The Emission of Alkali Atoms from Various Ammonia Catalysts

An apparatus is described for measuring the rate of emission of alkali as positive ions and as neutral atoms from the ammonia catalysts. The results show that when the temperature of the catalyst is first raised to the emission point the number of atoms leaving the surface far exceeds that of ions. In the course of several minutes heating the emissivity of atoms decays to a negligible value. Atom emission is much more persistent from catalysts containing Al2O3, also the temperature for equivalent ion currents is higher. Both atom and ion emission increase when sintering occurs, since the alkali can then diffuse more readily to the surface. The factors determining the ratio of atoms to ions are discussed. Under the conditions where the alkali concentration at the surface amounts to several hundredths of a molecular layer the presence of the adions will lower the work function of an appreciable fraction of the surface to a point below the ionization potential of potassium, thus permitting the potassium to escape as neutral atoms. When the alkali concentration is small, however, the work function is close to that of pure iron and the potassium can escape only in the form of ions.