Chemisorption on perfect surfaces: Hydrogen and nitrogen on tungsten and rhodium

Abstract

Chemisorption on highly perfect low index surfaces, prepared by low temperature field evaporation, is compared in simultaneous field emission measurements with chemisorption on the atomically rough regions which dominate the total electron current from the emitter. For the (110) plane of tungsten, the most densely packed of this lattice, it is found that contrary to some earlier reports, nitrogen does chemisorb after high exposures at room temperature. The nitrogen has no significant effect upon the work function for electrons; instead, chemisorption manifests itself by a change in the absolute emission intensity. Hydrogen also chemisorbs on W(110). However, at T?144 °K, this plane is populated only after the rougher, stepped regions have been substantially covered by a chemisorbed layer. At still lower temperatures, at T?38 °K, a layer of molecularly bound gas forms on exposure of W(110) to hydrogen. The layer is removed by warming to T<77 °K, without conversion into the chemisorbed state. This suggests that on the (110) plane, dissociative chemisorption of hydrogen is activated, and must occur over a barrier ? 3.5 kcal/mole. Chemisorption occurs preferentially on the stepped, atomically rough regions. It is by transport from these regions, and not by direct dissociation on W(110), that this surface is filled up. Measurements have also been made on the (111), (100), and (110) planes of rhodium at various temperatures. On all of these, hydrogen chemisorbs at a rate comparable to that on rough planes, and there is no indication that steps play an important role in the adsorption process.