<p>The role of NH₃ adsorption and diffusion in the pores of the catalyst on the ammonia synthesis rates at elevated temperatures is investigated through pulse feeding of the reactants over two different catalysts. One of the catalysts was a 1 wt% Ru/SBA-15, offering a very high and porous surface area. The second catalyst was a very low surface area Co₃Mo₃N catalyst with well-defined bulk and surface properties. By sending alternative pulses of H₂ and N₂ mixed with Ar, it was possible to differentiate the role of a porous support and observe surface events. Through a pulse feeding strategy, both reactor hydrodynamics and some of the surface events could be monitored. The transients of Ar in an empty reactor and a reactor packed with Co₃Mo₃N exhibited similar behaviours. On the other hand, delayed signals as tails in the derivative response curves over Ru/SBA-15 packed reactor indicated pore diffusion effects, even for Ar. Time derivatives of the mass spectrometer signals represent the rates, revealing system-based responses of the reactive species. Ammonia signal from Ru/SBA15 catalyst was congruent with the pulses during the initial period, becoming erratic suggesting cycles of pore condensation, saturation and desorption and finally reaching a steady value reflecting a steady desorption from the pores. On the other hand, ammonia signal from Co₃Mo₃N revealed NH₃ signals congruent with the pulses, consistent with the absence of effects related to adsorption and diffusion in pores.</p>