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.

Enhanced Light Absorption and Radiative Forcing by Black Carbon Agglomerates.

The climate models of the Intergovernmental Panel on Climate Change list black carbon (BC) as an important contributor to global warming based on its radiative forcing (RF) impact. Examining closely these models, it becomes apparent that they might underpredict significantly the direct RF for BC, largely due to their assumed spherical BC morphology. Specifically, the light absorption and direct RF of BC agglomerates are enhanced by light scattering between their constituent primary particles as determined by the Rayleigh-Debye-Gans theory interfaced with discrete dipole approximation and recent relations for the refractive index and lensing effect. The light absorption of BC is enhanced by about 20% by the multiple light scattering between BC primary particles regardless of the compactness of their agglomerates. The resulting light absorption agrees very well with the observed absorption aerosol optical depth of BC. ECHAM-HAM simulations accounting for the realistic BC morphology and its coatings reveal high direct RF = 3-5 W/m2 in East, South Asia, sub-Sahara, western Africa, and the Arabian peninsula. These results are in agreement with satellite and AERONET observations of RF and indicate a regional climate warming contribution by 0.75-1.25 °C, solely due to BC emissions.

Near 100% CO selectivity in nanoscaled iron-based oxygen carriers for chemical looping methane partial oxidation.

Chemical looping methane partial oxidation provides an energy and cost effective route for methane utilization. However, there is considerable CO2 co-production in current chemical looping systems, rendering a decreased productivity in value-added fuels or chemicals. In this work, we demonstrate that the co-production of CO2 can be dramatically suppressed in methane partial oxidation reactions using iron oxide nanoparticles embedded in mesoporous silica matrix. We experimentally obtain near 100% CO selectivity in a cyclic redox system at 750-935 °C, which is a significantly lower temperature range than in conventional oxygen carrier systems. Density functional theory calculations elucidate the origins for such selectivity and show that low-coordinated lattice oxygen atoms on the surface of nanoparticles significantly promote Fe-O bond cleavage and CO formation. We envision that embedded nanostructured oxygen carriers have the potential to serve as a general materials platform for redox reactions with nanomaterials at high temperatures.

Modulating Lattice Oxygen in Dual-Functional Mo-V-O Mixed Oxides for Chemical Looping Oxidative Dehydrogenation.

Oxygen chemistry plays a pivotal role in numerous chemical reactions. In particular, selective cleavage of C-H bonds by metal oxo species is highly desirable in dehydrogenation of light alkanes. However, high selectivity of alkene is usually hampered through consecutive oxygenation reactions in a conventional oxidative dehydrogenation (ODH) scheme. Herein, we show that dual-functional Mo-V-O mixed oxides selectively convert propane to propylene via an alternative chemical looping oxidative dehydrogenation (CL-ODH) approach. At 500 °C, we obtain 89% propylene selectivity at 36% propane conversion over 100 dehydrogenation-regeneration cycles. We attribute such high propylene yield-which exceeds that of previously reported ODH catalysts-to the involvement and precise modulation of bulk lattice oxygen via atomic-scale doping of Mo and show that increasing the binding energy of V-O bonds is critical to enhance the selectivity of propylene. This work provides the fundamental understanding of metal-oxygen chemistry and a promising strategy for alkane dehydrogenation.

Oxygen vacancy promoted methane partial oxidation over iron oxide oxygen carriers in the chemical looping process.

We perform ab initio DFT+U calculations and experimental studies of the partial oxidation of methane to syngas on iron oxide oxygen carriers to elucidate the role of oxygen vacancies in oxygen carrier reactivity. In particular, we explore the effect of oxygen vacancy concentration on sequential processes of methane dehydrogenation, and oxidation with lattice oxygen. We find that when CH4 adsorbs onto Fe atop sites without neighboring oxygen vacancies, it dehydrogenates with CHx radicals remaining on the same site and evolves into CO2via the complete oxidation pathway. In the presence of oxygen vacancies, on the other hand, the formed methyl (CH3) prefers to migrate onto the vacancy site while the H from CH4 dehydrogenation remains on the original Fe atop site, and evolves into CO via the partial oxidation pathway. The oxygen vacancies created in the oxidation process can be healed by lattice oxygen diffusion from the subsurface to the surface vacancy sites, and it is found that the outward diffusion of lattice oxygen atoms is more favorable than the horizontal diffusion on the same layer. Based on the proposed mechanism and energy profile, we identify the rate-limiting steps of the partial oxidation and complete oxidation pathways. Also, we find that increasing the oxygen vacancy concentration not only lowers the barriers of CH4 dehydrogenation but also the cleavage energy of Fe-C bonds. However, the barrier of the rate-limiting step cannot further decrease when the oxygen vacancy concentration reaches 2.5%. The fundamental insight into the oxygen vacancy effect on CH4 oxidation with iron oxide oxygen carriers can help guide the design and development of more efficient oxygen carriers and CLPO processes.

Methane adsorption and dissociation on iron oxide oxygen carriers: the role of oxygen vacancies.

We performed ab initio DFT+U calculations to explore the interaction between methane and iron oxide oxygen carriers for chemical looping reaction systems. The adsorption of CH4 and CHx (x = 0-3) radicals on α-Fe2O3(001), and the influence of oxygen vacancies at the top surface and on the subsurface on the adsorption properties of the radicals was investigated. The adsorption strength for CH4 and C radicals at the top of the α-Fe2O3(001) surface in the presence of oxygen vacancies is lower than that on the stoichiometric surface. However, for methyl (CH3), methylene (CH2) and methine (CH) radicals, it is correspondingly higher. In contrast, the oxygen vacancy formation on the subsurface not only increases the adsorption strength of CH3, CH2 and CH radicals, but also facilitates C radical adsorption. We found that oxygen vacancies significantly affect the adsorption configuration of CHx radicals, and determine the probability of finding an adsorbed species in the stoichiometric region and the defective region at the surface. With the obtained adsorption geometries and energetics of these species adsorbed on the surface, we extend the analysis to CH4 dissociation under chemical looping reforming conditions. The distribution of adsorbed CH4 and CHx (x = 0-3) radicals is calculated and analyzed which reveals the relationship between adsorbed CHx radical configuration and oxygen vacancies in iron oxide. Also, the oxygen vacancies can significantly facilitate CH4 activation by lowering the dissociation barriers of CH3, CH2 and CH radicals. However, when the oxygen vacancy concentration reaches 2.67%, increasing the oxygen vacancy concentration cannot continue to lower the CH dissociation barrier. The study provides fundamental insights into the mechanism of CH4 dissociation on iron based oxygen carriers and also provide guidance to design more efficient oxygen carriers.

Chemical Looping Technology: Oxygen Carrier Characteristics.

Chemical looping processes are characterized as promising carbonaceous fuel conversion technologies with the advantages of manageable CO2 capture and high energy conversion efficiency. Depending on the chemical looping reaction products generated, chemical looping technologies generally can be grouped into two types: chemical looping full oxidation (CLFO) and chemical looping partial oxidation (CLPO). In CLFO, carbonaceous fuels are fully oxidized to CO2 and H2O, as typically represented by chemical looping combustion with electricity as the primary product. In CLPO, however, carbonaceous fuels are partially oxidized, as typically represented by chemical looping gasification with syngas or hydrogen as the primary product. Both CLFO and CLPO share similar operational features; however, the optimum process configurations and the specific oxygen carriers used between them can vary significantly. Progress in both CLFO and CLPO is reviewed and analyzed with specific focus on oxygen carrier developments that characterize these technologies.

Formation of core-shell structured composite microparticles via cyclic gas-solid reactions.

This work reports a novel low-cost and environmental-friendly preparation strategy for core-shell structured composite microparticles and discusses its formation mechanism. Different from most conventional strategies, which involve coating or coating-like processes, this reported strategy uses irreversible solid-phase ionic diffusion in a gas-solid reaction cycle (e.g., reduction and oxidation of Fe) to gradually move the shell material from a core-and-shell material mixture microparticle to the surface. Without the need for solvent as do many conventional processes, this novel process only involves gas-solid reactions, which reduces environmental impact. To substantiate this conceived strategy, a micrometer-sized microparticle made up of a mixture of Fe2O3 and Al2O3 powders is first reduced by H2 and then oxidized by O2 over 50 cycles at 900 °C. These reactions are known to proceed mainly through the diffusion of solid-phase Fe cations. SEM and EDX analyses verify the formation of an Al2O3 core-Fe2O3 shell structure at the end of the 50 reaction cycles. If the cyclic reactions of a microparticle proceed mainly through the diffusion of gaseous-reactant-derived O anions such as the mixture of Fe2O3 and TiO2 instead of solid-phase Fe cation diffusion, no formation of the core-shell structure is observed in the resulting microparticle. These two opposing results underscore the dominating role of solid-phase ionic diffusion in the formation of the core-shell structure. A 2-D continuum diffusion model is applied to account for the inter-Fe-particle bridging and directional product layer growth phenomena during an oxidation reaction. The simulation further verifies the conceived core-shell formation strategy.

Reactive solid surface morphology variation via ionic diffusion.

In gas-solid reactions, one of the most important factors that determine the overall reaction rate is the solid morphology, which can be characterized by a combination of smooth, convex and concave structures. Generally, the solid surface structure varies in the course of reactions, which is classically noted as being attributed to one or more of the following three mechanisms: mechanical interaction, molar volume change, and sintering. Here we show that if a gas-solid reaction involves the outward ionic diffusion of a solid-phase reactant then this outward ionic diffusion could eventually smooth the surface with an initial concave and/or convex structure. Specifically, the concave surface is filled via a larger outward diffusing surface pointing to the concave valley, whereas the height of the convex surface decreases via a lower outward diffusion flux in the vertical direction. A quantitative 2-D continuum diffusion model is established to analyze these two morphological variation processes, which shows consistent results with the experiments. This surface morphology variation by solid-phase ionic diffusion serves to provide a fourth mechanism that supplements the traditionally acknowledged solid morphology variation or, in general, porosity variation mechanisms in gas-solid reactions.

Multirelaxation-time interaction-potential-based lattice Boltzmann model for two-phase flow.

The application of the lattice Boltzmann method in two-phase flows is often restricted by the numerical instability at low viscosities. In this work, a multirelaxation-time (MRT) lattice Boltzmann model (LBM) is developed using the interaction potential approach. With the MRT collision term and a general force term, the new MRT model is able to significantly enhance the numerical stability at low viscosities, without appreciable increase in computation time or memory use. Advanced force formulation using the multirange potential can also be readily incorporated into the current MRT scheme. Numerical tests are first performed in two dimensions under equilibrium conditions. The MRT model is able to reduce the lowest stable viscosity by an order of magnitude compared to the single relaxation time LBM. In addition, the spurious velocity at the gas-liquid interface can also be significantly decreased by tuning the adjustable relaxation parameters. Then two sets of three-dimensional simulations are conducted to investigate the buoyant rise of a gas bubble in a low-viscosity liquid. In particular, millimeter air bubble in water, which is difficult for traditional two-phase LBM due to both low viscosity and high-surface tension, is successfully simulated using the MRT technique developed in this study. The simulated bubble shape and velocity are compared with the experimental results and empirical correlations in the literature, and the satisfactory agreement proves the validity of the MRT-LBM for two-phase flows.

Electrical capacitance volume tomography: design and applications.

This article reports recent advances and progress in the field of electrical capacitance volume tomography (ECVT). ECVT, developed from the two-dimensional electrical capacitance tomography (ECT), is a promising non-intrusive imaging technology that can provide real-time three-dimensional images of the sensing domain. Images are reconstructed from capacitance measurements acquired by electrodes placed on the outside boundary of the testing vessel. In this article, a review of progress on capacitance sensor design and applications to multi-phase flows is presented. The sensor shape, electrode configuration, and the number of electrodes that comprise three key elements of three-dimensional capacitance sensors are illustrated. The article also highlights applications of ECVT sensors on vessels of various sizes from 1 to 60 inches with complex geometries. Case studies are used to show the capability and validity of ECVT. The studies provide qualitative and quantitative real-time three-dimensional information of the measuring domain under study. Advantages of ECVT render it a favorable tool to be utilized for industrial applications and fundamental multi-phase flow research.

Synthesis of iron-based chemical looping sorbents integrated with pH swing carbon mineral sequestration.

The previously developed pH swing carbon mineral sequestration immobilizes the gaseous CO2 into a thermodynamically stable solid, MgCO3, using Mg-bearing minerals such as serpentine. This mineral carbonation technology is particularly promising since it generates value-added solid products: high surface area silica, iron oxide, and magnesium carbonate, while providing a safe and permanent storage option for CO2. By carefully controlling the pH of the system, these solids products can be produced with high purity. This study focuses on the synthesis of iron oxide particles as a chemical looping sorbent in order to achieve the integration between carbon capture and storage technologies. Since the solubility of Fe in aqueous phase is relatively low at neutral pH, the effect of the weak acid and chelating agents on the extraction of Fe from serpentine was investigated. The synthesized iron-based chemical looping sorbent was found to be as effective as commercially available iron oxide nanoparticles at converting syngas into high purity H2, while producing a sequestration-ready CO2 stream.

Moving bed reactor setup to study complex gas-solid reactions.

A moving bed scale reactor setup for studying complex gas-solid reactions has been designed in order to obtain kinetic data for scale-up purpose. In this bench scale reactor setup, gas and solid reactants can be contacted in a cocurrent and countercurrent manner at high temperatures. Gas and solid sampling can be performed through the reactor bed with their composition profiles determined at steady state. The reactor setup can be used to evaluate and corroborate model parameters accounting for intrinsic reaction rates in both simple and complex gas-solid reaction systems. The moving bed design allows experimentation over a variety of gas and solid compositions in a single experiment unlike differential bed reactors where the gas composition is usually fixed. The data obtained from the reactor can also be used for direct scale-up of designs for moving bed reactors.