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For the first time, the detailed structure of χ-Al2O3 and η-Al2O3 surface has been established by implementing the NMR crystallography approach. The surface of η-Al2O3 has been demonstrated to be formed primarily by the (111) facets, while the χ-Al2O3 surface is a combination of (111) and (110) facets. This observation supports the block model of aluminum oxides previously proposed by Tsybulya and Kryukova [S. V. Tsybulya, G. N. Kryukova, Phys. Rev. B 77 (2008) 024112.]. The additional terminal OH groups, observed experimentally and not contributing to (111) and (110) theoretical calculations, are considered to be bonded to the tetrahedral aluminum sites. Their origin is related to the junctions of crystallographic faces of spinel building blocks, being a part of discussed model. Higher content of these terminal OH groups in χ-Al2O3 is a result of more junctions in the case of its more mosaic structure compared to η-Al2O3. [ABSTRACT FROM AUTHOR]

Abstract: 93Nb solid-state NMR spectra of a series of inorganic niobates with Nb in different oxygen coordination environments were measured. For all studied compounds the chemical shielding and quadrupole tensor parameters were determined using conventional and ultrahigh field NMR facilities, ultrahigh speed MAS, DQ STMAS, solid-echo and computer modeling. It has been demonstrated that the 93Nb isotropic chemical shift is sensitive to the coordination number of Nb sites. For the first time the 93Nb NMR chemical shift scale for NbO x polyhedra in solid materials has been proposed: for four-coordinated Nb sites, the isotropic shifts occur from −650 to −950ppm; five-coordinated Nb sites have the isotropic shifts in the range of –900 to –980ppm; for six-coordinated Nb sites the isotropic shifts vary from −900 to −1360ppm; the shifts from −1200 to −1600ppm are typical for seven-coordinated Nb sites; for eight-coordinated Nb sites the shifts are higher than −1400ppm. The possible correlation between the value of the isotropic chemical shift and the ionic character of the NbO x –MO y polyhedra association has been suggested. The magnitude of the 93Nb quadrupole coupling constant depends on the local symmetry of Nb sites and may vary from hundreds of kHz to hundreds of MHz. [Copyright &y& Elsevier]

Abstract: In this article we will discuss the potentiality of solid-state 51V NMR technique to characterize heterogeneous catalysts containing paramagnetic cations located on the surface or incorporated in the bulk of heterogeneous catalysts. 51V NMR data for a number of 3d and rare earth vanadates, where paramagnetic effects are caused by the presence of paramagnetic cations, are considered. We also summarize results available for vanadium bronzes, where some vanadium is in a paramagnetic V4+ state, and for some vanadium compounds, where closely spaced paramagnetic centers form diamagnetic pairs. The comprehensive 51V chemical shift scale (database) for paramagnetic solids was compiled. [Copyright &y& Elsevier]

• A new ammonium niobium oxalate phase with O-Nb-O bridges was found. • The combination of the XRD, NMR and DFT was efficient for structure refinement. • The well-developed protons and ligands mobility was detected in a new phase of niobium oxalate. Unusual structure transformation in NH 4 [NbO(C 2 O 4) 2 (H 2 O) 2 ∙nH 2 O was discovered and investigated by combination of techniques including NMR crystallography (variable temperature 1H, 2H, 13C and 93Nb solid-state NMR methods), X-Ray powder diffraction, simultaneous thermal analysis and DFT calculations. It has been revealed that the mixture of known oxalate phases NH 4 [NbO(C 2 O 4) 2 (H 2 O) 2 ∙3H 2 O (Phase I) and NH 4 [NbO(C 2 O 4) 2 (H 2 O) 2 ·2H 2 O (Phase II) treated at mild condition (temperature up to 100 °C in vacuo) induces an unknown crystal phase, catena-NH 4 [NbO 2/2 (C 2 O 4) 2 (H 2 O)] (Phase III). The new orthorhombic phase has polymeric structure consisted of 7-coordinated niobium fragments [NbO 2/2 (C 2 O 4) 2 (H 2 O)] connected by oxygen bridges (Nb O Nb) forming the infinite zigzag chains along the c → axis. The bridging oxygen atoms are connected to Nb symmetrically with respect to the 2-fold axis. Each of them forms two identical bonds Nb-O of 1.892(6) Å length, indicating their origination from the condensation process. The niobium coordination polyhedron can be crossed by the plane passing through the Nb, (O) 2/2 , (H 2 O) atoms. The oxalate groups lie in the opposite sides from the plane. In the new phase, 93Nb has unusually small nuclear quadrupole coupling constant mainly due to the developed thermal mobility of the crystal lattice. Variable temperature 93Nb, 2H and 13C NMR experiments together with DFT calculations confirm that the new Phase III has proton and ligand mobility. This mobility of the lattice correlates with the fact that hydrogen bonds network stabilizing the structure of Phases I and II practically disappear in the new Phase III. [ABSTRACT FROM AUTHOR]

Intensive phase transformations of alumina are known to occur at temperatures above 1000 °C. In the present work, high temperature behaviour of pure Al 2 O 3 and the carbon coated Al 2 O 3 @C sample with core-shell structure was comparatively studied using low-temperature nitrogen adsorption, transmission electron microscopy, powder X-ray diffraction (XRD) analysis and solid-state nuclear magnetic resonance (NMR). The solid-state NMR 27 Al method has allowed us to identify and estimate the concentration of all phases appeared during the transformation of pseudoboehmite γ-Al 2 O 3 into corundum α-Al 2 O 3 . The data obtained correlate well with the results of XRD analysis and low-temperature nitrogen adsorption. It is shown that the deposition of carbon coating with formation of core-shell Al 2 O 3 @C system stabilizes the size of oxide core and prevents the formation of corundum phase until the temperatures of 1350–1400 °C, which are close to the temperature of carbothermal reduction of alumina. The stabilization of the size of the oxide core nanoparticles was considered as a main factor preventing the formation of corundum phase at high temperatures. At the same time, the carbon coating does not affect the phase transformation of γ-Al 2 O 3 to δ -Al 2 O 3 . [ABSTRACT FROM AUTHOR]