NO2 adsorption on ultrathin theta-Al2O3 films: formation of nitrite and nitrate species.

Interaction of NO2 with an ordered theta-Al2O3/NiAl(100) model catalyst surface was investigated using temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The origin of the NO(x) uptake of the catalytic support (i.e., Al2O3) in a NO(x) storage catalyst is identified. Adsorbed NO2 is converted to strongly bound nitrites and nitrates that are stable on the model catalyst surface at temperatures as high as 300 and 650 K, respectively. The results show that alumina is not completely inert and may stabilize some form of NO(x) under certain catalytic conditions. The stability of the NO(x) formed by exposing the theta-Al2O3 model catalyst to NO2 adsorption increases in the order NO2 (physisorbed or N2O4) < NO2 (chemisorbed) < NO2- < NO3-.