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Hydrogenation of carbon dioxide over iron carbide prepared from alkali metal promoted iron oxalate
Applied Catalysis A, General 564 (2018) 243–249
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Hydrogenation of carbon dioxide over iron carbide prepared from alkali metal promoted iron oxalate
T
Muthu Kumaran Gnanamania, Hussein H. Hamdehb, Wilson D. Shafera,c, Shelley D. Hoppsa, ⁎ Burtron H. Davisa, a
Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, KY 40511, USA Department of Physics, Wichita State University, Wichita, KS 67260, USA c Asbury University, Natural Science and Allied Health Department, Wilmore, KY 40390, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Iron oxalate Hydrogenation Carbon dioxide Alkali metal Iron carbide
Iron carbide was prepared by the carburization of iron oxalate doped with various alkali metals in flowing CO at 400 °C. The addition of alkali metal (Li, Na, K, Rb or Cs) to iron oxalate was found to decrease the bulk iron carbides formation. However, the amount of carbon deposited on Fe as inferred from temperature programmed hydrogenation and high-resolution transmission electron microscopic studies show an increasing trend with the increase of the basicity of alkali added. The CO2 and H2 conversions of all alkali doped iron catalysts were initially higher but they declined over time irrespective of alkali. The benefits of addition of alkali to iron without any structural and reduction promoters, such as copper, is found to lower the selectivity for methane, higher olefins and oxygenates formation for CO2 hydrogenation.
1. Introduction Hydrogenation is considered as a potential route for recycling of carbon dioxide [1–3]. There are various ways (e.g., electrochemical [4,5], photocatalytic [6,7], heterogeneous [8–15], and homogeneous [16–19]) in which greenhouse gases can be hydrogenated into valueadded products. Hydrogenation of carbon dioxide using heterogeneous catalysts offers more advantages as it can be linked to an existing gasto-liquid technology that converts syngas (a mixture of carbon monoxide and hydrogen) into synthetic liquid fuels. In one approach, carbon dioxide is reduced into CO in the presence of hydrogen, known as reverse water-gas shift reaction, and the CO that is formed is subsequently involved in FT synthesis. Iron-based FischerTropsch (FT) catalysts are more suitable for this process due to its intrinsic reverse water-gas shift reaction. Secondly, Fe is the least expensive transition metal used commercially for carbon-carbon coupling reactions. In general, alkali metal promoter, such as K, is often added to Fe for syngas conversion in order to increase the catalyst stability and provide a higher C5+ selectivity [20–22]. In this context, different alkali promoters may affect the carburization of Fe differently. Earlier, Ribeiro et al. [23] investigated the promotional effect of alkali metals such as Li, Na, K, Rb, and Cs, on the carburization rate of Si-containing Fe
⁎
catalysts using an X-ray adsorption near edge spectroscopy (XANES) technique. The authors found that the carburization rate of FeSi increased in the following order: unpromoted < Li < Na < K = Rb = Cs. Xiong et al. [24] studied the effect of alkali promoters on the FT synthesis of carbon nanotube supported Fe catalysts. The authors revealed that Na and K promoters hinder Fe reduction. Recently, Li et al. [25] studied the role of alkali in iron-based FT synthesis catalysts using in situ X-ray diffraction (XRD) and X-ray photo electron spectroscopy (XPS) techniques. The authors showed that the addition of alkali on FeSi catalysts increased the strength of Fe-O bonds which makes the reduction of Fe2O3 difficult and it also increased C5+ formation. In the present investigation, the effects of addition of alkali metals on the carburization of iron oxalate were followed using X-ray diffraction (XRD), Mössbauer spectroscopy, temperature programmed hydrogenation (TPH) and high-resolution transmission electron microscopy (HRTEM) techniques. The catalysts were tested for hydrogenation of carbon dioxide using a 1 L continuous stirred tank reactor (CSTR). The reactivity and the product selectivity were compared for various alkalipromoted iron catalysts.
Corresponding author. E-mail address: [email protected] (B.H. Davis).
https://doi.org/10.1016/j.apcata.2018.07.034 Received 9 May 2018; Received in revised form 24 July 2018; Accepted 25 July 2018 Available online 26 July 2018 0926-860X/ © 2018 Elsevier B.V. All rights reserved.
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(45 SLh−1) at 400 °C for 6 h and then the reactor was cooled down to room temperature. The carburized catalyst then transferred in a glove box and embedded in wax without exposure to air. Finally, the catalyst chunks were added into a CSTR containing 310 gm of melted Polywax 3000 which had previously been purged with a nitrogen atmosphere. The reactor pressure was increased to 175 psig using a mixture of H2 and CO2 at a ratio of 3 to 1; then the reactor temperature was increased to 270 °C. Brooks mass flow controllers were used to control the flow rates of H2 and CO2. The conversions of CO2 and H2 were obtained by gas-chromatography (GC) analysis (Refinery Gas Analyzer from Agilent) of the reactor exit gas stream. The reaction products were collected in two traps maintained at different temperatures – a warm trap (100 °C) and a cold trap (5 °C). The organic phase condensed in the warm and cold traps were analyzed using HP 7890 GC with DB-5 capillary column, while the aqueous phase was analyzed using an SRI (Torrance CA) GC-TCD-8610C with a 6’ Poropak-Q stainless steel packed column. A 5673 N 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:
2. Experimental 2.1. Catalyst synthesis Iron oxalate {Fe(C2O4.2H2O), 98%] was obtained from Sigma Aldrich and used without further purification. An incipient-wetness impregnation method was used for doping of various alkali metals on iron oxalate by taking an appropriate amount (100 Fe:2 alkali, mol/ mol) of the respective alkali metal nitrate salt (Sigma Aldrich, 99%) dissolved in the required quantity of deionized water. The resultant solid was then dried in an air oven at 100 °C for 24 h. 2.2. Catalyst characterization The elemental analysis was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) technique using a Varian 720-ES analyzer. The materials were dissolved in a perchloric/ nitric acid mixture and the emission spectra of dissolved species (alkali) were compared to those of a series of standard solutions of known concentrations. Powder X-ray diffractograms of the freshly activated catalyst were recorded using a Philips X’Pert diffractometer with monochromatic Cu Kα radiation (λ = 1.5418). XRD scans were taken over the range of 2θ from 5-90°. The scanning step was 0.017°, the scan speed was 0.042 s−1. 57 Fe Mössbauer spectra were collected in a transmission mode by a standard constant acceleration spectrometer (MS-1200, Ranger Scientific). A radiation source of 50 mCi 57Co 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 analysis. In the case of used catalyst analysis, the end of slurry contain catalyst particles submerged with solvent polywax-3000 were used. All samples were investigated at 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 procedures are described elsewhere [26]. The temperature programmed hydrogenation (TPH) of various alkali metal promoted iron catalysts after activation was performed using an in-house system consisting of a furnace capable of operating at temperatures of up to 1200 °C, along with a thermal conductivity detector (TCD). The H2:He (1:3) gas mixture was used to hydrogenate iron carbides. The Origin Pro-8 (Data Analysis and Graphing Software) was used for baseline correction. Prior to TPH, the samples (∼1.0 g) were carburized in-situ in flowing CO (3 SLh-1, standard liter per hour) at 400 °C for 6 h. The temperature of the catalyst was dropped to 30 °C while purging with He and TPH was performed in a flow of 3 SLh−1 of H2:He and the temperature were increased to 850 °C at a ramp rate of 6.8 °C/min. The response to TCD for the consumption of H2 was monitored continuously. In-depth analysis of the nature of the iron particles and the carbons formed after carburization were obtained using a field emission analytical transmission electron microscope (JEOL JEM-2010 F) operated at an accelerating voltage of 200 kV. HR-TEM images were recorded under optimal focus conditions at typical magnifications of 100 K to 1.0 M. The electron beam had a point-to-point resolution of 0.5 nm. Gatan digital micrograph software was used for image processing. Samples were prepared on a lacy carbon copper grid and dispersed as powders after ultrasonic treatment in MeOH.
conversion = 100 x
selectivity = 100 x
nCO2 in−nCO2 out nCO2 in
nproduct out . carbon number nCO 2 in−nCO 2 out
where nCO2 in and nCO2 out are the numbers of moles of CO2 fed and unconverted, 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 total amount of CO2 consumed. 3. Results and discussion Elemental analysis of the added alkali to iron oxalates by ICP are provided in Table 1. The theoritical and experimentally obtained values for different alkali metals are in close agreement. The X-ray diffraction patterns of the freshly carburized catalysts after passivation are shown in Fig. 1. The carburization of iron oxalate at 400 °C in a CO atmosphere produced iron carbides. The undoped iron sample (100Fe) contains a mixture of iron carbide phases. The patterns are indexed to both Hägg (χ-Fe5C2, JCPDS 51-0997 [27]) as well as the theta carbides (θ-Fe3C, JCPDS 34-0001 [27]). This phase assignment is further verified from Mössbauer technique to be discussed later in this section. The doping of alkali to iron did not show any significant change in the pattern. This implies that the crystallinity of iron carbide phases is not influenced by the pressence of alkali under the present activation conditions. The phase composition of Fe is determined by 57Fe Mössbauer spectroscopy technique. Fig. 2A and B shows the Mössbauer spectra of the catalysts after carburization and reaction, respectively. The corresponding data are listed in Table 2. The quantification of various iron carbides including magnetite (Fe3O4) was performed based on the fittings obtained from a respective standard. As can be seen, the Table 1 Elemental analysis by ICP. Catalysts
Alkali (wt%) Theoretical
100Fe 100Fe:2Li 100Fe:2Na 100Fe:2K 100Fe:2Rb 100Fe:2Cs
2.3. Catalyst testing The hydrogenation of carbon dioxide was performed using a 1 L CSTR. Typically, 15.0 g of alkali metal promoted iron oxalate was taken in a fixed-bed tubular reactor. The sample was carburized in flowing CO
– 0.08 0.25 0.43 0.93 1.45
*
ICP analysis – 0.09 0.28 0.52 1.03 1.75
* Theoretical calculations are based on a molecular formula of Fe(C2O4) ⋅2H2O ANO3 (A stands for alkali). 244
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Table 2 Fe phase analysis with the freshly carburized and used iron catalysts using 57Fe Mössbauer spectroscopy. Iron species (%)a
Catalysts Fresh
100Fe 100Fe:2Li 100Fe:2Na 100Fe:2K 100Fe:2Rb 100Fe:2Cs a
Used
Fe3O4
χ-Fe5C2
θ-Fe3C
Fe3O4
χ-Fe5C2
θ-Fe3C
0 12 5 15 19 17
56 45 46 40 38 44
44 43 49 45 43 39
30 59 32 37 35 26
30 23 32 36 37 47
40 28 34 27 28 27
Uncertainty in these numbers vary by 3–5%.
alkali and/or binders. The majority of the phase is Hägg carbide along with magnetite (Fe3O4) [20]. In our case, iron oxalate decomposed and was carburized simultaneously in flowing CO at 400 °C. The resulting materials consisted of both Hägg and theta carbides. It is known that Hägg carbide prefers forming at a lower temperature of carburization than theta carbide. Table 2 shows about 10–19% of the Fe in the magnetite form for alkali-doped iron catalysts after carburization. The percent of magnetite is nil in the case of undoped iron whereas 100Fe:2 K catalyst had about 15% magnetite. The magnetite content in the carburized Rb and Cs-promoted catalysts is relatively higher compared to other catalysts studied in this work. The present data indicate
Fig. 1. XRD patterns of various alkali metal promoted iron catalysts after carburization.
carburization of iron oxalate (100Fe) at 400 °C yielded about equal amounts of Hägg (56%) and theta carbides (44%) and show no indication of Fe3O4. In general, a hematite (Fe2O3) is used for carburization to form iron carbide which is the active phase of iron for FT synthesis [20]. The temperature of carburization in those conditions ranged anywhere between 270–350 °C depending on the presence of
Fig. 2. Fe Mössbauer absorption spectrum of various alkali metal promoted iron catalysts performed at −253 °C after carburization (A); and after the reaction ((B). The fitted curves are shown as solid lines: black, total spectra; red, Fe3O4; blue, χ-Fe5C2; green, έ-Fe2.2C (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 245
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wrapped in multilayered graphitic sheets (shown by arrows). The thickness of graphitized carbon is ∼2.79 nm for undoped iron and 1.60 nm for Li-promoted iron catalysts, while Na-promoted iron exhibited a relatively higher thickness of about 3.20 nm. The thickness of graphitized carbon for K- and Rb-promoted iron catalysts are 3.84 and 4.07 nm, respectively and it tends to decrease for Cs-promoted iron to 3.20 nm. Overall, the thickness of graphitized carbon is relative higher in the case of alkali doped catalysts except Li than unpromoted iron. We have looked at different locations along the copper grid for Rb-promoted iron and similar results were obtained. Therefore, we concluded that Rb promotes CO dissociation that often produces more carbon deposition for iron (2CO → CO2 + C) than other alkali metals used in this study. Remember that the formation of bulk iron carbide did not vary much with nature of alkali. The conversions (CO2 or H2) for undoped and alkali-doped iron catalysts are shown in Fig. 6. The initial conversions for all alkali-doped iron catalysts are higher than the undoped iron. The Rb-doped iron catalyst exhibited higher CO2 and H2 conversions initially followed by Na, Cs and K-doped iron catalysts and then the undoped Fe. After about 100 h of synthesis, all iron catalysts, irrespective of alkali show more or less similar H2 and CO2 conversion. The initial higher CO2 conversions of Rb-promoted Fe could be attributed due to the presence of relatively higher carbon content. However, the activity has declined, like others, very steadily for Rb-promoted iron from 30% to 26% in 130 h. Lower olefins are important raw material for the polymer industry [30]. The production of lower olefins from CO2 is highly desirable. In this regard, the selectivity to lower olefins (C2-C4) are plotted against time for various iron catalysts as shown in Fig. 7. As we have seen that there is not significant differences in conversion, the selectivity to lower olefins are as high as about 30% for Rb,K and Cs-promoted iron catalysts compared to 18% Na, Li and undoped iron (5%). In conclusion, alkali addition to iron has shown a beneficial effect by producing ∼6 times more of the lower olefins than undoped iron. The selectivity to various FT products (methane, CO, olefins and oxygenates) for undoped and alkali doped iron catalysts at a similar CO2 conversion level is shown in Fig. 8 and Table 3. The methane selectivity decreased from 43% for a undoped iron catalyst to 29% for Na and 19% for K-doped iron catalysts but then the selectivity for Rb and Cs promoted iron catalysts increased to 23 and 27%, respectively. On the other hand, CO selectivity increases with increasing basicity of the added alkali up to Na and then dropped for K and leveled off for Rb and Cs-doped iron catalysts. Interestingly, the sum of olefins and oxygenates product selectivity for K, Rb and Cs promoted iron catalysts are much higher than any other products formed. Among the promoted catalysts, K-doped iron exhibits selectivity for olefins and oxygenates as high as 42% compared to undoped and Li-doped iron where both show less than 5%. Also, the chain growth probability factor (α) that determine the extent of formation of longer hydrocarbons or oxygenates was increased from 0.40 for undoped iron to 0.74 for Cs-doped iron catalysts. This indicates that alkali has a strong influence on the product distribution of iron for hydrogenation of CO2. A similar effect has been observed by others for syngas conversion using alkali-promoted iron catalysts [20,22,23]. In order to understand the stability of iron carbides under the reaction condition, the slurry containing catalyst sample was analyzed after reaction using Mössbauer technique. The spectral data are shown in Fig. 2 and a quantification of different iron phases are shown in Table 2. Undoped iron lost 30% of their initial iron carbide, whereas Li lost nearly 42% and Na, K, Rb and Cs lost 30, 26, 20 and 10%, respectively. The stability of bulk iron carbides was increased with the addition of alkali metals to iron. In general, hydrogenation of CO2 to hydrocarbons can follow two different pathways, namely the reverse water-gas shift (RWGS) and direct hydrogenation (DH). In RWGS route, CO2 is initially converted on Fe to CO in presence of hydrogen (i), and CO is subsequently involved in FT synthesis to make hydrocarbons (ii) or it can further
Fig. 3. Temperature programmed decarburization profiles of various alkali metal promoted iron catalysts after activation.
that the addition of alkali promoters decreased the iron carbides formation. Temperature programmed hydrogenation analyses of the freshly carburized catalysts are shown in Fig. 3. Hydrogen reacts with iron carbide and various forms of carbon deposited over iron and evolved methane. The TCD response measured as a function of temperature show at least three regions. Xu and Bartholomew [28] classified different carbons formed on iron catalysts as atomic or amorphous or bulk carbides or disordered or ordered graphitic surface carbons based on the temperature. According to these authors, those hydrogenating in the temperature range from 200 to 400 °C are classified as the lowtemperature carbon species, also known as atomic carbon or surface carbide. The middle region between 400 and 600 °C is considered to be amorphous carbons or β and finally the temperature above 600 °C is classified as carbidic carbon, or γ1, and graphitic carbons, or γ2. The unpromoted iron catalyst (100Fe) shows predominantly γ-type carbide and the graphitic carbon region. The addition of Li shifted the temperature of the peak maxima higher. On the other hand, the addition of Na or K promoters to iron caused a significant increase of the carbidic and amorphous carbons and alternatively, the graphitic carbon contents were decreased. Note that the freshly activated sample has some percent of magnetite (6–19%) in all alkali-promoted iron catalysts and its reduction to iron should take place in the temperature range between 450 and 600 °C. Therefore, some contribution in the values reported for amorphous carbon from magnetite reduction is expected in all those alkali-promoted iron catalysts. Nevertheless, the data indicate that the carbon deposited on iron was increased with an increase of basicity of alkali promoters. The temperature of peak maxima for amorphous carbon was shifted to a higher temperature with increase of basicity of alkali. Mössbauer investigations on the freshly activated catalysts infer that bulk iron carbides decreased by a few percent with the addition of alkali, and therefore the increase of carbon content, and particularly K, Rb or Cs containing Fe catalysts, the carbons could be derived mainly from carbon fringes associated to iron carbides. Results of HR-TEM examinations of carburized catalyst samples are shown in Figs. 4 and 5. The iron particles distributed very randomly for undoped and all alkali doped iron catalysts. Due to particles overlap the estimation of particle size distribution become difficult. The high resolution images shown in Fig. 4(b,d,F) all show the lattice fringes with a d-spacing of 0.20 nm correspond to iron carbide. It could be either Hägg or theta iron carbide. According to Datye et al. [29] with just diffraction pattern alone it is impossible to distinguish one phase from another. Hence, we denote those lattice fringes with d-spacing of 0.20 nm for iron carbide (FeCx). The HRTEM images show each iron particles 246
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Fig. 4. HRTEM images of 100Fe (a,b), 100Fe:2Li (c,d) and 100Fe:2Na (e,f) catalysts after carburization. (Arrow shows graphitized carbon).
affect CHx formation (v). In DH route, CO2 is hydrogenated directly to methane (vi). However, the extent to which the catalyst follow these pathways depends on the catalyst compositions (e.g., alkali loading, metal promoters, and nature of binders) and reaction conditions. Iron doped with more basic alkali metals such as K, Rb, and Cs stabilizes iron carbides, probably due to the enhanced dissociative adsorption of CO. Surprisingly, CO2 hydrogenation activity did not follow
dissociate into C and O (iii) as shown below (Scheme 1). The extent of carbons being formed over Fe depends mainly on two factors: (1) the ability of the catalyst to dissociate CO into C and O, and (2) the relative rates with which those carbons are being consumed, either by forming hydrocarbons or diffusing to form a bulk iron phase (iv). In this regard, alkalis play an important role on enhancing the dissociative adsorption of CO and suppressed the coverage of hydrogen that eventually could
Fig. 5. HRTEM images of 100Fe:2 K (a,b), 100Fe:2Rb (c,d) and 100Fe:2Cs (e,f) catalysts carburization. (Arrow shows graphitized carbon). 247
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Fig. 8. Comparison of FT products selectivity (after ∼100 h TOS) for various alkali metal promoted iron catalysts. (Reaction conditions: SV = 1.5 Temperature = 270 °C, Pressure = 175 psig, H2/CO2 = 3.0, SLh−1 g−1 metal oxalates).
carburization of iron oxalate doped with K, Rb and Cs all show much higher selectivity (30 mol%) for lower olefins. The alkali promoters are known to shift the Fe-FT products to a higher molecular weight hydrocarbons and more olefinic along with a drop in the overall reaction rates. In the literature, different mechanisms have been proposed; (1) site blocking effects, (2) through the metal electronic interactions, (3) through the space electronic interactions, (4) direct chemical interactions, and (5) alkali-induced surface reconstructions. Uner et al. [32] proposed another mechanism in which alkali promoters restricted the adsorption/desorption mobility of hydrogen on Ru/SiO2 catalysts. For example, potassium is proposed to facilitate CO dissociation through electron transfer from 4 s orbital of potassium to 2π* antibonding orbital of CO make more carbons on the surfaces of Fe. On the other hand, potassium was found to decrease the chemisorption of hydrogen on Fe. Both factors could have a strong influence on the reactivity and product selectivity for iron FT synthesis. Gaube and Klein [33] posulated a modified mechanism on alkalized iron surfaces, where alkali ions take part in the catalytic cycle. The difference in the promotional effect of alkali was explained in that study based on the solvation effect of alkali cations. The authors were observed promotional effects of alkali increase in the following order: H < < < Li < < Na < K∼Cs. More recently, Bui and de Klerk [34] speculated that the origin of a promotional effect of potassium to iron FT synthesis from the perspective of mobility of potassium formate due to its low melting point relative to high temperature FT synthesis. In the current work, K-, Rb- and Cs-promoted iron produce relatively a higher amount of C2-C4 olefins compared to unpromoted and Li, and Na-promoted iron catalysts. Addition of alkali promoters could potentially impact the hydrogen adsorption property of the iron carbide and this could lead to the enhanced olefins formation. However, the possibility of involvement of alkali metals in a catalytic cycle cannot completely be ruled out since alkali metals are known to interact with CO2 to form formate type intermediates [35].
Fig. 6. Conversion of CO2 and H2 over various alkali metal promoted iron catalysts. (Reaction conditions: T = 270 °C, 175 psig, H2/CO2 = 3.0, SV = 1.5 SLh−1 g−1 metal oxalates).
Fig. 7. The variation in lower olefins selectivity as a function of time on-stream. (Reaction conditions: T = 270 °C, 175 psig, H2/CO2 = 3.0, SV = 1.5 SLh−1 g−1 metal oxalates).
4. Conclusions
this trend. The results in the present study indicate that addition of alkali promoter to iron oxalate is more favorable for the improvement of productivity of lower olefins and oxygenates to some extent. Wang et al. [31] obtained similar data for hydrogenation reaction of carbon dioxide using various supported alkali doped iron catalysts and the authors concluded that modification of iron by K decreased hydrogenation ability and responsible for the enhanced selectivity to lower olefins. In our case, we noticed that iron carbides prepared by direct
Carburization of iron oxalate in a CO atmosphere was used to synthesize iron carbide. Mössbauer analysis of the freshly carbruized iron infer that the addition of alkali decreased the bulk iron carbide formation. Both Hägg and theta carbides were formed more or less to the same extent, irrespective of the nature of alkali. Temperature programmed hydrogenation analysis of the activated Fe catalysts suggest that Fe promoted with more basic alkali metal has more carbons 248
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Table 3 CO2 hydrogenation activity and selectivity of the iron catalysts. Catalysts
100Fe
100Fe:2Li
100Fe:2Na
100Fe:2K
100Fe:2Rb
100Fe:2Cs
TOS (h)
117
113
118
123
111
93
Conversion (%) CO2 H2
26.7 27.7
25.3 26.8
24.7 25.1
24.3 26.9
27.3 27.5
24.3 26.7
Product selectivity (mol%, CO free) C1 C2-C4 C5+
52.1 47.9 –
51.5 48.5 –
37.1 58.6 4.3
25.5 64.8 9.7
27.8 66.3 5.9
32.9 62.1 5.0
Chain growth probability (α value) Hydrocarbons 0.40 Oxygenates 0.42
0.34 0.40
0.70 0.69
0.71 0.69
0.72 0.68
0.74 0.74
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Scheme 1. Hydrogenation reactions of CO2 on a Fe-based FT catalyst.
deposited during activation. Rb-promoted iron exhibits initially higher activity for CO2 hydrogenation and the activity leveled off to a nearly constant value for other alkali as well as undoped iron catalysts after a certain time-on-stream. The stability of iron carbide during reaction was found to be increased with increase of the basicity of the added alkali. The addition of Na, K, Rb and Cs to iron catalyst results in lower methane selectivity and significantly enhanced the lower olefins formation. The chain-growth probability factor for hydrocarbons and oxygenates were increased with increase of the basicity of alkali added on iron for CO2 hydrogenation. Acknowledgments The authors gratefully acknowledge the financial support of this work by the Commonwealth of Kentucky. We would like to thank Dr. Dali Qian, CAER, University of Kentucky for assisting with TEM analyses. References [1] H. Yang, C. Zhang, P. Gao, H. Wang, X. Li, L. Zhong, W. Wei, Y. Sun, Catal. Sci. Technol. 7 (2017) 4580–4598. [2] S. Saeidi, N.A.S. Amin, M.R. Rahimpour, J. CO2 Util. 5 (2014) 66–81. [3] G. Melaet, W.T. Ralston, C.-S. Li, S. Alayoglu, K. An, N. Musselwhite, B. Kalkan, G.A. Somorjai, J. Am. Chem. Soc. 136 (2014) 2260–2263.
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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata
Hydrogenation of carbon dioxide over iron carbide prepared from alkali metal promoted iron oxalate
T
Muthu Kumaran Gnanamania, Hussein H. Hamdehb, Wilson D. Shafera,c, Shelley D. Hoppsa, ⁎ Burtron H. Davisa, a
Center for Applied Energy Research, University of Kentucky, 2540 Research Park Drive, Lexington, KY 40511, USA Department of Physics, Wichita State University, Wichita, KS 67260, USA c Asbury University, Natural Science and Allied Health Department, Wilmore, KY 40390, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Iron oxalate Hydrogenation Carbon dioxide Alkali metal Iron carbide
Iron carbide was prepared by the carburization of iron oxalate doped with various alkali metals in flowing CO at 400 °C. The addition of alkali metal (Li, Na, K, Rb or Cs) to iron oxalate was found to decrease the bulk iron carbides formation. However, the amount of carbon deposited on Fe as inferred from temperature programmed hydrogenation and high-resolution transmission electron microscopic studies show an increasing trend with the increase of the basicity of alkali added. The CO2 and H2 conversions of all alkali doped iron catalysts were initially higher but they declined over time irrespective of alkali. The benefits of addition of alkali to iron without any structural and reduction promoters, such as copper, is found to lower the selectivity for methane, higher olefins and oxygenates formation for CO2 hydrogenation.
1. Introduction Hydrogenation is considered as a potential route for recycling of carbon dioxide [1–3]. There are various ways (e.g., electrochemical [4,5], photocatalytic [6,7], heterogeneous [8–15], and homogeneous [16–19]) in which greenhouse gases can be hydrogenated into valueadded products. Hydrogenation of carbon dioxide using heterogeneous catalysts offers more advantages as it can be linked to an existing gasto-liquid technology that converts syngas (a mixture of carbon monoxide and hydrogen) into synthetic liquid fuels. In one approach, carbon dioxide is reduced into CO in the presence of hydrogen, known as reverse water-gas shift reaction, and the CO that is formed is subsequently involved in FT synthesis. Iron-based FischerTropsch (FT) catalysts are more suitable for this process due to its intrinsic reverse water-gas shift reaction. Secondly, Fe is the least expensive transition metal used commercially for carbon-carbon coupling reactions. In general, alkali metal promoter, such as K, is often added to Fe for syngas conversion in order to increase the catalyst stability and provide a higher C5+ selectivity [20–22]. In this context, different alkali promoters may affect the carburization of Fe differently. Earlier, Ribeiro et al. [23] investigated the promotional effect of alkali metals such as Li, Na, K, Rb, and Cs, on the carburization rate of Si-containing Fe
⁎
catalysts using an X-ray adsorption near edge spectroscopy (XANES) technique. The authors found that the carburization rate of FeSi increased in the following order: unpromoted < Li < Na < K = Rb = Cs. Xiong et al. [24] studied the effect of alkali promoters on the FT synthesis of carbon nanotube supported Fe catalysts. The authors revealed that Na and K promoters hinder Fe reduction. Recently, Li et al. [25] studied the role of alkali in iron-based FT synthesis catalysts using in situ X-ray diffraction (XRD) and X-ray photo electron spectroscopy (XPS) techniques. The authors showed that the addition of alkali on FeSi catalysts increased the strength of Fe-O bonds which makes the reduction of Fe2O3 difficult and it also increased C5+ formation. In the present investigation, the effects of addition of alkali metals on the carburization of iron oxalate were followed using X-ray diffraction (XRD), Mössbauer spectroscopy, temperature programmed hydrogenation (TPH) and high-resolution transmission electron microscopy (HRTEM) techniques. The catalysts were tested for hydrogenation of carbon dioxide using a 1 L continuous stirred tank reactor (CSTR). The reactivity and the product selectivity were compared for various alkalipromoted iron catalysts.
Corresponding author. E-mail address: [email protected] (B.H. Davis).
https://doi.org/10.1016/j.apcata.2018.07.034 Received 9 May 2018; Received in revised form 24 July 2018; Accepted 25 July 2018 Available online 26 July 2018 0926-860X/ © 2018 Elsevier B.V. All rights reserved.
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(45 SLh−1) at 400 °C for 6 h and then the reactor was cooled down to room temperature. The carburized catalyst then transferred in a glove box and embedded in wax without exposure to air. Finally, the catalyst chunks were added into a CSTR containing 310 gm of melted Polywax 3000 which had previously been purged with a nitrogen atmosphere. The reactor pressure was increased to 175 psig using a mixture of H2 and CO2 at a ratio of 3 to 1; then the reactor temperature was increased to 270 °C. Brooks mass flow controllers were used to control the flow rates of H2 and CO2. The conversions of CO2 and H2 were obtained by gas-chromatography (GC) analysis (Refinery Gas Analyzer from Agilent) of the reactor exit gas stream. The reaction products were collected in two traps maintained at different temperatures – a warm trap (100 °C) and a cold trap (5 °C). The organic phase condensed in the warm and cold traps were analyzed using HP 7890 GC with DB-5 capillary column, while the aqueous phase was analyzed using an SRI (Torrance CA) GC-TCD-8610C with a 6’ Poropak-Q stainless steel packed column. A 5673 N 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:
2. Experimental 2.1. Catalyst synthesis Iron oxalate {Fe(C2O4.2H2O), 98%] was obtained from Sigma Aldrich and used without further purification. An incipient-wetness impregnation method was used for doping of various alkali metals on iron oxalate by taking an appropriate amount (100 Fe:2 alkali, mol/ mol) of the respective alkali metal nitrate salt (Sigma Aldrich, 99%) dissolved in the required quantity of deionized water. The resultant solid was then dried in an air oven at 100 °C for 24 h. 2.2. Catalyst characterization The elemental analysis was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) technique using a Varian 720-ES analyzer. The materials were dissolved in a perchloric/ nitric acid mixture and the emission spectra of dissolved species (alkali) were compared to those of a series of standard solutions of known concentrations. Powder X-ray diffractograms of the freshly activated catalyst were recorded using a Philips X’Pert diffractometer with monochromatic Cu Kα radiation (λ = 1.5418). XRD scans were taken over the range of 2θ from 5-90°. The scanning step was 0.017°, the scan speed was 0.042 s−1. 57 Fe Mössbauer spectra were collected in a transmission mode by a standard constant acceleration spectrometer (MS-1200, Ranger Scientific). A radiation source of 50 mCi 57Co 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 analysis. In the case of used catalyst analysis, the end of slurry contain catalyst particles submerged with solvent polywax-3000 were used. All samples were investigated at 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 procedures are described elsewhere [26]. The temperature programmed hydrogenation (TPH) of various alkali metal promoted iron catalysts after activation was performed using an in-house system consisting of a furnace capable of operating at temperatures of up to 1200 °C, along with a thermal conductivity detector (TCD). The H2:He (1:3) gas mixture was used to hydrogenate iron carbides. The Origin Pro-8 (Data Analysis and Graphing Software) was used for baseline correction. Prior to TPH, the samples (∼1.0 g) were carburized in-situ in flowing CO (3 SLh-1, standard liter per hour) at 400 °C for 6 h. The temperature of the catalyst was dropped to 30 °C while purging with He and TPH was performed in a flow of 3 SLh−1 of H2:He and the temperature were increased to 850 °C at a ramp rate of 6.8 °C/min. The response to TCD for the consumption of H2 was monitored continuously. In-depth analysis of the nature of the iron particles and the carbons formed after carburization were obtained using a field emission analytical transmission electron microscope (JEOL JEM-2010 F) operated at an accelerating voltage of 200 kV. HR-TEM images were recorded under optimal focus conditions at typical magnifications of 100 K to 1.0 M. The electron beam had a point-to-point resolution of 0.5 nm. Gatan digital micrograph software was used for image processing. Samples were prepared on a lacy carbon copper grid and dispersed as powders after ultrasonic treatment in MeOH.
conversion = 100 x
selectivity = 100 x
nCO2 in−nCO2 out nCO2 in
nproduct out . carbon number nCO 2 in−nCO 2 out
where nCO2 in and nCO2 out are the numbers of moles of CO2 fed and unconverted, 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 total amount of CO2 consumed. 3. Results and discussion Elemental analysis of the added alkali to iron oxalates by ICP are provided in Table 1. The theoritical and experimentally obtained values for different alkali metals are in close agreement. The X-ray diffraction patterns of the freshly carburized catalysts after passivation are shown in Fig. 1. The carburization of iron oxalate at 400 °C in a CO atmosphere produced iron carbides. The undoped iron sample (100Fe) contains a mixture of iron carbide phases. The patterns are indexed to both Hägg (χ-Fe5C2, JCPDS 51-0997 [27]) as well as the theta carbides (θ-Fe3C, JCPDS 34-0001 [27]). This phase assignment is further verified from Mössbauer technique to be discussed later in this section. The doping of alkali to iron did not show any significant change in the pattern. This implies that the crystallinity of iron carbide phases is not influenced by the pressence of alkali under the present activation conditions. The phase composition of Fe is determined by 57Fe Mössbauer spectroscopy technique. Fig. 2A and B shows the Mössbauer spectra of the catalysts after carburization and reaction, respectively. The corresponding data are listed in Table 2. The quantification of various iron carbides including magnetite (Fe3O4) was performed based on the fittings obtained from a respective standard. As can be seen, the Table 1 Elemental analysis by ICP. Catalysts
Alkali (wt%) Theoretical
100Fe 100Fe:2Li 100Fe:2Na 100Fe:2K 100Fe:2Rb 100Fe:2Cs
2.3. Catalyst testing The hydrogenation of carbon dioxide was performed using a 1 L CSTR. Typically, 15.0 g of alkali metal promoted iron oxalate was taken in a fixed-bed tubular reactor. The sample was carburized in flowing CO
– 0.08 0.25 0.43 0.93 1.45
*
ICP analysis – 0.09 0.28 0.52 1.03 1.75
* Theoretical calculations are based on a molecular formula of Fe(C2O4) ⋅2H2O ANO3 (A stands for alkali). 244
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Table 2 Fe phase analysis with the freshly carburized and used iron catalysts using 57Fe Mössbauer spectroscopy. Iron species (%)a
Catalysts Fresh
100Fe 100Fe:2Li 100Fe:2Na 100Fe:2K 100Fe:2Rb 100Fe:2Cs a
Used
Fe3O4
χ-Fe5C2
θ-Fe3C
Fe3O4
χ-Fe5C2
θ-Fe3C
0 12 5 15 19 17
56 45 46 40 38 44
44 43 49 45 43 39
30 59 32 37 35 26
30 23 32 36 37 47
40 28 34 27 28 27
Uncertainty in these numbers vary by 3–5%.
alkali and/or binders. The majority of the phase is Hägg carbide along with magnetite (Fe3O4) [20]. In our case, iron oxalate decomposed and was carburized simultaneously in flowing CO at 400 °C. The resulting materials consisted of both Hägg and theta carbides. It is known that Hägg carbide prefers forming at a lower temperature of carburization than theta carbide. Table 2 shows about 10–19% of the Fe in the magnetite form for alkali-doped iron catalysts after carburization. The percent of magnetite is nil in the case of undoped iron whereas 100Fe:2 K catalyst had about 15% magnetite. The magnetite content in the carburized Rb and Cs-promoted catalysts is relatively higher compared to other catalysts studied in this work. The present data indicate
Fig. 1. XRD patterns of various alkali metal promoted iron catalysts after carburization.
carburization of iron oxalate (100Fe) at 400 °C yielded about equal amounts of Hägg (56%) and theta carbides (44%) and show no indication of Fe3O4. In general, a hematite (Fe2O3) is used for carburization to form iron carbide which is the active phase of iron for FT synthesis [20]. The temperature of carburization in those conditions ranged anywhere between 270–350 °C depending on the presence of
Fig. 2. Fe Mössbauer absorption spectrum of various alkali metal promoted iron catalysts performed at −253 °C after carburization (A); and after the reaction ((B). The fitted curves are shown as solid lines: black, total spectra; red, Fe3O4; blue, χ-Fe5C2; green, έ-Fe2.2C (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article). 245
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wrapped in multilayered graphitic sheets (shown by arrows). The thickness of graphitized carbon is ∼2.79 nm for undoped iron and 1.60 nm for Li-promoted iron catalysts, while Na-promoted iron exhibited a relatively higher thickness of about 3.20 nm. The thickness of graphitized carbon for K- and Rb-promoted iron catalysts are 3.84 and 4.07 nm, respectively and it tends to decrease for Cs-promoted iron to 3.20 nm. Overall, the thickness of graphitized carbon is relative higher in the case of alkali doped catalysts except Li than unpromoted iron. We have looked at different locations along the copper grid for Rb-promoted iron and similar results were obtained. Therefore, we concluded that Rb promotes CO dissociation that often produces more carbon deposition for iron (2CO → CO2 + C) than other alkali metals used in this study. Remember that the formation of bulk iron carbide did not vary much with nature of alkali. The conversions (CO2 or H2) for undoped and alkali-doped iron catalysts are shown in Fig. 6. The initial conversions for all alkali-doped iron catalysts are higher than the undoped iron. The Rb-doped iron catalyst exhibited higher CO2 and H2 conversions initially followed by Na, Cs and K-doped iron catalysts and then the undoped Fe. After about 100 h of synthesis, all iron catalysts, irrespective of alkali show more or less similar H2 and CO2 conversion. The initial higher CO2 conversions of Rb-promoted Fe could be attributed due to the presence of relatively higher carbon content. However, the activity has declined, like others, very steadily for Rb-promoted iron from 30% to 26% in 130 h. Lower olefins are important raw material for the polymer industry [30]. The production of lower olefins from CO2 is highly desirable. In this regard, the selectivity to lower olefins (C2-C4) are plotted against time for various iron catalysts as shown in Fig. 7. As we have seen that there is not significant differences in conversion, the selectivity to lower olefins are as high as about 30% for Rb,K and Cs-promoted iron catalysts compared to 18% Na, Li and undoped iron (5%). In conclusion, alkali addition to iron has shown a beneficial effect by producing ∼6 times more of the lower olefins than undoped iron. The selectivity to various FT products (methane, CO, olefins and oxygenates) for undoped and alkali doped iron catalysts at a similar CO2 conversion level is shown in Fig. 8 and Table 3. The methane selectivity decreased from 43% for a undoped iron catalyst to 29% for Na and 19% for K-doped iron catalysts but then the selectivity for Rb and Cs promoted iron catalysts increased to 23 and 27%, respectively. On the other hand, CO selectivity increases with increasing basicity of the added alkali up to Na and then dropped for K and leveled off for Rb and Cs-doped iron catalysts. Interestingly, the sum of olefins and oxygenates product selectivity for K, Rb and Cs promoted iron catalysts are much higher than any other products formed. Among the promoted catalysts, K-doped iron exhibits selectivity for olefins and oxygenates as high as 42% compared to undoped and Li-doped iron where both show less than 5%. Also, the chain growth probability factor (α) that determine the extent of formation of longer hydrocarbons or oxygenates was increased from 0.40 for undoped iron to 0.74 for Cs-doped iron catalysts. This indicates that alkali has a strong influence on the product distribution of iron for hydrogenation of CO2. A similar effect has been observed by others for syngas conversion using alkali-promoted iron catalysts [20,22,23]. In order to understand the stability of iron carbides under the reaction condition, the slurry containing catalyst sample was analyzed after reaction using Mössbauer technique. The spectral data are shown in Fig. 2 and a quantification of different iron phases are shown in Table 2. Undoped iron lost 30% of their initial iron carbide, whereas Li lost nearly 42% and Na, K, Rb and Cs lost 30, 26, 20 and 10%, respectively. The stability of bulk iron carbides was increased with the addition of alkali metals to iron. In general, hydrogenation of CO2 to hydrocarbons can follow two different pathways, namely the reverse water-gas shift (RWGS) and direct hydrogenation (DH). In RWGS route, CO2 is initially converted on Fe to CO in presence of hydrogen (i), and CO is subsequently involved in FT synthesis to make hydrocarbons (ii) or it can further
Fig. 3. Temperature programmed decarburization profiles of various alkali metal promoted iron catalysts after activation.
that the addition of alkali promoters decreased the iron carbides formation. Temperature programmed hydrogenation analyses of the freshly carburized catalysts are shown in Fig. 3. Hydrogen reacts with iron carbide and various forms of carbon deposited over iron and evolved methane. The TCD response measured as a function of temperature show at least three regions. Xu and Bartholomew [28] classified different carbons formed on iron catalysts as atomic or amorphous or bulk carbides or disordered or ordered graphitic surface carbons based on the temperature. According to these authors, those hydrogenating in the temperature range from 200 to 400 °C are classified as the lowtemperature carbon species, also known as atomic carbon or surface carbide. The middle region between 400 and 600 °C is considered to be amorphous carbons or β and finally the temperature above 600 °C is classified as carbidic carbon, or γ1, and graphitic carbons, or γ2. The unpromoted iron catalyst (100Fe) shows predominantly γ-type carbide and the graphitic carbon region. The addition of Li shifted the temperature of the peak maxima higher. On the other hand, the addition of Na or K promoters to iron caused a significant increase of the carbidic and amorphous carbons and alternatively, the graphitic carbon contents were decreased. Note that the freshly activated sample has some percent of magnetite (6–19%) in all alkali-promoted iron catalysts and its reduction to iron should take place in the temperature range between 450 and 600 °C. Therefore, some contribution in the values reported for amorphous carbon from magnetite reduction is expected in all those alkali-promoted iron catalysts. Nevertheless, the data indicate that the carbon deposited on iron was increased with an increase of basicity of alkali promoters. The temperature of peak maxima for amorphous carbon was shifted to a higher temperature with increase of basicity of alkali. Mössbauer investigations on the freshly activated catalysts infer that bulk iron carbides decreased by a few percent with the addition of alkali, and therefore the increase of carbon content, and particularly K, Rb or Cs containing Fe catalysts, the carbons could be derived mainly from carbon fringes associated to iron carbides. Results of HR-TEM examinations of carburized catalyst samples are shown in Figs. 4 and 5. The iron particles distributed very randomly for undoped and all alkali doped iron catalysts. Due to particles overlap the estimation of particle size distribution become difficult. The high resolution images shown in Fig. 4(b,d,F) all show the lattice fringes with a d-spacing of 0.20 nm correspond to iron carbide. It could be either Hägg or theta iron carbide. According to Datye et al. [29] with just diffraction pattern alone it is impossible to distinguish one phase from another. Hence, we denote those lattice fringes with d-spacing of 0.20 nm for iron carbide (FeCx). The HRTEM images show each iron particles 246
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Fig. 4. HRTEM images of 100Fe (a,b), 100Fe:2Li (c,d) and 100Fe:2Na (e,f) catalysts after carburization. (Arrow shows graphitized carbon).
affect CHx formation (v). In DH route, CO2 is hydrogenated directly to methane (vi). However, the extent to which the catalyst follow these pathways depends on the catalyst compositions (e.g., alkali loading, metal promoters, and nature of binders) and reaction conditions. Iron doped with more basic alkali metals such as K, Rb, and Cs stabilizes iron carbides, probably due to the enhanced dissociative adsorption of CO. Surprisingly, CO2 hydrogenation activity did not follow
dissociate into C and O (iii) as shown below (Scheme 1). The extent of carbons being formed over Fe depends mainly on two factors: (1) the ability of the catalyst to dissociate CO into C and O, and (2) the relative rates with which those carbons are being consumed, either by forming hydrocarbons or diffusing to form a bulk iron phase (iv). In this regard, alkalis play an important role on enhancing the dissociative adsorption of CO and suppressed the coverage of hydrogen that eventually could
Fig. 5. HRTEM images of 100Fe:2 K (a,b), 100Fe:2Rb (c,d) and 100Fe:2Cs (e,f) catalysts carburization. (Arrow shows graphitized carbon). 247
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Fig. 8. Comparison of FT products selectivity (after ∼100 h TOS) for various alkali metal promoted iron catalysts. (Reaction conditions: SV = 1.5 Temperature = 270 °C, Pressure = 175 psig, H2/CO2 = 3.0, SLh−1 g−1 metal oxalates).
carburization of iron oxalate doped with K, Rb and Cs all show much higher selectivity (30 mol%) for lower olefins. The alkali promoters are known to shift the Fe-FT products to a higher molecular weight hydrocarbons and more olefinic along with a drop in the overall reaction rates. In the literature, different mechanisms have been proposed; (1) site blocking effects, (2) through the metal electronic interactions, (3) through the space electronic interactions, (4) direct chemical interactions, and (5) alkali-induced surface reconstructions. Uner et al. [32] proposed another mechanism in which alkali promoters restricted the adsorption/desorption mobility of hydrogen on Ru/SiO2 catalysts. For example, potassium is proposed to facilitate CO dissociation through electron transfer from 4 s orbital of potassium to 2π* antibonding orbital of CO make more carbons on the surfaces of Fe. On the other hand, potassium was found to decrease the chemisorption of hydrogen on Fe. Both factors could have a strong influence on the reactivity and product selectivity for iron FT synthesis. Gaube and Klein [33] posulated a modified mechanism on alkalized iron surfaces, where alkali ions take part in the catalytic cycle. The difference in the promotional effect of alkali was explained in that study based on the solvation effect of alkali cations. The authors were observed promotional effects of alkali increase in the following order: H < < < Li < < Na < K∼Cs. More recently, Bui and de Klerk [34] speculated that the origin of a promotional effect of potassium to iron FT synthesis from the perspective of mobility of potassium formate due to its low melting point relative to high temperature FT synthesis. In the current work, K-, Rb- and Cs-promoted iron produce relatively a higher amount of C2-C4 olefins compared to unpromoted and Li, and Na-promoted iron catalysts. Addition of alkali promoters could potentially impact the hydrogen adsorption property of the iron carbide and this could lead to the enhanced olefins formation. However, the possibility of involvement of alkali metals in a catalytic cycle cannot completely be ruled out since alkali metals are known to interact with CO2 to form formate type intermediates [35].
Fig. 6. Conversion of CO2 and H2 over various alkali metal promoted iron catalysts. (Reaction conditions: T = 270 °C, 175 psig, H2/CO2 = 3.0, SV = 1.5 SLh−1 g−1 metal oxalates).
Fig. 7. The variation in lower olefins selectivity as a function of time on-stream. (Reaction conditions: T = 270 °C, 175 psig, H2/CO2 = 3.0, SV = 1.5 SLh−1 g−1 metal oxalates).
4. Conclusions
this trend. The results in the present study indicate that addition of alkali promoter to iron oxalate is more favorable for the improvement of productivity of lower olefins and oxygenates to some extent. Wang et al. [31] obtained similar data for hydrogenation reaction of carbon dioxide using various supported alkali doped iron catalysts and the authors concluded that modification of iron by K decreased hydrogenation ability and responsible for the enhanced selectivity to lower olefins. In our case, we noticed that iron carbides prepared by direct
Carburization of iron oxalate in a CO atmosphere was used to synthesize iron carbide. Mössbauer analysis of the freshly carbruized iron infer that the addition of alkali decreased the bulk iron carbide formation. Both Hägg and theta carbides were formed more or less to the same extent, irrespective of the nature of alkali. Temperature programmed hydrogenation analysis of the activated Fe catalysts suggest that Fe promoted with more basic alkali metal has more carbons 248
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Table 3 CO2 hydrogenation activity and selectivity of the iron catalysts. Catalysts
100Fe
100Fe:2Li
100Fe:2Na
100Fe:2K
100Fe:2Rb
100Fe:2Cs
TOS (h)
117
113
118
123
111
93
Conversion (%) CO2 H2
26.7 27.7
25.3 26.8
24.7 25.1
24.3 26.9
27.3 27.5
24.3 26.7
Product selectivity (mol%, CO free) C1 C2-C4 C5+
52.1 47.9 –
51.5 48.5 –
37.1 58.6 4.3
25.5 64.8 9.7
27.8 66.3 5.9
32.9 62.1 5.0
Chain growth probability (α value) Hydrocarbons 0.40 Oxygenates 0.42
0.34 0.40
0.70 0.69
0.71 0.69
0.72 0.68
0.74 0.74
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Scheme 1. Hydrogenation reactions of CO2 on a Fe-based FT catalyst.
deposited during activation. Rb-promoted iron exhibits initially higher activity for CO2 hydrogenation and the activity leveled off to a nearly constant value for other alkali as well as undoped iron catalysts after a certain time-on-stream. The stability of iron carbide during reaction was found to be increased with increase of the basicity of the added alkali. The addition of Na, K, Rb and Cs to iron catalyst results in lower methane selectivity and significantly enhanced the lower olefins formation. The chain-growth probability factor for hydrocarbons and oxygenates were increased with increase of the basicity of alkali added on iron for CO2 hydrogenation. Acknowledgments The authors gratefully acknowledge the financial support of this work by the Commonwealth of Kentucky. We would like to thank Dr. Dali Qian, CAER, University of Kentucky for assisting with TEM analyses. References [1] H. Yang, C. Zhang, P. Gao, H. Wang, X. Li, L. Zhong, W. Wei, Y. Sun, Catal. Sci. Technol. 7 (2017) 4580–4598. [2] S. Saeidi, N.A.S. Amin, M.R. Rahimpour, J. CO2 Util. 5 (2014) 66–81. [3] G. Melaet, W.T. Ralston, C.-S. Li, S. Alayoglu, K. An, N. Musselwhite, B. Kalkan, G.A. Somorjai, J. Am. Chem. Soc. 136 (2014) 2260–2263.
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