Adsorption States of NO2 over Water−Ice Films Formed on Au(111)

Abstract

The adsorption states of NO2 over amorphous and crystalline water−ice films formed on an Au(111) surface have been studied in an ultrahigh vacuum system by the temperature programmed desorption (TPD) technique and IR absorption−reflection spectroscopy (IRAS). The ice films are prepared by deposition of gas phase water on the Au substrate at <100 K for amorphous ice and at 140 K for crystalline ice. The surface of amorphous ice is characterized by the high density of free OH, while that of crystalline ice is characterized by grain boundaries and the lack of free OH. TPD for pure ice shows only one desorption peak of H2O, while after NO2 adsorption on it an additional weak H2O desorption peak appears at 185 K. This higher-temperature peak is attributable to decomposition of NO2−H2O adducts. IRAS measurements revealed that NO2 adsorbs on ice surfaces as N2O4 with D2h symmetry and that neither N2O4 isomers such as D-isomers nor NOx (x = 1, 2, and 3) species are produced in the temperature range of 90−140 K. Interaction of the ice surfaces with NO2 (N2O4) as well as orientation of N2O4 adsorbed on the ice surfaces are investigated as a function of temperature. Thermal decomposition of NO2 adsorbed on the water−ice formed on an Au surface is reconfirmed, which has been reported by Wang and Koel (J. Phys. Chem. A 1998, 102, 8573). A possible mechanism for the NO2 decomposition is proposed.