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Sep 4, 2018 - synergetic effect of Co doping on chemical looping reforming of ethanol. A variety of .... As mentioned above, the bimetallic structure perovskite.
catalysts Article

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

ID

, Zhehao Sun, Qian Zhang, Duyu Li and Dawei Tang *

Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education, Dalian University of Technology, Dalian 116024, China; [email protected] (L.L.); [email protected] (B.J.); [email protected] (Z.S.); [email protected] (Q.Z.); [email protected] (D.L.) * Correspondence: [email protected]; Tel.: +86-0411-8470-8460 Received: 13 August 2018; Accepted: 29 August 2018; Published: 4 September 2018

 

Abstract: This paper describes the synthesis of a series of La1.4 Sr0.6 Ni1−x Cox O4 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. La1.4 Sr0.6 Ni0.6 Co0.4 O4 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. Keywords: chemical looping reforming; perovskite; Ni-Co synergy

1. 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.

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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]. Catalysts 2018, 8, 372 2 of 12

Figure 1. Schematic diagram of chemical looping process. Figure 1. Schematic diagram of chemical looping process. Table 1. Reactions during fuel feed step and air feed step.

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 Step Reactions been studied. Kawi et al. [14] discovered that the Ni3+ in LaNiO3 could be readily reduced to Ni and Ni OH + H2the O →La 2CO +0.44H 2 H5 studied 2 3−d perovskite for ethanol distributed on La2O3 support. Morales et al. C[15] 0.6Sr CoO Fuel Feed Step CO + H2 O → CO2 + H2 steam reforming and have observed the movement of metallic cobalt from perovskite lattice to the Catalysts 2018, 8, x; doi: FOR PEER REVIEW

Air Feed Step

C2 H5 OH + 6NiO → 6Ni + 2CO2 + 3H2 O C + O2 → CO2 www.mdpi.com/journal/catalysts 2C + O2 → 2CO 2Ni + O2 → 2NiO

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 [1,8–10]. Nowadays, perovskite-type metal oxides as OCs with a general formula of ABO3 or A2 BO4 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 Ni3+ in LaNiO3 could be readily reduced to Ni and distributed on La2 O3 support. Morales et al. [15] studied the La0.6 Sr0.4 CoO3−d perovskite for ethanol steam reforming and have observed the movement of metallic cobalt from perovskite lattice to the

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surface of OCs. Wu et al. [16] developed a series of La1–x Cax NiO3 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 [17–19]. In this work, we propose a novel type of La1.4 Sr0.6 Ni1−x Cox O4 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 La1.4 Sr0.6 Ni1−x Cox O4 OCs are prepared by a coprecipitation method and are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), H2 temperature-programmed reduction (H2 -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. 2. Results and Discussion 2.1. 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 m2 /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◦ . The particle sizes decreased in the following sequence: La1.4 Sr0.6 NiO4 (14.5 nm) > La1.4 Sr0.6 Ni0.6 Co0.4 O4 (13.7 nm) > La1.4 Sr0.6 Ni0.2 Co0.8 O4 (12.8 nm). Table 2. Physical properties of oxygen carriers.

Sample

Surface Area (m2 /g)

Average Pore Diameter (nm)

Pore Volume (cm3 /g)

Ni Content a (wt.%)

Co Content a (wt.%)

Ni crystal Size after Reduction b (nm)

La1.4 Sr0.6 NiO4 La1.4 Sr0.6 Ni0.6 Co0.4 O4 La1.4 Sr0.6 Ni0.2 Co0.8 O4

5.3 6.1 6.7

40.3 44.1 41.9

0.02 0.03 0.03

16.87 10.32 4.26

0 5.99 11.14

14.5 13.7 12.8

a

Determined by ICP-OES. b Determined by the Scherrer’s equation from (111) planes 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 Co3 O4 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 Ni3+ ion is larger than that of Co3+ . The substitution of Co3+ 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 SrO2 and La2 O3 also formed, accompanying the decomposition of the perovskite structure. Table 3 lists the d spacings and diffraction angles of the Ni particles formed from the reduction of La1.4 Sr0.6 Ni1−x Cox O4 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 La2 O3 are shown in the diffraction profiles of reduced OCs, which are derived from the

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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 Catalysts 2018, 8, 372 4 of 12 via the reaction of La2O3 and CO2 [22]. Catalysts 2018, 8, x3.FOR PEER REVIEW Table Relationship between oxygen carrier and d spacings of Ni or Ni-Co alloy (111) planes at 2θ = 4 of 12

reduction of Co and Ni. With the reduction of Ni3+ and Co3+ , the La2 O3 would be uniformly mixed 44.20°–44.70° after reduction. with the that thethe LaLa promotion could significantly improve the the Ni-Co Ni-Co alloy. alloy.ItIthas hasbeen beendemonstrated demonstrated that promotion could significantly improve Metal Particle Ni (111) Co 0.4 -Ni Co 0.8 -Ni Co (111) sintering resistance andand cokecoke resistance in steam reforming process [10,22]. TheThe XRD patterns of the the sintering resistance resistance in steam reforming process [10,22]. XRD patterns of dOCs spacing/Å 2.026 2.031 2.050 2.052 reduced OCs alsoalso displayed evident diffraction peaks of Laof2 O CO , which are possibly generated via the reduced displayed evident diffraction peaks La 2 O 2 CO 3 , which are possibly generated 2 3 2θ/° 44.7 44.6 44.5 44.2 the the reaction of La COCO via reaction of2 O La32and O3 and 2 [22]. 2 [22]. Table 3. Relationship between oxygen carrier and d spacings of Ni or Ni-Co alloy (111) planes at 2θ = 44.20°–44.70° after reduction.

Metal Particle d spacing/Å 2θ/°

Ni (111) 2.026 44.7

Co0.4-Ni 2.031 44.6

Co0.8-Ni 2.050 44.5

Co (111) 2.052 44.2

Figure 2. XRD profiles freshoxygen oxygencarriers carriers and and (b) (b) reduced Figure 2. XRD profiles ofof(a)(a)fresh reducedoxygen oxygencarriers. carriers.

The images between of La1.4Sr 0.6Ni0.4Co0.6O4 after reduction are shown in Figure 3. The Ni-Co Table 3. TEM Relationship oxygen carrier and d spacings of Ni or Ni-Co alloy (111) planes at particles with lattice stripes after ◦ ◦ 2θ = 44.20 –44.70 after reduction. 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 formedCo after the reduction of the Metal Particle Ni (111) Co0.4 -Nithe alloy Co0.8is-Ni (111) Co-doped OCs. No apparent change was observed in terms of d spacing after reduction, suggesting 2.026 2.031 2.050 2.052 d spacing/Å Ni-Co alloy is very stable after reaction. The Ni-Co alloy particles were highly dispersed with the 2θ/◦ 44.7 44.6 44.5 44.2 particle sizes of approximately nm,fresh which is consistent with(b) the XRD results. Figure 2. XRD profiles 12 of (a) oxygen carriers and reduced oxygen carriers.

The TEM Sr 0.6Ni Ni0.4 Co0.60.6 after reduction are shown in Figure 3. The Ni-Co 0.4Co The TEM images images of of La La1.4 1.4Sr0.6 OO 4 4after reduction are shown in Figure 3. The Ni-Co particles with lattice stripes after the reduction Clear lattice lattice stripes stripes of of the the alloy alloy particles with lattice stripes after the reduction were were observed. observed. Clear appeared after after the the region region was was enlarged, The dd spacing spacing appeared enlarged, and and the the interstitial interstitial void void of of which which is is 0.203 0.203 nm. nm. The is between Ni (111) and Co (111), which indicates that the alloy is formed after the reduction of the the is between Ni (111) and Co (111), which indicates that the alloy is formed after the reduction of Co-doped OCs. OCs. No No apparent apparent change change was was observed observed in in terms terms of of dd spacing spacing after after reduction, reduction, suggesting suggesting Co-doped Ni-Co alloy is very stable after reaction. The Ni-Co alloy particles were highly dispersed with the the Ni-Co alloy is very stable after reaction. The Ni-Co alloy particles were highly dispersed with particle sizes of approximately 12 nm, which is consistent with the XRD results. particle sizes of approximately 12 nm, which is consistent with the XRD results. Figure 3. TEM profiles of reduced oxygen carriers.

The H2-TPR patterns of La1.4Sr0.6Ni1−xCoxO4 OCs 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 Sr2+ in place of La3+ forms unusual Co3+ or Ni3+ 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 Ni2+ or Co2+ to Ni0 or Co0, respectively. The Ni and Co metals peaks at HT merged into oneprofiles reduction peak, indicating that Ni-Co alloy is formed [21]. Figure 3. TEM of reduced oxygen carriers. Figure 3. TEM profiles of reduced oxygen carriers. Catalysts 8, x;patterns doi: FOR PEER The H2018, 2-TPR of LaREVIEW 1.4Sr0.6Ni1−xCoxO4

www.mdpi.com/journal/catalysts OCs are shown in of Figure 4. Generally, the valence The H -TPR patterns of La Sr Ni Co O OCs are shown in of Figure 4. Generally, valence 2+ in x 4 to be 2+, while partial introduction 1.4 0.6 is 1supposed −x of Srthe place of B site in 2an A2BO4 type perovskite 2+ of B site in an A BO type perovskite is supposed to be 2+, while partial introduction of Sr in place 3+ 3+ 3+ 2 4 of La forms unusual Co or Ni in these compounds [23,24]. The reduction process should be 3+ forms unusual Co3+ or Ni3+ in these compounds [23,24]. The reduction process should of La divided into two steps, i.e., from 3+ to 2+ and then from 2+ to 0. Moreover, the reduction temperature be divided twoare steps, from 3+ to 2+Therefore, and thenall from 2+ toobviously 0. Moreover, the two reduction ranges of Ni into and Co closei.e., to each other [25]. the OCs exhibited broad temperature ranges of Ni and Co are close to each other [25]. Therefore, all the OCs obviously 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 Ni2+ or Co2+ to Ni0 or Co0, respectively. The Ni and Co metals peaks at HT merged into one reduction peak, indicating that Ni-Co alloy is formed [21]. Catalysts 2018, 8, x; doi: FOR PEER REVIEW

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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 Ni2+ or Co2+ to Ni0 or Co0 , respectively. The Ni and Co metals peaks at HT merged into one reduction peak, indicating that Catalysts 2018, 8, x FOR PEER REVIEW 5 of 12 Ni-Co alloy is formed [21]. The peaks at LT are ascribed to the reduction of Ni3+ or Co3+ to Ni2+ or 2+ or 2+, respectively. peaks at LTThe are ascribed of was Ni3+ or Co3+ to Ni CoCo The splitting Co2+ ,The respectively. splittingtoofthe thereduction LT peaks observed for the doped perovskite samples. of the be LTdue peaks wasformation observed for thespinel Co doped This could be due to the This could to the of the typeperovskite structure, samples. such as SrLaCoO by the 4 , promoted formation of thethe spinel type structure, such SrLaCoO 4, five promoted by the presence of [26]. Sr when presence of Sr when molar ratio of La to Sr isaslower than in perovskite structure Thethe spinel molar ratio of La to Sr is lower than five in perovskite structure [26]. The spinel phase was not phase was not detected by XRD analysis, and this is probably due to its small particle size. Moreover, detected by XRD analysis, and this is probably due to its small particle size. Moreover, the reduction the reduction peaks of LT and HT tend to move towards lower temperatures with the amount of Co, peaks of LT and HT tend to move towards lower temperatures with the amount of Co, which may be which may be due to the small particle size of perovskite particles [21]. Smaller particles could provide due to the small particle size of perovskite particles [21]. Smaller particles could provide a higher a higher specific surface area (listed in Table 2) and higher specific surface energy, thus lowering the specific surface area (listed in Table 2) and higher specific surface energy, thus lowering the reduction reduction temperature Additionally, Valderrama al. [28] demonstrated substitution temperature [27]. [27]. Additionally, Valderrama et al.et[28] demonstrated that that Co Co substitution in in La0.8 Sr Ni Co O perovskite could promote the formation of positive holes and vacancies of lattice y yO 1− y 1−yCo 3 3 perovskite could promote the formation of positive holes and vacancies of lattice La0.2 0.8Sr0.2 Ni oxygen, therefore favoring thethe reduction thatananappropriate appropriate amount oxygen, therefore favoring reductionprocess. process. ItIt can can be concluded concluded that amount doping Coenhance can enhance the reducibility of active components the perovskite,thus thusimproving improving the doping of Coofcan the reducibility of active components inin the perovskite, the catalytic in aprocess. CLSR process. catalytic activity activity in a CLSR

Figure 4. TPR of fresh oxygen carriers.

Figure 4. TPR of fresh oxygen carriers.

2.2. Activity Tests of OCs

2.2. Activity Tests of OCs

Figure 5 displays the gas product distribution of the La1.4Sr0.6Ni1−xCoxO4 OCs during CLSR. The

Figure 5 displays the gas distribution of the La1.4 Sr0.6 Ni hydrogen concentrations of product all the three OCs increased dramatically within thex O first five during minutesCLSR. in 1−x Co 4 OCs the fuel feed step. The duration where hydrogen is produced is known ‘dead It is The hydrogen concentrations of all the threenoOCs increased dramatically withinasthe firsttime’. five minutes be anThe important indicator of no OCs’ redox properties [19]. Inis this period,asH‘dead 2 concentration in theconsidered fuel feed to step. duration where hydrogen is produced known time’. It is was almost zero. Only when the Ni ions in the perovskite lattice are sufficiently reduced to metallic considered to be an important indicator of OCs’ redox properties [19]. In this period, H2 concentration Ni can steam reforming of the ethanol and subsequent in situ water shift (WGS) occur [29,30].toInmetallic this was almost zero. Only when Ni ions in the perovskite latticegas are sufficiently reduced stage, the perovskite is gradually reduced, therefore generating the active Ni-Co alloy component. Ni can steam reforming of ethanol and subsequent in situ water gas shift (WGS) occur [29,30]. In this Afterwards, the reforming reaction of ethanol and WGS begins to play the dominated role in the stage, the perovskite is gradually reduced, therefore generating the active Ni-Co alloy component. steady stage of the fuel feed step, thus leading to a great rise in H2 concentration in seconds. Afterwards, the reforming of ethanol and WGS begins to play the dominated role in the steady Compared with the Lareaction 1.4Sr0.6NiO4, the Co doping samples showed shorter dead time due to the stage improved of the fuelreducibility, feed step, thus leading to with a great in results. H2 concentration Compared corresponding therise TPR De Lasa etinal.seconds. [31] investigated thewith the Lareactivity the Co doping samples showed shorterCLC dead time due thehave improved reducibility, and of Co-Ni/Al 2O3 OC in multicycle process, andtothey concluded that 1.4 Sr0.6 NiO 4 , stability corresponding with the TPR results. De LasaofetOC al. by [31] investigated thesupport reactivity and stability the Co promotion improved the reducibility affecting the metal interaction (MSI). of The average of La 1.4Sr0.6Ni 0.6Co 0.4O4have OC isconcluded 17.1%, which much than improved that of Co-Ni/Al in2 concentration multicycle CLC process, and they thatisthe Co higher promotion 2 O3 OCH 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 CO2 the reducibility of OC by affecting the metal support interaction (MSI). The average H2 concentration concentration lower CO concentration than counterparts of La1.4Sr0.6NiO4, indicating the of La1.4 Sr0.6 Ni0.6 Coand 0.4 O4 OC is 17.1%, which is much higher than that of La1.4 Sr0.6 NiO4 (14.3%) and enhanced WGS reactivity. Co are active for steam higher reforming ethanol; however, the La1.4 Sr0.6 Ni0.2 Co0.8 O4 (12.3%).Both The Ni Coand doping samples showed COof 2 concentration and lower La1.4Sr0.6Ni0.6Co0.4O4 showed superior performance in terms of H2 and CO2 concentrations. It is widely CO concentration than counterparts of La1.4 Sr0.6 NiO4 , indicating the enhanced WGS reactivity. Both Ni accepted that an effective catalyst for steam reforming of oxygenate compounds should not only be and Co are active for steam reforming of ethanol; however, the La1.4 Sr0.6 Ni0.6 Co0.4 O4 showed superior active in cleavage of C–C bond but also be active in WGS to remove CO formed on metal surface [32]. performance of H2 and CO It is widely that compared an effective catalyst 2 concentrations. Sinfelt etin al.terms [33] demonstrated that Ni possesses faster rate of C–Caccepted bond rupture with other for VIII group metals. Nevertheless, Ni has limited reactivity for WGS reaction, while Co possesses Catalysts 2018, 8, x; doi: FOR PEER REVIEW

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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 higher activity during the reaction Catalysts 2018, 8, x FOR PEER REVIEW 6 of 12 [34]. Furthermore, Co showed high dehydrogenation activity, Ni is conducive to C–C cleavage. Thus, higher activity is during the reaction [34]. Co showed Ni ethanol molecule dehydrogenated on Furthermore, Co site of Ni-Co alloy, high and dehydrogenation the C–C bond isactivity, subsequently is conducive to C–C cleavage. Thus, ethanol molecule is dehydrogenated on Co site of Ni-Co alloy, broken by the Ni site of Ni-Co alloy. This reaction route is fast, since the cleavage of the C–C bond and the C–C bond is subsequently broken by the Ni site of Ni-Co alloy. This reaction route is fast, in acetaldehyde is easier than that in ethanol molecule. Hence, La1.4 Sr0.6 Ni0.6 Co0.4 O4 possesses since the cleavage of the C–C bond in acetaldehyde is easier than that in ethanol molecule. Hence, improved capability of C–C rupture and moderate WGS activity due to the synergistic effect, therefore La1.4Sr0.6Ni0.6Co0.4O4 possesses improved capability of C–C rupture and moderate WGS activity due showing the highest H2 concentration among three samples. Methanation is highly undesirable since to the synergistic effect, therefore showing the highest H2 concentration among three samples. it decreases H2 yield. In our test conditions, methane is very low forthe all methane the samples, Methanation is highly undesirable since itthe decreases H2concentration yield. In our test conditions, considering the strong extremely exothermic nature of methanation reaction. The Co doping samples concentration is very low for all the samples, considering the strong extremely exothermic nature of showed high methane concentration in contrast with La Sr NiO . This is because Co is more active methanation reaction. The Co doping samples showed 1.4 high0.6 methane 4 concentration in contrast with in methanation than Ni La1.4Sr0.6NiOreactions 4. This is because Co[34]. is more active in methanation reactions than Ni [34].

Figure 5. Activity tests in fuel feed step and air feed step. (a) and (b) La1.4Sr0.6Ni0.2Co0.8O4; (c) and (d)

Figure 5. Activity tests in fuel feed step and air feed step. La1.4Sr0.6Ni0.6Co0.4O4; (e) and (f) La1.4Sr0.6NiO4. (c,d) La1.4 Sr0.6 Ni0.6 Co0.4 O4 ; (e,f) La1.4 Sr0.6 NiO4 .

(a,b) La1.4 Sr0.6 Ni0.2 Co0.8 O4 ;

The gas product concentrations in the air feed step are illustrated in Figure 5. The air feed step The gas product the air feed step illustrated in Figure 5. heat The air plays three roles inconcentrations a CLSR processinusing perovskite typeare OCs. Apart from providing andfeed as process in a conventional CLSR process, the air feedfrom step providing would alsoheat step eliminating plays threecoke rolesdeposition in a CLSR using perovskite type OCs. Apart thecoke perovskite structure [35]. It has been reported Ni ions thefeed perovskite lattice also and regenerate eliminating deposition as in a conventional CLSR that process, theinair step would would come out of the bulk in the fuel feed step and immerse back into the lattice in the air feed step regenerate the perovskite structure [35]. It has been reported that Ni ions in the perovskite lattice [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 Catalysts 2018, 8, x; doi: FOR PEER REVIEW

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would come out of the bulk in the fuel feed step and immerse back into the lattice in the air feed step [11], therefore the growth of Ni particles and maintaining its dispersion. The Catalysts 2018, 8, xsuppressing FOR PEER REVIEW 7 ofpeak 12 areas of C-containing gas products represent the coke deposition amounts. The coke deposition samples decreased in in thethefollowing La1.4Sr 0.6NiO4 > La1.4Sr0.6Ni0.2Co0.8O4 > of the three samples decreased following sequence: sequence: La 1.4 Sr0.6 NiO4 > La1.4 Sr0.6 Ni0.2 Co0.8 O4 > La1.4Sr0.6Ni0.6Co0.4O4, which is consistent with the order of Ni-Co alloy particle sizes listed in Table 2. La1.4 Sr0.6 Ni0.6 Co0.4 O4 , 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 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 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 temperature variation inThe theend air of feed correlates the heat of Niofand oxidation to and coke combustion. thestep air feed step is with signaled by therelease restoration O2 Co concentration coke combustion. The end of the air feed step is signaled by the restoration of O concentration to 21%. 2 21%. FigureFigure 6 shows the ethanol conversion and and steam conversion of the La1.4 Ni CoxO 6 shows the ethanol conversion steam conversion of the LaSr 1.4Sr Ni11−x 4 OCs xO 0.60.6 −xCo 4 OCs thefeed fuelstage. feed stage. Forthe all the perovskite OCs, theconversion conversion of of ethanol ethanol reached 78%, duringduring the fuel For all perovskite OCs, the reachedatatleast least 78%, while the conversion of steam was above25%. 25%. The conversion of Laof 1.4Sr 0.6NiO 4 was the lowest while the conversion of steam was above Theethanol ethanol conversion La Sr NiO was the 1.4 0.6 4 while thethe ethanol conversion of Laof 1.4Sr NiSr 0.6Co 0.4O4 Co OC reached almost lowest one one among amongthree threesamples, samples, while ethanol conversion La0.61.4 Ni O OC reached 0.6 0.6 0.4 4 minutes andand then leveled off. When further increasing the molar of ratio Co to of almost 100% 100%ininthe thefirst firstsixsix minutes then leveled off. When further increasing theratio molar 0.8, the ethanol conversion dropped to 95%, resulting from the insufficiency of Ni active centers for Co to 0.8, the ethanol conversion dropped to 95%, resulting from the insufficiency of Ni active centers C–C cleavage. Additionally, with the increase of Co ratio, the steam conversion also increased. Mei for C–C cleavage. Additionally, with the increase of Co ratio, the steam conversion also increased. et al. [37] calculated water adsorption and dissociation on the Rh (111), Ni (111) and Co (0001) surfaces Mei et using al. [37] calculated water adsorption and dissociation on the Rh (111), Ni (111) and Co (0001) Density functional theory (DFT), and their results show that water dissociation into hydroxyl surfaces using Density functional theory (DFT), and their results show water dissociation into and hydrogen atom on Co (0001) surface is both thermodynamically andthat kinetically feasible. hydroxyl and hydrogen atom on Co (0001) surface is both thermodynamically and kinetically feasible.

Figure 6. Ethanol steam conversion thefuel fuelfeed feed step. step. Figure 6. Ethanol andand steam conversion ininthe

2.3. Stability Tests

2.3. Stability Tests

Figure 7 shows the variation of ethanol conversion, CO2/CO ratio and H2 concentration of

Figure shows the variation of ethanol conversion, CO2 /CO anddecreases H2 concentration La1.4Sr70.6Ni 1−xCoxO4 OCs during multicycle tests at 650 °C. All OCsratio showed in ethanol of ◦ C. All OCs showed decreases in ethanol La1.4 Srconversion during multicycle tests at 650 x Owell 0.6 Ni1−x Coas 4 OCs as CO2/CO ratio after stability tests, while Co doping samples possessed more conversion as well as CO2than /COLaratio after while Coofdoping samples moderate variation 1.4Co0.6 NiOstability 4. The H2tests, concentration La1.4Sr0.6 Ni0.6Co0.4possessed O4 showed more an moderate variation than La1.4of Co0.3% The H2ofconcentration of Lawhile Ni0.6 inconspicuous decrease at4 .the end the stability test, the LaCo 1.4Sr 0.6O NiO 4 and 0.6 NiO 1.4 Sr0.6 0.4 4 showed La1.4Sr0.6Ni0.2Co 0.8O4 declined to 10.8% Since the CO an inconspicuous decrease of 0.3% at theand end11.3%, of therespectively. stability test, while the2/CO La1.4ratio Sr0.6has NiObeen 4 and as an indicator for WGS activity, the decrease in the ratio is associated with the loss of the La1.4 Srconsidered Ni Co O declined to 10.8% and 11.3%, respectively. Since the CO /CO ratio has been 0.6 0.2 0.8 4 2 synergistic effect of the Ni-Co alloy. It has been reported that the main obstacle for Ni-based OCs considered as an indicator for WGS activity, the decrease in the ratio is associated with the lossis of coke deposition andthe metal sintering the coke deposition eliminated in for the Ni-based air feed the synergistic effect of Ni-Co alloy.[7]. It Since has been reported that can the be main obstacle step, the OC deactivation in this work is mainly caused by metal sintering. As Ni is the active center OCs is coke deposition and metal sintering [7]. Since the coke deposition can be eliminated in the air for C–C bond cleavage, the decrease of ethanol conversion reflects the loss of active sites due to feed step, the OC deactivation in this work is mainly caused by metal sintering. As Ni is the active sintering. According to our previous investigation, the periodical movement of Ni into and out of the center for C–C bond cleavage, the decrease of ethanol conversion thethus lossimproving of active sites due perovskite lattice (self-regeneration capability) could regeneratereflects the OCs, the Ni to sintering. According ourIndeed, previous investigation, thework periodical movement of Niand intoCoand out of sintering resistanceto[11]. all OCs in our present exhibited high stability, doping the perovskite lattice (self-regeneration capability) could regenerate the OCs, thus improving the samples further improved the performance in stability tests. The promotion of the self-regenerationNi sintering resistance [11]. OCsbeinexplained our present work exhibited high stability, and Co doping effect caused by CoIndeed, dopingall could as follows. Co substitution in La0.8Sr 0.2Ni 1−yCo yO3 Catalysts 2018, 8, x; doi: FOR PEER REVIEW

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samples further improved the performance in stability tests. The promotion of the self-regeneration effect Catalysts caused2018, by 8,Co doping could be explained as follows. Co substitution in La0.8 Sr0.2 Ni1−8y Co x FOR PEER REVIEW of 12y O3 perovskite could promote the formation of bulk and surface defects, such as oxygen vacancies and perovskite could promote the formation of bulkbetween and surface defects, such as oxygen vacancies and threading dislocations, due to the radii difference Ni and Co [28]. Surface defects, especially threading dislocations, due to radii difference and Coatoms [28]. Surface defects, especially oxygen vacancies, could anchor Nithe particles either bybetween sharingNioxygen to form chemical bonds or oxygen vacancies, could anchor Ni particles either by sharing oxygen atoms to form chemical bonds by supplying valley sites to nest them, resulting in strong MSI [38]. Mawdsley et al. [39] demonstrated or by supplying valley sites to nest them, resulting in strong MSI [38]. Mawdsley et al. [39] that strong MSI is conducive to forming transient Ni-containing surface phases such as La2 NiO4−y demonstrated that strong MSI is conducive to forming transient Ni-containing surface phases such adjacent to LaFeO3 , which could perform as a provisional medium to carry Ni atoms into and out of as La2NiO4−y adjacent to LaFeO3, which could perform as a provisional medium to carry Ni atoms the perovskite the bulk defects resulted from Sr defects and Co resulted incorporation, as and evidenced into and lattice. out of Moreover, the perovskite lattice. Moreover, the bulk from Sr Co by TPR results, could form channels for Ni diffusion, and the high bulk oxygen mobility contributes incorporation, as evidenced by TPR results, could form channels for Ni diffusion, and the high bulk to the process drivingcontributes oxygen atoms from air back to perovskite oxygenfrom vacancies Mars-van oxygenofmobility to the process of driving oxygen atoms air backvia to the perovskite Krevelen redox cycle mechanism. oxygen vacancies via the Mars-van Krevelen redox cycle mechanism.

(a) Ethanol conversion, 2/CO and (c) H2 concentration during stability tests. FigureFigure 7. (a) 7. Ethanol conversion, (b) (b) COCO 2 /CO and (c) H2 concentration during stability tests.

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In conclusion, the CO2 /CO ratios for all the samples showed moderate decrease after the tests. La1.4 Sr0.6 Ni0.2 Co0.8 O4 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 La1.4 Sr0.6 Ni0.6 Co0.4 O4 (94.5%) was highest among three samples. La1.4 Sr0.6 Ni0.6 Co0.4 O4 exhibited highest average H2 concentration (17.2%) throughout the 50-cycle stability test due to the improved C–C rupture capability and tuning WGS activity. Meanwhile, La1.4 Sr0.6 Ni0.6 Co0.4 O4 showed the highest stability, and the H2 concentration of La1.4 Sr0.6 Ni0.6 Co0.4 O4 decreased by about 0.3% at the end of the stability test. The improved stability of La1.4 Sr0.6 Ni0.6 Co0.4 O4 is related to its having the strongest self-regeneration capability among three samples. 3. Materials and Methods 3.1. Preparation of OCs A nickel-based perovskite structure OC was synthesized by the co-precipitation technique. Stoichiometric amounts of Ni(NO3 )2 ·6H2 O (GR, 99%, Aladdin, Shanghai, China), La(NO3 )3 ·6H2 O (AR, 99%, Aladdin, Shanghai, China), Sr(NO3 )2 (99.97%, Aladdin, Shanghai, China) and Co(NO3 )2 ·6H2 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 La1.4 Sr0.6 Ni1−x Cox O4 (x = 0, 0.4 and 0.8) after the loading of Co. 3.2. 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 = 1.5406 Å) 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. H2 -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% H2 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 HNO3 to remove Ni species on surface. 3.3. Activity and Stability Tests The setup of our experimental system was described in our previous publications [17–19]. 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 N2 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: .

Xet =

X H2 O = .

.

net,in − net,out × 100% . net,in

X H2 O,in − X H2 O,out × 100% X H2 O,in

where n indicates the relevant molar flows (mol min–1 ).

(1)

(2)

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4. Conclusions A series of La1.4 Sr0.6 Ni1 −x Cox O4 perovskite OCs for hydrogen production by employing Co and Ni as the B-site components of perovskite were synthesized. The synergistic effect between Ni and Co, which is beneficial to enhancing the catalytic activity and improve the stability of perovskite OCs, was investigated. La1.4 Sr0.6 Ni0.6 Co0.4 O4 showed an average ethanol conversion of 92.4% and an average CO2 /CO ratio of 5.4 in a 30-cycle stability test. 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, therefore enhancing the sintering resistance. Moreover, the Co promotion also contributes to the improved WGS activity. Author Contributions: Experiment, L.L., Q.Z., D.L. and Z.S.; Data Curation, B.J., Q.Z. and Z.S.; Writing-Original Draft Preparation, L.L. and B.J.; Writing-Review & Editing, L.L. and B.J.; Supervision, D.T. and L.L.; Project Administration, D.T. and L.L.; Funding Acquisition, L.L. and D.T. Funding: This research was funded by [National Natural Science Foundation of China] grant number [51706030], [Fundamental Research Funds for Central Universities] grant number [DUT18JC11] and [China Postdoctoral Science Foundation] grant number [2017M611219]. Acknowledgments: We deeply appreciate the kind assistance from the Key Laboratory of Ocean Energy Utilization and Energy Conservation of Ministry of Education (China). Conflicts of Interest: The authors declare no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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