Rh-Based Intermetallic Rh–In/SiO₂ Catalyst for Parahydrogen-Induced Polarization

Heterogeneous catalysts are of considerable interest for their ability to produce gases with highly polarized nuclear spins by using parahydrogen. Rh catalysts are known for producing pronounced parahydrogen-induced polarization (PHIP) effects. This study illustrates that modification of Rh with In promotes the electronic/chemical state of Rh offering the new Rh–In bimetallic catalyst, which is more efficient in such processes compared to monometallic Rh. Both catalysts were tested in hydrogenations of propyne and propylene. We associate the 50-fold higher activity of Rh–In/SiO₂ in pairwise hydrogen addition compared to monometallic Rh/SiO₂ with the formation of the intermetallic Rh–In compound, which was confirmed by X-ray photoelectron spectroscopy and X-ray diffraction analysis. Intermetallic Rh–In/SiO₂ provided an ∼1400-fold NMR signal enhancement for propylene in propyne hydrogenation. It was also demonstrated that the activity of Rh–In/SiO₂ can be tuned by controlling the reductive/oxidative pretreatment.