ULTRADISPERSED RHODIUM RAFTS - THEIR EXISTENCE AND TOPOLOGY

Detailed studies have been performed on rhodium supported on alumina. Three techniques have been used in concert: chemisorption (using hydrogen and carbon monoxide), infrared spectroscopy (using adsorbed carbon monoxide), and ultra-high resolution electron microscopy. The first two techniques indicated that, under suitable reducing conditions, rhodium on alumina can not only be obtained easily in the atomically dispersed state, but that degrees of dispersion within the atomically dispersed state can be recognized and differentiated. These indications have been confirmed by electron microscopy and the existence of rafts of rhodium atoms of varying size has been shown directly. These rafts must be only one atom thick, as shown by a combination of chemisorption and microscopy on the same samples. Evidently, as we have shown that varying sizes of two-dimensional rafts occur, one can now define degrees of ultra-high dispersion. Such systems are, at the minimum, always atomically dispersed but there can be a variation in the average number of atoms in the rafts from one preparation to another. The ultimate state of ultra-high dispersion would be to have every metal atom separated from every other metal atom on the support. The smallest average raft size that we have been able to achieve so far is 1.5 nm, corresponding to each raft having an average of 8 Rh atoms. Under these conditions only two-dimensional rafts, one atom thick, could be detected; no three-dimensional cubic or spherical particles were seen on the photomicrographs. This is the best example we have been able to obtain, so far, of supported metals in the ultradispersed state. © 1979.