Unveiling the Surface Structure of ZnO Nanorods and H₂ Activation Mechanisms with ¹⁷O NMR Spectroscopy

Abstract

ZnO plays a very important role in many catalytic processes involving H2, yet the details on their interactions and H2 activation mechanism are still missing, owing to the lack of a characterization method that provides resolution at the atomic scale and follows the fate of oxide surface species. Here, we apply 17O solid-state NMR spectroscopy in combination with DFT calculations to unravel the surface structure of ZnO nanorods and explore the H2 activation process. We show that six different types of oxygen ions in the surface and subsurface of ZnO can be distinguished. H2 undergoes heterolytic dissociation on three-coordinated surface zinc and oxygen ions, while the formed hydride species migrate to nearby oxygen species, generating a second hydroxyl site. When oxygen vacancies are present, homolytic dissociation of H2 occurs and zinc hydride species form from the vacancies. Reaction mechanisms on oxide surfaces can be explored in a similar manner.

Emerging Applications of 17O Solid-State NMR Spectroscopy for Catalytic Oxides

“…At medium high temperatures with less significant diffusion, this problem may be solved by using a "control" sample without introducing the reactant molecules for comparison. 63 In this Perspective, the recent advances of 17 O solid-state NMR spectroscopy, as well as its applications on oxide catalysts, in particular, oxide nanomaterials and zeolites, are briefly overviewed. Owing to the recent developments in 17 O labeling, DNP techniques, and computational methods, 17 O solid-state NMR can now clarify the detailed chemistry of these oxide catalysts.…”

“…This new approach allows NMR parameters to be calculated precisely for periodic solids and thus have been extensively used and revolutionized the solid-state NMR research field. Successful predictions of NMR parameters have been obtained on binary ,,,,− and ternary − oxides, doped oxides, and zeolites …”

“…60 For example, only the top two layers of ceria nanorods were enriched after the material was exposed to 17 O-enriched H 2 O at a temperature equal to or lower than 100 °C. This method has been extended to label the surface of a variety of nanosized oxide catalysts, including TiO 2 , 61 ZnO, 62,63 and ZrO 2 . 64 However, the successful enrichment of surface species by applying these procedures relies on the reactivity of the surface of the nanosized oxide at low temperatures, and it may not work for all oxide catalysts.…”

“…For example, while applying Hubbard U corrections on all the O 2p and Zn 3d orbitals for ZnO gives good predictions for most of the 17 O NMR parameters of almost all oxygen species in ZnO nanorods, the calculated C Q s for surface water and hydroxyl species (5.3−8.2 MHz) are much larger than experimental values (2.5 MHz). 63 Removing Hubbard U corrections only on these surface species predicts C Q s of 0 MHz, which are too small. The values of δ iso s, on the other hand, are not sensitive to the applications of Hubbard U parameter in this case.…”

“…The values of δ iso s, on the other hand, are not sensitive to the applications of Hubbard U parameter in this case. 63 More detailed information concerning the development of GIPAW and the calculations of NMR parameters can be found in previous review papers 105,111−113 and will not be discussed further in this Perspective. 114 and recent research finds superior catalytic performance from oxide nanomaterials, 115 which is closely related to their detailed surface structure, for example, exposed facets.…”