Synthesis, characterization and activity of alumina-supported cobalt nitride for NO decomposition

Article abstract of DOI:10.1016/j.jssc.2007.06.031

A new type of transition metal nitride, viz. alumina-supported cobalt nitride, was synthesized for the first time by NH₃-temperature-programmed reaction, and its structure was characterized by BET, X-ray diffraction (XRD) and X-ray photoelectron spectroscopic (XPS) techniques. The supported cobalt nitride performs much better than its bulk counterpart for NO decomposition, owing to its small crystal size, high thermal stability and big surface area.

Full text of DOI:10.1016/j.jssc.2007.06.031

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Journal of Solid State Chemistry 180 (2007) 2635–2640
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Synthesis, characterization and activity of alumina-supported
cobalt nitride for NO decomposition
Ã
Zhiwei Yaoa, Aimin Zhua,b, Jing Chena, Xinkui Wanga, C.T. Auc, Chuan Shia,
aLaboratory of Plasma Physical Chemistry, Dalian University of Technology, Dalian 116024, China
bState Key Laboratory of Material Modification by Ion, Electron and Laser Beams, Dalian University of Technology, Dalian 116024, China
cDepartment of Chemistry, Center for Surface Analysis and Research, Hong Kong Baptist University, Kowloon Tong, Hong Kong
Received 22 January 2007; received in revised form 30 May 2007; accepted 4 June 2007
Available online 21 July 2007
Abstract
A new type of transition metal nitride, viz. alumina-supported cobalt nitride, was synthesized for the first time by NH3-temperature-
programmed reaction, and its structure was characterized by BET, X-ray diffraction (XRD) and X-ray photoelectron spectroscopic
(XPS) techniques. The supported cobalt nitride performs much better than its bulk counterpart for NO decomposition, owing to its small
crystal size, high thermal stability and big surface area.
r 2007 Published by Elsevier Inc.
Keywords: Alumina-supported cobalt nitride; Thermal stability; NO decomposition
1. Introduction
synthesis at 623 K under 0.1 MPa. Although activity was
high at the initial stage, there was deactivation due to Re3N
decomposition [16]. For the utilization of nitrides of Group
VII–VIII transition metals, a means of enhancing the
thermal stability remains as a meaningful challenge in the
field.
In this study, the supported cobalt nitride catalyst was
synthesized according to the following strategy: (i) to
enlarge the surface area of the catalyst by dispersing cobalt
nitride on g-Al2O3, (ii) to generate nanosized cobalt nitride
particles, and (iii) to enhance the thermal stability of cobalt
nitride by the interaction with g-Al2O3. The reaction of NO
direct decomposition was selected as a probe to study the
catalytic properties of the alumina-supported cobalt nitride
material.
Due to the fact that transition metal nitrides possess
chemical properties similar to those of noble metals, they
have received much attention. Various kinds of transition
metal nitrides such as monometallic VN, Mo2N, W2N
[1–3], and bimetallic such as Co3Mo3N, Ni2Mo3N,
Fe3Mo3N [4], as well as supported nitrides Mo2N/
g-Al2O3, Co3Mo3N/g-Al2O3 and Ni2Mo3N/g-Al2O3 [5,6]
have been synthesized. The application of these materials
has been found in catalytic reactions such as NH3 synthesis
[7], NO removal [8–12], hydrogenation (HYN), hydro-
desulfurization (HDS), hydrodenitrogenation (HDN) and
hydrodeoxygenation (HDO) [13]. Compared to the nitrides
of Group V–VI metals (e.g., V, Mo and W), the nitrides of
Fe, Co and Re (Group VII–VIII) have received far less
attention due to their poor thermal stability [14]. The
results of earlier observation indicated that Co4N was
unstable and undergoing a stepwise decomposition with N2
evolution below 700 1C: Co4N-Co3N+Co, Co3N-
Co2N+Co, Co2N-CoN+Co [15]. Kojima and Aika
reported the use of Re3N as catalysts for ammonia
2. Experimental
The Co3O4/g-Al2O3 precursor with Co loading of
30 wt% was prepared by method of wetness incipient
impregnation via stirring g-Al2O3 (SBETE250 m2/g, 40–60
mesh) in an aqueous cobalt nitrate (Co(NO3)2 ꢀ 6H2O)
solution, followed by drying at 120 1C for 12 h and
calcination at 500 1C for 3 h. The bulk cobalt nitride and
Ã
Corresponding author.
E-mail address: chuanshi@dlut.edu.cn (C. Shi).
0022-4596/$ - see front matter r 2007 Published by Elsevier Inc.
doi:10.1016/j.jssc.2007.06.031

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g-Al2O3 supported cobalt nitride catalysts were generated
by means of NH3-temperature-programmed reaction.
Typically, about 2.0 g of the oxide precursor was placed
in a microreactor and a flow of NH3 (150 ml/min) was
introduced into the system. Initially, the sample was
linearly heated from room temperature (RT) to 300 1C
over a period of 30 min, followed by a rise in temperature
from 300 to 450 1C at a rate of 0.67 1C/min, and a further
increase from 450 to 700 1C at a rate of 1.67 1C/min. The
temperature was kept at 700 1C for 2 h before cooling to
RT in a NH3 flow. The material was then purged with N2
for 10 min followed by passivation in 1% O2/99% N2 for
12 h. The cobalt metal and g-Al2O3 supported cobalt metal
catalysts were prepared from their corresponding oxide
precursors using H2 at 700 1C for 2 h. The material was also
passivated in 1% O2/99% N2 for 12 h before it was exposed
to air.
higher surface area is obtained for the alumina-supported
cobalt nitride compared with that of Co4N. The surface
area of the sample is in reasonable agreement with that
estimated from the average crystal size.
Fig. 1 shows the XRD patterns of bulk Co4N and Co4N/
g-Al2O3, as well as those of Co metal and g-Al2O3,
respectively. The XRD patterns of bulk Co4N and Co4N/
g-Al2O3 are consistent with those reported in the literature
for pure Co4N [17]. Nevertheless, compared to those of
pure g-Al2O3 the peaks of g-Al2O3 in Co4N/g-Al2O3 shift to
lower 2y angle. It indicates that there is significant
interaction between Co4N and g-Al2O3. Fang et al.
reported that the grain structure of Co4N is similar to that
of Co, which might easily lead to misinterpretation [15].
We also found that it is impossible to differentiate Co4N
from Co because the XRD pattern of Co3O4 reduced in H2
at 700 1C for 2 h looks exactly the same as that of Co4N
(Fig. 1).
In order to confirm the formation of Co4N during Co3O4
nitridation, temperature-programmed decomposition ex-
periments were conducted. There are clear discrepancies
between the decomposition profiles of Co4N and Co4N/
g-Al2O3 as shown in Fig. 2. Over the Co4N sample, we
detected an intense N2 peak at 620 1C and there was no
obvious desorption of NH3 and H2O in the entire
temperature range adopted for the study; this is a clear
indication that the N2 originated from the decomposition
of Co4N. In the case of Co4N/g-Al2O3, the N2 peak was at
420 1C and there was desorption of NH3 and H2O within
the 200–600 1C range. The former was strong whereas the
The BET surface areas of the passivated samples were
measured on an ASAP 2010 instrument. The N2 gas was
adopted for standard five-point BET surface area measure-
ments. X-ray diffraction (XRD) examination was performed
using an X-ray diffractometer (Rigaku D-Max Rotaflex)
˚
with CuKa radiation (l ¼ 1.5404 A). The particle size was
estimated according to the Scherrer formula. The tempera-
ture-programmed decomposition of the as-prepared nitrides
was examined on a flow system equipped with a mass
spectrometer (MS, HP G1800A). X-ray photoelectron
spectroscopic (XPS) investigation was conducted using an
AlKa source operated at 10 kV and 15 mA (Leybold
Heraeus-Shengyang SKL-12 with VG CLAM 4 MCD
analyzer); charging effects were corrected by means of adv-
entitious carbon (284.6eV) referencing.
The catalytic activity was evaluated using a quartz
microreactor (i.d. 4 mm). The temperature was measured
with a thermocouple placed adjacent to the catalyst outside
the reactor. A reaction gas mixture composed of 1000 ppm
NO and He as balance was fed through the catalyst (0.4 g)
which had been pretreated in pure He at 400 1C for 1 h. The
effluent gases were monitored by means of online GC with
a TCD detector. A molecular sieve 5A column was used to
separate H2, O2, N2 and NO. The amount of N2 produced
was used to calculate the conversion of NO to N2.
Co4N
Co
Co4N/γ-Al2O3
3. Results and discussion
γ-Al2O3
Table 1 summarizes the phase, specific surface area and
crystal size of bulk and alumina-supported cobalt nitride
samples prepared by ammonolysis of the corresponding
oxide precursors. It can be observed that significantly
20
40
60
80
100
2θ / °
Fig. 1. XRD patterns of Co4N, Co, Co4N/g-Al2O3 and g-Al2O3.
Table 1
Phase, BET surface areas and crystal size of catalysts
Metal oxide precursor
Phase identified after being nitrided in NH3
Surface area of nitride (m2 gꢁ1
)
Crystal size of the nitrided phase (nm)
Co3O4
Co3O4/g-Al2O3
Co4N
Co4N/g-Al2O3
1.3
148.4
25
14

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2637
designed for hydrogen detection. Nonetheless, the detec-
tion of the weak H2O desorption signal suggests that there
was only a small amount of oxygen on the surface of
Co4N/g-Al2O3 and there was NHx (x ¼ 1, 2, 3) dissocia-
tion. In addition, our NH3-TPD results (not shown) on
Co4N/g-Al2O3 revealed that the temperature (480 1C) for
N2 desorption in NH3-TPD investigation is close to that
detected in temperature-programmed decomposition reac-
tion (Fig. 2(b)). If one could accept that the N2 peak at
420 1C originated from NHx (x ¼ 1, 2, 3), the Co4N formed
on g-Al2O3 should be thermally stable and have kept its
structure reasonably well up to a temperature as high as
950 1C.
a
NH3(m/e=17)
620°C
N2(m/e=28)
The XPS Co 2p spectra of Co4N and Co4N/g-Al2O3 as
well as that of Co metal are shown in Fig. 3. The relative
intensities of spin–orbit doublet peaks are given by the
H2O(m/e=18)*10
200
ratio of their respective degeneracy, and the I(2p3/2)/I(2p1/2
)
0
400
600
800
1000
intensity ratio for the Co(2p3/2)/Co(2p1/2) doublet is 2/1. A
splitting energy of 15.2 eV is expected for the doublet. By
means of curve fitting, the cobalt oxidation states and the
corresponding distribution of cobalt species are estimated
(Table 2). As shown in Fig. 3(a), the binding energies of Co
2p1/2 and Co 2p3/2 for Co4N are 796.3 and 781.1 eV,
respectively. The value of Co 2p3/2 is consistent with that
reported by Milad et al. [17]. The value is higher than that
of Co2+ (779.970.4 eV) but slightly lower than that of
Co3+ (781.670.3 eV) [19]. Accordingly, we denote the Co
species of Co4N as Cod+, where 2odo3. For a Co sample
obtained via H2-treating a cobalt oxide sample after
passivation, there are two states of surface Co. The peaks
at binding energy of 777.9 and 793.1 eV are assigned to
Co0, and those at 780.5 and 795.7 eV, to Co3O4 (Fig. 3(d))
[20]. The results of XPS investigation indicated that of
metal cobalt and cobalt oxide coexist on the H2-reduced
cobalt oxide sample. As for the passivated Co4N/g-Al2O3
sample (Fig. 3(b)), the peaks at binding energy of 781.6 and
796.8 eV are closer to those of Co4N (781.1 and 796.3 eV)
in comparison to that of Co0 (778.0 and 793.0 eV) [20]. The
results indicated that the cobalt oxide precursor on g-Al2O3
was converted to Co4N but not Co0 during the ammono-
lysis process. In view of point that N2 peak shown in
Fig. 2(b) could be a result of Co4N decomposition, a
portion of the Co4N/g-Al2O3 sample was pretreated in a
flow of He at 500 1C for 3 h to eliminate surface NHx
(x ¼ 1, 2, 3). The Co 2p spectrum of the treated sample
(Fig. 3(c)) is similar to that of the un-treated one
(Fig. 3(b)). The results inferred that there was Co4N on
g-Al2O3 after the N2 desorption at 420 1C (Fig. 2(b)). In
other words, the evolution of N2 originated from NHx
(x ¼ 1, 2, 3) decomposition rather than from thermal
decomposition of Co4N.
Temperature /°C
b
420°C
N2(m/e=28)
NH3(m/e=17)
H2O(m/e=18)*10
600°C
200°C
0
200
400
600
800
1000
Temperature /°C
Fig. 2. Temperature-programmed decomposition profiles of (a) Co4N and
(b) Co4N/g-Al2O3 samples.
latter was weak. According to the work of Nagai et al. [18],
there was the formation of NHx (x ¼ 1, 2, 3) species on the
surface of supported nitride catalyst during cooling to RT
under NH3 after nitridation, and the adsorbed NHx (x ¼ 1,
2, 3) decomposed via sequential dehydrogenation, giving
ultimately N2 and H2. Thus, it is reasonable to deduce that
there were weakly and strongly adsorbed NHx (x ¼ 1, 2, 3)
species on Co4N/g-Al2O3 and the weakly adsorbed NHx
(x ¼ 1, 2, 3) desorbed as NH3 (g). It is not possible to be
confirmative whether the N2 peak at 420 1C is due to
decomposition of Co4N or dissociation of strongly
adsorbed NHx (x ¼ 1, 2, 3). The hydrogen originated
from NHx (x ¼ 1, 2, 3) decomposition desorbed as H2 (g)
or reacted with surface oxygen to give water. We could not
monitor H2 desorption because the MS equipment was not
The catalytic activity of Co4N and Co4N/g-Al2O3
catalysts for NO decomposition are shown in Fig. 4. It
can be observed that the effective temperature range for
NO decomposition is 200–450 1C for Co4N and 100–700 1C
for Co4N/g-Al2O3. In the cases of Cu-ZSM-5 and Pd/
Al2O3, the effective temperatures for high NO decomposition

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a
c
781.1
Co2p
Co2p
781.4
796.3
796.6
770
775
780
785
790
795
800
805
810
770
775
780
785
790
795
800
805
810
Binding Energy / eV
Binding Energy / eV
b
d
780.5
Co2p
Co2p
795.7
781.6
796.8
793.1
777.9
770
775
780
785
790
795
800
805
810
770
775
780
785
790
795
800
805
810
Binding Energy / eV
Binding Energy / eV
Fig. 3. Co 2p spectra of (a) Co4N, (b) Co4N/g-Al2O3, (c) Co4N/g-Al2O3 (treated in He at 500 1C for 3 h) and (d) Co in XPS study.
were 400–550 1C and above 500 1C, respectively, as re-
ported by Haneda et al. [21]. Compared to Cu-ZSM-5 and
Table 2
XPS parameters derived from curve-fitting the Co 2p3/2 envelope
Pd/Al2O3, the bulk and supported cobalt nitride catalysts
of the present study showed a wider temperature window
and a lower temperature of initial activity for NO
decomposition, and the Co4N/g-Al2O3 catalyst was of
higher catalyst activity, than the Co4N within the
temperature range of 100–250 1C. In addition, at tempera-
ture higher than 450 1C, the Co4N/g-Al2O3 catalyst
remained active and stable while the Co4N catalyst started
Catalyst
Co2+ or Co3+
B.E. (eV)
Cod+(2odo3)
Coo B.E.
(eV)
B.E. (eV)
Co4N
781.1
781.6
781.4
Co4N/g-Al2O3
Co4N/g-Al2O3 (500 1C
pretreated)
Co
780.5
777.9

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Co4N/γ-Al2O3
Co4N/γ-Al2O3
100
80
60
40
20
0
100
80
60
40
20
0
Co4N
Co/γ-Al2O3
100
200
300
400
500
600
700
0
5
10
15
20
25
30
Temperature /°C
Time/ h
Fig. 4. Variation of NO conversion to N2 as a function of reaction
temperature over Co4N and Co4N/g-Al2O3 catalysts. Reaction conditions:
NO ¼ 1000 ppm, gas flow rate ¼ 20 cm3/min, W/F ¼ 1.2 g/scm3.
Fig. 6. Time dependence of NO decomposition to N2 over Co4N/g-Al2O3
and Co/g-Al2O3 catalysts at 600 1C (reaction conditions: same as those of
Fig. 4).
of Table 1 into account, the activity of the two cobalt
nitride catalysts can be related to their surface area and
particle size. The thermal stability of cobalt nitride is
enhanced by having the nanoparticles dispersed on
g-Al2O3, and according to the XRD results, the dispersed
particles are stabilized via strong interaction between Co4N
and g-Al2O3. Fig. 6 shows the time dependence of NO
conversion to N2 over Co4N/g-Al2O3 and Co/g-Al2O3 at
600 1C. It can be seen that NO conversion was ca. 100%
throughout the entire period of 30 h over Co4N/g-Al2O3. It
is a clear indication that the alumina-supported cobalt
nitride is thermally and catalytically stable for NO
removal. Over Co/g-Al2O3, although a high NO conversion
of ca. 100% was attained, fast deactivation occurred after
20 h of on-stream time. As reported by Xiao et al. [22],
heavy accumulation of surface oxygen on well-dispersed
Co caused the deactivation of catalyst in NO decomposi-
tion. We take that the significant difference in activity
observed over the two samples is another piece of evidence
to show that the Co4N/g-Al2O3 synthesized is indeed
Co4N/g-Al2O3 rather than Co/g-Al2O3.
Co4N/γ-Al2O3
100
80
60
Co4N
40
20
0
0
5
10
15
20
25
30
Time/ h
Fig. 5. Time dependence of NO decomposition to N2 over Co4N and
Co4N/g-Al2O3 catalysts at 400 1C (reaction conditions: same as those of
Fig. 4).
4. Conclusions
to decompose with the evolution of N2 (dashed line in
Fig. 4). The stability of Co4N and Co4N/g-Al2O3 catalysts
for NO decomposition are shown in Fig. 5. It can be
observed that the Co4N catalyst showed an initial activity
of ca. 100% but deactivated after 10 h of on-stream
reaction: NO conversion to N2 decreased from ca. 100%
to ca. 51% within a period of 17 h. As for the Co4N/
g-Al2O3 catalyst, NO conversion to N2 stayed at ca. 100%
throughout the entire test period of 30 h. Taking the results
In conclusion, despite it is not possible to confirm by
means of XRD investigation whether cobalt oxide con-
verted to Co4N or reduced to Co metal in the ammonolysis
process, the results of XPS and temperature-programmed
decomposition investigation over bulk Co4N and Co4N/
g-Al2O3 provide proofs of the formation of Co4N on
g-Al2O3 after the nitridation of the Co3O4/g-Al2O3
precursor. Furthermore, the significant difference in
activity between Co4N/g-Al2O3 and Co/g-Al2O3 provided

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2640
supplementary evidence that the Co4N/g-Al2O3 synthesized
is indeed Co4N/g-Al2O3. It was observed that due to the
nanosize and good dispersion, the supported Co4N exhibits
thermal stability and surface area significantly higher than
that of bulk Co4N. As a consequence, the former showed
much better activity and stability for NO decomposition in
the temperature range 100–700 1C. It was noteworthy that
Co4N particles can also be stabilized by having them
dispersed on other supporters such as HZSM-5, SiO2,
CeO2 and so on. Comprehensive studies for the interaction
between metal nitrides and support materials might
provide new insights that could lead to enhanced thermal
stability of nitrides of Group VII–VIII transition metals.
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Acknowledgments
The work was supported by the National Natural
Science Foundation of China (No. 20573014) and the
Natural Science Foundation of Liao-ning Province
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