Mechanistic aspects of the role of chelating agents in enhancing Fischer-Tropsch synthesis activity of Co/SiO2 catalyst: Importance of specific interaction of Co with chelate complex during calcination

Article abstract of DOI:10.1016/j.jcat.2012.02.003

Co/SiO₂ catalysts (20 mass%) with enhanced activities for Fischer-Tropsch synthesis (FTS) were prepared by a newly developed two-step impregnation method using several chelating agents, and the effects of the calcination temperature and chelating agent on their FTS activities were investigated to clarify the role of the chelating agent during preparation of the catalyst. Their effects upon coordination environments of Co species were also studied by in situ Co K-edge quick XAFS in conjunction with ex situ characterization techniques. The FTS activity of the catalysts increased with the calcination temperature, 473-543 K, when the chelating agents with strong affinity to Co²⁺ such as trans-1,2-diaminocyclohexane-N,N,N′, N′-tetraacetic acid (CyDTA) were used for preparation. Such increase in the activity was accompanied with size reduction of Co species, while those in Co/SiO₂ prepared in the absence of the chelating agents were simply agglomerated during calcination. In situ quick XAFS suggested that size reduction of Co species was associated with specific interaction between small Co oxide clusters and chelate complexes during calcination. It was also speculated that these complexes were converted to surface silicate species after combustion of carbonaceous moieties which would work as anchoring sites of Co oxide clusters, preventing agglomeration of small Co₃O₄ species originated from the clusters during calcination at high temperatures.

Full text of DOI:10.1016/j.jcat.2012.02.003

Journal of Catalysis 289 (2012) 151–163
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Mechanistic aspects of the role of chelating agents in enhancing Fischer–Tropsch
synthesis activity of Co/SiO2 catalyst: Importance of specific interaction of Co
with chelate complex during calcination
a
a
a
a
Naoto Koizumi a, , Yukiya Ibi , Daichi Hongo , Yusuke Hamabe , Shigenobu Suzuki ,
⇑
Yasuhiko Hayasaka a, Takayoshi Shindo b, Muneyoshi Yamada c
a Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, Aoba 6-6-07, Aramaki, Aoba-ku, Sendai 980-8579, Japan
b Department of Engineering in Applied Chemistry, Graduate School of Engineering and Resource Science, Akita University, 1-1 Tegatagakuen-machi, Akita 010-8502, Japan
c Akita National College of Technology, 1-1, Iijima-Bunkyo-cho, Akita 011-8511, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Co/SiO2 catalysts (20 mass%) with enhanced activities for Fischer–Tropsch synthesis (FTS) were prepared
by a newly developed two-step impregnation method using several chelating agents, and the effects of the
calcination temperature and chelating agent on their FTS activities were investigated to clarify the role of
the chelating agent during preparation of the catalyst. Their effects upon coordination environments of Co
species were also studied by in situ Co K-edge quick XAFS in conjunction with ex situ characterization
techniques. The FTS activity of the catalysts increased with the calcination temperature, 473–543 K, when
the chelating agents with strong affinity to Co2+ such as trans-1,2-diaminocyclohexane-N,N,N0,N0-tetraace-
tic acid (CyDTA) were used for preparation. Such increase in the activity was accompanied with size reduc-
tion of Co species, while those in Co/SiO2 prepared in the absence of the chelating agents were simply
agglomerated during calcination. In situ quick XAFS suggested that size reduction of Co species was asso-
ciated with specific interaction between small Co oxide clusters and chelate complexes during calcination.
It was also speculated that these complexes were converted to surface silicate species after combustion of
carbonaceous moieties which would work as anchoring sites of Co oxide clusters, preventing agglomera-
tion of small Co3O4 species originated from the clusters during calcination at high temperatures.
Ó 2012 Elsevier Inc. All rights reserved.
Received 14 November 2011
Revised 9 February 2012
Accepted 10 February 2012
Available online 16 March 2012
Keywords:
Co/SiO2 catalyst
Fischer–Tropsch synthesis
Chelating agent
CyDTA
Quick XAFS
1. Introduction
per second) increases with decreasing size of the Co0 particles at
least up to 10 nm on conventional metal oxide support. Therefore,
Co-based catalysts are well known as typical Fischer–Tropsch
synthesis (FTS) catalysts, which synthesize mainly linear paraffins
with a wide range of carbon numbers from CO and H2. Nano-sized
Co0 particles formed on catalyst surface under FTS conditions have
been recognized to be the origin of their catalytic activity for hydro-
carbon formations. Iglesia [1] reported that turnover frequencies
(TOFs) of Co catalysts supported on SiO2, Al2O3, and TiO2 were sim-
ilar and almost independent of size of the Co0 particles when they
were larger than 10 nm. On the other hand, noticeable size effects
of the Co0 particles were found in a small particle range (2–
10 nm) for Co supported on carbon nanofiber [2,3] and ITQ-2 [4].
In these particle ranges, TOFs of these catalysts increased linearly
with increasing Co0 particle size. These studies demonstrated that
the Co-time yield (CTY; mole of CO converted per weight of Co
improvement of Co0 dispersion without loss of reducibility of Co
is important strategy for enhancing their FTS activities.
Co-based FTS catalysts are usually prepared by impregnation of
an aqueous solution containing Co(NO3)2ꢀ6H2O on support materi-
als followed by drying and calcination. During calcination, impreg-
nated Co nitrate is decomposed to form Co3O4 species, and these
precursor oxides are reduced by H2 to the Co0 particles prior to
FTS. Over the last two decades, many efforts have been devoted to
develop an effective preparation method for improving dispersion
of Co0 including, for example, adaptation of calcination procedures
[5–9] and exploring new Co precursors instead of conventional Co
nitrate [10–14]. Several researchers demonstrated that Co0 parti-
cles with smaller sizes were formed when the catalyst was pre-
pared using Co oxalate [10] and a mixed salt of Co nitrate and Co
acetate [11] as precursors. Unfortunately, however, use of these
precursors produced significant amounts of Co silicate-like species
simultaneously, which were hardly reduced to the metallic state
under normal conditions. Therefore, reducibility of Co was sup-
pressed greatly, resulting in only weak activity enhancements. In
⇑
Corresponding author. Present address: Earth and Mineral Sciences Energy
Institute, The Pennsylvania State University, C-211 Coal Utilization Laboratory,
University Park, PA 16802, USA.
E-mail address: nxk20@psu.edu (N. Koizumi).
0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2012.02.003

152
N. Koizumi et al. / Journal of Catalysis 289 (2012) 151–163
other words, trade-off relationships between dispersion of Co0 and
reducibility of Co were reported in these previous works. It has been
suggested that the formation of Co silicate-like species is associated
with strong interaction between Co precursors and surface hydroxy
groups during impregnation based on characterizations of the cat-
alysts with low Co loadings [12–14].
of its potential usefulness, however, in situ Co K-edge QXAFS of
the chelate-assisted Co catalysts has never been reported yet.
In this work, therefore, 20 mass% Co/SiO2 catalysts were pre-
pared by the newly developed two-step impregnation method
using several chelating agents, and the effects of the calcination
temperature and chelating agent on their FTS activities were inves-
tigated under pressurized conditions to deepen our understanding
of the role of the chelating agent. Their effects upon coordination
environments of Co species were also studied by in situ QXAFS in
conjunction with several ex situ characterization techniques
including XRD, XPS, FT-IR, and conventional XAFS. Based on these
results, mechanism for enhancing the FTS activity of Co/SiO2 in-
duced by the chelating agents is discussed.
On the other hand, some chelating agents are known to improve
activities of Co- and Ni-based catalysts, which are typically used for
hydrodesulfurization of oil fractions in their sulfided forms [15–
25]. In our previous works, these chelating agents were applied
to preparation of Co/SiO2 catalysts with enhanced FTS activities,
in which the catalyst was prepared by impregnation of the aqueous
Co nitrate solution containing one chelating agent (L; L–Co2+
= 1 mol molꢁ1) selected from those with different complex forma-
tion constants (KCo) followed by drying and calcination [26–29].
The FTS activity of these chelate-assisted catalysts showed a vol-
cano-type dependency upon the log(KCo) of the chelating agents,
and the maximum activity was attained by the catalyst prepared
using the chelating agent with medium affinity to Co2+, namely
nitrilotriacetic acid (NTA). The rate of CO conversion over the
NTA-modified catalyst was three times as high as that over the cat-
alyst prepared without using the chelating agents. Furthermore,
use of NTA improved dispersion of Co0 by a factor of 3, whereas
it hardly affected high reducibility of Co [27,29].
In spite of unique advantages of the co-impregnation method
using the chelating agents, however, Co loading of the catalyst
was limited to maximum of 5 mass% due to low solubility of the
chelating agent in the presence of Co nitrate. To increase Co load-
ing, a two-step impregnation method was newly developed in
which the catalyst was prepared by consecutive impregnation-dry-
ing steps using two kinds of impregnation solutions by turns; one
was an aqueous solution containing a chelating agent and the
other was an aqueous solution of Co nitrate, followed by calcina-
tion of the dried catalyst. By this two-step impregnation method,
Co loading of the catalyst was increased successfully up to
20 mass%, and their FTS activity was enhanced by 2- to 3-folds
compared with that of the catalyst prepared in the absence of
the chelating agent [30–33]. Interestingly, trans-1,2-diaminocyclo-
hexane-N,N,N0,N0-tetraacetic acid (CyDTA) with much higher affin-
ity to Co2+ than NTA was more effective for enhancing the FTS
activity in the two-step impregnation method. Besides, under con-
stant Co loading of 20 mass%, the FTS activity of the catalysts pre-
pared using CyDTA increased sharply with increasing CyDTA–Co2+
molar ratio from 0.06 to 0.25 and leveled off above these ratios
[30,32]. Therefore, it was suggested that only a part of Co2+ in
the catalysts participated in the complex formation with CyDTA,
which was associated with such large activity enhancement caused
by CyDTA. In other words, surplus Co would remain as Co nitrate,
and Co nitrate and the complex would coexist in the dried catalyst,
namely before calcination. These results imply importance of the
chemistries involved in drying and calcination of the catalyst for
activity enhancement caused by CyDTA [33]; however, details of
the mechanism have not been figured out yet.
2. Experimental
2.1. Preparation of catalysts
The catalysts studied in this work were prepared by the two-
step impregnation technique. Details of the preparation procedures
are reported elsewhere [30,32,33]. Briefly, SiO2 powder (BET sur-
face area: 224 m2 gꢁ1, average pore diameter: 15 nm, pore volume:
1.24 mL gꢁ1, particle size: 150–250 ꢂ 10ꢁ6 m) was impregnated
with an aqueous solution containing one chelating agent (L) se-
lected from those listed in Table 1 followed by pre-drying at
333 K for 2 h under flowing dry air. This sample was then dried at
393 K and 3 h in an electric oven under static air. The dried sample
was subsequently impregnated with an aqueous solution contain-
ing Co(NO3)2ꢀ6H2O (Wako Pure Chemicals, purity > 99.5%) followed
by the drying procedures under the same conditions. The sample
was finally calcined in an electric furnace at 723 K for 4 h with a
ramp rate of 1.0 K minꢁ1 under stagnant air conditions. Co loading
of the catalysts was 20 mass% (SiO2 weight basis), and the L–Co mo-
lar ratio was fixed at 0.25. The catalysts were also prepared by the
co-impregnation method in accordance with the procedures re-
ported previously (Co loading = 5 mass%, L–Co2+ molar ratio = 1)
[26,27]. Hereafter, these catalysts are simply denoted as Co/L/SiO2
(stepwise impregnation) and L–Co/SiO2 (co-impregnation), respec-
tively. Note that the ‘‘dried’’ and ‘‘calcined’’ catalysts in this work
are defined as the catalysts that underwent the 1st and 2nd impreg-
nation-drying cycles, and 1st and 2nd impregnation-drying cycles
followed by calcination, respectively.
2.2. Activity test
The FTS activity and selectivity of the catalysts were studied un-
der pressurized conditions using a high-pressure fixed-bed flow
reactor system. The reactor consisted of a stainless steel tube with
internal diameter of 7 mm and was placed in an electrically heated
oven. The gases, H2 (purity > 99.995%) and 33 vol.% CO/62 vol.% H2/
5 vol.% Ar (purity > 99.99995%), were used without further purifi-
cation. The flow rate and pressure of these gases were regulated
with mass flow controllers and a backpressure regulator. Typically,
0.1 g of the calcined catalyst mixed with glass beads was charged
into the reactor and then reduced in a stream of H2 under 773 K
Co K-edge XAFS is a powerful tool for investigating the specia-
tion and coordination environments of Co species in the Co-based
FTS catalysts because they usually lack in long-range order. In pre-
vious works, Co K-edge XAFS was mainly applied to structural
analysis of oxidized, reduced, and used catalysts regarding the pre-
cursor effects [34,35], support effects [36–38], promoter effects
[35,37,39,40] and deactivation mechanism of the catalysts [41–
43], while only a few papers have reported Co K-edge XAFS of
the catalysts during each process of catalyst preparation [35,44].
In our recent work, in situ Co K-edge quick XAFS (QXAFS) coupled
with temperature-programed oxidation (TPO) successfully dis-
closed the role of triethylene glycol (TEG) during preparation of
TEG-modified Co/SiO2 with enhanced FTS activities [44]. In spite
Table 1
Chelating agents used for preparation of the catalyst and their logarithmic complex
formation constants (logKCo).
Chelating agent
LogKCo
Iminodiacetic acid (IDA)
Nitrilotriacetic acid (NTA)
Ethylenediamine-N,N,N0,N0-tetraacetic acid (EDTA)
Triethylenetetramine-N,N,N0,N00,N000,N000-hexacetic acid (TTHA)
Trans-1,2-diaminocyclohexane-N,N,N0,N0-tetraacetic acid (CyDTA)
6.95
10.38
16.31
17.10
18.92

N. Koizumi et al. / Journal of Catalysis 289 (2012) 151–163
153
for 6 h. After H2 reduction, the catalyst was allowed to cool down
2.6. FT-IR
to ambient temperature. The feed gas CO/H2/Ar then flowed into
the system at the pressure of 1.1 MPa, and the catalyst was heated
to 503 K at ramp rates of 2.5 K minꢁ1 to 473 K followed by
1.0 K minꢁ1 to 503 K for the activity evaluation (W/F = 1.25 g-cat
h molꢁ1). Gaseous products were periodically sampled with com-
puter-controlled gas samplers and analyzed by two on-line GCs
after the reaction temperature reached 503 K. An on-line GC/TCD
(Shimadzu, GC-8A) was used for analysis of CO, CO2, and CH4, while
another on-line GC/FID (Shimadzu, GC-2014) was used for analysis
of C1–C7 hydrocarbons. Ar was used as an internal standard for GC/
TCD analysis. Liquid products were collected with an ice trap and
analyzed by an off-line GC/FID (Agilent Technologies, Agilent
6850A) after synthesis. In this work, the FTS activity of the cata-
lysts was expressed as the Co-time yield (CTY; mole of CO con-
verted per g-Co per second). Since the catalysts studied in this
work showed practically constant CO conversions above 7 h on
stream, the CTYs were calculated using the conversions obtained
at 7 h on stream. The product selectivity was calculated based on
molar amounts of the products accumulated during 20 h on
stream. The selectivity for hydrocarbon with carbon number n
(Cn selectivity) was defined on a carbon molar basis as indicated
below (Yn = molar yield of hydrocarbon).
Interaction between Co and the chelating agent in dried and cal-
cined catalysts was studied by FT-IR. The sample was diluted 100-
folds with KBr powder and pressed into self-supporting wafer.
FT-IR spectra were acquired using a Varian FTS-6000 FT-IR spec-
trometer in a transmission mode with spectral resolution and
accumulation of 4 cmꢁ1 and 1024, respectively, under ambient
conditions. The observed FT-IR spectrum was deconvoluted with
Gauss functions using IGOR Pro software (Wave Metrics).
2.7. XAFS measurements
Co K-edge XAFS of the catalysts were measured at BL14B2 in
SPring-8 synchrotron radiation facility (Harima, Japan) with ring
energy of 8 GeV by quick XAFS mode in a transmission setup.
The X-rays passed through a Si(111) double-crystal monochroma-
tor and focused onto the sample. The EXAFS data were collected in
a transmission mode using I0 and I ionization chambers filled with
100% N2 and 15% Ar/N2, respectively.
For ex situ measurements, the appropriate amount of the dried
and calcined samples were diluted 10-folds with polyethylene gly-
col powder using a mortar and pestle and then pressed into self-
,
!
supporting wafer (
the sample was adjusted so that
u
10 mm) at 25 MPa for 3 min. The amount of
t of wafer fell within the range
X
D
t
Cn selectivity ¼ nYn
nYn ꢂ 100
ð1Þ
n
of 0.7–1.0. XAFS measurements were carried out under ambient
temperature with 100 s dwell time unless otherwise stated. In
some experiments, the spectra were acquired at 17–20 K using a
He cryostat to improve the S–N ratio of EXAFS oscillation. Some
reference Co compounds including polycrystalline Co(NO3)2ꢀ6H2O,
2.3. H2 chemisorption and TPR measurements
Effects of the calcination temperature and chelating agent on
size of the Co0 particles formed in the reduced catalyst were stud-
ied by H2 chemisorption and TPR. Diameter of Co0 particles was
calculated assuming a hemispherical morphology of metallic parti-
cles and 1:1 hydrogen chemisorption on surface metallic Co atom.
Details of the procedures and the measurement conditions were
reported elsewhere [27,29].
Co3O4, Co(C2O4)ꢀ2H2O and
a-Co2SiO4 prepared from stoichiometric
mixture of Co3O4 and SiO2 were also used for XAFS measurements
under ambient conditions.
On the other hand, in situ QXAFS coupled with TPO was used to
monitor change in coordination structures of Co during calcination.
The dried sample was mixed with the equivalent amount of boron
nitride powder and then pressed into self-supporting wafer (
u
10 mm) under medium pressure (12 MPa, 2 min). Thickness of this
sample wafer was below 1 mm, which is indispensable for reduc-
ing diffusion resistance of gases into wafer. Sample wafer was
placed in an in situ XAFS cell made of quartz, and then the cell
was connected with a flow system equipped with mass flow con-
trollers. The sample was heated from 373 to 623 K at a ramp rate
of 1 K minꢁ1 followed by 10 K minꢁ1 in the range of 623–723 K un-
der flowing 20% O2/He (99.99995%, approximately 100 mL minꢁ1).
XAFS spectra were acquired at every 180 s. Hereafter, the XAFS
spectrum measured at T means that the spectrum was accumu-
lated during T 1.5 (15) K for T 6 623 K (623 K < T 6 723 K). Tail
gas from the in situ cell was analyzed simultaneously with an
on-line MS spectrometer during in situ QXAFS experiments.
2.4. XRD
XRD measurements were carried out using a MiniFlex powder
X-ray diffractometer (Rigaku). CuK
a radiation was used as the X-
ray source with the X-ray tube operating at 30 kV and 15 mA.
The observed diffraction peaks were assigned by referring to Joint
Committee on Powder Diffraction Standards (JCPDS) data. The
crystallite size of Co3O4 species was calculated by Debye–Scherrer
Eq. (2) using (311) diffraction peak of Co3O4.
Dhkl ¼ khklk=bhkl cos h
In this equation, k is the wavelength of the X-rays (k = 0.15405 nm),
and bhkl is the full width at half maximum (FWHM) of the diffraction
peak. The shape factor (khkl) was equal to 0.9 in this work.
ð2Þ
2.8. XAFS analysis
2.5. XPS
The observed Co K-edge XAFS were analyzed in a conventional
manner including background subtraction and normalization fol-
lowed by Fourier filtering using a Rigaku XAFS data analysis system
(REX2000). Contributions from coordination shells in the Fourier
XPS was used to study the effect of the chelating agent on size
(dispersion) of Co species formed in the dried and calcined samples.
The sample was mixed with boron nitride powder (Wako Pure
Chemicals purity > 99.5%) and pressed into self-supporting wafer.
XPS measurements were carried out on a Scienta ESCA200 spec-
transformed k3
v(k) were then inverse Fourier transformed with a
Hanning-type window function into k space. Structural parameters
of each coordination shell were determined by a non-linear least-
squares fitting in k space. Backscattering amplitude and phase shift
of a Co–O coordination shell were extracted from Fourier trans-
trometer with monochromatic AlKa line as an excited source. The
pass energy and energy step were 150 eV and 0.05 eV, respectively,
for all experiments. Charge shift was corrected using B1s binding
energy (190.1 eV) of boron nitride. Background was subtracted
from the spectrum using Shirley equation. The obtained spectrum
was further deconvoluted using Gaussian functions to quantify
chemical species present in subsurface region.
formed k3
v
(k) of Co(NO3)2ꢀ6H2O. Those of Co–O and Co–C coordi-
nation shells calculated by FEFF8.4 code [45] using the
Co(C2O4)ꢀ2H2O structure were also used in some fitting analysis.
Details of curve fitting analysis were reported elsewhere [44].

154
N. Koizumi et al. / Journal of Catalysis 289 (2012) 151–163
Pattern-fitting analysis was also carried out for Co K-edge XANES of
the catalysts to identify Co species in the catalysts. The R-factor de-
fined by the following Eq. (3) was used to evaluate the quality of
the fitting.
enhancement caused by the chelating agents. The increase in the
CTY implies that preparation of Co/SiO2 using the chelating agents
with the larger complex formation constants enhances dispersion
of Co during calcination, leading to the higher FTS activities after
H2 reduction.
Fig. 2 compares CH4 and C5+ selectivities over Co/SiO2 and
Co/CyDTA/SiO2 calcined at different temperatures. The C5+ selectiv-
ity over Co/CyDTA/SiO2 was lower than that over Co/SiO2 when the
catalysts were calcined at 523 K and lower temperatures. However,
it increased with increasing the calcination temperature and
reached 84 C-mol% at 723 K, which was comparable to that over
the catalyst prepared in the absence of the chelating agent. Con-
versely, the CH4 selectivity over Co/CyDTA/SiO2 decreased slightly
with the calcination temperature and finally was about 10 C-mol%.
h
.
i
X
X
2
2
Rf ð%Þ ¼
ðIobs ꢁ Ical
Þ
ðIobs
Þ
ꢂ 100
ð3Þ
3. Results
3.1. Effect of calcination temperature on FTS activity
The effect of the calcination temperature on the FTS activity of
the catalysts prepared using several chelating agents was investi-
gated in order to deepen our understanding of the origin of the
promoting effects induced by the chelating agents. In this experi-
ment, dried Co/L/SiO2 was calcined in an electric furnace at a ramp
rate of 1 K minꢁ1. Once furnace temperature reached a desired cal-
cination temperature, the calcined sample was allowed to cool
down to ambient temperature. The FTS activity of the calcined
sample was evaluated at 503 K and 1.1 MPa after H2 reduction at
773 K.
3.2. Impact of the calcination temperature on size of Co species
The catalysts calcined at different temperatures were then ana-
lyzed by ex situ XRD and XPS to confirm the effect of the chelating
agents on size (dispersion) of Co species formed in the calcined cat-
alyst. Co/NTA/SiO2 and Co/CyDTA/SiO2 were chosen as the chelate-
assisted catalysts for these experiments because of their typical
dependencies of the FTS activities upon the calcination temperature.
Fig. 1 illustrates CTYs over the catalysts prepared using the che-
lating agents including IDA, NTA, EDTA, TTHA, and CyDTA as a
function of the calcination temperature. The CTY decreased mono-
tonically with an increase in the calcination temperature when the
catalyst was prepared without using the chelating agent, namely
Co/SiO2. Such dependency upon the calcination temperature is
similar to those reported previously [46–48], indicating that Co
species are agglomerated to form large particles during calcination.
On the other hand, the CTYs over Co/IDA/SiO2 and Co/NTA/SiO2
were fairly constant and independent of the calcination tempera-
ture in the range studied here. These catalysts showed higher
activities than Co/SiO2 when calcined at 723 K. Furthermore, the
CTYs increased almost linearly with increasing the calcination tem-
perature in the range of 473–543 K when EDTA, TTHA, and CyDTA
were used for preparation of the catalyst. These results clearly
show that high calcination temperatures (>473 K) are crucial for
large activity enhancement induced by these chelating agents.
Conversely, the FTS activities of uncalcined Co/L/SiO2 were rather
lower than that of Co/SiO2. It is also stressed that the CTYs over
Co/L/SiO2 (L = EDTA, TTHA, and CyDTA) fall on the same line
regardless of the structure of the chelating agents. Since these che-
lating agents have larger complex formation constants with Co2+
(logKCo P 16) than others, strong affinity of the chelating agent
with Co2+ is another important factor for large activity
3.2.1. Ex situ XRD
Fig. 3A–C shows the effect of the calcination temperature on
XRD patterns of Co/SiO2, Co/NTA/SiO2, and Co/CyDTA/SiO2. These
figures also include the XRD pattern of polycrystalline Co3O4 as a
reference. No diffraction peaks were observed in the XRD pattern
of Co/SiO2 without calcination, while several diffraction peaks orig-
inated from Co3O4 species emerged when it was calcined at the
temperatures higher than 473 K. Crystallite size of Co3O4 species
calculated by Debye–Scherrer equation increased from 29 nm to
39 nm with increasing the calcination temperature as tabulated
in Table 2. Similarly, only the diffraction peaks of Co3O4 species
were observed in the XRD patterns of calcined Co/NTA/SiO2. How-
ever, crystallite size of Co3O4 species in this catalyst was smaller
than those in Co/SiO2 regardless of the calcination temperature
and was fairly constant in the temperature range studied here
(see Table 2). Furthermore, the diffraction peaks of Co3O4 were
barely observed in the patterns of Co/CyDTA/SiO2 only when it
was calcined at 523 K and 723 K. Crystallite size of Co3O4 species
in the catalyst calcined at 523 K was no more than 7 nm and in-
creased by only 2 nm even after calcination at 723 K as summa-
rized in Table 2. It is noted that the catalysts studied in this work
were calcined under stagnant air conditions, which resulted in
30
30
(a)
(b)
25
25
20
15
10
5
20
15
10
5
0
373
0
373
473
573
673
773
473
573
673
773
Calcination temperature / K
Calcination temperature / K
Fig. 1. Importance of the calcination temperature for enhancing the Co-time yield (CTY) over Co/SiO2 prepared by a two-step impregnation method using chelating agents
with various complex formation constants (see Table 1): Co/SiO2 (open diamond), Co/IDA/SiO2 (ꢂ), Co/NTA/SiO2 (+), Co/EDTA/SiO2 (closed circle), Co/TTHA/SiO2 (closed
triangle), Co/CyDTA/SiO2 (closed square) FTS: 503 K, 1.1 MPa, W/F = 1.25 g-cat h molꢁ1
.

N. Koizumi et al. / Journal of Catalysis 289 (2012) 151–163
155
100
80
60
40
20
0
I(Co2p3/2)/I(Si2p) for Co/SiO2, Co/NTA/SiO2, and Co/CyDTA/SiO2
are plotted as a function of the calcination temperature. As ex-
pected from the XRD results, the I(Co2p3/2)/I(Si2p) for calcined
Co/SiO2 decreased with increasing the calcination temperature
from 473 K to 523 K which indicates that Co species are agglomer-
ated to form large particles in this temperature range. On the other
hand, the I(Co2p3/2)/I(Si2p) for Co/L/SiO2 (L = NTA, CyDTA) changed
in different ways during calcination, that is, the I(Co2p3/2)/I(Si2p)
was almost constant and independent of the calcination tempera-
ture when NTA was used for preparation. More interestingly, the
I(Co2p3/2)/I(Si2p) for calcined Co/CyDTA/SiO2 increased sharply
with increasing the calcination temperature from 453 to 523 K
and then turned to slight decrease by further increase of the calci-
nation temperature. This sharp increase in the I(Co2p3/2)/I(Si2p)
suggests that relatively large Co species in dried Co/CyDTA/SiO2
turn into smaller ones during calcination. It is also highlighted that
the I(Co2p3/2)/I(Si2p) for Co/CyDTA/SiO2 decreases only slightly
even after calcination at high temperatures, 523–723 K. This is
consistent with the ex situ XRD results which showed that crystal-
lite size of Co3O4 species in calcined Co/CyDTA/SiO2 increased by
only 2 nm after calcination at 723 K (see Table 2).
373
473
573
673
773
Calcination temperature /K
Fig. 2. CH4 and C5+ selectivities over Co/SiO2 and Co/CyDTA/SiO2 calcined at
different temperatures: CH4 selectivity (diamond), C5+ selectivity (square), Co/SiO2
(open symbols), Co/CyDTA/SiO2 (closed symbols). Reaction conditions are the same
as those indicated in Fig. 1.
the formation of Co3O4 species with large crystallite size in the ab-
sence of the chelating agents. On the other hand, those with much
smaller size were observed in the catalyst calcined under N2 flow
(ca. 10 nm in 20 mass% Co/Al2O3) [6] or 1% NO/He flow (4–5 nm
in 18 mass% Co/SiO2) [7], respectively. In particular, size of Co3O4
species formed in the latter catalyst is even smaller than those
formed in the calcined catalyst prepared in the presence of CyDTA.
Therefore, optimization of the calcination conditions would be
promising for improving dispersion of Co3O4 species in calcined
Co/CyDTA/SiO2 further, and the effect of the calcination conditions
is now under investigation.
Hydrogen chemisorption capacity and TPR profiles were then
measured on several catalysts, and diameter of the Co0 particles
formed after H2 reduction at 773 K was calculated (see parentheses
in Table 2). Small Co0 particles with diameter of 5 nm were formed
in Co/CyDTA/SiO2 calcined at 523 K and grew slightly when cal-
cined at the higher temperature. Among the catalysts calcined at
723 K, diameter of the Co0 particles decreased in following order,
Co/CyDTA/SiO2 (7 nm) > Co/NTA/SiO2 (13 nm) > Co/SiO2 (30 nm).
These changing behaviors of the I(Co2p3/2)/I(Si2p) for Co/NTA/
SiO2 and Co/CyDTA/SiO2 were mostly consistent with those ob-
served in the FTS activity and crystallite size of Co3O4 species
formed in the corresponding catalysts. In this connection, it is
stressed that the I(Co2p3/2)/I(Si2p) for dried Co/L/SiO2 (L = NTA, Cy-
DTA) were smaller than that for dried Co/SiO2. This indicates that
relatively larger Co species are formed in these chelate-assisted cat-
alysts which would be related with the fact that uncalcined Co/L/
SiO2 showed the lower FTS activities than the catalyst prepared in
the absence of the chelating agent. However, a discrepancy also ar-
ose between crystallite size of Co3O4 species and I(Co2p3/2)/I(Si2p)
for calcined Co/NTA/SiO2; crystallite size of Co3O4 species in Co/
NTA/SiO2 calcined at 473 K was smaller than that in Co/SiO2 cal-
cined at the same temperature, while the I(Co2p3/2)/I(Si2p) for the
former sample was smaller than that for the later one. It is worth
noting that Co/NTA/SiO2 calcined at 473 K was a heterogeneous
mixture of black and purple powder which indicated that crystal-
line Co3O4 species (black powder) coexisted with other non-crystal-
line Co species (purple powder) in this sample. Since only
crystalline species are detected by XRD analysis, different sensitiv-
ities of XRD and XPS could cause such discrepancy mentioned
above.
3.2.2. Ex situ XPS
Since small Co species would escape from XRD analysis, XPS
was also used to evaluate size of Co species in the dried and cal-
cined catalysts. After deconvolution of the ex situ XPS spectrum,
the integrated intensity ratio of Co2p3/2 to Si2p, hereafter simply
denoted as I(Co2p3/2)/I(Si2p), was calculated and used for investi-
gating change in size of Co species during calcination. In Fig. 4,
3.3. Interaction between Co and chelating agent during calcination
Mechanistic aspects of the roles of the chelating agents (NTA
and CyDTA) during calcination were studied in view of interaction
(e)
(d)
(c)
(b)
(e)
(e)
(d)
(c)
103 counts
103 counts
103 counts
(d)
(c)
(b)
(b)
(a)
(a)
(a)
(A)
(B)
(C)
20
40
60
80
20
40
60
80
20
40
60
80
2 theta / deg.
2 theta / deg.
2 theta / deg.
Fig. 3. Effect of the calcination temperature on ex situ XRD patterns of Co/SiO2 (A), Co/NTA/SiO2 (B) and Co/CyDTA/SiO2 (C). Diffraction peaks of Co3O4 are indicated by open
circle: Polycrystalline Co3O4 (a), after drying at 383 K (b), after calcination at 473 K (c), 523 K (d) and 723 K (e).

156
N. Koizumi et al. / Journal of Catalysis 289 (2012) 151–163
and Co2+ in dried Co/NTA/SiO2 is similar to that of the [CoL]ꢁ com-
plex in the aqueous solution. This interaction was preserved during
calcination up to 493 K. Above this calcination temperature, the
band at 1590 cmꢁ1 disappeared from the FT-IR spectrum, indicat-
ing decomposition of the complex through oxidation and combus-
tion of NTA ligand during calcination at the high temperatures.
Table 2
Crystallite size of Co3O4 species formed in the calcined catalysts and diameter of Co0
particles formed after H2 reduction at 773 K.
Crystallite sizea (diameterb) (nm)
Calcination temperature (K)
473
523
723
Co/SiO2
Co/NTA/SiO2
Co/CyDTA/SiO2
29
23
–
35
20
7 (5)
39 (30)
25 (13)
9 (7)
3.3.3. Co/CyDTA/SiO2
FT-IR spectra of dried and calcined Co/CyDTA/SiO2 are illustrated
in Fig. 5C. Broad bands at 1652 cmꢁ1 and 1630 cmꢁ1 (dH2O) were
observed in the spectrum of the dried sample. The band at
1652 cmꢁ1 was also observed in the spectrum of the catalysts cal-
cined at 453–493 K, while the intensity of this band was gradually
reduced with the calcination temperature. On the other hand, a new
IR band emerged at 1590 cmꢁ1 in the spectrum of Co/CyDTA/SiO2
calcined at 453 K, and its intensity increased with the calcination
temperature up to 493 K. Again, it is confirmed that these IR bands
at 1652 and 1590 cmꢁ1 are observed only when CyDTA coexists
with Co nitrate by comparison with the spectrum of dried CyDTA/
a
Crystallite size of Co3O4 species calculated by Eq. (2).
Diameter of Co0 particles calculated by hydrogen chemisorption capacity and
b
reduction degree of Co.
1.5
1.0
0.5
0
After
drying
SiO2 (Fig. 5C-h). These bands were ascribed to
m
asCOOꢁ of the pro-
tonated CyDTA–Co complex and
m
asCOOꢁ of the CyDTA–Co com-
plex, respectively, as described below.
Assignment of IR bands for complexes of Co2+ with CyDTA in an
aqueous solution is rather complicated because two types of Cy-
DTA–Co complexes are formed depending on pH of the solution.
It is reported that a [CoL]2ꢁ (L = CyDTA) complex is predominant
in an aqueous solution under neutral and basic conditions, while
a protonated one, namely [CoHL]ꢁ, is stable in the aqueous solution
under low pH (63) conditions [49]. According to this previous
work, aqueous solutions containing CyDTA and Co nitrate with dif-
ferent pH of 2 and 11 (CyDTA–Co2+ molar ratio = 1) were prepared,
and their FT-IR spectra were acquired for reference purposes. The
reference spectrum of the solution with high pH (Fig. 5C-j) showed
373
473
573
673
773
Calcination temperature / K
Fig. 4. Impact of the calcination temperature on the I(Co2p3/2)/I(Si2p) ex situ XPS
intensity ratio for Co/SiO2 (open diamond), Co/NTA/SiO2 (+), and Co/CyDTA/SiO2
(closed square).
between Co and the chelating agents by means of ex situ FT-IR
spectroscopy. FT-IR spectroscopy is a suitable technique for this
purpose because it can easily distinguish chelate complexes from
corresponding free chelating ligands.
a weak band at 1590 cmꢁ1, which was attributed to asCOOꢁ of the
m
[CoL]2ꢁ complex. Meanwhile, two bands observed at 1740 cmꢁ1
and 1652 cmꢁ1 in the reference spectrum of the solution with
low pH (Fig. 5C-i) were assigned to
mCO and m
asCOOꢁ of the
[CoHL]ꢁ complex, respectively, in accordance with the literature
[51]. The characteristic band at 1652 cmꢁ1 was also observed in
the spectrum of dried Co/CyDTA/SiO2. Compared to the FT-IR spec-
tra of the reference solutions, it is likely that interaction between
CyDTA and Co2+ in the dried sample was similar to that of the
[CoHL]ꢁ complex in an acidic environment. This interaction is pre-
served to some extent during calcination up to 493 K because the
band at 1652 cmꢁ1 was still observed in the spectrum of the cata-
lyst calcined at 493 K. On the other hand, the emergence of the
new IR band at 1590 cmꢁ1 indicates the formation of different type
of interaction between Co and CyDTA during calcination, which is
similar to that of the [CoL]2ꢁ complex favorable in a neutral-basic
environment.
3.3.1. Co/SiO2
Fig. 5A illustrates FT-IR spectra of dried and calcined Co/SiO2. In
these spectra, only a broad band of dH2O was observed at
1631 cmꢁ1. The intensity of this band decreased with the calcina-
tion temperature, indicating that dehydration of the catalyst takes
place during calcination.
3.3.2. Co/NTA/SiO2
Fig. 5B demonstrates change in FT-IR spectra of Co/NTA/SiO2
during calcination. In the FT-IR spectrum of the dried sample, a
weak IR band was observed at 1590 cmꢁ1 in addition to the band
of dH2O at 1630 cmꢁ1. The band at 1590 cmꢁ1 was also observed
in the spectra of the catalysts calcined at 453–493 K, while it dis-
appeared from the spectrum when the catalyst was calcined at
523 K. In this figure, the spectrum of dried NTA/SiO2 is also in-
cluded (Fig. 5B-f), which showed no clear bands at around
1590 cmꢁ1. In other words, the IR band at 1590 cmꢁ1 was observed
only when NTA coexisted with Co nitrate.
Fig. 6 displays change in absorbance of the IR bands at 1590 and
1652 cmꢁ1 as a function of the calcination temperature. The inten-
sity of the band at 1590 cmꢁ1 increased with increasing the calci-
nation temperature in the range of 453–493 K where I(Co2p3/2)/
I(Si2p) for calcined Co/CyDTA/SiO2 increased sharply.
An aqueous solution containing NTA and Co nitrate with NTA–
Co2+ molar ratio of unity was then prepared and characterized by
FT-IR to study interaction between Co and NTA during calcination
of the catalyst (Fig. 5B-g). It is known that a [CoL]ꢁ (L = NTA) com-
plex is only stable Co species in the aqueous solution containing
Co2+ and NTA in a wide pH range (1 6 pH 6 14) [49]. The spectrum
of the reference solution adjusted to pH 10 showed a weak band at
3.4. Coordination environments of Co in dried and calcined catalysts
Coordination environments of Co in the dried and calcined cat-
alysts, and the effect of the chelating agent on them were then
studied by ex situ/in situ Co K-edge (Q)XAFS. Attentions were paid
to clarify temperature-dependent structural perturbations of the
complex formed in the chelate-assisted catalysts during calcina-
tion. Here, we present only (Q)XAFS results of Co/SiO2 and Co/Cy-
DTA/SiO2 because there was only minor difference between those
of Co/SiO2 and Co/NTA/SiO2.
1590 cmꢁ1 that was assigned to asCOOꢁ of the [CoL]ꢁ complex
m
[50]. Since the similar IR band was observed in the spectrum of
the dried sample, it is suggested that interaction between NTA

N. Koizumi et al. / Journal of Catalysis 289 (2012) 151–163
157
1.0
(A)
1.0
(B)
1.0
(C)
(a)
(b)
(a)
(b)
(a)
(b)
(c)
(d)
(c)
(d)
(c)
(d)
(e)
(e)
(h)
(e)
C=O
1800
1700
1600
1500
(f)
-1
Wavenumber / cm
(g)
(i)
(j)
1800
1700
1600
1500
1800
1700
1600
1500
-1
Wavenumber / cm-1
Wavenumber / cm
Fig. 5. Ex situ FT-IR spectra of Co/SiO2 (A), Co/NTA/SiO2 (B), and Co/CyDTA/SiO2 (C) after drying and calcination at different temperatures. This figure also includes some
reference spectra of dried L/SiO2 (L = NTA, CyDTA) and aqueous solutions containing Co(NO3)2ꢀ6H2O and L: After calcination at 523 K (a), 493 K (b), 473 K (c) and 453 K (d),
after drying at 383 K (e), dried NTA/SiO2 (f), Co(NO3)2ꢀ6H2O + NTA soln. (pH = 10.2) (g), dried CyDTA/SiO2 (h), Co(NO3)2ꢀ6H2O + CyDTA soln. (pH = 1.5) (i),
Co(NO3)2ꢀ6H2O + CyDTA soln. (pH = 10.7) (j).
3.0
2.5
2.0
1.2
1.0
0.8
(n 6 6) was observed at around 0.16 nm (not phase shift cor-
rected). The intensity of this peak was reduced with increasing
the calcination temperature, indicating that hydrating water is
eliminated from Co nitrate species. It would also result from
increasing thermal disorder caused by heating. On the other hand,
three peaks appeared at 429 K. These three peaks are characteristic
of Co3O4, and ascribed to the Co–O, Co–Co (Oh), and Co–Co (Td)
coordination shells of Co3O4 [29,33]. In other words, the appear-
ance of these three peaks is indicative of the formation of Co3O4
species. Fig. 8a displays change in partial pressures of CO2
(m/z = 44) and NO2 (m/z = 46) in tail gas from the in situ XAFS cell
monitored by on-line MS. As displayed in this figure, NO2 was ob-
served in tail gas simultaneously with the appearance of the Co3O4
peaks in the FT-EXAFS spectra. These results clearly show that Co
nitrate species were decomposed to form Co3O4 species at around
430 K.
0.6
0.4
0.2
0
1.5
1.0
0.5
0
360 385 410 435 460 485 510
Calcination temperature /K
Fig. 6. Change in absorbance of the IR bands at 1590 cmꢁ1 (open triangle) and
1650 cmꢁ1 (open circle) observed in the ex situ FT-IR spectra of Co/CyDTA/SiO2 after
drying and calcination at different temperatures (see Fig. 5). This figure also
illustrates change in the Co–C coordination numbers of the CyDTA–Co complex
formed in the same catalyst (closed square). These numbers were determined by
curve fitting analysis of in situ FT-EXAFS of Co/CyDTA/SiO2 during TPO (for more
detail see Section 3.4.2).
At high temperatures (>430 K), the intensities of the Co–Co
coordination peaks of Co3O4 species increased with increasing
the temperature up to approximately 500 K, indicating that
Co3O4 species are agglomerated in this temperature range.
3.4.2. Co/CyDTA/SiO2
3.4.2.1. Coordination environment of Co in dried catalyst. Ex situ Co
K-edge XANES of dried Co/CyDTA/SiO2 is shown in Fig. 9. General
features of this XANES spectrum were similar to those of polycrys-
talline Co(NO3)2ꢀ6H2O. However, there also existed small differ-
ence between them; a weak shoulder peak was observed at
around 7740 eV in the spectrum of dried Co/CyDTA/SiO2, while
such shoulder peak was absent from that of Co(NO3)2ꢀ6H2O [44].
Since sub-stoichiometric amount of CyDTA was used for prepara-
tion of Co/CyDTA/SiO2 (CyDTA–Co2+ molar ratio = 0.25), this differ-
ence could reflect coexistence of Co nitrate species and small
amount of the CyDTA–Co complex. Pattern-fitting analysis was
then performed using the XANES spectra of Co(NO3)2ꢀ6H2O and
dried Co–CyDTA/SiO2, namely the co-impregnation catalyst. Note
that all the Co in the co-impregnated catalyst is involved in the for-
mation of the 1:1 complex with CyDTA [28,29]. EXAFS analysis also
revealed that the coordination environment of Co in this co-
impregnated catalyst was similar to that of the protonated
complex. As illustrated in Fig. 9, the XANES spectrum of dried
3.4.1. Co/SiO2
3.4.1.1. Coordination environment of Co in dried catalyst. The coordi-
nation environment of died Co/SiO2 was studied by ex situ Co K-
edge XAFS in our previous work [44]. It showed that both XANES
and Fourier transformed k3
v(k), hereafter simply denoted as
FT-EXAFS, of this sample were similar to those of polycrystalline
Co(NO3)2ꢀ6H2O. Curve-fitting analysis confirmed that the coordina-
tion environment of Co species in dried Co/SiO2 was similar to that
of the hexaaqua Co (II) complex in which Co atom is surrounded by
six H2O molecules with a distorted octahedral structure [52].
3.4.1.2. Change in coordination environment of Co during calcina-
tion. Change in the coordination environment of Co during calcina-
tion was then studied by in situ Co K-edge QXAFS coupled with
TPO. Fig. 7a illustrates change in Co K-edge FT-EXAFS of Co/SiO2
during TPO. At low temperatures (366 6 T 6 426 K), only a broad
peak attributed to the Co–O coordination shell of Co(NO3)2ꢀnH2O

158
N. Koizumi et al. / Journal of Catalysis 289 (2012) 151–163
overlapping Co–O coordination shells of Co nitrate species and
(a)
the protonated complex by comparison with FT-EXAFS spectra of
Co(NO3)2ꢀ6H2O and dried Co–CyDTA/SiO2 (not shown here), while
a shoulder peak at around 0.23 nm (phase shift not corrected) was
originated from the Co–C coordination shell of the complex (indi-
cated by an arrow in Fig. 7b). The intensity of the Co–C coordina-
tion peak was almost constant up to 440–450 K, while it turned
to decline above this temperature. This coordination peak was
not observed in the FT-EXAFS spectrum acquired at 491 K. It is
noted that the intensity of the Co–O coordination peaks is fairly
constant in this temperature range. Therefore, it is unlikely that
disappearance of the Co–C coordination peak is due to increasing
thermal disorder induced by heating. To get a more quantitative
insight, the Co–C coordination numbers were calculated by curve
fitting analysis and are plotted against the calcination temperature
(see Fig. 6). In spite of large experimental errors, an overall trend is
clearly seen from this figure; the Co–C coordination number
started to decline at around 450 K and dropped almost zero at
around 480 K. Since the IR bands originated from the complexes
were still observed in the ex situ FT-IR spectra of Co/CyDTA/SiO2
calcined at 453–493 K, decrease in the Co–C coordination number
represents change in the coordination environment of the proton-
ated complex formed in the dried Co/CyDTA/SiO2 rather than com-
bustion of CyDTA ligands.
To confirm this point more explicitly, change in partial pres-
sures of CO2 and NO2 in tail gas during the QEXAFS experiment
of Co/CyDTA/SiO2 is shown in Fig. 8b. An intense peak of CO2 for-
mation was observed in the temperature range of 490–550 K,
which was accompanied with NO2 formation. Therefore, these
peaks are attributed to the combustion of CyDTA ligand and/or
its derivative. In other words, they are still present in the catalyst
in the cacination temperature range of 450–480 K. Since the Co–
C coordination number changed in a similar way to that observed
for the IR band at 1652 cmꢁ1 and decreased with increasing absor-
bance of the IR band at 1590 cmꢁ1 as illustrated in Fig. 6, change in
the coordination environment of the complex would be closely
related with an occurrence of different type of interaction between
Co and CyDTA as revealed by ex situ FT-IR. NO2 formation was
also observed at around 480 K, which should be attributed to
decomposition of Co nitrate species. This decomposition tempera-
ture is higher than that observed for Co/SiO2, but still lower
than that reported for thermal decomposition of polycrystalline
Co(NO3)2ꢀ6H2O [53]. Since the I(Co2p3/2)/I(Si2p) for dried Co/
CyDTA/SiO2 was smaller than that for dried Co/SiO2 as illustrated
in Fig. 4, it is suggested that agglomeration of Co nitrate species
in dried Co/CyDTA/SiO2 is responsible for the higher temperature
required for their decomposition during TPO.
12
10
8
6
4
623 K
2
0
Calcination
0.00
temperature
429 K
366 K
0.60
0.10
0.20
0.30
R / nm
0.40
0.50
(b)
12
10
8
6
4
711 K
529 K
2
0
Calcination
temperature
491 K
0.00
0.10
0.20
0.30
R / nm
370 K
0.60
0.40
0.50
Fig. 7. Change in Co K-edge FT-EXAFS of Co/SiO2 (a) and Co/CyDTA/SiO2 (b) during
calcination monitored by in situ QXAFS technique. The sample was heated from 373
to 623 K at a ramp rate of 1 K minꢁ1 followed by 10 K minꢁ1 in the range of 623–
723 K under flowing 20% O2/He (99.99995%, approximately 100 mL minꢁ1). XAFS
spectra were acquired at every 180 s.
Co/CyDTA/SiO2 was fitted well with those of Co(NO3)2ꢀ6H2O and
dried Co–CyDTA/SiO2. Pattern-fitting analysis also showed that
about 20% of the total Co in dried Co/CyDTA/SiO2 was involved in
the complex formation. This means that almost all CyDTA form
the complex with Co2+ in dried Co/CyDTA/SiO2 because a maxi-
mum amount of Co that can be involved in the 1:1 complex forma-
tion is one-fourth of the total Co in this sample.
3.4.2.2. Change in coordination environment of Co during calcina-
tion. In the FT-EXAFS of Co/CyDTA/SiO2 during TPO (Fig. 7b), two
peaks were observed in low and middle temperature ranges
(370 K 6 T < 491 K). A main peak was attributed to ill-resolved
120
430
120
(a)
(b)
525
m/z=44
90
90
m/z=46 ( 100)
m/z=46 ( 100)
60
60
480
30
30
m/z=44
0
0
400
500
600
700
400
500
600
700
Calcination temperature / K
Calcination temperature / K
Fig. 8. Change in partial pressures of CO2 (m/z = 44) ad NO2 (m/z = 46) in the tail gas from in situ XAFS cell monitored by the on-line MS spectroscopy; Co/SiO2 (a) and Co/
CyDTA/SiO2 (b).

N. Koizumi et al. / Journal of Catalysis 289 (2012) 151–163
159
were studied by activity test and in situ QXAFS in conjunction with
several ex situ characterization techniques. It was revealed that the
high calcination temperatures (>473 K) and large complex forma-
tion constants (logKCo P 16) of the chelating agent were crucial
for effective enhancement of both the FTS activity and dispersion
of Co3O4 species formed in the calcined catalyst. Finally, we discuss
mechanistic aspects of the role of the chelating agents, NTA and
CyDTA, based on the in situ and ex situ characterization results.
0.2
Fitting result
(Rf = 0.004%)
Co(NO3)2·6H2O
(
0.84)
Co(5)-CyDTA/SiO2 (dried)
4.1. Role of CyDTA
(
0.19)
4.1.1. Effect of CyDTA on surface structure of dried Co/CyDTA/SiO2
The ex situ FT-IR spectrum of dried Co/CyDTA/SiO2 indicated
the presence of interaction between Co and CyDTA, which was
similar to that of the protonated CyDTA–Co complex, namely
[CoHL]ꢁ (L = CyDTA), in the aqueous solution. Ex situ XANES anal-
ysis of the same sample also confirmed that about one-fourth of
the total Co in dried Co/CyDTA/SiO2 were involved in the complex
formation with CyDTA, while remaining species were present as Co
nitrate. This means that almost all CyDTA in the dried sample form
the [CoHL]ꢁ complex after drying due to its strong affinity with
Co2+ because the CyDTA–Co molar ratio of this catalyst is 0.25.
The formation of the protonated complex in the dried sample is
also reasonable in view of the solution chemistry in the Co2+–Cy-
DTA system [49] because the acidic Co nitrate solution with
approximate pH 3 was impregnated before drying. Although exact
coordination structures of the complexes in the aqueous solutions
are still unknown yet, it is reasonable to assume that the [CoL]2ꢁ
complex has a six (or seven) coordination structure according
to single crystal XRD studies on Ca[Fe(OH2)L]2ꢀ8H2O [54] and
[Na6(NO3)2(H2O)13][NiL]2 [55], while the protonated one has a five
(or six) coordination structure with one free carboxyl group. Fur-
thermore, our previous works on the co-impregnated Co–CyDTA/
SiO2 revealed that the complex formed in the impregnated and
dried samples had stronger affinity with surface hydroxy group
than Co nitrate [28] which indicates that the [CoHL]ꢁ complex in
dried Co/CyDTA/SiO2 interacts with the surface hydroxy group as
well. Since the PZC of SiO2 surface is around 2 [56], Si–OH species
would be a stable form in dried Co/CyDTA/SiO2. Therefore, it is un-
likely that the free carboxyl group of the protonated complex inter-
acts with the surface hydroxy group. Instead, it is probable that
unsaturated Co2+ of the [CoHL]ꢁ complex is coordinated with Si–
OH species as depicted in Scheme 1a. It is also stressed that,
according to the literature reported by Zhuravlev [57], the surface
O–H density of various amorphous SiO2 samples with maximum
7680 7700 7720 7740
7760 7780
Photon energy / eV
Fig. 9. Pattern-fitting result of the ex situ Co K-edge XANES of dried Co/CyDTA/SiO2
(solid line) using those of polycrystalline Co(NO3)2ꢀ6H2O and dried Co–CyDTA/SiO2
(broken lines). The Rf value defined by Eq. (3) is also indicated to show goodness of
the fitting.
In the FT-EXAFS of Co/CyDTA/SiO2 acquired at 529 K, three
peaks characteristic of Co3O4 species were observed. It is high-
lighted that the formation of Co3O4 species took place simulta-
neously with the combustion of CyDTA ligand. This means that
there is an obvious temperature gap between decomposition of
Co nitrate species and formation of Co3O4 species in Co/CyDTA/
SiO2. Such temperature gap was never observed during the QEXAFS
experiment of Co/SiO2. Once Co3O4 species were formed, the inten-
sities of the Co–Co coordination peaks of Co3O4 species increased
with the calcination temperature. However, their intensities were
no more than 50–60% of those observed in the QEXAFS spectra of
Co/SiO2 at the high temperatures. This indicates that agglomera-
tion of Co3O4 species was retarded in Co/CyDTA/SiO2, which is con-
sistent with ex situ XRD and XPS results.
4. Discussion
Our previous works demonstrated that the two-step impregna-
tion method using the chelating agents such as NTA and CyDTA im-
proved the FTS activity of 20 mass% Co/SiO2 effectively [30–33].
Use of CyDTA improved dispersion of Co0 as well, while it hardly
affects high reducibility of Co for the catalyst prepared in the ab-
sence of the chelating agents (96%) [33]. It is also worth noting that
only small amounts of CyDTA, namely the CyDTA–Co2+ molar ratio
of 0.25–0.5, were necessary for enhancing the FTS activity of the
catalyst prepared using CyDTA. This suggested that only a part of
Co2+ in the dried catalyst was involved in the complex formation
with CyDTA. In other words, surplus Co would remain as Co nitrate
before calcination. Seemingly, this is reminiscent of the results of
the catalysts prepared using the mixed salt of Co nitrate and Co
acetate [11] rather than those of the catalysts prepared by the
co-impregnation method using the chelating agents, while use of
the mixed salt as the precursor lowered reducibility of Co. Ex situ
XRD and Co K-edge EXAFS analyses of the calcined catalysts also
revealed that CyDTA had ability to decrease size of Co3O4 species
without forming detectable quantities of Co silicate-like species
[33], and provided indirect evidence that some kind of interaction
would be formed between Co nitrate and the complex during dry-
ing and/or calcination. However, formation mechanism of small
precursor oxides during calcination of chelate-assisted Co/SiO2
has not been fully understood yet.
degree of hydration is approximately 4.6–4.9 OH groups nmꢁ2
,
independent of the origin and structural characteristics of amor-
phous silicas. This surface density is comparable with that of the
complex formed in dried Co/CyDTA/SiO2 (approximately 2.4 com-
plexes nmꢁ2). Thus, it is also reasonable that the [CoHL]ꢁ complex
formed in dried Co/CyDTA/SiO2 spreads over support surface
through the formation of the coordination bonds with the surface
hydroxy groups. Conversely, strong affinity of the complex with
surface hydroxy group means that only a part of Co nitrate can
be involved in interaction with the surface hydroxy group, which
would facilitate agglomeration of Co nitrate compared to the cata-
lyst prepared without using the chelating agent. This is probably
why the I(Co2p3/2)/I(Si2p) for dried Co/CyDTA/SiO2 was smaller
than that for dried Co/SiO2, and the former sample showed the
lower FTS activity than the later one.
4.1.2. Interpretation of increase in I(Co2p3/2)/I(Si2p) for calcined
Co/CyDTA/SiO2
During calcination of dried Co/CyDTA/SiO2 at 453–523 K, sharp
increase in the I(Co2p3/2)/I(Si2p) was observed as illustrated in
Fig. 4 which suggests that relatively large Co species in this sample
In this work, 20 mass% Co/SiO2 were prepared by this two-step
impregnation method using several chelating agents, and the ef-
fects of the calcination temperature and chelating agent on their
catalytic activities and coordination environments of Co species

160
N. Koizumi et al. / Journal of Catalysis 289 (2012) 151–163
(a)
(b)
Scheme 1. Temperature-dependent structural perturbation of the protonated CyDTA–Co complex during calcination.
turn into smaller ones during calcination. However, interpretation
of such increase in the I(Co2p3/2)/I(Si2p) would not be so straight-
forward; it could be caused by removal of CyDTA from Co surface
during calcination instead of decrease in size of Co species. There-
fore, this phenomenon should be examined more carefully. In our
previous work on the co-impregnated CyDTA–Co/SiO2 [29], it was
revealed that the I(Co2p3/2)/I(Si2p) was practically constant during
calcination at 383–723 K, while a sharp exothermic peak due to
combustion of CyDTA ligand was observed at around 573 K by
DTA analysis. Besides, well-dispersed Co silicate-like species were
mainly observed in the catalyst calcined at 723 K. Since all the Co
in the co-impregnated catalyst is involved in the formation of the
1:1 complex with CyDTA, these results suggested that the proton-
ated CyDTA–Co complex formed in the dried sample was immobi-
lized on SiO2 surface during calcination at 383–573 K, probably
through the formation of coordination bond between the complex
and surface hydroxy group as discussed in Section 4.1.1. Above this
temperature, chelating ligand was removed from the catalyst sur-
face by combustion, which facilitated the formation of well-dis-
persed silicate-like species. These results of the co-impregnated
catalyst further imply that the complex formed in the stepwise-
impregnated Co/CyDTA/SiO2 is immobilized on SiO2 surface as well
during calcination, although surplus Co nitrate species coexist with
the complex in dried Co/CyDTA/SiO2, which may affect distribution
of the complex.
This phenomenon was then investigated from a different point
of view. In the in situ XANES spectra of Co/CyDTA/SiO2 during TPO,
a characteristic white line was observed at around 7720 eV when
the spectra were measured at 484 K and lower temperatures.
Above this temperature, the white line shifted toward higher en-
ergy side due to decomposition of Co nitrate species at 480 K as
shown in Fig. 8b. Pattern-fitting analysis showed that the XANES
spectra measured at 484 K and lower temperatures were fitted
with two reference spectra, namely polycrystalline Co(NO3)2ꢀ6H2O
and dried CyDTA–Co/SiO2. It also revealed that 20–28% of the total
Co in Co/CyDTA/SiO2 was involved in the complex formation which
confirmed that the protonated complex formed in dried Co/CyDTA/
SiO2 was preserved in this calcination temperature range. Further-
more, no CO2 formation was observed during this in situ XAFS
experiment except the one centered at 525 K (see also Fig. 8b).
Although these results are not direct evidence for immobility of
the complex during calcination, they ensure that CyDTA used for
preparation of Co/CyDTA/SiO2 is preserved as the complex during
clacination until combustion of CyDTA ligand at around 530 K.
Therefore, it would be reasonable to interpret sharp increase in
the I(Co2p3/2)/I(Si2p) for calcined Co/CyDTA/SiO2 as decrease in
size of Co species rather than removal of CyDTA ligand from Co
surface.
4.1.3. Mechanism for size reduction of Co during calcination of
Co/CyDTA/SiO2
In situ QEXAFS and ex situ FT-IR analyses indicated that the
coordination environment of the [CoHL]ꢁ complex formed in dried
Co/CyDTA/SiO2 was preserved during calcination up to ca. 450 K.
Above this temperature, the Co–C coordination number of the pro-
tonated complex decreased with increasing the calcination tem-
perature, and the Co–C coordination peak disappeared from the
QEXAFS spectra at 480–490 K. We suggest that decrease in the
Co–C coordination number is caused by cleavage of the coordina-
tion bonds between Co and COOꢁ moieties of CyDTA ligand as
illustrated in Scheme 1b.
On the other hand, tail gas analysis during the QEXAFS experi-
ment of Co/CyDTA/SiO2 further revealed that Co nitrate species
were decomposed in a similar temperature range, namely 460–
500 K. It is also important to emphasize that there was tempera-
ture difference of approximately 40 K between decomposition of
Co nitrate and the formation of Co3O4 species. Since in situ mea-
surements at high temperatures did not provide clear picture of
coordination structures of Co formed after decomposition of Co ni-
trate, ex situ Co K-edge EXAFS measurement was then performed
at low temperatures (17–20 K) using a He cryostat. FT-EXAFS spec-
tra of Co/CyDTA/SiO2 calcined at different temperatures are illus-
trated in Fig. 10. Three peaks characteristic of Co3O4 species
were clearly observed in the FT-EXAFS of Co/CyDTA/SiO2 calcined
at 523 K as expected from the QEXAFS result. Meanwhile, in addi-
tion to the Co–O coordination peak, only a weak peak ascribed to a
Co–Co coordination was observed when it was calcined at 473 K.
This indicates that Co nitrate species are decomposed to form
small Co oxide clusters (hereafter denoted as CoxOy), which are
preserved in this temperature range without forming Co3O4 spe-
cies. Since the formation of Co3O4 species took place simulta-
neously with the combustion of CyDTA ligand as illustrated in
Figs. 7b and 8b, it is suggested that these small oxide clusters
are stabilized by CyDTA ligand. Considering structural perturba-
tion of the [CoHL]ꢁ complex during calcination mentioned above,
it is speculated that the CoxOy clusters are stabilized through coor-
dination with COOꢁ moieties of the complex. The formation of
such coordination bonds is also confirmed by the ex situ FT-IR re-
sults, which indicated the formation of different type of interaction
between Co and CyDTA or its derivative at 450–490 K. Since the
[CoHL]ꢁ complexes formed in dried Co/CyDTA/SiO2 are distributed
uniformly over support through interaction with the surface hy-
droxy groups, the formation of such coordination bonds forms
some kind of carbon network over support surface, which would
not only prevent sintering of the CoxOy clusters but spread them
over SiO2 surface. In other words, such interaction between the
CoxOy cluster and carbon network is thought as the origin of size

N. Koizumi et al. / Journal of Catalysis 289 (2012) 151–163
161
[28,29]. It was considered that strong affinity of the complex with
the surface hydroxy group as well as its high stability was respon-
sible for selective formation of such silicate-like species. Girardon
et al. [59] and Trujillano et al. [14,60] also reported that the forma-
tion of surface silicate is facilitated by use of organic Co precursor or
the addition of organic ligands, respectively. To confirm the forma-
tion of Co silicate-like species in Co/CyDTA/SiO2 during calcination,
pattern-fitting analysis of in situ Co K-edge XANES spectra mea-
sured during TPO was carried out in two different ways; one was
to fit them with two XANES spectra of polycrystalline Co3O4 and
Co-Co (Oh)
Co-Co (Td)
3.0
Co-O
(d)
(c)
Co-Co
(b)
a-Co2SiO4, and the other was to use only one reference spectrum
(
0.5) (a)
of polycrystalline Co3O4. Fig. 11 illustrates typical pattern-fitting re-
sults of the XANES spectrum of Co/CyDTA/SiO2 acquired at 711 K.
The quality of fitting was always improved by involving contribu-
0
0.2
R/ nm
0.4
0.6
Fig. 10. Ex situ Co K-edge FT-EXAFS of Co/CyDTA/SiO2 calcined at different
temperatures in comparison with that of Co(NO3)2ꢀ6H2O. XAFS spectra were
tion of
a-Co2SiO4 for the spectra of Co/CyDTA/SiO2 measured in
the range of 548–711 K. Pattern-fitting analysis also revealed that
percentage of Co contributed to the silicate formation was 13–
23%. In situ XANES spectra of Co/SiO2 were, on the other hand, fitted
with only one reference spectrum of polycrystalline Co3O4. Contri-
bution from Co silicate was always negative when fitting analysis
was carried out using two reference spectra of polycrystalline
measured at low temperatures (17–20 K) using
a He cryostat; polycrystalline
Co(NO3)2ꢀ6H2O (a), dried Co/CyDTA/SiO2 (b), Co/CyDTA/SiO2 calcined at 473 K (c),
Co/CyDTA/SiO2 calcined at 523 K (d).
reduction of Co species observed in the similar calcination temper-
ature range (453–523 K, see Fig. 4).
Co3O4 and a-Co2SiO4. These results suggested that the protonated
In this connection, it is worth citing studies by Che et al. [15,58]
where they reported that dispersion of NiO species in calcined Ni/
SiO2 was enhanced by a two-step impregnation method using eth-
ylenediamine (en). In their preparation method, the aqueous solu-
tion containing [Ni(en)6]2+ complex was impregnated on SiO2 first.
This complex was converted into small Ni2+ species interacted with
SiO2 surface during 1st calcination. The aqueous solution of Ni ni-
trate was then impregnated on this sample followed by calcina-
tion. Their in situ Ni K-edge EXAFS results suggested that small
Ni2+ species interacted with SiO2 surface worked as anchoring sites
for preventing agglomeration of Ni oxide species during 2nd calci-
nation [58]. Different from en in their preparation method, the che-
late complex itself plays a significant role in enhancing dispersion
of Co in our two-step impregnation method.
complex in dried Co/CyDTA/SiO2 was converted into Co silicate-like
species after combustion of carbon network. These silicate species
would work as anchoring sites for Co3O4 species formed from the
CoxOy clusters, as suggested by Che et al. [15] and Lim et al. [61],
resulting in controlled agglomeration of Co3O4 species during calci-
nation at the high temperatures.
4.2. Role of NTA
As mentioned in 3.3.2, the ex situ FT-IR spectrum of dried Co/
NTA/SiO2 indicated the presence of interaction between Co and
NTA which is similar to that of the [CoL]ꢁ (L = NTA) complex in
the aqueous solution. However, in situ/ex situ XAFS did not provide
clear information about change in the coordination environment of
the NTA–Co complex during calcination. It is reasonable to assume
that only a small part of NTA is involved in the [CoL]ꢁ complex for-
mation due to its lower affinity with Co2+ compared to CyDTA, so
the presence of the complex would not cause clear differences in
the XANES and FT-EXAFS spectra. Presumably, the role of NTA is
similar to that of CyDTA. Only a small part of Co (CoxOy) can be sta-
bilized by the NTA complex during calcination, which works as the
dispersion enhancer. On the other hand, remaining Co nitrate spe-
cies are decomposed to form Co3O4 species followed by agglomer-
ation at high calcination temperatures. These two different effects
would offset each other, resulting in apparently constant I(Co2p3/
2)/I(Si2p) during calcination (Fig. 4).
4.1.4. Formation of Co silicate-like species during calcination and its
importance
Once Co3O4 species were formed in Co/CyDTA/SiO2, they started
to agglomerate as indicated by decrease in the I(Co2p3/2)/I(Si2p) in
the calcination temperature range of 523–723 K. However, agglom-
eration of Co3O4 species in Co/CyDTA/SiO2 was suppressed com-
pared to that observed during calcination of Co/SiO2. Since CyDTA
ligand is already removed from the catalyst surface by combustion,
a different mechanism should work for retarded agglomeration of
Co3O4 species. In this regard, it is worth noting that the complex
formed in co-impregnated Co–CyDTA/SiO2 was selectively con-
verted into Co silicate-like species after calcination at 723 K
(a)
(b)
Fitting result
(Rf = 0.011%)
Fitting result
(Rf = 0.019%)
0.2
0.2
Co3O4
0.90)
(
α-Co2SiO4
0.15)
(
7680 7700 7720 7740
7760 7780
7680 7700 7720 7740
7760 7780
Photon energy / eV
Photon energy / eV
Fig. 11. Pattern-fitting results of the in situ Co K-edge XANES spectrum of Co/CyDTA/SiO2 measured during TPO at 711 K. Fitting analysis was carried out using two reference
spectra of polycrystalline Co3O4 and -Co2SiO4 (a), and only one reference spectrum of polycrystalline Co3O4 (b).
a

162
N. Koizumi et al. / Journal of Catalysis 289 (2012) 151–163
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newly developed two-step impregnation method using several
chelating agents. The effects of the calcination temperature and
chelating agent on their FTS activities and surface structures were
studied to deepen our understanding of the role of the chelating
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(1) The FTS activity of Co/L/SiO2 prepared by a two-step impreg-
nation method using the chelating agent (L) was dependent
on the calcination temperature and increased with the calci-
nation temperature in the range of 473–543 K when the che-
lating agents with strong affinity with Co2+ such as CyDTA
were used for preparation.
(2) Ex situ XRD and XPS revealed that size of Co species in cal-
cined Co/CyDTA/SiO2 was reduced with the calcination tem-
perature in the range of 453–523 K, while those in calcined
Co/SiO2 were simply agglomerated to form Co3O4 species
with low dispersion in the catalyst calcined at 723 K. CyDTA
also had ability to suppress sintering of Co3O4 species during
calcination at high temperatures.
(3) Ex situ FT-IR and XAFS measurements also revealed that
about one-fourth of the total Co in dried Co/CyDTA/SiO2 were
involved in the formation of the [CoHL]ꢁ (L = CyDTA) com-
plex, while remaining species were present as Co nitrate. Ex
situ XAFS also indicated that there was no apparent interac-
tion between them.
(4) The results of in situ TPO-QEXAFS in conjunction with those
of ex situ FT-IR and low-temperature EXAFS suggested that
Co nitrate was decomposed during calcination of Co/CyDTA/
SiO2 at 460–500 K to form small CoxOy clusters which were
stabilized by the Co–COOꢁ bond formation between the clus-
ters and complexes. Such interaction was considered as the
origin of size reduction of Co species during calcination at
453–523 K.
(5) In situ TPO-QXAFS results of Co/CyDTA/SiO2 also suggested
that the complexes were converted to Co silicate-like species
after combustion of carbon network. It was speculated that
these silicate-like species worked as anchoring sites for pre-
venting agglomeration of Co3O4 species formed from the
CoxOy clusters during calcination at high temperatures.
(6) The results obtained in this work demonstrated that specific
interaction of the small Co oxide clusters from Co nitrate with
chelate complexes is crucial for large activity enhancement
induced by the chelating agent. Coexisting of both Co nitrate
and the complex before calcination is essential for prepara-
tion of Co/SiO2 catalyst by the two-step impregnation method
using the chelating agents.
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Acknowledgments
This research was supported by the Japan Society for the Pro-
motion of Science (JSPS), Grant-in-Aid for Scientific Research (S),
17106011, 2005. EXAFS measurement was performed at the
BL14B2 in the SPring-8 with the approval of JASRI (Proposal No.
2009B1835). We gratefully thank the staffs of SPring-8 for their
technical support and their kind help.
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