Cr3+-Co0.054Ni0.018Mg0.93O solid-solution catalysts for high-pressure syngas production: Effect of chromium on the reduction and catalysis

Article abstract of DOI:10.1021/cs400199b

Adding Cr to heat-tolerant Co_0.054Ni_0.018Mg _0.93O solid-solution catalysts for CH₄/CO ₂/H₂O reforming at 2.1 MPa allowed us to control the key properties determining the behavior of the supported catalysts, such as the reduction degree of the metal, metal particle size, and amount of exposed zerovalent metals. A small load of Cr [Cr/(Co + Ni) = 0.11 mol %] efficiently catalyzed Co²⁺ and Ni²⁺ reduction. This catalyst, which had abundant small alloy particles on the support material, resisted coking and oxidative deactivation, even under accelerated deterioration conditions. X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, extended X-ray absorption fine structure, and scanning transmission electron microscopy-energy-dispersive X-ray measurements indicated that Cr existed as a trivalent cation in a Cr_xCo_0.054Ni_0.018Mg _0.93O_1+1.5x solid solution, leading to the formation of a divalent cation vacancy and that Cr³⁺ was abundant on the surface of the catalyst particles after calcination at 1373 K. The Cr³⁺ destabilized Co²⁺ and Ni²⁺ in the solid solution, especially on the surface of the catalyst particles and, thus, promoted Co ²⁺ and Ni²⁺ reduction. However, excess Cr³⁺ induced sintering of the catalyst particles, resulting in aggregation of alloy particles and, thus, in an increased risk of coking.

Full text of DOI:10.1021/cs400199b

Research Article
pubs.acs.org/acscatalysis
Cr3+−Co0.054Ni0.018Mg0.93O Solid-Solution Catalysts for High-Pressure
Syngas Production: Effect of Chromium on the Reduction and
Catalysis
Katsutoshi Nagaoka,*,†,‡,§ Yosuke Abe,† Yusaku Hashimoto,† Takahiro Ishikawa,† Katsutoshi Sato,†,⊥
Yusaku Takita,† Toshiya Wakatsuki,∥ Masahiro Kunisu,¶ Eri Suda,¶ and Shin Inamoto¶
†Department of Applied Chemistry, Faculty of Engineering, Oita University, 700 Dannoharu, Oita 870-1154, Japan
‡JST CREST, Sanbancho 5, Chiyoda-ku, Tokyo 102-0075, Japan
§Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan
⊥Energy Technology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono,
Tsukuba, Ibaraki 305-8568, Japan
∥JAPEX Research Center, Japan Petroleum Exploration Co., Ltd., 1-2-1 Hamada, Mihama-ku, Chiba 261-0025, Japan
¶Toray Research Center, Inc., 3-7 Sonoyama 3-chome, Otsu, Shiga 520-8567, Japan
S
* Supporting Information
A B S T R A C T : A d d i n g C r t o h e a t - t o l e r a n t
Co0.054Ni0.018Mg0.93O solid-solution catalysts for CH4/CO2/
H2O reforming at 2.1 MPa allowed us to control the key
properties determining the behavior of the supported catalysts,
such as the reduction degree of the metal, metal particle size,
and amount of exposed zerovalent metals. A small load of Cr
[Cr/(Co + Ni) = 0.11 mol %] efficiently catalyzed Co2+ and
Ni2+ reduction. This catalyst, which had abundant small alloy
particles on the support material, resisted coking and oxidative
deactivation, even under accelerated deterioration conditions.
X-ray photoelectron spectroscopy, X-ray absorption near-edge
structure, extended X-ray absorption fine structure, and scanning transmission electron microscopy−energy-dispersive X-ray
measurements indicated that Cr existed as a trivalent cation in a CrxCo0.054Ni0.018Mg0.93O1+1.5x solid solution, leading to the
formation of a divalent cation vacancy and that Cr3+ was abundant on the surface of the catalyst particles after calcination at 1373
K. The Cr3+ destabilized Co2+ and Ni2+ in the solid solution, especially on the surface of the catalyst particles and, thus, promoted
Co2+ and Ni2+ reduction. However, excess Cr3+ induced sintering of the catalyst particles, resulting in aggregation of alloy
particles and, thus, in an increased risk of coking.
KEYWORDS: tailoring catalyst, solid-solution, chromium, methane, reforming, syngas production
feed gas. However, the risk of catalyst degradation due to
coking increases under such conditions.7,19,20 Noble metal-
based catalysts are less sensitive to carbon deposition, but are
expensive.7,8,21 Of the various supported metal catalysts,22−24
Ni and Co supported on MgO, formed by reduction of the
corresponding MgO solid solutions, are effective catalysts and
are resistant to coking and sintering during CH4 reforming with
H2O and CO2.11,19,25−27 The solid-solution preparation
method has advantages over general impregnation methods
because the former results in the formation of smaller metal
particles that interact more strongly with the MgO support.
However, after this catalyst is calcined at high temperature to
increase its thermal stability, the amount of zerovalent metal
INTRODUCTION
■
As petroleum resources continue to be depleted, the use of
natural gas (CH4) to produce chemicals and generate power
will become increasingly necessary.1 Reforming CH4 with CO2
(CH4 + CO2 ⇄ 2H2 + 2CO) or H2O (CH4 + H2O ⇄ 3H2 +
CO) to produce syngas (CO + H2) is attracting renewed
attention because advances in shale gas technology have
increased the global supply of recoverable CH42−6 and because
the process consumes CO2, a global warming gas.1 H2O
reforming of CH4 gives syngas with a higher H2/CO ratio than
does CO2 reforming, and these two processes can be used in
combination to adjust the H2/CO ratio.7−10 For example,
syngas with a H2/CO ratio of 2 is used for gas-to-liquid (GTL)
technology,11−13 such as the production of ultraclean liquid
fuels via the Fischer−Tropsch process.11−18 To increase the
efficiency of GTL processes, reforming should be performed at
high pressure (>2 MPa) with low O/C and H/C ratios in the
Received: November 15, 2012
Revised: May 22, 2013
© XXXX American Chemical Society
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exposed to the feed gas is low, owing to the poor reducibility of
Ni and Co. For example, only 2.9% of Ni atoms are reduced
when reduction is carried out at 1123 K for 0.5 h.28 The low
amount of zerovalent metal exposed leads to oxidative
deactivation of the catalysts at the low temperatures commonly
found at the inlet of reformers under real-world operation
conditions.28,29 Adding small amounts of a noble metal retards
Ni oxidation in the solid-solution catalysts,28,30,31 and the
resulting catalysts resist oxidative deactivation owing to
increased Ni2+ and Co2+ reduction and enhanced dissociative
adsorption of CH4;28 however, the use of even trace amounts of
noble metals is not economical.
As an alternative means to increase the amount of exposed
zerovalent metal in these catalysts, we previously explored the
addition of inexpensive 11 mol % Cr to Co + Ni and found that
the addition promotes Co2+ and Ni2+ reduction and retards
oxidative deactivation of a Co0.054Ni0.018Mg0.93O solid-solution
catalyst.29 In that research, a Co and Ni bimetallic catalyst was
used to control the kinetics of the reaction of CH4, CO2, and
H2O.32,33 However, the CoNi alloy particles are large (>50 nm)
and, thus, are subject to coking during long-term reactor
operation.34,35 Furthermore, we did not study the state or
location of the Cr.
In this study, we investigated the influence of Cr load on the
physicochemical properties of Cr−Co0.054Ni0.018Mg0.93O solid
solutions using X-ray photoelectron spectroscopy (XPS), X-ray
absorption near-edge structure (XANES) and extended X-ray
absorption fine structure (EXAFS) measurements along with
scanning transmission electron microscopy with energy-
dispersive X-ray (STEM−EDX) analysis. The results elucidated
the state and function of Cr during H2 activation. On the basis
of the results, we prepared a 0.11 mol % Cr−
Co0.054Ni0.018Mg0.93O solid-solution catalyst with abundant
small alloy particles on the catalyst particles. The catalyst
resisted coking and oxidative deactivation in CH4/CO2/H2O
reforming at high pressure (2.1 MPa), even under accelerated
degradation conditions.
catalysts were reduced in situ at 973 K for 1 h and cooled to
323 K under Ar. At 323 K, H2 was pulsed over the catalysts.
Subsequently, the catalysts were heated to 1123 K in an Ar
flow, and O2 was pulsed over them at 1123 K. H2 and O2
uptake amounts were measured with a thermal conductivity
detector for these experiments. To calculate the amount of
exposed zerovalent metal, we assumed that each metal site
chemisorbed one hydrogen atom. The total reduction degrees
of Co and Ni were calculated by assuming oxidation of Ni0 and
Co0 to NiO and CoO.
X-ray diffraction analysis was performed with a RINT-2000
X-ray diffractometer (Rigaku, Japan) with monochromatized
Cu Kα radiation. Crystallite sizes of MgO were calculated from
line broadening with the Scherrer equation.36
X-ray photoelectron spectra were obtained with an ESCA-
850 spectrophotometer (Shimadzu, Japan). The samples were
transferred to the chamber of the XPS instrument before and
after H2 activation. To investigate the state of the Cr in the bulk
as well as surface of the catalyst particles, Ar+ ion beam etching
of the samples was conducted at an energy of 2 keV and a
current of 20 mA. The binding energy was calibrated, assuming
that the Mg 2p core level was 51.5 eV .37 Throughout the
measurements, the Mg 2p peak did not split and retained the
same shape.
X-ray absorption spectra were measured at the Photon
Factory (Japan) on the BL9A and BL12C beamlines with a
Si(111) monochromator. The spectra of the catalysts were
measured before and after H2 activation. The activated catalysts
were again reduced in H2 at 973 K for 1 h. After the catalysts
were cooled to room temperature, their X-ray absorption
spectra were measured without exposure to air. X-ray
absorption spectra were recorded at the Cr K-edge (5988.8
eV) and analyzed with the Athena. The k3-weighted EXAFS
oscillation in the 2−10 Å−1 region was Fourier transformed. In
addition, the absorption spectra of MgCrO4 in MgO, 1.1 mol %
Cr/MgO, and MgCr2O4 were measured as references. For that
purpose, 10 wt % Cr/MgO was prepared by wet impregnation
of Cr(NO3)3·9H2O solutions and dried at 343 K overnight
before being calcined at 723 K for 5 h in flowing air. Powder X-
ray diffraction was used to confirm that Cr existed as MgCrO4
in the resultant 10 wt % Cr/MgO. In addition, a 1.1 mol % Cr/
MgO sample was prepared by the impregnation method and
further calcined in air at 1373 K for 5 h. MgCr2O4 was prepared
by a polymerizable complex method (based on polyester-
ification between citric acid and ethylene glycol) from
Cr(NO3)3·9H2O and Mg(NO3)2·6H2O and was calcined at
1173 K. Formation of MgCr2O4 was confirmed by X-ray
diffraction analysis.
EXPERIMENTAL SECTION
■
Preparation of Catalysts. Co3O4−NiO/MgO (Co/Ni =
3/1 mol/mol, 10 wt % total) was prepared by wet impregnation
of an aqueous solution of Co(NO3)3·6H2O (Wako Pure
Chemical Industries, Japan) and Ni(NO3)2·6H2O (Wako) with
MgO (JRC-MGO500A, Catalysis Society of Japan, Japan),
which had been precalcined at 1073 K for 5 h. Chromium, in
the form Cr(NO3)3·9H2O solutions (Wako) [Cr/(Co + Ni) =
0.11−61 mol %, refereed to x mol % Cr], was added
simultaneously with the Co and Ni precursors. The samples
were dried at room temperature and at 343 K overnight and
then calcined at 723 K for 5 h in flowing air. All the catalysts,
referred to as x mol % Cr−Co0.054Ni0.018Mg0.93O solid-solution
catalysts, were prepared by calcination at 1373 K for 5 h in
static air. The resulting powders were pressed into pellets at 1.2
MPa for 3 min. The pellets were crushed and sieved to obtain
grains with diameters between 250 and 500 μm. All the catalyst
grains were activated ex situ with H2 at 1173 K for 20 h.
Characterization of Catalysts. The specific surface areas
of the catalysts were measured with a BELSORP-mini (BEL
Japan, Japan) at 77 K with N2 as the analysis gas. The amount
of H2 chemisorbed at 323 K (a measure of the amount of
exposed zerovalent metal) and the amount of O2 absorbed at
1123 K (a measure of the total reduction degrees of Co and Ni)
were determined by pulse methods as follows. The activated
HAADF-STEM and EDX analysis were carried out with a
JEM-2100F microscope (JEOL, Japan) at an accelerating
voltage of 200 keV. The samples were thinned to <100 nm
by mechanical polishing and then subjected to Ar-ion milling,
after which cross sections were observed. A beam size of 1 nm
was used for EDX spot analysis.
Deposited carbon was quantified by a temperature-
programmed oxidation method. After the reforming reaction,
the catalyst was heated to 1273 K at a rate of 10 K min−1 in an
O2/Ar mixture (1/19, 30 mL min−1). COx gases derived from
deposited carbon were passed through a methanator and then
monitored with a flame ionization detector.
Tests of Catalyst Activity. A sample of each catalyst
(100−400 mg) was loaded into a tubular Inconel reactor
passivated with an aluminum diffusion coating. After reduction
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in situ with H2 at 973 K for 1 h, the catalyst was cooled to
reaction temperature, and the pressure was increased from 0.1
to 2.1 MPa. Then a 1/1/0.5 (mol/mol) CH4/H2O/CO2
mixture [1123 K, space velocity (SV) = 3000 h−1], a 1/0.75/
0.75 CH4/H2O/CO2 mixture (873 K, SV = 3000 h−1), or a 1/
1.5 CH4/H2O mixture (923 K, SV = 72 000 h−1) was passed
over the catalyst. The reaction products were analyzed by gas
chromatography with thermal conductivity detection (GC-8A,
Shimadzu, Japan). After the reaction, the reactor was purged
with Ar at the reaction temperature, and then the catalyst was
cooled to room temperature and used for further analysis.
zerovalent metal exposed remained nearly the same when the
Cr load was increased to 1.1 mol %, owing to the combined
effect of increasing the reduction degree of Co and Ni and
aggregation of the metal particles. Increasing the Cr load
further to ≥2.8 mol % drastically decreased the amount of
zerovalent metal exposed, owing to aggregation of the metal
particles. Hence, we concluded that Cr acted as a glue for
sticking the support materials to each other and thus induced
aggregation of the metal particles in the presence of excess Cr
at loads ≥0.28 mol %. To determine the mechanism by which
Cr promoted Co2+ and Ni2+ reduction, we characterized the
catalysts by means of XPS, XANES, EXAFS, and STEM−EDX
analyses.
RESULTS AND DISCUSSION
■
To determine the states of Cr, Cr 2p3/2 XP spectra of 11 mol
% Cr−Co0.054Ni0.018Mg0.93O solid-solution catalysts before and
after H2 activation were measured (Figure 2). On the other
Characterization of the Catalyst. The reduction degrees
of Co and Ni after H2 activation at 1173 K increased drastically
from 8 to 50% with the addition of 0.11 mol % Cr, and a Cr
load of ≥1.1 mol % resulted in almost complete reduction of
Co2+ and Ni2+ in the solid solution (Figure 1A). In addition, a
Figure 1. Influence of Cr load on physicochemical properties of the
solid-solution catalysts after reduction at 1173 K for 20 h: (A)
reduction degree of CoNi and specific surface area and (B) amount of
zerovalent metal exposed and average metal particle size.
Cr load of 0.11 mol % increased the specific surface area of the
catalysts from 17 to 20 m2 g−1. However, further increases in
the Cr load induced sintering of the catalysts: the specific
surface area of the catalysts decreased to 3 m2 g−1 at Cr loads of
≥2.8 mol %. Such catalyst sintering was facilitated by an
increase in the crystallite size of MgO (Supporting Information
Figure S1). In short, addition of Cr promoted reduction of
Co2+ and Ni2+ but induced sintering of the support material,
leading to aggregation of metal particles (Figure 1B): the
average metal particle size increases from 13 nm at 0 mol % Cr
to 96 nm at 61 mol % Cr. These results provide a reasonable
explanation for the observed changes in the amount of
zerovalent metal exposed as a function of increasing Cr load
(Figure 1B). The addition of 0.11 and 0.28 mol % Cr increased
the amount of zerovalent metal exposed from 1.4 to 8.2 and 11
μmol g−1, respectively, owing to a drastic increase in the
reduction degree of Co and Ni. No such increases in reduction
degree and amount of zerovalent metal exposed were reported
previously when noble metals were added to improve the
reducibility of Ni2+ or Co2+.28,30,31 In contrast, the amount of
Figure 2. X-ray photoelectron spectra of Cr in 11 mol % Cr−
Co0.054Ni0.018Mg0.93O solid-solution catalysts: (a) spectra of Cr 2p3/2
orbital before and after H2 activation; (b) depth profile of the atomic
ratio.
hand, XP spectroscopy was not sensitive enough for analysis of
the 0.11 and 1.1 mol % Cr catalysts. Without Ar+ etching, the
peak positions of the samples before and after H2 activation
were 577.2 and 577.1 eV, respectively. Furthermore, Ar+
etching and XPS measurements were repeated to investigate
the state of the Cr in the bulk of the catalysts particles and
atomic ratio of Cr/Mg. Irrelevance to the absence or presence
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of the H2 activation, the peak position was not shifted, even if
Ar+ etching was prolonged for 3000 s. On the other hand, peak
positions of Cr2O3 and K2CrO4 were 576.9 and 579.7,
respectively. These results indicate that Cr exists as a trivalent
cation in the bulk as well as at the surface of the catalyst
particles. Note that the Cr/Mg atomic ratio was decreased with
increase in Ar+ etching time and approached the value of the
total concentration, 0.09, determined by ICP-AES measure-
ments. Therefore, it was revealed that Cr3+ is abundant on the
surface of the 11 mol % Cr−Co0.054Ni0.018Mg0.93O solid-
solution catalyst before and after H2 activation.
Although we carried out XRD measurements to investigate
the structure of the phase including Cr, sensitivity of the
methods was not sufficient for all the samples. To obtain
information for the local structure of the Cr, we compared the
Cr K-edge XANES spectra of 0.11−11 mol % Cr−
Co0.054Ni0.018Mg0.93O solid-solution catalysts before and after
H2 activation with the spectra of Cr0 foil, 1.1 mol % Cr/MgO
calcined at 1373 K, MgCr2O4, and MgCrO4 (Figure 3). We
predicted by the phase equilibrium diagram39 for the Cr−Mg−
O system.
The peak positions in the XANES spectra of the 0.11, 1.1,
and 11 mol % Cr catalysts before H2 activation were similar to
the positions in the spectrum of the 1.1 mol % Cr/MgO,
although the pre-edge peak decreased with an increase in the
Cr load and disappeared for 11 mol % Cr catalyst. Here, it must
be recalled that XPS measurements (Figure 2) revealed that Cr
in 11 mol % Cr catalyst before H2 activation was trivalent.
Therefore, in combination with the results obtained by XANES
and XPS, it was indicated that Cr3+ is included in the solid
solution of 11 mol % Cr catalyst before H2 activation. After H2
activation at 1173 K, the 0.11 and 1.1 mol % Cr catalysts still
exhibited spectra similar to the spectrum of the 1.1 mol % Cr/
MgO except for the disappearance of the pre-edge band. Here,
0.11 and 1.1 mol % Cr catalysts before H2 activation were
prepared by calcination at 1373 K in static air after wet
impregnation of Co, Ni, and Cr precursors. Although the phase
equilibrium diagram for the MgO-Cr2O338 does not predict the
formation of Cr6+ at 1373 K, these results indicate that some of
the Cr3+, probably on the surface of the catalyst, was oxidized to
Cr6+, formation of which was predicted by the phase
equilibrium diagram39 for the Cr−Mg−O system, during the
cooling process after calcination. The Cr6+ was likely to be
reduced to Cr3+ during H2 activation.
In contrast, the spectrum of the 11 mol % Cr catalyst was
similar to the spectrum of the 1 mol % Cr/MgO only before H2
activation; after H2 treatment, the spectrum was nearly identical
to that of MgCr2O4. The valence of Cr in the compounds was
in agreement with the results of XPS measurements.
We compared k3-weighted Cr K-edge EXAFS oscillations
and their Fourier transforms for the 1.1−11 mol % Cr−
Co0.054Ni0.018Mg0.93O solid-solution catalysts with those for the
reference samples (Figure 4). Although EXAFS spectroscopy
was not sensitive enough for analysis of the 0.11 mol % Cr
catalyst, we speculate that the state of Cr in this sample was
similar to that in the 1.1 mol % Cr sample except for the Cr6+/
(Cr6+ + Cr3+) ratio, owing to the similarity of the XANES
spectra of the two catalysts. For the 1.1 mol % Cr catalyst
before and after H2 activation and the 11 mol % Cr catalyst
before H2 activation, the shape of the Cr K-edge EXAFS
oscillations were similar to that of 1.1 mol % Cr/MgO.
Furthermore, the first and second shell peaks in the FT of
EXAFS spectra for these three samples and 1.1 mol % Cr/MgO
were observed at the same position. These results support
formation of solid solutions, as indicated by Cr K-edge XANES,
where Cr3+ was isolated and substituted for Mg2+ in the MgO
lattice. The excess single positive charge of Cr3+ with respect to
Mg2+ was likely compensated for by the formation of a divalent
cation vacancy for the 1.1 and 11 mol % Cr catalysts before H2
activation. After H2 activation at 1173 K, the 1.1 mol % Cr
catalyst still existed as a solid solution.
Figure 3. Cr K-edge XANES for Cr−Co0.054Ni0.018Mg0.93O solid-
solution catalysts before and after H2 activation at 1173 K.
On the other hand, for the 11 mol % Cr catalyst after H2
activation, the shape of the Cr K-edge EXAFS oscillations was
identical to that of MgCr2O4. The first and second shell peaks
in the FT of EXAFS spectra for this sample were shifted to a
slightly shorter position compared wtih the 11 mol % Cr
catalyst before H2 activation and identical to those of MgCr2O4.
This means that a phase transition from a solid solution to a
composite oxide with spinel phase, that is, MgCr2O4, occurred
for the 11 mol % Cr catalyst during H2 activation. Excess Cr3+
reportedly forms electrically neutral MgCr2O4 clusters that
provide their own internal charge compensation.40
chose these reference samples by consulting phase equilibrium
38
diagrams for the MgO−Cr2O3 and Cr−Mg−O39 systems.
The phase equilibrium diagram38 for the MgO−Cr2O3 system
and the research reported by Thorp et al.40 indicated formation
of CrxMgO1+1.5x solid solution (<5 mol % Cr), where Cr3+ was
isolated and substituted for Mg2+ in the MgO lattice, and
MgCr2O4 (>5 mol % Cr) at 1373 K. Because peak positions of
1.1 mol % Cr/MgO calcined at 1373 K were different from
those of MgCr2O4, we believe that the former sample contains
mainly CrxMgO1+1.5x solid solution. On the other hand, at
temperatures lower than 1173 K, formation of MgCrO4 was
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Figure 4. . k3-Weighted Cr K-edge EXAFS oscillations (A) and their Fourier transforms (B) for Cr−Co0.054Ni0.018Mg0.93O solid-solution catalysts
before and after H2 treatment at 1173 K.
Figure 5. (A) HAADF−STEM image, (B) reconstructed overlay image of the maps shown in panels D−F, (C) Mg−K STEM−EDX map, (D) Cr−
K STEM−EDX map, (E) Co−K STEM−EDX map, and (F) Ni−K STEM−EDX map for the 11 mol % Cr−Co0.054Ni0.018Mg0.93O solid-solution
catalyst before H2 activation.
We used high-angle annular dark field (HAADF) STEM and
EDX elemental mapping to investigate the location of the Cr in
the 0.11−11 mol % Cr−Co0.054Ni0.018Mg0.93O solid-solution
catalysts before H2 activation. The samples were thinned to
<100 nm thickness by mechanical polishing and were then
subjected to Ar ion milling, after which cross sections were
observed. At this thickness, the surface and bulk of the catalyst
particles, which had diameters of >200 nm, were discriminable
in the STEM images. Mg, Co, and Ni were distributed
homogeneously throughout the surface and bulk of the particles
in the 11 mol % Cr catalyst (Figure 5), indicating that Co2+ and
Ni2+ were dissolved into the bulk of the MgO particles and that
a homogeneous solid solution was formed by calcination at
1373 K. These results conflict with our hypothesis that the
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Figure 6. (A) HAADF−STEM image, (B) reconstructed overlay image of the maps shown in panels D−F, (C) Mg−K STEM−EDX map, (D) Cr−
K STEM−EDX map, (E) Co−K STEM−EDX map, and (F) Ni−K STEM−EDX map for the 11 mol % Cr−Co0.054Ni0.018Mg0.93O solid-solution
catalyst after H2 activation.
Figure 7. (A) HAADF−STEM image, (B) reconstructed overlay image of the maps shown in panels D−F, (C) Mg−K STEM−EDX map, (D) Cr−
K STEM−EDX map, (E) Co−K STEM−EDX map, and (F) Ni−K STEM−EDX map for the 1.1 mol % Cr−Co0.054Ni0.018Mg0.93O solid-solution
catalyst after H2 activation.
addition of Cr promoted Co2+ and Ni2+ reduction by disturbing
the dissolution of Co2+ and Ni2+ into MgO. In contrast,
although Cr was present both on the surface and in the bulk of
the catalyst particles, a reconstructed overlay image of Cr, Co,
and Ni (Figure 5B) suggested that Cr was more abundant on
the surface, which was in agreement with the results of XPS.
The homogeneous distribution of Co2+ and Ni2+ and the
abundance of Cr on the surface of the 1.1 mol % Cr catalysts
before H2 activation are shown in Supporting Information
Figures S2 and S3. EDX spot analysis indicated that Cr was
abundant on the surface. For the 0.11 mol % Cr catalyst, a
homogeneous distribution of Co2+ and Ni2+ is shown in
Supporting Information Figure S4. Although even EDX spot
analysis was not sensitive enough for analysis of Cr, we
speculate that the location of the Cr in this sample was similar
to that in the 1.1 and 11 mol % Cr samples. Cr must have
existed as a trivalent cation in a CrxCo0.054Ni0.018Mg0.93O1+1.5x
solid solution, as indicated by the XPS, XANES, and EXAFS
measurements. Furthermore, the excess single positive charge
of Cr3+ with respect to Mg2+ was compensated for by the
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Figure 8. (A) HAADF−STEM image, (B) reconstructed overlay image of the maps shown in panels D−F, (C) Mg−K STEM−EDX map, (D) Cr−
K STEM−EDX map, (E) Co−K STEM−EDX map, and (F) Ni−K STEM−EDX map for the 0.11 mol % Cr−Co0.054Ni0.018Mg0.93O solid-solution
catalyst after H2 activation.
formation of a divalent cation vacancy. Hence, diffusion of the
Cr3+ from the surface to the bulk of the particles would not
have readily occurred, and therefore, Cr3+ would have remained
concentrated at the surface.
(Scheme 1). Before H2 activation (Scheme 1a), a
Cr0.00072Co0.054Ni0.018Mg0.93O solid solution existed. Because
Cr3+ was dissolved in the solid solution of Mg2+, Co2+, and Ni2+,
STEM and EDX elemental mappings of the 11 mol % Cr
catalyst after H2 activation (Figure 6) showed that Co and Ni
were present only on the surface of the catalyst particles,
indicating that upon activation, Co2+ and Ni2+ in the solid
solution moved from the bulk to the MgO surface, where they
were deposited as alloy particles. Because Cr was also visible at
the same position as Co and Ni, we can say that Co, Ni, and
trace amounts of Cr formed alloy particles. Furthermore, Cr
was apparently much more abundant on the surface than in the
bulk of the catalyst particles and was dispersed along the
surface. Such an abundance of Cr on the surface was in
agreement with the results of XPS. Chromium was also
observed in the bulk of the catalyst particles as clusters with
diameters of 10−20 nm. Considering these results in
conjunction with the results of Cr K-edge XANES and
EXAFS measurements, we can say that MgCr2O4 that formed
during H2 activation was distributed on the surface and in the
bulk of the catalyst particles. In addition, MgCr2O4 existed at
the catalyst particle boundaries, suggesting that Cr acted as a
glue to adhere the catalyst particles to each other (Figure 6),
thus inducing aggregation of the metal particles when the Cr
load was ≥0.28 mol %. The 1.1 mol % Cr catalyst also showed
CoNi particles only on the surface of the catalyst particles
(Figure 7). Unlike the 11 and 1.1 mol % Cr catalysts, the 0.11
mol % Cr catalyst showed Ni and Co in the bulk as well as on
the surface (Figure 8). This result was in agreement with the
results revealing incomplete reduction of Co and Ni for this
catalyst (Figure 1).
Scheme 1. Mechanism of the Promotion of Co2+ and Ni2+
Reduction by Cr3+ in 1.1 mol % Cr−Co0.054Ni0.018Mg0.93O:
Catalyst Composition (a) before, (b) during, and (c) after
H2 Activation
On the basis of the XPS, XANES, EXAFS, and STEM−EDX
results for the 1.1 mol % Cr−Co0.054Ni0.018Mg0.93O solid-
solution catalyst, we propose the following mechanism for the
promotion of Co2+ and Ni2+ reduction in the catalyst by Cr
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Research Article
divalent cation vacancies formed to neutralize the charge of the
solid solution. These phenomena destabilized Co2+ and Ni2+
(denoted M2+ hereafter) in the lattice of the MgO rock salt
structure. Cr3+ was more abundant on the particle surface than
in the bulk, indicating that M2+, especially on the particle
surface, was unstable, that is, easily reducible. At this stage,
some of the Cr6+ also existed because some of the Cr3+ was
oxidized during the cooling process after calcination at 1373 K.
The Cr6+ is not depicted in Scheme 1 because Cr6+ was easily
reduced in the subsequent reduction. When the catalyst was
exposed to H2 at 1173 K (Scheme 1b), M2+ in the surface-
unstable solid solution was reduced to its metallic state, M0,
thus decreasing the surface concentration of M2+ and
promoting M2+ diffusion from the bulk to the surface. The
Cr3+ in the bulk also enhanced M2+ diffusion. M2+ reduction
and regeneration were repeated until all the M2+ in the catalyst
was reduced (Scheme 1c). Because Cr reduced M2+ more
effectively at higher loads, M2+ reduction was insufficient at Cr
load of 0.11 mol %, but aggregation of CoNi alloy was
suppressed at the Cr load. In contrast, excess Cr loads
apparently glued the catalyst particles together and induced
sintering, which resulted in the aggregation of CoNi(Cr) alloy
particles. It should be noted that the mechanism by which Cr
promoted the reduction of M2+ is completely different from
that of conventional noble metals: H2 molecules dissociate to
hydrogen species on the surface of noble metal particles, and
the hydrogen species diffuse to the support surface and reduce
M2+.28,30,31
Figure 9. CH4 conversion vs time on-stream under (A) accelerated
coking conditions (CH4/CO2/H2O = 1/0.75/0.75, SV = 3000 h−1,
923 K) and (B) accelerated oxidation conditions (CH4/H2O = 1/1.5,
SV = 72 000 h−1, 873 K). The dashed line shows equilibrium
conversion.
Behavior of the Catalysts. Because 0−11 mol % Cr−
Co0.054Ni0.018Mg0.93O solid-solution catalysts showed stable
activity under practical syngas generation conditions at a H2/
CO ratio of ∼2 (2.1 MPa, CH4/H2O/CO2 = 1/1/0.5, 1123 K,
20 h; Supporting Information Figure S5) and only minor
coking (<0.1 wt %), we conducted tests under accelerated
deterioration conditions to evaluate the influence of Cr load on
catalyst activity. We plotted CH4 conversion vs time on-stream
at CH4/CO2/H2O = 1/0.75/0.75 (Figure 9A). Compared with
the conditions used for real-world GTL processes, the reaction
conditions studied here favored catalyst coking owing to the
lower O/C and H/C ratios and lower temperature,7 meaning
that reactions conducted under these conditions serve as an
accelerated coking test. For all the catalysts, we observed an
induction period of 5−6 h, corresponding to the time required
to replace the feed gas with the reactants in the cold trap
(volume ∼ 110 mL) at 2.1 MPa. After the induction period,
CH4 conversion reached approximately the equilibrium value.
Although coke deposition was negligible for the 0 and 0.11 mol
% Cr catalysts (<0.5 wt %), it increased with further increases
in the Cr load (1.5 wt % for 1.1 mol % Cr and 12 wt % for 11
mol % Cr). Apparently, coking was more favored on catalysts
with larger metal particles.34,35 These results indicate that for
the purposes of retarding coking, the Cr load should be ≤0.11
mol %, a value at which the average metal particle size was <20
nm.
indicate that the metals on the supports were oxidized by H2O
in the feed gas; that is, we ascribed the deactivation to Co0 and
Ni0 oxidation. This conclusion is supported by an observed
decrease in coking for all the catalysts (≤0.3 wt %). However,
adding a trace amount of Cr (0.11 mol %) improved catalytic
stability drastically, and the catalyst exhibited stable activity for
18 h. In contrast, increasing the Cr load to 11 mol % caused an
initial reduction in activity (3−12 h for the 1.1 mol % Cr
catalyst and 3−6 h for the 11 mol % Cr catalyst). We presume
that Cr0 on these catalysts was oxidized at the beginning of the
reaction and that some portion of the Co0 and Ni0 was
subsequently oxidized. These results thus indicate that the
optimum Cr load was 0.11 mol %; at this load, the metal
surface area was much higher than that at 0 mol % Cr, but Cr0
was not present in excess in the alloy particles.
CONCLUSIONS
■
Our results indicate that a small amount of Cr3+ [Cr/(Co + Ni)
= 0.11 mol %] in CrxCo0.054Ni0.018Mg0.93O1+1.5x solid-solution
catalysts efficiently catalyzed Co2+ and Ni2+ reduction and thus
tailored the CH4 reforming catalyst. Small alloy particles were
well dispersed on the catalyst support, and thus, the metal
surface area was high. The catalyst resisted coking and Co0 and
Ni0 oxidation, even under accelerated deterioration conditions.
However, these effects were limited to the relatively low Cr
loads; excess Cr induced sintering of the catalyst particles,
resulting in aggregation of alloy particles and thus in an
increased risk of coking. Catalysts tailored by means of the new
strategy described here, where composition of the surface layer
on thermally stable catalyst support is adjustable, should be
useful not only for reforming of other hydrocarbons but also for
other types of high-temperature reactions.
We next plotted CH4 conversion vs time on-stream at CH4/
H2O = 1/1.5 at 873 K (Figure 9B). Compared wtih real-world
GTL conditions, these conditions favored Co0 and Ni0
oxidation owing to the higher S/C and lower temperature,28,29
meaning that reactions conducted under these conditions serve
as an accelerated oxidative deactivation test. Again, a short
induction period of 2 or 3 h was observed for all the catalysts.
The 0 mol % catalyst was deactivated within 8 h, and the color
turned from ash to beige during deactivation. These results
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Research Article
(26) Chen, Y.-g.; Tomishige, K.; Fujimoto, K. Appl. Catal., A 1997,
161, L11−L17.
ASSOCIATED CONTENT
* Supporting Information
MgO crystallite size, STEM−EDX images, EDX spectra, and
catalytic activities. These materials are available free of charge
via the Internet at http://pubs.acs.org.
■
S
(27) Ruckenstein, E.; Wang, H. Y. Appl. Catal., A 2000, 204, 257−
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AUTHOR INFORMATION
Corresponding Author
*E-mail: nagaoka@oita-u.ac.jp.
Notes
■
(31) Nurunnabi, M.; Mukainakano, Y.; Kado, S.; Li, B.; Kunimori, K.;
Suzuki, K.; Fujimoto, K.; Tomishige, K. Appl. Catal., A 2006, 299,
145−156.
The authors declare no competing financial interests.
(32) Nagaoka, K.; Takanabe, K.; Aika, K. Appl. Catal., A 2004, 268,
151−158.
ACKNOWLEDGMENTS
■
(33) Takanabe, K.; Nagaoka, K.; Nariai, K.; Aika, K. J. Catal. 2005,
232, 268−275.
The authors thank M. Nomura, H. Abe, H. Nitani, Y. Niwa, and
J. Nishino at Photon Factory, KEK, Japan for their support and
guidance in the experiments. The authors thank Prof. K.
Teramura for fruitful discussion of XAFS analysis.
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