Designing atom-efficient, oxide-supported metal catalysts for selective hydrogenation reactions is challenging, and the application to parahydrogen-induced hyperpolarization (PHIP) for nuclear magnetic resonance (NMR) signal enhancements adds yet another level of complexity. A key requirement for the production of hyperpolarized molecules from parahydrogen is the pairwise addition to an unsaturated molecule. This is inherently inefficient over oxide-supported metal catalysts, as the facile and reversible dissociation of dihydrogen and rapid diffusion of hydrogen atoms across the metal surface, combined with the step-wise mechanism of addition of hydrogen to the unsaturated molecule, leads to the randomization of the parahydrogen singlet spin-correlation. We are pursuing several strategies to reduce the mobility of hydrogen ad-atoms: 1) minimization of the active metal particle size to limit the surface area available for diffusion and 2) blocking hydrogen diffusion on larger metal nanoparticles with an oxide layer, either by inducing strong metal support interactions or by depositing the oxide over the catalysts using atomic layer deposition. Single atoms of platinum supported on cerium dioxide (ceria) nanoparticle shapes were synthesized using atomic layer deposition, and the pairwise selectivity was shown to be improved relative to conventional supported nanoparticle catalysts. Unfortunately, the activity over the atomically dispersed catalysts was adversely affected. Interestingly, the pairwise selectivity and the stability of the single Pt atoms were shown to be sensitive to the exposed surface facet of the ceria nanoparticles. The highest pairwise selectivity was obtained for Pt catalysts supported on the (111) surface facets of ceria octahedra. The yield of hyperpolarized product can be increased by using a more active metal, such as rhodium, even though the pairwise selectivity was shown to be very low on rhodium supported on titanium dioxide (titania). A high-temperature reduction (773 K) of the titania-supported rhodium did improve the pairwise selectivity (compared to a low-temperature reduction at 473 K), likely due to restriction of hydrogen ad-atom diffusion, although a decrease in the hydrogen spillover from the metal to the support cannot be excluded as an alternative explanation.

Well-defined heterogeneous hydrogenation catalysts designed for parahydrogen-based nuclear magnetic resonance signal enhancement

Where thousands of chemistry professionals meet to share ideas and advance scientific and technical knowledge. Spring 2023 abstracts, posters, and presentations will launch at the end of March.

ACS Spring 2023

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.