Dzara, Michael J.; Campello, Arthur C.; Breidenbach, Aeryn T.; Strange, Nicholas A.; Park, James E.; Ambrosini, Andrea A.; Coker, Eric N.; Ginley, David S.; Lee, Young S.; Bell, Robert T.; Smaha, Rebecca W.
Material design is increasingly used to realize desired functional properties, and the perovskite structure family is one of the richest and most diverse: perovskites are employed in many applications due to their structural flexibility and compositional diversity. Hexagonal, layered perovskite structures with chains of face-sharing transition metal oxide octahedra have attracted great interest as quantum materials due to their magnetic and electronic properties. Ba4MMn3O12, a member of the “12R” class of hexagonal, layered perovskites, contains trimers of face-sharing MnO6 octahedra that are linked by a corner-sharing, bridging MO6 octahedron. Here, we investigate cluster magnetism in the Mn3O12 trimers and the role of this bridging octahedron on the magnetic properties of two isostructural 12R materials by systematically changing the M4+ cation from nonmagnetic Ce4+ (f0) to magnetic Pr4+ (f1). We synthesized 12R-Ba4MMn3O12 (M= Ce, Pr) with high phase purity and characterized their low-temperature crystal structures and magnetic properties. Using substantially higher purity samples than previously reported, we confirm the frustrated antiferromagnetic ground state of 12R-Ba4PrMn3O12 below TN ≈ 7.75 K and explore the cluster magnetism of its Mn3O12 trimers. Despite being atomically isostructural with 12R-Ba4CeMn3O12, the f1 electron associated with Pr4+ causes much more complex magnetic properties in 12R-Ba4PrMn3O12. In 12R-Ba4PrMn3O12, we observe a sharp, likely antiferromagnetic transition at T2 ≈ 12.15 K and an additional transition at T1 ≈ 200 K, likely in canted antiferromagnetic order. These results suggest that careful variation of composition within the family of hexagonal, layered perovskites can be used to tune material properties using the complex role of the Pr4+ ion in magnetism.
Thermochemical air separation to produce high-purity N2 was demonstrated in a vertical tube reactor via a two-step reduction–oxidation cycle with an A-site substituted perovskite Ba0.15Sr0.85FeO3–δ (BSF1585). BSF1585 particles were synthesized and characterized in terms of their chemical, morphological, and thermophysical properties. A thermodynamic cycle model and sensitivity analysis using computational heat and mass transfer models of the reactor were used to select the system operating parameters for a concentrating solar thermal-driven process. Thermal reduction up to 800 °C in air and temperature-swing air separation from 800 °C to minimum temperatures between 400 and 600 °C were performed in the reactor containing a 35 g packed bed of BSF1585. The reactor was characterized for dispersion, and air separation was characterized via mass spectrometry. Gas measurements indicated that the reactor produced N2 with O2 impurity concentrations as low as 0.02 % for > 30 min of operation. A parametric study of air flow rates suggested that differences in observed and thermodynamically predicted O2 impurities were due to imperfect gas transport in the bed. Temperature swing reduction/oxidation cycling experiments between 800 and 400 °C in air were conducted with no statistically significant degradation in N2 purity over 50 cycles.
A two-step solar thermochemical looping cycle based on Co3Mo3N/Co6Mo6N reduction/nitridation reactions offers a pathway for green NH3 production that utilizes concentrated solar irradiation, H2O, and air as feedstocks. The NH3 production cycle steps both derive process heat from concentrated solar irradiation and encompass 1) the reduction of Co3Mo3N in H2 to Co6Mo6N and NH3; and 2) nitridation of Co6Mo6N to Co3Mo3N with N2. Co3Mo3N reduction/nitridation reactions are examined at different H2 and/or N2 partial pressures and temperatures. NH3 production is quantified in situ using liquid conductivity measurements coupled with mass spectrometry (MS). Solid-state characterization is performed to identify a surface oxygen layer that necessitates the addition of H2 during cycling to prevent surface oxidation by trace amounts of O2. H2 concentrations of > 5% H2/Ar and temperatures >500 °C are required to reduce Co3Mo3N to Co6Mo6N and form NH3 at 1 bar. Complete regeneration of Co3Mo3N from Co6Mo6N is achieved at conditions of 700 °C under 25–75% H2/N2. H2 pressure-swings are observed to increase NH3 production during Co3Mo3N reduction. In conclusion, the results represent the first comprehensive characterization of and definitive non-catalytic production of NH3 via chemical looping with metal nitrides and provide insights for technology development.
Over the past few decades, inorganic nitride materials have grown in importance in part due to their potential as catalysts for the synthesis of NH3, a key ingredient in fertilizer and precursor to industrial chemicals. Of particular interest are the ternary (ABN) or higher-order nitrides with high metal-to-nitrogen ratios that show promise in enhancing NH3 synthesis reaction rates and yields via heterogeneous catalysis or chemical looping. Although metal nitrides are predicted to be numerous, the stability of nitrogen triple bonds found in N2, especially in comparison to the metal-nitrogen bonds, has considerably hindered synthetic efforts to produce complex nitride compounds. In this study, we present an exhaustive down-selection process to identify ternary nitrides for a promising chemical looping NH3 production mechanism. We also report on a facile and efficient two-step synthesis method that can produce well-characterized η-carbide Co3Mo3N/Fe3Mo3N or filled β-manganese Ni2Mo3N ternaries, as well as their associated quaternary, (Co,Fe)3Mo3N, (Fe,Ni)2Mo3N, and (Co,Ni)2Mo3N, solid solutions. To further explore the quaternary space, syntheses of (Co,Ni)3Mo3N (Ni ≤ 10 mol %) and Co3(Mo,W)3N (W ≤ 10 mol %) were also investigated. The structures of the nitrides were characterized via X-ray powder diffraction. The morphology and compositions were characterized with scanning electron microscopy. The multitude of chemically unique, but structurally related, nitrides suggests that properties such as nitrogen activity may be tunable, making the materials of great interest for NH3 synthesis schemes.
CuCr2O4 spinel is a candidate coating material for central receivers in concentrating solar power to protect structural alloys against high temperature oxidation and related degradation. Coating performance and microstructure of dip-coated and sintered coatings is dictated by the initial particle size of the CuCr2O4 and sintering temperature, but can be compromised by particle agglomeration. Here in this study, sub-micron particles were synthesised through the Pechini and modified Pechini sol–gel methods. Phase composition was confirmed via X-ray diffraction. Particle growth during calcination of the nanoparticles at different temperatures (650°C, 750°C, 850°C) and times (between 1 and 24 h) was measured via laser diffraction and scanning electron microscopy. The modified Pechini method displayed evidence of smaller particle sizes and greater agglomeration. The kinetics of particle growth observed are consistent with a diffusion limited inhibited grain growth model.
Ammonia (NH3) is an energy-dense chemical and a vital component of fertilizer. In addition, it is a carbon-neutral liquid fuel and a potential candidate for thermochemical energy storage for high-temperature concentrating solar power (CSP). Currently, NH3 synthesis occurs via the Haber-Bosch process, which requires high pressures (15-25 MPa) and medium to high temperatures (400-500 °C). N2 and H2 are essential feedstocks for this NH3 production process. H2 is generally derived from methane via steam reforming; N2 is sourced from air, after oxygen removal via combustion of hydrocarbons. Both processes consume hydrocarbons, resulting in the release of CO2. In addition, hydrocarbon fuels are burned to produce the heat and mechanical energy required to perform the NH3 reaction, further increasing CO2 emissions. Overall, the production of ammonia via the Haber-Bosch (H-B) process is responsible for up to 1.4% of the world’s carbon emissions. The development of a renewable pathway to NH3 synthesis, which utilizes concentrated solar irradiation as a process heat instead of fossil fuels and operates under low or ambient pressure, will result in a decrease (or elimination) of greenhouse gas emissions as well as avoid the cost, complexity, and safety issues inherent in high-pressure processes. Most current efforts to “green” ammonia production involve either electrolysis or simply replacing the energy source for H-B with renewable electricity, but otherwise leaving the process intact. The effort proposed here would create a new paradigm for the synthesis of NH3 utilizing solar-thermal heat, water, and air as feedstocks, providing a truly green method of production. The overall objective of the STAP (Solar Thermal Ammonia Production) project was to develop a solar thermochemical looping technology to produce and store nitrogen (N2) from air for the subsequent production of ammonia (NH3) via an advanced two-stage process. The goal is a cost-effective and energy efficient technology for the renewable N2 production and synthesis of NH3 from H2 (produced from H2O) and air using solar-thermal energy from concentrating sunlight, under pressures an order of magnitude lower than H-B NH3 production. Our process involves two looping cycles, which do not require catalysts and can be recycled. Over the course of the STAP project, we (1) developed and deeply characterized oxide materials for N2 separation; (2) developed a method for the synthesis of metal nitrides, producing a series of quaternary compounds that have been heretofore unreported; (3) modeled, designed, and fabricated bench-scale tube and on-sun reactors for the N2 production step and demonstrated the ability to separate N2 over multiple cycles in the tube reactor; (4) designed and fabricated a bench-scale Ammonia Synthesis Reactor (ASR) and demonstrated the proof of concept of NH3 synthesis via a novel looping process using metal nitrides over multiple cycles; and (5) completed a systems- and technoeconomic analysis showing the feasibility of ammonia production on a larger scale via the STAP process. The development of renewable, low-cost NH3 will be of great interest to the chemicals industry, particularly agricultural sectors. The CSP industry should be both an important customer and potential end-user of this technology, as it affords the capability of synthesizing a promising thermochemical storage material on-site. Since the NH3 synthesis step also requires H2, there will exist a symbiotic relationship between this technology and solar-thermochemical water-splitting applications. Green ammonia synthesis will result in the decarbonization of a hydrocarbon-intensive industry, helping to meet the Administration goal of industrial decarbonization by 2050. The resulting decrease in CO2 and related pollutants will improve health and well-being of society, particularly for those living in the vicinity of commercial production plants.
CO2-neutral ammonia production with concentrated solar technology is theoretically possible based on advanced solar thermochemical looping technology. STAP offers price stability achieving a target price <250 $/tonne NH3 without including the H2. The nitride cost is the most significant expense, accounting for more than the 50% of the total CapEx.
Solar Thermal Ammonia Production has potential to produce green ammonia using CSP, air, and water. Air separation to purify N2 was successfully demonstrated with BSF1585 in packed bed reactor; on-sun reduction reactor under construction. Metal nitrides (MNy) were successfully synthesized and characterized under both ambient and pressurized conditions. Co3Mo3N shown to successfully produce NH3 when exposed to pure H2 at pressures between 5 – 20 bar 600 – 750 °C. Ambient reaction experiments imply there may be a catalytic aspect as well. Technoeconomic and systems analyses show a path towards scale-up.
Gao, Xiang; Ermanoski, Ivan; De La Calle, Alberto; Ambrosini, Andrea A.; Stechel, Ellen B.
Ternary nitrides in the family A3BxN (A=Co, Ni, Fe; B=Mo; x=2,3) identified and synthesized. Experiments with Co3Mo3N in Ammonia Synthesis Reactor demonstrate cyclable NH3 production from bulk nitride under pure H2. Production rates were approx. constant in all the reduction steps with no evident dependence on the consumed solid-state nitrogen up to formation of 661. Material can be re-nitridized under pure N2 (or 10% H2/N2). Bulk N utilization per reduction step averaged between 25 – 40% of the total (2-3 hours). Rate equations and parameters extracted from data. NH3 selectivity exceeds gas phase equilibrium at higher temperatures (in a large excess of H2). Selectivity begins to decrease significantly above 650 C, N2 production rapidly increases above 650 C seemingly due to reaction that is zero order in H2 (thermal reduction of the nitride?). Poised to begin the systematics studies of relationships between materials and reactions.
In this report, we investigate the thermal reduction of the octahedral perovskite BaCe0.25Mn0.75O3(BCM) using in situ electron energy loss spectroscopy (EELS) in an aberration-corrected transmission electron microscope (TEM). The 12R-polytype of BCM is known to demonstrate high solar thermochemical hydrogen production capacity. In situ EELS measurements show that Mn is the active redox cation in BCM, undergoing thermal reduction from Mn4+to Mn3+during heating to 700 °C inside the TEM under a high vacuum. The progressive reduction of Mn4+during oxygen vacancy (Ov) formation was monitored as a function of temperature. Additionally, atomic-resolution scanning transmission electron microscopy identified two different types of twin boundaries present in the oxidized and reduced form of 12R-BCM, respectively. These two types of twin boundaries were shown, via computational modeling, to modulate the site-specific Ovformation energies in 12R-BCM. It is concluded that these types of atomic defects provide sites more energetically favorable for Ovformation during thermal reduction.
Solar Thermal Ammonia Production has the potential to synthesize ammonia in a green, renewable process that can greatly reduce the carbon footprint left by conventional Haber-Bosch reaction. Ternary nitrides in the family A3BxN (A=Co, Ni, Fe; B=Mo; x=2,3) have been identified as a potential candidate for NH3 production. Experiments with Co3Mo3N in Ammonia Synthesis Reactor demonstrate cyclable NH3 production from bulk nitride under pure H2. Production rates were fairly flat in all the reduction steps with no evident dependence on the consumed solid-state nitrogen, as would be expected from catalytic Mars-van Krevelen mechanism. Material can be re-nitridized under pure N2. Bulk nitrogen per reduction step average between 25 – 40% of the total solid-state nitrogen. Selectivity to NH3 stabilized at 55 – 60% per cycle. Production rates (NH3 and N2) become apparent above 600 °C at P(H2) = 0.5 – 2 bar. Optimal point of operation to keep selectivity high without compromising NH3 rates currently estimated at 650 °C and 1.5 - 2 bar. The next steps are to optimize production rates, examine effect of N2 addition in NH3 synthesis reaction, and test additional ternary nitrides.