Nanometer
WO
x species evidenced by
HRTEM
/
EDX
in
WO
3/
S
i
O
2 catalysts for the metathesis reaction
Authors: Poovarawam Narongrat , Suriye Kongkiat , Panpranot Joongjai , Praserthdam Piyasan , José Cadete Santos Aires Francisco 
Published: December 2016

The demand for propylene is growing rapidly in the chemical industry since it is used for the production of commercial products such as polypropylene, acrylonitrile, alcohol, acrylic acid, and other petrochemical products. The metathesis reaction (using feeds of ethylene and butenes) is among other methods (naphtha steam crackers, fluid catalytic crackers, propane dehydrogenation, Fischer–Tropsch reactions) a way of producing propylene. Rather recently, metathesis reaction over WO
3
/SiO
2
catalysts has received considerable attention by the industry to produce propylene [1] even though WO
3
/SiO
2
catalysts have long been considered and used in the metathesis of linear olefins [2]. Despite thorough characterization of the WO
3
/SiO
2
catalysts only a few works make use of transmission elec‐tron microscopy to study such catalysts [3]. In those studies particular interest has been taken to the morphology of the WO
3
rather large particles as well as to coking issues. Indeed TEM observations show the presence of large WO
3
crystallites (tens or hundreds of nm). It is generally admitted that two tungsten phases co‐exist : crystalline WO
3
and a surface phase in strong inter‐action with the support. The latter is supposed to be at the origin of the active sites but no TEM characterization of it has yet been performed to our knowledge. In this work we report HRTEM/EDX evidence of the co‐existence of highly divided WOx nanoparticles (< 2 nm) with WO
3
large crystallites.

Catalysts with different tungsten loadings (4, 6, 8, 10 wt%) were prepared by incipient wetness impregnation of an aqueous solution of ammonium metatungstate hydrate (Aldrich, 99.9%) over silica gel. Electron microscopy (HRTEM, EDX) was performed with a JEOL JEM 2010 TEM equipped with a LaB
6
gun, UHR pole‐piece and a Pentafet LinK‐Isis EDX spectrometer (Oxford Insts.).

In all samples irregular shaped polycrystalline (Figure 1a) and singlecrystal (Figure 1b‐d) large particles (10–50 nm) were observed. For both cases they result from the agglomeration of smaller crystallites. Together with these large particles, numerous smaller particles exist in every sample (Figure 2a). These nanopar‐ticles are generally smaller that 2 nm and even in the subnanometer range (Figure 2a‐c). The former can be isolated (Figure 2d) or tend to agglomerated and rearrange in larger structures after sometime under the electron beam (Figure 2b‐bottom, Figure 2e). The latter are rather difficult to detect and only the EDX spectra confirm the presence of W; diffraction contrast (Figure 2c) only appears after sometime under the electron beam. It is rather difficult to estimate the oxygen quantitative concentration on the WO
x
nanostructures since we are dealing with only a few atoms and above all the support is an oxide yielding a large contribution to the oxygen signal. The smaller nanoparticles (≤ 1 nm) present internal diffraction contrasts that can be attributed to a pseudo‐structure. However this is only a short‐range order and no finite periodicity was devised. For larger nanoparticles (1–2 nm) high resolution images (Figure 2d,e) show interplanary distances and angles between planes that can reasonably be attributed to a cfc structure. We cannot rule out however hcp structures in some cases. This is not inconsistent with the more classic orthorhombic or tetragonal structures for WO
3
since the values of the structural parameters a, b and c are very close to each other and since we expect these structures to be rather WO
x
(x<3) than WO
3
. These very small species may thus form the so‐called surface phase thought to be at the origine of the active sites for the metathesis reaction.[4]