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Flower-like Al2O3-supported iron oxides as an efficient catalyst for oxidative dehydrogenation of ethlybenzene with CO2
Journal of CO2 Utilization 17 (2017) 162–169
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Flower-like Al2O3-supported iron oxides as an efficient catalyst for oxidative dehydrogenation of ethlybenzene with CO2 Tehua Wanga , Lin Qib , Huiyi Luc,* , Min Jia,* a b c
College of Chemistry, Faculty of Chemical, Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116023, China School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, China Department of Pharmacy, The Second Affiliation Hospital of Dalian Medical University, Dalian 116027, China
A R T I C L E I N F O
Article history: Received 4 August 2016 Received in revised form 26 November 2016 Accepted 7 December 2016 Available online xxx Keywords: Flower-like alumina Iron oxide Ethylbenzene dehydrogenation Carbon dioxide
A B S T R A C T
An Al2O3 support with flower-like morphology was used as the support of iron oxide catalyst for oxidative dehydrogenation of ethylbenzene with CO2. A commercial Al2O3 was also used for comparison. These catalyst materials were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), N2 physisorption, H2 temperature programmed reduction (H2-TPR), cyclic voltammetry (CV), electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS) and CO2 temperature programmed desorption (CO2-TPD) techniques. It was found that the concentration of O2 anions on the flower-like Al2O3 surface was significantly higher than that on the commercial Al2O3 surface, leading to large amounts of highly dispersed FexOy clusters on flower-like Al2O3 in contrast with large Fe2O3 particles mainly present on commercial Al2O3. A possible formation mechanism of the small FexOy clusters was proposed. The catalytic results indicated that the highly dispersed FexOy clusters possessed an excellent catalytic activity, since the ethylbenzene conversion over Fe2O3/flower-like Al2O3 was 1.6 times higher than that over Fe2O3/commercial Al2O3. This work provides a new route for designing supported iron catalysts that are widely used, and flowerlike Al2O3 is expected to be a promising catalyst support for oxidative dehydrogenation of ethylbenzene with CO2. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction CO2 utilization in chemical industry has gained much attention for many years. Several CO2-based processes for oxidative dehydrogenation of ethylbenzene (CO2-ODEB) and other alkanes like C2-C4 have been developed to overcome the energy issues in the conventional processes [1]. Among them, dehydrogenation of ethylbenzene to styrene is one of the most important industrial processes. The conventional production of styrene is mainly via direct dehydrogenation process in the presence of a large quantity of superheated steam [2]. However, this process is highly energy consuming due to the loss of heat in the steam condensation. Additionally, the perpass conversion of ethylbenzene is low because of the equilibrium limitation. CO2-ODEB is one of the most prospective routes to produce styrene [3], in which the CO2 utilization can reduce the amount of energy required from 1.5 106 to 1.9 105 kcal/ton of styrene produced [4]. Furthermore, the
* Corresponding authors. E-mail addresses: [email protected] (H. Lu), [email protected] (M. Ji). http://dx.doi.org/10.1016/j.jcou.2016.12.005 2212-9820/© 2016 Elsevier Ltd. All rights reserved.
combination of reverse water-gas shift and ethylbenzene dehydrogenation is proposed to accelerate the reaction rate and alleviate the chemical equilibrium as well as enhance the styrene yield [1]. The effective catalysts for CO2-ODEB are mainly supported metal oxides, including Fe- [5–7], Cr- [8], V- [8–11], Ce-based oxides [12] and other mixed metal oxides [13,14], and the catalyst supports are usually Al2O3 [11], carbon materials [12], MgO [15], SiO2 [16], CeO2-ZrO2 [17], TiO2-ZrO2 [18] and spinel [6]. As we known, the support materials not only provide high surface areas to maximize the number of active sites but also alter the local microenvironments of active species. The physicochemical properties of the supports, such as specific surface area, thermal stability, acid-basic properties and redox properties, can dramatically influence the catalytic performance of the catalysts. Al2O3 has been widely utilized to prepare supported metal or metal oxide catalysts due to its large surface area and both thermal and chemical stability. Via physical and/or chemical modification focusing on Al2O3 surface, the properties of the active species of the catalyst can be tailored. For example, Liu et al. [11] developed a novel Ce0.6Zr0.4O2-Al2O3 support by a sol-gel method and prepared
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a supported vanadia catalyst for CO2-ODEB. It was found that the highly dispersed ceria and/or Ce1-xZrxO2 solid solution on the support surface was critical to the stabilization of highly dispersed V5+, which is responsible for the high activity and stability in CO2ODEB reaction. Sun et al. [19] proposed that the modification of Al2O3 surface with SO42 ions significantly modified the catalytic performance of Fe2O3/Al2O3 in propane dehydrogenation. These sulfate species strongly interacting with the Al2O3 support and Fe via the AlO S bond and the FeOS bond not only suppressed the formation of FexC species and thus the cracking reaction but also enhanced adsorption capacity of propane in reaction. On the other hand, morphology changes of Al2O3 supports also profoundly influence the catalytic performance of active centers of the catalysts. Al2O3 nanosheets was demonstrated to be a superior support of Pt-Sn catalyst for propane dehydrogenation [20]. It was found that the surface of Al2O3 nanosheets was rich in pentacoordinate Al3+ ions, which was beneficial to stabilizing highly dispersed Pt-Sn clusters at high temperature. Nanofibrous Al2O3 was used by Martínez et al. [21] to preparing Co-based catalysts for Fischer-Tropsch synthesis. The high surface area of nanofibrous Al2O3 was responsible for the high Co dispersion, and the large macroporosity of the resultant catalysts inhibited pore blocking caused by filling of liquid hydrocarbon and thus maintained the active sites accessible in the reaction. More recently, several supported metal catalysts using the Al2O3 with hierarchically flower-like structure assembled from nanoplates have been designed for the purpose of improving the catalytic efficiencies of metal nanoparticles, such as Co, Ni and Pd [22–24]. These novel catalysts embedded the metal nanoparticles into the nanoplates, leading to strong metal-support interaction and thereby high metal dispersion. In this paper, an Al2O3 material with flower-like morphology was synthesized by a hydrothermal method and used as the support to prepare iron–based catalyst. The effects of the flowerlike morphology on the surface properties of Al2O3 support, thereby the physicochemical properties of the Fe–based catalyst and the catalytic performance in CO2-ODEB will be discussed. 2. Experimental 2.1. Preparation of catalysts Flower-like Al2O3 support was prepared via a hydrothermal method [25]. In a typical synthesis, 18.75 g of Al(NO3)39H2O and 6.00 g urea were dissolved in 100 and 30 ml of deionized water, respectively. Then the two solutions were mixed by stirring for 15 min and transferred into a Teflon-lined stainless steel autoclave (200 ml), followed by hydrothermal treatment at 200 C for 24 h. The white solids were separated by filtration, washed and dried at 100 C for 12 h. Finally, flower-like Al2O3, labeled as FA, was obtained by calcination of the above solids at 600 C for 4 h in static air. The supported Fe2O3 catalysts were prepared by the incipient wetness method, via dipping fine powders of FA support into an aqueous solution of Fe(NO3)3. After drying at 120 C for 12 h, the catalysts were calcined at 600 C for 4 h in static air. The Fe/Al2O3 ratio in weight was 5/100. The obtained catalyst was labeled as Fe/ FA. For comparison, a commercial Al2O3, supplied by Sinopharm Chemical and labeled as CA, was also used as the support and the corresponding catalyst was labeled as Fe/CA. 2.2. Characterization techniques XRD patterns were acquired at room temperature in a Panalytical X’Pert Pro MPD diffractometer with Cu Ka radiation (0.15406 nm) at 40 kV and 40 mA. FESEM was recorded on a Zeiss
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Supra 55 microscope with an accelerating voltage of 20 kV. TEM analysis was carried out on a FEI Tecnai G2 20 S-TWIN. N2 physisorption experiments were performed on an automatic analyzer (3H-2000PS1, Beishide, China) at 196 C. All samples were outgassed under vacuum at 200 C for 2 h prior to measurement. The special surface areas were calculated by the BET method. The pore volumes were taken at the p/p0 = 0.98 single point. The pore size distribution were determined by BJH method using the desorption branch of the isotherms. H2-TPR and CO2-TPD experiments were carried out on a chemical adsorption instrument (PCA-1200, Builder, China). In H2-TPR process, 50 mg of sample was pretreated at 300 C for 30 min in N2 (50 ml/min) to remove the adsorbed water. After cooling to room temperature, TPR experiments were performed in 5 vol.% H2/N2 (30 ml/min) in the range of 100 to 900 C with a heating rate of 10 C/min. For CO2-TPD, 200 mg of sample was pretreated at 600 C for 30 min in He (30 ml/min) before the measurement. After cooling to 100 C, the sample was saturated by CO2 (30 ml/min) for 20 min. Thereafter, the sample was flushed at 100 C in the He stream for 1 h to remove the physically adsorbed CO2. CO2-TPD experiments were performed in the He stream (30 ml/min) in the range of 100–600 C with a heating rate of 10 C/ min. Cyclic voltammetry experiments were performed on an electrochemical workstation (CHI660B, Chenhua, Shanghai). All measurements were carried out at room temperature with a conventional three electrodes configuration consisting of a platinum wire as auxiliary electrode, a modified sample-paste carbon as working electrode and an Ag/AgCl (saturated KCl) reference electrode. KCl solution (0.1 mol/L) was used as supporting electrolyte. The modified sample-liquid paraffin-carbon powder paste was synthesized according the literature [16]. EPR spectra of the catalysts were recorded on a Bruker A200 spectrometer at room temperature or 173 C operated with a frequency of 9.41 GHz (X-band). XPS analysis of the samples was performed with a Thermo VG ESCALAB 250 Microprobe instrument using Al Ka radiation as the X-ray source. The binding energy of the element was calibrated using a C 1s photoelectron peak at 284.6 eV. 2.3. Catalytic test CO2-ODEB reaction was carried out in a fixed-bed quartz tubular reactor with an inner diameter of 4 mm at 550 C under an atmospheric pressure. Before the reaction, 0.3 g of catalyst was placed at the center of the reactor with quartz wool plugs and then purged with N2 at 550 C for 1 h. Reaction mixture gas of CO2 and EB vapours with a molar ratio of 20, produced by bubbling a CO2 flow (20 ml/min) through a liquid EB held at 50 C in a thermostat, was introduced into the reactor. The cooling products were analyzed by gas chromatography on Agilent-6890 equipped with a FID detector and capillary column of HP-5. The catalytic performance was evaluated only in terms of EB conversion because the selectivity of ST was always above 95%. 3. Results and discussion 3.1. Physical properties of the supports and the iron-based catalysts Fig. 1 shows the XRD patterns of Al2O3 supports and the ironbased catalysts. All samples mainly exhibit a series of diffraction peaks at 2u–37.1, 39.3 , 45.8 and 66.9 , corresponding to the (311), (222), (400) and (440) plane of a g-Al2O3 phase (JCPDS 100425), respectively. There are no obvious diffraction peaks of ironcontaining phases that can be detected over Fe/CA and Fe/FA
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Fig. 1. XRD patterns of the supports and the iron-based catalysts.
catalysts, meaning that the iron species exist in the form of high dispersion and/or smaller crystallites in the catalysts [8,26]. Fig. 2 provides a comparison of morphologies of two Al2O3 supports. As shown there, FA exhibits a flower-like structure assembled from nanoplatelets with a size of 800 170 36 nm
(length width thickness), while the CA only shows an undefined morphology. Subsequent to the loading of Fe2O3, the TEM images of Fe/FA and Fe/CA catalysts are displayed in Fig. 3. As seen in Fig. 3a, Fe/FA displays the lattice fringes with the interplanar spacing of 0.46 nm associated with the (111) facet of g-Al2O3. No iron oxide crystallite can be observed, indicating that the iron species are very well dispersed on the surface of FA. However, the observation in Fig. 3b indicates that there are Fe2O3 nanoparticles with diameter around 12 nm on Fe/CA. These Fe2O3 nanoparticles exhibit the lattice fringes with the interplanar spacing of 0.37 nm assigned to the (012) facet of a-Fe2O3 (JCPDS 33-0664). This phase is unsuccessfully detected by XRD probably due to the low content, which fails to meet the minimum detection limit of XRD. Above TEM results reveal that the iron species supported on Al2O3 with flower-like morphology have a higher dispersion than those on conventional CA support. The N2 adsorption-desorption isotherms and pore size distribution (PSD) curves of all materials are displayed in Fig. 4. From Fig. 4a, it can be seen that FA support shows a type IV isotherm with H3 type hysteresis loop, indicating the presence of slit-shaped
Fig. 2. FESEM images of (a) FA and (b) CA.
Fig. 3. TEM images of (a) Fe/FA and (b) Fe/CA.
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Fig. 4. N2 adsorption-desorption isotherms (a, b) and pore size distribution curves (c, d) of FA, Fe/FA, CA, and Fe/CA.
pores associated with the flower-like morphology [27,28]. In Fig. 4c, its narrow PSD centered around 3.5 nm together with a broad distribution consisting of interpenetrating meso- and macrostructures means the hierarchical porous structure. There are no significant variations in both isotherms and PSD curves after the iron oxides were loaded on FA. In Fig. 4b, the hysteresis loop of the isotherm of CA can be classified as type H2, which is typical of nearly tubular mesopore [21]. Fig. 4d shows that only a unimodal distribution centered around 8.0 nm is present in its PSD curve. Noteworthily, for the Fe/CA catalyst, it shows significant decreases in both the adsorbed volume of N2 for the isotherm and the pore volume for the PSD curve due to the loading of iron oxides. The textural data of these materials are summarized in Table 1. Both the BET surface area (SBET) and pore volume (V) of FA support are smaller than those of CA, and the textural data of the two Febased catalysts show some decreases in comparison with their corresponding original supports. It is worth noting that the drops in SBET and V of the flower-like materials are 15 m2/g and 0.08 cm3/ g, respectively, which are less than those of conventional materials. The slight variations in textural structure between FA and Fe/FA may be attributed to the hierarchical porosity derived from the flower-like morphology of the support, which can maintain the accessibility of internal pore surface of the catalyst to the N2 molecules, though iron oxides were located into the pore. On the
other hand, the small size of the highly dispersed iron species of Fe/ FA decreases the possibility of pore blocking. 3.2. Characterization of iron species H2-TPR was carried out to investigate the reducibility of iron species in the catalysts. As shown in Fig. 5, the profile of a-Fe2O3 shows three peaks, corresponding to the reduction steps of Fe2O3, i.e. Fe2O3 ! Fe3O4 (465 C), Fe3O4 ! FeO (647 C), and FeO ! Fe (755 C). Both Fe/CA and Fe/FA only show one broad reduction peak in the range from 250 to 600 C, assigned to the reduction of Fe3+ to Fe2+. However, there are differences between the two TPR profiles, in which the reduction temperature shifts from 467 C for Fe/CA to 409 C for Fe/FA. For supported metal oxides, smaller metal oxide
Table 1 Textural data of FA, Fe/FA, CA and Fe/CA. Sample
SBET (m2/g)
V (cm3/g)
FA CA Fe/FA Fe/CA
149 302 134 247
0.53 0.75 0.45 0.62 Fig. 5. H2-TPR profiles of a-Fe2O3, Fe/CA and Fe/FA.
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Fig. 6. Cyclic voltammogram of Fe/CA and Fe/FA.
particles generally display a stronger interaction with the surface of the support, which will hinder the reduction of the metal oxides. However, in spite of the higher dispersion of iron oxides over FA, the H2-TPR profiles indicate that the iron oxides of Fe/FA are more reducible than those of Fe/CA. This anomaly suggests that there are other competitive factors dominating the reduction process [29]. On the other hand, it is well known that the surface cations of metal oxides are more easily reduced than the bulk ones during the TPR process. Thus, the highly dispersed iron oxides of Fe/FA possess a higher surface/bulk ratio than those of Fe/CA, which is probably responsible for the lower reduction temperature. The absence of reduction Fe2+ ! Fe0 for both supported catalysts may be ascribed to the in situ formation of FeAl2O4 phase during TPR process [30]. CV experiments were also performed to study the reducibility of Fe3+ ions in Fe/CA and Fe/FA. As shown in Fig. 6, a reduction peak for Fe/CA at 528 mV is observed and attributed to the reduction of Fe3+ to Fe2+ [16]. In the case of Fe/FA, an easier reduction of iron species occurs at a less negative potential of 438 mV than that in Fe/CA. This result indicates that the Fe species in Fe/FA are more readily reduced than that in Fe/CA, in agreement with the results of H2-TPR. The EPR spectra of the iron-based catalysts shown in Fig. 7 were recorded at room temperature and low temperature of 173 C, respectively. At room temperature, a sharp EPR signal at g 4.3 was detected for both Fe/CA and Fe/FA simultaneously. This signal, the intensity of which becomes larger at 173 C (Fig. 7b) as predicted by Curie-Weiss Law, is typically assigned to isolated Fe3+ ions in distorted tetrahedral or octahedral environment [31]. In the g-Al2O3 structure, Fe3+ ions are likely diffused into lattice or
occupy the cation vacancies. The difference in ionic radii between Fe3+ (0.63 Å) and Al3+ (0.53 Å) is assumed to be responsible for the distortion of the local coordination sphere [32]. Thus the appearance of this signal indicates that there are some Fe3+ ions incorporated in g-Al2O3 framework in both catalysts. Because of the low content, their reduction peaks are not obvious in the H2TPR profiles. In addition to the signal of g4.3, Fe/CA shows another broad signal with low intensity around g2.4, which is similarly found in bulk iron oxides [33], indicating the presence of large Fe2O3 particles. The anti Curie-Weiss behavior of this signal extincted at low temperature is caused by the magnetic interaction between electron spins of neighboring Fe3+ ions [33]. Unlike Fe/CA, Fe/FA exhibits a broad and intensive signal at g 2.0, ascribed to Fe3+ ions in FexOy clusters [31,33]. Its global intensity slightly increases at low temperature, suggesting that a small amount of isolated Fe3+ ions in high symmetry also contribute to this signal. The EPR results reveal that the highly dispersed FexOy clusters are mainly present on Fe/FA surface, while large Fe2O3 particles are formed on Fe/CA surface, which is well in line with the observations of TEM. The surface compositions and surface atomic ratios for the supports and iron based catalysts were examined by XPS and the data are listed in Table 2. Compared with CA, FA shows a lower surface O/Al ratio, which can be explained by the removel of more surface hydroxyl group via dehydration process. After the loading of iron oxides, the surface O/Al ratios of both Fe/CA and Fe/FA show slight changes in contrast with the pristine supports. Nevertheless, the surface Fe/Al ratio of Fe/FA is 0.030, which is much lower than 0.047 for Fe/CA, suggesting that the Fe species of Fe/FA tend to be dispersed on the inner pore surface [29]. The binding energy (BE) of Al 2p and Fe 2p photoelectron peaks of the samples are also shown in Table 2. The BE values of Al 2p for CA and FA are 75.0 and 75.5 eV, respectively, which are much higher than that (74.0 eV) reported in the literature [34] and can be ascribed to the surface Al3+ ions with low coordination derived from the removel of hydroxyl groups on Al2O3 surface [35]. The higher Al 2p BE value for FA support means that its surface Al species exist in the state of more unsaturated coordination. The loading of iron oxides profoundly decreases the Al 2p BE value of FA by 2.5 eV against to 0.8 eV for CA, indicating that the interaction between iron oxide species and support in the Fe/FA catalyst is stronger than that in the Fe/CA catalyst. The higher Fe 2p BE value of Fe/FA listed in Table 2 further supports this point. Because of the inputs from Al2O3, Fe2O3, hydroxyl and oxygen anions, and strongly adsorbed H2O molecules [34], the O 1s BE values listed in Table 2 are very difficult to illustrate the coordination surroundings of iron species supported on different Al2O3 supports. So CO2-TPD were conducted to characterize the
Fig. 7. EPR spectra of Fe/FA and Fe/CA recorded at (a) room temperature and (b) 173 C.
T. Wang et al. / Journal of CO2 Utilization 17 (2017) 162–169 Table 2 XPS data for the supports and the iron-based catalysts. Sample
CA Fe/CA FA Fe/FA
Surface atomic ratio
Binding energy (eV)
O/Al
Fe/Al
Al 2p
Fe 2p
O 1s
1.59 1.66 1.34 1.38
– 0.047 – 0.030
75.0 74.2 75.5 73.0
– 712.8 – 713.0
530.5 531.9 530.8 532.0
Fig. 8. CO2-TPD of the supports and the iron-based catalysts.
surface basicity of the supports and iron-based catalysts and to uncover the interaction between iron oxides and Al2O3 supports. The corresponding curves are displayed in Fig. 8. Both CA and FA supports show two CO2 desorption peaks around 150 and 320 C,
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attributed to weak and medium basic sites on the surface, respectively. The peak area of the medium basic sites on FA surface is remarkably larger than that on CA, signifying the higher concentration of medium basic sites on FA surface. After the loading of iron species, it is clearly found that these medium basic sites on both supports are completely consumed, while the weak basic sites are almost unchanged, accompanied by the appearance of some strong basic sites associated with the iron species. These results clearly reveal that the iron species preferentially locate on the medium basic sites during the preparation process of the Febased catalysts. As proposed in the literature [36], two basic sites, hydroxyl ion site (-OH) and oxygen ion sites (O2), are usually present on the surface of dehydrated alumina. The Lewis basicity of O2 is generally stronger than that of OH [37,38]. Therefore, the weak basic sites shown in Fig. 8 can be ascribed to OH and the medium basic sites to O2 ions on Al2O3 surface, and the most important is that the concentration of surface O2 anions of FA support is higher than that of CA, which is consistent with the lower coordination of Al3+ ions on FA surface because low coordinate Al3+ ions and medium basic O2 anions are simultaneously formed at adjacent position on Al2O3 surface in the dehydration process. The dramatic decrease in Al 2p BE value in XPS and the disappearance of basic O2 in CO2-TPD for the Al2O3 supports after loading of iron oxides can be related to the strong interaction between iron species and surface O2 anions on Al2O3 surface. Based on the fact that the iron species of Fe/FA were highly dispersed and the results of XPS and CO2-TPD, we propose that O2 ions on the support surface play a key role in dispersing iron species. Fig. 9 shows the schematic of preparation processes of Fe/ FA and Fe/CA catalysts by the incipient wetness method. During the preparation process of Fe/FA, a high concentration of OH ions were produced from the surface O2 ions of FA support at the
Fig. 9. Schematic of preparation processes of Fe/CA and Fe/FA catalysts.
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interface via hydrolysis reaction [39]. These OH ions react with Fe3+ cations to form Fe(OH)3 precursors on the surface, covering the O2 sites and resulting in small FexOy clusters strongly interacting with the support surface. For CA, its surface O2 ions are insufficient and thereby a low concentration of OH ions is present. The direct pyrolysis of Fe(NO3)3 occurs during the subsequently thermal treatment, resulting in larger Fe2O3 particles. The above proposals are supported by the observation, as shown in the inserted pictures of Fig. 9, that the yellow color of Fe/ FA shallowed upon thermal decomposition while Fe/CA exhibited a reverse behavior. Therefore, we conclude that the formation of highly dispersed FexOy clusters on Fe/FA should be attributed to the high concentration of O2 anions on the surface of flower-like Al2O3. 3.3. Catalytic performance The catalytic results of CO2-ODEB over the Al2O3 supports and the supported catalysts are shown in Fig. 10. The EB conversion over FA and CA are similarly less than 10%, suggesting little contribution of the pure support to catalytic activity. However, a different scenario is observed in the case of supported iron oxide catalysts. During the 10 h of time on stream, the EB conversion over Fe/FA is almost 1.6-fold higher than that over Fe/CA due to the different forms of iron species on the catalyst surface. The highly dispersed FexOy clusters existing in Fe/FA surface provide a much higher density of active sites and result in the excellent catalytic activity. 4. Conclusions Flower-like Al2O3 support was synthesized and used to prepare supported iron oxide catalysts for CO2-ODEB. The properties of the flower-like surface and its effect on the dispersion of iron oxides and catalytic performance of the Fe2O3/Al2O3 catalysts were studied. There are more surface O2 anions present on the surface of flower-like Al2O3 than on commercial Al2O3. During the catalyst preparation process, the flower-like surface rich in O2 anions favored the formation of Fe(OH)3 precursors, which turned into highly dispersed FexOy clusters that strongly interacted with the flower-like surface upon calcination. Due to the high dispersion, the iron oxide species supported on flower-like Al2O3 exhibited a superior activity than those on commercial Al2O3 in CO2-ODEB reaction. Flower-like Al2O3 may be a promising support, instead of conventional Al2O3, to manufacture potential commercial catalysts for CO2-ODEB.
Fig. 10. Catalytic results of the supports and iron-based catalysts for CO2-ODEB.
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Flower-like Al2O3-supported iron oxides as an efficient catalyst for oxidative dehydrogenation of ethlybenzene with CO2 Tehua Wanga , Lin Qib , Huiyi Luc,* , Min Jia,* a b c
College of Chemistry, Faculty of Chemical, Environmental and Biological Science and Technology, Dalian University of Technology, Dalian 116023, China School of Materials Science and Engineering, Dalian University of Technology, Dalian 116023, China Department of Pharmacy, The Second Affiliation Hospital of Dalian Medical University, Dalian 116027, China
A R T I C L E I N F O
Article history: Received 4 August 2016 Received in revised form 26 November 2016 Accepted 7 December 2016 Available online xxx Keywords: Flower-like alumina Iron oxide Ethylbenzene dehydrogenation Carbon dioxide
A B S T R A C T
An Al2O3 support with flower-like morphology was used as the support of iron oxide catalyst for oxidative dehydrogenation of ethylbenzene with CO2. A commercial Al2O3 was also used for comparison. These catalyst materials were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), N2 physisorption, H2 temperature programmed reduction (H2-TPR), cyclic voltammetry (CV), electron paramagnetic resonance (EPR), X-ray photoelectron spectroscopy (XPS) and CO2 temperature programmed desorption (CO2-TPD) techniques. It was found that the concentration of O2 anions on the flower-like Al2O3 surface was significantly higher than that on the commercial Al2O3 surface, leading to large amounts of highly dispersed FexOy clusters on flower-like Al2O3 in contrast with large Fe2O3 particles mainly present on commercial Al2O3. A possible formation mechanism of the small FexOy clusters was proposed. The catalytic results indicated that the highly dispersed FexOy clusters possessed an excellent catalytic activity, since the ethylbenzene conversion over Fe2O3/flower-like Al2O3 was 1.6 times higher than that over Fe2O3/commercial Al2O3. This work provides a new route for designing supported iron catalysts that are widely used, and flowerlike Al2O3 is expected to be a promising catalyst support for oxidative dehydrogenation of ethylbenzene with CO2. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction CO2 utilization in chemical industry has gained much attention for many years. Several CO2-based processes for oxidative dehydrogenation of ethylbenzene (CO2-ODEB) and other alkanes like C2-C4 have been developed to overcome the energy issues in the conventional processes [1]. Among them, dehydrogenation of ethylbenzene to styrene is one of the most important industrial processes. The conventional production of styrene is mainly via direct dehydrogenation process in the presence of a large quantity of superheated steam [2]. However, this process is highly energy consuming due to the loss of heat in the steam condensation. Additionally, the perpass conversion of ethylbenzene is low because of the equilibrium limitation. CO2-ODEB is one of the most prospective routes to produce styrene [3], in which the CO2 utilization can reduce the amount of energy required from 1.5 106 to 1.9 105 kcal/ton of styrene produced [4]. Furthermore, the
* Corresponding authors. E-mail addresses: [email protected] (H. Lu), [email protected] (M. Ji). http://dx.doi.org/10.1016/j.jcou.2016.12.005 2212-9820/© 2016 Elsevier Ltd. All rights reserved.
combination of reverse water-gas shift and ethylbenzene dehydrogenation is proposed to accelerate the reaction rate and alleviate the chemical equilibrium as well as enhance the styrene yield [1]. The effective catalysts for CO2-ODEB are mainly supported metal oxides, including Fe- [5–7], Cr- [8], V- [8–11], Ce-based oxides [12] and other mixed metal oxides [13,14], and the catalyst supports are usually Al2O3 [11], carbon materials [12], MgO [15], SiO2 [16], CeO2-ZrO2 [17], TiO2-ZrO2 [18] and spinel [6]. As we known, the support materials not only provide high surface areas to maximize the number of active sites but also alter the local microenvironments of active species. The physicochemical properties of the supports, such as specific surface area, thermal stability, acid-basic properties and redox properties, can dramatically influence the catalytic performance of the catalysts. Al2O3 has been widely utilized to prepare supported metal or metal oxide catalysts due to its large surface area and both thermal and chemical stability. Via physical and/or chemical modification focusing on Al2O3 surface, the properties of the active species of the catalyst can be tailored. For example, Liu et al. [11] developed a novel Ce0.6Zr0.4O2-Al2O3 support by a sol-gel method and prepared
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a supported vanadia catalyst for CO2-ODEB. It was found that the highly dispersed ceria and/or Ce1-xZrxO2 solid solution on the support surface was critical to the stabilization of highly dispersed V5+, which is responsible for the high activity and stability in CO2ODEB reaction. Sun et al. [19] proposed that the modification of Al2O3 surface with SO42 ions significantly modified the catalytic performance of Fe2O3/Al2O3 in propane dehydrogenation. These sulfate species strongly interacting with the Al2O3 support and Fe via the AlO S bond and the FeOS bond not only suppressed the formation of FexC species and thus the cracking reaction but also enhanced adsorption capacity of propane in reaction. On the other hand, morphology changes of Al2O3 supports also profoundly influence the catalytic performance of active centers of the catalysts. Al2O3 nanosheets was demonstrated to be a superior support of Pt-Sn catalyst for propane dehydrogenation [20]. It was found that the surface of Al2O3 nanosheets was rich in pentacoordinate Al3+ ions, which was beneficial to stabilizing highly dispersed Pt-Sn clusters at high temperature. Nanofibrous Al2O3 was used by Martínez et al. [21] to preparing Co-based catalysts for Fischer-Tropsch synthesis. The high surface area of nanofibrous Al2O3 was responsible for the high Co dispersion, and the large macroporosity of the resultant catalysts inhibited pore blocking caused by filling of liquid hydrocarbon and thus maintained the active sites accessible in the reaction. More recently, several supported metal catalysts using the Al2O3 with hierarchically flower-like structure assembled from nanoplates have been designed for the purpose of improving the catalytic efficiencies of metal nanoparticles, such as Co, Ni and Pd [22–24]. These novel catalysts embedded the metal nanoparticles into the nanoplates, leading to strong metal-support interaction and thereby high metal dispersion. In this paper, an Al2O3 material with flower-like morphology was synthesized by a hydrothermal method and used as the support to prepare iron–based catalyst. The effects of the flowerlike morphology on the surface properties of Al2O3 support, thereby the physicochemical properties of the Fe–based catalyst and the catalytic performance in CO2-ODEB will be discussed. 2. Experimental 2.1. Preparation of catalysts Flower-like Al2O3 support was prepared via a hydrothermal method [25]. In a typical synthesis, 18.75 g of Al(NO3)39H2O and 6.00 g urea were dissolved in 100 and 30 ml of deionized water, respectively. Then the two solutions were mixed by stirring for 15 min and transferred into a Teflon-lined stainless steel autoclave (200 ml), followed by hydrothermal treatment at 200 C for 24 h. The white solids were separated by filtration, washed and dried at 100 C for 12 h. Finally, flower-like Al2O3, labeled as FA, was obtained by calcination of the above solids at 600 C for 4 h in static air. The supported Fe2O3 catalysts were prepared by the incipient wetness method, via dipping fine powders of FA support into an aqueous solution of Fe(NO3)3. After drying at 120 C for 12 h, the catalysts were calcined at 600 C for 4 h in static air. The Fe/Al2O3 ratio in weight was 5/100. The obtained catalyst was labeled as Fe/ FA. For comparison, a commercial Al2O3, supplied by Sinopharm Chemical and labeled as CA, was also used as the support and the corresponding catalyst was labeled as Fe/CA. 2.2. Characterization techniques XRD patterns were acquired at room temperature in a Panalytical X’Pert Pro MPD diffractometer with Cu Ka radiation (0.15406 nm) at 40 kV and 40 mA. FESEM was recorded on a Zeiss
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Supra 55 microscope with an accelerating voltage of 20 kV. TEM analysis was carried out on a FEI Tecnai G2 20 S-TWIN. N2 physisorption experiments were performed on an automatic analyzer (3H-2000PS1, Beishide, China) at 196 C. All samples were outgassed under vacuum at 200 C for 2 h prior to measurement. The special surface areas were calculated by the BET method. The pore volumes were taken at the p/p0 = 0.98 single point. The pore size distribution were determined by BJH method using the desorption branch of the isotherms. H2-TPR and CO2-TPD experiments were carried out on a chemical adsorption instrument (PCA-1200, Builder, China). In H2-TPR process, 50 mg of sample was pretreated at 300 C for 30 min in N2 (50 ml/min) to remove the adsorbed water. After cooling to room temperature, TPR experiments were performed in 5 vol.% H2/N2 (30 ml/min) in the range of 100 to 900 C with a heating rate of 10 C/min. For CO2-TPD, 200 mg of sample was pretreated at 600 C for 30 min in He (30 ml/min) before the measurement. After cooling to 100 C, the sample was saturated by CO2 (30 ml/min) for 20 min. Thereafter, the sample was flushed at 100 C in the He stream for 1 h to remove the physically adsorbed CO2. CO2-TPD experiments were performed in the He stream (30 ml/min) in the range of 100–600 C with a heating rate of 10 C/ min. Cyclic voltammetry experiments were performed on an electrochemical workstation (CHI660B, Chenhua, Shanghai). All measurements were carried out at room temperature with a conventional three electrodes configuration consisting of a platinum wire as auxiliary electrode, a modified sample-paste carbon as working electrode and an Ag/AgCl (saturated KCl) reference electrode. KCl solution (0.1 mol/L) was used as supporting electrolyte. The modified sample-liquid paraffin-carbon powder paste was synthesized according the literature [16]. EPR spectra of the catalysts were recorded on a Bruker A200 spectrometer at room temperature or 173 C operated with a frequency of 9.41 GHz (X-band). XPS analysis of the samples was performed with a Thermo VG ESCALAB 250 Microprobe instrument using Al Ka radiation as the X-ray source. The binding energy of the element was calibrated using a C 1s photoelectron peak at 284.6 eV. 2.3. Catalytic test CO2-ODEB reaction was carried out in a fixed-bed quartz tubular reactor with an inner diameter of 4 mm at 550 C under an atmospheric pressure. Before the reaction, 0.3 g of catalyst was placed at the center of the reactor with quartz wool plugs and then purged with N2 at 550 C for 1 h. Reaction mixture gas of CO2 and EB vapours with a molar ratio of 20, produced by bubbling a CO2 flow (20 ml/min) through a liquid EB held at 50 C in a thermostat, was introduced into the reactor. The cooling products were analyzed by gas chromatography on Agilent-6890 equipped with a FID detector and capillary column of HP-5. The catalytic performance was evaluated only in terms of EB conversion because the selectivity of ST was always above 95%. 3. Results and discussion 3.1. Physical properties of the supports and the iron-based catalysts Fig. 1 shows the XRD patterns of Al2O3 supports and the ironbased catalysts. All samples mainly exhibit a series of diffraction peaks at 2u–37.1, 39.3 , 45.8 and 66.9 , corresponding to the (311), (222), (400) and (440) plane of a g-Al2O3 phase (JCPDS 100425), respectively. There are no obvious diffraction peaks of ironcontaining phases that can be detected over Fe/CA and Fe/FA
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Fig. 1. XRD patterns of the supports and the iron-based catalysts.
catalysts, meaning that the iron species exist in the form of high dispersion and/or smaller crystallites in the catalysts [8,26]. Fig. 2 provides a comparison of morphologies of two Al2O3 supports. As shown there, FA exhibits a flower-like structure assembled from nanoplatelets with a size of 800 170 36 nm
(length width thickness), while the CA only shows an undefined morphology. Subsequent to the loading of Fe2O3, the TEM images of Fe/FA and Fe/CA catalysts are displayed in Fig. 3. As seen in Fig. 3a, Fe/FA displays the lattice fringes with the interplanar spacing of 0.46 nm associated with the (111) facet of g-Al2O3. No iron oxide crystallite can be observed, indicating that the iron species are very well dispersed on the surface of FA. However, the observation in Fig. 3b indicates that there are Fe2O3 nanoparticles with diameter around 12 nm on Fe/CA. These Fe2O3 nanoparticles exhibit the lattice fringes with the interplanar spacing of 0.37 nm assigned to the (012) facet of a-Fe2O3 (JCPDS 33-0664). This phase is unsuccessfully detected by XRD probably due to the low content, which fails to meet the minimum detection limit of XRD. Above TEM results reveal that the iron species supported on Al2O3 with flower-like morphology have a higher dispersion than those on conventional CA support. The N2 adsorption-desorption isotherms and pore size distribution (PSD) curves of all materials are displayed in Fig. 4. From Fig. 4a, it can be seen that FA support shows a type IV isotherm with H3 type hysteresis loop, indicating the presence of slit-shaped
Fig. 2. FESEM images of (a) FA and (b) CA.
Fig. 3. TEM images of (a) Fe/FA and (b) Fe/CA.
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Fig. 4. N2 adsorption-desorption isotherms (a, b) and pore size distribution curves (c, d) of FA, Fe/FA, CA, and Fe/CA.
pores associated with the flower-like morphology [27,28]. In Fig. 4c, its narrow PSD centered around 3.5 nm together with a broad distribution consisting of interpenetrating meso- and macrostructures means the hierarchical porous structure. There are no significant variations in both isotherms and PSD curves after the iron oxides were loaded on FA. In Fig. 4b, the hysteresis loop of the isotherm of CA can be classified as type H2, which is typical of nearly tubular mesopore [21]. Fig. 4d shows that only a unimodal distribution centered around 8.0 nm is present in its PSD curve. Noteworthily, for the Fe/CA catalyst, it shows significant decreases in both the adsorbed volume of N2 for the isotherm and the pore volume for the PSD curve due to the loading of iron oxides. The textural data of these materials are summarized in Table 1. Both the BET surface area (SBET) and pore volume (V) of FA support are smaller than those of CA, and the textural data of the two Febased catalysts show some decreases in comparison with their corresponding original supports. It is worth noting that the drops in SBET and V of the flower-like materials are 15 m2/g and 0.08 cm3/ g, respectively, which are less than those of conventional materials. The slight variations in textural structure between FA and Fe/FA may be attributed to the hierarchical porosity derived from the flower-like morphology of the support, which can maintain the accessibility of internal pore surface of the catalyst to the N2 molecules, though iron oxides were located into the pore. On the
other hand, the small size of the highly dispersed iron species of Fe/ FA decreases the possibility of pore blocking. 3.2. Characterization of iron species H2-TPR was carried out to investigate the reducibility of iron species in the catalysts. As shown in Fig. 5, the profile of a-Fe2O3 shows three peaks, corresponding to the reduction steps of Fe2O3, i.e. Fe2O3 ! Fe3O4 (465 C), Fe3O4 ! FeO (647 C), and FeO ! Fe (755 C). Both Fe/CA and Fe/FA only show one broad reduction peak in the range from 250 to 600 C, assigned to the reduction of Fe3+ to Fe2+. However, there are differences between the two TPR profiles, in which the reduction temperature shifts from 467 C for Fe/CA to 409 C for Fe/FA. For supported metal oxides, smaller metal oxide
Table 1 Textural data of FA, Fe/FA, CA and Fe/CA. Sample
SBET (m2/g)
V (cm3/g)
FA CA Fe/FA Fe/CA
149 302 134 247
0.53 0.75 0.45 0.62 Fig. 5. H2-TPR profiles of a-Fe2O3, Fe/CA and Fe/FA.
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Fig. 6. Cyclic voltammogram of Fe/CA and Fe/FA.
particles generally display a stronger interaction with the surface of the support, which will hinder the reduction of the metal oxides. However, in spite of the higher dispersion of iron oxides over FA, the H2-TPR profiles indicate that the iron oxides of Fe/FA are more reducible than those of Fe/CA. This anomaly suggests that there are other competitive factors dominating the reduction process [29]. On the other hand, it is well known that the surface cations of metal oxides are more easily reduced than the bulk ones during the TPR process. Thus, the highly dispersed iron oxides of Fe/FA possess a higher surface/bulk ratio than those of Fe/CA, which is probably responsible for the lower reduction temperature. The absence of reduction Fe2+ ! Fe0 for both supported catalysts may be ascribed to the in situ formation of FeAl2O4 phase during TPR process [30]. CV experiments were also performed to study the reducibility of Fe3+ ions in Fe/CA and Fe/FA. As shown in Fig. 6, a reduction peak for Fe/CA at 528 mV is observed and attributed to the reduction of Fe3+ to Fe2+ [16]. In the case of Fe/FA, an easier reduction of iron species occurs at a less negative potential of 438 mV than that in Fe/CA. This result indicates that the Fe species in Fe/FA are more readily reduced than that in Fe/CA, in agreement with the results of H2-TPR. The EPR spectra of the iron-based catalysts shown in Fig. 7 were recorded at room temperature and low temperature of 173 C, respectively. At room temperature, a sharp EPR signal at g 4.3 was detected for both Fe/CA and Fe/FA simultaneously. This signal, the intensity of which becomes larger at 173 C (Fig. 7b) as predicted by Curie-Weiss Law, is typically assigned to isolated Fe3+ ions in distorted tetrahedral or octahedral environment [31]. In the g-Al2O3 structure, Fe3+ ions are likely diffused into lattice or
occupy the cation vacancies. The difference in ionic radii between Fe3+ (0.63 Å) and Al3+ (0.53 Å) is assumed to be responsible for the distortion of the local coordination sphere [32]. Thus the appearance of this signal indicates that there are some Fe3+ ions incorporated in g-Al2O3 framework in both catalysts. Because of the low content, their reduction peaks are not obvious in the H2TPR profiles. In addition to the signal of g4.3, Fe/CA shows another broad signal with low intensity around g2.4, which is similarly found in bulk iron oxides [33], indicating the presence of large Fe2O3 particles. The anti Curie-Weiss behavior of this signal extincted at low temperature is caused by the magnetic interaction between electron spins of neighboring Fe3+ ions [33]. Unlike Fe/CA, Fe/FA exhibits a broad and intensive signal at g 2.0, ascribed to Fe3+ ions in FexOy clusters [31,33]. Its global intensity slightly increases at low temperature, suggesting that a small amount of isolated Fe3+ ions in high symmetry also contribute to this signal. The EPR results reveal that the highly dispersed FexOy clusters are mainly present on Fe/FA surface, while large Fe2O3 particles are formed on Fe/CA surface, which is well in line with the observations of TEM. The surface compositions and surface atomic ratios for the supports and iron based catalysts were examined by XPS and the data are listed in Table 2. Compared with CA, FA shows a lower surface O/Al ratio, which can be explained by the removel of more surface hydroxyl group via dehydration process. After the loading of iron oxides, the surface O/Al ratios of both Fe/CA and Fe/FA show slight changes in contrast with the pristine supports. Nevertheless, the surface Fe/Al ratio of Fe/FA is 0.030, which is much lower than 0.047 for Fe/CA, suggesting that the Fe species of Fe/FA tend to be dispersed on the inner pore surface [29]. The binding energy (BE) of Al 2p and Fe 2p photoelectron peaks of the samples are also shown in Table 2. The BE values of Al 2p for CA and FA are 75.0 and 75.5 eV, respectively, which are much higher than that (74.0 eV) reported in the literature [34] and can be ascribed to the surface Al3+ ions with low coordination derived from the removel of hydroxyl groups on Al2O3 surface [35]. The higher Al 2p BE value for FA support means that its surface Al species exist in the state of more unsaturated coordination. The loading of iron oxides profoundly decreases the Al 2p BE value of FA by 2.5 eV against to 0.8 eV for CA, indicating that the interaction between iron oxide species and support in the Fe/FA catalyst is stronger than that in the Fe/CA catalyst. The higher Fe 2p BE value of Fe/FA listed in Table 2 further supports this point. Because of the inputs from Al2O3, Fe2O3, hydroxyl and oxygen anions, and strongly adsorbed H2O molecules [34], the O 1s BE values listed in Table 2 are very difficult to illustrate the coordination surroundings of iron species supported on different Al2O3 supports. So CO2-TPD were conducted to characterize the
Fig. 7. EPR spectra of Fe/FA and Fe/CA recorded at (a) room temperature and (b) 173 C.
T. Wang et al. / Journal of CO2 Utilization 17 (2017) 162–169 Table 2 XPS data for the supports and the iron-based catalysts. Sample
CA Fe/CA FA Fe/FA
Surface atomic ratio
Binding energy (eV)
O/Al
Fe/Al
Al 2p
Fe 2p
O 1s
1.59 1.66 1.34 1.38
– 0.047 – 0.030
75.0 74.2 75.5 73.0
– 712.8 – 713.0
530.5 531.9 530.8 532.0
Fig. 8. CO2-TPD of the supports and the iron-based catalysts.
surface basicity of the supports and iron-based catalysts and to uncover the interaction between iron oxides and Al2O3 supports. The corresponding curves are displayed in Fig. 8. Both CA and FA supports show two CO2 desorption peaks around 150 and 320 C,
167
attributed to weak and medium basic sites on the surface, respectively. The peak area of the medium basic sites on FA surface is remarkably larger than that on CA, signifying the higher concentration of medium basic sites on FA surface. After the loading of iron species, it is clearly found that these medium basic sites on both supports are completely consumed, while the weak basic sites are almost unchanged, accompanied by the appearance of some strong basic sites associated with the iron species. These results clearly reveal that the iron species preferentially locate on the medium basic sites during the preparation process of the Febased catalysts. As proposed in the literature [36], two basic sites, hydroxyl ion site (-OH) and oxygen ion sites (O2), are usually present on the surface of dehydrated alumina. The Lewis basicity of O2 is generally stronger than that of OH [37,38]. Therefore, the weak basic sites shown in Fig. 8 can be ascribed to OH and the medium basic sites to O2 ions on Al2O3 surface, and the most important is that the concentration of surface O2 anions of FA support is higher than that of CA, which is consistent with the lower coordination of Al3+ ions on FA surface because low coordinate Al3+ ions and medium basic O2 anions are simultaneously formed at adjacent position on Al2O3 surface in the dehydration process. The dramatic decrease in Al 2p BE value in XPS and the disappearance of basic O2 in CO2-TPD for the Al2O3 supports after loading of iron oxides can be related to the strong interaction between iron species and surface O2 anions on Al2O3 surface. Based on the fact that the iron species of Fe/FA were highly dispersed and the results of XPS and CO2-TPD, we propose that O2 ions on the support surface play a key role in dispersing iron species. Fig. 9 shows the schematic of preparation processes of Fe/ FA and Fe/CA catalysts by the incipient wetness method. During the preparation process of Fe/FA, a high concentration of OH ions were produced from the surface O2 ions of FA support at the
Fig. 9. Schematic of preparation processes of Fe/CA and Fe/FA catalysts.
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interface via hydrolysis reaction [39]. These OH ions react with Fe3+ cations to form Fe(OH)3 precursors on the surface, covering the O2 sites and resulting in small FexOy clusters strongly interacting with the support surface. For CA, its surface O2 ions are insufficient and thereby a low concentration of OH ions is present. The direct pyrolysis of Fe(NO3)3 occurs during the subsequently thermal treatment, resulting in larger Fe2O3 particles. The above proposals are supported by the observation, as shown in the inserted pictures of Fig. 9, that the yellow color of Fe/ FA shallowed upon thermal decomposition while Fe/CA exhibited a reverse behavior. Therefore, we conclude that the formation of highly dispersed FexOy clusters on Fe/FA should be attributed to the high concentration of O2 anions on the surface of flower-like Al2O3. 3.3. Catalytic performance The catalytic results of CO2-ODEB over the Al2O3 supports and the supported catalysts are shown in Fig. 10. The EB conversion over FA and CA are similarly less than 10%, suggesting little contribution of the pure support to catalytic activity. However, a different scenario is observed in the case of supported iron oxide catalysts. During the 10 h of time on stream, the EB conversion over Fe/FA is almost 1.6-fold higher than that over Fe/CA due to the different forms of iron species on the catalyst surface. The highly dispersed FexOy clusters existing in Fe/FA surface provide a much higher density of active sites and result in the excellent catalytic activity. 4. Conclusions Flower-like Al2O3 support was synthesized and used to prepare supported iron oxide catalysts for CO2-ODEB. The properties of the flower-like surface and its effect on the dispersion of iron oxides and catalytic performance of the Fe2O3/Al2O3 catalysts were studied. There are more surface O2 anions present on the surface of flower-like Al2O3 than on commercial Al2O3. During the catalyst preparation process, the flower-like surface rich in O2 anions favored the formation of Fe(OH)3 precursors, which turned into highly dispersed FexOy clusters that strongly interacted with the flower-like surface upon calcination. Due to the high dispersion, the iron oxide species supported on flower-like Al2O3 exhibited a superior activity than those on commercial Al2O3 in CO2-ODEB reaction. Flower-like Al2O3 may be a promising support, instead of conventional Al2O3, to manufacture potential commercial catalysts for CO2-ODEB.
Fig. 10. Catalytic results of the supports and iron-based catalysts for CO2-ODEB.
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