Plasmonic Ammonia Synthesis with a AuRu Bimetallic Alloy

The Haber-Bosch process has had a significant impact on global food production, enabling ammonia synthesis for nearly all agricultural fertilizers. However, ammonia production generally requires high operating temperatures (exceeding 300 °C) and high pressures (100 atm), leading to over 3% of annual global greenhouse gas emissions. To promote sustainable chemistry, ammonia synthesis must be accomplished under milder conditions while also eliminating the greenhouse gas footprint. In this presentation, we describe bimetallic plasmonic nanoparticles that enable high-efficiency and sustainable nitrogen fixation at room temperature and pressure. We utilize AuRu metallic nanoparticles, where Au serves as the plasmonic optical absorber, and Ru acts as the catalyst for nitrogen bond-breaking. We develop a colloidal synthesis to create AuRu nanoparticles with controlled phases, including the fcc phase and hcp phase. We vary the Au and Ru composition from 1:0.1 to 1:0.3. We then load the nanoparticles onto a commercial MgO substrate at 5% mass-loading. Using UV-Vis spectroscopy, the nanoparticles show strong optical absorption at wavelengths of 520 nm. We use our bench-scale photoreactor to characterize the ammonia synthesis(N₂+3H₂→NH₃), using nitrogen and hydrogen as a feeding gas, at various temperatures, pressures, and under various illumination conditions. Our observations indicate an ammonia rate of 60 μmol/g/h under 100 mW/cm² visible light irradiation, and a 0.12% external quantum efficiency on our fcc AuRu_0.2 catalyst. By correlating the wavelength-dependent reactivity and electromagnetic hot carrier calculations, we have identified the potential contributions from phonon-assisted intraband and interband transitions in driving the photochemical transformation. Our study provides a solution for designing photocatalysts to drive nitrogen fixation under mild conditions and an understanding of the mechanisms that can inform future applications.