Silica Modified Alumina As Supports of Fe2O3 with High Performance

Subscriber access provided by University of South Dakota

Article

Silica modified alumina as supports of Fe2O3 with high performance in chemical looping combustion of methane Yu Kang, Ming Tian, Yuehan Wang, Yintong Wang, Chuande Huang, Yanyan Zhu, Lin Li, Guijin Wang, and Xiaodong Wang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02262 • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on September 3, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Silica modified alumina as supports of Fe2O3 with high performance in chemical looping combustion of methane Yu Kang,†,‡ Ming Tian,*,† Yuehan Wang,† Yintong Wang,† Chuande Huang,† Yanyan Zhu,§ Lin Li,† Guijin Wang,† Xiaodong Wang*,† †

Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457

Zhongshan road, Dalian 116023 (P. R. China). ‡

University of Chinese Academy of Sciences, No.19(A) Yuquan Road,

Shijingshan District, Beijing 100049 (P. R. China). §

College of Chemical Engineering, Northwest University, 229 Taibaibei Road,

Xi’an 710069 (P. R. China). *

Corresponding author: E-mail: [email protected]; [email protected].

1

ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Fe2O3/Al2O3 oxygen carriers (OCs) are considered to be promising due to their high reactivity in chemical looping combustion (CLC). However, iron species and supports suffered from severe sintering, leading to the deactivation of this OC during multiple redox reaction. In this work, a series of silica modified alumina were used as supports of Fe2O3 and found that the OC with the addition of 5% Si (Si-5) exhibited the highest performance for CLC of CH4 during 60 redox cycles. Si was discovered to be incorporated into Al rich coordination environment and the formed Si-O-Al structure inhibited phase transformation of γ-Al2O3 to α-Al2O3, which stabilized the support with high specific surface area. This led to the best dispersion of iron oxides during reaction (77%), confirmed by the formation of the highly dispersed ε-Fe2O3 phase identified in the CLC of CH4, which resulted in the largest amount of active FeAl2O4 (47%) in the reduction step. Key words: Chemical looping combustion; Conversion of CH4; Fe2O3/Al2O3 oxygen carrier; Dispersion of iron oxides; FeAl2O4;

2


Page 2 of 26

Page 3 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction Shale gas as a new unconventional energy source has attracted much attention in the world during recent decades due to its large recoverable reserves. Recent advances in exploration and drilling technologies make it more available in energy market.1-3 The utilization of shale gas to produce heat or electricity is accompanied by carbon dioxide (CO2) emission, one kind of greenhouse gas that is harmful to global climate. However, CO2 now gradually grows into an important chemical material in the clean energy era, which acts as a primary carbon source for hydrocarbon production driven by solar energy.4,5 Traditional combustion process of shale gas makes it difficult for CO2 capture due to the low concentration in the tail gas and thus high cost for gas separation.6,7 Therefore, a novel technique for CO2 capture with low energy penalty is required for further CO2 utilization.8 Chemical looping combustion (CLC) as a new alternative, has the potential to achieve CO2 capture with low cost and high energy utilization efficiency.9,10 In general, CLC consists of a two-step process. In the reduction period, metal oxides act as oxygen carriers (OCs) to provide lattice oxygen to combust fossil fuels such as shale gas. The produced CO2 and H2O could be then easily separated by condensation of water without much energy input. The reduced metal oxides restore to original state in the oxidization process by contacting with air.11,12 To achieve effective CO2 capture in CLC process, the selection of suitable OCs with high reactivity and stability is of significant importance.13,14 Typically, transition metal oxides are used as OCs during recent decades in CLC.15-21 Among them, Fe-based oxygen carriers are widely investigated due to the low cost, good mechanical strength and environmental compatibility.22 However, the deeper reduction of Fe2O3 beyond Fe3O4 would lead to the significant decrease of CO2 selectivity.23 Moreover, the re-oxidation of Fe or FeO if produced during reduction step to Fe2O3 will lead to the agglomeration of Fe2O3. Hence, only the transformation from Fe2O3 to Fe3O4 rather than FeO or Fe for pure Fe2O3 can be applicable in CLC process leading to the low oxygen transfer capacity (OTC).24 3



Various supports such as Al2O3, SiO2, MgAl2O4, TiO2, ZrO2, CeO2, YSZ, etc. have been found to be able to improve the dispersion of Fe2O3 thus the performance of iron oxides,25-29 and Fe2O3/Al2O3 OCs were the most reported.30-33 In chemical looping production of hydrogen, researchers proposed that the addition of Al2O3 would lead to the formation of FeAl2O4 which hindered the kinetic of the deeper reduction of FeO to Fe. In addition, weak oxidants of water (H2O) have to be used in order to produce hydrogen in the re-oxidation step which cannot completely oxidize FeAl2O4 to Fe2O3, leading to the decrease of kinetics and oxygen capacity thus the deactivation of Fe2O3/Al2O3.34 On the contrary, it was found in CLC of CH4 that the formation of FeAl2O4 (corresponding to Fe2+) could increase the OTC of Fe2O3/Al2O3 compared with pure Fe2O3 since iron oxide could be reduced beyond Fe3O4 (Fe2.67+).28,35 However, alumina support sintered severely during cyclic redox reaction under high temperature, which failed to support high loadings of Fe2O3 (usually higher than 30 wt%). As a result, the sintering and breakage of Fe2O3 occurred just as the case of pure Fe2O3. Obviously, both the sintering of Fe2O3 and Al2O3 would cause their interaction becoming weak, which is unfavorable for the formation of FeAl2O4 in the reduction step, leading to the decrease of OTC and thus the deactivation of OC during multiple redox cycles.36,37 It can be concluded that improving the sintering-resistance of Fe2O3 and supports during CLC conditions is highly desired for the supported Fe-based OCs. In order to alleviate the aggregation of iron species and supports and improve the performance of OCs, some dopants like rare earth metal, alkali and alkaline earth metal were doped into supports by some research groups. For instance, Gayán et al. found that the addition of Mg or Ca oxides in the γ-Al2O3 support could improve the redox stability of OCs by the formation of MgAl2O4 or CaAl2O4.38 As for the stability of iron species, it was reported that the dopants in supported Fe-based OCs could inhibit the phase separation of iron species and supports, leading to the higher redox stability of the OCs during reaction. Zachariah et al. incorporated alkali metal into Fe2O3/Al2O3 and revealed small amount of dopants could alleviate the sintering of iron oxides by inhibiting phase separation of iron oxide and alumina.39 Our group also 4


Page 4 of 26

Page 5 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


found that the addition of La led to the formation of LaAl12O19 hexaaluminates, which acted as a binder to inhibit the sintering and breakage of Fe2O3 very recently.40 In addition, the introduction of the dopants in the Fe-based OCs could enhance the mobility of the lattice oxygen, which prevented the outwards diffusion of iron species during the oxidation step, thus the agglomeration of Fe2O3. Bhavsar et al. demonstrated that the incorporation of La strongly improved the stability of Fe2O3/CeO2 OC during redox cycles, which was due to the creation of oxygen defects by the addition of La stabilizing the supported Fe particles and facilitating oxygen transport in the ceria lattice.41 Up to now, however, only rare earth metal and alkali (earth) metal as dopants were studied, and it is still highly desired to obtain a Fe-based OCs with high dispersion of iron species. In the present work, we introduced a series of Si-modified alumina and acted as supports of iron oxides to improve their dispersion in CLC process. It was found that the OC with the addition of 5% Si (Si-5) exhibited the highest performance with the oxygen converted (Ot) of 1.65 mmol/g and CO2 selectivity of 96% for CLC of CH4 after 60 redox cycles. This resulted from the best dispersion of iron oxides during reaction (77%), evidenced by the formation of the highly dispersed ε-Fe2O3 firstly identified in the CLC of CH4, which was favorable for the interaction between iron oxides and supports with high specific surface area thereby the largest amount of FeAl2O4 formed in the reduction step (47%).

Experiments Preparation of OCs Preparation of supports The pure Al2O3 and silica modified Al2O3 were prepared by sol-gel method with inorganic salt AlCl3 ·6H2O and tetraethoxysilane (TEOS) as Al and Si sources separately.42 The amounts Al and Si precursors are calculated based on the weight percentage of SiO2 in the supports. Taking 2 wt% silica modified Al2O3 as an example, 19.32 g AlCl3·6H2O, 70 ml C2H5OH, 72 ml H2O and 310 ml TEOS were mixed 5



together and stirred at 50 oC for 90 min. Then propylene oxide (PO) was dropped into the solution, followed by stirring for 10 min. The solution was allowed to gel within 3h. After aged for about 1 day, the wet gel was then transferred to a vacuum drying oven at 80 oC for 2 days. Finally the support was obtained by calcination at 1100 oC for 4 h.

Preparation of 40 wt% Fe2O3/silica doped Al2O3 Impregnation method was used to prepare 40 wt% Fe2O3/silica doped Al2O3. The mole of Fe(NO3)3·9H2O was derived from the calculation based on 40 wt% of Fe2O3 in the final OCs. Typically, 1 g of the support obtained above, 3.6667g of Fe(NO3)3·9H2O and deionized water with proper amount were mixed and stirred for 2 h, followed by bathing in water at 333 K for several hours until the water was evaporated completely. The sample was then placed into an oven at 393 K overnight. After calcined at 1173 K for 4 h, the 40 wt% Fe2O3/silica doped Al2O3 OCs (denoted as Si-x, x = 0, 2, 5, x indicates the mass fraction of silica) were obtained.

Reactivity tests The reactivity tests of materials were carried out by using a quartz fixed bed reactor system under atmospheric pressure at 1173 K. 0.2 g materials were loaded in the quartz reactor. The reduction step was performed in 5% methane balance helium with a flow rate of 15 mL/min for 3.5 min. 5% oxygen balance helium was used as oxidant medium with the same flow rate for 9 min. Between reduction and oxidation period, the reactor was flushed with helium for 5 min. The gas lines were designed as short as possible to reduce the dead volume. A five-way solenoid valve was used to shift gases automatically to run a long term cycle reaction. The gas products were measured by quadrupole mass spectrometer (MS) (IPI, GAM200). The MS signals were calibrated before each experiment by using four calibration gases with certain concentration balanced with helium. The detailed calculations of Ot, CO2 selectivity and CH4 conversion are provided in supporting information. 6


Page 6 of 26

Page 7 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Kinetic curves of different cycle for Si-0, Si-2 and Si-5 oxygen carriers.

Results and discussion Reactivity and stability test Figure 1 presents the kinetic curves of the 5th, 30th and 60th reduction process as well as the reaction rate represented by the slope of the curves in the last reduction with time on stream over Si-0, Si-2 and Si-5 OCs. In the 5th reduction step, all the samples behaved similarly with nearly the same rate of oxygen converted (Ot) (Figure 1a). After 30 cycles, the release rate of lattice oxygen for Si-0 was lower than that for Si-2 and Si-5 (Figure 1b). Their difference was more prominent in the 60th reduction (Figure 1c) wherein the maximum reaction rate for Si-5 reached 0.01 mmol/g·s, higher than that for Si-2 (0.0086 mmol/g·s) and Si-0 (0.0082 mmol/g·s) (Figure 1d). Moreover, the reaction rate for Si-5 was always larger than that for Si-0 after reaction time of 25 s. Obviously, Si-5 OC showed the highest reactivity after 60 cycles. Figure 2 and Table 1 compare the stability of the three prepared samples quantitatively. The active lattice oxygen of Si-0 available for CH4 combustion 7



Page 8 of 26

decreased significantly with the increasing number of cycles (Figure 2a), which dropped by as much as about 36% from 1.76 to 1.13 mmol/g (Table 1). The incorporation of silica could remarkably improve the stability of OCs. That is, the Ot of Si-2 dropped from 1.73 mmol/g in the 5th reduction to 1.55mmol/g in the last reduction, decreased by 10% while Si-5 behaved the most stable with a slight reduction of Ot by only 6%. These results implied that Si-5 exhibited the best redox stability. CH4 conversion and CO2 selectivity during 60 redox cycles were shown in Figure 2(b and c). It could be found that the CH4 conversion of Si-0 encountered a significant reduction from 82% in the 5th cycle to 53% in the last reduction (Figure 2b), decreased by almost 30%. In contrast, the conversion of CH4 for Si-2 dropped from 79% to 71% while Si-5 gave CH4 conversion from 80% to 75%, which was more stable than Si-0 and Si-2. The CO2 selectivity of these three samples was relatively stable (Figure 2c). However, Si-doped OCs showed slightly higher CO2 selectivity than Si-0 in the 60th cycle, indicating that the addition of silica could enhance the performance of OCs and Si-5 was the best candidate due to its better reactivity and redox stability than Si-0 and Si-2 for CLC of CH4. Table 1. Oxygen converted of OCs in the 5th and 60th cycle.

Oxygen transfer capacity (Ot) (mmol/g)

Decrease

5th cycle

60th cycle

Si-0

1.76

1.13

36%

Si-2

1.73

1.55

10%

Si-5

1.75

1.65

6%

Figure 2. Oxygen converted (Ot) (a), CH4 conversion (b) and CO2 selectivity (c) of Si-0, Si-2 and Si-5 OCs versus cycle numbers. 8


Page 9 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Characterizations of OCs BET characterization The textural properties of Si-0, Si-2 and Si-5 before and after 60 cycles are presented in Figure 3. It could be found that all the OCs presented a typical type IV isotherm with a hysteresis loop over a pressure range of 0.5 < P/Po < 1 before and after reaction, indicating mesoporous structure of materials. For fresh samples (Figure 3a), Si-2 and Si-5 with the pore size range of 5-25 nm possessed higher total pore volume than Si-0 did. After 60 cycles, the peak of pore size distribution for Si-0 almost completely disappeared (Figure 3b) and the specific surface area decreased from 22 m2/g before reaction to 6 m2/g after reaction (Table 2), suggesting the serious sintering of Si-0 after 60 cycles. In contrast, Si-2 and Si-5 OCs showed still relatively high total pore volume and a centered pore-size distribution, suggesting that the addition of Si significantly improved the textural properties of OCs during redox cycles. Although the specific surface area of all three OCs decreased after 60 redox cycles, silica modified Al2O3 samples still possessed larger specific surface area than Si-0 did and the specific surface area of Si-5 (37 m2/g) was almost 6 times of that of Si-0 (6 m2/g). Compared with those Fe2O3/Al2O3 prepared by co-precipitation or solution combustion in previous work, the specific surface area of Si-5 in this work was almost one order of magnitude larger than those after reaction. Such a large specific surface area could greatly improve the dispersion of iron oxides during CLC of CH4.

9



Page 10 of 26

Figure 3. Nitrogen adsorption-desorption isotherms, pore size distributions of fresh oxygen

carriers (a) and oxygen carriers after 60 cycles (b).

Table 2. The specific surface areas and crystal sizes of OCs.

Specific Surface Area (m2/g) Before After Reaction Reaction

Crystal Sizes (nm) Before Reaction

After 60 cycles Reaction

α-Fe2O3

α-Al2O3

α-Fe2O3

α-Al2O3

Si-0

22

6

49

36

55

49

Si-2

52

24

46

46

46

40

Si-5

79

37

36

-

-

-

XRD characterization Figure 4a shows the X-ray diffraction patterns of Si-0, Si-2 and Si-5 before and after reaction. For fresh Si-0, the peaks of α-Fe2O3 (JCPDS 01-084-0310) and α-Al2O3 (JCPDS 01-075-1865) were identified with the average crystal size of 49 nm and 36 nm (Table 2), respectively (calculated from Scherrer equation). After 60 cycles, the diffraction peaks of both the α-Fe2O3 and α-Al2O3 increased with the average crystal size of 55 nm and 49 nm, respectively, larger than those of fresh Si-0, indicating that the sintering of α-Fe2O3 and α-Al2O3 in Si-0 occurred after 60 cycles. For Si-2, α-Fe2O3 and α-Al2O3 were still observed with the similar average crystal size of 46 nm (Table 2) before reaction. After 60 cycles, in spite of slight increase of the diffraction peaks of α-Al2O3, the crystal sizes of α-Fe2O3 and α-Al2O3 were remarkably smaller than those of Si-0 with the crystal size of 46 nm and 40 nm, 10


Page 11 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


respectively, indicating that the addition of 2 wt% Si could alleviate the sintering of both Fe2O3 and the support. For fresh Si-5, the diffraction peaks of α-Fe2O3 and γ-Al2O3 (JCPDS 79-1456) instead of α-Al2O3 were observed since Si could inhibit the phase transformation of γ-Al2O3 to α-Al2O3 at high temperature.43-46 After 60 cycles, γ-Al2O3 was still observed accompanied with trace amount of α-Al2O3, indicating that the incorporation of 5 wt% Si could stabilize γ-Al2O3 and alleviate the sintering of the support during multiple redox cycles. It was worthwhile to note that a new phase attributed to ε-Fe2O3 (JCPDS 00-052-1449) was detected which was firstly identified during CLC of CH4.

Figure 4. (a) XRD patterns of OCs before reaction (-BR) and after 60 redox cycles (-AR). XRD

patterns of Si-0 (b), Si-2 (c) and Si-5 (d) after the re-oxidation time of (1) 0 min, (2) 1 min, (3) 3 min, (4) 5 min and (5) 9 min in the 5th redox cycle.

11



Page 12 of 26

In order to investigate the formation of ε-Fe2O3, X-ray diffraction patterns of all these three samples after different re-oxidation time in the 5th redox cycle were given in Figure 4(b, c, d). Those after the 5th reduction were also presented as reference samples. For Si-0, the oxidation for 1 min led to the decrease of Fe3O4 and FeAl2O4 and the appearance of α-Fe2O3, indicating that part of iron oxides in the lower state were oxidized. With the increase of oxidation time, α-Fe2O3 further increased and Si-0 was completely oxidized after oxidation for 9 min. The similar behaviors for Si-2 and Si-5 during the oxidation for 3 min were also observed. However, after the oxidation for 5 min, the conversion of Fe3O4 to ε-Fe2O3 besides α-Fe2O3 was observed, which was different from the case of Si-0 wherein Fe3O4 was only converted to α-Fe2O3 during the whole re-oxidation process. Generally, ε-Fe2O3 was an intermediate phase during the heat treatment of maghemite (γ-Fe2O3) to hematite (α-Fe2O3) and its appearance

suggested

low

agglomeration

of

iron

species.47,48

Our

BET

characterization results indicated that the Si-added OCs possessed higher specific surface area than Si-0 did (Table 2), which would significantly improve the dispersion of iron oxides during reaction. It could be inferred that the high dispersion of iron oxides played a key role in the formation of ε-Fe2O3. Moreover, the ε-Fe2O3 phase was only observed in Si-5 after 60 cycles (Figure 4a), indicating that iron oxides in Si-5 might exhibit the highest dispersion during redox reaction. The XRD results presented that both the Fe2O3 and support in Si-0 sintered severely during CLC of CH4. The addition of Si could alleviate the sintering of Al2O3 and Fe2O3, resulting in their smaller crystal size after 60 cycles. In particular, the incorporation of 5 wt% Si in the OC inhibited the phase transformation of γ-Al2O3 to α-Al2O3 accompanied by the formation of ε-Fe2O3, revealing that iron oxides in Si-5 might show the best dispersion during cyclic reaction.

SEM and TEM characterizations SEM images reveal the morphologies of the three materials before and after reaction, as displayed in Figure 5(a-f). Most particles with several hundred nanometer were observed accompanied with some small nanoparticles in Si-0 before reaction. 12


Page 13 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


After 60 cycles, these small ones disappeared, indicating serious sintering of Si-0. For fresh Si-2 and Si-5, it could be found that the particles with smaller sizes of less than 100 nm were observed. Though slight aggregations were observed after reaction, large amount of small nanoparticles still existed, particularly for Si-5, corresponding to the results of XRD and BET characterizations. TEM characterization with EDS-mapping was then conducted to identify the dispersion of Fe2O3 and Al2O3. The particle size decreased with the addition of Si in Figure 5(g-l). Large particles of iron species (green parts) and alumina (red parts) in Si-0 before reaction were observed (Figure S1). After 60 cycles, particles of alumina aggregated into larger clusters with particle size of hundreds of nanometer, indicating that Fe2O3 and Al2O3 in Si-0 showed poor dispersion. For Si incorporated OCs, the iron oxides and supports particles were well-distributed before reaction. Even after 60 cycles, these particles with uniform and small size could be also observed, particularly for Si-5, suggesting that Fe2O3 and support dispersed well in this OC during redox cycles.

13



Figure 5. SEM and TEM images of Si-0 (a, g), Si-2 (c, i), Si-5 (e, k) before reaction and Si-0 (b,

h), Si-2 (d, j), Si-5 (f, l) after 60 cycles of reaction.

57

Fe Mössbauer spectroscopy 57

Fe Mössbauer spectroscopy characterization was conducted to further confirm the

existence of ε-Fe2O3 and quantify the content of iron oxides in OCs. The results of Si-0, Si-2 and Si-5 after 60 cycles are shown in Figure 6 and the corresponding Mössbauer parameters are given in Table 3. For Si-0, a magnetic sextet and a superparamagnetic doublet were observed with relative area (A) of 81% and 19%, which were assigned to hematite (with large crystal size, α-Fe2O3-L) and superparamagnetic α-Fe2O3 (with smaller crystal size, α-Fe2O3-S), respectively.47,49 These two interactions were still detected for Si-2 sample but the A of α-Fe2O3-S increased to 32% while that of α-Fe2O3-L decreased to 68%, indicating that the addition of Si could significantly promote the dispersion of α-Fe2O3 during redox 14


Page 14 of 26

Page 15 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


cycles. Further increasing Si content to 5 wt% led to the A of α-Fe2O3-S reaching 27% while that of α-Fe2O3-L decreasing to as low as 23%, almost three and four times lower than those in Si-2 (68%) and Si-0 (81%) samples. It was also noted that besides the two species discussed above, three new sextets with the A of 50% were observed for Si-5 after 60 redox cycles, which were ascribed to the ε-Fe2O3 phase.50,51 Combined with the SEM and TEM (Figure 5f and 5l) results that iron oxide in Si-5 exhibited good dispersion after 60 cycles, the ε-Fe2O3 should be also attributed to highly dispersed iron oxides.47,48 Thus, iron oxides in Si-5 sample exhibited the highest dispersion (77% (27% α-Fe2O3-S + 50% ε-Fe2O3) vs. 32% α-Fe2O3-S for Si-2 and 19% α-Fe2O3-S for Si-0) after 60 cycles.

57

Fe Mössbauer spectroscopy results

exclusively confirmed that the addition of Si significantly improved the dispersion of Fe2O3 and the incorporation of 5 wt% Si led to the highest dispersion of iron oxides (77%) after 60 redox cycles.

Table 3. Room temperature 57Fe Mössbauer parameters of Si-0, Si-2 and Si-5 after the 60th oxidation.

Samples

Si-0

Si-2

Si-5

1

IS1

QS2

H3

A4

mm/s

mm/s

T

%

0.36

-0.20

50.5

81

Fe3+ in α-Fe2O3

0.30

0.53

-

19

Superparamagnetic α-Fe2O3

0.37

-0.20

50.5

68

Fe3+ in α-Fe2O3

0.27

0.72

-

32


0.36

-0.17

50.3

23

Fe3+ in α-Fe2O3

0.27

0.87

-

27


0.38

-0.28

41.8

25

ε-Fe2O3 (Fe1, Fe2)

0.36

-0.10

36.4

14

ε-Fe2O3 (Fe3)

0.24

-0.26

26.3

11

ε-Fe2O3 (Fe4)

-

-

-

50 (in total)

ε-Fe2O3

Assignment

Isomer shift; 2 Quadrupole splitting; 3 Hyperfine field; 4 Relative area

15



Figure 6. Room temperature 57Fe Mössbauer spectra of Si-0 (a), Si-2 (b) and Si-5 (c) after the 60th oxidation.

Discussion The role of silicon It was found that silica could stabilize Al2O3 with relatively high surface area and keep the OC stable during a long term cyclic reaction. To further understand the structure of Al2O3 and the role of Si in the supports, 27Al and 29Si MAS NMR of pure supports were conducted.

27

Al chemical shifts is closely related to aluminum

coordination. The two peaks displayed at 9-12 ppm and 64-80 ppm were ascribed to Al site in octahedral (VI) and tetrahedral (IV) coordination (Figure 7a).52 The probable content of certain Al site was derived from integration of corresponding peaks. For Si-0, high ratio of Al (VI) : Al (IV) = 6.14 indicated a large amount of α-Al2O3 formation since only octahedral coordination site exists in it.53 The content of Al (IV) increased with the addition of Si, leading to generation of γ-Al2O3 with spinel structure, in which tetrahedral and octahedral site coexists.53 The chemical 16


Page 16 of 26

Page 17 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


coordination of Si in 5 wt% Si/Al2O3 support was derived from

29

Si MAS NMR

spectra (Figure 7b). There was no peak at -107 ppm, assigning to the site with four silicon (4Si) in the coordination sphere, suggesting the absence of SiO2.54 The peak shifts at 80 and 90 ppm were ascribed to Si (4Al) and Si (3Al), respectively,55 indicating that Si incorporated into Al rich environment. Thus, the formation of Si-O-Al structure inhibited the transformation of γ-Al2O3 to α-Al2O3. As a consequence, much stable OC with high surface area was obtained in CLC process.

Figure 7. 27Al MAS NMR spectra for pure supports of OCs (a) and 29Si MAS NMR spectra of 5% Si doped Al2O3 (b).

The effect of FeAl2O4 For pure iron oxide, only reduction of Fe2O3 to Fe3O4 can be applicable for CLC of CH4 since further reduction to FeO or Fe would result in the significant drop of CO2 selectivity, and the re-oxidation of FeO or Fe to Fe2O3 could lead to the sintering and agglomeration of Fe2O3. The addition of Al2O3 support could enhance the oxygen transfer capacity (OTC) due to the formation of FeAl2O4 (reduction beyond Fe3O4 to Fe2+) in the reduction step for CLC of CH4.35,56,57 In addition, thermodynamic 17



Page 18 of 26

calculations also demonstrated that the formation of FeAl2O4 led to the remarkable decrease in Gibbs free energy of reaction between CH4 and Fe2O3 to form CO2 and H2O from -236 (equation 1) to -591 kJ/mol (equation 2), indicating that the formation of FeAl2O4 was favorable for the deeper reduction of Fe2O3 to Fe2+, resulting in larger oxygen converted than pure Fe2O3. CH4 (g) + 4Fe2O3 = CO2 (g) + 2H2O (g) + 8FeO

(1) △G = -236 kJ (1173K)

CH4 (g) + 4Fe2O3 + 8Al2O3 = CO2 (g) + 2H2O (g) + 8FeAl2O4

(2)

△G = -591 kJ (1173K) However, the sintering of supports in the OCs usually occurred during redox cycles under high temperature, which failed to provide an efficient surface for the dispersion of iron oxide leading to the sintering and breakage of Fe2O3.39 This made the interaction between iron oxides and Al2O3 weaker during reaction, resulting in the decrease in the formation of FeAl2O4. As a consequence, the supported Fe-based OCs suffered from deactivation during CLC of CH4 due to the decrease of the OTC.

Figure 8. XRD patterns of Si-0, Si-2 and Si-5 after the 60th reduction.

18


Page 19 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Indeed, our Si-0 sample sintered severely during 60 redox cycles (Figure 3a, 5b and 5h) with the specific surface area decreasing from 22 m2/g to only 6 m2/g (Table 2), which could not support 40 wt% Fe2O3 leading to their poor dispersion (19%) (Table 3). Figure 8 presented the X-ray diffraction patterns of Si-0, Si-2 and Si-5 after the 60th reduction. For Si-0, the peaks of Fe3O4 and α-Al2O3 were observed accompanied with the relatively lower intensity of peaks of FeAl2O4, indicating that the sintering of supports and Fe2O3 was unfavorable for the formation of FeAl2O4. For Si-doped OCs, the intensity of peaks of FeAl2O4 significantly increased, confirming that the improved dispersion of iron oxides (32% and 77% for Si-2 and Si-5, Table 3) and support was facile to the formation of FeAl2O4. Although some works reported that FeAl2O4 formed during the reduction step,22,35,56-59 to the best of our knowledge, none of them identified the amount of FeAl2O4 and its quantitative correlation with performance until now, which is important for the design of Fe-based OCs with high reactivity and redox stability for CLC of CH4. This might be due to the lack of an efficient characterization technique. In this work,

57

Fe Mössbauer spectroscopy was

employed to quantify the FeAl2O4 and the results were shown in Figure 9a and Table 4. It could be seen that two sextets and three doublets were observed for all the samples. These two sextets and the two doublets with IS values of 0.85~0.95 mm/s were assigned to magnetite Fe3O4 and Fe2+ ions in the FeAl2O4 while that with IS values of 0.28~0.31 mm/s were attributed to Fe3+ in Al2O3 support lattice.60-62 For Si-0, the relative area (A) of Fe3O4 reached as high as 75% and that of FeAl2O4 was only 15%, indicating that both the sintering of iron oxides and Al2O3 led to their weak interaction, thus the low amount of FeAl2O4 after 60 redox cycles. For Si-doped OCs, the A of FeAl2O4 significantly increased (43% and 47%), about three times of that for Si-0 and the largest amount of FeAl2O4 (47%) was obtained for Si-5, which was owing to the best dispersion of iron oxides in this OC after 60 redox cycles. In order to correlate FeAl2O4 with the reactivity of OCs, the amount of oxygen converted was given in Figure 9b. It could be found that oxygen converted increased with the enhancement of the amount of FeAl2O4, demonstrating that the formation of the larger amount of FeAl2O4 during reaction was responsible for the better performance of 19



Page 20 of 26

Si-doped OCs. The largest amount of FeAl2O4 formed during the reduction step for Si-5, thus it exhibited the best performance with the Ot of 1.65 mmol/g and CO2 selectivity of 96% after 60 redox cycles of CLC of CH4.

Table 4. Room temperature 57 Fe Mössbauer parameters of Si-0, Si-2 and Si-5 after the 60th reduction.

Samples

Si-0

Si-2

Si-5

IS1

QS2

H3

A4

mm/s

mm/s

T

%

0.29

-0.02

48.3

22

Fe3+ in Fe3O4

0.61

-0.03

44.6

53

Fe2+ and Fe3+ in Fe3O4

0.85

1.90

-

9

Fe2+(Oh) in FeAl2O4

0.94

1.01

-

6

Fe2+(Th) in FeAl2O4

0.31

0.51

-

10

Fe3+ in Al2O3

0.28

0.02

47.5

8

Fe3+ in Fe3O4

0.58

-0.06

41.8

35


0.92

1.89

-

12

Fe2+(Oh) in FeAl2O4

0.89

1.30

-

31

Fe2+(Th) in FeAl2O4

0.29

0.61

-

15

Fe3+ in Al2O3

0.30

-0.04

46.0

17

Fe3+ in Fe3O4

0.61

0.03

42.1

26


0.90

2.03

-

11

Fe2+(Oh) in FeAl2O4

0.88

1.36

-

36

Fe2+(Th) in FeAl2O4

0.28

0.54

-

10

Fe3+ in Al2O3

1

Isomer shift; 2 Quadrupole splitting; 3 Hyperfine magnetic field; indicate tetrahedral and octahedral site, respectively.

20


Assignment

4

Relative area; Th and Oh

Page 21 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. (a) Room temperature 57Fe Mössbauer spectra of Si-0 (1), Si-2 (2) and Si-5 (3) after the 60th reduction. (b) Oxygen converted and the amount of FeAl2O4 of OCs after 60 redox cycles.

Conclusion In this work, a series of silica modified alumina were prepared with sol-gel approach and for the first time used as supports of Fe2O3 in chemical looping combustion. 60 cyclic reactivity tests in a fixed bed reactor at 1173 K revealed that silica modified OCs exhibited higher oxygen converted (Ot), CH4 conversion and better redox stability compared with Fe2O3/Al2O3 OCs. The addition of Si inhibited phase transformation of γ-Al2O3 to α-Al2O3 by forming Si-O-Al structure. As a consequence, the obtained support with high specific surface area resulted in the significantly improved dispersion of iron oxides, which was easier to the formation of FeAl2O4 during the reduction step. Iron oxides in Si-5 exhibited the highest dispersion (77%) during 60 redox cycles, confirmed by the formation of ε-Fe2O3 during reaction which was firstly identified in CLC of CH4, leading to the largest amount of FeAl2O4 (47%). This accounted for its best performance with the Ot of 1.65 mmol/g and CO2 selectivity of 96% after 60 cycles. Our results in the present work provide a simple approach to obtain a Fe-based OC with highly dispersed iron species during the CLC of CH4.

Supporting information The supporting information contains all the characterization equipment, test 21



Page 22 of 26

conditions, mass spectroscopy calibration, results calculation methods and EDS element mappings of OCs.

Corresponding authors *

E-mail: [email protected];

*

E-mail: [email protected].

Acknowledgment We are grateful for the financial support provided by the National Science Foundation of China (NSFC) grants (21406225, 21573232 and 21676269) and Postdoctoral Science Foundation of China (2014M561261). We also express gratitude to prof. Xianchun Liu for his effort on MAS NMR characterization.

References [1]

Jia, C.; Zheng, M.; Zhang, Y. Unconventional hydrocarbon resources in China and the prospect of exploration and development. Petroleum Explor. Dev. 2012, 39, 139-146.

[2]

Kinnaman, T. C. The economic impact of shale gas extraction: A review of existing studies. Ecol. Econ. 2011, 70, 1243-1249.

[3]

Weber, C. L.; Clavin, C. Life cycle carbon footprint of shale gas: review of evidence and implications. Environ. Sci. Technol. 2012, 46, 5688-5695.

[4]

Wei, J.; Ge, Q.; Yao, R.; Wen, Z.; Fang, C.; Guo, L.; Xu, H.; Sun, J. Directly converting CO2 into a gasoline fuel. Nat. Commun. 2017, 8, 15174.

[5]

Buelens, L. C.; Galvita, V. V.; Poelman, H.; Detavernier, C.; Marin, G. B. Super-dry reforming of methane intensifies CO2 utilization via Le Chatelier’s principle. Science 2016, 354, 449-452.

[6]

Fan, L. S.; Zeng, L.; Wang, W.; Luo, S. W. Chemical looping processes for CO2 capture and carbonaceous fuel conversion – prospect and opportunity. Energy Environ. Sci. 2012, 5, 7254.

[7]

Huang, J.; Liu, W.; Yang, Y. Phase interactions in Mg-Ni-Al-O oxygen carriers for chemical looping applications. Chem. Eng. J. 2017, 326, 470-476.

[8]

Galvita, V. V.; Poelman, H.; Detavernier, C.; Marin, G. B. Catalyst-assisted chemical looping for CO2 conversion to CO. Appl. Catal., B 2015, 164, 184-191.

[9]

Miya, K.; Otomo, J. Improvements in reaction kinetics and stability of ilmenite as oxygen carrier by surface modification with calcium titanate in redox cycles of chemical-looping systems. Chem. Eng. J. 2017, 327, 257-267.

[10]

2–

Galinsky, N. L.; Huang, Y.; Shafiefarhood, A.; Li, F. Iron oxide with facilitated O transport for facile fuel oxidation and CO2 capture in a chemical looping scheme. ACS Sustainable Chem. Eng. 2013, 1, 364-373. 22


Page 23 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[11]

Chuang, S.; Dennis, J.; Hayhurst, A.; Scott, S. Development and performance of Cu-based oxygen carriers for chemical-looping combustion. Combust. Flame 2008, 154, 109-121.

[12]

Cheng, X.; Li, K.; Wang, H.; Zhu, X.; Wei, Y.; Li, Z.; Zheng, M.; Tian, D. Chemical looping combustion of methane in a large laboratory unit: Model study on the reactivity and effective utilization of typical oxygen carriers. Chem. Eng. J. 2017, 328, 382-396.

[13]

Arjmand, M.; Azad, A.-M.; Leion, H.; Lyngfelt, A.; Mattisson, T. Prospects of Al2O3 and MgAl2O4-supported CuO oxygen carriers in chemical-looping combustion (CLC) and chemical-looping with oxygen uncoupling (CLOU). Energy Fuels 2011, 25, 5493-5502.

[14]

Tian, M.; Wang, X. D.; Liu, X.; Wang, A. Q.; Zhang, T. Fe-substituted Ba-hexaaluminates oxygen carrier for carbon dioxide capture by chemical looping combustion of methane. AlChE J. 2016, 62, 792-801.

[15]

Song, Q. L.; Liu, W.; Bohn, C. D.; Harper, R. N.; Sivaniah, E.; Scott, S. A.; Dennis, J. S. A high performance oxygen storage material for chemical looping processes with CO2 capture. Energy Environ. Sci. 2013, 6, 288-298.

[16]

Donat, F.; Hu, W.; Scott, S. A.; Dennis, J. S. Characteristics of copper-based oxygen carriers supported on calcium aluminates for chemical-looping combustion with oxygen uncoupling (CLOU). Ind. Eng. Chem. Res. 2015, 54, 6713-6723.

[17]

Rydén, M.; Johansson, M.; Lyngfelt, A.; Mattisson, T. NiO supported on Mg–ZrO2 as oxygen carrier for chemical-looping combustion and chemical-looping reforming. Energy Environ. Sci. 2009, 2, 970.

[18]

Readman, J. E.; Olafsen, A. O.; Smith, J. B.; Blom, R. B. Chemical looping combustion using NiO/NiAl2O4: mechanisms and kinetics of reduction oxidation (Red-Ox) reactions from in situ powder X-ray diffraction and thermogravimetry experiments. Energy Fuels 2006, 20, 1382-1387.

[19]

Arjmand, M.; Azad, A.-M.; Leion, H.; Mattisson, T.; Lyngfelt, A. Evaluation of CuAl2O4 as an oxygen carrier in chemical-looping combustion. Ind. Eng. Chem. Res. 2012, 51, 13924-13934.

[20]

Jin, H.; Okamoto, T.; Ishida, M. Development of a novel chemical-looping combustion: synthesis of a solid looping material of NiO/NiAl2O4. Ind. Eng. Chem. Res. 1999, 38, 126-132.

[21]

de Diego, L. F.; Ortiz, M.; Adánez, J.; García-Labiano, F.; Abad, A.; Gayán, P. Synthesis gas generation by chemical-looping reforming in a batch fluidized bed reactor using Ni-based oxygen carriers. Chem. Eng. J. 2008, 144, 289-298.

[22]

Adanez, J.; Abad, A.; Garcia-Labiano, F.; Gayan, P.; de Diego, L. F. Progress in chemical-looping combustion and reforming technologies. Prog. Energy Combust. Sci. 2012, 38, 215-282.

[23]

Li, K.; Wang, H.; Wei, Y.; Yan, D. Direct conversion of methane to synthesis gas using lattice

[24]

Bohn, C. D.; Cleeton, J. P.; Müller, C. R.; Davidson, J. F.; Hayhurst, A. N.; Scott, S. A.; Dennis, J. S.

oxygen of CeO2–Fe2O3 complex oxides. Chem. Eng. J. 2010, 156, 512-518. The kinetics of the reduction of iron oxide by carbon monoxide mixed with carbon dioxide. AlChE J. 2010, 1016-1029. [25]

Azimi, G.; Leion, H.; Mattisson, T.; Rydén, M.; Snijkers, F.; Lyngfelt, A. Mn-Fe oxides with support of MgAl2O4, CeO2, ZrO2 and Y2O3-ZrO2 for chemical-looping combustion and chemical-looping with oxygen uncoupling. Ind. Eng. Chem. Res. 2014, 53, 10358-10365.

[26]

Zafar, Q.; Mattisson, T.; Gevert, B. Redox investigation of some oxides of transition-state metals Ni,Cu, Fe, and Mn supported on SiO2 and MgAl2O4. Energy Fuels 2006, 20, 34-44.

[27]

Galinsky, N. L.; Shafiefarhood, A.; Chen, Y.; Neal, L.; Li, F. Effect of support on redox stability of 23



iron oxide for chemical looping conversion of methane. Appl. Catal., B 2015, 164, 371-379. [28]

Li, F.; Sun, Z.; Luo, S.; Fan, L.-S. Ionic diffusion in the oxidation of iron—effect of support and its implications to chemical looping applications. Energy Environ. Sci. 2011, 4, 876.

[29]

Li, F. X.; Luo, S. W.; Sun, Z. C.; Bao, X. G.; Fan, L. S. Role of metal oxide support in redox reactions of iron oxide for chemical looping applications: experiments and density functional theory calculations. Energy Environ. Sci. 2011, 4, 3661.

[30]

Guo, Q.; Yang, M.; Liu, Y.; Yang, Q.; Zhang, Y. Multicycle investigation of a sol-gel-derived Fe2O3/ATP oxygen carrier for coal chemical looping combustion. AlChE J. 2016, 62, 996-1006.

[31]

Abad, A.; Adánez, J.; García-Labiano, F.; de Diego, L. F.; Gayán, P.; Celaya, J. Mapping of the range of operational conditions for Cu-, Fe-, and Ni-based oxygen carriers in chemical-looping combustion. Chem. Eng. Sci. 2007, 62, 533-549.

[32]

Huang, W. C.; Kuo, Y. L.; Su, P. C.; Tseng, Y. H.; Lee, H. Y.; Ku, Y. Redox performance of Na-modified Fe2O3/Al2O3 with syngas as reducing agent in chemical looping combustion process. Chem. Eng. J. 2018, 334, 2079-2087.

[33]

Deng, G.; Li, K.; Gu, Z.; Zhu, X.; Wei, Y.; Cheng, X.; Wang, H. Synergy effects of combined red muds as oxygen carriers for chemical looping combustion of methane. Chem. Eng. J. 2018, 341, 588-600.

[34]

Liu, W.; Ismail, M.; Dunstan, M. T.; Hu, W.; Zhang, Z.; Fennell, P. S.; Scott, S. A.; Dennis, J. S. Inhibiting the interaction between FeO and Al2O3 during chemical looping production of hydrogen. RSC Adv. 2015, 5, 1759-1771.

[35]

Cabello, A.; Abad, A.; García-Labiano, F.; Gayán, P.; de Diego, L. F.; Adánez, J. Kinetic determination of a highly reactive impregnated Fe2O3/Al2O3 oxygen carrier for use in gas-fueled Chemical Looping Combustion. Chem. Eng. J. 2014, 258, 265-280.

[36]

Rydén, M.; Cleverstam, E.; Johansson, M.; Lyngfelt, A.; Mattisson, T. Fe2O3 on Ce-, Ca-, or Mg-stabilized ZrO2 as oxygen carrier for chemical-looping combustion using NiO as additive. AlChE J. 2010, 2211-2220.

[37]

Zhang, J.; Guo, Q.; Liu, Y.; Cheng, Y. Preparation and characterization of Fe2O3/Al2O3 using the solution combustion approach for chemical looping combustion. Ind. Eng. Chem. Res. 2012, 51, 12773-12781.

[38]

Gayán, P.; de Diego, L. F.; García-Labiano, F.; Adánez, J.; Abad, A.; Dueso, C. Effect of support on reactivity and selectivity of Ni-based oxygen carriers for chemical-looping combustion. Fuel 2008, 87, 2641-2650.

[39]

Liu, L.; Zachariah, M. R. Enhanced performance of alkali metal doped Fe2O3 and Fe2O3/Al2O3 composites as oxygen carrier material in chemical looping combustion. Energy Fuels 2013, 27, 4977-4983.

[40]

Tian, M.; Wang, C.; Li, L.; Wang, X. High performance of La-promoted Fe2O3/α-Al2O3 oxygen carrier for chemical looping combustion. AlChE J. 2017, 63, 2827-2838.

[41]

Bhavsar, S.; Isenberg, N.; More, A.; Veser, G. Lanthana-doped ceria as active support for oxygen carriers in chemical looping combustion. Appl. Energy 2016, 168, 236-247.

[42]

Wu, X.; Shao, G.; Shen, X.; Cui, S.; Wang, L. Novel Al2O3–SiO2 composite aerogels with high specific surface area at elevated temperatures with different alumina/silica molar ratios prepared by a non-alkoxide sol–gel method. RSC Adv. 2016, 66, 5611-5620.

[43]

Mekasuwandumrong, O.; Tantichuwet, P.; Chaisuk, C.; Praserthdam, P. Impact of concentration and Si doping on the properties and phase transformation behavior of 24


Page 24 of 26

Page 25 of 26 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nanocrystalline alumina prepared via solvothermal synthesis. Mater. Chem. Phys. 2008, 107, 208-214. [44]

Wu, X.; Shao, G.; Cui, S.; Wang, L.; Shen, X. Synthesis of a novel Al2O3–SiO2 composite aerogel with high specific surface area at elevated temperatures using inexpensive inorganic salt of aluminum. Ceram. Int. 2016, 42, 874-882.

[45]

Jin, X.-H.; Gao, L.; Guo, J.-K. The structural change of diphasic mullite gel studied by XRD and IR spectrum analysis. J. Eur. Ceram. Soc. 2002, 22, 1307-1311.

[46]

Miller, J. B.; Ko, E. I. A homogeneously dispersed silica dopant for control of the textural and structural evolution of an alumina aerogel. Catal. Today 1998, 43, 51-67.

[47]

Tronc, E.; Chanéac, C.; Jolivet, J. P. Structural and magnetic characterization of ε-Fe2O3. J. Solid State Chem. 1998, 139, 93-104.

[48]

Ding, Y.; Morber, J. R.; Snyder, R. L.; Wang, Z. L. Nanowire structural evolution from Fe3O4 to ε-Fe2O3. Adv. Funct. Mater. 2007, 17, 1172-1178.

[49]

Kundut, T. K.; Mukherjee, M.; Chakravorty, D. Growth of nano-α-Fe2O3 in a titania matrix by the sol-gel route. J. Mater. Sci. 1998, 33, 1759-1763.

[50]

Popovici, M.; Gich, M.; Nizňanský, D. Optimized synthesis of the elusive ε-Fe2O3 phase via sol-gel chemistry. Chem. Mater. 2004, 16, 5542-5548.

[51]

Zboril, R.; Mashlan, M.; Barcova, K.; Vujtek, M. Thermally induced solid-state syntheses of γ-Fe2O3 nanoparticles and their transformation to α-Fe2O3 via ε-Fe2O3. Hyperﬁne Interactions 2002, 139/140, 597-606.

[52]

Hensen, E. J. M.; Poduval, D. G.; Magusin, P. C. M. M.; Coumans, A. E.; Veen, J. A. R. v. Formation of acid sites in amorphous silica-alumina. J. Catal. 2010, 269, 201-218.

[53]

Mardkhe, M. K.; Huang, B.; Bartholomew, C. H.; Alam, T. M.; Woodfield, B. F. Synthesis and characterization of silica doped alumina catalyst support with superior thermal stability and unique pore properties. J. Porous Mater. 2015, 23, 475-487.

[54]

Gonzalez-Carreño, T.; Sobrados, I.; Sanz, J. Formation of mullite and spinel phases from 29

27

SiO2-Al2O3 gels prepared by a spray pyrolysis technique. A Si and Al MAS NMR study. Chem. Mater. 2007, 3694-3703. [55]

Qin, Z.; Shen, B.; Yu, Z.; Deng, F.; Zhao, L.; Zhou, S.; Yuan, D.; Gao, X.; Wang, B.; Zhao, H.; Liu, H. A defect-based strategy for the preparation of mesoporous zeolite Y for high-performance catalytic cracking. J. Catal. 2013, 298, 102-111.

[56]

Cho, P.; Mattisson, T.; Lyngfelt, A. Comparison of iron-, nickel-, copper- and manganese-based oxygen carriers for chemical-looping combustion. Fuel 2004, 83, 1215-1225.

[57]

Moghtaderi, B.; Song, H. Reduction properties of physically mixed metallic oxide oxygen carriers in chemical looping combustion. Energy Fuels 2010, 24, 5359-5368.

[58]

Kidambi, P. R.; Cleeton, J. P. E.; Scott, S. A.; Dennis, J. S.; Bohn, C. D. Interaction of iron oxide with alumina in a composite oxygen carrier during the production of hydrogen by chemical looping. Energy Fuels 2012, 26, 603-617.

[59]

Ishida, M.; Takeshita, K.; Suzuki, K.; Ohba, T. Application of Fe2O3-Al2O3 composite particles as solid looping material of the chemical-loop combustor. Energy Fuels 2005, 19, 2514-2518.

[60]

Jastrzębska, I.; Szczerba, J.; Stoch, P.; Błachowski, A.; Ruebenbauer, K.; Prorok, R.; Śnieżek, E. Crystal structure and Mössbauer study of FeAl2O4. Nukleonika 2015, 60, 47-49.

[61]

Kamzin, A. S. Mössbauer investigations of Fe and Fe3O4 magnetic nanoparticles for hyperthermia applications. Phys. Solid State 2016, 58, 532-539. 25



[62]

Zhu, Y.; Wang, X.; Wang, A.; Wu, G.; Wang, J.; Zhang, T. Identification of the chemical state of Fe in barium hexaaluminate using Rietveld refinement and

57

Fe Mössbauer spectroscopy. J.

Catal. 2011, 283, 149-160.

TOC Graphic

Synopsis A small amount of Si doped into the supports drastically promotes the reactivity and stability of oxygen carriers in chemical looping combustion of methane.

26


Page 26 of 26

Silica Modified Alumina As Supports of Fe2O3 with High Performance

Recommend Documents