7 research outputs found
Effect of impregnation on the structure of niobium oxide/alumina catalysts studied by multinuclear solid-state NMR, FTIR, and quantum chemical calculations
Multinuclear solid-state 1H, 27Al, and 93Nb NMR experiments and DFT calculations were carried out for structura
Crystal plane dependent dispersion of cobalt metal on metastable aluminas
Metallic Co nanoparticles, widely used and studied as supported heterogeneous catalysts for Fischer-Tropsch synthesis (FTS), display catalytic properties that can vary significantly depending on their size and crystal structure. In this work, we used 59Co Internal Field NMR (59Co IF NMR) complemented by high-resolution transmission electron microscopy (HRTEM) to demonstrate the influence of strong metal-support interaction on two noticeably different metastable alumina phases - γ-Al2O3 and χ-Al2O3. According to 59Co IF NMR and HRTEM, the metallic particles supported on χ-Al2O3 were larger and displayed a significantly higher content of hcp Co phase, which are known to be more active and selective to C5+ in FTS. The 1H NMR chemical shifts of hydroxyl groups anchored to the (110) and (111) spinel crystal planes were calculated by DFT. It revealed that the hydroxyl coverage of γ-Al2O3 facilitates the dispersion of Co precursor over the surface of the support, ultimately leading to the formation of smaller metal Co nanoparticles on γ-Al2O3, than on χ-Al2O3
Possibilities of Mechanochemical Synthesis of Apatites with Different Ca/P Ratios
Apatite is widely used in medicine as a biomaterial for bone tissue restoration. Properties of apatite depend on its composition, including the Ca/P ratio. This paper shows what range of Ca/P ratio can be attained in apatite by the mechanochemical method of synthesis, providing fast formation of a single-phase product. The synthesis was carried out from a reaction mixture of CaHPO4 and CaO at different Ca/P ratios in the range of 1.17–2.10. The products were studied by PXRD, FTIR and NMR spectroscopy, HRTEM, and STA. In mixtures with a low initial Ca/P ratio (1.17–1.48), directly in the mill, the formation of calcium orthophosphate with whitlockite structure containing an HPO42− group and structural water is shown for the first time. This phosphate has structure similar to that of whitlockites of hydrothermal origin and differs from high-temperature β-tricalcium phosphate that has composition Ca3(PO4)3. A series of samples of apatite was obtained with varied composition, which depends on the initial Ca/P ratio. At Ca/P < 1.67, the formation of two types of calcium-deficient apatite was documented. At Ca/P > 1.67, the existence of two types of calcium-rich apatite is confirmed
Possibilities of Mechanochemical Synthesis of Apatites with Different Ca/P Ratios
Apatite is widely used in medicine as a biomaterial for bone tissue restoration. Properties of apatite depend on its composition, including the Ca/P ratio. This paper shows what range of Ca/P ratio can be attained in apatite by the mechanochemical method of synthesis, providing fast formation of a single-phase product. The synthesis was carried out from a reaction mixture of CaHPO4 and CaO at different Ca/P ratios in the range of 1.17–2.10. The products were studied by PXRD, FTIR and NMR spectroscopy, HRTEM, and STA. In mixtures with a low initial Ca/P ratio (1.17–1.48), directly in the mill, the formation of calcium orthophosphate with whitlockite structure containing an HPO42− group and structural water is shown for the first time. This phosphate has structure similar to that of whitlockites of hydrothermal origin and differs from high-temperature β-tricalcium phosphate that has composition Ca3(PO4)3. A series of samples of apatite was obtained with varied composition, which depends on the initial Ca/P ratio. At Ca/P 1.67, the existence of two types of calcium-rich apatite is confirmed
Effect of Impregnation on the Structure of Niobium Oxide/Alumina Catalysts Studied by Multinuclear Solid-State NMR, FTIR, and Quantum Chemical Calculations
Multinuclear solid-state <sup>1</sup>H, <sup>27</sup>Al, and <sup>93</sup>Nb NMR experiments and DFT calculations
were carried out
for structural characterization of alumina-supported niobium oxide
catalysts with high niobium content following an every stage in the
catalyst preparation. It was found that the first stage of the impregnation
procedure plays a key role in determining the catalyst structure and
acidity. In order to monitor the presence in catalysts of aluminum
niobate phase, AlNbO<sub>4</sub>, a series of <sup>27</sup>Al and <sup>93</sup>Nb NMR experiments was performed for several different individual
AlNbO<sub>4</sub> samples. Aluminum and niobium NMR parameters were
determined for AlNbO<sub>4</sub>, which crystal structure contains
two different crystallographic sites for each element. The compound
was investigated through a combination of experimental <sup>93</sup>Nb and <sup>27</sup>Al NMR spectroscopy methods at several magnetic
field strengths (9.4, 11.7, 19.4, and 21.1 T) and complemented by
ab initio quantum chemical calculations of NMR parameters for these
nuclei. The chemical shielding and the quadrupole coupling tensor
parameters were determined for both <sup>93</sup>Nb and <sup>27</sup>Al