A structural,thermogravimetric and ESR study of the RhO x -TiO2 system

Titanium dioxide samples containing a few per cent of rhodium oxide, heated in air at 1273 K, have been investigated in order to study the formation of solid solutions. The results, monitored by X-ray diffraction, thermogravimetry in a hydrogen stream and ESR spectroscopy, show that rhodium is present both as separate phase (Rh₂O₃) and in solid solution in the rutile structure. The incorporated rhodium is mainly present in the 3 + oxidation state, only a small fraction being present as Rh(II). Rh₂O₃ and the rhodium species in solid solution are reduced by hydrogen to metal in different temperature ranges. By combining thermogravimetric and analytical data, the average oxidation state of rhodium,n, has been evaluated. The variation ofn with the rhodium content has been accounted for on the basis of a strong metal-support interaction developing with the reduction. This interaction affects the metal dispersion.