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α-Fe2O3 catalyzed oxidative degradation of gaseous benzene: Preparation, characterization and its catalytic properties
Solid State Sciences 93 (2019) 79–86
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Co3O4/α-Fe2O3 catalyzed oxidative degradation of gaseous benzene: Preparation, characterization and its catalytic properties
T
Ying Xianga,b,c, Yi Zhua,b,c, Jun Lud, Chengzhu Zhua,b,c,∗, Mengyu Zhua,b,c, Qiaoqin Xiea,c, Tianhu Chena,c,∗∗ a
School of Resource and Environmental Engineering, Hefei University of Technology, Hefei, 230009, PR China Institute of Atmospheric Environment & Pollution Control, Hefei University of Technology, Hefei, 230009, PR China c Key Laboratory of Nanominerals and Pollution Control of Anhui Higher Education Institutes, Hefei University of Technology, Hefei, 230009, PR China d Center of Analysis & Measurement, Hefei University of Technology, Hefei, 230009, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: α-FeOOH α-Fe2O3 Co3O4 Benzene Catalytic oxidation degradation
Cobalt oxide/hematite (Co3O4/α-Fe2O3) composites were synthesized by using goethite as precursor via coprecipitation calcination method and employed in the catalytic degradation of benzene. The catalysts properties of the Co3O4/α-Fe2O3 composites were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption measurement (BET), X-ray photoelectron spectroscopy (XPS) and H2 temperature programmed reduction (H2-TPR) techniques, respectively. The results showed that the Co3O4/α-Fe2O3 composite had excellent catalytic oxidative performance on catalytic degradation of gaseous benzene, and its catalytic activity were influenced markedly by the ratio of Co3O4 and the calcination temperature. When the composite catalyst with the Co/Fe molar ratio of 0.6, calcined temperature at 500 °C and benzene initial concentration was 1000 mg/m3, the catalyzed oxidative temperature which benzene completely conversion and 91.7% CO2 selectivity was 400 °C. Large specific surface area, abundant surface oxygen species and strong redox properties which were ascribed to the interaction between Fe2O3 and Co3O4 play an important role in the catalyzed oxidative degradation of benzene.
1. Introduction Benzene which mainly derived from industrial processes, the cement concrete, furniture manufacturing and fossil fuel combustion is the representative pollutant in Volatile organic compound (VOCs) and is one of the most numerous found in indoor air [1,2]. It is toxic for human and caused such as headache, cough and cancer a series of serious health problems especially after a long exposure in benzene contained atmosphere environment [3]. Therefore, it is urgent to eliminate the emission of benzene in the air and develop appropriate treatment technologies to remove these VOCs pollutants. Many technologies have been explored to remove benzene before it emitted to atmospheric environment, such as adsorption, biological degradation, photocatalysis, plasma catalysis and catalytic oxidation degradation. Among them, catalytic oxidation is the most promising technology for removing high concentration of benzene due to its advantages of high efficiency and relatively low temperature [4,5]. In catalytic oxidation, it is common classified as noble metals and
∗
transition metal catalyst [6]. Although noble metals such as Pt-based catalysts and Au-based catalysts displayed a high catalytic activity at low temperature, the disadvantage of the exorbitant price and the easily sintering prohibit the general application [7–9]. Correspondingly, transition metal oxides and their nanocomposites are abundant in nature and have shown performance in various fields [10–18]. Transition metal oxides have shown good activity in some VOCs oxidation and are considered as promising alternative, which have attracted more attention in recent years [19]. Co3O4 with the advantage of low cost and great catalytic activity has been applied in wide range of reaction, such as VOCs oxidation, water oxidation and CO oxidation [20]. Cao et al. [21] found that mixing of Co3O4 with iron oxide improved the catalytic performance of iron oxide and pure Co3O4, and Biabani-Ravandi et al. [22] further indicated that these mixed oxide may be more active than the separate components. Goethite (α-FeOOH), as a mineral material, widely exists in the earth's crust with the feature of larger surface area and higher chemical
Corresponding author. School of Resource and Environmental Engineering, Hefei University of Technology, Hefei, 230009, PR China. Corresponding author. School of Resource and Environmental Engineering, Hefei University of Technology, Hefei, 230009, PR China. E-mail addresses: [email protected] (C. Zhu), [email protected] (T. Chen).
∗∗
https://doi.org/10.1016/j.solidstatesciences.2019.05.008 Received 21 February 2019; Received in revised form 1 May 2019; Accepted 6 May 2019 Available online 09 May 2019 1293-2558/ © 2019 Elsevier Masson SAS. All rights reserved.
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2.4. Catalytic activity evaluation
activity [23,24]. Ponomar et al. [25] investigated the properties of goethite transformation during high temperature treatment and found that the α-FeOOH could be turned into α-Fe2O3 by calcinations at 500 °C. While α-Fe2O3 has larger specific surface area and is beneficial to the catalytic oxidation [26]. Therefore, it often used as a loading material to improve the catalytic performance of the catalysts and increase CO2 selectivity [27]. In this work, the low-cost α-FeOOH as a precursor to employ as supporting material for Co3O4 by co-precipitation and calcination method, the Co3O4/α-Fe2O3 composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption measurement (BET), X-ray photoelectron spectroscopy (XPS) and H2 temperature programmed reduction (H2-TPR) techniques, respectively. The influence factors of composite catalysts preparation on catalytic oxidation degradation benzene, which was explored as the model pollutant for VOCs, and the CO2 selectivity to product and catalyst stability were systematically investigated, the reaction mechanism of catalytic oxidation degradation of benzene was also discussed.
Catalytic oxidation activity evaluation of benzene (1000 mg/m3 in dry air, 20% v/v O2) was performed in a home-made continuous flow quartz fixed-bed reactor with the inner diameter of 6 mm at the reaction temperature from 100 °C to 500 °C. In each test run, 0.35 g catalysts (20–40 mesh) were packed at the center of the reactor. The gas hourly space velocity (GHSV) was 48000 h−1, and the total flow ratio of the reaction was 500 mL/min. The outlet gases were analyzed by Trace 1300 gas chromatograph (GC) equipped with a flame ionization detector (FID) for the quantitative analysis of benzene after stabilizing for 20 min at each test temperature. A Fourier transform infrared spectrometer (Vertex 70) was used to analyzing the reactor outlet gas composition which provided convenience to calculate the CO2 selectivity. The degradation efficiency (η) of benzene and the CO2 selectivity (SCO2 ) were calculated as:
η=
2. Experimental sections
C0 − Ct × 100% C0
SCO2 =
2.1. Materials and chemicals
CCO2 × 100% 6 × (C0 − Ct )
where C0 was the inlet concentration of benzene, mg/m3; Ct was the outlet concentration of benzene, mg/m3; CCO2 was the outlet concentration of CO2, mg/m3.
Goethite was purchased from Zhejiang fine chemical factory, its particle size was less than 0.075 mm after crushing and grinding. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O) was purchased from Sigma Aldrich, Shanghai. All other chemicals and solvents were of analytical grade and used without any farther purification.
3. Results and discussion 3.1. Catalyst optimization
2.2. Catalyst preparation
Fig. 1a showed the conversion rate of benzene catalyzed by pure αFe2O3, pure Co3O4 and α-Fe2O3 mixed with different amount of Co3O4 composite catalysts calcined at 500 °C. It could be found that pure αFe2O3 exhibited poor catalytic oxidation activity of benzene and its conversion efficiency was only about 50% at the reaction temperature of 500 °C. As for pure Co3O4, it showed great catalytic activity at reaction temperature, and it was similarly reported by Lou [28]. In addition, the activity of composite catalysts increased first and then decreased with the increasing of the proportion of cobalt, when the composite catalyst at the Fe/Co molar ratio was 0.6, the composite catalyst showed an excellent catalytic activity with the lowest reaction temperature of benzene complete oxidation was approximately 400 °C. It was suggested that the accretion of appropriate amount of Co could considerably increase the catalytic activity of Fe-based catalyst. The catalytic oxidation of benzene over composite catalysts at different calcination temperature was showed in Fig. 1b. The results indicated that the composite had the highest catalytic activity when the calcining temperature reached 500 °C, and the activity decreased when further increased the temperature. It might be related to the decrease of surface area of the catalyst and it was good agreed with the research which found that α-FeOOH could get larger surface area by heating 500 °C [26]. From the above results, the composite catalyst containing the Fe/Co molar ratio of 0.6 and calcining at 500 °C exhibited the optimum activity on which the lowest reaction temperature with completely oxidation of benzene was approximately 400 °C. The reaction temperature of Co0.6FeOx (500) catalyst was much lower than some catalysts displayed in previous results (Table 1). Therefore, the following experiments were carried out with the Co3O4/α-Fe2O3 catalyst prepared under this optimum condition.
A certain amount of Co (NO3)2·6H2O was dissolved in 10 mL of deionized water and then dropwise added into 20 mL α-FeOOH (1.157 g) aqueous suspension solution. The mixture solution was stirred for 1 h and then aging 10 h at room temperature. After that, the sample was dried at 105 °C for 6 h, and then calcined at different temperature in air atmosphere for 3 h. Finally, the calcined sample was crushed and sieved to obtain 20–40 mesh particles to form Co3O4/α-Fe2O3 catalysts. The pure Co3O4 was also prepared by the same method without αFeOOH and the contrast α-Fe2O3 was prepared by calcining α-FeOOH at 500 °C in air atmosphere for 3 h. The Co3O4/α-Fe2O3 composite catalyst was denoted as the CoaFeOx(b), where a represents the Co/Fe molar ratio and b represents the calcination temperatures in the air atmosphere (b = 400 °C, 500 °C, 600 °C and 700 °C). 2.3. Catalyst characterizations The X-ray diffraction (XRD) patterns were recorded on a Rigaku D/ max diffractometer with Cu Kα radiation (40 kV and 40 mA), the initial angle was 15°, end angle was 80°, and the step size was 0.02° of 4°/min. The surface properties of catalysts (BET) were determined by a Quantachrome NOVA 3000e analyzer. Before the analysis, the catalysts were degassed at 105 °C for 24 h under a N2 flow for cleaning the surfaces and removing the adsorbed chemical species. The Scanning Electron Microscope (SEM) images were scanned by a Hitachi S-4800 N scanning electron microscope. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Escakab 250Xi X-ray operating at 10−9 Pa with Al Kα radiation (1486.6 eV), and the spectrum was corrected by using C1s at 284.6 eV. H2 temperature-programmed reduction (H2-TPR) measurement was carried out on a U-shaped quartz tube (d = 6 mm) containing 2.2 g catalyst. The samples was exposed to the 5% H2 and 95% Ar mixture with a flow rate of 20 mL/min and heated from 100 °C to 800 °C with a heating rate of 10 °C/min. The H2 content of the off-gas was analyzed by Hiden QIC-20 online mass spectrometer.
3.2. Characterization of catalysts 3.2.1. XRD It was found from Fig. 2a that the diffraction peaks of goethite calcined at 500 °C were in accordance with hematite (JCPDS NO.8980
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Fig. 1. (a) The effect of different Fe/Co molar radios and (b) calcined at different temperature on benzene degradation activity.
2810), and the peaks of cobalt samples observed at 2θ = 19.2, 31.2, 36.9, 38.6, 44.8, 59.5 and 65.3° could be assigned to pure cubic phase of Co3O4 spinel (JCPDS NO.42-1467) [34,35]. For the composite catalysts, both diffraction peaks of the two oxide phases (α-Fe2O3 and Co3O4) appeared and the intensity of Co3O4 signal increased proportionally with increasing Co ratio. The intensity of the peaks for composites exhibited a clear shrinkage compared with pure oxide samples, indicating that cobalt addition apparently reduced the crystallinity [36]. The low crystallinity could reduce the crystallite and generated more crystal defect which contributed to the electron transfer and produced more oxygen vacancies, the catalytic oxidation performance could be enhanced [37,38]. Fig. 2b manifested the XRD spectra of Co3O4/α-Fe2O3 composites at
Fig. 2. XRD patterns of the samples with (a) different Fe/Co molar radios and (c) different calcining temperatures.
different calcination temperature. When the temperature added from 400 °C to 600 °C, there were no other peaks for impurities could be observed and the intensity of diffraction lines corresponded to the αFe2O3 and Co3O4 phase enhanced gradually. In addition, when the calcination temperature increased up to around 700 °C, the peaks represented the α-Fe2O3 and Co3O4 phase disappeared, and some impurity peaks appeared. One of the diffraction peaks at 2θ values of 30.14°, 35.47°, 43.18°, 53.60°, 57.23° and 62.27° was corresponded to cubic Fe3O4 (JCPDS NO.75-1609). And the presence of diffraction peaks
Table 1 Catalytic conversion for oxidation of benzene reported in the literature. Catalysts
Preparation Method
Oxidation conditions
Benzene conversion
Reference
Co-Al
Co-precipitation
T100 = 470 °C
[29]
Pd-Ni/SBA-15
Co-precipitation
T100 = 400 °C
[30]
Fe2O3(3%)TiO2
Impregnation
T100 = 425 °C
[31]
CeCoOδ
Electrospinning
T100 = 475 °C
[32]
SSI-LaCoCe
Electrospinning
T100 = 500 °C
[33]
Co3O4/α-Fe2O3
Co-precipitation
516 ppm Benzene 36000 mLg−1h−1 1000 ppm Benzene 120000 mLg−1h−1 500 ppm Benzene 60000 mLg−1h−1 500 ppm Benzene 90000 mLg−1h−1 500 ppm Benzene 96000 mLg−1h−1 1000 mg/m3 Benzene 48000 h−1
T100 = 400 °C
This work
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Fig. 3. N2 adsorption desorption isotherms and (b) pore size distribution profiles of α-Fe2O3, Co3O4 and Co3O4/α-Fe2O3 composite.
at 2θ values of 30.14°, 35.47°, 43.18°, 53.60°, 57.23°, 62.27° and 74.12° corresponded to cubic CoFe2O4 (JCPDS NO.1-1121). The results indicated that the catalysts at high calcination temperature would generate agglomeration, resulting in the change of the crystal state. Fig. 4. SEM images of (a) α-Fe2O3, (b) Co3O4 and (c) Co3O4/α-Fe2O3.
3.2.2. BET The N2 adsorption desorption isotherms and pore size distribution of pure α-Fe2O3, Co3O4 and Co3O4/α-Fe2O3 composite were shown in Fig. 3 and Table 2. All samples showed type Ⅳ isotherms with H3 hysteresis loops, which was the significant feature of the presence of mesopores [39,40]. Compared with the specific surface area of pure αFe2O3 (SBET = 24.43 m2/g) and pure Co3O4 (SBET = 10.03 m2/g), Co3O4/α-Fe2O3 composite catalyst exhibited a remarkably higher value (SBET = 35.68 m2/g). The high surface area generally conducts to more exposure of active sites and activity enhancement [41].
3.2.3. SEM The pure α-Fe2O3 and pure Co3O4 presented the smooth rod-like structure (Fig. 4a) and the round-block structure (Fig. 4b), respectively. The Co3O4 with round-block structure was effectively loaded on the goethite in composite catalyst (Fig. 4c), which increased the specific surface area of the composite catalysts to a degree, and was beneficial to the adsorption of benzene on the composite catalysts [41]. 3.2.4. XPS XPS analysis was performed to get the information about the active oxygen species, the surface elemental composition and the metal oxidation states of the as-prepared catalysts. As shown in Fig. 5a, the XPS spectra of Fe 2p over as-prepared catalysts exhibited a spin-coupled doublet for the Fe 2p1/2 and Fe 2p3/2 at the binding energy of 724.98 eV and 710.93 eV, respectively. The peak distance between Fe 2p1/2 and Fe 2p3/2 was about 14eV and the satellite peaks was at about 718.66 eV, which indicated the existence of oxidation state of Fe3+ [42]. Moreover, the peak at 713.70 eV corresponded to the FeO species [43]. Above the results, there were Fe2+ and Fe3+ species on the surface of both pure and composite catalysts, and the Fe2+/Fe3+
Table 2 Specific surface area, pore volume and pore size of three catalysts. Catalyst
Specific surface area (m2/g)
Pore volume ( × 10−2 cm3/g)
Average pore size (nm)
Co3O4 α-Fe2O3 Co3O4/αFe2O3
10.03 24.43 35.68
3.53 6.85 9.18
13.57 11.21 12.52
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indicated that both Co2+ and Co3+ co-existed on the surface of the catalysts [44]. The M1 and M2 peaks were further deconvoluted into to four peaks, the peaks at the BE of 780.14–780.48 eV and 795.14–795.68 eV were assigned to the octahedral Co3+, on the other hands, the peaks at the BE of 781.88–781.89 eV and 796.99–797.41 eV corresponded to the tetrahedral Co2+ [45,46]. And the value of Co2+/ Co3+ in the composite catalyst is higher than pure Co3O4 catalysts (Table 3). The results indicated that the addition of Co leads to the electron transfer from Fe2O3 to Co3O4, and the oxygen vacancies are thus generated to maintain electrostatic balance. Fig. 5c showed the XPS results for O1s, three distinct peaks centered at the BE of 530.09 eV, 531.67 eV and 532.76 eV could be ascribed to the surface lattice oxygen (Olatt), adsorbed oxygen (Oads, eg., O2−, O22−, O−) and physically adsorbed oxygen species (Osur, eg., H2O, O2), respectively [47,48]. These amounts of surface O22− and O2 had a great effect on the catalytic activity [49], it indicated that the concentration of surface Oads and the molar ratio of surface Oads/Olatt over composite catalysts increased clearly compared with pure α-Fe2O3 (Table 3), which attributed to the interface effect between Fe and Co for improving the capability of oxygen activation and promoting oxygen transference [50]. Due to the formation of an interfacial oxide would lead to an energy shift [51,52], the typical peaks of Co3O4/α-Fe2O3 composite in XPS profiles shifted to higher energy values in comparison with the pure αFe2O3, it indicated that the mixture of cobalt oxides and ferric oxides existed in the Co3O4/α-Fe2O3 composite [53], the Fe had been interacted with Co on the interface of these metals and the degree of crystallinity of ferric oxides and cobalt oxides greatly decreased, which coincided with XRD results. Therefore, Co3O4/α-Fe2O3 with more surface active oxygen species exhibited higher catalytic activity than pure α-Fe2O3. 3.2.5. H2-TPR H2-TPR provided an effective way to investigate the reducibility of catalysts, the results of which were illustrated in Fig. 6. For pure αFe2O3, there were three main reduction peaks centered at about 403 °C、531 °C and 686 °C, corresponding to the reduction process of Fe2O3→Fe3O4→FeO→Fe [54]. And the Co3O4 displayed two typical reduction signals (the low temperature reduction named A, and the high temperature multiple reduction peaks named B1 and B2), attributing to the process of Co3O4→CoO→Co [55]. Compared with pure αFe2O3 sample, it could be found that the position of Co3O4/α-Fe2O3 composite reduction peaks obviously shifted to lower temperature and the intensity of the peaks centered at 477 °C and 652 °C evidently increased. Besides, the concentration of reducible oxygen species on composite catalysts also added according to the quantitative analysis of the H2-TPR profiles. Based the results, it is indicated that cobalt addition led to the interaction between Fe2O3 and Co3O4 promoted the concentration of reducible oxygen species, which ascribed to higher oxygen mobility and more oxygen vacancy. 3.3. Catalyst activity 3.3.1. Effect of initial concentration of benzene Fig. 7a manifested the degradation efficiency of benzene at different initial concentrations with Co3O4/α-Fe2O3 composite catalyst. It showed that increasing the initial concentrations from 500 to 2000 mg/ m3 led to continuous loss in the activity of the catalyst for benzene oxidation, the degradation rate at the reaction temperature of 400 °C decreased from 99.62% to 90.35%. This was due to the active sites of the catalysts surface were limited, enhanced benzene concentration could lead to insufficient active sites [56].
Fig. 5. XPS spectra of (a) Fe 2p, (b) Co 2p and (c) O1s of catalysts.
molar ratios calculated by the integration of the corresponding peak area changed on composite catalyst (Table 3). Co 2p spectra displayed two distinct peaks, named M1 and M2, and three satellite peaks, named S1, S2 and S3 (Fig. 5b), respectively. The energy difference between M1 and M2 peaks correspond to Co 2p3/2 and Co 2p1/2 spin-orbit-split components were around 15.2 eV,
3.3.2. Effect of GHSV Gas hourly space velocity (GHSV) was a significant parameter in the catalytic oxidation experiment. Therefore, the influence of GHSV on the 83
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Table 3 XPS results of α-FeOOH, Co3O4 and Co3O4/α-Fe2O3 composite. Catalyst
Olatt (%)
Oads (%)
Osur (%)
Oads/Olatt (%)
Co2+/Co3+ (%)
Fe2+/Fe3+ (%)
α-Fe2O3 Co3O4 Co3O4/α-Fe2O3
70.21 – 64.65
19.82 – 27.47
9.97 – 7.88
28.23 – 42.49
– 64.41 73.53
59.33 – 56.49
catalytic performance of the Co3O4/α-Fe2O3 composite catalyst was further investigated (Fig. 7b). When the temperature reached 400 °C, the catalytic conversion efficiency of benzene under different GHSVs (24000 h−1, 37000 h−1, 48000 h−1 and 58000 h−1) conditions was all exceeded 95%. Along the GHSV increasing from 24000 to 58000 h−1, the catalytic conversion efficiency of the composite decreased which appeared primarily in low temperature zone. This was due to the mass transfer rate between the gas and the outer surface of the sample became slower and the diffusion process was limited [57]. 3.4. The selectivity of CO2 The CO2 selectivity, which reflected the complete degradation of pollutants, to some extent represented the significant performance of a catalyst for catalytic of benzene [58]. Therefore, the CO2 selectivity of Co3O4/α-Fe2O3 catalyst which oxidized 1000 mg/m3 benzene for 20 min at different reaction temperature were inspected, and the results were illustrated on Fig. 8. The variation tendency of the CO2 selectivity was consistent with the benzene conversion. When the reaction temperature reached 400 °C, the selectivity of CO2 could reach about 91.7%, and CO2 was the main product of the benzene catalytic oxidation on the oxides. The results further indicated that the goethite loading on Co3O4 composite catalyst had excellent catalytic oxidation activity.
Fig. 6. H2-TPR profiles of α-Fe2O3, Co3O4, and Co3O4/α-Fe2O3 composite.
3.5. Paths of catalytic oxidation of benzene over Co3O4/α-Fe2O3 The pathway of catalytic oxidation of benzene over Co3O4/α-Fe2O3 should abide by the Mars-van Krevelen model [30,49,50,59]. As shown in Fig. 9, catalytic oxidation of benzene was though three ways: (1) benzene could be adsorbed on Fe-Co oxides and then oxidized by the release of oxygen from Co3O4, (2) benzene was adsorbed on Fe-Co oxides and oxidized by the active oxygen occurs at the Fe-Co interface, and (3) benzene was oxidized by the active oxygen species from gas oxygen molecules. The synergistic effect between Fe2O3 and Co3O4
Fig. 7. Effect of different (a) initial concentration and (b) GHSVs on conversion.
Fig. 8. The CO2 selectivity in the process of catalytic degradation of benzene by using Co3O4/α-Fe2O3 catalyst. 84
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stability for long-term benzene oxidation, and further proving it the potential for industrial application. 4. Conclusion In this paper, Co3O4/α-Fe2O3 composites were synthesized by using goethite as precursor via co-precipitation method and were evaluated of their catalytic activity for benzene oxidation degradation. The composite catalyst with the Co/Fe molar ratio of 0.6 and calcined at the temperature of 500 °C displayed the superior catalytic activity, on which the lowest reaction temperature with completely oxidation of benzene was 400 °C and the high selectivity to CO2 could even reached 91.7%. The characterization results showed that the best Co3O4/αFe2O3 composite catalyst had large specific surface area, high concentration of surface oxygen species and strong redox properties which were all beneficial to benzene oxidation. Moreover, Co3O4/α-Fe2O3 composite also exhibited a great stability that had no significant decrease in the catalytic activity for 144 h. The interaction between Fe2O3 and Co3O4 promoted the concentration of reducible oxygen species were responsible for the degradation of benzene.
Fig. 9. The mechanism of benzene catalytic degradation over Co3O4/α-Fe2O3 composite.
Acknowledgments The authors thank for the financial support from the Key University Science Research Project of Anhui Provincial Education Department of China (KJ2017ZD46) and National Natural Science Foundation of China (NSFC) of China (21177034 and 41572029) for support this study. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solidstatesciences.2019.05.008. References [1] M. Popova, A. Szegedi, Z. Cherkezova-Zhelva, I. Mitov, N. Kostova, T. Tsoncheva, Toluebe oxidation on titanium-and iron-modified MCM-41 materials, J. Hazard Mater. 168 (2009) 226–232. [2] H. Huang, H. Huang, Q. Feng, G. Liu, Y. Zhan, M. Wu, H. Lu, Y. Shu, D.Y.C. Leung, Catalytic oxidation of benzene over Mn modified TiO2/ZSM-5 under vacuum UV irradiation, Appl. Catal. B Environ. 203 (2017) 870–878. [3] G. Liu, R.L. Yue, Y. Jia, Y. Ni, J. Yang, H.D. Liu, Z. Wang, X.F. Wu, Y.F. Chen, Catalytic oxidation of benzene over Ce-Mn oxides synthesized by flame spray pyrolysis, Particuology 11 (2013) 454–459. [4] Z. Sihaib, F. Puleo, J.M. Garcia-Vargas, L. Retailleau, C. Descorme, L.F. Liotta, J.L. Valverde, S. Gil, A. Giroir-Fendler, Manganese oxide-based catalysts for toluene oxidation, Appl. Catal. B Environ. 209 (2017) 689–700. [5] M.H. Castano, R. Molina, S. Moreno, Cooperative effect of the Co-Mn mixed oxides for the catalytic oxidation of VOCs: influence of the synthesis method, Appl. Catal. Gen. 492 (2015) 48–59. [6] S.A.C. Carabineiro, X. Chen, M. Konsolakis, A.C. Psarras, P.B. Tavares, J.J.M. Orfao, M.F.R. Pereira, J.L. Figueiredo, Catalytic oxidation of toluene on Ce–Co and La–Co mixed oxides synthesized by exotemplating and evaporation methods, Catal. Today 244 (2015) 161–171. [7] Y. Ma, Y.Z. Li, M.Y. Mao, J.T. Hou, M. Zeng, X.J. Zhao, Synergetic effect between photocatalysis on TiO2 and solar light-driven thermocatalysis on MnOx for benzene purification on MnOx/TiO2 nanocomposites, J. Mater. Chem. A. 3 (2015) 5509–5516. [8] T. Barakat, J.C. Rooke, R. Cousin, J.F. Lamonier, J.M. Giraudon, B.L. Su, S. Siffert, Investigation of the elimination of VOC mixture over a Pd-loader V-doped TiO2 support, New J. Chem. 38 (2014) 2066–2074. [9] P. Yang, Z. Shi, S. Yang, R. Zhou, High catalytic performances of CeO2-CrOx catalysts for chlorinated VOCs elimination, Chem. Eng. Sci. 126 (2015) 361–369. [10] E.S. Jang, S.B. Khan, J. Seo, Y.H. Nam, W.J. Choi, K. Akhtar, H. Han, Synthesis and characterization of novel UV-curable polyurethane-clay nanohybrid: influence of organically modified layered silicates on the properties of polyurethane, Prog. Org. Coating 71 (2011) 36–42. [11] A. Khan, A.M. Asiri, M.A. Rub, N. Azum, A.A.P. Khan, S.B. Khan, M.M. Rahman, I. Khan, Synthesis, characterization of silver nanoparticle embedded polyaniline tungstophosphate- nanocomposite cation exchanger and its application for heavy metal selective membrane, Compos: Part B. 45 (2013) 1486–1492. [12] M.M. Rahman, A. Jamal, S.B. Khan, M. Faisal, Fabrication of chloroform sensor based on hydrothermally prepared low-dimensional β-Fe2O3 nanoparticles, Superlattice. Microst. 50 (2011) 369–376.
Fig. 10. Catalytic activity versus on-stream reaction time over the Co3O4/αFe2O3 for the oxidation of benzene at 450 °C.
could enhance the migration of oxygen vacancy to improve the catalytic activity [50] and played an important role in the whole catalytic process. 3.6. Catalyst stability The catalyst stability test was carried out at the condition with benzene in air and the GHSV of 48000 h−1, and the best Co3O4/αFe2O3 composite with 0.35 g was selected as the representative catalyst. The continuous test lasted over 144 h at the temperature of 450 °C which was higher than that required for complete conversion of benzene, and thus, allowed for the detection of concentration impact resistance. Fig. 10 showed the activity as a function with time on stream, it could be found that Co3O4/α-Fe2O3 catalyst was stable at the initial 72 h for complete oxidation of benzene with the concentration of 1000 mg/m3. When the concentration increased to 2000 mg/m3, the degradation rate still retained unchanged in another period of 72 h, indicating that the catalyst could stand the concentration impact resistance. Wu et al. [60] studied the effect of Fe2O3-modified Ti0.5Sn0.5O2 supported by CuO for CO catalytic oxidation, and they found the CO conversion had no obviously change for 18 h. Liu et al. [61] used the mesoporous Mn promoted Co3O4 oxides as catalyst for CO catalytic oxidation, and the sample displayed stable activity for 3000 min. Given the above results, Co3O4/α-Fe2O3 could exhibit great 85
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Co3O4/α-Fe2O3 catalyzed oxidative degradation of gaseous benzene: Preparation, characterization and its catalytic properties
T
Ying Xianga,b,c, Yi Zhua,b,c, Jun Lud, Chengzhu Zhua,b,c,∗, Mengyu Zhua,b,c, Qiaoqin Xiea,c, Tianhu Chena,c,∗∗ a
School of Resource and Environmental Engineering, Hefei University of Technology, Hefei, 230009, PR China Institute of Atmospheric Environment & Pollution Control, Hefei University of Technology, Hefei, 230009, PR China c Key Laboratory of Nanominerals and Pollution Control of Anhui Higher Education Institutes, Hefei University of Technology, Hefei, 230009, PR China d Center of Analysis & Measurement, Hefei University of Technology, Hefei, 230009, PR China b
A R T I C LE I N FO
A B S T R A C T
Keywords: α-FeOOH α-Fe2O3 Co3O4 Benzene Catalytic oxidation degradation
Cobalt oxide/hematite (Co3O4/α-Fe2O3) composites were synthesized by using goethite as precursor via coprecipitation calcination method and employed in the catalytic degradation of benzene. The catalysts properties of the Co3O4/α-Fe2O3 composites were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption measurement (BET), X-ray photoelectron spectroscopy (XPS) and H2 temperature programmed reduction (H2-TPR) techniques, respectively. The results showed that the Co3O4/α-Fe2O3 composite had excellent catalytic oxidative performance on catalytic degradation of gaseous benzene, and its catalytic activity were influenced markedly by the ratio of Co3O4 and the calcination temperature. When the composite catalyst with the Co/Fe molar ratio of 0.6, calcined temperature at 500 °C and benzene initial concentration was 1000 mg/m3, the catalyzed oxidative temperature which benzene completely conversion and 91.7% CO2 selectivity was 400 °C. Large specific surface area, abundant surface oxygen species and strong redox properties which were ascribed to the interaction between Fe2O3 and Co3O4 play an important role in the catalyzed oxidative degradation of benzene.
1. Introduction Benzene which mainly derived from industrial processes, the cement concrete, furniture manufacturing and fossil fuel combustion is the representative pollutant in Volatile organic compound (VOCs) and is one of the most numerous found in indoor air [1,2]. It is toxic for human and caused such as headache, cough and cancer a series of serious health problems especially after a long exposure in benzene contained atmosphere environment [3]. Therefore, it is urgent to eliminate the emission of benzene in the air and develop appropriate treatment technologies to remove these VOCs pollutants. Many technologies have been explored to remove benzene before it emitted to atmospheric environment, such as adsorption, biological degradation, photocatalysis, plasma catalysis and catalytic oxidation degradation. Among them, catalytic oxidation is the most promising technology for removing high concentration of benzene due to its advantages of high efficiency and relatively low temperature [4,5]. In catalytic oxidation, it is common classified as noble metals and
∗
transition metal catalyst [6]. Although noble metals such as Pt-based catalysts and Au-based catalysts displayed a high catalytic activity at low temperature, the disadvantage of the exorbitant price and the easily sintering prohibit the general application [7–9]. Correspondingly, transition metal oxides and their nanocomposites are abundant in nature and have shown performance in various fields [10–18]. Transition metal oxides have shown good activity in some VOCs oxidation and are considered as promising alternative, which have attracted more attention in recent years [19]. Co3O4 with the advantage of low cost and great catalytic activity has been applied in wide range of reaction, such as VOCs oxidation, water oxidation and CO oxidation [20]. Cao et al. [21] found that mixing of Co3O4 with iron oxide improved the catalytic performance of iron oxide and pure Co3O4, and Biabani-Ravandi et al. [22] further indicated that these mixed oxide may be more active than the separate components. Goethite (α-FeOOH), as a mineral material, widely exists in the earth's crust with the feature of larger surface area and higher chemical
Corresponding author. School of Resource and Environmental Engineering, Hefei University of Technology, Hefei, 230009, PR China. Corresponding author. School of Resource and Environmental Engineering, Hefei University of Technology, Hefei, 230009, PR China. E-mail addresses: [email protected] (C. Zhu), [email protected] (T. Chen).
∗∗
https://doi.org/10.1016/j.solidstatesciences.2019.05.008 Received 21 February 2019; Received in revised form 1 May 2019; Accepted 6 May 2019 Available online 09 May 2019 1293-2558/ © 2019 Elsevier Masson SAS. All rights reserved.
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2.4. Catalytic activity evaluation
activity [23,24]. Ponomar et al. [25] investigated the properties of goethite transformation during high temperature treatment and found that the α-FeOOH could be turned into α-Fe2O3 by calcinations at 500 °C. While α-Fe2O3 has larger specific surface area and is beneficial to the catalytic oxidation [26]. Therefore, it often used as a loading material to improve the catalytic performance of the catalysts and increase CO2 selectivity [27]. In this work, the low-cost α-FeOOH as a precursor to employ as supporting material for Co3O4 by co-precipitation and calcination method, the Co3O4/α-Fe2O3 composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption measurement (BET), X-ray photoelectron spectroscopy (XPS) and H2 temperature programmed reduction (H2-TPR) techniques, respectively. The influence factors of composite catalysts preparation on catalytic oxidation degradation benzene, which was explored as the model pollutant for VOCs, and the CO2 selectivity to product and catalyst stability were systematically investigated, the reaction mechanism of catalytic oxidation degradation of benzene was also discussed.
Catalytic oxidation activity evaluation of benzene (1000 mg/m3 in dry air, 20% v/v O2) was performed in a home-made continuous flow quartz fixed-bed reactor with the inner diameter of 6 mm at the reaction temperature from 100 °C to 500 °C. In each test run, 0.35 g catalysts (20–40 mesh) were packed at the center of the reactor. The gas hourly space velocity (GHSV) was 48000 h−1, and the total flow ratio of the reaction was 500 mL/min. The outlet gases were analyzed by Trace 1300 gas chromatograph (GC) equipped with a flame ionization detector (FID) for the quantitative analysis of benzene after stabilizing for 20 min at each test temperature. A Fourier transform infrared spectrometer (Vertex 70) was used to analyzing the reactor outlet gas composition which provided convenience to calculate the CO2 selectivity. The degradation efficiency (η) of benzene and the CO2 selectivity (SCO2 ) were calculated as:
η=
2. Experimental sections
C0 − Ct × 100% C0
SCO2 =
2.1. Materials and chemicals
CCO2 × 100% 6 × (C0 − Ct )
where C0 was the inlet concentration of benzene, mg/m3; Ct was the outlet concentration of benzene, mg/m3; CCO2 was the outlet concentration of CO2, mg/m3.
Goethite was purchased from Zhejiang fine chemical factory, its particle size was less than 0.075 mm after crushing and grinding. Cobalt nitrate hexahydrate (Co(NO3)2·6H2O) was purchased from Sigma Aldrich, Shanghai. All other chemicals and solvents were of analytical grade and used without any farther purification.
3. Results and discussion 3.1. Catalyst optimization
2.2. Catalyst preparation
Fig. 1a showed the conversion rate of benzene catalyzed by pure αFe2O3, pure Co3O4 and α-Fe2O3 mixed with different amount of Co3O4 composite catalysts calcined at 500 °C. It could be found that pure αFe2O3 exhibited poor catalytic oxidation activity of benzene and its conversion efficiency was only about 50% at the reaction temperature of 500 °C. As for pure Co3O4, it showed great catalytic activity at reaction temperature, and it was similarly reported by Lou [28]. In addition, the activity of composite catalysts increased first and then decreased with the increasing of the proportion of cobalt, when the composite catalyst at the Fe/Co molar ratio was 0.6, the composite catalyst showed an excellent catalytic activity with the lowest reaction temperature of benzene complete oxidation was approximately 400 °C. It was suggested that the accretion of appropriate amount of Co could considerably increase the catalytic activity of Fe-based catalyst. The catalytic oxidation of benzene over composite catalysts at different calcination temperature was showed in Fig. 1b. The results indicated that the composite had the highest catalytic activity when the calcining temperature reached 500 °C, and the activity decreased when further increased the temperature. It might be related to the decrease of surface area of the catalyst and it was good agreed with the research which found that α-FeOOH could get larger surface area by heating 500 °C [26]. From the above results, the composite catalyst containing the Fe/Co molar ratio of 0.6 and calcining at 500 °C exhibited the optimum activity on which the lowest reaction temperature with completely oxidation of benzene was approximately 400 °C. The reaction temperature of Co0.6FeOx (500) catalyst was much lower than some catalysts displayed in previous results (Table 1). Therefore, the following experiments were carried out with the Co3O4/α-Fe2O3 catalyst prepared under this optimum condition.
A certain amount of Co (NO3)2·6H2O was dissolved in 10 mL of deionized water and then dropwise added into 20 mL α-FeOOH (1.157 g) aqueous suspension solution. The mixture solution was stirred for 1 h and then aging 10 h at room temperature. After that, the sample was dried at 105 °C for 6 h, and then calcined at different temperature in air atmosphere for 3 h. Finally, the calcined sample was crushed and sieved to obtain 20–40 mesh particles to form Co3O4/α-Fe2O3 catalysts. The pure Co3O4 was also prepared by the same method without αFeOOH and the contrast α-Fe2O3 was prepared by calcining α-FeOOH at 500 °C in air atmosphere for 3 h. The Co3O4/α-Fe2O3 composite catalyst was denoted as the CoaFeOx(b), where a represents the Co/Fe molar ratio and b represents the calcination temperatures in the air atmosphere (b = 400 °C, 500 °C, 600 °C and 700 °C). 2.3. Catalyst characterizations The X-ray diffraction (XRD) patterns were recorded on a Rigaku D/ max diffractometer with Cu Kα radiation (40 kV and 40 mA), the initial angle was 15°, end angle was 80°, and the step size was 0.02° of 4°/min. The surface properties of catalysts (BET) were determined by a Quantachrome NOVA 3000e analyzer. Before the analysis, the catalysts were degassed at 105 °C for 24 h under a N2 flow for cleaning the surfaces and removing the adsorbed chemical species. The Scanning Electron Microscope (SEM) images were scanned by a Hitachi S-4800 N scanning electron microscope. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Escakab 250Xi X-ray operating at 10−9 Pa with Al Kα radiation (1486.6 eV), and the spectrum was corrected by using C1s at 284.6 eV. H2 temperature-programmed reduction (H2-TPR) measurement was carried out on a U-shaped quartz tube (d = 6 mm) containing 2.2 g catalyst. The samples was exposed to the 5% H2 and 95% Ar mixture with a flow rate of 20 mL/min and heated from 100 °C to 800 °C with a heating rate of 10 °C/min. The H2 content of the off-gas was analyzed by Hiden QIC-20 online mass spectrometer.
3.2. Characterization of catalysts 3.2.1. XRD It was found from Fig. 2a that the diffraction peaks of goethite calcined at 500 °C were in accordance with hematite (JCPDS NO.8980
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Fig. 1. (a) The effect of different Fe/Co molar radios and (b) calcined at different temperature on benzene degradation activity.
2810), and the peaks of cobalt samples observed at 2θ = 19.2, 31.2, 36.9, 38.6, 44.8, 59.5 and 65.3° could be assigned to pure cubic phase of Co3O4 spinel (JCPDS NO.42-1467) [34,35]. For the composite catalysts, both diffraction peaks of the two oxide phases (α-Fe2O3 and Co3O4) appeared and the intensity of Co3O4 signal increased proportionally with increasing Co ratio. The intensity of the peaks for composites exhibited a clear shrinkage compared with pure oxide samples, indicating that cobalt addition apparently reduced the crystallinity [36]. The low crystallinity could reduce the crystallite and generated more crystal defect which contributed to the electron transfer and produced more oxygen vacancies, the catalytic oxidation performance could be enhanced [37,38]. Fig. 2b manifested the XRD spectra of Co3O4/α-Fe2O3 composites at
Fig. 2. XRD patterns of the samples with (a) different Fe/Co molar radios and (c) different calcining temperatures.
different calcination temperature. When the temperature added from 400 °C to 600 °C, there were no other peaks for impurities could be observed and the intensity of diffraction lines corresponded to the αFe2O3 and Co3O4 phase enhanced gradually. In addition, when the calcination temperature increased up to around 700 °C, the peaks represented the α-Fe2O3 and Co3O4 phase disappeared, and some impurity peaks appeared. One of the diffraction peaks at 2θ values of 30.14°, 35.47°, 43.18°, 53.60°, 57.23° and 62.27° was corresponded to cubic Fe3O4 (JCPDS NO.75-1609). And the presence of diffraction peaks
Table 1 Catalytic conversion for oxidation of benzene reported in the literature. Catalysts
Preparation Method
Oxidation conditions
Benzene conversion
Reference
Co-Al
Co-precipitation
T100 = 470 °C
[29]
Pd-Ni/SBA-15
Co-precipitation
T100 = 400 °C
[30]
Fe2O3(3%)TiO2
Impregnation
T100 = 425 °C
[31]
CeCoOδ
Electrospinning
T100 = 475 °C
[32]
SSI-LaCoCe
Electrospinning
T100 = 500 °C
[33]
Co3O4/α-Fe2O3
Co-precipitation
516 ppm Benzene 36000 mLg−1h−1 1000 ppm Benzene 120000 mLg−1h−1 500 ppm Benzene 60000 mLg−1h−1 500 ppm Benzene 90000 mLg−1h−1 500 ppm Benzene 96000 mLg−1h−1 1000 mg/m3 Benzene 48000 h−1
T100 = 400 °C
This work
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Fig. 3. N2 adsorption desorption isotherms and (b) pore size distribution profiles of α-Fe2O3, Co3O4 and Co3O4/α-Fe2O3 composite.
at 2θ values of 30.14°, 35.47°, 43.18°, 53.60°, 57.23°, 62.27° and 74.12° corresponded to cubic CoFe2O4 (JCPDS NO.1-1121). The results indicated that the catalysts at high calcination temperature would generate agglomeration, resulting in the change of the crystal state. Fig. 4. SEM images of (a) α-Fe2O3, (b) Co3O4 and (c) Co3O4/α-Fe2O3.
3.2.2. BET The N2 adsorption desorption isotherms and pore size distribution of pure α-Fe2O3, Co3O4 and Co3O4/α-Fe2O3 composite were shown in Fig. 3 and Table 2. All samples showed type Ⅳ isotherms with H3 hysteresis loops, which was the significant feature of the presence of mesopores [39,40]. Compared with the specific surface area of pure αFe2O3 (SBET = 24.43 m2/g) and pure Co3O4 (SBET = 10.03 m2/g), Co3O4/α-Fe2O3 composite catalyst exhibited a remarkably higher value (SBET = 35.68 m2/g). The high surface area generally conducts to more exposure of active sites and activity enhancement [41].
3.2.3. SEM The pure α-Fe2O3 and pure Co3O4 presented the smooth rod-like structure (Fig. 4a) and the round-block structure (Fig. 4b), respectively. The Co3O4 with round-block structure was effectively loaded on the goethite in composite catalyst (Fig. 4c), which increased the specific surface area of the composite catalysts to a degree, and was beneficial to the adsorption of benzene on the composite catalysts [41]. 3.2.4. XPS XPS analysis was performed to get the information about the active oxygen species, the surface elemental composition and the metal oxidation states of the as-prepared catalysts. As shown in Fig. 5a, the XPS spectra of Fe 2p over as-prepared catalysts exhibited a spin-coupled doublet for the Fe 2p1/2 and Fe 2p3/2 at the binding energy of 724.98 eV and 710.93 eV, respectively. The peak distance between Fe 2p1/2 and Fe 2p3/2 was about 14eV and the satellite peaks was at about 718.66 eV, which indicated the existence of oxidation state of Fe3+ [42]. Moreover, the peak at 713.70 eV corresponded to the FeO species [43]. Above the results, there were Fe2+ and Fe3+ species on the surface of both pure and composite catalysts, and the Fe2+/Fe3+
Table 2 Specific surface area, pore volume and pore size of three catalysts. Catalyst
Specific surface area (m2/g)
Pore volume ( × 10−2 cm3/g)
Average pore size (nm)
Co3O4 α-Fe2O3 Co3O4/αFe2O3
10.03 24.43 35.68
3.53 6.85 9.18
13.57 11.21 12.52
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indicated that both Co2+ and Co3+ co-existed on the surface of the catalysts [44]. The M1 and M2 peaks were further deconvoluted into to four peaks, the peaks at the BE of 780.14–780.48 eV and 795.14–795.68 eV were assigned to the octahedral Co3+, on the other hands, the peaks at the BE of 781.88–781.89 eV and 796.99–797.41 eV corresponded to the tetrahedral Co2+ [45,46]. And the value of Co2+/ Co3+ in the composite catalyst is higher than pure Co3O4 catalysts (Table 3). The results indicated that the addition of Co leads to the electron transfer from Fe2O3 to Co3O4, and the oxygen vacancies are thus generated to maintain electrostatic balance. Fig. 5c showed the XPS results for O1s, three distinct peaks centered at the BE of 530.09 eV, 531.67 eV and 532.76 eV could be ascribed to the surface lattice oxygen (Olatt), adsorbed oxygen (Oads, eg., O2−, O22−, O−) and physically adsorbed oxygen species (Osur, eg., H2O, O2), respectively [47,48]. These amounts of surface O22− and O2 had a great effect on the catalytic activity [49], it indicated that the concentration of surface Oads and the molar ratio of surface Oads/Olatt over composite catalysts increased clearly compared with pure α-Fe2O3 (Table 3), which attributed to the interface effect between Fe and Co for improving the capability of oxygen activation and promoting oxygen transference [50]. Due to the formation of an interfacial oxide would lead to an energy shift [51,52], the typical peaks of Co3O4/α-Fe2O3 composite in XPS profiles shifted to higher energy values in comparison with the pure αFe2O3, it indicated that the mixture of cobalt oxides and ferric oxides existed in the Co3O4/α-Fe2O3 composite [53], the Fe had been interacted with Co on the interface of these metals and the degree of crystallinity of ferric oxides and cobalt oxides greatly decreased, which coincided with XRD results. Therefore, Co3O4/α-Fe2O3 with more surface active oxygen species exhibited higher catalytic activity than pure α-Fe2O3. 3.2.5. H2-TPR H2-TPR provided an effective way to investigate the reducibility of catalysts, the results of which were illustrated in Fig. 6. For pure αFe2O3, there were three main reduction peaks centered at about 403 °C、531 °C and 686 °C, corresponding to the reduction process of Fe2O3→Fe3O4→FeO→Fe [54]. And the Co3O4 displayed two typical reduction signals (the low temperature reduction named A, and the high temperature multiple reduction peaks named B1 and B2), attributing to the process of Co3O4→CoO→Co [55]. Compared with pure αFe2O3 sample, it could be found that the position of Co3O4/α-Fe2O3 composite reduction peaks obviously shifted to lower temperature and the intensity of the peaks centered at 477 °C and 652 °C evidently increased. Besides, the concentration of reducible oxygen species on composite catalysts also added according to the quantitative analysis of the H2-TPR profiles. Based the results, it is indicated that cobalt addition led to the interaction between Fe2O3 and Co3O4 promoted the concentration of reducible oxygen species, which ascribed to higher oxygen mobility and more oxygen vacancy. 3.3. Catalyst activity 3.3.1. Effect of initial concentration of benzene Fig. 7a manifested the degradation efficiency of benzene at different initial concentrations with Co3O4/α-Fe2O3 composite catalyst. It showed that increasing the initial concentrations from 500 to 2000 mg/ m3 led to continuous loss in the activity of the catalyst for benzene oxidation, the degradation rate at the reaction temperature of 400 °C decreased from 99.62% to 90.35%. This was due to the active sites of the catalysts surface were limited, enhanced benzene concentration could lead to insufficient active sites [56].
Fig. 5. XPS spectra of (a) Fe 2p, (b) Co 2p and (c) O1s of catalysts.
molar ratios calculated by the integration of the corresponding peak area changed on composite catalyst (Table 3). Co 2p spectra displayed two distinct peaks, named M1 and M2, and three satellite peaks, named S1, S2 and S3 (Fig. 5b), respectively. The energy difference between M1 and M2 peaks correspond to Co 2p3/2 and Co 2p1/2 spin-orbit-split components were around 15.2 eV,
3.3.2. Effect of GHSV Gas hourly space velocity (GHSV) was a significant parameter in the catalytic oxidation experiment. Therefore, the influence of GHSV on the 83
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Table 3 XPS results of α-FeOOH, Co3O4 and Co3O4/α-Fe2O3 composite. Catalyst
Olatt (%)
Oads (%)
Osur (%)
Oads/Olatt (%)
Co2+/Co3+ (%)
Fe2+/Fe3+ (%)
α-Fe2O3 Co3O4 Co3O4/α-Fe2O3
70.21 – 64.65
19.82 – 27.47
9.97 – 7.88
28.23 – 42.49
– 64.41 73.53
59.33 – 56.49
catalytic performance of the Co3O4/α-Fe2O3 composite catalyst was further investigated (Fig. 7b). When the temperature reached 400 °C, the catalytic conversion efficiency of benzene under different GHSVs (24000 h−1, 37000 h−1, 48000 h−1 and 58000 h−1) conditions was all exceeded 95%. Along the GHSV increasing from 24000 to 58000 h−1, the catalytic conversion efficiency of the composite decreased which appeared primarily in low temperature zone. This was due to the mass transfer rate between the gas and the outer surface of the sample became slower and the diffusion process was limited [57]. 3.4. The selectivity of CO2 The CO2 selectivity, which reflected the complete degradation of pollutants, to some extent represented the significant performance of a catalyst for catalytic of benzene [58]. Therefore, the CO2 selectivity of Co3O4/α-Fe2O3 catalyst which oxidized 1000 mg/m3 benzene for 20 min at different reaction temperature were inspected, and the results were illustrated on Fig. 8. The variation tendency of the CO2 selectivity was consistent with the benzene conversion. When the reaction temperature reached 400 °C, the selectivity of CO2 could reach about 91.7%, and CO2 was the main product of the benzene catalytic oxidation on the oxides. The results further indicated that the goethite loading on Co3O4 composite catalyst had excellent catalytic oxidation activity.
Fig. 6. H2-TPR profiles of α-Fe2O3, Co3O4, and Co3O4/α-Fe2O3 composite.
3.5. Paths of catalytic oxidation of benzene over Co3O4/α-Fe2O3 The pathway of catalytic oxidation of benzene over Co3O4/α-Fe2O3 should abide by the Mars-van Krevelen model [30,49,50,59]. As shown in Fig. 9, catalytic oxidation of benzene was though three ways: (1) benzene could be adsorbed on Fe-Co oxides and then oxidized by the release of oxygen from Co3O4, (2) benzene was adsorbed on Fe-Co oxides and oxidized by the active oxygen occurs at the Fe-Co interface, and (3) benzene was oxidized by the active oxygen species from gas oxygen molecules. The synergistic effect between Fe2O3 and Co3O4
Fig. 7. Effect of different (a) initial concentration and (b) GHSVs on conversion.
Fig. 8. The CO2 selectivity in the process of catalytic degradation of benzene by using Co3O4/α-Fe2O3 catalyst. 84
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stability for long-term benzene oxidation, and further proving it the potential for industrial application. 4. Conclusion In this paper, Co3O4/α-Fe2O3 composites were synthesized by using goethite as precursor via co-precipitation method and were evaluated of their catalytic activity for benzene oxidation degradation. The composite catalyst with the Co/Fe molar ratio of 0.6 and calcined at the temperature of 500 °C displayed the superior catalytic activity, on which the lowest reaction temperature with completely oxidation of benzene was 400 °C and the high selectivity to CO2 could even reached 91.7%. The characterization results showed that the best Co3O4/αFe2O3 composite catalyst had large specific surface area, high concentration of surface oxygen species and strong redox properties which were all beneficial to benzene oxidation. Moreover, Co3O4/α-Fe2O3 composite also exhibited a great stability that had no significant decrease in the catalytic activity for 144 h. The interaction between Fe2O3 and Co3O4 promoted the concentration of reducible oxygen species were responsible for the degradation of benzene.
Fig. 9. The mechanism of benzene catalytic degradation over Co3O4/α-Fe2O3 composite.
Acknowledgments The authors thank for the financial support from the Key University Science Research Project of Anhui Provincial Education Department of China (KJ2017ZD46) and National Natural Science Foundation of China (NSFC) of China (21177034 and 41572029) for support this study. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solidstatesciences.2019.05.008. References [1] M. Popova, A. Szegedi, Z. Cherkezova-Zhelva, I. Mitov, N. Kostova, T. Tsoncheva, Toluebe oxidation on titanium-and iron-modified MCM-41 materials, J. Hazard Mater. 168 (2009) 226–232. [2] H. Huang, H. Huang, Q. Feng, G. Liu, Y. Zhan, M. Wu, H. Lu, Y. Shu, D.Y.C. Leung, Catalytic oxidation of benzene over Mn modified TiO2/ZSM-5 under vacuum UV irradiation, Appl. Catal. B Environ. 203 (2017) 870–878. [3] G. Liu, R.L. Yue, Y. Jia, Y. Ni, J. Yang, H.D. Liu, Z. Wang, X.F. Wu, Y.F. Chen, Catalytic oxidation of benzene over Ce-Mn oxides synthesized by flame spray pyrolysis, Particuology 11 (2013) 454–459. [4] Z. Sihaib, F. Puleo, J.M. Garcia-Vargas, L. Retailleau, C. Descorme, L.F. Liotta, J.L. Valverde, S. Gil, A. Giroir-Fendler, Manganese oxide-based catalysts for toluene oxidation, Appl. Catal. B Environ. 209 (2017) 689–700. [5] M.H. Castano, R. Molina, S. Moreno, Cooperative effect of the Co-Mn mixed oxides for the catalytic oxidation of VOCs: influence of the synthesis method, Appl. Catal. Gen. 492 (2015) 48–59. [6] S.A.C. Carabineiro, X. Chen, M. Konsolakis, A.C. Psarras, P.B. Tavares, J.J.M. Orfao, M.F.R. Pereira, J.L. Figueiredo, Catalytic oxidation of toluene on Ce–Co and La–Co mixed oxides synthesized by exotemplating and evaporation methods, Catal. Today 244 (2015) 161–171. [7] Y. Ma, Y.Z. Li, M.Y. Mao, J.T. Hou, M. Zeng, X.J. Zhao, Synergetic effect between photocatalysis on TiO2 and solar light-driven thermocatalysis on MnOx for benzene purification on MnOx/TiO2 nanocomposites, J. Mater. Chem. A. 3 (2015) 5509–5516. [8] T. Barakat, J.C. Rooke, R. Cousin, J.F. Lamonier, J.M. Giraudon, B.L. Su, S. Siffert, Investigation of the elimination of VOC mixture over a Pd-loader V-doped TiO2 support, New J. Chem. 38 (2014) 2066–2074. [9] P. Yang, Z. Shi, S. Yang, R. Zhou, High catalytic performances of CeO2-CrOx catalysts for chlorinated VOCs elimination, Chem. Eng. Sci. 126 (2015) 361–369. [10] E.S. Jang, S.B. Khan, J. Seo, Y.H. Nam, W.J. Choi, K. Akhtar, H. Han, Synthesis and characterization of novel UV-curable polyurethane-clay nanohybrid: influence of organically modified layered silicates on the properties of polyurethane, Prog. Org. Coating 71 (2011) 36–42. [11] A. Khan, A.M. Asiri, M.A. Rub, N. Azum, A.A.P. Khan, S.B. Khan, M.M. Rahman, I. Khan, Synthesis, characterization of silver nanoparticle embedded polyaniline tungstophosphate- nanocomposite cation exchanger and its application for heavy metal selective membrane, Compos: Part B. 45 (2013) 1486–1492. [12] M.M. Rahman, A. Jamal, S.B. Khan, M. Faisal, Fabrication of chloroform sensor based on hydrothermally prepared low-dimensional β-Fe2O3 nanoparticles, Superlattice. Microst. 50 (2011) 369–376.
Fig. 10. Catalytic activity versus on-stream reaction time over the Co3O4/αFe2O3 for the oxidation of benzene at 450 °C.
could enhance the migration of oxygen vacancy to improve the catalytic activity [50] and played an important role in the whole catalytic process. 3.6. Catalyst stability The catalyst stability test was carried out at the condition with benzene in air and the GHSV of 48000 h−1, and the best Co3O4/αFe2O3 composite with 0.35 g was selected as the representative catalyst. The continuous test lasted over 144 h at the temperature of 450 °C which was higher than that required for complete conversion of benzene, and thus, allowed for the detection of concentration impact resistance. Fig. 10 showed the activity as a function with time on stream, it could be found that Co3O4/α-Fe2O3 catalyst was stable at the initial 72 h for complete oxidation of benzene with the concentration of 1000 mg/m3. When the concentration increased to 2000 mg/m3, the degradation rate still retained unchanged in another period of 72 h, indicating that the catalyst could stand the concentration impact resistance. Wu et al. [60] studied the effect of Fe2O3-modified Ti0.5Sn0.5O2 supported by CuO for CO catalytic oxidation, and they found the CO conversion had no obviously change for 18 h. Liu et al. [61] used the mesoporous Mn promoted Co3O4 oxides as catalyst for CO catalytic oxidation, and the sample displayed stable activity for 3000 min. Given the above results, Co3O4/α-Fe2O3 could exhibit great 85
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