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

Abstract

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 27Al RF field is applied. The mechanism of CP transfer and the Hartmann-Hahn (H-H) matching conditions were tested for 27Al 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 1H-27Al CP/MAS spectra strongly depends on 27Al carrier offset. Applied to γ-alumina, this method revealed that 1H-27Al CP/MAS at 60kHz 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-27Al CP/MAS is applied. Utilizing 2D 1H-27Al 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.