(631f) Tuning Strong Metal-Support Interactions of Pt/CeO2 and Rh/TiO2 Catalysts for Parahydrogen Enhanced Nuclear Magnetic Resonance Applications

Previous work has shown that strong metal-support interactions (SMSI) in heterogeneous catalysts can improve the pairwise selectivity of parahydrogen addition to unsaturated substrates. This preserves te antiparallel spin correlation of the protons and has potential to increase NMR signals by four orders of magnitude, which is important in magnetic resonance imaging applications. While SMSI can improve the pairwise selectivity, which is typically low for heterogeneous catalysts, it is often associated with a significant reduction in hydrogenation activity.

To better understand SMSI, we synthesized well-defined heterogeneous catalysts with ultra-low Pt loadings using a modified atomic layer deposition method and nanoparticle CeO₂ shapes with specific surface facets. The resulting isolated Pt species exhibit very different behavior under reducing conditions dependent on the ceria surface facet exposed. Strong electronic interactions between Pt and the CeO₂(111) surface facets of octahedra results in electron transfer from Pt to ceria and improves the stability of the catalysts, such that isolated Pt remains after reduction at 350 ^oC. In contrast, only Pt nanoparticles are observed after reduction on the CeO₂ cubes with (100) surface facets. The electron-deficient Pt on the ceria octahedra exhibits a significantly higher pairwise selectivity in the propene hydrogenation reaction, compared with the electron-rich Pt nanoparticles on the ceria cubes.

Since the hydrogenation activity is low over Pt/CeO₂ catalysts, we turned our attention to Rh/TiO₂, which has shown promising activity and pairwise selectivity in PHIP applications. Several Rh catalysts on different TiO₂ supports were synthesized to determine the influence of TiO₂ structure. The Rh catalyst supported on anatase TiO₂ exhibits activity at lower temperature compared with Pt/CeO₂, and induction of SMSI increases the pairwise selectivity while maintaining a reactant conversion an order of magnitude higher than for the Pt/CeO₂ catalysts.

This work reveals the importance of utilizing well-defined oxide supports when investigating structure-activity relationships.