Theoretical and experimental insights into applicability of solid-state 93Nb NMR in catalysis

Abstract

Ab initio DFT calculations of (93)Nb NMR parameters using the NMR-CASTEP code were performed for a series of over fifty individual niobates, and a good agreement has been found with experimental NMR parameters. New experimental and calculated (93)Nb NMR data were obtained for several compounds, AlNbO4, VNb9O25, K8Nb6O19 and Cs3NbO8, which are of particular interest for catalysis. Several interesting trends have been identified between (93)Nb NMR parameters and the specifics of niobium site environments in niobates. These trends may serve as useful guidelines in interpreting (93)Nb NMR spectra of complex niobium oxide systems, including amorphous samples and niobium-based multicomponent heterogeneous catalysts. Potential applications of (93)Nb NMR to study solid polyoxoniobates are discussed.

Structural and optical properties of ZnS/MgNb2O6 heterostructures

“…1(a) illustrates a unit cell for MN crystals with a columbite-type orthorhombic structure, a space group of (Pbcn) and point-group symmetry (D 2h ). In this unit cell, magnesium atoms (Mg) and niobium (Nb) atoms are coordinated to six oxygen (O) atoms which form distorted octahedral [MgO 6 ]/[NbO 6 ] clusters [16]. These octahedra are formed by 6-vertices, 6-faces and 12-edges.…”

Multinuclear Solid-State NMR Investigation of Hexaniobate and Hexatantalate Compounds

“…54 For 1 H spectra, Lorentz/ Gaussian (L/G) line shapes were used. For 23 Na spectra, the Qmas1/2 option was used corresponding to the central transition (broadened by eventual second-order quadrupolar effects) under the assumption of inf inite MAS rotation frequency (no spinning sidebands).…”

“…In particular, the alkali hexaniobate salts offer a unique class of compounds with multiple nuclei that can be probed by solidstate MAS NMR, namely 1 H, 6 Li, 7 Li, 17 O, 23 Na, 87 Rb, 133 Cs, and 93 Nb. Although several techniques were used to investigate hexaniobate and hexatantalate salts, only a few articles 20,36,46,54 have been published on 1 H, 6/7 Li, and 93 Nb solid-state MAS ( 93 Nb)) and not their isotropic chemical shift (δ iso ( 93 Nb)). The full interpretation of the NMR spectra was related to the corresponding XRD data.…”

“…93 Nb NMR chemical shifts were referenced to a saturated solution of NbCl 5 in acetonitrile at 0.0 ppm. Lapina et al 54 used this reference, whereas Hanna et al 55 used the following solution as a chemical shift reference: K[NbCl 6 ]/CH 3 CN. A solution of NbCl 5 in acetonitrile without KCl exhibits three peaks.…”

“…Again we stress the fact that the extracted δ CSA values were comparable to published data. 54,55 (iii) The clear separation of central transitions from the corresponding spinning sidebands is achieved at ultrafast MAS. At moderate MAS (30 kHz; see Figure 4B), all contributions are superimposed.…”