Investigation of the morphology and the active site distribution of Rh-graphimet Original Research Article

In order to gain information on the location and on the morphology of the metal content of Rh-graphimet, structural investigations were performed before and after medium-temperature reduction (MTR, 573 K, 1 h), by means of X-ray photoelectron spectroscopy, transmission electron microscopy, BET surface analysis, small-angle X-ray scattering and H2 titration. The active site distributions of the samples were characterized by 1-butene titration. It was established that the metal content of Rh-graphimet was situated in part in the interlayer space and in part on the surface of the graphite as finely-dispersed Rh crystallites. After MTR, the morphology of the surface Rh particles was retained and no significant differences in the structural parameters were observed. Nevertheless, the number of active sites in Rh-graphimet strongly depended on the temperatures of both pretreatment and the catalytic reaction. The main reactions of 1-butene for the pristine sample were cis- and trans-isomerization. Deactivation at 323 K affected the hydrogenation sites rather than the isomerization centres. For the catalytic activity of Rh-graphimet, the smallest Rh clusters of 1 nm were found to be of crucial importance and the loss of activity observed on MTR was correlated with the effect of sintering and encapsulation of Rh particles in the graphite host.