Enhanced lattice oxygen reactivity over Fe2O3/Al2O3 redox catalyst for chemical-looping dry (CO2) reforming of CH4: Synergistic La-Ce effect

Plasma Chemical Looping: Unlocking High-Efficiency CO2 Conversion to Clean CO at Mild Temperatures

We propose a plasma chemical looping CO2 splitting (PCLCS) approach that enables highly efficient CO2 conversion into O2-free CO at mild temperatures. PCLCS achieves an impressive 84% CO2 conversion and a 1.3 mmol g-1 CO yield, with no O2 detected. Crucially, this strategy significantly lowers the temperature required for conventional chemical looping processes from 650 to 1000 °C to only 320 °C, demonstrating a robust synergy between plasma and the Ce0.7Zr0.3O2 oxygen carrier (OC). Systematic experiments and density functional theory (DFT) calculations unveil the pivotal role of plasma in activating and partially decomposing CO2, yielding a mixture of CO, O2/O, and electronically/vibrationally excited CO2*. Notably, these excited CO2* species then efficiently decompose over the oxygen vacancies of the OCs, with a substantially reduced activation barrier (0.86 eV) compared to ground-state CO2 (1.63 eV), contributing to the synergy. This work offers a promising and energy-efficient pathway for producing O2-free CO from inert CO2 through the tailored interplay of plasma and OCs.

Structured Catalysts and Catalytic Processes: Transport and Reaction Perspectives

The structure of catalysts determines the performance of catalytic processes. Intrinsically, the electronic and geometric structures influence the interaction between active species and the surface of the catalyst, which subsequently regulates the adsorption, reaction, and desorption behaviors. In recent decades, the development of catalysts with complex structures, including bulk, interfacial, encapsulated, and atomically dispersed structures, can potentially affect the electronic and geometric structures of catalysts and lead to further control of the transport and reaction of molecules. This review describes comprehensive understandings on the influence of electronic and geometric properties and complex catalyst structures on the performance of relevant heterogeneous catalytic processes, especially for the transport and reaction over structured catalysts for the conversions of light alkanes and small molecules. The recent research progress of the electronic and geometric properties over the active sites, specifically for theoretical descriptors developed in the recent decades, is discussed at the atomic level. The designs and properties of catalysts with specific structures are summarized. The transport phenomena and reactions over structured catalysts for the conversions of light alkanes and small molecules are analyzed. At the end of this review, we present our perspectives on the challenges for the further development of structured catalysts and heterogeneous catalytic processes.

Perovskites as oxygen storage materials for chemical looping partial oxidation and reforming of methane

The traditional partial oxidation, dry reforming and steam reforming of methane technologies are separated into two reactors for execution by chemical looping technology, which can avoid the defects exposed in the traditional process (avoiding carbon accumulation, reducing costs, etc.). The key to chemical looping technology is to find suitable oxygen carriers (OCs), which can store and release oxygen to form a closed loop in the chemical looping. The purpose of this review is to summarize the current status of perovskite oxides for partial oxidation and reforming of methane in chemical looping, describe the structure, oxygen capacity, oxygen migration rate and common synthesis methods of perovskites in chemical looping. In addition, the effects of impregnation loading, ion doping, and structural morphology on the catalytic conversion of CH4 by perovskite OCs and the reaction mechanism on the OCs are also discussed.

Sustainable and Highly Efficient Recycling of Plastic Waste into Syngas via a Chemical Looping Scheme

Converting plastic waste into valuable products (syngas) is a promising approach to achieve sustainable cities and communities. Here, we propose for the first time to convert plastic waste into syngas via the Fe₂AlO_x-based chemical looping technology in a two-zone reactor. The Fe₂AlO_x-based redox cycle was achieved with the pyrolysis of plastic waste in the upper zone, followed by the decomposition and thermal cracking of hydrocarbon vapors, and the oxidation and water splitting in the lower zone (850 °C) enabled a higher carbon conversion (81.03%) and syngas concentration (92.84%) when compared with the mixed feeding process. The iron species could provide lattice oxygen and meanwhile act as the catalyst for the deep decomposition of hydrocarbons into CO and the accumulation of deposited carbon in the reduction step. Meanwhile, the introduced water would be split by the reduced iron and deposited carbon to further produce H₂ and CO in the following oxidation step. A high hydrogen yield of 85.82 mmol/g HDPE with a molar ratio of H₂/CO of 2.03 was achieved from the deconstruction of plastic waste, which lasted for five cycles. This proof of concept demonstrated a sustainable and highly efficient pathway for the recycling of plastic waste into valuable chemicals.

Chemical Looping Combustion: A Brief Overview

The current development of chemical looping combustion (CLC) technology is presented in this paper. This technique of energy conversion enables burning of hydrocarbon fuels with dramatically reduced CO2 emission into the atmosphere, since the inherent separation of carbon dioxide takes place directly in a combustion unit. In the beginning, the general idea of the CLC process is described, which takes advantage of solids (so-called oxygen carriers) being able to transport oxygen between combustion air and burning fuel. The main groups of oxygen carriers (OC) are characterized and compared, which are Fe-, Mn-, Cu-, Ni-, and Co-based materials. Moreover, different constructions of reactors tailored to perform the CLC process are described, including fluidized-bed reactors, swing reactors, and rotary reactors. The whole systems are based on the chemical looping concept, such as syngas CLC (SG-CLC), in situ Gasification CLC (iG-CLC), chemical looping with oxygen uncoupling (CLOU), and chemical looping reforming (CLR), are discussed as well. Finally, a comparison with other pro-CCS (carbon capture and storage) technologies is provided.

Hydrogen Yield from CO2 Reforming of Methane: Impact of La2O3 Doping on Supported Ni Catalysts

Development of a transition metal based catalyst aiming at concomitant high activity and stability attributed to distinguished catalytic characteristics is considered as the bottleneck for dry reforming of methane (DRM). This work highlights the role of modifying zirconia (ZrO2) and alumina (Al2O3) supported nickel based catalysts using lanthanum oxide (La2O3) varying from 0 to 20 wt% during dry reforming of methane. The mesoporous catalysts with improved BET surface areas, improved dispersion, relatively lower reduction temperatures and enhanced surface basicity are identified after La2O3 doping. These factors have influenced the catalytic activity and higher hydrogen yields are found for La2O3 modified catalysts as compared to base catalysts (5 wt% Ni-ZrO2 and 5 wt% Ni-Al2O3). Post-reaction characterizations such as TGA have showed less coke formation over La2O3 modified samples. Raman spectra indicates decreased graphitization for La2O3 catalysts. The 5Ni-10La2O3-ZrO2 catalyst produced 80% hydrogen yields, 25% more than that of 5Ni-ZrO2. 5Ni-15La2O3-Al2O3 gave 84% hydrogen yields, 8% higher than that of 5Ni-Al2O3. Higher CO2 activity improved the surface carbon oxidation rate. From the study, the extent of La2O3 loading is dependent on the type of oxide support.

Tuning the Support Properties toward Higher CO2 Conversion during a Chemical Looping Scheme

The chemical looping process is promising for CO₂ conversion because of the much higher CO₂ conversion efficiency than the photocatalytic and electrocatalytic processes. Conventional oxygen carriers have to include a high content of inert support, typically Al₂O₃, to avoid sintering, thus leading to a trade-off between reactivity and stability. Here, we propose the use of ion-conductive Gd_xCe_2-xO_2-δ (GDC) to prepare the supported oxygen carriers. The resulting Fe₂O₃/GDC materials achieve both high reactivity and stability. Fe₂O₃/Gd_0.3Ce_1.7O_2-δ shows high CO productivity (∼10.79 mmol·g^-1) and CO production rate (∼0.77 mmol·g^-1·min^-1), which are twofold higher than that of Fe₂O₃/Al₂O₃. The performance remains stable even after 30 cycles. The mechanism study confirmed the rate-limiting role of the oxygen-ion conductivity, and the GDC support enhanced the oxygen-ion conductivity of oxygen carriers during the redox reactions, thus leading to improved CO₂ splitting performance. A roughly linear relationship between the oxygen-ion conductivity and CO₂ yield is also obtained and verified in our testing conditions. This relation can be used to predict and select oxygen carriers with high CO₂ splitting performance.

Perovskites in the Energy Grid and CO2 Conversion: Current Context and Future Directions

CO2 emissions from the consumption of fossil fuels are continuously increasing, thus impacting Earth’s climate. In this context, intensive research efforts are being dedicated to develop materials that can effectively reduce CO2 levels in the atmosphere and convert CO2 into value-added chemicals and fuels, thus contributing to sustainable energy and meeting the increase in energy demand. The development of clean energy by conversion technologies is of high priority to circumvent these challenges. Among the various methods that include photoelectrochemical, high-temperature conversion, electrocatalytic, biocatalytic, and organocatalytic reactions, photocatalytic CO2 reduction has received great attention because of its potential to efficiently reduce the level of CO2 in the atmosphere by converting it into fuels and value-added chemicals. Among the reported CO2 conversion catalysts, perovskite oxides catalyze redox reactions and exhibit high catalytic activity, stability, long charge diffusion lengths, compositional flexibility, and tunable band gap and band edge. This review focuses on recent advances and future prospects in the design and performance of perovskites for CO2 conversion, particularly emphasizing on the structure of the catalysts, defect engineering and interface tuning at the nanoscale, and conversion technologies and rational approaches for enhancing CO2 transformation to value-added chemicals and chemical feedstocks.

Rare-earth-containing perovskite nanomaterials: design, synthesis, properties and applications

As star material, perovskites have been widely used in the fields of optics, photovoltaics, electronics, magnetics, catalysis, sensing, etc. However, some inherent shortcomings, such as low efficiency (power conversion efficiency, external quantum efficiency, etc.) and poor stability (against water, oxygen, ultraviolet light, etc.), limit their practical applications. Downsizing the materials into nanostructures and incorporating rare earth (RE) ions are effective means to improve their properties and broaden their applications. This review will systematically summarize the key points in the design, synthesis, property improvements and application expansion of RE-containing (including both RE-based and RE-doped) halide and oxide perovskite nanomaterials (PNMs). The critical factors of incorporating RE elements into different perovskite structures and the rational design of functional materials will be discussed in detail. The advantages and disadvantages of different synthesis methods for PNMs will be reviewed. This paper will also summarize some practical experiences in selecting suitable RE elements and designing multi-functional materials according to the mechanisms and principles of REs promoting the properties of perovskites. At the end of this review, we will provide an outlook on the opportunities and challenges of RE-containing PNMs in various fields.

Ternary Mixed Spinel Oxides as Oxygen Carriers for Chemical Looping Hydrogen Production Operating at 550 °C

Operating chemical looping at moderate temperatures circumvents the issue that the sintering of oxygen carrier materials is serious at typical operating conditions, 800-950 °C. However, lower temperatures can lead to deterioration on the reaction kinetics and thereby the low H₂ production rate and yield. Here, we present several doped spinel oxides consisting of earth-abundant elements for chemical looping water splitting. By virtue of the ability of the Cu dopant to improve the reduction of the Co-based binary spinel, the high reducibility of the dopants in the reduction period, as well as the phase reversibility in the water splitting period, Cu_0.25Co_0.25Fe_2.5O_y shows a high hydrogen yield (∼11.9 mmol g^-1) and an average hydrogen production rate (∼137.7 μmol g^-1 min^-1) at 550 °C, with negligible decays in repetitive redox cycles. The performance of this material is comparable to that of the state-of-the-art perovskites which usually contain rare-earth metals, enabling its potential in industrial implementation.

Facile one-step preparation of ordered mesoporous Ni–M–Al (M = K, Mg, Y, and Ce) oxide catalysts for methane dry reforming

Excellent DRM performances of a Ni–Y–Al oxide catalyst is related to the comprehensively advantageous textural properties, such as high Ni dispersion, large surface area, and small Ni nanoparticles inlaid in the honeycomb-like mesoporous skeleton.

Perspectives of multiscale rare earth crystal materials

Both multisize and multiweight effects are proposed to characterize multiscale rare earth crystal materials.