In situ formation of Ni nanoparticles supported on NiFe2O 4 by calcination

Article abstract of DOI:10.1016/j.jssc.2004.05.052

Ni/NiFe₂O₄ composites have been successfully prepared by calcination at 400°C in air of the mixture K₃[Fe(ox) ₃]·2.5H₂O and NiCl₂·6H ₂O with a molar ratio of 0.7-2. The products are featured by the Ni crystallite size of 15-34nm, BET surface area of 23-41m²/g and variable Ni loadings. TGA and FTIR results indicate in situ generation of CO molecules from decomposition of K₃[Fe(ox)₃]·2. 5H₂O. These in situ generated CO molecules account for the formation of Ni nanoparticles by reduction of Ni(II) ions.

Full text of DOI:10.1016/j.jssc.2004.05.052

ARTICLE IN PRESS
Journal of Solid State Chemistry 177 (2004) 4368–4371
Rapid Communication
In situ formation of Ni nanoparticles supported on NiFe O
2
4
by calcination
a,
Fengxi Chen, * Ziyi Zhong, Xiao-Jun Xu, and Yuxiang Chen
a
b
c
a
Department of Appalied Catalysis, Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island 627833, Singapore
Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut, Street, Philadelphia, PA 19104, USA
b
c
Southwest Petroleum Institute, No.8 Xindu Road, Xindu District, Chengdu 610500, China
Received 27 April 2004; received in revised form 26 May 2004; accepted 28 May 2004
Abstract
ꢀ
Ni/NiFe O composites have been successfully prepared by calcination at 400 C in air of the mixture K [Fe(ox) ] ꢁ 2.5H O and
2
4
3
3
2
NiCl
4
2
ꢁ 6H
2
O with a molar ratio of 0.7–2. The products are featured by the Ni crystallite size of 15–34 nm, BET surface area of 23–
1 m /g and variable Ni loadings. TGA and FTIR results indicate in situ generation of CO molecules from decomposition of
2
K [Fe(ox) ] ꢁ 2.5H O. These in situ generated CO molecules account for the formation of Ni nanoparticles by reduction of Ni(II)
3
3
2
ions.
r 2004 Elsevier Inc. All rights reserved.
Keywords: Supported Ni catalysts; Ferrites; Precursor; TG-FTIR; Thermal decomposition
1
. Introduction
[5], the Ni nanoparticles can also be expected to show
the scouting effects before ODNF reacts with CO2.
Particularly, the generation and re-generation of ODF
by H2 can be promoted by supporting fine nickel
particles via hydrogen spillover mechanism. This phe-
nomenon has already been observed in previous studies
CO2 decomposition is an important reaction to
mitigate the global greenhouse effects for environmental
protection. It has been demonstrated that Ni, Zn, Co,
Cu, etc. bearing oxygen deficient ferrites (ODF) are
highly efficient to decompose CO to carbon and oxygen
2
of the redox behaviors of ZrO or its mixed metal oxide
2
with little or no CO as a by-product [1–4]. It was also
reported that the mixed phase comprising a metallic
phase (e.g., a-Fe) and oxygen deficient Ni(II)-bearing
ferrites (ODNF) had a much higher reactivity toward
solid solutions (e.g., CeO –ZrO ) with the supported
2
Ni/NiO nanoparticles [6,7].
The objective of the present study is to prepare such
supported catalysts (i.e., Ni/NFs) by calcination. In
contrast to the previous methods [4,5], which consist of
two consecutive steps (i.e., preparation of NFs followed
2
CO decomposition than the single ODNF phase [5].
2
The a-Fe was formed while reducing the Ni(II)-bearing
ꢀ
ferrites (NiFe O , NF) in the H atmosphere at 300 C.
by H reducing), the calcination method can be carried
2
2
4
2
The higher activity of the mixed phase was assigned to
the scouting effects of the metallic phase prior to the
reaction between CO and ODNF [5].
out in ambient atmosphere, and the metallicand spinel
phases are directly formed.
2
Ni-loading NFs are desirable. Ni nanoparticles may
participate in all three sequences of the CO decom-
2. Experimental
2
position: H reduction to generate ODF, CO decom-
2
2
2.1. Preparation
position on ODF, and re-generation of used ferrites by
H (or methanation) [2]. As in the case of a-Fe/ODNFs
2
All reagents (FeCl ꢁ 6H O, K C O ꢁ H O and
3
2
2
2
4
2
NiCl ꢁ 6H O) are A.R. grade from Aldrich, and used
*
Corresponding author. Fax: +65-6316-6182.
2
2
E-mail address: chen fengxi@ices.a-star.edu.sg (F. Chen).
as received. Potassium iron (III) oxalate (PIO,
0022-4596/$ - see front matter r 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2004.05.052

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F. Chen et al. / Journal of Solid State Chemistry 177 (2004) 4368–4371
4369
1
K [Fe(ox) ]ꢁ 2.5H O) and NiCl ꢁ 6H O are used as iron
PXRD patterns are shown in Figs. 1b–e. Besides those
strong peaks attributable to the NF spinel phase
[JCPDS 86-2267], three extra peaks, indicated by
3
3
2
2
2
and nickel sources, respectively. A typical procedure is
given as follows: PIO was mixed with NiCl ꢁ 6H O at the
2
2
ꢀ
ꢀ
molar ratio, Mr; of PIO to Ni(II) of 0.7–2. The mixture
was ground in a ceramic mortar to get a dough-like
asterisks in Fig. 1b, were observed at 2y ¼ 44:5 ; 51.8
ꢀ
and 76.4 . They can be respectively indexed to the (111),
(200) and (220) reflections of the Ni(cubic) phase
[JCPDS 87-0712]. The Ni crystallite size was estimated
from its (111) peak to be in the range of 15–34 nm. The
ꢀ
precursor, followed by calcination in a 400 C muffle
furnace for 2.5 h. After naturally cooling down to room
temperature (r.t.), the resulting powder was dispersed into
distilled water with the solid to liquid ratio of 1 g/40 mL,
2
BET surface area, SBET; varies between 23 and 41 m /g.
According to the results shown in Table 1, a Ni/NFs
ꢀ
and stirred at 80 C hot plate and 600 rpm for 1 h to remove
2
nanocomposite with SBET of 41 m /g and the highest
the excess salts (e.g., unreacted starting materials). The
final products were recovered by filtration, and dried under
ambient conditions for subsequent characterizations.
loading (see Discussion below) of fine Ni particles (ca.
15 nm) can be formed under optimal conditions (i.e.,
ꢀ
Mr ¼ 2; 400 C, 2.5 h).
ꢀ
ꢀ
The relative peak intensities at 2y ¼ 35:8 ; 43.4 and
2
.2. Characterizations
ꢀ
6
3.0 are obviously intensified than those of the single
NF spinel phase, implying the co-existence of NiO(cu-
bic) [JCPDS 78-0429] in the products. The NiO(cubic)
phase, from which hydrogen has also been observed to
The phases present in the products were identified
from their powder X-ray diffraction (PXRD) patterns,
which were collected on Siemens D5005 with CuKa
˚
spill over into CeO –ZrO supports [6], may result from
2 2
radiation (l ¼ 1:5406 A) operated at 40 mA and 40 kV.
thermal decomposition of the Ni(II) salt, or partial
oxidation of the earlier formed metallicNi parti cl es in
ambient atmosphere. The measured XRD curves cannot
be well fitted with any combination of the spinel and
NiO phases with different weight ratios, suggesting that
other factors such as the distorted spinel lattice may also
contribute to the XRD profiles. In this case, the relative
The crystalline size was calculated using Scherrer
equation, Dh k l ¼ 0:9=Bh k l cos y; where Dh k l is the
crystallite size, l is the incidence wavelength of X-ray
radiation, Bh k l is full-width at half-height of the peak,
and y is the corresponding diffraction angle.
Thermal Analysis was conducted on Universal V2.5H
TA Instruments (Model: SDT 2960) in N2 (purity:
ꢀ
peak intensity of 2y ¼ 63:0 (existing in both the spinel
9
9.9995% from Soxal, flow rate: 90 mL/min). The
ꢀ
and NiO phases) to 2y ¼ 35:8 (unique in the spinel
thermogravimetry was connected with 70 cm stainless
steel tube to Bio-Rad FTIR spectrophotometer (Model:
Excalibur series, FTS 35000ARX), which allows for the
direct detection of the decomposition products during
phase), I
was taken to qualitatively describe the
3=35:8;
6
NiO content in the mixed phases. Similarly, I44:5=35:8 was
used to qualitatively describe the Ni content supported
on the NFs.
TGA. Before starting thermal analysis, the system was
ꢀ
flushed with N . The temperature was dwelled at 100 C
2
ꢀ
for 0.5 h and then increased to 1000 C with the heating
ꢀ
rate of 10 C/min. Thermal degradation profiles (i.e.,
TGA curves) and their first derivatives (differential
thermogravimetric analysis, DTG curves) were recorded
to determine the weight loss over a certain temperature
range and their corresponding peak position.
Adsorption isotherms were measured using nitrogen
gas as the adsorbate on Quantachrome Autosorb-6 at
7
3
7 K. Before measurement, the sample was outgassed at
ꢀ
00 C for 21 h. The BET surface area was calculated
using the BET equation in the range of relative pressures
between 0.04 and 0.4.
3
. Results and discussion
When the procedure described in Section 2.1 is
followed, Ni/NFs composites are prepared. Their
1
Potassium iron (III) oxalate (PIO, K
.5 and 3 depending on the extent of dryness) was prepared by the
3
[Fe(ox)
3
] ꢁ xH
2
O, x is between
Fig. 1. Powder XRD patterns of the products prepared under different
molar ratios of PIO to Ni(II). (a) N; (b) 2.0; (c) 1.3; (d) 1; (e) 0.7; (f)
0.5, and (g) 0. Asterisks in curve b indicate the peaks due to the
presence of the Ni(cubic) phase.
2
standard method and purified by repeated re-crystallization. For more
details, see Ref. [8].

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F. Chen et al. / Journal of Solid State Chemistry 177 (2004) 4368–4371
4370
Table 1
Influence of the molar ratio of PIO to Ni(II) on phase composition of the product
a
Molar ratio, Mr
Phase composition
N
Fe
2
1.3
1
0.7
0.5
0
NiO
O
3 4
NiFe
NiO
Ni
O
2 4
NiFe
NiO
Ni
2
O
4
NiFe
NiO
Ni
2
O
4
NiFe
NiO
2
O
4
NiFe
NiO
2
O
4
a-Fe
Ni
2
O
3
2 3
a-Fe O
Crystallite size of Ni (nm)
—
15
30
33
34
—
—
—
—
35
I63=35:8
0.77
—
1.48
0.17
41
1.66
0.19
31
2.07
0.14
35
2.63
0.10
23
2.80
—
I44:5=35:8
2
SBET (m /g)
61
14
a
ꢀ
Reaction parameters: 400 C, staticair, 2.5 h.
Table 2
Influence of temperature on phase composition of the product
a
ꢀ
Temperature ( C)
Phase composition
300
400
500
600
700
800
Fe(Ni)O
NiFe
NiO
2
O
4
NiFe
NiO
Ni
2
O
4
NiFe
NiO
2
O
4
NiFe
NiO
2
O
4
NiFe
NiO
2
O
4
I63=35:8
1.22
1.48
1.67
1.74
3.76
N
a
Reaction parameters: molar ratio of PIO to Ni(II) of 2, static air, 2.5 h.
The value of Mr and the calcination temperature
ꢀ
100
1.2
1.0
0.8
a
affect the phase composition of the product. At 400 C,
magnetite (Fe O , JCPDS 88-0866) or maghemite (g-
90
80
70
60
50
In 99.9995% N2
3
4
Fe O , JCPDS 25-1402) is formed from pure PIO. With
2
3
b
the addition of Ni(II), three features can be found from
Table 1. Firstly, I63=35:8 gradually increases with the
0.6
0.4
0.2
decrease of M ; implying the increase of NiO content.
r
Secondly, the metallic Ni phase appears with almost
constant content (I44:5=35:8: ca. 0.20) when Mr is 1.3–2.
However, its crystallite size increases from 15 to 34 nm
with the Ni(II) addition. Lastly, the metallicNi phase
gradually disappears with more Ni(II) addition (i.e.,
0.0
1000
ꢀ
0
200
400
600
800
Mro0:7). Instead, some new peaks (e.g., 2y 33.2 ) occur
Temperature (°C)
indicating the formation of a-Fe O [hematite, JCPDS
2
3
Fig. 2. Thermal analysis results of PIO in N
2
. (a) TGA curve and (b)
8
7-1166]. At the fixed M of 2, Ni nanoparticles are
r
ꢀ
DTG curve.
formed only at 400 C. The NiO formation is promoted
with increasing the calcination temperature, as shown by
the increase of the I63=35:8 value in Table 2. A FeO–NiO
Table 3
TGA results of PIO in 99.9995% N
2
ꢀ
solid solution is formed above 800 C.
Thermal analysis results of K [Fe(ox) ] ꢁ 2.5H O in
Peak position
ꢀ
76
98
292 455 550
780
2.9
3
3
2
(
C)
Fig. 2 and Table 3 shed some insight into mechanistic
aspects about the in situ formation of the Ni nanopar-
ticles. TGA curve in Fig. 2 includes PIO dehydration
Weight loss Experimental 76
(wt%)
98
9.7 13.7 8.4
23.2
4CO
ꢀ
before 100 C and decomposition of anhydrous PIO later
Calculated
9.2
2.5 H
11.2
2
3/4 (CO+CO )
Assignment
2
O
on. During dehydration, 2.5-coordinated water mole-
cules (ca. 9.2 wt%) are evolved. Two steps are involved
at early stages of decomposition of anhydrous PIO:
ꢀ
evolution of the CO molecules between 292 C and
ꢀ
Some evidences can be found from in situ FTIR
spectra (Fig. 3) supporting the generation of the CO and
CO molecules during PIO decomposition in N . Two
4
55 C and then the mixture of CO and CO between
2
ꢀ
ꢀ
ꢀ
550 C and 780 C. Weight loss above 800 C indicates
further pyrolysis of the K–Fe–C–O residues.
2
2
À1
peaks at 2324 and 2358 cm in Figs. 3b and c are due to

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F. Chen et al. / Journal of Solid State Chemistry 177 (2004) 4368–4371
4371
etc.) supported on the spinel phases; providing the
decomposition rate of the organic components and the
reducing rate of the targeted metal ions are delicately
balanced in kinetics.
(
a)
b)
(
4. Conclusions
(c)
Starting from iron(III) oxalate complex and Ni(II)
salt, the Ni/NFs nanocomposites are successfully pre-
pared with Ni crystallite size of 15–34 nm, SBET of 23–
4
2
1 m /g and various Ni loadings by calcination at 400 C
ꢀ
in ambient atmosphere. Thermal analyses show that the
in situ generated CO molecules from the decomposition
of the oxalate ligands are responsible for the formation
of metallicNi phase.
5
00
1000
1500
2000
2500
3000
3500
4000
-
1
Wavenumber (cm )
Fig. 3. FTIR spectra of decomposition products from TGA analyses
ꢀ ꢀ ꢀ
2
in N . (a) 100–150 C, (b) 220–320 C and (c) 370–470 C. The peak
marked by an arrow in curve c is due to the presence of CO.
Acknowledgments
the presence of CO in the decomposition products. A
2
small peak at ca. 2200 cm in Fig. 3c indicates the
FX Chen acknowledges the financial support by ICES
in-house project (project code: ICES/03-112002).
À1
presence of CO [9,10]. The ‘‘negative’’ transmittance in
the range of the water band for all curves in Fig. 3
indicates that the recorded amount of water is smaller
than in background spectrum. It should be related to
incomplete flushing of the TGA-FTIR system before the
collection of the background spectrum, or condensation
of the water product in the duct where it is carried from
TGA chamber to FTIR. (Note: relative humidity in
Singapore is quite high, usually 70–90%.) The lack of the
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2
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(
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(e.g.,
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6
8
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¨
¨
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2
4
(
phases, although the CO formation was also suggested
during thermal decomposition of the organic components.
The principle of this work should be applicable to a
wide range of metallicphases (e.g., Fe, Co, Cu, Cd, Zn,
[
[
˜