Chemical-Looping Steam Methane Reforming over a CeO2–Fe2O3

Article pubs.acs.org/EF

Chemical-Looping Steam Methane Reforming over a CeO2−Fe2O3 Oxygen Carrier: Evolution of Its Structure and Reducibility Xing Zhu,† Kongzhai Li,† Yonggang Wei,† Hua Wang,*,† and Lingyue Sun†,‡ †

State Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, and ‡Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming 650093, People’s Republic of China ABSTRACT: Chemical-looping steam methane reforming (CL-SMR) is a promising method for the co-generation of pure hydrogen and syngas on the basis of redox cycles via a gas−solid reaction using an oxygen carrier. The performance and life of the oxygen carrier play pivotal roles in determining the feasibility and economy of the CL-SMR process. The present research was focused on the evolution of the structure and reducibility of a CeO2−Fe2O3 oxygen carrier during the CL-SMR redox process to further understand the sustainability of the oxygen carrier. The investigated CeO2−Fe2O3 complex oxide exhibited satisfactory performance in the CL-SMR process because of the chemical interaction between Ce and Fe species. A Ce−Fe−O phase equilibrium based on a stable composition of CeO2, Fe3O4, and CeFeO3 formed in the recycled samples. Surface oxygen was removed, which was accompanied by an increase in the concentration of oxygen vacancies and a decrease in the surface area of the recycled samples; these effects resulted in an increase in the high-temperature reducibility and syngas selectivity of the samples. Oxygen mobility was intensified by the Ce−Fe chemical interaction via the formation of CeFeO3 and a micromorphological transformation. These properties counteracted the sintering of the materials and guaranteed the stability of the oxygen carrier in the CL-SMR process.

1. INTRODUCTION

oxidation

Hydrogen is one of the most important chemical feedstocks in modern industry and is used in methanol and ammonia production, petroleum refining, and high-purity metal extraction.1,2 In addition, hydrogen is widely accepted as being the most environmentally friendly energy carrier and is considered to be the ideal fuel for the future.3 Steam methane reforming (SMR, CH4 + H2O → CO + 3H2) combined with water−gas shift (WSG, CO + H2O → CO2 + H2) and pressure swing adsorption (PSA) processes is usually used for the large-scale production of hydrogen.1,4 Either the WGS or the PSA process is energy-consuming and requires high capital investment, which reduces the economic viability of the SMR process. The chemical-looping steam methane reforming (CL-SMR) process was developed to circumvent these problems.5,6 CL-SMR is a novel chemical looping technology that requires no post-reaction gas separation to produce high-purity hydrogen and syngas with a suitable H2/CO ratio.7,8 In this process, CH4 is converted into syngas with a H2/CO mole ratio of 2 via a solid oxygen carrier in the 1st step (i.e., the methaneconversion step) and H2O is decomposed over the reduced oxygen carrier to generate H2 and regenerates the reduced oxygen carrier in the second step (i.e., the water-splitting step). Syngas and hydrogen can be generated simultaneously via the CL-SMR process.7 The net reactions in the CL-SMR process can be illustrated as follows:

MexOy − δ + δ H 2O → MexOy + δ H 2

overall CH4 + H 2O → 3H 2 + CO

© 2014 American Chemical Society

(3)

where MexOy and Me denote the metal oxides (oxygen carriers) and metal, respectively, and MexOy−δ denotes the reduced metal oxides (reduced oxygen carriers). The overall reaction 3 represents the SMR process. In the CL-SMR process, gas separation and purification problems can be averted. The oxygen carrier is the key factor in the continuous generation of syngas and hydrogen via the CL-SMR redox process. Numerous metal oxides, including Fe3O4,9 WO3,10,11 Ni ferrites,12 (Zn, Mn) ferrites,13 Cu ferrites,14−16 and Ce-based oxides,17−23 have been investigated as oxygen carriers. Research results showed that pure hydrogen could be obtained via water splitting over reduced Fe species (FeO and/or Fe); however, the activity and reducibility of the oxidation states of the Fe species (Fe3O4 and/or Fe2O3) toward methane conversion were relatively low.8,24 CeO2 exhibited favorable performance in methane conversion (reaction 2), and its reactivity was obviously improved when it was doped with transition metals.8,25 Fe-Doped CeO2 exhibited the highest oxygen storage capacity among the Cebased oxides. Chemical interaction between the Ce and Fe species is responsible for the good oxygen mobility in Fe-doped CeO2. Abundant active sites for the activation of CH4 on the

reduction MexOy + δCH4 → MexOy − δ + δ(2H 2 + CO)

(2)

Received: February 21, 2013 Revised: January 24, 2014 Published: January 27, 2014

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collected using the TPR Win, version 1.50 software (Quantachrome Instruments Co.) in the range of 100−900 °C with a heating rate of 10 °C/min. CH4-IR experiments were performed on a fixed-bed reactor under a flow of 5% CH4/N2 (100 cm3/min) over a 0.5 g sample at 850 °C. Prior to the CH4-IR tests, the samples were dried in air at 300 °C for 2 h and pure N2 was subsequently flowed through the reactor at 300 °C for 1 h. The reactor was then heated to 850 °C at a heating rate of 10 °C/min. The gases evolved from the fixed-bed reactor was analyzed with an online non-dispersive infrared (NDIR) CO/CO2 detector (C600, by Shanghai Baoying Photoelectric Co.) after they were condensed and purified. 2.3. Investigation of CL-SMR Activity. Gas−solid reactions for both steps of the CL-SMR process were investigated in a fixed-bed reactor at atmospheric pressure. In the CH4 conversion step, Ce−Fe mixed oxides were packed into a quartz tube reactor with an inside diameter of 20 mm. Prior to the reaction, the sample was dried at 300 °C for 2 h in air and at 300 °C and for 1 h in a flow of pure N2. The CH4 isothermal reaction was conducted using CH4 (99.99% purity) flowing at 10 cm3/min over 1.8 g of Ce−Fe mixed oxides at 850 °C. The water-splitting step was conducted over the reduced sample from the CH4 conversion step at 850 °C. Pure N2 was introduced into the reactor to purge the off-gases when the CH4 conversion reaction was finished. The water-splitting reaction was performed over a sample reduced with a flowing mixture of steam/N2 gas. The steam was produced from distilled water injected into an electric furnace at 400 °C at a flow rate of 0.18 cm3/min and was subsequently carried out by N2 flowing at 50 cm3/min. Successive CL-SMR cycles: After the reaction between CH4 (10 cm3/min) and the sample (1.8 g) had proceeded for 20 min (CH4 conversion step), steam was introduced at a flow rate of 0.18 cm3/min in a N2 carrier gas (50 cm3/min) for 30 min over the reduced sample (water-splitting step). These two steps were separated by a purge with N2 (50 cm3/min) for 30 min. The temperature for the CL-SMR process was maintained at 850 °C. The gas evolved during the experiment was analyzed by an Agilent 7890A GC system equipped with HP-Plot 5A and HP-Plot-Q columns. The selectivity toward CO and H2 and the conversion of CH4 have been described in our previous work.25

surface of this Ce−Fe mixed oxide provided high selectivity for syngas production via the gas−solid reaction between the methane and oxide. In our previous study, we observed that CeO2−Fe2O3 allows for a successive redox cycle for the cogeneration of syngas and hydrogen.21 On the basis of these findings, we investigated the performance of a series of Ce1−xFexO2−δ (x = 0−0.6) oxygen carriers in the CL-SMR process.26 The CeO2−Fe2O3 complex oxide exhibited satisfactory activity in this redox process, which could be used to maximally explore the potential of Ce−Fe complex oxides in similar applications. In this work, the CeO2−Fe2O3 complex oxide with a molar ratio of Ce/Fe = 1 was prepared by co-precipitation and its redox properties in the CL-SMR process were studied using a fixed-bed reactor. Great attention is devoted to the relationship between the structure of Ce−Fe oxides and their activity in repeated redox processes (methane conversion/water splitting). In combination with X-ray diffraction (XRD), specific surface area obtained by the Brunauer−Emmett−Teller (BET) method, scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR), and methane isothermal reduction (CH4-IR) techniques, the relationship between the structure and activity of the oxygen carriers in the CL-SMR process was explored.

2. EXPERIMENTAL SECTION 2.1. Preparation of Oxygen Carriers. A CeO2−Fe2O3 mixed oxide (Ce/Fe molar ratio = 1) was prepared by chemical precipitation. The raw materials Ce(NO3)3·6H2O and/or Fe(NO3)·6H2O were used to prepare a solution with the appropriate mole ratio that was subsequently heated to 70 °C. A NaOH solution was added to the salt solution with stirring. When the pH of the solution reached 10−11, addition of the NaOH solution was stopped. The resulting solution was maintained at 70 °C and stirred for 2 h. The precipitate was filtered and washed with distilled water and ethanol after settling for 2 h. A loosely compacted conglomerate was obtained after the precipitate was allowed to naturally dry overnight and subsequently dried at 110 °C for 24 h. This object was calcined at 300 and 800 °C for 2 h at each temperature and then crushed into a powder. The sample was pressed under 10 MPa for 10 min to prepare granules with a 20−40-mesh size. 2.2. Oxygen Carrier Characterization. The phase, lattice parameters, and crystallite size of the materials were determine by XRD on a Japan Science D/max-R diffractometer equipped with a Cu Kα radiation source (λ = 0.154 06 nm) operated at a voltage of 40 kV and a current of 40 mA. The samples were scanned over the range of 10° ≤ 2θ ≤ 80°. The BET surface areas of the samples were measured by N2 adsorption at −196 °C on a Quantachrome NOVA 2000e analyzer. The morphology of the samples was observed by SEM on a Philip XL30 scanning electron microscope operated at an accelerating voltage of 30 kV. Samples for SEM were dusted onto an adhesive conductive carbon belt attached to a copper disk. Raman measurements were performed on a Renishaw Invia Raman imaging microscope using an excitation wavelength of 514.5 nm generated from an Ar ion laser with a power of 1 mW. The Raman data were collected over the range of 100−1800 cm−1 XPS spectra were collected on a PHI-5500 system equipped with a monochromatic Mg Kα (1253.6 eV) X-ray radiation source. The pressure in the analysis chamber was less than 10−7 Pa. The angle between the surface normal and the axis of the analyzer was 45°. The C 1s peak of carbon was fixed to 284.8 eV to set the binding-energy scale. The samples from the reaction were transferred to the XPS instrument under protection of an inert atmosphere. H2-TPR experiments were performed using 10% H2 in helium flowing at 75 cm3/min over 0.1 g of sample. Experimental data were

3. RESULTS AND DISCUSSION 3.1. Successive CL-SMR Cycle Performance. The performance of CeO2−Fe2O3 in successive CL-SMR processes (i.e., methane conversion and water splitting) for syngas and hydrogen generation was investigated; the results are presented in Figure 1. In the methane-conversion reaction (Figure 1b), a slight decrease (from 63 to 55%) in methane conversion was observed during the first 4 cycles, whereas the selectivity toward H2 and CO was approximately 90% during the entire test. Notably, the molar ratio of H2/CO varied in the range of 2.0 ± 0.3, which approached the theoretical value of 2.0 in reaction 1. These results indicate that the Ce−Fe sample exhibited favorable performance in the methane-conversion step via a gas−solid reaction.26 As established in our previous study, surface oxygen atoms likely result in total oxidation, whereas lattice oxygen atoms tend to result in selective oxidation.27 In this reaction, some surface oxygen atoms were removed because the reduced Fe species in the reduced sample were unable to reoxidize to their original state (i.e., Fe2O3) and some surface oxygen could therefore not be regenerated (see sections 3.4 and 3.5). Therefore, the repeated redox process resulted in decreased CH4 conversion and increased syngas selectivity. In the water-splitting reaction, the amounts of hydrogen produced fluctuated slightly during the repeated redox cycles; however, the amount of hydrogen produced did not obviously 755

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Fe2O3 are evident in the XRD patterns of a fresh sample. The lattice parameter of CeO2−Fe2O3 contracted slightly compared to that of CeO2 (0.5406 nm for the fresh sample of the mixed oxide and 0.5413 nm for pure CeO2), indicating that a small amount of Fe3+ cations (0.064 nm) substituted Ce4+ ions (0.101 nm) in the CeO2 lattice to form Ce-based solid solutions.31 The strong CeFeO3 characteristic peaks appear in the XRD pattern of the reduced sample obtained after the methane-conversion step proceeded for 20 min. The CeO2− Fe2O3 mixed oxide (Ce/Fe molar ratio = 1) produced by chemical co-precipitation provided good interfacial contact between Ce and Fe species, which may enhance the reactivity of the solid−solid reaction that forms CeFeO3. CeFeO3 in the reduced sample is likely formed via 3CeO2 + Fe2O3 + Fe → CeFeO3 and/or CeO2 + FeO → CeFeO3 reactions under a reductive atmosphere (CH4, CO, H2, N2, CO2, and H2O in the methane-conversion step) at 850 °C.16,25,32,33 In the case of the reoxidized sample obtained after the 1st cycle, the characteristic peaks in the XRD pattern of CeFeO3 become weak, accompanied by the intensification of the CeO2 peaks and the appearance of a Fe3O4 peak. These results indicate that most CeFeO3 was decomposed to CeO2 and Fe3O4 under the steam atmosphere via the reaction 3CeFeO3 + H2O → 3CeO2 + Fe3O4 + H2.33 The XRD pattern of the sample recovered after 10 repeated redox cycles is similar to that of the sample recovered after 1 cycle. CeFeO3 is known to be unstable, and further reduction will destroy its structure.32 After the 1st redox cycle, CeFeO3 contained in an oxygen carrier might be destroyed by further reduction and then regenerated on the interface of CeO2 and Fe2+ via 4CeO2 + Fe3O4 + Fe → 4CeFeO3 and/or CeO2 + FeO → CeFeO3 reactions during the oxidation step at a high temperature.34 CeFeO3, CeO2, and Fe3O4 in the 1st and 10th samples appeared to reach a Ce−Fe−O phase equilibrium under a steam atmosphere and maintain a stable composition during the redox cycling.35 Notably, the characteristic peaks in the XRD pattern were narrowed after redox treatments, suggesting that the crystal grains grew because of sintering (the specific surface area decreased in the order 14.7 → 1.3 → 0.4 m2/g for the fresh → 1st → 10th samples, respectively). The SEM images of different samples are shown in Figure 3 at magnifications of 10000× and 100000×. The grains of the Ce−Fe mixed oxides appeared to grow and adhere with each other, indicating that sintering of the oxygen carrier occurred during the redox process. In comparison to the fresh sample, which contained finely dispersed grains, the recycled samples transformed into a massive crystal with blurry crystal boundaries. The integration of the grains of the Ce−Fe oxide and the formation of CeFeO3 should greatly enhance the chemical interaction between the Ce and Fe species. This morphological transformation also contributes to the formation of CeFeO3. 3.3. Raman Spectra. Detailed surface information obtained from the Raman investigation (Figure 4) supports the XRD results and provides a supplement for the phase analysis. In the case of the fresh sample, a broad band attributed to the F2g vibration mode of the cubic ceria structure was detected at ca. 465 cm−1.36 Seven other bands (224, 243, 291, 409, 496, 612, and 1320 cm−1) correspond to hematite.37 In the case of the reduced sample, the Raman bands associated with Fe2O3 almost disappear, whereas new bands appear at approximately 282 and 426 cm−1. According to the XRD results, these new bands are likely associated with the Raman modes of the

Figure 1. Amounts of syngas and hydrogen produced during the (a) CL-SMR redox process and (b) methane-conversion performance over the CeO2−Fe2O3 oxygen carrier at 850 °C. The reaction times for the two steps in the redox process were fixed at 20 and 30 min, respectively.

decrease.26 No carbonaceous gas (i.e., CO or CO2) was detected during the water splitting, indicating that the Ce−Fe mixed oxide maintained a sustainable reducibility toward methane conversion without inducing methane cracking. The relatively high activity of the CeO2−Fe2O3 mixed oxide in catalysis or gas−solid reactions with hydrocarbons has been attributed to the strong interaction between Ce and Fe species.21,25,28−30 However, the effect of redox treatments on the Ce−Fe chemical interaction, which is very important to understand the stability of this material toward successive redox processes, has not been determined. We therefore performed XRD, BET, SEM, Raman, XPS, H2-TPR, and CH4-IR measurements to obtain details related to the CeO2−Fe2O3 mixed oxide during the redox process. 3.2. XRD and BET Surface Area. The XRD patterns of samples (fresh, reduced, 1st, and 10th) obtained in CL-SMR redox cycles over the CeO2−Fe2O3 mixed oxide are shown in Figure 2. Distinct fluorite-type cubic CeO2 and hexagonal α-

Figure 2. XRD patterns of samples (fresh, reduced, 1st, and 10th) obtained from CL-SMR redox cycles over the CeO2−Fe2O3 mixed oxide; 1st and 10th represent the samples obtained after 1 and 10 repeated redox cycles, respectively. 756

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Figure 5. XPS Ce 3d results of CeO2−Fe2O3 mixed oxide samples (fresh, reduced, 1st, and 10th) obtained after CL-SMR redox cycles.

as v, v′, v″, and v‴ and u, u′, u″, and u‴, where v and u represent the two different spin−orbit (3d5/2 and 3d3/2) contributions.39,40 Ce 3d5/2 multiplets are labeled v, and those of 3d3/2 are labeled u. In the case of Ce4+, the peaks labeled as v, v″, and v‴ (from highest to lowest binding energy) arise from Ce 3d5/2, whereas the peaks labeled u, u″, and u‴ arise from Ce 3d3/2. The v′ and u′ peaks associated with Ce3+ in the final state refer to Ce 3d5/2 and Ce 3d3/2, respectively. The Ce species in different samples were calculated according to the Ce3+ 3d5/2 (v′)/Ce4+ 3d5/2 area ratio (v, v″, and v‴), as shown in Table 1. In the case of the fresh sample, the v′ (885.9 eV)

Figure 3. SEM images of CeO2−Fe2O3 mixed oxide samples (fresh, 1st, and 10th) used as oxygen carriers in the CL-SMR redox process.

Table 1. XPS Ce 3d Results for CeO2−Fe2O3 Mixed Oxide Samples (Fresh, Reduced, 1st, and 10th) Obtained after CLSMR Redox Cycles Ce species molar percentage (%)

Figure 4. Raman patterns of CeO2−Fe2O3 mixed oxide samples (fresh, reduced, 1st, and 10th) obtained after CL-SMR redox cycles.

CeO2−Fe2O3 mixed oxide

Ce4+

Ce3+

fresh reduced 1st 10th

81.8 59.1 84.7 78.4

18.2 40.9 15.3 21.6

and u′ (904.0 eV) peaks indicate the presence of Ce3+ ions and the areas of these peaks increase in the spectra of the reduced sample. The molar percentage of Ce3+ in the fresh sample was ca. 18.2%, and it increased to 40.9% in the reduced sample. When the reduced sample was reoxidized by steam, most of the lattice oxygen was regenerated and the molar percentage of Ce3+ decreased to a low level. After 10 successive redox cycles, the atomic concentration of Ce3+ slightly increased because of the removal of accumulated surface lattice oxygen. The corresponding O 1s spectra of different samples used as oxygen carriers in the CL-SMR process are shown in Figure 6. In our previous study,39 the O 1s peak was fitted into three bands at ca. 529.5, 531.5, and 533.0 eV, labeled as the OI, OII, and OIII peaks, as shown in Table 2. These peaks correspond to the lattice oxygen (denoted as OI), oxygen defect (e.g., oxygen vacancies, denoted as OII), and surface-adsorbed oxygen (denoted as OIII). Lattice oxygen represents the majority of the oxygen species (58.4−70.8%) in all of the samples. The fresh sample shows a low OII percentage because of its lack of oxygen defects (see Figure 6 and Table 2). The percentage of oxygen defects in the reduced sample increased to a high level and slightly decreased after the sample was reoxidized by steam (1st sample). The high-temperature redox treatment under a weak reductive atmosphere resulted in a remarkable increase in the concentration of oxygen defects in

CeFeO3 perovskite structure. The Raman spectra of the two reoxidized samples (1st and 10th) are very similar, showing a strong peak corresponding to cubic CeO2 at 465 cm−1 and a broad peak at 666 cm−1 corresponding to the main Raman mode of Fe3O4.38 In the case of the XRD results (Figure 2), no Raman characteristic peaks associated with CeFeO3 are observed in the patterns of the recycled samples, indicating that CeO2 and Fe3O4 are the main surface species, whereas CeFeO3 is the inner species. This phenomenon is consistent with the product-layer diffusion-controlling mechanism of the water-splitting reaction, as reported by Kang et al.13 CeFeO3 is unstable under an oxidative atmosphere; it only exists in a sealed system or under an inert atmosphere. The combined XRD and Raman results lead to the conclusions that surface CeFeO3 in the reduced sample is reoxidized to CeO2 and Fe3O4 and that the formation of these products on the surface of the oxide prevents further oxidation of the inner CeFeO3. The structural consistency of the sample after the 1st and 10th cycles provides further evidence that the CeO2−Fe2O3 material maintained its structural stability during the redox processes. 3.4. XPS. The Ce 3d spectra of different samples obtained after they were used in the CL-SMR process are shown in Figure 5. The contributions to these complex peaks are labeled 757

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fresh sample, the samples used in the cycling experiments contain abundant oxygen defects and exhibit low surface areas. Given that less-active oxygen (i.e., bulk lattice oxygen or samples with a lower surface area) exhibits greater syngas selectivity, the removal of surface oxygen or a decrease in the surface area of a sample could enhance the syngas selectivity of the sample in the methane-conversion step.25 An abundance of oxygen vacancies in the materials could also enhance the mobility of their lattice oxygen (see sections 3.5 and 3.6). 3.5. H2-TPR. H2-TPR measurements were performed to investigate the reducibility of different samples, as shown in Figure 7. The H2-TPR profile of pure CeO2 usually shows two

Figure 6. XPS O 1s patterns of CeO2−Fe2O3 mixed oxide samples (fresh, reduced, 1st, and 10th) obtained after CL-SMR redox cycles.

Table 2. XPS O 1s Results for CeO2−Fe2O3 Mixed Oxide Samples (Fresh, Reduced, 1st, and 10th) Obtained after CLSMR Redox Cycles

Figure 7. H2-TPR patterns of CeO2−Fe2O3 mixed oxide samples (fresh, reduced, 1st, and 10th) obtained after being used as oxygen carriers for CL-SMR redox cycles.

oxygen species percentage (%) CeO2−Fe2O3 mixed oxide

OI

OII

OIII

fresh reduced 1st 10th

66.8 63.8 70.8 58.4

7.8 30.2 24.2 37.9

25.4 6.7 4.9 3.7

main peaks for the reduction of surface lattice (at ca. 570 °C) and bulk oxygen (at ca. 900 °C).26 The common stepwise process (Fe2O3 → Fe3O4 → Fe0) for the pure Fe2O3 was found to be divided into three steps (Fe2O3 → Fe3O4 → FeO → Fe0) in the Ce−Fe mixed oxide because of the chemical interaction between Ce and Fe species.31,42 On the basis of this finding, three peaks, labeled α, β, and γ in the hydrogen reduction profile of the CeO2−Fe2O3 mixed oxide, should originate from overlapping of the peaks associated with the three-step reduction of Fe2O3 with peaks attributed to CeO2 reduction.25 In the profile of the reduced sample, the first weak peak at ca. 470 °C is attributed to the reduction of residual surface Fe3+. On the basis of the XRD results (see Figure 2), the peak associated with the reduction of CeFeO3 should contribute to the main peak at ca. 620 °C. The profiles of samples used in 1 and 10 redox cycles are similar, with a main peak at ca. 840 °C. This strong peak is composed of overlapped reduction peaks of CeO2, CeFeO3, and Fe3O4 (see Figure 2). A comparison of the profiles of the fresh and reoxidized samples reveals three key points: (i) the reduction peak associated with surface oxygen in the fresh sample at low temperatures almost disappeared after the redox process treatments; (ii) the high-temperature (T > 600 °C) reducibility was enhanced in the case of the recycled samples; and (iii) the area of the hydrogen reduction peak in the profile of the recycled samples was relatively smaller than that in the profile of the fresh sample. The disappearance of the low-temperature reduction peak for the recycled samples is attributed to the removal of surface oxygen (see Figure 6 and Table 2). Lattice oxygen represented the majority of the oxygen species in the recycled samples, which resulted in enhanced high-temperature reducibility. Removal of the surface oxygen and a phase transformation (i.e., the appearance of CeFeO3 and Fe3O4)

the 10th sample. This result is in agreement with the conclusion reached on the basis of the XPS Ce 3d spectrum that surface lattice oxygen was removed by subsequent redox processes and that this oxygen removal resulted in an increased concentration of Ce3+ (see Figure 5 and Table 1). In this redox process, the oxygen carrier always functions under a reductive atmosphere (CH4, CO, and H2 in the methane-conversion reaction and H2 in the water-splitting reaction). Despite the influence of the specific surface area and sintering, surface lattice oxygen would be reduced by these reductive gases. On the basis of the XRD and Raman results, we concluded that the reduced sample contained greater concentrations of CeFeO3 with more abundant Ce3+ and oxygen vacancies, whereas the recycled samples contained lower concentrations of CeFeO3 with less abundant Ce3+ and oxygen vacancies. Given the good stability of CeFeO3 under a weakly reductive or inert atmosphere, the appearance of CeFeO3 in the Ce−Fe mixed oxide indicates the existence of Ce3+ and oxygen vacancies. The percentages of the OIII component in the recycled samples abruptly decreased to a low level, as compared to those in the fresh sample. The percentages of OIII in the samples were consistent with specific surface area (fresh > 1st > 10th; see Table 2) and morphology observed in SEM images. The results suggest that surface-adsorbed oxygen in the Ce−Fe mixed oxide is highly sensitive to the variation of the surface area of the material caused by sintering.41 In comparison to the 758

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high-temperature reducibility and syngas selectivity. Moreover, CeFeO3 formation (i.e., the formation of numerous additional oxygen vacancies) and the overlapping of grains during the further crystallization of materials could intensify the chemical interaction of Ce and Fe species and greatly improve the mobility of oxygen in the Ce−Fe mixed oxide. These properties can counteract both the sintering of the materials during the high-temperature treatment and the consequent decrease in oxygen storage capacity, thereby resulting in a material with stable activity toward the CL-SMR process.

resulted in a slight decrease in the amount of total hydrogen consumed. Surface oxygen contributes to the total oxidation of methane, whereas lattice oxygen can selectively oxidize methane to syngas. Thus, the removal of surface oxygen from the phase transformation in recycled samples should improve the syngas selectivity during the redox cycling (see Figure 1). Notably, the reducibility of the 10th sample, which contained a higher concentration of oxygen vacancies, remained at an acceptable level (see Tables 1 and 2) compared to that of the 1st sample. We concluded that an appropriate increase in the concentration of oxygen vacancies could somewhat enhance the mobility of the lattice oxygen to compensate for the loss of surface oxygen in the recycled samples. This compensation is possible because oxygen located within the bulk will transfer to the surface as the surface oxygen is consumed.43 Some CeFeO3 that remained in the recycled samples also played an important role in enhancing oxygen mobility. 3.6. CH4-IR. Methane isothermal reduction was performed to explore the reactivity of the recycled samples toward methane conversion. The kinetic curves of CO2 and CO volume fractions for the fresh, 1st, and 10th samples are shown in Figure 8. In the isothermal reaction between methane and

4. CONCLUSION The Ce−Fe mixed oxide exhibited excellent activity and stability in the production of pure hydrogen and high-quality syngas with a H2/CO ratio of ca. 2 during successive CL-SMR redox cycles. The performance of the Ce−Fe mixed oxide as an oxygen carrier in the CL-SMR process depends upon its phase composition, specific surface area, micromorphology, and distribution of oxygen species and oxygen vacancies, all of which are strongly influenced by the redox treatment. XRD and Raman results revealed that the CeO2−Fe2O3 complex oxide was transformed to CeFeO3 via solid−solid reactions during the methane-conversion step at 850 °C and that surface CeFeO3 was reoxidized to CeO2 and Fe3O4 during the water-splitting step. The Ce−Fe mixed oxide reached a Ce−Fe−O phase equilibrium and maintained a constant composition during the redox process. The grains increased in size, and this process was accompanied by the disappearance of crystal boundaries in the recycled samples, indicating that the materials were sintered. XPS results showed that a valence variation of Ce4+ → Ce3+ → Ce4+ in the mixed oxide could transform methane and steam into the desired products via oxygen exchange and that the Ce3+ concentration was increased by the redox treatment in a weakly reducing atmosphere at high temperatures. The formation of CeFeO3 was accompanied by the introduction of abundant oxygen vacancies in the Ce−Fe mixed oxide. The surface oxygen atoms of the reoxidized sample were removed, in conjunction with increased concentrations of bulk oxygen and oxygen vacancies, which promoted the high-temperature reducibility of the oxide. Oxygen mobility in the recycled samples was enhanced by the modified Ce−Fe chemical interaction via the formation of CeFeO3 and the further crystallization of materials. The CO selectivity and the oxygen release rate of the recycled samples were observed to be greatly enhanced. Because of these properties, the Ce−Fe mixed oxide could sustain the constant production of syngas and hydrogen via the CL-SMR process.

Figure 8. CH4-IR profiles of CeO2−Fe2O3 mixed oxide samples (fresh, 1st, and 10th) used as oxygen carriers in the CL-SMR redox process.

the solid oxygen carrier, active oxygen (surface-adsorbed oxygen and surface lattice oxygen) was first converted into deep-oxidation products (i.e., CO2 and H2O) and then lessactive oxygen (bulk lattice oxygen) was converted into partial oxidation products (i.e., CO and H2). Samples with different distributions of oxygen species provide compelling evidence that samples with lower concentrations of surface oxygen produced smaller amounts of CO2 at the beginning of the isothermal reaction, whereas samples with greater concentrations of surface oxygen produced larger amounts of CO2. The CO starting time was shifted to an earlier time with an increasing cycle number [fresh (60 min) > 1st (50 min) > 10th (40 min)]. Notably, the area of the CO peak also increased with an increasing cycle number. These phenomena confirm that the selectivity for methane conversion was greatly improved by redox process treatments over the Ce−Fe mixed oxide. In addition, favorable oxygen mobility was achieved in the recycled samples because the recycled samples exhibited high CO evolution rates and evolved large amounts of CO during this gas−solid reaction. In the CL-SMR redox process, phase transformation, crystallization of materials, and redox treatments under a weakly reductive atmosphere were observed to create a decrease in surface oxygen content, resulting in enhanced

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-871-65153405. E-mail: wanghuaheat@ hotmail.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (Projects 51204083, 51004060, 51104074, 513744004, and 51174105), the Applied Basic Research Program of Yunnan Province (2012FD016), the Candidate Talents Training Fund of Yunnan Province 759

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(2012HB009), and the Analysis and Testing Foundation of Kunming University of Science and Technology.

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dx.doi.org/10.1021/ef402203a | Energy Fuels 2014, 28, 754−760