Chemistry Reference
In-Depth Information
Synthesis 2
CH
4
+ 4 H
2
α−
MoC
1
-x
γ−
Mo
2
N
Synthesis 1
NH
3
Face Centered Cubic
Face Centered Cubic
MoO
3
CH
4
+ 4 H
2
β−
Mo
2
C
Ortho Rhombic
Synthesis 3
Hexagonal Close Pack
PRECURSOR
Fig. 2 Procedures for synthesis of Mo nitride and two types of Mo carbide from MoO
3
employing temperature programmed reaction, adapted with permission from ref. 52.
a direct carburization process from the oxide. The a-MoC
1
x
can be
formed in two steps, via aMo
2
N precursor formed by nitridation of the
MoO
3
in flowing ammonia. Secondly, the carbide is formed by TPR in a
mixture of methane and hydrogen. More recently, Bouchy et al.
99
re-
ported a method for the preparation of a pure a-MoC
1
x
by pre-reducing
the MoO
3
. The method claims its advantage and importance is due to its
simplicity.
Temperature Programmed Reaction. The conventional synthesis
technique to prepare molybdenum carbide is based on heating MoO
3
at a
slow rate in flowing methane/hydrogen mixtures. Typically, this is done
in a fixed-bed quartz reactor at atmospheric pressure. Conventionally, the
heating program is divided into sections. A fast heating temperature is
selected for the first part of the program, where no changes on the MoO
3
are expected, thus aiming to reduce the overall preparation time. It is
known that molybdenum oxide is stable at temperatures below 277 1C,
91
thus no carburization is happening up to that point. When approaching
277 1C the heating rate is typically decreased until reaching the working
temperature for the carburization. It has been reported that the actual
carburization process is taking place in two steps, with an intermediate
oxycarbide phase forming initially. The temperature for this to happen
depends on the carburizing mixture, heating rate, etc. Subsequently, the
carbide is formed from the oxycarbide phase.
15
Several carburizing media have been employed for the preparation of
the metal carbide, where mixtures of methane/hydrogen
91,92,94,100
and
ethane/hydrogen
94
are among the most commonly used. However, other
hydrocarbons such as propane,
15,101
acetylene
22
or butane
21
mixed with
hydrogen have been also investigated. Among alternative non-hydro-
carbon containing options are the use of carbon monoxide or mixtures of
carbon monoxide/carbon dioxide, and carbon monoxide/hydrogen.
102
High flow rates of the carburizing mixture have been reported as
