(781d) Applications of in-Situ/Operando Hyperpolarized NMR Spectroscopy to Molecular Single-File Diffusion and Heterogeneous Hydrogenation Catalysis

Nuclear spin hyperpolarization provides an NMR sensitivity enhancement of up to a factor of 10⁵, thereby extending the applicability of the technique to the study of nano-scale transport dynamics and reaction mechanisms. Two examples will be presented here: (1) characterization of anomalous molecular transport in single-file nanotubes and (2) hydrogenation catalysis on oxide supported metal nanoparticles. The ability to acquire NMR spectra with good signal-to-noise in a single scan obviates the need for signal averaging, thereby increasing the time-resolution for following molecular displacements, transport dynamics, chemical exchange, kinetics and mechanisms of catalysis.

 (1) One of the most interesting predictions of the analytical model for diffusion in single-file systems with finite file length and free exchange with the bulk phase at the file boundaries is a cross-over from single-file diffusion (SFD) to a center-of-mass (CM) diffusion regime with Fickian time-scaling of the mean squared displacement (MSD). No observation of such a cross-over in the time dependence in a molecular single-file system has been reported in the literature up to now. Simulations show that desorption barriers may shift the cross-over time, and for particles trapped between infinitely high barriers (i.e. blockages), the MSD obtains a maximum without entering the CM regime. More recent theoretical work has shown that the cross-over time  can also be shifted by particle-particle and particle-channel interactions. One or more of these factors could explain why the SFD-CM cross-over has not been observed. Hyperpolarized xenon-129 NMR, in combination with xenon-129 pulsed field gradient NMR, is providing new insights into this long-standing problem. Experimental results in several type of single-file nanotube systems will be presented.

(2) In the parahydrogen induced polarization (PHIP) effect, NMR-observable nuclear spin hyperpolarization is derived from the pure singlet spin order of parahydrogen by chemical hydrogenation reaction. PHIP was discovered in 1987 using Wilkinson’s catalyst in solution. Until recently, it was thought that heterogeneous hydrogenation catalysts would be ineffective for producing PHIP hyperpolarization because molecular hydrogen dissociates into atomic hydrogen upon chemisorption on metal surfaces. H-atom diffusion, spillover and even dissolution into the metal lattice, leads to random addition, in which H atoms are statistically unlikely to originate from the same H₂ molecule. Pair-wise addition, whereby two hydrogen atoms originate from the same molecule of dihydrogen, 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 carbon deposits. These factors, along with metal type, particle size, shape, morphology, dispersion, loading, and properties of the supports, including metal-support interactions (SMSI), may also be important in determining the favorability of pair-wise addition. PHIP NMR provides a unique tool to study pair-wise hydrogen addition, since only this pathway leads to hyperpolarized NMR signal enhancement. Representative results obtained by systematically varying the catalyst properties and reaction conditions will be presented.  

References

[1] K Hahn and J Karger, J. Phys. Chem. B 1998, 102, 5766-5771.

[2] M Dvoyashkin et al., Microporous and Mesoporous Materials, available online.

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

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

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

[6] Hagelin-Weaver, H. E. et al., J. Mol. Catal. A, 307, 29 (2009).

Acknowledgements

This work  is supported by the ACS-PRF #52258-ND5 and NSF CHE-0957641.