Inelastic neutron scattering investigations of the products of thermally and catalytically driven catalyst coking

Carbons which were deposited on catalyst surfaces under technical operation conditions were characterised by means of inelastic incoherent neutron scattering (INS). The graphiticity of pyrocarbon derived from methane at similar to 1200 degrees C during the synthesis of hydrogen cyanide from methane and ammonia as a product from thermally driven coking was measured before and after additional hydrogen treatment. The material appeared to be much closer to pure graphite than to (e.g.) carbon black. As counterparts from catalytically driven coke deposition and coke transformation carbons which were formed during the selective hydrogenation of acetylene to ethylene in the HCl-recycle gas stream of the vinyl chloride process at T < 200 degrees C were compared. They showed a much greater similarity to pure products from controlled chemical vapour deposition than to polymer-like carbons. Neutron scattering applied as a bulk technique selectively revealed the properties of the carbonaceous part of partly finely divided, strongly contaminated and highly absorbing deposits from the surfaces of technical catalysts via its proton dynamics. Contributions from inorganic contaminants were largely suppressed. SiO(2) catalyst supports showed strong vibrational bands at 65 and 122 cm(-1). The latter was attributed to a low wavenumber Si-OH torsional mode. INS seems uniquely suitable to extend the large body of Infrared and Raman work on cokes and on catalyst supports down to low energies revealing partly very strong and sharp vibrational bands. (C) 1999 Elsevier Science B.V. All rights reserved.