Foam-Structured NiO-MgO-Al2O3 Nanocomposites Derived from NiMgAl Layered Double Hydroxides In Situ Grown onto Nickel Foam: A Promising Catalyst for High-Throughput Catalytic Oxymethane Reforming

Article abstract of DOI:10.1002/cctc.201601167

Catalytic oxymethane reforming is an effective and efficient route to produce syngas, but the commonly used Ni catalysts suffer from coke deposition, Ni sintering, and heat-transfer limitations. A Ni-foam-structured NiO-MgO-Al₂O₃ nanocomposite catalyst was developed by thermal decomposition of NiMgAl layered double hydroxides (LDHs) in situ hydrothermally grown onto the Ni-foam. Originating from the lattice orientation effect and topotactic decomposition of the LDH precursor, NiO, MgO, and Al₂O₃ are highly distributed in the nanocomposite, and thus, this catalyst shows enhanced resistance to coke and sintering. At 700 °C and a gas hourly space velocity of 100 L g^?1 h^?1, 86.5 % methane conversion and selectivities of 91.8/88.0 % to H₂/CO are achieved with stability for at least 200 h. We believe this type of tailoring strategy and the as-obtained materials can open up new opportunities for future applications in other high-throughput and high-temperature reactions.

Full text of DOI:10.1002/cctc.201601167

Accepted Article
Title: Foam-Structured NiO-MgO-Al2O3 Nanocomposites Derived from
NiMgAl-LDHs In Situ Grown onto Ni-foam: A Promising Catalyst
for High-Throughput Catalytic Oxy-Methane Reforming
Authors: Ruijuan Chai, Qiaofei Zhang, Yakun Li, Songyu Fan, Zhiqiang
Zhang, Pengjing Chen, Guofeng Zhao, Ye Liu, and Yong Lu
This manuscript has been accepted after peer review and appears as an
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Link to VoR: http://dx.doi.org/10.1002/cctc.201601167
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ChemCatChem
10.1002/cctc.201601167
COMMUNICATION
Foam-Structured NiO-MgO-Al
2
O
3
Nanocomposites Derived from
NiMgAl-LDHs In Situ Grown onto Ni-foam: A Promising Catalyst
for High-Throughput Catalytic Oxy-Methane Reforming
Ruijuan Chai, Yakun Li, Qiaofei Zhang, Songyu Fan, Zhiqiang Zhang, Pengjing Chen, Guofeng Zhao,*
Ye Liu and Yong Lu*[a]
Abstract: The catalytic oxy-methane reforming is an effective and
effcient route to produce syngas, but the commonly-used Ni
catalysts suffer from coke deposition, Ni-sintering and heat transfer
uniform distribution of flexible and tunable metal ions in the
hydroxide layers, which can be well maintained in their oxidized
and/or reduced products due to their topotactic transformation.[
The work by Morioka et al. indicated that Ni-based catalysts
derived from NiCaAl-LDHs showed excellent catalytic
performance for COMR.[10] Zhang et al. prepared a series of
9]
limitation.
A
Ni-foam-structured NiO-MgO-Al
O
2 3
nanocomposite
catalyst was developed via thermal decomposition of NiMgAl layered
double hydroxides (LDHs) in situ hydrothermally grown onto the Ni-
foam. Originated from the lattice orientation effect and topotactic
2 3 2 3
NiO-MgO-Al O -La O mixed oxides also derived from LDHs and
decomposition of LDHs precursor, NiO, MgO and Al
2
O
3
are highly
such catalyst displayed both high activity and stability toward
COMR.[11] Their outstanding coking- and sintering-resistance
could be attributed to the presence of the highly dispersed nickel
species that can be well retained in the homogeneous mixed
oxides derived from LDHs precursor.
distributed in the nanocomposites, and thus this catalyst delivered
o
enhanced coke-/sintering-resistance. At 700 C and a gas hourly
-
1
-1
space velocity of 100 L g h , 86.5% methane conversion and
1.8%/88.0% selectivities to H /CO could be achieved with a stability
9
2

for at least 200 h. We believe this type of tailoring strategy and the
as-obtained materials can make open up new opportunities for future
Notably, COMR process is mildly exothermic with a ΔH 298
-1
of -36 kJ mol , but the concomitant complete oxidation of

applications in other high throughput and high temperature reactions. methane is strongly exothermic with a huge ΔH 298 of -802 kJ
-1
mol , which induces severe hot-spots in the catalyst bed and
then leads to catalyst deactivation as well as even safety
problems in the practical application. To alleviate the above
problems, the monolithic structured supports of reticulated-
metal-foam have been found to exhibit preferable properties to
the conventional oxide supports for strongly exo- or endo-
thermic reaction processes.[12] For example, Ni-foam, with many
advantages such as the high thermal-conductivity, gas
permeability and mechanical strength, is attracting increasing
interest in the development of promising catalysts for the high-
throughput exothermic reaction processes.[13-17] However,
traditional Ni-foam-structured catalysts obtained via wash-
coating, dip-coating or impregnation techniques, suffer from the
heterogeneous distribution and leaching of the active
components, detachment of coatings, and binder harmful
contamination.[18] On the other hand, the development of a
monolithic catalyst with satisfying resistance to the coke
deposition and sintering is still in its infancy.
2
Syngas (a mixture of CO and H ) production from methane is the
prerequisite process for methane industrial transformation into
fuels and other chemicals of particular importance.[1,2] The
present major production of syngas relies on steam reforming of
methane (SRM), but the strong C-H bonds in methane make this
route energy-costing and equipment-capital intensive, and the
H
2
/CO ratio from this route is only suitable for the downstream
processes requiring a H
2
-rich feed.[3] Compared with the SRM,
the catalytic oxy-methane reforming (COMR) could be a good
alternative owing to its high energy-efficiency, compact reactor
and suitable H /CO ratio straightforward for the downstream
2
methanol production and Fischer-Tropsch process.[4]
Oxides supported Ni-based catalysts are extensively
investigated due to their excellent performance and moderate
cost, but suffer from the critical problems of coke deposition and
Ni-sintering.[5] Therefore, many ways have being explored to
keep proper grain size against to sintering and avoid coke
deposition under the harsh reaction conditions, commonly
including the addition of promoters (alkali, alkaline earth and
rare earth metal oxides) or/and utilization of different supports
It is thus particularly desirable to seek an effective and
efficient route to develop an affordable catalyst with a unique
combination of satisfying activity, excellent coke-/sintering-
resistance and enhanced heat/mass transfer. To accomplish this
goal, a NiMgAl-LDHs layer was directly grown on a Ni-foam via
facile in situ hydrothermal reaction and NiMgAl-LDHs-derived
[
6,7]
(
SiO
exhibited the Ni-MgO/Al
and coke-resistance for COMR.
2
, Al
2
O
3
, ZrO
2
and Y
2
O
3
).
For instance, Ozdemir et al.
2
O catalyst with respect to sintering-
3
[
8]
Besides these attempts,
NiO-MgO-Al
2
O
3
nanocomposite catalyst was subsequently
layered double hydroxides (LDHs) evoked ever-increasing
interest due to their specific features, especially the highly
embedded onto the Ni-foam via thermal decomposition of the
NiMgAl-LDHs/Ni-foam precursor. This facile approach provided
an engineering of a NiO-MgO-Al
macro-meso-nano” triple-scale level of both porosity and
structure.
Figure1a presents a schematic illustration for fabrication of
the NiO-MgO-Al /Ni-foam catalyst derived from the Ni-foam-
structured NiMgAl-LDHs. We initially utilized the optical
photograph and scanning electron microscopy (SEM) to show
the flexibility in shape and size of a pristine Ni-foam (100 PPI),
and the three-dimensional open cell structure composed of
interconnected porous framework (Figure 1b). Via an in situ
2 3
O composite catalyst at a
“
[
a]
R. J. Chai, Dr. Y. K. Li, Dr. Q. F. Zhang, S. Y. Fan, Z. Q. Zhang, P.
J. Chen, Dr. G. F. Zhao, Prof. Dr. Y. Liu, Prof. Dr. Y. Lu
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes, School of Chemistry and Molecular Engineering, East
China Normal University, Shanghai 200062, P.R. China.
Fax: +86 21 62233424; E-mail: gfzhao@chem.ecnu.edu.cn
2
O
3
(G.F.Zhao); ylu@chem.ecnu.edu.cn (Y.Lu)
Supporting information for this article is given via a link at the end of
the document.
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ChemCatChem
10.1002/cctc.201601167
COMMUNICATION
decoration strategy, NiMgAl-LDHs nanosheets could be feasibly
grown onto the Ni-foam surface just by simply treating the Ni-
foam under the traditional LDHs synthesis conditions (to see
detailed information in Experimental section). Figure 1c
illustrates the X-ray diffraction (XRD) pattern of the
2 3
Subsequently, the representative NiO-MgO-Al O /Ni-foam
catalyst could be obtained from the NiMgAl-LDHs/Ni-foam
precursor (i.e., same as in Figure 1c,d) via thermal treatment at
o
700 C in He flow. Indeed, the XRD patterns clearly displayed
the formation of fcc-NiO phase (JCPDS No. 78-0429) and
meanwhile the complete disappearance of LDHs (Figure 2a).
The average grain size of NiO was calculated to be 16 nm by
Scherrer equation based on NiO (111) plane. Such small grains
were likely resulted from the lattice orientation effect and
topotactic decomposition of LDHs precursor.[19] Notably, MgO
representative
NiMgAl-LDHs/Ni-foam
catalyst
precursor
obtained in an aqueous mixture with Ni/Mg/Al=1/2/1. Besides the
characteristic diffraction peaks of metallic nickel phase of the Ni-
foam (JCPDS card, No. 04-0850), a series of characteristic
diffraction peaks corresponding to (003), (006), (012), (015),
(
018), (110) and (113) planes of the LDH phase (JCPDS card,
2 3
and Al O were not detectable, probably due to their small grain
No. 15-0087) were clearly detected. A weak NiCO (JCPDS card, size that is below the XRD detection limit of 2-3 nm. According
3
No. 78-0210) characteristic diffraction peak was observed, which
to the H
(H -TPR, based on H
2 3
estimated to be 10.2% while the corresponding MgO and Al O
contents were determined by inductively coupled plasma-atomic
emission spectroscopy (ICP-AES) to be 2.2 and 2.5%,
2
consumption in H
2
-temperature programmed reduction
was formed from the reaction between Ni2+ and CO
2-
+ NiO = Ni + H
O), the NiO content was
3
2
2
2
decomposed from urea. The transmission electron microscopy
TEM) image (Figure 1d) of such NiMgAl-LDHs/Ni-foam
(
exhibited obvious layer structures and their well-defined lattice
fringes with the lattice spacing of 0.24 nm, which is
corresponding to the (009) lattice planes of the NiMgAl-LDHs.
The SEM image clearly showed that the nanosheets of NiMgAl-
LHDs were oriented perpendicular to the surface of Ni-foam with
approximate thickness of 20-30 nm (Figure 1e), which uniformly
overspread on the Ni-foam and intercrossed with each other to
form hierarchical flower-like 0.5-2.0 μm microspheres (inset in
Figure 1e). More importantly, the NiMgAl-LDHs/Ni-foam showed
a promising mechanical strength, evidenced by the fact that the
nanosheets did not flake off from the Ni-foam even after 30-min
sonication treatment in distilled water. This feature of such
catalyst precursor would lay foundation for the desirable
robustness of the catalyst in the COMR process.
respectively. Moreover, the N
2
-Brunauer-Emmett-Teller specific
surface area of the catalyst was increased obviously to 30.3
2
2
-1
m /g from only 1.0 m g of the pristine Ni-foam. Such catalyst
sample exhibited the intermediate isotherms between Type II
and Type IV, suggesting the existence of mesopores and
macropores.[20] Also, the hysteresis loops in Figure 2b indicated
the merge of Type H3 loop, which was attributed to both the
stack of nanosheets (giving rise to slit-shaped pores) and the
removing of H
decomposition. Interestingly, as shown in Figure 2c, the LDHs-
derived NiO-MgO-Al nanocomposites in-situ structured on Ni-
2
O as well as interlayer anions during thermal
2
O
3
foam preserved well the original nanosheet morphology feature
of their corresponding Ni-foam-structured NiMgAl-LDHs (Figure
1e). Moreover, the corresponding energy dispersive X-ray (EDX)
element mapping (Figure 2d) clearly showed an as-expected
homogeneous distribution of Ni, Mg and Al elements in the layer
of NiO-MgO-Al O nanocomposites for such representative
2 3
catalyst sample, and their relative amounts by EDX were shown
in Figure S1. Figure S1 indicates a Ni/Mg/Al atom ratio of around
4
.6/1.0/1.1 in the NiO-MgO-Al
to that (3.4/1.0/1.3) determined by
measurements. Notably, the Ni/Mg/Al atom ratio in the NiO-
MgO-Al nanocomposites was much higher than that
Ni/Mg/Al=1/2/1) in the solution for LDHs growth on the Ni-foam,
2
O
3
nanocomposites, being close
H
2
-TPR and ICP
2
O
3
(
indicating that the Ni-foam participated in the NiMgAl-LDHs
growth by supplying Ni2+ ions.
H
2
-TPR experiment was performed to evaluate the
reducibility of the NiO-MgO-Al /Ni-foam catalyst. As shown in
2 3
O
Figure 3, the broad hydrogen consumption peaks could be
o
divided into four peaks in the range of 240-825 C, which were
assigned to the reduction of NiO species since no any reduction
of MgO and Al
2
O
3
took place. The first inconspicuous peak at
o
298 C was attributed to the reduction of free NiO tiny particles
o
on the support and the second peak at 389 C was assigned to
NiO species weakly interacting with the MgO/Al oxides. The
last two peaks related to the reduction of NiO species strongly
interacting with the MgO/Al oxides, especially the peak at
2
O
3
2
O
3
o
[14]
7
55 C. In general, the reduction temperature of the pure NiO
o
decomposed by Ni(NO
3
)
2
was at ca. 385 C, quite lower than the
main peak of our catalyst, suggesting the different chemical
circumstance of the NiO particles in NiO-MgO-Al
nanocomposites with strong interaction between NiO and
MgO/Al oxides. Moreover, in comparison with the H -TPR
result of the reference NiO-Al /Ni-foam catalyst in Ref. [13]
2 3
Figure 1. (a) Fabrication strategy of the NiO-MgO-Al O /Ni-foam catalyst
2
O
3
derived from NiMgAl-LDHs/Ni-foam precursor. (b) Optical photograph and
SEM image of the pristine Ni-foam wafers. XRD patterns (c), TEM images (d)
and SEM images (e) of the representative NiMgAl-LDHs/Ni-foam catalyst
precursor obtained in an aqueous mixture with Ni/Mg/Al=1/2/1.
2
O
3
2
2
O
3
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ChemCatChem
10.1002/cctc.201601167
COMMUNICATION
00 C and a GHSV of 100 L g h-1. Clearly, a slight decrease of
o
-1
7
CH conversion (from 86.5 to 81.8%) was observed as well as
4
the syngas selectivity (H
throughout 200 h testing.
2
/CO: 91.8/88.0 to 90.5/83.2%)
2 3
Figure 2. Representative NiO-MgO-Al O /Ni-foam catalyst derived from the
NiMgAl-LDHs/Ni-foam catalyst precursor obtained in an aqueous solution with
Ni/Mg/Al=1/2/1. (a) XRD pattern, (b) N adsorption/desorption isotherms and
2
Barrett-Joyner-Halenda (BJH) mesopore size distribution (inset), (c) SEM
image, and (d) SEM-EDX element mapping images.
Figure 4. (a) CH
dependent COMR using the representative NiO-MgO-Al
4
conversion and syngas selectivity for temperature-
/Ni-foam catalyst
2 3
O
same as in Figure 2, which was derived from the NiMgAl-LDHs/Ni-foam
catalyst precursor obtained in an aqueous solution with Ni/Mg/Al=1/2/1. (b)
CH
4
conversion and syngas selectivity for the COMR vs time on stream at 700
o
C using the same catalyst as in (a). Reaction conditions: 0.12 g catalyst, 0.1
-
1
-1
MPa, GHSV = 100 L g h , CH
Al /Ni-foam catalysts after pre-reduction and 200 h reaction testing. (d) XRD
pattern and (e) SEM image of the NiO-MgO-Al /Ni-foam catalyst after 200 h
4 2
/O = 1.8/1. (c) TGA curves of the NiO-MgO-
2 3
O
2 3
O
Figure 3. H
Figure 2 and the reference NiO-Al
2
-TPR of the representative NiO-MgO-Al
2 3
O /Ni-foam same as in
reaction testing. TEM images of (f) the reduced NiO-MgO-Al
catalyst and (g) the same one after 200 h reaction.
2 3
O /Ni-foam
2 3
O /Ni-foam in Ref. [13].
(
Figure 3), our representative NiO-MgO-Al
2
O
3
/Ni-foam showed
oxides.
Tracing the source of deactivation is of great importance for
catalyst development. As well known, coke deposition and
sintering of Ni nanoparticles are the main reasons for the
catalyst deactivation. Figure 4c shows the thermogravimetric
analysis (TGA) profiles. Based on the TGA results, the carbon
more strong interaction between NiO and MgO/Al
Figure 4a shows the temperature-dependent conversion and
selectivity for the COMR reaction using the representative NiO-
MgO-Al O /Ni-foam at a high gas hourly space velocity (GHSV)
2 3
of 100 L g h . Oxygen was totally consumed in the temperature
range studied. Such catalyst was capable of converting 86.5%
2
O
3
-1
-1
-1
amount on the used catalyst was estimated to be 0.3 gcar
g
cat
-1
-1
(
corresponding to a coking rate of 1.5 mgcar gcat h ), which
methane with up to 91.8%/88.0% selectivities to H
2
/CO at 700
existed mainly in the form of graphite carbon as evidenced by a
o
C. Tuning the Ni/Mg and (Ni + Mg)/Al ratios could not yield
better performance (Tables S1,S2). As shown in Figure 4a, with
the increase in reaction temperature from 600 to 800 oC,
methane conversion was monotonically increased as well as the
o
clear XRD peak at 2 of 26.6 (Figure 4d). In comparison with
the
previously
reported
Ni-foam-structured
2 3
NiO-Al O
nanocomposite catalyst, the coking rate for the present catalyst
was reduced by a factor of at least 4.[13] Normally, the acidic
nature of Al O enhanced the decomposition rate of CH and
2 3 4
4
syngas selectivity. A high CH conversion of 96.5% could be
achieved with very high syngas selectivity (H
9
conversion of 73.8% could also be obtained with acceptable
syngas selectivity (H , 86.6%; CO, 76.3%) even at a low
temperature of 600 oC. Moreover, our representative catalyst
showed promising stability. Figure 4b shows the CH conversion
and syngas selectivity for the COMR against time on stream at
2
, 95.1%; CO,
gave rise to carbon deposition, while high basicity was known to
o
4.7%) at a high temperature of 800 C, while a satisfying CH
4
inhibit carbon formation because of carbon gasification
promotion.[ Indeed, CO
21]
-temperature programmed desorption
2
2
(
CO
catalyst (Figure S2). The total amounts of desorbed CO
representative NiO-MgO-Al /Ni-foam and the reference NiO-
2
-TPD) results showed remarkably enhanced basicity of our
2
of the
4
2
O
3
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ChemCatChem
10.1002/cctc.201601167
COMMUNICATION
/Ni-foam were 48.2 and 19.9 μmol gcat-
1 [13]
,
respectively.
Al
2
O
3
ICP-AES, XRD, SEM/EDX, TEM, H
2 2
-TPR, CO -TPD, TGA and low
temperature N -sorption, and examined in the COMR reaction. The
2
Such basicity-dependent carbon resistance improvement was
further confirmed by the observation that the carbon deposit
amount indeed showed a decline trend with basicity of NiO-
details on the catalyst characterization and testing were placed in the
Supporting Information.
2 3
MgO-Al O /Ni-foam catalysts increased by tuning the Ni/Mg
ratios from 2.0 to 0.2 in the aqueous solution for LDH
compounds synthesis (to see supplementary text and Figure S3
in the Supporting Information).
Acknowledgements
In addition, it is generally accepted that carbon formation is
favourable on large Ni particles, because of their relatively low
Ni-Ni bond energy that facilitates carbon diffusion into the nickel
lattice for carbon formation.[6] Regarding our Ni-foam-structured
This work was funded by the NSF of China (21473057,
U1462129, 21273075, 21076083), and the “973 program”
(2011CB201403) from the MOST of China.
LDHs-derived NiO-MgO-Al
2
O
3
nanocomposite catalyst, the
Keywords: structured catalyst • Ni foam • layered double
hydroxides • catalytic oxy-methane reforming • syngas
nanosheet morphology was preserved very well even after 200 h
testing under harsh reaction conditions (Figure 4e). Interestingly,
fine spinal species were clearly observed on the 200-h tested
catalyst (Figure 4e), which are contributed to the Ni
nanoparticles (to see supplementary text and Figure S4 in the
Supporting Information). As previously noted, indeed, the LDHs-
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2
O
3
oxides
1828-1847.
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catalyst surface thereby preventing Ni nanoparticles from
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2 3
nanoparticles on the pre-reduced NiO-MgO-Al O /Ni-foam was
sized at 9.3 nm while behaving a slight increase to 15.2 nm after
underdoing 200 h reaction (Figure 4f,g). Such enhanced anti-
sintering property in turn would be helpful for kinetically
suppressing the carbon formation. By taking the advantages as
mentioned above, our NiO-MgO-Al O /Ni-foam catalyst behaved
2 3
promising stability in comparison with the early reported ones,
as illustrated in Table S3.
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[
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O
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3
high-throughput COMR reaction, providing unique combination
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thermal decomposition of NiMgAl LDHs in-situ hydrothermally
grown onto the Ni-foam.
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a versatile
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wafers (2 mm thick, diameter of 6 mm, 100 PPI; purchased from
Changsha Lyrun Material Co., Ltd, China) were obtained from their large
sheets by laser cutting. Typically, the Ni-foam (0.5 g) was degreased with
acetone, etched with 6.0 mol L-1 HCl for 30 min, rinsed with water, and
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2
011, 401, 170-175.
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016, 17, 248-251.
[
[
[
2
then dried in an oven. Ni(NO
mmol) and Al(NO ·9H O (3.00 mmol) were dissolved in a solution
consisting of 80 mL of H O and 39.96 mmol of (NH CO. The solution
3 2 2 3 2 2
) ·6H O (3.00 mmol), Mg(NO ) ·6H O (6.00
17] L. J. I. Coleman, E. Croiset, W. Epling, M. Fowler, R. R. Hudgins, Catal.
Lett. 2009, 128, 144-153.
3
)
3
2
2
2 2
)
18] a) J. H. Lunsford, Catal. Today 2000, 63, 165-174; b) L. X. Sang, B.
Sun, H. Y. Tan, C. X. Du, Y. T. Wu, C. F. Ma, Int. J. Hydrogen Energy
was magnetically stirred for 10 min at room temperature and transferred
into an autoclave pressure vessel. The Ni-foam was then immersed in
the solution and heated at 140 oC for 12 h. After hydrothermal process,
the samples were washed using distilled water, dried overnight at 100 C
to obtain NiMgAl-LDHs/Ni-foam. Finally the NiMgAl-LDHs/Ni-foam was
calcined in He at 700 oC for 2 h to get the corresponding NiO-MgO-
Al O /Ni-foam catalyst. All nitrate salts (A.R. grade) and urea (A.R.
2 3
grade) were directly used without further purification as received from
Sinopharm Chemical Reagent Co. Ltd. Catalysts were characterized by
2
012, 37, 13037-13043.
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015, 40, 7052-7063.
[
[
o
2
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[
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[
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ChemCatChem
10.1002/cctc.201601167
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Ni-foam-structured NiO-MgO-Al
2
O
3
Ruijuan Chai, Yakun Li, Qiaofei Zhang,
Songyu Fan, Zhiqiang Zhang, Pengjing
Chen, Guofeng Zhao,* Ye Liu, and
Yong Lu*
nanocomposite
obtainable
catalyst
via
facilely
thermal
decomposition of NiMgAl LDHs in-
situ grown onto the Ni-foam, is
highly active/stable for the high-
throughput catalytic oxy-methane
reforming.
Page No. – Page No.
Foam-structured NiO-MgO-Al
2 3
O
Nanocomposites Derived from
NiMgAl-LDHs In-situ Grown onto Ni-
foam: A Promising Catalyst for
High-throughput Catalytic Oxy-
methane Reforming
This article is protected by copyright. All rights reserved.