Structure of Rhodium/Titania in the Normal and the SMSI State as Determined by Extended X-ray Absorption Fine Structure and High Resolution Transmission Electron Microscopy

Abstract

Extended X-ray absorption fine structure (EXAFS) and high-resolution transmission electron microscopy (HRTEM) have been used to study the structure of a Rh/Ti02 catalyst. After reduction in H, at 473 K (when the catalyst is in the normal state) the metal particles contain on the average five rhodium atoms and are situated preferably on edges of the Ti0, crystallites but also on [loll and to a lesser extent on [OOl] anatase crystal faces. Reduction in H2 at 723 K leads to the SMSI state. Besides oxygen neighbors from the support, the rhodium metal atoms in the metal-support interface have Ti“’ neighbors at 3.4 and 4.3 A. These distances and their coordination numbers fit well with a model in which the metal particles rest on a TiO, suboxide. This indicates that the supporting oxide near the metal particle has been reduced to a suboxide of TiO,. In the SMSI state no indication for coverage has been found with either EXAFS or HRTEM. On the contrary, exposing the catalyst in the SMSI state to oxygen at 100 K resulted in changes in the EXAFS spectrum due to physisorption of oxygen. Consequently, in the SMSI state the particles are either not covered or are incompletely covered with TiO,. Since a Rh/AI2O3 catalyst under the same conditions became partly oxidized, it is evident that for the Rh/Ti02 catalyst oxidation has been suppressed. This is most probably the result of an electronic influence from the reduced supporting oxide. Even after oxygen admission at room temperature, the rhodium particles on the TiO, support remain in the metallic state. The TiO, suboxide in the vicinity of the metal particles starts to reoxidize and the metal-support interaction becomes weaker.