The project investigates a low-temperature, low-pressure photocatalytic alternative to the high temperature and pressure thermo-catalytic Haber-Bosch (H-B) commercial process for ammonia (NH3) production. Ammonia is a primary feedstock for fertilizer production, and its synthesis via the H-B process accounts for 1-2% of global energy consumption. The project will investigate a class of plasmonic metal particles as catalysts for activating the direct, gas-phase synthesis of NH3 from nitrogen (N2) and hydrogen (H2) at low temperatures and pressures utilizing natural ultra-violet and visible light. Photocatalytic synthesis of NH3 thus helps enable a future sustainable-energy path to meet global food needs based on harvesting energy from the sun rather than hydrocarbon resources.
The project is built on the hypothesis that plasmonic metal nanoparticles can activate direct, gas-phase ammonia synthesis at low temperatures and pressures when illuminated by UV-vis light of solar intensity. The defining characteristic of plasmonic metallic nanostructures is their strong resonant interaction with UV-vis light through the excitation of localized surface plasmon resonance (LSPR) which results in high rates of formation of high-energy electrons at the surface of the nanoparticles. The energetic electrons can induce chemical reactions on metals, including the activation of strong chemical bonds at low temperatures. The project will seek to enhance the plasmonic effect for NH3 synthesis by utilizing core-shell particles with a plasmonic metal core (Au, or Ag) and a thin shell (or even a small cluster) of a chemically more active metal (Ru, Rh, Pt, Pd, etc.). In the proposed bimetallic systems, the role of the plasmonic core is to efficiently harvest the energy of light and transfer it in the form of energetic electrons to the more active material. The role of the more active metal is to provide the catalytic sites for binding and dissociation of N2 using the energetic electrons. Further reduction of the N and NHx intermediates is fast on these metals. Beyond NH3 synthesis, the project will address a number of fundamental questions about the interaction of local light-induced electric fields on plasmonic metals and adsorbates. These questions are critical for controlling the hot-electron flow at nanoscales, which is of importance in the fields of photochemistry, photovoltaics and any application where photo-excitation and energetic charge harvesting play a role. The project will also employ a number of educational and outreach activities designed to promote learning via trickle-down of research findings through the curriculum to graduate, undergraduate, and even younger students while also involving a diverse group of younger students in conventional outreach programs and less conventional strategies aimed at improving the utilization of the World Wide Web in reaching students and the general public.