click on figure for more details (fig.1)
The active surface of a catalyst makes up a large part of its volume, so neutron scattering from the bulk material can provide information about the catalytic interface. For example, Zeolites are important catalysts in the petrochemical industry, and it would be interesting to know how absorbed molecules are attached and interact within the zeolite channels. Again, classical crystallography is difficult because all synthetic zeolites are polycrystalline materials.
The basic zeolite structure can be obtained with X-rays, but neutrons are much better for seeing the light atom of the hydrocarbon. Hydrogen itself has a large negative scattering length, and even greater contrast can be obtained by comparison with deuterated materials, since the deuterium hydrogen isotope has a large positive scattering length. For example, Fitch et al. have shown that relatively small numbers of molecules such as benzene adsorbed into sodium Y-zeolite can be located.
The diffraction pattern of the bare de-hydrated zeolite Na56Si136Al56O384 was first obtained (shown below, fig.1a), and the earlier single crystal structure confirmed by Rietveld refinement. Diffraction patterns were then obtained at 4K(to reduce disorder) with two different coverages of benzene: 1.1 and 2.6 molecules per supercage. The patterns obtained with benzene are clearly different (fig.1b) to that of the bare zeolite.
click on figure for more details (fig.1)
These differences then served to obtain the positions of the benzene molecules using `Fourier difference' techniques, which essentially produce a contour map of the differences between the bare and benzene loaded samples (fig.2a). The benzene positions were then refined using the Rietveld technique. In this way one can identify which atoms of the hydrocarbon interact with which atoms in the zeolite channels.
click on figure for more details (fig.2)
Figure 2a shows a simple example of such a difference Fourier map, showing benzene in sodium Y-zeolite. In figure 2b this information has been used to show schematically how the benzene molecules pack into the zeolite channels.
Since these channels and molecules are of a size that can be investigated by neutron small angle scattering, a further simple experiment can be performed. Neutron small angle scattering patterns are collected, again for the anhydrous and the loaded zeolite. In this case, the difference gives a direct measure of the size of hydrocarbon aggregates within the zeolite channels.
These measurements showed that at low coverages, benzene was distributed evenly throughout the supercages, with the molecule bonded by its pi-electron density to one of the sodium atoms. At higher coverages, clusters formed by aggregate of the molecules in adjacent supercages and occupation of the windows connecting them (fig. 2b).
The discovery of clustering molecules, and their interaction with certain sites in the zeolite cages, should help our understanding of how zeolites catalyse chemical reactions of adsorbed hydrocarbons.