# Two-Step Thermochemical Conversion of CO2 using a Novel Nd1-xSrxMnO3 Perovskite

Muhammad Usman, Giovanni Palmisano, Georgios Karanikolos, Stéphane Abanades, Khalid Al-Ali

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Muhammad Usman, Giovanni Palmisano, Georgios Karanikolos, Stéphane Abanades, Khalid Al-Ali. Two-Step Thermochemical Conversion of CO2 using a Novel Nd1-xSrxMnO3 Perovskite. 30th SolarPACES Conference, SolarPACES 2024, Solar Power & Chemical Energy Systems, Oct 2024, Rome Fondazione Primoli Università di Roma 3, Italy. hal-04820087

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# Two-Step Thermochemical Conversion of CO₂ using a Novel Nd₁-xSrxMnO₃ Perovskite

Muhammad Usman<fnref n="1" />,<fnref n="2" /> ID https://orcid.org/0000-0002-3515-6764, Giovanni Palmisano<fnref n="1" />,<fnref n="2" /> ID https://orcid.org/0000-0001-9838-6252, Georgios Karanikolos<fnref n="3" /> ID https://orcid.org/0000-0001-5823-7076, Stéphane Abanades<fnref n="4" /> ID https://orcid.org/0000-0002-6689-3652, and Khalid Al-Ali<fnref n="1" />,<fnref n="2" />,∗ ID https://orcid.org/0000-0002-8076-0047

**Abstract.** A two-step thermochemical splitting process of doped and undoped neodymium-based manganite perovskites was investigated using thermogravimetric analysis. The samples were synthesized using a modified Pechini method and characterized by XRD, SEM and EDS analysis before redox testing. The results showed that 40% Sr-doped Nd₀.₆Sr₀.₄MnO₃ outperformed the other samples, achieving 103.7 µmol-O₂/g during thermal reduction and 189.2 µmol-CO/g during thermal oxidation, with a re-oxidation yield of 91.2%.

**Keywords:** Thermochemical conversion, CO₂ Splitting, Perovskites, Neodymium-manganite

## 1. Introduction

The global energy infrastructure is experiencing a significant transformation towards a more renewable and greener sustainable future[1], [2], [3]. Thermochemical energy storage (TCES) systems, when integrated with concentrated solar power (CSP) plants, have the potential to store surplus solar energy for continuous supply of power during nocturnal periods by actively storing the solar energy diurnally [4], [5], [6]. These storage systems offer the advantage of utilizing the full solar spectrum (unlike photovoltaics, which only capture part of it) along with sequestering significant amounts of CO₂ by converting it into solar fuel, such as CO in a cyclic manner so as to minimize the greenhouse gas imprint on the environment[7], [8]. A two-step thermochemical energy storage system works by leveraging the reversibility of a chemical reaction, where energy input—typically from high-temperature solar heat collected by a CSP plant—drives the reaction forward. The products of the reaction are stored separately which can readily be combined to release back the energy during the energy input interlude i.e., nocturnal period when sun is not available. This process is known as the charging and discharging cycle. Among various available TCES systems, the two-step thermochemical conversion of CO₂ via perovskite materials stands out due to its higher reported fuel yields compared to other oxygen-exchange materials, such as ceria and metal oxides.

Generally, perovskites are ABO₃-type materials, where A is a 12-fold coordinated alkali, alkaline earth, or Lanthanide metal cation, and B is a 6-fold coordinated transition metal cation. Partial doping can occur at both the A and B sites, resulting in a AA'BB'O₃ structure, where A'

<footnote n="1">¹ Khalifa University of Science and Technology, Chemical and Petroleum Engineering, Abu Dhabi, UAE</footnote>

<footnote n="2">² Research and Innovation on CO₂ and H₂ (RICH) Center, Khalifa University, Abu Dhabi, UAE</footnote>

<footnote n="3">³ University of Patras, Chemical Engineering, Patras, Greece</footnote>

<footnote n="4">⁴Laboratoire Procédés, Matériaux et Energie Solaire France</footnote>

<footnote>*Correspondence: Khalid Al-Ali, khalid.alali@ku.ac.ae</footnote>
and B' represent dopant cations. Therefore, these materials represent a diverse combination of cations with tunable properties, offering immense potential with solutions to current day energy storage challenges. Equation 1 shows the first reaction in a two-step thermochemical CO₂ splitting process, known as thermal reduction. In this step, the perovskite material (ABO₃) is subjected to high temperatures under relatively low oxygen partial pressures. As a result, the perovskite material thermally reduces to oxygen deficient form i.e., ABO₃-δ where δ represents oxygen non-stoichiometry. In the subsequent step, as shown in Equation 2, the reduced perovskite is re-oxidized at relatively lower temperatures in a CO₂ environment.

$$ABO_3 + \Delta H_{\text{reduction}} \xrightarrow{\text{low } p_{O_2}} ABO_{3-\delta} + \frac{\delta}{2}O_2(g) \quad (1)$$

$$ABO_{3-\delta} + \delta(H_2O/CO_2) \xrightarrow{\text{exothermic process}} ABO_3 + \delta(H_2/CO) \quad (2)$$

However, despite the higher fuel yields, several challenges, such as slow kinetics and material sintering, still require further research attention. This study focuses on investigating a novel perovskite composition, Nd₁-xSrₓMnO₃ (where x = 0, 0.2, 0.4, 0.6, 0.8), in a two-step thermochemical redox process for converting CO₂ into CO, using a thermogravimetric analyzer.

# 2. Experimental section

## 2.1 Chemicals

Chemicals used for the synthesis of perovskite powders, including Nd₂O₃ (99.9%), Sr(NO₃)₂ (99.999%), Mn(NO₃)₂·4H₂O & (98-102%) & ethylene glycol (99%), were purchased from Glentham Chemicals. Citric acid (CA) (99%) was obtained from Sigma-Aldrich.

## 2.2 Material Synthesis

The perovskite powders were synthesized using a modified Pechini method [1], [2]. Metal salt precursors were mixed with a chelating agent i.e., citric acid (CA) and ethylene glycol (EG) as complexing agent in the following molar ratio: CA:M:EG = 1.5:1:1. Typically, the metal precursors were dissolved in deionized water with a fixed mass ratio of M:H₂O = 1:6.

The mixture was continuously stirred for 10 minutes and heated to 60°C, after which EG was added. The mixture was then heated to 120°C until a viscous colored sol was obtained. This sol was subjected to polyesterification to form a polymeric resin by heating it at 250°C for 120 minutes. The resulting resin was crushed into powder using an agate mortar. The powder was air-calcined in a muffle furnace at 1400°C for 6 hours, with a ramp rate of 5°C/min. After calcination, the product was again crushed into powder form using an agate mortar and subjected to further characterization and testing. Table 1 outlines the sample formulations.

## 2.3 Characterization

Various physico-chemical characteristics of the perovskite powder, including its crystalline structure, chemical composition, and morphology, were obtained and compared. The room temperature powder XRD pattern was recorded using a PANalytical Empyrean XRD diffractometer with Cu Kα radiation (wavelength λ = 0.15418 nm), a tube current of 35 mA, and a potential of 45 kV, in the 2θ range of 20° to 80°. The PXRD data were compared with the ICDD-OCD database. A JEOL JSM-7610F scanning electron microscope was used for evaluating the sample morphology, while Energy dispersive X-ray (EDX) spectroscopy was employed to study the composition of the synthesized samples.
## 2.4 Redox testing

The reactivity of the air-calcined perovskite samples was studied using thermogravimetric analysis (TGA) with a SETARAM SETSYS apparatus. Approximately 100 mg of the sample was placed in a platinum crucible. The samples were subjected to the thermal reduction step in the TGA at 1400°C with a heating ramp of 20 °C/min under an inert argon gas flow of 0.020 NL/min. with 99.999% purity and less than 2 ppm O₂. The temperature was kept constant at 1400 °C for 45 minutes. Subsequently, in the oxidation step, the temperature was lowered to 1050 °C with a ramp of 20 °C/min, and the gas environment was switched to 50% CO₂ in Ar at a total flow of 0.020 NL/min for 60 minutes, completing the first cycle. During the second cycle, the temperature was raised from 1050 °C to 1400 °C under argon environment, and thermal reduction was carried out as before for 45 minutes. A similar re-oxidation step was carried out, lowering the temperature from 1400 °C to 1050 °C under CO₂/Ar environment. During all these redox processes, mass changes in the perovskite samples were recorded. The mass changes during the thermal reduction/oxidation process, caused by the release or uptake of oxygen, were quantified in terms of nO₂ and nCO. Here, nO₂ represents the amount of oxygen produced per gram of perovskite during thermal reduction step, while nCO represents the amount of CO produced per gram the sample during the thermal oxidation step. Equations (3) and (4) show how nO₂ and nCO can be calculated from the thermogravimetric data.

$$n_{O_2} = \frac{\Delta m_{loss \ during \ TR}}{m_{sample} \cdot M_{O_2}} \quad (3)$$

$$n_{CO} = \frac{\Delta m_{gain \ during \ TO}}{m_{sample} \cdot M_{O}} \quad (4)$$

$\Delta m$ represents the mass loss/gain during thermal reduction/oxidation, $m_{sample}$ is the initial mass of the sample placed in the platinum crucible, $M_{O_2}$ and $M_O$ are the molecular masses of O₂ and O, respectively. Since the kinetics of oxidation step in perovskite samples are reported very slow and require extended operation times, the material recovery is quantified using the re-oxidation yield, as shown in Equation 5. This re-oxidation yield measures the extent to which the oxygen released by the perovskite material during thermal reduction is re-absorbed, indicating the relative completion of the oxidation process.

$$re - oxidation \ yield (\%) = \frac{1}{2} \left( \frac{n_{CO}}{n_{O_2}} \right) \times 100\% \quad (5)$$

## 3. Results and discussion

The following sections cover the characterization of the samples prior to the redox studies in the TGA, as well as the thermochemical redox yield. These results are used to explore the aspect of doping Sr as an A-site dopant on Nd₁-xSrₓMnO₃.

<table><caption>Table 1. Composition of synthesized samples as confirmed by EDS analysis</caption><thead><tr><th>S. No.</th><th>Sample Composition (as synthesized)</th><th>Sample ID</th><th>Sample composition (from EDS analysis)</th><th>*Micro-strain (ε)</th><th>*Average crystallite size D (nm)</th><th>PDF-5+ Reference pattern</th><th>#Phase</th></tr></thead><tbody><tr><td>1</td><td>NdMnO<sub>3</sub></td><td>NMXX</td><td>Nd<sub>0.93</sub>Mn<sub>1.07</sub>O<sub>3</sub></td><td>0.0045</td><td>45.26</td><td>04-07-0114, 04-016-2207</td><td>M O (44.9%) + Mono (55.1%)</td></tr><tr><td>2</td><td>Nd<sub>0.8</sub>Sr<sub>0.2</sub>MnO<sub>3</sub></td><td>NSM82X</td><td>Nd<sub>0.8</sub>Sr<sub>0.2</sub>Mn<sub>1.09</sub>O<sub>3</sub></td><td>0.0021</td><td>60.35</td><td>04-008-4443</td><td>S O (100%)</td></tr><tr><td>3</td><td>Nd<sub>0.6</sub>Sr<sub>0.4</sub>MnO<sub>3</sub></td><td>NSM64X</td><td>Nd<sub>0.57</sub>Sr<sub>0.43</sub>Mn<sub>1.05</sub>O<sub>3</sub></td><td>0.0029</td><td>96.55</td><td>04-026-9068</td><td>S O (100%)</td></tr><tr><td>4</td><td>Nd<sub>0.4</sub>Sr<sub>0.6</sub>MnO<sub>3</sub></td><td>NSM46X</td><td>Nd<sub>0.40</sub>Sr<sub>0.6</sub>Mn<sub>1.08</sub>O<sub>3</sub></td><td>0.0021</td><td>37.14</td><td>04-012-0580</td><td>S T (100%)</td></tr><tr><td>5</td><td>Nd<sub>0.2</sub>Sr<sub>0.8</sub>MnO<sub>3</sub></td><td>NSM28X</td><td>Nd<sub>0.18</sub>Sr<sub>0.82</sub>Mn<sub>0.96</sub>O<sub>3</sub></td><td>0.0012</td><td>26.34</td><td>04-016-9100</td><td>S T (100%)</td></tr></tbody></table>

<footnote>*Micro strain & average crystallite size are estimated by Williamson-Hall (WH) plot using XRD data.</footnote>

<footnote>#M: Multiphase, S: Single phase, O: Orthorhombic, Mono: Monoclinic, T: Tetragonal</footnote>
## 3.1 Crystalline Structure & Morphology/Composition

Room-temperature powder XRD data in Figure 1(a) show the crystalline perovskite structure of Sr-doped NdMnO₃. Figure 1(b) indicates the peak shift in the X-ray diffractogram towards higher angles with increasing Sr doping in Nd₁-xSrxMnO₃. Takalkar et. Al., (2021) reported a similar peak shift in La₁-xSrxMnO₃ samples, where successive Sr doping caused the XRD peaks to shift towards higher angles [11]. However, various other literature i.e., Gao et al., (2023) observed a shift towards lower angles due to lattice expansion, as Sr⁺² (with a larger ionic radii) substitutes Ce⁺³ (with a relatively smaller ionic radii) in Ce₁-xSrxMnO₃ [12]. Considering both explanations, since the Shannon's ionic Radii of Sr⁺² is larger than that of La or Nd in a La₁-xSrxMnO₃/Nd₁-xSrxMnO₃ perovskites, the successive substitution of Sr⁺² to replace Nd⁺³ can be attributed in terms of microstrain within the lattice rather than lattice expansion. Rietveld refinement indicates that NdMnO₃ is multiphasic with 44.9% Orthorhombic and 55.1% monoclinic phase. Rest all samples are single phasic orthorhombic for 2θ and 40% Sr doping. With successive Sr doping to 60 and 80%, the perovskite structure adopts a higher symmetry tetragonal structure instead of orthorhombic. Table 1 outlines the microstrain (ε) and the average crystallite size (D, in nm) determined using Wiliamson-Hall plot with XRD data for each sample (Figure 3). Among the doped samples, NSM64X (with 40% Sr doping) shows the highest microstrain and crystallite size. The increased microstrain within the lattice could potentially leverage a relatively facile Mn-O bond. Additionally, NSM64X has the highest average crystallite size, and may facilitate oxygen vacancies and charge movement within the lattice due to its relatively smaller grain boundaries compared to other samples.

<figure><img src="image_2.png" /><figcaption>Figure 1. (a) XRD diffractogram of Nd₁-xSrxMnO₃ samples. (b) Zoomed-in XRD pattern in the 2θ range of 32.5°-34.0° to visualize peak shift.</figcaption></figure>
Figure 2(a-f) presents SEM images of NdMnO₃ and other doped samples. NdMnO₃ shows a smooth morphology with compactly structured large grains. However, yield data (Figure 4) show minimal redox activity for the undoped sample, consistent with findings from various literature sources on similar systems [11], [13], [14], [15], [16]. With successive Sr doping, the morphology becomes rough and porous, as shown in Figure 2(b-e). Figure 2(c) shows that the 40% Sr-doped sample (Nd₀.₆Sr₀.₄MnO₃) exhibits the highest degree of roughness and porosity compared to other samples. As evident from Figure 2(d-f), any Sr doping above 40% leads to material sintering, where grains start breaking down and the material starts agglomerating, resulting in reduced porosity. This ultimately leads to longer grain boundaries posing resistance to charge movement and lower yields. Hence, microstrain/average crystallite sizes shown in figure 3 closely correspond to the SEM images (figure 2). Figure 2(f) shows SEM image of an air-calcined perovskite particle. Table 1 enlists the EDS composition data of the air-calcined perovskite samples before redox testing, which fairly corresponds with the intended stoichiometry of the samples.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** SEM images of (a) NdMnO₃ (b) NSM82X (c) NSM64X (d) NSM46X (e) NSM28X (f) air-calcined perovskite particles</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Figure 3.** Microstrain and average crystallite size of Nd₁-xSrₓMnO₃ samples</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Redox activity and re-oxidation yield of Nd₁-xSrₓMnO₃ samples</figcaption></figure>
## 3.2 Thermochemical Redox Activity

Figure 4 shows the thermochemical redox yield and re-oxidation yield of the perovskite samples. The temporal thermogravimetric mass variation (%) with the temperature program for the 2-step thermochemical redox activity of Nd₁-xSrxMnO₃ is illustrated in Figure 5. This complements the hypothesis that introducing Sr as an A-site dopant facilitates thermal reduction, i.e., more O₂ evolves with successive Sr substitution during the thermal reduction step, as supported by the literature [17]. During the first cycle of operation, the amount of O₂ evolved during thermal reduction increases with Sr doping, from 20% to 80% doping. The CO yield however is maximum for NSM64X. In the second cycle, the redox yield shows that 40% Sr doping produces the maximum O₂ yield (103.7 µmol-O₂/g) and CO yield (189.2 µmol-CO/g) during thermal reduction and oxidation steps, outperforming the other samples. These higher values of yield correlate with the crystalline structure and morphological characteristics discussed in Section 3.1.

<figure><img src="image_6.png" /><figcaption>**Figure 5.** Thermogravimetric data of Nd₁-xSrxMnO₃ samples</figcaption></figure>

Data from Figure 4 shows that NSM64X has the highest re-oxidation yield in the second cycle, at 91.2%. This indicates that 40% Sr-doped Nd₁-xSrxMnO₃ has higher thermal stabilization such that the re-oxidation reaction has reached almost 91% of completion, demonstrating superior oxidation kinetics for NSM64X. The yield and re-oxidation data for the 60% Sr-doped sample (NSM46X) show close proximity to the NSM64X, though slightly lower in yield and re-oxidation values. The thermal reduction O₂ yield of NSM46X is 101.7 µmol-O₂/g, while its thermal re-oxidation CO yield is 184.0 µmol-CO/g, with a re-oxidation yield of 90.5% during the second cycle of operation.

TGA data from Figure 5 show that during the first cycle of operation, undoped NdMnO₃ undergoes mass variations in two distinct steps around 950 °C. Typically, these changes could be attributed to the decomposition of the SrCO₃ phase, but since this sample is undoped, it can be inferred that NdMnO₃ decomposed into additional phases. In the second cycle, the re-oxidation yield of NdMnO₃ increases from 5.3% to 73.7%, which shows thermal stabilization of the material to some extent. However, the poor O₂ and CO yields suggest that NdMnO₃ is not suitable for potential application in 2-step thermochemical CO₂ splitting.
## 4. Conclusions

A series of neodymium-based perovskites were developed using the modified Pechini method, and their physico-chemical properties were characterized by XRD, EDS, and SEM. The activity of the perovskites was evaluated using thermogravimetric studies in a 2-step thermochemical process to calculate their redox and re-oxidation yields. The data revealed that the 40% Sr-doped sample (i.e., Nd<sub>0.6</sub>Sr<sub>0.4</sub>MnO<sub>3</sub>) outperformed the other samples in terms of redox and reoxidation yields, with the 60% Sr-doped sample (Nd<sub>0.4</sub>Sr<sub>0.6</sub>MnO<sub>3</sub>) being the next most promising candidate. Therefore, various compositions in the 40-60% Sr doping range should be tested experimentally to find the optimal Sr concentration. Additionally, the hypothesis of increased microstrain in Nd<sub>0.6</sub>Sr<sub>0.4</sub>MnO<sub>3</sub>, based on the Williamson-Hall plot, should be further elucidated using XPS studies to assess any changes in the oxidation state of the transition metal cation (Mn<sup>3+</sup>) for better understanding.

## Author contributions

Conceptualization: K.A., Experimental work: M.U., S.A.; Writing original-draft preparation: M.U., K.A.; Writing-Editing & Review & supervision: K.A., G.P., G.K.; All authors discussed the results and contributed to the final manuscript.

## Competing interests

The authors declare that they have no competing interests.

## Funding

This project was financially supported by Khalifa University of Science and Technology under grant number RC2-2019-007, which is attributed to the Research and Innovation Center on CO<sub>2</sub> and Hydrogen (RICH).

## Acknowledgement

The authors gratefully acknowledge the lab facilities at the RICH Center and Khalifa University Core Labs.

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