Abstract
Vanadia-promoted Rh/SiO2-catalysts have been prepared by impregnation followed by calcination at 573, 773, 973, and 1173 K. The structure and morphology of these materials in the oxidized state, and after low (523 K) and high (773 K) temperature reduction, were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM), microdiffraction, X-ray diffraction (XRD), and CO chemisorption. During calcination at temperatures ≥ 973 K, a RhVO4 phase is formed, which consists of well-crystallized rod-like particles after calcination at 1173 K. After reduction in H2, the catalysts consist of highly dispersed Rh° particles as judged from the electron micrographs. This high dispersion is presumably stabilized by interaction of the zerovalent Rh° with the promoter oxide V2O3. The well-crystallized sample (calcination at 1173 K) cannot be reduced at 523 K, but at 773 K the rhodium, originally present as RhVO4, is quantitatively reduced to small metal particles in contact with V2O3. In contrast to the high dispersion derived from TEM, CO chemisorption gave unexpectedly low CO/Rh ratios, which were even zero for the catalyst calcined at 1173 K. The CO/Rh ratios decreased with increasing calcination temperature (RhVO4 formation) at constant reduction temperature and with increasing reduction temperature at a given calcination temperature. It is suggested that the surface of the highly dispersed Rh° particles is decorated and blocked by VOx species. This effect, though more pronounced at higher reduction temperature, already occurs after reduction at 523 K. Highly dispersed Rh° particles are produced in the materials studied, particularly when a RhVO4 precursor phase was present, these particles being in intimate contact with a V2O3 promoter oxide. In extreme cases (calcination at 1173 K), the encapsulation of Rh° particles seems to be complete, so that no metal surface is exposed.
