Thermochemical Cycling Research Articles

Effect of diameter on the stability of granulated Ca(OH)2 during thermochemical cycling in a fixed bed reactor

One solution to reduce CO2 emissions and advance climate protection in the heat sector is the utilization of thermochemical energy storages (TCES) based on the material Ca(OH)2 to store waste heat or surplus electricity. In contrast to powdered materials, the material in granular form has a lower tendency to be affected by agglomeration effects and provides higher energy density and thermal conductivity. This results in enhanced durability, stability, and efficiency of the storage, making the storage material Ca(OH)2 in granular form more suitable for TCES applications. This work investigates the performance of a chemically unmodified granular sample over 15 storage cycles in a test fixed bed reactor and evaluates whether its performance and durability are already sufficient for potential applications without further modification. In addition, different granule samples with different diameters were also cycled to determine whether the selection of certain diameter sizes can improve the stability of the granules. Furthermore, different methods were used in the pre- and post-analyses to assess the stability of the samples. The comparison of three tested granular samples with different diameters reveals that the sample with a diameter of 2–4 mm has the highest durability and reactor performance. Despite a decrease in mechanical strength, this configuration could be suitable for applications with a small number of storage cycles, such as seasonal storages.

A review of methane-driven two-step thermochemical cycle hydrogen production

Solar thermochemical cycling is a promising approach to utilizing solar energy to produce H2 and CO, which can be further synthesized into liquid fuels via Fischer-Tropsch reactions, making storage and transportation easier. Despite the potential high theoretical efficiency, factors such as the high temperature and low oxygen partial pressure required for the reduction restrict the large-scale application of this technology. In the near-to-medium term, in order to accelerate the popularization and application of this technology, according to Le Chatelier's principle, the assisted reduction of methane can greatly reduce the demand for high-temperature materials and ultra-low oxygen partial pressure. However, the addition of methane will also bring additional challenges, such as carbon emissions and carbon deposition. Reducing the influence of factors such as material sintering and irreversible loss on system efficiency in the experiment will be essential for the development of this technology. This paper focuses on the methane-driven two-step thermochemical cycle hydrogen production process, summarizes its reaction mechanism, thermodynamic analysis, kinetic analysis, oxygen carrier material doping, and experimental research in detail, and comprehensively analyzes the effect of material properties and irreversible loss on the system. According to thermodynamic and kinetic analysis, rational selection and doping of metal oxygen carrier materials can effectively reduce the reaction temperature, improve gas production rates, and enhance structural stability. Analyzing and trying to reduce various irreversible losses of the system, such as reradiation energy loss, conduction energy loss, etc., and taking appropriate heat recovery measures will help to improve the system efficiency of the actual cycle. This paper would be valuable for the development of this technology and for achieving the goal of carbon neutrality.

Influence of structural changes on gas transport properties of a cycled CaO/Ca(OH)2 powder bulk for thermochemical energy storage

Thermochemical energy storage using reversible gas-solid reactions has a great potential to evolve into a vital part of a future energy system. A major obstacle for technical application, however, is caused by insufficient gas transport within storage reactors leading to ineffective charging and discharging as well as lower storage densities. Although significant changes in the solid bulk structure are reported throughout the literature, the relevant material property – the bulk permeability – is rarely experimentally studied. In the presented study, the evolution of the gas permeability of a technical grade CaO/Ca(OH)2 powder bulk is studied experimentally. To this end, the pressure drop at ambient conditions is recorded before thermochemical cycling as well as after each dehydration respectively rehydration reaction. The pressure gradient and thereby the permeability is determined based on Darcy's law. The permeability of the fresh Ca(OH)2 bulk after compaction due to the gas flow is 1·10−13 m2. Due to restructuring during thermochemical cycling the permeability increases to 1.6…3.3·10−12 m2 for the cycled Ca(OH)2 bulk and 9.2·10−12 m2 to 2.4·10−11 m2 for the cycled CaO bulk. Further analysis of fresh and cycled samples shows a general increase in particle size and decrease in specific surface area as well as a reduction in reactivity due to agglomeration of the powder. Noticeable is the inhomogeneity of the cycled bulk with powder mixed with larger agglomerates at the top of the bulk and highly porous but seemingly homogeneous solid material at the bottom. Based on the experimental results, gas transport within the agglomerates is identified as a major limiting factor within thermochemical reactors. It is therefore proposed to consider cycled CaO/Ca(OH)2 bulks on a macro- and mesoscale (respectively bulk and agglomerate) simultaneously if transport properties – and their impact on effective reaction dynamics – are relevant.

Redox Performance and Optimization of the Chemical Composition of Lanthanum–Strontium–Manganese-Based Perovskite Oxide for Two-Step Thermochemical CO2 Splitting

The effects of substitution at the A- and B sites on the redox performance of a series of lanthanum–strontium–manganese (LSM)-based perovskite oxides (Z = Ni, Co, and Mg) were studied for application in a two-step thermochemical CO2 splitting cycle to produce liquid fuel from synthesis gas using concentrated solar radiation as the proposed energy source and CO2 recovered from the atmosphere as the prospective chemical source. The redox reactivity, stoichiometry of oxygen/CO production, and optimum chemical composition of Ni-, Co-, and Mg-substituted LSM perovskites were investigated to enhance oxygen/CO productivity. Furthermore, the long-term thermal stabilities and thermochemical repeatabilities of the oxides were evaluated and compared with previous data. The valence changes in the constituent ionic species of the perovskite oxides were studied and evaluated by X-ray photoelectron spectroscopy (XPS) for each step of the thermochemical cycle. From the perspectives of high redox reactivity, stoichiometric oxygen/CO production, and thermally stable repeatability in long-term thermochemical cycling, Ni0.20-, Co0.35-, and Mg0.125-substituted La0.7Sr0.3Mn perovskite oxides are the most promising materials among the LSM perovskite oxides for two-step thermochemical CO2 splitting, showing CO productivities of 387–533 μmol/g and time-averaged CO productivities of 12.9–18.0 μmol/(min·g) compared with those of LSM perovskites reported in the literature.

A novel high-efficiency solar thermochemical cycle for fuel production based on chemical-looping cycle oxygen removal

Solar-driven two-step thermochemical cycling is a promising means to convert solar energy into storable and transportable chemical fuel, in which hydrogen or carbon monoxide is generated by continuous reduction and oxidation reactions. However, the high energy consumption of deoxygenation in the reduction step is an important factor restricting its efficiency improvement. In this work, we propose a fuel production system with thermochemical cycles coupled with chemical-looping cycles, which uses the chemical-looping cycle oxidation reaction at relatively low temperature to absorb the oxygen produced by the thermochemical cycle reduction reaction. The oxygen-carriers in the chemical-looping cycle can be reduced by adding reductants or direct heating with waste heat from the thermochemical cycle. The coupled system can remove the oxygen in the thermochemical cycle reduction reaction and reduce the energy consumption in this process. In addition to the hydrogen production, waste heat from the thermochemical cycle can also be used in the chemical-looping cycle to generate extra electricity. Theoretical calculation results show that in the oxidation temperature range of 900–1100 °C, the energy consumption for separating oxygen from inert gas after sweeping in the traditional thermochemical cycle accounts for 30–57% of the total energy consumption, while the chemical-looping cycle part in the coupled system accounts for 26–36%. The coupled system can improve the solar-to-fuel efficiency by 45.9% to 20.9% and improve the solar-to-electricity efficiency by 104.1% to 14.6% without heat recovery compared to traditional thermochemical cycles when the reduction temperature is 1500 °C. Under the conditions of 20% solid-state heat recovery and 90% gas-state heat recovery, the coupled system achieves a solar-to-fuel efficiency of 28.1%. In addition, we also put forward a vacuum pump, inert gas and chemical-looping cycles combined oxygen removal method. Since the vacuum pump has high efficiency and fast deoxygenation speed in the low vacuum interval, the system efficiency can be further improved. Our research can provide a new solution to the high energy consumption of deoxygenation in thermochemical cycles.

Structural stabilization of granular Ca(OH)2 by coating with nanostructured additives for thermochemical cycling in a fixed reaction bed

Thermochemical energy storage systems have a high potential to support a future energy supply based on renewables by providing comparably high storage capacities and option for long-term, cold and loss free storage, that allows a flexible utilization in several application areas. Due to its cost efficiency and abundance the reaction system Ca(OH)2/CaO is currently intensively researched. However, the handling of the material in its typically powdery form exhibits some major challenges under cyclic hydrothermal loading due to its high cohesivity and strong tendency towards agglomeration. To increase the bulk performance during cycling, various approaches to stabilize the particles are being investigated, most of them involve the addition of high fractions of thermochemically inert additives and thus high losses in energy density. This paper investigates the approach of coating pre-granulated Ca(OH)2 with nanostructured inorganic oxides by a low-cost mixing process followed by thermal treatment, which requires a low mass fraction of additives compared to the total material. The samples are comparatively analysed regarding their cycling stability, morphology as well as regarding their bulk performance and mechanical stability in a fixed bed reactor. It was found that the shells formed from coating materials increase the stability of the granules and, due to their permeability to the reactants, do not hinder the reaction. Especially the sample coated with nanostructured Al2O3 and thermally post-treated at 600 °C, has the greatest potential to reconcile cycling stability with structural integrity and mechanical stability. A constant conversion rate of around 88% could be measured over several cycles, which is also comparatively high. A constant conversion rate implies maintenance of structural integrity, which could also be confirmed by SEM micrographs.

Investigating the activity of Ca2Fe2O5 additives on the thermochemical energy storage performance of limestone waste

Activation energy for CaCO3 calcination reaction was reduced using Ca2Fe2O5 additives which improved the thermochemical cycling capacity of limestone waste.

Investigation of CO2 Splitting on Ceria-Based Redox Materials for Low-Temperature Solar Thermochemical Cycling with Oxygen Isotope Exchange Experiments

The surface exchange and bulk transport of oxygen are highly relevant to ceria-based redox materials, which are envisaged for the solar thermochemical splitting of carbon dioxide in the future. Experimental investigations of oxygen isotope exchange on CeO2-δ, Ce0.9M3+0.1O1.95-δ (with M3+ = Y, Sm) and Ce0.9M4+0.1O2-δ (with M4+ = Zr) samples were carried out for the first time utilizing oxygen-isotope-enriched C18O2 gas atmospheres as the tracer source, followed by Secondary Ion Mass Spectrometry (SIMS), at the temperature range 300 ≤ T ≤ 800 °C. The experimental K˜O and D˜O data reveal promising results in terms of CO2 splitting when trivalent (especially Sm)-doped ceria is employed. The reaction temperatures are lower than previously proposed/reported due to the weak temperature dependency of the parameters K˜O and D˜O. The majority of isotope exchange experiments show higher values of K˜O and D˜O for Sm-doped cerium dioxide in comparison to Y-doped and Zr-doped ceria, as well as nominally undoped ceria. The apparent activation energies for both K˜O and D˜O are lowest for Sm-doped ceria. Using Zr-doped cerium oxide exhibits various negative aspects. The Zr-doping of ceria enhances the reducibility, but the possible Zr-based surface alteration effects and dopant-induced migration barrier enhancement in Zr-doped ceria are detrimental to surface exchange and oxygen diffusion at lower temperatures of T ≤ 800 °C.

Cation-Deficient Ce-Substituted Perovskite Oxides with Dual-Redox Active Sites for Thermochemical Applications.

Identifying thermodynamically favorable and stable non-stoichiometric metal oxides is of crucial importance for solar thermochemical (STC) fuel production via two-step redox cycles. The performance of a non-stoichiometric metal oxide depends on its thermodynamic properties, oxygen exchange capacity, and its phase stability under high-temperature redox cycling conditions. Perovskite oxides (ABO3-δ) are being considered as attractive alternatives to the state-of-the-art ceria (CeO2-δ) due to their high thermodynamic and structural tunability. However, perovskite oxides often exhibit low entropy change compared to ceria, as they generally have one only redox active site, leading to lower mass-specific fuel yields. Herein, we investigate cation-deficient Ce-substituted perovskite oxides as a new class of potential redox materials combining the advantages of perovskites and ceria. We newly synthesized the (CexSr1-x)0.95Ti0.5Mn0.5O3-δ (x = 0, 0.10, 0.15, and 0.20; CSTM) series, with dual-redox active sites comprising Ce (at the A-site) and Mn (at the B-site). By introducing a cation deficiency (∼5%), CSTM perovskite oxides with both phase purity (x ≤ 0.15) and high-temperature structural stability under STC redox cycling conditions are obtained. Thermodynamic properties are evaluated by measuring oxygen non-stoichiometry in the temperature range T = 700-1400 °C and the oxygen partial pressure range pO2 = 1-10-4 bar. The results demonstrate that CSTM perovskite oxides exhibit a composition-dependent simultaneous increase of enthalpy and entropy change with increasing Ce-substitution. (Ce0.20Sr0.80)0.95Ti0.5Mn0.5O3-δ (CSTM20) showed a combination of large entropy change of ∼141 J (mol-O)-1 K-1 and moderate enthalpy change of ∼238 kJ (mol-O)-1, thereby creating favorable conditions for thermochemical H2O splitting. Furthermore, the oxidation states and local coordination environment around Mn, Ce, and Ti sites in the pristine and reduced CSTM samples were extensively studied using X-ray absorption spectroscopy. The results confirmed that both Ce (at the A-site) and Mn (at the B-site) centers undergo simultaneous reduction during thermochemical redox cycling.

Kinetics and Performance Study of Continuous Isothermal CeO2-Based Thermochemical Cycling for CO Production

Kinetics and Performance Study of Continuous Isothermal CeO2-Based Thermochemical Cycling for CO Production

Thermodynamic assessment of nonstoichiometric oxides for solar thermochemical fuel production

Two-step solar thermochemical cycling (STC) based on nonstoichiometric oxides is an ideal means of solar fuel (e.g., H2, CO) production. Screening of nonstoichiometric oxides with excellent thermodynamic performance is key to achieving high solar-to-fuel efficiency. However, application-driven materials assessment intended for reactor-level solar fuel production performance requires mimicking realistic operating conditions of on-sun tests in a laboratory setting, which makes oxides assessment and screening an onerous task to accomplish experimentally. In this work, a rapid assessment and screening model of nonstoichiometric oxides for two-step solar thermochemical cycling assuming fixed-bed flow pattern and quasi-equilibration of the solid with the flowing gas phase is developed, with solar-to-fuel efficiency being the target function of optimization. The model accounts for the thermodynamic parameters of oxide materials and typical operating conditions of experimental thermochemical cycling. This study employed the model to explore and compare two groups of typical nonstoichiometric oxides (CeO2- and (LaSr)MnO3-based) for their maximum efficiency under their uniquely optimized cycling conditions. The results show that CeO2 can reach a maximum efficiency of 12.9% at reduction temperature of 1500 °C, which is superior to other candidate materials, including 20 mol% Zr-doped CeO2 (10.1%), La0.6Sr0.4MnO3 (2.5%) and La0.8Sr0.2MnO3 (3.3%). Even when the reduction temperature is lowered to 1350 °C, ceria yields the highest efficiency amongst the candidate STC materials considered. The optimal cycling strategy depends on the inherent thermodynamic properties of the oxides. This approach serves as a framework for assessing the maximum efficiency and optimal conditions of candidate thermochemical materials within a range of constraints rather than comparing materials under arbitrary cycling conditions, which may inherently favor one material over another. For oxidation temperatures below 800–1000 °C, the model could be further improved by considering reaction kinetics.

Highly-selective CO2 conversion through single oxide CuO enhanced NiFe2O4 thermal catalytic activity

The solar-driven thermochemical CO2 conversion is an effective way to achieve the mission of carbon peaking and carbon neutrality. However, finding a material with excellent thermal stability, oxygen exchange capacity, and CO selectivity is one of the most important challenges in the thermochemical CO2 conversion approach. This study investigated the solar thermochemical CO2-splitting characteristics through the synergistic thermal catalytic activity of CuO and NiFe2O4 and the effect of SiC photothermal properties. The thermochemical redox cycling performance of CO2 conversion through thermally activated catalysts was experimentally investigated via thermogravimetric combined quantum chemical calculations to provide comprehensive insights into the physicochemical principles underlying the thermochemical catalytic CO2-splitting. The X-ray diffraction, scanning electron microscopy, and energy spectrum detector technics were developed to gain detail on the crystal structure, surface morphology, and chemical composition changes in the reacting media. The newly synthesized composite material consisting of 25%CuO and 75%NiFe2O4 (CNF-13) coated SiC porous skeleton resulted in higher CO2-splitting catalytic activity with an instantaneous CO production of 10.98mL ⋅ min‐1gCNF‐13−1 and direct CO2-to-CO conversion rate of 33.9% at 1000–1200 K reaction temperature. The thermochemical CO2-splitting activity and redox stability of CNF-13@SiC was promoted by the photothermal properties of SiC support and the formation of Cu2O and Fe3O4 new active material phases easing CO2 reduction at low energy. This study provided experimental and theoretical findings that can be used as guidance for the fundamental research and application of CO2 catalytic conversion into fuels through a solar thermochemical driven strategy.

Design and Analysis of Metal Oxides for CO2 Reduction Using Machine Learning, Transfer Learning, and Bayesian Optimization.

We aim to achieve resource recycling by capturing and using CO2 generated in a chemical production and disposal process. We focused on CO2 conversion to CO by the reverse water gas shift–chemical looping (RWGS-CL) reaction. This reaction proceeds in two steps (H2 + MOx ⇆ H2O + MOx–1; CO2 + MOx–1 ⇆ CO + MOx) via a metal oxide that acts as an oxygen carrier. High CO2 conversion can be achieved owing to a low H2O concentration in the second step, which causes an unwanted back reaction (H2 + CO2 ⇆ CO + H2O). However, the RWGS-CL process is difficult to control because of repeated thermochemical redox cycling, and the CO2 and H2 conversion extents vary depending on the metal oxide composition and experimental conditions. In this study, we developed metal oxides and simultaneously optimized experimental conditions to satisfy target CO2 and H2 conversion extents by using machine learning and Bayesian optimization. We used transfer learning to improve the prediction accuracy of the mathematical models by incorporating a data set and knowledge of oxygen vacancy formation energy. Furthermore, we analyzed the RWGS-CL reaction based on the prediction accuracy of each variable and the feature importance of the random forest regression model.

Two-step thermochemical cycles using fibrous ceria pellets for H2 production and CO2 reduction in packed-bed solar reactors

The thermochemical redox activity and performance of commercial-grade fibrous ceria pellets were determined including fuel production rates and yields from H2O and CO2 dissociation. Two solar reactors integrating the ceria pellets undergoing two-step thermochemical cycling (with temperature-swing between alternating redox steps) were experimentally tested. They consisted of packed-bed tubular and cavity-type solar reactors with direct or indirect heating of the reacting materials. The obtained fuel production rates compared favorably with the performance of other previously considered microstructured ceria materials such as templated foams, sintered felts, bioinspired or ordered macroporous morphologies. H2 production rates reached 2.3 mL.g−1.min−1 (with H2/O2 ratios approaching 2) in the indirectly-heated tubular reactor after reduction at 1400 °C and oxidation upon free cooling below 950 °C in steam atmosphere (57% molar content). The highest fuel production rate (~9.5 mL.g−1.min−1) and peak solar-to-fuel energy efficiency (~9.4%) were reached in the directly-irradiated cavity-type reactor (with a reduction step at 1400 °C under reduced pressure, and an oxidation step in pure CO2 below 950 °C). The reduction extent was favored at low pO2 (δ up to 0.056) and the fuel yields were thus improved notably (up to 315 μmol/g) by decreasing the total pressure below 0.1 bar during the reduction step. The fuel production rate increased when the oxidation temperature decreased (because the reaction was thermodynamically more favorable). An increase in CO2 partial pressure further promoted the oxidation kinetics (and decreased the pCO/pCO2 ratio, thus shifting the thermodynamic equilibrium toward CO product). The fibrous ceria structures exhibited stable morphologies with high fuel production performance similar to less scalable porous structures, thus offering the potential for versatile applications in packed-bed solar reactors.

A Novel Method for the Preparation of Fibrous CeO2–ZrO2–Y2O3 Compacts for Thermochemical Cycles

Zirconium-Yttrium-co-doped ceria (Ce0.85Zr0.13Y0.02O1.99) compacts consisting of fibers with diameters in the range of 8–10 µm have been successfully prepared by direct infiltration of commercial YSZ fibers with a cerium oxide matrix and subsequent sintering. The resulting chemically homogeneous fiber-compacts are sinter-resistant up to 1923 K and retain a high porosity of around 58 vol% and a permeability of 1.6–3.3 × 10−10 m² at a pressure gradient of 100–500 kPa. The fiber-compacts show a high potential for the application in thermochemical redox cycling due its fast redox kinetics. The first evaluation of redox kinetics shows that the relaxation time of oxidation is five times faster than that of dense samples of the same composition. The improved gas exchange due to the high porosity also allows higher reduction rates, which enable higher hydrogen yields in thermochemical water-splitting redox cycles. The presented cost-effective fiber-compact preparation method is considered very promising for manufacturing large-scale functional components for solar-thermal high-temperature reactors.

Synthesis and thermochemical redox cycling of porous ceria microspheres for renewable fuels production from solar-aided water-splitting and CO2 utilization

Porous ceria-based architected materials offer high potential for solar fuels production via thermochemical H2O and CO2-splitting cycles. Novel porous morphologies and micro-scale architectures of redox materials are desired to provide suitable thermochemical activities and long-term stability. Considering particle-based solar reactors, porous ceria microspheres are promising because of their excellent flowability and large surface area. In this work, such porous microspheres with perfect spherical shape, high density, and interconnected pore network were fabricated by a chemical route involving ion-exchange resins. The method involved the cationic loading of the resin in an aqueous medium followed by thermal treatment for oxide formation and porous microstructure stabilization. The utilization of these microspheres (∼150–350 μm in size) as redox materials for solar fuel production was investigated in packed-bed solar reactors (directly and indirectly irradiated). Superior redox performance was obtained for the pure ceria microspheres in comparison with other morphologies (powders and reticulated foams). Low pO2 values thermodynamically favored the reduction extent and associated fuel yield, whereas high pCO2 kinetically promoted the oxidation rate. The highest fuel production rate reached 1.8 mL/min/g with reduction step at 1400 °C, low total pressure (∼0.1 bar), and oxidation step below 1050 °C under pure CO2. Low pressure during reduction both improved reduction extent (oxygen under-stoichiometry δ up to 0.052) and associated fuel production yield (331 μmol/g CO). After 19 redox cycles (∼32 h under high-flux solar irradiation), the porous microspheres maintained their individual integrity (no agglomeration), spherical shape, and internal porosity, with great potential for stable fuel production capacity in particle-based solar reactors.

Combined Lithophile‐Siderophile Isotopic Constraints on Hadean Processes Preserved in Ocean Island Basalt Sources

AbstractDetection of Hadean isotopic signatures within modern ocean island basalts (OIB) has greatly influenced understanding of Earth's earliest history and long‐term dynamics. However, a relationship between two isotopic tools for studying early Earth processes, the short‐lived 146Sm‐142Nd and 182Hf‐182W systems, has not been established in this context. The differing chemical behavior of these two isotopic systems means that they are complementary tracers of a range of proposed early Earth events, including core formation, magma ocean processes, and late accretion. There is a negative trend between 142Nd/144Nd and 182W/184W ratios among Réunion OIB that is extended by Deccan continental flood basalts. This finding is contrary to expectations if both systems were affected by silicate differentiation during the lifetime of 182Hf. The observed isotopic compositions are attributed to interaction between magma ocean remnants and Earth's core, coupled with later assimilation of recycled Hadean mafic crust. The effects of this scenario on the long‐lived 143Nd‐176Hf isotopic systematics mirror classical models invoking mixing of recycled trace‐element enriched (sedimentary) and depleted (igneous) domains in OIB mantle sources.If the core provides a detectible contribution to the tungsten element budget of the silicate Earth, this represents a critical component to planetary‐scale tungsten mass balance. A basic model is explored that reconciles the W abundance and isotopic composition of the bulk silicate Earth resulting from both late accretion and core‐mantle interaction. The veracity of core‐mantle interaction as proposed here would have many implications for long‐term thermochemical cycling.

Geology and genesis of the Shalipayco evaporite-related Mississippi Valley-type Zn–Pb deposit, Central Peru: 3D geological modeling and C–O–S–Sr isotope constraints

The Shalipayco Zn–Pb deposit, in central Peru, is composed of several stratabound orebodies, the largest of which are the Resurgidora and Intermedios, contained in carbonate rocks of the Upper Triassic Chambara Formation, Pucara group. Petrography suggests that a single ore-forming episode formed sphalerite and galena within vugs, open spaces, and fractures. Three-dimensional (3D) geological modeling has allowed division of the Chambara Formation into four members (Chambara I, II, III, and IV) that better define lithological controls on sulfide formation. Diagenetic replacement of evaporite minerals with the organic matter (OM) presence likely generated secondary porosity and H2S accumulation by bacterial sulfate reduction (BSR), providing ground preparation for the later Zn–Pb mineralizing event. The least-altered host rocks have C–O isotope compositions of 1.8 ± 0.1‰ (VPDB) and 29.9 ± 2.1‰ (VSMOW), respectively, within the Triassic marine carbonate ranges. Early dolomite contains lighter C–O composition (1.1 ± 0.9 and 23.8 ± 2.9‰, respectively) consistent with OM decomposition during burial diagenesis. Post-mineralization calcite has still lighter C–O composition (− 5.1 and 13.3‰, respectively), suggesting meteoric water that had migrated through organic-rich strata. The strontium isotopes of Mitu group basalts (0.709654–0.719669) indicate it as a possible, but not the unique source of strontium and probably of other metals. Highly negative sulfide sulfur isotope values (− 23.3 to − 6.2‰ (VCDT)) indicate a major component of the ore sulfur derived ultimately from BSR. However, multiple lines of evidence suggest that preexisting H2S underwent thermochemical redox cycling prior to ore formation. The influx of hot metalliferous brines to dolomitized zones containing trapped H2S is the preferred model for ore deposition at Shalipayco.

Outstanding Properties and Performance of CaTi0.5Mn0.5O3–δ for Solar-Driven Thermochemical Hydrogen Production

<h2>Summary</h2> Variable valence oxides of the perovskite crystal structure have emerged as promising candidates for solar hydrogen production via two-step thermochemical cycling. Here, we report the exceptional efficacy of the perovskite CaTi_0.5Mn_0.5O_3–δ (CTM55) for this process. The combination of intermediate enthalpy, ranging between 200 and 280 kJ (mol-O)⁻¹, and large entropy, ranging between 120 and 180 J (mol-O)⁻¹ K⁻¹, of CTM55 create favorable conditions for water splitting. The oxidation state changes are dominated by Mn, with Ti stabilizing the cubic phase and increasing its reduction enthalpy. A hydrogen yield of 10.0 ± 0.2 mL g⁻¹ is achieved in a cycle between 1,350°C (reduction) and 1,150°C (water splitting) and a total cycle time of 1.5 h, exceeding all previous fuel production reports. The gas evolution rate suggests rapid material kinetics, and, at 1,150°C and higher, a process primarily limited by the magnitude of the thermodynamic driving force.

A Laser-Based Heating System for Studying the Morphological Stability of Porous Ceria and Porous La0.6Sr0.4MnO3 Perovskite during Solar Thermochemical Redox Cycling

Thermochemical processes are considered promising pathways to utilize solar energy for fuel production. Several physico-chemical, kinetic and thermodynamic properties of candidate oxides have been studied, yet their morphological stability during redox cycling under radiative heating is not widely reported. Typically when it is reported, it is for large-scale directly irradiated reactors (~1–10 kWth) aimed at demonstrating high efficiency, or in indirectly irradiated receivers where the sample surface is not exposed directly to extreme radiative fluxes. In this work, we aimed to emulate heat flux conditions expected in larger scale solar simulators, but at a smaller scale where experimentation can be performed relatively rapidly and with ease compared to larger prototype reactors. To do so, we utilized a unique infrared (IR) laser-based heating system with a peak heat flux of 2300 kW/m2 to drive redox cycles of two candidate materials, namely nonstoichiometric CeO2-δ and La0.6Sr0.4MnO3-δ. In total, 200 temperature-swing cycles using a porous ceria pellet were performed at constant pO2, and 5 cycles were performed for both samples by introducing H2O vapor into the system during reduction. Porous ceria pellets with porosity (0.55) and pore size (4–7 μm) were utilized because of their similarity to other porous structures utilized in larger-scale reactors. Overall, we observed that reaction extents initially decreased along with the decrease in reaction rates up to cycle 120 because of the change in structure and sintering. In the case of H2O splitting, ceria outperformed LSM40 in total H2 production because of the low pO2 during oxidation, where the oxidation of LSM40 is less favorable than that of ceria.