Hydrogen Production from Chemical Looping Reforming of Ethanol Using Ni / CeO 2 Nanorod Oxygen Carrier

Chemical looping reforming (CLR) technique is a prospective option for hydrogen production. Improving oxygen mobility and sintering resistance are still the main challenges of the development of high-performance oxygen carriers (OCs) in the CLR process. This paper explores the performance of Ni/CeO2 nanorod (NR) as an OC in CLR of ethanol. Various characterization methods such as N2 adsorption-desorption, X-ray diffraction (XRD), Raman spectra, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (TPR), and H2 chemisorption were utilized to study the properties of fresh OCs. The characterization results show the Ni/CeO2-NR possesses high Ni dispersion, abundant oxygen vacancies, and strong metal-support interaction. The performance of prepared OCs was tested in a packed-bed reactor. H2 selectivity of 80% was achieved by Ni/CeO2-NR in 10-cycle stability test. The small particle size and abundant oxygen vacancies contributed to the water gas shift reaction, improving the catalytic activity. The covered interfacial Ni atoms closely anchored on the underlying surface oxygen vacancies on the (111) facets of CeO2-NR, enhancing the anti-sintering capability. Moreover, the strong oxygen mobility of CeO2-NR also effectively eliminated surface coke on the Ni particle surface.


Introduction
Hydrogen is considered an efficient energy carrier that is environmentally benign [1].Chemical looping reforming (CLR) is a prospective alternative for hydrogen production due to its energy efficiency and inherent CO 2 capture [2,3].The fixed-bed reactor configuration CLR process (Figure 1) is performed by alternatively switching the feed gases; the oxygen carriers (OCs) are stationary and periodically exposed to redox atmosphere [4].The key to developing a CLR process is to screen high-performance OCs.Ni-based OCs have been widely investigated because of their ability for carbon-carbon and carbon-hydrogen bonds cleavage [5][6][7].Zafar et al. [8] prepared a series of OCs including Fe, Cu, Mn, and Ni supported by SiO 2 and MgAl 2 O 4 and concluded that NiO supported on SiO 2 exhibited high H 2 selectivity in CLR.Löfberg et al. [9] demonstrated that Ni plays two essential roles in CLR process, i.e., the anticipated activation of reactants as well as the regulation of the oxygen supply rate from solids.Dharanipragada et al. [10] reported that the Ni-ferrites OC suffers from the loss of oxygen storage capacity due to Ni sintering in chemical looping with alcohols.Improving oxygen mobility and sintering resistance remain the major challenges of the development of Ni-based OCs in the CLR process due to the high activation energy of oxygen anion diffusion in NiO (2.23 eV in CLR) and the low Tammann temperature of Ni (691 • C) [11][12][13][14].OCs in the CLR process due to the high activation energy of oxygen anion diffusion in NiO (2.23 eV in CLR) and the low Tammann temperature of Ni (691 °C) [11][12][13][14].Recent research has revealed that CeO2 exhibits excellent redox property due to abundant oxygen storage capacity and the strong capability to stabilize Ni nanoparticles because of strong metal support interaction (MSI) [15,16].CeO2 can readily release lattice oxygen under reducing conditions, thus creating oxygen vacancies which are associated with oxygen mobility [17].The MSI between the Ni and CeO2 could tune the physiochemical properties of Ni, contributing to high catalytic reactivity and stability [18].Jiang et al. [19] proposed that the oxygen vacancies of CeO2 could effectively eliminate surface coke deposition, activate steam, and shorten the "dead time" in the CLR process.Dou et al. [20] revealed that the surface oxygen originating from the CeO2 lattice can oxidize coke precursors, keeping the OC surface free of coke deposition.It has been reported that MSI and the mobility of lattice oxygen show strong dependence on the morphology of CeO2 [21].Lykaki et al. [22] have demonstrated CeO2 morphology dominates reducibility and oxygen mobility, which follow the sequence: nanorod (NR) > nanopolyhedra (NP) > nanocube (NC).Moreover, the apparent activation energy of these three CeO2 shapes for the CO oxidation in a hydrogen-rich gas shows the opposite trend, implying the potential highest water gas shift (WGS) activity of NR [23].
Therefore, in this work, a Ni/CeO2-NR OC for CLR of ethanol for hydrogen production is prepared by a hydrothermal method.The physicochemical properties are investigated by N2 adsorption-desorption, X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), Raman spectra, high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and H2 temperature-programmed reduction (H2-TPR).The performances of the Ni/CeO2-NR OC are tested in a packed-bed reactor and compared with the other reference bulk OC, i.e., Ni/CeO2.Recent research has revealed that CeO 2 exhibits excellent redox property due to abundant oxygen storage capacity and the strong capability to stabilize Ni nanoparticles because of strong metal support interaction (MSI) [15,16].CeO 2 can readily release lattice oxygen under reducing conditions, thus creating oxygen vacancies which are associated with oxygen mobility [17].The MSI between the Ni and CeO 2 could tune the physiochemical properties of Ni, contributing to high catalytic reactivity and stability [18].Jiang et al. [19] proposed that the oxygen vacancies of CeO 2 could effectively eliminate surface coke deposition, activate steam, and shorten the "dead time" in the CLR process.Dou et al. [20] revealed that the surface oxygen originating from the CeO 2 lattice can oxidize coke precursors, keeping the OC surface free of coke deposition.It has been reported that MSI and the mobility of lattice oxygen show strong dependence on the morphology of CeO 2 [21].Lykaki et al. [22] have demonstrated CeO 2 morphology dominates reducibility and oxygen mobility, which follow the sequence: nanorod (NR) > nanopolyhedra (NP) > nanocube (NC).Moreover, the apparent activation energy of these three CeO 2 shapes for the CO oxidation in a hydrogen-rich gas shows the opposite trend, implying the potential highest water gas shift (WGS) activity of NR [23].

Characterization of OCs
Therefore, in this work, a Ni/CeO 2 -NR OC for CLR of ethanol for hydrogen production is prepared by a hydrothermal method.The physicochemical properties are investigated by N 2 adsorption-desorption, X-ray diffraction (XRD), inductively coupled plasma optical emission spectroscopy (ICP-OES), Raman spectra, high resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and H 2 temperature-programmed reduction (H 2 -TPR).The performances of the Ni/CeO 2 -NR OC are tested in a packed-bed reactor and compared with the other reference bulk OC, i.e., Ni/CeO 2 .

Characterization of OCs
Predominant physicochemical properties of fresh Ni/CeO 2 -NR and CeO 2 -NR are tabulated in Table 1.The introduction of Ni species has a pronounced influence on the texture of CeO 2 -NR.The CeO 2 -NR support showed higher Brunauer-Emmett-Teller (BET) surface area than that of Ni/CeO 2 -NR.The average pore diameter and pore volume also exhibited the same trends.It has been reported that the BET surface of CeO 2 -NR decreased by 13-18% after the incorporation of 7.5 wt % CuO [22].The actual Ni content of Ni/CeO 2 -NR determined by ICP-OES was 9.7 wt %.Predominant physicochemical properties of fresh Ni/CeO2-NR and CeO2-NR are tabulated in Table 1.The introduction of Ni species has a pronounced influence on the texture of CeO2-NR.The CeO2-NR support showed higher Brunauer-Emmett-Teller (BET) surface area than that of Ni/CeO2-NR.The average pore diameter and pore volume also exhibited the same trends.It has been reported that the BET surface of CeO2-NR decreased by 13-18% after the incorporation of 7.5 wt % CuO [22].The actual Ni content of Ni/CeO2-NR determined by ICP-OES was 9.7 wt %.
The XRD profiles of fresh Ni/CeO2-NR and CeO2-NR OCs are shown in Figure 2. The reflection peaks at 28.5°, 33.1°, 47.5°, 56.3°, 59.1°, 69.4°, 76.7°, and 79.1° could be indexed to (111), ( 200), ( 220), (311), ( 222), (400), (331), (420), and (422) planes of a fluorite-structure CeO2 (Space group: Fm3m), respectively.There were no other impurity peaks of the hexagonal structure of Ce(OH)3 or Ce(OH)CO3 detected, indicating the excellent crystalline purity of CeO2.Two peaks at 44.5° and 51.8°, which could be respectively attributed to (111) and (200) facets of Ni, were observed for Ni/CeO2-NR.The mean crystal sizes of Ni and CeO2 were obtained from the Scherrer Equation (listed in Table 1).The crystal sizes of CeO2 for CeO2-NR and Ni/CeO2-NR were 14.8 and 16.1 nm, respectively, indicating that the incorporation of Ni would not significantly change the structure characteristics of CeO2 support.Similar crystallite sizes of CeO2-NR prepared by the hydrothermal method have been reported in other work [22,24].Raman spectroscopy was employed to characterize surface and bulk defects.As shown in Figure 3, both samples present four characteristic peaks.The appreciable peak at 462.8 cm −1 resulted from the first order F2g mode of the fluorite cubic structure.The Raman shift at 596.1 cm −1 could be assigned to the defect-induced band (D band).The relocation of O atom from the interior of tetrahedral cationic sub-lattice to the interior of ideally empty octahedral cationic sites (Frenkel interstitial sites) would lead to the deformation of the anionic lattice of CeO2 [25].The intensity of the D band is a gauge of the distortion of ionic lattice, which results in punctual defects and oxygen vacancies [22].Therefore, Raman spectroscopy was employed to characterize surface and bulk defects.As shown in Figure 3, both samples present four characteristic peaks.The appreciable peak at 462.8 cm −1 resulted from the first order F 2g mode of the fluorite cubic structure.The Raman shift at 596.1 cm −1 could be assigned to the defect-induced band (D band).The relocation of O atom from the interior of tetrahedral cationic sub-lattice to the interior of ideally empty octahedral cationic sites (Frenkel interstitial sites) would lead to the deformation of the anionic lattice of CeO 2 [25].The intensity of the D band is a gauge of the distortion of ionic lattice, which results in punctual defects and oxygen vacancies [22].Therefore, the value of I D /I F 2g is commensurate with the number of defect sites in ceria.The relative intensity of I D /I F 2g decreased after the incorporation of Ni, suggesting NiO species suppress the surface oxygen vacancy of CeO 2 -NR.Li et al. [26] reported that Vanadium atom would bond to the surface of CeO 2 -NR by generating V-O-Ce species, thus covering oxygen defects and stabilizing adjacent Ce atoms.The inconspicuous peak at 258.1 cm −1 results from the second order transverse acoustic mode (2TA) or doubly degenerate transverse optical mode (TO), which are Raman inactive in a perfect crystal [27].The peak at 1179.9 cm −1 may be related to the stretching mode of the short terminal Ce=O.Moreover, the Raman shift of Ni/CeO 2 -NR moved towards a low wavenumber in comparison with that of CeO 2 -NR, indicating the incorporation of Ni affects the symmetry of Ce-O bonds [21].Raman spectroscopy was employed to characterize surface and bulk defects.As shown in Figure 3, both samples present four characteristic peaks.The appreciable peak at 462.8 cm −1 resulted from the first order F2g mode of the fluorite cubic structure.The Raman shift at 596.1 cm −1 could be assigned to the defect-induced band (D band).The relocation of O atom from the interior of tetrahedral cationic sub-lattice to the interior of ideally empty octahedral cationic sites (Frenkel interstitial sites) would lead to the deformation of the anionic lattice of CeO2 [25].The intensity of the D band is a gauge of the distortion of ionic lattice, which results in punctual defects and oxygen vacancies [22].Therefore,  111) facets.The preferable exposure crystal facets of CeO 2 -NR are consistent with those of samples derived from the CeCl 3 precursor in other studies [28,29].Theory calculation has demonstrated that the (111) is the least reactive facet, followed by ( 100) and (110).In addition to the preferable crystal facets, there are other aspects degerming CeO 2 activity.Several 'dark pits' are shown in the box of Figure 4. Liu et al. [29] have reported that these dark pits are related to the surface reconstruction and defects; their study also concluded that these defects play a more important role in determining the CeO 2 activity than the exposure planes.Sayle et al. [30] have performed a molecular dynamic modeling to simulate the synthesis of CeO 2 -NR and discovered that the atomistic sphere model exhibits many steps on the (111) planes of CeO 2 -NR.The migration of oxygen in CeO 2 occurs by a vacancy hopping mechanism; therefore, the clusters of defects are conducive to oxygen transfer.If the diffusion rate of oxygen anions becomes adequately high, a consecutive oxygen flow is generated, resulting in high reducibility.The Ni particle size distribution, which is calculated from 52 particles, was inserted in Figure 4, and the average particle size was 8.1 nm (shown in Table 1).Shen et al. [31] have elucidated that the strong interfacial anchoring effect, which exists between the surface oxygen vacancies on (111) planes of CeO 2 -NR and the gold particles, only allows the gold particles to locally rotate or vibrate but not to migrate to form aggregates. Therefore, the strong MSI would significantly improve the Ni dispersion on the CeO 2 -NR surface, resulting in a small particle size of Ni.
of CeO2-NR by generating V-O-Ce species, thus covering oxygen defects and stabilizing adjacent Ce atoms.The inconspicuous peak at 258.1 cm −1 results from the second order transverse acoustic mode (2TA) or doubly degenerate transverse optical mode (TO), which are Raman inactive in a perfect crystal [27].The peak at 1179.9 cm −1 may be related to the stretching mode of the short terminal Ce=O.Moreover, the Raman shift of Ni/CeO2-NR moved towards a low wavenumber in comparison with that of CeO2-NR, indicating the incorporation of Ni affects the symmetry of Ce-O bonds [21].The TEM of CeO2-NR and Ni/CeO2-NR are illustrated in Figure 4.The Ni/CeO2-NR maintained the original nanorod shape after Ni incorporation.The CeO2-NR was approximately 14 nm in diameter and several hundreds nm in length.As shown in Figure 4a, the calculated interference fringe spacings (d) are 0.27 and 0.31 nm, revealing the CeO2-NR predominantly exposes the (100) and (111) facets.The preferable exposure crystal facets of CeO2-NR are consistent with those of samples derived from the CeCl3 precursor in other studies [28,29].Theory calculation has demonstrated that the (111) is the least reactive facet, followed by (100) and (110).In addition to the preferable crystal facets, there are other aspects degerming CeO2 activity.Several 'dark pits' are shown in the box of Figure 4. Liu et al. [29] have reported that these dark pits are related to the surface reconstruction and defects; their study also concluded that these defects play a more important role in determining the CeO2 activity than the exposure planes.Sayle et al. [30] have performed a molecular dynamic modeling to simulate the synthesis of CeO2-NR and discovered that the atomistic sphere model exhibits many steps on the (111) planes of CeO2-NR.The migration of oxygen in CeO2 occurs by a vacancy hopping mechanism; therefore, the clusters of defects are conducive to oxygen transfer.If the diffusion rate of oxygen anions becomes adequately high, a consecutive oxygen flow is generated, XPS was employed to investigate the valences of Ce and Ni cations.The XPS spectra of Ce 3d of Ni/CeO 2 -NR (shown in Figure 5a) could be deconvoluted into two spin-orbit series, i.e., 3d 5/2 (u) and 3d 3/2 (v).The multiplet splitting components labeled u 0 , u 1 , u 2 , v, v 1 and v 2 corresponds to the 3d 10 4f 0 state of Ce 4+ , while the u 0 and v 0 are related to 3d 10 4f 1 state of Ce 3+ .These two Ce species in Ni/CeO 2 -NR indicate the OC surface was partly reduced because of oxygen desorption and the formation of oxygen vacancies.It is widely accepted that oxygen vacancies are produced to maintain electrostatic balance once Ce 3+ exists in fluorite Ce (Equation 1).The percentage of Ce 3+ cations to the total Ce cations is determined by the area ratio of different Ce species in XPS spectra.The Ce 3+ ratio of Ni/CeO 2 -NR (16.9%) was lower than that of pure CeO 2 -NR (24.3%) reported in Lykaki et al. [22], suggesting NiO species could inhibit the formation of surface-unsaturated Ce 3+ .The decrease in surface Ce 3+ species of Ni/CeO 2 -NR was consistent in the trend of I D /I F 2g in the Raman result.The Ni 2p XPS spectra (illustrated in Figure 5b) were characterized by two spin-orbit groups, i.e., 2p 3/2 (855.2 and 856.4 eV) and 2p 1/2 (873.3eV), as well as a shake-up peak at 861.8 eV.The photoelectron peak of 2p 3/2 over Ni/CeO 2 -NR shifted towards high-binding energy in contrast to those over pure NiO (844.4 eV, reported in Lemonidou et al. [32]) and Ni/CeO 2 (854.5 eV, reported in Tang et al. [33]).This result implies there is an enhanced MSI between Ni and CeO 2 -NR.The Ni/Ce atom ratio of the outer surface of CeO 2 -NR (0.49) is higher than the nominal one (0.32), suggesting a Ni species enrichment from bulk to surface.
where δ represents an empty position derived from the removal of O 2− from an oxygen tetrahedral site (Ce 4 O).
Catalysts 2018, 8, x FOR PEER REVIEW 5 of 11 resulting in high reducibility.The Ni particle size distribution, which is calculated from 52 particles, was inserted in Figure 4, and the average particle size was 8.1 nm (shown in Table 1).Shen et al. [31] have elucidated that the strong interfacial anchoring effect, which exists between the surface oxygen vacancies on (111) planes of CeO2-NR and the gold particles, only allows the gold particles to locally rotate or vibrate but not to migrate to form aggregates. Therefore, the strong MSI would significantly improve the Ni dispersion on the CeO2-NR surface, resulting in a small particle size of Ni.XPS was employed to investigate the valences of Ce and Ni cations.The XPS spectra of Ce 3d of Ni/CeO2-NR (shown in Figure 5a) could be deconvoluted into two spin-orbit series, i.e., 3d5/2 (u) and 3d3/2 (v).The multiplet splitting components labeled u 0 , u 1 , u 2 , v, v 1 and v 2 corresponds to the 3d 10 4f 0 state of Ce 4+ , while the u0 and v0 are related to 3d 10 4f 1 state of Ce 3+ .These two Ce species in Ni/CeO2-NR indicate the OC surface was partly reduced because of oxygen desorption and the formation of oxygen vacancies.It is widely accepted that oxygen vacancies are produced to maintain electrostatic balance once Ce 3+ exists in fluorite Ce (Equation 1).The percentage of Ce 3+ cations to the total Ce cations is determined by the area ratio of different Ce species in XPS spectra.The Ce 3+ ratio of Ni/CeO2-NR (16.9%) was lower than that of pure CeO2-NR (24.3%) reported in Lykaki et al. [22], suggesting NiO species could inhibit the formation of surface-unsaturated Ce 3+ .The decrease in surface Ce 3+ species of Ni/CeO2-NR was consistent in the trend of   /  2 in the Raman result.The Ni 2p XPS spectra (illustrated in Figure 5b) were characterized by two spin-orbit groups, i.e., 2p3/2 (855.2 and 856.4 eV) and 2p1/2 (873.3 eV), as well as a shake-up peak at 861.8 eV.The photoelectron peak of 2p3/2 over Ni/CeO2-NR shifted towards high-binding energy in contrast to those over pure NiO (844.4 eV, reported in Lemonidou et al. [32]) and Ni/CeO2 (854.5 eV, reported in Tang et al. [33]).This result implies there is an enhanced MSI between Ni and CeO2-NR.The Ni/Ce atom ratio of the outer surface of CeO2-NR (0.49) is higher than the nominal one (0.32), suggesting a Ni species enrichment from bulk to surface.
where  represents an empty position derived from the removal of O 2− from an oxygen tetrahedral site (Ce4O).H2-TPR was performed to investigate MSI.As illustrated in Figure 6, the H2-TPR patterns of CeO2-NR are comprised of two reduction peaks.An inconspicuous peak at 262 °C, which can be assigned to the reduction of surface-adsorbed oxygen, was observed, while a broad peak ranging from 400 °C to 550 °C could be ascribed to the reduction of Ce 4+ to Ce 3+ [22,34].With regard to Ni/CeO2-NR, the TPR profiles consisted of three peaks.The small peak at 220 °C may have been attributable to the reduction of the NiO species, which slightly interacted with CeO2-NR supports.Zhang et al. [21] have reported that this NiO species is characterized by small radius and could incorporate into CeO2-NR surface.The second peak at 356 °C could be ascribed to the reduction of NiO species, with a strong interaction with CeO2-NR supports.The H2 consumption of the second H 2 -TPR was performed to investigate MSI.As illustrated in Figure 6, the H 2 -TPR patterns of CeO 2 -NR are comprised of two reduction peaks.An inconspicuous peak at 262 • C, which can be assigned to the reduction of surface-adsorbed oxygen, was observed, while a broad peak ranging from 400 • C to 550 • C could be ascribed to the reduction of Ce 4+ to Ce 3+ [22,34].With regard to Ni/CeO 2 -NR, the TPR profiles consisted of three peaks.The small peak at 220 • C may have been attributable to the reduction of the NiO species, which slightly interacted with CeO 2 -NR supports.Zhang et al. [21] have reported that this NiO species is characterized by small radius and could incorporate into CeO 2 -NR surface.The second peak at 356 • C could be ascribed to the reduction of NiO species, with a strong interaction with CeO 2 -NR supports.The H 2 consumption of the second peak was highest among the three peaks, indicating most of the NiO strongly interacted with the support.The third peak at 494 • C was also attributed to the reduction of Ce 4+ to Ce 3+ .However, the broad reduction peak of Ni/CeO 2 -NR shifted to a lower temperature than that of pure CeO 2 -NR, suggesting the introduction of Ni improves the reducibility of CeO 2 -NR.It has demonstrated that the promoted reduction behavior is caused by the generated Ni-Ce-O species which improves the deformation of CeO 2 [35].

Activity Tests of OCs
The activity of Ni/CeO 2 -NR was tested in CLR of ethanol and compared with Ni/CeO 2 .Figure 7 displays the performance of both OCs in CLR: H 2 selectivity and ethanol conversion at the fuel feed step as well as the concentration of CO, CO 2 and O 2 at the air feed step.The H 2 selectivity and ethanol conversion of both OCs remained relatively stable during the process.With regard to Ni/CeO 2 -NR, the average ethanol conversion remained at 88.0%, and the average H 2 selectivity was 78.9%.However, with the average ethanol conversion and H 2 selectivity of conventional Ni/CeO 2 at 72.9% and 60.5% respectively, the activity of Ni/CeO 2 -NR was superior to the Ni/CeO 2 .As the Ni content and other experimental conditions were the same for both OCs, the improved activity therefore resulted from the support.
promoted reduction behavior is caused by the generated Ni-Ce-O species which improves the deformation of CeO2 [35].The activity of Ni/CeO2-NR was tested in CLR of ethanol and compared with Ni/CeO2. Figure 7 displays the performance of both OCs in CLR: H2 selectivity and ethanol conversion at the fuel feed step as well as the concentration of CO, CO2 and O2 at the air feed step.The H2 selectivity and ethanol conversion of both OCs remained relatively stable during the process.With regard to Ni/CeO2-NR, the average ethanol conversion remained at 88.0%, and the average H2 selectivity was 78.9%.However, with the average ethanol conversion and H2 selectivity of conventional Ni/CeO2 at 72.9% and 60.5% respectively, the activity of Ni/CeO2-NR was superior to the Ni/CeO2.As the Ni content and other experimental conditions were the same for both OCs, the improved activity therefore resulted from the support.

Activity Tests of OCs
Various factors contributed to the enhanced catalytic activity.Notably, the CeO2-NR with high specific-to-volume ratio guarantees excellent Ni dispersion, as evidenced by TEM and TPR results.Such a one-dimensional nanostructure enables uniform and small Ni nanoparticles to be finely dispersed on supports, thus generating many accessible catalytic active sites.Additionally, the high concentration oxygen vacancies of CeO2-NR, which was proven via Raman and XPS analysis, can activate and produce OH groups from steam.H2 and CO2 can be generated via the reaction between Various factors contributed to the enhanced catalytic activity.Notably, the CeO 2 -NR with high specific-to-volume ratio guarantees excellent Ni dispersion, as evidenced by TEM and TPR results.Such a one-dimensional nanostructure enables uniform and small Ni nanoparticles to be finely dispersed on supports, thus generating many accessible catalytic active sites.Additionally, the high concentration oxygen vacancies of CeO 2 -NR, which was proven via Raman and XPS analysis, can activate and produce OH groups from steam.H 2 and CO 2 can be generated via the reaction between the formed OH groups as well as intermediate species, thus improving H 2 selectivity.It has been proposed that the interface of the metal/CeO 2 is the main site for steam reforming reaction [36].
The air feed process is highly oxygen consuming and is accompanied by the generation of CO 2 and CO due to coke oxidation and partial oxidation.As illustrated in Figure 7, all the samples exhibited CO 2 and CO evolution at the air feed stage.The integration areas of C-containing gas concentrations corresponded to the quantity of carbon deposition.Ni/CeO 2 -NR exhibited small peak areas compared with the Ni/CeO 2 OC, indicating that the carbon deposition on Ni/CeO 2 -NR was effectively suppressed.This is a result of the abundant oxygen vacancies and its strong oxygen storage capacity, which enhanced the oxygen mobility and thereby facilitated the removal of carbon deposition.In addition, the depleted CeO 2 -NR could be partially replenished by the air feed step.The O 2 concentration increased from 0%to 23% at the end of the air feed stage, indicating the end of coke elimination and replenishment of lattice oxygen.

Stability Tests
The durability of Ni/CeO 2 -NR and Ni/CeO 2 was tested in a 10-cycle CLR process.As shown in Figure 8, the H 2 selectivity of Ni/CeO 2 slowly declined with the increase of the running cycles, while the Ni/CeO 2 NR maintained its activity throughout the test.During the tests, the H 2 selectivity of Ni/CeO 2 -NR decreased from 81.7% to 79.2%, while Ni/CeO 2 exhibited a significant decrease in H 2 selectivity from 61.8% to 51.4%.The deactivation of OCs predominantly caused Ni sintering and carbon deposition.Notably, the Ni/CeO 2 -NR OC exhibited a more durable performance than the Ni/CeO 2 , revealing that the CeO 2 -NR supported metal was more resistant to deactivation.The superior durability of the Ni/CeO 2 -NR sample resulted from the strong MSI between Ni and CeO 2 -NR support, which improved the metal-sintering resistance.The abundant oxygen vacancies of CeO 2 -NR are essential units for the anchoring of metal particles.The essential role of CeO 2 is to disperse and stabilize Ni particles over its surface oxygen vacancies, which depend on the morphology of CeO 2 .As evidenced by XPS, the (111) plane of CeO 2 -NR plays an important role in anchoring the Ni particles.It has been proposed that the interfacial Au atoms which are located away from the particle perimeter (covered interfacial atoms) would closely interact with the underlying surface oxygen vacancies on the (111) facets of CeO 2 -NR [31].Because this interfacial region was not involved in the reforming reaction, this strong interaction would effectively stabilize the Ni particles on CeO 2 -NR.Additionally, small size Ni particles can lower the driving force for coke diffusion and thereby help to reduce the carbon deposition.Furthermore, the strong oxygen mobility of CeO 2 -NR is indispensable to removing the carbon deposition at the metal surface.It has been demonstrated that the oxygen deposited in CeO 2 lattice can react with the carbon species left over from the steam reforming reaction [37].Overall, CeO 2 -NR support not only promotes the anti-sintering capability of OCs but also reduces the carbon deposition, thus improving the durability of OCs.A comparison of several investigations regarding CLR over different Ni-based OCs is tabulated in Table 2.The Ni/CeO 2 -NR in this work showed higher average fuel conversion in long-term tests despite the low Ni loading, indicating the strong sintering resistance and high reforming activity of this OC.
Catalysts 2018, 8, x FOR PEER REVIEW 7 of 11 the formed OH groups as well as intermediate species, thus improving H2 selectivity.It has been proposed that the interface of the metal/CeO2 is the main site for steam reforming reaction [36].
The air feed process is highly oxygen consuming and is accompanied by the generation of CO2 and CO due to coke oxidation and partial oxidation.As illustrated in Figure 7, all the samples exhibited CO2 and CO evolution at the air feed stage.The integration areas of C-containing gas concentrations corresponded to the quantity of carbon deposition.Ni/CeO2-NR exhibited small peak areas compared with the Ni/CeO2 OC, indicating that the carbon deposition on Ni/CeO2-NR was effectively suppressed.This is a result of the abundant oxygen vacancies and its strong oxygen storage capacity, which enhanced the oxygen mobility and thereby facilitated the removal of carbon deposition.In addition, the depleted CeO2-NR could be partially replenished by the air feed step.The O2 concentration increased from 0%to 23% at the end of the air feed stage, indicating the end of coke elimination and replenishment of lattice oxygen.The durability of Ni/CeO2-NR and Ni/CeO2 was tested in a 10-cycle CLR process.As shown in Figure 8, the H2 selectivity of Ni/CeO2 slowly declined with the increase of the running cycles, while the Ni/CeO2 NR maintained its activity throughout the test.During the tests, the H2 selectivity of Ni/CeO2-NR decreased from 81.7% to 79.2%, while Ni/CeO2 exhibited a significant decrease in H2 selectivity from 61.8% to 51.4%.The deactivation of OCs predominantly caused Ni sintering and carbon deposition.Notably, the Ni/CeO2-NR OC exhibited a more durable performance than the Ni/CeO2, revealing that the CeO2-NR supported metal was more resistant to deactivation.The superior durability of the Ni/CeO2-NR sample resulted from the strong MSI between Ni and CeO2-NR support, which improved the metal-sintering resistance.The abundant oxygen vacancies of CeO2-NR are essential units for the anchoring of metal particles.The essential role of CeO2 is to disperse and stabilize Ni particles over its surface oxygen vacancies, which depend on the morphology of CeO2.As evidenced by XPS, the (111) plane of CeO2-NR plays an important role in anchoring the Ni particles.It has been proposed that the interfacial Au atoms which are located away from the particle perimeter (covered interfacial atoms) would closely interact with the underlying surface oxygen vacancies on the (111) facets of CeO2-NR [31].Because this interfacial region was not involved in the reforming reaction, this strong interaction would effectively stabilize the Ni particles on CeO2-NR.Additionally, small size Ni particles can lower the driving force for coke diffusion and thereby help to reduce the carbon deposition.Furthermore, the strong oxygen mobility of CeO2-NR is

Preparation of OCs
CeO 2 -NR was synthesized on the basis of our previous work with small modifications [38].Firstly, 1 g of CeCl 3 (99.9%,Aladdin, Shanghai, China) and 19.3 g of NaOH (97%, Aladdin) were dissolved in 50 mL deionized water and continuously stirred for 25 min.The resulting mixture was then put into a Teflon-lined autoclave and kept at 100 • C for 24 h.Subsequently, the sample was separated, washed, then dried at 85 • C for 10 h and finally calcined at 400 • C for another five hours.The Ni/CeO 2 -NR oxygen carrier was synthesized by a wet-impregnation method.Ni(NO 3 ) 2 •6H 2 O (98%, Aladdin) of 0.2 g, equivalent to 10 wt % Ni loading, was first dispersed in 5 mL deionized water, and the prepared CeO 2 -NR of 0.5 g was added into the solution.The mixture was stirred under sonication for three hours at 60 • C and then dried at 85 • C for 10 h.The as-synthesized sample was then calcined at 700 • C for two hours.
The other reference bulk Ni/CeO 2 was also synthesized by a wet-impregnation method, and the procedure has been reported in Xu et al. [39].The Ni content of this OC was also set to 10 wt %.

Characterization of OCs
The texuture of OCs were analyzed by a Micrometric Acusorb 2100E apparatus (Ottawa, ON, Canada) at 77 K.The OCs were degassed at 573 K for three hours before tests.
ICP-OES (Nijmegen, The Netherlands) was applied to measure the elemental composition.Before tests, the sample was solved by hydrofluoric acid solution.
XRD was performed, using a Shimadzu XRD-600 instrument (Kyoto, Japan), to identify the phase composition.The scanning 2θ degrees ranged from 10 • to 80 • , and the scanning rate was set to 4 • /min.A graphite-filtered Cu Kα radiation (λ = 1.5406Å) was applied as a radiation source.
Raman spectra were employed by a Renishaw Spectroscopy (Gloucestershire, UK) with a visible 514 nm Ar-ion laser under ambient condtions.During the mesurement, the flowing gas was He, and the test temperature was maintained at 300 • C.
TEM was conducted, using FEI Tecnai F30 (Cleveland, OH, USA), to investigate the morphology of the Ocs.The samples were first solved in ethanol under sonication, followed by dispersal on a copper grid-supported carbon foil and dried in air.
XPS was carried out by a ThermoFisher K-Alpha system with a 150 W Al Kα source (Waltham, MA, USA).The samples were placed on the holder, and the scanning step was set to 0.15 eV.H 2 -TPR was employed by a Micrometrics AutoChem 2920 instrument (Ottawa, ON, Canada).In a typical procedure, a sample of 90 mg was preheated at 450 • C for one hour in an Ar flow of 35 mL/min and subsequently cooled to 80 • C. A mixture of 10 vol % H 2 in Ar flow (35 mL/min) was then inserted, and the temperature was increased from 80 • C to 100 • C at a rate of 5 • C/min simultaneously.H 2 chemisorption was also performed by the same apparatus to analyze the Ni dispersion.The sample was first reduced at 700 • C in a H 2 /Ar flow (30 mL/min) for one hour, followed by cooling to 100 • C under Ar flow.Subsequently, H 2 pulses were introduced until the eluted peaks of successive pulses became steady.The Ni active surface area was obtained from the H 2 adsorbption volume considering the stoichiometric ratio H adsorbed /Ni surface = 1 and a surface area of 6.5 × 10 −20 m 2 per Ni atom [40].

Activity and Stability Tests
The performance of CLR of ethanol by Ni/CeO 2 -NR and reference conventional bulk OCs, i.e., Ni/CeO 2 , were conducted in a packed-bed reactor, whose schematic was described in our previous work [41,42].The tested OCs of 0.5 g were loaded at the center of the quartz reactor.During a fuel feed step, an ethanol solution (4 mL/h) with a steam to carbon ratio (S/C) of four was preheated to 150 • C and then inserted into the reactor in a N 2 flow of 180 mL/min.The reacter temperature of the fuel feed step was 650 • C.An Agilent 7890A (Santa Clara, CA, USA) chromatography with two detectors, thermal conductivity detector (TCD) and flame ionization detector (FID), was applied to verify the effluents.TCD with a TDX-01 column was applied to detect N 2 , H 2 , CO, CO 2 , and CH 4 ; the FID with a Porapak-Q column was applied to measure the concentration of C 3 H 8 O 3 , H 2 O, and CH 3 CHO.In an air feed step, the air flow was 100 mL/min, and the reaction temperature was also 650 • C. The exhausted gases were detected by another GC (Agilent 7890A) with a TCD detector.A 5A molecular sieve column was used to detect the O 2 , and the TDX-01 column was applied to detect CO 2 , CO, and N 2 .The durations of the fuel feed step and the air feed step were 60 and 10 min, respectively.A N 2 purge process of five minutes was performed between the fuel feed step and the air feed step to eliminate the residue gas in the reactor.
The conversion and H 2 selectivity were calculated as follows:

Conclusions
A Ni/CeO 2 -NR OC was synthesized by hydrothermal method and tested in CLR of ethanol process in this work.H 2 selectivity of 80% was achieved by Ni/CeO 2 -NR in a 10-cycle stability test.The characterization results show that the Ni/CeO 2 -NR possesses high Ni dispersion, abundant oxygen vacancies, and strong MSI.The small particle size and abundant oxygen vacancies contributed to the WGS reaction, thus improving the catalytic activity.The buried interfacial Ni atoms strongly anchored on the underlying surface oxygen vacancies on the (111) facets of CeO 2 -NR, therefore enhancing the anti-sintering capability.Moreover, the strong oxygen mobility of CeO 2 -NR also effectively eliminated surface coke on the Ni particle surface.

Figure 1 .
Figure 1.Schematic description of CLR of ethanol.

Catalysts 2018, 8 ,
x FOR PEER REVIEW 6 of 11 peak was highest among the three peaks, indicating most of the NiO strongly interacted with the support.The third peak at 494 °C was also attributed to the reduction of Ce 4+ to Ce 3+ .However, the broad reduction peak of Ni/CeO2-NR shifted to a lower temperature than that of pure CeO2-NR, suggesting the introduction of Ni improves the reducibility of CeO2-NR.It has demonstrated that the promoted reduction behavior is caused by the generated Ni-Ce-O species which improves the deformation of CeO2[35].
Figure 1.Schematic description of CLR of ethanol.

Table
[22,24]rystal sizes of CeO 2 for CeO 2 -NR and Ni/CeO 2 -NR were 14.8 and 16.1 nm, respectively, indicating that the incorporation of Ni would not significantly change the structure characteristics of CeO 2 support.Similar crystallite sizes of CeO 2 -NR prepared by the hydrothermal method have been reported in other work[22,24].

Table 2 .
Comparison of CLR over different OCs.