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carburization agent, the surface area of the obtained carbide was higher
due to the less aggressive conditions employed. Carburization tempera-
tures over 800 1C could lead to deposition of carbon on the carbide
surface.
Xiao et al. prepared Mo carbides by using butane
21
and acetylene
22
as
carbon source. The catalytic performance was tested for pyridine hydro-
denitrogenation (HDN), dehydrogenation of butane, and also methane
catalytic partial oxidation (CPO). The use of reactive hydrocarbons with
longer carbon chains or containing functional groups (double or triple
bonds) for the preparation of the carbide allowed a lower synthesis
temperature and thus increased the surface area of the produced carbide.
However, the formation of coke from acetylene or butane was also more
favoured at lower temperatures than in the case of methane. It was found
that an oxycarbide with fcc structure was formed at low temperatures
using butane. The formation of the b-Mo
2
C with hcp structure was
delayed until the temperature reached 950 1C. The catalytic activity was
tested for samples prepared at 550, 600 and 750 1C, where the highest
activity for pyridine HDN was achieved for the carbide prepared at 600 1C.
The authors argued that the lower activity for pyridine HDN with the
carbide prepared at 750 1C could be related to the hexagonal structure of
the carbide, which would possibly be less active than the cubic one for
this specific reaction. Effects from carbon deposition could also not be
excluded.
Acetylene as carburizing agent for the preparation of Mo
2
C was also
used by the same research group.
22
The temperatures used for the car-
burization were 450, 500, 550 and 630 1C. The materials were studied with
XRD, Raman, Scanning Electron Microscopy (SEM), specific surface area
measurements (BET method) and a CHN elemental analysis. The cata-
lytic performance for CPO was tested. The formation of the carbide was
observed when the temperature was above 500 1C. It was found that MoO
3
was firstly converted to MoO
2
and afterwards to a-MoC
1
x
. After treat-
ment at 850 1C in flowing N
2
the carbides were converted to a-MoC
1
x
and b-Mo
2
C. The authors concluded that acetylene was a good carbur-
izing agent for low-temperature preparation of a Mo carbide catalyst for
CPO.
22
The preparation of supported carbides has been considered as an al-
ternative to the bulk materials thus aiming to extend and achieve better
control of the surface area and pore size distribution. One of the most
widely employed supports has been g-alumina.
24,27,28,33,34,37,90
Typically,
molybdenum oxide, or its precursor, is impregnated onto the alumina
support before the carburizing treatment.
24,27,28,33,34,37
Other options
such as carbon nanotubes,
104
activated carbon,
105
silica,
85,106
titania
15,106
or zirconia
15,106
have also been investigated.
2.2 Metal nitrides
Transition metal carbides and nitrides are frequently studied together
due to their structural similarities.
9,10,97
The crystal structures for ni-
trides are compared with the ones for carbides in Fig. 1. The properties of
metal nitrides have attracted continuous attention related to their
