One-pot synthesis of a highly mesoporous Ni/MgAl2O4spinel catalyst for efficient steam methane reforming: influence of inert annealing

Article abstract of DOI:10.1039/d1cy00485a

A highly mesoporous Ni/MgAl₂O₄spinel catalyst was facilely synthesized by a scalable one-pot strategy and employed in the steam methane reforming (SMR) reaction. The influence of annealing the catalyst in a N₂atmosphere on

Full text of DOI:10.1039/d1cy00485a

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ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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DOI: 10.1039/D1CY00485A
Article type: Full Paper
One-Pot Synthesis of Highly Mesoporous Ni/MgAl2O4 Spinel Catalyst for
Efficient Steam Methane Reforming: Influence of Inert Annealing
Hyunjoung Kima,1, Ahmed Al-Shahat Eissaa,b,1, Seung Bo Kima, HongJin Leea, Woohyun
Kimc, Dong Joo Seoc, Kyubock Leea,* and Wang Lai Yoonc
a
Graduate School of Energy Science and Technology, Chungnam National University, 99
Daehak-ro, Yuseong-gu, Daejeon 34134, Republic of Korea
b Department of Chemistry, School of Science, South Valley University, Qena 83523, Egypt
c
Hydrogen Research Department, Korea Institute of Energy Research, 152 Gajeong-ro,
Yuseong-gu, Daejeon 34129, Republic of Korea
* Corresponding author: kyubock.lee@cnu.ac.kr
1 These authors contributed equally to this work.
Abstract
A highly mesoporous Ni/MgAl2O4 spinel catalyst was facilely synthesized by a scalable
one-pot strategy and employed in steam methane reforming (SMR) reaction. The influence of
annealing the catalyst in N2 atmosphere on its structural properties and catalytic performance
is fully demonstrated for the first time. Physical characterizations showed that subjecting the
catalyst to an inert-annealing process (e.g., in N2 atmosphere) before calcining it in air enhances
the specific surface area (SSA) and mesoporosity, improves the dispersion of Ni nanoparticles
(NPs) within the support matrix, boosts the interaction between the active phase and support,
and increases the content of the reduced Ni species on the catalyst surface. With its remarkable
structural properties, the catalyst exhibits high CH4 conversion efficiency and durability as well
as superb resistance to carbon deposition. These results reveal that the substructure and catalytic
activity of the catalyst can be tuned by controlling the annealing conditions during the synthesis
process. The straightforwardness of the synthesis strategy employed in this work makes it a
promising route for designing catalysts with high coke-resistance in reforming and other high-
temperature Ni-catalyzed reactions.
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DOI: 10.1039/D1CY00485A
Keywords
Steam methane reforming, one-pot synthesis, inert annealing, mesoporosity,
Ni/MgAl2O4 catalyst.
1. Introduction
The main source of energy is still fossil fuels, which are non-renewable and have
detrimental effects on our environment,1 due to the generation of huge amounts of greenhouse
gases upon the combustion of fossil fuels. On contrary, hydrogen (H2) is an attractive energy
carrier with high energy density (~ 122 kJ g–1), which can be utilized in fuel cells to generate
clean energy with high efficiency to operate portable devices and transportation systems.2 As a
result, the global demand for H2 is growing very fast. H2 production from natural gas is
positioned as a transition point between fossil fuel and renewable energy systems. SMR is one
of the most common and effective commercialized processes to produce H2, meeting almost
50% of the worldwide need for H2.3,4 SMR is a process that results in the formation of syngas
(a gaseous mixture of CO and H2) through a highly endothermic reaction between methane
(CH4) and water vapor (H2O).5,6 Syngas has long been used in energy production applications
as well as the synthesis of valuable chemicals through the Fischer-Tropsch reaction.7, 8
Noble metals such as ruthenium (Ru), rhodium (Rh), and platinum (Pt) provide high
catalytic activity suitable for the SMR reaction,9 however, their scarceness and high cost hinder
their commercial applications. Hence, the development of efficient noble-metal-free catalysts
is of utmost importance. Currently, nickel (Ni) is the most preferred catalyst for the SMR
reaction as well as dry methane reforming, due to its superb reactivity and low cost.10–13
However, the development of supported Ni-based catalysts with higher resistance against the
carbon deposition and thermal aggregation of Ni NPs during the long-term high-temperature
reaction conditions remains a major challenge.14 It is noteworthy that the durability of Ni-based
catalysts depends on the nature of support, the interaction between the metallic Ni phase and
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the support, and the size of Ni NPs.15 Indeed, carbon-deposition reactions (CH4 direct
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DOI: 10.1039/D1CY00485A
decomposition, the Boudouard reaction, etc.) happen more readily on large Ni crystallites,
which are created due to the high operating temperature required for proceeding the SMR
reaction, where the Ni NPs easily agglomerate.16 Therefore, reducing the Ni crystal size is
regarded as one of the most successful strategies for enhancing the catalyst stability. The
confinement of well-dispersed Ni-active phase within the mesopores of stable supports such as
alumina (Al2O3)4, 17–21 is an effective approach to minimize Ni agglomeration during the SMR
reaction, and hence maintaining high reactivity for prolonged periods under the harsh operating
conditions. The optimum support should be resistant to high temperature and must maintain
good metal dispersion during the reaction. In this regard, the magnesium aluminate spinel
(MgAl2O4) is a promising candidate as an efficient support in many catalytic applications,4, 22–
28 due to its high mechanical strength, high melting temperature (2408 K), admirable chemical
durability, and high thermal stability.29,30 More importantly, the incorporation of a basic metal
(Mg) into the substructure of the comparatively acidic Al2O3 inhibits the carbon deposition and
promotes the dispersion of the active Ni species within the support.31
In the present study, we report a one-pot synthesis strategy for producing Ni NPs
supported on highly mesoporous MgAl2O4 spinel catalyst, where mesoporous MgAl2O4 was
used as a support for the confinement of the well-dispersed Ni-active phase, due to its low
acidity, high sintering resistance, and robust interaction of the metal with MgAl2O4 support, all
giving rise to efficient catalysts for the SMR reaction. The catalyst was fabricated by ‘‘one-
pot’’ evaporation-induced self-assembly (EISA) method, then inert annealing in N2 atmosphere
at 800 °C for 2.0 h, was performed on the as-synthesized sample before normal calcination step
in air to remove the sacrificial template. The influence of inert annealing in N2 atmosphere on
the structural properties and catalytic activity of the catalyst is elucidated for the first time.
Through the careful investigation of the structure-performance relationship of the catalysts, it
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was found that the catalyst annealed in N2 atmosphere at 800 °C exhibited the sDuOpIe: 1r0i.o10r39C/DH1C4Y00485A
conversion efficiency and minimum carbon deposition, due to the high surface area, high
mesoporosity which facilitates the flow of reactants to the active sites, robust interaction of Ni
NPs with the MgAl2O4 support, and the homogeneous dispersion of the Ni NPs on the support.
2. Experimental
2.1 Materials
Aluminum iso-propoxide (Al(OiPr)3, ≥98%), magnesium nitrate hexahydrate
(Mg(NO3)2∙6H2O, ≥98%), and nickel nitrate hexahydrate (Ni(NO3)2∙6H2O, ≥98%) were
purchased from Sigma-Aldrich, USA, and used as sources of Al, Mg, and Ni, respectively.
Absolute ethanol (C2H5OH, ≥98%, Samchun, Korea) and nitric acid (HNO3, 60.0%, Samchun,
Korea) were used as solvents. Tri-block co-polymer Pluronic P123 (EO20PO70EO20, molecular
weight =5800 g mol–1, Sigma-Aldrich, USA) was used as a structure-directing agent. All of the
chemicals were utilized as received without any further purification.
2.2 Synthesis of catalysts
The catalysts were synthesized using EISA method as shown in the schematic
illustration of Fig. 1. Firstly, 6.32 g of P123 were added to a mixture consisting of 138.16 mL
of ethanol and 10.4 mL of HNO3 under a stirring rate of 700 rpm for 12.0 h. Secondly,
Al(OiPr)3, Mg(NO3)2∙6H2O, and Ni(NO3)2∙6H2O were added to the above solution and stirred
for additional 6.0 h. The mixing molar ratios of all of the components were fixed at 1 : 0.017 :
35 : 1.94 for (Al(OiPr)3 + Mg(NO3)2∙6H2O + Ni(NO3)2∙6H2O) : P123 : C2H5OH : HNO3. The
precursor solution was dried at 60 °C for 48.0 h in an oven, carbonized in N2 atmosphere at 800
°C for 2.0 h, calcined in air at the same temperature for another 2.0 h, then finally, reduced in
H2/N2 atmosphere at 800 °C for 2.0 h before the catalytic reaction. The obtained catalyst was
named O–Ni/MgAl2O4/Inert/Air, where ‘O’ refers to one-pot. For convenience, we named this
material O–NMA/Inert/Air, where N indicates Ni, M indicates Mg, and A indicates aluminate
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(Al2O4) (Fig. 1c). Another sample was fabricated using the same procedure except without
DOI: 10.1039/D1CY00485A
annealing in N2 atmosphere, and named O–NMA/Air (Fig. 1b). A third sample was prepared
by loading Ni, via a conventional incipient wetness impregnation method, where the dried gel
obtained by EISA method was calcined in air at 800 °C for 2.0 h, then impregnated with Ni.
Finally, the solid was calcined again in air at 800 °C for 2.0 h, then reduced in H2/N2 atmosphere
at 800 °C for 2.0 h, before the SMR reaction. This sample was named as I–NMA, where ‘I’
indicates impregnation (Fig. 1a). The weight percent (wt%) of Ni in all catalysts was fixed at
18 wt%. The obtained powder was pelletized under 200 bar, then ground and sieved to obtain
particles of 149–177 μm size for use in the catalytic reaction.
2.3 Physical characterization of catalysts
A Brunauer–Emmett–Teller (BET) analysis was performed using BELSORP-mini
(MicrotracBEL, Japan). The samples were pretreated in a vacuum at 300 °C for 3.0 h, then
subjected to nitrogen adsorption-desorption at –196 oC. The pore-size distribution was analyzed
according to the Barrett-Joymer-Halenda (BJH) theory. Inductively coupled plasma atomic
emission spectroscopy (ICP-AES) analysis was performed to determine the chemical
composition of the as-synthesized catalysts using the Avio500 (Perkin-Elmer, USA). X-ray
diffraction (XRD) analysis was performed using an AXS D8 diffractometer (Bruker, USA) at
Cu Kα wavelength, 40 kV, and 40 mA. The crystallite size of Ni0 NPs was calculated using
Scherrer equation:2
Ni0 crystallite size = (K  λ) / (β  cos θ)
where K is a shape factor, λ is the wavelength of the X-ray source, β is the full width at half-
maximum in radian, and θ is the peak position in radians. Transmission electron microscopy
(TEM) analysis was performed using JEM-2100F (JEOL, Japan) at 200 kV at the National
Nanofab Center in Korea. The reduction properties of the catalysts were analyzed by hydrogen-
temperature programmed reduction (H2-TPR) using the BET-CAT (MicrotracBEL, Japan). The
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o
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DOI: 10.1039/D1CY00485A
analysis was performed at temperatures ranging from 50 to 1000 C under 5% H2/Ar
o
atmosphere after the pretreatment at 200 C under He atmosphere. For determining the
reduction degree, the amount of consumed H2 for reducing each catalyst was calculated in the
range 50 – 800 °C. 800 °C was selected as the pre-reduction step before the SMR reaction, then
the reduction degree was calculated from integrating the peak areas under the TPR profiles at
800 °C and 1000 °C according to the following equation:
Reduction degree (%) = H2 amount consumed at 800 °C / H2 amount consumed at 1000 °C.
The dispersion of active Ni0 was estimated by using H2-chemisorption method (Auto chem II
2920 Micromeritics). Before the estimation, the samples were reduced under 5 vol% H2/Ar at
800 °C for 3 h. The amount of chemisorbed hydrogen molecules on Ni0 was measured at 50 °C
under the pressure from 0 mmHg to 600 mmHg. For calculating the dispersion of Ni0 (D (%))
under the assumption of the stoichiometry between hydrogen molecules and Ni0 on the surface
of the catalyst to be 1:1, the following equation was used:
푉퐻2,퐶ℎ푒푚 × 푆퐹 × 푀푊푁푖
퐷 (%) =
× 100
푊푠푎푚푝푙푒 × 푊퐹푁푖 × 푀푉퐻2,푆푇푃
where VH2,Chem is the volume of chemisorbed hydrogen (mL) on Ni0 under the standard
temperature and pressure (STP) conditions. The dimensionless value, stoichiometry factor, SF is
1. Molecular weight of Ni, MWNi, is 58.69 g/mol. The weight of the sample (g) is denoted as
Wsample. The weight fraction of Ni, WFNi, is the same (18%) for all samples. The molar volume of
H2 under STP conditions, MVH2,STP, is 22,414 mL/mol.
X-ray photoelectron microscopy (XPS) signals were recorded using a Thermo Scientific K-
Alpha XPS system (Thermo Fisher Scientific, UK) equipped with a micro-focused
monochromatic Al Kα X-ray source (1486.6 eV). A 400 μm X-ray beam was used at 6 mA ×
12 kV. The spectra were acquired in the constant analyzer energy mode with a pass energy of
400 eV. Thermo Scientific Avantage software (Thermo Fisher Scientific, UK) was used for
digital acquisition and data processing. Thermogravimetric analysis (TGA) was performed to
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determine the amount of deposited cokes using the Labsys TGA EVO (Setaram
DOI: 10.1039/D1CY00485A
Instrumentation, France) at temperatures up to 1000 oC in air. The coke deposits on the surfaces
of the catalysts were observed by the field-emission scanning electron microscopy (FESEM, S-
4800, Hitachi Co., Japan). Raman spectra of the used catalysts were collected on a LabRAM
HR-800 UV-Visible-NIR system equipped with a charge-coupled detector (CCD) and an
InGaAs array detector, with a laser wavelength of 514 nm and 1800 Grating, an ND 10% filter,
a time of 8 s, and a wavenumber range of 1000 ~ 2000 cm–1.
2.4 Catalytic activity measurements
Catalytic reactions were conducted in a fixed-bed reactor (using a quartz tube of 4 mm
internal diameter). Prior to the reaction, the calcined powder was in-situ reduced at 800 °C for
2.0 h under a flow rate of 50 sccm (H2/N2=1/4) to obtain the active phase (Ni0). The flow was
then switched to the reactant mixture (H2O/CH4=3/1) at T= 800 °C and P= 1.0 atm. Water was
injected by a Micro pump (NP-KX-200, DONGSUNG science, Korea), wherein it was
o
evaporated at 180 C, then mixed with the other gases. The total gas hourly space velocity
–1
(GHSV) was increased from 10,000 to 40,000 mL gcat h–1 (catalyst weight = 0.085 g).
Catalytic reactions in the accelerated deactivation mode, which promote the formation of coke,
were performed under the conditions of H2O/CH4= 1, T= 700 °C, P= 1.0 atm., and GHSV=
10,000 mL gcat–1 h–1. The reaction temperature was controlled with a thermocouple located at
the center of the catalyst bed. Next, the reactants and products were quantified by on-line gas
chromatography (GC) using a HP 6890 GC system (Agilent Technologies Inc., USA) equipped
with a thermal conductivity detector. The CH4 conversion and H2/CO ratio were calculated
using the following equations:
[
]
[
]
CH4 – CH4
표푢푡
푖푛
CH4 conversion (%) =
× 100
[
]
푖푛
CH4
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Where [CH4]in and [CH4]out are the inlet and outlet volume flow of CH4 gas,
DOI: 10.1039/D1CY00485A
respectively.
[
H2
]
표푢푡
2/퐶푂 =
H
[CO]표푢푡
Where [H2] and [CO] are the outlet volume of H2 and CO gases, respectively.
3. Results and discussion
3.1 Characterization of as-prepared catalysts
BET analyses were performed to determine the SSA and pore size distribution (PSD)
for the synthesized materials before and after the reduction process, using N2
adsorption/desorption isotherms (Fig. 2). As shown in Fig. 2a, b, all the as-synthesized samples
presented type-IV isotherms with conspicuous H1-shaped hysteresis loops between the
adsorption and desorption segments at higher P/P0 values (˃0.6), which ordinarily are related to
capillary condensation and evaporation in mesoporous nanomaterials.33 Moreover, the hysteresis
loops displayed by the synthesized catalysts exhibited a high degree of steepness, indicating the
existence of abundant mesopores with premium uniformity within the substructure of the
catalysts.32 These valuable characteristics of the isotherms indicate that the as-prepared catalysts
had optimum mesoporous structure, which is very fundamental for boosting the catalytic activity
of a material. The shift in the loop position to higher relative pressures in the case of I-NMA in
Fig. 2b is due to the increase in the size of its mesopores. As shown in Table S1, the catalyst
fabricated via the impregnation route (I–NMA) had the minimum BET SSA among the
examined samples both before and after the reduction process (156.17 and 98.93 m2 g–1). When
the one-pot strategy was applied for synthesizing the material (O–NMA/Air), the BET SSA
before the reduction enhanced greatly from 156.17 to 211.18 m2 g–1 (Table S1). This suggests
that the synthesis method highly influenced the textural characteristics of the material. When
the material was heated in N2 atmosphere for 2.0 h at 800 °C before calcining in air (O–
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NMA/Inert/Air), the BET SSA enlarged from 211.18 to 253.08 m2 g–1, indicatiDnOgI: t1h0.e103g9r/De1aCtY00485A
influence of the inert carbonization in increasing the SSA of the material. After the reducing
process, the BET SSAs of all materials were decreased; however, the catalyst that had been
carbonized in N2 atmosphere (O–NMA/Inert/Air) maintained the maximum BET SSA (122.49
m2 g–1). This clearly demonstrates the importance of this step in promoting the SSA of the
catalyst, and hence strengthening its SMR performance, where the high BET SSA provides
more accessible active sites during an SMR reaction.
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Fig. 1 – Schematic diagram for the synthetic procedures of mesoporous Ni/MgAl2O4 catalysts.
Since the O–NMA/Air sample was subjected to a calcination temperature of 800 °C for
4.0 h (2.0 h in air and 2.0 h in H2/N2 atmosphere), meanwhile, the sample O–NMA/Inert/Air was
subjected to the same temperature but for 6.0 h (2.0 h in N2, 2.0 h in air, and 2.0 h in H2/N2
atmosphere), an additional sample was fabricated by maintaining the calcination temperature for
4.0 h in air (O–NMA/Air4h). When the as-prepared sample was annealed in air for 4.0 h instead
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of 2.0 h, the BET SSA decreased from 211.18 to 187.49 m2 g–1 and from 104.37 to D8O3I.:4105.10m39/Dg1CY00485A
before and after the reduction process, respectively (Fig. S1 and Table S1). Subjecting the
material to a longer calcination process offers a greater probability of agglomeration of the NPs,
and thus diminishing its SSA as well. Consequently, the O–NMA/Air4h catalysts displayed a
smaller SSA than the O–NMA/Air2h sample. Under the established conditions, both O–
NMA/Inert/Air and O–NMA/Air4h were subjected to 800 °C for 6 h; however, O–NMA/Inert/Air
had a higher BET SSA (122.49 m2 g–1) than O–NMA/Air4h (83.45 m2 g–1), demonstrating the
importance of the inert annealing in promoting the SSA of the material.
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Fig. 2 – N2 adsorption/desorption isotherms (a, b) and BJH pore-size distributions (c, d)
DOI: 10.1039/D1CY00485A
of mesoporous Ni/MgAl2O4 catalysts before (a, c) and after reduction (b, d).
The presence of mesopores was emphasized by the PSD curves, obtained from the
desorption branches of the isotherms (Fig. 2c, d). All samples exhibit a sharp PSD with one
narrow peak existing within the conventional range for uniform mesoporous materials,
demonstrating that all samples have a homogenous mesoporous structure.34,35 This optimum
porosity is very beneficial for improving the catalytic activity.32 The PSD analysis showed that
the pore diameters of all calcined catalysts were predominately around 7.65, 8.01, and 6.58 nm
for I–NMA, O–NMA/Air, and O–NMA/Inert/Air, respectively, assuring that the high SSAs of
the herein as-fabricated materials were due to the mesoporosity (mesopores size: 2.0–50.0 nm
based on the IUPAC definition). After reducing the materials, the predominant pore diameter
(d) of the I–NMA catalyst increased sharply from 7.65 to 17.63 nm; meanwhile, it slightly
increased in the case of the O–NMA/Air and O–NMA/Inert/Air catalysts (Fig. 2c, d and Table
S1), indicating that the latter materials had a stable porous structure compared with the former
one. The increase in the pore size of I–NMA catalyst after reduction can be explained as
follows: In the one-pot synthetic method, Ni was added before fabricating the support and
hence it became like a main part of the support. This resulted in strong interaction of Ni NPs
with the support, where the Ni NPs were fixed within the pores of the support. As a result, the
obtained catalysts prepared by one-pot strategy (O–NMA/Air and O–NMA/Inert/Air) had a
stable pore structure with robust interaction with the Ni NPs, which prevents their
agglomeration. On the other hand, the impregnation method involves two steps for synthesizing
the catalyst: the first step involves synthesizing the support, and second step involves
impregnating the support with Ni. In that case, the formed Ni NPs were not strongly interacted
with the support, and hence easily agglomerated during the reduction step, which resulted in
deteriorating the pores of the support, where the pores cannot sustain the extension in the size
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of Ni NPs, and therefore they deteriorated and increased in size. This has been conDfOirIm: 10e.1d03f9r/oD1mCY00485A
the XRD data where the size of Ni NPs was maximum in the case of I–NMA catalyst and
minimum O–NMA/Inert/Air catalyst (Table 1). This observation clarifies the importance of
one-pot method in preparing the catalyst in the terms of obtaining a stable porous structure,
strong interaction of the active phase with the support, and small size of the NPs.
Furthermore, the pore volume of O–NMA/Inert/Air was the highest among the
synthesized catalysts before (0.5814 cm3 g–1) and after (0.3433 cm3 g–1) the reduction process,
which is very helpful for enhancing the mass transfer during the SMR reaction. On the other
hand, increasing the annealing time in air from 2.0 h to 4.0 h resulted in ‘d’ growing from 8.01
to 8.48 nm and from 17.08 to 19.88 nm before and after the reduction process, respectively
(Fig. S1 and Table S1). This again emphasizes that the materials prepared by the one-pot
method had a more durable porous structure than the materials prepared by two-step methods,
where the value of ‘d’ did not change dramatically after performing the reduction process in the
H2/N2 atmosphere. The durable porous structure in the case of the O–NMA/Inert/Air catalyst is
essential for maintaining the high reactivity of the catalyst during the SMR reaction. From the
BET data, it can be concluded that the O–NMA/Inert/Air catalyst had a superb texture structure,
which could greatly contribute to boosting its catalytic activity towards the SMR.
Powder X-ray diffraction (XRD) analysis was performed to determine the crystal
structure and the nature of the phases existing in the as-prepared samples (Fig. 3). All samples
showed clear diffraction peaks indicating the high crystallinity. Before the reduction process,
all calcined samples showed comparable patterns regardless of the preparation method, except
I-NMA catalyst, which showed two distinct peaks at 2θ of 43.29° and 62.85°, which are
assigned to the NiO phase (PDF#01-1239). On the other hand, O–NMA/Inert/Air displayed a
very weak NiO-related peak at 2θ of 43.29°, while O–NMA/Air did not show any peaks for
NiO phase. The absence of apparent diffraction peaks for NiO phase in O–NMA/Air and O–
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NMA/Inert/Air reflects the excellent dispersion of NiO within the substructure of the support.
DOI: 10.1039/D1CY00485A
The high intensity of peaks related to the spinel phases is due to the overlapping between the
diffraction peaks of the MgAl2O4 (PDF#21-1152) and NiAl2O4 (PDF#10-0339) crystal phases,
since both phases have approximately the same diffraction angles, as can be clearly seen from
the reference peaks in Fig. 3. After reducing the materials, the peaks related to the NiO phase
in the I-NMA catalyst disappeared. In addition, two prominent diffraction peaks positioned at
2θ of 51.85° and 76.38°, were discerned in all of the catalysts, which are attributed to the
formation of the cubic Ni phase (PDF#65-2865). More importantly, the peak located at 2θ of
44.49° showed the maximum intensity among all peaks in all catalysts, owing to the formation
of the cubic Ni phase. These observations confirm the success of the reduction process for all
samples. It can be clearly seen from the XRD patterns of the catalysts that the intensities of the
diffraction peaks related to the spinel phases pronouncedly decreased after completion of the
reduction process (Fig. 3b). This was due to the reduction of Ni from the NiAl2O4 spinel phase,
which corroborates the existence of NiAl2O4 spinel species in the synthesized catalysts. Further
confirmation will be provided by the H2-TPR analysis (see below).
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Fig. 3 – PXRD patterns of mesoporous Ni/MgAl2O4 catalysts synthesized by different strategies,
DOI: 10.1039/D1CY00485A
before (a) and after (b) reduction.
TEM images of the reduced samples are shown in Fig. 4. It is obvious that all the
catalysts show mesoporous textures. The Ni NP size distributions reveal that the O–
NMA/Inert/Air catalyst had smaller Ni NPs (~ 26.73 nm) distributed on the MgAl2O4 support
relative to the I–NMA and O–NMA/Air catalysts (NPs size ~ 49.83 and 38.29 nm,
respectively). This microscopic observation is in accordance with the calculated Ni crystallite
sizes by using Scherrer equation based on the XRD results, namely the smallest Ni crystallite
size for the O–NMA/Inert/Air catalyst as summarized in Table 1. Ni reduction degree and Ni
dispersion were calculated from the results of H2-TPR and H2-chemisorption, respectively
(Table 1). The I–NMA, O–NMA/Air, and O–NMA/Inert/Air catalysts showed reduction
degrees of 94, 88, and 86%; respectively, indicating that Ni species in O–NMA/Inert/Air
catalyst are more difficult to reduce than the other two catalysts. This confirms that Ni species
in the latter catalyst has a strong interaction with the support, which will be discussed further
in detail. The H2-chemisorption was performed to evaluate the dispersion degree of Ni active
sites for the various catalysts. The O–NMA/Inert/Air catalyst exhibited the maximum degree
of Ni dispersion (3.35%) among the examined catalysts, meanwhile I–NMA catalyst showed
the minimum one (1.22%). The O–NMA/Air catalyst showed Ni dispersion degree between
both I–NMA and O–NMA/Inert/Air catalysts (2.38%). These results prove that Ni species in
I–NMA that weakly interacted with the support were reduced to large Ni0 NPs, meanwhile,
they were reduced into small Ni0 NPs in O–NMA/Inert/Air, due to the strong interaction with
the support. These results also confirm that quantity of Ni species that were reduced to Ni0 NPs
in I–NMA was higher than those in O–NMA/Inert/Air, due to the more robust interaction of Ni
species with support in the latter one. This indicates that both the synthetic approach and the
calcination step in N2 atmosphere had great influences on the Ni dispersion degree within the
support matrix, which would have a fundamental role on the catalytic activity during the SMR
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reaction. The number of Ni0 active sites was calculated by considering the dispersion of Ni and
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DOI: 10.1039/D1CY00485A
the reduction degree.41 It is clear from Table 1 that O–NMA/Inert/Air displayed the highest
number of Ni0 actives sites among the as-prepared catalysts (8.68  10–7 mol). Since O–
NMA/Inert/Air showed the highest surface area, lowest NPs size, highest Ni dispersion, and
the largest number of Ni0 active sites, it was expected that this catalyst will show the best
catalytic activity among the herein studied catalysts.
Fig. 4 – TEM images of the as-synthesized catalysts: (a) I-NMA, (b) O–NMA/Air, and (c)
O–NMA/Inert/Air. The inset shows the NPs size distribution of the various catalysts.
The results of the ICP-OES analysis of the calcined samples are shown in Table S2. All
the as-synthesized catalysts had nearly the same contents of Ni (18.2 ± 0.1 wt%), Mg (7.7 ± 0.1
wt%), and Al (34.2 ± 0.3 wt%), in agreement with the theoretical values. The excellent
agreement between the experimental and theoretical values of the metal contents confirms that
all of the Ni, Mg, and Al injected into the solution during the fabrication process was recovered
in the solid catalyst after finishing the preparation process. Considering this paramount
observation, the difference in the performances of the catalysts can be attributed to the
difference in the catalysts’ substructure and composition, such as BET SSA, nature of phases
present, and extent of interaction between Ni active phase and support. This again highlights
the essential role of inert annealing in modifying the catalyst substructure, where catalyst
properties were greatly improved which would lead to much enhancement in the catalytic
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performance towards the SMR reaction as will be discussed in the catalytic performance
DOI: 10.1039/D1CY00485A
section. Especially, the high dispersion and small size of the Ni NPs are known to prevent the
deposition of carbon species on the catalyst surface during the SMR reaction,4 leading to
sustained high stability for long periods.
Fig. 5 – (a) H2–TPR and (b) XPS spectra of mesoporous Ni/MgAl2O4 catalysts synthesized
by different strategies.
Another important parameter for the high stability of catalysts is the strong metal-
support interaction. This strong interaction plays a fundamental role in preventing Ni sintering
during the SMR process, thus maintaining the catalytic activity for prolonged periods with high
stability. H2-TPR is an efficient technique for identifying the reducibility of metal-oxide-based
supported catalysts and the degree of interaction between the active phase and support. H2-TPR
results suggest that the interaction of Ni NPs with the support is stronger in the O–
NMA/Inert/Air catalyst than that in I–NMA and O–NMA/Air catalysts (Fig. 5a). I–NMA
catalyst exhibited three reduction peaks at temperatures of 490, 735, and 836 °C, indicating the
presence of different reducible Ni species with different interactions with the spinel support.
The presence of a reduction peak at 490 °C in this sample is due to the reduction of bulk NiO
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species, which weakly bounded with the support,31 in agreement with the XRD data (Fig. 3a).
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DOI: 10.1039/D1CY00485A
The reduction peak at 735 °C, was related to the reduction of highly dispersed NiO species
which strongly interact with the support. 37,38 On the other hand, the presence of the reduction
peak at higher temperature (836 °C) was ascribed to the reduction of the NiAl2O4 spinel,4,31,39,40
further confirming the existence of the NiAl2O4 phase in all catalysts, as had been demonstrated
from the XRD analysis. When the one-pot strategy was applied for synthesizing the catalyst
(O–NMA/Air), the reduction peak at 490 °C disappeared, and instead, a small shoulder
appeared at a higher temperature (581 °C). The absence of reduction peaks in this catalyst at
temperatures below 500 °C suggests the absence of the bulk NiO species, in agreement with
the XRD data (Fig. 3a). The remaining two reduction peaks, which are attributed to the
reduction of highly dispersed NiO and NiAl2O4, respectively shifted to higher temperatures of
752 and 859 °C, respectively. More importantly, the reduction peaks at 859 °C had the
maximum intensity. All of these observations emphasize the great enhancement in the
dispersion and interaction of Ni species with the support when the one-pot strategy is employed
instead of the impregnation strategy, which indicates the importance of selecting the suitable
route for synthesizing the SMR catalysts. In the case of the O–NMA/Inert/Air catalyst, there
was only one very intense peak at 862 °C, which was ascribed to the reduction of NiAl2O4.40 It
is noteworthy to observe that the reduction peak in this catalyst had the maximum intensity and
the maximum reduction temperature (862 °C). In contrast to the I–NMA and O–NMA/Air
samples, which showed clear and intense peaks below the temperature of 800 °C, the O–
NMA/Inert/Air catalyst showed only very weak peaks below 800 °C, which attributed to the
reduction of the very small content of NiO existing in this catalyst as has been shown from the
XRD (Figs. 3a) where there was a very small peak for NiO. Taking into account the
aforementioned observations from the H2-TPR profiles, it can be concluded that the majority
of Ni species in the O–NMA/Inert/Air catalyst strongly linked to the mesoporous framework
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of the spinel support. The H2-TPR profiles of the O–NMA/Air and O–NMA/air4h samples are
DOI: 10.1039/D1CY00485A
presented in Fig. S2. Increasing only the calcination time from 2.0 to 4.0 h resulted in shifting
the reduction peaks from 752 and 859 °C to 684 and 834 °C, respectively, indicating the weaker
interaction between the Ni species and support in O–NMA/air4h catalyst. This demonstrates the
great importance of inert annealing in upgrading the structural properties of the catalyst.
The XPS analysis was performed to define the chemical states and electronic structures
of the elements existing in the surface region of the catalysts (Fig. 5b). The XPS spectra of Ni
2p of the reduced catalysts show two main peaks at binding energies of 855.7 ± 0.1 and 873.2
± 0.1 eV along with a shake-up satellite peak at around 861.5 eV± 0.1. In addition, there is a
noticeable peak at 852.3 ± 0.1 eV, which is ascribed to the metallic Ni species.42 The binding
energies of the Ni species in the catalysts are shown in Table S3. It is clear that the binding
energies are almost same in all catalysts, indicating that the surface Ni was not influenced by
altering the synthetic strategy, and its electronic environment was not changed. The existence
of the Ni 2p3/2 peak and satellite peak at around 855.7 and 861.5 eV, respectively, for all samples
is a typical characteristic of Ni species in the NiAl2O4 spinel.43,44 The binding energy of Ni 2p3/2
at around 855.7 eV is attributed to the Ni2+ species robustly interacted with the support,45
suggesting that Ni0 and Ni2+ moieties coexisted on the surface of the reduced catalysts. This
demonstrates that some proportion of Ni2+ was reduced to Ni0, while the rest proportion
remained as Ni2+. Taking into account the existence of the oxidation state of the surface Ni in
the form of Ni2+ and the fact that the binding energy of Ni 2p3/2 in pure NiAl2O4 is 856.0 eV,31
which is very close to 855.7 eV, it can be concluded that NiAl2O4 spinel species indeed exist
within the surface of prepared catalysts, which agrees well with the XRD and H2-TPR
measurements. The binding energy of Ni 2p3/2 in pure NiO is about 854.4 eV 31,44 which is much
smaller than the value observed herein (855.7 eV) for the reduced catalysts. This indicates that
the Ni 2p3/2 species exist in the form possessing strong interaction with the support which is
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NiAl2O4 instead of NiO species. This further confirms the presence of NiAl2O4 spinel species
DOI: 10.1039/D1CY00485A
in the herein synthesized materials. To clearly show the contents of reduced Ni species present
within the surface region in the different catalysts, the deconvolution of the Ni XPS spectra was
performed, and the data are shown in Fig. S3. It is obvious that the O–NMA/Inert/Air catalyst
had the maximum peak intensity for Ni0 among the synthesized catalysts, indicating that it
possessed the maximum quantity of metallic Ni on its surface, which is very beneficial for
enhancing catalytic activity towards the SMR reaction. This further confirms the fundamental
role played by the inert annealing step in upgrading the catalyst substructure where it led to
increasing the content of reduced Ni species on the catalyst surface, which is very crucial for
boosting the catalytic activity.
3.2 Catalytic performance
The SMR activities of the developed catalysts and the commercial catalyst at different
GHSVs are plotted in Fig. 6 for the reaction conditions H2O/CH4= 3, T= 800 °C, and P= 1.0
–1
atm. At the GHSV of 10,000 mL gcat h–1, all catalysts showed similar CH4 conversion
efficiencies until the stream time of 120.0 min. As the feed flow rate was increased from 10,000
–1
to 40,000 mL gcat h–1, the CH4 conversions of the as-prepared catalysts as well as the
commercial catalysts decreased accordingly. As shown from Fig. 6, when the GHSV was
increased from 10,000 to 40,000 mL gcat–1 h–1, the drop in the CH4 conversion efficiency was
minimum for the O–NMA/Inert/Air catalyst: from 97.6 to 82.0%. The I–NMA and commercial
catalysts showed conversion decreases from 97.5 and 98.0% to 68.8 and 67.0%, respectively.
The deterioration in the catalytic conversion with increasing the gas feed-flow rate is attributed
to the reduced reactant habitation time on the catalyst surface and the restricted access to active
sites as a result of the increment in the number of reactants.43 Moreover, coke formation on the
surfaces of catalysts is enhanced by increasing the GHSV.46 While the conversion efficiency
remained almost constant as the GHSV was maintained constant, the O–NMA/Air catalyst
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displayed a rapid degradation in its conversion efficiency at 40,000 mL gcat–1 h–1. It is clear that
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DOI: 10.1039/D1CY00485A
the O–NMA/Inert/Air catalyst maintained the highest conversion efficiency among the tested
catalysts, indicating the best catalytic performance during the SMR reaction.
Fig. 6 – CH4 conversions of mesoporous Ni/MgAl2O4 catalysts and commercial catalyst at
different GHSVs ranging from 10,000 to 40,000 mL gcat–1 h–1.
Although the I–NMA and O–NMA/Inert/Air catalysts showed similar catalytic activity
at the beginning of the reaction, they exhibited different activity with increasing the reaction
time and GHSV: I–NMA showed a CH4 conversion efficiency of 68.8% at the stream time of
–1
480 min and GHSV of 40,000 mL gcat h–1, whereas O–NMA/Inert/Air showed a CH4
conversion efficiency of 82.0% at the same stream time. This result demonstrates the
importance of selecting the proper synthetic strategy for designing an efficient catalyst for the
SMR reaction. On the other hand, although the O–NMA/Air and O–NMA/Inert/Air catalysts
were prepared by the same one-pot strategy except for the annealing step in the N2 atmosphere
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in the case of O–NMA/Inert/Air, they displayed different catalytic conversion efDfiOcI:i1e0n.1c03ie9/sD1aCtY00485A
the stream time of 480 min and GHSV of 40,000 mL gcat–1 h–1 (53.7 and 82.0% for O–NMA/Air
and O–NMA/Inert/Air, respectively), thus elucidating the significance of the inert annealing in
boosting the reactivity of the catalyst during the SMR process. The O–NMA/Inert/Air catalyst
displayed the highest H2/CO ratios (Fig. S4) among the examined catalysts at various GHSVs
ranging from 10,000 to 40,000 mL gcat–1 h–1, which further confirms that this catalyst offered
the best catalytic performance in the SMR reaction. In addition, Table S4 shows the comparison
of SMR catalytic activity and stability of our catalyst (O–NMA/Inert/Air c) with that of similar
composition reported in literature. It is obvious that O–NMA/Inert/Air catalyst is one of the top
performing catalysts towards the SMR reaction. Base on the above physical characterization
of the different catalysts, the superior catalytic performance of O–NMA/Inert/Air catalyst can
be attributed to: (i) the large SSA with optimum mesoporosity and high pore volume, which
drive to supplying more accessible Ni active sites for the gaseous reactants, (ii) the small size
and high dispersion of Ni NPs throughout the catalyst infrastructure, (iii) the strong interaction
between the Ni NPs and support, and hence suppressing the catalyst sintering and increasing
the resistance to carbon deposition, and (iv) the high concentration of metallic Ni on the
catalyst’s surface as has been elucidated from the XPS analysis. The enrichment of catalyst
surface with Ni0 could provide more catalytic active sites for the reaction between CH4 and H2O.
The most important objective of this study was to develop SMR catalyst with high
resistance to coke formation under harsh operating conditions. We performed a catalytic
reaction in the accelerated mode of coke formation under the conditions of H2O/CH4= 1, T=
700 °C, P= 1.0 atm., and GHSV= 10,000 mL gcat–1 h–1 for 7.0 hours (Fig. S5). Under these SMR
reaction conditions, I–NMA, O–NMA/Air, O–NMA/Inert/Air, and the commercial catalyst
showed CH4 conversions of 57.0, 59.3, 72.3, and 70.5%, respectively, which remained almost
constant over the entire 7.0 h duration of the SMR reaction. The O–NMA/Inert/Air catalyst had
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the highest CH4 conversion efficiency among all the as-synthesized catalysts (Fig. S5a). Fig.
DOI: 10.1039/D1CY00485A
S5b shows that the O–NMA/Inert/Air catalyst displayed the highest H2/CO ratio, close to 3,
which is the theoretical H2/CO ratio. It also showed the highest value for carbon balance that
would be ideally 1.0, meaning the least probability of coke deposition among the tested catalysts
under the accelerated reaction conditions (Fig. S5c). The high durability of the O–
NMA/Inert/Air catalyst is ascribed to the stable mesoporous substructure and the strong
interaction between the active phase and support, as was confirmed from the BET, H2-TPR,
and H2-chemisorption measurements. The excellent mesoporous structure facilitates the
accessibility of the gaseous reactants to quickly reach the active sites existing within the catalyst
substructure; meanwhile, the strong interaction of metallic Ni with the support inhibits the
growth of NPs during the SMR reaction, thereby suppressing the coke formation, which easily
occurs on larger NPs.
3.3. Characterization of used catalysts
The deposition of carbon in the form of filaments over the surfaces of all catalysts is
shown in Fig. 7. As seen from Fig. 7a, the commercial catalyst displayed a very high density of
coke, which almost fully covered its surface, revealing its weak resistance to carbon deposition
and hence susceptibility to deactivation. On the other hand, the as-synthesized catalysts,
especially O–NMA/Inert/Air, showed less affinity for carbon deposition, indicating their better
resistance to coke formation. Raman spectroscopy was employed to identify the nature of
carbon deposited on the surface of the catalysts after the SMR reaction (Fig. 8a). There were
two distinct peaks in the Raman spectra at wavelengths of ~1344 and 1585 cm–1, corresponding
to the D and G bands.32 The first peak (D) is ascribed to the structural defects and lattice
distortions in the sp3-hybridized carbon systems, and the second peak (G) is due to the in-plane
vibrations of the sp2-bonded carbon atoms with a high degree of symmetry and graphitization.
The intensity of the D band relative to the that of G band (ID/IG) has been used to characterize
the degree of crystallinity and/or the existence of disorders in the carbon structure.32 The I–
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NMA and O–NMA/Inert/Air catalysts showed high ID/IG ratios of 1.12 and 1.09, respectively,
DOI: 10.1039/D1CY00485A
comparing to 0.71 for the O–NMA/Air catalyst. This demonstrated that the carbon deposited
on the surface of the former catalysts was amorphous and contained high numbers of defects,
meanwhile that deposited on the surface of the latter one had a high degree of crystallinity. The
high graphitic degree of the deposited carbon could densely wrap the metallic Ni active sites,
and hence leading to weakening the catalytic performance. It is noteworthy to observe that the
carbon deposited on the surface of the O–NMA/Inert/Air catalyst had a very low degree of
graphitization (ID/IG= 1.09) comparing to O–NMA/Air (0.71), which is advantageous in that
the Ni-active sites were not fully blocked by the carbon deposited on its surface, meaning that
the high catalytic activity could be sustained for longer periods over the O–NMA/Inert/Air
catalyst than would be possible over the O–NMA/Air catalyst, indicating the significant role of
the inert annealing in modifying the catalyst structure.
Fig. 7 – SEM images of the studied catalysts after the SMR reaction. The green arrows
refer to the carbon filaments formed on the catalyst surface after the reaction.
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DOI: 10.1039/D1CY00485A
Fig. 8 – Raman spectra (a) and TGA (b) plots of mesoporous Ni/MgAl2O4 catalysts after
the SMR reaction.
To determine the quantities of carbon species deposited on the surfaces of the used
catalysts after performing the SMR reaction at 700 °C for 7.0 h (H2O/CH4= 1 and GHSV=
10,000 mL gcat–1 h–1), a TGA analysis was done under air atmosphere, and the results are shown
in Fig. 8b. The weight loss below 200 °C is attributed to the desorption of adsorbed moisture
from the catalysts; meanwhile, the weight increase in the temperature range from 200 to about
400 °C is ascribed to the oxidation of metallic Ni species during the TGA experiments.47 The
weight-loss stage over 400 until 700 °C was due to the oxidation of carbon species deposited
on the catalysts during the SMR reaction.48 It is obvious that the weight loss was maximum in
the commercial catalyst and minimum in O–NMA/Inert/Air, in well agreement with the
FESEM data, where the amount of deposited carbon was very high in the former catalyst and
very low in the latter one.
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DOI: 10.1039/D1CY00485A
Fig. 9 – XRD plots of mesoporous Ni/MgAl2O4 catalysts fabricated by different strategies
after the SMR reaction.
In order to get insights about the changes in the size of NPS after the SMR reaction, the
XRD analysis of the used catalyst was done and the results are shown in Fig. 9. The calculated
sizes of Ni NPs after the SMR reaction were found to be 52.34, 40.41, and 28.21 nm for I–
NMA, O–NMA/Air, and O–NMA/Inert/Air, respectively comparing to 49.83, 38.29, and 26.73
nm, respectively for the same catalysts before the SMR reaction. It is obvious that there was a
small increase in the size of the Ni NPs after performing the SMR reaction, indicating the strong
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interaction between the Ni NPs and support, which is essential for minDimOI:iz10i.n10g39/Dth1CeY00485A
agglomeration of the NPs. It is also clear from the above figure that only I–NMA showed a
clear carbon peak at 2θ of 26.1°, originated from the severe coke deposition, meanwhile the
other catalysts did not show any obvious peaks for carbon.
4. Conclusions
Annealing the Ni/MgAl2O4 spinel catalyst in N2 atmosphere before calcining it in air
had a significant influence on its physicochemical properties. This step affords the following
important advantages in terms of catalyst structure: 1) enhancing the SSA and mesoporosity
leading to increased accessibility of reactants to active sites; 2) improving the dispersion of Ni
NPs within the support and minimizing their size, and therefore, lowering the probability of
coke formation, which easily occurs on large metallic NPs; 3) strengthening the interaction
between the active phase (Ni) and the support, and thus preventing the catalyst sintering under
the harsh working condition; 4) increasing the content of reduced Ni on the catalyst surface,
and thereby providing more active sites for occurring the SMR reaction. As a result, the
Ni/MgAl2O4 spinel catalyst subjected to an inert carbonization process before the calcination
step in air displayed high CH4 conversion efficiency, high durability, and excellent resistance
for carbon deposition. The insights obtained from this study may have broad applications in
developing Ni-based catalysts for various catalytic reactions such as water-gas shifting,
reforming of hydrocarbons and oxygenates, CO2 conversion, etc.
Acknowledgments
This research was funded by the framework of the research and development program
of the Korea Institute of Energy Research (C0-2406). This research was also supported by
Korea Electric Power Corporation (R20XO02-16).
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DOI: 10.1039/D1CY00485A
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