Study of the CO2 decomposition over doped Ni-ferrites

Article abstract of DOI:10.1016/j.jpcs.2007.02.022

The H₂ reduced NiFe_2-xCr_xO₄ can be used to decompose CO₂ to C repeatedly. A series of nanocrystalline Ni-ferrite doping different contents of Cr³⁺ were synthesized by mixed ions co-precipitation method and characterized by XRD, BET and TEM. The results showed that their crystallite sizes were 1-2 nm and BET surface area changed from 220 to 285 m²/g. The evaluation of the activity and stability indicated that Ni-ferrite with 4 wt% Cr³⁺ dopant could be used repeatedly as many as 60 times and was transformed to Fe_yNi_1-y (05C₂ gradually during the cycle decomposition of CO₂ to carbon, especially for no Cr³⁺ sample. After the 60th reaction, although NiFe₂O₄ phase just remained 2.1 wt%, the decomposition activity of Ni-ferrite with 4 wt% Cr³⁺ was still 60% of initial activity. This fact suggests that nanocrystalline Fe_yNi_1-y (02. The results from scanning electron microscopy (SEM), TEM and XRD show that the deposited carbon from CO₂ decomposition consisted of amorphous, crystallite and carbon nanotubes.

Full text of DOI:10.1016/j.jpcs.2007.02.022

ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 68 (2007) 1330–1335
www.elsevier.com/locate/jpcs
Study of the CO decomposition over doped Ni-ferrites
2
a
Ling Juan Ma , Lin Shen Chen , Song Ying Chen
b,
Ã
a
a
Institute of Catalysis, Zhejiang University (Xixi Campus), Hangzhou 310028, China
b
Measurement and Analysis Center, Zhejiang University (Xixi Campus), Hangzhou 310028, China
Received 21 August 2006; received in revised form 12 February 2007; accepted 12 February 2007
Abstract
The H reduced NiFe Cr O can be used to decompose CO to C repeatedly. A series of nanocrystalline Ni-ferrite doping different
2
2
2ꢀx
were synthesized by mixed ions co-precipitation method and characterized by XRD, BET and TEM. The results
x
4
3+
contents of Cr
showed that their crystallite sizes were 1–2 nm and BET surface area changed from 220 to 285 m /g. The evaluation of the activity and
2
3
+
stability indicated that Ni-ferrite with 4 wt% Cr
Fe gradually during the cycle decomposition of CO
Ni1ꢀy (0oyo1) alloy and Fe
0th reaction, although NiFe phase just remained 2.1 wt%, the decomposition activity of Ni-ferrite with 4 wt% Cr was still 60% of
initial activity. This fact suggests that nanocrystalline Fe Ni
dopant could be used repeatedly as many as 60 times and was transformed to
3
+
y
5
C
2
2
to carbon, especially for no Cr sample. After the
3
+
6
2 4
O
(0oyo1) alloy from the cycle reaction can contribute to the
y
1ꢀy
decomposition of CO . The results from scanning electron microscopy (SEM), TEM and XRD show that the deposited carbon from CO
decomposition consisted of amorphous, crystallite and carbon nanotubes.
r 2007 Elsevier Ltd. All rights reserved.
2
2
Keywords: A. Inorganic compounds; C. Electron microscopy; C. X-ray diffraction; D. Microstructrure
1
. Introduction
as follows:
MFe O þ H ! MFe O
þ H O;
(1)
(2)
2
4
2
2
4ꢀd
2
The cycles of CO to O in atmosphere are very essential
2
2
for survival of plants as well as animals. If people are
forced to work and live in a space that is airtight such as
space shuttle, spaceport or submarine cabin, O2 is
necessary for life sustaining. A major problem in the space
is how to clear away CO and supplement O consumed.
MFe O
2
þ CO ! C þ MFe O ,
4ꢀd
2
2
4
electrolysis
H2O ꢀ! H2 þ O2.
In reaction (1), MFe O is activated or reduced to
(3)
2
4
2
2
MFe O
2
by H and water is produced. In reaction (2),
2
Obviously, one of the most efficient methods is to
decompose CO to C and O . If we can utilize CO to
produce O by a series chemical reaction, it would be very
4ꢀd
CO2 is decomposed to carbon by oxygen-deficient
2
2
2
MFe O derived from the reaction (1). At the same
2
4ꢀd
time, MFe O captures the oxygen from CO2 and
2
useful in dealing with the CO as a ‘green house’ gas.
2
2
4ꢀd
transforms itself to MFe O . In the third step, water
In the early 1990s, it was reported [1] that CO could be
2
2
4
produced in reaction (1) is electrolyzed to O and H , where
2
reduced to C efficiently nearly 100% at 290 1C on the
oxygen-deficient magnetite. Subsequently, some research-
ers [2–6] found that the oxygen-deficient spinel structure
2
H is cycled to activate MFe O again. A large number of
2
2
4
bivalent metal ferrites (MFe O ; M ¼ Fe, Ni, Co, Cu, Zn,
2
4
Mg and Mn) have been studied. In our previous study [7],
it was found that pure NiFe O had a very good initial
MFe O
2
(M ¼ transition metals) could also decompose
4ꢀd
CO to C at about 300 1C by 100%. The cycle decomposi-
2
4
2
activity but almost lost its all activity after 18 reaction
cycles. On the other hand, for NiFe O sample doped
tion of CO to O including a series of chemical reactions is
2
2
2
4
3
+
Ã
proper content of Cr , the repeat reaction numbers could
reach 35 times and total deposited carbon was about
25.3%.
Corresponding author. Tel.: +86 571 88273191;
fax: +86 571 88273690.
E-mail address: chenls@zju.edu.cn (L.S. Chen).
0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jpcs.2007.02.022

ARTICLE IN PRESS
L.J. Ma et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1330–1335
1331
2
Fig. 1. Apparatus for CO decomposition.
It is well known that carbon nanotubes can be grown
from the catalytic decomposition of hydrocarbon over
small metal particles such as iron, cobalt, nickel and some
of their alloys [8–11]. Recently, it was reported that carbon
per step. Rietveld refinements were performed on a PC
using JADE 6.0 software. The crystallite size was obtained
using Scherrer formula. The TEM pictures were obtained
from the JEM-2010(HR). The surfaces of the specimens
were examined using a SIRION scanning electron micro-
scopy (SEM). The BET specific surface area was deter-
mined by N2 adsorption at 77 K using a Coulter
OMNISORP-100 instrument.
nanotubes could be formed from CO decomposition for
2
freshly reduced CuFe O at 400–500 1C of temperature
2
4
[12], however, the morphology of the deposited carbon has
not been investigated in detail yet.
In this paper, the activity and microstructure of NiFe O
2
4
3
+
doping different amounts of Cr
as well as the
2.3. Decomposition of CO2
morphology of carbon deposited on the catalyst surface
are studied systematically. The number of reaction cycles
The reaction cycle, which involves CO decomposition to
2
(
the amount of decomposed CO is as high as 63 mmol/g if
reactions (1) and (2) above) can reach above 60 times and
C on NiFe2 Cr O
ꢀx
and reduction of NiFe Cr O to
4ꢀd 2ꢀx x 4
x
2
NiFe Cr O
2ꢀx
system shown in Fig. 1. 2.0 g catalyst sample was placed in
x
4ꢀd
by H , was conducted in a reaction
2
3
NiFe O is doped with 4 wt% Cr . Some of the deposited
+
2
4
carbon after 60 cycle reaction was found to look like
carbon nanotubes.
a quartz tube reactor (V ¼ 47 ml) heated with an electric
2
furnace. After evacuation, H gas was fed and passed over
2
the samples for 2 h at a flow rate of 60 ml/min at 310 1C.
After the catalyst was activated and the reactor was again
2
. Experimental
evacuated, pure CO gas was fed into reaction system. The
2
2
.1. Samples preparation
quantity of the CO initially injected to the reaction cell
2
(n1) was estimated by the change of the pressure gauge 1,
NiSO ꢁ 6H O, Fe (SO ) ꢁ xH O, Cr(NO ) ꢁ 9H O and
using the following equation: n ¼ DPV=RT, where
DP ¼ P –P , P and P2 are initial and final pressures
4
2
2
4 3
2
3 3
2
NaOH (A.R.) were used as starting materials. NiFe O was
2
4
+
1
1
2
1
2
prepared by mixed ions co-precipitation from [Ni ] ¼
read from pressure gauge 1, respectively, V flask volume
(V ¼ 340 ml), T reactor temperature and R gas constant.
3
+
1
ratio of Ni and Fe
.0 M, [Fe ] ¼ 1.2 M, [NaOH] ¼ 3.0 M, where the molar
1
2
+
3+
was 1:2. Before precipitating, the
The quantity of CO remained after decomposition (n )
2
2
two metal ion solutions were mixed thoroughly. Then, the
mixture was precipitated with NaOH keeping the tempera-
ture in the range 50–70 1C and at pH 11–14. The
suspension was filtered and washed carefully until all
was estimated by DP the change of the pressure gauge 2,
2
using the same equation. The amounts of decomposed CO2
equal to (n –n ).
1
2
The procedure is repeated again until the goal required is
reached or the activity of CO2 decomposition loses
completely.
ꢀ
2ꢀ
NO and SO4 was removed. The precipitate was dried at
3
1
Cr -doped NiFe O was prepared using the above
20–150 1C and calcined at 350 1C for 3 h in air. Similarly,
3+
2
4
procedures.
.2. Characterization
XRD data for Rietveld analysis were obtained with a
3. Results and discussion
3
+
2
3.1. Characterization of NiFe O doping different Cr
2 4
Figs. 2 and 3 show the XRD profiles of NiFe O with
2
4
3
+
Rigaku PC 2550 X-ray powder diffractometer, using Cu
K radiation and a power of 40 kV ꢂ 300 mA a diffracting
different contents of Cr (0, 2, 4, 6 and 8 wt%) calcined at
350 and 600 1C, respectively. Obviously, the shapes of the
diffraction peaks in the two figures are quite different. The
peaks are more dispersive for samples calcined at 350 1C
a
beam graphic monochrometer over the range 2y ¼ 15–1301
with a step interval of 2y ¼ 0.021 and a count time of 1 s

ARTICLE IN PRESS
L.J. Ma et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1330–1335
1332
Table 1
1
1
500
200
3+
2 4
The cell parameter and BET area of NiFe O doped with different Cr
3
+
Cr content
2 wt%
240
8
wt%Cr3+
6wt%Cr3+
0
4 wt%
258
6 wt%
272
8 wt%
285
9
6
3
00
00
00
2
BET area [m /g (350 1C)] 220
Cell parameter [nm
600 1C)]
0.83395(2) 0.83394(1) 0.83375(2) 0.83317(3) 0.83245(2)
4
2
wt%Cr3+
wt%Cr3+
(
0
wt%Cr3+
2
During the reduction by H (reaction (1)), O at surface
ꢀ
2
2
0
30
40
50
60
70
and those migrated to the surface can react with the
adsorbed H to form H O which leave oxygen empty sites
in their structures. In order to keep the electrical neutrality
2
θ (°)
2
2
3+
Fig. 2. XRD profiles of NiFe O with different contents of Cr calcined
2 4
3
+
2+
3+
at 350 1C.
of the compound, Fe
are reduced to Fe
when Fe
accept electrons released by the reaction. As a result,
NiFe O is transformed to oxygen-deficient structure
2
4
2
+
NiFe O . Subsequently, Fe
in NiFe O
is oxidized
4ꢀd
by CO and itself is reduced to C (reaction (2)). At the same
2
4ꢀd
2
3
3
2
2
1
1
500
000
500
000
500
000
2
2
ꢀ
time, O fill the empty sites formed by H reduction.
2
Then, oxygen-deficient NiFe O
is gradually returned to
the normal NiFe O . Expectedly, if there is over-reduction
2
4ꢀd
3
3
+
+
2
4
8
6
wt%Cr
wt%Cr
such as reduction at too high temperature, by too high H2
pressure and lasting too long time, metal Ni and Fe rather
than oxygen-deficient NiFe O
2
spinel structure will be destroyed as follows:
will be produced and
4ꢀd
3
3
+
+
4
2
wt%Cr
wt%Cr
NiFe2O4 þ H2 ! FeyNi1ꢀyð0oyo1Þ þ H2O;
(4)
(5)
500
3
+
0
wt%Cr
Fe Ni ð0oyo1Þ þ CO ! Fe O þ Fe C ðNiÞ.
y
1ꢀy
2
3
4
5
2
0
2
+
20
30
40
50
θ (°)
60
70
80
Ni metal appears first since Ni
3
is easier to reduce
than Fe . On the other hand, Ni metal has strong
+
2
ability to adsorb and dissociate H , which can promote
2
3
+
2 4
Fig. 3. XRD profiles of NiFe O with different contents of Cr calcined
3
+
the reduction of some amount of Fe to form metal Fe.
Then, Fe Ni
(0oyo1) alloy is formed. Certainly,
at 600 1C.
y
1ꢀy
the crystallite size of Fe Ni
(0oyo1) could be very
1ꢀy
y
small in initial stage. Additionally, experimental results
also indicated that the small Fe Ni
(0oyo1) crystals
(
become wider with the increase of Cr
Fig. 2) than for those calcined at 600 1C (Fig. 3) and
3+
contents. The
y
1ꢀy
have the ability to react with CO to produce iron carbide
results indicate that their crystallite sizes are smaller for the
samples calcined at lower temperature and decrease with
2
such as Fe C . Therefore, with the increase of reaction
5
2
3
the increase of Cr dopant.
+
cycle number, the content of NiFe O phase decreases
2
4
gradually accompanying the increase of the content
and crystallite size of Fe C and Fe Ni
crystalline phase. As a result, CO decomposition activity
goes down gradually since the carbide has no activity to
decompose CO2.
The BET specific surface area and cell parameter of
3
+
(0oyo1)
1ꢀy
samples with various Cr
dopants are given in Table 1.
5
2
y
3
+
Clearly, with the increasing of the Cr dopant, the BET
area of samples (calcined at 350 1C) increases and the cell
parameter (calcined at 600 1C) decreases. It has been
2
3
+
reported that Cr
and should be located in octahedral site (B site) [13].
Being AB O type of spinel structure, the oxygen
has entered into the cell of NiFe O
2 4
3.2. CO decomposition activity
2
2
4
closed packing of NiFe O4 is fcc. A view perpen-
2
Fig. 5 shows the curves of decomposed CO vs. number
2
dicular to the close-packed layers (the octahedral or
2
of cycle reaction for NiFe O and NiFe
Cr0.08O4.
2ꢀ0.08
Obviously, NiFe O has higher initial CO decomposition
2
4
+
(
the Fe are located in octahedral sites (B site), the other
1 1 1) face) is shown in Fig. 4. Ni
+
and a half of
2 4 2
3
activity, but decreases rapidly. While for NiFe2
Cr0.08O , there is still about 60% of the initial activity at
ꢀ0.08
3
+
half of Fe
are located in tetrahedral sites (A site).
4
Octahedral and tetrahedral cations are in green and purple,
respectively.
the 60th reaction cycle. The total amount of decomposed
CO is as high as 63 mmol/g.
2

ARTICLE IN PRESS
L.J. Ma et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1330–1335
1333
Fe(Ni)(B site)
O
Fe(A site)
H2
H O
2
C
CO2
Fig. 4. A view perpendicular to the close-packed layers (the octahedral or (1 1 1) face).
3
2
2
1
1
0000
5000
0000
5000
0000
a
b
3
3
2
2
1
.5
.0
.5
.0
.5
I(obs)
I(cal)
I(dif)
C
FeNi alloy
Fe C
5
2
NiFe O4
2
5000
0
20
30 40 50 60 70 80 90 100 110 120 130
θ (°)
2
0
10
20
30
40
50
60
Number of cycle
4
Fig. 6. XRD Rietveld pattern of NiFe2ꢀ0.08Cr0.08O after 60 reaction
cycles. I (obs): observation; I (cal): calculation; I (dif): difference.
2
Fig. 5. Quantity amount of decomposed CO vs. number of cycle: (a)
NiFe2ꢀ0.08Cr0.08
O
4
2 4
and (b) NiFe O .
NiFe O is almost transformed to Fe Ni (0oyo1) alloy
1ꢀy
2
4
y
In the XRD pattern, as shown in Fig. 6, in addition to
the carbon peak (JCPDS File no. 50-1083) the small peaks
of NiFe O appear along with the strong peaks of Fe Ni
1ꢀy
and carbide Fe C .
5 2
Figs. 7(a) and (b) are the TEM micrographs of fresh
sample and sample after 60 cycles which look quite
different. The fresh one shows nano-ferrite particles
(Fig. 7(a)), while used one (Fig. 7(b)) shows various
crystallites or particles of deposit carbon ferrites, Fe–Ni
alloy, ferrous carbide and carbon. Figs. 8(a) and 8(b) show
the SEM micrograph and EDS analysis of sample after 60
cycles and indicate that most of the deposited carbon on
2
4
y
(
0oyo1) alloy and carbide. Table 2 lists the crystallite size
and the content of phases obtained from fitting analysis of
all XRD profiles (Rietveld analysis). The crystallite size of
Fe Ni
y
(0oyo1) alloys and carbide is 10 and 9 nm,
1ꢀy
respectively. XRD analysis reveals that after many cycles
the content of NiFe O is just only about 2.1 wt%, and
2
4

ARTICLE IN PRESS
L.J. Ma et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1330–1335
1334
the surface of catalyst was porous amorphous (Fig. 8(a))
which cannot be detected by XRD. EDS analysis confirms
the results of phase composition obtained by Rietveld
analysis. After 60 reaction cycles, spinel phase abundance
position is still about 60% which can come from
nanocrystalline Fe Ni
This fact that NiFe O loses its activity just after 15 cycle
(0oyo1) formed.
y
1ꢀy
2
4
reactions, while NiFe2
Cr0.08O still keeps its activity as
4
ꢀ0.08
high as 60% initial activities after 60 cycles can be
is only about 2.1wt% while the activity of CO decom-
2
3
+
3+
explained as follows. Although both Fe
+3 valence ions, the acidity is stronger for Cr than Fe
and Cr
3
are
3+
+
Table 2
Average crystalline size and phase abundance of NiFe
by virtue of its ability to attract an electron pair since their
˚
ion radii are 0.615 and 0.645 A [14], respectively. Further-
2
O
4
doped 4 wt%
3+
Cr after 60 cycles
more, the strength and stability of Cr–O–Fe covalent bond
are stronger than Fe–O–Fe [15]. When some content of
Crystal phase
3
Cr enter into the bulk structure of NiFe O and replace
+
2
4
Crystal carbon Fe
y
Ni1ꢀy Fe
5
C
2
NiFe
2
O
4
3+
a part of Fe , Cr–O bonds would distribute uniformly in
B sites of the spinel structure and could strengthen the
structure stability of NiFe O . After some reaction cycles,
Crystallite size (nm) 18.6
Phase abundance (wt%) 34.2
10
28.8
9
34.9
/
2.1
2
4
3
+
small amount of Cr
transform to the form of Cr O ,
2 3
R
R
wp
6.62%
4.17%
which is well known as a stabilizer and prevents high
temperature from sintering. The existence of Cr O on
the surface of the samples can also decrease the effect of
exp
2
3
R
wp ¼ weighted residual factor, Rexp ¼ expected residual factor.
Fig. 7. TEM picture of fresh (a) and after 60 cycles (b).
Fig. 8. SEM micrograph (a) and EDS analysis (b) for sample after 60 cycles.

ARTICLE IN PRESS
L.J. Ma et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1330–1335
1335
4. Conclusions
3+
1) When doped with 4 wt% content of Cr , the cycle
(
reaction life can be prolonged as many as 60 times, and
the total amount of decomposed CO reaches more
than 63 mmol/g.
2
(
(
2) It was found that some of the deposited carbon looks
like carbon nanotubes, which formed at 300 1C of low
temperature from CO2.
3) Ferrites have been transformed to Fe Ni
y
(0oyo1)
1ꢀy
alloy and Fe C during cycle decomposition process.
5
2
Fe Ni
y
(0oyo1) alloy can decompose CO to C
1ꢀy
completely.
2
3
4) Cr can enter into the cell of NiFe O and should be
+
(
2 4
located in octahedral site by co-precipitation and
3
+
calcination. With the increase of Cr
surface areas increase and crystallite size decreases.
content BET
Acknowledgment
Fig. 9. TEM picture of deposited carbon.
This work was supported by the National Natural
Science Foundation of China (No. 20277033).
over-reduction as well as the reaction rate of Ni–Fe alloy
and C and the collapsing rate of Ni–Fe particles so that the
cycle reaction life can be prolonged.
3
.3. The morphology of deposited carbon
References
The results in Table 2 indicate that some of the deposited
[
[
1] Y. Tamaura, M. Tabata, Nature 346 (1990) 255.
2] T. Kodama, K. Tominaga, M. Tabata, et al., J. Am. Ceram. Soc. 75
carbon is crystallite. Furthermore, as displayed in Fig. 9,
they look like having the structure of shot carbon
nanotubes. It was estimated from TEM pictures that the
diameters of carbon nanotubes are about 2–30 nm and the
lengths range from 20 to 200 nm. As is well known, the
carbon nanotube has a large potential application in many
fields, such as adsorbent and catalyst support, etc. This is
because of its special properties, high surface area, high
electrical conductivity, as well as excellent adsorption
ability. If it is possible to make carbon nanotubes from
CO2 decomposition, we can simultaneously clear away
CO2 and generate O2 and produce carbon nanotubes,
which can also be used as high-performance adsorbent for
harmful gas in airtight cabins. To the best of our
knowledge, there are few reports about carbon nanotubes
(
1992) 1287.
[3] M. Tabata, Y. Nishida, T. Kodama, et al., J. Mater. Sci. 28 (1993)
71.
4] K. Akanuma, K. Nishizawa, T. Kodama, et al., J. Mater. Sci. 28
1993) 860.
5] C.L. Zhang, S. Li, L.J. Wang, et al., Mater. Chem. Phys. 62 (2000)
4.
[6] C.L. Zhang, S. Li, L.J. Wang, et al., Mater. Chem. Phys. 62 (2000)
2.
9
[
[
(
4
5
[
[
7] M.S. Fu, L.S. Chen, S.Y. Chen, Chem. J. Chin. Univ. 26 (2005) 2279.
8] J. Guinot, M. Audier, M. Coulon, L. Bonnetain, Carbon 19 (1981)
9
[9] T. Baird, J.R. Fryer, B. Grant, Carbon 12 (1974) 591.
5.
[10] E. Boellaard, P.K. Debokx, A.J.H.M. Kock, J.W. Geus, J. Catal. 96
1985) 481.
(
[
[
[
[
11] G.A. Jablonski, F.W. Geurts, A. Sacco, Carbon 30 (1992) 87.
12] M.H. Khedr, A.A. Farghali, Appl. Catal. B—Environ. 61 (2005) 219.
13] D. Kedem, T. Rothem, Phys. Rev. Lett. 38 (1969) 165.
14] The Shannon and Prewitt ionic radii Table.
from CO decomposition at about 300 1C. The formation
2
mechanism has not been understood yet, so more
investigation is required.
[15] H. Topsoe, M. Boudart, J. Catal. 31 (1973) 346.