Shining Light on Sustainable Ammonia: Photocatalytic Breakthroughs with Ru-Loaded Defective Pyrochlore K2Ta2O6-x under Low Pressure

Photocatalytic ammonia synthesis is a sustainable and innovative approach for producing ammonia (NH₃) under ambient conditions, and offers a promising alternative to the traditional Haber-Bosch process, which is energy-intensive and relies heavily on fossil fuels. The method leverages the power of light and photocatalysts to drive the chemical reaction to convert nitrogen from the air and hydrogen into ammonia. The process is attracting considerable interest due to its potential for reducing carbon emissions and harnessing renewable energy sources. The efficiency of the process heavily depends on the availability of effective photocatalysts that can absorb a broad spectrum of light, have high quantum efficiency, and possess excellent stability under reaction conditions. Currently, the rates of photocatalytic ammonia synthesis and the yields are relatively low compared to the industrial Haber-Bosch process, necessitating further research and development.  Achieving high selectivity for ammonia synthesis in the presence of other possible reactions (like water splitting) and preventing the reverse reaction (ammonia decomposition) are significant challenges. Extensive research in photocatalytic ammonia synthesis is focused on developing novel materials and catalysts, and exploring new reaction conditions that can enhance the absorption of light, increase catalytic activity, and improve overall efficiency. To this account, a new study published in Angewandte Chemie International Edition led by Professor Lu Li from the Jilin University and conducted by Zihan Zhao, Ruike Tan, Yuxiang Kong, Zihao Zhang, Shou Qiu, and Xiaoyue Mu, the researchers investigated the photocatalytic ammonia synthesis capability of Ru-loaded defective pyrochlore K₂Ta₂O_6-x.  The team synthesized potassium tantalates with pyrochlore and perovskite structures, followed by a high-temperature solid-state reduction to introduce oxygen vacancies and low-valent Ta. Ruthenium nanoparticles were deposited on these structures to create the final photocatalyst. They confirmed the phase purity, structural integrity, and successful deposition of Ru nanoparticles using X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy. These techniques demonstrated the effective modification of the electronic structure and surface morphology of the catalysts.

The authors evaluated the photocatalytic activity under visible light irradiation at low pressure in a batch reactor. They measured the rate of ammonia production to assess the efficiency of the photocatalysts. They found that Ru-loaded K₂Ta₂O_6-x has significantly higher photocatalytic rate for ammonia synthesis compared to the best previously reported photocatalyst, indicating a 2.8-fold increase in efficiency. It also demonstrated a 3.7-fold increase in intrinsic activity over perovskite-type K₂Ta₂O_6-x, attributed to better photoexcited charge separation efficiency and a more favorable conduction band position. The study revealed that the Schottky barrier formed at the interface between K₂Ta₂O_6-x and Ru facilitated improved photoexcited charge separation and efficient electron transfer, aiding in the activation of N₂ molecules for the synthesis of ammonia. Moreover, the presence of oxygen vacancies and the modification of the electronic structure were crucial for extending visible light absorption and enhancing the photocatalytic activity. The oxygen vacancies acted as sites for electron trapping, contributing to the high performance of the catalyst. Furthermore, the catalyst demonstrated excellent stability and could be reused for multiple cycles without significant loss of activity, indicating its robustness for practical applications.  Additionally, the researchers compared the photocatalytic performance of Ru-loaded K₂Ta₂O_6-x with other tantalates and found it superior, underscoring the effectiveness of the pyrochlore structure and the synergistic effect of Ru deposition and oxygen vacancies. They also provided insights into the mechanisms underlying the enhanced photocatalytic performance, including efficient charge separation, electron transfer dynamics, and the activation of nitrogen molecules through detailed characterization and kinetic studies. The authors used UV/Vis diffuse reflectance spectroscopy to analyze the band gap and light absorption properties. The reduced band gap and extended absorption in the visible range for Ru-loaded K₂Ta₂O_6-x confirmed its superior visible-light-driven photocatalytic capabilities.

From an engineering perspective, Professor Lu Li and his team demonstrated the critical role of material design in the development of efficient and scalable photocatalytic systems for ammonia synthesis. The Ru-loaded defective pyrochlore K₂Ta₂O_6-x represents a significant step forward in the application of semiconductor materials for green chemical synthesis. Its ability to operate under low pressure and at lower temperatures than the Haber-Bosch process, coupled with its high efficiency and stability, makes it a promising candidate for decentralized ammonia production. This aligns with the growing interest in integrating renewable energy sources with chemical production processes, contributing to the development of sustainable industrial ecosystems. Furthermore, the research highlights the importance of understanding the interplay between material structure, electronic properties, and catalytic performance. The introduction of oxygen vacancies and the deposition of ruthenium nanoparticles not only modify the electronic structure and optical properties of K₂Ta₂O_6-x but also optimize the interaction between the photocatalyst and nitrogen molecules. These modifications lead to enhanced photocatalytic activity by facilitating the activation of N₂ and the efficient transfer of photoexcited electrons to the adsorbed nitrogen, thereby lowering the energy barrier for ammonia synthesis.