W and two-dimensional WO₃/W nanocrystals produced by controlled self-sustaining reduction of sodium tungstate

Abstract

The influence of calcium fluoride (CaF 2 ) on combustion characteristics of Na 2 WO 4 1 3 Mg system and microstructure of the produced W and WO 3 /W crystals is investigated. The results of thermodynamic analysis and experimental investigations show that CaF 2 simultaneously enhances the conversion of Na 2 WO 4 toward tungsten and binds sodium through the formation of NaF phase. The examination of the microstructure of quenched combustion products and differential scanning calorimetry analysis indicate that at early stages of combustion, a part of Na 2 WO 4 is reduced by Mg to tungsten, whereas another part reacts with CaF 2 forming CaWO 4 and NaF. Subsequent magnesium reduction of CaWO 4 significantly increases the overall temperature of the combustion process. Such modification in reaction mechanism coupled with postcombustion processing (e.g., acid/basic treatment) of the product allows us to produce either pure tungsten nanocrystals or tungsten oxide-tungsten nanostructures consisting of two-dimensional WO 3 nanoflakes assembled on a W core. It is found that CaF 2 does not influence the sizes of tungsten nanocrystals. However, since the addition of CaF 2 leads to the increase of overall reaction temperature, it facilitates the formation of W particles with equilibrium crystal shape by faceting process.

Solution Combustion Synthesis of Nano-Crystalline Metallic Materials: Mechanistic Studies

“…In some modeled experiments, combustion of gels is performed in a wedge of a massive copper block. While the combustion wave propagates along the wedge, the heat losses increase to a point, leading to the arrest of the reaction front. , The layer-by-layer studies of the quenched sample allow us to determine the dynamics of microstructural transformation in the combustion wave.…”

Facile Fabrication of WO3 Nanoplates Thin Films with Dominant Crystal Facet of (002) for Water Splitting

“…32 Perry et al 33 have synthesized nanoscale WO 3 •H 2 O platelets by a crash precipitation method. Manukyan et al 34 have reported that WO 3 •H 2 O nanosheets can be obtained by acid treatment of CaWO 4 sheets, which were produced by controlled selfsustaining reduction of Na 2 WO 4 at room temperature. However, the morphologies of the platelets and the nanosheets obtained by these two methods are not uniform.…”

Oxidative coupling of methane—comparisons of MnTiO3–Na2WO4 and MnOx–TiO2–Na2WO4 catalysts on different silica supports

“…The lattice d-spacings of 0.20 and 0.27 nm corresponded to the crystal plane of MnTiO 3 [1 1 0] 47 and TiO 2 [1 0 1] 47 , respectively. The WO 3 /W had a lattice d-spacing of 0.21 nm, relating to the lattice plane of WO 3 /W [2 0 1] 48 . The Mn 2 O 3 and Mn 3 O 4 had a lattice d-spacing of 0.23 nm, corresponding to the crystal plane of Mn 2 O 3 [1 1 1] 49 and Mn 3 O 4 [0 1 0] 50 .…”