Biradicals (orange), introduced into the pores of mesoporous alumina (green), can be irradiated with high-power and high-frequency microwaves (purple), emitted from a gyrotron and transmitted to the sample via a wave-guide, to induce dynamic nuclear polarization of surrounding nuclear spins. This DNP enhances the intensity of NMR signals (red compared to blue) and permits the acquisition of 2D spectra where structural information can be obtained (top left).
Mesoporous alumina (Al2O3) presents itself as a technologically important material due to its high surface area and catalytic properties. However, the characterization of this material remains elusive to various techniques. Using a hyperpolarization technique called dynamic nuclear polarization (DNP) it was possible to study the surface of this material by nuclear magnetic resonance (NMR) spectroscopy.
The specific design of improved surface catalysts relies on knowledge of the surface structure and how this relates to function. A variety of techniques are available for the characterisation of surfaces. As always, each technique will exhibit specific benefits but will also suffer from specific limitations. So far, solid-state nuclear magnetic resonance (SSNMR) spectroscopy has been largely overlooked by the surface characterisation community. This is possibly due to the major specific limitation of NMR: the sensitivity of the technique. This deterrent is accentuated when considering surfaces, which usually constitute only a small fraction of the studied sample. However, one of the first world-wide installations of a high-field DNP-MAS NMR system, here at INAC, has facilitated the acquisition of high-resolution NMR spectra with greatly enhanced signal-to-noise ratios. DNP involves the transfer of the substantial spin-polarization of unpaired electrons to many nuclei. By this approach the net polarization of the nuclei is enlarged significantly. Since the extent of polarization relates directly to the intensity of the observable NMR signal the use of DNP heralds the imagination and implementation of previously unrealistic surface experiments.
Owing to a true sensitivity enhancement resulting from DNP experiments on mesoporous alumina, corresponding to a time-saving factor of ~32000 (compared to conventional NMR), it was possible to record NMR spectra that aided in obtaining structural information about the surface of the system. In particular, two-dimensional correlation spectra were recorded (4h with DNP, > 15 years without!) that detailed the inter-connectivity between different surface-aluminium coordination states.
This work was conducted in collaboration with UCCS (Lille).
For more information see: Daniel Lee et al., J. Am. Chem. Soc., 134, 18491-4 (2012)
Maj : 04/03/2014 (1003)