Application of 1H and 27Al magic angle spinning solid state NMR at 60 kHz 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 (60 kHz) 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 27A1 RF field is applied. The mechanism of CP transfer and the Hartmann -Hahn (H -H) matching conditions were tested for 27A1 RF fields equal to 3.3 and 8.3 kHz. It has been found that double quantum (DQ) CP/MAS is the best choice for H -H set with RF =3.3 kHz. It has been also proved that the quality of 111-27A1 CP/MAS spectra strongly depends on 27AI carrier offset. Applied to y -alumina, this method revealed that 111-27A1 CP/MAS at 60 kHz is extremely useful for mapping the distribution of hydroxyl groups on the surface. Indeed, the Alv sites, which are not easily detected with Single Pulse Experiment (SPE), are clearly seen when 1H -27A1 CP/MAS is applied. Utilizing 2D 111-27A1 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.