Emergence of Similar Nitrogen-Containing Surface Environments during Ammonia Synthesis over Molybdenum-Based Catalysts

Abstract The recent discovery of molybdenum oxynitride (MoO2–x/Mo2N) being an effective plasmonic catalyst demonstrated potential for light-driven ammonia synthesis under ambient pressure. However, its preparation requires high-temperature ammonia annealing to introduce lattice nitrogen, increasing both cost and process complexity. Here, we investigate molybdenum oxide (MoO3–x), the precursor to synthesize MoO2–x/Mo2N, as a catalyst for ammonia synthesis. We show that molybdenum oxide undergoes continuous nitrogen incorporation during the reaction, accompanied by enhanced catalytic activity. Time-dependent X-ray photoelectron spectroscopy reveals the emergence of nitrogen-containing surface environments. Thermal treatment is sufficient to induce this evolution, which approaches a relatively stable state after several hours. Notably, both molybdenum oxide and pre-synthesized MoO2–x/Mo2N develop similar nitrogen-containing surface chemical environments under reaction conditions. These findings demonstrate that the catalyst surface dynamically evolves during ammonia synthesis and that nitrogen incorporation is closely associated with enhanced catalytic activity.