Choosing and controlling the size distribution of Ru-supported nanoparticles are crucial steps to reach a high activity in ammonia synthesis under low temperature and pressure. A promoted reaction is expected on surfaces exhibiting polydispersed Ru nanoparticles, where small and larger particles give rise to a catalytic cooperation process. In this work we implement new strategies for the synthesis of supported RuO2 nanoparticles with broad size distributions. Catalysts with high heterogeneity of Ru sizes were successfully prepared and they shown enhanced activity in ammonia synthesis, as long as a sufficient concentration of small particles was reached. Indeed, catalytic activity drastically decreased for low concentrations of 2-3 nm Ru particles. A model is proposed for the deposition and growth of RuO2 colloidal nanoparticles on alumina, providing insights in processes associated to other synthesis methods. According to this model, firstly attached RuO2 particles behave as nucleation points for further deposition of hydrated RuO2, while remaining RuO2 nanoparticles in suspension are successively deposited in a constantly changing surface, triggering growth of supported clusters.