Solar-Driven Ammonia Synthesis over a Silver-Only Antenna–Reactor Catalyst

Photocatalytic ammonia synthesis is a potential alternative to Haber-Bosch but is limited by low solar-to-ammonia (STA) efficiency. We report a sunlight-driven, plasmonic antenna-reactor catalyst using just 0.1 wt % Ag deposited on a honeycomb AAO plate to form a ∼200 nm active layer. AAO pores trap/recycle light for photon-cascade harvesting, while >10 nm Ag "antennas" concentrate near-fields onto adjacent ≤5 nm Ag "reactors" enriched in low-coordination sites, directly coupling light capture to bond activation. Under concentrating natural sunlight (∼4 suns), the reactor self-heats to ∼150 °C at 0.5 MPa (1:3 N₂/H₂) and achieves a high metal-normalized rate of 5.6 × 10^-3 s^-1, exceeding industrial Fe at 450 °C/9 MPa on a metal-normalized basis. Under simulated sunlight (200 °C, 0.4 MPa), it reaches 159 mmol g_Ag^-1 h^-1 for 200 h, with STA = 0.149% and AQE = 1.34% (575 nm). Density functional theory (DFT) and experiments support plasmon-assisted H₂ dissociation and associative N₂ hydrogenation (NNH*, N₂H_<i>x</i>), overturning Ag's presumed inertness.