Effects of CeO2, ZrO2, and Al2O3 Supports on Iron Oxygen Carrier for

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Effects of CeO2, ZrO2 and Al2O3 Supports on Iron Oxygen Carrier for Chemical Looping Hydrogen Generation Shiwei Ma, Shiyi Chen, Ahsanullah Soomro, and Wenguo Xiang Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01141 • Publication Date (Web): 06 Jul 2017 Downloaded from http://pubs.acs.org on July 6, 2017

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Effects of CeO2, ZrO2 and Al2O3 Supports on Iron Oxygen Carrier for Chemical Looping Hydrogen Generation Shiwei Ma, Shiyi Chen, Ahsanullah Soomro, Wenguo Xiang* Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing210096, China ABSTRACT: Fe2O3 is an excellent active metal oxide for chemical looping hydrogen generation (CLHG) with high conversion of CO to CO2 in reduction stage, and high H2 mole fractions in subsequent steam oxidation stage, especially for its low cost and abundance in nature. However, supports are generally used to improve its reactivity and stability and to eliminate its carbon deposition. In this paper, Fe-based oxygen carriers are prepared by co-precipitation method with three supports, i.e. CeO2, ZrO2 and Al2O3. The reactivity, carbon deposition, redox stability and sintering characteristics of the oxygen carriers are analyzed to investigate the effects of supports as well as the fundamental mechanism. The results show that the properties of the oxygen carriers highly rely on the support and its interaction with iron oxide. The oxygen carrier supported on Al2O3 exhibites poor reactivity and stability, and the oxygen carrier supported on ZrO2 leads to much carbon deposition, decreasing H2 purity, despite its high reactivity and stability. Nevertheless, the oxygen carrier supported on CeO2 demonstrates good reactivity and stability with no carbon deposition observed, and the reducible support CeO2 counteractes the negative effect originating from sintering and guarantees the reactivity and stability of Fe2O3/CeO2 due to its oxygen mobility property and the oxygen mobility enhancement originating from the formation of Fe-Ce solid solution and perovskite-type CeFeO3. Overall, the reducible CeO2 is a

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potential support to improve the redox characteristics of iron oxygen carrier in CLHG, and the Fe2O3/CeO2 exhibites the highest reactivity at 850 °C. In addition, all three oxygen carriers are characterized by SEM images, EDX analysis and XRD patterns before and after the redox cycles. KEY WORDS: Chemical looping; Hydrogen; Carbon deposition; Iron oxide; Support

1. INTRODUCTION Hydrogen is an ideal solution for the supply of energy in future. Also, hydrogen was an important industrial raw material which can be widely used in the production of ammonia, gasoline, methanol, ethanol, etc.1 Steam methane reformation (SMR) is commercially proven and is the predominant technology for hydrogen production at present.2, 3 Nevertheless, the process for SMR is complex and expensive with low efficiency, more importantly, CO2 emission into the environment accompanies with it. Chemical looping hydrogen generation (CLHG) is a promising hydrogen production technology developed from chemical looping combustion (CLC). It can not only produce high purity hydrogen but also separate CO2 inherently. The technology consists of three reactors: a fuel reactor (FR), a steam reactor (SR) and an air reactor (AR), as is shown in Figure 1. The oxygen carrier, mostly iron oxide, circulates among these reactors, transforming fuels into hydrogen. Take CO as fuel, the reactions in these three reactors can be described as follows. In the FR, Fe2O3 is reduced into Fe or FeO, which is shown in R1 to R3. 3Fe 2 O 3 + CO → 2Fe 3 O 4 + CO 2

(R1)

Fe 3 O 4 + CO → 3FeO + CO 2

(R2)

Fe 3 O 4 + 4CO → 3Fe + 4CO 2

(R3)


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In the SR, Fe or FeO is oxidized into Fe3O4, releasing H2, as listed in R4 and R5. 3FeO + H 2 O → Fe 3 O 4 + H 2

(R4)

3Fe + 4H 2 O → Fe 3 O 4 + 4H 2

(R5)

In the AR, Fe3O4 is further oxidized and Fe2O3 is regenerated, as can be seen in R6. 4Fe 3 O 4 + O 2 → 6Fe 2 O 3

(R6)

The hydrogen produced in CLHG with CO as fuel and Fe2O3 as oxygen carrier can reach high purity above 99% (dry), the reason why the purity cannot achieve 100% is the carbon deposition generated in the FR.4 The related reaction is called as Boudouard reaction, see below in R7. 2CO → CO 2 + C

(R7)

The carbon deposition will then react with H2O in the SR, leading to the production of CO and CO2, contaminating the generated hydrogen, as shown in R8 and R9. C + H 2 O → CO + H 2

(R8)

CO + H 2 O → CO 2 + H 2

(R9)

Many factors can influence carbon deposition, such as reaction pressure, temperature, composition and concentration of fuel gas, and the available quantity of the lattice oxygen supplied by the oxygen carrier.5 CO2 can be introduced into the FR to reduce the carbon deposition and increase the H2 purity, while CO2 would cut down the fuel gas conversion.6, 7 In addition, steam can also be used to decrease the carbon deposition.5, 8, 9 However, the steam is always used in CLC rather than in CLHG since it would reduce the H2 yield in the SR. Although many metal oxides, such as NiO, Fe2O3, CuO, CoO and Mn3O4, can be used in CLC, only Fe2O3 is most appropriate candidate for CLHG in view of thermodynamics.10, 11 In addition


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to the active metal oxides, supports are also very important and indispensable part to the oxygen carriers for high reactivity, favorable stability, and resistance to sintering, agglomeration, attrition and carbon deposition. There are many kinds of inert supports that can be used in the synthesis of oxygen carriers, like Al2O3, MgAl2O4, TiO2, SiO2, ZrO2, YSZ (yttrium-stabilized zirconia) and perovskites. Besides, the synthesis method is also very critical to the properties of an oxygen carrier, and the most used preparation methods are mechanical mixing, freeze granulation, spray drying, co-precipitation, sol-gel, solution combustion, etc.3 Though many investigations have been focused on the effects of different inert supports for CLC and CLR(Chemical Looping Reforming) regarding to the Fe-based oxygen carriers,9, 12-15 those for CLHG are limited. The essential difference between CLHG and CLC/CLR lies in that Fe2O3 must be reduced into Fe or FeO in CLHG rather than Fe3O4, and that H2O is introduced into the process as an oxidant to turn Fe or FeO into Fe3O4 with H2 generated. However, Fe and FeO could facilitate carbon deposition, which can be easily eliminated in CLC with Fe2O3 generally reduced to Fe3O4.16, 17 In addition, FeO could react with some supports, leading to the generation of unreactive compound, decreasing the effective quality of iron oxide and shortening the service life of the oxygen carrier.18, 19 So the selection of the supports is an issue of crucial importance for the Fe-based oxygen carriers used in CLHG. Chen et al.20 concluded that the H2 yield of Fe-based oxygen carrier supported by Al2O3 was superior to that of TiO2 in CLHG since TiO2 could react with the reduced Fe2O3 to form unreactive FeTiO3; besides, the quality of the carbon deposition seemed to have no obvious relationship with the support materials. Ma and coworkers21 investigated the effects of different supports on H2 generation and carbon deposition for


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Fe-based oxygen carriers in CLHG. The results showed that the reactivity and H2 yield for the investigated oxygen carriers followed the order: Fe2O3/MgAl2O4 > Fe2O3/ZrO2 > Fe2O3/YSZ > Fe2O3/Al2O3 > Fe2O3/SiO2, and the order of H2 purity was identical with that of H2 yield as a result of carbon deposition. Moreover, the addition of dopants can facilitate the reactivity and stability of the oxygen carrier; for example, adding some potassium into iron ore could not only increase the H2 yield, but also decrease the carbon deposition.22 Oxide ion and electronic conductivity of the supports are also important factors that affects the performance of oxygen carriers in CLHG. Kosaka et al.23 investigated the redox reaction of iron oxide for H2 storage with various supports, ZrO2, CeO2, YSZ and GDC (gadolinia-doped ceria), which had different oxide ion and electronic conductivities. The results showed that oxide ion conductors YSZ and GDC could accelerate the reduction of Fe2O3 to Fe for H2 storage, while CeO2 and GDC would distinctly boost the reduction kinetics from FeO to Fe; furthermore, all these three supports could improve the steam-iron reaction rates compared with ZrO2. As a catalytic material, CeO2 has been widely used in redox catalysis field, such as automotive exhaust abatement, water gas shift and catalytic methane oxidation. The promotion role of CeO2 for the active metal phase of the catalyst is due to its property of storing and releasing oxygen, i.e., the facile and reversible release of lattice oxygen via the conversion between Ce4+ and Ce3+, which could be used to optimize the electronic and structural characteristics and accelerate the reduction of active metal.24 CeO2 has been used in CLC as a reducible support. It is concluded by Bhavsar et al.25 that CeO2, as a chemically active support for Ni-based and Fe-based oxygen carriers in CLC, was able to significantly accelerate the oxidation and reduction kinetics and


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enlarge the time window for total oxidation of methane compared to conventional, nonreducible inert supports Al2O3 and SiO2; what was striking was the fact that CeO2 allowed for complete Nibased carrier conversion with H2 or CH4 as fuel. Liu et al.26 found that the oxygen carrier Fe2O3/Al2O3 with CeO2 additive provided faster oxygen transfer rates from the bulk to the surface to obtain higher average reaction rates in CLC with CO as fuel, for the vacancies inspired by CeO2 additive could transfer oxygen from Fe2O3 quickly to the surface of the oxygen carrier by vacancy diffusion or even through an oxygen tunnel formed by vacancies. Similarly, the performance of CH4 combustion in CLC can also be improved with CeO2 additive to iron oxide.27 Furthermore, oxygen carriers supported by gadolinia doped-ceria or lanthana-doped ceria showed even higher reactivity and oxygen capacity in CLC.28, 29 For CLR, the oxygen carrier Fe2O3/CeO2 could maintain high catalytic activity and selectivity during the conversion of CH4 to synthesis gas due to the oxygen mobility enhancement by the solid solution between Fe2O3 and CeO2.30, 31 There are also some researches on chemical-looping steam methane reforming (CL-SMR) with the oxygen carrier Fe2O3/CeO2. Zhu et al.32, 33 showed that Fe2O3/CeO2 was a promising oxygen carrier in CL-SMR for its high redox activity, desired product yield of syngas and hydrogen. It was also found in their another research work34 that CeO2 modified Fe2O3 was a promising material for the chemical hydrogen storage and production, and Ce addition could not only lower the hydrogen reduction temperature, but also decrease the reaction temperature of water splitting reaction and accelerate the hydrogen production process. In addition, Ce could be used to eliminate the carbon deposition due to its redox properties and oxygen buffering capacity. Zheng et al.35 reported that CeO2 could reduce the carbon deposition for CLR of CH4. Sun et al.36


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prepared the oxygen carrier Fe2O3/Al2O3 modified by CeO2, and found that the CeO2 additive could effectively inhibit carbon deposition or Fe3C formation in CLHG. It makes sense to develop a new oxygen carrier that can not only convert more fuel gas into hydrogen, but also have an outstanding ability to decrease or eliminate the carbon deposition and obtain hydrogen of high purity. Based on the storing and releasing oxygen property of CeO2 and above findings, it is reasonable for us to expect excellent performance when CeO2 serves as the support for the Febased oxygen carriers in CLHG. The present work investigated the impacts of supports on the redox reactivity and stability, carbon deposition, H2 yield and purity for CLHG using iron oxide in a batch fluidized bed. Apart from CeO2, the common inert supports Al2O3 and ZrO2, which are frequently encountered and perform well in the chemical looping process3,14, are also included for comparison.

2. EXPERIMENTAL SECTION 2.1. Oxygen Carrier Synthesis. The Fe-based oxygen carriers were prepared by coprecipitation, and the mass ratio of Fe2O3 and the inert support was kept 6:4. The calculated amounts of precursor nitrates for Fe2O3, Fe(NO3)3·9H2O (AR, Nanjing Chemical Reagent Co., Ltd), and the metal oxide supports, Ce(NO3)3·6H2O (99.5%, Aladdin), Al(NO3)3·9H2O (AR, Greagent), and Zr(NO3)4·5H2O (AR, Greagent), were dissolved in deionized water. The solution was sufficiently blended and heated in a magnetic stirrer until the temperature reached 70 °C. Then a 25%~28% ammonia solution was gradually introduced into the mixture to increase the PH of the solution to 9. It was then aged for 12h under room temperature. Afterwards, the resulting precipitate was filtered and dried at 110 °C for 24 h. The solids obtained were subsequently


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subjected to decomposition at 350 °C for 2h and sintered at 900 °C for 2h in a muffle oven. Then the particles were crushed and sieved to get powder in the size range of 0.150~0.250 mm for experiments. 2.2. Oxygen Carrier Characterization. X-ray powder diffraction (XRD) was implemented to study the crystal phase compositions before and after reactivity tests. XRD patterns were recorded using a Rigaku SmartLab X-ray diffractometer with Cu-Kα radiation operating at 3 kW. The sample scans were performed in a step-scan mode between an angle range of 20~90° (2θ), with a step size rate of 0.1°/s. Scanning electron microscopy (SEM) and Energy Dispersive X-Ray Spectrometer (EDX) were performed using an Ultra Plus (Carl Zeiss AG) microscope instrument to study the morphology and the element distribution of the oxygen carriers. 2.3. Experimental Reactor and Procedure. A laboratory-scale batch fluidized bed reactor was used in the test as shown in Figure 2. This system was comprised of three parts, a gas distribution system, a fluidized bed reactor, and an off-gas treatment system. In the gas distribution system, distilled water was pumped into a steam generator and heated into steam, whereas CO, N2 and O2 were sent into the reactor through the mass flow controllers. The reactor was a quartz fluidized bed, which had a length of 750 mm and an inner diameter of 22 mm with a porous quartz plate placed 150 mm above the bottom. The quartz tube was heated by an electric furnace with a temperature controller. The off-gas was first condensed to remove water, then dried by silica gel, and finally sent into the gas analyzer. In each test, the oxygen carrier was 10g, which was placed on the porous quartz plate, and the furnace was heated to the desired temperature under nitrogen atmosphere of 0.35 L/min, then the


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particles were exposed to alternating reducing and oxidizing atmosphere. Table 1 shows a summary of the experimental procedure in a cycle. The experiments were conducted at isothermal condition with a total flow rate of 0.5 L/min, while it was 0.6 L/min for the steam oxidation stage. The minimum fluidized velocity umf of the particles was 0.018 m/s at 850 °C, and the gas velocity ug corresponding to the flow rate of 0.5 L/min was 0.101 m/s, i.e., 5.6 umf, sufficient to fluidize the oxygen carrier. In order to investigate the carbon deposition of the oxygen carriers, and to avoid CO contamination from the reduction stage, the reactor was blown by nitrogen between reduction stage and steam oxidization stage for about 40 min until no CO or CO2 was observed. 2.4. Data Evaluation. The total volumetric flow rate of the off-gas Fout is defined as:

Fout =

FN2 ,in

(1)

1 − X CO − X CO2 − X H2

Where FN2 ,in is the volumetric flow rate of the inlet nitrogen, XCO , X CO and X H are the 2

2

measured concentrations of CO, CO2 and H2 respectively. The volumetric flow rate of CO2 in the off-gas is:

FCO2 = Fout XCO2

(2)

The volumetric flow rate of H2 in the off-gas for the steam oxidization stage is:

FH2 = Fout X H2

(3)

The H2 yield in the steam oxidizing stage is: t

VH2 = ∫ FH2

(4)

0

The gas yield of carbonaceous gases, including CO and CO2, in the steam oxidization stage is: t

(

VCOx = ∫ Fout X CO2 + X CO 0

)


(5)

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The H2 purity in the steam oxidization stage is:

αH = 1−

VCOx

(6)

Fout

2

The conversion rate of oxygen carrier is defined as: ROX =

mox − m mox − mred

(7)

Where mox is the mass of the oxygen carrier in full oxidation, mred is the mass in full reduction, and m is the instantaneous mass. When ROX is 11.1%, the Fe2O3 in the oxygen carrier is all converted into Fe3O4; when ROX reaches 33.3%, the iron is all in the state of FeO; and ROX = 100% indicates that Fe2O3 is thoroughly reduced to metallic Fe. The weight loss of the oxygen carrier in the CO reduction stage is: t

Fout X CO2

0

22.4

∆m = 16∫

(8)

3. RESULTS AND DISCUSSION 3.1. Effect of Supports on the H2 Yield. Figure 3 shows the H2 yield in 10 redox cycles for oxygen carriers with different supports at 850°C. After three cycles, the H2 yields for the oxygen carriers Fe2O3/ZrO2 and Fe2O3/CeO2 came to a stable state, decreasing slightly with the cycle number, which indicated that the structure of the oxygen carriers tended to stabilize. However,

there was obviously a loss in the H2 yield for Fe2O3/Al2O3, which decreased by 19.0% from the 3rd cycle to the 10th cycle. The H2 yield followed the sequence: Fe2O3/ZrO2 > Fe2O3/CeO2 > Fe2O3/Al2O3. And the average values of the H2 yields from the 3rd to the 10th cycles were 1.6L, 1.3L, 0.6L, respectively. Fe2O3/ZrO2 showed high H2 yield due to the high chemical and thermal stability of ZrO2. As for Fe2O3/CeO2, its poor performance likely resulted from the


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solid-state migration of iron cations and subsequent enrichment on the oxygen carrier surface, which led to agglomeration and lowered lattice oxygen accessibility.15 As a note of caution, however, although CeO2 demonstrated reducibility relative to ZrO2 and Al2O3, the oxygen carrying capacity was negligible compared with active metal oxides, such as Fe2O3 and NiO.24 The poor H2 yield for Fe2O3/Al2O3 could be attributed to the generation of inactive spinel FeAl2O4 and the agglomeration phenomenon.37-39 Further more, carbon deposition is another important factor which have a serious impact on the H2 purity and should be sufficiently considered.

3.2. Carbon Deposition. Carbon deposition could be removed both in the steam oxidation stage and air oxidation stage. Nevertheless, the attention was paid to the steam oxidation stage in this paper, which exerted direct influence on the H2 purity. In the condition described in section 2.3, CO and CO2 could not be detected by the gas analyzer in the steam oxidation stage, that is to say, the concentrations of CO and CO2 were beyond the detection limit (0.01% in volume) of the gas analyzer. Therefore, 100% CO (flow rate 0.5L/min) was sent into the fluidized bed instead of 30% CO in the reduction stage to investigate and distinguish the resistance of the oxygen carriers toward carbon deposition, and the reduction condition are same for all the three oxygen carriers. However, for the oxygen carrier Fe2O3/CeO2 and Fe2O3/Al2O3, carbonaceous gases, i.e., CO and CO2, were beyond the detection limit of our instrument in the steam oxidation stage even with 100% CO. It seems that the result doesn’t coincide with the previous studies,20-22, 36 which suggested obvious carbon deposition; the reason should be that the purging time was generally fixed and too short to oxide the carbon deposition by the lattice oxygen. However, the purging


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time was long enough to guarantee zero CO and CO2 at the end of the nitrogen blowing stage in this paper, so the true quantity, other than the relative amount of the carbon deposition, can be obtained. On the other hand, the high lattice oxygen conductivity in Fe2O3/CeO2 and the lack of metallic iron on the surface of Fe2O3/Al2O3 also helps to eliminate the carbon deposition.15, 26 Considerable carbonaceous gases could be detected in the steam oxidation stage for Fe2O3/ZrO2 as depicted in Figure 4. It can be seen that carbonaceous gases only exited in the first 3 mins. And Figure 4 just illustrates the first 5 mins of the steam oxidation stage since the foucus of this section is the carbonaceous gases and the resulting decrease in H2 purity. The maximum concentration of CO2 is just about 0.02%, nevertheless, the peak value for CO reaches approximately 1.0%, much more than that of CO2, indicating that the conversion rate of CO for the water gas shift reaction was very low without catalyst. Also, high temperature (850 °C) was disadvantageous to the water gas shift reaction, which is exothermic reaction. Figure 5 shows the H2 purity and carbonaceous gases contamination in the steam oxidation stage for Fe2O3/ZrO2, which is calculated from the gases concentrations illustrated in Figure 4 by removing the nitrogen gas in the off-gas. Since the CO2 concentration, just 0.08% at most, was much lower than those of H2 and CO, the concentration curves for H2 and CO are almost invariably symmetrical. It is also worth noting that there are two troughs and peaks in the concentration curves of H2 and CO in the first 1.2 mins, during which the H2 and CO concentration increased continually. The shape of the curves just originates from the relative concentration of H2 and CO. Nevertheless, the concentration curves of the initial time in Figure 5 cannot reflect the gas concentration in the continous CLHG plants.


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Kang et al.40 found that ZrO2 supported oxygen carrier showed more carbon deposition than that of CeO2. The reason for low carbon deposition on Fe2O3/CeO2 is that CeO2 can provide oxygen species from the lattice oxygen to oxidize the surface carbon and produce CO or CO2. However, for Fe2O3/ZrO2, plenty of Fe or FeO originating from deep reduction promote the Boudouard reaction, leading to much carbon deposition. The gas analyzer could not detect CO or CO2 generated from the carbon deposition for Fe2O3/Al2O3 probably due to the sintering of the particle surface and the resulting reduction of active sites Fe or FeO. The fuel cell with a polymeric electrolyte membrane (PEM) has been chosen by most of the automotive companies as the power source for future vehicles,41 however, the level of CO contamination for the hydrogen used in a PEM fuel cell must be below ∼50 ppm, so as not to poison the platinum clusters on the anode.42 The average H2 purity for Fe2O3/ZrO2 over the whole steam oxidation stage corresponding to Figure 5 was 99.80%. Therefore, only the hydrogen yielded by Fe2O3/Al2O3 and Fe2O3/CeO2 can meet the requirement. To sum up, Fe2O3/CeO2 is the most suitable oxygen carrier for hydrogen production with the consideration of H2 yield and purity in CLHG. Furthermore, carbonaceous gases, i.e. CO and CO2, could be detected with faint concentration in the air oxidation stage, which were relatively close to the detection limit of our instrument. This illustrated that carbon deposition was almost completely consumed in the steam oxidation stage. Since the little carbonaceous gas generated in the air oxidation stage has no impact on the H2 purity and almost no influence on the CO2 capture efficiency, there is no need for further measurement or research on the small quantity of carbon deposition gasified in the air oxidation


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stage for the time being.

3.3. Reactivity and Stability. The CO2 concentration in the reduction stage of the 10th cycle is shown in Figure 6 to illustrate the reactivity and oxygen transfer capacity of different oxygen carriers. As can be seen in Figure 6, the reactivity of the oxygen carriers in the initial time of the reduction stage followed the sequence: Fe2O3/Al2O3 > Fe2O3/ZrO2 > Fe2O3/CeO2. The CO2 concentration for Fe2O3/Al2O3 reached the peak value first at about 8 min and then decreased significantly. However, the CO2 concentration for Fe2O3/ZrO2 and Fe2O3/CeO2 continued to climb and that of Fe2O3/ZrO2 get the highest peak value 17.0%. The CO2 concentration for Fe2O3/CeO2 took the most time (about 12min) to reach the maximum and then decreased so slow that it exceeded that of Fe2O3/ZrO2 at 12 ~ 25 min, even though it suffered from agglomeration problem. This phenomenon further illustrated the excellent storing and releasing oxygen property of CeO2, which transferred the oxygen species from bulk to surface, promoting the reduction of Fe2O3.26 Also, the formation of CeFeO3 and solid solution between CeO2 and Fe2O3 could promote the oxygen mobility and the reduction of the oxygen carrier.33 In conclusion, Fe2O3/ZrO2 and Fe2O3/CeO2 showed better reactivity and oxygen transfer capacity, and the change of the CO2 concentration for Fe2O3/CeO2 exhibited much bigger inertia. Figure 7 illustrates the conversion rates of oxygen carriers for the reduction stage in the 10th cycle at 850 °C. The reactivity of the investigated oxygen carriers can be further understood. As shown in Figure 7, the ROX of all the investigated oxygen carriers reached above 33.3%, indicating that the iron oxide in all oxygen carriers was reduced partly to Fe. The conversion rates


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at the end of the reduction stage followed the sequence: Fe2O3/ZrO2 > Fe2O3/CeO2 > Fe2O3/Al2O3, consistent to those of H2 yield in Figure 3. For Fe2O3/ZrO2, the ROX was 68.0% at the end of reduction stage, which means a mole fraction of 52% iron was in the form of Fe and 48% in FeO. However, it was not excluded that some Fe2O3 were only reduced to Fe3O4 or even in its original state in the interior of the particles. It can also be seen from Figure 7 that Fe2O3/Al2O3 got the highest conversion rate in the initial reduction stage when the ROX was below 11.1% in the reduction process of Fe2O3 to Fe3O4, corresponding to Figure 6. Afterwards, the ROX of both Fe2O3/ZrO2 and Fe2O3/CeO2 exceeded that of Fe2O3/Al2O3 as it increased from 11.1% to 33.3% in the reduction process of Fe3O4 to FeO. Meanwhile, the gap between Fe2O3/Al2O3 and other two oxygen carriers widened as the ROX grew further. In addition, Figure 7 also shows that the difference between the ROX values of Fe2O3/ZrO2 and Fe2O3/CeO2 first rose, and then declined, which can be speculated from the comparison of CO2 concentration for Fe2O3/ZrO2 and Fe2O3/CeO2 in Figure 6, and it finally kept almost constant after the ROX went above 33.3%. Overall, the oxygen loss rates of all the investigated oxygen carriers declined as CO reduction stage proceeded. The H2 concentration in steam oxidation stage for three Fe-based oxygen carriers is depicted in Figure 8 to investigate the effect of supports on the reactivity of H2 generation. The Fe2O3/CeO2 obtained the highest H2 concentration in the initial steam oxidation stage until the H2 concentration come to a steady state at about 30%. The H2 concentration for Fe2O3/ZrO2 got peak value similiar to Fe2O3/CeO2, while the H2 concentration at the initial stage was almost the same for Fe2O3/ZrO2 and Fe2O3/Al2O3 and the former exceeded the latter due to higher ROX value which


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also led to longer peak duration for Fe2O3/ZrO2 than Fe2O3/CeO2. On the whole, most of the H2 was generated in the first 10 mins for all the three Fe-based oxygen carriers. Though Fe2O3/CeO2 did not achieve the highest ROX value, it got the maximum H2 concentration in the initial steam oxidation stage and obtain high peak value of H2 concentration as Fe2O3/ZrO2, indicating the excellent reactivity of Fe2O3/CeO2 in the steam-iron reaction due to the promotion of CeO2.23 The H2 concentration fluctuations may be due to the fluctuations of water flow rate from the peristaltic pump. The stability of Fe2O3/CeO2 was further researched, for Fe2O3/CeO2 showed the best comprehensive performance, by comparing the CO2 concentration of the reduction stage in redox cycles. As can be seen in Figure 9, the peak values of CO2 concentration for Fe2O3/CeO2 decreased from 16.6% to 14.9% from the 1st to 3rd cycle, but those for the 3rd and 10th cycles were very close to each other, and the CO2 concentration curves were almost the same, which indicated the stability of reactivity for Fe2O3/CeO2. It can also be found in Figure 9 that the CO2 concentrations at the end of the reduction stage in the 3rd and 10th cycles were higher than that in the 1st cycle, and that in the 10th cycle was the highest. This phenomenon can be attributed to the increasing amount of perovskite CeFeO3 and Fe-Ce solution along with redox cycles, which could promote the reactivity of Fe2O3/CeO2, counteract the negative impact originating from sintering and enrichment of iron oxide on the surface of the oxygen carrier and guarantee the stability of Fe2O3/CeO2.26, 33 The stability of Fe2O3/CeO2 was further investigated in view of the H2 concentration in the steam oxidation stage of the 1st, 3rd and 10th cycles, as shown in Figure 10. The H2 concentration


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in the steam oxidation in the 1st cycle was only slightly higher than that in the 3rd and 10th cycle. However, in general, all the peak values of H2 concentration for the three investigated cycles were about 30% considering the H2 concentration fluctuations, and the H2 concentration curves for the 3rd and 10th cycles were very close to each other, indicating the high reactivity stability of Fe2O3/CeO2 again. In addition, it can also be found in Figure 10 that the H2 concentration decreased sharply at about 10 min, and the H2 yield was mainly concentrated in the first 10 mins. It can be calculated that the H2 yield in the first 10 mins accounted for 78.0%, 80.8% and 81.1% of the total yield in the 1st, 3rd and 10th cycles, respectively. This property can be utilized to improve the efficiency and compactness of the reactors for CLHG.

3.4. Effect of Temperature on Reactivity for Fe2O3/CeO2. Furthermore, the effect of temperature on H2 yield for Fe2O3/CeO2 from 750 °C to 950 °C was investigated, as is illustrated in Figure 11. The tests were carried out for 3 cycles at every temperature point and the H2 yield in the 3rd cycle was taken for comparison. It can be seen that the H2 yield increased at first and then dramatically decreases afterwards with the rise of the temperature, obtaining the highest H2 yield 1.3 L/min at 850 °C. The lower H2 yields below 850 °C were due to the restriction of lower temperature on the reaction kinetics, while the lower H2 yields above 850 °C can be attributed to the well-known significantly lower thermal stability of CeO2. The grains of the Ce−Fe mixed oxides appeared to grow and adhere with each other at high temperature, leading to sintering due to the lack of thermal stability for CeO2.33, 40 Figure 12 shows the CO2 concentration in reduction stage for Fe2O3/CeO2 in the 3rd cycle at different temperatures. The peak value of the CO2 concentration increased as the temperature


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raised and got its maximum at 850 °C, then it decreased at high temperature. Moreover, the CO2 concentration achieved the peak value in increasingly short time and then decreased more and more rapidly below 900 °C. The peak value dropped sharply and the CO2 concentration growth rates reduced at the beginning of the reduction stage as the temperature increased further to 950 °C owing to the sintering problem. The H2 concentration in steam oxidation stage for Fe2O3/CeO2 in the 3rd cycle at different temperatures is depicted in Figure 13. As can be seen, the peak values are all about 30% at all temperatures except 950 °C at approximately 25%. Moreover, the peak duration is the longest at 800 °C and 850 °C, and the main reaction period is the first 10 mins in which most of the H2 was generated, for instance, the H2 yield at 850 °C in the first 10 mins accounted for 80.8% of the total yield. From the results and discussion above, it is clearly noted that the oxygen carrier Fe2O3/CeO2 showed the best reactivity and the highest H2 yield at 850 °C.

3.5. Morphology and Components. In order to achieve a better understanding of the physical and chemical evolution of the oxygen carriers, the SEM, EDX and XRD analysis of Fe2O3/CeO2, Fe2O3/ZrO2 and Fe2O3/Al2O3 were conducted in view of morphology and components of the oxygen carriers, as shown in Figure 14, 15 and 16, respectively. As can be seen in Figure 14, grains with a size of 0.2~0.5 µm over the surface of the particles led to similiar porous structure for all the three kinds of fresh oxygen carriers except for slight sintering on the particle surface of Fe2O3/CeO2. However, the SEM images were quite different from each other after 10 redox cycles. For Fe2O3/CeO2 in Figure 14(b), small grains agglomerated together and serious sintering occurred after redox cycles, blocking the pores and preventing


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sufficient contact between reactant gas and oxygen carrier. It seems that all the grains on the particle integrated into a dense entity. Nevertheless, the oxygen carrier Fe2O3/CeO2 still kept high reactivity and stable H2 yield along with the redox cycles due to the oxygen mobility property of the reducible support CeO2,23, 33, 43 as verified by Figure 3 and Figure 10. Therefore, it is resonable to add some other inert material that shows high resistance to sintering to promote the reactivity of Fe2O3/CeO2, which will be studied in our later researches. Concerning Fe2O3/ZrO2 shown in Figure 14(d), larger grains with high sphericity and a size of 0.5~1.0 µm were generated with good porous structure due to the high melting point and chemical stability of ZrO2. The porosity of the material was preserved at the micrometer scale, suggesting that intraparticle diffusion was not significantly affected by cycling, resulting in the highest H2 yield as can be seen in Figure 3. In Figure 14(f), much larger grains with a size of about 2 µm were generated owing to sintering phenomenon, while there are much bigger holes on the surface of the particle compared with the fresh sample in Figure 14(e), which could counteract the sintering effect to a certain extent. To determine the surface composition changes over redox cycles, the EDX analysis of various oxygen carriers before and after 10 redox cycles were conducted as shown in Figure 15, and the scanned area is all about 100 µm × 100 µm. The average mole fraction of Fe element on the surface of the oxygen carrier particles corresponding to Figure 15 are demonstrated in Table 2, in which nFe , nCe , nZr and nAl are the atom moles of Fe, Ce, Zr and Al, respectively. For the fresh samples shown in Figure 15(a), (c) and (e), all the elements were distributed relatively uniformly on the surface of particles. The much lighter color of the Ce elemental mapping illustrated the lower mole concentration of Ce compared to Zr and Al elements. As can be seen in Table 2, the


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Fe mole ratios are all fairly close to the theory values, indicating the uniform distribution of iron oxide and the supports. After 10 cycles, the elemental mappings are completely different as can be seen in Figure 15(b), (d) and (f). The Ce element can hardly be detected in Figure 15(b), iron oxide is likely to have been enriched on the oxygen carrier surface since EDX detects elemental compositions within a few micrometers from the sample surface.15 What’s more, Table 2 shows that the average mole fraction of Fe element went up to 99.23%, consistent with the elemental mapping of Ce in Figure 15(b). For the Fe2O3/ZrO2 in Figure 15(d), the isolated regions enriched with Zr are identified. As to the Fe2O3/Al2O3 in Figure 15(f), it is similiar to that in fresh state in Figure 15(e). However, it can been seen in Table 2 that the mole fraction of Fe element on the surface of the particles all increased by about 10%, indicating that the enrichment of iron oxide occurred on both Fe2O3/ZrO2 and Fe2O3/Al2O3 as well,44 while the enrichment degree was much less serious than that of Fe2O3/CeO2. Figure 16 shows XRD patterns of various Fe-based oxygen carriers. In Figure 16(a), only Fe2O3 and CeO2 existed for the fresh sample. After air oxidation in the 10th cycle, the intensity of CeO2 peaks decreased seriously, indicating an decrease in the crystal size of CeO2. However, the intensity of Fe2O3 peaks increased, and its characteristic diffraction peak corresponding to (110) face of Fe2O3 narrowed (see the inset in Figure 16(a)) after cycling, suggesting an increase in the crystal size of Fe2O3. Also, the characteristic peak shifted to lower 2θ angles due to the expansion of cell, which indicates the incorporation of Ce4+ ions into the Fe2O3 lattice to form Fe-Ce solid solution,34 enhancing the Fe-Ce interaction during the redox process and contributing to the decrease of the CeO2 peak intensity as well. The XRD pattern of Fe2O3/CeO2 in reduction state


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demonstrated not only the existence of Fe and FeO, but also CeFeO3 perovskite, which could intensify the oxygen mobility and promote the reactivity of Fe2O3/CeO2.15, 33 For the Fe2O3/ZrO2 in Figure 16(b), two phases of ZrO2, monoclinic (Baddelyite) and tetragonal zirconia, were present in the fresh sample. However, the tetragonal zirconia disappeared after redox cycles. Narwankar et al.45 found that the phase transition from the tetragonal to the monoclinic phase could take place at the reaction temperature of 1123 K when ZrO2 was mixed with Fe2O3. Fe2O3 and tetragonal ZrO2 peaks are more intense after redox cycles, which corresponds to larger crystallite sizes. As to the Fe2O3/Al2O3 in Figure 16(c), Fe2O3 and Al2O3 peaks are less intense for the sample in the oxidation state of the 10th cycle, suggesting an decrease in their crystallite sizes. In addition, an inert species, FeAl2O4, with high intensity peak, was found in the reduction state. Therefore, the support Al2O3 was not inert. Indeed, it has been found to participate actively in the oxidation and reduction processes, principally through the formation of hercynite and various solid solutions.44 Although the use of alumina as support has a positive effect on the oxygen transport capacity of Fe-based oxygen carrier and still almost reaches complete combustion of H2 and CO into H2O and CO2 if FeAl2O4 is formed in CLC,46 the presence of alumina retards the rate of reduction and may limit the extent of conversion of the iron oxide to Fe in CLHG, since much stronger reducing and oxidizing conditions were required to decompose the solid solutions to free the active iron oxides.44 Also, the generated FeAl2O4 does not react noticeably with steam.47 Hence, the stoichiometric yield of hydrogen was reduced with the addition of Al2O3, and Al2O3 is not suitiable as support for Fe-based oxygen carrier in CLHG.


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4. CONCLUSIONS Three Fe-based oxygen carriers, supported by Al2O3, ZrO2 and CeO2, were prepared by coprecipitation and investigated in a batch fluidized bed at 850°C using CO as fuel for CLHG. The conclusions were drawn as below. (1) The properties of the oxygen carriers, e.g. reactivity and stability, mainly depended on the support and its interaction with iron oxides; (2) The redox reactivity and H2 yield for the oxygen carriers followed the sequence: Fe2O3/ZrO2 > Fe2O3/CeO2 > Fe2O3/Al2O3, however, the carbon deposition resistance of Fe2O3/CeO2 and Fe2O3/Al2O3 were much better than that of Fe2O3/ZrO2, and no carbonaceous gas was observed in the steam oxidation stage even with 100% CO as the fuel gas in the reduction stage; Fe2O3/CeO2 was most suitable oxygen carrier for CLHG with consideration of H2 yield and carbon deposition, and it exhibited the highest reactivity at 850 °C; (3) Fe2O3/CeO2 suffered sintering in redox cycles, but it still kept high reactivity, redox stability and H2 yield due to the oxygen mobility property of CeO2 and the oxygen mobility enhancement originating from the formation of Fe-Ce solid solution and CeFeO3 perovskite, which could counteract the sintering problem and guarantee the stability of the oxygen carrier; (4) SEM images showed the serious sintering of the Fe2O3/CeO2 and also demonstrated the better sintering resistance for Fe2O3/Al2O3, especilly Fe2O3/ZrO2; EDX analysis illustrated the enrichment of Fe2O3 on the surface of all three oxygen carriers, especilly Fe2O3/CeO2 with mole fraction of Fe element at 99.23%. (5) XRD analysis verified the formation of Fe-Ce solid solution and perovskite-type CeFeO3 that could guarantee the reactivity and stability of Fe2O3/CeO2, and FeAl2O4 in Fe2O3/Al2O3 that retarded the rate of reduction and may limit the extent of conversion of the iron oxide to Fe in CLHG.


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

ACKNOWLEDGEMENTS The authors gratefully express thanks to the National Natural Science Foundation of China (51576042), the National Program on Basic Research Project (2016YFB0600802), and the Natural Science Foundation of Jiangsu (BK20160672) for financial support of this research.

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Figure 1. Schematic diagram of CLHG process with Fe2O3 as the oxygen carrier

Figure 2. Schematic diagram of the experimental setup


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2.5 Fe2O3/CeO2 Fe2O3/ZrO2

H2 Yield (L)

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0.12 0.08 0.04

CO2 contamination (%)

Figure 3. Effect of cycle number on H2 yield for oxygen carriers with different supports at 850 °C

0.00

t=1.2 min 0

1

2 Time (min)

3

4

-0.04

5

Figure 4. Carbonaceous gases and H2 concentrations in the first 5 mins of the steam oxidation stage for Fe2O3/ZrO2 at 850 °C 25

0.24

20

0.16

CO CO2

95

15

90

10

t=1.2min

5

85

0.08

0.00

-0.08

CO2 contamination in H2 (%)

H2

CO contamination in H2 (%)

100

H2 purity (%)

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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0

80 0

1

2

3

4

5

-0.16

Time (min)

Figure 5. H2 purity and carbonaceous gases contamination in the first 5 mins of the steam oxidation


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stage for Fe2O3/ZrO2 at 850 °C Fe2O3/CeO2

20

CO2 Concentration (%)

Fe2O3/ZrO2 Fe2O3/Al2O3 15

10

5

0 0

5

10

15

20

25

30

35

Time (min)

Figure 6. CO2 concentration in reduction stage for three Fe-based oxygen carriers in the 10th cycle at 850 °C Conversion rates 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

Energy & Fuels

75 Fe2O3/CeO2 Fe2O3/ZrO2

60

Fe2O3/Al2O3

45 ROX=33.3%

30 ROX=11.1%

15 0 0

5

10

15 20 Time (min)

25

30

35

Figure 7. Conversion rates in reduction stage for three Fe-based oxygen carriers in the 10th cycle at 850 °C


Energy & Fuels

Fe2O3/CeO2

H2 Concentration (%)

40

Fe2O3/ZrO2

15

Fe2O3/Al2O3

10

30

1

2

20

10

0 0

5

10 Time (min)

15

20

Figure 8. H2 concentration in steam oxidation stage for three Fe-based oxygen carriers in the 10th cycle at 850 °C

CO2 Concentration (%)

20 1st Cycle 3rd Cycle 10th Cycle

15

10 9

5

8 7

0

6

0

29

10

31

33

35

20 Time (min)

30

Figure 9. CO2 concentration in reduction stage for Fe2O3/CeO2 in the 1st, 3rd and 10th cycles at 850 °C

1st Cycle 3rd Cycle 10th Cycle

40 H2 Concentration (%)

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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30

20

10

0 0

5

10 Time (min)

15

20

Figure 10. H2 concentration in steam oxidation stage for Fe2O3/CeO2 in the 1st, 3rd and 10th cycles at


Page 33 of 37

850 °C

H2 Yield (L)

1.50

Fe2O3/CeO2

1.25

1.00

0.75 750

800

850

900

950

Temperature (°C)

Figure 11. Effect of temperature on H2 yield for Fe2O3/CeO2

CO2 concentration (%)

0.20

750 °C 800 °C 850 °C 900 °C 950 °C

0.15

0.10

0.05

0.00

0

5

10

15

20

25

30

35

Time (min)

Figure 12. CO2 concentration in reduction stage for Fe2O3/CeO2 at different temperatures 750 °C 800 °C 850 °C 900 °C 950 °C

40

H2 concentration (%)

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

30

20

10

0 0

5

10

15

20

Time (%)

Figure 13. H2 concentration in steam oxidation stage for Fe2O3/CeO2 at different temperatures


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

(a) Fe2O3/CeO2—Fresh

(b) Fe2O3/CeO2—After 10 redox cycles

(c) Fe2O3/ZrO2—Fresh

(d) Fe2O3/ZrO2—After 10 redox cycles

(e) Fe2O3/Al2O3—Fresh

(f) Fe2O3/Al2O3—After 10 redox cycles

Figure 14. SEM images of various oxygen carriers before and after 10 redox cycles


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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

(a) Fe2O3/CeO2—Fresh

(b) Fe2O3/CeO2—After 10 redox cycles

(c) Fe2O3/ZrO2—Fresh

(d) Fe2O3/ZrO2—After 10 redox cycles

(e) Fe2O3/Al2O3—Fresh

(f) Fe2O3/Al2O3—After 10 redox cycles

Figure 15. EDX analysis of various oxygen carriers before and after 10 redox cycles


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

(a) Fe2O3/CeO2

(b) Fe2O3/ZrO2

(c) Fe2O3/Al2O3 Figure 16. XRD patterns of various oxygen carriers


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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

Table 1. Experimental procedure in a cycle No.

Experimental stage

Time(min)

Gas flow rate(L/min)

1

CO reduction

35

2

Nitrogen blowing

~ 40

3

Steam oxidization

20

N2:0.35; H2O(l):0.20mL/min

4

Nitrogen blowing

5

N2:0.5

5

Air oxidization

15

N2:0.42; O2:0.08

6

Nitrogen blowing

10

N2:0.5

N2:0.35; CO:0.15 N2:0.5

Table 2. Mole fraction of Fe element on the surface of various oxygen carriers Oxygen carrier

Fe2O3/CeO2

Fe2O3/ZrO2

Fe2O3/Al2O3

Fe atom ratio

nFe ( nFe +nCe )

nFe ( nFe +nZr )

nFe ( nFe +nAl )

Theory value

76.33%

69.75%

48.88%

Fresh sample

77.95%

69.59%

52.51%

Sample after 10 cycles

99.23%

80.31%

64.31%


Effects of CeO2, ZrO2, and Al2O3 Supports on Iron Oxygen Carrier for

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