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 [ l o l l 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. IntroductionIn heterogeneous metal catalysis the support is used to provide a large surface area to facilitate the preparation of well-dispersed catalysts and to prevent sintering of the small supported metal particles, in order to preserve their state of high dispersion. It is often found that support materials modify the chemical reactions of the metal catalyst. Examples are shape selectivity induced by a zeolitic support and bifunctional catalysis of metal particles dispersed on an acidic support, where the metal component catalyzes the hydrogenation/dehydrogenation reactions and the acidic support facilitates isomerization of olefinic compounds. In addition, the support may have a more direct influence on the chemical properties of supported metal particles, especially after reduction at high (>650 K) temperature. Thus, it is well-known that, for metals dispersed on certain transition-metal oxides, the capacity to adsorb hydrogen or carbon monoxide drastically diminishes when the catalyst is reduced at high, rather than low temperatures (<650 and usual >450 K) even though the particle size remains unchange~l-~ Non-transition-metal oxides like A1,03 and SiOz do not influence the capacity to adsorb gasses; the decrease in adsorption after reduction at high temperature of the metal particles dispersed on these supports can be accounted for by ...

Reduction of a highly dispersed 2.85 wt % Rh/Ti02 catalyst at 473 K after previous calcination at 623 K resulted in EXAFS whose primary contributions are due to nearest rhodium (average coordination number of 3.1 and distance of 2.67 A) and oxygen neiphbors (coordination 2.5 and distance 2.71 A). These oxygen neighbors originated at the metalsupport interface. The average rhodium-rhodium coordination number did not change in the SMSI state produced by reducing the catalyst at 673 K. However, the average coordination distance contracted by 0.04 A with an accompanying decrease of the Debye-Waller factor of the Rh-Rh bond of 0.0012 A2. This is due tQ the fact that in the SMSI state the surface of the metal particles is not covered with chemisorbed hydrogen. The SMSI state leads to a structural reorganization of the support in the vicinity of the rhodium metal particles. This can be concluded from the appearance of a Rh-Ti bond at 3.42 A in the SMSI state coupled with the fact that the average coordination number of the rhodiumsupport oxygen bonds does not increase. Other types of rhodium-oxygen bonds could not be detected with EXAFS in this state. Thus, these results provide no evidence for coverage of the metal particle by a suboxide of TiOz in the SMSI state.

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An alumina-supported rhodium catalyst has been studied with EXAFS. After reduction and evacuation, oxygen was admitted at 100 and 300 K. EXAFS spectra of the catalyst after oxygen admission at 100 K indicated the beginning of oxidation. At 300 K only a small part of the rhodium particles remained metallic and this metallic "kernel" was partly covered with rhodium oxide. In the rhodium metal to rhodium oxide interface the same 2.7-A Rho-0' distances are present as in the metal-support interface. A model is presented that explains the observed formation of the rhodium metal-rhodium oxide interface.

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