Tailoring Hybrid Nonstoichiometric Ceria Redox Cycle for Combined

Jun 26, 2016 - Abstract: A solar cavity-receiver containing a reticulated porous ceramic (RPC) foam made of pure CeO2 has been experimentally investig...
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Tailoring Hybrid Nonstoichiometric Ceria Redox Cycle for Combined Solar Methane Reforming and Thermochemical Conversion of H2O/ CO2 Mahesh M. Nair and Stéphane Abanades* Processes, Materials and Solar Energy Laboratory, PROMES-CNRS (UPR 8521), 7 Rue du Four Solaire, 66120 Font-Romeu, France S Supporting Information *

ABSTRACT: Combining partial oxidation of methane with H2O/CO2 splitting under solar thermal conditions presents a very promising strategy for producing solar fuels. In order to achieve this, the development of stable and efficient redox catalysts is necessary, among which ceria (CeO2) seems to be one of the most promising for lattice oxygen transfer. In this study, CeO2 was used for splitting CO2 and H2O using concentrated solar energy with reaction temperatures in the range 900−1100 °C. The experimental studies in a solar-driven thermogravimetric system indicated that both CH4 induced reduction and CO2 induced oxidation of CeO2−δ followed close reaction orders with activation energies of 109 and 36 kJ mol−1, respectively. The results were compared with those obtained from hydrothermal templating and surfactant induced self-assembly. To our knowledge, such materials are studied for the first time for CH4 induced fuel production via solar thermochemical redox cycles. Enhanced reaction rates and stability upon cycling were observed for materials synthesized by hydrothermal and self-assembly methods. Experiments were also carried out to deduce the effect of various inert materials (MgO and Al2O3) as promotional agents. Higher reduction rate and maximum nonstoichiometry (δ = 0.431) during reduction at 1000 °C were observed in the case of MgO promoted CeO2. In addition, the amount of evolved CO was found to be the highest (δ = 0.402), indicating almost complete reoxidation. The achieved nonstoichiometry and the resulting fuel productivity are more than 10 times higher than the reported values for thermal reduction of ceria. Studies were also performed in a solar reactor prototype, enabling both partial ceria reduction with methane, followed by oxidation with H2O/CO2. Typically, MgO and Al2O3 promoted ceria were tested under packed bed conditions and compared with commercial ceria for syngas production. In this case, significant enhancement in the system efficiency was observed for MgO promoted CeO2.

1. INTRODUCTION Solar energy is the largest exploitable alternative to fossil fuels with enormous potential. However, it must be harnessed and converted into storable and transportable forms such as hightemperature heat (thermal storage), power (photovoltaics or solar thermal), or chemical fuels (H2, syngas, liquid fuels). A wide variety of strategies are being explored on this regard including solar cells, semiconductor photocatalysis, etc.1,2 The efficiency of these methods mostly relies on the visible region of the solar spectrum. Besides, thermochemical conversion of solar energy into fuels or feedstock such as syngas (CO + H2) has gained considerable interest over recent years.3−5 In this method, the entire solar spectrum can be effectively utilized while the redox properties of metal oxides are exploited to convert H2O and CO2 into H2, CO, or syngas.6 Two-step redox processes for splitting H2O and/or CO2 can be divided into two categories, namely, stoichiometric and nonstoichiometric cycles. Thermochemical redox reactions proceed via discrete transition between two valence states for stoichiometric oxides (which generally occurs with phase transitions), while formation of oxygen vacancies in the oxide lattice during solid-state reactions occurs for nonstoichiometric oxides. Thus, the development of efficient and stable redox materials remains one of the major bottle necks for the effective commercialization of this technology. Alternatively, the maximum temperature of the redox cycles can be decreased by using a carbonaceous reducing agent (such as carbon or methane).7−9 © 2016 American Chemical Society

Several aspects are to be considered for selecting appropriate redox materials such as solid phase stability during the operating conditions, resistance to carbon deposition, formation of metal carbide during the reduction step with CH4 in the case of a carbothermal process, etc. One of the most widely investigated materials for solar thermochemical fuel production is ceria (CeO2).3,10−13 The inbuilt unique redox properties of CeO2 make it an extremely promising material for solar thermochemical fuel production, with high potential for commercial applications.14 The additional advantages encompass high oxygen storage and release capacities, crystallographic stability during Ce4+ to Ce3+ reduction, and rapid kinetics during thermochemical cycles. First, ceria is partially reduced to a nonstoichiometric oxide at high temperatures exceeding 1400 °C, in an endothermic thermal reduction step. In a subsequent low temperature exothermic oxidation, the reduced ceria is oxidized with H2O and/or CO2, producing H2 and/or CO, along with the stoichiometric oxide. Recent studies indicated that significant attention must be paid with regard to the stoichiometry of chemical reactions involved and the presence of volatile compounds in the gas phase at equilibrium.15 One way to enhance the fuel production is to increase the extent of Received: May 3, 2016 Revised: June 23, 2016 Published: June 26, 2016 6050

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monitor continuously the amount of oxygen released/absorbed by the material during redox reactions. The experimental setup used is depicted in Figure 1. In comparison, studies were also

reduction (δ) at a lower temperature. Introduction of 4+ valence dopants (Zr, Hf) with a lower ionic radius than Ce4+ in the CeO2 fluorite structure significantly helped to overcome this issue by improving the reducibility of ceria.16−20 However, slower reoxidation kinetics compared to pure CeO2 was also observed in this case. Studies performed by Le Gal et al. indicated that ceria with dopant concentration of 25 mol % Zr4+ was the optimal composition for reoxidation with steam.21 The structural modifications induced by the substitution of Ce4+ by Zr4+ appear to be responsible for the enhancement of the reduction processes. The reduction/oxidation processes induce expansion/contraction of the lattice parameter and strongly modify the material microstructure. The sintering process induces a structural modification of the ceria-zirconia solid solution, which promotes the reduction in the bulk and induces formation of mesoporosity, resulting in a high efficiency of the Ce4+-Ce3+ redox cycle at moderate temperatures.22 The reduction occurs concurrently at the surface and in the bulk of the solid solution. The availability of bulk Ce(IV) sites for the redox processes at moderate temperatures, even after extensive sintering, makes these materials highly attractive for processes requiring an efficient redox couple with a high thermal stability. A computer simulation study of the Ce4+/ Ce3+ reduction process in CeO2-MO (M = Ca, Mn, Ni, Zn) and CeO2-M2O3 (M = Sc, Mn, Y, Gd, La) mixed oxides showed that the process is enhanced with increasing metal dopant content (the enhancement being more pronounced for divalent dopants).23 For a fixed dopant content, larger dopant ions favor the reduction of cerium. These results are explained by the higher oxygen vacancy concentration and the larger dopant size being effective in accommodating the greater relaxation or elastic strain associated with the larger Ce3+ species formed upon reduction. Alternatively, performing ceria reduction in the presence of a reducing agent such as methane is found to be effective.24−27 This highly endothermic process is expressed as CeO2 + δCH4 → CeO2 − δ + δCO + 2δ H 2

Figure 1. Schematic representation of the experimental setup used for solar-driven thermogravimetric analysis of CeO2 redox cycles and photograph of solar concentrating system and reactor.

performed under similar conditions using CeO2 synthesized via hydrothermal and self-assembly methods and clearly indicated that such differences in the synthesis strategies modified the redox performance of the materials. This performance modification can be attributed to the variations in structure and morphology. Such materials were previously found to be advantageous for a variety of applications in comparison with their bulk counterparts.28−32 Further, the effect of various inert promotional agents, such as Al2O3 and MgO, on the redox properties of CeO2 was studied. Finally, a different reactor configuration consisting of a porous SiC packed bed to support CeO2 and MgO or Al2O3 promoted CeO2 was also used for H2O and CO2 splitting redox cycles.

(1)

The reduced metal oxide thus produced can be used for splitting H2O or CO2 in the second step. CeO2 − δ + δCO2 (H 2O) → CeO2 + δCO (H 2)

(2)

2. EXPERIMENTAL SECTION

Hence, this two-step cyclic process has a noticeable incentive for the production of carbon-free hydrogen for fuel cells and/or syngas. The coupling of partial oxidation of CH4 with CeO2 redox cycle enables one to perform the entire reactions under isothermal or near isothermal conditions because the temperature of the reduction step is greatly decreased and both reactions can thus proceed at the same temperature. Also, additional H2 and CO are produced during the CeO2 reduction step in the presence of CH4 with a 2:1 ratio desired for conversion to liquid fuels via Fischer−Tropsch reactions or methanol synthesis. So far, few studies were performed by coupling partial oxidation of CH4 with H2O/CO2 splitting over CeO2.24−27 Most of these studies focused on using pure or doped CeO2. However, the effects of surface structure, morphology, and porosity on the performance of these materials are rarely exploited, especially in the case of CeO2 redox cycles coupled with partial oxidation of CH4. In the present study, the kinetic analysis of the reduction and reoxidation of commercial CeO2 in the presence of CH4 and CO2, respectively, was performed using a customized solar thermogravimetric device in order to

2.1. Materials Synthesis. CeO2, MgO, and Al2O3 were purchased from Aldrich (99.9%). CeO2 was used as received, and for MgO and Al2O3 promoted materials, appropriate quantities of materials (50 wt %) were ground well in a mortar before performing the tests for promoting the mixture homogenization. All of these materials were calcined at 1000 °C for 2 h in air before the solar experiments to warrant their structural and chemical stability after this pretreatment step and to remove the impurities such as moisture that got adsorbed on the surface of the material. A recently developed one-pot hydrothermal templating method was also used for the synthesis of CeO2 (CeO2-HT in the following).33,34 Typically, 3.5 g of glucose was dissolved in 20 mL of water. Next, 1.4 g of Ce(NO3)3·6H2O (CeO2 purity > 99.5%, total remaining rare earth oxides < 0.05%) was added to this solution and kept stirring for 1 h. The entire homogeneous solution was transferred to a Teflon-lined stainless steel autoclave and was hydrothermally treated at 180 °C for 24 h. The resulting black precipitate was filtered, dried at 80 °C, and calcined under air at 1000 °C for 2 h. CeO2 was also synthesized by following the self-assembly method (CeO2-SA in the following) as described by Yuan et al. for the synthesis of ordered mesoporous (Ce,Zr)O2.35 Nonionic surfactant Pluronic P123 was used as the soft template. In a typical synthesis, 1 g of P123 and 4.34 g of Ce(NO3)3·6H2O were dissolved in 10 mL of 6051

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Energy & Fuels ethanol and stirred for 2 h at room temperature. The clear homogeneous solution was aged at 40 °C for 48 h and subsequently dried at 100 °C for 24 h. The resultant dried gel was ground well and calcined at 1000 °C for 2 h, under air. 2.2. Characterization. Structural and morphological characterizations of the synthesized materials before and after redox cycles were performed. X-ray diffraction (XRD) patterns were recorded with a Philips PW 1820 diffractometer with the Cu Kα radiation (αCu = 0.15418 nm, angular range = 20−100 2θ, steps = 0.02 2θ, recording time = 2 s). For the used/cycled materials, the whole sample powder was collected after thermochemical cycles and mixed in a mortar before XRD analysis. The powder morphology and pore structure were studied by using SEM (FESEM, Hitachi S4800). 2.3. Thermochemical Activity Measurements. The experimental setup is shown schematically in Figure 1 and was previously developed for investigating high-temperature solid−gas thermochemical reactions in a controlled atmosphere.36 Concentrated solar energy is used to drive the thermochemical reactions. The high-temperature process heat is supplied by a horizontal axis solar furnace consisting of a sun-tracking heliostat reflecting the incident solar irradiation toward a 2 m diameter parabolic dish concentrator. The solar thermogravimetric reactor consists of a cavity receiver with the aperture (15 mm diameter) positioned at the focal point of the concentrator for the optimal access and absorption of concentrated solar radiation. The cavity made of graphite is lined with a surrounding insulation layer and separated from the atmosphere using a glass window. Concerning the solar thermogravimetric measurements, the sample to be analyzed was placed in an alumina crucible (12 mm i.d., 15 mm o.d., 10 mm height) connected to a microbalance inside a cylindrical lining tube made of alumina (25 × 20 mm diameter). Temperature above the reacting sample during redox cycles was measured by placing a B-type thermocouple inside the alumina tube. It corresponds to the reaction temperature because the cavity-type solar reactor configuration provides homogeneous temperature distribution in the reacting zone inside the absorber tube where the sample is placed.36 The sample is mainly heated by radiation from the nearby hot tube walls (radiative heat transfers are prevailing because of the high temperatures) with a radiative thermal equilibrium established in this zone. The absence of thermal gradient inside the reacting powder is presumed given the very limited size of the crucible. Purge gas (Ar, 99.999% purity, O2 content below 2 ppm), CH4 (99.95% purity), and CO2 (99.995% purity) flow rates were regulated by using electronic mass flow controllers. Total flow rate was maintained constant (0.5 L·min−1) to keep the gas residence time constant. The reduction extent (δ) in eq 1 is calculated as follows

δ = (Δm/m)· (MCeO2 /MO)

(3)

Figure 2. Representative wide angle powder XRD patterns of (a) fresh CeO2, CeO2−δ after CH4 induced reduction, and CeO2 after 2 redox cycles; (b) Al2O3 and MgO promoted CeO2 before and after 3 redox cycles; (c) CeO2-HT and CeO2-SA before and after 3 redox cycles.

where Δm is the mass loss (or gain) obtained during ceria reduction (or oxidation), m is the initial amount of sample used for analysis, MCeO2 is the molecular weight of CeO2, and MO is the molecular weight of the oxygen atom. The same equation is used for estimating the amount of CO evolved in eq 2 with the mass loss replaced with mass gain during oxidation.

after oxidation. Thus, complete reoxidation was evidenced for the cycled material. Part of ceria was completely converted into the Ce2O3 phase after reduction, as confirmed by the XRD pattern of the reduced ceria, while a fraction of CeO2 was partially reduced to nonstoichiometric ceria. The concomitant presence of partially reduced CeO2 and Ce2O3 in the recovered sample after reduction yields global composition CeO2−δ (with δ < 0.5). In the case of MgO and Al2O3 promoted materials (50 wt %), respective diffraction peaks of the inert material were also observed. This confirms that the interactions between the individual compounds remained negligible and the materials retained their phase composition during the initial precalcination process. For these materials, diffraction patterns matching well with the fresh materials were also observed after cycling. It can be concluded that the phase purity of these materials was unaffected under the thermochemical reaction conditions that involved reduction in the presence of CH4 and oxidation in the

3. RESULTS AND DISCUSSION 3.1. Physicochemical Characterization. Initial studies were dedicated to perform the physicochemical characterization of the fresh/used materials. Wide angle X-ray diffraction patterns of the materials used in this study are provided in Figure 2. All the observed reflections for the commercial, hydrothermal, and self-assembled materials match well with the highly crystallized and pure phase of the cubic fluorite-type CeO2 structure. XRD patterns for CeO2 reduced in the presence of CH4 and CeO2 after three successive redox cycles reveal that, while partial structural changes resulting from the formation of Ce2O3 phase took place during reduction, the structure returned back to the initial cubic fluorite structure 6052

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Energy & Fuels presence of CO2 at 1000 °C. Similar results were observed in the case of CeO2 redox materials synthesized by hydrothermal and self-assembly methods. The scanning electron microscopy (SEM) images of commercial CeO2 along with the ones synthesized by hydrothermal and self-assembly methods are provided in Figure 3. In the case of commercial CeO2, closely agglomerated

of the ceria-based materials, commercial CeO2 was investigated initially. Reduction of CeO2 was first monitored as a function of temperature during a nonisothermal run in the presence of CH4 (40% in Ar, CH4 injection at about 400 °C during temperature rise) in order to identify the temperature at which the reaction commences. As a result, ceria reduction with CH4 occurs slightly from ∼600 °C with a steep rate increase from ∼800 °C as the temperature rises. The obtained reduction profile after a blank run correction is provided in the Supporting Information (Figure SI1). Isothermal redox cycles were then performed. The two-step cycle consists of partial oxidation of CH4, resulting in the formation of reduced CeO2−δ, followed by CO2 induced reoxidation. Heat delivered from concentrated solar energy was used to increase the temperature of the reaction chamber where the sample is placed, under a flow of Ar (0.5 L· min−1). Purging was done with Ar to avoid ceria reduction during heating up to the operation temperature (at ∼80−100 °C/min) before introducing CH4 under isothermal conditions. This was confirmed by the mass loss that remained negligible until CH4 was injected. When the temperature reached 1000 °C, a continuous flow of CH4 (0.2 L·min−1, 40% CH4 in Ar) was introduced under isothermal conditions and was retained until the oxygen releasing step from ceria was completed, as evidenced by the steep mass loss curve as a function of time. Afterward, the entire CH4 remaining in the reactor was removed using a vacuum pump and Ar refilling, and CO2 was subsequently introduced (0.2 L·min−1, 40% CO2 in Ar) at the same temperature (1000 °C) in order to reoxidize the partially reduced ceria (CeO2−δ), thereby completing the cycle as shown in Figure 4a. On the whole, CO and H2 are produced from CH4 and CO2. In order to check the stability of this material, the entire process of reduction and reoxidation was performed again as mentioned above. During the initial reduction step in the first cycle, approximately 3.5% mass loss was observed, corresponding to a δ value of 0.378 (which also corresponds to the amount of released O from ceria in mol·mol−1). The resulting global ceria reduction yield is thus 76% assuming that a δ of 0.5 corresponds to complete ceria reduction into Ce2O3. The quantity of CO produced during the CO2-splitting step was calculated from the mass gain observed during the reoxidation of CeO2−δ, which amounted to 0.365 mol·mol−1. Clearly, the negligible difference observed between the mole values of O and CO evolved during reduction and reoxidation, respectively, indicated almost 100% reoxidation for this material. Even though significant variation in the reduction extent was not observed during the reduction in the presence of CH4 in the second cycle (δ = 0.372), the amount of CO evolved was found to decrease to 0.322 mol·mol−1 during the reoxidation step. From this observation, it can be concluded that stability of commercial CeO2 will be diminished during the long run. The amounts of evolved product gases during each cycle are compiled in Table 1, along with the reduction yields (δ corresponds to oxygen nonstoichiometry in CeO2−δ). In order to investigate the reaction kinetics for commercial CeO2 during CH4 induced reduction and reoxidation in the presence of CO2, the kinetics of both reduction and reoxidation over this material were monitored as a function of the temperature and reacting gas concentration. The corresponding reduction and reoxidation profiles as a function of time are shown in Figure 5. Reduction was carried out at 900, 1000, and 1050 °C, and the concentration of CH4 in the feed was 20, 40, and 60% (balanced with Ar and using a total flow rate of 0.5 L· min−1). The values of reaction rates for each temperature were

Figure 3. Representative SEM images of fresh CeO2 materials depicting the structural and morphological variations based on the differences in synthesis methods.

dense particles with uneven morphologies and a nearly uniform size distribution were observed. In the case of hydrothermally synthesized CeO2 (CeO2-HT), even though agglomerated domains were visible, the particles were uniformly sized with a more or less spherical morphology. Regions containing broken particles were also visible, indicating that the interior of these particles was hollow to some extent. This was also confirmed by the fact that a similar mass of these CeO2-HT materials occupied almost twice the volume occupied by commercial CeO2. The synthesis of these hollow CeO2 particles involved the use of carbonaceous spheres obtained from glucose, as hard templates. CeO2 formation took place on the surface of these carbon spheres under hydrothermal conditions. Thermal removal of the carbon templates generally results in the formation of well-defined oxide hollow spheres, as observed in various studies for a variety of oxide compositions.33,34 The deviations, however, observed in the present study can be attributed to the high temperature treatment (1000 °C) aimed at the stabilization of the CeO2 structure. It must be noted that such high temperature pretreatment cannot be avoided in the present study because it corresponds to the temperature chosen for performing redox experiments. SEM analyses were also performed to determine the morphology of CeO2 synthesized by the template assisted self-assembly method using pluronic P123 as soft template (CeO2-SA). In this case, extremely small-sized particles were agglomerated, leading to the formation of largely porous material with a pore size distribution in the macro/meso range. Two types of macropores were observed with larger pores of ∼300 nm and smaller pores of ∼150 nm. In addition, mesopores of ∼40 nm were observed throughout the material. This porous CeO2 was synthesized by following the cooperative self-assembly method as reported by Yuan et al., for the synthesis of ordered mesoporous (Ce,Zr)O2.35 As mentioned above, the deviation of structural order in the present study resulted from the inevitable higher calcination temperature during materials synthesis (1000 °C). 3.2. Parametric Analysis of Ceria Redox Cycles via Solar Thermogravimetry. Regarding the redox performance 6053

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Figure 4. Time-dependent solar thermochemical reduction and reoxidation profiles for (a) CeO2 (black), CeO2-HT (red), and CeO2-SA (blue), and (b) CeO2-MgO (green) and CeO2-Al2O3 (yellow). Temperature was fixed at 1000 °C. Reduction experiments were performed in the presence of CH4 and oxidation in the presence of CO2 (with reacting gas mole fractions of 40% in Ar).

Figure 5. (a) CeO2 reduction (CO2 was used as the oxidizing agent, 40% in Ar at 1000 °C) and (b) reoxidation (CH4 was used as the reducing agent, 40% in Ar at 1000 °C) profiles as a function of time with varying temperature and concentration of CH4/CO2 in the feed.

determined from the slope of the curves provided in Figure 5a in the part corresponding to constant reaction rate (almost linear evolution of the mass loss). The reaction order (α) with respect to CH4 was obtained from a logarithm plot of the reaction rate versus the partial pressure of CH4. The reaction rates were further plotted against the partial pressure of CH4 raised to the power α, and a linear dependence was observed between the reaction rates and the partial pressure of CH4, as shown in eq 4 and the Supporting Information (Figure SI2). r = k(PCH4)0.62

(4)

Rate constants (k = A·exp(−Ea/RT)) were determined by performing the linear regression of eq 4, the values of which were found to vary directly with the inverse temperature. The values of activation energy (Ea) and pre-exponential factor (A) were thus obtained from the Arrhenius plot as given in Figure 6. The values are compiled in Table 2.

Figure 6. Arrhenius plot for CH4 induced reduction and CO2 induced reoxidation of CeO2 under solar thermochemical conditions.

Table 1. Reduction Extent (δ) and CO Released during Isothermal Solar Redox Cycling of CeO2 at 1000 °C δ (O) evolved (mol·mol−1) sample

cycle I

cycle II

commercial CeO2 CeO2-Al2O3 CeO2-MgO CeO2-HT CeO2-SA

0.378 0.331 0.431 0.346 0.379

0.372 0.264 0.394 0.349 0.363

δ (CO) evolved (mol·mol−1)

cycle III

cycle I

cycle II

0.333 0.350 0.338 0.369

0.365 0.320 0.402 0.341 0.374

0.322 0.236 0.346 0.347 0.367 6054

reduction yield (%)

cycle III

time (s)

cycle I

cycle II

cycle III

0.350 0.344 0.342 0.360

2984 4232 2560 2010 2998

76 66 86 69 76

74 53 79 70 73

66 70 68 74

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cycle, it was still higher than that observed in the case of pure CeO2. Interestingly, during the third cycle, the reduction extent further reduced to 0.350 mol·mol−1, but, however, the reoxidation step yielded a stable CO amount (0.344 mol· mol−1) that agrees well with the oxidation step during the second cycle, confirming complete oxygen replenishment during ceria oxidation. In addition, Al2O3 (50 wt %) was used as a promotional agent for CeO2 (Figure 4b). However, differently from the case of MgO, the presence of Al2O3 lowered the reactivity of pure CeO2, which can be attributed to sintering. The rates of reduction and reoxidation were found to be the lowest in this case among the series of materials considered in the present study. However, the reduction extent and the amount of evolved CO were similar to those observed for other materials, except during the second cycle as given in Table 1. After completing the first cycle, the system was not purged, leaving some CO2 in the system, which hindered the CH4 induced reduction in the second cycle. This explains the lower values of evolved O and CO during the second cycle. The system was purged with Ar before the third cycle, and hence values of evolved O and CO in the third cycle became higher than the ones observed during the first cycle. The ceria reduction capability and amount of CO production during cycles were not significantly altered by Al2O3 addition, which denotes the negligible interaction between individual compounds (in particular, the possible formation of the stable CeAlO3 compound that keeps Ce in its Ce(III) oxidation state was not detected, although it may alter the long-term redox activity of ceria). The results obtained in this study clearly indicate that addition of MgO significantly enhanced the extent of CeO2 reduction and hence the amount of CO released. Also, the rates of reduction and reoxidation were found to be improved markedly. Further experiments were performed to investigate the effect of structural and morphological variations in the redox performance of CeO2 under solar thermochemical conditions. Agglomerated CeO2 particles with near spherical morphology were obtained by performing one-pot hydrothermal templating (CeO2-HT). Here, carbonaceous spheres synthesized from glucose acted as a hard template, on the surface of which CeO2 was formed under hydrothermal conditions. The template was further removed by performing calcination at 1000 °C. This method was previously employed for the successful synthesis of various hollow oxide compositions.33,34 To our knowledge, such CeO2 particles are studied for CH4 induced solar thermochemical fuel production for the first time. The extent of reduction and reoxidation was found to be slightly lower than that for pure CeO2 and MgO promoted CeO2, with comparable amounts of released oxygen and CO as evidenced from Figure 4 and Table 1. Interestingly, this material was found to be highly stable with respect to the amount of O and CO evolved, as evidenced by the three cycles of reduction and reoxidation performed in this study. Additionally, even though the reaction rate during the first cycle was comparable with MgO promoted CeO2, the rate was enhanced during the subsequent cycles of operation and the highest among the series of materials used in this study. These results indicate that the morphology of the materials plays an important role in determining the redox cycling efficiency of CeO2. To further elucidate the effect of porosity on the performance of CeO2, reduction and reoxidation experiments were performed using porous CeO2 (CeO2-SA) synthesized by following the

Table 2. Kinetic Parameters Obtained for CH4 Induced Reduction and CO2 Induced Reoxidation over CeO2 under Solar Thermochemical Conditions step

Ea (kJ mol−1)

A (g·s−1)

reduction reoxidation

109 ± 6 36 ± 4

6 × 10−2 5 × 10−5

Similar studies were performed to elucidate the kinetics of the reoxidation step in the presence of CO2. In this case, temperature of initial reduction in the presence of CH4 was fixed at 1000 °C. Reoxidation was also performed at 900, 1000, and 1070 °C and a CO2 concentration of 20, 40, and 60%, as shown in Figure 5b. The reaction rate was determined as the slope of the mass increase evolution, which is almost linear before slowing down when the oxidation reaction approaches completion (Figure 5b). The reaction order (α) with respect to CO2 was obtained from a logarithm plot of the reaction rate versus the partial pressure of CO2. The rate equation used to determine the rate constants is given below: r = k(PCO2)0.53

(5)

The values of activation energy (Ea) and pre-exponential factor (A) were determined as mentioned before in the case of reduction (Figure 6 and Table 2). A noticeable difference between the values of Ea and pre-exponential factors was observed for reduction and reoxidation. Activation energy for oxidation is consistently lower than that for reduction. The values of activation energies obtained in the present study were compared with the ones previously observed for CeO 2 reduction and reoxidation. The value for reduction was found to be significantly lower when compared with the thermal reduction of CeO2 (reduction with CH4 is consistently easier than purely thermal reduction), and the activation energy for reoxidation was found to be similar.37 As mentioned above, these initial experiments with commercial CeO2 indicated loss of stability during multiple cycles. In particular, the amount of evolved CO decreased during the reoxidation step in the second cycle, most probably due to sintering. Therefore, in order to improve the resistance to sintering, the effect of the presence of promotional agents along with CeO2 was studied first. Hence, MgO and Al2O3, which are inert under the operating conditions, were used. The presence of such inert materials could hinder the high temperature sintering during prolonged exposure, by reducing the contact between the active phases. First, 50 wt % of MgO was mixed with CeO2 by grinding and was used for CH4 induced solar thermochemical CO2 splitting. The reduction and reoxidation profile obtained during three successive cycles is shown in Figure 4b. The weight loss during the reduction step for these materials was found to be higher (4%) compared to commercial CeO2. Reduction extent and CO evolution amounts of about 0.431 and 0.402 mol·mol−1 were obtained during the first reduction and reoxidation steps, respectively. These values were higher than that observed in the case of pure CeO2, indicating that the reduction extent and amount of evolved CO increased in the presence of MgO. Interestingly, this high δ value approaches the theoretical maximum value of 0.5 for complete reduction and the reduction extent is approximately 10 times higher than that observed previously for ceria-based materials under solar thermal reduction.10,38 Even though the reduction extent and CO amounts decreased slightly to 0.394 and 0.346 mol·mol−1 during the second redox 6055

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Energy & Fuels cooperative self-assembly method using P123 as the soft template as reported by Yuan et al.35 Here again, such materials are utilized for the first time for CH4 induced solar thermochemical fuel production. The CH4 induced reduction and reoxidation in the presence of CO2 were found to proceed with higher rates when compared to the case of commercial CeO2 as shown in Figure 4. The reduction extent was found to be closely similar to the one of commercial CeO2 materials, however, with improved stability. Even though the reaction rates were lower than those for hollow materials, these materials outperformed the commercial counterpart with reaction rates similar to MgO promoted CeO2. SEM analysis was performed over the materials recovered after redox cycles (Figure 7). Commercial CeO2 was found to

Figure 8. Schematic representation of the packed bed solar reactor used for solar thermochemical analysis of CeO2 redox cycles.

mixed with SiC particles (about 3.5 g of CeO2 powder mixed with 8.8 g of SiC particles). This layout was used in order to increase heat transfer within the particle bed, facilitate gas circulation through the bed, and alleviate bed densification resulting from downward gas flow. The reaction temperature (temperature of the particle bed) was measured directly via a Pt−Rh thermocouple settled within the tube and immersed in the particle bed. The bed was first heated in Ar up to the targeted operation temperature (1000 °C). The feed gas consisting of reactant gases (40% CH4 in Ar during reduction) was fed from the top of the tube, and the product gases exiting from the reactor outlet at the bottom were monitored using a gas analyzer (GEIT GAS 3100P SYNGAS analyzer). A total gas flow rate of 0.5 L·min−1 (0.3 L·min−1 Ar and 0.2 L·min−1 CH4 or CO2) was used during cycles. Liquid water (0.2 mL·min−1) was injected with a peristaltic pump via a capillary stainless steel tube to supply steam during the water-splitting redox cycles. First experiments were performed with commercial CeO2 using H2O and CO2 as the oxidants at a constant bed temperature of 1000 °C. Representative gas evolution profiles observed during the reduction step under CH4 and reoxidation using H2O during the first cycle are provided in the Supporting Information (Figure SI3). During all the reduction steps of ceria with CH4, H2 and CO were the main gaseous products, but small quantities (approximately 1%) of CO2 and acetylene were also observed. This indicates the occurrence of some unwanted side reactions to a small extent. During the reduction steps, peak concentrations of about 18% H2 and 6% CO were measured. These values remained definitely constant during the second and third cycles when using CH4 as the reducing agent of ceria (Figure 9a). Likewise, H2 was the main species measured during ceria oxidation with H2O (Figure 9b), but side products including mainly CO and CO2 were detected (Figure SI3). This denotes the presence of solid carbon that is oxidized when H 2 O is injected. The plateau region in the H 2 concentration profile after 300 s (Figure 9a) can be attributed to the thermal decomposition of CH4 leading to the formation of residual H2 and deposition of carbon. This side reaction also explains the presence of acetylene in the gas products during ceria reduction with CH4 and the CO/CO2 formation during ceria oxidation with H2O. The reaction of CH4 decomposition

Figure 7. Representative SEM images of various CeO2 materials after reduction and reoxidation cycles at 1000 °C.

undergo extensive sintering during the two redox cycles, resulting in the formation of very large particles in comparison with the fresh ones. Even though sintering was observed to a smaller extent in the case of CeO2-HT, this material mostly retained its particle size and morphology during the three cycles. Probably, the observed sintering might have occurred among the broken particles. In the case of CeO2-SA, no significant variation in the particle size and porosity was observed. Accordingly, the superior performance of this material can be attributed to the enhanced structural stability. Also, the presence of larger pores provided easier access of gas species to active sites and enabled faster diffusion of reactants and products. In the case of Al2O3 promoted material, large CeO2 particles were observed, which denotes sintering similar to the case of commercial CeO2. In contrast, for MgO promoted materials, the CeO 2 particle size remained considerably lower in comparison with pure CeO2 and Al2O3 promoted CeO2. 3.3. Experimental Study in a Packed Bed Solar Reactor. Solar thermochemical fuel production over ceria was performed in a packed bed solar reactor configuration with online gas analysis for gas species detection and identification. This reactor was heated by concentrated solar energy delivered by a 2 m diameter parabolic concentrator. The reactor was based on a cavity receiver (45 mm height, 40 mm width, and 55 mm depth) crossed by a vertical alumina tube and closed at the front by a glass window.39 Inside the tube, a porous SiC foam (20 mm diameter, 20 mm length, 30 ppi) was used to support the redox material with minimal pressure drop. Regarding the packed bed experimental setup (Figure 8), the SiC reticulated porous foam was settled inside the alumina absorber tube, above which a packed bed was placed. This packed bed consists of a bottom layer of inert SiC particles (about 2.5 g, 1 mm mean diameter) and an upper layer of the reactive ceria sample 6056

DOI: 10.1021/acs.energyfuels.6b01063 Energy Fuels 2016, 30, 6050−6058

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Figure 10. CO evolution profiles as a function of time over commercial CeO2 in a packed bed reactor under solar thermochemical reaction conditions. CH4 was used as the reducing agent, and CO2 was used as the oxidizing agent. Both reduction and reoxidation were performed at 1000 °C.

However, this side reaction is not a detrimental issue for the whole cycle since the oxidation step with H2O/CO2 produces further H2/CO from solid carbon. Decreasing the temperature of ceria reduction with CH4 would be one possible solution to lower the carbon deposition and to favor the syngas production, at the expense of slower reaction kinetics. In short, this study showed the relevant coupling of partial oxidation of methane with ceria redox cycle for syngas production in a solar reactor prototype.

Figure 9. H2 and CO evolution profiles as a function of time over commercial CeO2 in a packed bed reactor under solar thermochemical reaction conditions (solid lines for H2, dotted lines for CO). (a) CH4 was used as the reducing agent and (b) H2O was used as the oxidizing agent. Both reduction and reoxidation were performed at 1000 °C.

4. CONCLUSIONS Ceria-based materials appear to be promising for producing syngas by coupling partial oxidation of methane with CO2/H2O splitting during solar thermochemical redox cycles. In this study, different strategies to modify the reactivity of CeO2 were analyzed. The results clearly indicate that MgO can be used as a promotional agent to enhance the efficiency of CeO2 for CO2 splitting. Significant enhancement regarding the extent of evolved oxygen, reaction rate, and stability was observed for MgO promoted CeO2. In addition, structural and morphological variations modified the efficiency of CeO2. Typically, CeO2 synthesized by self-assembly and hydrothermal methods were more reactive and stable than their commercial bulk counterpart. Among the series of the materials used in this study, CeO2 synthesized via hydrothermal and self-assembly and MgO promoted CeO2 were the most active with regard to the reaction rate and amount of CO evolved. Finally, experiments performed in a packed bed solar reactor indicated repeatable syngas production through redox cycling, and the occurrence of side reactions and carbon deposition ascribed to CH4 decomposition. However, such a solar reactor prototype can be optimized for the production of syngas. On the basis of the enhanced performance of porous CeO2 evidenced in this study, it would be interesting to utilize mesostructured CeO2 synthesized by a hard templating method. Since these materials exhibit well ordered mesoporosity, their implementation in a packed bed configuration could be relevant with replacement of inert SiC surface promoting CH4 decomposition, by such active materials promoting partial oxidation of CH4 to syngas via ceria redox cycle.

yielding H2 and solid carbon along with acetylene byproduct formation was previously observed using the same solar reactor configuration with a packed bed of SiC particles.39 During the reoxidation step, the H2 evolution started when H2O was injected and lasted about 3 min, which indicates that the reaction was rapid. Further tests using CO2 as the oxidant indicated high yield to CO (Figure 10). However, in this case, the reaction took a longer time to reach completion, suggesting that ceria oxidation with CO2 is slower in comparison with H2O. Similarly to the solar thermogravimetry analysis, experiments were performed using MgO and Al2O3 promoted CeO2 under packed bed reactor conditions at 1000 °C. The corresponding H2 and CO production profiles per unit mass of CeO2 are given in the Supporting Information (Figure SI4). During the oxidation step with H2O, the maximum CO concentration was about 1%/g in the case of MgO promoted ceria, with a peak H2 concentration of about 7%/g. When MgO was used as a promoter, the amounts of CO and H2 produced during the redox cycles were significantly higher than those for pure ceria. Regarding the use of Al2O3 as the promotional agent, the amount of H2 decreased and the amount of CO remained the same as for pure ceria. These results indicate that addition of MgO significantly favored the undesirable CH4 decomposition reaction during the reduction step. CH4 decomposition is a heterogeneous reaction generally favored by the presence of active sites offered by the reactive ceria particles and by the high surface area offered by inert supports such as SiC porous foam, SiC particles, and promoting agents used in this study. 6057

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(21) Le Gal, A.; Abanades, S.; Bion, N.; Le Mercier, T.; Harlé, V. Energy Fuels 2013, 27, 6068−6078. (22) Fornasiero, P.; Balducci, G.; Di Monte, R.; Kašpar, J.; Sergo, V.; Gubitosa, G.; Ferrero, A.; Graziani, M. J. Catal. 1996, 164, 173−183. (23) Balducci, G.; Islam, M. S.; Kašpar, J.; Fornasiero, P.; Graziani, M. Chem. Mater. 2003, 15 (20), 3781−3785. (24) Krenzke, P. T.; Davidson, J. H. Energy Fuels 2014, 28, 4088− 4095. (25) Jeong, H. H.; Kwak, J. H.; Han, G. Y.; Yoon, K. J. Int. J. Hydrogen Energy 2011, 36, 15221−15230. (26) Otsuka, K.; Wang, Y.; Nakamura, M. Appl. Catal., A 1999, 183, 317−324. (27) Otsuka, K.; Wang, Y.; Sunada, E.; Yamanaka, I. J. Catal. 1998, 175, 152−160. (28) Xu, M.; Kong, L.; Zhou, W.; Li, H. J. Phys. Chem. C 2007, 111, 19141−19147. (29) Yen, H.; Kleitz, F. J. Mater. Chem. A 2013, 1, 14790−14796. (30) Nair, M. M.; Kaliaguine, S.; Kleitz, F. ACS Catal. 2014, 4, 3837− 3846. (31) Rudisill, S. G.; Venstrom, L. J.; Petkovich, N. D.; Quan, T.; Hein, N.; Boman, D. B.; Davidson, J. H.; Stein, A. J. Phys. Chem. C 2013, 117, 1692−1700. (32) Malonzo, C. D.; De Smith, R. M.; Rudisill, S. G.; Petkovich, N. D.; Davidson, J. H.; Stein, A. J. Phys. Chem. C 2014, 118, 26172− 26181. (33) Titirici, M. M.; Antonietti, M.; Thomas, A. Chem. Mater. 2006, 18, 3808−3812. (34) Cui, X.; Antonietti, M.; Yu, S. H. Small 2006, 2, 756−759. (35) Yuan, Q.; Liu, Q.; Song, W. G.; Feng, W.; Pu, W. L.; Sun, L. D.; Zhang, Y. W.; Yan, C. H. J. Am. Chem. Soc. 2007, 129, 6698−6699. (36) Levêque, G.; Abanades, S. Sol. Energy 2014, 105, 225−235. (37) Le Gal, A.; Abanades, S. Int. J. Hydrogen Energy 2011, 36, 4739− 4748. (38) Ackermann, S.; Sauvin, L.; Castiglioni, R.; Rupp, J. L. M.; Scheffe, J. R.; Steinfeld, A. J. Phys. Chem. C 2015, 119, 16452−16461. (39) Abanades, S.; Kimura, H.; Otsuka, H. Int. J. Hydrogen Energy 2014, 39, 18770−18783.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.6b01063. CeO2 reduction profile as a function of temperature and time, reaction rates as a function of partial pressures, gas evolution profiles of CeO2 in a packed bed reactor, and gas evolution profiles of Al2O3 and MgO promoted CeO2 in a packed bed reactor (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Airbus Foundation. The authors also thank E. Beche for technical support in XRD analysis and O. Prevost for technical support during solar reactors design and manufacturing.

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REFERENCES

(1) Kubacka, A.; Fernandez-García, M.; Colon, G. Chem. Rev. 2012, 112, 1555−1614. (2) Roldan-Carmona, C.; Malinkiewicz, O.; Soriano, A.; Espallargas, G. M.; Garcia, A.; Reinecke, P.; Kroyer, T.; Dar, M. I.; Nazeeruddin, M. K.; Bolink, H. J. Energy Environ. Sci. 2014, 7, 994−997. (3) Chueh, W. C.; Falter, C.; Abbott, M.; Scipio, D.; Furler, P.; Haile, S. M.; Steinfeld, A. Science 2010, 330, 1797−1801. (4) McDaniel, A. H.; Miller, E. C.; Arifin, D.; Ambrosini, A.; Coker, E. N.; O’Hayre, R.; Chueh, W. C.; Tong, J. Energy Environ. Sci. 2013, 6, 2424−2428. (5) Demont, A.; Abanades, S. J. Mater. Chem. A 2015, 3, 3536−3546. (6) Kodama, T.; Gokon, N. Chem. Rev. 2007, 107, 4048−4077. (7) Levêque, G.; Abanades, S. Energy Fuels 2014, 28, 1396−1405. (8) Aoki, A.; Ohtake, H.; Shimizu, T.; Kitayama, Y.; Kodama, T. Energy 2000, 25, 201−218. (9) Adinberg, R.; Epstein, M. Energy 2004, 29, 757−769. (10) Furler, P.; Scheffe, J.R.; Steinfeld, A. Energy Environ. Sci. 2012, 5, 6098−6103. (11) Gibbons, W. T.; Venstrom, L. J.; De Smith, R. M.; Davidson, J. H.; Jackson, G. S. Phys. Chem. Chem. Phys. 2014, 16, 14271−14280. (12) Hao, Y.; Yang, C. K.; Haile, S. M. Phys. Chem. Chem. Phys. 2013, 15, 17084−17092. (13) Furler, P.; Scheffe, J. R.; Marxer, D.; Gorbar, M.; Bonk, A.; Vogt, U.; Steinfeld, A. Phys. Chem. Chem. Phys. 2014, 16, 10503−10511. (14) Montini, T.; Melchionna, M.; Monai, M.; Fornasiero, P. Chem. Rev. 2016, 116 (10), 5987−6041. (15) Rager, T. Chem. Commun. 2012, 48, 10520−10522. (16) Scheffe, J. R.; Jacot, R.; Patzke, G. R.; Steinfeld, A. J. Phys. Chem. C 2013, 117, 24104−24114. (17) Hao, Y.; Yang, C. K.; Haile, S. M. Chem. Mater. 2014, 26, 6073− 6082. (18) Abanades, S.; Le Gal, A.; Cordier, A.; Peraudeau, G.; Flamant, G.; Julbe, A. J. Mater. Sci. 2010, 45, 4163−4173. (19) Meng, Q. L.; Lee, C. I.; Ishihara, T.; Kaneko, H.; Tamaura, Y. Int. J. Hydrogen Energy 2011, 36, 13435−13441. (20) Le Gal, A.; Abanades, S. J. Phys. Chem. C 2012, 116, 13516− 13523. 6058

DOI: 10.1021/acs.energyfuels.6b01063 Energy Fuels 2016, 30, 6050−6058