Hydrothermal Formation of W/Mo-Oxides: A Multidisciplinary Study of Growth and Shape

Abstract

The hydrothermal formation of mixed nanoscale W/Mo-oxides with the hexagonal tungsten bronze (HTB) structure has been investigated by in situ EDXRD (energy dispersive X-ray diffraction). Compared to the binary oxide systems, they display intermediate kinetics with a nucleation-controlled mechanism dominated by the slow growing tungsten component. Furthermore, the thermal stability of nanostructured W/Mo-HTB compounds has been monitored through combined in situ X-ray absorption spectroscopy (XAS) and XRD in reducing and oxidizing atmospheres. Their transformation into other mixed nanostructures was only observed above 300 °C in O 2 -and H 2 -containing atmospheres. In addition, the shape of nanoscale hexagonal W/Mo-oxides can be expanded into a variety of morphologies via the use of alkali chlorides as hydrothermal additives. The alkali cations exert a two-fold role as internal stabilizers and external shape control agents. Their mobility within the channels of the W/Mo-oxide host framework has been investigated by solid state NMR spectroscopy.

On the origin of the spontaneous formation of nanocavities in hexagonal bronzes (W,V)O3

“…6 Moreover, the selection of the adequate alkali cation additives during the hydrothermal process of synthesis, allows both the crystal morphology control as well as the structure stability. 7 In this sense, it has been proposed that the HTB framework cannot be maintained without traces of stabilizing ions or molecules in the hexagonal channels.…”

“…The existence of h-WO3 with only residual occupation of the hexagonal tunnels 7 and, therefore, with accessible active acid sites, led to the idea of modifying the composition of the structural framework with the addition of vanadium 8 and transform the material into a bifunctional catalyst with acidic and redox properties simultaneously. The efficiency of these materials was tested in the transformation of glycerol to acrylic acid.…”

A high-temperaturein situcell with a large solid angle for fluorescence X-ray absorption fine structure measurement

“…This cell also has no water-cooling system because the Kapton window temperature was kept at less than 373 K due to the high thermal conductivity of Al, even at a sample temperature of 703 K. The system can be applied to reaction analysis of solid oxide, bimetallic catalyst for fuel cells, [19][20][21][22] and in situ analysis of W oxide photo/electrochromic materials. 54,55 These powder samples are loaded into a small container made of BN or graphite and fixed to the heater. The cell is thus universally applicable to many systems in the measurements of fluorescence XAFS under the reaction conditions.…”

Targeted Synthesis and Environmental Applications of Oxide Nanomaterials

“…[20] This synthetic route can subsequently be tuned by using alkali chlorides as additives in the above-mentioned hydrothermal procedure. [16,21] They exert a dual function: on the one hand, they stabilize the wide interior of the hexagonal channel system through intercalation and on the other hand, the morphology of the products can be steered with the alkali cation. The use of LiCl and NaCl gives rise to nanorod formation, whereas very small nanorods organize into cylinders that furthermore form spherical aggregates in the presence of KCl, RbCl and CsCl (Fig.…”

“…1). [21] Further parameter investigations of the hydrothermal system showed that the formation of the resulting hierarchical W/Mo-oxide architectures depends on the presence of the larger alkali cations K + , Rb + and Cs + . The question how this morphological variety can arise from a single hydrothermal protocol inspired us to investigate the reaction pathway with in situ methods.…”