Shaped Ceria Nanocrystals Catalyze Efficient and Selective Para‐Hydrogen‐Enhanced Polarization

Abstract

Intense para-hydrogen-enhanced NMR signals are observed in the hydrogenation of propene and propyne over ceria nanocubes, nano-octahedra, and nanorods. The well-defined ceria shapes, synthesized by a hydrothermal method, expose different crystalline facets with various oxygen vacancy densities, which are known to play a role in hydrogenation and oxidation catalysis. While the catalytic activity of the hydrogenation of propene over ceria is strongly facet-dependent, the pairwise selectivity is low (2.4% at 375 °C), which is consistent with stepwise H atom transfer, and it is the same for all three nanocrystal shapes. Selective semi-hydrogenation of propyne over ceria nanocubes yields hyperpolarized propene with a similar pairwise selectivity of (2.7% at 300 °C), indicating product formation predominantly by a non-pairwise addition. Ceria is also shown to be an efficient pairwise replacement catalyst for propene.

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

“…PHIP is a simple and robust method offering high polarization rates using relatively inexpensive instrumentation. The ease of separation of the hyperpolarized product from the polarizing agent is a key advantage of PHIP by heterogeneous catalysis . However, hyperpolarization levels afforded by supported metals and metal‐oxide catalysts have up to now been limited to only a few percent owing to the low pairwise selectivity of hydrogenation on metal surfaces.…”

“…The activation barrier for this path is estimated at approximately 3 eV (calculated for acetylene) . Our previous study of propene (PE) hydrogenation showed that stepwise addition yields a facet‐independent 2–3 % pairwise selectivity over CeO 2 . Given that the lowest energy transition state for PY hydrogenation is one that promotes purely random addition, it follows that any NMR signal enhancement that might be observed in the PHIP spectrum must originate from either stepwise addition or concerted addition via the four‐membered‐ring transition state.…”

“…It is almost negligible at 250 °C, maximizes at 300 °C, and then decreases at 350 °C. At 300 °C, pairwise selectivities of (8.1±0.5) % on rods, (2.6±0.2) % on cubes, and (1.6±0.5) % on octahedra (assuming 90 % syn addition) were obtained . Clearly, distinct surface chemistry on the various facets renders significantly different pairwise selectivities.…”

“…Crystallinity was confirmed by X‐ray diffraction (XRD, see Figure S3 in the Supporting Information of Ref. ) The Brunauer–Emmett–Teller (BET) surface areas of the nanorod, nanocube, and nano‐octahedra samples were 60, 21, and 4 m 2 g −1 , respectively, in reasonable agreement with the surface areas estimated from the nanocrystal size distributions observed in the TEM images (see Ref. ).…”

Hyperpolarized NMR Spectroscopy: d‐DNP, PHIP, and SABRE Techniques

“…However, the contributions of pairwise hydrogen addition with such catalysts were comparable to those for supported metal catalysts. In 2015 and 2016 Bowers and co‐workers studied the hydrogenation reactions of propene and propyne over CeO 2 nanocrystals of different morphologies (nanocubes, nano‐octahedra, and nanorods) . In the hydrogenation of propene, the percentage of pairwise hydrogen addition was 2.4 % with all of the catalysts, that is, there was no observed dependence on the shape of the nanocrystals.…”