Investigation of (Photo) Electrocatalytic Conversion of N2 to NH3 under Ambient Conditions Using Hybrid Hollow Plasmonic Nanostructures

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Ammonia is one of the widely produced chemicals in the world. It is used in agricultural fertilizer, energy, and in the pharmaceutical industry. The current method for producing ammonia involves heating the reactants to very high temperatures and at high pressures, which requires large amounts of energy. With support from the Macromolecular, Supramolecular, and Nanochemistry Program in the Division of Chemistry, Professor Mostafa El-Sayed at the Georgia Institute of Technology and his students are exploring a photo-electrochemical approach that would produce ammonia from nitrogen and water at atmospheric pressure and room temperature. Their approach makes use of hybrid nanoparticles that are 10,000 times smaller than a grain of salt. Each particle consists of a plasmonic nanoparticle, surrounded by a semiconductor shell and catalytic metal. Exposure to light excites the electrons in the nanoparticle. These electrons are then transferred to the semiconductor and the catalyst, where the chemical conversion to ammonia occurs. The team's discoveries could extend beyond ammonia synthesis and impact the broader field of nanocatalysis, which is widely used in chemical production, sustainable energy, and materials chemistry. The project is training the next generation of scientists and is engaging students at historically black colleges and universities (HBCUs) in the Atlanta metropolitan area. Professor El-Sayed and his group also showcase their research through lab tours and public presentations at STEM career fairs and local schools. Various metal (e.g., Au, Pd, and Ru) nanocages, hybrid double shell (e.g., Ag-Au, Au-Pd, Au-Ru) nanocages, and Au nanorattles where a solid Au plasmonic nanoparticle is placed inside the hollow nanocatalyst are synthesized. The conversion of nitrogen (N2) to ammonia (NH3) under ambient conditions is then explored using these nanocatalysts in a (photo) electrochemical system, which measures the reaction rate and catalytic efficiency. In-situ surface-enhanced Raman spectroscopy and atomic force microscopy are performed to study the full reaction mechanism during electrochemical nitrogen reduction reaction. Transition catalytic metals (e.g., Ru, Pd) are utilized as dopants, co-catalysts, and plasmon enhancers with semiconductor and plasmonic metals (Au) to study their effects on the charge carrier recombination and the dynamics of electron injection using ultrafast pump-probe spectroscopy. These experiments help to design an active photo-electrocatalyst with longer recombination time, therefore facilitating the photo-electrochemical nitrogen reduction reaction. The group's extensive experience in nanoparticle synthesis, bench scale photo-electrochemical testing and state of the art spectroscopy is complemented by theoretical studies to gain a fundamental understanding of photo-electrochemical nitrogen fixation processes that enable nitrogen-based fertilizer production.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.