Nanoscaled Oxygen Carrier-Driven Chemical Looping for Carbon Neutrality: Opportunities and Challenges

ConspectusClimate change poses unprecedented challenges, demanding efforts toward innovative solutions. Amid these efforts, chemical looping stands out as a promising strategy, attracting attention for its CO2 capture prowess and versatile applications. The chemical looping approach involves fragmenting a single reaction, often a redox reaction, into multiple subreactions facilitated by a carrier, frequently a metal oxide. This innovative method enables diverse chemical transformations while inherently segregating products, enhancing process flexibility, and fostering autothermal properties. An intriguing facet of this novel technique lies in its capacity for CO2 utilization in processes like dry reforming and gasification of carbon-based feeds such as natural gas and biomass. Central to the success of chemical looping technology is a profound understanding of the intricacies of redox chemistry within these processes. Notably, nanoscaled oxygen carriers have proven effective, characterized by their extensive surface area and customizable structure. These carriers hold substantial promise, enabling reactions under milder conditions.This Account offers a concise overview of the mechanisms, benefits, opportunities, and challenges associated with nanoscaled carriers in chemical looping applications, with a focus on CO2 utilization. We delve into the nuances of redox chemistry, shedding light on ionic diffusion and oxygen vacancy─two key elements that are crucial in designing oxygen carriers. This discussion extends to nanospecific factors such as the particle size effect and gas diffusivity. Through the application of density functional theory simulations, insights are drawn regarding the impact of nanoparticle size on syngas production in chemical looping. Interestingly, nanosized iron oxide (Fe2O3) carriers exhibit elevated syngas selectivity and constrained CO2 formation at the nanoscale. Moreover, the reactivity enhancement of mesoporous SBA-16 supported Fe2O3 over mesoporous SBA-15 supported Fe2O3 is elucidated through Monte Carlo simulations that emphasize the superiority of the 3-dimensional interconnected porous network of SBA-16 in enhancing gas diffusion, thereby amplifying reactivity compared to the 2-dimensional SBA-15. Furthermore, we explore prevalent nanoscaled carriers, focusing on their amplified performance in CO2 utilization schemes. These encompass the integration of nanoparticles with mesoporous supports to enhance surface area, the adoption of nanoscale core-shell architectures to enhance diffusion, and the dispersion of nanoscaled active sites on microsized carriers to accelerate reactant activation. Notably, our mesoporous-supported Fe2O3 nanocarrier facilitates methane dissociation and oxidation by reducing energy barriers, thereby promoting methane conversion. The Account proceeds to outline key challenges and prospects for nanoscaled carriers in chemical looping, concluding with a glance into future research directions. We also shine a spotlight on our research group's efforts in innovating oxygen carrier materials, supplemented by discussions on indispensable elements that are essential for successful scale-up deployment.

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

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.

Strengthening the Connection between Science, Society and Environment to Develop Future French and European Bioeconomies: Cutting-Edge Research of VAALBIO Team at UCCS

The development of the future French and European bioeconomies will involve developing new green chemical processes in which catalytic transformations are key. The VAALBIO team (valorization of alkanes and biomass) of the UCCS laboratory (Unité de Catalyse et Chimie du Solide) are working on various catalytic processes, either developing new catalysts and/or designing the whole catalytic processes. Our research is focused on both the fundamental and applied aspects of the processes. Through this review paper, we demonstrate the main topics developed by our team focusing mostly on oxygen- and hydrogen-related processes as well as on green hydrogen production and hybrid catalysis. The social impacts of the bioeconomy are also discussed applying the concept of the institutional compass.

Intensification of Chemical Looping Processes by Catalyst Assistance and Combination

Chemical looping can be considered a technology platform, which refers to one common basic concept that can be used for various applications. Compared with a traditional catalytic process, the chemical looping concept allows fuels’ conversion and products’ separation without extra processes. In addition, the chemical looping technology has another major advantage: combinability, which enables the integration of different reactions into one process, leading to intensification. This review collects various important state-of-the-art examples, such as integration of chemical looping and catalytic processes. Hereby, we demonstrate that chemical looping can in principle be implemented for any catalytic reaction or at least assist in existing processes, provided that the targeted functional group is transferrable by means of suitable carriers.

Enhanced and environment-friendly chemical looping gasification of crop straw using red mud as a sinter-resistant oxygen carrier

Syngas production from biomass gasification is a promising technology, which is widely used in the chemical industry. Crop straw and red mud are typical agricultural and industrial wastes, respectively, which are cheap and widespread; however, they cause serious environmental pollution due to the open burning of straw and the toxicity and alkalinity of red mud. In the present work, we converted crop straw into syngas by chemical looping gasification using red mud as a sinter-resistant oxygen carrier. The reactivity of red mud, the syngas yields, and the air pollutant emissions under different conditions were systematically investigated through a thermo-gravimetric analyzer and mass spectrometer. Compared with pure Fe₂O₃, red mud can promote the syngas yields from crop straw gasification owing to the presence of inert Al₂O₃ and SiO₂. Red mud can effectively reduce the emission of air pollutants owing to the presence of alkaline components such as CaO and Na₂O. As the Fe₂O₃/fuel mass ratio increases, the syngas yield increases and the air pollutant emissions simultaneously reduce; whereas the syngas yield and the air pollutant emissions decrease with increasing heating rate. After calcination at high temperature, the structure of red mud remains stable with slight agglomeration, and can be easily regenerated. Therefore, the promising results provide a breakthrough for efficient utilization and disposal of both crop straw and red mud.

Facile CO2 separation and subsequent H2 production via chemical-looping combustion over ceria-zirconia solid solutions

A novel chemical-looping combustion scheme is proposed, where facile gas separation via steam condensation enables the production of sequestrable CO2 from alkanes, such as CH4, and pure H2 from H2O. This cycle consists of two steps, namely, (1) the endothermic reduction of a ceria-based solid solution via the complete oxidation of CH4, followed by (2) the exothermic oxidation of the reduced metal oxide via H2O splitting. Relative to iron oxide-based materials and undoped ceria, ceria-zirconia solid solutions possess favorable partial molar enthalpic and entropic properties; this promotes selective production of complete combustion products, H2O and CO2, during the reforming reaction. Thermodynamic predictions suggest that the complete oxidation of CH4 is possible by increasing the Zr content to 20 mol%, operating below 600 °C, increasing total pressure, or reducing the amount of delivered reactant. Furthermore, any H2, CO, or unreacted CH4 that may persist is thermodynamically favored to oxidize if exposed to unreacted oxide downstream, as is typical for a packed-bed or downer reactor configuration. Experiments were performed to validate the thermodynamic trends using isothermal thermogravimetry coupled with residual gas analysis, which confirmed that high selectivity towards H2O and CO2 is attainable for methane-driven reduction of Ce0.9Zr0.1O2; selectivities greater than 0.70 were observed at initial reaction extents. Importantly, metal oxide oxidation via H2O splitting and selective production of H2 (or CO if CO2 is the delivered oxidant) is also thermodynamically favored at the operating conditions considered for the first step. This work ultimately presents a viable avenue for the carbon-neutral conversion of CH4 (or other alkanes) to H2 if a renewable energy resource, such as solar energy, is leveraged to supply process heat.

Advanced Chemical Looping Materials for CO₂ Utilization: A Review

Combining chemical looping with a traditional fuel conversion process yields a promising technology for low-CO₂-emission energy production. Bridged by the cyclic transformation of a looping material (CO₂ carrier or oxygen carrier), a chemical looping process is divided into two spatially or temporally separated half-cycles. Firstly, the oxygen carrier material is reduced by fuel, producing power or chemicals. Then, the material is regenerated by an oxidizer. In chemical looping combustion, a separation-ready CO₂ stream is produced, which significantly improves the CO₂ capture efficiency. In chemical looping reforming, CO₂ can be used as an oxidizer, resulting in a novel approach for efficient CO₂ utilization through reduction to CO. Recently, the novel process of catalyst-assisted chemical looping was proposed, aiming at maximized CO₂ utilization via the achievement of deep reduction of the oxygen carrier in the first half-cycle. It makes use of a bifunctional looping material that combines both catalytic function for efficient fuel conversion and oxygen storage function for redox cycling. For all of these chemical looping technologies, the choice of looping materials is crucial for their industrial application. Therefore, current research is focused on the development of a suitable looping material, which is required to have high redox activity and stability, and good economic and environmental performance. In this review, a series of commonly used metal oxide-based materials are firstly compared as looping material from an industrial-application perspective. The recent advances in the enhancement of the activity and stability of looping materials are discussed. The focus then proceeds to new findings in the development of the bifunctional looping materials employed in the emerging catalyst-assisted chemical looping technology. Among these, the design of core-shell structured Ni-Fe bifunctional nanomaterials shows great potential for catalyst-assisted chemical looping.

Combined Ceria Reduction and Methane Reforming in a Solar-Driven Particle-Transport Reactor

We report on the experimental performance of a solar aerosol reactor for carrying out the combined thermochemical reduction of CeO₂ and reforming of CH₄ using concentrated radiation as the source of process heat. The 2 kW_th solar reactor prototype utilizes a cavity receiver enclosing a vertical Al₂O₃ tube which contains a downward gravity-driven particle flow of ceria particles, either co-current or counter-current to a CH₄ flow. Experimentation under a peak radiative flux of 2264 suns yielded methane conversions up to 89% at 1300 °C for residence times under 1 s. The maximum extent of ceria reduction, given by the nonstoichiometry δ (CeO_2-δ), was 0.25. The solar-to-fuel energy conversion efficiency reached 12%. The syngas produced had a H₂:CO molar ratio of 2, and its calorific value was solar-upgraded by 24% over that of the CH₄ reformed.

Perovskite nanocomposites as effective CO2-splitting agents in a cyclic redox scheme

We report iron-containing mixed-oxide nanocomposites as highly effective redox materials for thermochemical CO₂ splitting and methane partial oxidation in a cyclic redox scheme, where methane was introduced as an oxygen "sink" to promote the reduction of the redox materials followed by reoxidation through CO₂ splitting. Up to 96% syngas selectivity in the methane partial oxidation step and close to complete conversion of CO₂ to CO in the CO₂-splitting step were achieved at 900° to 980°C with good redox stability. The productivity and production rate of CO in the CO₂-splitting step were about seven times higher than those in state-of-the-art solar-thermal CO₂-splitting processes, which are carried out at significantly higher temperatures. The proposed approach can potentially be applied for acetic acid synthesis with up to 84% reduction in CO₂ emission when compared to state-of-the-art processes.

Perovskite-structured AMnxB1−xO3 (A = Ca or Ba; B = Fe or Ni) redox catalysts for partial oxidation of methane

High oxygen carrying capacity, lack of loosely bound lattice oxygen, and preferential surface segregation of Ba make BaMn_xB_1−xO₃ (B = Ni or Fe) based redox catalysts suitable for chemical looping reforming of methane with high syngas yield and coke resistance.