Long-range diffusion of K promoter on an ammonia synthesis catalyst surface - Ionization of excited potassium species in the sample edge fields

The potassium ions K+ which diffuse out to the surface from the K promoted ammonia synthesis iron catalyst material do not desorb from there but diffuse rapidly along the surface until they reach the edges of the sample. This is shown by angular distributions of the ion emission at the normal operating temperatures of 900-1100 K, The ionic emission at low field strengths, of the order of 2-50 V cm(-1) has a minimum or even a zero signal in the direction of the surface normal. Instead of desorbing from the surface, the ions interact strongly with the surface and give electronically excited states K* on the surface, which diffuse rapidly along the surface over a distance of several millimeters to the edges of the sample. A detailed model is proposed for this process, based on recent kinetic results. At the edges, ions are formed in the stronger electric field just outside the surface giving lobes along the surface. With the highest field strengths used, these lobes are transformed into strongly peaked distributions at 45-70 from the normal, with a strong minimum in the normal direction. From the open surface, only clusters K-n and neutral atoms K can be emitted. Trajectory calculations show that ions, which are emitted from the edges of the sample with higher then thermal energy, appear in the experimentally observed angular range, Their excess energy may be derived from the work function difference between the catalyst sample and its Ta holder. (C) 1999 Academic Press.