Catalyst systems, neutron scattering

In this paper we discuss how neutron scattering spectroscopy can be applied to the study of the structure and dynamics of adsorbed molecules. Since reviews of elastic and inelastic neutron scattering from adsorbed films have recently appeared (1.-3), our purpose here is not to present a comprehensive survey of every adsorbed system investigated by neutron scattering. Rather, we shall be concerned primarily with two questions which are basic to the characterization of adsorbed species on catalysts and which have been central to the discussion of this symposium. These are the extent to which the neutron scattering technique can be used to determine 1) the orientation and position of an adsorbed molecule and 2) the strength and location of the forces bonding a molecule to a surface. 

The pore diameters of MFI-type zeolites are comparable to the size of many commercially important molecules, such as aromatics or linear or branched hydrocarbons [1]. Thus, the study of the difiusion of reactive molecules in the channel system of zeolite catalysts is of considerable interest for the understanding of the catalyst performance. A variety of methods has been developed and applied to the measurement of diffii-sion coefficients, amongst others gravimetric techniques [2], neutron scattering [3], NMR [4] and Frequency Response [5]. The FTIR technique offers the possibility to study sorption and sorption kinetics under conditions close to those of catalytic experiments. By the use of an IR microscope, single crystals have become accessible to the FTIR technique. 

Because the transition rate between the ortho- and para- states is low except in the presence of magnetic catalysts, we can equilibrate the gas at a given temperature in the presence of a magnetic catalyst. For low temperatures, in the presence of a catalyst, the system will become virtually entirely para while for high temperatures it will tend to 25% para and 75% ortho. We can therefore do neutron scattering experiments with different predetermined ratios of the two forms so that both cross-sections can be extracted. 

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