The structure and properties of methane adsorbed on graphitized carbon black determined by neutron diffraction

Neutron diffraction patterns from methane-d4 on the graphitized carbon black, Vulcan III, have been measured over a range of coverage and temperature. At coverages up to 0·9 monolayers the methane forms a two dimensional layer in registry (√3 × √3) with the basal planes of graphite that make up the surface. At higher coverages a more compressed structure is formed. The melting points of the √3 × √3 layer at different coverages have been measured. The diffraction patterns contain contributions from cross interference between substrate and adsorbate. A theoretical analysis of this effect is presented and has been used to determine the distance of methane from the surface. The methane-carbon to surface-carbon distance is 3·35 ± 0·1 Å and, at low temperatures (20 K), the methane molecule is in a tripod configuration on the surface. The cross interference effect gives new information about the structure of the carbon black itself. It is a strained, defective material with a mean defect length of 38 Å and a mean particle size (normal to the surface) of 85 Å. The mean defect free length in the adsorbed layer is also 85 Å and this, too, is a property of the surface of the substrate.