Application of H and Al magic angle spinning solid state NMR at 60kHz for studies of Au and Au-Ni catalysts supported on boehmite/alumina.

In this work for the first time we show the power of solid state NMR spectroscopy in structural analysis of alumina and catalysts supported on the alumina surface employing very fast (60kHz) magic angle spinning (MAS) technique. In the methodological part we demonstrate that under such MAS condition, cross-polarization (CP) from proton to aluminum is an efficient process when a very weak Al RF field is applied. The mechanism of CP transfer and the Hartmann-Hahn (H-H) matching conditions were tested for Al RF fields equal to 3.3 and 8.3kHz. It has been found that double quantum (DQ) CP/MAS is the best choice for H-H set with RF =3.3kHz. It has been also proved that the quality of H-Al CP/MAS spectra strongly depends on Al carrier offset. Applied to γ-alumina, this method revealed that H-Al CP/MAS at 60kHz is extremely useful for mapping the distribution of hydroxyl groups on the surface. Indeed, the Al sites, which are not easily detected with Single Pulse Experiment (SPE), are clearly seen when H-Al CP/MAS is applied. Utilizing 2D H-Al CP/MAS HETCOR experiment it was possible to assign the proton positions and to correlate them with aluminum centers. Studies of mono- (Au) and bi- (Au-Ni) metallic catalysts supported on boehmite/alumina carrier employing 1D and 2D HETCOR experiments clearly show that distributions of hydroxyl groups for both systems are dramatically different.