Understanding Automotive Exhaust Catalysts Using a Surface Science Approach: Model NO_x Storage Materials

摘要

The structure-reactivity relationships of model BaO-based NO_x storage/reduction catalysts were investigated under well controlled experimental conditions using surface science analysis techniques. The reactivity of BaO toward NO2, CO2, and H2O was studied as a function of BaO layer thickness [0 < θ_(BaO) < 30 monolayer (ML)], sample temperature, reactant partial pressure, and the nature of the substrate the NO_x storage material was deposited onto. Most of the efforts focused on understanding the mechanism of NO2 storage either on pure BaO, or on BaO exposed to CO2 or H2O prior to NO2 exposure. The interaction of NO2 with a pure BaO film results in the initial formation of nitriteitrate ion pairs by a cooperative adsorption mechanism predicted by prior theoretical calculations. The nitrites are then further oxidized to nitrates to produce a fully nitrated surface. The mechanism of NO2 uptake on thin BaO films (<4 ML), BaO clusters (<1 ML) and mixed BaO/Al2O3 layers are fundamentally different: