AVS 69 Session HC+SS-ThA: Closing in on Reality & HC Discovery Reception

Producing hyperpolarized molecules is important for increasing signal intensities in nuclear magnetic resonance (NMR) or magnetic resonance imaging (MRI) applications, and one efficient strategy is to add a parahydrogen molecule, where the nuclei have antiparallel spins, to an unsaturated substrate. The requirements for the production of a hyperpolarized molecule are that the added hydrogens must come from the same hydrogen molecule, i.e. a pairwise addition, the spins must be preserved, and the hydrogens in the generated product must be inequivalent. While this is efficient over homogeneous organometallic catalysts, heterogeneous catalysts would be preferred to facilitate separation of hyperpolarized product from the catalyst and allow continuous operation. However, over typical heterogeneous catalysts, i.e. oxide-supported metal nanoparticles, the pairwise addition of parahydrogen is challenging due to facile and reversible dissociation of dihydrogen, rapid diffusion of hydrogen atoms across the metal surface, step-wise addition of hydrogen atoms to the unsaturated molecule, and spillover of hydrogen from the active metal to the oxide support, as these are all mechanisms that can lead to a rapid loss in the singlet spin-correlation of the original parahydrogen molecule. Therefore, the pairwise selectivity in hydrogenation reactions over supported metal catalysts is often very low (< 1%).

To limit diffusion of hydrogen across the metal surface and improve the pairwise selectivity in the hydrogenation of propene, the metal particle size was first reduced to the limit, i.e. single atoms on the support. Single atoms on an oxide support are indeed more selective to pairwise addition of parahydrogen than larger nanoparticles of the same metal, but the activity is low and stability is an issue during reaction conditions. Another approach is to limit diffusion by blocking metal sites with an oxide overlayer. This was done by inducing strong metal-support interactions via a high-temperature reduction of titania-supported catalysts. The structure of the titania support, anatase versus rutile, influenced the metal-support interactions, and active metals, such as Rh and Ir, exhibited different behavior in the pairwise selective addition of parahydrogen to propene. However, in all cases, the high temperature reduction increased the pairwise selectivity regardless of whether geometric (migration of titania over metal) or electronic metal-support interactions were induced. Preliminary data reveal that oxide layers deposited by ALD can also improve the pairwise selectivity in hydrogenation reactions.