Chemical Looping Process Research Articles

Evaluation of redox activity of brownmillerite-structured Ca2Fe2O5 oxygen carrier for chemical looping applications

The brownmillerite-structured Ca2Fe2O5 oxygen carrier has shown its great potential in chemical looping processes, due to it can be completed regeneration in a H2O or CO2 atmosphere without the air reactor. However, the low reaction reactivity of Ca2Fe2O5 restricts its application. In this study, Ca2Fe2O5 oxygen carrier was prepared by sol-gel method. The reduction and oxidation kinetics of Ca2Fe2O5 were evaluated in H2 and CO2 atmospheres, respectively. The reduction of Ca2Fe2O5 in H2 atmosphere in good agreement with the random nucleation and growth model with Ea and A of 53.82 kJ/mol and 2.65 s−1, respectively. The dimension of the model increases with conversion (A1.5 → A2 → A3 → A4). The oxidation of reduced Ca2Fe2O5 in CO2 atmosphere can be described by the zero-order contraction model with Ea and A of 11.66 kJ/mol and 0.05 s−1, respectively. The kinetics analysis showed that both the reduction of H2 and the oxidation of CO2 are one-step reactions, as evidenced by the fact that only Fe0 and Fe3+ phases were detected in semi-in situ XRD analysis. It was inferred that the release and recovery of lattice oxygen is from inside to outside for Ca2Fe2O5 oxygen carrier in the redox process. By reducing the migration energy barrier of lattice oxygen between bulk and surface would be an effective means to improve the reactivity of Ca2Fe2O5 oxygen carriers.

Improving attrition resistance of oxygen carriers by biomass ash in chemical looping process

Chemical looping combustion (CLC) relies on cyclic circulation of oxygen carriers (OCs) between fuel reactor and air reactor to transfer lattice oxygen and heat. OCs’ attrition is an important issue hindering large scale application of CLC technology. Attrition is caused by cracks and abrasion, and method to stop crack propagation is an effective way to mitigate attrition. Biomass ash contains inert and alkali metal components, and it may have the potential to increase the reactivity and reinforce the skeleton structure of OCs. The effects of rape straw ash, corn straw ash and bagasse ash on the reactivity, cumulative attrition rate of red mud OCs were evaluated, and their effects on skeleton structure were analyzed, and finally, the strengthening mechanism of OCs skeleton structure was proposed. The results showed that the rape straw ash rich in K and Ca components promoted the reactivity of OCs. Although the Si-rich bagasse ash decreased the reactivity of OCs, it improved the attrition resistance of OCs. Corn straw ash rich in K and Si components was an effective skeleton strengthening material, which improved both the reactivity and the attrition resistance. The cumulative attrition rate of red mud was 2.74%, and the cumulative attrition rate of CSA sample was 0.35%, which was 87.32% lower than that of OC sample, indicating fine attrition resistance. The fiber structure could strengthen the skeleton of OC, therefore, decreasing attrition rate.

Reaction kinetics of a NiO-based oxygen carrier with ethanol to be applied in chemical looping processes

The use of bio-ethanol in chemical looping combustion (CLC) and reforming (CLR) has the potential to produce energy and/or hydrogen and, as a negative-emissions technology (NET), to remove CO2 from the atmosphere at low cost following the principles of bio-energy with CO2 capture and storage (BECCS). The determination of the rate of the chemical processes involving the fuel conversion with the oxygen carrier is required for the modelling and design of a chemical looping unit. In this work, a kinetic study of ethanol conversion is performed considering a NiO-based material as the oxygen carrier. Combined experiments in TGA and fixed bed were conducted. TGA tests confirmed that the oxygen carrier was reduced by a ethanol-containing gaseous stream at temperatures higher than 773 K. An apparent reaction kinetics for the NiO reduction with ethanol was determined, which might be used in simplified fuel reactor models. However, tests performed in a fixed bed revealed that products derived of the ethanol decomposition, such as CH4, CO, H2 were responsible for NiO reduction, instead of the direct reduction with ethanol. High temperature kinetics of processes involved in ethanol conversion, namely dehydration, dehydrogenation and decomposition, were determined. Both, non-catalytic and Ni-catalytic reactions were of relevance under chemical looping conditions. A reaction pathway was described to be used in detailed fuel reactor models of chemical looping units.

Co3O4-CeO2 for enhanced syngas by low-temperature methane conversion with CO2 utilization via a catalytic chemical looping process

Fabricating effective transition metal-based oxygen carriers (OCs) in a low-temperature methane chemical looping conversion is of considerable importance but remains challenging. Herein, a series of Co3O4-CeO2 metal oxides were synthesized and used as OCs in a low-temperature methane conversion with CO2 utilization via chemical looping systems. Co3O4/CeO2–3 (Co/Ce ratios =7/3) showed the better performance with CH4 conversion above 70%, the highest hydrogen yield and CH4 conversion up to 55.73 mmol/g and 94.09% at 700 °C. The enhancement on the performance of Co3O4/CeO2–3 for hydrogen production is attributed to the interaction between Co3O4 and CeO2 particles at the interfaces, which could cause charge redistribution. Compared to pristine Co3O4, this structure endows Co3O4-CeO2 with higher CH4 conversion performance. Moreover, the resulting Co0 in the reduced OCs is beneficial for catalytic methane decomposition toward C and H2, followed by C gasification into CO during CO2 atmosphere with CO2 conversion as high as 81%. In the proposed chemical looping processes, catalytic methane decomposition step is active for hydrogen production, coupling with CO2 conversion toward CO to adjust the H2/CO ratio of syngas.

Process Optimization and CO2 Emission Analysis of Coal/Biomass Gasification Integrated with a Chemical Looping Process

Biomass gasification is an attractive technology and one of the pathways for producing hydrogen. Due to the variable seasons and low calorific value of biomass, the addition of coal in the gasifier is suggested because coal has a high calorific value and carbon-to-hydrogen ratio. In general, the gaseous product obtained in gasification always contains a high amount of carbon dioxide, therefore, the co-gasification of biomass and coal should integrate with the calcium looping carbon dioxide capture process to provide purified hydrogen. In this work, the model of the co-gasification of biomass and coal integrated with the calcium looping carbon dioxide capture process was developed through an Aspen Plus simulator. The developed model was used to analyze the performance of this process. The sensitivity analysis demonstrated that increasing the gasification temperature, steam-to-feed (S/F) ratio, calcium oxide-to-feed (CaO/F) ratio, and regenerator temperature could improve hydrogen production. Next, further optimization was performed to identify the optimal operating condition that maximizes hydrogen production. The results showed that the optimal operating temperature of the gasifier is 700 °C with an S/F mass ratio of 2 and coal to biomass (C/B) mass ratio of 0.75:0.25. However, the carbonator and regenerator temperatures should be 450 °C and 950 °C, respectively, with a CaO/F mass ratio of 3. Under these operating conditions, the maximum H2 content and H2 yield can be provided as 99.59%vol. (dry basis) and 92.38 g hydrogen/kg biomass feeding. The other results revealed that the energy efficiency and carbon capture efficiency of this process are 42.86% and 99.99%, respectively, and that the specific emission of released CO2 is 80.77 g CO2/MJ.

Novel chemical looping oxidation of biomass-derived carbohydrates to super-high-yield formic acid using heteropolyacids as oxygen carrier

In recent years, formic acid (FA) shows great potential in fuel cells and hydrogen storage. For FA production from renewable resources, catalytic aerobic oxidation of biomass to FA has been reported. However, the yield of by-product CO2 is fairly high, limiting the FA selectivity and causing greenhouse gas emission. Herein, a novel chemical looping oxidation of biomass-derived carbohydrates to FA was first proposed. A heteropolyacid H8PV5Mo7O40 (HPA-5) was selected as an excellent oxygen carrier. Oxidation of carbohydrates to FA and re-oxidation of reduced HPA-5 (HPA-5red) by O2 were separately performed, by which the FA yield from glucose achieved 95.4%. The extremely high yield of FA is attributed to two reasons. On one hand, the chemical looping process avoids over-oxidation caused by the hydroxyl radicals formed in the re-oxidation of HPA-5red. On the other hand, abundant HPA-5 promotes the glucose oxidation to FA, suppressing the acid-catalyzed decomposition and the oxidation toward saccharic acids. Besides, HPA-5red can be completely re-oxidized by O2 and the chemical looping oxidation has stably performed well in five chemical cycles. This oxidation strategy also shows outstanding FA yields from xylan, cellulose and raw biomass.

Metal impregnation on steel converter slag as an oxygen carrier

AbstractOxygen carriers used in chemical looping processes operated with biofuel are affected by the inorganic matter of the fuel. It is therefore expected that the lifetime of the oxygen carrier is limited, and preferably low‐cost oxygen carriers should be used. Oxygen carriers based on iron ore or steel manufacturing waste products are available in significant quantities at low cost. However, it is common for these types of materials that their reactivity is low. This study investigates the effect of adding small amounts of more reactive elements into steel converter slag, also called LD slag.Slag particles were wet impregnated with 2 or 5 wt.% of Ni, Cu, Mn, or Ce. The new material's morphology was evaluated using X‐Ray Diffraction and SEM‐EDS. Changes in reactivity towards CO, CH4 and the model tar molecule benzene were evaluated using a bench‐scale laboratory fluidized bed reactor.It was observed that even small amounts of either Ni, Cu, or Mn could increase reactivity toward CO. Both Cu and Mn formed phases with LD slag that released oxygen via CLOU (chemical looping with oxygen uncoupling) and increased the conversion of methane and benzene. Ni and Ce doping also increased methane conversion but had only a minor effect on the benzene conversion. © 2023 The Authors. Greenhouse Gases: Science and Technology published by Society of Chemical Industry and John Wiley & Sons Ltd.

Production of Acetaldehyde via Oxidative Dehydrogenation of Ethanol in a Chemical Looping Setup.

A novel chemical looping (CL) process was demonstrated to produce acetaldehyde (AA) via oxidative dehydrogenation (ODH) of ethanol. Here, the ODH of ethanol takes place in the absence of a gaseous oxygen stream; instead, oxygen is supplied from a metal oxide, an active support for an ODH catalyst. The support material reduces as the reaction takes place and needs to be regenerated in air in a separate step, resulting in a CL process. Here, strontium ferrite perovskite (SrFeO3-δ) was used as the active support, with both silver and copper as the ODH catalysts. The performance of Ag/SrFeO3-δ and Cu/SrFeO3-δ was investigated in a packed bed reactor, operated at temperatures from 200 to 270 °C and a gas hourly space velocity of 9600 h-1. The CL capability to produce AA was then compared to the performance of bare SrFeO3-δ (no catalysts) and materials comprising a catalyst on an inert support, Cu or Ag on Al2O3. The Ag/Al2O3 catalyst was completely inactive in the absence of air, confirming that oxygen supplied from the support is required to oxidize ethanol to AA and water, while Cu/Al2O3 gradually got covered in coke, indicating cracking of ethanol. The bare SrFeO3-δ achieved a similar selectivity to AA as Ag/SrFeO3-δ but at a greatly reduced activity. For the best performing catalyst, Ag/SrFeO3-δ, the obtained selectivity to AA reached 92-98% at yields of up to 70%, comparable to the incumbent Veba-Chemie process for ethanol ODH, but at around 250 °C lower temperature. The CL-ODH setup was operated at high effective production times (i.e., the time spent producing AA to the time spent regenerating SrFeO3-δ). In the investigated configuration with 2 g of the CLC catalyst and 200 mL/min feed flowrate ∼5.8 vol % ethanol, only three reactors would be required for the pseudo-continuous production of AA via CL-ODH.

Electrodriven Chemical Looping Ammonia Synthesis Mediated by Lithium Imide

The chemical looping process for ammonia synthesis (CLAS) has advantages in avoiding competitive adsorptions of nitrogen and hydrogen and optimizing the thermodynamics and kinetics of a reaction step by step. Nitrogen carriers with favorable thermodynamic and kinetic properties are critical for such a process, especially under mild conditions. Recent studies have shown that alkali- and alkaline-earth-metal imides are attractive nitrogen carriers; however, the equilibrium concentration of NH3 in the hydrogenation of imides to ammonia and hydrides is low at atmospheric pressure. Moreover, the reaction rate of nitrogen fixation on the hydrides is slow. Here, we show that an electrodriven CLAS (ECLAS) mediated by a lithium imide is feasible in a LiCl–NaCl–KCl eutectic electrolytic cell with a nickel-foam electrode. We demonstrate that the reactions of lithium imide hydrogenation and lithium hydride nitridation are both promoted by the input of electric energy. The average ammonia production rate over such an electrodriven CLAS is ca. 8 times higher than that of a thermal-driven CLAS.

Dynamic oxygen migration and reaction over ceria-supported nickel oxides in chemical looping partial oxidation of methane

The oxygen species over oxygen carriers determine the reaction performance in chemical looping partial oxidation of methane (CLPOM). This paper describes the dynamic migration and reaction of oxygen species over ceria-supported NiO oxygen carrier for CLPOM. At initial reaction stage, Ni-O species could be consumed rapidly and cause the full oxidation of methane to CO2. Afterwards, the active Ni-O-Ce species dominates the methane partial oxidation to promote the syngas yield. It is found that CO formation rate is linearly related with the content of oxygen species over Ni-O-Ce. Furthermore, the lattice oxygen from CeO2 would migrate through the bulk to complement the consumed Ni-O-Ce oxygen species. Eventually, 5NiO/(40CeO2-Al2O3) exhibit nearly three times as high as methane reaction rate of CeO2/Al2O3 and two times as high as CO formation rate of 5NiO/Al2O3. This work provides the comprehensive understanding of transport and reaction mechanism of oxygen species for chemical looping processes.

The effect of microscopic phenomena on the performance of iron-based oxygen carriers of chemical looping hydrogen production

The scope of this work was to investigate and understand the microscopic effects on the performance of the active catalytic system in the chemical looping process. The inhibiting and accelerating effects of the different materials have been extensively studied. The ZrO2 doped with MgO and Y2O3 showed a chemical inertness and the formation of a porous morphology for an enhanced cyclic stability. Testing in large-scale application indicates that supporting material with higher yttrium content in ZrO2 has an excellent performance for 100 cycles. The HT-XRD and DSC analyzes clearly show microscopic phenomena play a fundamental role in the process and correspond to the phase transition. Stabilization of the cubic/tetragonal crystal structure leads to suppression of the phase transition and exhibits highest activity. The work reveals new fundamental principles for the design of OC in chemical looping hydrogen and understanding the influence of microscopic effects on the stability of oxygen carriers.

Kinetic Relevance of Surface Reactions and Lattice Diffusion in the Dynamics of Ce–Zr Oxides Reduction–Oxidation Cycles

Reduction–oxidation cycles in oxides are ubiquitous in oxygen storage and transport, chemical looping processes, and fuel cells. O-atom addition and removal are mediated by coupling reactions of oxidants and reductants at surfaces with diffusion of O-atoms within oxide crystals, with either or both processes as limiting steps. CeO2–ZrO2 solid solutions (CZO) are ubiquitous in practice. They are used here to illustrate general experimental strategies and reaction–diffusion formalisms for nonideal systems that enable assessments of the kinetic relevance of the steps that mediate O-atom addition and removal in these materials; these experiments are described within the context of models that describe the driving forces for reaction and diffusion rigorously in terms of oxygen chemical potentials (μO). These strategies assess the rate consequences of varying the fluid phase redox potential, through changes in the identity and pressures of the reactants and products used in redox cycles (O2; CO/CO2; H2/H2O; N2O/N2), of introducing dispersed metal nanoparticles that capture and react lattice O-atoms in CZO using CO or H2, and of imposing intervening dwells without reaction within redox cycles. O-removal rates depend on reductant pressures, even when CO/CO2 and H2/H2O ratios are chosen to maintain the same surface μO if surface reactions were quasi-equilibrated. These data, taken together with significant rate enhancements in O-removal when Pt nanoparticles are present at CZO crystal surfaces and with similar rates before and after inert dwells, demonstrate that reduction rates by both CO and H2 are limited by surface reactions without the presence of consequential spatial gradients in μO within CZO crystals. In contrast, O-addition rates to partially reduced CZO crystals are similar for N2O and O2 reactants and are not affected by the presence of Pt nanoparticles; O-addition rates are significantly higher after intervening inert dwells during CZO oxidation, indicative of spatial gradients in μO, which relax during nonreactive periods. These methods and models, illustrated here for CZO redox cycles at conditions relevant to oxygen storage practice, allow systematic assessments of the kinetic relevance of lattice diffusion and surface reactions for systems that use solids for the reversible storage and release of atoms, irrespective of the identity of the solids or the atoms (e.g., O, H, N, and S).

Simulation analysis of bio-oil reforming for co-production syngas and high purity H2 or CO2 with chemical looping processes

Chemical looping allows for the production of different products, such as syngas, high-purity hydrogen and high concentration CO2, through a single process without the need for separation. In this study, a detailed analysis is investigated by utilizing Aspen Plus software to explore the feasibility of bio-oils efficient utilization in four chemical looping processes. The Sorption Enhanced Steam Reforming (SESR) process for syngas production with CO2 capture can be modified into an autothermal process by combusting a fraction of the syngas, leading to a syngas yield of 11.04 (mol/mol fuel) and a CO2 capture capacity of 4.18 (mol/mol fuel). The Calcium and Chemical Looping Reforming (CaL-CLR) process is a unique approach to attaining the greatest syngas yield of 11.31 (mol/mol fuel) and capturing the highest CO2 of 5.40 (mol/mol fuel) simply by adjusting the flow rate of NiO oxygen carrier to realize autothermic reforming without compromising the syngas output. Nevertheless, the integration of NiO and air reactor (AR) will lead to increased financial and operational outlays. For co-production of syngas and high-purity hydrogen, the Three-Reactors Chemical Looping Hydrogen (TR-CLH) process can produce high-purity hydrogen with 5.70 (mol/mol fuel) under autothermal conditions, but the syngas yield and purity are low. The Chemical Looping Reforming and Water Splitting (CLRWS) process has the potential to create a substantial syngas yield, up to 9.63 (mol/mol fuel), as well as a considerable flow rate of high-purity hydrogen, 8.42 (mol/mol fuel). It is clear that this process is more successful than other processes available; however, the excessively high temperature of the fuel reactor (FR) impedes its further utilization.

Reducing the cost of low-carbon hydrogen production via emerging chemical looping process

A thorough techno-economic analysis where inherent carbon capture is examined against state-of-the-art blue hydrogen production configurations for large (100,000 Nm3/h) and very large (333,000 Nm3/h) capacities. Advanced solvent-based technologies based on post-combustion capture and auto-thermal reformer combined with a gas heated reformer are simulated with process flowsheet software and compared with the emerging chemical looping process. A network of dynamically operated packed bed reactors has been designed and modelled using an in-house code and key parameters generating uncertainties in the results have been examined in a sensitivity analysis. The chemical looping reforming process presents a higher net reforming efficiency than the benchmark cases (8.2 % higher at large scale and 1.5 % higher at very large scale) ranged 75.4–75.7 % while the specific energy for CO2 avoidance is negative in the range of −0.78 to −0.85 MJ/kgCO2. In the carbon capture cases, the chemical looping reforming in packed beds technology generated a levelised cost of hydrogen of 168.9 £/kNm3H2 for the large scale and 159.1 £/kNm3H2 for the very large scale, with the values for the benchmark cases being higher at 196.4 and 166.6 £/kNm3H2, respectively while the levelised cost of hydrogen values are 1 % higher in the benchmark cases where carbon emission price is accounted for. The carbon capture ratio is 99.9 % for the chemical looping reforming cases compared to 90–91 % for the benchmark ones, thus providing a significant foreground for the scale-up and implementation of chemical looping reforming technologies for hydrogen production.

Modelling of multiphase reactive flows in a full-loop coal-direct chemical looping combustor

The coal-direct chemical looping (CDCL) process is a promising clean coal combustion technology, yet the internal multiphase reactive flows in the full-loop system are not clear. In this paper, for the first time, a multi-fluid CFD model of gas–solid-powder reactive flow is developed to describe the gas-oxygen carrier-coal reactive flow in a full-loop CDCL unit, based on Eulerian-Eulerian-Lagrangian framework. The model is validated by comparing with experimental data in terms of flow and reaction behaviours. The model predicts the unique gas-oxygen carrier-coal flow in the full-loop moving-bed CDCL unit, including multiple fluidisations, mixing and separation between the oxygen carrier and coal. The mean residence time of coal particles is around 13 s ∼ 20 s through statistics of discrete particles at the reducer outlet. The high temperature in the CDCL system is observed in the AR riser, cyclone and the top section of the fuel reactor (FR); whereas the low temperature is observed at the bottom of the FR and the l-valve, as a collected result of endothermic and exothermic reactions. A less than 5 mol%, leaks of O2 from the combustor to the reducer can be captured, which is caused by the unbalanced pressure distribution. Overall, over 90 vol% CO2 concentration and 100 % coal carbon conversion are achieved at the FR outlet, which indicates a high CO2 separation efficiency and coal carbon conversion of this CDCL system. This work provides a useful tool for designing and optimising CLC processes.

High-purity hydrogen production from real biomass pyrolysis vapors via a chemical looping process

High-purity H2 could be obtained from biomass pyrolysis vapors via chemical looping with Fe–Al–Ni oxides as the oxygen carrier.

Impact of the Sr content on the redox thermodynamics and kinetics of Ca1-xSrxMnO3-δ for tailored properties.

CaMnO3-δ-based perovskites find application in a variety of thermochemical cycles, e.g. oxygen partial pressure adjustment, chemical looping processes, and thermochemical energy storage. The applicability of these materials is governed by their thermodynamic and kinetic properties. Therefore, tunability of these properties is desirable to adapt the material to the required conditions. In this study, the effect of Sr content in Ca1-xSrxMnO3-δ on thermodynamics and kinetics is investigated by thermogravimetric analysis. The thermodynamics are measured in the temperature range of 873 K to 1473 K with oxygen partial pressures of 1 × 10-4 bar to 0.8 bar. The oxidation kinetics were characterized in the temperature range from 473 K to 673 K with oxygen partial pressures of 0.01 bar to 1 bar. The reduction kinetics were very rapid in the temperature range of 873 K to 1023 K, with the measured rates limited by the constraints of the measurement device. The results show that with increasing Sr content the structural changes of the material decrease the reduction enthalpy and the oxidation activation energy. This not only leads to a tunability of material properties, but can also be used to predict changes of these properties when only the structural changes are known.

Chemical looping oxidative propane dehydrogenation controlled by oxygen bulk diffusion over FeVO4 oxygen carrier pellets

The oxygen distribution and evolution within the oxygen carrier exert significant influence on chemical looping processes. This paper describes the influence of oxygen bulk diffusion within FeVO4 oxygen carrier pellets on the chemical looping oxidative propane dehydrogenation (CL-ODH). During CL-ODH, the oxygen concentration at the pellet surface initially decreased and then maintained stable before the final decrease. At the stage with the stable surface oxygen concentration, the reaction showed a stable C3H6 formation rate and high C3H6 selectivity. Therefore, based on Fick’s second law, the oxygen distribution and evolution in the oxygen carrier at this stage were further analyzed. It was found that main reactions of selective oxidation and over-oxidation were controlled by the oxygen bulk diffusion. C3H8 conversion rate kept decreasing during this stage due to the decrease of the oxygen flux caused by the decline of oxygen gradient within the oxygen carrier, while C3H6 selectivity increased due to the decrease of over-oxidation. In addition, reaction rates could increase with the propane partial pressure due to the increase of the oxygen gradient within the oxygen carrier until the bulk transfer reached its limit at higher propane partial pressure. This study provides fundamental insights for the diffusion-controlled chemical looping reactions.

Understanding C-H activation in light alkanes over Cu-MOR zeolites by coupling advanced spectroscopy and temperature-programmed reduction experiments.

The direct activation of methane to methanol (MTM) proceeds through a chemical-looping process over Cu-oxo sites in zeolites. Herein, we extend the overall understanding of oxidation reactions over metal-oxo sites and C-H activation reactions by pinpointing the evolution of Cu species during reduction. To do so, a set of temperature-programmed reduction experiments were performed with CH4, C2H6, and CO. With a temperature ramp, the Cu reduction could be accelerated to detect changes in Cu speciation that are normally not detected due to the slow CH4 adsorption/interaction during MTM (∼200 °C). To follow the Cu-speciation with the three reductants, X-ray absorption spectroscopy (XAS), UV-vis and FT-IR spectroscopy were applied. Multivariate curve resolution alternating least-square (MCR-ALS) analysis was used to resolve the time-dependent concentration profiles of pure Cu components in the X-ray absorption near edge structure (XANES) spectra. Within the large datasets, as many as six different CuII and CuI components were found. Close correlations were found between the XANES-derived CuII to CuI reduction, CH4 consumption, and CO2 production. A reducibility-activity relationship was also observed for the Cu-MOR zeolites. Extended X-ray absorption fine structure (EXAFS) spectra for the pure Cu components were furthermore obtained with MCR-ALS analysis. With wavelet transform (WT) analysis of the EXAFS spectra, we were able to resolve the atomic speciation at different radial distances from Cu (up to about 4 Å). These results indicate that all the CuII components consist of multimeric CuII-oxo sites, albeit with different Cu-Cu distances.

Oxygen migration performance of LaFeO3 perovskite-type oxygen carriers with Sr doping.

A perovskite-type metal oxide with the formula of ABO3 exhibits excellent redox reaction properties for the chemical looping process, where A is usually an alkaline earth metal and B is a transition metal. The oxygen transfer property plays an important role in the performance of oxygen carriers. In this study, the difference in oxygen vacancy formation energy and oxygen migration barrier between the surface and the bulk in the LaFeO3 (ABO3 type) perovskite is investigated via density functional theory (DFT). The effect of Sr doping on the A-site is considered. The results show that the covalency of the interaction between Fe and O at the surface is higher than that in the bulk, and the Sr doping on the A-site enhances this covalency, leading to a lower oxygen vacancy formation energy. The oxygen migration barrier is also different with the distance from the surface. A smaller deviation of the surrounding lattice atoms from their original place in the crystal at a deeper layer leads to a higher oxygen migration energy. Meanwhile, Sr doping increases the lattice constant, leading to larger migration space thus lowering the migration barrier. In addition, Sr doping can weaken the difference in the oxygen migration barrier between near the surface and in the bulk.