Chemisorption of Nitric Oxide on Tungsten

Abstract

The chemisorption of NO on polycrystalline W has been investigated using a combination of ultrahigh-vacuum techniques: flash-desorption mass spectroscopy and field-emission microscopy. It has been found that NO chemisorbs nondissociatively on W at room temperature. Dissociation occurs at elevated temperature with an activation energy of about 47 kcal/mole (197 kJ/mole). Kinetic evidence indicates that dissociation involves the interaction of an adsorbed NO molecule with a neighbor empty site. At high NO coverage, dissociation is rate controlling for the liberation of ω-mode nitrogen near 1000°K. Heating of an NO-shadowed W field-emission tip above ∼700°K results in the formation of a receding boundary which remains sharp during the entire course of its backward motion over the initially covered region. This behavior is consistent with a model involving dissociation of NO at the boundary where empty sites are available. For monolayer NO (1.4×1015 molecules cm−2) on polycrystalline W, Δφ=+1.85±0.10 eV; this corresponds to an average surface dipole moment of 0.35-D per chemisorbed NO molecule at full coverage.