Mass spectrometric study of heterogeneous chemical and photochemical reactions of NOx on alumina taken as a model substance for mineral aerosol

The interaction of NOx (N2O, NO and NO2) with γ-Al2O3 taken as a mineral dust model at 225–360 K is studied by mass spectrometry at low pressures corresponding to typical concentrations of these pollutants in the atmosphere. It is found that, at low coverage (<0.01 monolayer), adsorbed NOx molecules are strongly bonded to surface centers and cannot be removed by evacuation at room temperature. According to temperature-programmed desorption data, desorption energies of N2O and NO fall in the intervals of 120–140 and 90–135 kJ mol−1, respectively. N2O adsorption is accompanied by partial dissociation into N2 and O2, whereas NO2 adsorption leads to complete dissociation to NO and O2. Adsorption of N2O and NO2 is accompanied by chemiluminescence with a maximum emission at 420 nm. NO adsorption does not result in appreciable dissociation. Upon UV-irradiation of Al2O3 in a N2O flow, partial N2O decomposition into N2 and O2 and parallel N2O desorption from the surface are observed, while in the case of a NO flow, only photoinduced NO desorption is detected. It is assumed that the dissociative adsorption of N2O and NO2, chemiluminescence and photoinduced N2O dissociation occur via the capture by the adsorbed molecules of electrons from surface F- and F+-centers of Al2O3 to give rise to Oads2− and/or Oads− species. The photodesorption efficiency of N2O and NO correlates with absorption spectrum of Al2O3 in the UV range and may be associated with light-induced decomposition of complexes formed by these molecules with surface centers. Thermodesorption of preadsorbed N2O and NO from Al2O3 occurs predominantly without decomposition of these molecules, whereas the major desorption product of NO2 is NO. The desorption-rate maxima and the product distribution depend on the initial surface coverage indicating inhomogeneity of the adsorption centers.