NiAl2O4

Article Cite This: Ind. Eng. Chem. Res. 2019, 58, 13163−13173

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Influence of Synthesized Method on the Cycle Stability of NiO/ NiAl2O4 during Chemical Looping Combustion of Biomass Pyrolysis Gas Yan Sun, Enchen Jiang,* Xiwei Xu,* Jiamin Wang, Ren Tu, and Fuping Fan

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College of Materials and Energy, South China Agricultural University, Guangzhou 510640, China ABSTRACT: The industrial application of NiO/NiAl2O4 oxygen carrier is always restricted by its poor cycle stability. The cycle stability of NiO/NiAl2O4 synthesized by different methods was compared based on the change of activity, structure, and composition of oxygen carrier after several cycles. The results showed that the oxygen carrier obtained by the coprecipitation method exhibits better cycle stability, and the average CH4 conversion and CO2 selectivity of coprecipitated NiO/NiAl2O4 could reach 99% and 75%, respectively. The results of X-ray diffraction, scanning electron microscopy, and Brunauer−Emmett−Teller investigations proved that the phase separation of NiAl2O4 and the agglomeration of NiO were the main reasons for the decrease of cycle stability. The oxygen carrier synthesized by the coprecipitation method possessed high stability and activity after 10 cycles of chemical looping combustion, for it displayed a more proper distribution of NiAl2O4 and NiO. The research supplied a better choice for a synthesized highly active and stable oxygen carrier and explained the mechanism of the cycle stability decrease. NiO/Al2O3 was a potential option for CLC.13 However, an extensive observation was proposed in many literature reports that NiO/Al2O3 would form NiAl2O4 at high temperature (800−1400 °C) or after redox cycles, which would decrease the stability and reactivity of OCs.13−16 Shang17 and Boukha18 also found that NiAl2O4 formation was unavoidable in high temperature CLC. Several authors studied the performance of the oxygen carrier NiO/NiAl2O4 directly to avoid the formation of NiAl2O4 in the active part of NiO and Al2O3.17−20 Table 1 lists the performance of NiO/NiAl2O4 and some other Ni based OCs with different supports. It is clear that NiO/NiAl2O4 obtained a stable performance during cycle reaction. However, phase separation of NiAl2O4 to NiO and Al2O3 would aggravate the NiO sintering and decrease the stability of OCs. It is widely accepted that the structure of the oxygen carrier NiO/NiAl2O4 has an important influence on its properties and stability.13 However, the formation of the oxygen carrier structure is closely related to its synthesis method. In our previous studies,27 it was found that different synthetic methods influenced the distribution of inert carriers, active metals, and redox properties. However, the influence of the preparation method on the cyclic stability was not investigated and was rarely mentioned in previous studies.

1. INTRODUCTION Increasing CO2, especially from the combustion of fossil fuels, has caused a huge problem for the climate.1 Biomass is one of the most abundant renewable energy sources in the world. The utilization of biomass is beneficial for reducing the emission of CO2. However, during the utilization of the biomass, biogas is one of the main products. Biomass pyrolysis gas (BPG), which includes H2, CH4, CO, CO2, and a small amount of C2H4 and C2H6, is the byproduct of the pyrolysis of biomass for bio-oil or biochar. BPG is an unavoidable product of the utilization of biomass, but its lower C/H ratio determined that supplying heat or synthesizing chemical products at low cost2,3 is not a good choice for usinge these gases. Chemical looping combustion (CLC) is considered as a potential option for dealing with BPG.4 During the CLC procedure, CO2 could be captured internally and the reduced oxygen carriers (OCs) could be used to produce H2. However, the complex gas component is a severe challenge to the OCs. Based on the standard Gibbs free energies of reactions as a function of temperature, only a few metals are suitable for chemical looping reactions. Ni-based materials are some of the most extensive materials in the literature, due to the high reactivity of nickel.5,6 However, Ni-based OCs are restricted by carbon deposition and sintering. Also, the wide application of Ni-based OCs is also restricted by the toxicity and expensive price.7 Therefore, some inert materials were used by researchers to improve the properties of Ni-based oxygen carriers.8−12 It is noteworthy that Al2O3 attracts extensive attention for its high thermostability, good porous structure, and low cost. Both lab scale experiment and large scale application illustrated that © 2019 American Chemical Society

Received: Revised: Accepted: Published: 13163

May 8, 2019 June 28, 2019 June 28, 2019 June 28, 2019 DOI: 10.1021/acs.iecr.9b02532 Ind. Eng. Chem. Res. 2019, 58, 13163−13173

Article

Industrial & Engineering Chemistry Research

Table 1. Comparison of Ni Based Oxygen Carriers with Different Supports in Performance of Chemical Looping Reaction support

method

applied

CH4 conv/%

cycles

changes

ref

SBA-16 Al2O3 ZrO2 NiAl2O4 NiAl2O4

impregnation impregnation impregnation calcination spray drying

CLSR CLSR CLSR CLSR CLC

100 85−100 75 99 100

16 20−40 20 10 500

surface decreased activities decreased activities decreased no obvious change no obvious change

21 22−24 24 25 26

Figure 1. Scheme of the fixed bed reactor.

mixture was dried and calcined at the same conditions as the sample NiAl. The sample of NiO/NiAl2O4 obtained from the impregnation method was named “Imp OCs”. 2.2.4. Preparation of NiO/NiAl2O4 via Precipitation Method: Pcp OCs. The sample NiAl was used as support in the precipitation method as well. After the support was soaked in the nickel nitrate solution at 65 °C with stirring for 30 min, the precipitant solution (0.5 mol/L (NH4)2·CO3) was added to change the pH to 8. Then the sediment was dried and calcined at the same conditions as the sample NiAl. The sample of NiO/ NiAl2O4 obtained from the precipitation method was named “Pcp OCs”. 2.3. OC Characterization. X-ray diffraction (XRD) was used to analyze the crystalline phases of the OC samples. The data were obtained by a D8 Venture (Bruker) with Cu Kα radiation. The samples were scanned with a step of 10°/min, and the 2θ range was 5−80°. The particle size of the sample was calculated with the Scherrer formula. X-ray photoelectron spectroscopy (XPS) was used to analyze the binding energies of Ni 2p3/2. The data were obtained by a Thermo-Scientific spectrometer with 5 × 10−9 vacuum and monochromatized Al Kα X-rays (1486.6 eV). The final patterns were corrected by the Shirley and linear baseline. Scanning electron microscopy (SEM; Hitachi-S4800 FESEM) was used to investigate the surface morphology of the fresh OCs and the spent OCs. N2 adsorption/desorption was used to determine the specific surface areas and pore volumes of the samples. In order to collect the adsorption/desorption isotherms, samples were degassed at 300 °C for 8 h, and then the data were obtained at 77 K using a Micromeritics ASAP 2460 analyzer. The OC oxidation ability and temperature were determined with a STA449C Jupiter thermogravimetric analyzer (TGA). An approximately 15 mg sample was placed in a Al2O3 crucible, the mixed gas, which contains N2 (30 mL/min) and air (30 mL/ min), was introduced to the thermogravimetric analyzer with the

Therefore, the cycle stability of oxygen carriers synthesized in three different ways was investigated in this paper. The oxygen carrier performance was evaluated by CH4 conversion and CO2 selectivity. The material composition and structural evolution of the oxygen carrier were analyzed by X-ray diffraction (XRD), Xray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The influence of material distribution on structure and stability is further analyzed.

2. METHODS AND EXPERIMENT 2.1. Model Gas Composition. We chose a model biomass pyrolysis gas as the raw material; it is based on the result of the rice husk pyrolysis gas composition analysis.28 The composition is 4.01 vol % H2, 37.9 vol % CO, 38.5 vol % CO2, 15.8 vol % CH4, 1.91 vol % C2H4, and 1.88 vol % C2H6. 2.2. OCs Preparation. OCs which consisted of 60 wt % NiAl2O4 and 40 wt % NiO were prepared via impregnation (Imp), precipitation (Pcp), and coprecipitation (Cop) methods. The synthesis methods followed our previous research.27 The typical steps followed are in sections 2.2.1−2.2.4. 2.2.1. Synthesis of NiAl2O4: NiAl OCs. A mixture solution of nickel nitrate (8.72 g of Ni(NO3)2·6H2O) and aluminum nitrate (22.50 g of Al(NO3)3·9H2O), in which the mole ratio of Ni/Al is 2.0, was precipitated by a solution of 0.5 mol/L (NH4)2CO3. A kind of colloid was obtained until the pH reached 4−5. Then the spinel of NiAl2O4 was obtained after drying (65 °C,12 h) and calcining (400 °C, 2 h, and then 900 °C, 4 h). The sample of NiAl2O4 spinel was named “NiAl”. 2.2.2. Preparation of NiO/NiAl2O4 via Coprecipitation Methods: Cop OCs. The coprecipitation method is same as the NiAl2O4 preparation, but the mole ratio of Ni/Al is 4.1:1.0. The sample of NiO/NiAl2O4 obtained from the coprecipitation method was named “Cop OCs”. 2.2.3. Preparation of NiO/NiAl2O4 via Impregnation Method: Imp OCs. The sample NiAl was used as support in the impregnation method. The NiAl support was soaked in the nickel nitrate solution at 65 °C with stirring for 30 min. Then the 13164

DOI: 10.1021/acs.iecr.9b02532 Ind. Eng. Chem. Res. 2019, 58, 13163−13173

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Figure 2. Trends for the evolution of gas production in each cycle of different OCs.

temperature increased from 30 to 900 °C at a rate of 10 °C/min, and then the temperature was kept at 900 °C for 10 min. 2.4. Chemical Combustion of Biomass Gasification Gas. The process of chemical looping combustion of biomass gasification gas (CLC-BPG) contained reduction and oxidation

of OCs. The OC was reduced via the CH4 and CO in the BPG and produced CO2 products, and then it was oxidized via air. The scheme of the fixed bed reactor is shown in Figure 1 2.4.1. Oxidation of BPG via OCs. A 5 g sample of oxygen carrier was placed in the quartz tube. Then the tube reactor was 13165


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Figure 3. Average CH4 conversions and CO2 selectivities of different OCs.

that (i) CH4 was partly oxidized to produce CO due to consumption of lattice oxygen in OCs according to eq 3. Meanwhile, NiO was reduced to Ni. (ii) Metal Ni played the role of catalyst which promoted the increase of CO and H2 due to the CMR (CO2 methane reforming) and catalytic cracking according to eqs 4 and 5:

put into the tube furnace; the reaction tube was purged with N2 before the furnace temperature was increased to 800 °C. After that, the BPG model gas (40 vol % model BPG/60 vol % N2) was introduced into the fixed reactor. Each gas flow rate was controlled by an independent mass flow controller, the total mass flow was controlled at 100 mL/min (room temperature), and the oxidation process lasted for 45 min. 2.4.2. Recovery of OCs via Air Oxidation. After the CLC of BPG, the reactor was purged by N2 for 10 min, and then air was introduced via another mass flow controller with N2 as the carrier gas. Both air and N2 flow rates were kept at 50 mL/min. The OC was oxidized for 15 min, and the reactor was purged by N2 for 10 min before another cycle. The samples were collected after cooling to room temperature in the N2 atmosphere. The gas products were collected by a gas bag, and then analyzed by a gas chromatograph (Agilent 6842A GC). The column was HP-PLOT/Q (30 m × 0.53 mm × 40 μm), and N2 was employed as a carrier gas. CH4 conversion and CO2 selectivity was calculated based on the analysis results of the GC according to the following equations: Mc =

Cs =

CH4 + NiO/NiAl 2O4 → Ni + CO + 2H 2O + Al 2O3 (3) Ni

CH4 + CO2 → 2CO + 2H 2 Ni

CH4 → 2H 2 + C

(5)

When the oxygen was completely exhausted, the carbon formed from carbon deposition during CH4 catalytic cracking took eqs 6 and 7. NiΔ

C + H 2O ⎯⎯→ CO + H 2 NiΔ

C + CO2 ⎯⎯→ 2CO

V0CH4Fv0 − V1CH4Fv1

(6) (7)

The main reaction during the CLC-BPG was varied with the content of lattice O in OCs. Therefore, the CO2 selectivity and CH4 conversion were chosen as the index to analysis the oxygen supplied content and oxygen migration rate of oxygen carrier obtained by different methods. The oxygen carrier will gradually release oxygen during the reaction, and the CO2 selectivity will gradually decrease with the reaction time. It can be seen from Figure 2 that the CO2 content was gradually stabilized after 15 min, which indicates that the oxidation reaction was basically completed. Therefore, the oxygen supply amount and oxygen supply rate of the oxygen carrier are evaluated by calculating the average selectivity of CO2 in the first 15 min of the reaction. For NiAl in Figure 2a, 20% H2 and 30% CO were obtained in the first cycle with the trend increasing gradually. Meanwhile, the CO2 relative content was deceased rapidly from 50 to 10% in the first 20 min, while the CH4 relative content was maintained at 1%. It is clear that NiAl shows poor reactivity. However, the content of CO2 increased from the first cycle to the 10th cycle, which indicated more and more lattice oxygen released with cycles. It is evidence that the structure of NiAl2O4 gradually changed. Blas4 found NiO detached from the spinel structure during the cyclic reaction, which caused the spinel structure transformation. After loading NiO, the gas evolution trend during CLC via Pcp OCs and Imp OCs was similar to that for Cop OCs in the first cycle. However, the highest CO2 content decreased with the

V0CH4Fv0 V1CO2Fv1 − V0CO2Fv0 (V0CH4 + V0CO + 2V0C2H4 + 2V0C2H6)Fv0

Mc is the methane conversion rate; Cs is the CO2 selectivity; V1x is the gas concentration of x in the outlet gas; V0x is the gas concentration of x in the inlet gas; Fv0 is the gas flow rate of the inlet gas; Fv1 is the gas flow rate of the outlet gas.

3. RESULTS AND DISCUSSION 3.1. Cycle Stability Properties of OCs on Fixed Bed. Figure 2 shows the trends for the evolution of H2, CO, CH4, and CO2 during the CLC of BPG via three different OCs in 10 cycles. CO2 is the main product in the initial stage. Obviously, when the lattice oxygen in the oxygen carrier is sufficient, the CO2 selectivity is very high. Equations 1 and 2 are the complete combustion processes of CH4 and CO during CLC-BPG. CH4 + lattice O in NiO and NiAl 2O4 → CO2 + 2H 2O (1)

CO + lattice O in NiO and NiAl 2O4 → CO2

(4)

(2)

However, CO and H2 contents significantly figure in the CLCBPG’s increased reaction time. This can be explained by the facts 13166


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Figure 4. TPR results in the H2 reduction reaction between different OCs.

and broad. Meanwhile, the peak around 850 °C became much weaker. These suggest that the H2 consumption of NiO increased as the cycles increased. It is possible that the NiO separated from NiAl2O4, and the NiO content became more and more after 10 cycles. This was consistent with the increase of CO2 selectivity during the gas experiment. However, the H2 consumption of NiO in Cop OCs is not obvious. In the study of Gil-Calvo,20 it has been found that NiAl2O4 will be divided into NiO and Al2O3 during the reduction, while both NiO and Al2O3 will form NiAl2O4 again in air atmosphere with high temperature. Thus, the multiple cycle process may form an equilibrium between NiAl2O4 decomposition and regeneration. The composition change of Cop OCs was not obvious after multiple cycles, which illustrated the structural composition is closer to equilibrium status. Moreover, the NiO reduction temperature peaks of Imp OCs, Pcp OCs, and NiAlOCs increased from 400−550, 420−560, and 450−550 °C to 450−620, 460−610, and 450−610 °C, respectively after 10 cycles. Meanwhile, the NiAl2O4 reduction peak of those three OCs increased from 830, 850, and 850 °C to >950, >950, and 940 °C, respectively. Those induced that the lattice oxygen would not release in low temperature, which presents that lattice oxygen needs more energy to migrate. In addition, the peak of NiO of the three OCs was increased significant after 10 cycles, which further proved that NiO may separate from NiAl2O4.30 The changes of Cop OCs between the first cycle sample and 10th cycle sample are not obvious. Both samples showed three divided peaks around 550 °C, including peaks at 420/450, 500/ 520, and 570/600 °C (first cycle sample/10th cycle sample), which corresponded to the weak, medium, and strong interactions between NiO and the support, respectively. It is obvious that the medium strength peak significantly decreased after 10 cycles, while the weak one increased. Meanwhile, the peak of NiAl2O4 became broad. These suggested the NiO in Cop OCs was easier to be reduced after 10 cycles, and the cycle stability of Cop OCs was higher than that of OCs synthesized with other methods during redox reaction. In conclusion, NiO separating from NiAl2O4 played the key role in the reduce of OC stability. The composition property of reduced OCs was analyzed via air-TGA. The reduced OCs were oxidized during air-TGA, the Ni particles were recovered to NiO, and then the NiO combined with Al2O3 to form NiAl2O4 (Ni → NiO + Al2O3 → NiAl2O4).4 It is acceptable that the reduced property is related to the oxidation property. The oxygen supplied amount of the OCs in

cycle times. Especially the highest CO2 content obtained via Pcp OCs in the first 5 min decreased from 90% (during the first three cycles) to 70% (during the last two cycles), while the highest CO2 content obtained via Imp OCs decreased from 90% (in the first three cycles) to 80% (at the last four cycles). Meanwhile, the highest CO2 content is still maintained at 90% after 10 cycles for Cop OCs. It is worth noting that both Imp OCs and Pcp OCs exhibit decreasing trends for CO2 content with the cycle times, but the decrease intensity and rate of CO2 via Imp OC is slightly deeper and faster than those via Pcp OC. Moreover, the Cop OC shows a slight increase after the sixth cycle. It is possible that most of NiO may be concentrated on the surface for Pcp OCs and Imp OCs after several cycles. The accumulation of NiO results in agglomeration and causes a decrease in activity. For the Cop OC, the NiO concentrates in the interior; the accumulation of internal nickel oxide may produce a porous structure,26 which not only accelerates the release of lattice oxygen, but also provides more active sites. Those induced the cycle stability of OCs following the order Cop OCs > Imp OCs > Pcp OCs > NiAl OCs during CLC-BPG. This means that the OC obtained from the coprecipitation was more stable during the cycle reaction. The CH4 conversion and CO2 selectivity of different OCs in each cycle are shown in Figure 3. It can be found from Figure 3a that the CH4 conversion of OCs followed the order Imp > Cop > NiAl > Pcp. The CH4 conversion for Imp and Cop OCs was 100 and 99%, respectively, while it was only 60% for Pcp OCs. Figure 3b shows the CO2 selectivity for Imp and Pcp OCs decreased with the cycle times. The decrease of cycle stability can be explained by the fact that the structure of OCs became dense with the cycle times, which inhibited the immigration of lattice oxygen.29,30 It is worth noting that the CO2 selectivity of Cop OCs was stable during the 10 cycles, further indicating that the OC synthesis with the Cop method possessed stable structure and activity. 3.2. Role of Structure and Composition on Stability of OCs. 3.2.1. Reaction Properties. Hydrogen temperatureprogrammed reduction (H2-TPR) was used to evaluate the relationship between the composition and temperature. In Figure 4a, all OCs show a weak broad peak around 500 °C and a strong peak around 850 °C, which correspond to the reduction of NiO and NiAl2O4, respectively.31 It worth noting that the Cop OCs showed a stronger peak around 500 °C than the other OCs, which illustrated that Cop OCs contained more NiO. After 10 cycles, except for Cop OCs, the peak at 500 °C became strong 13167


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Figure 5. TGA results of different OCs after first (a) and 10th cycles (b).

Table 2. TGA Calculated Parameters of Different OCs after First and 10th Cycles OCs after first cycle

OCs after 10th cycle

OC category

oxygen amt (theor)/%

temp/°C

rate/%·°C−1

amt/%

temp/°C

rate/%·°C−1

amt/%

NiAl OCs Pcp OCs Cop OCs Imp OCs

9.04 13.96 13.96 13.96

387 345 375 370

0.63 0.97 1.53 1.21

4.3 7.8 12.2 12.0

540 570 540 575

0.60 0.75 1.13 0.98

6.5 7.5 10.5 8.8

Figure 6. XRD patterns (a) and particle sizes (b) of different OCs after first and 10th cycles.

the reduced stage could be determined via the weight changes during the oxidation stage. For example, in the methane oxidation stage, NiO lost oxygen and converted into Ni, and the

metal Ni was recovered to NiO in the air oxidation stage. Thus, the weight changes during the air oxidation stage could be used to calculate the oxygen supplied amount. However, the oxygen 13168


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Figure 7. Ni 2p3/2 XPS patterns of different OCs after first and 10th cycles.

methods may have more effective Ni and a more favorable morphological structure for the oxygen release reaction. It should be noted that the weight gain of Cop OCs, Imp OCs, and Pcp OCs decreased from 12.2, 12.0, and 7.8% to 10.5, 8.8, and 7.5 respectively after 10 cycles of reaction. It is possible that the oxygen involved in NiAl2O4 was more and more difficult to release around 800 °C. In contrast, the oxygen obtained from air is the highest (12.2 and 10.5%) for Cop OCs after the air-TGA test, which further proves that the cycle stability of Cop OCs is the highest. 3.2.2. Composition Properties. Figure 6 shows the XRD spectra of used oxygen carriers after the first and 10th cycles. It can be seen that the characteristic peaks of oxygen carriers are much sharper after several cycles. Meanwhile, the intensities of characteristic peaks at 18.8 and 32.1° corresponding to NiAl2O4 became much stronger after 10 cycles, indicating that the crystallite size and crystallinity of crystal became larger and higher. It is also worth noting that the NiAl OCs show an obvious NiO characteristic peak after 10 cycles, which is consistent with the TPR results. It further illustrates that the redox process leads to a significant phase separation of NiAl2O4 support, which is not beneficial for the stability of carrier oxygen.33 The changes of the other three OCs were not obvious. However, it is worth noting that, after 10 cycles of reaction, the peak position of NiAl2O4 in the oxygen carrier shows a tendency to shift to higher angle, while the 2θ angle of Al2O3 (ICSD 89/ 59522) is higher than that of NiAl2O4 (ICSD 73/0239), about 0.5°. Blas4 also finds the shift tendency represents that the NiAl2O4 shifts toward γ-Al2O3, which proves that the three OCs also showed phase separation. But it can be seen that Cop OCs exhibit the slightest shift, which also indicates that the structure of Cop OCs is more stable. The phase separation and particle size change of oxygen carriers synthesized with different methods are further analyzed. The results are shown in Figure 6b. It can be found that the grain size of NiO in the oxygen carrier increases significantly after 10 cycles. The increment of NiO size in different oxygen carriers follows the order NiAl > Imp > Cop > Pcp. Although the size of NiO in Pcp only increases

supplied amount was influenced by the agglomeration or sintering of Ni, because the sintered NiO cannot supply all of the oxygen during the reaction time. Air-TGA could directly reflect the OC changes in structure and reactivity via the oxidation temperature and weight changes. The oxygen recovery performance of the oxygen carrier can be evaluated by the oxygen recovery amount of the oxygen carrier in the air-TGA. Figure 5 shows the weight change of the oxygen carrier obtained by different synthesis methods in the CO-TGA test after the first and 10th reaction cycles. Details of the changes of the parameters are listed in Table 2. In Figure 5, the mass gain peak of the oxygen carrier can be clearly distinguished. After the first cycle, the mass gain peak of the OCs appears in the range 300−500 °C, and after the 10th cycle, the mass gain peak appears in the range 400−700 °C. Since the reduction product of the oxygen carrier is Ni, the reason for the weight gain of the sample is the transition of Ni to NiO. Compared with the first cycle sample curves in Figure 5a, it is obvious that the 10th cycle sample of different OCs shows the same changes in the oxidation temperature and the oxidation rate, with shifts to higher oxidation temperature with lower oxidation rate. Noticeably, the oxidation temperature of NiAl OCs and Cop OCs increased by 153 and 165 °C, respectively, which is less than those of Pcp OCs (175 °C) and Imp OCs (195 °C). It suggests that, after several cycles, the redox activity of OCs significantly decreased. However, the influence for coprecipitation OCs is slight. Otherwise, the reduction property of OCs mainly depended on its reduction degree during the oxidation reaction.32 By comparing with the theoretical oxygen amount, both NiAl and Pcp OCs exhibit poor oxygen release capacity. Since NiO is the main source of released oxygen, it can be inferred that the effective Ni content in the two oxygen carriers was low; the reason may be that the structure of tightly aggregated morphology affects the transmission and release of oxygen while strengthening the interaction between metal oxides. Cop and Imp OCs showed good oxygen release performances, and the oxygen releasing content can reach 87.41 and 85.98% of the theoretical one in the first cycle, which indicates that the oxygen carrier obtained by the Cop and Imp 13169


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Figure 8. Pore size distributions and N2 absorption/desorption isotherms of OCs sample after first (a) and 10th (b) reaction cycles.

by 1.3 nm, its NiO grain size of is much larger than those of the other oxygen carriers. The larger NiO grain size is the key factor for decreasing the redox activity of the oxygen carrier and restricting oxygen release. For the other OCs, the grain size of NiAl2O4 and NiO increased by 7.8 and 6.0 nm in the Cop carriers and by 11.8 and 16.9 nm in the Imp carriers, respectively. The increase of NiO size can be explained by two facts: (1) phase separation of NiAl2O4 leads to the increase of NiO content in the oxygen carrier, which promotes the aggregation and crystallization of the NiO particles; (2) the loaded NiO may agglomerate after several cycles. Above all, the results of XRD indicate that the composition of the OC synthesis with the coprecipitation method exhibits more stability than the other samples. XPS spectra of the OCs used in the first and 10th cycles are presented in Figure 7. They are consistent with the XRD results that the spinel NiAl2O4 and NiO can be seen in the oxygen carrier. Especially for the NiAl sample, the proportion of NiO in carrier oxygen increased from 15 to 40% after 10 cycles, which indicated that the phase separation occurred in oxygen carriers and more NiO appeared on the surface of OCs. For these NiO/NiAl2O4 OCs, the content ratio of NiAl2O4 to NiO is according to the order Cop (2.56) > Pcp (2.03) > Imp (1.87) after the first cycle. This illustrates that the NiO in the Pcp and Imp OCs mainly dispersed on the surface of the oxygen carrier, while the NiO evenly dispersed in the bulk in the Cop OCs. After the 10th cycle, the content ratio of NiAl2O4 to NiO in different oxygen carriers follows the order Cop (2.06) > Pcp (1.18) > Imp (1.13). Although the order is the same with the first cycle, the surface NiO content in the Pcp, Imp, and Cop oxygen carriers increased from 43.0, 44.6, and 28.1% to 55.9, 56.9, and 32.7% respectively. The content of NiO on the surface of Cop oxygen carriers only increases by 4.6% via the analysis of XPS, indicating that it is difficult for the migration of NiO from inside to surface after 10 cycles. However, the content of NiO on the surface of Pcp and Imp oxygen carrier increased by 12.9 and 12.3%, respectively. Therefore, the increase of surface NiO was mainly due to the phase separation of NiAl2O4, which further validates the results of XRD. 3.2.3. Structure Properties. The effects of cycles of reactions on the structure of the OCs were studied by SEM and Brunauer−Emmett−Teller (BET) analysis, respectively. Figure 8 shows the BET analysis results of the OCs experiencing the first and 10th cycles of reactions, and Table 3 lists the details of the structural parameters of the OCs gone through different cycles of reactions.

Table 3. Structure Parameters of OC Samples after First (a) and 10th (b) Cycles of Reaction no. of cycle

OC name

first

NiAl Pcp Cop Imp NiAl Pcp Cop Imp

10th

surf. area/m2·g−1 vol/cm3·mg−1 78.60 45.79 57.85 51.60 44.75 13.87 25.11 18.44

2.21 1.52 2.54 1.76 2.08 0.59 1.06 0.93

pore size/nm

particle size/nm

11.74 12.05 16.11 11.98 16.86 19.36 18.10 28.47

76.34 131.04 116.28 103.71 134.08 432.46 272.71 325.29

By comparison with previous studies,27 it could be found that the surface area and structure of the OCs did not change significantly after the first reaction. Figure 8a shows the nitrogen adsorption and desorption of different OCs after the first reaction, in which NiAl OCs exhibit the highest adsorption amount; this may due to that the NiAl spinel was not loading NiO and experiencing secondary calcination, so it exhibits good pore structure. NiAl OCs show type IV adsorption isotherm and H1 hysteresis loop, and the adsorption isotherm shows a rapid increase in adsorption amount in the range from 0.8 to 0.9 P/P0, indicating that most of the pores in the OCs were mesoporous. The pore size analysis shows that the pore size of NiAl mainly concentrates around 10 nm. The type of hysteresis loop indicated that the OCs are a stacked through-hole structure. Imp and Pcp OCs show the same structure as NiAl, but their maximum adsorption amount is lower than that of NiAl, which may be due to that the loading of NiO causes the partial pore structure to be blocked, resulting in lower specific surface area and pore volume. Cop OC was also not calcined secondarily, so its specific surface area could reach 57.85 m2·g−1. It should be noted that the Cop OC exhibits a rapid increase in the adsorption capacity at higher partial pressures (>0.9), indicating that its pore size is larger. As seen from Table 3, the average pore diameter of Cop OC is 16.11 nm, which is larger than those of the other three OCs. This may be due to the difference in material composition in the bulk phase, which leads to larger stacking channels. Figure 8b shows the BET analysis results of different OCs after 10 cycles. It can be seen that the maximum adsorption capacity of the OCs is significantly decreased, and the average pore diameter of the OCs is increased to 20 nm. However, the adsorption isotherms and hysteresis loop types of the OCs did not change significantly, indicating that the composition and the structure of the OCs remained the same as 13170


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Figure 9. SEM images of different OCs after first cycle: (a) NiAl; (c) Imp; (e) Pcp; (g) Cop. SEM images of different OCs after 10th cycle: (b) NiAl; (d) Imp; (f) Pcp; (h) Cop.

before. However, by comparing specific parameters, it can be found that the surface areas of NiAl, Imp, Pcp, and Cop decreased by about 43.1, 64.3, 69.7, and 56.6%, respectively. This indicates that the pore structure of the OCs is obviously blocked after repeated cycles, which is mainly due to the Ni

agglomeration and sintering under high temperature conditions, resulting in blockage of the OC pores. In addition, it can be seen from the change of the particle size that the particle sizes of the OCs show different growths, and the particle diameter growth rates of NiAl, Imp, Pcp, and Cop are 173, 330, 234, and 313%, 13171


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For Cop NiO/NiAl2O4 OCs, the average particle size increased from 30 to 80 nm after 10 cycles. It can be seen that there is no obvious agglomeration in the sample. Since NiO in the Cop oxygen carrier is uniformly distributed in the system, only 100 nm of NiO particles appears on the surface after 10 cycles, and the overall structure of the oxygen carrier does not change significantly. In addition, the results of the CH4 cycle reaction show that the stability of the structure also maintains a stable reaction performance. The oxygen supply amount and rate are basically stable during the 10 cycles. Although the agglomeration of the surface is not obvious, the BET results indicate there is also a corresponding aggregation phenomenon in the bulk phase, so the specific surface area is significantly reduced in the case where the pore volume and pore diameter change slightly. This also indicates that the phase separation of NiAl2O4 also causes agglomeration of Cop OCs, but its better material dispersion decreased the effect on its reactivity. This indicates that the Cop OC maintains a good morphology structure after cycles, which further proves that the stability of Cop OCs is the highest.

respectively. The phenomenon of particle agglomeration in the OCs is also further proved. SEM characterization could reflect the change of the structure and morphology of the OCs. The morphologies of four OCs used in the first and 10th cycles are presented in Figure 9, respectively. It can be seen that the NiAl OC is mainly composed of particles with 30 nm average particle diameter. Although some sintering particles are present on the surface of the OCs, most of the particles still distribute evenly. However, the average particle size of NiAl2O4 particles increased to 50 nm after 10 cycles, and more sintering particles with a long strip shape appeared on the surface of the OCs. In addition, it can be found that some small particles are attached around the sintered particles, which may indicate that the sintered particles are gradually agglomerated by small particles. It can be seen from the change of NiAl that the fine particles in the patterns are NiAl2O4 spinel, and the massive or layered large particles formed by agglomeration are NiO particles; the same change of structure is also found in the research of Remiro.31 In addition, the characteristic structural changes of the NiAl carrier after several cycles indicate that the precipitated NiO gradually aggregates on the surface of the carrier, indicating that it can provide more oxygen in the reaction. Therefore, in the reaction results shown in Figure 2a, it can be found that the CO2 yield in the product gradually increases as the number of cycles increases. However, the aggregation of the NiO particles causes the surface structure of the oxygen carrier to be destroyed, which directly confirms the results of BET. In the pattern of Imp OCs, the particle size is about 30 nm after the first cycle, and some densely massive particles with block shape could be observed in the OCs. It is possible that NiO is well bonded to the surface of NiAl2O4. After 10 cycles, the average particle diameter in the Imp OCs increased from 30 to 80 nm, and some massive particles about 500 nm could be observed clearly in the OCs. This indicates that the agglomeration speed of small particles in Imp OCs is faster than that in the NiAlOCs. It is possible that the NiO in the OCs is widely dispersed on the surface, and it is easier to contact each other and agglomerate into bulks. In addition, Many new NiO particles were produced via the phase separation of NiAl2O4, which may accelerate the sintering of the particles. For the Pcp OCs, the particle size of the OCs is around 80 nm after the first cycle, and obvious agglomeration of particles could be found in the pattern. The agglomerated bulk size is about 200 nm, whose shape is similar to the agglomerated particles in Figure 9d. After 10 cycles, obvious large bulks appeared in the OCs. It was possible that the particles gradually agglomerated together and formed a platelike structure during the reaction. The formation of platelike structure can be explained by two reasons: (1) the rapid agglomeration and compaction of the particles may be due to the uneven distribution of NiO caused by the synthesis method; (2) the NiO produced from the phase separation of NiAl2O4 may further accelerate the agglomeration. For the OCs obtained by the Imp and Pcp methods, due to the secondary load process, the supported NiO is mostly concentrated on the surface of the NiAl2O4 spinel, so the surface has an obvious sintering phenomenon, which directly affects the reactivity and pore structure of the oxygen carrier. As the number of cycles increases, the Ni element can gradually migrate to the surface and aggregate, which can also explain why the oxygen supply capacity of the oxygen carrier gradually decreases, as the number of cycles increases.

4. CONCLUSION In this article, the cyclic stabilities of NiO/NiAl2O4 synthesized in three different ways were compared. The gas experiment results showed that Pcp OCs and Imp OCs show good redox performance in the previous cycles, but their performance gradually decreased with cycle times. The Cop OC exhibited good stable performance after 10 cycles, and its average CH4 conversion and average CO2 selectivity were 99% and 75%, respectively. The structural analysis results indicated that synthesis method had a significant influence on the distribution of NiO. The distributed pattern of NiO was uneven for Pcp OCs and Imp OCs, while the NiO distributed uniformly in the Cop OCs. The results of XRD and XPS indicated that the phase separation of NiAl2O4 is one of the key points causing stability decrease. The phase separation for Pcp OCs and Imp OCs was severer than that for Cop OCs. The large scale application of NiO/NiAl2O4 synthesized with the coprecipitation method in the field of CLCBPG is worthy to be further investigated in the future.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (E.J.). *E-mail: [email protected] (X.X.). ORCID

Xiwei Xu: 0000-0003-0607-402X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

The authors acknowledge support from the National Science Foundation of China (Grant Nos. 51706075 and 51576071); the National Natural Science Foundation of China (NSFC) and Royal Society (RS), China (Grant No. 51711530230); and the Science and Technology Planning Project of Guangdong Province, China (Grant Nos. 2016A020210073 and 2015B020237010). 13172


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