Low Temperature H₂O and NO₂ Coadsorption on θ-Al₂O₃/NiAl(100) Ultrathin Films

The coadsorption of H(2)O and NO(2) molecules on a well-ordered, ultrathin theta-Al(2)O(3)/NiAl(100) film surface was studied using temperature programmed desorption (TPD), infrared reflection absorption spectroscopy (IRAS), and X-ray photoelectron spectroscopy (XPS). For H(2)O and NO(2) monolayers adsorbed separately on the theta-Al(2)O(3)/NiAl(100) surface, adsorption energies were estimated to be 44.8 and 36.6 kJ/mol, respectively. Coadsorption systems prepared by sequential deposition of NO(2) and H(2)O revealed the existence of coverage and temperature-dependent adsorption regimes where H(2)O molecules and the surface NO(x) species (NO(2)/N(2)O(4)/NO(2)(-),NO(3)(-)) form segregated and/or mixed domains. Influence of the changes in the crystallinity of solid water (amorphous vs crystalline) on the coadsorption properties of the NO(2)/H(2)O/theta-Al(2)O(3)/NiAl(100) system is also discussed.