Semihydrogenation of Propyne over Cerium Oxide Nanorods, Nanocubes, and Nano-Octahedra: Facet-Dependent Parahydrogen-Induced Polarization

The pairwise selectivity of hydrogenation is a fundamental quantity in hydrogenation catalysis that underpins the NMR signal enhancement achievable by parahydrogen‐induced polarization (PHIP). Herein, we show how the crystal facet dependence of the pairwise selectivity of the semihydrogenation of propyne, when interpreted in the context of recent DFT calculations, can reveal new details about the hydrogenation mechanism. The pairwise selectivity of propyne hydrogenation is strongly shape dependent, which reflects the surface atom arrangements exposed on the different facets of CeO2 nanocrystals. In this first demonstration of a catalyst shape dependence of PHIP, an unprecedented pairwise selectivity of 8.1 % is observed over oxygen‐deficient facets of rods, whereas oxygen‐rich octahedra facets deliver only 1.6 % selectivity. The PHIP data are consistent with a concerted addition pathway through a six‐membered‐ring transition state, as predicted in the DFT study. About‐face: Herein, we show how the crystal facet dependence of the pairwise selectivity of the semihydrogenation of propyne, when interpreted in the context of recent DFT calculations, can reveal new details about the hydrogenation mechanism. Parahydrogen‐induced polarization correlates with oxygen vacancy density of CeO2 nanocrystal facets: {1 1 0}>{1 0 0}>{1 1 1}.