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Reverse water gas shift reaction catalyzed by Fe nanoparticles with high catalytic activity and stability
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JIEC-2153; No. of Pages 5 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
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Reverse water gas shift reaction catalyzed by Fe nanoparticles with high catalytic activity and stability Dae Han Kim, Sang Wook Han, Hye Soo Yoon, Young Dok Kim * Department of Chemistry, Sungkyunkwan University, Suwon 440-746, South Korea
A R T I C L E I N F O
Article history: Received 1 May 2014 Received in revised form 4 July 2014 Accepted 25 July 2014 Available online xxx Keywords: CO2 Reverse water gas shift reaction Fe
A B S T R A C T
Unsupported Fe-oxide nanoparticles were used as catalysts for reverse water gas shift (RWGS) reaction at 600 8C, which showed a high catalytic activity and stability. Using transmission electron microscopy, nanoparticles of Fe-oxide was found to be resistant toward agglomeration during the RWGS reaction. Xray photoelectron spectroscopy and X-ray diffraction studies revealed that C and O formed by the reaction between Fe-oxide surface and reagent and product (CO and CO2) of the RWGS reaction diffused into the bulk of Fe-oxide nanocatalysts. As a consequence, structure of catalytically active surface, consisting of metallic Fe, was maintained during the RWGS reaction, resulting in a long-term stability of catalytic activity. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction CO2 is the most widely known global warming gas, and recently, much attention has been paid to CO2 capture and sequestration [1– 3]. In particular, synthesis of valuable chemicals out of CO2 has been of great interest. Conversion of CO2 to CO has been extensively studied, since CO is a syngas, which can be used as a starting material for production of valuable chemicals using already well-known catalytic reactions such as Fischer–Tropsch process [3–8]. Diverse heterogeneously catalyzed reactions such as CO2 reforming of CH4 and reverse water gas shift (RWGS) can be utilized for synthesis of CO from CO2 [9–12]. In RWGS reaction, CO2 reacts with H2 to form H2O and CO. RWGS has been attracting attention, since it is one of the key process in CAMERE (carbon dioxide hydrogenation to form methanol via a reverse-water gas shift) reaction: in the CAMERE process, RWGS reaction is followed by methanol synthesis using CO2/CO/H2 [8]. Chemical reactions using CO2 as reagents are generally highly endothermic and therefore require high operating temperatures (typically above 600 8C). Therefore, catalysts for those reactions should show not only high initial catalytic activity but also stability at elevated temperatures. Noble metals such as Pt can be used as
* Corresponding author. Tel.: +82 31 299 4564; fax: +82 31 290 7075. E-mail address: [email protected] (Y.D. Kim).
catalysts [11]; however, due to its high cost and limited natural abundance, other alternative materials should be considered as catalysts. For RWGS reaction, Cu and Zn-based catalysts have been considered [8–10]. However, long-term stabilities of these catalysts at high temperature warrant further studies in the future. In the case of ZnO-based catalysts for RWGS reaction at 600 8C, bare ZnO showed decrease in activity with reaction time, whereas alumina-supported ZnO showed a higher stability of catalytic activity [8]. In the present work, Fe-based unsupported catalysts were used for RWGS reaction at 600 8C. We show that these catalysts are highly stable at 600 8C under conditions of RWGS reaction. The origin of high stability of Fe-based catalysts will be discussed. Experimental Catalyst preparation Fe-oxide nanoparticles were synthesized in aqueous solution without any surfactants. In this method, ferric and ferrous ions are mixed with a molar ratio of 3:2 in highly basic solution at room temperature [13,14]. Reagents, ferric chloride (FeCl3, 97%, Aldrich) and ferrous chloride (FeCl2, 98%, Aldrich), were mixed with 25 ml of distilled water. Then, 20 ml of ammonium hydroxide (28% NH3) was added to the mixture and the resulting solutions were stirred for 10 min. Herein, black precipitates were immediately formed, which were filtrated and washed with distilled water for several
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The RWGS reaction experiment using the Fe-oxide nanoparticle was carried out in a fixed-bed quartz reactor (internal diameter = 31 mm, length = 530 mm) under atmospheric pressure. Typically, 0.2 g of each sample was loaded on quartz wool and placed in the center of the quartz reactor. Before RWGS reaction, the sample was calcined and reduced with air and H2 gas, respectively. Calcination was carried out in air with a flow rate of 30 ml/min at 700 8C for 3 h, and then H2 gas was injected into the reactor with a flow rate of 30 ml/min at 600 8C for 6 h. After these pre-treatments, the reagent gas mixture of CO2 and H2 was continuously fed into the reactor with a 1:1 ratio, and the total feed flow was kept constant at 20 ml/min. The quartz reactor was heated from room temperature to 600 8C with a heating rate of 2 8C/min by temperature controller. The temperature of the sample was kept at 600 8C for about 19 h. The gases which passed through the sample were injected to gas chromatograph (HP6890, HEWLETT PACKARD) equipped with a fused silica capillary column (Carboxen 1010 PLOT, 30 m 0.32 nm, SUPELCO), flame ionization detector (FID) and thermal conductivity detector (TCD). ZnO is a well-known catalyst for RWGS reaction at high temperature, and thus it was also used for RWGS reaction in order to compare the catalytic activity of ZnO (nanopowder, <100 nm, Aldrich) with that of the synthesized Fe-oxide catalyst. Catalyst characterization The physical structure of the Fe-oxide catalyst was analyzed by Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH), and the elemental composition of the catalyst was analyzed by inductively coupled plasma optical emission spectrometry (ICPOES, Agilent 7500, Agilent). The surface composition and the chemical state of each sample were analyzed by X-ray photoelectron spectroscopy (XPS). XPS measurements were performed in an ultra-high vacuum (UHV) chamber (base pressure of 4.0 10 10 Torr) equipped with a concentric hemispherical analyzer (CHA, PHOIBOS-Has 3500, SPECS) and a dual Al/Mg Xray source. All XPS spectra were acquired at room temperature using the Mg Ka-line (1253.6 eV). The morphological images of ZnO powder were obtained by field emission scanning electron microscopy (FESEM, JSM-7000F, JEOL). The particle size and distribution and surface morphology of the Fe-oxide catalyst were analyzed by high resolution transmission electron microscopy (HRTEM, JEM-3010, JEOL). An X-ray diffraction (XRD) pattern of each sample was obtained using XRD spectrometer (Ultima IV, RIGAKU) equipped with Cu-Ka (l = 0.15406 nm) X-ray source.
Intensity (arb. unit)
Catalytic activity test
Fe(III) 711.5 eV)
a)
740
730
720
Fe(II )
( 710.3 eV)
710
700
b) Intensity (arb. unit)
times to remove impurities. Lastly, the precipitates were dried in furnace at 80 8C for 6 h.
545 540 535 530 525 520 515 510
Binding enrgy (eV) Fig. 1. (a) Fe 2p and (b) O 1s spectra of the as-prepared Fe-oxide nanoparticles.
34% and this value became 32% after reaction time of 1150 min. Feoxide showed a high catalytic reactivity for the RWGS reaction and a long-term stability. It is noteworthy that 38% conversions of CO2 and H2 are expected at 600 8C based on thermodynamics consideration. Conventional magnetite powder (<5 mm, 95%, Aldrich) was also tested for RWGS reaction under the same experimental conditions and had reactivity for RWGS reaction with a CO2 conversion and a CO selectivity of 21.3% and 0.85, respectively (data not shown). Due to dissimilarity in the physical properties such as particle size of the synthesized and conventional Fe-oxide, it is hard to directly compare the reactivity of both catalysts for RWGS reaction at 600 8C. At least, one can say that our Fe-oxide catalysts are better than the conventional magnetite sample in terms of selectivity for RWGS. In contrast to the results using Fe-oxide catalysts, ZnO did not show any reactivity for RWGS reaction under the same experimental conditions as those used for Fe-oxide. In a previous study,
50
CO2 conversion (%)
2
a)
40
in equilibrium state
30 20
Fe-oxide ZnO
10
Results and discussion
0
Fig. 1(a) shows Fe 2p core level XPS spectrum of as-prepared catalyst, whereas Fig. 1(b), O 1s spectrum of the same sample. The Fe 2p3/2 peak was centered at about 711 eV, which corresponds to the Fe(III) oxidation state. A shoulder at lower binding energies can be attributed to the Fe(II) species [15,16]. O 1s peak was centered at 530 eV, which agrees well with the O 1s state of Fe-oxide [15,16]. The Fe-oxide sample displayed in Fig. 1 was used as a catalyst for the RWGS reaction, and the results of the reaction experiments are summarized in Fig. 2(a). Fig. 2 shows conversion of CO2, whereas Fig. 2(b) that of H2. CO2 conversion was initially 35%, and this value drastically decreased to 31% within first 60 min. Then, the CO2 conversion was stable within an error bar of 1% for the next 1100 min. The initial H2 conversion using Fe-oxide catalyst was
50
H2 conversion (%)
0
200
400
600
800 1000 1200
b)
40 30 Fe-oxide ZnO
20 10 0 0
200
400
600
800 1000 1200
Time (min) Fig. 2. (a) CO2 and (b) H2 conversion of in the RWGS reaction using the Fe-oxide catalysts and ZnO plotted as a function of reaction time.
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bare ZnO showed an initial CO2 conversion of 30% at 600 8C [8], and different results of this previous work comparing to those of the present study can be most likely attributed to different experimental conditions. Promoted or supported ZnO showed reactivity at lower temperatures, e.g., Cu–ZnO catalyst showed a CO2 conversion of 10% at 240 8C [9].
Fig. 3. TEM images of the Fe-oxide before (a) and (b) and after the RWGS reaction (c) in Fig. 2.
3
Fig. 3(a) and (b) shows TEM images of the as-prepared Fe-oxide catalysts. One can notice that the Fe-oxide catalysts consisted of nanoparticles with a mean size of 10–20 nm. Fig. 3(c) shows TEM images of Fe-oxide catalysts taken after the RWGS reaction for comparison with those of Fe-oxide before reaction; the morphology of the catalyst was almost not changed. In contrast to the results of Fe-oxide, ZnO nanoparticles agglomerated forming large micrometer-scale particles by just pre-treatments at high temperatures (Fig. 4). Fe-oxide nanoparticles were more stable at high temperatures and therefore resistant toward agglomeration at experimental conditions of RWGS, which could be one of the reasons of high stability of catalytic activity of Fe-oxide observed in
Fig. 4. SEM images of bare (a) and (b) and pre-treated ZnO (c).
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Fe(II) (710.3 eV)
Bare Fe3C (708.5 eV)
Before RWGS reaction Fe
(706.8 eV)
After RWGS reaction
716
714
712
710
708
706
704
Binding energy Fig. 5. XPS Fe 2p3/2 spectra of the Fe-oxide: data of the as-prepared sample, before and after the RWGS reaction are compared.
Fig. 2. In contrast, unsupported ZnO immediately experiences reduction of surface area at the same reaction conditions as those of Fe-oxide (Fig. 4), resulting in almost no catalytic activity for RWGS reaction. In order to shed light on the change in the surface structure of Fe-oxide catalysts during the RWGS reaction, XPS studies were carried out, and the results are displayed in Fig. 5. Each Fe 2p3/2 spectrum in Fig. 5 was fitted by using four different components, which correspond to Fe(III) at 711.5 eV, Fe(II) at 710.3 eV, Fe3C at 708.5 eV and metallic Fe at 706.8 eV after Shirley-background subtraction using CASA-XPS software [15,16]. Each component consists of linear combination of Gaussian and Lorentzian functions with a ratio of 7:3, and during fitting, the full-width of the half-maximum of each component and its position were kept constant, whereas the intensity of each peak was varied. In the as-prepared sample, one can see Fe(III) as the majority species with some Fe(II), which can be identified by a shoulder at lower binding energy. When the catalysts were pre-treated at 600 8C without reaction experiments, appearance of Fe3C and metallic Fe could be observed. This result implies that the active catalyst surface consists of a mixture of Fe(III), Fe(II), Fe3C and metallic Fe. Formation of Fe3C could stem from reaction between impurity carbon of the catalysts and Fe. After RWGS reaction, the XPS Fe 2p spectrum did not much change, even though the metallic Fe component slightly decreased in intensity (Table 1). This result is in line with the results in Fig. 2, in which the catalytic activity of this catalyst was very stable with a reactivity change below 10% during more than 1100 min. In contrast the results of XPS, XRD patterns of the catalyst taken before and after RWGS reaction were completely different (Fig. 6). The as-prepared sample showed no peak in XRD, revealing an amorphous nature of this sample. After pre-treatments of the sample, the catalysts showed pronounced XRD peaks corresponding to metallic Fe [17,18]. After the RWGS reaction for longer than 1100 min, the XRD spectrum showed complex patterns, and the new peaks arising after the reaction can be attributed to Fe-oxide and Fe3C [18,19]. XPS is a surface sensitive technique monitoring 3–5 nm thick slabs from the surface topmost layers. XRD is in contrast sensitive to the deeper layers. Pre-treated sample before the RWGS reaction shows Fe(III), Fe(II), Fe3C and metallic Fe in the XPS spectrum; however, only metallic Fe can be observed in the XRD pattern
Fig. 6. XRD data of the Fe-oxide: data of the as-prepared sample, after pretreatment, before and after the RWGS reaction are compared (*: Fe (JCPDS 870721), ^: Fe-oxide (JCPDS 19-0629),*: Fe3C (JCPDS 35-0772)).
Intensity (arb. unit)
Intensity (arb. unit)
Fe(III) (711.5 eV)
295
Before RWGS reaction
After RWGS reaction
290
285
280
275
Binding energy (eV) Fig. 7. XPS C 1s spectra of the Fe-oxide before and after the RWGS reaction.
[17,18]. This result shows that pre-treated sample of Fe was mostly reduced, and oxide and carbide of Fe only remained on the surface part of the sample. During the RWGS reaction, bulk structure of our catalysts was converted from metallic Fe to Fe-oxide and Fe3C, indicating that most likely metallic Fe reacted with reagent or product gases such as CO and CO2 to form Fe-oxide and Fe3C [17–19]. In this context, it is worth mentioning that dissociated chemisorption of CO on Fe surface was found previously [20,21]. Even though the bulk structure of the catalyst was completely changed upon the RWGS reaction, surface structure of catalyst remained nearly unchanged, as one can see from XPS results. The C 1s XPS spectra of the Fe-oxide catalysts before and after RWGS reaction shown in Fig. 7 also support the aforementioned explanation. Herein, the amount of carbon species on the catalyst surface was not much changed even after RWGS reaction for about 1100 min. Consequently, one can suggest that reaction of Fe with CO or CO2 could yield atomic C or O on the surface, which mostly do not remain on the surface, yet diffuse into the bulk of catalyst nanoparticles, forming Fe-oxide or carbide. This leads to the situation that the surface structure of the catalysts can remain almost unchanged during reaction, resulting in stable catalytic activity with almost no deactivation during the reaction. In a hightemperature catalytic reaction, coke formation on the surface can cause detrimental deactivation of the catalysts, and in our experiments, C and O, which can contaminate catalyst surface, seem to diffuse into deeper layers, avoiding change in the surface structure of catalyst during the reaction. Conclusion
Table 1 Relative peak areas of each component in Fig.5 are summarized.
Bare Before RWGS reaction After RWGS reaction
Iron oxide (FeOx)
Iron carbide (Fe3C)
Metallic iron (Fe)
100% 63.0% 70.0%
– 15.3% 15.2%
– 21.7% 14.8%
Fe-oxide nanoparticles were synthesized and used as catalysts for RWGS reaction at 600 8C, and the catalytic activity was high and stable for about 19 h. We found that Fe-oxide nanoparticles did not experience significant agglomeration during the reaction, maintaining high catalytic activity for long time. More importantly, atomic carbon and oxygen forming as a consequence of dissociative chemisorption of CO or CO2 seem to diffuse into the bulk of
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the nanoparticles forming Fe-oxide, and Fe-carbide, yet the surface structure of catalyst consisting of metallic Fe remained nearly unchanged after RWGS reaction, which can cause almost no deactivation of the catalytic activity. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (2012R1A1B3000992), and by the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Education, Science and Technology) (No. 2012M1A8A1056409). References [1] [2] [3] [4] [5]
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JIEC-2153; No. of Pages 5 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Reverse water gas shift reaction catalyzed by Fe nanoparticles with high catalytic activity and stability Dae Han Kim, Sang Wook Han, Hye Soo Yoon, Young Dok Kim * Department of Chemistry, Sungkyunkwan University, Suwon 440-746, South Korea
A R T I C L E I N F O
Article history: Received 1 May 2014 Received in revised form 4 July 2014 Accepted 25 July 2014 Available online xxx Keywords: CO2 Reverse water gas shift reaction Fe
A B S T R A C T
Unsupported Fe-oxide nanoparticles were used as catalysts for reverse water gas shift (RWGS) reaction at 600 8C, which showed a high catalytic activity and stability. Using transmission electron microscopy, nanoparticles of Fe-oxide was found to be resistant toward agglomeration during the RWGS reaction. Xray photoelectron spectroscopy and X-ray diffraction studies revealed that C and O formed by the reaction between Fe-oxide surface and reagent and product (CO and CO2) of the RWGS reaction diffused into the bulk of Fe-oxide nanocatalysts. As a consequence, structure of catalytically active surface, consisting of metallic Fe, was maintained during the RWGS reaction, resulting in a long-term stability of catalytic activity. ß 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Introduction CO2 is the most widely known global warming gas, and recently, much attention has been paid to CO2 capture and sequestration [1– 3]. In particular, synthesis of valuable chemicals out of CO2 has been of great interest. Conversion of CO2 to CO has been extensively studied, since CO is a syngas, which can be used as a starting material for production of valuable chemicals using already well-known catalytic reactions such as Fischer–Tropsch process [3–8]. Diverse heterogeneously catalyzed reactions such as CO2 reforming of CH4 and reverse water gas shift (RWGS) can be utilized for synthesis of CO from CO2 [9–12]. In RWGS reaction, CO2 reacts with H2 to form H2O and CO. RWGS has been attracting attention, since it is one of the key process in CAMERE (carbon dioxide hydrogenation to form methanol via a reverse-water gas shift) reaction: in the CAMERE process, RWGS reaction is followed by methanol synthesis using CO2/CO/H2 [8]. Chemical reactions using CO2 as reagents are generally highly endothermic and therefore require high operating temperatures (typically above 600 8C). Therefore, catalysts for those reactions should show not only high initial catalytic activity but also stability at elevated temperatures. Noble metals such as Pt can be used as
* Corresponding author. Tel.: +82 31 299 4564; fax: +82 31 290 7075. E-mail address: [email protected] (Y.D. Kim).
catalysts [11]; however, due to its high cost and limited natural abundance, other alternative materials should be considered as catalysts. For RWGS reaction, Cu and Zn-based catalysts have been considered [8–10]. However, long-term stabilities of these catalysts at high temperature warrant further studies in the future. In the case of ZnO-based catalysts for RWGS reaction at 600 8C, bare ZnO showed decrease in activity with reaction time, whereas alumina-supported ZnO showed a higher stability of catalytic activity [8]. In the present work, Fe-based unsupported catalysts were used for RWGS reaction at 600 8C. We show that these catalysts are highly stable at 600 8C under conditions of RWGS reaction. The origin of high stability of Fe-based catalysts will be discussed. Experimental Catalyst preparation Fe-oxide nanoparticles were synthesized in aqueous solution without any surfactants. In this method, ferric and ferrous ions are mixed with a molar ratio of 3:2 in highly basic solution at room temperature [13,14]. Reagents, ferric chloride (FeCl3, 97%, Aldrich) and ferrous chloride (FeCl2, 98%, Aldrich), were mixed with 25 ml of distilled water. Then, 20 ml of ammonium hydroxide (28% NH3) was added to the mixture and the resulting solutions were stirred for 10 min. Herein, black precipitates were immediately formed, which were filtrated and washed with distilled water for several
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Please cite this article in press as: D.H. Kim, et al., J. Ind. Eng. Chem. (2014), http://dx.doi.org/10.1016/j.jiec.2014.07.043
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The RWGS reaction experiment using the Fe-oxide nanoparticle was carried out in a fixed-bed quartz reactor (internal diameter = 31 mm, length = 530 mm) under atmospheric pressure. Typically, 0.2 g of each sample was loaded on quartz wool and placed in the center of the quartz reactor. Before RWGS reaction, the sample was calcined and reduced with air and H2 gas, respectively. Calcination was carried out in air with a flow rate of 30 ml/min at 700 8C for 3 h, and then H2 gas was injected into the reactor with a flow rate of 30 ml/min at 600 8C for 6 h. After these pre-treatments, the reagent gas mixture of CO2 and H2 was continuously fed into the reactor with a 1:1 ratio, and the total feed flow was kept constant at 20 ml/min. The quartz reactor was heated from room temperature to 600 8C with a heating rate of 2 8C/min by temperature controller. The temperature of the sample was kept at 600 8C for about 19 h. The gases which passed through the sample were injected to gas chromatograph (HP6890, HEWLETT PACKARD) equipped with a fused silica capillary column (Carboxen 1010 PLOT, 30 m 0.32 nm, SUPELCO), flame ionization detector (FID) and thermal conductivity detector (TCD). ZnO is a well-known catalyst for RWGS reaction at high temperature, and thus it was also used for RWGS reaction in order to compare the catalytic activity of ZnO (nanopowder, <100 nm, Aldrich) with that of the synthesized Fe-oxide catalyst. Catalyst characterization The physical structure of the Fe-oxide catalyst was analyzed by Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH), and the elemental composition of the catalyst was analyzed by inductively coupled plasma optical emission spectrometry (ICPOES, Agilent 7500, Agilent). The surface composition and the chemical state of each sample were analyzed by X-ray photoelectron spectroscopy (XPS). XPS measurements were performed in an ultra-high vacuum (UHV) chamber (base pressure of 4.0 10 10 Torr) equipped with a concentric hemispherical analyzer (CHA, PHOIBOS-Has 3500, SPECS) and a dual Al/Mg Xray source. All XPS spectra were acquired at room temperature using the Mg Ka-line (1253.6 eV). The morphological images of ZnO powder were obtained by field emission scanning electron microscopy (FESEM, JSM-7000F, JEOL). The particle size and distribution and surface morphology of the Fe-oxide catalyst were analyzed by high resolution transmission electron microscopy (HRTEM, JEM-3010, JEOL). An X-ray diffraction (XRD) pattern of each sample was obtained using XRD spectrometer (Ultima IV, RIGAKU) equipped with Cu-Ka (l = 0.15406 nm) X-ray source.
Intensity (arb. unit)
Catalytic activity test
Fe(III) 711.5 eV)
a)
740
730
720
Fe(II )
( 710.3 eV)
710
700
b) Intensity (arb. unit)
times to remove impurities. Lastly, the precipitates were dried in furnace at 80 8C for 6 h.
545 540 535 530 525 520 515 510
Binding enrgy (eV) Fig. 1. (a) Fe 2p and (b) O 1s spectra of the as-prepared Fe-oxide nanoparticles.
34% and this value became 32% after reaction time of 1150 min. Feoxide showed a high catalytic reactivity for the RWGS reaction and a long-term stability. It is noteworthy that 38% conversions of CO2 and H2 are expected at 600 8C based on thermodynamics consideration. Conventional magnetite powder (<5 mm, 95%, Aldrich) was also tested for RWGS reaction under the same experimental conditions and had reactivity for RWGS reaction with a CO2 conversion and a CO selectivity of 21.3% and 0.85, respectively (data not shown). Due to dissimilarity in the physical properties such as particle size of the synthesized and conventional Fe-oxide, it is hard to directly compare the reactivity of both catalysts for RWGS reaction at 600 8C. At least, one can say that our Fe-oxide catalysts are better than the conventional magnetite sample in terms of selectivity for RWGS. In contrast to the results using Fe-oxide catalysts, ZnO did not show any reactivity for RWGS reaction under the same experimental conditions as those used for Fe-oxide. In a previous study,
50
CO2 conversion (%)
2
a)
40
in equilibrium state
30 20
Fe-oxide ZnO
10
Results and discussion
0
Fig. 1(a) shows Fe 2p core level XPS spectrum of as-prepared catalyst, whereas Fig. 1(b), O 1s spectrum of the same sample. The Fe 2p3/2 peak was centered at about 711 eV, which corresponds to the Fe(III) oxidation state. A shoulder at lower binding energies can be attributed to the Fe(II) species [15,16]. O 1s peak was centered at 530 eV, which agrees well with the O 1s state of Fe-oxide [15,16]. The Fe-oxide sample displayed in Fig. 1 was used as a catalyst for the RWGS reaction, and the results of the reaction experiments are summarized in Fig. 2(a). Fig. 2 shows conversion of CO2, whereas Fig. 2(b) that of H2. CO2 conversion was initially 35%, and this value drastically decreased to 31% within first 60 min. Then, the CO2 conversion was stable within an error bar of 1% for the next 1100 min. The initial H2 conversion using Fe-oxide catalyst was
50
H2 conversion (%)
0
200
400
600
800 1000 1200
b)
40 30 Fe-oxide ZnO
20 10 0 0
200
400
600
800 1000 1200
Time (min) Fig. 2. (a) CO2 and (b) H2 conversion of in the RWGS reaction using the Fe-oxide catalysts and ZnO plotted as a function of reaction time.
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bare ZnO showed an initial CO2 conversion of 30% at 600 8C [8], and different results of this previous work comparing to those of the present study can be most likely attributed to different experimental conditions. Promoted or supported ZnO showed reactivity at lower temperatures, e.g., Cu–ZnO catalyst showed a CO2 conversion of 10% at 240 8C [9].
Fig. 3. TEM images of the Fe-oxide before (a) and (b) and after the RWGS reaction (c) in Fig. 2.
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Fig. 3(a) and (b) shows TEM images of the as-prepared Fe-oxide catalysts. One can notice that the Fe-oxide catalysts consisted of nanoparticles with a mean size of 10–20 nm. Fig. 3(c) shows TEM images of Fe-oxide catalysts taken after the RWGS reaction for comparison with those of Fe-oxide before reaction; the morphology of the catalyst was almost not changed. In contrast to the results of Fe-oxide, ZnO nanoparticles agglomerated forming large micrometer-scale particles by just pre-treatments at high temperatures (Fig. 4). Fe-oxide nanoparticles were more stable at high temperatures and therefore resistant toward agglomeration at experimental conditions of RWGS, which could be one of the reasons of high stability of catalytic activity of Fe-oxide observed in
Fig. 4. SEM images of bare (a) and (b) and pre-treated ZnO (c).
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Fe(II) (710.3 eV)
Bare Fe3C (708.5 eV)
Before RWGS reaction Fe
(706.8 eV)
After RWGS reaction
716
714
712
710
708
706
704
Binding energy Fig. 5. XPS Fe 2p3/2 spectra of the Fe-oxide: data of the as-prepared sample, before and after the RWGS reaction are compared.
Fig. 2. In contrast, unsupported ZnO immediately experiences reduction of surface area at the same reaction conditions as those of Fe-oxide (Fig. 4), resulting in almost no catalytic activity for RWGS reaction. In order to shed light on the change in the surface structure of Fe-oxide catalysts during the RWGS reaction, XPS studies were carried out, and the results are displayed in Fig. 5. Each Fe 2p3/2 spectrum in Fig. 5 was fitted by using four different components, which correspond to Fe(III) at 711.5 eV, Fe(II) at 710.3 eV, Fe3C at 708.5 eV and metallic Fe at 706.8 eV after Shirley-background subtraction using CASA-XPS software [15,16]. Each component consists of linear combination of Gaussian and Lorentzian functions with a ratio of 7:3, and during fitting, the full-width of the half-maximum of each component and its position were kept constant, whereas the intensity of each peak was varied. In the as-prepared sample, one can see Fe(III) as the majority species with some Fe(II), which can be identified by a shoulder at lower binding energy. When the catalysts were pre-treated at 600 8C without reaction experiments, appearance of Fe3C and metallic Fe could be observed. This result implies that the active catalyst surface consists of a mixture of Fe(III), Fe(II), Fe3C and metallic Fe. Formation of Fe3C could stem from reaction between impurity carbon of the catalysts and Fe. After RWGS reaction, the XPS Fe 2p spectrum did not much change, even though the metallic Fe component slightly decreased in intensity (Table 1). This result is in line with the results in Fig. 2, in which the catalytic activity of this catalyst was very stable with a reactivity change below 10% during more than 1100 min. In contrast the results of XPS, XRD patterns of the catalyst taken before and after RWGS reaction were completely different (Fig. 6). The as-prepared sample showed no peak in XRD, revealing an amorphous nature of this sample. After pre-treatments of the sample, the catalysts showed pronounced XRD peaks corresponding to metallic Fe [17,18]. After the RWGS reaction for longer than 1100 min, the XRD spectrum showed complex patterns, and the new peaks arising after the reaction can be attributed to Fe-oxide and Fe3C [18,19]. XPS is a surface sensitive technique monitoring 3–5 nm thick slabs from the surface topmost layers. XRD is in contrast sensitive to the deeper layers. Pre-treated sample before the RWGS reaction shows Fe(III), Fe(II), Fe3C and metallic Fe in the XPS spectrum; however, only metallic Fe can be observed in the XRD pattern
Fig. 6. XRD data of the Fe-oxide: data of the as-prepared sample, after pretreatment, before and after the RWGS reaction are compared (*: Fe (JCPDS 870721), ^: Fe-oxide (JCPDS 19-0629),*: Fe3C (JCPDS 35-0772)).
Intensity (arb. unit)
Intensity (arb. unit)
Fe(III) (711.5 eV)
295
Before RWGS reaction
After RWGS reaction
290
285
280
275
Binding energy (eV) Fig. 7. XPS C 1s spectra of the Fe-oxide before and after the RWGS reaction.
[17,18]. This result shows that pre-treated sample of Fe was mostly reduced, and oxide and carbide of Fe only remained on the surface part of the sample. During the RWGS reaction, bulk structure of our catalysts was converted from metallic Fe to Fe-oxide and Fe3C, indicating that most likely metallic Fe reacted with reagent or product gases such as CO and CO2 to form Fe-oxide and Fe3C [17–19]. In this context, it is worth mentioning that dissociated chemisorption of CO on Fe surface was found previously [20,21]. Even though the bulk structure of the catalyst was completely changed upon the RWGS reaction, surface structure of catalyst remained nearly unchanged, as one can see from XPS results. The C 1s XPS spectra of the Fe-oxide catalysts before and after RWGS reaction shown in Fig. 7 also support the aforementioned explanation. Herein, the amount of carbon species on the catalyst surface was not much changed even after RWGS reaction for about 1100 min. Consequently, one can suggest that reaction of Fe with CO or CO2 could yield atomic C or O on the surface, which mostly do not remain on the surface, yet diffuse into the bulk of catalyst nanoparticles, forming Fe-oxide or carbide. This leads to the situation that the surface structure of the catalysts can remain almost unchanged during reaction, resulting in stable catalytic activity with almost no deactivation during the reaction. In a hightemperature catalytic reaction, coke formation on the surface can cause detrimental deactivation of the catalysts, and in our experiments, C and O, which can contaminate catalyst surface, seem to diffuse into deeper layers, avoiding change in the surface structure of catalyst during the reaction. Conclusion
Table 1 Relative peak areas of each component in Fig.5 are summarized.
Bare Before RWGS reaction After RWGS reaction
Iron oxide (FeOx)
Iron carbide (Fe3C)
Metallic iron (Fe)
100% 63.0% 70.0%
– 15.3% 15.2%
– 21.7% 14.8%
Fe-oxide nanoparticles were synthesized and used as catalysts for RWGS reaction at 600 8C, and the catalytic activity was high and stable for about 19 h. We found that Fe-oxide nanoparticles did not experience significant agglomeration during the reaction, maintaining high catalytic activity for long time. More importantly, atomic carbon and oxygen forming as a consequence of dissociative chemisorption of CO or CO2 seem to diffuse into the bulk of
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the nanoparticles forming Fe-oxide, and Fe-carbide, yet the surface structure of catalyst consisting of metallic Fe remained nearly unchanged after RWGS reaction, which can cause almost no deactivation of the catalytic activity. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (2012R1A1B3000992), and by the Korea CCS R&D Center (KCRC) grant funded by the Korea government (Ministry of Education, Science and Technology) (No. 2012M1A8A1056409). References [1] [2] [3] [4] [5]
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