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

Abstract

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 (δ_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 ¹H → ¹⁷O HETCOR DNP experiments allow us to discriminate three bridging (Al_n)-μ₂-OH and two terminal (Al_n)-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.