Strong Size Effects in Supported Ionic Nanoparticles: Tailoring the Stability of NO x Storage Catalysts

Based on a well-defined model-catalyst approach, we study the particle size dependent properties of NO _x storage materials. The single-crystal based model systems are prepared on an ordered Al₂O₃ film, on which BaO nanoparticles are grown under ultrahigh-vacuum (UVH) conditions. Particle size and density are characterized by scanning tunneling microscopy (STM). The interaction with NO₂ is probed by molecular beam (MB) methods in combination with time-resolved IR reflection absorption spectroscopy (TR-IRAS). It is found that both, the stability and the formation kinetics of alumina supported barium nitrate nanoparticles show a strong dependence on particle size. Very small BaO particles are rapidly converted into nitrates, however, the resulting aggregates exhibit a strongly reduced thermal stability. Surface and bulk nitrate and nitrate features are identified by means of vibrational spectroscopy. It is concluded that the size dependencies are related to the formation and decomposition of surface-related BaNO _x species the decomposition temperature of which can be tuned over an exceptionally large temperature interval. It is suggested that the stability of these surface NO _x species is strongly modified by the underlying support.