Pechini synthesis and characterization of molybdenum carbide and nickel molybdenum carbide

Article abstract of DOI:10.1016/j.jssc.2008.06.050

Carbides, such as η-Ni₆Mo₆C, are considered as low-cost substitutes for noble metal catalysts for present applications in hydrodesulfurization and for a possible future sulfur-tolerant fuel cell anode catalyst. Most synthesis methods set the carbon content of the carbides by a carbon-based atmosphere or solid carbon in the synthesis. We show here that β-Mo₂C and η-Ni₆Mo₆C can be synthesized using a Pechini process, simply by heating metal acetates mixed with citric acid and ethylene glycol in one step under H₂ with the only source of carbon being the precursor solution. The β-Mo₂C forms when heating a Mo-acetate precursor at 850°C. When using Ni- and Mo-acetates, β-Mo₂C forms at 700°C and lower temperatures, while η-Ni₆Mo₆C forms during heating at 800-900°C. The η-Ni₆Mo₆C has a surface area of 95.5m ²g^-1 and less than 10m²g^-1 when prepared at 800 and 900°C, respectively. Some Ni₃C, Ni, and NiC impurities are also present in the nanostructured η-Ni₆Mo ₆C that was prepared at 900°C. The η-Ni₆Mo ₆C materials made by the Pechini process are compared with those made from a traditional synthesis, using metal organic precursors at 1000°C under CO/CO₂ mixtures with a carbon activity of 0.011. Our results imply that H₂ and the Pechini process can be used to achieve carbon activities similar to those obtained by methods using gaseous or solid carbon sources.

Full text of DOI:10.1016/j.jssc.2008.06.050

ARTICLE IN PRESS
Journal of Solid State Chemistry 181 (2008) 2741–2747
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Pechini synthesis and characterization of molybdenum carbide and nickel
molybdenum carbide
Ã
1
Arnold M. Stux , Christel Laberty-Robert , Karen E. Swider-Lyons
Chemistry Division, Naval Research Laboratory, Washington, DC 20375-5342, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
6 6
Carbides, such as Z-Ni Mo C, are considered as low-cost substitutes for noble metal catalysts for
present applications in hydrodesulfurization and for a possible future sulfur-tolerant fuel cell anode
Received 28 September 2007
Received in revised form
catalyst. Most synthesis methods set the carbon content of the carbides by a carbon-based atmosphere
13 May 2008
or solid carbon in the synthesis. We show here that
Pechini process, simply by heating metal acetates mixed with citric acid and ethylene glycol in one step
under H with the only source of carbon being the precursor solution. The -Mo C forms when heating a
Mo-acetate precursor at 850 1C. When using Ni- and Mo-acetates, -Mo C forms at 700 1C and lower
temperatures, while -Ni Mo C forms during heating at 800–900 1C. The -Ni Mo C has a surface area
of 95.5 m g and less than 10 m g when prepared at 800 and 900 1C, respectively. Some Ni C, Ni, and
NiC impurities are also present in the nanostructured -Ni Mo C that was prepared at 900 1C. The
-Ni Mo C materials made by the Pechini process are compared with those made from a traditional
synthesis, using metal organic precursors at 1000 1C under CO/CO mixtures with a carbon activity of
.011. Our results imply that H and the Pechini process can be used to achieve carbon activities similar
to those obtained by methods using gaseous or solid carbon sources.
2 6 6
b-Mo C and Z-Ni Mo C can be synthesized using a
Accepted 14 June 2008
Available online 1 July 2008
2
b
2
Keywords:
b
2
Pechini process
Molybdenum
Nickel
Z
6
6
Z
6
6
2
ꢀ1
2 ꢀ1
3
Z
6
6
Carbide
Z
6
6
Electrocatalyst
Hydrodesulfurization
2
0
2
&
2008 Published by Elsevier Inc.
1
. Introduction
atoms as the center of Mo octahedra with a Mo–C substructure of
four other similar octahedra surrounding the central octahedron
tetrahedrally so that they are all face-sharing. Another interpene-
trating structure is superimposed comprising a diamond lattice of
Transition metal carbides were traditionally researched for
their mechanical hardness and high melting points for steel
hardening [1], but they are also now known as catalysts for
ammonia synthesis and decomposition, hydrogenolysis, isomer-
ization, methanation, and hydroprocessing [2]. Some molybde-
num- and tungsten-carbides can be similar to platinum in their
catalytic properties [3,4]. Catalytic activity of the carbides arises
from the carbon atoms, which comprise up to 50% of their crystal
structure, and which increase the metal–metal distance thus
increasing the d-band electron density at the Fermi level of the
transition metals [5]. The ability of some carbides to mimic Pt has
created interest in using them as a noble-metal-free replacement
for the Pt/carbon electrocatalyst used in fuel cells [2,6,7].
1
1 1
carbon atoms displaced by
. These other displaced C atoms are
2 2
2
then surrounded by tetrahedra of Ni atoms.
Transition metal carbides are traditionally synthesized by
several different methods [10]: (a) carbothermal reduction of
metal oxides with graphite in an inert environment, (b) electro-
chemical synthesis, (c) arc melting with graphite, (d) thermal
decomposition of diethylnitriamine oxometal compounds, and
(e) reduction of oxides by various means, such as H
2
, mixtures of
H
2
and hydrocarbons as the carburizing gas, or another source of
carbon. A critical feature of these processes is that the carbon
activity of the environment is thermodynamically favorable for
the formation of the carbide phases.
The
Mo
metals and are catalytically active for hydrodesulfurization (HDS)
Z
2
-12 carbides of molybdenum Ni
, are specific examples of carbides that mimic noble
6 6 6 6
Mo C, Co Mo C, and
Co
6
6
C
Z
-Ni
trisethylene diamine molybdate (Ni(en)
He-diluted H to 650 1C, and then converted to the carbide by
further heating at 10001C under CO/CO gas with a carbon activity
6
Mo
6
C can be made by a two-step process whereby nickel
3
MoO is heated in
4
)
[
8]. The Fd 3¯ m structure of
Z
-Ni
6
Mo
-Ni
6
C is depicted by a ball and
Mo C structure has carbon
2
stick model in Fig. 1 [9]. The
Z
6
6
2
of 0.011 [6,11,12]. Numerous methods exist for preparing molybde-
num carbides. Molybdenum carbide preparation is possible from
heating oxide precursors, e.g., MoO
peroxide solutions [13] and also via nitride precursors [14].
The -Mo C and -Mo C phases have been prepared under a
3
, in ethylene glycol or
Ã
Corresponding author.
E-mail address: Arnold.Stux@nrl.navy.mil (A.M. Stux).
1
Present address: Pierre and Marie Curie University, Paris, France.
b
2
Z
2
0
022-4596/$ - see front matter & 2008 Published by Elsevier Inc.
doi:10.1016/j.jssc.2008.06.050

ARTICLE IN PRESS
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A.M. Stux et al. / Journal of Solid State Chemistry 181 (2008) 2741–2747
2 2 2 2
glycol (HOCH CH OH) and citric acid (HOC(COOH)(CH COOH) )
were added in a 1:1:1 M ratio to the Mo-acetate dimer solution.
This solution was heated to 85 1C in a beaker with vigorous
Citric acid + Ethylene Glycol +
Molybdenum (II) acetate dimer mixing
in aqueous solution
Metal-citrate complex
evaporating at 75–85 °C
6 6
Fig. 1. Ball and stick representation of the Z-Ni Mo C crystal structure.
Gel precursor – heating at 850 °C
propane/H
with post-treatment of H
been synthesized by heating ammonium heptamolybdate
Mo 24(NH O) mixed with an active carbon under H
2
whereby the carbon serves as a source of methane [16]. Recently,
Yu et al. have described a Pechini-based process to make
nanocrystalline TiC that is embedded in a mesoporous carbon
2
mixture at 650 1C, with the
b
-Mo
2
C phase forming
2
[15]. Carbon-supported
b
-Mo C has
2
under H2
(
7
O
4
)
6
ꢁ 4H
2
2
composite [17]. Like the Mo C in Ref. [16], the TiC is formed in situ
by a carbothermal reduction process within the carbon matrix.
We present herein the Pechini process as a new, one-step
Crystalline Mo C
2
synthesis of
citric acid, ethylene glycol and H
Z
-Ni
6
Mo
6
C and
b
-Mo
2
C, by using metal acetates,
2
, and no solid carbon as a carbon
source. This sol–gel method offers the abilities to control surface
and bulk properties, to cast the material in any shape, and to form
coatings, which provide advantages over other traditional proces-
sing routes. In the Pechini process [18], polybasic chelates are
formed between a-hydroxyl carboxylic acids, containing at least
one hydroxyl group, and metal cations. The chelates undergo
Citric acid + Ethylene Glycol +
Molybdenum (II) acetate dimer + Nickel
acetate mixing in aqueous solution
polyesterification on heating with the polyfunctional alcohol
(ethylene glycol) to form a large metal–organic complex. Further
heating produces a viscous resin, then a rigid transparent glassy
gel, and finally a fine oxide powder. Citrate gel precursors offer
chemical uniformity and control of the composition [19].
Coordination of the metals in the metal–organic complexes
remains unchanged on polymerization and molecular level mixing
is retained at the resin stage.
Metal-citrate complex
evaporating at 75–85 °C
We develop this Pechini process for carbide formation by
observing the thermal events that occur upon heating the
precursors in the presence of H
2
. The resulting materials, made
under pure H , are characterized by X-ray diffraction (XRD),
2
Gel precursor heating at 600-900 °C
scanning electron microscopy (SEM), elemental analysis, and the
Brunauer-Emmett-Teller (BET) method for surface area measure-
under H2
ments. The materials are compared with our
000 1C under CO/CO mixtures.
6 6
Z-Ni Mo C made at
1
2
2
. Experimental section
2.1. Materials and synthesis
Crystalline Ni Mo C
6
6
b
-Mo
cially available molybdenum (II) acetate dimer (Mo
Aldrich). An aqueous solution was prepared by dissolving the Mo-
acetate dimer into 50 mL H
O to form a ꢂ1 mM solution. Ethylene
2
C was synthesized by the Pechini process from commer-
2
(OCOCH ) ,
3 4
Fig. 2. Schematic diagram of the synthetic process of (A)
-Ni Mo C by the Pechini process.
2
b-Mo C and (B)
2
Z
6
6

ARTICLE IN PRESS
A.M. Stux et al. / Journal of Solid State Chemistry 181 (2008) 2741–2747
2743
stirring on a hotplate. The light green aqueous solution became
brown and then reddish brown. Within a few hours of heating and
stirring, the solution became redder and then reverted back to
light green. The solution was evaporated on the hotplate for over a
few hours at 75 1C until there was only ꢂ10 mL left. The resulting
viscous green solution was pipetted into an alumina crucible and
heated in a tube furnace (alumina tube, i.d. ¼ 2.0 cm) at a ramp
were determined from their d-spacings, as obtained from Bruker’s
EVA program, and using a least-squares analysis program for
lattice parameter refinement, UnitCell for OSX (v0.3) [20]. The
lattice parameters were corrected to those of an internal silicon
standard, which was refined from the d-spacings of the measured
data using UnitCell, and then corrected to the NIST standard value
˚
for Si (a ¼ 5.43104 A). The correction percentage for the measured
ꢀ
1
ꢀ1
rate of 2 1C min under 9 mL min flowing H
2
. The temperature
vs. standard Si value was used to adjust the refined lattice
of the samples was held at 850 1C (1023 K) for 14 h and then
6 6
parameter of the cubic Z-Ni Mo C.
ꢀ
1
cooled at 5 1C min . The H
cooled down to less than 100 1C, when it was opened to the
atmosphere. The H was 99.95% pure with less than 0.001%
2
flow was turned off when the samples
The XRD patterns were also used to determine domain sizes,
through analysis of the most intense diffraction peak with the
Scherrer equation (e.g., the scattering angle at 43.51 2
2
y for the
hydrocarbons. The synthetic process for Mo
tically in Fig. 2A.
2
C is shown schema-
Z
-Ni Mo C). The particle morphology of both materials was also
characterized with SEM (Leo Supra 55) with 1 and 2 kV voltage
6
6
Z
-Ni
6
Mo
6
C was synthesized by the Pechini process using
ꢁ xH O, Alfa
operating conditions and an in-lens detector.
Mo-acetate dimer and nickel(II) acetate (Ni(OCOCH
3
)
2
2
Surface areas were determined by nitrogen physisorption using
a Micromeritics ASAP2010 Accelerated Surface Area and Porosi-
metry Analyzer. The sample was degassed at 150 1C for at least
12 h prior to characterization. Specific surface areas were
determined using the multipoint BET method.
Aesar). The Mo- and Ni-acetates were dissolved in a 1:1 M ratio of
Mo and Ni to form a 250 mL (40 mM) aqueous solution. Ethylene
glycol and citric acid (1:1 M ratio) were added to form a 3:2 M
ratio of ethylene glycol to nickel. The solution was heated to 85 1C
in a beaker with vigorous stirring on a hotplate and was
evaporated on the hotplate for over a few hours at 75 1C until
the contents were viscous. The resulting reddish brown viscous
solution was pipetted into an alumina crucible and heated in a
Elemental analyses were performed by Desert Analytics, Inc.,
Tucson, AZ.
ꢀ
1
3. Results
tube furnace with a ramp rate of 2 1C min
to 600–900 1C
873–1173 K) for 12–14 h under 9 mL min flowing H and cooled
flow was turned off when the samples
ꢀ1
(
2
ꢀ1
down at 5 1C min . The H
2
Table 1 summarizes the appearance and crystal phases of the
cooled down to less than 100 1C, when it was opened to the
atmosphere. The synthesis is shown schematically in Fig. 2B.
materials made from the Mo-acetate and Ni-Mo-acetates at
various temperatures. The compound synthesized from
Mo-acetate using ethylene glycol, citric acid, and heating under
Standards of
carburization using a method adapted from the patent literature
11]. Nickel Ni(en) MoO was placed in a tube furnace (alumina
tube, i.d. ¼ 3.81 cm) under a combination of flowing H and Ar at
4.3 mL min each (2.1 cm min linear flow rate) and heated at
6 6
Z-Ni Mo C were also prepared by gas-phase
H
2
(process in Fig. 2A) matches well to JCPDS pattern no. 72-1683
[
3
4
for the hexagonal -phase of Mo C, as shown in Fig. 3.
b
2
The compounds synthesized from Ni- and Mo-acetate mixtures
with ethylene glycol and citric acid (process in Fig. 2B) at 800 and
2
ꢀ1
ꢀ1
2
ꢀ
1
15 1C min to 650 1C (923 K), where the temperature was held for
900 1C under H
2
have XRD patterns that match to the JCPDS
ꢀ
1
3
0 min. The gas was switched to pure Ar at 50 mL min (4.3 cm
pattern 80-0337 of the
lattice parameters were 10.8570.01 and 10.86070.009 A for
Z
-12 phase for Ni
6
Mo
6
C (Fig. 3). Their
˚
ꢀ1
min
linear flow rate) and heated to 1000 1C (1273 K) at
ꢀ
1
15 1C min , at which point the gases were switched to a mixture
materials heated to 800 and 900 1C, respectively, in good
ꢀ1
˚
of Ar, CO, and CO
2
at 61 and 20 mL min , respectively, to impose a
agreement with that of Newsam et al. [12] (10.8932 A). The
carbon activity 0.011. The reaction was run for 3 h, after which the
reactor was allowed to cool to ambient temperature under flowing
materials heated to 800 1C were dull-colored flakes and those
heated at 900 1C were shiny flakes. Both samples exhibit
ꢀ
1
Ar at 61 mL min
.
additional small scattering angle peaks at ca. 451 and 591 2
which are assigned to Ni C, and are most prevalent in the sample
heated at 900 1C. The suggested presence of Ni C in this sample
y,
3
3
2.2. Characterization
and lack of Mo C or Mo impurities implies the volatilization of
2
traces of Mo. The XRD data for the sample heated at 900 1C
The materials were characterized for structure, porosity, and
composition. Differential thermal analysis (DTA, Rheometric
Scientific, Inc., STA1500) was carried out using a mixture of Ar
suggest the presence of Ni peaks at 77.51 and 94.41 2
also NiC impurities because of peaks at 51.81, 75.41, and 92.41 2
The coexistence of either Ni, NiC, or Ni C is likely a result of a
slight excess of Ni-acetate that was added to compensate for
commercial Ni(CH COO) O being a hydrate. Analysis of the
ꢁ xH
XRD peak widths indicate that the -Ni Mo C heated at 800 and
900 1C and the Mo C formed at 700 1C all have crystal domain
y. There are
y.
3
2
and 5% H (MG Industries, analyzed to 99.95% purity). XRD
measurements were made both with and without an internal Si
3
2
2
ꢀ
1
standard (CuK
a
radiation: 40 kV, 40 mA; 1.21 2
y
min step size;
Z
6
6
Bruker D8 Advance). The lattice parameters of the
Z-Ni
6
Mo
6
C
2
Table 1
Results from different heat treatments of Pechini precursors plus
Z
-Ni
6
Mo
6
C made by carbothermal reduction
Phase(s) present by XRD
Metal acetate precursor(s) Heating conditions
Appearance
Dull metal
(
a)
Mo (II) acetate dimer
850 1C, H
2
, 12 h
, 12 h
b
2
-Mo C
(b)
Ni(II) acetate, Mo (II) acetate dimer
Ni(II) acetate, Mo (II) acetate dimer
Ni(II) acetate, Mo (II) acetate dimer
Ni(II) acetate,Mo (II) acetate dimer
Tri(ethylene diamine) nickel molybdate
900 1C, H
800 1C, H2, 12 h
700 1C, H
600 1C, H
1000 1C, CO/CO
2
Z
Z
b
b
Z
-Ni
6
Mo
Mo
6
C+Ni
3
C+trace NiC+Ni
Shiny
-Ni
6
6
C+trace Ni
3
C
Discolored shiny
Dull metal
Dull metal
Shiny
2
, 12 h
, 12 h
-Mo
-Mo
2
C+trace
C
6 6 3
Z-Ni Mo C+Ni C
2
2
2
6 6
-Ni Mo C

ARTICLE IN PRESS
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A.M. Stux et al. / Journal of Solid State Chemistry 181 (2008) 2741–2747
sizes of 30 nm; the Ni
are about 15 nm in diameter. The samples derived from Ni- and
Mo-acetates and heated at 600 and 700 1C for 12 h under H are
dull colored and predominantly have distorted -Mo
structure. There is a trace of Ni C even as low as 700 1C as
indicated by a small peak at 451 2 . This small presence of Ni
-Ni Mo C to maintain the
3
C impurities in the 900 1C-treated sample
2
a
b
2
C
3
y
3
C
likely coexists with
stoichiometric ratio.
b
-Mo
2
C and
Z
6
6
Fig. 4 displays the DTA of both Mo-acetate and mixed Mo- and
Ni-acetates under 2.5% H in Ar. In the thermogram of the Mo-
2
acetate precursor above 250 1C, there is only one endotherm at
ꢂ600 1C, while there are three for the Mo- and Ni-acetate
precursor mixture. The thermogram for the mixed Mo- and
Ni-acetate exhibits a small endotherm at 380 1C, a slightly larger
one at 475 1C, and a sharp one at 730 1C. The endotherm at 730 1C
can be attributed to formation of
lower temperatures are attributed to the formation of
Z
-Ni
6
Mo
6
C, while those at
-Mo C and
C. The thermal analysis is consistent with the observation of
C impurities by XRD for the samples heated at 700 1C and
-Ni Mo C above 730 1C
C below 730 1C are in agreement with the XRD results
b
2
Ni
Ni
3
3
above. The reduction and formation of
and -Mo
in Table 1.
The morphology of the
with the Pechini precursors at 850 1C under H
Z
6
6
b
2
b
-Mo
2
C formed by heating Mo-acetate
is shown in Fig. 5.
Fig. 3. XRD patterns of compounds made from Mo-acetate and mixtures of Ni-
2
and Mo-acetates.
This compound has a low surface area (non-porous but granular)
2
0
0
20
10
-
-
-
-
20
40
60
80
0
-10
-20
-30
-40
-
-
-
100
120
140
-50
Mo- acetate
Ni- Mo- acetates
-60
0
200
400
600
800
1000
0
200
400
600
800
1000
Temperature (°C)
Temperature (°C)
ꢀ1
2
Fig. 4. Differential thermal analysis of the metal acetate precursors with ethylene glycol and citric acid under 2 1C min 2.5% H /Ar flow for the (A) Mo compound
and (B) Ni–Mo compound.
Fig. 5. SEM image of Mo-acetate gel heated under H
2
at 850 1C magnified at (A) 5000 ꢃ and (B) 50,000 ꢃ .

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2745
Fig. 6. SEM images of compounds made from Ni- and Mo- organics and the Pechini and CO/CO
2
processes at 5000 ꢃ and 50,000 ꢃ : (A,B) from the Pechini process at 700 1C;
(
C,D) Pechini process at 800 1C; (E,F) Pechini process at 900 1C; and (G,H) CO/CO process at 1000 1C.
2
with some micrometer- and submicrometer-sized particles and a
flake-like habit.
that in the pure Mo-compound, shown in Fig. 5, with low surface
area (non-porous but granular) and micrometer- and submicrom-
eter-sized particles. The same Ni- and Mo-acetates heated to
The SEM images of the various compounds made from Ni- and
Mo-acetate precursors are shown in Fig. 6. The
that formed at 800 and 900 1C have more porosity than the
predominantly Mo C phases that formed at lower tempera-
tures from similar starting resins. The -Mo C (with traces of
C) formed by heating the Ni- and Mo-acetate mixture
at 700 1C (Figs. 6A and B) appears almost identical to
Z
-Ni
6
Mo
6
C phases
800 1C (
with some nanoparticles and porosity in some regions (Figs. 6C
and D). The compound made at 900 1C under H , ( -Ni Mo C with
a small amount of Ni C) is highly porous (Figs. 6E and F). As in
Table 2, the -Ni Mo C prepared at 800 1C has a surface area of
while the sample prepared at 900 1C has less than
Z
-Ni
6
Mo
6
C with a trace of Ni
3
C) have ꢂ10 mm domains
2
2
Z
6
6
b
2
3
Z
-Ni
under H
6
Mo
6
Z
6
6
2
ꢀ1
2
95.5 m g

ARTICLE IN PRESS
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Table 2
Surface area properties and percent elements present of different heat treatments
of Pechini precursors of -Ni Mo C, Ni(II) acetate+Mo (II) acetate dimer with
ethylene glycol and citric acid
Elemental analysis in Table 2 for the 900 1C-heated sample
shows a slight excess of carbon in the powder compared with the
theoretical values of 1.27% C, 37.5% Ni, and 61.2% Mo. The Ni
content is lower than the expected value implying excess Mo, yet
no Mo was observed by XRD. The elemental analysis for the
Z
6
6
2
ꢀ1
a
b
c
c
Treatment (1C)
Surface area (m g
)
C (1350 1C)
H
Mo
Ni
8
00 1C-heated sample indicated a large excess of Ni and carbon
which may be attributed to Ni C impurities. There is little excess
of Ni and C present in the 900 1C-heated sample where there
should be excess Ni and C based on the presence of Ni C and Ni
impurities shown by XRD. The higher carbon content in the
800 1C-heated sample when compared with theoretical values
suggests that a large amount of carbon is adsorbed on the surface
and may partially account for the higher surface area. The slightly
higher carbon content of the 900 1C-heated sample vs the
theoretical values indicates that less carbon is adsorbed on its
surface.
9
00
00
4.5
2.2
o0.06
0.28
66.6
26.6
31.1
60.1
3
8
95.5
13.0
a
b
c
3
Surface areas were determined from BET.
Carbon content for the heat-treated samples was measured at 1350 1C.
The analytical error was 75% for metals.
4. Discussion
The results herein demonstrate that metal carbides can form
when heating acetate gels under only H
with citric acid and ethylene glycol under H
A reaction of Ni- and Mo-acetates with citric acid and ethylene
glycol under similar conditions forms -Ni Mo
C at ꢂ800 1C. The
endotherm at 730 1C for the as-prepared Ni–Mo acetate gel
mixture likely corresponds to a reaction with H
At 900 1C, the as-prepared Ni–Mo acetate gel mixture forms
-Ni Mo C with traces of Ni C, NiC, and Ni, suggesting that the
Ni C was partially converted to Ni and NiC at 900 1C. The presence
of some Ni in the 900 1C-treated sample is consistent with the
decomposition of Ni C nanoparticles to Ni at 900 1C under H [21].
The Ni C and other impurities coexisting with the predominant
Mo C are present as nanograins, as indicated by XRD.
The reaction of Ni- and Mo-acetates predominantly forms
-Ni Mo C at 800 and 900 1C but mostly -Mo C at 600 and
00 1C. No other phases beyond -Mo C, Ni C, NiC, -Ni Mo C,
2
. Heating Mo-acetate
2
creates -Mo C.
b
2
Z
6
6
2
.
Z
6
6
3
3
3
2
3
Z-Ni
6
6
Z
7
6
6
b
3
2
b
2
Z
6
6
and Ni are observed in any of the samples prepared with Ni- and
Mo-acetates. If the endotherm corresponding to -Mo C for-
mation from the Ni–Mo acetate gel mixture is at 475 1C, then it is
b
2
at a lower temperature than the endotherm at 600 1C for
formation using only the Mo-acetate. Formation of the
b
-Mo
2
C
C
b
-Mo
2
phase at this lower temperature in the presence of Ni-acetate
2
+
suggests that Ni ions promote the formation of
The surface area and porosity is clearly dependent on the
reaction temperature. The -Mo C phases resulting from the 600
to 700 1C heat treatments are non-porous with some granularity.
Larger pores formed in the -Ni Mo C at 900 1C than at 800 1C,
2
b-Mo C.
b
2
Z
6
6
according to SEM, but the 800 1C-treated sample has a larger
surface area from its micro and mesopores. When heated to
9
00 1C, these micro and mesopores collapse to give a lower surface
2
ꢀ1
area, less than 10 m g . The decrease in surface area with the
00 1C heat treatment is a result of sintering the -Ni Mo C phase.
9
Z
6
6
Fig. 7. N
2 6 6
-sorption isotherms for the prepared Z-Ni Mo C samples via the Pechini
The type of emitted gases may play a role in the initial formation
of pores during decomposition of organics, but the pore size at
high temperature is mostly because of the sintering process. The
precise identity of the gases has not been determined, but
process with different heat treatments of (A) 800 1C and (B) 900 1C.
2
ꢀ1
1
0 m g . The drastic decrease in surface area can be attributed to
previous work on carbon-supported Mo
2
C suggests that a
the sintering of -Ni Mo C. The uptake of N from the adsorption
Z
6
6
2
combination of CO, CO , H O, and CH is evolved [16]. The
2
2
4
isotherm is greater for the 800 1C-heated than the 900 1C-heated
sample as in Fig. 7. The isotherms indicate greater mesoporosity
for the 800 1C-heated sample. The median pore volume of the 800
and 900 1C-heated samples for pores over 5 nm is 0.05 and
morphology of initial pores is expected to affect the pore size at
higher temperatures. Tailoring the surface of these materials was
not addressed in this study because it is dependent on the final
application.
The high specific surface area of the Z-Ni Mo C sample at
6 6
800 1C shows that the Ni–Mo system has promise in the field
3
ꢀ1
0
.002 cm g , respectively. The broad pore size distribution is
similarly shaped for both materials with a larger pore volume for
the 800 1C-heated sample.
of catalysis. The sorption and surface area measurements

ARTICLE IN PRESS
A.M. Stux et al. / Journal of Solid State Chemistry 181 (2008) 2741–2747
2747
complement the pore size obtained by SEM. Based on our
comparison of synthesis methods, the CO/CO mixture is not the
best process for carbide synthesis, and too high a reaction
temperature leads to a decreased specific surface area.
agreement with other results [13,14,16,21,22], we show that the
synthetic processes can have a large effect on the surface areas
and bulk properties. The catalytic activity of materials prepared by
an optimized process based on the one presented here can be the
subject of a future investigation.
2
The carbon source for the carbides is presumed to be from the
2
precursors. The H gas for the experiment is pure enough that it
cannot be a significant source of the carbon. While the differences
in compositions between the bulk and surface may be unclear, we
can assume, based on the elemental analysis and surface area
results, that at least as much carbon is present closer to the
surface as is in the bulk. The higher carbon content for both 800
5
. Conclusions
Bulk fcc
step Pechini process by heating metal acetate citrate gels with
ethylene glycol under only H . The synthesis of -Ni Mo C and its
surface area are sensitive to temperature, with -Mo C favored at
temperatures of 600 and 700 1C and -Ni Mo C at 800 and 900 1C.
The impurities in -Ni Mo C formed at 900 1C were found to be
Ni, Ni C, and NiC. In the other process presented here using
Ni- and Mo-organics under CO/CO , the carbon source is fixed at
.011 by the gaseous mixture. That we could form the carbides
without a fixed carbon source suggests stability of the -Ni Mo
and -Mo C carbides over a wide range of carbon activities and
Z-Ni Mo C and hcp b-Mo C were synthesized in a one-
6 6 2
and 900 1C heat treatments of
6 6
Z-Ni Mo C when compared with
2
Z
b
6
6
theoretical values implies excess carbon adsorbed to the surface.
Each of the carbon sources in the Pechini process, acetate,
ethylene glycol and citric acid, may play a role in forming
carbides. In the metal acetates before dissolution, the carbon that
is bonded to the oxygen in the acetate moiety is in close proximity
to the metal center. After dissolution of all reactants, the citric acid
forms a chelation complex with the ethylene glycol and the metal
acetates, keeping the metal atoms separated. For the formation of
2
Z
6
6
Z
6
6
3
2
0
Z
6
6
C
b
2
6 6
Z-Ni Mo C, we expect an almost 20:1 M equivalent of carbon to
continued opportunity for the fabrication of high surface area
carbide catalysts using ‘‘green’’ chemistry with few hydrocarbons.
This method shows promise in forming fibers or films.
Mo to form carbon byproducts based on reaction (1), but some of
the excess carbon may remain in solid form and adsorb to the
surface.
Mo
þ 3HOCðCOOHÞðCH
=3 -Ni Mo
ðsÞ þ 39 2=3 gaseous carbon products
As the organic complex is heated, the organics are presumed to
decompose to CO , CO, CH , and H O in the presence of H to
create a reducing, carburizing environment. Based on expected
Pechini chemistry, water is initially removed by heating to form a
viscous gel, Mo and Ni are chelated and polyesterification takes
2
ðOCOCH
3
Þ
4
þ 2NiðOCOCH
3
Þ
2
2
þ 3HOCH
2 2
CH OH
2
COOHÞ
!
Acknowledgments
1
Z
6
6
C
(1)
The authors thank the financial support of the Naval Research
Laboratory through the Office of Naval Research. We are grateful
to our colleagues, Dr. Michelle Johannes, for providing the ball-
2
4
2
2
6 6
and-stick figure of Z-Ni Mo C in Fig. 1 and Drs. Jeffrey Long and
Christopher Chervin for assistance with BET measurements.
place to form a network. Under the flow of H
decomposed and the carbon byproducts, CO, CO
presumed to form. In the sample treated at 900 1C, presence of Ni
and NiC, ostensibly from decomposition of Ni C, may involve
some decomposition to elemental C. If this is the case, carbon
forming from Ni C decomposition cannot be a carbon source to
form CH because -Ni Mo C is formed at a lower temperature
than the formation of Ni C and/or its decomposition. If any CH
2
, this network is
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2
, and CH are
4
[
[
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(
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þ 3HOCðCOOHÞðCH COOHÞ
-Mo
ðsÞ þ 31 gaseous carbon products
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2
3
Þ
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2 2
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2
2
!
(
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(2)
[
[
1
(
Z
2
-Ni
6 6
Mo C in
[
[
[
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