Disclosing the synergistic mechanism in the catalytic activity of different-sized Ru nanoparticles for ammonia synthesis at mild reaction conditions

•Ru nanoparticles with heterogeneous sizes present high activity in ammonia synthesis.•The release of active sites blocked by H atoms is very slow on small Ru nanoparticles.•Hydrogen activation barrier and adsorption strength are lower on larger particles.•H atoms can migrate from large to small particles and cause a catalytic cooperation.•The hydrogenation of adsorbed NHx and the release of active sites might be promoted.