Solid solution for catalytic ammonia synthesis from nitrogen and hydrogen gases at 50 °C

The Haber-Bosch process, while crucial for global food production, consumes significant energy and generates substantial CO₂ emissions. Improving ammonia synthesis efficiency at lower temperatures is vital for sustainable production. Current iron-based catalysts are effective above 350 °C, resulting in low yields despite high pressure. Lowering the reaction temperature significantly improves yields but conventional catalysts lose their activity below 100-200 °C. While homogeneous catalytic systems have shown promise at room temperature, they often employ non-reusable reagents. This research aims to address the long-standing challenge of developing efficient low-temperature catalysts for ammonia synthesis by exploring the potential of materials with high electron-donating capabilities at low temperatures. The authors hypothesize that materials capable of stable electron donation at low temperatures could overcome the kinetic barrier to ammonia formation, paving the way for more sustainable and energy-efficient ammonia production. The study investigates the use of calcium hydride (CaH₂) as a starting point due to its simplicity and well-known properties as a dehydrating agent. The limitations of existing catalysts, especially the loss of activity at low temperatures, are analyzed to justify the research. The focus is on overcoming the challenges associated with N₂ dissociation, a key step in ammonia synthesis that is usually enhanced by electron donation into the π* orbitals of N₂.

The paper reviews existing ammonia synthesis catalysts and their limitations, highlighting the energy consumption and CO₂ emissions associated with the Haber-Bosch process. It discusses previous research on homogeneous catalytic systems that operate at lower temperatures but often use non-reusable reagents. The authors also cite studies demonstrating the electron donation role in N₂ dissociation, a key step in ammonia synthesis. Previous attempts to use Ru-deposited CaH₂ showed improvements but still exhibited activity loss below 150-200°C, similar to conventional catalysts. This loss of activity is related to the temperature at which H₂ desorption begins, suggesting a need for materials that release hydrogen atoms at lower temperatures.

The study involved the synthesis of a new catalyst material, Ru nanoparticle-deposited cubic CaFH solid solution. The preparation of CaFH involved heating a mixture of CaH₂ and BaF₂ in a flow of H₂ at 340 °C. The formation of the cubic CaFH solid solution was confirmed using XRD, which showed an asymmetrical diffraction peak attributable to the (200) plane of the cubic structure. The material's characteristics were further analyzed using H₂-TPD, XRD, XPS, and DFT computations. Ru nanoparticles were then deposited on the CaFH solid solution to create the catalyst (Ru/CaFH). Two methods were used for the deposition of Ru nanoparticles, both resulting in similar catalytic activity. The catalytic performance was evaluated in a fixed-bed reactor under a flow of N₂-H₂ at varying temperatures and pressures. Ammonia formation rates were measured using both mass spectrometry and ion chromatography, with consistent results obtained from both methods. The apparent activation energy was determined from the temperature-dependent reaction rates. Further mechanistic studies employed ammonia synthesis from N₂ and D₂ to determine the reaction pathway and the role of hydrogen from the CaFH bulk. FT-IR spectroscopy was used to study N₂ adsorption on the catalyst and evaluate its electron-donating ability. DFT calculations were performed to estimate the work function of the CaFH solid solution. The paper also includes detailed descriptions of the synthesis and characterization techniques used, such as XRD, H₂-TPD, XPS, SEM, TEM, FT-IR, and DFT calculations, with specific parameters and conditions provided.

The key findings of the study are: 1. The novel cubic CaFH solid solution catalyst exhibits significantly enhanced activity for ammonia synthesis at low temperatures. 2. Ammonia production was successfully demonstrated at 50 °C, with a remarkably low activation energy of 20 kJ mol⁻¹, less than half of that reported for conventional catalysts. 3. The catalyst shows excellent stability over extended periods and at higher temperatures, maintaining its high activity with consistent reaction rates. 4. H₂-TPD and DFT computations reveal that the introduction of F⁻ anions into CaH₂ weakens Ca²⁺-H⁻ bonds, lowering the hydrogen release temperature and increasing electron-donating power. 5. DFT calculations confirmed that the work function of CaFH with hydrogen vacancies is significantly lower than that of CaH₂, further supporting its enhanced electron-donating capability. 6. Isotopic studies using N₂ and D₂ show that hydrogen from the CaFH bulk is involved in the early stages of ammonia formation, demonstrating the unique mechanism of this catalyst. 7. FT-IR spectroscopy confirms that the catalyst has an exceptionally strong electron donation capability, leading to efficient N₂ dissociation, which explains its high activity. The catalyst outperforms several existing catalysts, including Ru/BaH₂-BaO and Cs-doped MgO, in terms of both reaction rate and catalyst weight efficiency. Quantitative data on ammonia formation rates at various temperatures (50, 75, 100, and 125 °C) are provided, showing a clear increase with temperature. The catalyst weight efficiency at 200 °C is also quantified and compared favorably to other high-performance catalysts.

The findings demonstrate that the introduction of F⁻ into CaH₂ to form the CaFH solid solution effectively enhances the catalytic performance for low-temperature ammonia synthesis. The weak Ca²⁺-H⁻ bonds in CaFH, combined with the high electron-donating capacity resulting from the electron repulsion between electrons and F⁻, facilitates the dissociation of N₂ and subsequent hydrogenation to form ammonia. This work challenges the existing understanding of low-temperature ammonia synthesis and introduces a new class of catalysts with remarkable performance and stability. The extremely low activation energy suggests a novel reaction mechanism that warrants further investigation. The findings have significant implications for developing more sustainable and energy-efficient ammonia production technologies, potentially reducing reliance on the high-temperature, high-pressure Haber-Bosch process.

This research successfully demonstrates a novel, highly active, and stable catalyst for low-temperature ammonia synthesis. The cubic CaFH solid solution, achieved by a simple method of heating CaH₂ and BaF₂, exhibits significantly improved catalytic performance compared to conventional catalysts. The low activation energy and high stability suggest a new paradigm for efficient and sustainable ammonia production, with potential applications in green ammonia synthesis technologies. Future research could focus on optimizing the catalyst composition and exploring other solid solutions to further enhance catalytic activity and broaden its applicability.

While the catalyst shows exceptional performance, the study's scope is limited to a specific catalyst composition and reaction conditions. Further investigation is needed to explore the effect of different Ru loadings, Ca:Ba ratios in the CaFH solid solution, and other reaction parameters on the catalytic activity. The long-term stability under industrial conditions also needs to be assessed to determine its practical applicability.