Rapid synthesis of magnetite catalysts incorporated with M (Cu, Ni, Zn, and Co) promoters for high temperature water gas shift reaction

Article abstract of DOI:10.1039/c4nj00854e

A rapid one step direct synthesis of high temperature water gas shift (HT-WGS) catalysts, magnetite incorporated with various promoters like Cu, Ni, Zn, and Co, has been shown. The key achievements in this work, in terms of catalyst preparation, are as follows: (1) direct synthesis of magnetite, a catalytically active phase in HT-WGS reaction, (2) easy incorporation of various transition metal promoters like Cu, Ni, Zn, and Co, (3) rapid preparation without any washing step by using an ample amount of solvent, and (4) a simplified procedure by avoiding longer digestion time and any specific pH value. To achieve this, an amino acid, glycine, was used as a complexing agent as well as a fuel for the auto-combustion process. The zwitterionic nature of glycine favors the easy incorporation of the promoter metal ions. In addition, the reducing atmosphere created by glycine during its burning makes it possible to prepare magnetite directly. During the HT-WGS reaction, the activity follows the order Cu-FAG > Ni-FAG > Zn-FAG > Co-FAG. The optimization of the Cu promoter is also carried out. At 12.5 mol% of Cu loading the catalytic activity reached a CO conversion value of 87% at 400 °C. This journal is

Full text of DOI:10.1039/c4nj00854e

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Rapid synthesis of magnetite catalysts
incorporated with M (Cu, Ni, Zn, and Co)
promoters for high temperature water gas
shift reaction
Cite this: New J. Chem., 2014,
38, 4872
Vijayanand Subramanian,a Dae-Woon Jeong,a Won-Bi Han,a Won-Jun Jang,a
Jae-Oh Shim,a Jong Wook Baeb and Hyun-Seog Roh*a
A rapid one step direct synthesis of high temperature water gas shift (HT-WGS) catalysts, magnetite
incorporated with various promoters like Cu, Ni, Zn, and Co, has been shown. The key achievements in
this work, in terms of catalyst preparation, are as follows: (1) direct synthesis of magnetite, a catalytically
active phase in HT-WGS reaction, (2) easy incorporation of various transition metal promoters like Cu,
Ni, Zn, and Co, (3) rapid preparation without any washing step by using an ample amount of solvent, and
(4) a simplified procedure by avoiding longer digestion time and any specific pH value. To achieve this,
an amino acid, glycine, was used as a complexing agent as well as a fuel for the auto-combustion
process. The zwitterionic nature of glycine favors the easy incorporation of the promoter metal ions. In
addition, the reducing atmosphere created by glycine during its burning makes it possible to prepare
magnetite directly. During the HT-WGS reaction, the activity follows the order Cu-FAG 4 Ni-FAG 4
Zn-FAG 4 Co-FAG. The optimization of the Cu promoter is also carried out. At 12.5 mol% of Cu loading
the catalytic activity reached a CO conversion value of 87% at 400 1C.
Received (in Montpellier, France)
24th May 2014,
Accepted 28th July 2014
DOI: 10.1039/c4nj00854e
www.rsc.org/njc
For the preparation of metal promoted iron oxide based HT-WGS
catalysts, various chemical methods such as precipitation/impreg-
1. Introduction
Recent development of fuel cell technology renewed the interest nation, coprecipitation and sol–gel have been adopted.9–13 Also, the
in the water gas shift (WGS) reaction for initial CO clean up and method of preparation influences strongly the overall performance
additional H2 generation.1–3 The WGS reaction comprises two of the catalysts.10,11 However, in the existing synthesis methods,
steps with different temperature regions due to the thermo- there are some drawbacks in terms of catalyst preparation. Some of
dynamic limitation: high temperature shift (HT-WGS) and low the common problems are promoter loss through the formation of a
temperature shift (LT-WGS). In general, HT-WGS is carried out at soluble complex (copper ammonium complex), a lot of liquid waste
350–550 1C, using iron oxide stabilized with chromium catalysts, during the washing step to remove alkali metal ions, usage of costly
for CO clean up at the kinetically favorable region.4,5
chemicals, critical adjustments of pH, and requirement of a longer
In the Fe–Cr catalyst, chromium is mainly added as a textural calcination time etc.10,11 Hence, simplification of synthesis to avoid
stabilizer to prevent sintering. It is proven that iron oxide suffers or modify any of the steps mentioned above during catalyst prepara-
from severe sintering in the absence of chromium.4 Furthermore, tion is highly inevitable.
various promoters have been used in the Fe–Cr formulation to
An amino acid (glycine) assisted auto-combustion method
enhance the overall catalytic performance.6–8 However, the environ- finds importance in the preparation of very fine porous
mental concerns related to chromium urged the research on the powders of various materials.14–17 This value added glycine is
HT-WGS catalysts free of chromium. The addition of copper and economically feasible, since it is one of the cheapest amino
aluminum, as structural and textural promoters, enhanced the acids. It has both a carboxylic acid group at one end and an
catalytic performance with reasonably good stability.9–11
amino acid group at the other end. Thus, it is called as a
zwitterionic compound.18 Due to this nature, it can effectively
bind metal ions of different sizes. This particular aspect favors
the catalyst preparation where more than one metal promoter
is required to be incorporated homogeneously into the matrix.
Also, it can act as a fuel for the combustion reaction through
oxidation by nitrate ion(s).15 Hence, one can control the overall
a Department of Environmental Engineering, Yonsei University, 1 Yonseidae-gil,
Wonju, Gangwon 220-710, South Korea. E-mail: hsroh@yonsei.ac.kr;
Fax: +82-33-760-2571; Tel: +82-33-760-2834
b School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon,
Gyeonggi 440-746, South Korea
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Table 1 Characteristics of MFAG catalysts (M = Cu, Ni, Zn, and Co)
oxidation or reduction by controlling the ratio of glycine to
metal nitrate(s).
Crystallite sizea (nm)
BET surface
area (m2 gÀ1
In the iron oxide based HT-WGS catalysts, magnetite is the
active phase in HT-WGS. Therefore, before the catalytic reac-
tion, an activation step is required to reduce the initial hema-
tite phase to the catalytically active magnetite phase. During
this process extreme care should be taken to avoid the metallic
iron formation through over reduction. The metallic iron is
known to catalyze the unwanted methanation reaction by
consuming valuable H2. Hence, during the activation process
an appropriate ratio of ‘R’, (CO + H2)/(CO2 + H2O) should be
followed.4 Also, an excess amount of steam is used generally to
avoid any over-reduction. However, this leads to a higher
operational cost for plant operation. Therefore, any measure
to avoid the activation step, both in terms of following any
specific factor ‘R’ and usage of excess steam, is highly useful for
the overall catalytic process, in terms of operational cost. There
are few reports available in the literature on the active phase
magnetite preparation of the HT-WGS catalysts to avoid the
aforesaid problems.19–22
Hence, the objective of the present study is to synthesize
directly the active phase magnetite, incorporated with metal
promoters. To simplify the synthesis procedure, an amino acid
(glycine) assisted auto-combustion reaction was utilized for the
catalyst preparation. Various metal promoters like Cu, Ni, Zn,
and Co together with Al were chosen to show the generality of
the synthetic strategy in terms of effective incorporation of
promoter metal ions. Above all, the as prepared catalysts were
tested over the waste derived simulated synthesis gas by using a
very high gas hourly space velocity value of 48 269 hÀ1. Since the
catalysts were prepared in its active phase magnetite, there was
no separate activation step before the reaction. All the results
based on the HT-WGS reaction with catalyst characterization by
BET, XRD, and TPR were presented in this paper.
Catalyst
)
Fresh
Used
Cu-FAG
Ni-FAG
Zn-FAG
Co-FAG
09.1
17.1
17.1
27.6
30.2
24.4
29.3
19.7
35.4
27.9
30.5
27.8
a
Calculated from XRD measurements.
an auto-combustion reaction was observed, resulting in the
formation of fluffy mass of the corresponding oxides. The
sample codes of the various metal promoter incorporated Fe/Al
oxide catalysts (MFAG) are given in Table 1.
2.1. Characterization
The BET surface area was measured by nitrogen adsorption at
À196 1C using an ASAP 2010 (Micromeritics). The XRD patterns
were recorded using
a Rigaku D/MAX-IIIC diffractometer
(Ni filtered Cu-Ka radiation, 40 kV, 50 mA). The crystallite size
was estimated using the Debye–Scherrer equation.23–26 Tempera-
ture programmed reduction (TPR) experiments were carried out
using an Autochem 2910 (Micromeritics). Typically, 0.1 g of the
precalcined sample was loaded into a quartz reactor. The TPR was
performed using 10% H2 in Ar with a heating rate of 10 1C minÀ1
from 20 to 1000 1C. The sensitivity of the detector was calibrated
by reducing the known weight of NiO.27–30
2.2. Catalytic reaction
Activity tests were carried out from 350 to 550 1C under atmo-
spheric pressure in a fixed-bed micro-tubular quartz reactor with
an inner diameter of 4 mm. The catalyst charge was 70 mg.
T-union was employed at the exit of the quartz reactor to install
a thermocouple. A thermocouple was inserted into the catalyst bed
to measure the reaction temperature. Prior to each catalytic
measurement, the catalyst was heated from room temperature to
400 1C at a heating rate of 4.6 1C minÀ1 and then the temperature
was maintained for 1 h. Afterwards, the temperature was
decreased to 350 1C. The simulated reformed gas consisted of
17.02 vol% CO, 9.55 vol% CO2, 1.03 vol% CH4, 13.14 vol% H2,
55.20 vol% H2O, and 4.06 vol% N2, which represents a typical
syngas from waste gasifier that might enter the WGS reactor in a
waste gasification system. The feed H2O/(CH4 + CO + CO2) ratio
was intentionally fixed at 2.0 to avoid coke formation.31–33 A GHSV
value of 48 269 hÀ1 was used to screen the catalysts in this study.
Water was fed using a syringe pump and was vaporized at 180 1C
upstream of the reactor. The product gas was chilled, passed
through a trap to condense the residual water, and then analyzed
on-line using an Agilent micro-gas chromatograph (Agilent 3000).
2. Experimental
A series of transition metal loaded catalysts have been synthe-
sized through auto-combustion reaction using the amino acid
glycine (98.5%, Aldrich) as a fuel. Precursors used in this study
were Fe(NO3)3Á9H2O (98%, Aldrich), Al(NO3)3Á9H2O (98%,
Aldrich), Cu(NO3)3ÁxH2O (99%, Aldrich), Ni(NO3)2Á6H2O (99%,
Aldrich), Zn(NO3)2Á6H2O (98%, Aldrich), and Co(NO3)2Á6H2O
(98%, Aldrich). In a typical synthesis, total moles of metals were
fixed as 0.01 moles. During the synthesis procedure, the ratio of
M : Fe : Al = 5 : 85 : 10 (M = Cu, Ni, Zn, and Co) was maintained.
To optimize the Cu promoter, the mole percent of Cu was
varied from 2.5 to 15.0. Initially, based on the mole percent, the
corresponding weights of metal nitrates were dissolved in a
minimum amount of water. To that, the calculated weight of
equal mole percent of glycine, dissolved in a minimum amount
of water, was added. After mixing completely, the solution was
3. Results and discussion
3.1. Glycine as a fuel
kept in a hot plate at 240 1C for 30 min. At this stage, water was Glycine is one of the cheapest amino acids which has both the
evaporated slowly, resulting in gel formation due to metal amino acid and carboxylic acid group (zwitterion). Hence, it
complexation with glycine. Once the gelation was completed, favors the efficacious complexation of metal ions of various
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sizes, by taking advantage of the zwitterionic nature. In addi-
tion, glycine is considered to be an excellent fuel for the
solution combustion synthesis (SCS).18 The nature of combus-
tion reaction relies on the ratio (f) between the oxidizer
(O; metal nitrates) and fuel (F; glycine). When f (=O/F) is unity,
the energy released during combustion is maximum.34 In
general, while burning in air one mole of glycine tends to give
five moles of product gas as mentioned in eqn (1).35 Therefore,
by adjusting the ratio of glycine to nitrate (metal) the nature of
combustion can be controlled. Also, the fuel amount for a
particular reaction product can be calculated theoretically
based on the stoichiometric redox reaction.36 This provides
the possibility of obtaining the products of interest, such as
metal oxides, metals, and metal alloys.
2CH2NH2COOH + 2O2 - N2 + 4CO2 + 5H2
(1)
In the case of HT-WGS reaction, an iron oxide based catalyst
having a magnetite phase is proven to be a catalytically active
phase. In the magnetite phase, structurally ((Fe3+)A[Fe2+-
Fe3+]BO4), a redox couple between Fe2+ and Fe3+ is present, in
the octahedral (B) site. The existence of a redox couple in the
catalyst is highly desirable for the regenerative mechanism, a
well known operating mechanism in HT-WGS reaction.13 As
discussed earlier, before the catalytic reaction
a crucial
reduction step is needed to reduce the as prepared iron oxide
from the hematite phase to the catalytically active magnetite
phase. As shown in eqn (1), during decomposition, glycine
releases H2 as one of the gaseous products. Hence, in the
present work, by making use of the in situ generation of a
controlled reducing atmosphere, a catalytically active magne-
tite phase was synthesized through the glycine assisted SCS
method. Additionally, various metal promoters like Cu, Ni, Zn,
and Co have also been incorporated to enhance the catalytic
performance. In the catalyst formulation, Al is also added as a
textural promoter. In the present study, for an efficient synth-
esis of various metal promoters incorporated catalysts, max-
imum spent time was less than an hour. All the as prepared
catalysts were subjected to XRD, BET, and TPR analysis to
identify their characteristic properties. Furthermore, all the
catalysts were evaluated for their catalytic activities in HT-WGS
reaction using waste derived simulated synthesis gas.
Fig. 1 XRD patterns of MFAG catalysts (M = Cu, Ni, Zn, and Co) (a) before
reaction and (b) after reaction.
It is clearly evident from Fig. 1(b) that there was no change in
the structure after the HT-WGS reaction, as the XRD patterns
are similar to those of the fresh catalysts. However, all the XRD
patterns in Fig. 1(b) are sharper than those in Fig. 1(a) due to
the particle growth at higher temperature during the HT-WGS
reaction. The crystallite size of used samples supports the
aforementioned factor.
The reduction behavior of as prepared catalysts was
obtained through TPR measurements as shown in Fig. 2. The
catalyst, Cu-FAG, exhibits a low temperature peak at 196 1C due
3.2. Effect of metal promoters (Cu, Ni, Zn, and Co)
To identify the phase purity and structural details, all catalysts
were subjected to XRD measurements. Fig. 1(a) shows the XRD
patterns of MFAG catalysts before the HT-WGS reaction. All the
patterns can be indexed to the phase pure spinel with a cubic
lattice, as it matches well with the standard pattern (ICDD =
89-4319). This ascertains the versatility of the synthetic meth-
odology adapted for the rapid synthesis of metal incorporated
magnetite based HT-WGS catalysts. Also, the peak broadening
reveals the nano-sized nature of the catalysts. The calculated
crystallite size, as mentioned in Table 1, follows an increasing
order: Cu-FAG 4 Zn-FAG 4 Ni-FAG 4 Co-FAG. Fig. 1(b) depicts
XRD patterns of the MFAG catalysts after the HT-WGS reaction.
Fig. 2 TPR patterns of MFAG catalysts (M = Cu, Ni, Zn, and Co).
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to the reduction of Cu2+ to Cu0. Another peak observed at 421 1C highest catalytic performance. A general point of view over the
can be assigned to the reduction of Fe3+ to Fe3O4, based on the TPR pattern of the Cu-FAG catalyst suggests that there is the
literature.20 Whereas, in all the other three catalysts a single peak lack of synergism, as it shows two distinguished peaks corre-
is observed between 370–390 1C. It is well known from the sponding to the transformation of Cu and iron oxide. However,
literature that Co and Ni exhibit a single broad peak between the superior catalytic response infers that there is some inter-
200–600 1C for the reduction of their respective oxides to metal.37 action between Cu and iron oxide in the matrix. Moreover, it is
In the present case also a single peak is evident for Co-FAG and well known that Cu alone is a good catalyst for WGS reaction.39
Ni-FAG at 387 1C and 378 1C, respectively. In both cases there is Among the various metals, Cu possesses a moderate energy for
no separate peak related to the magnetite formation, suggesting CO adsorption and desorption.40 In addition to this, Cu can
the interaction of iron oxide with the promoters. As a result, it is increase the mobility of lattice oxygen and the hydroxyl group,
most likely that the reduction of metal oxides (metal: Co or Ni) which in turn enhances the electronegativity. This character-
and Fe3+ overlaps in this case. Whereas, the peak observed at istic feature of Cu results in the highest catalytic activity.41
371 1C for Zn-FAG can be assigned to the reduction of Fe3+ to Another interesting observation in the present study is the
Fe3O4, since ZnO to Zn formation generally takes place at higher absence of any methanation reaction while the catalyst Ni-
temperature, as it is an irreducible oxide.38
FAG was used in the HT-WGS reaction (Fig. 4). Although Ni is
Fig. 3 depicts the catalytic response of MFAG catalysts in a good promoter in WGS reaction it is known for activation of
HT-WGS reaction. The selectivity towards CH4 and CO2 over the methanation reaction.42 In the present study, next to Cu-FAG,
MFAG catalysts is shown in Fig. 4. All the catalysts showed the catalyst Ni-FAG showed a moderate performance without
100% selectivity to CO2. During the HT-WGS, each catalyst any side reaction. This particular observation suggests that
showed its unique response during the catalytic reaction. there is a collective behavior between the promoter Ni and
Among those, a Cu promoted catalyst (Cu-FAG) exhibited the the iron oxide matrix in the CO adsorption and O–H bond
breaking steps. The TPR of Ni-FAG (Fig. 2) also confirms this by
exhibiting a single peak as a result of synergism between Ni and
iron oxide. As shown in Fig. 3, when compared to Co-FAG, the
catalyst Zn-FAG exhibited higher conversion in the whole range
of reaction temperature. This particular observation is very
different from the literature where Co is known to promote
WGS reaction better than Zn.7 In the present case, a plausible
explanation can be given based on two reasons: (1) ZnO is an
amphoteric oxide and hence it can behave as a basic oxide.43 In
the WGS reaction basic oxides favor the O–H bond dissociation
step, which is considered to be an important step that takes
place at first during the reaction.44 (2) Structurally Zn has
strong preference to the tetrahedral (A) site in the spinel.45 It
is proposed that the expansion of the tetrahedral site and
contraction of the octahedral site result in an improved electron
Fig. 3 CO conversion as a function of reaction temperature over MFAG hopping mechanism.46 The incorporation of Zn can possibly
catalysts (M = Cu, Ni, Zn, and Co) (H2O/(CH4 + CO + CO2) = 2.0; GHSV =
48 269 hÀ1).
result in the expansion of the ‘A’ site. As a result, the covalency
effect in terms of Fe3+ 2 Fe2+ exchange can be modified
resulting in an enhanced electron hopping mechanism. Co
prefers to occupy the octahedral (B) site in the spinel.45 This
would result in the partial replacement of the Fe ion (Fe2+ or
Fe3+) depending on the oxidation state of Co, as it can exist in
Co2+ and Co3+ states. With the partial replacement and/or
displacement of Fe, the change in the overall redox mechanism
has to be taken into consideration. A similar replacement due to
Zn in the A site is also possible. However, it does not have any
direct impact based on the oxidation state of the Fe ion, since the
‘A’ site is not a catalytically active surface site.46 Nevertheless, the
scenario may change with the systematic replacement of either
the Zn or Co promoter. That requires a systematic study, which
is beyond the scope of the present study. Overall, a rapidly
synthesized iron oxide incorporated with various metal promo-
ters together with aluminum follows the CO conversion in the
Fig. 4 Selectivity to CO2 and CH4 with reaction temperature over MFAG
order Cu-FAG 4 Ni-FAG 4 Zn-FAG 4 Co-FAG, under the
present reaction conditions.
catalysts (M = Cu, Ni, Zn, and Co) (H2O/(CH4 + CO + CO2) = 2.0; GHSV =
48 269 hÀ1).
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3.3. Effect of Cu loading
According to the reaction results for HT-WGS, Cu exhibited the
highest catalytic performance. Hence, to optimize the promo-
tional effect of Cu, catalysts with various Cu mole percent
values were prepared. The corresponding catalyst composition
and sample codes are given in Table 2.
Fig. 5(a) depicts the XRD patterns of Cu2.5FAG–Cu15.0FAG
catalysts before the HT-WGS reaction. All the catalysts exhibit the
pattern corresponding to magnetite (ICDD = 89-4319). The
absence of any other phases other than magnetite suggests that
the catalysts are phase pure. Although the phase can be assigned
to magnetite there is a possibility of the existence of g-Fe2O3, at
the surface, a structural analogue of magnetite. It differs from
magnetite with the characteristics of cation deficiency. Hence, the
phase formed in the present study can be either cation deficient
magnetite and/or g-Fe2O3. In such a case also the transformation
Fig. 6 TPR patterns of Cu(X)FAG catalysts (X = 2.5, 5.0, 10.0, 12.5, and
15.0).
to magnetite is easier through the topotactic transformation.47
After the HT-WGS reaction XRD patterns are similar to fresh
catalysts as shown in Fig. 5(b). This ascertains that there was no
structural alternation after the HT-WGS reaction.
Table 2 Sample code, composition of the M : Fe : Al catalyst where M =
Cu, and surface area
Fig. 6 depicts TPR profiles of Cu2.5FAG–Cu15.0FAG. All the
samples show two major peaks. The low temperature peak
(170–240 1C) can be assigned to the reduction of Cu2+ to Cu0.
Whereas the high temperature peak (420–440 1C) is due to the
reduction of Fe3+ to Fe3O4 based on the literature.20 In the case
of CuO reduction, a shoulder peak is observed together with the
main peak. This particular feature, in the case of Cu, is due to
the reduction of highly dispersed or isolated surface species of
Cu present in the catalyst system.48,49
Catalyst
M : Fe : Al
Cu2.5FAG
Cu5.0FAG
Cu10.0FAG
Cu12.5FAG
Cu15.0FAG
02.5 : 87.5 : 10.0
05.0 : 85.0 : 10.0
10.0 : 80.0 : 10.0
12.5 : 78.5 : 10.0
15.0 : 75.0 : 10.0
To investigate the effect of Cu loading on the catalytic
reaction, all catalysts were subjected to HT-WGS reaction.
Fig. 7 shows that the CO conversion increases with an increase
in Cu loading up to 12.5 mol%. At 350 1C, Cu2.5FAG showed
18% CO conversion whereas it reached 69% for Cu10.0FAG.
The catalyst Cu12.5FAG exhibited the highest performance. It
showed a CO conversion value of 81% at 350 1C. Furthermore,
the CO conversion reached the highest value of 87% at 400 1C.
At 550 1C, interestingly, the activity reached the equilibrium
conversion. A decrease in the CO conversion profile was seen
Fig. 7 CO conversion as
a function of reaction temperature over
Fig. 5 XRD patterns of Cu(X)FAG catalysts (X = 2.5, 5.0, 10.0, 12.5, and
15.0) (a) before reaction and (b) after reaction.
Cu(X)FAG catalysts (X = 2.5, 5.0, 10.0, 12.5, and 15.0) (H2O/(CH4 + CO +
CO2) = 2.0; GHSV = 48 269 hÀ1).
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A rapid, simple, facile, and cost effective synthetic methodology
was proposed for the direct synthesis of magnetite catalysts for
high temperature water gas shift reaction. One of the cheapest
amino acid, glycine, was used for the synthesis. The zwitterionic
nature of glycine favored the complexation of various metal ions.
Glycine served the role of fuel in solution combustion synthesis.
Also, glycine favored the direct formation of magnetite, by
creating an in situ reducing atmosphere during the synthesis.
The versatility of the approach was proven by showing incorpora-
tion of various transition metal promoters such as Cu, Ni, Zn,
and Co. A structural promoter Al was also included in all the
catalyst formulations. The X-ray diffraction measurement of
fresh and used catalysts shows a peak assigned to magnetite,
suggesting the phase formation and stability through retainment
of the phase after the high temperature water gas shift reaction.
There is no additional peak corresponding to any of the transi-
tion metal promoters, which confirms that the promoters occupy
the lattice well. Among the various promoters copper showed a
reasonably good conversion at a very high gas hourly space
velocity value of 48629 hÀ1. Moreover, during the optimization
of Cu loading, the catalyst with 12.5 mol% of Cu showed a
maximum CO conversion value of 87% at 400 1C. There was no
specific reduction step before the high temperature water gas
shift reaction. This reveals that the present method offers
economical feasibility by avoiding the passage of excess of steam
during the reduction step. In conclusion, a single step rapid
synthesis method was proposed for the preparation of the active
phase magnetite catalyst incorporated with various transition
metal promoters for high temperature water gas shift reaction.
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23 H.-S. Roh, D.-W. Jeong, K.-S. Kim, I.-H. Eum, K. Y. Koo and
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24 D.-W. Jeong, H. S. Potdar, K.-S. Kim and H.-S. Roh, Bull.
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25 D.-W. Jeong, H. S. Potdar and H.-S. Roh, Catal. Lett., 2012,
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This research was supported by Basic Science Research Program
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Science, ICT and Future Planning
(2013R1A1A1A05007370).
26 H.-S. Roh, H. S. Potdar, D.-W. Jeong, K.-S. Kim, J.-O. Shim,
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27 W.-J. Jang, D.-W. Jeong, J.-O. Shim, H.-S. Roh, I. H. Son and
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28 D.-W. Jeong, H. S. Potdar, J.-O. Shim, W.-J. Jang and
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29 D.-W. Jeong, V. Subramanian, J.-O. Shim, W.-J. Jang,
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