ARTICLE pubs.acs.org/EF
CO2 and H2O Splitting for Thermochemical Production of Solar Fuels Using Nonstoichiometric Ceria and Ceria/Zirconia Solid Solutions Alex Le Gal, Stephane Abanades,* and Gilles Flamant PROcesses, Materials, and Solar Energy laboratory (PROMES-CNRS), 7 Rue du Four Solaire, 66120 Font Romeu, France ABSTRACT: The solar thermochemical splitting of CO2 and H2O with ceria and Zr-doped ceria for CO and H2 production is considered. The two-step process is composed of the thermal reduction of the ceria-based compound followed by the oxidation of the nonstoichiometric ceria with CO2/H2O to generate CO/H2, respectively. As a reference, the reactivity of pure undoped ceria was first characterized during successive thermochemical cycles using a thermobalance. Then, Zr0.25Ce0.75O2 was synthesized using different soft chemical synthesis routes to evaluate the influence of the powder morphology on the reactivity during the reduction and the oxidation steps. The reduction yield of ceria was significantly improved by doping with Zr as well as the CO/H2 production yields, but the kinetic rates of the oxidation step for doped ceria were lower than for pure ceria. CO and H2 production of 241 and 432 μmol/g, respectively, have been measured. A kinetic analysis of the CO2-splitting step allowed one to estimate the activation energy that ranged between 83 and 103 kJ/mol depending on the synthesis route of Zr0.25Ce0.75O2. The powder morphology played an important role on the materials cyclability. In contrast to pure ceria, Zr-doped ceria showed possible deactivation when cycling at 1400 °C, and the influence of the synthesis route on the thermal stability was evidenced. The thermally resistant powders with porous morphology ensured stable reactivity during cycling. The Zr-doped ceria synthesized via pechini process produced the largest amounts of CO/H2 during successive cycles.
1. INTRODUCTION In the current energetic context, hydrogen technologies become of primary interest. Indeed, dihydrogen is a molecule with a high energetic density (120 MJ/kg), and consequently, it can be used as an efficient way of energy storage under chemical form. Due to its chemical instability, dihydrogen does not exist naturally on Earth, and so, it must be synthesized with the use of a primary energy source. To make the transition toward a future hydrogen economy, the intermediate storage of the produced hydrogen in the form of a synthetic hydrocarbon fuel that is already compatible with current distribution infrastructures and combustion engines is of major concern. The precursor to a renewable liquid fuel is the synthesis gas (syngas) that must be produced by combining H2 with CO. Using solar power to produce the syngas directly from H2O and captured CO2 provides a promising path toward sustainable energy conversion into solar fuels. Within the framework of concentrated solar energy conversion, the H2/CO production via two-step thermochemical cycles is considered. The process consists of a thermal dissociation of H2O and/or CO2 using a metal oxide catalyst. It proceeds in two distinct steps: (1) the catalyst is first thermally reduced at high temperature (above 1400 °C) obtained with solar concentrating systems; (2) the activated form of the catalyst then reacts in contact with water vapor or carbon dioxide to be reoxidized and at the same time splits the H2O and CO2 reactants. This second exothermic step occurs at lower temperatures (below 1000 °C), and the catalyst is regenerated for its recycling in the first step. Mx Oy f Mx Oy-1 þ 1=2O2
ð1Þ
Mx Oy-1 þ H2 OðCO2 Þ f Mx Oy þ H2 ðCOÞ
ð2Þ
r 2011 American Chemical Society
Such a process allows H2O/CO2 dissociation at much lower temperatures than direct thermolysis (without catalyst).13 Two distinct types of catalysts are investigated for the splitting of H2O and/or CO2: the volatile redox pairs such as ZnO/Zn or SnO2/ SnO4,5 that produce gaseous species during the high-temperature reduction and the nonvolatile redox pairs such as Fe3O4/ FeO,6,7 ferrite redox pair,813 or CeO2/Ce2O31419 that remain in condensed state during the entire process. This study focuses on the cerium oxide redox pair. The system based on ceria was previously proposed by Abanades and Flamant14 who demonstrated the high reactivity of Ce(III) species during the hydrolysis step. The main drawback is the partial sublimation during the reduction that occurs because of the high reduction temperature (2000 °C) and that induces material losses and consequently reactivity decrease during cycling. The lowering of the reduction temperature below 1400 °C is thus required to avoid ceria sublimation, and the insertion of Zr as a dopant is proposed to improve the reduction yield of ceria at such temperatures. Indeed, Zr allows a decrease in the reduction temperature because of lattice deformation (induced by the smaller size of Zr4+ than Ce4+). Zr-doped ceria shows a lower reduction temperature than pure ceria and thus an improved catalytic performance during cycling.16 This study aims to investigate the influence of the synthesis method on the powder reactivity with H2O and CO2 in order to optimize the reactivity and the cyclability of the material. Actually, the powder morphology directly affects the material reactivity. Powder characteristics, such as large specific surface area enabling optimized solidgas Received: July 4, 2011 Revised: August 30, 2011 Published: August 30, 2011 4836
dx.doi.org/10.1021/ef200972r | Energy Fuels 2011, 25, 4836–4845
Energy & Fuels
ARTICLE
Figure 1. Diffraction pattern of Zr0.25Ce0.75O2 synthesized by different routes.
exchange, small particle size limiting the diffusion phenomena, or material morphology resisting sintering, enhance the reactivity. In addition to pure ceria synthesized by coprecipitation and used as a reference material, several soft chemical synthesis routes were applied to produce solid solutions of Ce0.75Zr0.25O2. These powders were then characterized by XRD and SEM/EDS, and their reactivity with H2O/CO2 was investigated by thermogravimetric analysis (TGA) to highlight the influence of the synthesis route and of the material composition. Kinetic parameters were derived from experimental results for the CO2 splitting reaction.
Table 1. Lattice Parameters and Crystallite Size Estimation Calculated from XRD Patterns lattice material
crystallite
crystallite size
size estimation estimation after (nm) cycling (nm)
CeO2 (JCPDF 81-0792) 5.4124 CeO2 coprecipiation
5.414 ( 0.002
13
90
Zr0.25Ce0.75O2
5.380 ( 0.002
5
54
Zr0.25Ce0.75O2 pechini
5.390 ( 0.002
6
21
Zr0.25Ce0.75O2 solgel Zr0.25Ce0.75O2
5.370 ( 0.002 5.401 ( 0.002
5 4
27 47
coprecipitation
2. EXPERIMENTAL SECTION 2.1. Synthesis Methods. Four soft chemical synthesis routes were selected for their ease of implementation, the suitable control of all the parameters, and their low environmental impact. They are the coprecipitation of hydroxides, the hydrothermal-assisted synthesis method, the pechini synthesis, and the solgel synthesis using alkoxide precursors. All these methods are wet chemical routes in order to favor material deposition on porous ceramic supports in the next step of materials integration in solar chemical reactors. The amounts of reactants are given hereafter for the synthesis of Zr0.25Ce0.75O2. 2.1.1. Coprecipitation of Hydroxides. Concerning the coprecipitation of hydroxides, 10.19 g of Ce(NO3)3 3 6H2O and 2.65 g of ZrO(NO3)2 3 6H2O were dissolved into demineralized water (0.5 L) under stirring (5 g of material is targeted). NH4OH (28%) was dripped into the solution with stirring until pH = 9. The obtained solution was then stirred for 12 h. The resulting precipitate was separated by centrifugation (10 min at 5000 rpm) and washed with water to eliminate the residues of synthesis. It was then dried at 90 °C in an oven, crushed, and calcined at 800 °C in air for 2 h to obtain a yellow powder (4.85 g). 2.1.2. Hydrothermal Treatment. Regarding the hydrothermal synthesis derived from Pan et al.,20 2.25 103 mol of Ce(NO3)3 3 6H2O and 7.5 104 mol of ZrO(NO3)2 3 6H2O were dissolved into 20 mL of demineralized water. CTABr (cetyltrimethylammonium bromide; 1 103 mol) was then dissolved in this solution under stirring. A transparent gel was obtained and NH4OH (4 mL, 28%) was dripped into the solution under stirring (1 h). The solution was finally enclosed in a pressurized vessel and heated at 100 °C for 24 h to obtain the powder (0.38 g). No calcination is needed because the material crystallizes with the self-induced high pressure.
parameter a (Å)
hydrothermal
2.1.3. Pechini Method. In the pechini method, metallic nitrates were dissolved in demineralized water (10.18 g of Ce(NO3)3 3 6H2O and 2.65 g of ZrO(NO3)2 3 6H2O) under stirring. A separate solution was prepared by mixing ethylene glycol (83 mL) and nitric acid (14.3 mL, 65%) heated at 50 °C to initiate polyesterification. The solution of metallic nitrates was then added under stirring and heated at 80 °C for 3 h. A viscous gel was obtained after cooling. It was heated at 600 °C for 4 h to obtain about 5 g of the oxide powder with a low density. 2.1.4. SolGel Method. This synthesis is extracted from Rossignol et al.21 In the solgel method, 4.69 103 mol of Zr(OC3H7)4 was dissolved into 20 mL of 1-propanol (solution 1) and 1.41 102 mol of Ce(NO3)3.6H2O was dissolved into 20 mL of demineralized water (solution 2). The solution 2 was mixed into solution 1 for hydrolyzing the Zr alkoxide and stirred for a few hours. The precipitate was separated by centrifugation (10 min at 5000 rpm) and washed with water. It was finally dried at 90 °C and calcined at 800 °C for 2 h to recover a yellow powder (2.55 g). 2.2. Characterization. 2.2.1. X-ray Diffraction. The synthesized powders were characterized by X-ray diffraction with a Phillips PW 1820 using the Cu Kα1 (αCu = 0.15418 nm, angular range = 2075° 2θ, steps = 0.02° 2θ, recording time = 2 s). The XRD pattern shown in Figure 1 confirms that all the synthesized materials present a cubic fluorite structure like the pure ceria (JCPDF 81-0792). A peak shift toward high angle 2θ is observed, which is in agreement with the 4837
dx.doi.org/10.1021/ef200972r |Energy Fuels 2011, 25, 4836–4845
Energy & Fuels
ARTICLE
Figure 2. SEM pictures of Zr0.25Ce0.75O2 synthesized by different methods after calcination at 800 °C during 2 h. (a) Coprecipitation of hydroxides; (b) solgel; (c) pechini; (d) hydrothermal treatment. formation of a solid solution of ceria/zirconia. Indeed, the atomic substitution of some cerium atoms by smaller zirconium atoms results in a lattice shrinking. Lattice parameters and crystallite sizes are listed in Table 1. The lattice parameters of Ce0.75Zr0.25O2 are consistent with the values reported previously.22,23 The variations in lattice constants may be due to differences in Zr/Ce ratios and oxygen nonstoichiometry. The powders have similar crystallite size after synthesis. The crystallite size increases significantly after cycling except for the powder synthesized via pechini. 2.2.2. SEM/EDS Analysis. The powder microstructure was studied using a FESEM Hitachi S4500. Figure 2 presents SEM images of Zr0.25Ce0.75O2 obtained by the different synthesis after calcination at 800 °C during 2 h. Powder morphology can be compared. The ones synthesized by coprecipitation and by hydrothermal treatment are composed of agglomerates of some μm length. A higher magnification allows observing spherical particle of some tens of nanometers of diameter for the coprecipitation synthesis. The material obtained from alkoxide precursors (solgel) presents a morphology of sintered blocks of tens of micrometers. Concerning doped ceria synthesized via pechini method, the powder is formed of highly porous particles of about one hundred micrometers. This powder is very light referring to the micrometric pores. This morphology appears as the most interesting one for the targeted application considering the high surface area that enhances solid/gas exchange and that should improve the reactivity. Nevertheless, the low density of the material can be an obstacle during processing because of the small amount that can be coated on porous substrate. To complement the SEM analysis, a chemical elementary analysis was realized by EDS to estimate the chemical composition of the powders. The accelerating voltage used was 23 keV with an acquisition time of 100 s and an aperture angle of 30°. Results are in agreement with the atomic ratio of Zr0.25Ce0.75O2 for doped ceria (theoretical atomic composition: 66.7% O, 8.3% Zr, 25% Ce) except for the hydrothermal synthesized
material that presents an oxygen deficient composition (54.6% O, 13.3% Zr, 32.1% Ce).
3. REACTIVITY TESTING After structural and chemical characterization of powders, the catalytic activity was investigated using a thermobalance (Setaram Setsys Evo). The same experimental conditions were used for all the analyses. About 100 mg of material was introduced in a platinum crucible hung on the balance with platinum suspensions and placed inside the furnace chamber. Then, residual air in the TGA chamber was eliminated by a preliminary pumping in order to operate in inert atmosphere and to improve the reduction yield by decreasing the oxygen partial pressure. The sample was heated in an argon flow, and the mass variation was registered continuously. The mass variation can be directly correlated to the oxygen release during the reduction step or to the reoxidation of the material during the CO2/H2O splitting step. During one typical experiment, three successive cycles (reduction and reoxidation) were done. The reduction step consists of heating the powder up to 1400 °C with a 20 °C/min heating rate and of maintaining a temperature plateau during 45 min. This reduction temperature was chosen because it will be the operation temperature of a typical solar reactor. Although a higher reduction yield could be obtained at a higher temperature, the main issues would be linked to the reactant losses by sublimation and to constraints of thermal stability for the materials of the solar reactor. The mass loss registered when the temperature exceeds 800 °C is directly correlated to the oxygen released from the oxide. Then, the temperature is lowered for the second reoxidation step, and H2O or CO2 is injected inside the furnace chamber to react with the oxygen-deficient material and to 4838
dx.doi.org/10.1021/ef200972r |Energy Fuels 2011, 25, 4836–4845
Energy & Fuels
ARTICLE
Figure 3. TG analysis of CeO2 synthesized by coprecipitation of hydroxides during three consecutive CO2 and H2O-splitting cycles.
produce H2 or CO. Concerning CO2-splitting tests, the CO2 stream is injected from the auxiliary gas inlet and mixed with argon, the carrier gas (total flow rate: 20 mL/min at normal condition). The mixture (50% CO2) then flows down toward the sample and exits the chamber by the bottom. Regarding H2Osplitting tests, the water vapor is produced with a steam generator connected to the thermobalance (40 mL/min Ar with 80% relative humidity RH at 40 °C). The Ar/H2O mixture is injected from the bottom of the furnace, and the gas flows up toward the exit located at the top of the chamber. This oxidation step occurs at isothermal conditions during 35 min. Three different temperatures for the oxidation step (1200, 1100, 1000 °C) were tested during the successive cycles to point out the temperature influence and to derive kinetic parameters. The mass variations are converted to the mole amount of O2 released (reduction step) per gram of ceria or doped-ceria using the following equation (eq 3). nO2 ¼ Δmloss =ðMO2 3 mceria Þ
ð3Þ
with Δmloss, the mass variation measured by TGA; MO2, the molecular weight of O2; and mceria, the mass of ceria used during the experiment. The reduction yield (Xred) is then calculated using eq 4. X red ¼ nO2 =nO2;max
ð4Þ
with nO2;max ¼ ð1 δÞ=4M ceria
ð5Þ
where nO2,max is the maximum amount of O2 (mol/g) that could be released if Ce4+ was completely reduced to Ce3+, δ is the stoichiometric coefficient of Zr in ZrδCe1-δO2 (δ = 0 for pure ceria), and Mceria is the molecular weight of ceria depending on the Zr content. Xred thus quantifies the fraction of Ce atoms in the oxidation state Ce3+.
The H2 or CO amount (mol/g) is calculated with eq 6: nH2 ¼ Δmgain =ðMO 3 mceria Þ
ð6Þ
with Δmgain, the mass gain during the reoxidation step, and MO, the molecular weight of O atom. The oxidation yield (α) is calculated with the mass variation of the reoxidation step and the reduction yield. α¼
nH2 ð1 δÞ 1 : X red : 2 M ceria
ð7Þ
Concerning the CO2 splitting experiments, the same expression is used to calculate the yield but nH2 is substituted by nCO. 3.1. Reactivity of Pure Nonstoichiometric Ceria. As a reference, experiments were conducted with pure undoped ceria synthesized by coprecipitation of hydroxides. Three successive cycles were carried out to investigate the reactivity of ceria during CO2 or H2O-splitting cycle. The results are compared with those obtained with Zr-doped ceria in the next part. Figure 3 represents the thermogravimetric analysis of CeO2 during cycling experiments. The mass variation during the reduction step starts at a temperature higher than 1100 °C with a fast mass decrease during the heating ramp and then a slower mass evolution during the plateau at 1400 °C. Concerning the reoxidation step, the reduced material totally reacts with the water vapor or CO2 with a fast reaction rate. In both experiments, the temperature has an influence on the reoxidation yield. The lower the temperature, the higher are the CO/H2 production yields. The shape of the first reoxidation peak (at 1200 °C) shows that an equilibrium occurs between oxidation and reduction because the mass increases rapidly as soon as steam is injected, but then, a slight mass decrease is observed due to the reduction even during the CO2/H2O injection. This can be explained by thermodynamic limitation, i.e., the exothermic reaction is not favored by a 4839
dx.doi.org/10.1021/ef200972r |Energy Fuels 2011, 25, 4836–4845
Energy & Fuels
ARTICLE
Table 2. Quantity of O2 Released and H2 or CO Produced from CeO2 during Three Successive Thermochemical Cycles CO2-splitting
H2O-splitting α (%)
nO2 released (μmol/g)
Xred (%)
H2 produced (μmol/g)
98
86.0
75
5.2
125
83.3
102
102.0
58
4.0
128
110.3
99.0
66
4.6
144
109.1
nO2 released (μmol/g)
Xred (%)
1st cycle
57
3.9
2nd cycle
50
3.5
3rd cycle
53
3.7
105
CO produced (μmol/g)
α (%)
Figure 4. Equilibrium phase composition and theoretical reduction yield predicted by thermodynamics for the CeO2 system (1 mol of CeO2, P = 1 bar in Ar, Ar/Ce ratio = 100).
temperature increase. Consequently, the oxidation competes with the reduction at 1200 °C, and the optimal temperature for the exothermic step is 1000 °C. Table 2 lists the amounts of O2 released and H2 or CO produced. The theoretical maximum amount of oxygen released per gram of ceria (corresponding to the total reduction of Ce4+ to Ce3+) is 1.45 mmol (4.65% of mass loss) and the H2 or CO theoretical production is 2.9 mmol/g. During the reduction step at 1400 °C, the amount of oxygen released is about 53 μmol/g for the CO2-splitting cycle and about 66 μmol/g for the H2Osplitting cycle, corresponding to an average reduction yield of 3.7% and 4.6%, respectively. These values are low, which justifies the study on the reduction yield improvement. The small discrepancy between the two experiments can be explained by (1) a different oxygen partial pressure during the reduction step induced by the different carrier gas flow rates used and (2) the TG configuration that differs between the H2O and the CO2 splitting experiments (upward flow for H2O and downward flow for CO2). The amount of oxygen released is relatively stable during cycling, which denotes that the thermal treatment has no significant impact on the reduction yield at this temperature. Values of α higher than 100% are obtained because the reduced species that did not react during a previous oxidation at a higher temperature can be oxidized at a lower temperature. The oxidation step does not show any diffusion limitation in the case of pure ceria. This suggests a reduction restricted to the particle surface (negligible bulk reduction consistent with the low
reduction yield) and a surface-controlled regime in the oxidation step with CO2 and H2O. The experimental results concerning ceria reduction can be compared with thermodynamic predictions. The reduction of CeO2 was thus simulated using HSC Chemistry program for equilibrium composition calculation in the temperature range of 2002000 °C in an inert gas at atmospheric pressure (Figure 4). Thermodynamics predicts the start of Ce2O3 formation at about 1600 °C while nonstoichiometric phases of ceria, chiefly CeO1.83 and CeO1.72, are formed at lower temperatures. The theoretical reduction yield (defined by eq 4) increases with temperature up to about 42% at 2000 °C. At 1400 °C (reduction temperature in the TG tests), the theoretical reduction yield predicted by thermodynamics is 18%, which is higher than the reduction yield obtained experimentally. This means that kinetic limitations are the controlling factors during the reduction of ceria at this temperature. 3.2. Catalytic Activity of Zr-Doped Ceria during CO2Splitting Cycle. 3.2.1. Thermogravimetric Analysis. The reactivity of Zr-doped ceria was first investigated during CO2-splitting thermochemical cycles. The same experimental conditions as for pure ceria experiments were used in order to evaluate the influence of Zr doping. The influence of the synthesis route on the catalytic activity was also studied. Figure 5 shows the thermogravimetric analysis of Ce0.75Zr0.25O2 synthesized by four different methods. It represents the mass evolution during the thermal cycling with CO2 injection during the low temperature step. A sharp mass 4840
dx.doi.org/10.1021/ef200972r |Energy Fuels 2011, 25, 4836–4845
Energy & Fuels
ARTICLE
Figure 5. TGA of CO2-splitting experiments during three successive cycles using Ce0.75Zr0.25O2 synthesized via four different soft chemistry routes.
Table 3. Quantity of O2 and CO Released from Ce0.75Zr0.25O2 during Three Successive CO2-Splitting Cycles As a Function of the Synthesis Route reduction (1400 °C) 1st cycle
2nd cycle
CO2-splitting 1st cycle (1200 °C)
3rd cycle
μmol O2/g Xred/% μmol O2/g Xred/% μmol O2/g Xred/%
μmol CO/g
α/%
2ndcycle (1100 °C) μmol CO/g
α/%
3rd cycle (1000 °C) μmol CO/g
α/%
coprecipitation
137
11.7
96
8.2
74
6.3
139
51.7
132
68.7
99
66.7
pechini
145
12.4
116
9.9
154
13.2
142
49.0
215
92.7
241
78.2
solgel
193
16.5
74
6.4
47
4.0
148
48.4
83
56.1
46
48.9
hydrothermal
114
9.8
97
8.3
90
7.8
152
66.7
145
74.7
126
70.0
decrease (not represented here) is observed below 800 °C for all the powders, which corresponds to the water desorption and to the thermal degradation of organic compounds remaining from the synthesis. Then, it appears that the reduction starts at temperature around 1050 °C. For the powders synthesized via pechini process and hydrothermal treatment, the shape of the first peak of CO production is similar to the one for pure ceria. A competition between the reduction and oxidation reactions occurs at 1200 °C, whereas the other powders (coprecipitation and solgel synthesis) show a slower kinetic of reoxidation without any mass decrease ascribed to the competition with reduction. The temperature of the CO2-splitting step has an influence on the kinetics for the four powders: the higher the temperature, the faster the CO2 dissociation contrarily to the reaction with pure ceria. The shape of the second and the third peak of reoxidation can be decomposed in two parts, a fast mass increase during the first minutes of CO2 injection (reaction-controlled regime) followed by a slower constant evolution (diffusion-controlled regime). The fast reaction-controlled regime tends to disappear during cycling, except for the powder synthesized via pechini process, which can be cycled repeatedly without any reactivity losses regardless of the CO2-splitting temperature. Table 3 reports the quantity of O2 and CO released during the three CO2-splitting cycles. During the first reduction step, the reduction yield varies from 9.8% for the hydrothermal synthesis to 16.5% for the solgel synthesis (193 μmol of O2 per gram). After the first cycle, all the powders except the pechini one show a
decrease of the reduction yield. Thus, the successive thermal treatments impact the reduction yield of Zr-doped ceria, except for the powder synthesized by pechini method that shows stable microstructure. Indeed, the pechini powder is the only one that does not sinter micro- and macroscopically, and the reduction is not limited by diffusion. Concerning the reoxidation, the highest CO2-splitting yield is obtained at 1100 °C. This temperature is a compromise regarding the high CO2-splitting kinetic rate while the competitive reactions between reduction and oxidation are not observed. The amounts of produced CO range between 139 and 241 μmol/g for the powders synthesized by coprecipitation and pechini, respectively. These results are in agreement with a previous study in which the optimal temperature of H2Osplitting was found to be 1050 °C.16 SEM imaging was performed after the cycling experiments (Figure 6) and confirmed that the pechini powder saves a porous morphology after thermal treatment, supporting the cycling capability of the material. The three other powders are macroscopically sintered (hard agglomeration), as observed on SEM pictures. These observations are consistent with the evolution of the crystallite size of the powders (Table 1). The increase of crystallite size for the powder synthesized via pechini is much lower than for the other powders. The low sintering after repeated cycles due to the stable microstructure in the case of the pechini powder is thus confirmed. 3.2.2. Kinetic Investigation of the CO2-Splitting Reaction. Kinetic parameters of the CO2-splitting step were derived from 4841
dx.doi.org/10.1021/ef200972r |Energy Fuels 2011, 25, 4836–4845
Energy & Fuels
ARTICLE
Figure 6. SEM pictures of Zr0.25Ce0.75O2 synthesized via (a) coprecipitation, (b) solgel, (c) pechini, and (d) hydrothermal treatment after three thermochemical cycles.
Figure 7. Master plot analysis of the CO2-splitting reaction at 1100 °C with Zr0.25Ce0.75O2 synthesized by coprecipitation (the kinetic models used for this study are detailed by Gotor et al.24).
the TG analysis at isothermal conditions using an Arrhenius type equation (eq 8). dα Ea ¼ k 3 f ðαÞ ¼ A 3 exp f ðαÞ dt RT 3
ð8Þ
where α is the fractional conversion, f(α) is a function depending on the reaction mechanism, k is the rate constant, A is the preexponential factor, Ea is the activation energy, R is the gas constant, and T is the temperature. The CO2-splitting reaction can be generally decomposed in two phases: a surface reaction followed by a diffusion rate-limiting step. 4842
dx.doi.org/10.1021/ef200972r |Energy Fuels 2011, 25, 4836–4845
Energy & Fuels
ARTICLE
Figure 8. Arrhenius plot for the CO2-splitting reaction with Zr0.25Ce0.75O2 synthesized by (a) coprecipitation, (b) solgel, (c) hydrothermal treatment, and (d) pechini process.
The kinetic model, f(α), is a function describing the reaction mechanism and the main limitation of the reaction rate. The ratelimiting mechanism was identified via a master plot analysis that consists of comparing the normalized rate data in differential form to the theoretical functions of known solid-state kinetic models.24 For pure ceria, the experimental data was best represented by a second order kinetic rate model with f(α) = (1 α)2, even after the peak oxidation rate (i.e., from the beginning of the rate decrease to the end of the oxidation step). This suggests that the reaction was limited by a surface chemical mechanism described by the second order reaction model in the case of ceria. The weak ceria reduction was the result of a surface-limited reduction at 1400 °C, and thus, the CO2-splitting step was also restricted to the surface with no diffusion limitation. Inversely, the experimental data for Zr-doped ceria agreed well with a diffusion model from the peak oxidation rate (α = 0.15) to the end of the oxidation step, which means that the bulk reduction of ceria combined to the powder sintering led rapidly to a diffusion-limited regime during the CO2-splitting. Figure 7 illustrates the master plot analysis of the CO2-splitting step at 1100 °C using Zr0.25Ce0.75O2 synthesized by coprecipitation where the diffusion models (D2, D3, D424) show the best correlation with the experimental data (the χ2 values for the fit expressions of the diffusion models are around 0.02 for α ranging between 0.2 and 0.6). The diffusion control was delayed in the case of the powder synthesized via pechini method (diffusion control observed for α > 0.35), which led to a higher oxidation yield than for the other powders. The powder synthesized by pechini did not show strong sintering after repeated cycles, and the master plot analysis confirmed the correlation between sintering and reactivity. The porous morphology increased the surface area and thus improved the yield of CO produced by surface reaction during the fast oxidation step. The diffusion limitation only occurred when the surface sites had reacted. This explains why the powder had no reactivity decrease during cycling.
Table 4. Kinetic Parameters of the CO2-Splitting Reaction with Zr0.25Ce0.75O2 for Different Synthesis Routes Ea (kJ/mol) coprecipitation
82.7
pechini
88.1
solgel
96.7
hydrothermal
103.3
Figure 9. TGA of H2O-splitting experiments during three successive cycles using Ce0.75Zr0.25O2 synthesized via four different soft chemistry routes.
The kinetic constant k was then calculated from the isothermal TG data corresponding to the reaction-controlled step. The activation energy was the slope of the logarithmic plot of k as a function of 1/RT (Figure 8). As a result, Ea was found to vary from 82.7 kJ/mol for the material synthesized by coprecipitation of hydroxides to 103.3 kJ/mol for the powder synthesized by hydrothermal treatment (Table 4). These values are higher than the previously obtained activation energy of H2O-splitting with 4843
dx.doi.org/10.1021/ef200972r |Energy Fuels 2011, 25, 4836–4845
Energy & Fuels
ARTICLE
Table 5. Quantity of O2 and H2 Released from Ce0.75Zr0.25O2 during Three Successive H2O-Splitting Cycles As a Function of the Synthesis Route reduction 1400 °C 1st cycle μmol O2/g
H2O-splitting
2nd cycle
Xred/%
μmol O2/g
Xred/%
3rd cycle μmol O2/g
1st cycle
Xred/%
μmol H2/g
2nd cycle α/%
μmol H2/g
α/%
3rd cycle μmol H2/g
α/%
coprecipitation
208
17.7
119.8
10.2
118.8
10.1
267
64.2
236
98.5
212.5
89.4
pechini
246.8
21
231.5
19.8
177.5
15.2
432.5
87.6
345.2
74.5
354.1
99.7
solgel hydrothermal
226.2 255.4
19.3 21.8
165.9 155.8
14.2 13.3
82.3 98.2
7.0 8.4
319.2 360.5
70.5 70.5
161.8 188.4
48.7 60.4
159.6 202.4
96.9 103.0
Zr-doped ceria (Ea = 51 kJ/mol for H2O splitting).16 Kinetics of CO2/H2O splitting for pure ceria cannot be identified from previous experiments because the kinetic rate decreases when the temperature increases due to the thermodynamic control above 1000 °C. The difference in the reoxidation kinetics between undoped ceria and Zr-doped ceria is due to the different reaction mechanisms. Indeed, the reaction remains restricted chiefly to the surface for undoped ceria, thus leading to short reaction duration, while the bulk also participates to the reaction in the case of ceriazirconia, which increases the overall reaction duration. Thus, the difference is due to the presence of Zr that increases oxygen mobility in the lattice structure. 3.3. Catalytic Activity of Zr-Doped Ceria during H2OSplitting Cycle. The reactivity of the synthesized materials was also studied during H2O-splitting for hydrogen production. Experimental conditions were similar to the ones of CO2splitting, except that water was injected using a steam generator coupled to the TGA (80% RH at 40 °C with 40 mL/min Ar). The thermogravimetric analysis is represented in Figure 9. As stated previously, the discrepancy in the reduction yield compared with the CO2-splitting experiments may be explained by the different Ar flow rate above the sample. Concerning the rate profile for the H2O-splitting step, a fast kinetic is observed during H2O-splitting at 1200 °C but the reverse reduction reaction takes place simultaneously (mass decrease during steam injection). The H2Osplitting rate decreases at 1100 and 1000 °C with a surface reaction-controlled regime during the first minutes of steam injection followed by a diffusion-controlled regime. The loss in reaction rate may be due both to the decrease of surface area and to the particle growth induced by sintering, leading to longer diffusion lengths. The reduction yield decreases during cycling, whatever the synthesis route, but this reactivity decrease is moderate for Zrdoped ceria synthesized via the pechini method (Table 5), which denotes an improved thermal stability. In addition, the pechini powder is also the one that produced the largest amount of hydrogen (432 μmol/g) at 1200 °C (345 μmol/g at 1100 °C and 354 μmol/g at 1000 °C). The H2O-splitting yield (calculated with eq 7) reaches nearly completion during the third cycle, whatever the synthesis route. However, the cycling results proved that the synthesis route has a significant influence on the catalytic activity and that the powder morphology can improve thermal resistance enabling stable reactivity during cycling. The pechini powder is the only one that keeps a porous morphology after cycling and that does not sinter macroscopically, and it thus shows the most stable CO/H2 production during repeated cycles.
4. CONCLUSION The catalytic activity of pure nonstoichiometric ceria and Zrdoped ceria was studied for CO2- and H2O-splitting by thermochemical cycles. The Zr doping of ceria improved the reduction yield from about 5% for pure undoped ceria up to about 20% for Ce0.75Zr0.25O2, and the CO or H2 production was also improved markedly. Conversely, the cycling experiments highlight that Zrdoped ceria may exhibit a reduction yield decrease when cycling, depending on the synthesis route because of a lower thermal resistance inducing sintering, whereas undoped ceria does not show any reactivity decrease during cycling at the expense of low CO/H2 productivities. The synthesis route determines the morphology of powders. As a result, the material synthesized by pechini method shows a porous morphology that is resistant to thermal treatment and that avoids deactivation during cycling. Therefore, this Zr-doped ceria allows for the improvement of the catalytic activity of pure ceria while maintaining its cycling capability. Finally, the kinetic parameters for CO2-splitting were identified, which evidenced the enhanced reactivity of Zr-doped ceria with respect to H2O-splitting. Further work will be devoted to the materials shaping and to the development of a solar chemical reactor integrating supported materials to investigate the materials performances under solar irradiation conditions. ’ AUTHOR INFORMATION Corresponding Author
*Tel: (+33) (0)4 68 30 77 30. Fax: (+33) (0)4 68 30 29 40. E-mail:
[email protected].
’ ACKNOWLEDGMENT This study was funded by ANR (project contract No. ANR-09JCJC-0004-01) and CNRS (Interdisciplinary Energy Program, DISCO2 project). Authors thank the center for materials characterization of Perpignan University for SEM/EDS analysis. ’ REFERENCES (1) Nakamura, T. Sol. Energy 1977, 19, 467–475. (2) Ihara, S. Int. J. Hydrogen Energy 1978, 3, 287–296. (3) Lede, J.; Lapicque, F.; Villermaux, J. Int. J. Hydrogen Energy 1983, 8, 675–679. (4) Chambon, M.; Abanades, S.; Flamant, G. Int. J. Hydrogen Energy 2009, 34 (13), 5326–5336. (5) Abanades, S.; Chambon, M. Energy Fuels 2010, 24 (12), 6667–6674. (6) Tofighi, A.; Sibieude, F. Int. J. Hydrogen Energy 1984, 9, 293–296. 4844
dx.doi.org/10.1021/ef200972r |Energy Fuels 2011, 25, 4836–4845
Energy & Fuels
ARTICLE
(7) Charvin, P.; Abanades, S.; Flamant, G.; Lemort, F. Energy 2007, 32 (7), 1124–1133. (8) Han, S. B.; Kang, T. B.; Joo, O. S.; Jung, K. D. Sol. Energy 2007, 81, 623–628. (9) Tamaura, Y.; Steinfeld, A.; P., Ehrensberger, K. Energy 1995, 20, 325–330. (10) Kodama, T.; Gokon, N.; Yamamoto, R. Sol. Energy 2008, 82, 73–79. (11) Agrafiotis, C.; Roeb, M.; Konstandopoulos, A. G.; Nalbandian, L.; Zaspalis, V. T.; Sattler, C.; Stobbe, P.; Steele, A. M. Sol. Energy 2005, 79, 409–421. (12) Kaneko, H.; Kodama, T.; Gokon, N.; Tamaura, Y.; Lovegrove, K.; Luzzi, A. Sol. Energy 2004, 76, 317–322. (13) Tamaura, Y.; Kojima, N.; Hasegawa, N.; Inoue, M.; Uehara, R.; Gokon, N.; Kaneko, H. Int. J. Hydrogen Energy 2001, 26, 917–922. (14) Abanades, S.; Flamant, G. Sol Energy 2006, 80, 1611–1623. (15) Abanades, S.; Le Gal, A.; Cordier, A.; Peraudeau, G.; Julbe, A. J. Mat. Sci. 2010, 45 (15), 4163–4173. (16) Le Gal, A.; Abanades, S. Int. J. Hydrogen Energy 2011, 36 (8), 4739–4748. (17) Kaneko, H.; Ishihara, H.; Taku, S.; Naganuma, Y.; Hasegawa, N.; Tamaura, Y. J. Mat. Sci. 2008, 43, 3153–3161. (18) Kaneko, H.; Tamaura, Y. J. Phys. Chem. Solids. 2009, 70, 1008–1014. (19) Chueh, W. C.; Haile, S. M. Philos. Trans. R. Soc., A 2010, 368, 3269–3294. (20) Pan, C.; Zhang, D.; Shi, L. J. Solid State Chem. 2008, 181, 1298–1306. (21) Rossignol, S.; Madier, Y.; Duprez, D. Catal. Today 1999, 50, 261–270. (22) Kaspar, J.; Fornasiero, P. Catalysis by ceria and related materials; Trovarelli, A., Ed.; 2002; Chapter 6: Structural properties and thermal stability of ceria-zirconia and related materials, Imperial College Press: London; pp 217241. (23) Kim, D. J. J. Am. Ceram. Soc. 1989, 72, 1415–1421. (24) Gotor, F. J.; Criado, J. M.; Malek, J.; Koga, N. J. Phys. Chem. 2000, 104, 10777–10782.
4845
dx.doi.org/10.1021/ef200972r |Energy Fuels 2011, 25, 4836–4845
