Definitive adsorption states of intermediates on Ru nanocatalysts for progress of ammonia synthesis discovered by modulation excitation spectroscopy under reaction conditions

•In-situ IR observation combined with MES (IR-MES) was applied to NH3 synthesis reactions at 400 °C under 0.1 MPa.•Time-variable adsorption states of intermediates on Ru nanoparticles were observed.•Transient steps were clarified by IR-MES and DFT calculations.•A critical step has been proposed for the advancement of NH3 synthesis at lower temperatures.