Carbon Dioxide Reforming of Methane using an Isothermal Redox Membrane Reactor

Synthesizing High-Volume Chemicals from CO2 without Direct H2 Input

Decarbonizing the chemical industry will eventually entail using CO₂ as a feedstock for chemical synthesis. However, many chemical syntheses involve CO₂ reduction using inputs such as renewable hydrogen. In this review, chemical processes are discussed that use CO₂ as an oxidant for upgrading hydrocarbon feedstocks. The captured CO₂ is inherently reduced by the hydrocarbon co-reactants without consuming molecular hydrogen or renewable electricity. This CO₂ utilization approach can be potentially applied to synthesize eight emission-intensive molecules, including olefins and epoxides. Catalytic systems and reactor concepts are discussed that can overcome practical challenges, such as thermodynamic limitations, over-oxidation, coking, and heat management. Under the best-case scenario, these hydrogen-free CO₂ reduction processes have a combined CO₂ abatement potential of approximately 1 gigatons per year and avoid the consumption of 1.24 PWh renewable electricity, based on current market demand and supply.

High redox performance of Y0.5Ba0.5CoO3−δ for thermochemical oxygen production and separation

The efficient production and separation of oxygen is essential for numerous energy-intensive industrial applications in the fuel and mineral processing sectors.

Solar Energy on Demand: A Review on High Temperature Thermochemical Heat Storage Systems and Materials

Among renewable energies, wind and solar are inherently intermittent and therefore both require efficient energy storage systems to facilitate a round-the-clock electricity production at a global scale. In this context, concentrated solar power (CSP) stands out among other sustainable technologies because it offers the interesting possibility of storing energy collected from the sun as heat by sensible, latent, or thermochemical means. Accordingly, continuous electricity generation in the power block is possible even during off-sun periods, providing CSP plants with a remarkable dispatchability. Sensible heat storage has been already incorporated to commercial CSP plants. However, because of its potentially higher energy storage density, thermochemical heat storage (TCS) systems emerge as an attractive alternative for the design of next-generation power plants, which are expected to operate at higher temperatures. Through these systems, thermal energy is used to drive endothermic chemical reactions, which can subsequently release the stored energy when needed through a reversible exothermic step. This review analyzes the status of this prominent energy storage technology, its major challenges, and future perspectives, covering in detail the numerous strategies proposed for the improvement of materials and thermochemical reactors. Thermodynamic calculations allow selecting high energy density systems, but experimental findings indicate that sufficiently rapid kinetics and long-term stability trough continuous cycles of chemical transformation are also necessary for practical implementation. In addition, selecting easy-to-handle materials with reduced cost and limited toxicity is crucial for large-scale deployment of this technology. In this work, the possible utilization of materials as diverse as metal hydrides, hydroxides, or carbonates for thermochemical storage is discussed. Furthermore, special attention is paid to the development of redox metal oxides, such as Co₃O₄/CoO, Mn₂O₃/Mn₃O₄, and perovskites of different compositions, as an auspicious new class of TCS materials due to the advantage of working with atmospheric air as reactant, avoiding the need of gas storage tanks. Current knowledge about the structural, morphological, and chemical modifications of these solids, either caused during redox transformations or induced wittingly as a way to improve their properties, is revised in detail. In addition, the design of new reactor concepts proposed for the most efficient use of TCS in concentrated solar facilities is also critically considered. Finally, strategies for the harmonic integration of these units in functioning solar power plants as well as the economic aspects are also briefly assessed.

Thermodynamic limits of countercurrent reactor systems, with examples in membrane reactors and the ceria redox cycle

Countercurrent reactors can be utilized in chemical reaction systems which involve either a reaction between flows of different phases, or reactions between flows separated by a selective permeable membrane. This idea is quite similar in nature to a countercurrent heat exchanger, where the inlet of one participating flow is exposed to the outlet of the opposite flow. A countercurrent configuration can therefore improve the reaction conversion extent and transport properties. Here we formulate a straightforward approach in terms of an exchange coordinate, in order to determine an upper bound of species exchange in such systems, subject to the second law of thermodynamics and conservation of mass. The methodology is independent of the specifics of reactor design and can be generally applied to determine the maximum thermodynamic benefit of using a countercurrent reactor. We then demonstrate the analysis for a number of thermochemical fuel production routes; membrane thermolysis of carbon dioxide, dry methane reforming across a membrane, reverse water gas shift across a membrane, and the thermochemical ceria cycle.

Dry Reforming in a Milli-Scale Reactor Driven by Simulated Sunlight

In this study, a directly irradiated, milli-scale chemical reactor with a simple nickel catalyst was designed for dry reforming of methane for syngas. A milli-scale reactor was used to facilitate rapid heating, which is conducive to combating thermal transience caused by intermittent solar energy, as well as reducing startup times. Milli-scale reactors also allow for a distributed and modular process to produce chemicals on a more local scale. In this setup, the catalyst involved in the reaction is located directly in the focal area of the solar simulator, resulting in rapid heating. The effects of mean residence time and temperature on conversion and energy efficiency were tested. The process, which is intended to store thermal energy as chemical enthalpy, achieved 10% thermal-to-chemical energy conversion efficiency at a mean residence time of 0.028 s, temperature of 1000 °C, and molar feed ratio of 1:1 CO2:CH4. A significant portion of the thermal energy input into the reactor was directed toward sensible heating of the feed gas. Thus, this technology has potential to achieve solar-to-chemical efficiency with the integration of recuperative heat exchange.

Hydrogen-assisted Carbon Dioxide Thermochemical Reduction on La0.9 Ca0.1 FeO3-δ Membranes: A Kinetics Study

Kinetics data for CO₂ thermochemical reduction in an isothermal membrane reactor is required to identify the rate-limiting steps. A detailed reaction kinetics study on this process supported by an La_0.9 Ca_0.1 FeO_3-δ (LCF-91) membrane is thus reported. The dependence of CO₂ reduction rate on various operating conditions is examined, such as CO₂ concentration on the feed side, fuel concentrations on the sweep side, and temperatures. The CO₂ reduction rate is proportional to the oxygen flux across the membrane, and the measured maximum fluxes are 0.191 and 0.164 μmol cm^-2  s^-1 with 9.5 mol% H₂ and 11.6 mol% CO on the sweep side at 990 °C, respectively. Fuel is used to maintain the chemical potential gradient across the membrane and CO is used to derive the surface reaction kinetics. This membrane also exhibits stable performances for 106 h. A resistance-network model is developed to describe the oxygen transport process and the kinetics data are parameterized using the experimental values. The model shows a transition of the rate limiting step between the surface reactions on the feed side and the sweep side depending on the operating conditions.

Solar-Driven Thermochemical Splitting of CO2 and In Situ Separation of CO and O2 across a Ceria Redox Membrane Reactor

Splitting CO2 with a thermochemical redox cycle utilizes the entire solar spectrum and provides a favorable path to the synthesis of solar fuels at high rates and efficiencies. However, the temperature/pressure swing commonly applied between reduction and oxidation steps incurs irreversible energy losses and severe material stresses. Here, we experimentally demonstrate for the first time the single-step continuous splitting of CO2 into separate streams of CO and O2 under steady-state isothermal/isobaric conditions. This is accomplished using a solar-driven ceria membrane reactor conducting oxygen ions, electrons, and vacancies induced by the oxygen chemical potential gradient. Guided by the limitations imposed by thermodynamic equilibrium of CO2 thermolysis, we operated the solar reactor at 1,600°C, 3·10-6 bar [Formula: see text] and 3,500 suns radiation, yielding total selectivity of CO2 to CO + ½O2 with a conversion rate of 0.024 μmol·s-1 per cm2 membrane. The dynamics of the oxygen vacancy exchange, tracked by GC and XPS, further validated stable fuel production.

Design principles of perovskites for solar-driven thermochemical splitting of CO2

Perovskites are attractive redox materials for thermo/electrochemical fuel synthesis. To design perovskites with balanced redox energetics for thermochemically splitting CO2, the activity of lattice oxygen vacancies and stability against crystal phase changes and detrimental carbonate formation are predicted for a representative range of perovskites by electronic structure computations. Systematic trends in these materials properties when doping with selected metal cations are described in the free energy range defined for isothermal and temperature-swing redox cycles. To confirm that the predicted materials properties root in the bulk chemical composition, selected perovskites are synthesized and characterized by X-ray diffraction, transmission electron microscopy, and thermogravimetric analysis. On one hand, due to the oxidation equilibrium, none of the investigated compositions outperforms non-stoichiometric ceria - the benchmark redox material for CO2 splitting with temperature-swings in the range of 800-1500 °C. On the other hand, certain promising perovskites remain redox-active at relatively low oxide reduction temperatures at which ceria is redox-inactive. This trade-off in the redox energetics is established for YFeO3, YCo0.5Fe0.5O3 and LaFe0.5Ni0.5O3, identified as stable against phase changes and capable to convert CO2 to CO at 600 °C and 10 mbar CO in CO2, and to being decomposed at 1400 °C and 0.1 mbar O2 with an enthalpy change of 440-630 kJ mol-1 O2.

High Redox Capacity of Al-Doped La1-x Srx MnO3-δ Perovskites for Splitting CO2 and H2 O at Mn-Enriched Surfaces

Perovskites are attractive candidates for the solar-driven thermochemical redox splitting of CO₂ and H₂ O into CO and H₂ (syngas) and O₂ . This work investigates the surface activity of La_1-x Sr_x Mn_1-y Al_y O_3-δ (0≤x≤1, 0≤y≤1) and La_0.6 Ca_0.4 Mn_0.6 Al_0.4 O_3-δ . At 1623 K and 15 mbar O₂ , the oxygen non-stoichiometry of La_0.2 Sr_0.8 Mn_0.8 Al_0.2 O_3-δ increases with the strontium content and reaches a maximum of δ=0.351. X-ray photoelectron spectroscopy analysis indicates that manganese is the only redox-active metal at the surface. All La_1-x Sr_x Mn_1-y Al_y O_3-δ compositions exhibit surfaces enriched in manganese and depleted in strontium. We discuss how these compositional differences of the surface from the bulk lead to the beneficially higher reduction extents and lower strontium carbonate concentrations at the aluminum-doped surfaces. Using first principles calculations, we validate the experimental reduction trends and elucidate the mechanism of the partial electronic charge redistribution upon perovskite reduction.