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Hydrogenation of carbon dioxide over Co-Fe bimetallic catalysts Muthu Kumaran Gnanamani, Gary Jacobs, Hussein H Hamdeh, Wilson D. Shafer, Fang Liu, Shelley D Hopps, Gerald A. Thomas, and Burtron H. Davis ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.5b01346 • Publication Date (Web): 24 Dec 2015 Downloaded from http://pubs.acs.org on December 25, 2015

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Hydrogenation of carbon dioxide over Co-Fe bimetallic catalysts Muthu Kumaran Gnanamania, Gary Jacobsa, Hussein H. Hamdehb, Wilson D. Shafera, Fang Liua, Shelley D. Hoppsa, Gerald A. Thomasa, Burtron H. Davisa* a

Center for Applied Energy Research, University of Kentucky, 2540 Research Park Dr., Lexington, KY, 40511, USA b

Wichita State University, Department of Physics, Wichita, KS, 67260, USA

Abstract A series of Co-Fe bimetallic catalysts was prepared, characterized and studied for the hydrogenation of carbon dioxide reaction. The catalyst precursors were prepared via an oxalate co-precipitation method. Monometallic (Co or Fe) and bimetallic (Co-Fe) oxalate precursors were decomposed under N2 flow at 400°C and further pretreated in a CO flow at 250°C. The catalysts (before decomposition of the oxalates or after activation) were characterized by BET, TGA-MS, X-ray diffraction, CO-TPR, SEM, HR-TEM, and Mössbauer spectroscopy techniques. The hydrogenation reaction of CO2 was performed using Co-Fe bimetallic catalysts pretreated in situ in a fixed-bed catalytic micro-reactor operating in the temperature range of 200-270°C and a pressure of 0.92 MPa. With increasing Fe fraction, the selectivity to C2-C4 for Co-Fe catalyst increased at all operating conditions.

The alcohol selectivity was found to increase with

increasing iron content of the Co-Fe catalyst up to 50% but then it dropped with further addition of iron. Among the three different activation conditions, the CO pretreated Co-Fe (50Co50Fe) catalyst exhibited a much lower selectivity for methane. Addition of 1 wt% Na or 1.7 wt% K to 50Co50Fe catalyst increases its olefinic (C2-C4) and oxygenates selectivities. Keywords: Co-Fe bimetallic catalyst; oxalates co-precipitation method; carbon dioxide (CO2) hydrogenation; iron carbides; Mössbauer spectroscopy; thermogravimetry; cobalt carbide; oxygenates.

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* Corresponding author. Tel.: +1 859 257 0251; fax: +1 859 257 0302; email: [email protected]

1. Introduction Cobalt and iron are the two elements being used by the industry to produce fuels and chemicals from syngas (H2+CO) via Fischer-Tropsch synthesis (FTS).1-3 However, the active form of cobalt and iron for FT synthesis are different.4 Many have agreed that carbide is the main active form of iron whereas metallic cobalt is the main active form of cobalt for FTS.5,6 Under typical FTS conditions using H2 and CO2, cobalt tends to selectively produce methane and a small quantity of lower hydrocarbons (C2-C4) in comparison with iron, the latter of which yields a regular FT product spectrum.

This difference between iron and cobalt regarding

selectivity for CO2 hydrogenation is mainly due to the lack of reverse water-gas shift (RWGS) activity exhibited by cobalt; RWGS is assumed to be a reaction step in the primary pathway for the conversion of CO2 to higher hydrocarbons.

Therefore, most of the studies for CO2

hydrogenation have focused on using Fe-based FTS catalysts7-10 and few papers have been published in the open literature that have focused on using cobalt.11,12 Early studies of CO2 hydrogenation have explored the kinetics and mechanisms of Nibased catalysts13-17. Falconer and Zagli14 have studied adsorption and methanation of carbon dioxide for a Ni/SiO2 catalyst. The authors were found that carbon dioxide dissociatively adsorbs on Ni at elevated temperatures to yield CO and O. Moreover, the activated adsorption of CO2 on Ni produced a very high H2:CO surface ratio during state-state hydrogenation of CO2 and this favors methane formation over higher hydrocarbons.

G.D. Weatherbee and C.H.

Bartholomew17 compared intrinsic rates and product distributions for CO2 hydrogenation on Co/SiO2, Fe/SiO2 and Ru/SiO2 catalysts at 450-650 K, 140-1030 kPa, and a range of space


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velocities. The authors found that the selectivity to methane is very high on Ru/SiO2 and decreases in the order Ru/SiO2, Ni/SiO2, Co/SiO2, Fe/SiO2. Russell and Miller18 reported Cuactivated, Co catalysts for the synthesis of higher hydrocarbons from CO2 and H2. The authors showed that there was no liquid formation when the catalyst contained no alkali but did yield small amounts of liquid hydrocarbon when alkali was present. Stowe and Russell19 have studied cobalt, iron and their alloys as catalysts for the hydrogenation reaction of carbon dioxide. The authors did not find any relation between composition and catalytic activity; however, they mentioned that the observed activity can be explained based on the atomic moment and lattice distance. Owen et al.20 showed that the addition of alkali metals like Na and K to cobalt enhanced C5+ selectivity for the hydrogenation of carbon dioxide at elevated temperatures and atmospheric pressure. Our recent study21 with sodium promoted carburized cobalt supported on silica for the hydrogenation reaction of CO2 revealed that both sodium as well as the partially carburized form of cobalt are responsible for the high alcohol selectivity (73.2%) observed. Satthawong et al.22,23 have recently reported a significant bimetallic promotion of C2+ hydrocarbons synthesis in CO2 hydrogenation on Fe-Co catalysts. The authors claimed that the addition of a small amount of Co with Fe leads to a dramatic improvement in selectivity to C2+ hydrocarbons. In their work, Fe-Co bimetallic catalysts supported over γ-Al2O3 were activated in the presence of hydrogen to obtain the active form of the catalyst. However, the carburized form of iron is known to be more active and selective toward higher hydrocarbons for CO and CO2 hydrogenation reactions.5,24 Hence, it is of interest to study such hydrogenation reactions using Co-Fe bimetallic catalysts in a carburized form.


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In the present work, we explored the possibility of keeping iron and cobalt in proximity in a carburized form and to test for the hydrogenation activity of carbon dioxide. For this, Co-Fe bimetallic oxalates were prepared by co-precipitating cobalt and iron in various proportions using oxalic acid as a precipitating agent. Pretreatment of Co-Fe catalyst in a CO flow at 250°C yields various phases such as FeCx, CoC2, FeCo alloy and cobalt containing Fe3O4 (i.e., CoFe2O4) in addition to CoO, Fe3O4 and metallic cobalt and iron. The effect of temperature and the fraction of Fe in Co-Fe catalysts were investigated for the activity and selectivity of hydrogenation of carbon dioxide. Different pretreatment conditions (H2, syngas, or CO) were followed in order to explore the differences in the activity and selectivity for CO2 hydrogenation between metallic and carburized forms of the Co-Fe catalyst. 2. Experimental 2.1 Preparation of Catalysts 2.1.1 Preparation of Co-Fe bimetallic oxalates Co-Fe catalysts with varying atomic ratios [100Co, 90Co10Fe, 50Co50Fe, 25Co75Fe, 100Fe] were prepared by an oxalate route.25 A solution containing appropriate amounts of cobalt nitrate [Co(NO3)2.6H2O, Aldrich, >98%] and ferrous sulfate [FeSO4.7H2O, Aldrich, >98%] was made having an excess in oxalic acid [H2C2O4.2H2O, Aldrich, >99%] at slightly

warm

conditions (35-40°C). Precipitation was virtually instantaneous, and the mixture was stirred vigorously at ~35°C for at least 60 min. After removal of the supernatant liquid, the precipitated metal oxalate was filtered and then dried in air at 100°C. Only air dried oxalate samples at room temperature were used for TGA-MS analysis to study the decomposition of metal oxalates. For the purpose of comparison, 1 wt.% Na or equivalent K (1.7 wt.%) was introduced to the dried


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oxalate metal precursor (50Co50Fe) by conventional incipient-wetness impregnation to obtain the alkali promoted 50Co50Fe catalyst. 2.1.2 Decomposition of metal oxalates and pre-activation The decomposition of metal oxalates (CoxFe1-x.C2O4.2H2O) was performed using a fixedbed plug-flow reactor with 2.5 g of oxalate precursor under N2 flow at 400°C for 2 h. The resulting sample was subjected to passivation under flowing 1%O2 in N2 at room temperature (25°C) for 3 h before being used for the purpose of characterization by methods such as BET, Xray diffraction, and CO-TPR. There was a very small exotherm (~5°C) associated with the passivation, which was deemed complete after the sample had once again returned to room temperature. This was the standard procedure for performing ex-situ characterization of any air sensitive activated transitional metals such as Co, Fe, Ru as well as transitional metal carbides like cobalt carbides, iron carbides, etc., protecting against air by oxidation of the outermost surface layers, and thus impeding the further modification of the metals or carbonaceous species present on the solid surface.26 After decomposition in N2 atmosphere, Co-Fe samples were activated under CO flow at 250°C for 20 h to obtain the catalyst in a carburized form. Three different activation methods (H2, syngas, CO) were tested and among the three, CO activation was employed for all of the catalysts prepared with different atomic ratios of Co to Fe. The 50Co50Fe catalyst was tested for the other two activation conditions (i.e., using syngas and H2). The details of the three activation conditions followed in this work are described below. (i) CO activation: For CO activation, 2.5 grams of the dried metal oxalates precursor (CoxFe1x.C2O4.2H2O)

was initially decomposed in N2 (3.0 slph) at 400°C for 2 h followed by pretreating

under CO flow (3.0 slph, H2:CO:N2) at 250°C for 20 h.


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(ii) syngas activation: With syngas pretreatment, 2.5 grams of the dried metal oxalates precursor (CoxFe1x.C2O4.2H2O)


under syngas flow (3.0 slph, H2:CO:N2) at 300°C for 20 h. (iii) H2 activation: For H2 reduction treatment, 2.5 grams of the dried metal oxalates precursor (CoxFe1x.C2O4.2H2O)


under H2 flow (15.0 slph, H2:N2) at 350°C for 20 h. The samples after different activation methods were subjected to passivation as described earlier in this section, and used for the purpose of characterization such as XRD, HR-TEM and SEM techniques. 2.2 Catalyst Characterization The compositions of the dried Co-Fe bimetallic oxalate samples at 100°C were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a Varian 720-ES analyzer. The materials were dissolved in a perchloric/nitric acid mixture and the emission spectra of dissolved species (Co, Fe and alkali) were compared to those of a series of standard solutions of known concentrations. Carbon analyses were made using a Leco CHN 628 analyzer. The sample was combusted at 1,223 K in oxygen and the carbon, as CO2, was determined using an IR detector. A Micromeritics Tri-Star system was used to determine BET surface area and porosity of decomposed and passivated Co-Fe samples. Prior to the measurement, the sample temperature was slowly ramped to 160°C and evacuated for 24 h to approximately 50 mTorr. Temperature-programmed reduction of CO profiles of N2-decomposed and passivated mono metallic (Co or Fe) and bimetallic (CoFe) samples were recorded using a Zeton-Altamira


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AMI-200 unit equipped with a thermal conductivity detector. The TPR was performed by using a 10%CO/He gas mixture (referenced to helium) at a flow rate of 30 cm3 min-1. The samples were heated from 50 to ~500°C using a heating ramp rate of 10°C/min and held for 2 h. A liquidnitrogen trap was used to continuously remove the CO2 produced during reduction/carburization. Thermogravimetric analysis of mono and bimetallic metal oxalates was performed using a Netzsch TG/QMS (STA 449 F3 TG-DSC/DTA) instrument coupled with a QMS 403C Aeolos instrument either under Ar or air flow (50 cc/min) by increasing temperature from 25°C to 500°C at a heating rate of 10°C/min. The evolved gases such as H2O, CO and CO2 were monitored using the QMS 403C unit. Powder X-ray diffractograms of freshly activated Co-Fe samples were recorded after passivation using a Philips X’Pert diffractometer with monochromatic Cu Kα radiation (λ = 1.5418). XRD scans were taken over the range of 2θ from 10 - 90°. The scanning step was 0.01, the scan speed was 0.0025 s-1, and the scan time was 4 s. The morphology of as-prepared metal oxalates and samples after CO pretreatment followed by passivation in 1%O2 in N2 at room temperature was studied by scanning electron microscopy (SEM). The scanning electron microscope (Hitachi S-4800 model) was operated at 300 kV in the SE display mode. For characterization prior to analysis, samples were gold coated in a sputter coating unit (Edwards Vacuum Components Ltd., Sussex, England).

In-depth

analysis of individual particles of carburized CoFe samples were obtained using a field emission analytical transmission electron microscope (JEOL JEM-2010F) operated at an accelerating voltage of 200 kV. HRTEM images were recorded under optimal focus conditions at typical magnifications of 100-500 K. The electron beam had a point-to-point resolution of 0.5 nm.


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Gatan Digital Micrograph software was used for image processing. Samples were prepared on lacy carbon copper grids and dispersed as powders. 55

Fe Mössbauer spectra were collected in transmission mode by a standard constant

acceleration spectrometer (MS-1200, Ranger Scientific). A radiation source of 50 mCi

57

Co in

Rh matrix was used and the spectra were obtained using a gas detector. Following activation, each sample was carefully fixed into wax inside a glove box before being transported for Mössbauer analysis.

All samples were investigated at room temperature (25°C) and low

temperature (-253°C) using a closed cycle refrigerator, typically over a velocity range of ±10 mm/s.

Structural analysis of each sample was carried out by least-squares fitting of the

Mössbauer spectra to a summation of hyperfine sextets. Details about the least-squares fitting procedure are described elsewhere.27 2.3 Catalyst testing The experiments were conducted using a fixed-bed reactor (stainless steel having a length of 17 cm and an inside diameter of 1.6 cm). Typically, 2.5 grams of the dried metal oxalate (60100 µm, after activation the weight of the catalyst sample is in the range of 0.9 to 1.2 grams depending on whether the catalyst was pretreated in flowing H2 or CO) were diluted with 7.5 grams of powdered glass beads in the size range of 40-100 µm, and loaded onto a quartz wool plug in the reactor. The temperature of the catalyst bed was monitored by placing a K-type thermocouple in the middle of the catalyst bed. After activation, a gas mixture, containing 71.5% H2 and 28.5% CO2 at 2.0 nL/h/g catalyst, was fed to the reactor, which was maintained at a temperature of 200°C and a pressure of 0.92 MPa. Brooks mass flow controllers were used to control the flow rates of H2 and CO2. The conversions of CO2 and H2 were obtained by gaschromatography (GC) analysis (micro-GC equipped with thermal conductivity detector) of the


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reactor exit gas mixture. The reaction products were collected in two traps maintained at different temperatures - a hot trap (150°C), and a cold trap (5°C). The products were separated into different fractions for quantification. The liquid products condensed in the hot trap were analyzed using a HP 7890 GC with DB-5 capillary column, while the aqueous phase was analyzed using a SRI (Torrance CA) GC-TCD-8610C with a 6’ Poropak-Q stainless steel packed column. A 5973N MSD coupled to the 6890 GC from Agilent was employed for qualitative analysis of various oxygenated compounds. The conversion and selectivity reaction parameters are defined as: = 100

= 100

−

. "# $%#

& '( − &

where ") are the numbers of moles of CO2 fed and not-consumed, respectively. The selectivity is defined as the percentage of moles of CO2 consumed to form a particular Cn product (hydrocarbon, CO or oxygenate), normalized by the amount of CO2 consumed. The hydrogenation of carbon dioxide is an exothermic reaction and the heat of reaction will give rise to a temperature difference between the inside and boundary of the catalyst particle while the reaction is proceeding. Recognizing that transport phenomena can disguise results and lead to inaccurate conclusions, Anderson’s criterion was used to check the intra-particle heat transport effect. The detailed calculation is shown in the supporting information section (page11). The criterion is obeyed for the reaction of hydrogenation of carbon dioxide over all the catalysts tested in this work. Hence, intraparticle heat effects can be neglected. The Prater number (β) is also useful to evaluate the temperature gradient existing within a catalyst particle


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for any exothermic reaction. The Prater number (β) calculated in this case varied from 0.03 to 0.36 for various catalysts and the value for any particular catalyst does not change appreciably by an increase in reaction temperature (supporting information section, page-12). Based on these values, the maximum temperature difference (Tmax) one could expect on this catalyst surface was 2°C. However, this is based on the assumption that the concentration of reactants near to a catalyst surface (Cs ≈ Cbulk) would be the same as that of the bulk composition (i.e., no internal and external diffusional limitation). 3.0 Results and Discussion 3.1 Characterization of Co-Fe bimetallic oxalates Table S1 shows the elemental compositions of Co, Fe, C, O and S in the Co-Fe bimetallic oxalate precursor. Experimental values are in close agreement with theoretical values, particularly for Co and Fe, which suggests that complete precipitation of both metals (Co and Fe) occurred. Samples that contain Fe have traces of residual sulfur which increased from 100 to 255 ppm with increasing iron content of the catalyst. The BET results of Co-Fe bimetallic samples after decomposition in N2 at 400°C are displayed in Table 1. As the fraction of Fe increased, the BET surface area and pore volume of the catalysts increased. The 25Co75Fe sample exhibited a maximum surface area of 22.5 m2/g and its single point pore volume was 0.166 cm3/g. However, the 50Co50Fe sample displayed the largest pore diameter (35.4 nm). It is apparent that addition of iron in Co-Fe caused an increase in the BET surface area largely due to an increase in porosity of the catalyst. In order to determine whether samples were solid solutions or only a mixture of the two oxalates, X-ray diffraction was used. The X-ray diffractograms (XRD) of Co-Fe bimetallic oxalate samples after drying in air at 100°C are shown in Figure 1. The XRD patterns of all


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samples reveal diffraction lines corresponding to metal oxalate dihydrate [MC2O4.2H2O]. The major diffraction line at 2θ in the range of 17-19° corresponds to the (111) plane of metal oxalate dihydrate (MC2O4.2H2O), whose peak position (Figure 1, zoomed portion) is slightly shifted to lower 2θ with increasing fraction of Fe in Co-Fe bimetallic oxalates. As indicated in Figure S1, the interplanar distance (d) for the (111) plane of pure iron(II)oxalate dihydrate [100Fe] was 2.44 Å. The d-spacing decreased slightly from 2.44 to 2.42 Å when the cobalt fraction was increased from 0 to 0.5 and, in line with this, the d-value obtained for the pure cobalt (II)oxalate dihydrate was 2.39 Å. This shift may be attributed to the distribution of the Co2+ ions in the lattice of iron oxalate in the formation of solid solutions.28 The SEM micrographs of monometallic and bimetallic oxalates are shown in Figure 2. The 100Co sample contained rod-like particles (2A) on the order of about 4-8 µm in length and about 0.1-0.3 µm in width. However, the 50Co50Fe sample displayed a mixture of both rod-like and cubic morphology (2B), and the particle sizes were 10-20 µm in length and 1-4 µm in width. The 25Co75Fe sample displayed a mixture of disc-like and cubic morphology, (2C) and the pure iron oxalate sample exhibited a cubic morphology (2D) with particle sizes ranging from 10 to 20 µm in length and 1 to 5 µm in width. By increasing the fraction of Fe in Co-Fe, the sample morphology changed from a rod-like structure to a cubicone, in agreement with an earlier report on the synthesis and thermal investigation of several bimetallic oxalates.29 The TG curves of the various Co-Fe bimetallic oxalates in He are shown in Figure 3. The initial weight loss that occurs below 120°C is associated with the loss of physically absorbed water (3A). The actual weight loss (~20%) associated with the removal of water from the metal oxalate dihydrate complex occurred in the temperature range of 150°C to 230°C [Eqn. (i) (ii) (iii)]. As noted in Figure 3B, the loss of weight due to the process of dehydration would remain


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more or less the same for all samples, even though the temperature of the peak maximum shifted towards higher temperature with increasing iron content in Co-Fe (Figure 3D). This indicates that the removal of water is more difficult if water is associated with Fe than Co. CoC2O4.2H2O

CoC2O4 + 2H2O

(i)

FeC2O4.2H2O

FeC2O4 + 2H2O

(ii)

Co1−nFen.C2O4.2H2O CoC2O4 3FeC2O4 3Co1−n FenC2O4

Co1−n Fen.C2O4 + 2H2O CoO + CO + CO2 Fe3O4 + 4CO + 2CO2 2Co1−n Fe3n/2O4 + Co1−n O + CO2 + CO+ 4C

(iii) (iv) (v) (vi)

Total weight loss ranged from 30-40% for the mono-metallic oxalate complexes in the temperature range of 220°C and 420°C, which is slightly lower than the theoretical value of 46% corresponding to the formation of their respective partially reduced metal oxides as shown in Eqn. (iv) and (v). In the case of bimetallic oxalates, formation of CoO or cobalt ferrites may be dominant depending upon the composition; however, in either case, it is very likely that carbon deposition could occur on this partially reduced metal [Eqn. (vi)]. The reaction products for the decomposition of cobalt or iron oxalates in inert atmosphere is controversial.30 The evolved gas that comes out as decomposed products for metal oxalates was analyzed using a QMS-403C mass analyzer. The main decomposed product found within the temperature range of 280-450°C was carbon dioxide (Figure 3C) as well as some carbon monoxide and methane. Irrespective of composition, all samples except pure iron oxalate started evolving CO2 as low as 220°C, and the peak maximum appeared in the range of 380-400°C for each sample. This indicates that the decomposition process does not appear to be affected by the bimetallic ratio. 3.2 Characterization of carburized Co-Fe bimetallic samples


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CO activation was followed by analyzing the activated/passivated Co-Fe samples by XRD.

Figure 4 shows a typical X-ray diffractogram of 50Co50Fe catalyst sample after

decomposition and carburization. The decomposed 50Co50Fe sample mainly exhibits spinel cobalt ferrite [CoFe2O4, JCDPS file no. 00-002-1045], whose reflections are 30.08° 35.44° 56.98° 62.59°, bimetallic CoFe [bcc CoFe alloy, JCPDS file no. 48-1818], and a carburized form of iron (FeCx). Introducing CO at 250°C after decomposition of 50Co50Fe resulted in the slow disappearance of the cobalt ferrite phase with time, and two sets of peaks appeared; one set of reflections was due to a CoO phase [JCPDS file no. 43-1004] and another was ascribed to an iron carbide phase FeCx as confirmed by Mössbauer spectroscopy (to be discussed). After 20 h of CO pretreatment, most of the cobalt ferrite was converted to iron carbide (FeCx), Co2C, CoO and CoFe alloy phases. As shown in Figure 4, the reflection at 44° (2θ) increased with increasing time of exposure of the 50Co50Fe catalyst to CO, indicating an increase in the iron carbide phase (FeCx). This indicates that the inhomogeneity in the cobalt and iron phases occurs after the catalyst is exposed to CO. The alloy character of Co-Fe was later confirmed by analyzing the bimetallic samples using Mössbauer spectroscopy, which will be discussed later in this section. The CO-TPR profiles of monometallic (Co or Fe) and bimetallic Co-Fe samples after decomposition of oxalates in nitrogen at 400°C are shown in Figure 5. As expected, none of these samples exhibited the low temperature carburization peak corresponding to the absence of a reduction step of Co3O4 to CoO or Fe2O3 to Fe3O4. The 100Co sample displays a two peak profile centered at 360° and 380°C, which can be assigned to the conversion of CoO to Co metal and Co metal to CoCx, respectively. A further increase in the temperature could cause deposition of more carbon on the surface of the catalyst, resulting in a tailing in the TPR pattern. However, the 100Fe sample exhibited only a weak signal at 400°C, ascribed to the reduction of Fe3O4 to


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FeCx30 with further increases in temperature of carburization producing additional carbon on the iron catalyst. Adding a small amount of Fe to Co (10Fe90Co) caused a significant increase in the carburization rates of Co-Fe bimetallic catalysts and, furthermore, the peak maximum for the 50Co50Fe catalyst shifted to 30°C lower temperature than that of the 100Co sample, indicating that cobalt associated with iron carburizes at a lower temperature than pure cobalt. However, due to the complexity of the presence of various phases after decomposition of the 50Co50Fe bimetallic oxalate (Figure 4), delineating individual carburization steps during CO-TPR is a daunting task. Nevertheless, the carburization profiles of Co-Fe bimetallic samples are significantly different from the monometallic (Co or Fe) catalysts. Figure 6 shows HR-TEM images of CoFe samples after pretreatment in CO at 250°C, followed by passivation. All catalyst samples showed a mixed phase, mainly consisting of Fe3O4 (CoFe2O4), χ-Fe5C2, FeCo, Co2C, CoO, and Co and Fe metals. As expected, there is little to no carbon deposition at the edges of particles on both 100Co and 100Fe catalyst samples. However, the 25Co75Fe and 50Co50Fe samples displayed carbon fringes around the mixed-metal particles (bimetallic alloy - FeCo) which indicate that the presence of Fe in Co enhanced the carburization rate of cobalt, and vice-versa. The low temperature carburization method (250°C) was followed to carburize all the monometallic and bimetallic samples in order to avoid carbon growth over the active sites of the catalyst. The particle size distribution of 25Co75Fe and 50Co50Fe samples after carburization was in the range of 20-40 nm, whereas monometallic samples (100Co and 100Fe) exhibited particles much larger in size that ranged from 40-100 nm. In order to assess the nature of iron phases present after carburization, the Mössbauer spectroscopy technique was applied to all carburized samples.

Table 2 summarizes room

temperature MES results for the carburized CoFe bimetallic samples. Magnetite was found to be


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the highest fraction in the 90Co10Fe sample with the rest of the iron being in the doublet state. By increasing the iron content in the CoFe catalyst, the fraction of FeCo alloy and χ-carbide in the catalyst also increased. To resolve the doublet pattern of Fe, all the samples were subjected to MES analysis at -253°C, and the corresponding data are displayed in Table 3. A comparison of the MES spectra of the carburized 50Co50Fe sample obtained at two different temperatures is shown in Figure 7. For the room temperature MES measurement, the 50Co50Fe sample exhibits a doublet (30%) which was resolved at -253°C into a magnetite phase. Irrespective of the composition of Co and Fe, an average of about 40% of the Fe was present as a magnetite phase and the 50Co50Fe sample contained more carburized iron (χ-carbide: 21%, ε’-carbide: 22%) than any other sample investigated. The 100Fe sample displayed two different iron carbides, namely χ-carbide (25%) and ϴ-Fe2C (17%). Stanfield and Delgass31 observed a variation in the magnetic hyperfine field of the Mössbauer spectra of FeCo alloys as a function of cobalt loading. The authors compared magnetic hyperfine field data, which passes through a maximum with increasing Co loading for the supported catalysts, with results on bulk alloys reported by Johnson et al.32 and concluded that FeCo bimetallic alloy particles were formed with Fe and Co being in intimate contact. In the current context, the magnetic hyperfine field (H, kG) for the Hägg carbide (χ-carbide) increased from 249 to 255 by increasing the proportion of cobalt from 0 to 50 (Table S2). However, the magnetic hyperfine field for the low temperature iron carbides (i.e., epsilon, ε’-carbide) did not change from sample to sample. On the other hand, the value of H for the magnetite phase shown in Table S2 decreased continuously with increasing cobalt content. The results suggest that cobalt is possibly located in magnetite as cobalt ferrite as well as to some extent in the lattice of Hägg carbide after carburization. Further study is needed to establish the precise location of cobalt in the Hägg carbide phase, as well as its stability during reaction.


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The XRD patterns of carburized CoFe samples are shown in Figure 8. The 100Co catalyst exhibits a peak at 44.2° corresponding to metallic cobalt (Co, fcc), and peaks corresponding to cobalt carbide, Co2C, and partially reduced cobalt (CoO) are also observed. Figure 8 reveals that with an increase in Fe loading, lines corresponding to metallic iron, CoFe alloy and FeCx phases gradually appeared as those corresponding to Co(fcc), CoO and Co2C phases diminished. The 100Fe sample profile displays lines corresponding to a magnetite phase, as well as a broad feature at around 42-45° (2 theta), which might be due to the presence of much smaller particles of metallic iron and iron carbides. On the other hand, cobalt rich samples (90Co10Fe and 100Co) exhibited mainly a partially reduced state of cobalt (i.e., CoO), which indicates that substitution of Fe with Co enhanced the reduction and carburization behavior of cobalt, which leads to a surface structure that can be tuned to control the activity and selectivity for the hydrogenation of carbon dioxide. 3.3 Effect of pretreatment conditions To study the effect of the method of pretreatment of Co-Fe bimetallic oxalate on the formation of various phases, the catalysts containing the different ratios of Co and Fe were activated in H2 at 350°C for 15 h or using syngas at 300°C for 20 h. The detailed procedures of individual activation conditions are provided in Section 2.1.2. The XRD patterns of the H2pretreated Co-Fe samples are shown in Figure 9. After H2 pretreatment, except for the case of pure Co and pure Fe, all other samples exhibited distinct lines for Co-Fe alloy. The particle sizes of the reduced samples of 25Co75Fe and 50Co50Fe were measured using WINFIT software and diameters were in the range of 150-200 nm. In the case of syngas pretreatment, cobalt rich samples (100Co and 90Co10Fe) exhibited mainly partially reduced cobalt (CoO) and metallic


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cobalt (Co, fcc) along with a fraction of cobalt carbide. The 50Co50Fe sample after syngas pretreatment consists of cobalt carbide and bimetallic carbide [Fe(Co)Cx] phases. The effect of pretreatment conditions on the particle sizes of iron carbide (FeCx) and CoFe alloy were followed using SEM. The 50Co50Fe catalyst was subjected to three different activation conditions (H2, syngas and CO), separately. Figure S2 shows the SEM image of the 50Co50Fe sample after various pretreatment conditions. A significant decrease in particle size was observed when using syngas and CO pretreatments. Figure 10 shows MES spectra for the 50Co50Fe sample pretreated at different conditions. The dotted lines are the experimental values and the black solid lines are fitted curves. The H2 pretreated 50Co50Fe catalyst exhibits one single pattern corresponding to a bimetallic alloy of CoFe. The enhanced hyperfine field for Fe suggests that on average the signal comes from FeCo and not from pure Fe. The reported hyperfine field for α-Fe is 330 kG.33 The hyperfine field measured here is larger than 340 kG (Table S2). According to the literature, the mean

57

Fe hyperfine magnetic field of Fe-Co alloys increases with increasing cobalt

concentration in Fe up to 30% with further addition of Co to Fe causing the hyperfine magnetic field to drop. In this study, we observed the hyperfine magnetic field of 351 kG for 25Co75Fe and 342 kG for 50Co50Fe catalyst samples indicating that both H2 pretreated CoFe catalysts must be in CoFe form. The syngas pretreated 50Co50Fe catalyst contained 58% χ-carbide, 8% ε’-carbide, 26% ϴ-carbide (Fe2C) and 8% Fe3O4. As shown in Table 3 and Figure 10, the CO pretreated catalyst exhibited equal amounts of χ-carbide (21%) and ε’-carbide (22%) in addition to the FeCo alloy and Fe3O4. As summarized in Table S2, the observed magnetic hyperfine field for the iron phases demonstrates conclusively that cobalt was located in metallic and oxide


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phases of Fe and one cannot ruled out the possibility for its presence in any of the iron carbides present. 3.4 Catalytic performance The Thiele modulus was calculated for the reaction of hydrogenation of CO2 over mono and bimetallic CoFe catalysts at different temperatures as shown in Figures S3 and S4. The details regarding the Thiele modulus calculation are provided in the supporting information section (pages 5-10). In all these cases, the Thiele modulus was found to be less than 0.01, giving a conservative estimate of the effectiveness factor of approximately unity. It indicates that the conversions of CO2 and H2 measured in this study using 100Co, 90Co10Fe, 50Co50Fe, 25Co75Fe and 100Fe catalysts with particle diameter in the range of 0.006 to 0.01 cm were not limited by internal pore diffusional resistances. Duplicate experiment was performed for three catalysts (100Co, 50Co50Fe and 100Fe) to establish the reproducibility of the data. The conversions of H2 and CO2 for the three catalysts are shown in Figure S5. The standard deviation of the measured conversions was often well below 5% of the measured value (Figure S6). The standard deviation of the measured selectivity (Figure S7) was less than 6% for 100Co and 50Co50Fe catalysts, whereas that of the 100Fe catalyst was slightly higher than 10%.

This underscores that the data obtained during

experimentation were reliable. The conversions of CO2 and H2 for the carburized Co-Fe catalysts at various reaction temperatures are shown in Figure 11. The activity of the catalyst was compared at similar reaction conditions. The 100Co catalyst exhibited much higher hydrogenation activity for CO2 at all temperatures compared with other Fe substituted carburized Co-Fe catalysts.

With

increasing Fe content, the hydrogenation activity of CO2 for Co-Fe decreased (temperature range


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of 200 to 270°C). With increasing reaction temperature, CO2 conversion for the carburized CoFe catalysts increased as expected. The CO pretreated 100Fe catalyst displayed the lowest CO2 conversion among all the catalysts tested, and its conversion increased from 3 to 18% with increasing temperature from 220 to 270°C. Likewise, the hydrogen conversion (%) for the carburized 100Fe catalyst increased from 0.5 to 24% with increasing reaction temperature from 220 to 270°C. It appears from Figure 11 that the H2/CO2 usage ratio of 100Co catalyst seems higher than the other Fe-substituted CoFe catalysts. In other words, increasing Fe content not only suppresses the CO2 conversion rates, but also controls the selectivity of the catalysts with respect to the rate of H2 consumption. The product selectivity for the hydrogenation reaction of CO2 was compared among various Co-Fe catalysts at three different temperatures (220°C, 250°C and 270°C). The % CO2 conversions as well as product selectivities of all the catalysts are included in the same figure (Figure 12) for ease of comparison. At 220°C, methane selectivity decreased from 93% to 51% with increasing the fraction of Fe from 0 to 0.5; however, no further changes were noted with increasing the iron fraction further to 1.0.

Figure 12 shows that the selectivity of lower

hydrocarbons (C2-C4) increased sharply from 4 to 20% by just introducing 0.1 fraction of Fe to the Co-Fe catalyst. This indicates that introducing even a small fraction of Fe to Co had a beneficial effect on producing lower hydrocarbons, i.e., C2-C4.

As the iron content was

increased further, the selectivity of the catalysts for C2-C4 hydrocarbons increased but only to a lesser extent. This trend can be seen even at higher reaction temperatures (250°C and 270°C). The selectivity to CO appears to be more prominent only at lower reaction temperatures. With increasing the iron fraction up to 0.5, the selectivity of the CoFe catalyst to CO also increased from nearly 0 to 28%. However, further increases in Fe content in the catalyst did not result in


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further appreciable changes in the selectivity to CO. For the same catalyst composition (e.g., 50Co50Fe), the selectivity for CO was higher at lower reaction temperatures than at higher temperatures. Oxygenates are an integral part of the reaction products of iron FT synthesis. Hence, it is of interest to compare the selectivity of oxygenates among Co-Fe samples with varying Fe content. Figure 12 shows the variation in the selectivity of oxygenates (C-%) with increasing fraction of Fe in Co-Fe samples at three different reaction temperatures. The oxygenate content increased from near zero to 4.5% by increasing the fraction of Fe in Co-Fe from 0 to 0.5. Further increases in the iron fraction from 0.5 to 1.0 decreased the selectivity to oxygenates to 1%. The results show that 50Co50Fe seems to be the optimum level of Co and Fe for hydrogenating CO2 to produce hydrocarbons and oxygenates. Coville et al34 have prepared a series of Fe:Co/TiO2 bimetallic catalysts by the co-impregnation technique and tested these catalysts for the hydrogenation of carbon monoxide. These authors found that the 5:5 Fe:Co catalyst showed higher activity and more selective towards lower olefins and oxygenates than catalysts prepared with either 2.5:7.5 or 7.5:2.5 Fe:Co ratios. The authors attributed the higher activity of the 5:5 Fe:Co/TiO2 catalyst to Fe-Co interactions and enhanced reducibility of the mixtures as well as to changes in the Co phase with catalyst composition. In the present study, the CO-pretreated 50Co50Fe catalyst was more selective toward C2+ hydrocarbons and oxygenates formed by the hydrogenation reaction of carbon dioxide. The distribution of alcohols plotted against catalyst composition is shown in Figure S8. As the fraction of Fe in Co-Fe increased from 0 to 1.0, the selectivity to methanol increased from 53% to 82% and, at the same time, the ethanol content decreased from 47% to 18%. The production of n-propanol from CO2 was noticed only using a Co-Fe catalyst having higher Fe


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contents. However, 100Fe did not produce a measurable amount of n-propanol even at lower temperatures. Table 4 summarizes activity and selectivity of hydrogenation of carbon dioxide using Co, Fe and bimetallic Co-Fe catalysts. For the purpose of comparison, literature results for the same reaction using alumina and silica supported mono and bimetallic Co-Fe catalysts are included. In the present catalytic system, the beneficial effect of introducing Fe to Co is seen in terms of an increase in CO2 conversion as well as C2+ product selectivity. Addition of 1 wt% Na or 1.7 wt% K (equal atomic ratios) to 50Co50Fe catalyst not only suppressed CO2 and H2 conversions but also decreased methane selectivity to 14.1% and 10.3% for Na- and K-promoted 50Co50Fe catalysts, respectively.

The Na-promoted 50Co50Fe catalyst exhibited much higher C2-C4

hydrocarbon and oxygenate selectivities compared to the K-promoted 50Co50Fe catalyst. Overall, both Na- and K-promoted 50Co50Fe catalysts (Na and K) displayed a tendency to form higher C5+ hydrocarbons from CO2 compared to the un-promoted 50Co50Fe catalyst. Earlier, Bartholomew et al.17 performed the hydrogenation of carbon dioxide reaction over Co/SiO2,Fe/SiO2 and Ru/SiO2 catalysts. The cobalt based catalyst produced about 89% methane, 10% CO and a very small amount (65%) dominated in the temperature range of 210 to 240°C; however, further increases in the reaction temperature from 240 to 270°C caused the selectivity of oxygenates to shift from acetic acid towards ethanol and acetaldehyde. Moreover, only traces of acids were found in the products obtained during the reaction of carbon dioxide using the other two catalysts (i.e., 50Co50Fe and 1.0%Na-50Co50Fe).

Reaction

temperatures above 240°C may increase the hydrogenation activity of the FeCx phase, which eventually could boost alcohol formation through an acetaldehyde intermediate.


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Scheme 1 A plausible mechanism for the formation of acetic acid over alkalized metal surfaces A plausible mechanism is proposed as shown in Scheme 1 for the formation of acetic acid from CO2 at lower reaction temperature using K-promoted 50Co50Fe catalyst.

In the

presence of potassium (Cycle-A), the 50Co50Fe catalyst promotes the reverse water-gas shift reaction, which produces carbon monoxide from CO2. At appropriate conditions, the acyl group may be formed after the insertion of CO into the M-CH3 bond. The resulting species may be further attacked by the potassium oxo species forming an intermediate species (e.g., potassium carboxylate). In the next step, depending upon whether the intermediate species would be 23 ACS Paragon Plus Environment

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attacked by either H2O or H2, the product selectivity may switch from acetic acid to ethanol. At lower reaction temperature, the catalyst preferentially produced acetic acid from CO2 rather than ethanol. As the reaction temperature increased, the selectivity for acetic acid decreased slowly and at the same time the ethanol content increased. This indicates that the hydrogenation activity of the catalyst seems to increase with increasing reaction temperature and thus the adsorbed potassium carboxylate species becomes hydrogenated above 240°C to form acetaldehyde and/or ethanol.

Under these reaction conditions, acetaldehyde is not as stable as the carboxylate

species; hence, it would be easily converted to ethanol. On the other hand, in the absence of potassium, a regular CO insertion pathway could be operating and be responsible for producing ethanol from CO2 (Cycle-B). Hattori et al.35 observed the direct formation of acetic acid from H2/CO2 over Ag-Rh(0.2-1)/SiO2 catalyst at 200°C, 2 MPa, and H2/CO2=1/2. Carbon monoxide was a main product with 96% selectivity; however, acetic acid was produced with 2.4% selectivity. The authors proposed that direct insertion of CO2 to surface methyl species on Rh led to acetate formation followed by hydrogenation to acetic acid. However, the insertion of CO2 to M-alkyl bond might not be the favorable pathway (direct CO2 insertion) for acetic acid formation over cobalt or iron based catalysts due to the fact that cobalt and iron have high hydrogenation activity for CO2 compared to Rh.36 Hence, CO2 appears to initially convert to CO by the reverse water-shift reaction over bimetallic CoFe catalysts which is then inserted into the M-alkyl bond as proposed in Scheme 1, resulting in formation of acetic acid or acetaldehyde or ethanol. At present, we do not have any experimental evidence to show that potassium is involved in a catalytic path by forming potassium carboxylate under our experimental conditions. Therefore, the mechanism described above is only speculative and a detailed theoretical work is needed to address this issue.


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The olefinic selectivity (O/P) for the lower hydrocarbons (C2-C4) was compared between unpromoted and Na- or K-promoted 50Co50Fe catalysts after carburizing using CO. As shown in Figure S9, the O/P ratio of lower hydrocarbons (C2-C4) increased with increasing reaction temperature for Na- and K-promoted 50Co50Fe catalysts. The catalyst without any alkali, i.e., 50Co50Fe, did not produce significant amounts of olefins from CO2. For an equal molar concentration, K promoted 50Co50Fe catalyst produced twice the amount of olefins as the Na promoted catalyst. This indicates that K inhibits the hydrogenation property of the active sites of the Co-Fe catalyst much more so than Na. In order to compare the fraction of carbides present in the catalyst with activity and oxygenates selectivity for hydrogenation of CO2, the sum of the two active iron carbides, χ and ε’, were plotted against the rate of CO2 conversion and total selectivity to oxygenates, as shown in Figure 14. By increasing the total iron carbide content (cobalt associated with either carbide) from 0.27 to 0.44, the selectivity to oxygenates increased from 0.5 to 5.4% and the rate of CO2 conversion increased from 2.0 to 3.4 mmol/h/g cat. The cobalt rich Co-Fe sample after CO activation (90Co10Fe) did not yield reliable MES data to resolve the presence of various iron phases; thus, data are not provided for this catalyst. Therefore, a comparison was made between the amounts of iron carbides with activity parameters for the remaining three iron containing CoFe catalysts. The amount of iron carbide and total oxygenates selectivity for the three catalysts decrease in the following order 50Co50Fe > 25Co75Fe

> 100Fe.

The present data demonstrate that the activity and product distribution for the hydrogenation reaction of carbon dioxide on unsupported Co-Fe catalysts depend strongly on the catalyst composition and method of activation. Although the CO-pretreated cobalt catalyst (100Co) exhibited higher hydrogenation activity for CO2, introducing Fe to Co had a beneficial


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effect in terms of selectivity to obtain products other than methane (i.e., C2-C4, CO and oxygenates). The catalyst containing 50:50 (Co:Fe) composition was found to be the optimum for hydrogenation of carbon dioxide to produce higher hydrocarbons and oxygenates. Further increases in Fe content did not result in significant changes in methane selectivity at various reaction temperatures.

Though intimate mixing of Co and Fe were clearly evident in H2-

pretreated 50Co50Fe catalyst, which formed a bimetallic CoFe alloy as confirmed by XRD and Mössbauer spectroscopy techniques, it was still inferior to the carburized 50Co50Fe catalyst from the point-of-view of selectivity.

Surprisingly, the 50Co50Fe bimetallic alloy catalyst

exhibited a higher selectivity for carbon monoxide from CO2 than the 50Co50Fe catalyst either activated in syngas or CO. Mössbauer spectroscopy analyses of freshly CO activated Co-Fe samples suggest that the iron carbide fraction increased with increasing Fe content in CoFe catalysts. The fraction of iron carbides present in the catalyst correlate well with the oxygenates selectivity and the rate of conversion of CO2 in Fe rich Co-Fe samples, indicating that the active sites of the carburized Co-Fe catalysts must be associated with iron carbide phases. Addition of Na (1.0 wt%) or K (1.7 wt%) to 50Co50Fe catalyst suppressed methane formation and increased the production of lower olefins, CO, and oxygenates from CO2. 4.0 Conclusions A series of unsupported, Co-Fe bimetallic catalysts was prepared by following an oxalate route. The catalysts were characterized and studied for the CO2 hydrogenation reaction after carburization using CO. An increase in d-spacing for the (111) plane of the metal oxalates with increasing Fe fraction suggests that Fe incorporated into the cobalt lattice to form a Co-Fe bimetallic oxalate. The separation of Co and Fe is clearly evident upon decomposition of Co-Fe bimetallic oxalates under flowing N2 at 400°C and after pretreatment in CO flow at 250°C.


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Partially reduced cobalt oxide (CoO), CoFe alloy, FeCx, Co2C, metallic Co, and Fe co-exist after CO pretreatment of CoFe samples. The bimetallic CoFe alloy is the only phase identified with the 50Co50Fe catalyst after reduction in H2 at 350°C. The CO carburized CoFe samples contained particles smaller than syngas or H2 pretreated Co-Fe samples. The

55

Fe Mössbauer

study suggests that cobalt was present in close proximity to Fe in oxidic and metallic phases of iron. However, one cannot rule out the possibility of the presence of cobalt in any of the iron carbides present. Further study is ongoing to establish the structure and location of cobalt in iron carbides at lower concentrations. The CO2 hydrogenation activity of the CO-pretreated Co-Fe catalyst was found to decrease with increasing Fe content from 0 to 50%. The 50Co50Fe catalyst exhibited a fairly high selectivity towards C2-C4 hydrocarbons (20-25%), and oxygenates (4.05.0%), and a lower selectivity to methane (50-60%) compared to the CO-pretreated monometallic catalysts (i.e., Fe or Co). Further increases in Fe content of the catalyst did not significantly alter CO2 hydrogenation activity and selectivity. The CO-pretreated 50Co50Fe sample exhibited a fairly low selectivity for methane (47.1%) compared to syngas- or H2pretreated 50Co50Fe catalysts (69.2% and 58.5%). An even better selectivity to oxygenates (5.1 or 2.2%) and lower selectivity to methane (30.6 or 14.4%) from CO2 was found for the 1%Na- or 1.7%K-promoted 50Co50Fe catalyst. Potassium may be involved in the catalytic cycle for forming acetic acid, acetaldehyde and ethanol from CO2.

Acknowledgements This research was conducted with financial support from the Commonwealth of Kentucky. References


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Table 1 BET surface area and porosity data. BJH Pore Pore Diameter BJH Pore Pore Volume (single point, Volume (single point, Diameter, cm3/g) (cm3/g) nm) (nm) 100Co 6.0 0.028 0.027 18.6 30.2 90Co10Fe 14.7 0.072 0.071 19.4 22.4 50Co50Fe 13.8 0.121 0.124 35.4 43.8 25Co75Fe 22.5 0.166 0.166 29.6 32.2 100Fe 20.5 0.114 0.113 22.2 27.4 # BET surface area measured after the decomposition of CoFe oxlates in N2 at 400°C for 2 h Catalyst Description#

BET SA (m2/g)

followed by passivation using 1%O2 in N2 at 25°C.

Table 2 Summary of phase identification of iron from Mössbauer spectroscopy analysis of CO carburized CoFe bimetallic samples performed at room temperature (25°C). Catalyst description

#

90Co10Fe 50Co50Fe 25Co75Fe 100Fe 50Co50Fe (syngas) 50Co50Fe (H2)

RT Fractions of Fe# Fe3O4 FeCo 0.51 0 0.12 0.19 0.16 0.30 0.35 0 0 0 0 1

χ-carbide 0 0.18 0.28 0.28 0.48 0

έ-Fe2.2C 0 0.22 0.06 0.03 0.06 0

θ-Fe2C 0 0 0 0.18 0.23

Average uncertainties in Fe fraction is ±0.03


doublet (unknown) 0.49 0.29 0.20 0.16 0.23 0

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Table 3 Summary of phase identification of iron from Mössbauer spectroscopy analysis of carburized CoFe bimetallic samples performed at -253°C. Catalyst description 90Co10Fe 50Co50Fe 25Co75Fe 100Fe 50Co50Fe (syngas) 50Co50Fe (H2) #

20K Fractions of Fe# Fe3O4 0.40 0.39 0.56 0.08 -

FeCo 0.17 0.25 0 0 -

χ-carbide 0.21 0.27 0.25 0.58 -

έ-Fe2.2C 0.22 0.09 0.02 0.08 -

θ-Fe2C 0 0 0.17 0.26 -

Average uncertainties in Fe fraction is ±0.03

Table 4 Catalytic performance of mono (Co or Fe) and bimetallic (CoFe) catalysts for the hydrogenation f carbon dioxide reaction. Catalyst

Activ.

T (°C) 220 270 220 270 220 270 220 270

P (MPa) 0.92 0.92 0.92 0.92 0.92 0.92 0.92 0.92

% CO2 conv. 24.3 36.6 14.9 27.2 2.4 21.8 7.3 22.7

CH4 96.5 96.3 53.7 70.8 58.9 64.7 28.9 35.0

CO 1.0 32.1 13.9 25.8 17.3 60.6 42.1

Selectivity, mol% C2 C3 C4 1.9 0.5 0.1 1.9 0.5 0.1 7.2 2.0 0.4 8.9 3.5 0.8 12.6 2.1 0.3 9.5 5.5 1.9 3.7 3.3 2.1 9.1 5.3 2.3

C5+ 0.2 0.4 0.9

Oxy. 1.0 0.2 4.4 1.6 0.3 1.2 1.5 5.3

100Coa

CO

50Co50Fea

CO

100Fea

CO

1%Na50Co50Fea

CO

1.7%K50Co50Fea

CO

220 270

0.92 0.92

10.2 22.6

14.1 30.8

83.6 53.0

0.1 6.1

1.2 3.7

0.6 2.0

0.1 1.1

0.4 3.3

Co/SiO2* Fe/SiO2* FeCo/Al2O3b

H2 H2 H2

205 291 300

1.11 1.11 1.10

11.2 9.9 25

89.0 39.9 44.0

10.7 53.0 13.0

0.3 3.8

0.04 2.0

0.7 43 (C2+)

0.5

-

FeCo/K(1.0) /Al2O3b

H2

300

1.10

31.0

13.0

18.0

*

69 (C2+)

Data from Ref.[17]. a(Reaction conditions: T=220, 270°C, P= 0.92 MPa, H2:CO2=2.5, SV= 2.0 nl/h/g cat). bData from Ref [22,23].


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4000 100Co 90Co10Fe 50Co50Fe 25Co75Fe 100Fe

111 3000

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

2000

1000

0 20

30

40

50

60

2 theta (θ)

Figure 1 XRD patterns of Co-Fe bimetallic oxalate samples after drying in air at 100°C.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 2 SEM images of Co-Fe bimetallic oxalates after dried in air at 100°C: (A) 100Co, (B) 50Co50Fe, (C) 25Co75Fe, and (D) 100Fe.


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2.0


(A)

60

(B)

100Co 90Co10Fe 50Co50Fe 25Co75Fe 100Fe

1.5

dW

weight loss (%)

80

1.0

0.5

40 0.0

20 100

200 300 Temperature (C)

400

100

500

600

6e-9

400

500

4e-8

600 (D)

(C)

100Co 90Co10Fe 50Co50Fe 25Co75Fe 100Fe temperature

500 400 300

4e-9 200 2e-9

Temperature (C) H2 O response (a.u.)

8e-9

300

Temperature (C)

1e-8 temperature 100Co 90Co10Fe 50Co50Fe 25Co75Fe 100Fe

200

3e-8

2e-8

200 1e-8 100

0 0

500

1000

1500

2000

2500

400 300

100

0

500 Temperature (C)

0

CO2 response (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

0

3000

0 0

500

time (sec)

1000

1500

2000

2500

3000

time (sec)

Figure 3 TGA (A,B) and MS (C,D) curves for the decomposition of Co-Fe bimetallic oxalates in flowing He.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 4 X-ray diffractograms of 50Co50Fe catalyst after exposure to CO at 250°C for different durations.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 5 CO-TPR profiles of Co-Fe samples after decomposition in N2 at 400°C for 2 h.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 6 HRTEM images of carburized (CO treatment at 250°C for 20 h followed by a passivation under flowing 1%O2 in He at room temperature for 6h) Co-Fe bimetallic samples.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 7 Mössbauer spectra of CO pretreated 50Co50Fe bimetallic sample analyzed at two different temperatures. The fitted curves are shown in solid lines: black, total spectra; red, oxide; blue, χ-Fe5C2; green, ε-Fe2.2C; dark blue, FeCo. Average uncertainties in Mössbauer parameters, ±0.03 Fe fraction, ±5 kG Hyperfind magnetic field, ±0.03 mm/s Isomer shift, ±0.05 mm/s Quadrupole splitting.


ACS Catalysis

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Figure 8 XRD patterns of the Co-Fe bimetallic samples after activation in CO at 250°C for 20 h.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 9 XRD patterns of the Co-Fe bimetallic samples after activation in: (A) H2 at 350°C for 15 h, (B) syngas at 300°C for 20 h.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 10 Mössbauer spectrum of 50Co50Fe bimetallic sample measured at -253°C after different pretreatment conditions (H2 activation at 350°C, syngas activation at 300°C, CO activation at 250°C). The fitted curves are shown as solid lines: black, total spectra; red, oxide; blue, χ-Fe5C2; green, ε-Fe2.2C. Average uncertainties in Mössbauer parameters, ±0.03 Fe fraction, ±5 kG Hyperfine magnetic field, ±0.03 mm/s Isomer shift, ±0.05 mm/s Quadrupole splitting.


ACS Catalysis

CO2 conversion (%)


30

20

10

0 200

210

220

230

240

250

260

270

280

Temperature (°C) 60 50 H2 conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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40

100Co 90Co10Fe 50Co50Fe 25Co75Fe 100Fe

30 20 10 0 200

220

240

260

280

Temperature (°C)

Figure 11 Effect of temperature on the conversions (CO2 and H2) of various Co-Fe bimetallic catalysts (Reaction conditions: P= 0.92 MPa, H2:CO2=2.5, SV= 2.0 nl/h/g cat).


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Figure 12 Effect of Fe fractions in CoFe bimetallic catalysts on the product selectivity for hydrogenation reaction of carbon dioxide at different temperatures. (Reaction conditions: P= 0.92 MPa, H2:CO2=2.5, SV= 2.0 nl/h/g cat).


ACS Catalysis

Distribution of oxygenates (%)

80 Methanol Acetaldehyde Ethanol Acetic acid n-Propionic acid

60

40

20

0 200

210

220

230

240

250

260

270

280

Temperature (°C)

10

4

8 3 6 50Co50Fe

2

4 1 2

25Co75Fe 100Fe

0 0.20

0.25

0.30

0.35

0.40

0.45

0 0.50

CO2 conversion rate (mmol/h/g cat.)

Figure 13 Effect of temperature on the distribution of oxygenates obtained from the hydrogenation reaction of CO2 using 1.7%K-50Co50Fe catalyst. (Reaction conditions: P= 0.92 MPa, H2:CO2=2.5, SV= 2.0 nl/h/g cat).

Selectivity to oxygenates (C-%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Fraction of FeCo carbide (χ+ε χ+ε ')

Figure 14 Effect of variation in the iron carbide content on CO2 hydrogenation activity and the selectivity for oxygenates over carburized Co-Fe catalysts.


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