSR and 57Fe Mössbauer analyses.

Acknowledgements The authors would like to acknowledge the financial supports from the National Research Foundation of Korea (NRF) grant funded by the Korea government (Project #: NRF2016M3D3A1A01913253 and 2017R1D1A1B03028214). This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) with Project numbers of 20132010201750.

Supporting Information Additional information such as pore size distribution from N2 adsorption-desorption, XRD, XPS, TEM, XAFS (including XANES fitting) and Mössbauer analyses of the fresh and used 23


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FeZr catalysts are included in supplementary material. This additional material is available free of charge via the Internet at http://pubs.acs.org.

Notes and references (1) Diehl, F.; Khodakov, A. Y. Oil Gas Sci. Technol. 2009, 64, 11-24. (2) Xiang, H. F.; Jewell, L. L.; Conille, N. J. ACS Catal. 2015, 5, 2640-2658. (3) Fu, T. J.; Li, Z. H. Chem. Eng. Sci. 2015, 135, 3-20. (4) James, O. O.; Chowdhury, B.; Mesubi, M. A.; Maity, S. RSC Adv. 2012, 2, 7347-7366. (5) Braganca, L. F. F. P. G.; Ojeda, M.; Fierro J. L. G., Pais da Silva, M.I. Appl. Catal., A 2012, 423-424, 146-153. (6) Shi, B.; Davis, B. H. Appl. Catal., A 2004, 277, 61-69. (7) Khodakov, A. Y.; Chu, W.; Fongarland, P. Chem. Rev. 2007, 107, 1692-1744. (8) Schulz, H. Appl. Catal., A 1999, 186, 3-12. (9) Cheng, X.; Yang, H.; Tatarchuk, B. J. Catal. Today. 2016, 273, 62-71. (10) Jin, Y.; Datye, A. K. J. Catal. 2000, 196, 8-17. (11) Li, S.; Krishnamoorthy, S.; Li, A.; Meitznera, G. D.; Iglesia, E. J. Catal. 2002, 206, 202217. (12) Cano, L. A.; Cagnoli, M. V.; Bengoa, J. F.; Alvarez, A. M.; Marchetti, S. G. J. Catal. 2011, 278, 310-320. (13) Merkache, R.; Fechete, I.; Maamache, M.; Bernard, M.; Turek, P.; Al-Dalama, K.; Garin, F. Appl. Catal., A 2015, 504, 672-681. (14) Jung, J. S.; Kim, S. W.; Moon, D. J. Catal. Today. 2012, 185, 168-174. (15) Rosen, J.; Hutchings, G. S.; Jiao, F. J. Catal. 2014, 310, 2-9. (16) Shu, P.; Ruan, J.; Gao, C.; Li, H.; Che, S. Micropor. Mesopor. Mater. 2009, 123, 314-323. (17) Ahn, C. I.; Koo, H. M.; Jin, M.; Kim, J. M.; Kim, T.; Shu, Y.; Yoon, K. J.; Bae, J. W. 24


Page 24 of 35

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Micropor. Mesopor. Mater. 2014, 188, 196-202. (18) Ahn, C. I.; Koo, H. M.; Cho, J. M.; Roh, H. S.; Lee, J. B.; Lee, Y. J.; Jang, E. J.; Bae, J.W. Appl. Catal., B 2016, 180, 139-149. (19) Ahn, C. I.; Lee, Y. J.; Um, S. H.; Bae, J.W. Chem. Commun. 2016, 52, 4820-4823. (20) Tuysuz, H.; Liu, Y.; Weidenthaler, C.; Schuth, F. J. Am. Chem. Soc. 2008, 130, 1410814110. (21) Sun, S.; Zhao, X.; Yang, M.; Wu, L.; Wen, Z.; Shen, X. Sci. Rep. 2016, 6, 19564-19573. (22) Li, Q.; Du, Y.; Li, X.; Lu, G.; Wang, W.; Geng, Y.; Liang, Z.; Tian. X. Sens. Actuators B 2016, 235, 39-45. (23) Fei, S.; Xia, K.; Tian, X.; Mei, P.; Yan, H.; Cheng, H. Int. J. Hydrogen Energy 2016, 41, 5652-5660. (24) Ang, W. A.; Gupta, N.; Prasanth, R.; Hng, H. H.; Madhavi, S. J. Mater. Res. 2013, 28, 824-831. (25) Mou, X.; Wei, X.; Li, Y.; Shen, W. Cryst. Eng. Comm. 2012, 14, 5107-5120. (26) Jeong, M. H.; Lee, D. H.; Bae, J. W. Int. J. Hydrogen Energy 2015, 40, 2613-2620. (27) Chen, K.; Fen, Y.; Hu, Z.; Yan. Q. Catal. Lett. 1996, 36, 139-144. (28) Zielinski, J.; Zglinickab, I.; Znaka, L.; Kaszkur, Z. Appl. Catal., A 2010, 381, 191-196. (29) Rosmaninho, M. G.; Moura, F. C. C.; Souza, L. R.; Nogueira, R. K.; Gomes, G. M.; Nascimento, J. S.; Pereira, M. C.; Fabris, J. D.; Ardisson, J. D.; Nazzarro, M. S.; Sapag, K.; Araujo, M. H.; Lago, R. M. Appl. Catal., B 2012, 115-116, 45-52. (30) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G.E. Handbook of X-ray Photoelectron Spectroscopy.; Physical Electronics Division, Perkin-Elmer Corporation; 1979; p 76 and p 100. (31) den Breejen, J. P.; Radstake, P. B.; Bezemer, G. L.; Bitter, J. H.; Froseth, V.; Holmen, A.; de Jong, K. P. J. Am. Chem. Soc. 2009, 131, 7197-7203. 25


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(32) Borg, O.; Dietzel, P. D. C.; Spjelkavik, A. I.; Tveten, E. Z.; Walmsley, J. C.; Diplas, S.; Eri, S.; Holmen, A.; Rytter, E. J. Catal. 2008, 259, 161-164. (33) Koh, T.; Koo, H. M.; Yu, T.; Lim, B.; Bae, J. W. ACS Catal. 2014, 4, 1054-1060. (34) Bae, J. W.; Park, S. J.; Lee, Y. J.; Park, H. G.; Kim, Y. B.; Lee, D.H.; Kim, B. W.; Park, M. J. Catal. Lett. 2011, 141, 799-807. (35) Kang, S. H.; Bae, J. W.; Cheon, J. Y.; Lee, Y. J.; Ha, K. S.; Jun, K. W.; Lee, D. H.; Kim, B.W. Appl. Catal., B 2011, 103, 169-180. (36) Zhang, X.; Su, H.; Yang, X. J. Mol. Catal. A: Chem. 2012, 360, 16-25. (37) Rohr, F.; Lindvag, O. A.; Holmen, A.; Blekkan, E. A. Catal. Today 2000, 58, 247-254. (38) Enache, D. I.; Roy-Auberger, M.; Revel, R. Appl. Catal., A 2004, 268, 51-60. (39) Prakash, R.; Mishra, A. K.; Roth, A.; Kubel, C.; Scherer, T.; Ghafari, M.; Hahn, H.; Fichtner, M. J. Mater. Chem. 2010, 20, 1871-1876. (40) Xu, K.; Sun, B.; Lin, J.; Wen, W.; Pei, Y.; Yan, S.; Qiao, M.; Zhang, X.; Zong, B. Nature Commun. 2014, 5, 5783. (41) Li, J.; Cheng, X.; Zhang, C.; Chang, Q.; Wang, J.; Wang, X.; Lv, Z.; Dong, W.; Yang, Y.; Li, Y. Appl. Catal., A 2016, 518, 131-141. (42) Bezemer, G. L.; Bitter, J. H.; Kuipers, H. P. C. E.; Oosterbeek, H.; Holewijn, J. E.; Xu, X.; Kapteijn, F.; van Dillen, A. J.; de jong, K. P. J. Am. Chem. Soc. 2006, 128, 3956-3964. (43) Ahmed, M. H.; Byrne, J. A.; McLaughlin, J.; Ahmed, W. J. Biomater. Nanobiotechnol. 2013, 4, 194-203. (44) Urbonaite, S.; Halldahl, L.; Svensson, G. Carbon 2008, 46, 1942-1947. (45) Ferrari, A, C.; Robertson, J. Phys. Rev. B 2000, 61, 14095-14107. (46) Al-Dossary, M.; Fierro, J, L, G.; Spivey, J, J. Ind. Eng. Chem. Res. 2015, 54, 911-921. (47) Zhang, C.; Zhao, G.; Liu, K.; Yang, Y.; Xiang, H.; Li, Y. J. Mol. Catal. A: Chem. 2010, 328, 35-43. 26


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(48) Pour, A. N.; Housaindokht, M. R.; Tayyari, S. F.; Zarkesh, J.; Alaei, M. R. J. Mol. Catal. A: Chem. 2010, 330, 112-120. (49) Eliason, S. A.; Bartholomew, C. H.; in Bartholomew, C. H.; Fuentes G. A., Eds.; Catalyst Deactivation, Elsevier: Amsterdam, 1997; p 517. (50) Xu, J.; Bartholomew, C. H. J. Phys. Chem. B 2005, 109, 2392-2403. (51) Jian, W.; Qingjie, G.; Ruwei, Y.; Zhiyong, Wen.; Chuanyan, F.; Lisheng, G.; Hengyong, X.; Jian, S. Nature Commun. 2017, 8, 15174. (52) Park, J, C.; Yeo, S, C.; Chen, D, H.; Lim, J, T.; Yang, J, I.; Lee, H, T.; Hong, S, J.; Lee, H, M.; Kim, C, S.; Jung, H. J. Mater. Chem. A 2014, 2, 14371-14379. (53) Xin, Z.; Yongan, N.; Xiangdong, M.; Yao, L.; Jiupeng, Z. CrystEngComm 2013, 15, 8166-8172. (54) Perez-Cabero, M.; Taboada, J, B.; Guerrero-Ruiz,A.; Overweg, A, R.; Rodriguez-Ramos, I. Phys. Chem. Chem. Phys. 2006, 8, 1230-1235. (55) Cristina, G.; Alexander, K.; Iris, F.; Cristina, H.; goestar, K.; markus, A. J. Mater. Chem. 2011, 21, 16963-16967. (56) Mazzucco, S.; Wang, Y.; Tanase, M.; Picher, M.; Li, K.; Wu, Z.; Irle, S.; Sharma, R. J. Catal. 2014, 319, 54-60. (57) Yang, C.; Zhao, H.; Hou, Y.; Ma, D. J. Am. Chem. Soc. 2012, 134 (38), 15814-15821. (58) Peng, Z.; Cong, X.; Rui, G.; Xi, L.; Mengzhu, L.; Weizhen, L.; Xinpu, F.; Chunjiang, J.; Jinglin, X.; Ming, Z.; Xia, W.; Yong, W, L.; Qianwen, Z.; Xiao, D, W.; Ding, M. Angew. Chem. Int. Ed. 2016, 55, 9902-9907. (59) Liu, X, W.; Zhao, S.; Meng, Y.; Peng, Q.; Albert, K, D.; Huo, C, F.; Yang, Y.; Li, Y, W.; Xiao, D, W. Sci. Rep. 2016, 6, 26184.

Figure Captions 27


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Figure 1. TEM image (20 nm scale-bar) of (A-1) KIT-6 and (A-2) FeZr(0.3), (B) Pore size distribution of the fresh FeZr catalysts obtained from N2 adsorption-desorption isotherms, (C) Powder XRD patterns of the fresh FeZr catalysts, (D) TPR profiles of the fresh FeZr catalysts and (E) XPS spectra of Fe 3p3/2 on the fresh FeZr catalysts Figure 2. CO conversion with time on stream on the FeZr catalysts for 60 h on stream Figure 3. Normalized XANES fitting results of (A-1) FeZr(0) and (A-2) FeZr(0.3) before and after FTS reaction with the references of Fe foil, FeO, Fe3O4, and the Fe2O3, (B) XPS spectra of C 1s of the used FeZr catalysts, (C) TPSR patterns (m/z = 15) of the used FeZr catalysts and (D) 57Fe Mössbauer spectra with reference samples of FeO, Fe2O3 and χ-Fe5C2 of the used (D-1) FeZr(0) and (D-2) FeZr(0.3) Figure 4. Representative TEM images (20 nm scale-bar) of the fresh and used FeZr(0) and FeZr(0.3)

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Table 1. Surface and bulk characteristics of the ordered mesoporous Fe2O3-ZrO2 catalysts N2 sorptionb

XRD H2-TPR Raman Mössbauer XPS (fresh→used) crystallite Reduction surface Notationa Sg Pd Pv BE (eV) of BE (eV) of degree ID/IG size of IZr/IFe χ-Fe5C2 (%)f 2 3 (m /g) (nm) (cm /g) Fe 2p3/2 Zr 3d5/2 c d e Fe2O3 (nm) (%) ratio 53.2 FeZr(0) 0 110.8 7.2 0.3 32.4 33.6 710.3→712.2 0.907 n.a. FeZr(0.1) 0.13 127.9 5.4 0.2 22.8 35.8 710.3→712.9 181.6→182.7 0.6→1.3 0.843 75.7 FeZr(0.3) 0.25 147.6 5.8 0.3 18.8 45.8 711.0→712.3 182.2→182.1 0.7→0.6 0.845 FeZr(0.5) 0.50 147.9 5.1 0.2 15.9 44.4 n.a. 711.1→712.6 182.4→182.8 1.2→1.7 0.835 0 FeZr(1) 1.0 209.5 6.7 0.4 11.8 53.5 711.0→712.3 182.0→182.2 2.2→3.3 1.069 a FeZr(x) represents the ordered mesoporous mixed metal oxides of Fe2O3- ZrO2, where x represents the molar ratios of Zr to Fe metal (Zr/Fe). b Specific surface area, average pore diameter, and pore volume were measured by N2 adsorption- desorption analysis and they were denoted as Sg, Pd, and Pv, respectively. c Crystallite size of the Fe2O3 magnetite was calculated using the most intense diffraction peak at 2θ = 33.2o with the help of Scherrer equation. d Degree of reduction (%) of the fresh FeZr catalysts was calculated by using the amount of hydrogen consumption below 600 oC divided by the theoretical amount of hydrogen consumption. e Intensity ratio of the IZr/IFe was calculated by using an atomic sensitivity factor (ASF) and integrated area of each XPS peaks, and IZr/IFe = (peak area of Zr / peak area of Fe) x (SZr/SFe), where S represents ASF value on the fresh and used FeZr catalysts. f Mössbauer analysis was not analyzed (n.a.) on the FeZr(0.1) and FeZr(0.5) due to its similar properties of carbon species (ID/IG ratio) with Raman analysis. Zr/Fe molar ratio

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Table 2. Catalytic activity and product distribution on the ordered mesoporous Fe2O3-ZrO2 catalystsa CO Product distribution (C-mol%) ChainCO Olefin conversion growth selectivityb Notation conversion to CO2 probability CH4 C2-C4 C5-C10 C11+ (C-mol%) (C-mol%) (C-mol%) (ɑ)c FeZr(0) 5.6 46.5 7.6 14.1 78.3 72.4 NA FeZr(0.1) 88.3 42.2 9.6 14.1 22.4 53.9 42.6 0.917 FeZr(0.3) 82.1 41.2 4.6 9.8 13.3 72.3 53.6 0.928 FeZr(0.5) 77.0 41.4 9.9 14.1 20.6 55.4 39.6 0.903 FeZr(1) 44.8 38.3 9.4 13.8 19.6 57.2 37.5 0.918 a Catalytic performance data were obtained using the theveraged values at a steady-state under the reaction conditions of T = 300 oC, P = 2.0 MPa, WHSV = 8000 L/(kgcat·h), and H2/CO molar ratio of 2. b Olefin selectivity (C-mol%) was calculated in the C2-C4 light hydrocarbons. c Chain growth probability (ɑ) of waxy hydrocarbons formed by the FTS reaction was obtained from the Anderson-Schulz-Flory (ASF) distribution in the range of C12-C30 hydrocarbons.

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Figure 1. TEM image (50 nm scale bar) of (A-1) KIT-6 and (A-2) FeZr(0.3), (B) Pore size distribution of the fresh FeZr catalysts obtained from N2 adsorption-desorption isotherms, (C) Powder XRD patterns of the fresh FeZr catalysts, (D) TPR profiles of the fresh FeZr catalysts and (E) XPS spectra of Fe 3p3/2 on the fresh FeZr catalysts 31


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Figure 2. CO conversion with time on stream on the FeZr catalysts for 60 h on stream

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Figure 3. Normalized XANES fitting results of (A-1) FeZr(0) and (A-2) FeZr(0.3) before and after FTS reaction with the references of Fe foil, FeO, Fe3O4, and the Fe2O3, (B) XPS spectra of C 1s of the used FeZr catalysts, (C) TPSR patterns (m/z = 15) of the used FeZr catalysts and (D) 57Fe Mössbauer spectra with reference samples of FeO, Fe2O3 and χ-Fe5C2 of the used (D-1) FeZr(0) and (D-2) FeZr(0.3) 33


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Figure 4. Representative TEM images (20 nm scale bar) of the fresh and used FeZr(0) and FeZr(0.3)

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GRAPHICAL ABSTRACT Highly ordered mesoporous Fe-Zr bimetal oxides for an enhanced activity of CO hydrogenation to hydrocarbons with their structural stability

Jae Min Cho, Sae Rom Lee, Jian Sun, Noritatsu Tsubaki, Eun Joo Jang, Jong Wook Bae*

Highly ordered mesoporous Fe2O3-ZrO2 mixed bimetal oxides (FeZr) without any additional chemical promoters were firstly applied for CO hydrogenation to hydrocarbons. The improved catalytic activity and stability of the ordered mesoporous FeZr(0.3) were mainly attributed to the facile formation of active iron carbide species such as χ-Fe5C2 without any significant structural collapses by the strongly interacted iron nanoparticles with ZrO2 structural promoter.

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