Applied Catalysis B: Environmental 291 (2021) 120074
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Applied Catalysis B: Environmental
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NixAl1O2-δ mesoporous catalysts for dry reforming of methane: The special
role of NiAl2O4 spinel phase and its reaction mechanism
Shuangshuang Zhang a, Ming Ying a, Jun Yu b, *, Wangcheng Zhan a, Li Wang a, Yun Guo a,
Yanglong Guo a, *
a
Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, School of Chemistry and Molecular Engineering, East China University of Science and
Technology, Shanghai, 200237, PR China
Research Institute of Applied Catalysis, School of Chemical and Environmental Engineering, Shanghai Institute of Technology, Shanghai, 201418, PR China
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
Dry reforming of methane
NixAl1O2-δ catalysts
NiAl2O4 spinel phase
Coke resistance
The structure evolution of surface NiAl2O4 spinel phase in NixAl1O2-δ mesoporous catalysts, synthesized by the
citric acid sol-gel method, was systematically investigated. Small-size Ni nanoparticles, obtained by partial
reduction from NiAl2O4 spinel in NixAl1O2-δ catalysts with low Ni contents at high temperature, can effectively
inhibit the carbon formation from kinetics, while the irreducible NiAl2O4 counterpart can participate in elimi­
nation of carbon deposition. The constructed structure of Ni0-NiAl2O4 interfaces, produced by exsolution of Ni
from NiAl2O4 spinel, is responsible for its high long-term stability and excellent resistance to coking and sintering
for dry reforming of methane (DRM) reaction. The structure sensitivity and kinetic compensation effect of CH4
dissociation on Ni0 active sites are observed. DRM reaction proceeds via a Langmuir-Hinshelwood mechanism
accompanied by an additional redox mechanism. It is noteworthy that the active oxygen species generated by
filling the oxygen vacancies of NiAl2O4 spinel by CO2 provide another rapid redox route to eliminate carbon
species.
1. Introduction
Dry reforming of methane (DRM) with carbon dioxide, which can
consume two primary greenhouse gases (CH4 and CO2) into syngas (H2
+ CO) with low H2/CO molar ratio, has received considerable attentions
[1–4]. The resultant syngas with low H2/CO ratio is more appealing in
Fischer-Tropsch process for long-chain hydrocarbons and in the syn­
thesis of highly valuable oxygenated chemicals, such as acetic acid,
oxo-alcohols, ethyl acetate and dimethyl ether [5–7].
Noble metal catalysts, such as Ru, Rh, Pd and Pt, possess high cata­
lytic activity and excellent coke resistance for DRM reaction, but their
high price and limited availability restrict the large-scale commercial
applications. Therefore, low-cost nickel-based catalysts with abundant
resources have become the main research objects. Among them, NiAl2O3 system has been studied more, however carbon deposition and
sintering of metal Ni particles are the vital reasons for deactivation of NiAl2O3 catalysts [8–10]. Moreover, over Ni-Al2O3 system, NiAl2O4 spinel
phase is inevitably formed by calcination at high temperatures.
Although NiAl2O4 spinel phase is not considered to be catalytically
active for DRM reaction [11–14], the presence and role of NiAl2O4 phase
has been disputed continuously, which is variously described as having
an inert, deactivating, or stabilizing effects over Ni-Al2O3 system [8,15,
16]. Some researchers reported that the formation of surface NiAl2O4
spinel can effectively inhibit carbon deposition [14,17,18]. Ross et al.
[19] elucidated that nickel sites, derived from nickel oxide and surface
NiAl2O4 spinel, had different intrinsic activities for the steam reforming
of methane reaction. Recently, Littlewood et al. pointed out that the
presence of NiAl2O4 would affect the active sites, although it was not
clear [20]. It should be noted that the above speculations about the
ability of NiAl2O4 to suppress carbon deposition were essentially due to
the generation of tiny and anti-sintering Ni◦ , but it did not involve the
contribution of NiAl2O4 spinel itself.
NiAl2O4 spinel is known to present a partially inverted structure, in
which the tetrahedral sites are preferentially occupied by trivalent cat­
ions, while both of divalent and trivalent cations fill in the octahedral
sites [21]. Due to the partially inverted structure of NiAl2O4 spinel, the
oxygen vacancies are formed for the charge neutral balance on NiAl2O4
[22,23]. For DRM reaction, CO2 is the only object in the feedstock for
* Corresponding authors.
E-mail addresses: yujun@sit.edu.cn (J. Yu), ylguo@ecust.edu.cn (Y. Guo).
https://doi.org/10.1016/j.apcatb.2021.120074
Received 26 December 2020; Received in revised form 24 February 2021; Accepted 25 February 2021
Available online 1 March 2021
0926-3373/© 2021 Elsevier B.V. All rights reserved.
S. Zhang et al.
Applied Catalysis B: Environmental 291 (2021) 120074
carbon elimination. If CO2 cannot be activated sufficiently to the carbon
removal, the residual carbon will be gradually inert. Thus, improving
the ability of CO2 activation is extremely important to develop a catalyst
with high coke-resistant ability. Some researchers pointed out that ox­
ygen vacancy was one of the key factors to promote CO2 adsorption and
activation [24–26], which was favorable to suppress carbon deposition.
Additionally, thermodynamic calculations by Giehr et al. predicted that
the reaction of NiAl2O4 with solid carbon was thermodynamically
favored [27]. Therefore, we propose that the surface oxygen vacancies
on NiAl2O4 spinel may be involved in the reaction pathway of carbon
elimination in DRM reaction. On the other hand, the studies on the
structure evolution of NiAl2O4 spinel before and after DRM reaction are
lacked in the previous works, which should be propitious to systemati­
cally understand the spinel-based system.
Herein, a series of NixAl1O2-δ mesoporous catalysts with surface
NiAl2O4 spinel phase were synthesized by the citric acid sol-gel method,
and the size of surface Ni nanoparticles was adjusted by the amount of Ni
source. The catalytic performance for DRM reaction over NixAl1O2-δ
catalysts were tested at 800 and 650 ◦ C. Various characterization tech­
nologies (such as in situ DRIFTS, XPS, TPSR, pulse and kinetic tests) were
performed to a deeper understanding of the specific role of NiAl2O4
spinel phase and its reaction mechanism of DRM reaction over
NixAl1O2-δ catalysts.
XCO2 =
H2
FH2, out
=
CO
FCO, out
( )
TOFCH4 S-1 =
(3)
XCH4 × VCH4 × MNi
mcat × WNi × RNi × DNi
(4)
where XCH4 was CH4 conversion under the reaction conditions: GHSV
=600,000 mL/(gcat h, CH4/CO2/N2 = 1.5/1.5/7, 400− 650 ◦ C, 1 atm,
VCH4 was CH4 gas molar flow rate in unit of mol/s, MNi was atomic
weight of Ni, mcat was mass of catalyst, WNi was Ni content in the
catalyst determined by ICP-OES, RNi was reduction degree of Ni2+ in the
catalyst, and DNi was dispersion of Ni, which were calculated by the
surface exposed Ni atoms. The surface exposed Ni atoms were consid­
ered as active sites and their amount was measured by H2 pulse chem­
isorption experiments that assuming Hads/Nisurf stoichiometry of 1/1.
2.1. Catalyst preparation
NixAl1O2-δ catalysts were prepared by the citric acid sol-gel method,
and citric acid (C6H8O7) was employed as a chelating agent. The prep­
aration procedure was described as follows: 0.02 mol Al(NO3)3⋅9H2O
and the required amounts of Ni(NO3)2⋅6H2O were dissolved in 60 mL
deionized water at 70 ◦ C under vigorous stirring. Then citric acid was
added at a molar ratio of citric acid to total metal ions = 1:1. The mixed
solution was then continuously stirred at 70 ◦ C for a few hours until a
green gel was formed. The resultant gel was dried at 110 ◦ C for 12 h and
170 ◦ C for 4 h and then calcined at 800 ◦ C for 6 h at a heating rate of 2
◦
C/min in flowing air atmosphere to obtain NixAl1O2-δ (x = 0.01, 0.025,
0.05, 0.1, 0.2) catalysts. For comparison, Al2O3 and NiO were synthe­
sized as the same method, named as Al2O3-Gel and NiO-Gel. And the
stoichiometric Ni0.5Al1O2 (Ni/Al = 0.5) sample was also synthesized.
Finally, the calcined sample was pressed, crushed, and sieved to
0.25− 0.45 mm (40–60 mesh).
2.3. Catalyst characterization
The N2 adsorption-desorption isotherms were measured on a
Micromeritics ASAP 2020 M surface area and porosity analyzer at − 196
◦
C. All samples were degassed at 180 ◦ C for 1 h prior to analysis. The
specific surface area was calculated by Brunauer-Emmett-Teller (BET)
method, and the pore size and pore volume were determined using BJH
method applied to the desorption branch of the isotherms [28,29].
The actual Ni and Al contents of the synthesized NixAl1O2-δ catalysts
were determined using an inductively coupled plasma optical emission
spectroscopy (ICP-OES) (Agilent 725). Each sample was dissolved in a
mixed solution of aqua regia and hydrofluoric acid and then diluted
prior to measurement.
Powder X-ray diffraction (XRD) patterns were collected on a Bruker
D8 Focus diffractometer with Cu Kα radiation (λ = 1.54056 Å) for 2θ =
10− 80◦ operated at 40 kV and 40 mA. The average crystalline size of Ni
of NixAl1O2-δ catalysts was calculated by the Scherrer equation based on
the diffraction peak broadening.
Field emission transmission electron microscopy (TEM) images of
the reduced and spent samples were recorded on a Tecnai G2 F20 STWIN electron microscope operated at a working voltage of 200 kV. The
samples were prepared by ultrasonic dispersion in ethanol, evaporating
a drop of the resultant suspension onto copper grids with carbon film
TEM grids. High-angle annular dark-field scanning transmission elec­
tron microscopy (HAADF-STEM) and energy dispersive X-ray (EDX)
mapping were performed on a Tecnai G2 F20 S-TWIN electron micro­
scope to further visualize Ni distribution in ultra-thin sections (prepared
as above) of the selected sample.
Raman spectra was collected to analyze the existence forms of Ni
species by a Raman spectrometer (Renishaw invia reflex) equipped with
a 325 nm Ar-ion laser beam under ambient condition. Each sample was
analyzed more than three times at different locations. In addition,
Raman spectra were obtained over the spent catalysts after DRM reac­
tion from a Raman spectrometer equipped with a 532 nm Ar-ion laser
2.2. Catalytic performance test
Dry reforming of methane (DRM) with CO2 was carried out in a fixed
bed reactor at atmospheric pressure with a quartz tube (inner diameter
of 4 mm). The catalyst (50 mg) diluted with quartz sand (300 mg, 40–60
mesh) was loaded in the center of reactor tube. The thermocouple close
to the catalyst bed was used to monitor the reaction temperatures. Prior
to the reaction, the catalyst was first reduced in situ at 800 ◦ C in a flow of
10 vol % H2/Ar (50 mL/min) for 2 h, followed by Ar flow (50 mL/min)
flushing at this temperature for 1 h. Then, the temperature of reactor
tube was adjusted to the required value (800 ◦ C or 650 ◦ C), and the gas
flow was switched to a typical reactants (15 % CH4, 15 % CO2 balanced
with N2, N2 was used as an internal standard). The total flow rate of gas
mixture is 30− 60 mL/min. The reactor effluents were analyzed by an
online gas chromatography equipped with TCD and FID detectors. On
the basis of the molar flow at the inlet and outlet of the reactor, the
conversion and product ratio were calculated, according to the
following equations:
FCH4, in − FCH4, out
× 100%
FCH4, in
(2)
where Fi, in was the flow rate of each component (i) in the inlet feed, and
Fi, out was the flow rate of each component (i) in the outlet feed.
Kinetic experiments were performed by using less amount of catalyst
and high GHSV to control the conversion lower than 20 % in order to
eliminate the thermal and diffusion effects. In the kinetics studies, the
reactant feed flows of CH4 (30 % in N2) and CO2 (30 % in N2) were 50
mL/min, respectively, and the reaction temperature range was 400− 650
◦
C.
To determine the turnover frequency (TOF) of DRM reaction, the
catalytic performance in the kinetics-controlled regime were analyzed.
TOFs in term of CH4 conversion rate (molecules per second) per active
site was calculated using the following equation:
2. Experimental
XCH4 =
FCO2, in − FCO2, out
× 100%
FCO2, in
(1)
2
S. Zhang et al.
Applied Catalysis B: Environmental 291 (2021) 120074
Table 1
Physicochemical properties of Al2O3 support and NixAl1O2-δ catalysts.
Catalysts
Ni/Al
ICP
a
Al2O3-Gel
Ni0.01Al1O2-δ
n.a.
0.012
Ni0.025Al1O2-
0.026
δ
Ni0.05Al1O2-δ
0.047
Ni0.1Al1O2-δ
0.098
Ni0.2Al1O2-δ
0.203
Ni0.5Al1O2
0.495
a
b
c
d
e
f
g
b
XPS
n.a.
0.015/0.016/
0.014
0.023/0.027/
0.032
0.044/0.056/
0.071
0.054/0.071/
0.066
0.078/0.099/
0.081
0.173/0.204/N/
A
Nia wt
%
SBETc (m2/
g)
Vpc (cm3/
g)
Dca
(nm)
α0d Å
Crystalline size of Nie
(nm)
Mean size of Nif
(nm)
Dispersion of Nig
(%)
n.a.
1.1
81.1
109.6
0.12
0.11
4.1
3.3
7.893
7.903
n.a.
n.a.
n.a.
2.5/3.1
n.a.
13.0
2.3
105.2
0.13
3.6
7.916
4.1/4.6
6.5/7.9
12.2
4.5
111.6
0.12
3.3
7.918
7.4/7.7
11.5/11.8
10.1
8.8
92.9
0.11
3.8
7.948
13.4/15.5
14.6/15.8
9.4
16.2
65.2
0.11
4.8
7.997
18.6/20.4
19.5/21.3
7.3
30.0
60.8
0.10
4.7
8.054
22.5/n.a.
n.a.
n.a.
Determined by ICP-OES.
Obtained from XPS results, calcined/reduced/spent catalysts.
Determined from nitrogen physisorption.
Lattice parameter (a0) calculated from XRD pattern.
Evaluated from XRD pattern by Scherrer equation, reduced/spent catalysts.
Obtained from TEM images of reduced/spent catalysts.
Calculated from H2 pulse chemisorption by assuming H/Ni of 1.
beam.
UV–vis diffuse reflectance spectroscopy (UV–vis-DRS) analysis was
conducted over NixAl1O2-δ and NiO-Gel via a PerkinElmer Lambda 950
UV–vis NIR spectrophotometer equipped with an integrating sphere.
The scanning range was 200− 800 nm at medium scanning speed. Pol­
ytetrafluoroethylene (PTFE) was used as the reference spectra.
Temperature-programmed reduction of H2 (H2-TPR) was performed
on a P X 200 instrument with a thermal conductivity detector (TCD). The
U-type quartz tube reactor was loaded with 50 mg catalyst (40–60
mesh). The sample was pretreated in N2 with a flow rate of 40 mL/min at
300 ◦ C for 1 h, and followed by cooling to room temperature, then
heated from room temperature to 800 ◦ C at a ramp rate of 10 ◦ C/min in
10 % H2/N2 mixture gases with a flow rate of 40 mL/min and main­
tained at 800 ◦ C for another 2− 3 h. A high purity of CuO was employed
as a standard sample to quantify H2 consumption amount.
The X-ray photoelectron spectroscopy (XPS) spectra were carried out
on a Thermo Scientific ESCALAB 250Xi using Al Kα (hν = 1486.6 eV)
radiation as the excitation source under ultra-high vacuum (6.7 × 10− 8
Pa). In addition, all binding energies (BE) were determined with respect
to the C1 s line (284.6 eV) originating from adventitious carbon. The
reduced and spent catalysts were collected with protection of Ar atmo­
sphere at cooling stage.
Pulse chemisorption of H2 was employed using a AutoChem II 2920
chemisorption analyzer associated with a Hiden HPR20 mass spec­
trometer. The sample was first reduced with 10 vol % H2/Ar (40 mL/
min) at 800 ◦ C for 2 h and purged in a flow of high purity of He (40 mL/
min) for another 1 h, then cooled down to 50 ◦ C. Thereafter, H2 pulses
(loop volume of 0.5173 mL) were injected for chemisorption until the
elution peak of the continuous pulse remained constant. The metallic Ni
dispersion was calculated assuming Hads:Nisurf stoichiometry of 1:1
[30–32].
Temperature programmed surface reaction of CH4 (CH4-TPSR) was
carried out in the same chemisorption analyzer as pulse chemisorption
of H2. The catalyst (50 mg) was pre-reduced in 10 % H2/Ar (40 mL/min)
at 800 ◦ C for 2 h and purified in high purity of He (40 mL/min) for 1 h,
then cooled to 50 ◦ C. The TPSR tests were performed using a 50 mL/min
mixture of 1 % CH4/He at a rate of 10 ◦ C/min from 50 to 800 ◦ C. CH4
was identified from the fragmentation pattern at m/z = 15 to avoid the
interference of the oxygen fragmentation pattern at m/z = 16 from CO2
and H2O [33]. In addition, the products (H2, H2O, CO, O2, CO2) were
identified from the fragmentation pattern at m/z = 2, 18, 28, 32 and 44,
respectively. CO2-TPSR was performed with the same procedures as
CH4-TPSR except for the different feed gas of 1 % CO2/He.
Pulse experiment of CH4 was employed using the same chemisorp­
tion analyzer as pulse chemisorption of H2. The sample was first reduced
with 10 % H2/Ar (40 mL/min) at 800 ◦ C for 2 h and purged in a flow of
high purity of He (40 mL/min) for another 1 h, then cooled down to the
required temperature. Thereafter, CH4 pulses (loop volume of 0.5173
mL) were injected for CH4 dissociation. The MS signals of m/z = 2, 15,
18, 28, 32, 44, 40 (corresponding to H2, CH4, H2O, CO, O2, CO2, Ar)
were recorded. CO2-Pulse was performed with the same procedures as
CH4-Pulse except for the different pulse gas.
Diffuse reflectance infrared fourier transform spectroscopy (DRIFTS)
spectra were recorded on a Nicolet Nexus 6700 FT-IR spectrometer
equipped with a MCT detector, and the sample cell was fitted with ZnSe
windows and a heating chamber. The DRIFTS spectra were obtained
with a resolution of 8 cm− 1 and 64 scans. The testing sample was prereduced with 10 % H2/N2 at 800 ◦ C for 2 h in an external furnace and
then cooled to room temperature. Prior to the measurements, the sample
was charged into the IR cell and further reduced in H2 at 650 ◦ C for 1 h.
Subsequently, the system was purged with pure Ar and cooled down to a
desired temperature. After the background spectrum was recorded at a
certain temperature, Ar was replaced by the mixed gas (10 mL/min) of
15 vol % CH4 or/and 15 vol % CO2 in Ar and in situ DRIFTS spectra of the
samples were taken at a certain time. And DRIFTS spectra of the
adsorbed species on the catalyst surface at a specific time were collected.
Temperature programmed oxidation of O2 (O2-TPO) was performed
on the same chemisorption analyzer as pulse chemisorption of H2. 20 mg
of the spent sample was pretreated in high purity of He (40 mL/min) at
150 ◦ C for 60 min, then cooled down to 50 ◦ C. After the baselines of CO2,
CO and H2O signals in MS profiles maintained stable, the sample was
then exposed to a 3 % O2 in He flow (40 mL/min) from 50 to 800 ◦ C with
a heating ramp of 10 ◦ C/min. The MS signals of CO2 (m/z = 44), CO (m/z
= 28) and H2O (m/z = 18) were recorded.
The quantity of coke deposition over the spent catalyst was deter­
mined by thermogravimetric and differential thermal analysis (TG/
DTA) using PerkinElmer Pyris Diamond TG/DTA. 5 mg of the spent
catalyst was heated in air flow of 100 mL/min from room temperature to
800 ◦ C at a heating rate of 20 ◦ C/min.
O2-TPD profile of the catalyst (50 mg) was tested on the same
chemisorption analyzer as pulse chemisorption of H2. After pretreated in
high purity of He (40 mL/min) at 150 ◦ C for 60 min, the catalyst was
cooled down to 50 ◦ C, and then O2 (40 mL/min) was introduced into the
flow system for 30 min. The catalyst was heated from 50 to 800 ◦ C at a
3
S. Zhang et al.
Applied Catalysis B: Environmental 291 (2021) 120074
Fig. 1. (a) XRD patterns of NixAl1O2-δ catalysts calcined at 800 ◦ C for 6 h; (b) The partial enlarged drawing of gray area in Fig. 1a; (c) XRD patterns of NixAl1O2-δ
catalysts reduced at 800 ◦ C for 2 h; (d) XRD patterns of the spent NixAl1O2-δ catalysts after DRM reaction at 800 ◦ C.
heating rate of 10 ◦ C/min under He flow. The MS signal of O2 (m/z = 32)
was recorded.
A designed experiment, marked as Carbon-TPR, was performed,
similar to Soot-TPR experiment in soot combustion reaction, in which
graphite carbon (collected from CH4 thermal cracking at 800 ◦ C) and the
calcined Ni0.05Al1O2-δ catalyst with a weight ratio of 1:10 were ground
in a mortar for 10 min to obtain tight contact mixtures. The tight
catalyst-carbon mixture was pre-treated under He flow at 100 ◦ C with a
flow rate of 40 mL/min to remove the contaminant, then the tempera­
ture increased from 100 to 800 ◦ C at a heating rate of 10 ◦ C/min.
Additionally, the above catalyst was replaced by the calcined stoichio­
metric Ni0.5Al1O2 catalyst or calcined Al2O3-Gel for this experiment. The
same quantity of carbon was used for each experiment. During the
heating procedure, He flow was replaced by pure CO2 flow, labeled as
CO2-TPR.
SBET was observed over non-stoichiometric catalysts with a decrease in
Ni content, with values in the range of 65.2-111.6 m2/g. An increase in
SBET was attributed to the decrease of NixAl1O2-δ crystallites size, which
was consistent with the results of XRD and TEM. In addition, no marked
differences in the pore volume, around 0.10− 0.13 cm3/g in all catalysts,
were observed.
The structural properties of the calcined and reduced catalysts were
investigated by XRD, and the corresponding patterns are depicted in
Fig. 1. As shown in Fig. 1a and b, the main diffraction peaks of the
calcined Ni0.5Al1O2 were in good agreement with JCPDS 10-0339 data
of the face-centred cubic structure of NiAl2O4 (space group Fd-3 m).
Thus, the diffraction peaks at 2θ values of 19.1, 31.4, 37.0, 45.0, 59.7,
65.5, and 77.7 were assigned to (111), (220), (311), (400), (511), (440)
and (533) diffraction planes of NiAl2O4 spinel, respectively. For
comparative purposes, the pattern of the as-synthesized Al2O3-Gel was
also added, which was in good agreement with the γ-phase Al2O3
(JCPDS 10-0425). With an increase in Ni content (namely increasing Ni/
Al ratio), the (440) diffraction peaks of all calcined NixAl1O2-δ catalysts
gradually shifted to lower angles (Fig. 1b). This phenomenon was
attributed to incorporation of Ni2+ into Al2O3 lattice to form NiAl2O4,
and the corresponding lattice parameter (a0) of NixAl1O2-δ increased
from 7.903 to 8.054 Å (Table 1) with the enhancement of Ni/Al ratio.
The a0 value (8.054 Å) for Ni0.5Al1O2 accorded with those reported in
the literature for the same spinel NiAl2O4 prepared by the ceramic
method at high temperature [34] and the sol-gel method at 900 ◦ C [35].
Furthermore, the absence of NiO phase with diffraction peaks at 2θ
values of 43.3◦ and 62.9◦ (JCPDS 71-1179) revealed that Ni2+ species
reacted with alumina to form NiAl2O4, or existed in the amorphous form
or were highly dispersed with tiny crystallites in γ-Al2O3 matrices. As
shown in Fig. 1a, with an increase in Ni content, the diffraction peaks of
the calcined NixAl1O2-δ catalysts gradually became stronger and nar­
rower, indicating the increasing crystallization degree and size of
3. Results and discussion
3.1. Characterization of the calcined and reduced catalysts
The Ni and Al contents of NixAl1O2-δ catalysts were determined by
ICP-OES (Table 1), in which the Ni/Al ratio was close to the mole ratio of
feedstock. Fig. S1a and b depict N2 adsorption-desorption isotherms and
the pore size distributions of NixAl1O2-δ catalysts. The isotherms were
identified as type IV with an obvious hysteresis loop in the relative
pressure range of 0.4 to 0.8, which was characteristic of mesoporous
materials. As shown in Table 1, the mean pore sizes of these catalysts
varied irregularly in the range of 3.3–4.8 nm. As shown in Fig. S1b, the
pore size distributions of the catalysts with low Ni contents (Ni0.01,
Ni0.025, Ni0.05) were narrow and the main peaks were located in the
range of 3.3–3.6 nm. The BET surface area (SBET) of the stoichiometric
Ni0.5Al1O2 catalyst was 60.8 m2/g. However, a significant increase in
4
S. Zhang et al.
Applied Catalysis B: Environmental 291 (2021) 120074
Fig. 2. H2-TPR profiles of the calcined NixAl1O2-δ catalysts.
Table 2
Reducibility and surface compositions of NixAl1O2-δ catalysts before/after reduction and after DRM reaction.
Catalysts
H2 uptake a mmol/gcat
Degree of reduction (%)
Reduction peak temperature (◦ C)
Al2O3-Gel
Ni0.01Al1O2-δ
Ni0.025Al1O2-δ
Ni0.05Al1O2-δ
Ni0.1Al1O2-δ
Ni0.2Al1O2-δ
Ni0.5Al1O2
n.a.
0.19 (0.08)
0.39 (0.21)
0.77 (0.59)
1.50 (1.28)
2.76 (2.51)
5.11 (4.82)
n.a.
42.6
54.1
76.4
85.5
90.9
94.3
n.a.
796.2
795.7
796.4
793.9
794.4
783.2
a
b
Ni0/(Ni0+Ni2+)b
Osurf/Olatt b
Calcined
Reduced
Spent
Calcined
Reduced
Spent
n.a.
0
0
0
0
0
0
n.a.
0.017
0.065
0.193
0.362
0.340
0.160
n.a.
0.019
0.371
0.315
0.565
0.845
n.a.
0.58
0.48
0.39
0.31
0.29
0.20
0.16
n.a.
0.49
0.42
0.38
0.37
0.34
0.32
n.a.
0.52
0.43
0.40
–
–
n.a.
Values in parentheses were obtained from H2-TPR.
Obtained from XPS results.
broad bands at 460 and 500 cm− 1, while crystalline NiAl2O4 spinel
possessed two characteristic bands located at 375 and 600 cm− 1.
Therefore Ni species in the calcined NixAl1O2-δ catalysts mainly existed
in the form of NiAl2O4 spinel. As shown in Fig. 3b, the characteristic
bands at 375 and 600 cm− 1 belonging to NiAl2O4 spinel were still
observed over the reduced NixAl1O2-δ catalysts, suggesting the existence
of remaining NiAl2O4 spinel in the reduced catalysts, which was
consistent with the reduction degree obtained from H2-TPR analysis.
The coordination and the oxidation state of the surface nickel species
of the calcined NixAl1O2-δ catalysts were studied by UV–vis-DRS. As
presented in Fig. 3c, the attribution of the bands was carried out by
comparison with NiO-Gel sample. The band of NiO-Gel centred at
220− 345 nm was ascribed to O2− →Ni2+ ligand to metal electron charge
transfer [41], and the bands located at 380 and 720 nm accompanied by
420 and 465 nm represented the octahedrally coordinated Ni2+ species
in NiO lattice [42]. For the spectra of all NixAl1O2-δ catalysts, the bands
of NiO were absent, while the bands at 550 nm and in the range of
580− 640 nm were observed, which was related to the tetrahedrally
coordinated Ni2+ species in the NiAl2O4 lattice [28,35,42,43]. In a word,
Ni2+ species in NixAl1O2-δ catalysts were present as Ni2+ species occu­
pying both octahedral and tetrahedral positions of NiAl2O4 spinel lat­
tice, namely partially inverted spinel structure [41,44]. Wu and
Heracleous et al. [45,46] reported that tetrahedrally coordinated nickel
ions were hard to reduce while octahedral nickel ions were readily
reduced species. Combining with H2-TPR and UV–vis-DRS, it was
legitimately deduced that the difficult reduction of nickel species was
assigned to the tetrahedrally coordinated nickel ions in NixAl1O2-δ
catalyst, which increased with a decrease in Ni content. In addition, the
band at about 374 nm belonging to d-d transitions of octahedral Ni2+
ions shifted toward lower wavelength (blue shift from 374 to 364 nm)
with the gradual decline of Ni content, which was associated with the
decrease of metal oxide nanoparticle size [47].
XPS was performed to provide information about the oxidation state
and the chemical environment of the surface nickel species of the
NixAl1O2-δ crystallites.
As shown in Fig. 1c, over the reduced NixAl1O2-δ catalysts, new
diffraction peaks centered at 2θ values of ca. 44.5◦ , 51.8◦ and 76.4◦ were
observed, corresponding to (111), (200), and (220) reflections for
metallic Ni (JCPDS 04-0850), respectively. These peak intensities
increased with the enhancement of Ni content. The mean size of Ni◦
crystallites was determined by applying the Scherrer equation to Ni
(200) signal at 2θ = 51.8◦ . Results listed in Table 1 showed that Ni
crystalline size increased with the enhancement of Ni content.
Fig. 2 compares H2-TPR profiles of NixAl1O2-δ catalysts, meanwhile,
the experimental and theoretical H2 uptakes, and the corresponding
reduction peak temperature are listed in Table 2. The stoichiometric
Ni0.5Al1O2 catalyst exhibited a broad reduction profile with a distinct
reduction peak at 783 ◦ C, typically connecting with reduction of
NiAl2O4 spinel phase to metallic Ni and Al2O3 [36]. The reduction peak
temperature varied from 794 to 796 ◦ C over all non-stoichiometric
NixAl1O2-δ catalysts, which was attributed to reduction of NiAl2O4
spinel in NixAl1O2-δ. In addition, a small shoulder peak was observed at
317− 407 ◦ C over all catalysts. For comparison, Al2O3-Gel showed a
broad reduction peak with low intensity at 300− 800 ◦ C (Fig. S2a),
which was attributed to reduction of surface oxygen species such as
hydroxyl groups on alumina [37]. Thus, the small shoulder peak at
300− 400 ◦ C should be assigned to reduction of the surface oxygen
species on NixAl1O2-δ. The reduction degree of catalyst was enhanced
from 42.6 to 94.3 % with an increase in Ni/Al ratio from 0.01 to 0.5
(Table 2). This result suggested that NiAl2O4 spinel in the catalyst could
not be completely reduced at 800 ◦ C even if the reduction time was
extended to 3 h. Furthermore, the lower the Ni content was, the more
difficult the catalyst was to reduce.
To identify the existent forms of Ni over these catalysts, the calcined
and reduced catalysts were characterized by Raman spectra, and the
results are shown in Fig. 3a, b. As shown in Fig. 3a, two bands located at
375 and 600 cm− 1 were clearly observed over all NixAl1O2-δ catalysts.
As reported in previous studies [38–40], crystalline NiO exhibited two
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Fig. 3. Raman spectra of the calcined NixAl1O2-δ catalysts (a) and the reduced NixAl1O2-δ catalysts (b); UV–vis-DRS spectra (c) of the calcined NixAl1O2-δ catalysts.
calcined and reduced catalysts (Fig. 4). The Ni 2p3/2 spectra of the
calcined NixAl1O2-δ catalysts were comprised of two peaks, namely the
primary peak corresponding to Ni2+ ions and its satellite peak posi­
tioned within a binding energy range of 855.7–856.1 eV and
861.8–862.5 eV, respectively. In generally, the binding energy (BE) of Ni
2p3/2 of pure NiO is ~854 eV, while the BE of Ni 2p3/2 of NiAl2O4 spinel
appears at ~856.0 eV [48]. Because the difference between the main
peak and its satellite peak (6.3 ± 0.3 eV) matched with the reference
value (6.3 ± 0.3 eV), the surface Ni2+ species of the calcined NixAl1O2-δ
was associated with NiAl2O4 spinel [28]. On the other hand, as seen
from Table 1, the surface Ni/Al ratios of NixAl1O2-δ (x < 0.1) were close
to the ratios given by ICP-OES, while the surface Ni/Al ratios of
NixAl1O2-δ (x ≥ 0.1) were much lower than those given by ICP-OES,
indicating that a nickel-deficient aluminium-rich phase existed on the
surface of NixAl1O2-δ, especially the catalysts with higher Ni contents.
In relation to XPS spectra of the reduced catalysts, the BE of new line
was centred at 852.5 eV (Fig. 4), which was correlated with the metallic
nickel (Ni◦ ) reduced from NiAl2O4 spinel [49]. Correspondingly, due to
exsolution of Ni from NiAl2O4, the surface Ni/Al ratios of the reduced
catalysts were slightly higher than those of the calcined catalysts
(Table 1). Moreover, the principal peak (~856 eV) corresponding to
Ni2+ ions still remained in the reduced catalysts. It was further corrob­
orated that the surface NiAl2O4 species in NixAl1O2-δ catalysts could not
be completely reduced after the reduction treatment, that is to say,
NiAl2O4 and metallic Ni were co-existed in the reduced catalysts, which
was in agreement with the results obtained by H2-TPR and Raman
spectra. On the other hand, both of Ni (111) (lattice distance of 0.204
nm) and NiAl2O4 (111) (interplanar spacing of 0.465 nm) were identi­
fied by HRTEM image of the reduced Ni0.05Al1O2-δ catalyst (Fig. S3).
Fig. S4 shows the O 1s XPS spectra of the calcined and reduced
NixAl1O2-δ catalysts. The spectra of O 1s were fitted with two Gassian
peaks, in which the peak with BE at 530.8 eV corresponded to the sur­
face lattice oxygen of NiAl2O4 or Al2O3 [50], and the peak with BE at
531.8 eV was attributed to the surface adsorbed oxygen species, related
2−
to the groups: –OH in water and CO– in CO2−
3 [25]. Wherein, CO3
species derived from carbonate species trapped by oxygen vacancies
[24]. The relative proportion of the surface adsorbed oxygen species to
the lattice oxygen (Osurf/Olatt) was calculated on the basis of XPS data
shown in Table 2. For all calcined and reduced catalysts, the lower the Ni
content was, the higher the proportion of the surface adsorbed oxygen
species was. This phenomenon may be because the decrease of Ni con­
tent reduces the formation of surface NiAl2O4, which leads to more
exposure of Al2O3 with hydroxyl groups on the surface. In addition,
compared with the calcined counterpart, the increased Osurf/Olatt shown
in Table 2 for the reduced catalyst could be an index, which indicated
that another possible reason for the formation of oxygen vacancies
might be induced by reduction of NiAl2O4 in NixAl1O2-δ catalyst, since
the removal of surface oxygen might leave the surface oxygen vacancies.
TEM images of the reduced catalysts are shown in Fig. S5, and their
Ni nanoparticle sizes are tabulated in Table 1. The majority of
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Fig. 4. XPS spectra of Ni 2p3/2 region of NixAl1O2-δ catalysts: For each spectra, at the bottom spectra the catalysts were calcined at 800 ◦ C for 6 h, in the middle
spectra the catalysts were reduced with 10 % H2/Ar at 800 ◦ C for 2 h and at the top spectra the catalysts were used for DRM reaction at 800 ◦ C for 50 h. (a)
Ni0.5Al1O2, (b) Ni0.2Al1O2-δ, (c) Ni0.1Al1O2-δ, (d) Ni0.05Al1O2-δ, (e) Ni0.025Al1O2-δ, (f) Ni0.01Al1O2-δ.
Fig. 5. The structure evolution of NixAl1O2-δ catalyst before and after DRM reaction.
nanoparticle sizes were concentrated in the range of 1− 3 nm, 4− 8 nm,
8− 13 nm, 13− 16 nm and 16− 23 nm over Ni0.01Al1O2-δ, Ni0.025Al1O2-δ,
Ni0.05Al1O2-δ, Ni0.1Al1O2-δ and Ni0.2Al1O2-δ, respectively. Correspond­
ingly, Ni nanoparticle sizes of these reduced catalysts were measured to
be 2.5 nm, 6.5 nm, 11.5 nm, 14.6 nm and 19.5 nm, respectively
(Table 1). On the other hand, HAADF-STEM image and EDX elemental
mapping of the reduced Ni0.05Al1O2-δ shown in Fig. S6 revealed that a
number of aggregated Ni nanoparticles with 8− 12 nm were relatively
uniformly dispersed on the catalyst surface, which was consistent with
the results shown in Fig. S5e. The above results indicate that Ni nano­
particle sizes of NixAl1O2-δ catalysts can be effectively adjusted by
adding different content of Ni source using the citric acid sol-gel method.
In summary, based on the above-mentioned characterizations of the
calcined and reduced catalysts, the structure evolution of NiAl2O4 spinel
phase is clarified (shown in the left part of Fig. 5). That is, Ni2+ species in
the calcined NixAl1O2-δ exist in the form of NiAl2O4 spinel. The
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Fig. 6. Long-term performances for DRM reaction at 800 ◦ C over NixAl1O2-δ catalysts and the synthetic Al2O3-Gel support. Reaction conditions: GHSV = 108,000
mL/(h⋅gcat), CH4/CO2/N2 = 1.5/1.5/7, 1 atm. (a) Ni0.2Al1O2-δ, (b) Ni0.1Al1O2-δ, (c) Ni0.05Al1O2-δ, (d) Ni0.025Al1O2, (e) Ni0.01Al1O2-δ, (f) Al2O3-Gel. The black dashed
lines in all figures represented the equilibrium conversion of CH4 at 800 ◦ C.
subsequent formation of active Ni0 species from NiAl2O4 spinel by
reduction at high temperature plays a key role in boosting the metalsupport interaction, which gains smaller and more dispersed active Ni
crystallites over NixAl1O2-δ with low content of Ni (Table 1). In addition,
it is noted that Ni2+ species in the calcined NixAl1O2-δ can’t be
completely reduced, namely, Ni0 species and NiAl2O4 phase are coexisted in the form of Ni0-NiAl2O4/Al2O3 over the reduced NixAl1O2-δ
catalysts.
3.2. Long-term performance for DRM reaction at 800 ◦ C and
characterization of the spent catalysts
The long-term performance of NixAl1O2-δ catalysts for DRM reaction
at 800 ◦ C are shown in Fig. 6. The conversions of CH4 and CO2 were
stable over all catalysts during 50 h stability test. Except for
Ni0.01Al1O2-δ, other NixAl1O2-δ catalysts showed a similar activity, in
which CH4 conversion reached in the range of 91–92 %, less than CH4
equilibrium conversion of 97.5 % at 800 ◦ C. Higher CO2 conversion of
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Fig. 7. Long-term performances for DRM reaction at 650 ◦ C over Ni0.05Al1O2-δ (a) and Ni0.025Al1O2-δ (b) catalysts. Reaction conditions: GHSV =36,000 mL/(h⋅gcat),
CH4/CO2/Ar = 1.5/1.5/7, 1 atm. The black dashed line in all figures represented the equilibrium conversion of CH4 at 650 ◦ C.
96.5–97.8 % was obtained than CH4 conversion, and H2/CO ratio was
steady around 0.95 less than 1, suggesting that the side reaction of
reverse water-gas shift (RWGS reaction (H2 + CO2 → CO + H2O))
simultaneously took place [51]. Over Ni0.01Al1O2-δ (Fig. 6e), due to its
low Ni content, the relative low conversions of CH4 and CO2 were ob­
tained at 88 % and 91.8 %, respectively. For reference, Al2O3-Gel was
tested at 800 ◦ C for 20 h, only about 0.25 % CH4 conversion caused by
methane thermal cracking was obtained (Fig. 6f). It indicates that
metallic Ni species are the active sites for initiating DRM reaction.
As shown in Fig. 1d, the diffraction peak of Ni◦ crystallite at 2θ =
44.5◦ and 51.8◦ marginally strengthened after DRM reaction, suggesting
that growth of Ni◦ grains took place during DRM reaction due to high
reaction temperature (800 ◦ C) [52]. Although Ni◦ mean particle sizes of
the spent catalysts were slightly larger than that of the reduced catalysts
(Table 1), the presence of NiAl2O4 phase retarded aggregation of Ni◦
obviously. In addition, the diffraction peak attributed to graphite carbon
appeared at 2θ of 26.2◦ over the spent Ni0.1Al1O2-δ and Ni0.2Al1O2-δ
(Fig. 1d), indicating that these two catalysts suffered from serious car­
bon deposition during DRM reaction.
As seen from Ni 2p3/2 XPS spectra of the spent catalysts shown in
Fig. 4, the intensity of the peak at ~852.5 eV was further enhanced,
compared with the corresponding reduced catalysts. And the surface
Ni◦ /(Ni◦ +Ni2+) ratios of the spent NixAl1O2-δ catalysts were higher than
those of the reduced ones (Table 2). These results implied that the sur­
face NiAl2O4 species in the reduced NixAl1O2-δ further suffered from
reduction under DRM reaction. It is in good agreement with the result
demonstrated by Littlewood et al. that NiAl2O4 spinel phase is unstable
under DRM reaction conditions and slowly undergoes reduction to
metallic Ni and Al2O3 [20]. But on the other hand, NiAl2O4 phase still
existed clearly on the spent catalysts, which was attributed to the very
slow reduction of NiAl2O4 under DRM reaction conditions.
Observing TEM images of the spent NixAl1O2-δ catalysts shown in
Fig. S5b, d, f, h, j and Table 1, Ni particle sizes of the spent catalysts
continued to grow slightly. In addition, a number of long whisker-like
filamentous carbontube deposits was observed over NixAl1O2-δ (x ≥
0.05). These results are in agreement with the results reported by Kim
et al. [53] that the formation of filamentous carbon is significantly
influenced by Ni particle size and proceeds mostly over the metal par­
ticles larger than 7 nm.
The amount of deposited carbon over the spent catalysts was deter­
mined by TG/DTA analysis (Fig. S7). For comparison, TG/DTA curves of
Al2O3-Gel support are displayed in Fig. S2b. A continuous weight loss of
11.3 % occurred from 200 to 800 ◦ C over Al2O3-Gel support, implying
removal of surface hydroxy groups on γ-Al2O3 (Fig. S2c) [54]. The
reduced and spent catalysts of Ni0.01Al1O2-δ and Ni0.025Al1O2-δ
exhibited the similar TG/DTA curves with no exothermic peaks shown in
Fig. S7d, e. It indicates that Ni0.01Al1O2-δ and Ni0.025Al1O2-δ with small
Ni nanoparticle sizes have superior coking resistance ability during DRM
reaction at high temperature. With an increasing in Ni content, a notable
weight loss at high temperature (>600 ◦ C) was observed over the spent
NixAl1O2-δ (x ≥ 0.05), accompanied by a distinct exothermic peak
around 650 ◦ C. This exothermic peak was attributed to combustion of
deposited carbon on the spent catalysts during TG tests. Obviously,
carbon deposition was conspicuous over the catalysts with high content
of Ni. As shown in Fig. S7f, the carbon formation rate of NixAl1O2-δ was
quantified as the following order: Ni0.2Al1O2-δ (3.65) > Ni0.1Al1O2-δ
(1.37) > Ni0.05Al1O2-δ (0.46) > Ni0.025Al1O2-δ (0.0) = Ni0.01Al1O2-δ
(0.0). These results coincide with those of the previous reports that
small-size Ni particles are relatively effective to suppress carbon depo­
sition in DRM reaction [55,56].
O2-TPO was employed to quantify the types of deposited carbon
species as shown in Fig. S8. Single prominent peak in the range of
300− 700 ◦ C, was observed over the spent NixAl1O2-δ (x ≥ 0.05).
Compared with the references [57,58], the carbon formed during DRM
reaction was classified as graphite-like (Cγ) carbon, and the amount of
deposited carbon significantly increased with an increase in Ni content,
which was consistent with the above characterizations.
3.3. Long-term performance for DRM reaction at 650 ◦ C and
characterization of the spent catalysts
Combining with the catalytic activity and coking resistance, both
Ni0.05Al1O2-δ and Ni0.025Al1O2-δ were further chosen to evaluate the
long-term performance for DRM reaction at 650 ◦ C (reaction tempera­
ture for the most severe carbon deposition) for 100 h. As shown in Fig. 7,
a slight drop of CH4 and CO2 conversions (from 68.7 % and 80.4 % to
61.7 % and 73.8 %, respectively) as a function of time was observed over
Ni0.05Al1O2-δ, indicating the slight deactivation of Ni0.05Al1O2-δ during
DRM reaction at 650 ◦ C. However, Ni0.025Al1O2-δ exhibited more stable
conversions of CH4 (69 %) and CO2 (82 %). XRD patterns of these two
spent catalysts shown in Fig. S9a exhibited the similar Ni intensity to the
reduced ones, suggesting that Ni nanoparticles did not aggregate under
DRM reaction at 650 ◦ C. In addition, a characteristic peak of graphitic
coke at 2θ = 26.2◦ appeared over Ni0.05Al1O2-δ. The formation of carbon
deposits over Ni0.05Al1O2-δ and Ni0.025Al1O2-δ was further confirmed by
TGA/DTA, and the degree of coking over Ni0.05Al1O2-δ catalyst was
more serious than that over Ni0.025Al1O2-δ (Fig. S9c, d), which was
consistent with the catalytic activity shown in Fig. 7 and XRD results
shown in Fig. S9a. Furthermore, Raman spectra (Fig. S9b) and O2-TPO
profiles (Fig. S9e, f) also confirmed the formation of carbon nanotubes
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Fig. 8. Kinetics experiments of NixAl1O2-δ catalysts. (a) Effect of Ni nanoparticle size on TOFCH4 at different reaction temperatures; (b) Arrhenius plots of ln TOFCH4
vs. 1000/T; (c) Relationship between Ea for dissociation of CH4 and Ni nanoparticle size over NixAl1O2-δ catalysts; (d) Relationship between ln A and Ea over
NixAl1O2-δ catalysts. The inset in (c) was the relationship between ln A and Ni nanoparticle size over NixAl1O2-δ catalysts.
and encapsulating carbon on the spent catalyst surface [59]. The carbon
formation rate over Ni0.05Al1O2-δ and Ni0.025Al1O2-δ during DRM reac­
tion at 650 ◦ C determined from TG results was 2.1 and 0.59 mgC⋅g−cat1
h− 1, respectively (Fig. S9c, d).
TEM images and Ni 2p3/2 XPS spectra of these two spent catalysts are
shown in Fig. S10. Compared with the spent Ni0.025Al1O2-δ catalyst,
there were many carbon species covered on the surface of the spent
Ni0.05Al1O2-δ catalyst, while part of Ni active sites were coated by
encapsulating carbon. XPS results showed that the surface Ni◦ /
(Ni◦ +Ni2+) ratio of Ni0.025Al1O2-δ increased after DRM reaction at 650
◦
C, indicating that the surface NiAl2O4 species were further reduced
under DRM reaction, which was similar to DRM reaction at 800 ◦ C.
Conversely, the surface Ni◦ /(Ni◦ +Ni2+) ratio of the spent Ni0.05Al1O2-δ
was even lower than that of the reduced one, which was attributed to the
carbon covering and coating of Ni active sites on the surface of the spent
Ni0.05Al1O2-δ [60].
In short, based on the above-mentioned characterizations of the
spent catalysts, the structure of Ni0-NiAl2O4/Al2O3 is further evolved by
suffering from reduction of Ni2+ species in NiAl2O4 phase to active
metallic Ni0 under DRM reaction, especially at higher reaction temper­
ature. And the migration and merging of the exsolved Ni species onto
initial Ni nanoparticles under DRM reaction leads to a slight increase in
the size of Ni nanoparticles (shown in the lower part of Fig. 5).
As reported by many researchers, one of the important factors
affecting carbon deposition is the size of Ni nanoparticles [61–64], and
the carbon deposition can significantly decrease when the size of Ni
nanoparticles is less than 5− 6 nm [65] or 7− 10 nm [53,66–68]. In this
context, Ni0.025Al1O2-δ catalyst with Ni particle size of 6.5 nm exhibits
good catalytic activity and coking resistance for DRM reaction.
3.4. Kinetics study of DRM reaction over NixAl1O2-δ catalysts
In order to obtain the intrinsic kinetics information of CH4 dissoci­
ation in DRM reaction on Ni active sites, kinetics experiments were
performed to investigate dissociation of CH4 over NixAl1O2-δ catalysts,
which is shown in Fig. 8. TOFCH4 values at different reaction tempera­
tures, calculated according to Eq. (4) and Table S1, are listed in Table S2.
TOFCH4 value increased with the elevation of the reaction temperature
over all catalysts. TOFCH4 value obtained over NixAl1O2-δ with different
amounts of Ni (or dimensions of Ni) was not fixed at the same reaction
temperature. The relationship between TOFCH4 value and Ni nano­
particle size over NixAl1O2-δ catalysts at different reaction temperatures
are shown in Fig. 8a. The plots of TOFCH4 value versus Ni nanoparticle
size exhibited volcanic curve, and the highest TOFCH4 value was ob­
tained over Ni0.05Al1O2-δ with Ni nanoparticle size of 11.5 nm. It has
been reported that one reaction can be defined as a structure sensitive
reaction when the reaction activity of TOF is changed with the metal
particle size [69,70]. Therefore, the volcanic curve of TOFCH4 value
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Fig. 9. EPR curves (a) and O2-TPD profiles (b) of Ni0.05Al1O2-δ and Ni0.5Al1O2 catalysts; Carbon-TPR profiles of the calcined Ni0.05Al1O2-δ (c), the calcined Ni0.5Al1O2
(d) and the calcined Al2O3-Gel (e); CO2-TPR profiles of Ni0.05Al1O2-δ and Ni0.5Al1O2 catalysts (f).
versus Ni nanoparticle size provides the direct evidence for the structure
sensitive reaction of CH4 dissociation in DRM reaction over NixAl1O2-δ
catalysts.
DRM reaction was performed in the kinetics region, in which the
activation energy (Ea) and the pre-exponential factor (A) further eluci­
dated the influence of Ni nanoparticle size on TOFCH4 value of CH4
dissociation. The plots of ln TOFCH4 versus 1000/T are shown in Fig. 8b,
which shows a good linear relationship for each catalyst. The Ea and A
for CH4 dissociation calculated based on Fig. 8b according to Arrhenius
formula over NixAl1O2-δ catalysts were Ni0.01 (89.7 kJ/mol, 6.54 × 105)
> Ni0.025 (72.1 kJ/mol, 8.08 × 104) > Ni0.05 (42.0 kJ/mol, 1.49 × 103)
> Ni0.1 (41.1 kJ/mol, 7.82 × 102) > Ni0.2 (40.5 kJ/mol, 6.64 × 102).
Both Ea and A decreased with an increase in Ni nanoparticle size. The
plot of Ea versus Ni nanoparticle size indicated a relatively good linear
relationship with the correlated coefficient (R2) of 0.98555 (Fig. 8c)
when Ni nanoparticle size was less than 11.5 nm, but Ea was poor linear
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Fig. 10. TPSR profiles of Ni0.05Al1O2-δ catalyst from 50 to 800 ◦ C and maintained at 800 ◦ C for 30-60 min: (a) CH4-TPSR profiles of the reduced Ni0.05Al1O2-δ catalyst
with 1 % CH4/He, (b) CO2-TPSR profiles of the reduced Ni0.05Al1O2-δ catalyst with 1 % CO2/He, (c) CO2-TPSR profiles of the coked Ni0.05Al1O2-δ catalyst (coking by
CH4 at 800 ◦ C for 60 min) with 1 % CO2/He.
versus Ni nanoparticle size (SNi > 11.5 nm) with R2 of 0.8926. The plot
of ln A versus Ni nanoparticle size showed the similar trends to that of Ea
vs. Ni nanoparticle size (the inset in Fig. 8c).
Based on the Arrhenius equation of k = A⋅exp (-Ea/RT), the
enhancement of Ea makes the reaction difficult to happen, while the
increase in A can speed up the reaction rate. The kinetic compensation
effect between Ea and A takes the form of a sympathetic linear corre­
lation between Ea and ln A for a class of the related heterogeneous
catalytic reactions or catalysts [71,72]. The general equation for
compensation effect [73,74] is shown below: A = A0⋅exp (b⋅Ea) or ln A =
b⋅Ea + ln A0, where A0 and b are two constants. The plot of ln A versus Ea
obtained from CH4 dissociation over different Ni nanoparticle sizes is
shown in Fig. 8d. Ln A was well linear with Ea for CH4 dissociation in
DRM reaction and the equation was ln A = 1.15672 Ea + 0.13792 with
R2 of 0.99064. The linear relationship between Ea and ln A proved the
presence of the kinetic compensation effect (KCE) in CH4 dissociation in
DRM reaction over NixAl1O2-δ catalysts, which was responsible for the
volcanic curve of the plots of TOFCH4 versus Ni nanoparticle size shown
in Fig. 8a.
(Ov). As shown in Fig. 9a, both Ni0.05Al1O2-δ and Ni0.5Al1O2 catalysts
exhibited a strong axial signal with g value of 2.002, assigned to the
single of electron O−2 radical trapped by Ov [75,76], which confirmed the
presence of Ov in NiAl2O4 spinel. In O2-TPD profiles shown in Fig. 9b,
the peaks of α and β were attributed to the loosely bound surface oxygen
species and tightly captured oxygen species by Ov, respectively [77].
And γ peak located at 780 ◦ C was observed over Ni0.05Al1O2-δ and
Ni0.5Al1O2, which was related to the easily migrated surface lattice ox­
ygen species on NiAl2O4. Interestingly, we found that the calcined
Ni0.05Al1O2-δ (in the form of NiAl2O4/Al2O3) could suffer from slow
reduction under pure Ar atmosphere at 800 ◦ C for 2 h or under H2 at­
mosphere at 500 ◦ C for 2 h, accompanied by the change of catalyst color
from sky blue to black, indicating removal of surface lattice oxygen
species. This black sample after treatment under Ar at 800 ◦ C or H2 at
500 ◦ C was named as defective Ni0.05Al1O2-δ catalyst. NiAl2O4 spinel
would slowly undergo reduction for long-term treatment at high tem­
perature in an inert gas, which was also reported by Littlewood et al.
[20]. While, the color of this defective Ni0.05Al1O2-δ catalyst could re­
turn back to sky blue under a flowing CO2 at 800 ◦ C for 2 h, which
suggested that the lost surface lattice oxygen species were replenished
by CO2 (shown in the upper part of Fig. 5). However, the black exsolved
Ni◦ species on the surface of the reduced Ni0.05Al1O2-δ catalyst could not
change color under CO2 flow at 800 ◦ C. In addition, Carbon-TPR was
used to investigate the availability and mobility of surface oxygen spe­
cies for carbon (collected from CH4 thermal cracking at 800 ◦ C) elimi­
nation, and the Carbon-TPR profiles of Ni0.05Al1O2-δ and Ni0.5Al1O2 are
shown in Fig. 9c, d. MS signals of CO, CO2, H2 and H2O were observed in
3.5. Mechanism study of DRM reaction over Ni0.05Al1O2-δ catalyst
To insight into the coking resistance behavior and DRM reaction
mechanism, Ni0.05Al1O2-δ catalyst with relatively easy to deposit carbon
was selected for a series of characterizations, such as EPR, O2-TPD,
Carbon-TPR, TPSR, Pulse and in situ DRIFTS.
EPR measurements were employed to explore the oxygen vacancies
12
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Applied Catalysis B: Environmental 291 (2021) 120074
Ni0.05Al1O2-δ > Al2O3-Gel > without catalyst. This result evidently re­
veals the promotional role of NiAl2O4 spinel over catalyst for oxidation
of carbon species, that is, the oxygen vacancy of NiAl2O4 spinel provides
crucial active sites for CO2 activation. The absorbed CO2 in the oxygen
vacancy is dissociated to CO and oxygen species, resulting in carbon
oxidation via a rapid redox route. Therefore, the redox mechanism, in
which oxygen vacancies have emerged as important active sites for
assisting in carbon elimination, has been proposed.
Coking reactions via CH4-TPSR were carried out in order to explore
activation of CH4 over the reduced Ni0.05Al1O2-δ catalyst, as shown in
Fig. 10a. The decomposition of CH4 started at 343 ◦ C and a small amount
of H2 was produced simultaneously. The consumption of CH4 acceler­
ated at 350 ◦ C and reached a maximum at 550 ◦ C. Then the reaction
declined gradually and elevated again at 783 ◦ C. It must be pointed out
that small amount of CO2 (located at 339 ◦ C) and CO (in the range of
325− 800 ◦ C) were concurrently produced with the decomposition of
CH4. Considering that there was no O2 in the feed, the reactive oxygen
species on the catalyst surface, which reacted with carbon species from
dissociation of CH4, were responsible for production of CO2 and CO.
In order to study adsorption and activation of CO2, CO2-TPSR pro­
files of the reduced Ni0.05Al1O2-δ catalyst are shown in Fig. 10b. the
desorption of CO2 started from 140 ◦ C, and the dissociation of CO2 to CO
accelerated at 700 ◦ C and reached a maximum at 799 ◦ C. The desorption
of CO2 in TPSR process should come from the pre-absorbed CO2 at the
low temperature, which was proved by the experiment of CO2 pulse at
50 ◦ C (shown in Fig. S11a). In addition, CO2-TPSR of the coked
Ni0.05Al1O2-δ is shown in Fig. 10c. CO2 involved in eliminating carbon
species on the coked Ni0.05Al1O2-δ was less than 400 ◦ C, while large
amount of CO was formed in the temperature range of 400− 700 ◦ C with
a maximum at 492 ◦ C, which indicated that CO2 had the ability to
eliminate carbon species.
Fig. S11b shows CH4-TPSR profiles of Ni0.05Al1O2-δ with preadsorption of CO2. Compared with CH4-TPSR profiles, a small con­
sumption of CH4 and desorption of CO2 at 100 ◦ C were observed in
Fig. S11b, speculating that the pre-adsorption of CO2 presumably
facilitated the adsorption of CH4 at low temperature. Compared TPSR
experiments with different gas atmospheres, it can be inferred that DRM
reaction over NixAl1O2-δ is initiated by decomposition of CH4, and then
CO2 participates in the reaction pathway by reacting with active carbon
species from CH4 decompostion to avoid carbon accumulation.
Pulse experiments were performed at 800 ◦ C and 650 ◦ C over
Ni0.05Al1O2-δ catalyst to investigate effects of the individual surface
species and further propose the reaction mechanism. Fig. 11a shows the
results of CH4 pulse at 800 ◦ C over Ni0.05Al1O2-δ. CH4 was completely
consumed within the initial ten pulses, simultaneously, H2, CO, and a
trace amount of CO2 were detected as the gas-phase products. The yields
of these products gradually diminished with the increase of pulse
numbers, accompanied by the decrease in the consumption of CH4. It is
suggested that CH4 is dissociated into CHx (x = 0–3) and H2 over nickel
active sites, followed by reacting with active oxygen species to form CO
mainly. With consumption of these oxygen species, the formation of CO
falls, and the conversion of CH4 decreases continuously as the carbon
species are accumulated on the catalyst surface. Since there is no oxygen
in the feed of CH4 pulse, the only oxygen source may come from the
surface OH groups or/and adsorbed H2O on the interface between Ni
active sites and support, where the surface oxygen is activated with
atomic hydrogen spilled from Ni active sites [78,79]. The results of CH4
pulse at 650 ◦ C is shown in Fig. S11c, in which the evolution of CH4, H2,
CO and CO2 were similar to those of CH4 pulse at 800 ◦ C, except for the
shorter period of pulse signals. The yields of H2, CO and CO2 decreased
sharply over the initial four pulses. It indicated that surface oxygen
species were insufficient to react with surface carbon species at the low
temperature of 650 ◦ C. In addition, the results of CO pulse at 650 ◦ C
(shown in Fig. S11d) showed that CO could decompose into CO2 and C
species, which provided another way to produce carbon species on the
catalyst surface. Therefore, the surface Ni active sites were quickly
Fig. 11. Pulse reaction of Ni0.05Al1O2-δ catalyst at 800 ◦ C: (a) CH4 pulse of the
reduced Ni0.05Al1O2-δ catalyst with 15 % CH4/Ar, (b) CO2 pulse of the reduced
Ni0.05Al1O2-δ catalyst with 1 % CO2/He, (c) CO2 pulse of the coked
Ni0.05Al1O2-δ catalyst (coking by CH4 at 800 ◦ C for 60 min). Red squares in (a)
represented CH4 pulse at room temperature. Red squares in (b) represented CO2
pulse at room temperature.
the temperature range of 200− 800 ◦ C. Two distinctive peaks of CO and
CO2 were identified with the former being surface adsorbed oxygen
species (200− 550 ◦ C), while the latter being the easily migrated surface
lattice oxygen or surface oxygen species tightly trapped by Ov on
NiAl2O4 (550− 800 ◦ C) [24,25,77], since the latter was not observed
over Al2O3-Gel (Fig. 9e). Fig. 9f shows carbon oxidation under CO2 flow
as a function of temperature. The carbon oxidation activity under the
flowing CO2 was improved in the following order: Ni0.5Al1O2 >
13
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Applied Catalysis B: Environmental 291 (2021) 120074
Fig. 12. In-situ DRIFTS spectra of (a) CH4 or/and CO2 adsorbed on the reduced Ni0.05Al1O2-δ catalyst at 50 ◦ C; (b) Temperature-programmed decomposition of CH4;
(c) Temperature-programmed activation of CO2; (d) Adsorption and activation of CO2 at 300 ◦ C, followed by introduction of CH4 or H2 at 150 ◦ C; (e) Temperatureprogrammed reaction of CH4 + CO2 on the reduced Ni0.05Al1O2-δ catalyst from 50 to 650 ◦ C.
covered by carbon accumulation, leading to the rapid decline in the
conversion of CH4 at 650 ◦ C.
Fig. 11b shows the results of CO2 pulse at 800 ◦ C over Ni0.05Al1O2-δ.
Ni0.05Al1O2-δ catalyst had a limited dissociation ability of CO2, mean­
while, a trace amount of H2 balanced dynamically on the reduced
catalyst surface was consumed. It indicated that H adatom took part in
the CO2 activation process. When the CO2 pulse experiment was carried
out over the catalyst of Ni0.05Al1O2-δ coked by CH4 (Fig. 11c), the
dissociation of CO2 was enhanced by reacting with C/CHx species,
meanwhile, CO2 conversion and CO generation decreased gradually
14
S. Zhang et al.
Applied Catalysis B: Environmental 291 (2021) 120074
along with consumption of C/CHx species during the subsequent pulses.
It was obvious that H species created by the dynamic balance (CHx →
CHx-1 + H*, x = 1/2/3; 2H* → H2) participated in CO2 activation and
consumed sequentially by subsequent pulses.
In situ DRIFTS measurements were performed to study the active
intermediates formed by CH4 or/and CO2 adsorption, as shown in
Fig. 12. For CH4 adsorption over the reduced Ni0.05Al1O2-δ catalyst
(Fig. 12a, b), the IR band appeared at 3016 cm− 1 was attributed to
absorbed CH4, and the bands at 1270 and 1300 cm− 1 corresponded to
gaseous CH4 [60,80–82]. Simultaneously, the IR band appeared at 1354
cm− 1 was assigned to the symmetric deformation vibrations of adsorbed
CH3 species [83], and the bands at 1331 and 1342 cm− 1 were belonged
to the deformation vibration of CHx species. With an increase in tem­
perature, the intensity of gaseous methane decreased slowly, due to the
decomposition of CH4. For CO2 adsorption over the reduced
Ni0.05Al1O2-δ catalyst (Fig. 12a, c), the IR bands at 1431, 1543 and 1655
cm− 1 were characteristic of monodentate and bidentate carbonates over
Ni0.05Al1O2-δ, respectively [60]. The bands at 1376 and 1597 cm− 1
corresponded to the formate species [84]. The intensity of formates
decreased rapidly with an increase in temperature, while the intensity of
carbonates decreased slowly until it vanished at 500 ◦ C. After CH4 or H2
was introduced to the catalyst surface with pre-adsorption of CO2
(Fig. 12d), the carbonates (1431, 1543, 1655 cm− 1) were depleted along
with the increase of the intensity of formates (1376, 1597 cm− 1),
demonstrating that the surface carbonates were converted to formates
assisted by the spilled hydrogen. In addition, the IR bands in Fig. 12c
arose at 3626/3599 cm− 1 and 3726/3707 cm− 1 were assigned to OH
groups on the catalyst surface [84], which were formed by reaction of H
species with carbonates and decomposition of formate species. When
CO2 and CH4 were introduced together (Fig. 12a, black line), the in­
tensities of carbonates, formates, OH groups and CHx species increased
in comparison with only introduction of CH4 or CO2. It indicates that the
adsorption/activation of methane and carbon dioxide can be mutually
reinforced. In comparison with CO2 adsorption, the frequencies of sur­
face OH group species formed by coadsorption of CO2 and CH4 varied
from 3726 and 3707 cm− 1 to 3730 and 3703 cm− 1, respectively (the
inset in Fig.12a). This shift may be caused by presence of hydrogen
bonding interaction between H species of adsorbed CH4 and O species of
surface hydroxy groups. With raising the temperature, the adsorbed
CHx, carbonates, formates and OH group species on the surface were
consumed by reacting with each other (Fig. 12e). Notably, the bands at
1640 and 3000− 3700 cm− 1 corresponded to the absorbed water were
detected dramatically above 450 ◦ C due to RWGS reaction [85].
To sum up, in situ DRIFTS results reveal that DRM reaction proceeds
via adsorption and activation of CH4 and CO2 on the surface of
NixAl1O2-δ catalysts, i.e., via a Langmuir-Hinshelwood mechanism as
reported by the reference [86]. It is generally accepted that CH4 can be
dissociated on the surface metallic Ni active sites through thermal
decomposition, forming adsorbed CHx and H species. Simultaneously,
CO2 activation could be evoked via formation of carbonates and for­
mates adsorbed on the support or the interface between metallic Ni
active sites and the support with the help of H species.
On the basis of the results obtained by the above-mentioned char­
acterizations, the elementary reaction steps outlined in [Eqs. (5)–(15)]
seem appropriate to describe the reaction mechanism of DRM reaction
over NixAl1O2-δ catalysts:
Fig. 13. Schematic diagram of reaction mechanism of DRM reaction over
NixAl1O2-δ catalyst.
Step 5: CO2* + H* → HCOO* H-assisted CO2 activation
Step 6: HCOO* → CO* + OH*
(10)
Step 7: CHx* + OH* → CO* + H* Oxidation of CHx fragments
(11)
Step 8: H* + H* → H2(g) Desorption of H2
(12)
Step 9: CO* → CO(g) Desorption of CO
(13)
Step 10: OH* + H* → H2O*
(14)
Step 11: H2O* → H2O(g) Desorption of H2O
(15)
Herein, * denotes an adsorption site on the catalyst surface, Ov
represents the oxygen vacancy on NiAl2O4. The schematic diagram of
reaction mechanism of DRM reaction over NixAl1O2-δ catalyst is pro­
posed in Fig. 13.
4. Conclusions
A series of NixAl1O2-δ mesoporous catalysts with surface NiAl2O4
spinel phase were synthesized by the citric acid sol-gel method, in which
the metallic Ni nanoparticle size can be properly controlled by varying
Ni content in the preparation step of sol-gel process. Small-size Ni
nanoparticles, obtained by partial reduction from NiAl2O4 spinel in
NixAl1O2-δ catalysts with low Ni contents at high temperature, can
effectively inhibit the carbon formation from kinetics, while the irre­
ducible NiAl2O4 counterpart can participate in the elimination of carbon
deposition. The constructed structure of Ni◦ -NiAl2O4 interfaces pro­
duced by the exsolution of Ni from NiAl2O4 spinel is responsible for its
high long-term stability and excellent resistance to coking and sintering
for DRM reaction at 800 and 650 ◦ C.
This work indicates that CH4 dissociation on Ni active sites is a
structure-sensitive reaction due to the volcanic curve of the plot of
TOFCH4 versus Ni nanoparticle size over NixAl1O2-δ catalysts. The results
of kinetics and activation energy show that there is a kinetic compen­
sation effect (KCE) between the activation energy (Ea) and the preexponential factor (A) in DRM reaction over NixAl1O2-δ catalysts. CH4TPSR and CH4-Pulse demonstrate that highly reactive oxygen species are
present on NixAl1O2-δ surface. CO2-TPSR and CO2-Pulse of the coked
catalysts indicate that CO2 activation can be assisted by H species
stemmed from CH4 dissociation. In-situ DRIFTS and designed experi­
ments reveal that DRM reaction over NixAl1O2-δ catalysts proceeds via a
Langmuir-Hinshelwood mechanism accompanied by an additional
redox mechanism, in which the active oxygen species generated by
filling the oxygen vacancies of NiAl2O4 spinel by CO2 provides another
rapid redox route to eliminate carbon species. This work provides a deep
insight into the specific role of NiAl2O4 phase, its structure evolution and
reaction mechanism of DRM reaction over Ni-Al2O3 catalyst system,
Step 1: CH4(g) → CHx* + (4-x)H*, x = 0–3 CH4 dissociation on metal Ni
active sites
(5)
Step 2: CO2 + Ov → CO* + O* Oxygen vacancy promoted CO2 activation(6)
Step 3: CHx* + O* (NiAl2O4) → H* + CO* (Ov on NiAl2O4) CHx oxidation
(7)
Step 4: CO2(g) → CO2* CO2 adsorption as carbonates
(9)
(8)
15
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S. Zhang et al.
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CRediT authorship contribution statement
Shuangshuang Zhang: Conceptualization, Methodology, Data
curation, Investigation, Writing - original draft. Ming Ying: Methodol­
ogy, Validation. Jun Yu: Supervision, Writing - review & editing.
Wangcheng Zhan: Validation, Visualization. Li Wang: Formal analysis.
Yun Guo: Resources, Funding acquisition. Yanglong Guo: Supervision,
Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
This work was supported by the National Key Research and Devel­
opment Program of China (2016YFC0204300), the National Natural
Science Foundation of China (21808142), and the Fundamental
Research Funds for the Central Universities.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.apcatb.2021.120074.
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