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

Abstract

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 nitrite/nitrate 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: in these systems initially nitrites are formed only, and then converted to nitrates at longer NO₂ exposure times. These results clarify the contradicting mechanisms presented in prior studies in the literature. After the formation of a nitrate layer the further conversion of the underlying BaO is slow, and strongly depends on both the sample temperature and the NO₂ partial pressure. At 300 K sample temperature amorphous Ba(NO₃)₂ forms that then can be converted to crystalline nitrates at elevated temperatures. The reaction between BaO and H₂O is facile, a series of Ba(OH)₂ phases form under the temperature and H₂O partial pressure regimes studied. Both amorphous and crystalline Ba(OH)₂ phases react with NO₂, and initially form nitrites only that can be converted to nitrates. The NO₂ adsorption capacities of BaO and Ba(OH)₂ are identical, i.e., both of these phases can completely be converted to Ba(NO₃)₂. In contrast, the interaction of CO₂ with pure BaO results in the formation of a BaCO₃ layer that prevents to complete carbonation of the entire BaO film under the experimental conditions applied in these studies. However, these “carbonated” BaO layers readily react with NO₂, and at elevated sample temperature even the carbonate layer is converted to nitrates. The importance of the metal oxide/metal interface in the chemistry on NO_x storage-reduction catalysts was studied on BaO(<1 ML)/Pt(111) reverse model catalysts. In comparison to the clean Pt(111), new oxygen adsorption phases were identified on the BaO/Pt(111) surface that can be associated with oxygen atoms strongly adsorbed on Pt atoms at the peripheries of BaO particles. A simple kinetic model developed helped explain the observed thermal desorption results. The role of the oxide/metal interface in the reduction of Ba(NO₃)₂ was also substantiated in experiments where Ba(NO₃)₂/O/Pt(111) samples were exposed to CO at elevated sample temperature. The catalytic decomposition of the nitrate phase occurred as soon as metal sites opened up by the removal of interfacial oxygen via CO oxidation from the O/Pt(111) surface. The temperature for catalytic nitrate reduction was found to be significantly lower than the onset temperature of thermal nitrate decomposition.