Dopant Incorporation in Ceria for Enhanced Water-Splitting Activity

Article pubs.acs.org/JPCC

Dopant Incorporation in Ceria for Enhanced Water-Splitting Activity during Solar Thermochemical Hydrogen Generation Alex Le Gal and Stéphane Abanades* Processes, Materials, and Solar Energy Laboratory (PROMES-CNRS), 7 Rue du Four Solaire, 66120 Font-Romeu, France ABSTRACT: Ceria has emerged as an attractive candidate for solar thermochemical hydrogen production; however, the necessary temperatures for CeO2 reduction to Ce2O3 are too high for conventional solar concentrating systems, while the reduction to nonstoichiometric CeO2−δ below 1500 °C shows restricted chemical yield. Doping ceria with another metal can improve the reactivity at lower temperatures. This study focuses on the doping of ceria with different metals such as tantalum or trivalent lanthanides (La, Sm, and Gd) to form binary oxides and on the doping of ceria−zirconia solid solutions to form ternary oxides. Ceria materials doped with tantalum show a high reducibility, but the structural evolution during thermal treatment leads to the formation of a secondary phase that hinders the water dissociation reaction. Besides, the doping with trivalent lanthanides results in an improved thermal stability during consecutive cycles, while the hydrogen production is unchanged compared to ceria. Concerning ternary oxides, the addition of 1% gadolinium to ceria−zirconia solid solutions results in the production of 338.2 μmol (7.58 mL) of hydrogen per gram during one cycle with the O2-releasing step at 1400 °C and the H2-generation step at 1050 °C. This production is higher than the one observed for undoped ceria−zirconia. The addition of lanthanum enhances the thermal stability of ceria−zirconia solid solution, thus leading to stable reactivity during repeated cycles.

Endothermal step: MxCe1 − xO2 → MxCe1 − xO2 − δ + δ /2O2

1. INTRODUCTION The development of sustainable alternatives to fossil fuel resources has become a predominant research area since the link between climate change and rise in CO2 atmospheric concentration has been evidenced. The increase of fossil resource prices induced by stock decrease and impending shortage also stimulates the development of new energetic solutions. Concentrated solar energy can be used to produce electricity or synthetic fuels with high conversion efficiencies and without any greenhouse gas emissions. This study focuses on the investigation of innovative materials and their reactivity optimization in view to produce hydrogen using solar thermochemical cycles. This hydrogen production path via direct solar thermal energy conversion should present higherenergy conversion efficiencies than electrolysis using solar electricity.1 Solar thermochemical hydrogen production using mixed metal oxides, like ceria-based materials, is based on a two-step process that leads to water dissociation. During the first endothermal step, the metal oxide is thermally reduced at about 1400 °C, and this reduction proceeds via the formation of oxygen vacancies and the release of gaseous O2, resulting in the subsequent change in stoichiometry. The second exothermal step proceeds at a lower temperature and corresponds to the metal oxide oxidation with water to generate hydrogen. The principle of thermochemical cycles using doped ceria is represented by the following reactions. © 2012 American Chemical Society

(1)

Exothermal step: MxCe1 − xO2 − δ + δ H 2O → MxCe1 − xO2 + δ H 2

(2)

The oxide is not consumed during the hydrogen production process, and the net reaction is equivalent to the water dissociation reaction. In this way, solar energy is directly converted and stored under chemical form. Several metal oxides can be used for this process such as zinc oxide,2−6 tin oxide,7,8 iron oxide9−11 or cerium oxide.12−15 Cerium oxide is an interesting candidate because it remains in a condensed phase during the high-temperature step. Previous studies on this material showed that pure cerium dioxide (ceria) presents a low reducibility at 1400 °C,16 which is mainly the result of oxygen mobility in the nonstoichiometric fluorite structure. Consequently, doped ceria was investigated to improve the reduction yield at 1400 °C and, thereby, the hydrogen production yield per gram of material. Ceria−zirconia solid solution was proposed to improve markedly the hydrogen production because of the increased reducibility of the mixed oxide.16,17 Similarly, other dopants may also be considered to improve the reactivity of ceria. Several studies about the Received: March 5, 2012 Revised: May 15, 2012 Published: June 5, 2012 13516

dx.doi.org/10.1021/jp302146c | J. Phys. Chem. C 2012, 116, 13516−13523

The Journal of Physical Chemistry C

Article

Figure 1. Oxygen evolution during the reduction step of pure ceria at 1420 °C.

addition of dopants in ceria (Gd,18−20 Sm,21 or La19,22) have already been carried out for other applications such as catalytic automotive depollution to improve the oxygen mobility and the oxygen storage capacity of ceria. For example, doping with trivalent lanthanide increases the oxygen mobility by creating oxygen vacancies. The doping of ceria with trivalent lanthanides has not been experimentally investigated in view of thermochemical hydrogen production via two-step water splitting. This study deals with the doping of ceria to improve the material reactivity during thermochemical water-splitting cycles for hydrogen production. Binary oxides incorporating trivalent lanthanides (Sm0.1Ce0.9O1.95, Gd0.1Ce0.9O1.95, La0.1Ce0.9O1.95, and La0.25Ce0.75O1.875) and pentavalent cation (Ta0.1Ce0.9O2+δ, Ta0.25Ce0.75O2+δ, Ta0.5Ce0.5O2+δ) are investigated, along with ternary oxides based on ceria−zirconia (Zr0.23La0.02Ce0.75O2−δ, Zr0.25Gd0.05Ce0.7O2−δ, Zr0.25Gd0.01Ce0.74O2−δ, Zr0.23Y0.02Ce0.75O2−δ). These materials are experimentally tested during water-splitting cycles for hydrogen production, and the performances are compared to the ones of previously tested materials.

the mass variation was registered continuously. The successive steps of the thermochemical cycle were realized at different temperatures (1400 °C for the reduction and lower temperature for the hydrolysis), and water vapor was injected during the hydrolysis step (80% of relative humidity in Ar at 40 °C). The mass variation associated to the temperature program corresponds to the oxygen release during reduction or to the oxygen uptake during water splitting, which allows calculating the reduction yield and the hydrogen production yield. The reduction yield (Xred) corresponds to the mole ratio of reduced cerium Ce3+ to the total amount of cerium atoms. It is calculated as the amount of O2 released during the reduction step to the theoretical maximum amount of O2 that could be released if Ce4+ was completely reduced to Ce3+. Regarding the reoxidation yield α, it corresponds to the amount of hydrogen produced divided by the theoretical maximum amount of hydrogen that can be produced after the preceding reduction step.

3. RESULTS AND DISCUSSION 3.1. Pure Ceria As Reference Material. As a reference, the water-splitting activity of pure ceria was investigated during thermochemical cycles. A given mass of a commercial powder (8 g, CeO2; Aldrich 99.9%) was placed in an alumina crucible and heated up to 1420 °C (10 °C/min) in argon sweeping gas (0.2 NL/min) in a tubular furnace. The reduction temperature was then maintained for 100 min to complete the O2-releasing step. The concentration of oxygen released was measured online with an oxygen trace analyzer. Figure 1 represents the evolution of oxygen production (instantaneous rate and cumulated amount) during the reduction reaction. The start of the O2-releasing reaction occurs at 1090 °C, and the peak in O2 production is observed just before the beginning of the temperature plateau. The final amount of oxygen released was 80 μmol/g at the end of the temperature plateau (δ = 0.03 in CeO2−δ), corresponding to a reduction yield of 5.5% (Ce3+/ Cetotal). This reduction yield is in agreement with the one measured previously in TGA.16 On the basis of a reoxidation yield of 100%, the amount of hydrogen produced from 1 g of ceria is 160 μmol (3.6 mL/g). The partially reduced ceria presents a high reactivity with water during the hydrolysis step, but the weak reduction yield limits strongly the amount of produced hydrogen.16 The doping of ceria is considered to improve the reactivity, especially the reduction yield of ceria, during thermochemical water-splitting cycles. Experimental results have been reported concerning solar hydrogen production using doped ceria with Zr, Hf, Mg, Sc, Cu, Ni, Mn, Fe, and Pr,23−29 and the relevant results are summarized in Table 1.

2. EXPERIMENTAL SECTION The materials were synthesized by coprecipitation of hydroxides with metallic nitrate or chloride precursors. These precursors were weighed and dissolved into distilled water under magnetic stirring. The pH of the solution was adjusted with a base to obtain a precipitate (hydroxides) that was then separated by centrifugation. This precipitate was washed with distilled water three times and dried in an oven at 80 °C. Finally, the powder was calcined at 800 °C in air for two hours to obtain the final oxide. Then the materials were characterized by X-ray diffraction (XRD) using a Philips PW 1820 diffractometer with Cu Kα radiation (αCu = 0.15418 nm, angular range = 20−75° 2θ, steps = 0.02° 2θ, recording time = 2 s). XRD data were used to confirm the formation of solid solutions by calculating the lattice parameters and to observe the crystallographic evolution after hydrogen production cycles. The activity of the synthesized materials during thermochemical cycles was investigated by thermogravimetric analysis performed in a controlled atmosphere (TGA, SETARAM Setsys Evo 1750) coupled with a steam generator. About 100 mg of material was introduced in a platinum crucible hung to the balance with platinum suspensions and placed inside the furnace chamber. Then, residual air in the TGA chamber was eliminated by a preliminary pumping to operate in inert atmosphere and to improve the reduction yield because of the decreased oxygen partial pressure. The sample was heated in an argon flow (40 mL/min) with a heating rate of 20 °C/min, and 13517



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Table 1. Summary of Results on Solar Hydrogen Production from Doped Ceria dopantpercentage

reduction temperature, °C

Zr-10% Zr-25% Zr-50% Hf-10% Sc-10% Mg-10% Ni-10% Fe-10% Mn-10% Cu-10% Pr-10%

1400 1400 1400 1500 1500 1500 1500 1500 1500 1500 1500

hydrogen production mL/g per cycle (μmol/g) 5.76 6.67 5.95 4.50 4.06 3.89 2.73 1.94 3.77 0.97 3.96

(257) (298) (266) (201) (181) (174) (122) (87) (168) (43) (177)

references 17 17 17 28 28 28 24, 26, 27 24, 25 24 24 29

Figure 2. Diffraction patterns of Ta-doped ceria synthesized via coprecipitation of hydroxides.

The best dopant appears to be the zirconium yielding a maximum hydrogen production up to 6.67 mL/g during one cycle with an activation temperature of 1400 °C.17 Consequently, the Zr-doped ceria leads to a significantly higher amount of produced H2 than for the other considered dopants, while the reduction temperature is lower (1400 °C instead of 1500 °C) to avoid any sublimation issue of ceria. The optimal composition is a substitution of cerium with 25% of zirconium. The addition of Zr improves the reducibility, but the reactivity during hydrogen production may decrease during cycling because of diffusion limitations associated to material sintering, depending on the morphology of the powder and thus on the synthesis method that plays an important role in the hydrogen production.16 Consequently, the limitation of the particle growth induced by sintering that leads to longer diffusion lengths has to be addressed. 3.2. Doping with Pentavalent Tantalum. Previous work on Ta-doped ceria showed a high material reducibility in air,27 but the water-splitting step was not thoroughly investigated. Hence, Ta-doped ceria was synthesized and characterized during water-splitting cycles. The tantalum is a pentavalent cation, and the mixed oxide presents an excess of negative charges in the anionic network (incorporation of extra O2−), which may improve the ceria reducibility and the hydrogen production. Indeed, the charge excess induced by the pentavalent cation Ta5+ can be readily neutralized by reducing the cerium cation, and a large quantity of oxygen neighboring Ce3+ can be released. Three compositions, Ta0.1Ce0.9O2+δ, Ta0.25Ce0.75O2+δ, Ta0.5Ce0.5O2+δ (with δ = 0.05, 0.125, 0.25 for 10%, 25%, 50% of Ta, respectively), were synthesized by coprecipitation of hydroxides using cerium nitrate and tantalum chloride (TaCl5) precursors. The diffraction patterns (Figure 2) show that the mixed oxides present a fluorite-type structure before thermal treatment (TaxCe1−xO2+x/2). The lattice parameters calculated from the diffraction patterns (Table 2) confirm the insertion of tantalum inside the fluorite structure and the substitution of cerium atoms. The lattice parameter decrease can be attributed to the smaller ionic radius of Ta5+ (0.070 nm) compared to Ce4+. The water-splitting activity of the Ce-based material doped with 10% of Ta and previously reduced at 1450 °C was characterized using a device composed of a tubular quartz furnace and a hydrogen analyzer (catharometer). The powder (1 g) was placed in an alumina crucible and heated in argon sweeping gas (0.2 NL/min) to the desired temperature. Then, water was injected inside the tubular furnace with a peristaltic

Table 2. Lattice Parameters of Ta-Doped Ceria materials

lattice parameters (Å)

Ta 10% Ta 25% Ta 50%

5.404 (±0.002) 5.401 (±0.002) 5.382 (±0.002)

pump (the mole fraction of steam water in the flue gas was 38.3%) to produce hydrogen. Figure 3 represents the time

Figure 3. Hydrogen production during the hydrolysis step with Ta0.1Ce0.9O2+δ at three different temperatures.

course of hydrogen production at different temperatures with Ta0.1Ce0.9O2+δ after a reduction step at 1450 °C in an inert atmosphere (Xred = 10.5%). The hydrogen production reaches 142 μmol/g at 1040 °C after 20 min of reaction. The maximum production is obtained at the highest temperature, but the hydrolysis yield reaches only 54.8% at 1040 °C on the basis of the reduction yield. The XRD data of this material after the reduction step and after the hydrolysis step are represented in Figure 4. A peak shift toward low angles after the reduction step confirms that the material is partially reduced. Indeed, the ionic radius of the Ce3+ cation (0.114 nm) is larger than that of Ce4+ (0.097 nm), which induces structural deformation (expansion), increase of lattice parameter, and consequently a peak shift toward low angles. After the hydrolysis step, XRD reveals a partial reoxidation because a peak shift toward high angles is observed compared to the reduced species. The peak shifts after each 13518



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Figure 6. TGA of two consecutive thermochemical cycles with 25% and 50% Ta-doped ceria.

Figure 4. Diffraction patterns of Ta 0.1 Ce 0.9 O 2+δ before the thermochemical cycle, after the reduction step, and after the hydrolysis step.

XRD analysis (Figure 7) of the 25% Ta-doped ceria shows characteristic peaks associated to the CeTaO4 compound after

cycle step may also be attributed to differences in oxidation state and oxygen nonstoichiometry. Several thermochemical cycles were performed in TGA with the 25% Ta-doped ceria (Figure 5). The mass loss

Figure 7. Diffraction patterns of Ta0.25Ce0.75O2+δ before the thermochemical cycle, after the reduction step, and after the hydrolysis step. Figure 5. TGA of three consecutive thermochemical cycles with 25% Ta-doped ceria.

the reduction step. Actually, the cerium is reduced to Ce3+, but it also forms a new stable species with tantalum that does not react with water during the hydrolysis step. This is confirmed by XRD analysis after the hydrolysis step since CeTaO4 is still present. The small hydrogen production is thus attributed to the part of reduced cerium that is not found under CeTaO4 form. The incorporation of a limited amount of Ta (10%) improves the hydrogen production compared to pure ceria, but the performances are, however, not improved compared to ceria−zirconia. Concerning the addition of higher amounts of Ta (25%, 50%), the pentavalent cation does not enhance the hydrogen production because of the formation of a stable CeTaO4 phase during the high-temperature reduction step. The positive charge excess induced by the tantalum incorporation in ceria is neutralized during the reduction step by the reduction of Ce4+ to Ce3+, but the stable phase of CeTaO4, detected after the reduction step, does not react with steam for hydrogen production. 3.3. Doping with Trivalent Lanthanides. The production of hydrogen from ceria doped with trivalent lanthanides has never been experimentally investigated. La-, Sm-, and Gddoped ceria were thus considered for thermochemical hydrogen production.

corresponding to the reduction of the material starts around 700 °C, and the reduction yield reaches 38% after 45 min at 1400 °C during the first cycle. Weak mass gains are then observed during the hydrolysis steps at 1300, 1100, and 900 °C, corresponding to 56, 74, and 75 μmol/g of hydrogen produced, respectively. The hydrogen production is lower at 1300 °C because of a competition between the oxidation and the reduction reaction. During the next hydrolysis at 1100 and 900 °C, the amount of hydrogen is stable. The reoxidation yield is thus very small (∼10%), and the material doped with 10% of Ta exhibits the best performances in comparison with higher fractions of Ta. Indeed, the hydrogen production with the materials doped with 25% and 50% of Ta was also studied by TGA. Two successive thermochemical cycles were done with a temperature of 1400 °C for the reduction step and 1050 °C for the hydrolysis step. Figure 6 represents the mass variation during two thermochemical cycles with Ta 0.25 Ce 0.75 O 2+δ and Ta0.5Ce0.5O2+δ. The reduction yield is 36.5% and 88.4% with the 25% and 50% Ta-doped ceria, respectively. Regarding hydrogen generation, the 25% Ta-doped ceria produces 75 and 69 μmol/g during the two successive cycles, while the 50% Tadoped ceria presents a negligible hydrogen production in spite of the high reduction yield. 13519



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The synthesis of lanthanide-doped ceria was realized with nitrate precursors via coprecipitation of hydroxides, and the powder was calcined at 800 °C for 2 h in air. The studied materials are Sm0.1Ce0.9O1.95, Gd0.1Ce0.9O1.95, La0.1Ce0.9O1.95, and La0.25Ce0.75O1.875. The XRD patterns are presented in Figure 8 confirming the formation of the fluorite-type structure.

Figure 9. TGA of three consecutive thermochemical cycles with Ladoped ceria.

A similar behavior is observed when doping with other lanthanides such as Gd3+ or Sm3+. The mass variations during two consecutive thermochemical cycles are represented in Figure 10. Compared to pure ceria, the reduction yields are slightly lower as well as the hydrogen production (Table 5). This result is explained by the substitution of a part of reducible Ce4+ by nonreducible lanthanides that do not facilitate the reduction of cerium. The crystallite size is increased after cycling due to sintering effect regardless of the material considered (Table 3). Ternary oxides were then studied to conjugate the effect of the different dopants. Because zirconium incorporation increases oxygen mobility in the lattice structure (thereby improving the ceria reducibility) and lanthanides enhance the thermal stability, ternary oxides were synthesized and characterized during thermochemical cycles for hydrogen production. 3.4. Doped Ceria/Zirconia Solid Solutions. Previous work about the enhancement of the oxygen storage capacity with ternary oxides Ce0.6Zr0.4−xMxO2−x/2 (M = Y3+, La3+, Ga3+) showed that the increase of the redox properties is associated to oxygen desorption at lower temperature and creation of oxygen vacancies.30 This improvement is due to the lower binding energy of lattice oxygen and the incorporation of trivalent cations. These types of materials were not investigated before in view of solar hydrogen production. Thus, ceria−zirconia solid solutions doped with lanthanum, gadolinium, and yttrium were prepared by coprecipitation of hydroxides. Different compositions were synthesized, Zr0.23La0.02Ce0.75O2−δ, Zr 0 . 2 5 Gd 0 . 0 5 Ce 0 . 7 O 2 − δ , Zr 0 . 2 5 Gd 0 . 0 1 Ce 0 . 7 4 O 2 − δ , and Zr0.23Y0.02Ce0.75O2−δ (δ = 0.005, 0.01, and 0.025 for 1%, 2%, and 5% of trivalent cation, respectively), and their reactivity was compared to the one of undoped ceria−zirconia. The diffraction patterns of the ternary oxides are represented in Figure 11. All the synthesized compounds have a fluoritetype structure similar to the one of pure ceria. The reactivity of the ternary oxides was investigated with the thermobalance during two consecutive thermochemical watersplitting cycles. Figure 12 represents the mass variations of these materials along with the ones of ceria−zirconia solid solution. Except for Zr0.25Gd0.05Ce0.7O2−δ, all the compounds exhibit a high reactivity with water during the first hydrolysis step. In addition, the amount of hydrogen produced is greatly improved in comparison to pure ceria. However, a reactivity decrease is observed for all the materials during the second

Figure 8. XRD patterns of lanthanide-doped ceria.

Table 3 lists the values of lattice parameters identified from XRD data. Compared to the diffraction pattern of pure ceria Table 3. Lattice Parameters and Crystallite Sizes of Doped Ceria Calculated from XRD Data materials CeO2 Gd0.1Ce0.9O1.95 Sm0.1Ce0.9O1.95 La0.1Ce0.9O1.95 La0.25Ce0.75O1.875

lattice parameters (Å) 5.415 5.422 5.428 5.436 5.475

(±0.002) (±0.002) (±0.002) (±0.002) (±0.002)

crystallite size (before cycling) (nm)

crystallite size (after cycling) (nm)

12.5 28.5 28.6 12.7 12.2

68.3 104.3 97.4 78.8 59.7

synthesized by the same way, a peak shift to low 2θ angle is observed when cerium atoms are substituted by doping cations. This corresponds to a higher lattice parameter confirming that the dopants are inserted inside the crystallographic structure because the trivalent cation size is larger than that of tetravalent cerium (rCe4+ = 0.097 nm < rGd3+ = 0.105 nm < rSm3+ = 0.108 nm < rLa3+ = 0.116 nm). The reactivity of these materials was then studied via TGA. First, La-doped ceria was investigated during three consecutive thermochemical cycles with a reduction temperature of 1400 °C and three different hydrolysis temperatures of 1200, 1100, and 1000 °C, respectively. The mass variations of La0.1Ce0.9O1.95 and La0.25Ce0.75O1.875 during thermochemical cycles are represented in Figure 9. The reduction yield decreases when the amount of lanthanum increases, and consequently, the amount of hydrogen produced is lower with La0.25Ce0.75O1.875. The hydrogen production is relatively constant from one cycle to another, which means that the materials do not deactivate during cycling at high temperature. Table 4 gathers the amounts of oxygen released and hydrogen produced during successive thermochemical cycles. The hydrogen production is lower when doping with lanthanum. Although previous works describe an improvement of the oxygen storage capacity by doping ceria with lanthanum,22 the hydrogen production is not enhanced compared to pure ceria. 13520



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Table 4. Oxygen and Hydrogen Production during Successive Thermochemical Cycles with La-Doped Ceria O2 released during reduction (1400 °C) 1st cycle

CeO216 La 10% La 25%

2nd cycle

H2 produced during oxidation 2nd cycle (1100 °C)

1st cycle (1200 °C)

3rd cycle

3rd cycle (1000 °C)

μmol O2/g

Xred (%)

μmol O2/g

Xred (%)

μmol O2/g

Xred (%)

μmol H2/g

α (%)

μmol H2/g

α (%)

μmol H2/g

α (%)

75 99 88

5.2 7.5 8.0

58 60 45

4.0 4.5 4.1

66 63 51

4.6 4.8 4.7

125 117 107

83.3 59.4 60.8

128 123 108

110.3 103.1 111.7

144 137 111

109.1 107.9 107.8

Figure 10. TGA of two consecutive thermochemical cycles with Smand Gd-doped ceria.

Figure 11. Diffraction patterns of ceria-based ternary oxides synthesized via coprecipitation of hydroxides.

hydrolysis step, which can be attributed to diffusion limitations in the bed of particles packed in the crucible. Indeed, the particles confined within the Pt-crucible tend to pack and sinter after the first thermal cycle, which enhances the diffusion effects. As a result, the kinetic rate of the second hydrolysis was lowered, and the mass uptake during the second hydrolysis reaction was composed of a fast initial step followed by a diffusion-controlled step. This denotes a rate-limiting transition between the surface reaction and the bulk reaction, and additional H2 could have been further produced with a longer duration of the hydrolysis reaction to reach completion. Table 6 lists the amounts of oxygen and hydrogen produced during thermochemical cycles with the ternary oxides, which shows that the gravimetric amounts of O2 and H2 produced from Zr-doped ceria are much higher than with undoped ceria (Table 4), although there is a smaller amount of available Ce in the ceria−zirconia. The presence of Zr increases oxygen bulk mobility in the lattice structure. The bulk thus participates in the hydrolysis reaction in the case of ceria−zirconia, which explains the increased overall reaction duration. Consequently, bulk oxygen diffusion needs to be promoted. The addition of 1% of Gd enhances the material performance during watersplitting cycles compared to ceria−zirconia obtained from an identical synthesis route (coprecipitation method). Indeed, reduction yields of 17% and 14.9% were measured at 1400 °C, and a hydrogen production up to 338.2 μmol/g was obtained

Figure 12. TGA of ceria-based ternary oxides during two consecutive thermochemical cycles.

(Table 6). The material performances decrease when doping with a larger quantity of Gd. The doping with La results in a better thermal stability than for ceria−zirconia because the hydrogen production yield during the second cycle reaches 94.2% instead of 70.2% with the undoped ceria−zirconia material. The doping with yttrium does not provide any enhancement of the reactivity during the cycles.

Table 5. Oxygen and Hydrogen Production during Successive Thermochemical Cycles with Sm- and Gd-Doped Ceria O2 released (1400 °C) 1st cycle Sm0.1Ce0.9O1.95 Gd0.1Ce0.9O1.95

H2 produced (1050 °C) 2nd cycle

1st cycle

2nd cycle

nO2 (μmol/g)

Xred (%)

nO2 (μmol/g)

Xred (%)

nH2 (μmol/g)

α (%)

nH2 (μmol/g)

α (%)

65 62

4.8 4.7

56 53

4.2 4.0

108.6 101.6

83.5 81.3

107.1 101.4

95.6 95.7

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Table 6. Oxygen and Hydrogen Production during Successive Thermochemical Cycles with Doped Ceria−Zirconia Solid Solutions O2 released (1400 °C) 1st cycle Zr (25%) Zr−La (23/2%) Zr−Y (23/2%) Zr−Gd (25/1%) Zr−Gd (25/5%)

H2 produced (1050 °C) 2nd cycle

1st cycle

2nd cycle

(μmol/g)

Xred (%)

(μmol/g)

Xred (%)

(μmol/g)

α (%)

(μmol/g)

α (%)

179.4 160.3 163.5 200.0 164.0

15.3 13.7 14.0 17.0 14.0

155.3 128.9 142.0 174.0 84.2

13.3 11.0 12.1 14.9 7.2

323.9 271.6 300.8 338.2 167.2

91.9 84.7 92.0 84.5 50.9

218.0 242.8 178.5 224.0 117.3

70.2 94.2 62.8 64.4 69.6

The lattice parameter of the different synthesized Zr-doped materials is reported in Table 7 to show the lattice contraction

the improvement of the reactivity can thus be ascribed to the enhanced thermal resistance compared to ceria−zirconia.

Table 7. Lattice Parameters and Crystallite Sizes of Doped Ceria−Zirconia Calculated from XRD Data

4. CONCLUSION Several doping metals were considered to increase the reactivity of ceria during hydrogen production from two-step thermochemical water-splitting cycles. The doping with a pentavalent cation such as tantalum led to a sharp increase of the reduction yield, but structural changes after high-temperature thermal treatment limited the hydrogen production. Trivalent lanthanide incorporation did not show better performance than for undoped ceria, in contrast to previous works on oxygen storage capacity for catalytic applications at lower temperatures. Nevertheless, the addition of such dopants in ceria−zirconia solid solutions led to interesting results, and unprecedented water-splitting activity was achieved. Indeed, the doping of ceria−zirconia solid solution with lanthanum enhanced the thermal stability (thus the ability to resist sintering), which is a key feature to avoid reactivity decrease during cycling. Gadolinium incorporation also resulted in a higher hydrogen production than for ceria−zirconia. The results also evidenced that the limitation of the crystallite growth during cycling is essential to improve the reactivity of doped ceria. The Zr-doped ceria results in a better thermal resistance and stability, which can be further improved by adding Gd or La. The amount of H2 produced from doped ceria−zirconia activated at 1400 °C was substantially improved in comparison to previous studies on different doped-ceria compounds activated at 1500 °C (H2 production improvement of approximately 40−50% minimum with respect to CeO2−MOx binary oxides). In addition, the activation temperature (1400 °C for the O2-releasing step) was lowered, which is compulsory for avoiding ceria sublimation. Further investigation on ternary oxides should be carried out to optimize the material composition and the stoichiometry to maximize the hydrogen production without any reactivity decrease during cycling.

materials

lattice parameters (Å)

crystallite size (before cycling) (nm)

crystallite size (after cycling) (nm)

Zr0.25Ce0.75O2 Zr0.25Gd0.01Ce0.74O1.995 Zr0.25Gd0.05Ce0.7O1.975 Zr0.23La0.02Ce0.75O1.99 Zr0.23Y0.02Ce0.75O1.99

5.376 5.386 5.389 5.394 5.378

7.3 7.0 7.6 7.4 8.0

48.7 19.4 35.1 31.6 37.2

of ceria due to the atomic substitution of some cerium atoms by zirconium atoms with smaller ionic radius (0.084 nm). The diffraction patterns of materials after cycling are presented in Figure 13. The characteristic peaks are sharpened after cycling,

Figure 13. Diffraction patterns of ceria-based ternary oxides after cycling.

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which suggests that crystallite sizes are larger after thermal treatment. The increase of crystallite size after cycling is confirmed in Table 7, but the material sintering is strongly alleviated when considering ceria−zirconia solid solutions. The addition of zirconia into pure ceria is already known to enhance the reactivity during water-splitting cycles, and it also limits the crystallite size growth after thermal treatment (48.7 nm for Zr0.25Ce0.75O2 after cycling versus 68.3 nm for CeO2). This study brings out that the limitation of crystallite growth and ability to resist sintering can be further optimized by doping ceria−zirconia with Gd, La, or Y (Table 7). The addition of 1% of Gd leads to the lowest crystallite growth (crystallite size of 19.4 nm after cyclic thermal treatment), and

AUTHOR INFORMATION

Corresponding Author

*Tel.: +33 4 68 30 77 30. Fax: +33 4 68 30 29 40. E-mail address: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was funded by ANR (project contract N° ANR-09JCJC-0004-01) and CNRS (Interdisciplinary Energy Program). 13522



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