To mitigate global warming and achieve a sustainable society, innovative technologies for efficient CO2 utilization are required. Integrated CO2 capture and reduction (CCR) using dual-function materials (DFMs) is favorable owing to its potentially low energy consumption, capital investment, and processing costs. Although numerous studies have focused on catalytic science, continuous and steady-state CCR operations have not been sufficiently addressed from an engineering perspective. In this study, a circulating fluidized bed (CFB) system is investigated for continuous CCR to syngas (CO + H2). In the CFB system, transition-metal-free DFM (Na/Al2O3) particles are circulated between two bubbling fluidized-bed reactors. The DFM captures CO2 in one reactor (CO2 capture reactor) and reduces the captured CO2 to CO by the reaction with H2 in the other reactor (H2 reactor). The effluent gas concentrations from both reactors reach steady state and are maintained for over 8 h. For the product gas from the H2 reactor, the CO2 conversion and CO selectivity exceed 80 % and 99 %, respectively. However, the H2 conversion is <20 %, indicating a potential challenge for the CFB system for integrated CCR. Furthermore, this study confirms that the H2/CO ratio for syngas can be controlled by adjusting the experimental conditions (particularly, the H2 flow rate). Consequently, the CFB system can be modified to facilitate the interaction between H2 gas and the DFM particles.@en

Recently, carbon capture and reduction (CCR) technology has gained interest to directly convert CO₂ to value-added products without requiring purification of CO₂ and its subsequent transportation. CCR to methanol in one dual function material (DFM) poses mechanistic and kinetic challenges. To counteract this, a process combining Na/Al₂O₃ as a capture component and Cu/ZnO/Al₂O₃ (CZA) as methanol synthesis catalyst was developed to allow CCR to methanol. With a 5 vol% CO₂ flow for capture and subsequent H₂ stream combined with a temperature swing, a methanol selectivity of 26 % was achieved at 9 bar. Further investigation found that Na/Al₂O₃ significantly increased methanol yield, while a stacked configuration of Na/Al₂O₃ followed by CZA significantly outperformed a mixed configuration of the two catalysts. With further investigation of operation at higher pressure and surface mechanism, an effective CCR to methanol process using two affordable yet readily available catalysts can be realized.

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Carbon capture and utilization (CCU) technologies, such as CO₂ methanation, generally require energy-intensive CO₂ capture and separation processes prior to catalytic CO₂ conversion. In contrast, integrated CO₂ capture and reduction (CCR) technologies that use dual function materials (DFM) can directly convert low-concentration CO₂ in flue gas or atmosphere into high-concentration CH₄ or CO. In this study, we demonstrate a circulating fluidized bed (CFB) approach to enable continuous operation of CCR. In the CFB approach, the DFM (Na/Ni/Al₂O₃) circulates between two bubbling fluidized beds to enable steady-state cyclic operation of (1) selective capture of CO₂ in flue gas/air and (2) hydrogenation of the captured CO₂. We succeeded in the continuous synthesis of CH₄ with high CO₂ capture efficiency (>88 %) and high H₂ conversion (>85 %) yielding mainly CH₄ (selectivity > 99 %) as the product at high concentration (>20 % CH₄) using 2 % CO₂/N₂ as the model flue gas.

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Integrated CO2 capture and conversion (ICCC) using dual-function materials (DFMs) is one of the key technologies for addressing critical global environmental and energy issues. DFMs generally consist of alkali or alkaline earth metals for CO2 capture and transition metal catalysts for CO2 conversion. In this study, we studied the ICCC to CO using transition-metal-free DFMs to demonstrate their potential to directly produce syngas from atmospheric-level CO2. Among the DFMs prepared herein, Na/Al2O3 exhibited excellent performance and achieved a CO2 conversion exceeding 90% and CO selectivity exceeding 95% at a reaction temperature of 450-500 °C. Na/Al2O3 maintained its capture and conversion capacity throughout a 50-cycle stability test without significant deactivation. Furthermore, in the scale-up experiments using Na/Al2O3 DFM, a syngas-like mixture an H2/CO molar ratio of 3.3 (48.1 vol% H2 and 14.5 vol% CO) was directly obtained from 400 ppm CO2. These results suggest that ICCC using the transition-metal-free Na/Al2O3 DFM may be practicable provided the CO2 capture capacity of the DFM is further improved while maintaining the aforementioned advantages.

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A desirable process for realizing a low-carbon society is the direct conversion of dilute CO2 from flue gases or air into highly concentrated hydrocarbons without a need for separate CO2 capture and purification processes. In this study, we investigated the performance of integrated CO2 capture and reduction to CH4 over Ni-based dual-functional catalysts promoted with Na, K, and Ca. Ni/Na-γ-Al2O3 exhibited the highest activity for integrated CO2 (5% CO2) capture and reduction, achieving high CO2 conversion (>96%) and CH4 selectivity (>93%). In addition, very low-concentration CO2 (100 ppm CO2) was successfully converted to 11.5% CH4 at the peak point (>1000 times higher concentration than that of the supplied CO2) over Ni/Na-γ-Al2O3. The Ni-based dual-functional catalyst exhibited a high CO2 conversion exceeding 90%, even when 20% O2 was present during CO2 capture. Furthermore, an increased operation pressure had positive impacts on both CO2 capture and CH4 formation, and these advantageous effects were also observed when CO2 concentration was at the level of atmospheric CO2 (100-400 ppm). As the pressure increased from 0.1 to 0.9 MPa, CH4 production capacity with 400 ppm CO2 was enhanced from 111 to 160 μmol gcat-1. This approach in combination with the efficient catalyst shows encouraging potential for CO2 utilization, enabling direct air capture-conversion to value-added chemicals.

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