Influence of Metal Particle Size on Oxidative CO2 Reforming of

DOI: 10.1002/cctc.201500003
Full Papers
Influence of Metal Particle Size on Oxidative CO2
Reforming of Methane over Supported Nickel Catalysts:
Effects of Second-Metal Addition
Annabathini Geetha Bhavani,[a] Won Young Kim,[b] Jin Woo Lee,[b] and Jae Sung Lee*[a]
Oxidative CO2 reforming of methane is studied over Ni/AlCeZrOx catalyst with addition of a second metal element (Mg, Co,
La, Mn, or Fe) aiming to improve the performance of the catalyst and increase their resistance to coking. Addition of Mg
produces the best catalyst showing high CH4 and CO2 conversions as well as high selectivities to synthesis gas with no
severe coke deposition. Addition of Co, La, and Mn also exhibits significant improvement, whereas Fe brings only a moderate
improvement. The high activity is accompanied by high stabili-
ty and low amount of deposited coke. The high activity and
stability are attributed mainly to nanosized metallic particles,
which accompany high amounts of available surface oxygen,
and high basicity of the surface. The basicity of the catalyst
promotes adsorption and dissociation of CO2, and surface
oxygen can oxidize surface carbon intermediates to regenerate
active metal sites before they turn to coke to sustain catalytic
cycle.
Introduction
Methane reforming to synthesis gas (syngas) is an important
industrial reaction that provides the raw material for many processes in the chemical industry including industrial syntheses
of ammonia, methanol, and oxoalcohols; the Fischer–Tropsch
synthesis; in fuel cells and in direct reduction of iron ore.
Syngas typically consists of H2 and CO in different proportions.
A great deal of research has been conducted for a wide range
of natural gas reforming processes such as steam reforming,
CO2 (dry) reforming, partial oxidation, and oxidative CO2 reforming (autothermal reforming). The oxidative CO2 reforming
of CH4 is an attractive route to recycle CO2 as a feedstock,
which is the most important greenhouse gas. It involves both
exothermic partial oxidation and endothermic CO2 reforming,
making the process thermo-neutral. Oxidative CO2 reforming
also produces syngas with a low H2/CO ratio suitable for the
Fischer–Tropsch process and methanol synthesis. The most serious problem of standard CO2 reforming of methane is formation of coke that blocks the catalyst surface and eventually
leads to process shutdown.[1] The addition of oxygen to the
CO2 reforming process increases the methane conversion and
reduces the carbon deposition on the catalyst surface.[2]
2 CH4 þ O2 þ CO2 ! 3 H2 þ 3 CO þ H2 O
[a] Dr. A. G. Bhavani,+ Prof. J. S. Lee
School of Energy and Chemical Engineering
Ulsan National Institute of Science & Technology (UNIST)
50 UNIST-gil, Ulsan, 689-798 (Korea)
E-mail: [email protected]
[b] W. Y. Kim,+ Prof. J. W. Lee
Department of Chemical Engineering
Pohang University of Science and Technology (POSTECH)
San 31, Hyoja-Dong, Pohang 790-784 (Korea)
[+] These authors contributed equally to this work
ChemCatChem 2015, 7, 1445 – 1452
ð1Þ
Most transition metals catalyze the reforming reactions,[3]
but for economic reasons, Ni is the most commonly employed
catalyst material, although it is more prone to sintering and
coke formation than noble metals. These shortcomings of
nickel are made up by modifications with promoters[4] and
proper supports. Catalyst surface basicity is believed to help
suppress carbon deposition by promoting the dissociation of
CO2[4b, 5] on the catalyst surface. However, catalyst deactivation
is still a most serious problem for CO2 reforming of methane
over Ni-based catalysts.
The main objective of this research is to develop a coke-resistant Ni-based catalyst with high catalytic performance and
stability in oxidative CO2 reforming of methane. To design
a better Ni catalyst, the effects of a second metal element (Mg,
Co, La, Mn, or Fe) incorporation to Ni/AlCeZrOx catalyst were
studied. The most important effect of the second metal addition was the reduced metal particle sizes leading to increased
fractions of exposed metal (dispersion), larger oxygen release
under inert or reducing atmosphere, the increased number of
basic sites. As a result, the catalysts show greatly improved activity of reforming, resistance to sintering,[4b, 5b] and stability in
the catalytic reaction.[6] The influence of the metal particle size
was discussed on the properties of the catalysts and their catalytic performance in oxidative CO2 reforming of methane.
Results
Physical properties of the catalysts
The catalysts studied here employed a common composition
of Ni supported on Al2O3–CeO2–ZrO2 (Ni/ACZ) as the base catalyst. The three oxides comprising the support have been commonly used for methane reforming catalysts individually and
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Table 1. Metal compositions and BET surface areas of the catalysts.
Catalyst
Ni/AlCeZrOx
(Ni/ACZ)
Ni-Mg/AlCeZrOx
(Ni–Mg/ACZ)
Ni-Co/AlCeZrOx
(Ni–Co/ACZ)
Ni-Mn/AlCeZrOx
(Ni–Mn/ACZ)
Ni-La/AlCeZrOx
(Ni–La/ACZ)
Ni-Fe/AlCeZrOx
(Ni–Fe/ACZ)
Ni
Metal oxide composition [wt %]
2nd metal
Al
Ce
Zr
Surface area [m2 g¢1]
AsAfter
synthesized calcination[a]
10
0
10
2
78
8.6
21.1
10
10
10
2
68
39.7
58.9
10
10
10
2
68
27.4
47.8
10
10
10
2
68
24.1
44.5
10
10
10
2
68
25.4
46.6
10
10
10
2
68
11.5
29.2
[a] Calcination at 550 8C.
of 5.6 nm, and Ni/ACZ (without the 2nd metal) has
the largest size of 17.8 nm. The surface area values
correlate to the particle sizes of Ni0 determined from
XRD line broadening. Thus Ni–Mg/ACZ has the highest surface area and the smallest Ni0 crystallite size,
whereas Ni/ACZ and Ni–Fe/ACZ exhibit lower surface
areas and large Ni0 crystallite sizes (Table 2). The support has dominantly ZrO2 phase and some Ce–ZrO2
phase. No Al2O3 phase was detected probably because of its poor crystallinity.
We chose the representative Ni/ACZ, Ni–Co/ACZ,
and Ni–Mg/ACZ catalysts to visualize their possible
differences in metal particle morphology and size distribution after the reduction by high-resolution (HR)
TEM as shown in Figure 2. The differences in reducibility and basicity detected for these samples as dis-
in binary oxide forms, and the ternary ACZ oxide used here is
an optimized composition (weight ratio of Al2O3/CeO2/ZrO2 =
10:2:78) for high resistance to thermal sintering and coke deposition in our previous works.[4–5] Various second metals were
added to this Ni/ACZ catalyst. In Table 1 compositions and surface areas of both as-synthesized and calcined catalysts are
shown. The calcination at 550 8C for 4 h decomposed the precursors to form mixed metal oxides, which increased the BET
surface areas of all catalysts. The base Ni/ACZ catalyst had
a surface area of 21.1 m2 g¢1 and addition of a second metal
oxide (M = Mg, Co, Mn, La, or Fe) led to a considerable increase
in surface areas. Among all catalysts, Mg-incorporated catalyst
had the highest surface area of 58.9 m2 g¢1 and Fe incorporated catalyst showed the lowest of 29.2 m2 g¢1.
In Figure 1, the XRD patterns of reduced catalysts are shown
Figure 1. XRD patterns of reduced catalysts. a) Ni–Co/ACZ, b) Ni–Mg/ACZ,
at 600 8C for 1 h in H2 flow. Main broad diffraction lines are atc) Ni– La/ACZ, d) Ni–Mn/ACZ, e) Ni–Fe/ACZ, and f) Ni/ACZ catalysts.
tributed to the ZrO2 support corresponding to (111), (2 0 0),
Table 2. Properties of the reduced catalysts.
(2 2 0), (3 11), (2 2 2), and (4 0 0)
planes of a cubic fluorite phase
O2 release
CO2 desorbed
Coke
H2
Metal
Catalyst
Particle
(PDF-ICDD 88-2391). All the cataconsumption
[mmol g¢1][d]
[mmol g¢1][e]
[wt %][f]
dispersion
size of
lysts exhibit ZrO2, CeO2–ZrO2
[mmol g¢1][c]
Ni [%][b]
Ni [nm][a]
solid solution (Ce–ZrO2), and Ni0
Ni/ACZ
17.8
0.7
904.4
14.6
7.1
11.4
phases predominately. The most
Ni–Mg/ACZ
5.6
9.7
3801.7
128.5
97.80
0.05
Ni–Co/ACZ
7.4
3.8
2911.5
89.4
59.18
0.19
intense reflection of metallic
0
Ni–Mn/ACZ
7.6
3.5
2841.3
87.2
51.14
0.27
nickel (Ni ) is found for Ni–Mg/
Ni–La/ACZ
7.9
3.6
2701.8
88.1
54.87
0.24
ACZ catalyst, and the intensity
Ni–Fe/ACZ
15.2
1.5
1263.2
27.4
16.67
8.7
decreases in the order: Ni–La/
[a] Particle size was estimated by applying the Scherrer equation to peaks of the XRD patterns. [b] Measured
ACZ > Ni–Mn/ACZ > Ni–Fe/ACZ >
by CO–TPD, [c] H2–TPR (for calcined catalysts), [d] O2–TPD (for calcined catalysts), and [e] CO2–TPD. [f] Measured
Ni/ACZ. This clearly indicates
by thermogravimetric analysis after 24 h on stream.
that the nature of the second
metal plays a unique role on the
cussed below could be related to their different morphologies.
crystallinity of phases. The Ni–Co/ACZ catalyst (Figure 1 a) disThe HRTEM images reveal that Ni/ACZ contains the most conplays diffraction peaks of Co0 at 36 and 43.28. Addition of
trasted metal particles with approximately 10–20 % of the parmetal species did not affect the cubic phase structure of
ticles less than 10 nm in size (Figure 2 a). The fraction of smaller
ZrO2.[5b] The Ni0 crystallite sizes calculated by applying the
metallic particles increases to approximately 40–60 % with the
Scherrer equation to XRD peak width[7] are shown in Table 2.
addition of Co (Ni–Co/ACZ) as seen in Figure 2 b. The addition
The Mg-incorporated sample has the smallest Ni particle size
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remarkably with second-metal
incorporation to the highest
value 3801.7 mmol g¢1 for Ni–
Mg/ACZ. Consequently, the
order of H2 consumption is:
Ni/ACZ < Ni–Fe/ACZ < Ni–Mn/
ACZ < Ni–La/ACZ < Ni–Co/
ACZ < Ni–Mg/ACZ.
The fractions of exposed
metal (dispersion) of the catalysts were estimated by CO
Figure 2. HRTEM images of reduced a) Ni/ACZ, b) Ni–Co/ACZ, and (c) Ni–Mg/ACZ catalysts.
pulse chemisorption on reduced catalysts assuming one
CO molecule adsorbed per exposed Ni atom and the results
of Mg (Ni–Mg/ACZ) gives 100 % of nanosized metallic particles
are listed in Table 2. With the same level of metal content, Ni–
( … 3–5 nm), which are evenly distributed on the support.
Mg/ACZ has the highest metal dispersion of 9.7 %, whereas Ni/
ACZ has the lowest metal dispersion of 0.7 %. Other catalysts
Adsorption and redox properties
of Ni-Co/ACZ, Ni–La/ACZ, Ni–Mn/ACZ have similar metal dispersions of 3.5–3.8 %, but Ni–Fe/ACZ catalyst has a particularly
The reducibility of oxide catalysts was investigated by temperlow 1.5 % metal dispersion. The estimated metal dispersions by
ature-programmed reduction (TPR) in the temperature range
CO chemisorption show a good correlation with the sizes of
25–900 8C as shown in Figure 3, and amounts of H2 consumed
metal crystallites by XRD analysis. The rate of catalytic reacby individual catalysts are listed in Table 2. The presence of sigtions is enhanced on the smaller metal crystallites of high disnificant consumption of H2 during the TPR experiments conpersion because of the high active metal surface areas.
firms the formation of NiO species during the calcination treatThe amounts of oxygen liberated from the structure of the
oxide catalysts are shown in Figure 4 A as O2 temperature-programmed desorption (TPD) results. The process is not a typical
desorption of adsorbed species from the top-most surface because the oxygen is released thermally from the sub-surface
region of the catalyst as well. It represents redox property of
the metal oxide and is related to the amount of oxygen available during the catalytic CO2 reforming of methane, which
may promote the continuous removal of carbonaceous deposits from active sites to sustain the catalyst turnover. As seen in
Figure 4 A, all the catalysts give one broad O2 signal at approximately 200–580 8C, and the amounts of desorbed O2 are listed
in Table 2. The Ni/ACZ catalyst exhibits an amount of desorbed
O2 of 14.6 mmol g¢1. Addition of Mg shifts the desorption peaks
to slightly lower temperatures indicating that Mg can promote
the thermal reduction of nickel oxide in the catalyst that exhibits 128.5 mmol g¢1 of desorption. The O2 liberation decreases
with addition of Co (89.4 mmol g¢1), La (88.1 mmol g¢1), Mn
Figure 3. H2–TPR profiles of calcined a) Ni–Mg/ACZ, b) Ni–Co/ACZ, c) Ni–La/
(87.2 mmol g¢1), and further drops drastically over Fe addition
ACZ, d) Ni–Mn/ACZ, e) Ni–Fe/ACZ, and f) Ni/ACZ catalysts.
(27.4 mmol g¢1) to Ni/ACZ. The behavior is closely related to the
amounts of H2 consumption in TPR. But the amounts of O2 libment. Consequently, to obtain active metallic Ni nanosized pareration are less than 10 % of H2 consumption, indicating that
ticles, an appropriate activation procedure is necessary. All the
oxygen is liberated from its chemisorbed state on the surface
metal-incorporated catalysts display the reduction peaks at apand from capping or other structural oxygen near the surface
proximately 450 and 600 8C. The different reduction temperain the range of our testing temperatures. Its strong correlation
tures of NiO species are related to different interactions with
with the metal dispersion also suggests that the oxygen is rethe support: NiO in a weak contact with support is reduced at
leased during O2–TPD from near-surface layers of nickel.
low temperatures and that with the stronger interaction at
The basicity of the support is an important factor to suphigher temperatures.[8] In contrast, the TPR profile of the Ni/
press carbon formation on the catalyst surface. Normally CO2
ACZ catalyst was characterized by a single reduction peak cenfavors adsorption at basic sites. Carbon deposition is favored
tered at approximately 570 8C. Clearly, metal incorporation
on acidic supports such as SiO2, and Lewis base supports such
leads to low-temperature reduction. As seen in Table 2, Ni/ACZ
as ZrO2 and MgO have been reported to reduce carbon depohas a low H2 consumption of 904.4 mmol g¢1, which increases
sition.[9] Lewis bases have a high affinity for the chemisorption
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strength basic sites and the other broad peak at approximately
800 8C related to strong basic sites like carbonate species. Interestingly Ni–Fe/ACZ and Ni/ACZ catalysts exhibit a single
medium strength peak significantly shifted to high temperatures and no strong basic peak. These peaks are associated to
the amount of CO2 that interacts with the basic centers of the
catalysts with different strengths. The amount of basic sites titrated by CO2 adsorption on Ni/ACZ catalyst is low
(7.1 mmol g¢1), whereas Ni–Mg/ACZ catalyst shows the highest
amount (97.8 mmol g¢1). The Ni–Co/ACZ, Ni–Mn/ACZ, and Ni–
La/ACZ catalysts exhibit similar amounts of 59.1, 51.1, and
54.8 mmol g¢1, respectively. Therefore, the addition of the
second metal brings marked increase in the amount of basic
sites, especially for the Ni–Mg/ACZ catalyst containing strongly
basic MgO.
Oxidative CO2 reforming of methane
Figure 4. A) O2–TPD profiles of calcined catalysts and B) CO2–TPD profiles of
reduced catalysts. a) Ni–Mg/ACZ, b) Ni–Co/ACZ, c) Ni–La/ACZ, d) Ni–Mn/ACZ,
e) Ni–Fe/ACZ, and f) Ni/ACZ catalysts.
and dissociation of CO2 and it
has been suggested that adsorbed oxygen reacts with deposited carbon to form CO,
thereby reducing coke formation.[10] In our previous report,[5a]
2 wt % of basic promoters added
to the nanosized Ni and Co
metal particles on ZrO2 support
decreased the acidity and increased the basicity of the catalyst, which led to negligible coke
formation. The basicity of the
samples was evaluated from
CO2–TPD profiles in Figure 4 B.
There is a small peak near 100 8C
attributed to CO2 bound to
weaker basic sites, but CO2–TPD
profiles are composed of mainly
two predominate peaks, that is,
at approximately 200 8C assigned
to CO2 bound to mediumChemCatChem 2015, 7, 1445 – 1452
The activity and stability over a 24 h period was investigated
with 0.1 g catalyst amount and a feed mole ratio of CH4/CO2/
O2/N2 = 1 : 0.8 : 0.2 : 2 at 800 8C and atmospheric pressure. The
catalyst activity for CH4 and CO2 conversions are shown in Figure 5 A. In general, CH4 conversions are higher than CO2 conversions mainly by the side reaction of methane combustion.
The unmodified Ni/ACZ catalyst starts from low initial CH4 and
CO2 conversions of 44.8 and 54.6 %, which drop to 11.5 and
11.1 %, respectively, in 24 h of continuous reaction. Addition of
a second metal to the Ni/ACZ catalyst leads to marked effects
depending upon the nature of the metal incorporated. Among
the five (Ni–second metal) combinations studied here, the Ni–
Mg/ACZ catalyst shows the highest initial CH4 and CO2 conversions of 98.3 and 93.8 %, respectively. The nearly complete
Figure 5. A) Conversions of CH4 and CO2. B) Selectivities of H2 and CO at 800 8C; a) Ni–Mg/ACZ, b) Ni–Co/ACZ,
c) Ni–La/ACZ, d) Ni–Mn/ACZ, e) Ni–Fe/ACZ, and f) Ni/ACZ catalysts.
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methane conversion even at 800 8C is particularly remarkable.
The Ni–Mn/ACZ, Ni–La/ACZ, and Ni–Co/ACZ catalysts also
show high initial activities with CH4 conversions of 80–83 %
and CO2 conversions of 82–85.1 %, which decrease slightly in
the first 5 h but remain relatively stable until the end of the
24 h on stream. In contrast, Ni–Fe/ACZ catalyst shows the
lowest activity and stability that are only slightly better than
those of the unmodified Ni/ACZ catalyst.
The product selectivities for H2 and CO are shown in Figure 5 B. The selectivity loss is mainly caused by side reactions
such as coking, reverse water gas shift, and combustion reactions. Coking is the result of two reactions, that is, methane
decomposition and disproportionation of CO (Boudouard reaction). Both reactions reduce the yield of syngas. The reverse
water gas shift reaction between the reactant CO2 and the reforming product H2 makes the selectivity of H2 lower than that
of CO. Finally, combustion of H2 and CO to H2O and CO2 also
reduces the yield of syngas.
The Ni/ACZ catalyst exhibits a H2 selectivity of 21.6 % initially
and 5.6 % finally, whereas CO selectivity starts from 24.9 % then
drops to 7.7 %. The second-metal incorporation to Ni/ACZ catalyst significantly improved the syngas selectivity and its stability. The best selectivity was observed for Ni–Mg/ACZ catalyst
with stable approximately 90 % of H2 selectivity and approximately 80 % for CO selectivity. Other second-metal-modified
catalysts give somewhat lower selectivities, which also decrease significantly with time on stream.
Discussion
As presented, incorporation of a second metal to Ni/ACZ has
profound effects on activity, selectivity, and stability (or coke
deposition) of the catalyst in oxidative CO2 reforming of methane at 800 8C and atmospheric pressure. According to the
extent of the effects, catalysts could be classified into three
groups. Ni–Mg/ACZ shows a unique behavior of the most desired effects of the highest activity, syngas selectivity, and stability (least coke deposition). The Ni–Fe/ACZ represents the
worst case showing marginal improvement with respect to the
performance of unmodified Ni/ACZ catalyst. The other three
catalysts, Ni–Co/ACZ, Ni–La/ACZ, and Ni–Mn/ACZ show more
or less similar behavior exhibiting significantly improved performance by the second-metal addition, but not as good as in
the case of Ni–Mg/ACZ. The performance of the catalysts could
be ordered as follows: Ni/ACZ < Ni–Fe/ACZ ! Ni–Mn/ACZ < Ni–
La/ACZ < Ni–Co/ACZ ! Ni–Mg/ACZ. Here we discuss the relation between the catalyst performance and different physical
properties brought about by the introduction of a second
metal to Ni/ACZ catalyst.
There are three parameters that represent the catalyst performance, that is, activity, syngas selectivity, and stability, and
the stability could be mainly related to coke deposition. The
amounts of coke formed on the catalysts during the 24 h of
oxidative CO2 reforming of methane are listed in Table 2. Clearly the amounts of coke deposited are in good correlation with
the stability of the catalyst. The initial performance of activity
and syngas selectivity may be determined by the physical
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properties including reducibility (availability of surface oxygen)
represented by H2–TPR and O2–TPD, surface basicity by CO2–
TPD, and metal particle sizes that determine the metal dispersion or active metal surface area. Thus, the highest initial activity of Ni–Mg catalyst could be attributed to relatively easy reducibility, high basic site density, and good metal dispersion
(high metallic surface area). Notably, the values of these parameters summarized in Table 2 are highly correlated to each
other. Obviously the smaller metal particles give higher dispersion and larger metal surface areas. The reducibility probed by
H2–TPR and O2–TPD is also proportional to the metal surface
area because oxygen is released from the surface and lattices
of near-surface layers. The second metal seems to change the
temperature of O2 release or reduction, but the amount of desorbed oxygen is proportional to the metal surface area. The
basic property was measured by TPD of CO2 adsorbed on the
surface. There is some difference in peak temperatures and
peak shapes among the catalysts, which should be related to
their basic strength and distribution owing the different nature
of the second metal. However the amount of desorbed CO2 is
proportional to the metal dispersion because CO2 chemisorbs
only on the surface. Hence, those changes in physical properties of the catalysts upon introduction of the second metal
could be represented by the single parameter, that is, particle
size (or dispersion) of the metals in the catalysts. Hence, the
metal particle size could be a single descriptor that accounts
for the performance of the catalysts in the oxidative CO2 reforming of CH4 over the nickel-based catalysts.
Thus, addition of a second metal induces a marked reduction of nickel particle sizes, and the extent of the effect differs
depending on the nature of the added second metals. The
chemical state of these metals under the reaction conditions is
also different. Easily reducible Co and Fe exist as individual
metal form or alloy with nickel. In contrast, Mn, La, and Mg
mainly form their oxides.[11] In addition, Co, Fe, and Mn are
able to form alloys with Ni because of their similar atomic
size.[12] The oxide or alloy formation could be a mechanism to
form smaller Ni particles.
The maintenance of the initial activity of the catalysts or stability could also be discussed in terms of the effect of particle
size on coking. TEM images in Figure 6 show three typical catalysts of Ni/ACZ with diameter … 15 nm, Ni–Co/ACZ with diameter … 8 nm, and Ni–Mg/ACZ with diameter … 5 nm, after continuous reforming reaction for 24 h. They show different degrees of coking; Ni/ACZ of heavy carbon deposition covering
the catalyst surface with 11.4 wt % of carbon, Ni–Co/ACZ (similarly for Ni–La/ACZ and Ni–Mn/ACZ) of medium coking with
0.19–0.27 wt % of carbon, and finally Ni–Mg/ACZ showing little
coke formation of only 0.05 wt %. The most of the second
metals employed here have already been reported to promote
the activity and stability of many Ni-based CO2 reforming of
CH4. However, Mg is unique in its superior promotional effects,
which separate it from other second metals of Co, Mn, and La.
To understand the relation between the metal particle size
and the stability of the catalyst, the reaction mechanism of oxidative CO2 reforming of methane is considered, which has
been well established.[12] The initial step of methane reforming
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reduction part of the catalytic
cycle is also accelerated by the
basic sites, which accounts for
the correlation between the
number of the basic sites and
the performance of the catalysts.
The stability of the catalysts is
directly related to the particle
size of the active metal (Ni). The
metal particle sizes determined
by XRD analysis and HRTEM
images show a good agreement
with the metal dispersion (surface area) measured by CO
chemisorption. Herein we propose a model (Figure 6) that
Figure 6. HRTEM images of spent a) Ni/ACZ, b) Ni–Co/ACZ, and c) Ni–Mg/ACZ catalysts. Scale bars = 20 nm. d–f) A
shows schematically the influmodel to illustrate how the metal particle sizes influence the coke formation over the catalysts in oxidative CO2
ence of metal particle size on
reforming of CH4. Carbon species (black); metal particle (blue); adsorbed oxygen (red), which removes carbon specatalyst deactivation by coking.
cies by reaction to form CO.
Figure 6 d represents a large-size
metal particle, for example
> 15 nm. The carbon species
formed on the metal particle by reaction (1) is on the average
occurs over a reduced metallic site (*) by decomposition of
far away from the periphery of the metal particle/support and
CH4, whereas CO2 dissociation occurs at the metal-support
it is difficult to react with adsorbed oxygen formed there from
boundary by the formation of surface carbonates.
CO2 by reaction (2). Then carbon species, if not reacted away
CH4 ! CH3 * ! CH2 * ! CH* ! C*
ð1Þ
rapidly, tend to form polymeric carbons that accumulate on
the
metal particles leading to activity loss with time on stream.
*
*
ð2Þ
CO2 þ ! CO þ O
The large Ni particles also provide a high density of carbon
atoms on their surface leading to coking. This is the case of Ni/
Hydrogen is liberated as a gaseous hydrogen product. If eleACZ and Ni–Fe/ACZ catalysts with high amount of coke formamental carbon is exposed to the reaction conditions long
tion (Table 2). However, catalyst activity tends to be stable and
enough, it tends to be polymerized to form coke. However, if
carbon formation rate significantly decreases if the particles
oxygen atom is supplied in time, the carbon could be removed
are < 10 nm in size (Figure 6 e) as exemplified by Ni–Co/ACZ,
as CO before polymerization.[13]
Ni–La/ACZ, and Ni–Mn/ACZ catalysts with only a minor deactiC* þ O* ! CO þ 2*
ð3Þ
vation. If the particle sizes are even smaller, approximately
5 nm (Figure 6 f), most of the carbon species get closer to the
particle periphery and can react readily with adsorbed oxygen
Thus, oxygen atoms from the activated CO2 or gaseous O2
to form CO [reaction (3)], and hence there is negligible carbon
migrate to the metal surface to oxidize the carbon atoms or
deposition on the catalyst surface as for the best Ni–Mg/ACZ
CHx intermediates derived from CH4 decomposition. The efficatalyst. If the carbon formation on the catalyst surface is supciency of this step is essential for the catalysts to have high acpressed, the clean metallic sites are now available for further
tivity and stability with minimal coke formation in methane remethane decomposition to continue the catalytic cycles.
forming reactions. The source of oxygen is mainly CO2 as reacHence, the kinetic suppression of the coke deposition is essention (2), yet other oxygen-containing species are available such
tial to obtain high activity, selectivity, and stability of the pracas H2O and O2 in the reactor. Since all these steps require actitical catalysts for oxidative CO2 reforming of CH4, and synthesis
vation of gaseous molecules by the catalyst, it would be easier
to use surface and bulk oxygen contained in the catalyst.[14]
of nanosized Ni particles is the most important requirement
for the purpose.[16]
The H2–TPR and O2–TPD experiments reveal that these oxygen
species are readily available at the reaction conditions. Hence,
the amounts of surface and lattice oxygen available on the surConclusions
face and from the bulk measured by H2–TPR and O2–TPD exhibit a strong correlation with activity, selectivity, and stability
A Ni/ACZ catalyst was modified by various second metals such
of the catalysts. Now the used oxygen species has to be reas Mg, Co, La, Mn, and Fe, for oxidative CO2 reforming of
plenished by reactant molecules and, in particular, activation
methane with a molar feed ratio of CH4/CO2/O2/N2 = 1:0.8:0.2:2
of CO2 would be a critical step in CO2 reforming reactions to
at 800 8C under atmospheric pressure. Among the modified
catalysts, Ni–Mg/ACZ exhibited the highest CO2 conversions,
sustain the turnover of the catalytic cycle. The basic sites provide the adsorption sites for acidic CO2 molecules.[15] This CO2
syngas selectivity, and stability with negligible coke formation.
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That catalyst also demonstrated the highest fractions of exposed metal (dispersion), the largest oxygen release under
inert or reducing atmosphere, the largest number of basic
sites, and the smallest metal particle sizes. The formation of
the highly dispersed nanosized metallic nickel particles was
considered the most important factor contributing to the excellent performance. The small Ni particles reduce the distance
between carbon precursors and oxygen atoms formed at the
periphery of the Ni particles/support to react away the carbon
atoms to regenerate clean metal surface before they form
coke and stop the catalyst turnover.
Experimental Section
Multi-component catalysts were prepared by the precipitation
method with Ni as the main active metal and a second metal from
Mg, Co, Mn, La, and Fe deposited on the surface of Al2O3–CeO2–
ZrO2 (ACZ) support. Thus the precursors of Ni(NO3)2·6 H2O,
Co(NO3)2·6 H2O, Mn(NO3)2·4 H2O, ZrOCl2·8 H2O, La(NO3)2·6 H2O,
Al(NO3)3·9 H2O, Ce(NO3)3·6 H2O, Mg(NO3)2, and Fe(NO3)·6 H2O dissolved in deionized water were used as reagents and a solution of
NaOH was used as a precipitating reagent. The precipitation was
performed at a room temperature with continuous stirring and
NaOH was added dropwise to obtain optimum pH of 9.5–10. The
composition and code of each catalyst are listed in Table 1. The obtained cake was washed and dried at 150 8C for 24 h. The dried
cake was powdered and calcined at 550 8C for 4 h and the calcined
powdered catalyst underwent reduction in flowing H2 at 600 8C for
1 h before the catalytic reforming reaction.
The BET surface area and pore-size distribution of the samples
were obtained from the N2 adsorption–desorption isotherms measured at ¢196 8C using Micromeritics 2010 sorptometer. The crystal
phases were analyzed for air-exposed catalysts after reduction by
using the powder X-ray diffraction (XRD) patterns recorded on
a DX-1000 diffractometer using Cu Ka radiation between 208 and
808 with a continuous mode. The voltage and anode currents were
40 kV and 25 mA, respectively, and the scan step was 0.068 s¢1.
TPR/TPD analysis was performed on an Auto Chem 2910 (Micromeritics, USA) apparatus equipped with a TC detector. Prior to TPR
experiments, a sample of approximately 30 mg was thermally treated under air stream at 300 8C to remove water and other contaminants. The TPR profile was obtained by heating the sample in
a 10 % H2/Ar flow (50 mL min¢1). For TPD of CO2, the sample was
treated in flowing H2 at 600 8C for 1 h, cooled down to 25 8C, and
saturated in flow (20 mL min¢1) of a gas mixture containing
10 vol % of CO2 in helium until no adsorption of probe molecules
was observed. Then the sample was purged in the helium flow
until a constant baseline level was attained. Desorption was performed with a linear heating rate (10 8C min¢1) in a flow of He
(20 mL min¢1). Traces of H2O and O2 in helium (grade 5.0) used as
the carrier gas were removed by appropriate traps (Alltech). The
CO2–TPD spectra were obtained from the mass spectrometer
signal of m/z = 44. More detailed procedures of these characterization techniques are reported in our previous work.[4b, 5a] HRTEM
studies were conducted on the Ni/ACZ, Ni–Co, and Ni-Mg catalysts
to examine the particle sizes, their distribution, and morphology.
All fresh catalysts were calcined in air at 550 8C for 4 h and then reduced in H2 (1 bar) at 600 8C for 1 h before HRTEM measurements.
A 10 mg powder sample was dispersed pure ethanol (5 mL) and
kept in an ultrasonic bath for 20 min. Droplets of the sample were
then deposited onto a carbon-covered copper grid and left to dry
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at room temperature. A JEOL 2100F transmission electron microscope with a field-emission gun and a point resolution of 0.19 nm
were used. To measure carbon deposition, TGA was conducted on
a thermogravimeter (Mettler Toledo, TG-SDTA 851 instrument)
using air as a carrier gas and with a heating rate of 108 min¢1. Prior
to each TGA experiment, all samples were stored over anhydrous
CaCl2 in a desiccator for at least 24 h to ensure a complete dehydration.
Catalytic oxidative CO2 reforming of CH4 reactions was performed
in a tubular fixed-bed flow reactor at atmospheric pressure, and
feed gases were controlled by mass-flow controllers. Product
stream was led to gas chromatography through a chillier. Typically
a 100 mg sample of the catalyst was loaded, preheated in air at
500 8C for 30 min and then activated in flowing H2 at 600 8C for
1 h. The reactant gases consisted (in mole ratio) of CH4/CO2/O2 =
1:0.8:0.2 at 800 8C under atmospheric pressure, which were fed
into the reactor at gas hourly space velocity of 243 000 h¢1. To determine conversion and selectivity, the products were analyzed
after 30 min of steady-state operation by an on-line GC (Agilent
Technologies 7890A, 60/80 Carboxen 1000 column) with a TC detector. The conversions of CH4 and CO2 were calculated from the
consumed reactants and the selectivities to H2 and CO were determined from their concentrations defined as follows:[4b, 5a]
H2 selectivity ð%Þ ¼ 100
ðH2 Þout
2½ðCH4 Þin¢ðCH4 Þout¤
CO selectivity ð%Þ ¼ 100
ðCOÞout
ðCH4 Þin¢ðCH4 Þout þ ðCO2 Þin¢ðCO2 Þout
Acknowledgements
This research was supported by Brain Korea Plus Program of Ministry of Education, and Korea Center for Artificial Photosynthesis
(KCAP, No. 2009-0093880), a basic research program
(2012R1A2A2A01002879) funded by the Ministry of Science, ICT
and Future Planning (MSIP) through the National Research Foundation of Korea.
Keywords: carbon · nanoparticles · nickel · oxidation ·
supported catalysts
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Received: January 1, 2015
Published online on March 25, 2015
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