Adsorption, desorption, and surface diffusion kinetics of NH3 on MgO(100)

Abstract

Laser-induced thermal desorption (LITD) techniques were used to study the adsorption, desorption, and surface diffusion kinetics of NH3 on MgO(100) single-crystal surfaces. Isothermal LITD adsorption measurements revealed that the sticking coefficient of NH3 on MgO(100) was constant vs coverage and decreased with increasing surface temperature. The adsorption kinetics were consistent with a mobile precursor intermediate. In addition, the saturation NH3 coverage on MgO(100) was strongly dependent on temperature, and decreased by a factor of 5 as temperature increased from 130 to 165 K. Isothermal and linear temperature ramp LITD experiments indicated that the desorption kinetics of NH3 from MgO(100) could be modeled using a first-order rate law with a coverage-dependent desorption activation energy. The coverage dependence of the desorption activation energy was consistent with repulsive lateral interactions between NH3 adsorbates on the MgO(100) surface. The surface diffusion of NH3 on MgO(100) was also examined using LITD techniques. In contrast to recent predictions of NH3 surface diffusion on MgO(100), no evidence of NH3 surface mobility (D≤10−9 cm2/s) was observed for temperatures as high as 165 K.