Elsevier

Applied Surface Science

Volume 489, 30 September 2019, Pages 80-91
Applied Surface Science

Full length article
Combustion synthesized A0.5Sr0.5MnO3-δ perovskites (where, A = La, Nd, Sm, Gd, Tb, Pr, Dy, and Y) as redox materials for thermochemical splitting of CO2

https://doi.org/10.1016/j.apsusc.2019.05.284Get rights and content

Highlights

  • A0.5Sr0.5MnO3-δ perovskites were synthesized using solution combustion method.

  • PXRD and EDS analysis confirms phase pure composition of each A0.5Sr0.5MnO3-δ perovskite.

  • SEM analysis indicate agglomerated larger irregularly shaped porous particles.

  • PrSM shows maximum nO2 = 144.8 μmol/g·cycle and nCO = 252.3 μmol/g·cycle.

  • A0.5Sr0.5MnO3 perovskites shows significantly higher nO2 and nCO than CeO2.

Abstract

In this paper, A0.5Sr0.5MnO3 perovskites (where, A = La, Nd, Sm, Gd, Tb, Pr, Dy, and Y) were examined towards thermochemical CO2 splitting (CS) reaction. The solution combustion synthesis (SCS) method was employed for the preparation of the A0.5Sr0.5MnO3 perovskites, in which glycine was used as the fuel. The characterization of the as-prepared and reacted A0.5Sr0.5MnO3 perovskites was accomplished by means of powder X-ray diffractometer (PXRD), scanning electron microscope (SEM), and energy dispersive X-ray spectroscopy (EDS). A Seteram Setsys Evolution TGA set-up was utilized to estimate the amounts of O2 released (nO2) and CO produced (nCO) by each SCS synthesized A0.5Sr0.5MnO3 perovskite in multiple thermochemical cycles. Obtained TGA results confirmed that all the A0.5Sr0.5MnO3 perovskites attained thermal and redox stability from second thermochemical cycle. It was also understood that all the A0.5Sr0.5MnO3 perovskites shows significantly higher nO2 and nCO as compared to the phase pure CeO2. Among all the A0.5Sr0.5MnO3 perovskites investigated, PrSM shows maximum nO2 = 144.8 μmol/g·cycle and nCO = 252.3 μmol/g·cycle with an average CO/O2 molar ratio of 1.74. The experimental findings also indicate that the fuel production aptitude of all the A0.5Sr0.5MnO3 perovskites can be upsurged if longer CS reaction time is employed.

Introduction

Conversion of thermodynamically stable H2O and CO2 into synthetic liquid fuels via a renewable and environmentally sustainable process, such as metal oxide (MO) based solar thermochemical cycle, is essential to fulfill the forthcoming energy demand of the civilization [1]. The employment of the above-mentioned process can also aid to reduce the consumption of fossil fuels and emission of CO2, which ultimately results into a drop in the risk associated with the global warming. Several non-volatile MOs have been considered for this process including iron oxide [2,3], Ni-ferrite [4,5], Zn-ferrite [6,7], Co-ferrite [8,9], Mn-ferrite [10], Sn-ferrite [11], Ni-Zn-ferrite [12], Ni-Mn-ferrite [13], CeO2 [14,15], ceria-zirconia solid solution [16,17], and other doped ceria materials [18,19]. Among these MOs, the ceria based redox materials are increasingly more attractive because of their elevated oxygen storage capacity (OSC), faster reaction kinetics, and exceptional material stability during multiple thermochemical cycles.

As the thermal reduction (TR) temperatures required for the perovskite oxides (ABO3) are lower than the other MOs investigated so far, in recent years, ABO3 have been examined as an alternative for the ceria based redox materials [20]. The two-step TR and H2O (WS) and CO2 splitting (CS) reactions associated with the perovskite oxides are shown below:ABO3xABO3y+12O2gABO3y+H2OABO3x+H2gABO3y+CO2ABO3x+COg

Following is the list of key perovskites scrutnized until now for the thermochemical splitting of H2O and CO2: (LaSr)MnO3-δ [[21], [22], [23], [24], [25], [26]], (YSr)MnO3-δ [26,27], (LaSr)FeO3-δ [26], (LaSr)CoO3-δ [26], (LaCa)MnO3-δ [26,27], (BaSr)FeO3-δ [26], (BaSr)(FeCo)O3-δ [26], (LaSr)(CoFe)O3-δ [26], (LaSr)(FeMn)O3-δ [27], (LaBa)(Mn)O3-δ [27], (LaSr)(MnAl)O3-δ [27,28], and (LaSr)(CrCo)O3-δ [29].

The above mentioned list clearly indicate that most of the studies were directed towards varying the dopants associated with the B-site by keeping the A-site mainly stable at Lanthanum (La). According to us, the exploration of the influence of variation in the metal cations associated with the A-site of the perovskite is also essential. Furthermore, in most of the previous examinations, the redox reactivity of the perovskite oxides was investigated by performing only one thermochemical cycle. The solar thermochemical community is largely interested towards identifying the best MOs based on their ability towards stable amounts of O2 released (nO2) and H2 (nH2) or CO produced (nCO) in multiple thermochemical cycles. Thus, in this study, the TR and CS aptitude of A0.5Sr0.5MnO3-δ (where, A = La, Nd, Sm, Gd, Tb, Pr, Dy, and Y) was estimated by performing 10 thermochemical CS cycles. The A0.5Sr0.5MnO3-δ perovskites were synthesized by using a solution combustion synthesis (SCS) method. Physical properties of the derived perovskites were estimated by using multiple analytical techniques and the nO2 and nCO by each material was assessed by performing numerous thermogravimetric experiments.

Section snippets

Solution combustion synthesis of A0.5Sr0.5MnO3-δ

The A0.5Sr0.5MnO3-δ perovskites were synthesized by employing a solution combustion synthesis (SCS) approach (Fig. 1). The abbreviations provided to all the A0.5Sr0.5MnO3-δ perovskites are listed in Table 1. Measured quantities of metal nitrates (obtained from Sigma Aldrich) with stoichiometric glycine were dissolved into 50 ml of deionized water. This mixture was continuously agitated at room temperature to achieve a uniform solution. The prepared solution was preheated by placing it on the

Characterization of A0.5Sr0.5MnO3-δ perovskites

The phase composition of the annealed A0.5Sr0.5MnO3-δ perovskite powders was characterized by using a PANalytical XPert MPD/DY636 powder X-ray diffractometer (PXRD). The PXRD patterns reported in Fig. 3a specify phase pure composition of each A0.5Sr0.5MnO3-δ perovskite with absence of any metal or metal oxide impurities. As the crystal ionic radii of metal cation dopants decreases i.e., La (117.2 pm) > Pr (113.0 pm) > Nd (112.3 pm) > Sm (109.8 pm) > Gd (107.8 pm) > Tb (106.3 pm) > Dy

Summary and conclusions

The La, Nd, Sm, Gd, Tb, Pr, Dy, and Y doped strontium‑manganese perovskites were successfully prepared by using solution combustion synthesis method. The powders obtained after combustion were further calcined up to 1000 °C in air. The physico-chemical analysis of the calcined powders was conducted by utilizing the various analytical techniques. The PXRD and EDS analysis confirms phase pure composition of each A0.5Sr0.5MnO3-δ perovskite. Moreover, the SEM analysis indicate agglomerated larger

Acknowledgements

This publication was made possible by the NPRP grant (NPRP8-370-2-154) from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of author(s).

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