Effect of Surface Diffusion on Rates of Adsorption

Abstract

SOME striking results on the rates of adsorption of nitrogen on tungsten have recently been obtained by a flash filament technique¹. The rate of adsorption is expressed in terms of a ‘sticking factor’, s, the probability that a molecule striking the surface shall remain there. It is found that the sticking factor at 300° K. remains constant independent of surface coverage, θ, until there is about one nitrogen atom for every four tungsten atoms. As the temperature is increased by steps to 1,100° K., s both decreases for any given θ and also starts to fall off at lower surface coverages. It would seem that the decrease of s with rising temperature would not be expected from the usual picture of activated adsorption. Furthermore, although the decrease of rate with increasing θ may be influenced by repulsive forces between the adsorbed atoms, the decrease might be supposed to take place at a fixed θ, that is, at a certain distance between adsorbed atoms, and not to change with temperature as was found. Similar behaviour has been observed earlier for adsorption on catalysts, namely, a constant rapid rate of adsorption extending up to high surface coverages followed by a swift decrease as saturation is reached. A rate proportional to exp(−aθ) has been proposed².