Probing the surface of γ-Al2O3 by oxygen-17 dynamic nuclear polarization enhanced solid-state NMR spectroscopy

gamma-Al2O3 is an important catalyst and catalyst support of industrial interest. Its acid/base characteristics are correlated to the surface structure, which has always been an issue of concern. In this work, the complex (sub-)surface oxygen species on surface-selectively labelled -Al2O3 were probed by O-17 dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP-SENS). Direct O-17 MAS and indirect H-1-O-17 cross-polarization (CP)/MAS DNP experiments enable observation of the (sub-)surface bare oxygen species and hydroxyl groups. In particular, a two-dimensional (2D) O-17 3QMAS DNP spectrum was for the first time achieved for -Al2O3, in which two O(Al)(4) and one O(Al)(3) bare oxygen species were identified. The O-17 isotropic chemical shifts ((cs)) vary from 56.7 to 81.0 ppm and the quadrupolar coupling constants (C-Q) range from 0.6 to 2.5 MHz for the three oxygen species. The coordinatively unsaturated O(Al)(3) species is characterized by a higher field chemical shift (56.7 ppm) and the largest C-Q value (2.5 MHz) among these oxygen sites. 2D H-1 O-17 HETCOR DNP experiments allow us to discriminate three bridging (Al-n)-(2)-OH and two terminal (Al-n)-(1)-OH hydroxyl groups. The structural features of the bare oxygen species and hydroxyl groups are similar for the -Al2O3 samples isotopically labelled by O-17(2) gas or (H2O)-O-17. The results presented here show that the combination of surface-selective labelling and DNP-SENS is an effective approach for characterizing oxides with complex surface species.