Model NOx storage systems: Storage capacity and thermal aging of BaO/θ-Al2O3/NiAl(100)

The NOx storage properties of a BaO/θ-Al2O3/NiAl(100) model system, with a BaO coverage of ∼2 monolayer equivalent (MLE), was studied. X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) techniques were used to investigate NO2 adsorption and reaction on the BaO/θ-Al2O3/NiAl(100) surface. These results were compared with those of the θ-Al2O3/NiAl(100) support material, a thermally aged BaO/θ-Al2O3/NiAl(100) model system, and a realistic BaO (20 wt%)/γ-Al2O3 high-surface area counterpart. At T>300 KT>300 K, adsorbed NO2 is converted to nitrates on all of the surfaces studied. Nitrates residing on the alumina sites of the model catalyst surfaces are relatively weakly bound and typically desorb within 300–600 K, leading to NO(g) evolution; while nitrates associated with the baria sites are significantly more stable and desorb within 600–850 K, resulting in NO(g) or NO(g) + O2(g) evolution. NOx uptake by the baria sites of the BaO/θ-Al2O3/NiAl(100) model system was found to be as much as five-fold greater than that of the θ-Al2O3/NiAl(100) support material. Thermal aging of a BaO/θ-Al2O3/NiAl(100) surface at 1100 K before NOx uptake experiments brings about a significant (>70%) reduction in the NOx storage capacity of the model catalyst surface.