Mobility and Adsorption of Hydrogen on Tungsten

The surface diffusion of hydrogen on W field emitters has been investigated by making use of the low vapor pressure and high sticking coefficient of H2 at liquid helium temperature to obtain unilateral contamination. Average values of the energy of desorption of H2 have also been obtained. It is found that spreading occurs in three distinct phases. At very high initial coverages a moving boundary migration occurs below 20°K, corresponding to migration of physically adsorbed H2 on top of the chemisorbed layer, precipitation at its edge, and further migration over the newly covered region. For initial deposits corresponding to ∼1 monolayer spreading sets in at ∼180°K with a boundary moving radially outward from the close-packed 110 faces. The activation energy of this process is ∼6 kcal and corresponds to migration over the smooth (close-packed) regions of the tip followed by precipitation of chemisorbate at trap sites on the rough regions. At coverages too low to permit saturation of traps diffusion out of the latter becomes rate controlling so that boundary free diffusion with E≈9.5 kcal and at even lower coverages with E∼16 kcal is observed. The heat of adsorption of H2 at extremely low coverage is found to be 64 kcal, but drops rapidly with increasing coverage. These results lead to the conclusion that surface heterogeneity is largely the result of surface topography and is inherent on all but the close-packed faces of W. The activation energies for diffusion are found to be 10 to 20% of the binding energy, depending on the nature of the sites. It is also seen from the low-temperature spreading that ∼75% of the chemisorbed layer can be formed below 20°K with effectively zero activation energy, and a large probability factor. The bearing of these results on the mechanism of some catalytic reactions is discussed.