Chemistry Reference
In-Depth Information
Preparation procedures. Boudart et al.
90-93
developed the method that
later became common for the preparation of metal carbides. MoO
3
or its
corresponding precursor were employed as starting materials. The car-
bide is usually prepared by gas-solid reactions between the solid metal
oxide and a gaseous carbon source. Mixtures of molecular hydrogen and
the carbon source are employed to produce the carbide through a tem-
perature programmed procedure. Depending on the reactivity of the se-
lected carburizing mixture, the major variables are the heating ramp rate,
final temperature and the content of hydrogen in the mixture.
15,91,94
Al-
ternatives to temperature programmed reactions have also been investi-
gated. Wang et al.
95
recently reported the formation of the carbide in a
single step by the reaction of hexamethylenetetramine and a mixed salt
precursor containing molybdenum. The synthesis of bulk and supported
molybdenum carbide after heat treatment in argon at 700 1C was pre-
sented. The catalytic activity for HDS of these materials was tested,
showing promising results. Patel et al.
96
studied the synthesis of Mo
2
C
via a solution route. The precursors employed were ammonium molyb-
date tetrahydrate and sucrose. The chemicals were dissolved in water and
the solution was dried for several days. The final step was pyrolysis of the
obtained powders at 800 or 1200 1C to form the final carbides.
An alternative route to high surface area metal carbides was developed
by Ledoux et al.
97
This method entails reacting volatile metal oxide spe-
cies with solid carbon, e.g. Mo
2
C was prepared by reacting MoO
3
vapor
with activated carbon, under what the authors describe as vacuum con-
ditions, where the low residual pressure was composed of the metal oxide
vapor and CO as one of the products of the reaction. The resulting car-
bide had a surface area of 147 m
2
/g. A similar route using WO
3
gave
almost pure WC. These materials were initially proposed as isomeriza-
tion catalysts,
14,18
and were also investigated in other reactions, e.g.
dehydrogenation.
79
The formation of two different geometric structures of molybdenum
carbide is reported (b-Mo
2
C and a-MoC
1
x
),
52,91,92,95,98
depending
upon the preparation conditions. As seen in Fig. 1, b-Mo
2
C represents a
hexagonal closed packed structure (hcp), whereas the carbide a-MoC
1
x
shows a face centred cubic (fcc) structure. Figure 2 illustrates alternative
routes to different structures. It is known that b-Mo
2
C can be obtained by
Fig. 1 Crystal structures of carbides and nitrides. Reproduced with permission from ref. 98.
