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

γ-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 γ-Al2O3 were probed by 17O dynamic nuclear polarization surface-enhanced NMR spectroscopy (DNP-SENS). Direct 17O MAS and indirect 1H-17O cross-polarization (CP)/MAS DNP experiments enable observation of the (sub-)surface bare oxygen species and hydroxyl groups. In particular, a two-dimensional (2D) 17O 3QMAS DNP spectrum was for the first time achieved for γ-Al2O3, in which two O(Al)4 and one O(Al)3 bare oxygen species were identified. The 17O isotropic chemical shifts (δcs) vary from 56.7 to 81.0 ppm and the quadrupolar coupling constants (CQ) 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 CQ value (2.5 MHz) among these oxygen sites. 2D 1H → 17O HETCOR DNP experiments allow us to discriminate three bridging (Aln)-μ2-OH and two terminal (Aln)-μ1-OH hydroxyl groups. The structural features of the bare oxygen species and hydroxyl groups are similar for the γ-Al2O3 samples isotopically labelled by 17O2 gas or H217O. 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.