Nanostructured molybdenum chalcogenides: synthesis, structure and catalytic properties
Abstract
The catalytic properties of nanostructured molybdenum oxides and sulfides were
investigated. Several synthesis methods were tested in order to determine a reliable
and reproducible way to produce structurally and chemically homogeneous products.
The structure of synthesized molybdenum oxides and sulfides was thoroughly studied
by electron microscopy techniques, X-ray diffraction and X-ray photoelectron spectroscopy
among others. It was found that the synthesis method of choice produced
high yield, single phase, single crystalline molybdenum oxide nanoparticles. These
oxide crystals are susceptible to reduction and sulfidation, which enable their use as
precursor in the synthesis of molybdenum sulfides. The sulfidation of molybdenum
oxide nanocrystals produced highly textured molybdenum sulfide nanostructures.
Pseudo one-dimensional structures were identified in these samples. The structural
model proposed for such structures implies the presence of sulfur atoms decorating
the edges. This is particularly important since these extra-sulfur atoms may lead to
an electronic structure different from the bulk molybdenum sulfide. The catalytic
properties of these compounds were studied with the model reaction of hydrodesulfurization
(HDS) of dibenzothiophene (DBT). DBT is considered an appropriate
compound for the investigation of the activity and reaction mechanisms of HDS
catalysts, and it is widely used in the literature, which facilitates the comparison
with reported data. The nanostructured molybdenum compounds showed typical to
high activity values, however they presented enhanced selectivity. The oxides were
more efficient in the desulfurization of DBT, while the sulfides for hydrogenation.