Hydrogen Production from Chemical Looping Steam Reforming of Ethanol over Perovskite-Type Oxygen Carriers with Bimetallic Co and Ni B-Site Substitution

This paper describes the synthesis of a series of La1.4Sr0.6Ni1−xCoxO4 perovskite OCs using co-precipitation method by employing Co and Ni as the B-site components of perovskite and the synergetic effect of Co doping on chemical looping reforming of ethanol. A variety of techniques including N2 adsorption-desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM) and H2 temperature-programmed reduction (TPR) were employed to investigate the physicochemical properties of the fresh and used OCs. The activity and stability in chemical looping reforming were studied in a fixed bed reactor at 600 ◦C and a S/C ratio of three. The synergetic effect between Ni and Co was able to enhance the catalytic activity and improve the stability of perovskite OCs. La1.4Sr0.6Ni0.6Co0.4O4 showed an average ethanol conversion of 92.4% and an average CO2/CO ratio of 5.4 in a 30-cycle stability test. Significantly, the H2 yield and purity reached 11 wt.% and 73%, respectively. The Co doping was able to significantly improve the self-regeneration capability due to the increase in the number of oxygen vacancies in the perovskite lattice, thereby enhancing the sintering resistance. Moreover, Co promotion also contributes to the improved WGS activity.


Introduction
Hydrogen is an environmentally benign fuel source which mitigates the global dependency on fossil fuels [1,2].Chemical looping steam reforming (CLSR) is a novel hydrogen production technology which significantly differs from conventional steam reforming and other hydrogen production technologies.As illustrated in Figure 1, the oxygen carriers (OCs) were circulated between a fuel feed stage and an air feed stage in a CLSR process.The oxidation reaction in the air feed stage is exothermic, and the generated heat can be supplied for the following fuel feed reaction that converts fuel into hydrogen, thus reducing the energy consumption (Table 1) [3].Meanwhile, carbon deposition formed on the OCs can be eliminated during the air feed step.These advantages of CLSR enable it to be an economical and efficient approach for hydrogen production.
reaction.At the subsequent air feed stage, the active metal would be reversibly oxidized into the perovskite lattice, and the OC could be regenerated.The structural transformation of perovskite may inhibit the growth of metal particles [12].Additionally, the lattice distortion in perovskite would generate a large number of oxygen vacancies which can oxidize the formed carbon, thus improving the resistance to coke deposition [13].The structural and self-regeneration characteristics of perovskite could be regulated via partially substituting A or B sites components.Various types of A and B cations in perovskite structure have been studied.Kawi et al. [14] discovered that the Ni 3+ in LaNiO3 could be readily reduced to Ni and distributed on La2O3 support.Morales et al. [15] studied the La0.6Sr0.4CoO3−dperovskite for ethanol steam reforming and have observed the movement of metallic cobalt from perovskite lattice to the Table 1.Reactions during fuel feed step and air feed step.
Step Reactions

Fuel Feed
Step The selection of high-performance OCs is a key issue for CLSR.Among various metal oxides, Ni-based OCs have attracted much attention due to their strong ability to rupture C-C bonds, excellent redox property and low cost.However, its application is still limited by the severe coke deposition and metal sintering.Many previous investigations have committed to solving the deactivation of OCs [4,5].Single component metal nanoparticles are easy to agglomerate at high temperature, thus leading to severe active phase sintering.The redox property and mobility of metal nanoparticles could be modified by doping another metal, and the synergistic effect between two different metals would affect the catalytic activity of OCs [6].According to this principle, some bimetallic OCs have been prepared and investigated in steam reforming, such as Ni-Fe, Cu-Co, Cu-Ni and Co-Ni [7].The Cu-Co and Cu-Ni-based OCs exhibit high hydrogen selectivity and ethanol conversion, but show poor stability after multi-cycle reaction.In comparison, Ni-Co-based OCs show higher stability, which is due to the strong interaction between Co and Ni.Another efficient way to improve sintering resistance is to accommodate the Ni nanoparticles in unique-structure supports such as hydrotalcite, perovskite and montmorillonite [1,[8][9][10].Nowadays, perovskite-type metal oxides as OCs with a general formula of ABO 3 or A 2 BO 4 have been drawing much attention.Such structures are easy to interact with transition metals, which is beneficial to improve the dispersion of active component.The most significant property of perovskite is its self-regeneration ability in CLSR process [11].At the fuel feed stage, the perovskite would be reduced, and the active metal nanoparticles could generate from the perovskite lattice, which is responsible for the catalytic reaction.At the subsequent air feed stage, the active metal would be reversibly oxidized into the perovskite lattice, and the OC could be regenerated.The structural transformation of perovskite may inhibit the growth of metal particles [12].Additionally, the lattice distortion in perovskite would generate a large number of oxygen vacancies which can oxidize the formed carbon, thus improving the resistance to coke deposition [13].
The structural and self-regeneration characteristics of perovskite could be regulated via partially substituting A or B sites components.Various types of A and B cations in perovskite structure have been studied.Kawi et al. [14] discovered that the Ni 3+ in LaNiO 3 could be readily reduced to Ni and distributed on La 2 O 3 support.Morales et al. [15] studied the La 0.6 Sr 0.4 CoO 3−d perovskite for ethanol steam reforming and have observed the movement of metallic cobalt from perovskite lattice to the surface of OCs.Wu et al. [16] developed a series of La 1-x Ca x NiO 3 catalysts for hydrogen production by glycerol steam reforming.Ni and Co are commonly used as active B-site metals in perovskite OCs.However, as a single metal component, Ni or Co still suffers from severe metal sintering when it is generated from the perovskite [11].As mentioned above, the bimetallic structure perovskite can enhance the interaction of two metals, thus improving the dispersion of metal nanoparticles and suppressing metal sintering [17][18][19].
In this work, we propose a novel type of La 1.4 Sr 0.6 Ni 1−x Co x O 4 perovskite OC for hydrogen production by employing Co and Ni as the B-site components of perovskite.The synergistic effect between Ni and Co is conducive to the enhancement of the catalytic activity and the stability of perovskite OCs.The La 1.4 Sr 0.6 Ni 1−x Co x O 4 OCs are prepared by a coprecipitation method and are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), H 2 temperature-programmed reduction (H 2 -TPR) and inductively coupled plasma optical emission spectroscopy (ICP-OES).The reactivity and stability of the proposed OCs are performed in a fixed-bed reactor.

Characterization of OCs
The physical properties of fresh OCs are listed in Table 2.The BET areas of these prepared OCs are lower than 10 m 2 /g, which is consistent with the inherent characteristics of perovskite-type structures.The Ni or Ni-Co alloy particle sizes after reduction were calculated from the (111) plane at 44.7  Figure 2a illustrates the XRD profiles of the fresh OCs.All samples possessed clear perovskite diffraction peaks and no obvious NiO and Co 3 O 4 peaks were detected, indicating that pure perovskite-type OCs had been formed.The diffraction peaks of perovskite shifted to a higher angle with the doping of cobalt at B site.This is because that the radius of Ni 3+ ion is larger than that of Co 3+ .The substitution of Co 3+ results in decreases in the crystal size and d spacing, therefore shifting the 2θ to a higher degree [20].Zhao et al. [21] also observed that the main diffraction peaks of the perovskite phase shift to a higher 2θ value when cobalt ions are doped into the B site.The shift of diffraction angles suggests that cobalt and nickel are both incorporated into the perovskite lattice.The XRD profiles of the reduced OCs are exhibited in Figure 2b.After reduction, the perovskite structure disappeared, and the active component in perovskite is transformed into the metal phase before it migrates out of the perovskite lattice.The SrO 2 and La 2 O 3 also formed, accompanying the decomposition of the perovskite structure.3+ and Co 3+ , the La 2 O 3 would be uniformly mixed with the Ni-Co alloy.It has been demonstrated that the La promotion could significantly improve the sintering resistance and coke resistance in steam reforming process [10,22].The XRD patterns of the reduced OCs also displayed evident diffraction peaks of La 2 O 2 CO 3 , which are possibly generated via the reaction of La 2 O 3 and CO 2 [22].with the Ni-Co alloy.It has been demonstrated that the La promotion could significantly improve the sintering resistance and coke resistance in steam reforming process [10,22].The XRD patterns of the reduced OCs also displayed evident diffraction peaks of La2O2CO3, which are possibly generated via the reaction of La2O3 and CO2 [22].The H2-TPR patterns of La1.4Sr0.6Ni1−xCoxO4OCs are shown in of Figure 4. Generally, the valence of B site in an A2BO4 type perovskite is supposed to be 2+, while partial introduction of Sr 2+ in place of La 3+ forms unusual Co 3+ or Ni 3+ in these compounds [23,24].The reduction process should be divided into two steps, i.e., from 3+ to 2+ and then from 2+ to 0. Moreover, the reduction temperature ranges of Ni and Co are close to each other [25].Therefore, all the OCs obviously exhibited two broad reduction peaks at 300 °C~450 °C (LT, low temperature) and 500 °C~600 °C (HT, high temperature).The peaks at HT are attributed to the reduction of Ni  with the Ni-Co alloy.It has been demonstrated that the La promotion could significantly improve the sintering resistance and coke resistance in steam reforming process [10,22].The XRD patterns of the reduced OCs also displayed evident diffraction peaks of La2O2CO3, which are possibly generated via the reaction of La2O3 and CO2 [22].The H2-TPR patterns of La1.4Sr0.6Ni1−xCoxO4OCs are shown in of Figure 4. Generally, the valence of B site in an A2BO4 type perovskite is supposed to be 2+, while partial introduction of Sr 2+ in place of La 3+ forms unusual Co 3+ or Ni 3+ in these compounds [23,24].The reduction process should be divided into two steps, i.e., from 3+ to 2+ and then from 2+ to 0. Moreover, the reduction temperature ranges of Ni and Co are close to each other [25].Therefore, all the OCs obviously exhibited two broad reduction peaks at 300 °C~450 °C (LT, low temperature) and 500 °C~600 °C (HT, high temperature).The peaks at HT are attributed to the reduction of Ni 2+ or Co 2+ to Ni 0 or Co 0 , respectively.The Ni and Co metals peaks at HT merged into one reduction peak, indicating that Ni-Co alloy is formed [21].4. Generally, the valence of B site in an A 2 BO 4 type perovskite is supposed to be 2+, while partial introduction of Sr 2+ in place of La 3+ forms unusual Co 3+ or Ni 3+ in these compounds [23,24].The reduction process should be divided into two steps, i.e., from 3+ to 2+ and then from 2+ to 0. Moreover, the reduction temperature ranges of Ni and Co are close to each other [25].Therefore, all the OCs obviously exhibited two broad reduction peaks at 300 • C~450 • C (LT, low temperature) and 500 • C~600 • C (HT, high temperature).The peaks at HT are attributed to the reduction of Ni 2+ or Co 2+ to Ni 0 or Co 0 , respectively.The Ni and Co metals peaks at HT merged into one reduction peak, indicating that Ni-Co alloy is formed [21].The peaks at LT are ascribed to the reduction of Ni 3+ or Co 3+ to Ni 2+ or Co 2+ , respectively.The splitting of the LT peaks was observed for the Co doped perovskite samples.This could be due to the formation of the spinel type structure, such as SrLaCoO 4 , promoted by the presence of Sr when the molar ratio of La to Sr is lower than five in perovskite structure [26].The spinel phase was not detected by XRD analysis, and this is probably due to its small particle size.Moreover, the reduction peaks of LT and HT tend to move towards lower temperatures with the amount of Co, which may be due to the small particle size of perovskite particles [21].Smaller particles could provide a higher specific surface area (listed in Table  The peaks at LT are ascribed to the reduction of Ni 3+ or Co 3+ to Ni 2+ or Co 2+ , respectively.The splitting of the LT peaks was observed for the Co doped perovskite samples.This could be due to the formation of the spinel type structure, such as SrLaCoO4, promoted by the presence of Sr when the molar ratio of La to Sr is lower than five in perovskite structure [26].The spinel phase was not detected by XRD analysis, and this is probably due to its small particle size.Moreover, the reduction peaks of LT and HT tend to move towards lower temperatures with the amount of Co, which may be due to the small particle size of perovskite particles [21].Smaller particles could provide a higher specific surface area (listed in Table 2) and higher specific surface energy, thus lowering the reduction temperature

Activity Tests of OCs
Figure 5 displays the gas product distribution of the La1.4Sr0.6Ni1−xCoxO4OCs during CLSR.The hydrogen concentrations of all the three OCs increased dramatically within the first five minutes in the fuel feed step.The duration where no hydrogen is produced is known as 'dead time'.It is considered to be an important indicator of OCs' redox properties [19].In this period, H2 concentration was almost zero.Only when the Ni ions in the perovskite lattice are sufficiently reduced to metallic Ni can steam reforming of ethanol and subsequent in situ water gas shift (WGS) occur [29,30].In this stage, the perovskite is gradually reduced, therefore generating the active Ni-Co alloy component.Afterwards, the reforming reaction of ethanol and WGS begins to play the dominated role in the steady stage of the fuel feed step, thus leading to a great rise in H2 concentration in seconds.Compared with the La1.4Sr0.6NiO4, the Co doping samples showed shorter dead time due to the improved reducibility, corresponding with the TPR results.De Lasa et al. [31] investigated the reactivity and stability of Co-Ni/Al2O3 OC in multicycle CLC process, and they have concluded that the Co promotion improved the reducibility of OC by affecting the metal support interaction (MSI).The average H2 concentration of La1.4Sr0.6Ni0.6Co0.4O4OC is 17.1%, which is much higher than that of La1.4Sr0.6NiO4(14.3%) and La1.4Sr0.6Ni0.2Co0.8O4(12.3%).The Co doping samples showed higher CO2 concentration and lower CO concentration than counterparts of La1.4Sr0.6NiO4,indicating the enhanced WGS reactivity.Both Ni and Co are active for steam reforming of ethanol; however, the La1.4Sr0.6Ni0.6Co0.4O4showed superior performance in terms of H2 and CO2 concentrations.It is widely accepted that an effective catalyst for steam reforming of oxygenate compounds should not only be active in cleavage of C-C bond but also be active in WGS to remove CO formed on metal surface [32].Sinfelt et al. [33]    showing the highest H 2 concentration among three samples.Methanation is highly undesirable since it decreases H 2 yield.In our test conditions, the methane concentration is very low for all the samples, considering the strong extremely exothermic nature of methanation reaction.The Co doping samples showed high methane concentration in contrast with La 1.4 Sr 0.6 NiO 4 .This is because Co is more active in methanation reactions than Ni [34].

Activity Tests of OCs
higher activity during the reaction [34].Furthermore, Co showed high dehydrogenation activity, Ni is conducive to C-C cleavage.Thus, ethanol molecule is dehydrogenated on Co site of Ni-Co alloy, and the C-C bond is subsequently broken by the Ni site of Ni-Co alloy.This reaction route is fast, since the cleavage of the C-C bond in acetaldehyde is easier than that in ethanol molecule.Hence, La1.4Sr0.6Ni0.6Co0.4O4possesses improved capability of C-C rupture and moderate WGS activity due to the synergistic effect, therefore showing the highest H2 concentration among three samples.Methanation is highly undesirable since it decreases H2 yield.In our test conditions, the methane concentration is very low for all the samples, considering the strong extremely exothermic nature of methanation reaction.The Co doping samples showed high methane concentration in contrast with La1.4Sr0.6NiO4.This is because Co is more active in methanation reactions than Ni [34].The gas product concentrations in the air feed step are illustrated in Figure 5.The air feed step plays three roles in a CLSR process using perovskite type OCs.Apart from providing heat and eliminating coke deposition as in a conventional CLSR process, the air feed step would also regenerate the perovskite structure [35].It has been reported that Ni ions in the perovskite lattice would come out of the bulk in the fuel feed step and immerse back into the lattice in the air feed step [11], therefore suppressing the growth of Ni particles and maintaining its dispersion.The peak areas of C-containing gas products represent the coke deposition amounts.The coke deposition of the three The gas product concentrations in the air feed step are illustrated in Figure 5.The air feed step plays three roles in a CLSR process using perovskite type OCs.Apart from providing heat and eliminating coke deposition as in a conventional CLSR process, the air feed step would also regenerate the perovskite structure [35].It has been reported that Ni ions in the perovskite lattice  2. It has been reported that small-sized Ni particles would increase the saturation concentration of coke deposition, which leads to a low driving force for coke diffusion over the active phase [36].The temperature variation in the air feed step correlates with the heat release of Ni and Co oxidation and coke combustion.The end of the air feed step is signaled by the restoration of O 2 concentration to 21%.
Figure 6 shows the ethanol conversion and steam conversion of the La 1.4 Sr 0.6 Ni 1−x Co x O 4 OCs during the fuel feed stage.For all the perovskite OCs, the conversion of ethanol reached at least 78%, while the conversion of steam was above 25%.The ethanol conversion of La 1.4 Sr 0.6 NiO 4 was the lowest one among three samples, while the ethanol conversion of La 1.4 Sr 0.6 Ni 0.6 Co 0.4 O 4 OC reached almost 100% in the first six minutes and then leveled off.When further increasing the molar ratio of Co to 0.8, the ethanol conversion dropped to 95%, resulting from the insufficiency of Ni active centers for C-C cleavage.Additionally, with the increase of Co ratio, the steam conversion also increased.Mei et al. [37] calculated water adsorption and dissociation on the Rh (111), Ni (111) and Co (0001) surfaces using Density functional theory (DFT), and their results show that water dissociation into hydroxyl and hydrogen atom on Co (0001) surface is both thermodynamically and kinetically feasible.samples decreased in the following sequence: La1.4Sr0.6NiO4> La1.4Sr0.6Ni0.2Co0.8O4> La1.4Sr0.6Ni0.6Co0.4O4,which is consistent with the order of Ni-Co alloy particle sizes listed in Table 2.
It has been reported that small-sized Ni particles would increase the saturation concentration of coke deposition, which leads to a low driving force for coke diffusion over the active phase [36].The temperature variation in the air feed step correlates with the heat release of Ni and Co oxidation and coke combustion.The end of the air feed step is signaled by the restoration of O2 concentration to 21%.
Figure 6 shows the ethanol conversion and steam conversion of the La1.4Sr0.6Ni1−xCoxO4OCs during the fuel feed stage.For all the perovskite OCs, the conversion of ethanol reached at least 78%, while the conversion of steam was above 25%.The ethanol conversion of La1.4Sr0.6NiO4was the lowest one among three samples, while the ethanol conversion of La1.4Sr0.6Ni0.6Co0.4O4OC reached almost 100% in the first six minutes and then leveled off.When further increasing the molar ratio of Co to 0.8, the ethanol conversion dropped to 95%, resulting from the insufficiency of Ni active centers for C-C cleavage.Additionally, with the increase of Co ratio, the steam conversion also increased.Mei et al. [37] calculated water adsorption and dissociation on the Rh (111), Ni (111) and Co (0001) surfaces using Density functional theory (DFT), and their results show that water dissociation into hydroxyl and hydrogen atom on Co (0001) surface is both thermodynamically and kinetically feasible.

Stability Tests
Figure 7 shows the variation of ethanol conversion, CO2/CO ratio and H2 concentration of La1.4Sr0.6Ni1−xCoxO4OCs during multicycle tests at 650 °C.All OCs showed decreases in ethanol conversion as well as CO2/CO ratio after stability tests, while Co doping samples possessed more moderate variation than La1.4Co0.6NiO4.The H2 concentration of La1.4Sr0.6Ni0.6Co0.4O4showed an inconspicuous decrease of 0.3% at the end of the stability test, while the La1.4Sr0.6NiO4and La1.4Sr0.6Ni0.2Co0.8O4declined to 10.8% and 11.3%, respectively.Since the CO2/CO ratio has been considered as an indicator for WGS activity, the decrease in the ratio is associated with the loss of the synergistic effect of the Ni-Co alloy.It has been reported that the main obstacle for Ni-based OCs is coke deposition and metal sintering [7].Since the coke deposition can be eliminated in the air feed step, the OC deactivation in this work is mainly caused by metal sintering.As Ni is the active center for C-C bond cleavage, the decrease of ethanol conversion reflects the loss of active sites due to sintering.According to our previous investigation, the periodical movement of Ni into and out of the perovskite lattice (self-regeneration capability) could regenerate the OCs, thus improving the Ni sintering resistance [11].Indeed, all OCs in our present work exhibited high stability, and Co doping samples further improved the performance in stability tests.The promotion of the self-regeneration effect caused by Co doping could be explained as follows.Co substitution in La0.8Sr0.2Ni1−yCoyO3In conclusion, the CO 2 /CO ratios for all the samples showed moderate decrease after the tests.La

Preparation of OCs
A nickel-based perovskite structure OC was synthesized by the co-precipitation technique.Stoichiometric amounts of Ni(NO 3 ) 2 •6H 2 O (GR, 99%, Aladdin, Shanghai, China), La(NO 3 ) 3 •6H 2 O (AR, 99%, Aladdin, Shanghai, China), Sr(NO 3 ) 2 (99.97%,Aladdin, Shanghai, China) and Co(NO 3 ) 2 •6H 2 O (99.99%, Aladdin, Shanghai, China) were dissolved in deionized water under stirring.Then, ammonia of 1M was added dropwise to the solution under stirring at 50 • C, and maintaining a pH of about 8.5.Once the precipitation began to form, continuous stirring was carried out at 50 • C for one hour.Subsequently, the suspension was aged, filtrated and washed repeatedly until the pH was near neutral.After that, it was dried at 110 • C for about 15 h and calcined at 900 • C for 5 h.Finally, the fresh OCs were ground to 0.20-0.45 in diameter.The samples were denoted as La 1.4 Sr 0.6 Ni 1−x Co x O 4 (x = 0, 0.4 and 0.8) after the loading of Co.

Characterization of OCs
XRD (Shimadzu XRD-6000 powder diffractometer, Kyoto, Japan) was used to identify the crystal phases of fresh and reduced OCs, where Cu Kα radiation (λ = 1.5406Å) served as the X-ray source.TEM (FEI Tecnai G2, Hillsboro, OR, USA) was employed to investigate the morphology of the reduced OCs.The samples were ground and applied on a Cu grid coated with carbon film.H 2 -TPR (Quantachrome OBP-1, Boynto Beach, FL, USA) was applied to ascertain the interaction of the fresh metal-support OCs.The samples were first heated at 450 • C in a He flow and then cooled at 90 • C. Subsequently, a flow of 10% H 2 in He was introduced at 20 mL/min.Meanwhile, the temperature increased to 800 • C at 10 • C/min.ICP-OES was employed to analyze the accurate elemental composition of the OCs.Prior to measurements, the samples were processed with the HNO 3 to remove Ni species on surface.

Activity and Stability Tests
The setup of our experimental system was described in our previous publications [17][18][19].A mixture of OC of 1 g and quartz sand was loaded into a quartz tubular reactor (Φ15 × 800 mm).During the fuel feed step, a mixture of steam and ethanol with a steam to carbon (S/C) ratio of 3 was introduced into the reactor in a N 2 flow (300 mL/min).In the air feed step, an air flow of 600 mL/min was fed to eliminate carbon deposition and regenerate the OCs at 600 • C. The oxidation reactions ended when the oxygen concentration went back to ca.21 vol%.
The ethanol conversion and steam conversion were calculated in the following equations: n indicates the relevant molar flows (mol min -1 ).

Figure 1 .
Figure 1.Schematic diagram of chemical looping process.

Figure 1 .
Figure 1.Schematic diagram of chemical looping process.

Figure 2 .
Figure 2. XRD profiles of (a) fresh oxygen carriers and (b) reduced oxygen carriers.The TEM images of La1.4Sr0.6Ni0.4Co0.6O4after reduction are shown in Figure3.The Ni-Co particles with lattice stripes after the reduction were observed.Clear lattice stripes of the alloy appeared after the region was enlarged, and the interstitial void of which is 0.203 nm.The d spacing is between Ni (111) and Co (111), which indicates that the alloy is formed after the reduction of the Co-doped OCs.No apparent change was observed in terms of d spacing after reduction, suggesting Ni-Co alloy is very stable after reaction.The Ni-Co alloy particles were highly dispersed with the particle sizes of approximately 12 nm, which is consistent with the XRD results.

Figure 3 .
Figure 3. TEM profiles of reduced oxygen carriers.
2+ or Co 2+ to Ni 0 or Co 0 , respectively.The Ni and Co metals peaks at HT merged into one reduction peak, indicating that Ni-Co alloy is formed[21].

Figure 2 .
Figure 2. XRD profiles of (a) fresh oxygen carriers and (b) reduced oxygen carriers.

Figure 3 .
Figure 3. TEM profiles of reduced oxygen carriers.

Figure 3 .
Figure 3. TEM profiles of reduced oxygen carriers.

2
) and higher specific surface energy, thus lowering the reduction temperature [27].Additionally, Valderrama et al. [28] demonstrated that Co substitution in La 0.8 Sr 0.2 Ni 1−y Co y O 3 perovskite could promote the formation of positive holes and vacancies of lattice oxygen, therefore favoring the reduction process.It can be concluded that an appropriate amount doping of Co can enhance the reducibility of active components in the perovskite, thus improving the catalytic activity in a CLSR process.Catalysts 2018, 8, x FOR PEER REVIEW 5 of 12 Catalysts 2018, 8, x; doi: FOR PEER REVIEW www.mdpi.com/journal/catalysts [27].Additionally,Valderrama et al. [28]  demonstrated that Co substitution in La0.8Sr0.2Ni1−yCoyO3perovskite could promote the formation of positive holes and vacancies of lattice oxygen, therefore favoring the reduction process.It can be concluded that an appropriate amount doping of Co can enhance the reducibility of active components in the perovskite, thus improving the catalytic activity in a CLSR process.
Figure5displays the gas product distribution of the La1.4Sr0.6Ni1−xCoxO4OCs during CLSR.The hydrogen concentrations of all the three OCs increased dramatically within the first five minutes in the fuel feed step.The duration where no hydrogen is produced is known as 'dead time'.It is considered to be an important indicator of OCs' redox properties[19].In this period, H2 concentration was almost zero.Only when the Ni ions in the perovskite lattice are sufficiently reduced to metallic Ni can steam reforming of ethanol and subsequent in situ water gas shift (WGS) occur[29,30].In this stage, the perovskite is gradually reduced, therefore generating the active Ni-Co alloy component.Afterwards, the reforming reaction of ethanol and WGS begins to play the dominated role in the steady stage of the fuel feed step, thus leading to a great rise in H2 concentration in seconds.Compared with the La1.4Sr0.6NiO4, the Co doping samples showed shorter dead time due to the improved reducibility, corresponding with the TPR results.De Lasa et al.[31] investigated the reactivity and stability of Co-Ni/Al2O3 OC in multicycle CLC process, and they have concluded that the Co promotion improved the reducibility of OC by affecting the metal support interaction (MSI).The average H2 concentration of La1.4Sr0.6Ni0.6Co0.4O4OC is 17.1%, which is much higher than that of La1.4Sr0.6NiO4(14.3%) and La1.4Sr0.6Ni0.2Co0.8O4(12.3%).The Co doping samples showed higher CO2 concentration and lower CO concentration than counterparts of La1.4Sr0.6NiO4,indicating the enhanced WGS reactivity.Both Ni and Co are active for steam reforming of ethanol; however, the La1.4Sr0.6Ni0.6Co0.4O4showed superior performance in terms of H2 and CO2 concentrations.It is widely accepted that an effective catalyst for steam reforming of oxygenate compounds should not only be active in cleavage of C-C bond but also be active in WGS to remove CO formed on metal surface[32].Sinfelt et al.[33] demonstrated that Ni possesses faster rate of C-C bond rupture compared with other VIII group metals.Nevertheless, Ni has limited reactivity for WGS reaction, while Co possesses

Figure 5
Figure 5 displays the gas product distribution of the La 1.4 Sr 0.6 Ni 1−x Co x O 4 OCs during CLSR.The hydrogen concentrations of all the three OCs increased dramatically within the first five minutes in the fuel feed step.The duration where no hydrogen is produced is known as 'dead time'.It is considered to be an important indicator of OCs' redox properties [19].In this period, H 2 concentration was almost zero.Only when the Ni ions in the perovskite lattice are sufficiently reduced to metallic Ni can steam reforming of ethanol and subsequent in situ water gas shift (WGS) occur [29,30].In this stage, the perovskite is gradually reduced, therefore generating the active Ni-Co alloy component.Afterwards, the reforming reaction of ethanol and WGS begins to play the dominated role in the steady stage of the fuel feed step, thus leading to a great rise in H 2 concentration in seconds.Compared with the La 1.4 Sr 0.6 NiO 4 , the Co doping samples showed shorter dead time due to the improved reducibility, corresponding with the TPR results.De Lasa et al. [31] investigated the reactivity and stability of Co-Ni/Al 2 O 3 OC in multicycle CLC process, and they have concluded that the Co promotion improved the reducibility of OC by affecting the metal support interaction (MSI).The average H 2 concentration of La 1.4 Sr 0.6 Ni 0.6 Co 0.4 O 4 OC is 17.1%, which is much higher than that of La 1.4 Sr 0.6 NiO 4 (14.3%) and La 1.4 Sr 0.6 Ni 0.2 Co 0.8 O 4 (12.3%).The Co doping samples showed higher CO 2 concentration and lower CO concentration than counterparts of La 1.4 Sr 0.6 NiO 4 , indicating the enhanced WGS reactivity.Both Ni and Co are active for steam reforming of ethanol; however, the La 1.4 Sr 0.6 Ni 0.6 Co 0.4 O 4 showed superior performance in terms of H 2 and CO 2 concentrations.It is widely accepted that an effective catalyst for
of the bulk in the fuel feed step and immerse back into the lattice in the air feed step [11], therefore suppressing the growth of Ni particles and maintaining its dispersion.The peak areas of C-containing gas products represent the coke deposition amounts.The coke deposition of the three samples decreased in the following sequence: La 1.4 Sr 0.6 NiO 4 > La 1.4 Sr 0.6 Ni 0.2 Co 0.8 O 4 > La 1.4 Sr 0.6 Ni 0.6 Co 0.4 O 4 , which is consistent with the order of Ni-Co alloy particle sizes listed in Table

Figure 6 .
Figure 6.Ethanol and steam conversion in the fuel feed step.

Figure 6 .
Figure 6.Ethanol and steam conversion in the fuel feed step.

Figure 7
Figure 7 shows the variation of ethanol conversion, CO 2 /CO ratio and H 2 concentration of La 1.4 Sr 0.6 Ni 1−x Co x O 4 OCs during multicycle tests at 650 • C. All OCs showed decreases in ethanol conversion as well as CO 2 /CO ratio after stability tests, while Co doping samples possessed more moderate variation than La 1.4 Co 0.6 NiO 4 .The H 2 concentration of La 1.4 Sr 0.6 Ni 0.6 Co 0.4 O 4 showed an inconspicuous decrease of 0.3% at the end of the stability test, while the La 1.4 Sr 0.6 NiO 4 and La 1.4 Sr 0.6 Ni 0.2 Co 0.8 O 4 declined to 10.8% and 11.3%, respectively.Since the CO 2 /CO ratio has been considered as an indicator for WGS activity, the decrease in the ratio is associated with the loss of the synergistic effect of the Ni-Co alloy.It has been reported that the main obstacle for Ni-based OCs is coke deposition and metal sintering [7].Since the coke deposition can be eliminated in the air feed step, the OC deactivation in this work is mainly caused by metal sintering.As Ni is the active center for C-C bond cleavage, the decrease of ethanol conversion reflects the loss of active sites due to sintering.According to our previous investigation, the periodical movement of Ni into and out of the perovskite lattice (self-regeneration capability) could regenerate the OCs, thus improving the Ni sintering resistance [11].Indeed, all OCs in our present work exhibited high stability, and Co doping

Table 2 .
Physical properties of oxygen carriers.

Surface Area (m 2 /g) Average Pore Diameter (nm) Pore Volume (cm 3 /g) Ni Content a (wt.%) Co Content a (wt.%) Ni crystal Size after Reduction b (nm)
Table 3lists the d spacings and diffraction angles of the Ni particles formed from the reduction of La 1.4 Sr 0.6 Ni 1−x Co x O 4 OCs.The diffraction peaks of Ni (111) and Co (111) are located at 44.7 • and 44.2 • , respectively.The d spacing of Ni (111) increased with the amount of Co.Moreover, the diffraction angles also shift between Ni (111) and Co (111).The above results indicate Ni and Co are reduced from the OCs in the state of solid solution.The particle sizes calculated from the (111) plane are shown in Table 2, which are about 10 nm for all the samples.The weak peaks of La 2 O 3 are shown in the diffraction profiles of reduced OCs, which are derived from the reduction of Co and Ni.With the reduction of Ni
1.4 Sr 0.6 Ni 0.2 Co 0.8 O 4 possessed the highest average ratio of 5.4 during the stability test due to the strong WGS capability of Co promotion.The average ethanol conversion of La 1.4 Sr 0.6 Ni 0.6 Co 0.4 O 4 (94.5%) was highest among three samples.La 1.4 Sr 0.6 Ni 0.6 Co 0.4 O 4 exhibited highest average H 2 concentration (17.2%) throughout the 50-cycle stability test due to the improved C-C rupture capability and tuning WGS activity.Meanwhile, La 1.4 Sr 0.6 Ni 0.6 Co 0.4 O 4 showed the highest stability, and the H 2 concentration of La 1.4 Sr 0.6 Ni 0.6 Co 0.4 O 4 decreased by about 0.3% at the end of the stability test.The improved stability of La 1.4 Sr 0.6 Ni 0.6 Co 0.4 O 4 is related to its having the strongest self-regeneration capability among three samples.
,in − X H 2 O = X H 2 O,in − X H 2 O,out X H 2 O,in