NO and NO₂ Adsorption on Barium Oxide:  Model Study of the Trapping Stage of NOx Conversion via Lean NOx Traps

The use of NO x traps is one strategy being pursed to enable the implementation of more fuel-efficient lean-burn gasoline engines. Materials development to enhance NO x 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 NO x . In this work, we have focused on the adsorption of NO and NO 2 on in-situ evaporated thin films of barium oxide, the primary storage material in lean NO x traps, to accentuate the details of the trapping stage of NO x 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 NO 2, 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 NO 2 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 NO x trapping is offered, that accounts for the NO x storage capacity benefits resulting from NO oxidation over noble-metal sites.