Ru-promoted perovskites as effective redox catalysts for CO₂ splitting and methane partial oxidation in a cyclic redox scheme

Abstract

The current study reports A_xA′_1−xB_yB′_1−yO_3−δ perovskite redox catalysts (RCs) for CO₂-splitting and methane partial oxidation (POx) in a cyclic redox scheme. Strontium (Sr) and iron (Fe) were chosen as A and B site elements with A′ being lanthanum (La), samarium (Sm) or yttrium (Y), and B′ being manganese (Mn) or titanium (Ti) to tailor their equilibrium oxygen partial pressures (P_O2s) for CO₂-splitting and methane partial oxidation. DFT calculations were performed for predictive optimization of the oxide materials whereas experimental investigation confirmed the DFT-predicted redox performance. The redox kinetics of the RCs improved significantly by 1 wt% ruthenium (Ru) impregnation without affecting their redox thermodynamics. Ru-impregnated LaFe_0.375Mn_0.625O₃ (A = 0, A′ = La, B = Fe, and B′ = Mn) was the most promising RC in terms of its superior redox performance (CH₄/CO₂ conversion >90% and CO selectivity ∼95%) at 800 °C. Long-term redox testing over Ru-impregnated LaFe_0.375Mn_0.625O₃ indicated a stable performance during the first 30 cycles followed by an ∼25% decrease in the activity during the last 70 cycles. Air treatment was effective to reactivate the redox catalyst. Detailed characterizations revealed the underlying mechanism of the redox catalyst deactivation and reactivation. This study not only validated a DFT-guided mixed oxide design strategy for CO₂ utilization but also provides potentially effective approaches to enhance redox kinetics and long-term redox catalyst performance.

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1. Introduction

Carbon dioxide (CO₂) levels in the Earth's atmosphere are growing at an alarming rate. Meanwhile, the total anthropogenic CO₂ emissions in 2019 have already surpassed the Paris Accord's 2040 target by a factor of 2.¹ As such, cost-effective CO₂ capture and utilization technologies are highly desirable.^2,3 Converting CO₂ to carbon monoxide (CO) is advantageous for a wide range of applications including the production of esters, alcohols, and acids.⁴ However, breaking the C=O bond in CO₂ is highly energy-intensive.⁵ Electrochemical and photochemical reductions of CO₂ have been explored but these methods still need significant improvements to address low CO₂ conversion, slow electron transfer rates, low product selectivity, and/or low photon efficiency.^6–9 Thermochemical conversion of CO₂ to CO represents another promising alternative, in which an oxygen carrier, also known as a redox catalyst (RC), is first thermally decomposed. CO is then produced by exposing the (partially) reduced RC to CO₂ whereupon the RC is re-oxidized by abstracting an oxygen atom from the CO₂ molecule.^10–17 A key challenge involved in thermochemical methods is that the decomposition of the RC requires a very high temperature (≥1100 °C),^10,18 which can be lowered to ≤950 °C by introducing reducing agents such as hydrogen (H₂) and methane (CH₄) to facilitate CO₂ splitting in an open-loop hybrid redox process (HRP).^19–24 Using CH₄ is attractive since inherently separated CO and syngas with a H₂/CO ratio of ∼2 can be obtained via the HRP (Fig. 1). In this two-step process, the lattice oxygen released from the RC (represented as ABO_3−δ1) during CH₄ exposure produces syngas via the partial oxidation of CH₄ (POx) as indicated by R1 in Table 1. Subsequently, CO is produced by exposing the (partially) reduced RC (ABO_3−δ2) to CO₂ in the second step (R2), in which the RC is re-oxidized by abstracting an oxygen atom from the CO₂ molecule, thereby completing the redox loop. As such, syngas from the POx step is suitable for the synthesis of methanol and liquid fuels, whereas CO from the splitting step can be used separately as a feedstock for chemical production, without the need for complex syngas conditioning and separation steps.^20,23

Fig. 1 A simplified schematic of the hybrid redox process.

Table 1 List of key reactions during the methane POx and CO₂-splitting steps (R1 and R2) as well as side reactions in the methane POx step (R3 and R4)

Partial oxidation: R1	CH4 + 1/(δ2 − δ1)ABO3−δ1 ⇋ 2H2 + CO + 1/(δ2 − δ1)ABO3−δ2
CO2 splitting: R2	CO2 + 1/(δ2 − δ1)ABO3−δ2 ⇋ CO + 1/(δ2 − δ1)ABO3−δ1
H2 combustion: R3	2H2 + 1/(δ2 − δ1)ABO3−δ1 ⇋ 2H2O + 1/(δ2 − δ1)ABO3−δ2
CO combustion: R4	2CO + 1/(δ2 − δ1)ABO3−δ1 ⇋ 2CO2 + 1/(δ2 − δ1)ABO3−δ2

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Iron/iron oxide redox pairs are commonly used as the redox catalyst.^25–33 However, studies have shown that they generally lead to low syngas selectivity and CO₂ conversion.^25,34,35 On the other hand, mixing Fe with other metal oxides has been shown to improve redox properties.^{22,24,36–39} For example, it has been reported that CeO₂/Fe₂O₃ ⁴⁰ and Al₂O₃/Fe₂O₃ ⁴¹ mixed oxides favour the partial oxidation of CH₄ to syngas in a chemical looping dry reforming scheme (CLDRS).^42–53 In addition, studies have also shown that promoting Fe-containing materials with other metals such as Ni, Cu, Mn, and Co is an effective approach to acquiring high syngas yields.^54–56 Although tremendous efforts have been made in developing RCs, their optimization still largely relies on heuristics and trial-and-error. Meanwhile, the emergence of mixed oxides has expanded the design space for RCs^28,57–59 thus making it impractical to screen out the promising RCs given the infinitely many possible compositions of mixed oxides. Therefore, effective computational tools are highly desirable to narrow down the design space for RC development and optimization.

The current study investigates the redox performance and long-term stability of perovskite-structured RCs in the context of the HRP. The perovskite materials were chosen due to their unique flexibility in terms of composition, structure and mixed ionic and electronic conductivities.⁶⁰ This leads to highly tunable redox thermodynamic and kinetic properties, which are critical for the proposed reactions (R1 and R2).

The size and electronic configurations of the A- and B-site atoms have a great influence on the structure, reducibility, and ionic and electronic conductivities of the mixed oxides.^24,61 Based on the DFT screening, four different perovskite oxides, i.e. Sr_0.625La_0.375Fe_0.625Ti_0.375O_3−δ (SLFT), Sr_0.5Sm_0.5Fe_0.625Ti_0.375O_3−δ (SSFT), Sr_0.625Y_0.375Fe_0.5Ti_0.5O_3−δ (SYFT), and LaFe_0.375Mn_0.625O_3−δ (LFM) were selected. Impregnation with 1 wt% Ru was found to be highly effective in enhancing the redox kinetics for most of these oxide RCs. Ru-LFM, which exhibited the best overall performance, was further characterized and tested for long-term stability. Temperature-programmed reduction/oxidation (TPR/TPO), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) were performed to determine the phase, stability, redox kinetics, and the deactivation/reactivation mechanisms.

2. Experimental and computational details

2.1. Computational methods

Optimal equilibrium oxygen partial pressures (P_O2s) for the desirable redox catalysts were calculated via a Gibbs free energy minimization approach reported previously.^22,55 We have also reported DFT calculations to screen A_xA′_1−xB_yB′_1−yO_3−δ oxides in the context of chemical looping air separation and CO₂-splitting.²⁴ Briefly, first-principles simulations were performed at the DFT level implemented by the Vienna ab initio simulation package (VASP) with the frozen-core all-electron projector augmented wave (PAW) model and Perdew–Burke–Ernzerhof (PBE) functional. Kinetic energy cut-off was 450 eV with the convergence criteria of 0.01 eV Å⁻¹ (force) and 105 eV (energy). Gaussian smearing of 0.1 eV was applied for optimizations. Gamma k-point was used for the 2 × 2 × 2 A_xA′_1−xB_yB′_1−yO_3−δ perovskite supercells. Each supercell would contain 8 × (5–δ) or 40–8δ atoms. The GGA + U approach with U_eff = 4, 3.9, and 3 was used for Fe, Mn, and Ti. For efficient computation, only FM phase magnetic ordering was applied for all the impregnated structures, given that magnetic ordering has a relatively small influence on oxygen vacancy formation and migration.

2.2. Redox catalyst synthesis

The RCs were synthesized by the modified Pechini method.⁶² Nitrate precursors of the metal cations were used for Sr, Sm, Fe, Mn, La, and Y. Titanium butoxide was used as a precursor for Ti. Firstly, the nitrate precursors were mixed in a beaker to the desired stoichiometric ratio with citric acid at a molar ratio to the metal ions of 2.5 : 1. Water was then added, and the resulting solution was stirred using a magnetic stirrer at 300 rpm for 30 minutes at 40 °C. For titanium butoxide, ethanol was added at a 3 : 1 weight ratio to the titanium precursor. This was followed by the addition of ethylene glycol as the chelating agent at a 1.5 : 1 molar ratio to citric acid. This solution was then heated at 80 °C to form a viscous paste, which was then heated overnight at 120 °C. The resulting dried solid was calcined at 900 °C for 10 hours under an oxidative environment. Finally, the desired perovskite was formed and subjected to further characterizations or cyclic testing. 1 wt% of ruthenium (Ru) was also impregnated over the perovskite oxides (SLFT, SSFT, SYFT, and LFM) via the wet incipient method. The impregnated sample was then calcined at 900 °C for 10 hours under an oxidative environment.

2.3. Cyclic redox tests

For cyclic redox testing, the calcined perovskites were crushed and sieved to obtain a 250–450 μm range of particles. Approximately 0.4 g of the resulting particles was loaded into a quartz U-tube (4 mm inner diameter), which was placed inside an electric furnace. Connected to the U-tube within the electric furnace were two gas lines. The inlet was configured to have gases introduced from a panel controlled by a computer for automated cyclic valve switching. The outlet contained product gases from the redox cycles which were then delivered to a quadrupole mass spectrometer (Cirrus 2, MKS) for analysis. Samples in the U-tube were pre-treated by exposing them to 30 sccm of H₂ + CO₂ + Ar mixture at 900 °C. The redox performance of the RCs was assessed by exposing the pre-treated samples to 5 redox cycles of CH₄/CO₂ isothermally at 800, 850, 900, and 950 °C, respectively. A fixed flow rate of argon (22.5 sccm) was used throughout the initial temperature ramp to the isothermal temperature (during cyclic redox testing). The redox cycle consisted of a 3 min injection of both CH₄ and CO₂ at 2.5 sccm during the CH₄-POx and CO₂-splitting steps, respectively. Between the two steps, there was a 2-minute argon purge to remove any leftover gases from each step. For long-term testing, 1.5 sccm was used for CH₄ and CO₂.

2.4. Characterization of the redox catalyst

Temperature-programmed reduction/oxidation (TPR/TPO) experiments were performed in a thermogravimetric analyzer (TGA) to study the reduction/oxidation behaviour and CH₄/CO₂ activation. For this type of testing, 10 mg of the sample was loaded into a small crucible within the TGA setup. During the TPR experiments, samples were exposed to 200 sccm of 5 vol% H₂ or CH₄ (balance argon) by simultaneously increasing the temperature from 25 °C to 1000 °C at various heating rates (19, 23, and 27 °C min⁻¹). A similar procedure was used for TPO experiments but with 5 vol% CO₂. Temperature-programmed oxidation of the post-100 cycles RC was also performed in a packed bed by exposing it to 5 vol% oxygen and increasing the temperature from 25 °C to 900 °C at 10 °C min⁻¹. The post-100 cycles RC was also treated in the air for 1 hour at 900 °C for its reactivation. X-ray diffraction (XRD) was performed to identify the phases within the as-prepared and spent forms of the catalysts. The XRD was performed on a Rigaku SmartLab X-ray diffractometer with Cu Kα radiation at 40 kV and 44 mA. The characterization was done via a step size method, having a residence time of 2 seconds, and varying the 2θ angle by 0.05° increments from 20° to 80°. High-resolution transmission electron microscopy (TEM) images were also obtained using a field-emission scanning transmission electron microscope (STEM) (JEOL 2010 F) operating at 200 kV. To determine the surface elemental composition of the samples, X-ray photoelectron spectroscopy (XPS) was used. The Kratos Analytical Axis Ultra (monochromatic Al Kα) was operated at 10 mA and 15 kV. The results from XPS were analyzed by CasaXPS software, with an adventitious peak of carbon at 284.8 eV.

3. Results and discussion

3.1. Screening of perovskite oxide redox catalysts

The selection of RC is critical to the HRP since it serves as an oxygen source (POx step) and sink (CO₂-splitting step).²² From a thermodynamic standpoint, a redox pair (ABO_3−δ1/ABO_3−δ2) with a high equilibrium partial pressure of oxygen (P_O2) can lead to the over-oxidation of syngas (see R3 and R4 in Table 1) whereas a low P_O2, although favorable for CO₂ splitting, will result in low CH₄ conversions.^22,24Fig. 2a summarizes the optimal P_O2 region for the partial oxidation of methane to syngas and Fig. 2b illustrates the relationship between P_O2 and CO₂ conversion at different operating temperatures.

Fig. 2 Thermodynamic analysis of (a) methane POx and (b) CO₂-splitting reactions as a function of P_O2 and temperature.

Our previous studies indicate that a P_O2 value between 10⁻²¹ and 10⁻¹⁷ atm is optimal for the efficient POx and CO₂-splitting steps within a temperature range of 750 to 900 °C.^22,24 As can be seen from Fig. 2a, the equilibrium P_O2 for the Fe/FeO redox pair, a frequently investigated RC, is notably higher than this high performance region. Therefore, fine-tuning the redox properties of the oxide materials is necessary to obtain an optimal P_O2. In a recent study, we performed high-throughput DFT calculations to investigate the oxygen chemical potential of 2401 A_xA′_1−xB_yB′_1−yO_3−δ perovskites at 950 °C by calculating the Gibbs free energy associated with oxygen uptake/release between discrete oxygen non-stoichiometric levels (δs, see Fig. 2).²⁴ The dopants considered include Ca, K, Y, Ba, La, and Sm for the A-site and Co, Cu, Mn, Mg, Ni, and Ti for the B-site. The current study focuses on several DFT-predicted compositions that are suitable for HRP application. The specific compositions include Sr_0.625La_0.375Fe_0.625Ti_0.375O_3−δ (SLFT), Sr_0.5Sm_0.5Fe_0.625Ti_0.375O_3−δ (SSFT), Sr_0.625Y_0.375Fe_0.5Ti_0.5O_3−δ (SYFT), and LaFe_0.375Mn_0.625O_3−δ (LFM). As shown in Fig. 2a, DFT calculations covered δ ranges with a relatively low resolution due to the limited supercell size for practical calculations. Generally, equilibrium P_O2 monotonically decreases with increasing δ. As such, all four materials of interest cover the optimal P_O2 region within a viable oxygen non-stoichiometry range for the perovskite oxides, i.e., the discrete data points predicted by DFT cover the entire span of the desirable P_O2 region for all four materials. As shown in Fig. 2b, the corresponding P_O2 values of the RCs at various δ ranges also favour high CO₂ conversions (see the ESI† for calculation details). The following sections focus on the experimental evaluation and characterization of these redox catalysts.

3.2. Phase characterization and redox performance of oxygen carriers

XRD patterns of the as-synthesized A_xA′_1−xB_yB′_1−yO_3−δ RCs are shown in Fig. 3. An orthorhombus-structured perovskite phase was identified for all the A_xA′_1−xB_yB′_1−yO_3−δ compositions, expect for SYFT, which contained a YFeO₃ phase due to the difficulty in incorporating yttrium into the parent SrFeO₃ perovskite structure.⁶³ The redox performance of the as-synthesized RCs is shown in Fig. 4a. As can be seen, CH₄ conversion varied between 38 and ∼97% whereas CO selectivity remained above 80% at 950 °C during the CH₄-POx step. This confirms that the DFT-based materials screening resulted in RCs with suitable P_O2 levels for CH₄ partial oxidation without over-oxidizing syngas products. CO₂ conversion also varied between 58 and ∼98% at 950 °C. It may be noted, from Fig. 2a, that both CH₄ and CO₂ conversions can be higher than 90% at 950 °C from a thermodynamic standpoint. The lower experimental CH₄ and CO₂ conversions (Fig. 4a) are likely to be due to the slow kinetics of the methane POx step, which can also affect the subsequent CO₂-splitting step. To confirm this, the RCs were impregnated with 1 wt% ruthenium (Ru), which was shown to be active for methane activation.^64–66 The redox performance of Ru-impregnated RCs is shown in Fig. 4c. A significant increase in CH₄ and CO₂ conversions was observed for all the samples. In particular, near 100% CH₄/CO₂ conversions and CO selectivity were observed for Ru-impregnated SSFT and LFM at 950 °C. The effect of Ru was less significant on SLFT, possibly due to the strong metal support interactions. The improvement in the redox performance over Ru-impregnated RCs further confirms that the DFT calculations were effective in predicting the redox thermodynamics of the mixed oxides. Given the improved redox kinetics of Ru-impregnated RCs, we also investigated the redox performance at low temperatures (800–900 °C). The redox performance of the as-synthesized impregnated/non-impregnated RCs at low temperatures is also shown in Fig. 4(a and c). As can be seen, CO selectivity was above 80% in general and above 90% at higher temperatures during the CH₄-Pox step. However, relatively low CO selectivity was observed for non-impregnated RCs at 800 °C.

Fig. 3 XRD patterns of the as-synthesized A_xA′_1−xB_yB′_1−yO_3−δ (PDF for YFeO₃: 01-086-8583; PDF for all others: 04-007-9930).

Fig. 4 Redox performance of as-prepared (a) and (b) and Ru-impregnated (c) and (d) Sr_xA_1−xFe_yB_1−yO_3−δ at 750, 800, 850, and 900 °C; CH₄ and CO₂ conversions, and CO selectivity (a) and (c), and H₂/CO ratio (b) and (d).

On the other hand, CH₄ and CO₂ conversions varied from 5 to 97% and 20 to ∼100% for non-impregnated and impregnated RCs, respectively. Meanwhile, the H₂/CO ratio was near 2 for all the RCs (impregnated and non-impregnated) except at selected conditions for SYFT and SLFT, as shown in Fig. 2(b and d). The H₂/CO ratio was much lower than 2 for SLFT and Ru-SLFT at 800 °C. This may indicate the formation of SrCO₃ in the CO₂-splitting step and subsequent “dry reforming” of the carbonate by methane. On the other hand, the H₂/CO ratio for Ru-SYFT varied between 2 and 3, indicating that a fraction of CH₄ was thermally cracked, probably due to the phase segregation indicated in the XRD spectra of SYFT (Fig. 3). Out of all the Ru-doped RCs, Ru-LFM was the most interesting: CH₄ and CO₂ conversions were improved by up to more than 10-fold at 800 °C. Due to its excellent redox performance and coke-resistant nature at 800 °C, Ru-LFM was selected for further investigation and characterization.

3.3. Characterization of 1 wt% Ru-LFM

Temperature-programmed reduction/oxidation experiments were conducted to compare the redox kinetics of impregnated and non-impregnated LFM. The shifts of the DTG peaks to significantly lower temperatures during CH₄-TPR and CO₂-TPO (Fig. 5) confirmed the ease of CH₄/CO₂ activation over Ru-LFM compared to that over LFM. The activation energies (E_a), which were calculated using the Kissinger method,⁶⁷ also decreased from 149 and 201 kJ mol⁻¹ to 41 and 78 kJ mol⁻¹ for CH₄ POx ad CO₂-splitting, respectively (Fig. S1 and S2 in the ESI†). On the other hand, the activation energies during the H₂ reduction largely remained unchanged for LFM (76 kJ mol⁻¹) and Ru-LFM (83 kJ mol⁻¹). This confirms the role of Ru in accelerating the kinetics for CH₄ and CO₂ conversions and that the redox thermodynamics of the original perovskite would not be affected by Ru impregnation. Meanwhile, XRD patterns of the impregnated and non-impregnated LFM (Fig. S3†) are nearly identical. This confirms that (a) Ru was not incorporated into the original perovskite phase; and (b) the absence of Ru-containing phases further confirmed its high dispersion on the surface. Further XRD analysis of the post-CH₄ and post-CO₂ Ru-LFM samples (Fig. 6) indicated that no additional phases were present. Small shifts in 2θ angles can be observed between the main peaks of the post-CH₄ step sample and post-CO₂ treated sample. This corresponds to the lattice oxygen release and uptake of Ru-LFM, while maintaining its perovskite structure. Under our typical operating conditions, lattice oxygen corresponding to 0.4–0.6 wt% of the sample is reversibly removed in each cycle, although the maximum oxygen capacity corresponds to ∼6 wt% of the sample weight (Fig. S4†).

Fig. 5 DTG peaks of Ru-LFM during H₂/CH₄-TPR and CO₂-TPO.

Fig. 6 XRD spectra of post-CH₄ and post-CO₂ Ru-LFM samples after 5 redox cycles.

3.4. Long-term stability test over Ru-LFM

Ru-LFM was exposed to 100 redox cycles to evaluate its long-term stability. The results of the redox experiments (Fig. 7) indicated that both CH₄ and CO₂ conversions were ≥85% up to cycle 30. However, ∼25% performance loss was observed between the 31st and 100th cycles. Meanwhile, the H₂/CO ratio (Fig. S5†) and CO selectivity remained at ∼2 and 95%, respectively. To reveal the reason for deactivation, XRD analysis of the post-100 cycles Ru-LFM sample was performed, which indicated the presence of a Mn₂O₃ phase (Fig. 8a). This suggests that there was some phase segregation that may have caused a slight deactivation. EDS mapping (Fig. 9) of the post-100 cycles Ru-LFM sample also confirmed such phase segregation.

Fig. 7 CH₄/CO₂ conversions and CO selectivity for long-term tests with Ru-LFM.

Fig. 8 (a) XRD spectra of fresh, post-100 cycles, and post-O₂ TPO Ru-LFM samples and (b) gas profiles during O₂-TPO.

Fig. 9 EDS mappings of fresh, post-100 cycles, and air-treated Ru-LFM samples.

Our recent study showed that the activity of a LaNi_0.5Fe_0.5O₃ based RC can be recovered with periodic air treatment,²⁰ which removes the deposited carbon. We attempted a similar strategy to reactivate the post-100 cycles Ru-LMF sample. Unlike LaNi_0.5Fe_0.5O₃, which was deactivated by carbon deposition, the product gas analysis from the packed bed during O₂-TPO indicated an oxygen uptake but with trace amounts of CO_x (Fig. 8b). This indicates that Ru-LFM was highly coke resistant and corroborated the 2 : 1 H₂/CO ratio we observed throughout the experiments. On the other hand, the XRD analysis of the fresh, post-100 cycles, and oxidized Ru-LFM samples in Fig. 8a indicated that the air oxidation step helped Mn₂O₃ to reincorporate into the mixed oxide phase, since no Mn₂O₃ peak was observed for the post-oxidation sample. Meanwhile, EDS mapping (Fig. S6†) of the post-O₂ TPO sample indicated that the distribution of metals (Fe, Mn, and La), although being more homogeneous than the post-100 cycles sample, still exhibited some degree of segregation. To confirm the effectiveness of air treatment in redistributing the segregated Mn, we further oxidized the post-100 cycles sample in the air for 60 minutes at 900 °C. EDS mapping of the air-treated sample (Fig. 9) indicated that the longer treatment with air was effective in promoting the homogeneous distribution of the cations, thereby restoring the initial activity of Ru-LFM. To further clarify, 20 additional cycles were performed. As can be seen from Fig. 7, the redox performance of the air-treated sample improved significantly where CH₄ and CO₂ conversions were ≥80% for the 20 additional cycles. This indicates that deep oxidation in air is an effective approach to reverse the deactivation of the redox catalyst. XPS analysis was also performed on the Ru-LFM samples (Fig. 10). Compared to the as-prepared sample, the post-100 cycles sample exhibited peak shifts of ∼0.42 eV for Mn 2p_3/2 and ∼1.29 eV for Fe 2p_3/2, both of which were towards the lower binding energy. After exposing the fresh redox catalyst to CH₄, the average oxidation state of the near-surface Fe decreased from approximately +3.4 to +2.1 and the oxidation state of Mn decreased from +3.3 to +2.6, based on peak fitting. After ending 100 cycles in the CO₂ step, the oxidation state of Fe was around +2.5 and Mn was approximately +2.7. In addition, both Mn⁴⁺ and Fe⁴⁺ shown in the fresh catalyst were not observed in the post-100 cycles sample. These observations indicated that the perovskite structure was not fully re-oxidized under the CO₂ environment in the oxidation step. This is consistent with the DFT calculations, which indicated that the perovskite oxides would operate under certain oxygen vacancy ranges. The oxidation state changes for the near-surface B-site cations also correspond well to the redox reactions performed under HRP. Meanwhile, extended operation under oxygen-deficient conditions may have led to the phase segregation of Mn₂O₃. The XPS results also corroborate the earlier finding in a way that the air treatment would likely reactivate the catalyst because air would re-oxidize both Fe and Mn to higher oxidation states than CO₂ re-oxidation alone. This would promote the formation of the original LFM phase by reincorporating the segregated Mn₂O₃.

Fig. 10 XPS analysis of fresh, post-CH₄, and post-100 cycles Ru-LFM samples: (a) manganese^68–70 and (b) iron.^71–75

It is noted that the redox experiments were conducted within a temperature range of 800–950 °C. This temperature range is comparable to typical methane catalytic reforming conditions. The high CO₂ conversions, 85–100% for some of the RCs such as Ru-LFM reported in this study, compares favourably with other recent studies, such as membrane-based electroreduction, photoreduction and, thermochemical CO₂ conversion to CO^76,77 as well as the typical CO₂ splitting methods, such as thermochemical CO₂ splitting and dry reforming of methane.^25,48

4. Conclusion

In this study, perovskite-structured redox catalysts (RCs) with the general formula A_xA′_1−xB_yB′_1−yO_3−δ (A/A′ = Sr, La, Y, Sm, Y; B/B′ = Fe, Mn, Ti) were investigated for CO₂-splitting and methane partial oxidation (POx) in a hybrid redox scheme. The specific oxide compositions, i.e., Sr_0.625La_0.375Fe_0.625Ti_0.375O_3−δ (SLFT), Sr_0.5Sm_0.5Fe_0.625Ti_0.375O_3−δ (SSFT), Sr_0.625Y_0.375Fe_0.5Ti_0.5O_3−δ (SYFT), and LaFe_0.375Mn_0.625O_3−δ (LFM), were selected on DFT-based materials screening, which identified perovskite oxides with optimal equilibrium oxygen partial pressures (P_O2s) for CO₂-splitting and methane partial oxidation. The experimental investigations confirmed the redox properties of the RCs for the proposed reactions. In addition, impregnation of the RCs with 1 wt% Ru was found to be highly effective in enhancing the reaction kinetics without altering the intrinsic redox thermodynamics of the oxide materials. For instance, impregnating 1 wt% Ru in LFM decreased the activation energies for the CH₄-POx and CO₂-splitting reactions by ∼73% and 62%, respectively, when compared to the as-prepared LFM. XRD and TEM analysis indicated uniform elemental distribution for the as-prepared LFM and satisfactory Ru dispersion on the surface. Long-term redox testing over Ru-LFM indicated a stable performance for 30 cycles, followed by a ∼25% decrease in the activity between the 31st and 100th cycles. Further characterization indicated that their deactivation resulted from the segregation of Mn, in the form of Mn₂O₃, from the LFM perovskite phase. Air treatment was shown to effectively promote the reincorporation of Mn into the perovskite phase, thereby reversing the deactivation of the redox catalyst. The DFT-guided mixed oxide design strategy, which was experimentally validated in this study, can be highly effective in further optimization of the redox catalysts for CO₂ utilization. This study also reports effective approaches to enhance redox kinetics and long-term redox catalyst performance.

Author contributions

Sherafghan Iftikhar: methodology, redox catalyst synthesis, data collection and analysis, and writing – original draft and revision; William Martin: catalyst synthesis and data collection and analysis; Xijun Wang: DFT calculations and data analysis; Junchen Liu: data collection and analysis; Yunfei Gao: data analysis; Fanxing Li: supervision, conceptualization, methodology, and writing – revision and editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Science Foundation (CBET – 1923468), the U.S. Department of Energy (Award DE-FE0031703), and the North Carolina State University Kenan Institute for Engineering, Technology and Science. The characterization work was performed in part at the Duke University Shared Materials Instrumentation Facility (SMIF). It was also performed in part at the Analytical Instrumentation Facility (AIF) at North Carolina State University, which is supported by the State of North Carolina and the National Science Foundation (award number ECCS-2025064). Both the SMIF and AIF are members of the North Carolina Research Triangle Nanotechnology Network (RTNN), a site in the National Nanotechnology Coordinated Infrastructure (NNCI).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nr04437d

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