Synthetic ammonia has been the primary worldwide source of agricultural fertilizer over the last century and is a promising carbon-free energy carrier for sustainable transportation. Despite its global importance, synthetic ammonia produced with the Haber−Bosch process is extremely energy-and resource-intensive. Here we demonstrate a three-step chemical looping strategy to produce ammonia using only light, natural gas, nitrogen, and water. Titanium nitride nanoparticles were utilized as plasmonic antennas to assist the transformation of magnesium-based nanomaterials through oxide, metallic, and nitride phases under optical illumination. All reactions were performed and monitored in situ using frequencymodulated rotational spectroscopy, which allowed the experiments to take advantage of the rotational spectra's unique sensitivity to isotopic labeling to monitor and verify key reaction intermediates. This validation of a light-driven process for the synthesis of ammonia demonstrates an innovative route toward photosynthetic production of essential chemical commodities.