NO and NO₂ Adsorption on Barium Oxide: Model Study of the Trapping Stage of NO<i>_x</i> Conversion via Lean NO<i>_x</i> Traps

Abstract

The use of NOx traps is one strategy being pursed to enable the implementation of more fuel-efficient lean-burn gasoline engines. Materials development to enhance NOx storage capacity and sulfur tolerance will be necessary for performance improvement. Progress in these areas will benefit from a more detailed understanding of the base metal oxide−precious metal surface chemistry involved in the trapping, release, and reduction of NOx. In this work, we have focused on the adsorption of NO and NO2 on in-situ evaporated thin films of barium oxide, the primary storage material in lean NOx traps, to accentuate the details of the trapping stage of NOx conversion using these systems. X-ray photoelectron spectroscopy has been used to identify the species formed and their relative abundance following room-temperature adsorption. Annealing experiments were performed to follow changes in adsorbed species with temperature. For NO, our results are consistent with nitrites forming as a result of molecular adsorption. In the case of NO2, nitrates are favored at high exposure and appear to form via a nitrite intermediate. We propose that as coverage increases nitrates form via trimer formation involving two surface nitrites and an additional molecularly adsorbed NO2 resulting in a complex in which all nitrogen centers are nitratelike. In light of results presented, an alternative and more detailed interpretation of the mechanism of NOx trapping is offered, that accounts for the NOx storage capacity benefits resulting from NO oxidation over noble-metal sites.