CeO2 oxygen carrier via

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Enhanced hydrogen generation for Fe2O3/CeO2 oxygen carrier via rare-earth (Y, Sm, and La) doping in chemical looping process Shiwei Ma, Shiyi Chen, Zhenghao Zhao, Ahsanullah Soomro, Min Zhu, Jun Hu, Mudi Wu, and Wenguo Xiang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02758 • Publication Date (Web): 23 Oct 2018 Downloaded from http://pubs.acs.org on October 29, 2018

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Enhanced hydrogen generation for Fe2O3/CeO2 oxygen carrier via rare-earth (Y, Sm, and La) doping in chemical looping process Shiwei Ma, Shiyi Chen, Zhenghao Zhao, Ahsanullah Soomro, Min Zhu, Jun Hu, Mudi Wu, Wenguo Xiang* Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China ABSTRACT: Fe2O3/CeO2 exhibits desirable redox performance in chemical looping hydrogen generation (CLHG) because of its favorable lattice oxygen conductivity originating from ceria. Meanwhile, rare earths can bring about even more improvement on the reactivity of Fe2O3/CeO2. Herein, we synthesized Fe2O3/CeO2 doped with three rare earths, Y, Sm, and La, respectively by co-precipitation approach. The redox performance and fundamental mechanisms were investigated to study the influence of the rare earth additives on the Fe2O3/CeO2 oxygen carrier for CLHG. It was shown that the Fe2O3/Ce0.8Sm0.2O1.9 demonstrated the highest redox reactivity and the purity of generated hydrogen could be up to 100% (with the detection limit at 0.01% in volume). The reactivity was ranked as Fe2O3/Ce0.8Sm0.2O1.9 > Fe2O3/Ce0.8La0.2O1.9 > Fe2O3/Ce0.8Y0.2O1.9 > Fe2O3/CeO2. The rare earths were incorporated into CeO2 and increased the concentration of oxygen vacancies, promoting the oxygen mobility and reactivity of these oxygen carriers. Specifically, no bleed-out of rare earths from doped CeO2 was observed after redox cycles for Sm and Y, and both rare earths could suppress the outward diffusion of iron cations in the particle and subsequent enrichment on the surface, improving the redox stability of oxygen

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carriers. However, La would bleed out from Ce0.8La0.2O1.9 and generate a stable perovskite LaFeO3, which resulted in the outward migration of iron cations and reduced the quantity of active iron oxides, exerting a detrimental effect on the redox reactivity of Fe2O3/Ce0.8La0.2O1.9. KEY WORDS: Chemical looping; Hydrogen; Fe2O3/CeO2; Rare earth 1. INTRODUCTION Hydrogen is widely used in chemical engineering industry and regarded as the best candidate for the future as well. However, it must be generated from other primary energies.1 Chemical looping hydrogen generation (CLHG) was recently developed, which could convert carbonaceous fuels into highly pure hydrogen.2 It is a three-reactor configuration including a fuel reactor (FR), a steam reactor (SR) and an air reactor (AR), which can be referred to in our previous literature.3 Although Fe2O3 is a satisfactory oxygen carrier for CLHG in terms of kinetics as well as thermodynamics, 4, 5 a support is indispensable for the improvement on its activity and recyclability.6-9 CeO2 has been extensively applied as a catalyst on account of its oxygen storage capacity.10, 11 Though CeO2 is inappropriate to be an oxygen carrier in itself due to the low oxygen transport capacity,12 its potential as a support is encouraging and promising for the promotion of oxygen mobility in oxygen carriers.12-16 Specifically, CeO2 has the capacity to promote the lattice oxygen conductivity, which could impede the carbon deposition 17, 18 and improve the activity of oxygen carrier.12, 16, 19 Therefore, our group studied the CeO2-supported Fe2O3 for CLHG based on its superior performance.3, 20, 21 The results showed that these oxygen carriers exhibited desirable activity and recyclability with hydrogen of high purity generated. It is worthy to mention that doping CeO2 with rare earth can further promote the redox performance


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by improving the lattice oxygen conductivity.22-24 The rare earth cations would increase the oxygen vacancy concentration to keep charge neutrality and nonstoichiometric compensation, which is a key factor to facilitate the oxygen mobility from bulk lattice to surface.25, 26 In addition, the thermal stability of CeO2 can also be enhanced by rare earth doping.27, 28 The oxygen mobility plays a crucial role for the reducibility property of oxygen carriers.14 CeO2 doped by Gd or Sm has been demonstrated to have the highest oxygen mobility in the CeO2-rare earth oxide systems, because of their low association enthalpy between doping cations and oxygen vacancies.29-31 Furthermore, Sm is superior to Gd regarding to the enhancement on the oxygen mobility owing to its higher concentration of oxygen vacancy defects, which could bring about low energetic path for oxygen mobility,29, 32-38 although there are also some controversial findings that the Gd doped CeO2 possesses the best oxygen mobility.39 Anjaneya et al.40 reported that the doping of Sm into CeO2 exhibited the highest ionic conductivity as well as the lowest activation energy. In particular, Yahiro et al.30, 41 claimed that the oxygen mobility of Ce0.8Sm0.2O1.9 was 1.5-2.0 times more than that of Ce0.8Gd0.2O1.9 at 800 °C. La-doped CeO2 system has also been reported for its superior catalytic properties. BuenoLópez27 found that the La-doped CeO2 catalyst still kept satisfactory lattice oxygen conductivity after calcination at 1000 °C. The La additive could promote the thermostability and catalytic activity of CeO2 in catalyzed diesel soot combustion. Katta et al.28 claimed that the Ce-La sample had a higher oxygen storage capacity, better thermostability and soot oxidation reactivity than the Ce-Zr catalyst, which could be ascribed to the abundant oxygen vacancies and lattice defects in Ce-La sample. Balducci et al.42 indicated that La doping facilitated the Ce4+/Ce3+ reduction


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process more effectively, which was a key feature of CeO2 for its oxygen buffering capability, than other trivalent dopants such as Sc, Mn, Y and Gd, owing to the large radius of La3+ beneficial to accommodate the elastic strain caused by Ce3+. For the Y-doped CeO2, He et al.43 found that the doping could bring about more lattice defects and active sites, resulting in better catalytic performance for the CH3SH decomposition. It is worth noting that doping CeO2 with rare earth (such as Y3+,44 Sm3+,26, 36, 45 and La3+ 46) has been extensively investigated for catalytic application like CH4, CO and volatile organic compounds oxidation, and so on. As for the doped CeO2 as supports for active metal oxides in chemical looping process, Bhavsar et al.47 found that La-doped CeO2 could increase the oxygen transfer capacity and improve the activity and recyclability of iron oxygen carrier. Moreover, La doped Fe2O3/Al2O3 can significantly improve the activity and recyclability for the chemical looping combustion (CLC) and partial oxidation of CH4 by CO2 due to the generation of LaHexaaluminate.48, 49 Hedayati et al.50 indicated that the metal oxides with Ce0.9Gd0.1O1.9 as support were more active than those with CeO2 in CLC, demonstrating satisfactory sintering and agglomeration resistance. In addition, the gadolinia-doped ceria (GDC) could clearly increase the rate of Fe2O3 reduction, especially for the step from FeO to Fe, with the oxygen vacancies and oxide anion mobility in GDC enhancing the oxygen removal from iron oxides.14 Though the rare earth doping can considerably improve the oxygen mobility property of oxygen carriers, the oxygen ion conductivity is strongly dependent on the species of rare earths.38, 39, 41

However, few investigations concerning the issue have been conducted related to the

chemical looping process. The main characteristic lies in that the source of the oxygen ion in the


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chemical looping process is mainly the active metal oxide and the doped CeO2 just acts as the support.14, 16, 19 Nevertheless, the direct oxygen source for the doped ceria used in conventional catalyst fields, e.g., three way automotive catalysis, fluid catalytic cracking and solid oxide fuel cells,34, 51 is the CeO2 itself. In this work, the rare earths Y, Sm and La were selected as the dopants for the Fe2O3/CeO2 oxygen carrier, and the rare earth doped Fe2O3/CeO2 was synthesized by co-precipitation approach with the Fe2O3 loading at 60 wt%.20 The dopant concentration in CeO2 were selected as 20 at% since it corresponds approximately to the maximum oxygen conductivity,52-54 and defects interaction and vacancy traps will reduce the oxygen mobility with further increasing dopant concentration.52, 55, 56 Moreover, the redox performance and fundamental mechanisms were investigated to study the influence of the rare earth additives on the Fe2O3/CeO2 oxygen carrier for CLHG. 2. EXPERIMENTAL SECTION 2.1. Oxygen Carriers Preparation. The rare earths doped Fe2O3/CeO2 oxygen carriers were synthesized using co-precipitation approach as we did previously.3, 21 The Y, Sm and La doped Fe2O3/CeO2 oxygen carriers, with the Fe2O3 loading as 60 wt% and the mole ratio of rare earths to CeO2 as 20 at%, were labeled as Fe2O3/Ce0.8Y0.2O1.9, Fe2O3/Ce0.8Sm0.2O1.9 and Fe2O3/Ce0.8La0.2O1.9, respectively. The Fe2O3/CeO2 oxygen carrier with no doping was also prepared for comparison. The size of the obtained particles was 150 ~ 250 μm. 2.2. Oxygen Carriers Characterization. Hydrogen temperature-programmed reduction (H2TPR) was conducted using BELCAT-B (MicrotracBEL Japan, Inc.). A sample of 50 mg was


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reduced in 10% H2/Ar at 30 ml/min, and the temperature range is 100 ~ 950 °C at a rate of 10 °C/min. X-ray powder diffraction (XRD) was performed on Rigaku Ultima IV with Cu-Kα radiation at a rate of 0.1°/s. Scanning electron microscopy (SEM) and Energy Dispersive X-Ray Spectrometer (EDX) were carried out on Ultra Plus (Carl Zeiss AG). Raman spectra was performed on Thermo Fisher DXR with the laser wavelength at 532 nm, and XPS spectra was collected on Thermo Fisher ESCALAB 250 with Al Kα (1486.6 eV) X-ray radiation. 2.3. Experimental Reactor and Procedure. A fluidized bed was applied in the experiment, which was consistent with that in our previous literatures.3, 21 The sample weight was 10 g in each test and the temperature was maintained at 850 °C. Table 1 depicts the experimental procedure. In order to study the carbon deposition and avoid the contamination of H2 by the residual CO or CO2, the reactor was swept with N2 after the reduction stage for ca. 40 min until no CO and CO2 can be detected (the detection limit of the gas analyzer was 0.01% in volume). The steam oxidation stage was terminated after the H2 concentration was less than 1.0%. 2.4. Date Evaluation. The volume flow rate of the off-gas Fout is:

Fout 

FN2 ,in 1  X CO  X CO2  X H2

(1)

Where FN2,in is the flow rate of inlet N2, X is the concentration of corresponding gas. The flow rates of CO2 and H2 are:

FCO2  Fout X CO2

(2)

FH2  Fout X H2

(3)

Then the H2 yield can be defined as:


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t

VH2   FH2

(4)

0

The total yield of CO and CO2 in steam oxidization stage is:



t

VCO x   Fout X CO2  X CO 0



(5)

Then the H2 purity can be calculated by:

H  1 

VCO x

(6)

Fout

2

The conversion rate of oxygen carrier is: ROX 

mox  m mox  mred

(7)

Where mox and mred are masses of the oxygen carrier totally oxidized and reduced, respectively, and m is the instantaneous mass. The weight loss of oxygen carrier in reduction stage is: t

Fout X CO2

0

22.4

m  16

(8)

3. RESULTS AND DISCUSSION 3.1. Oxygen Carriers Characterization. Figure 1 illustrates XRD patterns of the fresh rare earth doped Fe2O3/CeO2 oxygen carriers. Figure 1b and c show the magnified diffraction peaks for the (321) crystal face of Fe2O3 and (220) crystal face of CeO2, respectively. The label B could stand for both CeO2 and Ce-Y/Sm/La solid solutions with cubic fluorite structure. For Fe2O3/CeO2, Fe2O3/Ce0.8Y0.2O1.9 and Fe2O3/Ce0.8Sm0.2O1.9, only CeO2 and Fe2O3 were demonstrated with no diffraction peaks ascribed to Y2O3 and Sm2O3, indicating Y and Sm were totally incorporated into the CeO2 lattice.40, 43 However, the XRD plot shows obvious diffraction peaks of perovskite LaFeO3 for the Fe2O3/Ce0.8La0.2O1.9 sample, which is completely different from those of Fe2O3/Ce0.8Y0.2O1.9


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and Fe2O3/Ce0.8Sm0.2O1.9. The phenomenon suggests the strong interaction between La and Fe, providing the driving force for the La bleed-out, given that La can be totally incorporated into CeO2 even in Ce0.5La0.5O2-δ.57 Furthermore, Bhavsar et al.47 claimed that it is difficult to stabilize La in CeO2 with the presence of Fe2O3 beyond ca. 10 mol% for La in the LaxCe1-xO2-x/2 oxides. Considering that the radii of the host iron and cerium cations (Fe3+=0.78 Å, Ce4+=0.97 Å58) are smaller than those of the doping rare earth cations (Y3+ = 1.03 Å, Sm3+ = 1.08 Å and La3+ = 1.15 Å40, 59), a shift of the diffraction peaks for Fe2O3 and CeO2 to lower 2θ position could be caused. As can be seen Figure 1b, the diffraction peaks shifted slightly to lower 2θ position, indicating the incorporation of rare earth dopants into the Fe2O3 lattice.60 The substitution of Fe with Ce can be excluded for the decomposition of hematite-structured Fe-Ce solid solution at high temperature.61 A minor shift extent was observed for the (321) face of Fe2O3 in the Fe2O3/Ce0.8La0.2O1.9 sample, probably owing to that the La cation was inclined to generate perovskite LaFeO3 rather than the hematite-like solid solution. Moreover, Qin et al.60 found that the impurity phase LaFeO3 could even be formed in 2% La-doped Fe2O3. In Figure 1c, the shift extent for CeO2 increased with the radii of rare earth cations from Y to La via Sm. Specifically, the peak shift for the (220) reflection of CeO2 can hardly be observed due to the similar size of Ce4+(0.97 Å) and Y3+(1.03 Å) cation. In addition, the diffraction peaks of Fe2O3 and CeO2 for rare earth doped Fe2O3/CeO2 oxygen carriers were much shorter and broader than those of Fe2O3/CeO2, as demonstrated in Figure 1b and c. This phenomenon suggests the distortion of Fe2O3 and CeO2 lattice for the doping of rare earth cations, resulting in smaller crystallite sizes, as depicted in Table 2. Table 2 depicts the structural characterizations (originated from XRD data) for the fresh rare


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earth doped Fe2O3/CeO2 oxygen carriers. It can be seen that both the lattice parameters of CeO2 and Fe2O3 increased with the dopants from Y3+ (1.03 Å) to Sm3+ (1.08 Å), and further expansion of the lattice for CeO2 was found with the La3+ (1.15 Å) dopant cation. However, a decrease in the lattice constant for Fe2O3 was detected due to the La bleed-out and subsequent generation of LaFeO3. As to the crystallite size, both that of CeO2 and Fe2O3 in the rare earth doped Fe2O3/CeO2 oxygen carriers were smaller than that of undoped one.40, 43 The Ce-O-Y/Sm/La bonds could prevent the CeO2 crystallite growth.62 On the other hand, the rare earth dopants had a tendency to migrate towards the Fe2O3 surface, inhibiting its crystallite growth.60 Furthermore, both the crystallite sizes of CeO2 and Fe2O3 decreased from Y3+ to Sm3+, however, followed by an increase for La3+. Nevertheless, this variation rule for the crystallite size of CeO2 was contrary to the result from previous literatures.40, 43 It was probably caused by the presence of Fe2O3 in this work, which led to different allocation proportions of rare earth between CeO2 and Fe2O3 in various rare earth doped Fe2O3/CeO2 oxygen carriers, especially for La. Figure 2a displays the Raman spectra of prepared rare earth doped Fe2O3/CeO2 oxygen carriers, and the amplification for the range of 350-550 cm-1 is shown in Figure 2b to illustrate the shift of Raman bands for Fe2O3 and CeO2. The band at 466 cm−1 represents CeO2,63, 64 and six other bands (224, 243, 291, 409, 496, 612 cm-1) corresponds to Fe2O3.65 The band for CeO2 shifted to lower frequencies due to its lattice expansion induced by rare earth doping,43, 66 while this phenomenon for Fe2O3/Ce0.8Y0.2O1.9 can hardly be observed due to the similar size of Ce4+(0.97 Å) and Y3+(1.03 Å) cation. Moreover, the band for CeO2 attenuated and broadened significantly after the addition of rare earth cations, which was reported to be associated with the


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decrease of crystalline size and the appearance of more oxygen vacancies compared with Fe2O3/CeO2.61, 64 The Fe2O3 bands also attenuated after rare earth doping due to the diminution of crystalline size, as clearly reflected by the band at 243 cm-1.61, 63 In addition, the Fe2O3 bands, especially for that of Fe2O3/Ce0.8Sm0.2O1.9, shifted to lower wavenumbers, which was the evidence of the strain or defects on the surface of fresh samples due to the incorporation of rare earths.61 Therefore, the results derived from the Raman spectra agreed well with the XRD data. However, it should be mentioned that two prominent bands at 637 cm−1 and 434 cm−1, corresponding to the perovskite LaFeO3,67 cannot be observed, which indicated that the LaFeO3 should exist mainly in the bulk of particles. Figure 3 illustrates the H2-TPR profile of fresh rare earth doped Fe2O3/CeO2 oxygen carriers. Generally, the reduction of pure Fe2O3 would go through Fe2O3 → Fe3O4→ Fe. However, the interaction between Fe2O3 and CeO2 makes the reduction process to be Fe2O3 → Fe3O4 → FeO → Fe.65, 68 The peaks, α, β and γ, in Figure 3 should mainly correspond to the three stages, and the peak δ could relate to the reduction of bulk lattice oxygen. Moreover, the H2 consumption originating from the reduction of rare earth doped CeO2, including its surface lattice oxygen and the bulk of doped CeO2,26, 43 overlapped with that of Fe2O3 and had an impact on the TPR profile, however, the impact was very limited with respect to that of Fe2O3.12, 69 In Figure 3, the positions of α peak for all oxygen carriers, except Fe2O3/Ce0.8Y0.2O1.9, were almost the same, probably owing to the favorable porous structure of fresh oxygen carriers and the much higher reactivity of the first step Fe2O3 → Fe3O4 than that of latter steps Fe3O4 → FeO → Fe.14, 70 The change rules of β and γ peaks with the rare earth species were identical, and the corresponding temperatures of


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the reduction peaks were ranked as follows: Fe2O3/Ce0.8Sm0.2O1.9 < Fe2O3/Ce0.8La0.2O1.9 < Fe2O3/Ce0.8Y0.2O1.9 < Fe2O3/CeO2. The results suggested that the rare earth doping could improve the reduction reactivity and the promotion effect was tightly related to the rare earth species. The mechanism was that rare earth doping in CeO2 could create oxygen vacancies and enhance the oxygen conductivity from bulk to the surface of particles, resulting in high reduction activity.47, 50 Therefore, the “active” support with oxygen mobility property was remarkably beneficial to the reactivity of oxygen carriers.14 Furthermore, previous investigations40, 59 have demonstrated that the oxygen ionic conductivities of rare earth doped CeO2 followed the sequence: Ce0.8Sm0.2O1.9 > Ce0.8La0.2O1.9 > Ce0.8Y0.2O1.9 > CeO2. They claimed that the dopant size was closely related to the total ionic conductivity and the oxygen mobility increased with the dopant size and reached a maximum for Sm. 3.2. Effect of Rare Earth Doping on H2 Generation. Figure 4 depicts the H2 yield for the rare earth doped Fe2O3/CeO2 oxygen carriers with redox cycles. All of them tended to stabilize from the 6th cycle on, indicating the structural stabilization of these samples. The H2 yield followed the sequence: Fe2O3/Ce0.8Sm0.2O1.9 > Fe2O3/Ce0.8La0.2O1.9 > Fe2O3/Ce0.8Y0.2O1.9 > Fe2O3/CeO2, which was consistent with the H2-TPR results, and the average H2 yield from the 6th to the 12th cycle were 0.885, 0.826, 0.760, and 0.638 L, respectively. Furthermore, the H2 yield for Fe2O3/Ce0.8Sm0.2O1.9 was 38.7% higher than that of Fe2O3/CeO2. The Fe2O3/Ce0.8Sm0.2O1.9 exhibited the highest H2 yield for its most enhancement on oxygen mobility relative to Fe2O3/CeO2, since the Ce0.8Sm0.2O1.9 had the minimal association enthalpy between the dopant cations and oxygen vacancies and the lattice strain was minimal as well, leading to high oxygen


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mobility.40 In particular, the H2 yield for all samples climbed up in the initial cycles, then decreased and gradually stabilized with slight decline in the following cycles, indicating an activation process in the initial redox cycles. Attention should also be paid to the carbon deposition for CLHG. No CO or CO2 could be observed in the generated H2 for all samples with the detection limit at 0.01% in volume, indicating the satisfactory carbon deposition resistance for the rare earth doped Fe2O3/CeO2 oxygen carriers. Furthermore, the Fe2O3/Ce0.8Sm0.2O1.9 was superior to the other oxygen carriers in terms of H2 yield and the redox performance of these samples will be elaborated below. Figure 5 shows the evolution of CO2 concentration in the reduction stage of the 12th cycle. The whole reduction process can be divided into three steps, with ca. 5.0 min and 14.0 min as the time nodes. In the first step, the CO2 concentration of all oxygen carriers obtained their peak values in 5.0 min. The Fe2O3/Ce0.8Sm0.2O1.9 got the highest peak value at 17.8%, while the peak value for Fe2O3/CeO2 was only 13.8%. In the second step, the CO2 concentration for Fe2O3/Ce0.8Sm0.2O1.9 was higher than that of the others. Moreover, the reactivity order was the same with that of H2 yield in the first two steps. In the last step, all the CO2 concentration curves were almost overlapped with each other. Specifically, the CO2 concentrations for the rare earth doped oxygen carriers were no higher than that of Fe2O3/CeO2 in 15 ~ 22 min, and then, however, exceeded it afterwards. This could be ascribed to the higher reduction extent and excellent oxygen mobility property of the rare earth doped oxygen carriers, respectively.40, 59 In addition, a large number of oxygen vacancies could be generated in the last step due to the reduction of the supports for all oxygen carriers, and so the oxide ion conductivity would increase.71 This


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phenomenon was beneficial to narrow the gaps among the CO2 concentrations of these samples, especially for Fe2O3/CeO2, due to its extremely low oxygen vacancy concentration in the initial reduction stage.14 Moreover, the CeO2 and rare earth doped ones also had electronic conductivity in reducing atmosphere due to the variation of Ce valence,72, 73 which could contribute the reduction of FeO to Fe.14 In conclusion, the rare earth doping into the Fe2O3/CeO2 can greatly increase the conversion of CO to CO2 based on the enhanced oxide ion conductivity and the Fe2O3/Ce0.8Sm0.2O1.9 showed the highest oxygen transfer capacity and reduction reactivity. Figure 6 demonstrates the evolution of conversion rate ROC in the reduction stage of the 12th cycle. The ROC for all samples exceeded 33.3%, suggesting that the Fe2O3 was reduced beyond FeO. As for the Fe2O3/Ce0.8Sm0.2O1.9, the final ROC at 54.9% demonstrated 67.6% of iron was in FeO state and 32.4% in metallic Fe state on the whole. Moreover, the corresponding turning point time for the ROC values at 11.1% and 33.3% was ca. 5.0 min and 14.0 min, respectively, which was consistent to the time nodes for the CO2 concentration in Figure 5. Hence, the phase composition of the iron oxides was very critical for the reduction reactivity of these samples. Since the reduction reactivity is high in the phase transition from Fe2O3 to Fe3O4 and slows down for further reduction from Fe3O4 to Fe,14, 70 the rising rate of the ROC for all oxygen carriers dropped after 5.0 min as demonstrated in Figure 6. Furthermore, the ROC gap between Fe2O3/CeO2 and the rare earth doped ones widened as the reduction stage proceeded. Overall, the ROC of all rare earth doped samples were much higher than that of Fe2O3/CeO2 and the Fe2O3/Ce0.8Sm0.2O1.9 obtained the highest ROC after ca. 5.0 min. The evolution of H2 concentration in the steam oxidation stage is depicted in Figure 7. All


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oxygen carriers climbed to their peak values rapidly in ca. 2.0 min, then declined and completed the H2 generation process in 10.0 min. The Fe2O3/Ce0.8Sm0.2O1.9 got the highest peak value at 44.5%, while the peak value for Fe2O3/CeO2 was only 37.2%. Moreover, the H2 concentration curves was obviously a stepwise process, which can be divided into two steps with ca. 3.3 min as the cut-off point, and most of the H2 was generated in the first step. Taking the Fe2O3/Ce0.8Sm0.2O1.9 as an example, it took 10.0 min to complete the steam oxidation, while the H2 yield at 3.3 min was 0.674 L, making up 76.2% of the total yield. Considering that the difference of the steam oxidation reactivity between Fe → FeO and FeO → Fe3O4 was unapparent,14 it indicated that the rate-determining step shifted from chemical reaction to gas diffusion in the oxygen carriers in view of reaction kinetics. Above all, the rare earth doping favored the oxidation reactivity of FeO/Fe,14 and the Fe2O3/Ce0.8Sm0.2O1.9 achieved the best performance. 3.3. Morphology and Components. XRD, SEM, EDX, XPS and Raman spectra analysis were performed regarding to the morphology and components of the rare earth doped Fe2O3/CeO2 oxygen carriers, as shown in Figures 8-12. Figure 8 illustrates the XRD patterns of all samples before and after redox cycles. The label F could stand for both CeFeO3 and Fe-Ce-Y/Sm/La solid solutions with perovskite structure. Only Fe2O3 and CeO2 were demonstrated in all samples of oxidation state, except Fe2O3/Ce0.8La0.2O1.9, which also contained LaFeO3. For the oxygen carriers in reduction state, FeO, Fe and CeFeO3 can be detected in each sample. Furthermore, CeO2 existed in all rare earth doped samples in reduction state but Fe2O3/CeO2,20 probably owing to the shielding effect of Fe element enriched


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on the particle surface, as shown in Table 3 below. In addition, Fe3O4 can be found in the reduction state of Fe2O3/CeO2, indicating its poor oxygen mobility relative to the rare earth doped ones. Moreover, the perovskite LaFeO3 was even observed in both reduction and oxidation state for the Fe2O3/Ce0.8La0.2O1.9, demonstrating its high chemical stability.47 Therefore, the La doping herein decreased the oxygen capacity of the oxygen carrier, although it increased the oxygen vacancies in CeO2 and enhanced the oxygen mobility of Fe2O3/Ce0.8La0.2O1.9. However, optimum La doping at a lower concentration without La bleed-out from CeO2 can be more beneficial to the chemical looping process.47 Specifically, the intensity of the CeFeO3 diffraction peaks for Fe2O3/CeO2 was much weaker than the other samples due to the segregation of Fe cations and subsequent enrichment on the particle surface. The enlarged views (not shown here) of Figure 8 indicated that both of the diffraction peaks for Fe2O3 and CeO2 were narrowed and heightened for all oxidized samples after cycles (except for CeO2 in Fe2O3/CeO2 and Fe2O3/Ce0.8La0.2O1.9 for its low content on the particle surface), suggesting the increase of crystallite size and sintering of the oxygen carriers. It is worth mentioning that the diffraction peak for CeO2 in the cycled Fe2O3/Ce0.8La0.2O1.9 shifted to higher 2θ angle on account of the contraction of cell, which probably indicated the bleeding of La during the redox cycles. Nevertheless, no shift for Fe2O3 and CeO2 peaks of the other oxygen carriers could be observed, which certified that the phase composition was stable all through the redox cycles. Figure 9 depicts the SEM images for different oxygen carriers. Grains with a size of 0.1 ~ 0.2 μm over the particle surface led to a porous structure for all fresh samples. No sintering was observed in the rare earth doped oxygen carriers. However, the grains of fresh Fe2O3/CeO2


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agglomerated together with slight sintering, leading to lower specific surface area. This phenomenon indicated that the rare earth doping could improve the high temperature resistance of CeO2.27, 28 After cycles, serious sintering appeared and the grains grew up and aggregated together tightly in all oxygen carriers,20, 74 blocking the pores and inhibiting adequate contact between reactant gas and the samples. Furthermore, these samples performed diverse microstructures after cycles. For both Fe2O3/Ce0.8Sm0.2O1.9 in Figure 9f and Fe2O3/Ce0.8La0.2O1.9 in Figure 9h, the particles were composed of grains of 0.2 ~ 1.0 μm with no porous structure. Concerning the Fe2O3/CeO2 in Figure 9b, the grain size was smaller, ca. 0.2 ~ 0.5 μm. Moreover, there appeared many micro pores in Fe2O3/CeO2. In fact, however, the pores were very shallow and the grains combined into a dense entity on the surface of the particles. As for the Fe2O3/Ce0.8Y0.2O1.9 in Figure 9d, the grain size varied in a wide range between 0.2 ~ 2 μm with no porous structure as well. However, much larger grains than the other samples occupied most area of the particle surface, leading to less active sites for reaction with reactant gas.75 In conclusion, although the rare earth doped Fe2O3/CeO2 showed server sintering as Fe2O3/CeO2, the reactivity was greatly improved as shown in Figures 4-7 owing to their high oxygen mobility. The high temperature resistance enhanced by rare earth doping was limited, and it was reasonable to employ some other support with anti-sintering property to further improve the redox reactivity and stability. Figure 10 displays the EDX analysis of different oxygen carriers. Table 3 presents the content of Fe element on the particle surface corresponding to the scanning areas in Figure 10. For all fresh samples, the distribution of Fe and Ce elements were homogeneous and the Fe content approximated to the theoretical values, as shown in Table 3. After cycles, the Fe content on


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particle surface kept almost constant for Fe2O3/Ce0.8Y0.2O1.9 and Fe2O3/Ce0.8Sm0.2O1.9, as certified in Table 3. However, inhomogeneous distribution of Fe and Ce on the surface can be observed in Figure 10d and Figure 10f (more scanning times led to the heavier color in Figure 10f than that in Figure 10e), which was detrimental to the contact between them and therefore the reactivity of both samples. As for Fe2O3/CeO2, the Fe mole fraction increased to 99.78%, and the remaining Ce on the surface was also inhomogeneously distributed as depicted in Figure 10b. This phenomenon should originate from the outward migration of Fe cations, and then the iron oxide crystallite aggregated and sintered as the reduction progressed.76 The sintering of iron oxide on the surface could serve as the barrier for oxygen mobility, resulting in low reactivity of the Fe2O3/CeO2. Concerning the Fe2O3/Ce0.8La0.2O1.9, the remaining Ce was unevenly distributed as shown in Figure 10h, with the mole fraction of Fe increasing to 87.69%, which was more than that of its fresh sample, however, less than that of cycled Fe2O3/CeO2. This can be attributed to the bleed-out of La from the doped CeO2. Ma et al.76 found that the enrichment of Fe element and subsequent sintering on the particle surface could be caused by the slower diffusion of inward O anions than that of outward Fe cations. Therefore, the rare earth doping into CeO2, which can increase the O diffusion towards the level of Fe diffusion, suppressed the outward diffusion of Fe cations. The bleed-out of La from CeO2 may decline the total oxygen mobility capacity, resulting in Fe enrichment on the particle surface. Above all, the Fe2O3/Ce0.8Y0.2O1.9 and Fe2O3/Ce0.8Sm0.2O1.9 were suitable oxygen carriers to suppress the outward diffusion of Fe cations. The O 1s spectra of cycled samples in Figure 11 was fitted into three bands, marked as OI,


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OII, and OIII bands, respectively. The position for the bands varied among different samples due to different electronic structures of the oxygen species influenced by rare earth dopants.77 These fitted bands represent lattice oxygen (OI), oxygen vacancies (OII), and surface-adsorbed oxygen (OIII), respectively.65, 78 The oxygen species percentages were calculated as shown in Table 4. The lattice oxygen (OI) was the most (53.3-73.4%) for all samples.65 The rare earth doping led to a sharp increase in the quantity of oxygen vacancies (OII), and the order was as follows: Fe2O3/Ce0.8Sm0.2O1.9 > Fe2O3/Ce0.8Y0.2O1.9 > Fe2O3/Ce0.8La0.2O1.9 > Fe2O3/CeO2, which further certified the excellent oxygen mobility of the Fe2O3/Ce0.8Sm0.2O1.9. The low OII component content for Fe2O3/Ce0.8La0.2O1.9 can be ascribed to the bleed-out of the La dopant, and the relatively high oxygen mobility of it was originated partly from the oxygen vacancies in Ladoped CeO2 and partly from the perovskite LaFeO3. In addition, the OIII component content was ranked as follows: Fe2O3/CeO2 > Fe2O3/Ce0.8Sm0.2O1.9 > Fe2O3/Ce0.8La0.2O1.9 > Fe2O3/Ce0.8Y0.2O1.9, which was in accordance with the specific surface area,78 as reflected by the SEM images of cycled samples in Figure 9. Figure 12 illustrates the Raman spectra comparison after redox cycles. The bands intensity of CeO2 at 466 cm-1 for Fe2O3/Ce0.8Y0.2O1.9 and Fe2O3/Ce0.8Sm0.2O1.9 were much weaker and wider than that of Fe2O3/Ce0.8La0.2O1.9, even though the CeO2 content on the surface of the latter was less than that of the former as demonstrated in Table 3. This phenomenon suggested the diminution of CeO2 crystallite or presence of defects.79, 80 For Fe2O3/CeO2, the band for CeO2 cannot be observed, indicating almost no CeO2 existed on the surface, which agreed with the result from the EDX analysis in Table 3.


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4. CONCLUSIONS The Fe2O3/CeO2 oxygen carriers, doped with rare earths, Y, Sm and La, respectively and prepared by co-precipitation method, were studied on a fluidized bed for CLHG with respect to the redox performance and fundamental mechanisms. The following results were concluded. (1) The Fe2O3/Ce0.8Sm0.2O1.9 demonstrated the highest redox reactivity and the purity of the generated hydrogen could be up to 100% (with the detection limit at 0.01% in volume), and the reactivity of the rare earth doped oxygen carriers was ranked as Fe2O3/Ce0.8Sm0.2O1.9 > Fe2O3/Ce0.8La0.2O1.9 > Fe2O3/Ce0.8Y0.2O1.9 > Fe2O3/CeO2. (2) The rare earths were incorporated into CeO2 and increased the concentration of oxygen vacancies, promoting the oxygen mobility and reactivity of these oxygen carriers. Furthermore, the concentration of oxygen vacancies in cycled samples followed the sequence: Fe2O3/Ce0.8Sm0.2O1.9 > Fe2O3/Ce0.8Y0.2O1.9 > Fe2O3/Ce0.8La0.2O1.9 > Fe2O3/CeO2. (3) No bleed-out of rare earths from doped CeO2 was observed after redox cycles for Sm and Y, and both rare earths could suppress the outward diffusion of iron cations in the particle and subsequent enrichment on the surface, improving the redox stability of oxygen carriers. (4) The rare earth La would bleed out from Ce0.8La0.2O1.9 and generate a stable perovskite LaFeO3, which resulted in the outward migration of iron cations and reduced the quantity of active iron oxides, exerting a detrimental effect on the reactivity of Fe2O3/Ce0.8La0.2O1.9. (5) It is reasonable to add some other anti-sintering support to further enhance the redox performance of Fe2O3/Ce0.8Sm0.2O1.9.

AUTHOR INFORMATION Corresponding Author 19 ACS Paragon Plus Environment

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*Tel.: +86-25-8379 5545; fax: +86-25-8771 4489; E-mail address: [email protected] (Wenguo Xiang)

ACKNOWLEDGEMENTS The work was supported by the National Natural Science Foundation of China (51576042), and the Natural Science Foundation of Jiangsu (BK20160672).

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dc-electrical conductivity of rare-earth doped caria. Appl. Phys. A: Mater. Sci. Process. 1989, 49 (3), 225-232. (57) Singh, K.; Kumar, K.; Srivastava, S.; Chowdhury, A. Effect of rare-earth doping in CeO2 matrix: Correlations with structure, catalytic and visible light photocatalytic properties. Ceram. Int. 2017, 43 (18), 17041-17047. (58) Zhao, L.; Bishop, S. R.; Hyodo, J.; Ishihara, T.; Sasaki, K. XRD and Raman spectroscopy study of Fe solubility in cerium oxide. ECS Trans. 2013, 50 (40), 53-58. (59) Fu, Y.; Chen, S.; Huang, J. Preparation and characterization of Ce0.8M0.2O2−δ (M=Y, Gd, Sm, Nd, La) solid electrolyte materials for solid oxide fuel cells. Int. J. Hydrogen Energy 2010, 35 (2), 745-752. (60) Qin, L.; Guo, M.; Cheng, Z.; Xu, M.; Liu, Y.; Xu, D.; Fan, J. A.; Fan, L. Improved cyclic redox reactivity of lanthanum modified iron-based oxygen carriers in carbon monoxide chemical looping combustion. J. Mater. Chem. A 2017, 5 (38), 20153-20160. (61) Gu, Z.; Li, K.; Qing, S.; Zhu, X.; Wei, Y.; Li, Y.; Wang, H. Enhanced reducibility and redox stability of Fe2O3 in the presence of CeO2 nanoparticles. RSC Adv. 2014, 4 (88), 47191-47199. (62) Kuntaiah, K.; Sudarsanam, P.; Reddy, B. M.; Vinu, A. Nanocrystalline Ce1-xSmxO2−δ (x=0.4) solid solutions: Structural characterization versus CO oxidation. RSC Adv. 2013, 3 (21), 7953-7962. (63) Li, K.; Haneda, M.; Ning, P.; Wang, H.; Ozawa, M. Microstructure and oxygen evolution of Fe-Ce mixed oxides by redox treatment. Appl. Surf. Sci. 2014, 289, 378-383. (64) Li, D.; Li, K.; Xu, R.; Wang, H.; Tian, D.; Wei, Y.; Zhu, X.; Zeng, C.; Zeng, L. Ce1-xFexO2-δ catalysts for catalytic methane combustion: Role of oxygen vacancy and structural dependence.


Energy & Fuels 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

Catal. Today 2017, http://dx.doi.org/10.1016/j.cattod.2017.12.015. (65) Zhu, X.; Li, K.; Wei, Y.; Wang, H.; Sun, L. Chemical-looping steam methane reforming over a CeO2-Fe2O3 oxygen carrier: Evolution of its structure and reducibility. Energy Fuels 2014, 28 (2), 754-760. (66) Arunkumar, P.; Preethi, S.; Suresh Babu, K. Role of iron addition on grain boundary conductivity of pure and samarium doped cerium oxide. RSC Adv. 2014, 4 (84), 44367-44376. (67) Mahapatra, A. S.; Mitra, A.; Mallick, A.; Shaw, A.; Greneche, J. M.; Chakrabarti, P. K. Modulation of magnetic and dielectric property of LaFeO3 by simultaneous doping with Ca2+ and Co2+-ions. J. Alloy. Compd. 2018, 743, 274-282. (68) Li, K.; Wang, H.; Wei, Y.; Yan, D. Direct conversion of methane to synthesis gas using lattice oxygen of CeO2-Fe2O3 complex oxides. Chem. Eng. J. 2010, 156 (3), 512-518. (69) Li, H.; Li, K.; Wang, H.; Zhu, X.; Wei, Y.; Yan, D.; Cheng, X.; Zhai, K. Soot combustion over Ce1-xFexO2-δ and CeO2/Fe2O3 catalysts: Roles of solid solution and interfacial interactions in the mixed oxides. Appl. Surf. Sci. 2016, 390, 513-525. (70) 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 (1), 34-44. (71) Wu, Y.; Lin, C. The microstructures and property analysis of aliovalent cations (Sm3+, Mg2+, Ca2+, Sr2+, Ba2+) co-doped ceria-base electrolytes after an aging treatment. Int. J. Hydrogen Energy 2014, 39 (15), 7988-8001. (72) Kosacki, I.; Suzuki, T.; Petrovsky, V.; Anderson, H. U. Electrical conductivity of nanocrystalline ceria and zirconia thin films. Solid State Ionics 2000, 136 (SI), 1225-1233.


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Energy & Fuels

(73) Kharton, V. V.; Figueiredo, F. M.; Navarro, L.; Naumovich, E. N.; Kovalevsky, A. V.; Yaremchenko, A. A.; Viskup, A. P.; Carneiro, A.; Marques, F.; Frade, J. R. Ceria-based materials for solid oxide fuel cells. J. Mater. Sci. 2001, 36 (5), 1105-1117. (74) Dharanipragada, N. V. R. A.; Meledina, M.; Galvita, V. V.; Poelman, H.; Turner, S.; Van Tendeloo, G.; Detavernier, C.; Marin, G. B. Deactivation Study of Fe2O3-CeO2 during redox cycles for CO production from CO2. Ind. Eng. Chem. Res. 2016, 55 (20), 5911-5922. (75) Qin, L.; Cheng, Z.; Guo, M.; Xu, M.; Fan, J. A.; Fan, L. Impact of 1% lanthanum dopant on carbonaceous fuel redox reactions with an iron-based oxygen carrier in chemical looping processes. ACS Energy Lett. 2016, 2 (1), 70-74. (76) Ma, Z.; Xiao, R.; Chen, L. Redox reaction induced morphology and microstructure evolution of iron oxide in chemical looping process. Energy Convers. Manage. 2018, 168, 288-295. (77) Bagus, P. S.; Ilton, E.; Nelin, C. J. Extracting chemical information from XPS spectra: A perspective. Catal. Lett. 2018, 148 (7), 1785-1802. (78) Li, K.; Wang, H.; Wei, Y.; Yan, D. Transformation of methane into synthesis gas using the redox property of Ce-Fe mixed oxides: Effect of calcination temperature. Int. J. Hydrogen Energy 2011, 36 (5), 3471-3482. (79) Li, K.; Wang, H.; Wei, Y.; Yan, D. Partial oxidation of methane to syngas with air by lattice oxygen transfer over ZrO2-modified Ce-Fe mixed oxides. Chem. Eng. J. 2011, 173 (2), 574-582. (80) Ilieva, L.; Pantaleo, G.; Sobczak, J. W.; Ivanov, I.; Venezia, A. M.; Andreeva, D. NO reduction by CO in the presence of water over gold supported catalysts on CeO2-Al2O3 mixed support, prepared by mechanochemical activation. Appl. Catal. B: Environ. 2007, 76 (1-2), 107-114.


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(a)

C

A B C

A

A A

C

A

A BA CB

A

A B

A B B

A B A

A

B A

20

A

30

AB

AA

B

AC B

A

A B

A

A

AB

AA

AB B

50 2θ (°)

B A

AB B

Fe2O3/CeO2

B AB

40

B A

Fe2O3/Ce0.8Y0.2O1.9

B

A A

CB A

B

A

Fe2O3/Ce0.8Sm0.2O1.9

A A

AA

(c)

(b)

C—LaFeO3

Fe2O3/Ce0.8La0.2O1.9

B C

B

Intensity

B—CeO2

A—Fe2O3

B

AA

60

B A

AB B

80 53

70

55 2θ (°)

47 48 2θ (°)

Figure 1. (a) XRD patterns of prepared rare earth doped Fe2O3/CeO2 oxygen carriers. (b) Amplification of the (321) reflection for Fe2O3. (c) Amplification of the (220) reflection for CeO2. A

B

A


300

B

A


200

A—Fe2O3 B—CeO2

(b) A

A

A

100

A—Fe2O3 B—CeO2

A

Intensity

(a)

Intensity

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

Page 30 of 38



Fe2O3/Ce0.8Y0.2O1.9

Fe2O3/Ce0.8Y0.2O1.9

Fe2O3/CeO2

Fe2O3/CeO2

400 500 600 Raman Shift (cm-1)

700

800 350

400

450 Raman Shift (cm-1)

500

550

Figure 2. (a) Raman spectra of prepared rare earth doped Fe2O3/CeO2 oxygen carriers; (b) Amplification of Raman spectra in the range of 350-550 cm-1


Page 31 of 38

β

α

γ

δ

Intensity



Fe2O3/Ce0.8Y0.2O1.9

Fe2O3/CeO2

200

300

400

500 600 700 Temperature (°C)

800

900

Figure 3. H2-TPR patterns of prepared rare earth doped Fe2O3/CeO2 oxygen carriers. 1.4 Fe2O3/CeO2 Fe2O3/Ce0.8Y0.2O1.9

1.2

Fe2O3/Ce0.8Sm0.2O1.9 Fe2O3/Ce0.8La0.2O1.9

H2 Yield (L)

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

Energy & Fuels

1.0

0.8

0.6

0.4

2

4

6 8 Cycle Number

10

12

Figure 4. Effect of redox cycle on H2 yield for rare earth doped Fe2O3/CeO2 oxygen carriers.


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20

CO2 Concentration (%)

Fe2O3/CeO2 Fe2O3/Ce0.8Y0.2O1.9

15


10

5

0

t=14.0 min

t=5.0 min 0

5

10

15 Time (min)

20

25

30

Figure 5. CO2 concentration in the reduction stage for rare earth doped Fe2O3/CeO2 oxygen carriers in the 12th cycle. 60

Conversion rate of oxygen carriers/%

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

Page 32 of 38

Fe2O3/CeO2 Fe2O3/Ce0.8Y0.2O1.9 45


ROC=33.3%

30

ROC=11.1%

15

t=5.0 min

0 0

5

10

t=14.0 min 15 Time/min

20

25

30

Figure 6. Conversion rates in the reduction stage for rare earth doped Fe2O3/CeO2 oxygen carriers in the 12th cycle.


Page 33 of 38

50

Fe2O3/CeO2

H2 Concentration (%)

Fe2O3/Ce0.8Y0.2O1.9 40


30 20 10 t=3.3 min

0 0

2

4 6 Time (min)

8

10

Figure 7. H2 concentration in the steam oxidation stage for rare earth doped Fe2O3/CeO2 oxygen carriers in the 12th cycle. A—Fe2O3 B—CeO2 C—Fe3O4 D—FeO E—Fe F—CeFeO 3

CD

C

F D

F D

F

A B A

B B

A

A

B

AA

BA

A B

D

F

F F B

B A

AB B

A

30

A

A

A

40

B

AA

AB

50 2θ (°)

60

B A

30

A

A

30

B

F

G

D

E F

D

F

Oxidation after cycles

A

A

B

20

A B

F

B A

A

AA B AB

A B A

40

B

A

50 2θ/°

A BA B A A

60

A B AA AB

A

F

B A AB B

A B AB AA

A

50 2θ (°)

B A

60

AB B

70

80

B A AB B

B A

70

A

B

A

80 20

（c）Fe2O3/Ce0.8Sm0.2O1.9

GB F

B

G

F G

D

A B

B

AB B

Reduction after cycles

E D G D

B

Fresh sample A

D

A—Fe2O3 B—CeO2 D—FeO E—Fe F—CeFeO3 G—LaFeO3

Reduction after cycles FB F

E F

（b）Fe2O3/Ce0.8Y0.2O1.9

Intensity

B

F D

B

40

F G

FD

D

Fresh sample A A

F

B

F B

A B

A

A—Fe2O3 B—CeO2 D—FeO E—Fe F—CeFeO3

F F

B A

20

F

Oxidation after cycles

A

（a）Fe2O3/CeO2

E

FB F

A B

B

80

F D

E

A

AB B

70

Reduction after cycles D

B

Fresh sample B

A

20

E F

Oxidation after cycles A

A

F

Intensity

Intensity

F F

A—Fe2O3 B—CeO2 D—FeO E—Fe F—CeFeO3

Reduction after cycles

E

F

Intensity

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

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F E G

D G

F G

Oxidation after cycles A

G A B G A

30

A

B

A

A

A BG

B

AA

B A AB B Fresh sample

B A

40

G

A

50 2θ (°)

A BA GB

AA

B A AB B

60

（d）Fe2O3/Ce0.8La0.2O1.9

Figure 8. XRD patterns of rare earth doped Fe2O3/CeO2 oxygen carriers.


70

80

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Page 34 of 38

（a）Fe2O3/CeO2—Fresh

（b）Fe2O3/CeO2—After cycles

（c）Fe2O3/Ce0.8Y0.2O1.9—Fresh

（d）Fe2O3/Ce0.8Y0.2O1.9—After cycles

（e）Fe2O3/Ce0.8Sm0.2O1.9—Fresh

（f）Fe2O3/Ce0.8Sm0.2O1.9—After cycles

（g）Fe2O3/Ce0.8La0.2O1.9—Fresh

（h）Fe2O3/Ce0.8La0.2O1.9—After cycles 34

ACS Paragon Plus Environment

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Energy & Fuels

Figure 9. SEM images of rare earth doped Fe2O3/CeO2 oxygen carriers before and after cycles.

（a）Fe2O3/CeO2—Fresh

（b）Fe2O3/CeO2—After 8 cycles

（d）Fe2O3/Ce0.8Y0.2O1.9—After 8 cycles

（c）Fe2O3/Ce0.8Y0.2O1.9—Fresh

（e）Fe2O3/Ce0.8Sm0.2O1.9—Fresh

（f）Fe2O3/Ce0.8Sm0.2O1.9—After 8 cycles


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（h）Fe2O3/Ce0.8La0.2O1.9—After 8 cycles

（g）Fe2O3/Ce0.8La0.2O1.9—Fresh

Figure 10. EDX analysis of rare earth doped Fe2O3/CeO2 oxygen carriers before and after cycles.

O

Intensity

Intensity

O

O

O

O

O

526

528

530 Binding Energy (eV)

532

534

526

(a) Fe2O3/CeO2

528 530 Binding Energy (eV)

532

534

532

534

(b) Fe2O3/Ce0.8Y0.2O1.9 O

O

O

O

Intensity

Intensity

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

Page 36 of 38

O O

526


532

526

534

(c) Fe2O3/Ce0.8Sm0.2O1.9


(d) Fe2O3/Ce0.8La0.2O1.9

Figure 11. XPS O 1s spectra of the rare earth doped Fe2O3/CeO2 oxygen carriers after cycles.


Page 37 of 38

A—Fe2O3 A

A

A

A

B—CeO2

Fe2O3/Ce0.8La0.2O1.9 A

B


Intensity

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

Energy & Fuels

Fe2O3/Ce0.8Y0.2O1.9

Fe2O3/CeO2

100

200

300

400 500 600 Raman shift (cm-1)

700

800

Figure 12. Raman spectra of rare earth doped Fe2O3/CeO2 oxygen carriers after cycles. Table 1. Experimental procedure in a cycle No.

Experimental stage

Time(min)

Gas flow rate(L/min)

1

CO reduction

30

N2:0.40; CO:0.10

2

Nitrogen blowing

~ 40

N2:0.50

3

Steam oxidization

~ 10

N2:0.40; H2O(l):0.20mL/min

4

Nitrogen blowing

5

N2:0.50

5

Air oxidization

10

N2:0.40; O2:0.10

6

Nitrogen blowing

10

N2:0.50

Table 2. Structural characterizations of prepared rare earth doped Fe2O3/CeO2 oxygen carriers. Lattice constant (Å)

Crystallite size (nm)

Fe2O3 Samples

CeO2

a

c

CeO2

Fe2O3

Fe2O3/CeO2

5.410±0.001

5.038±0.001

13.745±0.003

34.7±1.8

68.2±7.4

Fe2O3/Ce0.8Y0.2O1.9

5.409±0.003

5.041±0.002

13.752±0.005

16.7±0.9

40.1±4.1


5.425±0.002

5.044±0.002

13.752±0.006

13.3±0.6

36.5±3.6

Fe2O3/ Ce0.8La0.2O1.9

5.434±0.001

5.040±0.001

13.750±0.003

26.2±1.0

52.7±4.7


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Page 38 of 38

Table 3. Mole fraction of Fe element on the surface of various oxygen carriers Oxygen carrier

Fe2O3/CeO2

Fe2O3/Ce0.8Y0.2O1.9 Fe2O3/Ce0.8Sm0.2O1.9 Fe2O3/Ce0.8La0.2O1.9

Fe atom ratio

nFe/(nFe+nCe)

nFe/(nFe+nCe+nY)

nFe/(nFe+nCe+nSm)

nFe/(nFe+nCe+nLa)

Theory value

76.38%

75.07%

76.42%

76.18%

Fresh sample

70.54%

79.32%

70.95%

73.53%

98.78%

71.09%

76.88%

87.69%

Sample after cycles Table 4. XPS O 1s results for rare earth doped Fe2O3/CeO2 oxygen carriers after cycles Oxygen species percentage (%) Oxygen carrier

OI

OII

OIII

Fe2O3/CeO2

53.3

3.5

43.2

Fe2O3/Ce0.8Y0.2O1.9

68.1

14.3

17.6


56.9

18.9

24.2


73.4

4.8

21.8


CeO2 oxygen carrier via

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