Nb⁵⁺-Induced-CeO₂ Lattice Strain for Robust and Scalable Solar-Driven Ammonia Synthesis

Ammonia (NH₃) underpins fertilizer production and is emerging as a renewable-energy carrier. Solar-driven gas-phase N₂/H₂ conversion requires catalysts that couple efficient light utilization with structural stability under H₂-rich conditions. Here, Nb⁵⁺ incorporation into CeO₂ contracts the lattice and induces compressive microstrain, suppressing excessive oxygen-sublattice reduction and preserving Ru dispersion. In situ NAP-XPS and computational results show that Nb-CeO₂ maintains an electron-enriched Ru working state under reaction conditions, which is further promoted by illumination. Accordingly, 5 wt% Ru/Nb-CeO₂ achieves an NH₃ synthesis rate of 7.4 mmol g^-1 h^-1 at 350°C and 0.1 MPa and remains stable for 144 h. Kinetic analyses and in situ DRIFTS indicate that the electronically promoted Ru state alleviates N₂-activation limitations and facilitates subsequent *NH_x hydrogenation and NH₃ desorption. Scale-up synthesis affords a 3 wt% Ru/Nb-CeO₂-L catalyst stable for 480 h and operable under outdoor concentrated sunlight. A screening-level analysis estimates a catalyst-and-feedstock cost of 286-322 USD t_NH3 ^-1 under the lower-bound electricity scenario, 42%-51% below the selected Cs-Ru/MgO benchmark within the same framework. These results identify lattice strain as a promising strategy for simultaneously stabilizing the oxygen sublattice and electronically promoting Ru for efficient solar-driven ammonia synthesis.