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Synthesis and characterization of iron-based catalyst on mesoporous titania for photo-thermal F-T synthesis
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 8 7 0 e8 7 7
Available online at www.sciencedirect.com
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Synthesis and characterization of iron-based catalyst on mesoporous titania for photo-thermal F-T synthesis Shiyong Yu, Tao Zhang, Yanhong Xie, Qinghua Wang, Xuechuan Gao, Renfei Zhang, Yulong Zhang, Haiquan Su* School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, Inner Mongolia, China
article info
abstract
Article history:
In this paper, a series of iron-based FischereTropsch catalysts were prepared with meso-
Received 31 March 2014
porous titanium dioxide as catalyst carrier. And the F-T catalytic activities of iron sup-
Received in revised form
ported on the mesoporous TiO2 with different doping concentrations were also
26 September 2014
investigated in photo-thermal driven FischereTropsch synthesis. Transmission Electron
Accepted 27 October 2014
Microscopy (TEM), Scanning Electron Microscopy (SEM), Low temperature N2 adsorption-
Available online 20 November 2014
desorption, X ray powder diffraction (XRD), Temperature Programmed Reduction (TPR) were used to characterize the catalysts, and the catalytic performance were also investi-
Keywords:
gated for Photo-thermal driven FischereTropsch synthesis under the condition of atmo-
Mesoporous TiO2
spheric pressure at the temperature of 220 C, n (H2)/n (CO) ¼ 2:1 and GHSV ¼ 1500 h1. It is
Doping
shown that, for different doping concentration of iron, the conversion of CO, the selectivity
Iron-based
of methane and the selectivity of C2þ changes, resulting in the difference performance of
Photo-thermal FTS
Photo-thermal Fischer Tropsch synthesis reaction. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen has been considered to be the most potential strategic energy and clear energy in 21st century. Hydrogen has been proposed as the perfect fuel to address energy demands against the environment pollution while reducing carbon dioxide and other greenhouse gas emissions. The conversion of syngas (CO þ H2 mixtures) into liquids, and more specifically clean fuels and chemical feedstock via FischereTropsch synthesis, is currently of increasing interest. Generally speaking, the FischereTropsch synthesis (FTS) can be indicated as nCO þ (2n þ 1)H2 / CnH(2nþ2) þ nH2O.
FischereTropsch synthesis is a process that with syngas as raw materials by using catalyst (mainly Fe based catalyst) and under the proper condition to synthesize liquid fuels with high paraffin hydrocarbons obtained [1]. There has been an increased attention in the development of gas to liquid (GTL) technology in the recent decade. The main motivator for this conversion are the increased availability of natural gas in remote locations for which no nearby markets exist, environmental pressure to minimize the flaring of associated gas, the growing demand for middle distillate transportation fuels (gasoil and kerosene) especially in the AsiaePacific regions, and improvements in the cost effectiveness of GTL technology, resulting from the development of more active catalyst and improved reactor design. FischereTropsch reactor being
* Corresponding author. Tel./fax: þ86 04714992981. E-mail address: [email protected] (H. Su). http://dx.doi.org/10.1016/j.ijhydene.2014.10.121 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 8 7 0 e8 7 7
the heart of GTL conversion processes has great significance in the economics of the overall plant. We can see in Scheme 1, natural gas, coal and biomass were transformed into syngas, and the syngas is made into liquid energy by photo-thermal FT synthesis. On another aspect of catalytic active center, iron, nickel, and cobalt are the most active metals for the hydrogenation of carbon monoxide. Vannice et al. [2] showed that the molecular average weight of hydrocarbons produced by FT synthesis decreased in the following sequence:
Fe > Co > Rh > Ni > Ir > Pt > Pd. Cobalt and iron are the metals which were proposed by Fischer and Tropsch as the first catalysts for syngas conversion. Both cobalt and iron catalysts have been used in the industry for hydrocarbon synthesis. Cobalt catalysts operate at a very narrow range of temperatures and pressures; an increase in temperature leads to a spectacular increase in methane selectivity. Iron-based catalysts have been used in FTS of hydrocarbons from syngas since 1923 [3]. Iron catalysts seem to be more appropriate for conversion of biomassderived syngas to hydrocarbons than cobalt systems because they can operate at lower H2/CO ratios [4]. Fe-based catalysts have been widely investigated recently because of their low cost, high activity, and capability, and they operate over a wide temperature range (220 Ce350 C) to produce diesel and wax in low-temperature FTS or gasoline components in high-temperature FTS [1,5], the use of iron-based catalyst is more attractive for FTS with low H2/CO ratio syngas derived from coal or biomass [6]. Because the support is very important for the properties of the final FischereTropsch catalyst, the activity is indirectly affected by the support nature. TiO2 is commonly used catalyst support for metals and their complexes. Mesostructured titania, combining high surface area, uniform pore size and accessible open frameworks is a promising material as photocatalyst and catalyst support for hydrogen production [7]. The detailed advantages of the mesoporous structure with nanocrystalline TiO2 walls include shorter distance of the photogenerated electron hole pairs to reach the photocatalyst surface and higher surface to deposit metals
871
or other compounds in order to modulate its activity. Both aspects are beneficial for improving the photocatalytic efficiency of TiO2 since the electron hole pair recombination rate, which is one of its main limitations, may be drastically reduced [8]. In addition, TiO2 is used for its well-known ability to interact with metals [9]. The so-called strong metal-support interaction (SMSI) effect, firstly introduced by Tauster et al. [10] and observed for group VIII metals over reducible support materials deeply affects the properties of a catalyst [11e14]. Photo-thermal catalysis reaction is not a simple combination of photo-catalysis and thermal-catalysis, but there is a Photo-thermal catalytic coupling effect. There are many researches on photo-thermal catalysis, most of them are about the degradation of pollutants [15e17]. In the past few decades, the traditional process of FischereTropsch synthesis has been comparative maturity, and the agenda for how to explore a new and cleaner synthesis method will been brought up. But there are few researches about Photo-thermal F-T. In this paper, we combined light energy into F-T process to attempt a new synthesis method of the coal to liquid. A series of iron doped catalyst are prepared with mesoporous titanium dioxide as catalyst carrier. And the effect for different concentration of iron in Photo-thermal driven FischereTropsch Synthesis were also investigated.
Experimental section Materials preparation All starting materials were commercially available products： glucose and urea (99.0%) were produced by Tianjin Beilian Fine Chemicals Development Co. Ltd. Titanium butoxide(TBOT) (99.0%) produced by Aladdin-reagent. Ethanol absolute(99.7%) was purchased in Tianjin FengChuan chemical reagent Technology Co., Ltd. Iron(III) nitrate nonahydrate(98.5%) was purchased in Sinopharm Chemical Reagent Co., Ltd.
Synthesis of mesoporous TiO2 The mesoporous TiO2 powders were obtained by a hydrothermal reaction without any surfactants. In a typical experiment, 27.024 g of glucose and 27.027 g of urea were dissolved in 150 ml deionized water and 365 ml of ethanol absolute under mild stirring for 2 h, and the transparent solution was obtained. A 5.4 ml sample of titanium butoxide mixed with 10 ml ethanol absolute was then slowly added into this solution, and the milky-white solution obtained was transferred into a Teflon-lined autoclave and heated at 180 C for 24 h. The resulting slurry was then centrifuged and washed with deionized water and ethanol absolute before drying in a vacuum oven. The powder identified as mTiO2.
Synthesis of Fe(x%)-mTiO2 catalysts
Scheme 1 e The schematic diagram of reaction mechanism.
The mTiO2 supported catalysts were prepared by the incipient wetness impregnation method. Iron nitrate (Fe(NO3)3$9H2O) solution was used as the iron source. In the synthesis of the catalysts, 2 g mTiO2 was impregnated with iron nitrate
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Table 1 e The weight of Fe(NO3)3·9H2O for different iron content of the catalyst. Fe:Ti atomic ratio 5% 10% 15% 20% 30%
Fe(NO3)3$9H2O 0.5058 1.0117 1.5175 2.0234 3.0351
g g g g g
solution under continuous stirring. The impregnated catalysts were aged overnight, and dried in the ambient air of 50 C and then calcined at 450 C for 2 h and 550 C for 2 h respectively. Finally, the solid product identified as Fe(x%)-mTiO2. Table 1 shows the weight of Fe(NO3)3·9H2O for different iron content of the catalyst.(Fe:Ti is atomic ratio).
Photothermal Fischer Tropsch synthesis Catalyst filling 1 g catalyst as prepared was dissolved in 20 ml ethanol absolute, and then coated on the surface of custom-built quartz
glass evenly. After natural air-dry, the glass coated with catalysts into a reaction kettle with card slot, and finally sealed the reaction kettle.
Activity tests The photo-thermal F-T tests were carried out in a stainlesssteel fixed-bed tank reactor with up-flow under atmospheric pressure. The reactor coated with a zone of heating and the temperature of the reactor was controlled to ±2 C by a programmable controller. The reaction temperature was measured by a thermocouple which was placed into the bottom of the reactor. Gas hourly space velocity (GHSV) was kept constant at 1500 h1 for all tests. The lines after the reactor outlet were kept at 100 C in order to avoid premature condensation. The cold trap at 1 C was located immediately after the reactor for collecting heavy hydrocarbons and the medium range hydrocarbons/water. Reactants and gaseous product streams were analyzed by using an online gas chromatograph. The FID channel was configured to analyze the hydrocarbons from C1 to C4. The TCD channel was dedicated to analyze CO2, CO, N2 and H2. Catalyst samples were activated in situ under a hydrogen stream under atmospheric pressure by increasing the temperature from room temperature to 350 C at a rate of 2.5 C/ min. The samples were kept at 350 C for 500 min to maximize their reduction. H2 to CO ratio is 2:1 of syngas in the tests. Nitrogen as the carrier gas in the tests and the tests were performed at 220 C.
Characterization N2 physisorption isotherms were measured at 77 K on a Quantachrome apparatus. Samples were firstly degassed at 200 C for 4 h. The BET equation and BJH model were applied for surface area and pore size distribution determinations, respectively. Powder X-ray diffraction (XRD) patterns were obtained on a MAC Science diffractometer using CuKa radiation. Wide-angle patterns were recorded from 2q ¼ 5 e 90 , using a step size of 0.0167 and a step time of 5 s. Scanning Electron Microscopy (SEM) was conducted on a Hitachi S-3400N instrument.
Results and discussion Catalysts characterization XRD was used to research the phase structure of the asprepared Fe-doped mesoporous TiO2 powders. Fig. 1 shows
Table 2 e The composition and textural properties of catalysts. 5%
Fig. 1 e XRD patterns from the catalysts at calcined state.
10%
15%
20%
30%
Surface area (m2/g) 77.419 90.274 91.616 108.391 119.953 0.36 0.384 0.263 0.227 0.225 Pore volume (cm3/g) Pore diameter Dv(d/nm) 3.816 6.541 7.797 7.768 7.758 Crystallite size (nm) 37.046 37.068 37.054 37.078 37.093
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Fig. 2 e The pore size distributions and N2 adsorptionedesorption isotherms of samples.
the effect of Fe concentration on phase structures of the mesoporous TiO2 powders prepared by the maceration method. It can be seen that the diffraction peaks of 5% sample is similar to the peaks of TiO2 anatase phase. However, the other Fe-doping samples have effect on the crystallization of TiO2. It shows that with Fe-doping concentration increasing, the peak intensity of anatase at 25.4 slightly decreases.
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Therefore, it is reasonable to deduce that the larger the amount of Fe-doping, the poorer the crystallization of the TiO2 powders. It is interesting to note that iron oxides or FexTiyOz phases are not found in the XRD patterns of 5% [18,19]. There are two reasons responsible for this. One probable reason is that the concentration of Fe-doping is so low that it cannot be ˚ ) and detected by XRD. The other is that the radii of Ti4þ(0.68 A 3þ ˚ Fe (0.64 A) ions are very similar and all the iron ions may be insert into the structure of titanium, and locate at interstices or occupy some of the titanium lattice sites, forming an ironetitanium oxide solid solution [19]. With the increasing of Fe-doping concentration from 10% to 30%, the diffraction peaks of the Fe2TiO5(PDF card No.00-003-0374) gradually came into view. The physical properties, including BET surface area, porous size and volume, obtained from the isotherm data are listed in Table 2. The nitrogen adsorptionedesorption isotherms and the pore size distributions of Fe(x%)-mTiO2 were shown in Fig. 2. As shown in the Fig. 2 and Table 2, the Fe(5%)-mTiO2 and Fe(10%)-mTiO2 samples has a very broad pore size distribution and small pore volume, which results in a low surface area. For the other three samples seen in Fig. 2, the pore size obtained from the desorption branch of the isotherms exhibits a very narrow distribution in the mesoporous region between 7 nm and 8 nm, which implies the uniformity of the pore size and verifies a good quality of the synthesized nanocrystal. These results indicate that with increasing the doping level of Fe, the surface area of samples increase and the pore volume
Fig. 3 e XPS spectra of Fe(x%)-mTiO2 samples: (A) survey, (B)Fe 2p peaks, and (C) Ti2p peaks.
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Fig. 4 e H2-TPR profiles of Fe(5%) -mTiO2, Fe(10%)-mTiO2, Fe(15%) -mTiO2, Fe(20%)-mTiO2 and Fe(30%) -mTiO2. of samples decrease, while the crystallite size changes slightly. N2 adsorptionedesorption isotherms of the Fe(5%)mTiO2, Fe(10%)-mTiO2, Fe(15%)-mTiO2, Fe(20%)-mTiO2 and Fe(30%)-mTiO2 simples shown in Fig. 2. The N2 adsorptionedesorption isotherm of the Fe(5%)-mTiO2 and Fe(10%)mTiO2 samples corresponds to the IUPAC type V pattern [20]. It is clear that Fe(5%)-mTiO2 and Fe(10%)-mTiO2 samples has no mesoporous characteristic due to the absence of hysteresis
loop. This observation was consistent with the surface area dates listed in Table 2. For the Fe(15%)-mTiO2, Fe(20%)-mTiO2 and Fe(30%)-mTiO2 catalysts, the isotherms exhibit typical curve of type V pattern with H2 hysteresis loop according to IUPAC classification, indicating the existence of ink-bottletype pore structure with a narrow entrance and a large cavity [20,21]. Fig. 3(A)show XPS survey spectrum of the Fe-mTiO2. The samples contain Ti, O, Fe, and C elements. The C element was ascribed to the adventitious hydrocarbon of the XPS instrument. To investigate the chemical states of both ferrum and titanium, high-resolution XPS spectra of both Ti2p and Fe2p were analyzed. An XPS spectrum of the Fe2p core level are shown in Fig. 3(B). The Fe signal was also observed with the low levels of doping (5%). The binding energies located at approximately 711.0 eV and 724.9 eV were assigned to Fe3þ2p3/ 3þ 2 and Fe 2p1/2, respectively. The Fe elements in the samples existed mainly in the þ3 oxidation state(Fe3þ). The Fe3þ ions could be incorporated into the TiO2 lattice to form the TieOeFe bonds present in the Fe(x%)-mTiO2 samples treated by calcinations. This result coincides with the Fe2TiO5 shown in XRD date. In Fig. 3(C), we can see that, both Ti2p1/2 and Ti2p3/2 spin-orbital splitting photoelectrons were located at 458.0 eV and 464.0 eV, indicating the presence of Ti4þ. The examination of the hydrogen reduction behavior of our catalysts by TPR-H2 measurements provided additional information about chemical composition of the sample and surface properties acquired during its preparation. H2-TPR curves of our samples are shown in Fig. 4. The reduction of
Fig. 5 e SEM images of different molar ratio of Fe to Ti (a) (b) Fe(5%)-mTiO2; (c) (d)Fe(10%)-mTiO2; (e) (f)Fe(15%)-mTiO2; (i) (j) Fe(30%) -mTiO2.
Fig. 6 e Effect of Fe content on the Photo-thermal FischereTropsch reaction over mTiO2. (A) CH4 selectivity, (B) C2H4 selectivity, (C) C2H6 selectivity, (D) C3H6 selectivity, (E) C3H8 selectivity, (F) C4H8 selectivity, (G) C4H10 selectivity, (H) CO conversion, and (I) Carbon efficiency.
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iron species in hydrogen was postulated as a three-step mechanism 3Fe2O3 / 2Fe3O4 / 6FeO / 6Fe at the temperature range 300e600 C [22]. We can attribute the hydrogen consumption observed at the temperature range 300e400 C to the first stage of reduction of hematite to magnetite: Fe2O3 / Fe3O4, and the next broad peak at 500e600 C to the two-step magnetite reduction to metallic iron. The appearance of wu¨stite phase(FeO), as an intermediate of hematite reduction in hydrogen was experimentally confirmed at a temperature above 570 C [22]. With the Fe content doping increase, the reduction temperature of catalysts are shifted towards a higher temperature. It may be suggested that the calcined catalyst possess fine particles and large aggregates containing non-uniformly distributed iron oxides. Fig. 5 shows SEM images of synthesized samples after calcined for 2 h at 450 C and 550 C, respectively. From Fig. 5, we can see that the agglomeration of synthesized composite powders was observed which was ascribed to Fe(III) deposits. Analysis of SEM images (see Fig. 5a, b,c,d,e and f) shows that 5%, 10% and 15% Fe-doped mTiO2 catalyst contains a higher number of irregular shaped particles. The particles didn't possess any definite shape. The images showed large aggregates in addition to small particles dispersed on them. The SEM images of Fe(20%)-mTiO2 and Fe(30%)-mTiO2 are shown in Fig. 5(g,h) and (i,j) respectively. They also indicated tiny crystallites on large agglomerates. Both the tiny crystallites and large aggregates were assigned to Fe2TiO5 which is in support of the XRD pattern that revealed the formation of Fe2TiO5 phase. Fig. 6 shows the effect of the Fe amount incorporated into the titanium dioxide framework. Under the setting conditions, the series catalysts are unstable to CH4 selectivity. As to the reason for this phenomenon, we can't figure out clearly at the moment, and we will do further research in the future. When the molar ratio of Fe to Ti is about 22%, the selectivity of C2H4 is very well, and reaches about 20%. It is generally high for C2H6 selectivity for all catalyst samples, and the selectivity is all above 40%. With the molar ratio of Fe to Ti is about 20%, it displays the best selectivity to C2H6. While the series catalysts are unstable to the selectivity of C3 products, and it didn't presents the regularity in the relationship between the iron content and the C3 selectivity at current research stage. Fig. 6(H) presents the CO conversion of different Fe content catalysts. It can be seen that the CO conversion are in the range of 75%e85% at this condition, and the maximum conversion of CO for samples with molar ratio of Fe to Ti is 15%. In Fig. 6(I) we can see the carbon efficiency is above 60% at this condition, and the maximum efficiency of carbon for catalysts with molar ratio of Fe to Ti is 20%.
Conclusions Mesostructured titania (mTiO2) with well surface area and crystalline framework has been synthesized by sol-thermal synthesis method. The different content iron-based catalysts with mesostructured titania as carrier were prepared by impregnation method. The prepared catalysts, which are mainly Fe2TiO5 phase identified by XRD, were evaluated via Photo-thermal F-T synthesis. As a whole, Fe(20%)-mTiO2
sample is the best catalyst in all samples, and for this catalyst, it has a higher selectivity than others, particularly for C2H4, C2H6, C4H8 and C4H10. Moreover, Fe(20%)-mTiO2 has a high CO conversion in the range of 75%e85%. Up till the present moment, due to there are less research on Photo-thermal F-T. The operation condition and the reaction mechanism of Photo-thermal F-T are still not very clear, and more detailed researches are under progress.
Acknowledgments The authors are grateful to the financial aid from the Specialized Research Fund for the Doctoral Program of Higher Education (Grant Nos. 20131501110001), the National Natural Science Foundation of China (Grant Nos. 20961006), and Inner Mongolia Natural Science Foundation (Grant Nos. 2014MS0206).
references
[1] Huo CF, Li YW, Wang JG, Jiao HJ. Insight into CH4 formation in iron-catalyzed Fischer-Tropsch synthesis. J Am Chem Soc 2009;131:14713e21. [2] Vannice MA. The catalytic synthesis of hydrocarbons from H2/CO mixtures over the group VIII metals:I. The specific activities and product distributions of supported metals. J Catal 1974;37:449e61. [3] Leckel D. Diesel production from Fischer-Tropsch: the past, the present, and new concepts. Energy Fuels 2009;23:2342e58. [4] Khodakov AY, Chu W, Fongarland Pascal. Advances in the development of novel cobalt Fischer-Tropsch catalysts for synthesis of long-chain hydrocarbons and clean fuels. Chem Rev 2007;107:1692e744. [5] de Smit E, Cinquini F, Beale AM, Safonova OV, van Beek W, Sautet P, et al. Stability and reactivity of ecq iron carbide catalyst phases in FischerTropsch synthesis: controlling mC. J Am Chem Soc 2010;132:14928e41. [6] Dry ME. High quality diesel via the FischereTropsch process: a review. J Chem Technol Biotechnol 2001;77:43e50. lvez P. Enhanced [7] Serrano DP, Calleja G, Pizarro P, Ga photocatalytic hydrogen production by improving the Pt dispersion over mesostructured TiO2. Int J Hydrogen Energy 2014;39:4812e9. [8] Zhang RY, Elzatahry Ahmed A, Al-Deyab Salem S, Zhao DY. Mesoporous titania: from synthesis to application. Nano Today 2012;7:344e66. [9] Adhikari S, Fernando SD, Haryanto A. Hydrogen production from glycerin by steam reforming over nickel catalysts. Renew Energy 2008;33:1097e100. [10] Tauster SJ, Fung SC, Garden RL. Strong metal-support interactions. Group 8 noble metals supported on TiO2. J Am Chem Soc 1978;100:170e5. [11] Vannice MA, Garten RL. Metal-support effects on the activity and selectivity of Ni catalysts in CO/H2 synthesis reactions. J Catal 1979;56:236e48. [12] Wu T, Yan Q, Wan H. Partial oxidation of methane to hydrogen and carbon monoxide over a Ni/TiO2 catalyst. J Mol Catal A Chem 2005;226:41e8. [13] Raj KJA, Prakash MG, Mahalakshmy R, Elangowan T, Viswanathan B. Selective hydrogenation of acetophenone over nickel supported on titania. Catal Sci Technol 2012;2:1429e36.
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[14] Men Y, Yang M. SMSI-like behavior and Ni promotion effect on NiZnAl catalysts in steam reforming of methanol. Catal Commun 2012;22:68e73. [15] Li YZ, Huang JC, Peng T, Xu J, Zhao XJ. Photothermocatalytic synergetic effect leads to high efficient detoxification of benzene on TiO2 and Pt/TiO2 nanocomposite. Chem Cat Chem 2010;2:1082e7. [16] Liu YL, Shu W, Chen KQ, Peng ZY, Chen W. Enhanced photothermocatalytic synergetic activity toward gaseous benzene for MoþC-Codoped titanate nanobelts. ACS Catal 2012;2:2557e65. [17] Li YZ, Sun Q, Kong M, Shi WQ, Huang JC, Tang JW, et al. Coupling oxygen ion conduction to photocatalysis in mesoporous nanorod-like ceria significantly improves photocatalytic efficiency. J Phys Chem C 2011;115:14050e7. [18] Zhou MH, Yu JG, Cheng B, Yu HG. Preparation, photocatalytic activity of Fe-doped mesoporous titanium dioxide
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Synthesis and characterization of iron-based catalyst on mesoporous titania for photo-thermal F-T synthesis Shiyong Yu, Tao Zhang, Yanhong Xie, Qinghua Wang, Xuechuan Gao, Renfei Zhang, Yulong Zhang, Haiquan Su* School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, Inner Mongolia, China
article info
abstract
Article history:
In this paper, a series of iron-based FischereTropsch catalysts were prepared with meso-
Received 31 March 2014
porous titanium dioxide as catalyst carrier. And the F-T catalytic activities of iron sup-
Received in revised form
ported on the mesoporous TiO2 with different doping concentrations were also
26 September 2014
investigated in photo-thermal driven FischereTropsch synthesis. Transmission Electron
Accepted 27 October 2014
Microscopy (TEM), Scanning Electron Microscopy (SEM), Low temperature N2 adsorption-
Available online 20 November 2014
desorption, X ray powder diffraction (XRD), Temperature Programmed Reduction (TPR) were used to characterize the catalysts, and the catalytic performance were also investi-
Keywords:
gated for Photo-thermal driven FischereTropsch synthesis under the condition of atmo-
Mesoporous TiO2
spheric pressure at the temperature of 220 C, n (H2)/n (CO) ¼ 2:1 and GHSV ¼ 1500 h1. It is
Doping
shown that, for different doping concentration of iron, the conversion of CO, the selectivity
Iron-based
of methane and the selectivity of C2þ changes, resulting in the difference performance of
Photo-thermal FTS
Photo-thermal Fischer Tropsch synthesis reaction. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen has been considered to be the most potential strategic energy and clear energy in 21st century. Hydrogen has been proposed as the perfect fuel to address energy demands against the environment pollution while reducing carbon dioxide and other greenhouse gas emissions. The conversion of syngas (CO þ H2 mixtures) into liquids, and more specifically clean fuels and chemical feedstock via FischereTropsch synthesis, is currently of increasing interest. Generally speaking, the FischereTropsch synthesis (FTS) can be indicated as nCO þ (2n þ 1)H2 / CnH(2nþ2) þ nH2O.
FischereTropsch synthesis is a process that with syngas as raw materials by using catalyst (mainly Fe based catalyst) and under the proper condition to synthesize liquid fuels with high paraffin hydrocarbons obtained [1]. There has been an increased attention in the development of gas to liquid (GTL) technology in the recent decade. The main motivator for this conversion are the increased availability of natural gas in remote locations for which no nearby markets exist, environmental pressure to minimize the flaring of associated gas, the growing demand for middle distillate transportation fuels (gasoil and kerosene) especially in the AsiaePacific regions, and improvements in the cost effectiveness of GTL technology, resulting from the development of more active catalyst and improved reactor design. FischereTropsch reactor being
* Corresponding author. Tel./fax: þ86 04714992981. E-mail address: [email protected] (H. Su). http://dx.doi.org/10.1016/j.ijhydene.2014.10.121 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 8 7 0 e8 7 7
the heart of GTL conversion processes has great significance in the economics of the overall plant. We can see in Scheme 1, natural gas, coal and biomass were transformed into syngas, and the syngas is made into liquid energy by photo-thermal FT synthesis. On another aspect of catalytic active center, iron, nickel, and cobalt are the most active metals for the hydrogenation of carbon monoxide. Vannice et al. [2] showed that the molecular average weight of hydrocarbons produced by FT synthesis decreased in the following sequence:
Fe > Co > Rh > Ni > Ir > Pt > Pd. Cobalt and iron are the metals which were proposed by Fischer and Tropsch as the first catalysts for syngas conversion. Both cobalt and iron catalysts have been used in the industry for hydrocarbon synthesis. Cobalt catalysts operate at a very narrow range of temperatures and pressures; an increase in temperature leads to a spectacular increase in methane selectivity. Iron-based catalysts have been used in FTS of hydrocarbons from syngas since 1923 [3]. Iron catalysts seem to be more appropriate for conversion of biomassderived syngas to hydrocarbons than cobalt systems because they can operate at lower H2/CO ratios [4]. Fe-based catalysts have been widely investigated recently because of their low cost, high activity, and capability, and they operate over a wide temperature range (220 Ce350 C) to produce diesel and wax in low-temperature FTS or gasoline components in high-temperature FTS [1,5], the use of iron-based catalyst is more attractive for FTS with low H2/CO ratio syngas derived from coal or biomass [6]. Because the support is very important for the properties of the final FischereTropsch catalyst, the activity is indirectly affected by the support nature. TiO2 is commonly used catalyst support for metals and their complexes. Mesostructured titania, combining high surface area, uniform pore size and accessible open frameworks is a promising material as photocatalyst and catalyst support for hydrogen production [7]. The detailed advantages of the mesoporous structure with nanocrystalline TiO2 walls include shorter distance of the photogenerated electron hole pairs to reach the photocatalyst surface and higher surface to deposit metals
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or other compounds in order to modulate its activity. Both aspects are beneficial for improving the photocatalytic efficiency of TiO2 since the electron hole pair recombination rate, which is one of its main limitations, may be drastically reduced [8]. In addition, TiO2 is used for its well-known ability to interact with metals [9]. The so-called strong metal-support interaction (SMSI) effect, firstly introduced by Tauster et al. [10] and observed for group VIII metals over reducible support materials deeply affects the properties of a catalyst [11e14]. Photo-thermal catalysis reaction is not a simple combination of photo-catalysis and thermal-catalysis, but there is a Photo-thermal catalytic coupling effect. There are many researches on photo-thermal catalysis, most of them are about the degradation of pollutants [15e17]. In the past few decades, the traditional process of FischereTropsch synthesis has been comparative maturity, and the agenda for how to explore a new and cleaner synthesis method will been brought up. But there are few researches about Photo-thermal F-T. In this paper, we combined light energy into F-T process to attempt a new synthesis method of the coal to liquid. A series of iron doped catalyst are prepared with mesoporous titanium dioxide as catalyst carrier. And the effect for different concentration of iron in Photo-thermal driven FischereTropsch Synthesis were also investigated.
Experimental section Materials preparation All starting materials were commercially available products： glucose and urea (99.0%) were produced by Tianjin Beilian Fine Chemicals Development Co. Ltd. Titanium butoxide(TBOT) (99.0%) produced by Aladdin-reagent. Ethanol absolute(99.7%) was purchased in Tianjin FengChuan chemical reagent Technology Co., Ltd. Iron(III) nitrate nonahydrate(98.5%) was purchased in Sinopharm Chemical Reagent Co., Ltd.
Synthesis of mesoporous TiO2 The mesoporous TiO2 powders were obtained by a hydrothermal reaction without any surfactants. In a typical experiment, 27.024 g of glucose and 27.027 g of urea were dissolved in 150 ml deionized water and 365 ml of ethanol absolute under mild stirring for 2 h, and the transparent solution was obtained. A 5.4 ml sample of titanium butoxide mixed with 10 ml ethanol absolute was then slowly added into this solution, and the milky-white solution obtained was transferred into a Teflon-lined autoclave and heated at 180 C for 24 h. The resulting slurry was then centrifuged and washed with deionized water and ethanol absolute before drying in a vacuum oven. The powder identified as mTiO2.
Synthesis of Fe(x%)-mTiO2 catalysts
Scheme 1 e The schematic diagram of reaction mechanism.
The mTiO2 supported catalysts were prepared by the incipient wetness impregnation method. Iron nitrate (Fe(NO3)3$9H2O) solution was used as the iron source. In the synthesis of the catalysts, 2 g mTiO2 was impregnated with iron nitrate
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Table 1 e The weight of Fe(NO3)3·9H2O for different iron content of the catalyst. Fe:Ti atomic ratio 5% 10% 15% 20% 30%
Fe(NO3)3$9H2O 0.5058 1.0117 1.5175 2.0234 3.0351
g g g g g
solution under continuous stirring. The impregnated catalysts were aged overnight, and dried in the ambient air of 50 C and then calcined at 450 C for 2 h and 550 C for 2 h respectively. Finally, the solid product identified as Fe(x%)-mTiO2. Table 1 shows the weight of Fe(NO3)3·9H2O for different iron content of the catalyst.(Fe:Ti is atomic ratio).
Photothermal Fischer Tropsch synthesis Catalyst filling 1 g catalyst as prepared was dissolved in 20 ml ethanol absolute, and then coated on the surface of custom-built quartz
glass evenly. After natural air-dry, the glass coated with catalysts into a reaction kettle with card slot, and finally sealed the reaction kettle.
Activity tests The photo-thermal F-T tests were carried out in a stainlesssteel fixed-bed tank reactor with up-flow under atmospheric pressure. The reactor coated with a zone of heating and the temperature of the reactor was controlled to ±2 C by a programmable controller. The reaction temperature was measured by a thermocouple which was placed into the bottom of the reactor. Gas hourly space velocity (GHSV) was kept constant at 1500 h1 for all tests. The lines after the reactor outlet were kept at 100 C in order to avoid premature condensation. The cold trap at 1 C was located immediately after the reactor for collecting heavy hydrocarbons and the medium range hydrocarbons/water. Reactants and gaseous product streams were analyzed by using an online gas chromatograph. The FID channel was configured to analyze the hydrocarbons from C1 to C4. The TCD channel was dedicated to analyze CO2, CO, N2 and H2. Catalyst samples were activated in situ under a hydrogen stream under atmospheric pressure by increasing the temperature from room temperature to 350 C at a rate of 2.5 C/ min. The samples were kept at 350 C for 500 min to maximize their reduction. H2 to CO ratio is 2:1 of syngas in the tests. Nitrogen as the carrier gas in the tests and the tests were performed at 220 C.
Characterization N2 physisorption isotherms were measured at 77 K on a Quantachrome apparatus. Samples were firstly degassed at 200 C for 4 h. The BET equation and BJH model were applied for surface area and pore size distribution determinations, respectively. Powder X-ray diffraction (XRD) patterns were obtained on a MAC Science diffractometer using CuKa radiation. Wide-angle patterns were recorded from 2q ¼ 5 e 90 , using a step size of 0.0167 and a step time of 5 s. Scanning Electron Microscopy (SEM) was conducted on a Hitachi S-3400N instrument.
Results and discussion Catalysts characterization XRD was used to research the phase structure of the asprepared Fe-doped mesoporous TiO2 powders. Fig. 1 shows
Table 2 e The composition and textural properties of catalysts. 5%
Fig. 1 e XRD patterns from the catalysts at calcined state.
10%
15%
20%
30%
Surface area (m2/g) 77.419 90.274 91.616 108.391 119.953 0.36 0.384 0.263 0.227 0.225 Pore volume (cm3/g) Pore diameter Dv(d/nm) 3.816 6.541 7.797 7.768 7.758 Crystallite size (nm) 37.046 37.068 37.054 37.078 37.093
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Fig. 2 e The pore size distributions and N2 adsorptionedesorption isotherms of samples.
the effect of Fe concentration on phase structures of the mesoporous TiO2 powders prepared by the maceration method. It can be seen that the diffraction peaks of 5% sample is similar to the peaks of TiO2 anatase phase. However, the other Fe-doping samples have effect on the crystallization of TiO2. It shows that with Fe-doping concentration increasing, the peak intensity of anatase at 25.4 slightly decreases.
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Therefore, it is reasonable to deduce that the larger the amount of Fe-doping, the poorer the crystallization of the TiO2 powders. It is interesting to note that iron oxides or FexTiyOz phases are not found in the XRD patterns of 5% [18,19]. There are two reasons responsible for this. One probable reason is that the concentration of Fe-doping is so low that it cannot be ˚ ) and detected by XRD. The other is that the radii of Ti4þ(0.68 A 3þ ˚ Fe (0.64 A) ions are very similar and all the iron ions may be insert into the structure of titanium, and locate at interstices or occupy some of the titanium lattice sites, forming an ironetitanium oxide solid solution [19]. With the increasing of Fe-doping concentration from 10% to 30%, the diffraction peaks of the Fe2TiO5(PDF card No.00-003-0374) gradually came into view. The physical properties, including BET surface area, porous size and volume, obtained from the isotherm data are listed in Table 2. The nitrogen adsorptionedesorption isotherms and the pore size distributions of Fe(x%)-mTiO2 were shown in Fig. 2. As shown in the Fig. 2 and Table 2, the Fe(5%)-mTiO2 and Fe(10%)-mTiO2 samples has a very broad pore size distribution and small pore volume, which results in a low surface area. For the other three samples seen in Fig. 2, the pore size obtained from the desorption branch of the isotherms exhibits a very narrow distribution in the mesoporous region between 7 nm and 8 nm, which implies the uniformity of the pore size and verifies a good quality of the synthesized nanocrystal. These results indicate that with increasing the doping level of Fe, the surface area of samples increase and the pore volume
Fig. 3 e XPS spectra of Fe(x%)-mTiO2 samples: (A) survey, (B)Fe 2p peaks, and (C) Ti2p peaks.
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Fig. 4 e H2-TPR profiles of Fe(5%) -mTiO2, Fe(10%)-mTiO2, Fe(15%) -mTiO2, Fe(20%)-mTiO2 and Fe(30%) -mTiO2. of samples decrease, while the crystallite size changes slightly. N2 adsorptionedesorption isotherms of the Fe(5%)mTiO2, Fe(10%)-mTiO2, Fe(15%)-mTiO2, Fe(20%)-mTiO2 and Fe(30%)-mTiO2 simples shown in Fig. 2. The N2 adsorptionedesorption isotherm of the Fe(5%)-mTiO2 and Fe(10%)mTiO2 samples corresponds to the IUPAC type V pattern [20]. It is clear that Fe(5%)-mTiO2 and Fe(10%)-mTiO2 samples has no mesoporous characteristic due to the absence of hysteresis
loop. This observation was consistent with the surface area dates listed in Table 2. For the Fe(15%)-mTiO2, Fe(20%)-mTiO2 and Fe(30%)-mTiO2 catalysts, the isotherms exhibit typical curve of type V pattern with H2 hysteresis loop according to IUPAC classification, indicating the existence of ink-bottletype pore structure with a narrow entrance and a large cavity [20,21]. Fig. 3(A)show XPS survey spectrum of the Fe-mTiO2. The samples contain Ti, O, Fe, and C elements. The C element was ascribed to the adventitious hydrocarbon of the XPS instrument. To investigate the chemical states of both ferrum and titanium, high-resolution XPS spectra of both Ti2p and Fe2p were analyzed. An XPS spectrum of the Fe2p core level are shown in Fig. 3(B). The Fe signal was also observed with the low levels of doping (5%). The binding energies located at approximately 711.0 eV and 724.9 eV were assigned to Fe3þ2p3/ 3þ 2 and Fe 2p1/2, respectively. The Fe elements in the samples existed mainly in the þ3 oxidation state(Fe3þ). The Fe3þ ions could be incorporated into the TiO2 lattice to form the TieOeFe bonds present in the Fe(x%)-mTiO2 samples treated by calcinations. This result coincides with the Fe2TiO5 shown in XRD date. In Fig. 3(C), we can see that, both Ti2p1/2 and Ti2p3/2 spin-orbital splitting photoelectrons were located at 458.0 eV and 464.0 eV, indicating the presence of Ti4þ. The examination of the hydrogen reduction behavior of our catalysts by TPR-H2 measurements provided additional information about chemical composition of the sample and surface properties acquired during its preparation. H2-TPR curves of our samples are shown in Fig. 4. The reduction of
Fig. 5 e SEM images of different molar ratio of Fe to Ti (a) (b) Fe(5%)-mTiO2; (c) (d)Fe(10%)-mTiO2; (e) (f)Fe(15%)-mTiO2; (i) (j) Fe(30%) -mTiO2.
Fig. 6 e Effect of Fe content on the Photo-thermal FischereTropsch reaction over mTiO2. (A) CH4 selectivity, (B) C2H4 selectivity, (C) C2H6 selectivity, (D) C3H6 selectivity, (E) C3H8 selectivity, (F) C4H8 selectivity, (G) C4H10 selectivity, (H) CO conversion, and (I) Carbon efficiency.
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iron species in hydrogen was postulated as a three-step mechanism 3Fe2O3 / 2Fe3O4 / 6FeO / 6Fe at the temperature range 300e600 C [22]. We can attribute the hydrogen consumption observed at the temperature range 300e400 C to the first stage of reduction of hematite to magnetite: Fe2O3 / Fe3O4, and the next broad peak at 500e600 C to the two-step magnetite reduction to metallic iron. The appearance of wu¨stite phase(FeO), as an intermediate of hematite reduction in hydrogen was experimentally confirmed at a temperature above 570 C [22]. With the Fe content doping increase, the reduction temperature of catalysts are shifted towards a higher temperature. It may be suggested that the calcined catalyst possess fine particles and large aggregates containing non-uniformly distributed iron oxides. Fig. 5 shows SEM images of synthesized samples after calcined for 2 h at 450 C and 550 C, respectively. From Fig. 5, we can see that the agglomeration of synthesized composite powders was observed which was ascribed to Fe(III) deposits. Analysis of SEM images (see Fig. 5a, b,c,d,e and f) shows that 5%, 10% and 15% Fe-doped mTiO2 catalyst contains a higher number of irregular shaped particles. The particles didn't possess any definite shape. The images showed large aggregates in addition to small particles dispersed on them. The SEM images of Fe(20%)-mTiO2 and Fe(30%)-mTiO2 are shown in Fig. 5(g,h) and (i,j) respectively. They also indicated tiny crystallites on large agglomerates. Both the tiny crystallites and large aggregates were assigned to Fe2TiO5 which is in support of the XRD pattern that revealed the formation of Fe2TiO5 phase. Fig. 6 shows the effect of the Fe amount incorporated into the titanium dioxide framework. Under the setting conditions, the series catalysts are unstable to CH4 selectivity. As to the reason for this phenomenon, we can't figure out clearly at the moment, and we will do further research in the future. When the molar ratio of Fe to Ti is about 22%, the selectivity of C2H4 is very well, and reaches about 20%. It is generally high for C2H6 selectivity for all catalyst samples, and the selectivity is all above 40%. With the molar ratio of Fe to Ti is about 20%, it displays the best selectivity to C2H6. While the series catalysts are unstable to the selectivity of C3 products, and it didn't presents the regularity in the relationship between the iron content and the C3 selectivity at current research stage. Fig. 6(H) presents the CO conversion of different Fe content catalysts. It can be seen that the CO conversion are in the range of 75%e85% at this condition, and the maximum conversion of CO for samples with molar ratio of Fe to Ti is 15%. In Fig. 6(I) we can see the carbon efficiency is above 60% at this condition, and the maximum efficiency of carbon for catalysts with molar ratio of Fe to Ti is 20%.
Conclusions Mesostructured titania (mTiO2) with well surface area and crystalline framework has been synthesized by sol-thermal synthesis method. The different content iron-based catalysts with mesostructured titania as carrier were prepared by impregnation method. The prepared catalysts, which are mainly Fe2TiO5 phase identified by XRD, were evaluated via Photo-thermal F-T synthesis. As a whole, Fe(20%)-mTiO2
sample is the best catalyst in all samples, and for this catalyst, it has a higher selectivity than others, particularly for C2H4, C2H6, C4H8 and C4H10. Moreover, Fe(20%)-mTiO2 has a high CO conversion in the range of 75%e85%. Up till the present moment, due to there are less research on Photo-thermal F-T. The operation condition and the reaction mechanism of Photo-thermal F-T are still not very clear, and more detailed researches are under progress.
Acknowledgments The authors are grateful to the financial aid from the Specialized Research Fund for the Doctoral Program of Higher Education (Grant Nos. 20131501110001), the National Natural Science Foundation of China (Grant Nos. 20961006), and Inner Mongolia Natural Science Foundation (Grant Nos. 2014MS0206).
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