(583fq) Development of in-Situ Parahydrogen Induced Polarization Techniques for Elucidating the Kinetics and Mechanism of Heterogeneous Hydrogenation Catalysis

Parahydrogen Induced Polarization (PHIP) [1,2,3] NMR provides a unique spectroscopic tool to study pair-wise hydrogen addition in hydrogenation catalysis, since only this pathway leads to hyperpolarized NMR signal enhancement. Pair-wise addition, whereby two hydrogen atoms originate from the same molecule of dihydrogen, thereby retaining their nuclear spin correlation, is not typically detected as a distinct process. Surface catalysis can be affected by the presence of reactants, intermediates, products and byproducts such as carbon deposits [4]. These factors, along with metal type, particle size, shape, morphology, dispersion and loading, as well as the properties of the supports, including metal-support interactions (SMSI) [5], may also be important in determining the favorability of pair-wise addition.

In this study, we prepared a variety of transition metal nanoparticle (Pt, Pd, Ir, etc.) catalysts supported on a series of different oxides (Al₂O₃, TiO₂, etc.). The catalysts were prepared by precipitation of metal salts onto the oxide supports from an aqueous solution. Active surface area, dispersion and particle size of the catalysts were characterized with CO chemisorption measurements on a Quantachrome ChemBET 3000 instrument. [6] For PHIP experiments, a home-built mini-reactor incorporating an oven and temperature control unit was installed on top of the 9.4 Tesla superconducting NMR magnet. The precise control of the gas mixture (propylene, p-H₂and carrier gas) was achieved using Mass Flow Controllers (MFCs) and the hyperpolarized propane product was delivered through tubing from the reactor down the magnet bore to the NMR probe for NMR detection. The PHIP signal enhancement was optimized as a function of the NMR flip angle, reactor temperature, gas composition and flow rate. We find that high dispersion (90%), low loading (1%) and small particle size (< 1 nm) Pt/TiO₂ yield very strong signal enhancement of hyperpolarized propane. By comparing the NMR spectra obtained using normal-H₂ and p-H₂, we were able to deduce the contributions from random or pairwise additions and the corresponding reaction orders and activation energies.  

Our results provide new insight into the surface processes, reaction intermediates and conditions for optimizing heterogeneous PHIP catalysts .  The use of such catalysts to generate hyperpolarized liquids or gases is advantageous for in-vivo biomolecular studies because there is negligible leaching of toxic metals into the hyperpolarized product stream.

References

[1] Bowers, C.R. and Weitekamp, D.P. Phys. Rev. Lett. 57, 2645-2648 (1986)

[2] Bowers, C.R. Sensitivity Enhancement Utilizing Parahydrogen, Encyclopedia of Nuclear Magnetic Resonance: Supplementary Volume, John Wiley & Sons, 750-770 (2002).

[3] Koptyug, I.V. et al., J. Am. Chem. Soc. 5580-5586 129 (2007).

[4] Bond, G. C. Appl. Catal., A. 149 (1), 3-25 (1997).

[5] Zhivonitko, V. V. J. Phys. Chem. C. 115 (27), 13386-13391 (2011).

Acknowledgements

This project is supported by the ACS-PRF #52258-ND5.