(191f) Structure-Dependent Metal-Support Interactions in Titania-Supported Rhodium Catalysts and Their Influence on Catalytic Properties

Our group focuses on the synthesis and characterization of well-defined heterogenous catalysts to identify structure-dependent metal-support interactions and evaluate how they influence catalytic properties, such as activity, selectivity, and stability, in different reactions. Titania [titanium(IV) oxide] is an important catalyst support due to its unique reduction-oxidation properties. However, it has three stable or meta-stable structures that can be of importance under conditions relevant for catalysis. Therefore, to determine the influence of titania structure on supported rhodium catalysts, we deposited low loadings of rhodium (0.4% by weight to maximize metal-support interactions) on anatase, brookite and rutile titania nanoparticles with carefully matched average particle sizes (to the extent possible). Anatase titania nanoparticles with different average particle sizes were also included, since the largest particle size (15 nm) investigated had a small rutile impurity according to X-ray diffraction analysis. Rutile titania nanorods were also synthesized to provide a well-defined (110) surface but were shown to be unstable during reducing conditions despite its rutile phase. A simple incipient wetness impregnation method yielded 1.5-2.0 nm Rh particles on the supports. Characterizations of these catalysts after reduction in hydrogen at 500 °C indicated migration of TiO_x over rhodium on the anatase and brookite but not on the rutile titania nanoparticles. It was also shown that the oxidation state of rhodium after the different reduction treatments varies with titania structure plus particle size. After reduction at 200 °C, more metallic rhodium was present at the surface of the 15-nm anatase titania support compared with rhodium on brookite or rutile titania, or smaller anatase titania nanoparticles. More metallic rhodium likely explains why this is the most active catalyst in the hydrogenation of propene. This study reveals that rhodium-titania interactions depend sensitively on the TiO₂ structure, particle size and stability, and can vary significantly with pretreatment conditions.