Chemistry Reference
In-Depth Information
beneficial to obtain carbides with higher surface areas. Such behaviour
has been attributed to the eciency of water removal.
103
Thermodynamics are useful for selecting hydrocarbon type, tempera-
ture of operation, as well as hydrogen content in the carburizing mix-
ture.
91
Hydrogen is mixed with the hydrocarbon, aiming to prevent the
formation of separate carbon phases, to increase the surface area of
the formed carbide and to favour the formation of the mixed oxygen-
containing carbide, i.e. oxycarbide.
94
The temperature for the formation
of the material should not be too high (to avoid coke formation), but it
should be high enough to ensure the transformation of the precursor to
the carbide.
Thermodynamics predicts that the application of C
2
þ
carbiding
agents such as butane or ethane allow employing a lower carbiding
temperature. Carbides with a higher BET surface area are obtained when
less aggressive parameters are used in the preparation. However, coke
deposition is also favoured at lower temperatures when using butane
compared to methane, as indicated in Fig. 3. Thus, the final temperature
level should be carefully selected according to the composition and re-
activity of the carburizing mixture.
21
Hanif et al.
94
reported that the use of methane/hydrogen or ethane/
hydrogen as carburizing mixtures first caused reduction of the MoO
3
to
MoO
2
. Afterwards, at the appropriate temperature an oxycarbide with fcc
structure was formed. Finally, when the preparation temperature reached
the appropriate value a b-Mo
2
C phase with hcp structure was obtained.
Characterization techniques such as temperature programmed reduction
(TPR) coupled with high-resolution transmission electron microscopy
(HRTEM), x-ray diffraction (XRD) or physical surface area measurements
(N
2
-physisorption by Brunauer-Emmet-Teller method, BET) measure-
ments were used to follow the structural changes during the carbide
formation. As expected, it was found that when ethane was used as
100
"C
1
"
CH
4
= 2 H
2
+ C
C
2
H
6
= 3 H
2
+ 2 C
C
4
H
10
= 5 H
2
+ 4 C
80
"C
2
"
60
"C
4
"
40
20
0
-200
0
200
400
600
800
1000
1200
Temperature [°C]
Fig. 3 Decomposition and carbon formation as function of temperature for CH
4
,C
2
H
6
and C
4
H
10
. Conversions estimated using HSC 5.11 software.
