Adsorption and reaction of NO2 on ordered alumina films and mixed baria-alumina nanoparticles: Cooperative versus non-cooperative reaction mechanisms

In order to obtain insights into the mechanism and kinetics of adsorption and reaction of NO2 on aluminum and barium oxide surfaces, we have performed systematic adsorption experiments on single-crystal-based model materials. As a model surface, we use BaO containing nanoparticles grown by physical vapor deposition (PVD) of Ba under ultrahigh vacuum (UHV) conditions on an ordered Al2O3 film on NiAl(110) and subsequent oxidation and annealing. The growth behavior, the morphology and the chemical composition of the three-dimensional mixed barium aluminum oxide particles (BaAl2xO1+3x) formed via this procedure have previously been characterized by scanning tunneling microscopy (STM) and high-resolution photoelectron spectroscopy (HR-PES). In order to monitor adsorption and reaction processes on these systems, we perform time-resolved infrared reflection absorption spectroscopy (TR-IRAs) during exposure to a molecular beam (MB) of NO2. In a first step, the interaction of NO2 with the Al2O3/NiAl(110) model support is probed. At 100 K, only molecular adsorption occurs in form of the D-2h dimer (N2O4) in all coverage regions from the submonolayer up to multilayers. At 300 K, a slow surface reaction occurs, initially leading to the formation of surface nitrites and, subsequently, of surface nitrates in bridging adsorption geometry. On the BaAl2xO1+3x, particles on Al2O3/NiAl(110), NO2 shows a very different and strongly temperature-dependent behavior. At 100 K, molecular adsorption of the D-2h dimer (N2O4) is accompanied by a highly efficient reaction channel, leading to the formation of surface nitrites and nitrates. With the surface temperature increasing to 300 K, however, the reaction probability decreases by several orders of magnitude. In contrast to reaction at 100 K, surface nitrites in flat-lying adsorption geometry are the only primary product, indicating a temperature-dependent change in the reaction mechanism from a cooperative to a non-cooperative pathway. With increasing exposure, the nitrite coverage increases and, finally, the surface nitrites are converted into bridging and monodentate Surface nitrates. The latter reaction shows a complex kinetics, including an initial induction period. For temperatures of 400 K and above, nitrate formation becomes more efficient, eventually resulting in the formation of ionic nitrates. The vibrational properties of these ionic species sensitively depend on the reaction temperature, indicating the formation of well-defined structures in a narrow temperature interval around 500 K. The mechanism of the NOx storage process and the vibrational assignments of the related surface species are critically discussed on the basis of the present results. (C) 2008 Elsevier Inc. All rights reserved.