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.

According to our previous results, a broad size distribution of Ru-supported nanoparticles is required for a high activity in ammonia synthesis under mild reaction conditions, since a synergy in the catalytic activity was observed between particles of different sizes. In this work we reach a more controlled heterogeneity of sizes by mixing different ruthenia colloidal suspensions, before impregnation on alumina, which improves the catalytic activity. Kinetic studies, ‘in situ’ characterizations and H/D isotopic exchange experiments were separately performed on catalysts containing small Ru nanoparticles and catalysts containing larger particles, in order to elucidate the mechanism of catalytic cooperation. Clear evidence is presented of a mechanism involving the migration of H atoms from large to small Ru nanoparticles, which is expected to improve the catalytic performance by promoting the hydrogenation of adsorbed NHx species and the release of active sites.

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