Efficient CO₂ conversion to CO using chemical looping over Co–In oxide

Abstract

CO₂ conversion to CO by reverse water-gas shift using chemical looping (RWGS-CL) can be conducted at lower temperatures (ca. 723–823 K) than the conventional catalytic RWGS (>973 K), and has been attracting attention as an efficient process for CO production from CO₂. In this study, Co–In₂O₃ was developed as an oxygen storage material (OSM) that can realize an efficient RWGS-CL process. Co–In₂O₃ showed a high CO₂ splitting rate in the mid-temperature range (723–823 K) compared with previously reported materials and had high durability through redox cycles. Importantly, the maximum CO₂ conversion in the CO₂ splitting step (ca. 80%) was much higher than the equilibrium conversion of catalytic RWGS in the mid-temperature range, indicating that Co–In₂O₃ is a suitable OSM for the RWGS-CL process.

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Limiting anthropogenic global warming to a specific level requires reaching at least net zero CO₂ emissions.^1,2 For this purpose, syntheses of liquid fuels using renewable electricity and captured CO₂, which are, respectively, designated as Sun-to-Fuel (STF) and Power-to-Liquid (PtL),³ have been proposed as technologies to replace fossil fuels by carbon-neutral energy resources. The CO₂-based fuels produced by PtL processes can supply renewable energy to the transportation sector.^4,5 One feasible route for PtL processing from captured CO₂ is the Fischer–Tropsch (FT) method.⁶ Nevertheless, technologies for direct synthesis from CO₂ are currently in an early stage. Consequently, in the near future, CO converted from captured CO₂ might be a useful intermediate for PtL fuel processing. Reverse water-gas shift (RWGS) is a representative method of converting CO₂ to CO. It is an endothermic equilibrium-limited reaction.⁷

CO₂ + H₂ ↔ CO + H₂O ΔH⁰₂₉₈ = 42.1 kJ mol⁻¹ (1)

Mallapragada et al. estimated that the route consisting of RWGS followed by FT synthesis is a more efficient route for STF than other routes such as biomass gasification, algae-derived oils, and direct photosynthesis.⁸ However, conventional catalytic RWGS requires high reaction temperatures and separation of gaseous products to achieve high conversion and a suitable H₂/CO ratio syngas. Moreover, side reactions (eqn (2) and (3)) also occur, which complicate the separation of the outlet gas and reduce the process efficiency.⁷

CO + 3H₂ ↔ CH₄ + H₂O ΔH⁰₂₉₈ = −206.5 kJ mol⁻¹ (2)

CO₂ + 4H₂ ↔ CH₄ + 2H₂O ΔH⁰₂₉₈ = −165.0 kJ mol⁻¹ (3)

Reverse water-gas shift via chemical looping (RWGS-CL) can overcome the shortcomings of conventional catalytic processing.^9–12 In RWGS-CL, CO₂ is converted to CO in a separated redox reaction on oxygen storage materials (OSMs), as shown in eqn (4) and (5).

Reduction of OSM: MO_x + δH₂ ↔ MO_x−δ + δH₂O (4)

Oxidation of OSM: MO_x−δ + δCO₂ ↔ MO_x + δCO (5)

In this process, CH₄ formation is avoided because CO/CO₂ and H₂/H₂O flows are separated. Therefore, RWGS-CL processes present benefits of driving the reaction toward CO production and of simplifying gas separation. A study of solar-to-syngas process efficiency revealed that RWGS-CL processing can reduce the energy demand for separation by 77% compared to conventional catalytic RWGS processing.⁹ The redox performance of the OSM is an important factor that makes this process feasible. Recently reported results have shown that Cu-modified In₂O₃ (Cu–In₂O₃) exhibits a higher CO₂ splitting rate¹² than earlier reported OSMs such as perovskite-type oxides^11,13–19 and Fe-based oxides^20–26 at low temperatures (673–823 K). Actually, RWGS-CL on Cu–In₂O₃ proceeds by the redox of In(0) ⇆ In(III) with the formation and decomposition of a Cu–In alloy. The high CO₂ splitting rate on Cu–In₂O₃ is attributable to fast migration of oxide ions in the Cu–In alloy and the preferential oxidation of the interface of alloy–In₂O₃. In terms of the CO₂ decomposition rate, Cu–In₂O₃ is a promising OSM, but it presents issues related to durability over numerous cycles. In addition, because of its high reducibility, the rate of reverse reaction in eqn (5) is also high, resulting in a low CO concentration (CO/CO + CO₂) in the product gas.

Here we propose a novel material for this purpose. The most desirable OSM for this purpose was found to be Co-modified In₂O₃ (Co–In₂O₃). In fact, Co–In₂O₃ showed a higher CO₂ splitting rate, even at low temperatures (723–823 K) under an atmosphere with high CO concentration.

To elucidate the mechanism of CO₂ splitting on this OSM, various characterizations were undertaken using powder X-ray diffraction (XRD), scanning transmission electron microscopy with energy-dispersive X-ray spectrometry (STEM-EDX) and in situ X-ray absorption fine structure (XAFS) measurements.

Co-supported indium-based oxides used for this study were prepared using a complex polymerization method and an impregnation method. The procedures are described in the ESI.† The procedures for redox tests, i.e. CO₂ splitting and regeneration, are also described in the ESI.† The series of reduction and oxidation steps was defined as one cycle for isothermal RWGS-CL. The amount of reduction and oxidation (redox) was defined as the amount of oxygen atoms released or backfilled in each step. The maximum CO₂ conversion of the oxidation reaction (eqn (5)) was also evaluated. Electronic states of samples were evaluated using in situ In and Co K-edge X-ray absorption fine structure (XAFS) spectroscopy at the BL14B2 beamline of SPring-8 in Japan. XAFS measurements were conducted in transmission mode. Details of these characterisations are described in the ESI.†

First, we conducted screening tests for oxides that achieve high RWGS-CL performance. Subsequently, Co–In oxide (Co–In₂O₃) was developed. Fig. 1 presents a comparison of the CO₂ splitting rate with previously reported materials (please see also ESI†). The Co–In₂O₃ showed very high oxidation rates of 127.6, 191.0, 280.2, 365.7, and 429.5 μmol min⁻¹ g⁻¹, respectively, in the middle-temperature and low-temperature ranges of 723, 748, 773, 798, and 823 K, exceeding those of previously reported perovskite oxides,^11,13–19 Fe-based oxides,^20–26 and In-based oxides.¹² Therefore, it is considered that Co–In₂O₃ is kinetically the most promising oxide for RWGS-CL.

Fig. 1 Comparison of average CO₂ splitting rates on Co–In₂O₃ (this study) and on various oxides in earlier reports: The purple triangle plot represents the maximum CO₂ conversion rate during temperature-programmed oxidation by CO₂ over La_0.6Ca_0.4Fe_0.4Mn_0.6O₃⁽¹⁵⁾.

The results of RWGS-CL cycling tests for 10 cycles by Co–In₂O₃ are presented in Fig. 2. The detailed RWGS-CL results and the confirmation of the outlet gas in the re-oxidation step are shown in Table S1 and Fig. S1 (ESI†). Co–In₂O₃ showed a reduction amount of 6.39 mmol g⁻¹ and oxidation amount of 2.73 mmol g⁻¹ in the first cycle and a redox amount of 3.25 ± 0.05 mmol g⁻¹ after the second cycle. The amount of reduction that was not backfilled by CO₂ in the first cycle was found to be the reduction of Co by crystal structure analysis, as discussed in detail later, which indicates that in Co–In₂O₃, the redox of Co is not involved in the reaction after the reduction of the first cycle. The only redox species is indium. In contrast to the high redox performance of Co–In₂O₃, In₂O₃ reportedly has a redox amount of about 1.64 mmol g⁻¹ and an oxidation rate of 93.4 μmol min⁻¹ g⁻¹ at 773 K.¹² Although the redox species is indium in both oxides, Co–In₂O₃ shows higher redox capacity and a higher oxidation rate than those of In₂O₃. Consequently, the results indicate that Co can enhance the redox performance of indium. Moreover, in terms of stability, Cu–In₂O₃, which has high RWGS-CL performance and the same redox mechanism as Co–In₂O₃, shows an oxidation rate of 161.8 μmol min⁻¹ g⁻¹ in the first cycle, but the rate decreases to 85.5 μmol min⁻¹ g⁻¹ in the fifth cycle.¹² However, Co–In₂O₃ showed an oxidation rate of 293.2 μmol min⁻¹ g⁻¹ in the second cycle, and still shows a high oxidation rate of 281.8 μmol min⁻¹ g⁻¹ in the tenth cycle, with no noticeable decrease in the rate, maintaining high stability. These results demonstrate that Co can improve the redox performance of indium, especially its oxidation performance, and can stabilize the redox of indium.

Fig. 2 RWGS-CL stability test for Co–In₂O₃.

Next, the maximum CO₂ conversion in the oxidation step of Co–In₂O₃ and previously reported In-based oxides was compared with the equilibrium conversion of conventional RWGS.²⁷ The results are depicted in Fig. 3. Actually, Cu–In₂O₃, which is reported to have high redox kinetics, was found to be thermodynamically disadvantageous when compared to conventional RWGS. This finding indicated that Cu–In₂O₃ as unsuitable for industrial applications. Regarding In₂O₃, a maximum CO₂ conversion of 70–90% was achieved in the medium temperature region (at 673 K), which is much higher than the equilibrium conversion of conventional RWGS. Nevertheless, concern persists that In(0), which is derived from the reduction of In₂O₃, is liquefied in the medium temperature region (at 673 K) because of its low melting point. This issue engenders challenges for the industrialization of the RWGS-CL process. However, Co–In₂O₃ also achieved a high maximum CO₂ conversion of 70–80% in the medium temperature region, while suppressing the melting of indium because the redox of indium progresses with the formation and decomposition of Co–In alloy (as discussed later) with a high melting point.²⁸ To summarize the discussion presented above, Co–In₂O₃ can improve the redox kinetics of indium and can achieve high stability, while maintaining the maximum CO₂ conversion. Additionally, compared to materials reported earlier, Co–In₂O₃ has higher performance and is an industrially more promising material.

Fig. 3 Comparison between equilibrium conversion of conventional RWGS and maximum CO₂ conversion of RWGS-CL materials during the oxidation step.

Next, the structural and electronic states of the Co–In₂O₃ in RWGS-CL were evaluated using XRD, STEM-EDX, and XAFS measurements. In the XRD of the as-prepared Co–In₂O₃ (see Fig. S2 in ESI†), the diffraction peaks of Co₃O₄ and In₂O₃ were observed, respectively, with no peak shift in each representative diffraction peak, indicating that the complexation of Co and In did not occur. The SEM image (Fig. S3 in ESI†) showed particles of several hundred nanometres. The EDS image showed non-uniform dispersion of Co species and uniform dispersion of In species. The XRD profile of the reduced sample (Fig. S2, ESI†) shows diffraction peaks of Co–In alloy (CoIn₂), Co metal, and In₂O₃. The results demonstrated that a part of the Co reduced by hydrogen formed an alloy with indium, which was also reduced. The XRD profile of the re-oxidized sample (Fig. S2, ESI†) showed diffraction peaks of Co metal and In₂O₃, indicating that CO₂ oxidizes only indium in the Co–In alloy and not in Co metal. In other words, the RWGS-CL in the Co–In₂O₃ is established by the redox of indium with the formation and decomposition of the Co–In alloy. The SEM images obtained after reduction and re-oxidation (Fig. S3, ESI†) showed particles of several thousand nanometres in size. The EDS images showed sparsely and heterogeneously dispersed Co on the In species. The Co species dispersed sparsely and heterogeneously on the indium species were regarded as involved in the alloy formation with indium.

The Co K-edge and In K-edge XANES spectra of Co–In₂O₃ during RWGS-CL are shown in Fig. 4. Several spectral changes at the In K-edge (Fig. 4(A and B)) represent the redox of indium during RWGS-CL. In addition, it is particularly noteworthy that the Co K-edge XANES spectra of Co–In₂O₃ (Fig. 4(C)) were peculiar. After the reduction, the spectrum was similar to that of Co-foil, but showed slightly different features (blue line in Fig. 4(C)). This spectrum indicated the formation of Co–In alloy in addition to Co metal. When the reduced Co–In₂O₃ was exposed to CO₂, the spectrum shifted to that of Co-foil (red lines in Fig. 4(C)). Results of XAFS measurements support the redox of indium with the formation and decomposition of Co–In alloys (detailed in the ESI† text). This redox mechanism might have contributed to the high redox performance of indium, especially its oxidation performance.

Fig. 4 The XANES spectra of Co–In₂O₃ during (A) H₂ reduction and (B) CO₂ oxidation on In K-edge, and (C) H₂ reduction and CO₂ oxidation on the Co K-edge.

To investigate details of the difference in oxidation performance of Co–In₂O₃, kinetic analysis of the isothermal solid-state reaction proposed by Hancock and Sharp^29–31 was applied for the RWGS-CL oxidation step. Details of the method based on the kinetic model are described along with the fitting process in the ESI.† For In₂O₃, oxidation by CO₂ is known to proceed in the nucleation model.¹² In this model, the nuclei of the reaction are generated in the particles at the initial stage. The reaction proceeds as the nuclei grow and mutually collide continuously.^32–34 For In₂O₃, a rapid decrease in the reaction rate has been reported because of a decrease in the nucleation region in the initial stage of oxidation. Regarding the Co–In₂O₃, the oxidation reaction kinetics fitted best to the zero-order model.^29–31 This model is a reaction-order model in which the differential oxidation rate is kept constant irrespective of the degree of oxidation. Fig. 5 shows the differential oxidation rate behaviour of Co–In₂O₃. As presented in Fig. 5, the oxidation of Co–In₂O₃ progresses, while maintaining the differential oxidation rate up to solid conversion of about 50–60%, as expressed by the zero-order model. These results suggest that, in Co–In₂O₃, surface reaction sites with CO₂ do not decrease with the progress of oxidation, i.e., the oxide ions on the surface oxidized by CO₂ are regarded as migrating quickly to the bulk.

Fig. 5 Transition of differential oxidation rate on Co–In₂O₃.

The rate constant k on the oxidation side at each temperature was calculated for the oxidation reaction of Co–In₂O₃. An Arrhenius plot was obtained (Fig. S6, ESI†). The reported apparent activation energy on the oxidation side of In₂O₃ is 85.9 kJ mol⁻¹,¹² whereas 48.5 kJ mol⁻¹ in Co–In₂O₃, indicating that the co-existence of Co reduces the activation barrier of the oxidation reaction of indium by about half.

Completion of quick oxidation in the Co–In₂O₃ might be realized by fast oxygen ion migration in the Co–In alloy. The schematic diagrams are portrayed in Fig. S7 (ESI†). Specifically, as found from structural analyses, Co–In₂O₃ gives Co–In alloys dispersed on In₂O₃ after the reduction. When the oxidation starts, CO₂ is adsorbed and dissociated on the Co–In alloy. Then oxide ions are supplied to the Co–In alloy surface. They quickly migrate from the Co–In alloy surface to the interface between the Co–In alloy and In₂O₃. This rapid migration of oxide ions into the bulk might engender the rapid growth of the In₂O₃. This peculiar oxidation mechanism gives Co–In₂O₃ a higher oxidation rate than those of materials described in earlier reports.

Conflicts of interest

The authors have no conflicts of interest.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d2cc00208f

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