93 Nb Solid State NMR of High Surface Area Niobium Oxides
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0984-MM14-03
93
Nb Solid State NMR of High Surface Area Niobium Oxides
Luis J. Smith and Xuefeng Wang Carlson School of Chemistry and Biochemistry, Clark University, 950 Main St, Worcester, MA, 01610
ABSTRACT The local environment of niobium in oxides reflects the perturbations in bond strength that affect the acidity of oxygen atoms in the structure. To understand the relationship between metal environment and properties, 93Nb solid state NMR has been used to measure the electric field gradient and chemical shift anisotropy for layered niobates with either alkali cations or protons at the material surface. In order to determine these parameters, a variety of techniques have been applied to extract information for the multiple environments located in these oxides. Variable offset cumulative echo spectra were collected on static samples at multiple magnetic fields, 4.7 T, 9.4 T and 14.1 T. RAPT enhanced QPASS data were collected at 9.4 T to extract quadrupolar-coupling information without the influence of chemical shift anisotropy. Data from KCa2Nb3O10 and an acid exchanged form were collected and two distinct quadrupolar environments were observed. Acid exchange altered the isotropic chemical shift but did not significantly affect the electric field gradient or the chemical shift anisotropy.
INTRODUCTION The local coordination and symmetry of a surface species affects the reactivity of that site. Research has found that variations in the metal-oxygen bond lengths of early transition metal oxides influence the acidity of the material[1]. An understanding of what factors control the distortions needed to alter acidity will be necessary to intelligently tailor the heterogeneous catalytic properties of novel materials. To properly correlate structure with properties, it is necessary to expand from a local view of bonding to a perspective that includes the structure of the metal oxide polyhedra that make up a material. However, such information can be difficult to extract from amorphous or nanoscopic systems. The lack of long range order due to disorder in amorphous systems or due to the physical lack of space in nanoscopic materials prevents thorough characterization via diffraction methods. High surface area crystalline, layered compounds can serve as models for the bond perturbations that affect high surface area solid acid niobates, as they have crystalline structures but thin layers exposing the majority of the niobium to the interface with the interlayer gallery. NMR is sensitive to the local environment of a nucleus and as such can observe distributions present in a solid. In particular, the chemical shift anisotropy (CSA) and electric field gradient (EFG) are sensitive to the bonding and symmetry of the local structure of a nucleus. Quantification of these values in a sample can be troublesome due to the overlap
between the powder patterns of different environments in a material. Such a difficulty is encountered with 93Nb (I= 9/2), which has a large quadrupolar moment and can have large CSA interactions. H
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