A neutron scattering study of adsorbed gases and some model compounds

Abstract

Neutron scattering theory and spectrometers are described and the relevance of neutron scattering to surface science discussed. Low frequency vibrational modes of hydrogeneous ligands were studied, mainly by incoherent inelastic neutron scattering. Complexes containing ethylene were investigated and their torsional vibrations assigned. The in-phase and out-of-phase torsions have been observed in the case of complexes containing more than one ethylene ligand, and models have been proposed for the interaction between the ethylene groups. The experimentally observed frequencies have been expressed in terms of constants derived from these models. π-bonded complexes (π-allyls, norbornadienes and cyclobutadiene iron tricarbonyl) have been studied, their low frequency modes assigned and their effective torsional force constants calculated. However, there remains an anomaly in the assignments of the norbornadiene complexes. Studies of square planar Pd and Pt ammines have shown that there is just a single torsional mode in each case. It has previously been suggested that the NH(_3) ligandsin trans complexes (particularly Pt) could couple via the "d" orbitals. No evidence of such coupling was found. From the momentum transfer dependence of the i.n.s. spectrum, and the temperature dependence of the i.r. spectrum, evidence for a significant metal-metal interaction in cis-Pt(NH(_3))2Cl(_2) was obtained. In the case of sulfur complexes containing NH(_3) and NH(_2) groups, the torsions have been assigned. Comparison of their i.n.s. spectra with that of disilver sulfamide, has shown that the latter does not contain an NH(_2) group. C(_2)H(_4) adsorbed on a silver exchanged zeolite (Ag-13X) at two different coverages was studied. From the observed i.n.s. intensities and shifts on deuteration (C(_2)D(_4) and trans-C(_2)D(_2)H(_2)) assignments of the vibrational modes of the ethylene relative to the surface was possible, in the higher coverage case interaction between ethylenes on two different sites was observed. The i.n.s. spectrum of ethylene adsorbed on the sodium form of 13X was indicative of a much more weakly held species and an analysis of the quasi-elastic scattering confirmed this. I.N.S. spectra of C(_2)E(_2) and C(_2)D(_2) adsorbed on Ag-13X were obtained and the acetylene-surface vibrations were assigned. From the shifts on deuteration it appears that the adsorbed molecule is non-linear. The i.n.s. spectra of C(_2)H(_2) adsorbed on Na-13X was too poorly resolved for definite conclusions to be drawn. Hydrogen adsorbed (at 200ºC) on Pt black was studied in the region 0 → 4000 cm(^-1). The adsorption conditions produced just one surface state (6) and the adsorption was shown to be dissociative. A vibration of the hydrogen, relative to the surface, was observed at 403 cm(^-1). In fact the spectra are best interpreted as being due to co-adsorbed H(^2)O and H. Higher energy peaks were obtained (including one assigned to the V(_2) vibration of water) but could not be definitely assigned. This project is not yet complete and further experimental work is required before full assignments can be made.