Influence of TiO2 Structure on Metal-Support Interactions in Rh/TiO2 Catalysts Probed By Propylene Hydrogenation and Other Techniques

Interactions between rhodium metal and varying TiO₂ nanoparticle supports of similar sizes were analyzed to determine the dependence of resulting electronic interactions and reaction activity on catalytic structure. Nanoparticles of anatase (ANP), rutile (RNP), and brookite (BNP) were included in the study. Following an initial reduction at 200 °C, rhodium supported on ANP had higher activity in the propylene hydrogenation test reaction when compared to rhodium catalysts supported on BNP and RNP. The higher activity is likely attributable to a more electron-rich metallic rhodium species being present on the ANP support compared to other supports as revealed through X-ray Photoelectron Spectroscopy (XPS). A decrease in CO uptake and propylene hydrogenation activity was recorded in all Rh/TiO₂ catalysts following a reduction at 500 °C after which the activity could be effectively recovered by reoxidation at 400 °C and re-reduction at 200 °C in the rhodium catalysts supported on ANP and BNP. This suggests that ANP and BNP both result in the migration of a TiO_x layer over the rhodium nanoparticles, while a rutile TiO₂ support does not, at least under the conditions that are commonly used to induce strong interactions between an active metal and support. Additionally, while high reduction temperatures of BNP and ANP both cause TiO_x migration, the electronic interactions between the active rhodium metal and the different TiO₂ structures vary greatly. This study has revealed the importance of carefully matching the particle sizes of the active metal and supports when examining metal support interactions as they can be critically dependent on the stability and structure of the TiO₂ support.