Parahydrogen induced polarization (PHIP) is a nuclear magnetic resonance (NMR) signal enhancement technique with potential applications in clinical magnetic resonance imaging (MRI). PHIP-NMR can provide unique information about hydrogenation mechanisms that is not available from any other analytical technique. This is due to the fact that PHIP-NMR signals are observed only when the two hydrogen atoms that are transferred to the unsaturated substrate originate from the same parahydrogen (H₂) molecule, i.e., when the hydrogenation occurs by pairwise addition.

Clifford R. Bowers and Helena E. Hagelin-Weaver, University of Florida, Gainesville, USA, and colleagues used PHIP-NMR to investigate the selective hydrogenation of propyne to propene over shaped CeO₂ nanocrystals, an industrially important reaction to remove alkynes from alkene streams. The dominant reaction path for semihydrogenation of propyne over ceria {1 1 1} proceeds via a low-energy six-membered-ring transition state. CeO₂ nanocrystals show a cubic fluorite crystal structure. Ceria nanorods are enclosed predominantly by {1 1 0} and {1 0 0} facets, nano-octahedra and nanocubes expose (111) and (100) facets, respectively. A facet-structure dependence of the total activity and pairwise selectivity were observed.

Whereas the CeO₂ octahedra are most active in the hydrogenation of propyne, pairwise selectivity is significantly higher over the rods. In contrast, the pairwise selectivity of propene hydrogenation over CeO₂ was previously shown to be facet-independent. These observations, which are consistent with recent DFT calculations, show the potential of PHIP-NMR for the study of heterogeneous hydrogenation reactions.

• Semihydrogenation of Propyne over Cerium Oxide Nanorods, Nanocubes, and Nano-Octahedra: Facet-Dependent Parahydrogen-Induced Polarization,
  Evan W. Zhao, Yan Xin, Helena E. Hagelin-Weaver, Clifford R. Bowers,
  ChemCatChem 2016.
  DOI: 10.1002/cctc.201600270