Noteworthy performance of La_1−xCa_xMnO₃ perovskites in generating H₂ and CO by the thermochemical splitting of H₂O and CO₂

Abstract

Perovskite oxides of the composition La_1−xCa_xMnO₃ (LCM) have been investigated for the thermochemical splitting of H₂O and CO₂ to produce H₂ and CO, respectively. The study was carried out in comparison with La_1−xSr_xMnO₃, CeO₂ and other oxides. The LCM system exhibits superior characteristics in high-temperature evolution of oxygen, and in reducing CO₂ to CO and H₂O to H₂. The best results were obtained with La_0.5Ca_0.5MnO₃ whose performance is noteworthy compared to that of other oxides including ceria. The orthorhombic structure of LCM seems to be a crucial factor.

---

Increasing interest in sustainable energy sources and climate change has encouraged research in the production of H₂ from H₂O. Similarly, there is much interest in the reduction of CO₂ to CO by simple means. One of the significant methods being explored for the generation of H₂ and CO is the thermochemical splitting of H₂O and CO₂, respectively, by using oxide materials. The basic reactions of thermochemical H₂O and CO₂ splitting cycles employing metal oxides MO_n are the following:

Endothermal step: MO_n → MO_n−δ + δ/2O₂ (1)

Exothermal step: MO_n−δ + δCO₂ → MO_n + δCO

MO_n−δ + δH₂O → MO_n + δH₂ (2)

For this purpose, ZnO, ferrites, CeO₂ and related oxides have been employed.^1–10 Recently, oxide perovskites such as La_1−xSr_xMnO₃ and La_1−xSr_xFeO₃ were employed for thermochemical fuel production,^11–13 and results have been encouraging. Thus, Sr and Mn doped LaAlO₃ splits H₂O and CO₂ better than CeO₂.¹⁴ Because of the contemporary relevance of this subject, we considered that it is important to investigate other perovskite oxides for the thermochemical splitting of H₂O and/or CO₂ with the expectation that they may produce superior results. The materials employed by us for this purpose belong to the La_1−xCa_xMnO₃ (LCM) family which possesses Mn³⁺ and Mn⁴⁺ ions and has paramagnetic insulator properties at high temperatures unlike paramagnetic metals having La_1−xSr_xMnO₃ (LSM) compositions. Furthermore, LCMs are orthorhombic while LSMs are rhombohedral. We carried out measurements on LCMs in comparison with LSMs and ceria. Besides, we also tested the activity of La_1−xMnO₃ and La_1−xSr_xFeO₃. We have studied the reduction of CO₂ by thermogravimetric analysis and the reduction of H₂O by employing an experimental set-up fabricated locally.

La_1−xCa_xMnO₃ perovskites with x = 0.35, 0.5, 0.65 (designated here as LCM 35, LCM 50 and LCM 65) possess an orthorhombic structure (refer to ESI,† Fig. S1) with the lattice parameters decreasing slightly with the increasing Ca content. This is because, the content of the smaller Mn⁴⁺ (ionic radius 0.54 Å) increases over that of Mn³⁺ (ionic radius 0.65 Å) upon the substitution of trivalent La with bivalent Ca (refer to ESI,† Fig. S2).¹⁵ We calculated the atomic % of La, Ca and Mn present in LCMs (refer to ESI,† Table S1 and Fig. S3) by EDS analysis. The composition of LCMs was established from the background corrected La3d (∼836 eV), Ca2p (∼286 eV) and Mn2p (∼642 eV) XP survey scan spectra (refer to ESI,† Fig. S4, eqn (3)). The calculated ratios among La, Ca and Mn in LCM 35 and LCM 50 are 0.66/0.37/1 and 0.5/0.5/1, respectively (refer to ESI,† Table S2).

Thermogravimetric analysis of the three LCM compositions shows that upon heating above 1000 °C, O₂ production increases with the Ca²⁺ or Mn⁴⁺ content. The mass loss due to reduction starts around 1000 °C and reaches a plateau after 1400 °C. The O₂ released from LCM 35, LCM 50 and LCM 65 is 109, 316 and 653 μmol g⁻¹, respectively (Fig. 1a). It is to be noted that the amount of O₂ evolved by LCM 50 is greater than that evolved by La_0.5Sr_0.5MnO₃ (LSM 50), the value in the case of the latter being 201 μmol g⁻¹. Ceria evolves only 63 μmol g⁻¹ of O₂ (Fig. 1b). Thus, the amount of O₂ evolved by LCM 50 is 1.6 times that of LSM 50 and five times that of ceria. We studied the splitting of CO₂ by the LCM compounds. A reversible mass gain is observed in all the compounds during the CO₂ injection at 1100 °C (Fig. 1a).

Fig. 1 Representative TGA of thermochemical CO₂ splitting of (a) La_1−xCa_xMnO₃ (x = 0.35, 0.5, 0.65); (b) La_0.5Ca_0.5MnO₃ (LCM 50) and La_0.5Sr_0.5MnO₃ (LSM 50) in comparison with CeO₂. Reduction and oxidation temperatures are 1400 °C and 1100 °C, respectively.

A detailed study was carried out by changing the calcium content systematically. CO produced by LCM 35, LCM 50 and LCM 65 is 175, 525 and 810 μmol g⁻¹, respectively, over the same time scale. The CO production activity of LCM 50 is also about 1.6 times and 5 times more than that of LSM 50 and ceria, respectively (Table 1).

Table 1 TGA results for O₂ and CO yield of associated samples

	O2 released (μmol g^−1)	CO produced (μmol g^−1)
La0.65Sr0.35MnO3	100	137.5
La0.5Sr0.5MnO3	201.3	325
La0.65Ca0.35MnO3	109.4	175
La0.5Ca0.5MnO3	315.6	525
La0.35Ca0.65MnO3	653.1	350
Ceria	62.5	112

---

The kinetics of CO production appears to become sluggish at a high Ca content. The reoxidation peak of LCM 65 can be deconvoluted in two parts depending on its shape. The initial straight increase in the mass (394 μmol g⁻¹) corresponds to the reaction-controlled regime followed by the slower constant gain (450 μmol g⁻¹) corresponding to the diffusion-controlled regime (Fig. 1a). The fuel production activity of LCM 50 was tested during three successive cycles (Fig. 2) in the temperature range of 1400–1000 °C. This study has revealed reversibility in both O₂ evolution and CO production, though the kinetics of reoxidation appears to become slow on repeated cycling. We have carried out the reduction of the LCM 50 compound at 1350 °C followed by reoxidation at 1000 °C (Fig. 2 and refer to ESI,† Table S3; cycle 3). We found that the amount of O₂ evolved (237 μmol g⁻¹) and CO produced (481 μmol g⁻¹) over the temperature range of 1350–1000 °C is more than that evolved by the activity of Sr, Mn doped LaAlO₃, although the experimental set-up in the two cases is not identical.¹⁴ As-synthesized LCM 50 is composed of few micron size particles as observed from FESEM images. No significant change in the morphology of LCM 50 was observed after TGA analysis (refer to ESI,† Fig. S5).

Fig. 2 TGA of three consecutive thermochemical CO₂ splitting cycles of La_0.5Ca_0.5MnO₃ (LCM 50) performed in the temperature range of 1400–1000 °C.

In order to understand the superior performance of LCM 50, we prepared a perovskite of the composition Sm_0.5Sr_0.5MnO₃ (SmSM 50) and carried out TG analysis. SmSM 50 is also an orthorhombic perovskite (refer to ESI,† Fig. S6) with the tolerance factor (t = 0.982) comparable with that of orthorhombic LCM 50 (t = 0.985).¹⁶ Interestingly, both LCM 50 and SmSM 50 produce comparable amounts of O₂ and CO during TG analysis in the temperature range of 1400–1100 °C, unlike LSM 50 (Fig. 3). This indicates that the orthorhombic perovskites show superior activity than the rhombohedral perovskites. This could be due to the distortion in octahedral MnO₆ in orthorhombic perovskites compared to the rhombohedral one.^15,16

Fig. 3 Representative TGA of thermochemical CO₂ splitting of Sm_0.5Sr_0.5MnO₃ (SmSM 50) in comparison with that of La_0.5Ca_0.5MnO₃ (LCM 50). Reduction and oxidation temperatures are 1400 °C and 1100 °C, respectively.

We investigated the splitting of water by employing the experimental set-up shown in Fig. 4 (refer to ESI,† details are provided in the reactivity tests part). The H₂O splitting activity of LCM 50 was tested in comparison with that of LSM 50 with the help of the set-up presented in Fig. 4. The sample was placed inside the tubular furnace and heated up to 1400 °C under a constant N₂ flow.

Fig. 4 Experimental set-up for thermochemical water splitting.

As shown in Fig. 5a, O₂ evolution starts at around 1000 °C, and approaches the plateau region at 1400 °C. Both LCM 50 and LSM 50 show the complete O₂ production within 30 min after reaching 1400 °C. The amount of O₂ produced in the case of LCM 50 and LSM 50 is 272 and 193 μmol g⁻¹, respectively, close to the values obtained from TGA measurements discussed earlier. The H₂O splitting activity was tested by introducing steam in a continuous N₂ flow. The splitting temperature of H₂O is 1000 °C. Fig. 5b shows the rate of H₂ production. Production of H₂ starts immediately after the entry of H₂O in the gas stream. The amount of H₂ produced in a span of 100 min is 407 and 308 μmol g⁻¹ for LCM 50 and LSM 50, respectively. Notably, H₂ production is not complete even after 100 min and exhibits an extended tail over a long duration. The slow kinetics of H₂ evolution can arise due to factors such as a decrease in chemical diffusivity, changes in the surface reaction constant, steam concentrations as well as the intrinsic thermodynamic driving forces.¹³

Fig. 5 (a) Oxygen and (b) hydrogen evolution profiles of La_0.5A_0.5MnO₃ (A = Sr, Ca). Reduction and oxidation temperatures are 1400 °C and 1000 °C, respectively.

We tested the CO₂ splitting activity of La_1−xMnO₃ (x = 0.1, 0.2) as it contains a high concentration of Mn⁴⁺ (Fig. 6a).¹⁷ But phase transformation from the rhombohedral to the orthorhombic structure takes place under the high temperature reaction conditions (refer to ESI,† Fig. S7). Therefore, La_1−xMnO₃ cannot be used for this purpose. We also investigated the activity of La_1−xSr_xFeO₃ (x = 0.3) inspired by the literature data available for its H₂ production activity in membrane reactors.^12,18 The O₂ evolution of La_0.7Sr_0.3FeO₃ starts at a low temperature, with the total amount of O₂ produced being 360 μmol g⁻¹ at 1300 °C (Fig. 6b). Unfortunately, the CO production yield is low (20% of reduction yield).

Fig. 6 Representative TGA of thermochemical CO₂ splitting of (a) La_1−xMnO₃ (x = 0.1, 0.2; performed in the temperature range of 1400–1100 °C) and (b) La_0.7Sr_0.3FeO₃ (performed in the temperature range of 1300–1100 °C).

Besides having different structures, LCM and LSM compositions possess Mn³⁺ and Mn⁴⁺ ions in different electronic states. While two Mn ions with distinctly different charges are present in LCMs, there is fast electron transfer from Mn³⁺ to Mn⁴⁺ in LSMs giving rise to magnetism and mettalicity.¹⁹

Conclusions

The present results show that La_1−xCa_xMnO₃ is an excellent material for the thermochemical splitting of both H₂O and CO₂. The performance of La_1−xCa_xMnO₃ is superior to that of CeO₂ as well as that of the corresponding La_1−xSr_xMnO₃ composition. The amount of O₂ and CO evolved by La_0.5Ca_0.5MnO₃ is 1.6 times and 5 times more than that evolved by La_0.5Sr_0.5MnO₃ and the current state-of-the-art material ceria, respectively. Our investigation also indicates the superior performance of orthorhombic perovskite over the rhombohedral one in thermochemical fuel production. In conclusion, it would be profitable to attempt high temperature fuel production using LCM perovskites.

Acknowledgements

SD thanks CSIR for a fellowship. BSN acknowledges DST for a fellowship.

Notes and references

1. E. Bilgen and C. Bilgen, Int. J. Hydrogen Energy, 1982, 7, 637.
2. S. Abanades, P. Charvin, F. Lemont and G. Flamant, Int. J. Hydrogen Energy, 2008, 33, 6021.
3. A. Steinfeld, Int. J. Hydrogen Energy, 2002, 27, 611.
4. Y. Tamaura, A. Steinfeld, P. Kuhn and K. Ehrensberger, Energy, 1995, 20, 325.
5. T. Kodama, N. Gokon and R. Yamamoto, Sol. Energy, 2008, 82, 73.
6. H. Kaneko, T. Kodama, N. Gokon, Y. Tamaura, K. Lovegrove and A. Luzzi, Sol. Energy, 2004, 76, 317.
7. Y. Tamaura, N. Kojima, N. Hasegawa, M. Inoue, R. Uehara, N. Gokon and H. Kaneko, Int. J. Hydrogen Energy, 2001, 26, 917.
8. W. C. Chueh, C. Falter, M. Abbott, D. Scipio, P. Furler, S. M. Haile and A. Steinfeld, Science, 2010, 330, 1797; S. G. Rudisill, L. J. Venstrom, N. D. Petkovich, T. Quan, N. Hein, D. B. Boman, J. H. Davidson and A. Stein, J. Phys. Chem. C, 2013, 117, 1692.
9. H. Kaneko, T. Miura, H. Ishihara, S. Taku, T. Yokoyama, H. Nakajima and Y. Tamaura, Energy, 2007, 32, 656; A. L. Gal and S. Abanades, J. Phys. Chem. C, 2012, 116, 13516; M. Kang, X. Wu, J. Zhang, N. Zhao, W. Wei and Y. Sun, RSC Adv., 2014, 4, 5583.
10. A. L. Gal, S. Abanades, N. Bion, T. L. Mercier and V. Harle, Energy Fuels, 2013, 27, 6068; A. L. Gal, S. Abanades and G. Flamant, Energy Fuels, 2011, 25, 4836; J. R. Scheffe, R. Jacot, G. R. Patzke and A. Steinfeld, J. Phys. Chem. C, 2013, 117, 24104; S. Abanades and G. Flamant, Sol. Energy, 2006, 80, 1611.
11. J. R. Scheffe, D. Weibel and A. Steinfeld, Energy Fuels, 2013, 27, 4250.
12. A. Demont, S. Abanades and E. Beche, J. Phys. Chem. C, 2014, 118(24), 12682.
13. C. K. Yang, Y. Yamazaki, A. Aydin and S. M. Haile, J. Mater. Chem. A, 2014, 2, 13612.
14. A. H. McDaniel, E. C. Miller, D. Arifin, A. Ambrosini, E. N. Coker, R. O. Hayre, W. C. Chueh and J. H. Tong, Energy Environ. Sci., 2013, 6, 2424.
15. S. Faaland, K. D. Knudsen, M. A. Einarsrud, L. Rormark, R. Hoiera and T. Grande, J. Solid State Chem., 1998, 140, 320.
16. P. M. Woodward, T. Vogt, D. E. Cox, A. Arulraj, C. N. R. Rao, P. Karen and A. K. Cheetham, Chem. Mater., 1998, 10, 3652.
17. A. Arulraj, R. Mahesh, G. N. Subbanna, R. Mahendiran, A. K. Raychaudhuri and C. N. R. Rao, J. Solid State Chem., 1996, 127, 87.
18. L. Nalbandian, A. Evdou and V. Zaspalis, Int. J. Hydrogen Energy, 2009, 34, 7162.
19. C. N. R. Rao, J. Phys. Chem. B, 2000, 104, 5877; V. B. Shenoy and C. N. R. Rao, Philos. Trans. R. Soc., A, 2008, 366, 63.

---

Footnote

† Electronic supplementary information (ESI) available: Details of perovskite synthesis, reactivity testing methods and additional figures. See DOI: 10.1039/c4cp04578e

---