Surface potential measurements on nickel and iron films during the chemisorption of ammonia, nitrogen, and hydrogen

Abstract

The surface potential changes Δφ during the adsorption of ammonia, nitrogen, and hydrogen on evaporated films of nickel and iron have been studied. After little change of potential at low coverage, large positive Δφ values were obtained when ammonia was adsorbed at −78 °C to high coverages on both metals. Since this potential increased with time and the adsorption was not reversible, chemisorption with dissociation was probable. The initial zero value of Δφ was explained by the NH 2 radical and H atom having dipole moments of approximately the same magnitude but opposite in sign, the latter on a fully covered surface giving a Δφ of −0.4 volt. The positive potential was a consequence of an activated transformation of the H adatom from its interstitial surface site to one outside the surface between adjacent chemisorbed NH 2 radicals with change of sign (and magnitude) of its dipole, as previously found in the CO + H, and O 2 + H systems. At 20 °C and above, a surface equilibrium was set up between chemisorbed NH 2, NH, and H radicals on nickel films as evidenced by the displacement in the NH 2 direction (with increase in +Δφ) by increasing either the hydrogen pressure or the ammonia pressure in the gas phase, and in the reverse direction by evacuation (thereby desorbing some hydrogen), by increase in temperature, or by working at low coverages. There was no evidence of the formation of the N-atom complex NiN, but with iron, a surface layer of FeN was formed and had a Δφ of −0.4 volt for a “fully” covered surface. With this latter metal, a surface equilibrium between NH 2, NH, N, and H radicals was set up. Thus, with high coverages of NH 3, Δφ was positive because of the presence of adsorbed “protonic” hydrogen atoms, but its bonding energy was less than on nickel, and hydrogen was spontaneously desorbed at 20 °C, thereby causing Δφ to drift in a negative direction.