Adsorption state of NO on Mo(1 0 0) surface studied by ESDIAD

Abstract

The adsorption states of nitric oxide and oxygen on a Mo(1 0 0) surface have been studied by electron stimulated desorption ion angular distribution (ESDIAD) and low energy electron diffraction (LEED). On a NO-exposed Mo(1 0 0) surface, only O + ions are detected by ESD and the ESDIAD pattern has a bright center spot perpendicular to the surface and four off-normal spots in the [0 0 1], [0 1 0], [ 00 1 ¯ ] and [ 0 1 ¯ 0 ] directions with a very weak intensity. The ESDIAD patterns of the NO- and O 2-exposed surfaces have the same shape but different intensities. The center spot is hazy and square at low exposure of NO below 0.5 L and becomes circular with increasing exposure. For low exposure, oxygen atoms adsorb at fourfold hollow and on-top sites, and for higher exposure, they mainly adsorb at on-top sites. Some oxygen atoms adsorb at threefold hollow sites coordinated to two first-layer molybdenum atoms and one second-layer molybdenum atom. On the O 2-exposed surface, the intensities of the center and off-normal spots decrease monotonically with increasing annealing temperature. Oxygen atoms diffuse into the bulk in the temperature range of 300–800 K. On the NO-exposed surface, these intensities decrease by a two-step process and disappear at around 1200 K. Nitrogen atoms adsorb at the sites where the diffusion of the oxygen atoms into the bulk is suppressed, such as fourfold hollow sites. The NO- and O 2-exposed surfaces show the following structures with increasing temperature: a high background up to 850 K, a streak structure in the temperature range of 900–1100 K, (2 × 1) in the range of 1200–1300 K, (2 × 2 + 4 × 2) in the range of 1350–1500 K, (√5 × √5) R26°33′′ in the range of 1550–1650 K, c(4 × 4) in the range of 1700–1800 K and a clean structure above 1850 K. Oxygen atoms begin to form the surface oxide above 800 K on or under the surface.