ABSORPTION, ADSORPTION, AND DESORPTION STUDIES OF THE OXYGEN/RH(111) SYSTEM USING O-2, NO, AND NO2

The adsorption of oxygen on the Rh(111) surface using O-2, NO, and NO2 has been studied using thermal desorption of O-2, helium diffraction, and time-resolved specular helium scattering. At all surface temperatures, T-S, the surface coverage of oxygen saturates with theta(0)(sat) = 0.5 ML (monolayers). At T-S > 375 K, additional subsurface oxygen is absorbed. The subsurface O will segregate to and desorb from the Rh(111) surface at T-S > 650 K. The rate of subsurface deposition varies with the source of O, with NO2 >> NO > O-2. For absorption of O, E(a) = 4.3 +/- 0.7 kcal/mol. The difference in enthalpy between the surface and subsurface O is 4.3 +/- 0.3 kcal/mol. For theta(0) < 0.15 ML, O-2 desorption occurs with second order kinetics, with E(a) = 56 +/- 2 kcal/mol. The shape of O-2 desorption peaks at total (surface + subsurface) O similar to 0.5 ML is independent of the source of O, surface or subsurface. Rate analysis indicates that the interatomic interactions between coadsorbed O species are approximately the same magnitude as the interactions between adsorbed and absorbed O species.