Adjusted interactions of nickel nanoparticles with cobalt-modified MgAl2O4-SiC for an enhanced catalytic stability during steam reforming of propane

Article abstract of DOI:10.1016/j.apcata.2017.09.031

Steam reforming of propane (SRP) for the stable production of hydrogen-rich reformates was investigated using the Ni-supported on the cobalt-modified SiC-embedded MgAl₂O₄ support (denoted as NCMAS). The adjusted interactions of the Ni nanoparticles with the cobalt-modified SiC-embedded MgAl₂O₄ (MgAl₂O₄-SiC) and its crystallite size distributions largely altered the catalytic activity and stability of NCMAS. The introductions of SiC on the NCMAS, where SiC has a higher thermal conductivity, also increased the dispersion of smaller MgAl₂O₄ grains with the less formations of inactive NiAl₂O₄ species, which resulted in the higher catalytic activity with smaller formations of unreformed light hydrocarbons. The positive roles of cobalt promoter on the MgAl₂O₄-SiC matrices were mainly attributed to the suppressed aggregation of nickel nanoparticles by their strong and intimate interactions with the cobalt-modified MgAl₂O₄. The effects of cobalt promoter at an optimal 5wt%Co in the MgAl₂O₄-SiC (NCMAS(5)) enhanced the oxidation-resistance of the nickel nanoparticles with less formations of inactive metal aluminates by being reversibly re-reduced under the SRP reaction conditions. These phenomena further lessen coke depositions by intimately interacting with highly dispersed oxophilic cobalt or cobalt aluminate species. The optimal NCMAS(5) was applied to derive kinetic parameters using Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanisms, and the reasonable activation energy of 73 kJ/mol and optimal operating parameters to maximize hydrogen production by SRP reaction was estimated in terms of the reaction conditions such as space velocity, feed ratio and reaction temperature.

Full text of DOI:10.1016/j.apcata.2017.09.031

Accepted Manuscript
Title: Adjusted interactions of nickel nanoparticles with
cobalt-modified MgAl O -SiC for an enhanced catalytic
2
4
stability during steam reforming of propane
Authors: Kyung Soo Park, Minji Son, Myung-June Park, Dae
Hyun Kim, Jeong Hwa Kim, So Hyun Park, Joon-Hwan Choi,
Jong Wook Bae
PII:
S0926-860X(17)30472-6
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2017.09.031
APCATA 16425
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
1-8-2017
18-9-2017
25-9-2017
Please cite this article as: Kyung Soo Park, Minji Son, Myung-June Park, Dae
Hyun Kim, Jeong Hwa Kim, So Hyun Park, Joon-Hwan Choi, Jong Wook Bae,
Adjusted interactions of nickel nanoparticles with cobalt-modified MgAl2O4-SiC for
an enhanced catalytic stability during steam reforming of propane, Applied Catalysis
A, General https://doi.org/10.1016/j.apcata.2017.09.031
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Submitted to the Applied Catalysis A: General
Adjusted interactions of nickel nanoparticles with cobalt-modified MgAl
2
O
4
-SiC
for an enhanced catalytic stability during steam reforming of propane
1
2
2,3,
1
1
Kyung Soo Park , Minji Son , Myung-June Park *, Dae Hyun Kim , Jeong Hwa Kim ,
1
4
1,
So Hyun Park , Joon-Hwan Choi , Jong Wook Bae **
1School of Chemical Engineering, Sungkyunkwan University (SKKU), Suwon 16419,
Republic of Korea
2Department of Energy Systems Research, Ajou University, Suwon 16499, Republic of Korea
3Department of Chemical Engineering, Ajou University, Suwon 16499, Republic of Korea
4Functional Ceramics Department, Korea Institute of Materials Science (KIMS), Changwon,
Gyeongnam 642-831, Republic of Korea
----------------------------------------------------------------------------------------------------------------
*
Corresponding author (M.-J. Park): Tel.: +82-31-219-2383; Fax: +82-31-219-1612; E-mail
address: mjpark@ajou.ac.kr
*Corresponding author (J.W. Bae): Tel.: +82-31-290-7347; Fax: +82-31-290-7272; E-mail
address: finejw@skku.edu
*
1

Graphical Abstract
Highlights


Adjusted interactions of nickel nanoparticles with cobalt-modified MgAl
catalytic stability during steam reforming of propane
Kyung Soo Park, Minji Son, Myung-June Park*, Dae Hyun Kim, Jeong Hwa Kim, So Hyun
Park, Joon-Hwan Choi, Jong Wook Bae**
2
O -SiC for an enhanced
4

◀ Steam reforming of propane (SRP) was studied over the Ni-supported on Co-modified
MgAl
2
4
O @SiC (NCMAS). ◀ Adjusted interactions of Ni nanoparticles largely altered the
activity and stability. ◀ Cobalt promoter showed the suppressed Ni aggregation with
intimate interactions with CMAS support. ◀ Kinetic parameters using LHHW model
were properly derived for SRP reaction.
ABSTRACT
Steam reforming of propane (SRP) for the stable production of hydrogen-rich reformates
was investigated using the Ni-supported on the cobalt-modified SiC-embedded MgAl
2
O
4
support (denoted as NCMAS). The adjusted interactions of the Ni nanoparticles with the
2

cobalt-modified SiC-embedded MgAl
2
O
4
(MgAl
2
4
O -SiC) and its crystallite size distributions
largely altered the catalytic activity and stability of NCMAS. The introductions of SiC on the
NCMAS, where SiC has a higher thermal conductivity, also increased the dispersion of
smaller MgAl
in the higher catalytic activity with smaller formations of unreformed light hydrocarbons. The
positive roles of cobalt promoter on the MgAl -SiC matrices were mainly attributed to the
suppressed aggregation of nickel nanoparticles by their strong and intimate interactions with
the cobalt-modified MgAl . The effects of cobalt promoter at an optimal 5wt%Co in the
MgAl -SiC (NCMAS(5)) enhanced the oxidation-resistance of the nickel nanoparticles
2
O
4
grains with the less formations of inactive NiAl
2
O
4
species, which resulted
2
O
4
2
O
4
2
O
4
with less formations of inactive metal aluminates by being reversibly re-reduced under the
SRP reaction conditions. These phenomena further lessen coke depositions by intimately
interacting with highly dispersed oxophilic cobalt or cobalt aluminate species. The optimal
NCMAS(5) was applied to derive kinetic parameters using Langmuir-Hinshelwood-Hougen-
Watson (LHHW) mechanisms, and the reasonable activation energy of 75 kJ/mol and optimal
operating parameters to maximize hydrogen production by SRP reaction was estimated in
terms of the reaction conditions such as space velocity, feed ratio and reaction temperature.
Keywords: Steam reforming of propane (SRP); Nickel nanoparticle; metal-support
interaction; Co-modified on SiC-embedded MgAl
2
4
O ; Metal aluminates.
NOMENCLATURE AND ACRONYMS
E
F
Activation energy, J/mol
Molar flow rate, mol/h
Objective function
F
obj
k
Forward reaction rate constant
Kad
Adsorption equilibrium constant, bar–1
K
eq
Reaction equilibrium constant
3

MARR
MG
P
Mean of absolute relative residuals
Mixture of gas
Pressure, bar
r
Reaction rate, gmol/(gcat·min)
Relative standard deviation of individual errors
Steam reforming
RSDE
SRM
WGS
Water gas shift
1
. Introduction
Due to significant depletion of fossil fuel reservoirs and its combustion emissions causing
environmental problems recently [1], possible utilizations of alternative energy resources
have been largely attracted. Hydrogen, as a new clean fuel due to its high energy efficiency
and zero emission of SOx, NOx, and COx, can be effectively utilized for an alternative
cleaner energy resource through the applications of fuel cell power plants or vehicles
compared with the conventional power generation systems. The fuel cells can be operated by
the fuel processing technologies through steam reforming, autothermal reforming, dry
reforming or their combined reforming of hydrocarbons sources [2]. Among them, the steam
reforming reaction of light alkanes, which has been known as the most economical route for
the syngas production [3], seems to be an appropriate process for a hydrogen-rich syngas
production. Even though natural gas (or shale gas) has been used for steam reforming
reaction, propane seems to be beneficial for a fuel processor application as well due to its
easy replacement of the well-constructed infrastructures of liquefied petroleum gas (LPG)
with an easy-liquefying nature and a higher capacity for hydrogen production per unit volume
than that of methane. The main reactions of steam reforming of propane (SRP) are composed
with steam reforming, water-gas shift and methanation reaction with an unfavorable
4

Boudouard reaction for a coke formation, and the SRP reaction has highly endothermic and
irreversible natures by showing a complete conversion at the temperature ranges of 500 - 800
oC thermodynamically [4,5].
To suppress the catalyst deactivation by carbon depositions originated mainly from thermal
hydrocarbon decompositions, substantial efforts have been endeavored to minimize the coke
formations which are mandatory for a robust operation of fuel processor [6-8]. In general, the
group VIII transition metals such as Rh, Ru and Ir and so on have been widely used because
of their superior catalytic activity and stability with less coke formations during steam
reforming reactions [3,8-15]. However, the supported Ni-based catalysts are preferred
industrially due to its low cost, and many efforts were made to develop coke-resistant Ni-
based reforming catalysts. In addition, those catalysts are generally suffered from the fast
deactivation due to its facile aggregations of Ni nanoparticles by transforming to less active
nickel aluminates or by forming less active sites via irreversible carbon depositions [7,8]. To
overcome those problems, Ni-based catalysts were modified by controlling the crystallite
sizes of nickel nanoparticles [16] with a higher dispersion, by modifying the supports with
the addition of lanthanide, alkali or rare earth metals [8,15,17-20] to decrease the carbon
depositions and by adding small amounts of noble metals such as Rh and Ru to promote in-
situ reducibility of Ni nanoparticles [21,22]. In addition, some bimetallic catalysts containing
the active Ni and Co species on the various supporting materials have been also largely
reported to enhance the catalytic activity and to lessen coke formations as well [13,23-26]. A
possible formation of Ni-Co alloy can generate a higher electron density state of the
supported metallic Ni nanoparticles resulted in enhancing the catalytic activity due to the
enhanced resistances to metal aggregations and coke formations [25]. In addition, to modify
metal-support interactions, various metal oxides with high surface area and thermal stability
such as MgO, MgAl
2
O
4
, Al
2
O
3
, and TiO
2
and so on haven been used to suppress permeant
5

deactivations [27]. Among them, a spinel-type MgAl
high resistance for coke depositions through stronger interactions with nickel nanoparticles
11,14,16,24]. In addition, SiC material has been also reported to have a better activity
compared to Al due to its high thermal conductivity and chemical inertness [28].
In this present study, some positive effects of oxophilic cobalt promoter on the SiC-
embedded MgAl support in terms of Ni and Co distributions with its metal-support
2
O
4
has been widely studied due to its
[
2
O
3
2
O
4
interactions for an enhanced oxidation-resistance of nickel nanoparticles with less formation
of inactive nickel aluminates as well as coke precursors were investigated to elucidate the
synergetic effects of the metallic Ni and Co promoter for the SRP reaction. In addition, an
optimal catalyst selected from activity test was further investigated to derive proper kinetic
parameters using Langmuir-Hinshelwood-Hougen-Watson (LHHW) models, and the best
reaction conditions were also proposed to maximize a hydrogen productivity from SRP
reaction.
2
2
. Experimental
.1. Catalyst preparations and activity measurements
SiC-embedded MgAl
a sol-gel method using a gelation agent of propylene oxide based on the previously reported
work [29]. For more details, two metal nitrate precursors such as Al(NO ∙9H O (Daejung,
8%) and Mg(NO ∙6H O (Alfar Aesar, 98-102%) were simultaneously dissolved in an
anhydrous ethanol (Daejung, 99.9%) with vigorous stirring, and it was poured into a slurry of
2
O
4
(denoted as MgAl
2
O
4
-SiC) supporting materials were prepared by
3
)
3
2
9
3
)
2
2
β-SiC (SICAT, where SiC was coated with SiO
x
C outer surface layers with a surface area of
y
2
~
25.2 m /g) at a fixed MgAl
2
O
4
/β-SiC weight ratio of 7/3, 5/5 and 3/7 at a stoichiometric
MgO/Al
2
O
3
molar ratio of 1. After complete mixing of above mixtures for 20 minutes,
propylene oxide (Samchun, 99.0%) as a gelating agent was added to the above mixture, and it
was transformed to a gel after stirring for 10 minutes. As-prepared gel was aged at room
6

o
temperature for 6 h then it was dried in 80 C for 60 h. The dried xerogel was further calcined
o
at T = 700 C for 3 h under air atmosphere. The finally obtained MgAl
2
O
4
-SiC powder was
to (β-SiC+MgAl ), and
support itself. The cobalt-modified MAS (denoted as CMAS)
supports were further prepared at a fixed weight ratio of MgAl /SiC = 7/3 as similar
preparation method with the MAS-7 support except for the addition of cobalt promoter. The
desired amount of cobalt nitrate precursor (Co(NO ∙6H O, Samchun, 97%) such as 1, 5 and
wt%Co based on the weight of MgAl was additionally dissolved into the ethanol
solution containing Mg and Al precursors. The cobalt-modified MAS-7 supports are denoted
denoted as MAS-y, where y represents a weight ratio of MgAl
MA represents MgAl
2
O
4
2
O
4
2
O
4
2
O
4
3
)
2
2
8
2
O
4
as CMAS(x), where x represents the wt% of the cobalt metal in the matrices of MgAl
the further preparation of nickel supported catalysts, the nickel was introduced on the
2
4
O . For
CMAS(x) support by conventional wet impregnation method at a fixed amount of 10 wt%Ni.
The nickel precursor of Ni(NO ∙6H O (Samchun, 98%) was previously dissolved in
3
)
2
2
methanol solvent, and the CMAS support was subsequently added to the above solution. The
slurry was vigorously stirred for 1 h and the methanol solvent was completely evaporated at
o
5
0 C in a vacuum evaporator (NE-2001, EYELA). After the evaporation step, the sample
o
o
was dried in an oven kept at 80 C overnight, and it was calcined at 700 C for 3 h under air
environment. The as-prepared final catalysts were denoted as NCMAS(x) with the different
cobalt wt% using the MAS-7 supports having a weight ratio of MgAl
2
4
O /SiC = 7/3.
Catalytic activity of the SRP reaction on the NCMAS was measured in a fixed bed tubular
inconel reactor having an outer diameter of 9.5 mm in size. A 0.15 g of NCMAS catalyst was
placed on the α-alumina ball-bed, and thermocouple was placed just over the catalyst-bed to
monitor the temperature variations at a fixed reaction temperature controlled by electric
o
furnace. Prior to the SRP reaction, the catalyst was reduced at 700 C for 3 h under a flow of
5
vol% H
2
balanced with N
2
. Subsequently, the reactor temperature was cooled down to the
7

o
reaction temperature of 650 C and steam was fed to the catalyst-bed with a steam to carbon
ratio of H O/C (S/C) = 3. For the catalytic activity tests, the SRP reaction was carried out at
an ambient pressure and weight hourly space velocity (SV) of 33600 L/(kgcat∙h) based on dry
gases using mixed gas compositions of C /N /H O = 2/8/18 (mole). For the kinetic
2
3
H
8
2
2
analysis with accurate parameter estimations on the finally selected optimal NCMAS(5),
additional SRP reactions were carried out at a wide range of reaction conditions of T = 550 –
o
7
50 C, S/C molar ratio = 3 – 7, and SV = 168000 – 336000 L/(kgcat∙h) with 0.02 g catalyst
diluted with 0.18 g SiC (total 0.2 g catalyst-bed weight). The effluent gases from reactor exit
were passed over a cold trap, and they were in-situ analyzed by a gas chromatography
(
Younglin, YL6100, GC) equipped with a Carboxen 1000 packed column to measure the
inflammable gases such as H , N , CO, CO , CH using a thermal conductivity detector
TCD) as well as a flame ionization detector (FID) with a GS-GasPro capillary column to
analyze the formed hydrocarbons (mainly, C -C paraffin). Conversion of propane (C
and product distribution (selectivity) were calculated using the following equations based on
the carbon balances with the help of an internal standard gas of N
2
2
2
4
(
1
3
3
8
H )
2
.
Number of carbon in propane in the gas phase product
) x 100
Sum of the number of carbon in the gas phase products
Conversion of C
3
H
8
(mol %) = (1 -
mols of each product
Sum of the products mols in gas phase
Selectivity (mol %) = (
The C
on stream, and the selectivity of products was calculated in terms of a dry mole basis just
after excluding the concentration of internal standard N gas.
.2. Catalyst characterization
Wide-angle powder X-ray diffraction (XRD) patterns of the fresh and used NCMAS were
) x 100
3
8
H conversion and product distribution were presented for an averaged value for 12 h
2
2
analyzed to verify the crystalline phases and sizes of NiO, Ni, and spinel-type species such as
3
MgAl
2
O
4
NiAl
2
O
4
and CoAl
2
O
4
before and after the SRP reaction using a X’Pert Powder
8

(PANalytical) instrument operated with a Cu-Kα radiation of 0.15406 nm at 40 kV and 100
o
o
mA in a scanning range of 15 - 80 with its rate of 4 /min. The full width at half maximum
(FWHM) values of the deconvoluted NiO and Ni diffraction peaks at a Bragg angle of 2θ =
o
o
4
3 and 51.9 were respectively used for the calculations of their crystallite sizes. The proper
differentiation peak among overlapped diffraction peaks of NiO with spinel structured
NiAl and MgAl was further applied by using Scherrer constant of 0.9 with the
2
O
4
2
O
4
assumption of spherical shape of crystallites. N adsorption-desorption analysis was carried
2
o
out at -196 C using a constant volume sorption equipment (Tristar II, Micromeritics) after
o
the fresh NCMAS samples were degassed in a vacuum condition at 350 C for 3 h. The
specific surface area, pore volume and average pore diameter of the fresh NCMAS were
calculated by using the equations derived from Brunauer-Emmett-Teller (BET) model and
Barrett-Joyner-Halenda (BJH) method just from a desorption isotherm.
H
2
-chemisorption analysis was carried out to verify the dispersion of active nickel metals on
the NCMAS using an ASAP 2020C (Micromeritics) instrument. A 0.2g sample was loaded in
an u-shaped quartz reactor, and it was pre-reduced under a flow of 30 mL/min of 10vol%H
2
o
balanced with Ar at 700 C for 3 h. Under a vacuum condition, all adsorbed gases were
o
evacuated while it was cooled down to the 35 C and kept at that temperature for 1 h to keep
the vacuum level of 0.02 mmHg. After the complete evacuation, H
2
uptakes were determined
through two successive adsorption experiments with a pulse injection of small amount of H
2
gas, and the adsorption stoichiometry of hydrogen to metal was assumed as one for H/metal,
where two metals of Ni and Co species are possibly assumed as adsorption sites.
The outermost surface compositions and oxidation states of the supported Ni nanoparticles
on the NCMAS were also characterized by X-ray photoelectron spectroscopy (XPS) using a
VG Multilab 2000 ESCA system (Themo Fisher scientific) equipped with a Mg/Al anode as
an X-ray source. For the XPS analysis, the fresh and used NCMAS catalysts were previously
9

pressed into a thin pellet, and each binding energy (BE) was corrected by the reference BE of
C1s at 284.6 eV. To further verify local surface distributions of Ni and Co nanoparticles, the
intensity ratios of the Ni to Al species (denoted as INi/IAl), Co to Al species (ICo/IAl) and Co to
Ni species (ICo/INi) on the fresh and used NCMAS were further calculated by comparing the
integrated areas of each peak of Ni 2p3/2, Co 2p3/2 and Al 2p appeared at ~ 855, 741 and 74
eV, respectively. To minimize the surface re-oxidation of the metallic Ni and Co species
under air-exposure as well as to lessen the coke depositions, the used catalyst was collected
after sufficiently cooling down to room temperature under a reductive condition and a
subsequent partial oxidation at the same temperature under a flow of 1%O
The fresh NCMAS catalysts were also characterized by Fourier transform infrared analysis
FT-IR, Perkin Elmer Frontier) using a TGS detector to verify the strengths of the M-O bonds
2
balanced with N
2
.
(
of the spinel-type metal aluminates by mixing it with 100 times larger amount of KBr, which
was previously pelletized into a thin pellet with diameter of 13 mm. The background data of
the FT-IR were subtracted from sample data to obtain the final FT-IR spectra.
The reduction profiles and amount of H
2
consumptions on the fresh NCMAS catalysts were
measured by temperature programmed reduction (TPR) experiments using a BELCAT-M
(Bel Japan) instrument. Previously, a 30 mg of a sample was placed into a quartz tubular
o
reactor, and it was pretreated under a flow of 30 mL/min of Ar at 500 C for 1 h to eliminate
o
any contaminants and water adsorbed. After cooling it down to 100 C, 30 mL/min of 10
vol% H
2
balanced with Ar was fed to the reactor to obtain stable TCD signal. The pretreated
o
o
sample was heated up to 1000 C at a ramping rate of 10 C/min, and the effluent gases were
passed over a 5A molecular sieve followed by measuring the consumed amounts of H
2
by a
TCD analyzer. In addition, two different successive temperature programmed (TP)
techniques were further performed using the same instrument for the above TPR experiment
to measure the reduction-oxidation resistances as well as to confirm the possible formation of
10

the inactive spinel-type metal aluminates. For measuring the reduction-oxidation resistances
o
of the NCMAS catalysts, a 30 mg of sample was reduced at 700 C for 3 h and it was cooled
o
down to 550 C and kept at that temperature whereas a mild oxidant of CO
2
with 99.999%
o
purity was fed to the reduced sample subsequently for 1 h. After cooling it down to 100 C,
the sample was purged by 10 vol%H
2
/Ar and a second successive TPR analysis from 100 to
000 C at a ramping rate of 10 C/min was conducted to verify the extents of the reoxidation
of active metal nanoparticles by the presence of CO product possibly formed during SRP
reaction, which was denoted as R-CO -TPR method. For further elucidating a possible
o
o
1
2
2
formation of some less active metal aluminates at a higher temperature, a 30 mg of sample
was previously carried out through the same TPR experiment and it was cooled down to 100
o
o
o
C again followed by increasing the temperature up to 800 C at a ramping of 10 C/min
under a flow of CO oxidant (temperature-programmed oxidation with CO , CO -TPO). After
the above TPO experiment, TPR analysis was carried out again on the sample successively
denoted as TPR-TPO(CO )-TPR method). From the results of two additional TP techniques,
2
2
2
(
2
the oxidation-reduction behaviors of supported metals with their reversibility and the possible
formations of less active spinel-type aluminates from Ni and Co nanoparticles can be
estimated under a harsh oxidation condition in the presence of the oxidant products such as
CO
2
and H
2
O. The extent of oxidation-reduction reversibility from the TPR-TPO(CO
2
)-TPR
analysis was represented as a re-reducibility (denoted as D
on the NCMAS catalysts.
R
) of the supported active metals
The surface morphologies and size distributions of metal crystallites on the fresh and used
NMA, NMAS and NCMAS(5) were also characterized by transmission electron microscopy
(TEM) using a TECNAI G2 instrument operating at 200 kV. The outermost surface Ni and
Co distributions and spinel-type metal aluminates on the selected fresh and used NMA,
NMAS and NCMAS(5) were also analyzed by using scanning transmission electron
11

microscopy (STEM) using a high angle annular dark field (HAADF) mode imaging analysis
as well as energy dispersive spectroscopy (EDS) mapping techniques.
Temperature programmed surface reaction by using H (TPSR) of the used NCMAS was
2
performed to verify the amount of coke formations on the surfaces by using a BELCAT-M
instrument, connected with a quadruple mass spectrometer (QMS-200, Balzers Prisma). For
the TPSR analysis, a 15 mg of the used sample was loaded in a reactor and it was pretreated
o
under a flow of Ar at 500 C for 1 h. After the pretreatment, the temperature decreased to 50
oC followed by flowing 10vol%H
/Ar and the hydrogenation of surface carbon precursors
2
o
o
was carried out by increasing the temperature up to 700 C at a ramping rate of 15 C/min to
analyze the hydrogenated coke precursors in the form of CH
4
species assigned to MS signal
of m/z =15.
3
. Results and Discussion
3
.1. Bulk and surface properties of the NCMAS catalysts
The bulk and surface properties of the fresh NCMAS such as textural properties from N
adsorption-desorption, crystallite sizes from XRD and surface areas of the supported metals
from H -chemisorption are summarized in Table 1 and supplementary Table S1. The
2
2
original surface areas of the CMAS supports having cobalt promoter concentration of 1 -
2
8
wt%Co were found to be in the range of 90 – 210 m /g (supplementary Table S2). They
2
were decreased after 10wt%Ni impregnation to the values of 97 – 180 m /g on the NCMAS
due to a possible deposition on the CMAS support inner walls and outer pore mouths as well
[14]. The positive contributions of cobalt promoter for an increased surface area of the
2
2
NCMAS (180 m /g on the NCMAS(1)) compared with that of 98 m /g on the NAMS can be
possibly attributed to the well-dispersed cobalt promoter in the matrices of the MgAl
embedded with the less porous SiC [30,31]. The small amount of cobalt promoter seems to
selectively present in the matrices of the MgAl structures, which can possibly generate
2
O
4
2
O
4
12

some inter-particular pores during the calcination step as well. It can also cause the much
stronger metal-support interactions. However, the decreased surface area on the NMAS
2
compared with the value of 112 m /g on the NMA can be originated by using the smaller
2
surface area of SiC with 25.2 m /g. This observation was supported by the surface area of the
NMAS prepared at different MgAl
2
O
4
/(MgAl
2
O
4
+SiC) ratios, and it was steadily decreased
2
2
with an increase of SiC content from 98 m /g on the NMAS-7 to 59 m /g on the NMAS-3 as
summarized in supplementary Table S1. The variations of pore volumes on the fresh
NCMAS were similar with those of surface areas, and the much larger pore volume was
3
3
observed on the NCMAS(1) with the value of 0.83 cm /g compared to that of 0.45 cm /g on
the NMAS-7. In addition, the average pore diameter was found to be much larger on the
NMA with 20 nm in size than others in the range of 13 – 17 nm. The much smaller pore
diameters on the NCMAS compared to that of the NMA seem to be attributed to the well-
dispersed cobalt-promoted MgAl
pores possibly [30,31] by stronly interacting with the SiO
pore size distribution and N adsorption-desorption isotherms on the fresh NCMAS as shown
2
O
4
grains on the SiC support by forming the inter-particular
C
x y
outer layers of the SiC. The
2
in supplementary Figure S1 revealed the regular pore size distributions with typical type IV
hysteresis isotherms, which represents typical mesoporous structures. The SiC-embedded
MgAl
2
4
O supports having a higher thermal conductivity and chemically inertness showed
more porous characters than the SiC itself since SiC has an intrinsically lower surface area. It
can also make a high dispersion of nickel nanoparticles and a facile heat transfer for the
endothermic SRP reaction. The addition of cobalt promoter on the MAS enhanced the surface
area by forming the much smaller MgAl
2
4
O grains at a lower content of cobalt promoter,
which can be beneficial for an enhanced catalytic activity as well.
The wide-angle XRD patterns of the fresh and used NCMAS are displayed in Figure 1, and
the crystalline phases can be assigned to the characteristic diffraction peaks of NiO, -SiC,
13

Al
14,31]. Any diffraction peaks assigned to cobalt oxides (CoO and Co
observed due to its small amount and possible peak overlapping with Al
2
O
3
, spinel-type oxides such as NiAl
2
O
4
, MgAl
2
O
4
and CoAl
2
O
4
on all the fresh NCMAS
) were not clearly
or MgAl [31].
and
[
3
O
4
2
O
3
2
O
4
However, the intense peak intensity of spinel-type mixed metal oxides such as NiAl
2
O
4
o
MgAl
2
O
4
appeared at 2 = 19.1 and 36.9 on the NMA and NMAS-7 compared to NCMAS
suggests the preferential formations of those spinel-type crystallite as shown in Figure 1 and
supplementary Figure S2. The crystallite sizes of the spinel-type NiAl were significantly
decreased after the addition of cobalt promoter on the cobalt-promoted NCMAS below 10 nm
in size as shown in Figure 1. However, the crystallite sizes of spinel-type MgAl were not
significantly altered with the sizes of 5.9 – 7.5 nm as summarized in supplementary Table
S1. We believe that the increased amounts of the embedded Co promoter in the MgAl -SiC
matrix can partially form CoAl species, which can further prevent the formation of
inactive NiAl crystallites on the NCMAS. In addition, the peak intensity of NiO was
2
O
4
2
O
4
2
O
4
2
O
4
2
O
4
found to be similar and its crystallite sizes were found to be in the range of 4.1 – 7.1 nm on
the fresh NCMAS without any significant variations. It suggests that the sub nano-sized NiO
crystallites can be well-dispersed on the MgAl
2
O
4
-SiC with its smaller crystallite sizes of
inactive NiAl by strongly interacting with CMAS supports compared with the NMA and
2
O
4
NMAS. These observations also suggest that the better dispersions of NiO nanoparticles with
their strong interactions on the NCMAS can enhance the number of active sites with an
increased activity and less aggregations [32].
3
.2. Reduction behaviors and redox properties of the NCMAS catalysts
The metal-support interactions between Ni nanoparticles and supports on the fresh NCMAS
were precisely verified by TPR experiments by two different successive reduction-oxidation
TPR analyses. The reduction patterns of the fresh NCMAS with its degrees of re-reduction
o
after an oxidation with CO
2
at 550 and 800 C are respectively displayed in Figure 2 and
14

supplementary Figure S3. The main broad reduction peak of active metals on the NCMAS
o
was found to be at the maximum temperatures around 750 – 790 C (Figure 2(A)), which can
be assigned to the reduction of NiO which were strongly interacted with CMAS supports
[32,33]. With a decrease of MgAl
2
O
4
to (β-SiC+MgAl
2
4
O ) ratio on the NMAS without the
addition of Co promoter (Figure S3), the maximum reduction peaks were shifted to lower
o
o
temperature from 749 C on the NMAS-7 to 719 C on the NMAS-3, which was much lower
o
than 753 C on the NMA. This can be attributed to the weak interactions of Ni nanoparticle
on the MgAl
2
O
4
-SiC surfaces compared to that of MgAl
2
O
4
itself due to a higher dispersion
of smaller MgAl
2
O
4
grains [14]. The weak metal-support interactions between NiO and MAS
supports favored the facile reduction of NiO to metallic nickel nanoparticles, which was
o
supported by the decreased reduction temperature of 719 C on the NMAS-3 with a larger
NiO crystallite size of 8.3 nm compared with that of 5.1 nm on the NMAS-7 with that of 753
oC as summarized in supplementary Table S1. The maximum reduction peaks at lower
o
temperatures of ~ 400 C with a small peak intensity (Figure 2(A)) can be assigned to the
partial reduction of the segregated NiO nanoparticles or the possible reductive natures of
cobalt promoter on the MgAl
weaker interactions between NiO and MAS support partially including spinel-type NiAl
or MgAl phases [31,33] with its less dispersions and smaller surface areas compared to
the MA support as summarized in Table 1 and supplementary Table S2. This weak
2
O
4
-SiC surfaces [30]. These results were attributed to the
2
O
4
2
O
4
interaction largely influenced to form larger sizes of Ni nanoparticles after SRP reaction on
the NMAS-7 with its crystallite size of 18.6 nm compared with the cobalt-promoted NCMAS
in the size ranges of 10.8 – 16.8 nm and the NMA with 15.3 nm due to its weak interaction
natures as summarized in Table 1. TPR patterns on the NCMAS after adding the cobalt
promoter are precisely displayed in Figure 2(A). The main reduction peaks located in the
o
ranges of T= 700 – 800 C can be originated from the strongly interacted NiO nanoparticles
15

by partially forming the spinel-like crystalline structures with Al
2
O
3
or MgAl
2
4
O supports
o
[
32,33]. The lower temperature reduction peaks at around 400 C and weak shoulder peaks at
o
5
00 – 650 C possibly suggest the reductions of weakly interacted NiO nanoparticle with the
CMAS supports. Interestingly, with an incorporation of cobalt promoter on the NMAS-7, the
higher temperature reduction peaks was steadily increased with an increase of cobalt content
o
o
from 749 C on the NMAS-7 to 765 – 786 C on the NCMAS, except for the NCMAS(8)
o
with the value of 748 C. It seems to be attributed to the newly formed interactions between
Ni and Co nanoparticles, which can stabilize the Ni nanoparticles through the stronger Ni-Co
or spinel-type Ni-CoAl
2
O
4
interactions, which can be easily formed on the Co/Al
2
3
O catalytic
systems [34,35]. However, the large amount of cobalt promoter on the NCMAS(8) can also
facilitate the facile reduction of Ni nanoparticles through an intimate interaction between the
Ni-Co crystallites by the well-known H
2
spillover mechanisms due to the much easier
species [35,36],
reducing natures of cobalt oxides than that of Ni or spinel-type NiAl
2
O
4
which were further verified by XPS analyses in the next section. Therefore, the size growths
of NiO nanoparticles can be not only attributed to the textural properties during the catalyst
preparation but also to the chemical properties such as the metal-support interactions. It is
interesting that the addition of cobalt promoter on the NMAS-7 can generate synergy effects
by forming active and strongly interacted Ni-Co nanoparticles as well as by partially forming
less active spinel-type metal aluminates, which can finally increase the catalytic activity and
stability with less aggregation of Ni nanoparticles.
To further verify the redox properties of Ni nanoparticles on the NCMAS, two separate TPR
o
experiments such as the TPR after CO
2
treatment at 550 C 1 h (R-CO
2
-TPR) to elucidate
-TPO up to 800
outermost surface reoxidation properties (Figure 2(B)) and the TPR after CO
oC (TPR-TPO(CO
2
2
)-TPR) to confirm the re-reducibility of the NCMAS (Figure 2(C)) were
carried out. For the R-CO
2
-TPR method, the resistance to a mild CO oxidant and extent of
2
16

irreversible oxidation of the reduced Ni nanoparticles showed significantly different
behaviors of the NCMAS catalysts. The NCMAS containing small amount of cobalt
o
promoter showed much lower reduction temperatures below 450 C, and the temperatures
were shifted a lower one with an increase of cobalt contents. This observation suggests that
the outermost surface Ni nanoparticles can be easily reduced even under a possible re-
oxidative SRP reaction conditions, and cobalt promoter can enhance the reducibility by
intimately interacting with Ni nanoparticles. Therefore, the reduction peaks in the
o
temperature ranges of 436 – 446 C on the NCMAS were found to be slightly lower than 448
o
o
C on the NMAS-7. Interestingly, the multiple reduction peaks in the ranges of 300 – 600 C
were clearly observed on the NMA and NMAS-7 compared with one broad reduction
patterns on the NCMAS. This observation suggests the heterogeneous size distributions of Ni
nanoparticles with different interactions with the MAS supports, especially on the NMA as
shown in Figure 2(B). Therefore, the contribution of the cobalt promoter on the NCMAS can
make oxidation-resistant Ni nanoparticles with a homogeneous size distribution compared
with the NMA and NMAS-7, which can be also attributed to the strongly interacted Ni
nanoparticles with the CMAS supports by partially forming thermally stable spinel-type
metal aluminates [14,24,28,30,36]. In addition, the cobalt promoter can be slightly segregated
on the outer surfaces of the NCMAS during SRP reaction, which seems to mainly present in
the form of cobalt aluminates [37]. These migrated cobalt nanoparticles can enhance the
reducibility of Ni nanoparticles due to its higher H spillover natures [38] and its highly
2
oxophilic characters of the distorted cobalt defect surfaces [39-41] with less coke deposition
natures [37,42]. As shown in supplementary Figure 2(B), these hypothesis was clearly
o
supported by the observed higher reduction temperature peak located at 787 C on the
CMAS(8) support, which was attributed to the reduction of the spinel-type of cobalt
aluminates [35,37]. These imbedded-cobalt nanoparticles cannot be fully reduced again after
17

CO
with MgAl
To confirm the re-reducibility of surface Ni nanoparticles with their interactions with
CMAS supports, TPR-TPO(CO )-TPR experiments were carried out and TPR patterns are
displayed in Figure 2(C). The re-reducibility (D
2
treatment (R-CO
2
-TPR), which suggests that the cobalt oxides can be strongly interacted
2
O
4
support by possibly forming spinel-type CoAl
2
O
4
phases.
2
R
) on the NCMAS was calculated using the
st
nd
o
consumed amounts of H
2
for the 1 and 2 TPR run up to 800 C, and they are summarized
in Table 1 with their corresponding amounts in supplementary Table S2. Interestingly, only
nd
NMA showed delayed reduction peak positions for 2 TPR run, which suggests an abundant
formation of less active nickel aluminate species compared to other catalysts. The reduction
o
peaks appeared at 500 - 700 C assigned to the weakly interacting NiO nanoparticles with the
supports were disappeared by possibly being transformed to hardly reducible spinel-type
NiAl
2
O
4
phases on the NMA and NMAS-7 with lower D
R
values of 0.77 and 0.75,
respectively. The NMAS-7 showed less amount of hydrogen consumptions after CO
2
-TPO
due to a possible formation of much bigger NiO nanoparticles, which can be originated from
its weak interactions between Ni and the MAS support. However, the cobalt-promoted
NCMAS showed a much higher D
NCMAS(5), which suggest an strong resistance of Ni nanoparticles to the irreversible
oxidation by CO with a less formation of inactive spinel-type NiAl phases. However, the
simultaneous formations of CoAl
R
values above 0.91 with a maximum value of 0.99 on the
2
2
O
4
2
O
4
phases on the CMAS surfaces seem to be possible since
o
the lower temperature shifts of the reduction peaks above 900 C on the NCMAS(5) and
NCMAS(8) were clearly observed, which can be attributed to the larger amounts of cobalt
promoter to selectively form spinel-type CoAl
2
4
O as confirmed by the TPR patterns of the
CMAS(8) in Figure 2(B). In addition, the M-O bond strengths on the fresh NCMAS were
also characterized and the FT-IR spectra are displayed in Figure 2(D). The peaks appeared at
-1
the wavenumbers of 692 and 820 cm can be attributed to the stretching modes of M-O
18

bonds in the metal aluminate structures such as the MgAl
Al or MgO as well [43,44]. The wavenumbers of those peaks shifted to slightly higher
ones on the NCMAS(1) and NCMAS(5) compared to the NMA and NCMAS(8). This
observation can be possibly attributed to the strongly interacted MgAl particles by being
2
O
4
or CoAl
2
O
4
and segregated
2
O
3
2
O
4
incorporated with the highly dispersed cobalt promoter. Therefore, the stabilized Ni
nanoparticles with their smaller crystallite sizes on the NCMAS after optimal amount of
cobalt incorporation are responsible for the larger metallic surface areas measured by H
2
chemisorption as summarized in Table 1. On the fresh NCMAS(5), the largest metallic
2
surface area of the supported metals was observed with 1.38 m /gcat, which was much larger
2
than that of NMAS of 0.64 m /gcat. The cobalt-promoted NCMAS also showed the larger
2
surface areas in the range of 1.22 – 1.38 m /gcat. We believe that the larger surface area of
2
1
.29 m /gcat on the fresh NMA can be significantly decreased during SRP reaction due to its
structurally unstable natures of MgAl
2
O
4
support by forming inactive NiAl
2
4
O preferentially.
In brief summary of reducibility, the cobalt promoter on the NCMAS with an optimal
concentration can enhance the re-reducibility with less aggregation of Ni nanoparticles by an
intimate interaction with active Ni nanoparticles as well as by strongly interacting with the
spinel-type MgAl
2
4
O on the CMAS supports, which finally resulted in showing a better
catalytic activity and stability during SRP reaction on the NCMAS(5).
3
.3. Roles of cobalt promoter and its surface distributions on the NCMAS catalysts
The oxidation states of Ni and Co species and their interactions with the CMAS supports
with surface concentrations on the fresh and used NCMAS were measured by XPS analyses,
and the results are displayed in Figure 3 and summarized in Table 1 with supplementary
Table S1 and Figure S4. The binding energies of Ni 2p3/2 in Figure 3(A-1) were increased
with an increase of cobalt content from 855.0 eV on the NMAS-7 to 855.5 eV on the
NCMAS(1) and NCMAS(5), and it was decreased again to 855.0 eV on the NCMAS(8),
19

2
+
where these BEs are corresponding to Ni species, which were much higher BEs than that of
the NMA with the value of 854.1 eV as summarized in Table 1. This observation can be
attributed to the stronger interaction of Ni nanoparticles with the supports due to the partially
formed and well-distributed spinel-type MgAl
2
O
4
or CoAl
2
4
O phases. These XPS results are
also in line with the results of TPR as well. In addition, the BEs of Co 2p3/2 were also slightly
increased with increasing the cobalt contents from 780.6 to 780.9 eV (assigned to Co2+
species) on the NCMAS(1) and NCMAS(8), respectively. This observation also suggests that
the intimate interactions between Ni and Co nanoparticles at an optimal cobalt content on the
NCMAS by possibly donating electrons from the cobalt species such as CoO or CoAl
2
4
O to
the supported Ni nanoparticles, which can make the Ni nanoparticles with a higher electron
density [13,23-26] on the NCMAS compared to the unpromoted NMA or NMAS-7. The
synergy effects of the Ni nanoparticles having a higher electron density can effectively
prohibit the tendency of re-oxidation and aggregations during reforming traction [23,25],
which are also well mathced with the results of the two successive TPR experiemtns as
swhon in in Figure 2. The changes of BEs of Ni nanoparticles seem to be attrbuted to the
strong interactions with cobalt promoters instead of aluminum species since the BEs of Al 2s
on the fresh cobalt-promoted NCMAS were not significantly altered in the range of 73.7 –
7
3.8 eV as summarized in Table S1 and Figure 3(B-1). Therefore, those BEs of Ni 2p3/2
were much largely altered according to the absence of cobalt promoters between the fresh and
used catalysts, especially on the used NMA and NMAS. All BEs of Ni 2p3/2, Al 2p and Co
2
8
p
3/2 revealed the presence of the oxidized metal oxide forms of NiO, Al
2
O
3
and CoAl
2
O
4
at
55.5, 73.7 – 74.6 and 781 eV, respectively. Based on the shifts of BEs, the cobalt-promoted
NCMAS seems to have the small amounts of spinel-type metal aluminates such as the
NiAl or CoAl . The calculated differences (delta, ) of BEs between the fresh and used
NCMAS are summarized in Table 1 and supplementary Table S1. The delta values of Ni
2
O
4
2
O
4
20

2
p
3/2 on the cobalt-promoted NCMAS were found to be in the range of 0 – 0.6 eV, which
were much smaller than the NMA and NMAS, which showed the respective values of 1.6 and
.8. In addition, the BE changes of Al 2p (Figure 3(B-2)) were also found to be smaller on
0
the cobalt-promoted NCMAS than the NMA and NMAS (Table S1). It strongly suggests that
the stable preservation of Ni nanoparticles on the NCMAS at an optimal amount of cobalt
promoter, especially on the NCMAS(1) and NCMAS(5), and it can be attributed to the less
formation of inactive NiAl
and highly dispersed spinel-type MgAl
The positive effects of the spinel-type metal aluminate formations can efficiently prohibit the
2
O
4
phases with the help of the presence of the thermally stable
2
O
4
and CoAl phases on the NCMAS [12,43-45].
2
O
4
aggregation of the Ni nanoparticles and enhance their re-reducibility under oxidative CO
2
environment as confirmed by two successive TPR experiments.
In addition, the relative outermost surface compositions were calculated by using the ratios
of the integrated areas of the each Ni 2p3/2, Co 2p3/2 and Al 2p peak of the fresh and used
NCMAS, and the results are summarized in Table 1 and supplementary Table S1. The
outermost surface concentrations of Ni species (INi/IAl) on the fresh NCMAS was found to be
in the range of 0.32 – 0.49, where the values were found to be smaller on the NCMAS(5) and
larger on the NMAS and NCMAS(8). It reveals the well dispersion of Ni nanoparticles on the
NCMAS(5) compared to other catalysts. The values of ICo/IAl and ICo/INi were steadily
increased with an increase of cobalt contents on the fresh NCMAS in the range of 0.11 – 0.19
and 0.31 – 0.40, respectively. Interestingly, these values were largely altered on the used
NCMAS due to the different extents of aggregations of the Co and Ni nanoparticles by
partially forming the less active spinel-type metal aluminates. The INi/IAl values on the used
NCMAS were monotonously increased with an increased cobalt contents in the range of 0.54
–
1.53, and the smallest value was observed on the NMA with that of 0.40, which can be
attributed to the preferential formation of the thermally stable and less active NiAl phases
2
O
4
21

[12,36,45]. The observed larger INi/IAl values on the cobalt-promoted NCMAS suggest the
preferential distributions of Ni nanoaprticles on the outer surfaces with their smaller
crystallite sizes [20], which were also increased with an increase of cobalt contents due to the
2
decrased surface area of CMAS supports from 180 to 97 m /g (Table 1). We believe that the
thermally stable Ni nanoaprticles distributed on the outer surfaces having less aggreagtion
natures through the strong interactions with the CMAS suppports can be responsible for a
higher catalytic activity and stability on the NCMAS [14,30,35]. Therefore, the values of ICo/-
IAl and ICo/INi on the used NCMAS were further compared, and the values of ICo/IAl were also
increased with an increase of cobalt content in the range of 0.28 – 0.88 (Table S1), which
were also found to be larger than that of the fresh NCMAS due to the possible migrations of
cobalt species to the outermost surfaces of the CMAS supports or its preferential segregations
during SRP reaction. The much larger changes of the ICo/IAl from 0.12 to 0.87 on the
NCMAS(5) (Table S1) can be possibly originated from the preferential segregations of the
cobalt oxides to the outer surfaces of the CMAS(5) supports by partially forming thermally
stable CoAl
2
4
O phases as well [30,35]. We believe that the larger variations of ICo/Al values
on the used NCMAS(5) and NCMAS(8) were attributed to the higher saturated
concentrations of the surface cobalt species, which can be further affected by the extents of
CoAl
2
4
O formations on the NCMAS structures resulted in altering the aggregations of the
surface Ni nanoparticles possibly. This hypothesis was previously confirmed by two
successive TPR experiments (Figure 2), and it was also further supported by the calculated
ICo/INi ratios on the used NCMAS as summarized in Table 1. The larger ICo/INi values of 0.77
and 0.58 on the used NCMAS(5) and NCMAS(8) compared with that of 0.57 on the
NCMAS(1) strongly suggest that the intimate interactions between the segregated Co
nanoparticles and Ni nanoparicles on the outermost surfaces of the NCMAS(5). Although the
excess amount cobalt promoter on the NCMAS(8) can induce an easy reduction of NiO
22

nanoparticles as shown in Figure 2(A), the relatively lower dispersion of Ni nanoparticles
due to their heterogeneous dispersions can possibly cause the significant aggregations of
active metals compared with the NCMAS(5) relatively. In addtion, the previously modified
MgAl
2
4
O -SiC at an optimal amount of cobalt promoter can effectively prohibit the selective
formations of less active spinel-type NiAl
2
O
4
phases by interacting with Ni nanoparticles
o
strongly. It seems to be possible since a melting point of Ni metal (1455 C) is slighly lower
o
than Co metal (1495 C), which was also supproted by re-reducibility of the NCMAS
confirmed by two seccesive TPR experiments (Figuer 2(C)). The Ni nanoparticles, which are
intimately interacted with an oxophilic cobalt species on the porous CMAS supports, can also
reduce the coke depositions by easily removing the surface coke precursors by the roles of
oxophilic cobalt species having a higher CO affinity as well [37,39,41,42]. In brief
2
summary, the NCMAS(5) seems to show a higher catalytic activity and stability with a less
deactivation rate due to the newly formed intimate interactions of Co-Ni nanoparticles with a
less formation of inactive spinel-type NiAl
2
4
O phases compared with the NMAS, which was
well correlated with the results of two successive TPR and XPS analyses.
3
.4. Catalytic activities and deactivation behaviors of the NCMAS catalysts
The catalytic activity and stability on the NCMAS for 12 h on stream are displayed in
Figure 4(A) and supplementary Figure S5 and the results are summarized in Table 1 at the
o
reaction conditions of T = 650 C, P = 0.1 MPa, and SV = 33600 L (total feed gases)/(kgcat∙h).
All the NCMAS showed higher conversions of C
3
8
H above 99% by approaching the
equilibrium conversion of ~100% at the present reaction conditions (Table 1), except for the
NMA of 86.8%. The NMA showed a fast deactivation with a deactivation rate of 1.68 %/h
from a complete conversion of propane. It can be said that the Ni nanoparticles strongly
interacted with the MA support can be easily transformed to the less active NiAl
2
4
O phases
possibly as confirmed by the ratios of the INi/IAl from XPS analysis. While using the
23

MgAl
2
4
O -SiC (MAS) support of the NMAS, the deactivation rate was suppressed
significantly with 0.03 %/h by maintaining a higher conversion above 99.5%. It can be
attributed to the facile formations of the much larger Ni nanoparticles (largest size of 18.6 nm
from XRD analysis in Table 1) with small reduction nature of NiO, which can possibly form
less amounts of inactive NiAl
2
4
O phases. However, the simultaneous increase of coke
depositions can be preferential on the NMAS-7 due to the nature of the larger metallic nickel
crystallite sizes for graphitic whisker carbon formations [7,8,46]. When decreasing the ratios
of the MgAl
S1, C conversion was dramatically decreased to 77.4% on the NMAS-3, which strongly
supported the positive effects of inert material of SiC by giving a weaker metal-support
interaction with a facile reduction of NiO compared to the MgAl support itself.
2
O
4
to (β-SiC+MgAl
2
4
O ) as summarized in supplementary Figure S5 and Table
3
H
8
2
O
4
Interestingly, the cobalt-promoted NCMAS showed much higher conversions and stability
with the almost non-deactivation and the highest activity was observed on the NCMAS(5)
with a deactivation rate of 0.01 %/h. The observed higher conversion on the cobalt-promoted
NCMAS, especially on the NCMAS(5), was induced from the preferentially surface-enriched
cobalt oxides by partially forming the spinel-type CoAl
2
4
O during SRP reaction. It can
effectively generate the strongly interactied Ni nanoparticles with a less formation of coke
precursors with the help of the oxophilic cobalt characters compared to the NMAS [37,41].
However, on the NCMAS(1) and NCMAS(8), smaller metallic surface area of Ni
nanoparticles can cause a little bit lower catalytic activity due to an excess cobalt content due
to the small contributions of cobalt promoter with its intrinsically lower content and possible
uneven cobalt size distributions, respectively. In terms of the product distributions as
summarized in Table 1, the lower conversions of C
NMAS-5 (Table S1) were related with a lower H selectivity due to the enhanced CH
byproduct formation and lower water-gas shift (WGS) reaction activity (CO + H
3
H on the NMA and NMAS-3 and
8
2
4
2
O = CO +
2
24

H
2
) with a higher CO concentration above 20%, which can be attributed to the preferential
formations of less active spinel-type NiAl phases. The observed higher H selectivity on
the NMAS-7 above 70.5% seems to be attributed to the facile reduction of NiO confirmed by
TPR analysis. However, the cobalt-promoted NCMAS(5) and NCMAS(8) showed higher H
selectivities above 71% with smaller amounts of CH and CO concentration below 3.1 and
2.4%, respectively. This observation can be attributed to the stable preservation of the
2
O
4
2
2
4
1
smaller Ni nanoparticles through the intimate and strong interactions of Ni species with the
surface-enhanced cobalt species, where the cobalt oxides can act as an effective WGS
catalyst as well as reforming catalysts of hydrocarbons at a relatively lower temperature due
to its fast oxygen transfer characters [39,46].
To further verify the additional effects of coke depositions to catalytic stability, TPSR
analysis on the used NCMAS for 12 h reaction was carried out and the hydrogenation
patterns are displayed in Figure 4(B). Generally, the deposited coke precursors can be
categorized with three characteristic TPSR peaks such as α, ß, and γ phases, where each peak
according to the increasing hydrogenation temperature orders clearly indicates the active
atomic carbon species (carbidic, α) and amorphous surface methyl chains (ß) with the bulk
o
metal carbides (γ) or heavy cokes above 500 C, respectively [47,48]. By measuring the CH
fragments (m/z = 15) formed using GC/MS, the relative amounts of cokes and their types
were compared. The amounts of coke formed (mainly in the heavy coke phases with a
4
o
maximum hydrogenation temperature at ~ 600 C) were followed by the orders of NMAS-7
>> NCMAS(1) > NMA > NCMAS(5)  NCMAS(8). Although the NMAS-7 showed a stable
activity for 12 h of the SRP reaction, the amount of carbon depositions was found to be
largest on the NMAS-7, which can be attributed to the presence of larger Ni nanoparticles
having a weaker metal-support interaction due to the contributions of the Ni-SiC interactions
7,8,46]. In addition, the less coke deposited NMA still showed a lower activity (C
[
3
H
8
25

conversion of 86.8 %) and it was mainly induced from the large amounts of less active
NiAl formation. However, all the cobalt-promoted NCMAS showed a higher resistance to
coke depositions at a higher amount of cobalt promoter above 5wt% due to its oxophilic
characters which can activate CO to produce active O and subsequently can remove the coke
2
O
4
2
precursors formed [37,41,42] through the preferential oxidation of them. It was also
attributed to the intimately dispersed Ni-Co nanoparticles on the outer surfaces with an
enhanced thermal stability by preserving smaller sizes of Ni nanoparticles. Interestingly, the
roles of cobalt promoter was found to be more dominant on the NCMAS(8) with the smallest
coke depositions, however, it showed a little bit lower activity than NCMAS(5) due to their
heterogeneous size distributions of active metals (Ni and Co) on the surface confirmed by
XPS analysis. Therefore, it suggests that the intimate interactions between Co and Ni
nanoparticles seem to be more important to obtain a higher catalytic activity with less coke
deposition. On the selected highly active NCMAS(5), a long-term stability was further
performed to verify the roles of cobalt promoter and the results are displayed in Figure 4(C).
o
The C
3
H
8
conversion at 650 C and SV = 33600 mL/(gcat·h) was maintained during the SRP
reaction for 100 h on stream with steady product distributions. The selectivity of H
2
was
found to be ~71.3% with 2.5% of hydrocarbon formation (mainly CH ) and a lower CO
4
selectivity around 13.6%, which almost approach the equilibrium values as summarized in
Table 1. These superior SRP activities on the NCMAS(5) seem to be originated by the
synergy effects of cobalt promoter with strongly interacted smaller Ni nanoparticles without
any significant deactivation for 100 h on stream.
The surface morphologies and phases of the selected NMA, NMAS-7, and NCMAS(5)
before and after SRP reaction were characterized by TEM analysis, and their images are
displayed in Figure 5. The crystallite sizes of metal oxides on the fresh catalysts (Figure
5
(A)-(C)) showed similar sizes with their average ones of 10 nm (relatively larger ones on
26

the NMAS-7 with heterogeneous size distributions), which are well matched with the results
of XRD analysis. The magnified TEM images as shown in Figure 5(A )– (C ) similarly
m
m
revealed the larger crystallite sizes on the NMAS compared with the NMA and NCMAS(5).
Interestingly, the SiC phases were also observed on all the fresh NCMAS, which seems to be
originated from the β-SiC with its original phases of the coated SiO
x
C
y
species on the bulk
SiC [49]. The spinel-type phases of MgAl or NiAl on the NMA and NCMAS(5) were
2
O
4
2
O
4
also observed in the vicinity of the smaller Ni nanoparticles. After SRP reaction, all the
NCMAS showed the aggregated Ni nanoparticles in the average sizes above 10 nm, however,
it was more significant on the NMAS-7 with some aggregated particles larger than 50 nm
with heterogeneous size distribution as shown in Figure 5(D)-(F). The much larger crystallite
formation on the NMAS-7 was mainly attributed to the much weaker interactions of Ni
nanoparticles on the NMAS as confirmed by XRD and TPR analysis. In addition, the opaque
black images covered on the Ni nanoparticles on the NMAS as shown in the inset figure of
Figure 5(E) compared with the NCMAS(5) (inset figure in Figure 5(F)) revealed the harder
coke depositions on the Ni nanoparticle surfaces. On the most active NCMAS(5), the
aggregated Ni nanoparticles showed a characteristic d-spacing value of the smaller metallic
Ni nanoparticles without any coke depositions on the metal surfaces. The smaller Ni
nanoparticles were intimately interacted with the spinel-type metal oxides such as the
MgAl
2
O
4
or CoAl
2
4
O phases. As confirmed by XPS results, Ni nanoparticles on the
NCMAS(5) have a much higher electron density than that of the NMA and NMAS-7, which
can be attributed to the intimate interactions between Ni and Co nanoparticles for an easy re-
reducibility of Ni nanoparticles to suppress the significant coke depositions [37,41]. These
intimate interactions of Ni-Co were further characterized by STEM-HAADF-EDS mapping
technique on the fresh and used NCMAS(5), and the images are displayed in Figure 6. The
Ni and Co species were detected from the X-ray signals of the Ni-K and Co-K edge upon the
27

collision with electrons to the fresh and used catalysts. Each Ni-K and Co-K edge signal and
the signals overlaid on the STEM-HAADF images are displayed with the line plots of the
selected signals in Figure 6, respectively. The metal distributions of Ni and Co species on the
fresh NCMAS(5) revealed highly dispersed Ni nanoparticles on the NCMAS(5) surfaces as
well as the highly-dispersed Co nanoparticles as shown in Figure 6(A). It was originated
from the impregnation steps since the active Ni nanoparticles were subsequently impregnated
on the previously prepared cobalt-promoted MgAl
2
O
4
-SiC support. On the used NCMAS(5),
the Ni nanoparticles were aggregated significantly with much larger sizes as shown in Figure
6
4
(B). Interestingly, based on the line plots of the Ni-K and Co-K displayed in the Figure 6(B-
), the embedded cobalt nanoparticles on the CMAS(5) support were preferentially migrated
to the vicinity of the aggregated Ni nanoparticles due to the similar crystallographic natures
of the Co and Ni nanoparticles. These intimate distributions also seem to suppress the
significant aggregations of the supported Ni nanoparticles by forming new strong interactions
on the cobalt-modified MgAl
2
4
O -SiC. This observation can be a direct evidence for an
intimate interaction of Ni-Co nanoparticles on the NCMAS(5) during SRP reaction to support
our hypothesis suggested from TPR, XPS and XRD results.
In summary, some correlations of catalyst deactivation with C
3
8
H conversion on the
NCMAS were derived in terms of the surface properties from XPS analysis such as the BE
changes of the Ni 2p3/2 and Al 2p species (delta values) between the fresh and used NCMAS.
The small variations of those BEs were well correlated with the less deactivation and higher
conversion of C
smallest variations of BEs of Ni 2p3/2 and Al 2p before and after SRP reaction, which was
mainly attributed to the formation of thermally stable spinel-type MgAl and CoAl
3
H
8
as shown in Figure 7(A). The most active NCMAS(5) showed the
2
O
4
2
O
4
phases. Based on the above results, the schematic diagrams of the suppressed aggregation
phenomena of the Ni nanoparticles on the representative NMAS-7 and NCMAS(5) are
28

proposed in the Figure 7(B). The Co promoter on the NCMAS(5) can be homogenously
distributed in the matrix of the MgAl -SiC before SRP reaction, however, the Co
2
O
4
nanoparticles seem to be preferentially migrated to the outer surfaces by strongly interacting
with the aggregated Ni nanoparticles by a close interaction on the vicinity of Ni nanoparticles
during SRP reaction. The aggregated Co nanoparticles can suppress the aggregation of Ni
nanoparticles and reduce the coke depositions by forming a new strong interaction with the
cobalt-modified MgAl
was dominant on the NMA and NMAS-7 having no cobalt promoter since there is no
previous modification of MgAl surfaces. Therefore, the simple and efficient modification
methods of the MgAl -SiC support with small amount of cobalt promoter can generate the
2
O
4
-SiC. The preferential formation of the less active NiAl
2
O
4
phases
2
O
4
2
O
4
synergetic effects by forming segregated cobalt oxides with an intimate interaction of Ni
nanoparticles, which can reduce the aggregation of Ni nanoparticles with less coke
depositions on the NCMAS(5) during SRP reaction. In addition, the selected best NCMAS(5)
was further investigated to derive kinetic parameters based on the Langmuir-Hinshelwood-
Hougen-Watson (LHHW) mechanisms in the following section.
3
.5. Estimation of kinetic parameters and optimal conditions on the NCMAS(5) catalyst
SRP reaction is generally composed of three main overall reactions such as steam reforming
(SRM), water gas shift (WGS), and CO methanation with a minor Boudouard reaction for a
coke formation as follows, and the schematic reaction pathways related with major overall
reactions such as SRM, WGS and methanation are presented in the Scheme 1. For the kinetic
data acquisitions for a highly endothermic SRP reaction, 0.02 g catalyst diluted with 0.18 g
SiC (total 0.2 g catalyst-bed weight) was tested to minimize the temperature gradients in the
catalyst-bed as shown in supplementary Figure S6. In addition, the criteria for mass- and heat
transfer limitations [55] were further calculated using the experimental data (summarized in
29