Structure and dynamics of ammonia adsorbed on graphitized carbon black. Part 2.—Neutron diffraction

Abstract

Neutron diffraction experiments determining the structure of the adsorbate in the system ammonia on Graphon are described. The experiments were done over a range of temperatures from 80 K to just above the melting point of bulk ammonia at ≈ 200 K, and a range of adsorbed volumes of vapour equivalent to surface coverages between 0.25 and 6 statistical layers. These ranges overlap those used for the isotherm measurements described in the previous paper.

The results show that, on cooling to temperatures well below its bulk melting point, most of the ammonia desorbs from the surface to form bulk crystallites. Formation of the crystallites takes place over a wide range of temperature and is very sensitive to the amount of ammonia present. Melting point curves determined by neutron diffraction are shown to be closely connected with the capacity of the surface to adsorb ammonia. The results are interpreted in terms of an equilibrium between bulk solid and adsorbed fluid. Analysis of the shapes of the melting point curves and comparison of these with the isotherm measurements establishes a clear link between adsorption and heterogeneous nucleation.

Addition of methane to the surface makes it less able to adsorb ammonia and the ammonia crystallites are stable at correspondingly higher temperatures.