The dry reforming of methane through the nonstoichiometric ceria (CeO2–CeO2−δ) redox cycle was examined theoretically and experimentally for converting high‐temperature, solar process heat to syngas. The aforementioned cycle is composed of: (1) endothermic reduction of ceria and simultaneous partial oxidation of methane and (2) exothermic oxidation of the reduced ceria and simultaneous reduction of CO2. In both steps, chemical equilibrium calculations indicate that isothermal operation is thermodynamically favorable under a wide range of conditions. The influence of the total amount of reactive gas, the operating temperature, and the inclusion of gas‐ or solid‐phase heat exchangers on the cycle performance was determined through a holistic process model. A theoretical solar‐to‐fuel conversion efficiency, defined as the ratio of the difference between the calorific value of syngas (H2 and CO) produced and methane converted to the total solar radiative input energy, of more than 45 % was predicted with no heat recuperation. Experimental validation was subsequently demonstrated in a packed‐bed‐type solar reactor using the high‐flux solar simulator at the University of Florida at three discrete isothermal temperatures, namely, 950, 1035, and 1120 °C. Upon the completion of the reduction at each temperature, the bed‐averaged oxygen nonstoichiometry equaled 0.07, 0.21, and 0.24, which yielded methane conversions of 9, 41, and 51 %, respectively. At 1120 °C, the extrapolated solar‐to‐fuel conversion efficiency was 9.82 %.

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Thermodynamic data for several LaMnO 3 -based perovskites indicates that in the high oxygen partial pressure (pO 2 ) range (e.g., 10 −7 to 10 −3 atm), where isothermal thermochemical redox cycling is viable, they can undergo larger changes in oxidation state than ceria for a given change in pO 2 . This suggests that these materials may be more optimal for isothermal operation than ceria and offers the potential for more efficient H 2 /CO production via thermochemical splitting of H 2 O/CO 2 . To investigate this hypothesis, we developed a thermodynamic process model to predict the solar-to-fuel efficiencies of La 1−x (Sr,Ca) x Mn 1−y Al y O 3 perovskites and compared results to ceria and Zr-doped ceria. The calculations were performed for isothermal or near-isothermal cycling from 1473 to 1773 K. Four methods of lowering the reduction pO 2 were considered: inert gas sweeping, mechanical vacuum pumping, electrochemical oxygen pumping, and thermochemical oxygen pumping. Considering a reduction pO 2 of 10 −6 atm and a gas-phase heat recovery effectiveness of 95%, the calculations showed that the perovskites outperformed ceria and Zr-doped ceria during isothermal operation in terms of fuel production and efficiency regardless of the pO 2 reduction method. For example, at 1773 K, the calculated efficiencies were 35.17% for La 0.6 Sr 0.4 Mn 0.6 Al 0.4 O 3 and 28.26% for ceria when implementing thermochemical oxygen pumping. Other methods of lowering the reduction pO 2 resulted in lower efficiencies, where electrochemical oxygen pumping > inert gas sweeping > vacuum pumping. Small temperature swings using inert gases to lower the pO 2 resulted in the highest efficiencies overall. For example, with a reduction temperature of 1773 K and a temperature swing of 100 K, the efficiency of the ceria-based cycle was 35.18% and with a temperature swing of 300 K, the efficiency of the La 0.6 Ca 0.4 MnO 3 cycle was 40.75%. Importantly, in the case of inert gas sweeping, the efficiency of the ceria-based cycle exceeds that of the candidate materials when the temperature swing is low. The theoretical calculations within this work show that perovskites have the potential for improved solar-to-fuel efficiencies during isothermal or near-isothermal redox cycling beyond those achievable by ceria.

Technologies that facilitate the conversion of CH4 and/or CO2 with concentrated sunlight provide a viable strategy for storing solar energy in the form of liquid fuels and reducing anthropogenic greenhouse gas emissions. Herein, a scalable prototype receiver‐reactor is developed to experimentally demonstrate the chemical‐looping, dry reforming of methane over ceria with simulated concentrated solar radiation. Optimal operating conditions are identified by investigating wide ranges of parameters like temperature, gas flowrate, inlet CH4 concentration, initial oxygen nonstoichiometry, and particle size. Ultimately, a selectivity to H2 and CO of greater than 0.93 is observed at reactant conversions of 0.69 and 0.88 for CH4 and CO2, respectively. As a result, the calorific value of the products relative to the reactants is upgraded, and a solar‐to‐fuel conversion efficiency of 10.06% is attained, higher than the previously reported record of 7%. Near‐perfect selectivity to syngas is achieved by operating with low reactant residence times, and if reactions were initiated over oxygen‐deficient ceria. Reactant conversion is enhanced through a reduction in particle size, which enables more rapid kinetics via an increase in surface oxygen availability. Stable performance is demonstrated over 10 consecutive redox cycles under conditions that maximized efficiency for the system presented herein.

The two-step metal oxide redox cycle is a promising and thermodynamically attractive means of solar fuel production. In this work, we describe the development of a high-temperature tubular reactor in which the fundamental thermodynamic and kinetic behavior of thermochemical materials can be readily assessed. This reactor system is capable of operating at temperatures up to 1873 K, total pressures ranging from vacuum to ambient, and oxygen partial pressures (pO2) as low as 10−29 atm. Compared to off-the-shelf systems like thermogravimetric analyzers or indirect conductivity-based measurement systems, this system has three inherent benefits: (1) the flexibility to control the sample morphology (e.g., powder, packed bed, reticulated porous ceramic, or pellet), (2) the potential for a well-developed and characterized flow, and (3) the ability to readily customize the system on demand (e.g., easy integration with a steam generator to control and operate at very low pO2). The reactor system and experimental methods were validated by performing isothermal relaxation experiments with undoped ceria, wherein the sample environment was rapidly altered by stepwise changes in the delivered H2O vapor concentration, and comparing measured oxygen nonstoichiometries with accepted data available in the literature. Data were measured at temperatures from 1173 to 1473 K and pO2 from 4.54  × 10−18 to 1.02 × 10−9 atm. The measured equilibrium data displayed strong agreement with the literature and the expected trends were preserved. Kinetic data were extracted by first transforming reactant concentrations measured downstream of the reaction zone using a tanks-in-series mixing model to account for gas dispersion. Next, a mechanistic kinetic model distinguishing surface and bulk species concentrations was fit to the data to extract pertinent thermodynamic and kinetic parameters. The model assumed a two-step reaction mechanism mediated by the formation of an intermediate hydroxyl species on the surface. Activation energies and defect formation enthalpies and entropies for the forward and reverse reactions were found to be in good agreement with previous modeling efforts, providing further validation of the use of this system to explore thermodynamic and kinetic behavior of emerging thermochemical materials.