Activity study of NiO-based oxygen carriers in chemical looping steam methane reforming

Article abstract of DOI:10.1016/j.cattod.2015.10.027

This study evaluates the performance of NiO-based oxygen carriers supported on ZrO₂, TiO₂, SiO₂, Al₂O₃ and NiAl₂O₄ as conventional reforming catalysts at low temperature (650 °C)

Full text of DOI:10.1016/j.cattod.2015.10.027

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Contents lists available at ScienceDirect
Catalysis Today
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Activity study of NiO-based oxygen carriers in chemical looping steam
methane reforming
Andy Antzaraa, Eleni Heracleousb,c, Lishil Silvesterd, Dragomir B. Bukurd,e
,
Angeliki A. Lemonidoua,c,∗
a Department of Chemical Engineering, Aristotle University of Thessaloniki, University Campus, University Box 425, 54124 Thessaloniki, Greece
b School of Science & Technology, International Hellenic University (IHU), 14th km Thessaloniki–Moudania, 57001 Thessaloniki, Greece
c Chemical Process and Energy Resources Institute, Center of Research and Technology Hellas, CPERI/CERTH, 6th km Charilaou-Thermi Road, P.O. Box 361,
57001 Thessaloniki, Greece
d Chemical Engineering Program, Texas A and M University at Qatar, P.O. Box 23874, Doha, Qatar
e Artie McFerrin Department of Chemical Engineering, Texas A and M University, 3122 TAMU, College Station, TX 77843, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 July 2015
Received in revised form 7 October 2015
Accepted 19 October 2015
Available online xxx
This study evaluates the performance of NiO-based oxygen carriers supported on ZrO2, TiO2, SiO2, Al2O3
and NiAl2O4 as conventional reforming catalysts at low temperature (650 ◦C) as well as oxygen transfer
materials (OTMs) for chemical looping steam methane reforming. Conventional reforming experiments
with pre-reduced OTMs demonstrated satisfactory performance of NiO/Al2O3, NiO/ZrO2 and NiO/NiAl2O4
at 650 ◦C, with less than 8% relative decrease in conversion after 10 h time-on-stream. On the other
hand, NiO/SiO2 and NiO/TiO2 were found to have low activity (<50% initial CH4 conversion) and deacti-
vated rapidly, eventually dropping to less than 10% CH4 conversion after 2 h on stream. The three most
promising materials were tested under chemical looping steam methane reforming conditions for twenty
consecutive redox cycles in a fixed bed flow unit. NiO/ZrO2 exhibited good activity with initial CH4 con-
version higher than 80% and had very good stability. Deactivation was minimal after 20 consecutive redox
cycles, corresponding to 20 h exposure to CH4/steam. NiO/Al2O3 and NiO/NiAl2O4 demonstrated similar
high initial activity, but also high deactivation, leading to methane conversion of 59 and 63% conversion
respectively at the end of the test.
Keywords:
Oxygen transfer materials
Nickel oxide
Support
Nickel aluminate
Alumina
Silica
Titania
Zirconia
© 2015 Elsevier B.V. All rights reserved.
Chemical looping steam methane reforming
Hydrogen
1. Introduction
strongly endothermic reforming reaction [2]. It becomes therefore
a necessity to introduce new environmentally friendly, low-energy
Hydrogen is an important raw material with wide use in the
chemical and petrochemical industry. The current industrial major
technology for large scale production of hydrogen is steam reform-
ing of natural gas, which although a mature process, still remains
very energy intensive. This thermodynamically limited reaction is
industrially operated at temperatures higher than 800 ◦C in the
presence of supported nickel-based catalysts [1]. High operating
temperatures negatively affect the economic and environmental
performance of the process, due to requirements for expensive
materials such as high alloy nickel–chromium steel for the tubu-
lar reformer, irreversible carbon formation in the reactor, high
energy consumption and CO2 emissions resulting from demands of
consumption technologies with high hydrogen efficiency. In this
context, research efforts have focused on new concepts for hydro-
gen production that operate at milder conditions and have lower
carbon footprint, such as membrane assisted reforming [3–5],
sorption enhanced reforming (SER) [6–9] and chemical looping
reforming (CLR) [10–15].
Chemical looping methane reforming provides a novel route
mainly for syngas production. It is based on the same principles
as chemical looping combustion (CLC) [16–18] where the fuel is
oxidized by oxygen provided by a solid oxygen transfer material
(OTM) instead of direct mixing with air. The chemical looping sys-
tem mainly consists of two interconnected fluidized bed reactors
(an air reactor and a fuel reactor) with the oxygen transfer material
circulating between the two. The OTM, typically a metal oxide, is
reduced by the fuel in the first reactor and is then reoxidized by
air in the second reactor. In the case of chemical looping reform-
ing, the CH4/OTM ratio is kept high in order to favor partial rather
∗
Corresponding author at: Department of Chemical Engineering, Aristotle Uni-
versity of Thessaloniki, University Box 425, 54124 Thessaloniki, Greece.
E-mail address: alemonidou@cheng.auth.gr (A.A. Lemonidou).
http://dx.doi.org/10.1016/j.cattod.2015.10.027
0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Antzara, et al., Activity study of NiO-based oxygen carriers in chemical looping steam methane
reforming, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.10.027

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than total oxidation of the fuel, leading to CO/H2 production. In
order to shift the product of the reaction to H2, a hybrid chemical
reforming process with steam or CO2 addition has been proposed.
In this case, the hydrogen yield increases due to the additional
occurrence of reforming reactions in the fuel reactor. This how-
ever also increases the thermal demands of the process – since the
reforming reactions are endothermic – and therefore low H2O/C
or CO2/C ratios are preferred [19]. Taking this a step further, the
combination of chemical looping steam methane reforming with
sorption enhanced reforming in a single process has been sug-
gested to further enhance hydrogen production, reduce the energy
requirements of the process and produce a ready for sequestration
CO2 stream [19,20]. In the combined process, the OTM is mixed
with a CaO-based CO2 sorbent material that captures in situ the
produced CO2 and drives the equilibrium of reactions to the product
side, resulting in increased H2 yield and purity. The exothermic CaO
carbonation provides in situ the necessary heat for the endother-
mic reactions in the fuel reactor, while the heat released by the
highly exothermic OTM reoxidation in the second reactor is uti-
lized for regeneration of the saturated CO2 sorbent, minimizing
the total requirements of the overall process. Therefore in order
to efficiently transfer the heat from the OTM to the sorbent and
maintain a low amount of solids circulating between the two reac-
tors, which results in reduction to the overall operating cost of this
novel process, the OTM should have a high enough Ni loading. Our
recent thermodynamic-based calculations on the combined sorp-
tion enhanced chemical looping steam reforming process showed
that a decrease of up to 55% of energy demands is possible com-
pared to conventional steam reforming [20].
One of the challenges in chemical looping processes is the iden-
tification of effective oxygen transfer materials. A suitable oxygen
carrier should be able to undergo multicycle redox reactions at
intermediate to high operating temperatures and have sufficiently
high oxygen transport capacity, high mechanical strength, low
tendency for fragmentation, attrition and agglomeration and low
production cost. Different metal oxides have been proposed in lit-
erature as possible candidates, such as CuO, NiO, Fe2O3 and CoO
[21–25]. NiO appears to be the most promising for reforming pro-
cesses, due to the excellent catalytic properties of reduced Ni for
steam methane reforming and water gas shift reactions [24,26,27].
Even though NiO has a high oxygen transfer capacity, it exhibits
poor re-oxidation over multiple reduction and oxidation cycles
in bulk form due to nickel agglomeration, which affects its cyclic
performance and renders it unsuitable for chemical looping appli-
cations. Therefore, NiO is commonly supported on refractory oxides
for increased stability. Among the supporting inert materials, Al2O3
has received the highest attention due to its high thermal and
mechanical strength and favorable fluidization properties [28–30].
However in NiO–Al2O3 systems there is always formation of mixed
NiAl2O4, a resilient phase that reduces at temperatures above
850 ◦C, “neutralizing” a significant fraction of NiO. De Diego et al.
[26] reported results from evaluation of Ni-based OTMs supported
on different types of alumina in a TGA and in a batch fluidized bed
reactor unit. It was shown that the type of alumina (␣-, ␥- and
␪-Al2O3) affects the reducibility of NiO by methane, with NiO on
␥-Al2O3 demonstrating the lowest activity due to extensive for-
mation of NiAl2O4. Better results were obtained with the lower
surface areas ␣- and ␪-aluminas. Only a few studies have utilized
other types of supports, such as SiO2, TiO2 or ZrO2 [22,29,31,32].
In the previous study [33], we performed preliminary evalua-
tion of NiO supported on alumina and zirconia as potential OTMs
for chemical looping steam methane reforming. NiO supported
on ZrO2 exhibited very stable redox behavior during twenty CH4
reduction/air oxidation cycles with almost complete reduction of
NiO to metallic Ni in every cycle (94%). In contrast, a low degree
of reduction was obtained initially with NiO/Al2O3 (51%) which
however gradually increased to 85% during the first eight cycles
and then remained stable during the remaining twelve cycles. It
was postulated that the cyclic reduction/oxidation of the material
leads to restructuring of the surface and reduction of a NiAl2O4
species that form during re-oxidation of the reduced OTM. In this
work, we extend our study by investigating the effect of additional
three supports (SiO2, TiO2 and NiAl2O4), focusing on extensive
testing of all materials in a fixed bed flow unit under conventional
reforming and multiple chemical looping steam methane reform-
ing cycles. Reaction tests have been complemented by detailed
characterization of physicochemical properties of the OTMs before
and after reaction to identify crucial properties that determine
their performance in the targeted reaction.
2. Experimental
2.1. Preparation of oxygen carriers
NiO oxygen carriers with a constant loading of 40 wt% of NiO
supported on commercial Al2O3, SiO2, TiO2 and ZrO2 (Saint Gobain-
NORPRO) and one lab-synthesized NiAl2O4 were prepared via wet
impregnation. For the preparation of the NiAl2O4 support, a co-
precipitation method was followed. Stoichiometric amounts of
Ni(NO3)2·6H2O (Merck) and Al(NO3)3·9H2O (Panreac) were dis-
solved in distilled H2O and were stirred at room temperature until
a transparent green solution was obtained. A 1 M NH4OH solution
was added dropwise until the pH reached ∼8 and the precipita-
tion occurred. The precipitate particles were then filtered, washed
with distilled water and dried overnight at 100 ◦C. Calcination of
the dried powder was performed at 900 ◦C for 11 h in air flow.
For the preparation of the NiO-supported OTMs, the supports
were first crushed and sieved in order to obtain a particle size of
100 < dp < 500 ␮m. Nickel nitrate (Ni(NO3)2·6H2O, Merck) was used
as a precursor salt. The required amount of precursor in order to
achieve 40 wt% NiO loading was dissolved in de-ionized water and
was mixed with the appropriate amount of support in a round bot-
tom flask. The mixture was heated slowly in a rotary evaporator
for 1 h under stirring at 70 ◦C. The solvent was then evaporated at
80–85 ◦C under reduced pressure. The solids were dried overnight
at 100 ◦C and were calcined at 650 ◦C for 4 h in air flow.
2.2. Characterization of oxygen carriers
Surface areas and pore volumes of the samples were determined
by N2 adsorption at 77 K, using the multipoint BET analysis method,
with an Autosorb-1 Quantachrome flow apparatus. Prior to the
measurements, the samples were dehydrated in vacuum at 250 ◦C
overnight.
X-ray diffraction (XRD) patterns were obtained with a Siemens
D500 diffractometer and Cu-K␣ radiation with radiation wave-
length of 0.15406 nm. An aluminum holder was used to support
the samples during the measurements. The intensity data were col-
lected over a 2ꢀ range of 5–80◦ with a step of 0.04◦ and a scanning
rate of 2 s/point. The crystallite size of metallic Ni in used samples
was determined from the XRD patterns, using the Debye–Scherrer’s
equation:
0.64 ∗ ꢁ
D =
(1)
(ˇ − ˇ0)[cos(ꢀ)]
where D is the crystallites size (nm), ꢁ the radiation wavelength
(Cu-K␣, ꢁ = 0.15406 nm), ˇ is the full width at half of the maxi-
mum of the peak (radians), ˇ0 is an appropriate correction for the
peak broadening due to the diffractometer itself (ˇ0 ≈ 0.129◦) and
ꢀ is the angular position of the peak. Fitting of the peaks was per-
formed using Lorentzian function. In order to have more accurate
results only the two high intensity peaks of Ni at 44.4◦ and 51.8◦
Please cite this article in press as: A. Antzara, et al., Activity study of NiO-based oxygen carriers in chemical looping steam methane
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Table 1
3
Textural properties of the five 40 wt% NiO OTMs and the pure supports.
BET surface area (m2/g)
Pore volume (cm3/g)
Al2O3
40% NiO/Al2O3
ZrO2
40% NiO/ZrO2
SiO2
40% NiO/SiO2
TiO2
40% NiO/TiO2
NiAl2O4
40% NiO/NiAl2O4
203
117
93
22
100
55
39
16
81
50
0.58
0.33
0.28
0.15
0.47
0.27
0.31
0.15
0.27
0.19
flow controllers and are pre-mixed before entering the reactor. An
Ultra-Fast Liquid Chromatography (UFLC) pump (Shimadzu) is used
for admission of distilled water (steam) to the reactor through a
pre-heater. The fixed bed quartz reactor (10 mm internal diameter),
with a coaxial thermocouple for temperature monitoring, is heated
electrically by a tubular furnace, with three independently con-
trolled temperature zones. A fritted quartz disk placed in the center
of the reactor is used to hold the catalytic bed. Gaseous products
exiting the reactor are cooled to condensate the unreacted steam
and the gas phase products are analyzed with an online gas chro-
matograph (Agilent Technologies, 7890A) equipped with thermal
conductivity detector. Two columns (Molecular Sieve and Poraplot)
in series by pass configuration are used for the analysis of the outlet
gas stream. The Molecular Sieve column is used for separation of
H2, CH4 and CO, while the Poraplot column is used for detection
of CO2. The CO and CO2 concentrations in the reactor exit are also
monitored by a gas analyzer (Madur, Mamos-200).
For the evaluation of the conventional steam reforming activity
of the OTMs, about 50 mg of material diluted with a predetermined
amount of quartz particles was loaded in the reactor. The amount
of diluent varied between 55 and 75% of the weight of the OTM,
based on the specific density of each material, in order to achieve a
catalytic bed of constant length over all materials and a Gas Hourly
Space Velocity of 100,000 h−1. The sample was initially heated to
650 ◦C under He flow, followed by in situ reduction in 33% H2/He
for 1 h at 650 ◦C in order to obtain the metallic form of nickel. After
the reduction, reforming was performed by switching to CH4 and
steam flow, with S/C molar ratio of 3. The reaction was conducted
isothermally at 650 ◦C for 10 h.
Fig. 1. Schematic layout of the fixed bed bench-scale flow unit.
2ꢀ positions were used, while the low intensity Ni peak at 76.4◦
was excluded from the calculations of the average crystallite size
in order to minimize the errors related to non-proper fitting.
H2-temperature programmed reduction was performed for all
materials in an AutoChem II 2920 unit equipped with a thermal
conductivity detector (TCD). Prior to the measurements, the sam-
ples were pre-treated in air up to 650 ◦C to ensure that they were in
fully oxidized form. Temperature was then lowered to room tem-
perature (RT) in He flow. Temperature programmed reduction from
RT to 1000 ◦C was performed with a heating rate of 10 ◦C/min under
10% H2/Ar flow and maintaining the final temperature of 1000 ◦C
for 30 min.
Temperature Programmed Oxidation (TPO) experiments were
performed in a specially designed lab scale flow unit coupled with
a mass analyzer (Omnistar GSD 320) in order to determine the
amount of coke deposited on the materials after reaction. The used
sample was loaded in a U-shaped quartz reactor, equipped with a
coaxial thermocouple for temperature monitoring. The sample was
initially pretreated for 30 min at 250 ◦C under He flow followed by
cooling to room temperature. TPO was performed under a flow of
20% O2/He with a heating rate of 10 ◦C/min from RT to 900 ◦C. The
outlet gas composition was monitored online with the mass spec-
trometer analyzer. Quantitative analysis of the evolved CO2 was
based on (m/z) 44.
In order to determine the surface oxidation state of Ni after
reforming, the NiO oxygen carriers were characterized by XPS.
Characterization of the fresh samples after reduction in H2 at 650 ◦C
for 1 h was also conducted for comparison. The XPS measurements
were performed with a Kratos Analytical AXIS ultra DLD instru-
ment equipped with a monochromatic Al X-ray source. XPS spectra
were recorded with pass energy 20 eV, step 0.1 eV and X-ray power
150 W. Deconvolution of the peaks was performed with a Kratos
Vision software (Version 2.2.10) using Shirley background subtrac-
tion and mixed Gaussian–Lorentzian functions. Binding energies
were referenced to C 1s at 284.6 eV.
For the evaluation of the chemical looping steam reforming
activity, about 50 mg of material diluted with a predetermined
amount of quartz particles (in order to maintain the Gas Hourly
Space Velocity constant at 100,000 h−1) was loaded in the reactor.
The materials were initially exposed to CH4/steam in their oxidized
state for 1 h at 650 ◦C (S/C ratio of 3). During the first 10 min of the
reduction/reforming step a small external H2 flow (5% of the total
flow) was added in order to facilitate NiO reduction. After 1 h, the
temperature of the reactor was raised to 850 ◦C under He flow and
then the feed was switched to air for re-oxidation for 20 min and
the cycle was then repeated.
The performance of the OTMs is expressed in terms of CH4 con-
version (XCH4 ) which was calculated with the following equation:
COout + CO2,out
XCH (%) =
× 100
(2)
4
CH4,out + COout + CO2,out
2.3. Evaluation of conventional and chemical looping steam
reforming activity in a flow unit
3. Results and discussion
The evaluation of the NiO-OTMs’ conventional and chemical
looping steam reforming activity was performed at atmospheric
pressure in a bench scale laboratory flow unit. The unit consists
of liquid and gas feed section, a fixed bed reactor and the product
analysis section (Fig. 1). The incoming gases are controlled by mass
3.1. Characterization of oxygen carriers
The surface area and pore volume of the NiO supported oxy-
gen carriers, along with those of the pure supports, are presented
in Table 1. Upon impregnation of the supports with NiO, both the
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40% NiO/Al2O3
A: NiO
Al2O3 support
A: NiO
B: γ-Al2O3
40% NiO/ZrO2
ZrO2 support
A
C: NiAl2O4
A
A
A
A
A
C
A
C
A
B
B
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2θ (°)
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2θ (°)
40% NiO/TiO2
TiO2 support
40% NiO/SiO2
SiO2 support
A: NiO
B: NiTiO3
A: NiO
A
A
A
B
B
A
A
B
A
B
B
A
A
B
B
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2θ (°)
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2θ (°)
40% NiO/NiAl2O4
NiAl2O4 support
A: NiO
B: NiAl2O4
A
A
A
B
B
B
A
B
B
B
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2θ (°)
Fig. 2. XRD patterns of 40% NiO supported OTMs and respective pure supports.
surface area and the pore volume decreased. Considering the high
NiO loading (40 wt%), it can be deduced that for the Al2O3, SiO2
and NiAl2O4 supported OTMs the decrease in surface area is almost
exclusively due to the replacement of the high surface area of the
supports with NiO. In the case of zirconia and titania supports, the
reduction in surface area is much higher, indicating blockage of the
pores by NiO and/or poor stability of the supports under the high
temperature calcination conditions.
The X-ray diffraction patterns of calcined OTMs and the respec-
tive as received pure supports are shown in Fig. 2(a)–(e). In the
case of the alumina supported OTM, the presence of both NiO
and the mixed nickel aluminate (NiAl2O4) crystal phases is evi-
dent, indicating a strong interaction between alumina and nickel
oxide. Similar results are obtained for titania, where formation
of NiTiO3 is observed. On the contrary, only the pure NiO crys-
tal phase is detected when ZrO2 and SiO2 are used as supports.
Therefore, interaction between these refractory oxides and nickel
oxide can be characterized as rather weak. The diffractogram of
the synthesized NiAl2O4 support displays high intensity peaks of
the pure nickel aluminate phase, indicating successful preparation.
The highly crystalline nature of this support differentiates NiAl2O4
from the nearly amorphous alumina support, as well as the other
low crystallinity refractory oxides used. The NiO/NiAl2O4 oxygen
transfer material exhibits clear diffractions attributed to NiO, in
addition to those of NiAl2O4.
The reducibility of the NiO oxygen carriers supported on differ-
ent oxides was investigated by H2 temperature programmed reduc-
tion and the TPR profiles are presented in Fig. 3. NiO/Al2O3 exhibits
two main peaks with a maximum at 322 ◦C and 729 ◦C. The first
peak is ascribed to the reduction of NiO without or with low interac-
tion with the support to Ni0 metal phase [34,35]. The second peak at
∼730 ◦C is attributed to the reduction of nickel in NiAl2O4 [36–38],
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5
100
90
80
70
60
50
40
30
20
10
0
CH equilibrium conversion
4
NiO/Al2O3
NiO/ZrO2
NiO/SiO2
NiO/TiO2
NiO/Al2O3
NiO/ZrO2
NiO/SiO2
NiO/TiO2
NiO/NiAl2O4
0
50 100 150 200 250 300 350 400 450 500 550 600
Time (min)
NiO/NiAl2O4
NiAl2O4
Fig. 4. Methane conversion as a function of time for supported NiO oxygen carriers
(T = 650 ◦C, S/C ratio = 3, GHSV = 100,000 h−1).
the “free” NiO with the individual supports as well as to different
extent of sintering of nickel oxide at the high calcination tempera-
ture, which may significantly account for the shift of the maximum
reduction temperature of these NiO species [41].
100 200 300 400 500 600 700 800 900 1000
Temperature (°C)
Fig. 3. H2 temperature-programmed reduction profiles for the five NiO-OTMs and
NiAl2O4 support.
3.2. Performance evaluation in conventional steam methane
reforming
confirming the XRD findings that indicate that part of NiO is bound
in the nickel aluminate phase. Compared with the TPR profile of the
as-synthesized pure NiAl2O4 support, nickel aluminate on alumina
reduces at ∼70 ◦C lower temperature. This could be due to the co-
existence of nickel aluminate with “free” NiO in the NiO/Al2O3 OTM.
Li and Chen [38] observed that in a series of NiO/Al2O3 catalysts the
reduction of nickel aluminate shifted to lower temperatures with
increasing NiO loading. This was attributed to the presence of more
easily reducible NiO species that may facilitate nucleation of other
Ni nuclei that could originally interact strongly with the support,
thus enhancing the reduction of these more strongly interacting
NiO species. On the contrary the NiAl2O4 peak in the TPR profile of
the NiO/NiAl2O4 OTM appears at higher temperature (∼780 ◦C). In
addition, the shift of the NiO reduction peak to higher temperature
and the shoulder at 625 ◦C indicate a stronger interaction between
free NiO and the NiAl2O4 compared to ␥-Al2O3 support. The reduc-
tion profile of NiO/TiO2 presents similar features, with two low
temperature peaks at 362 and 394 ◦C, attributed to NiO reduction
that has different degree of interaction with the titania surface and
a high temperature one ascribed to the reduction of bulk NiTiO3 to
Ni0 [39,40]. In the presence of TiO2, NiO seems to reduce at a higher
temperature than on Al2O3 (394 ◦C vs 322 ◦C). NiTiO3 on the other
hand is reduced at lower temperatures than NiAl2O4, pointing out
to a weaker interaction of NiO with TiO2. This is also supported by
the fact that the peak attributed to the reduction of free NiO has
higher intensity in TiO2 than in Al2O3 and shows that more NiO
is reducible at low temperatures on titania. NiO/SiO2 demonstrates
only one large reduction peak with a maximum at 415 ◦C due to the
reduction of NiO to Ni metal. Although the XRD of NiO/ZrO2 showed
the presence only of NiO, the TPR profile presents two clear reduc-
tion peaks at 394 ◦C and 474 ◦C. The reduction peak at low tempera-
ture corresponds to bulk NiO with low interaction with the support,
while the reduction peak at higher temperature could be attributed
to the fixed NiO interacting with (but not chemically bonded to) the
ZrO2 support. In summary, the temperature for free NiO reduction
to metallic Ni0 decreases in the order; SiO2 > ZrO2 = TiO2 > Al2O3.
These differences could be attributed to the different interaction of
3.2.1. Activity testing
For an oxygen carrier to be suitable for use in chemical loop-
ing steam methane reforming, it is a prerequisite for the material
to exhibit good reforming activity in its reduced form. Therefore,
the materials were pre-reduced in situ and were then tested as
conventional reforming catalysts in steam methane reforming at
650 ◦C with S/C ratio of 3. It should be noted that the developed
oxygen transfer materials were tested at relatively low tempera-
ture (650 ◦C), as we intend to use them in the combined chemical
looping reforming process with in situ CO2 capture with CaO-based
sorbents [20,42]. Based on a recent thermodynamic analysis of
the sorption enhanced chemical looping reforming process per-
formed by our group [20], the optimum temperature for effective
CO2 capture by CaO-based sorbents is ∼650 ◦C. In Fig. 4, methane
conversion is plotted as a function of time-on-stream for the five
oxygen carriers. The Ni catalysts supported on titania and silica
have low activity; initial CH4 conversion is low (less than 50% CH4
conversion) and decreases rapidly, eventually dropping below 10%
CH4 conversion after less than 2 h on stream.
On the other hand, NiO/ZrO2, NiO/Al2O3 and NiO/NiAl2O4 have a
satisfactory activity, with high initial methane conversion ranging
from 80 to 87%. Both NiO/Al2O3 and NiO/ZrO2 were relatively stable
with approximately 8% relative decrease in conversion after 10 h on
stream. NiO/NiAl2O4 OTM had lower initial activity (∼80% conver-
sion) compared to NiO/Al2O3, but it exhibited the highest stability
with almost no deactivation. After 10 h of testing it had the same
conversion as NiO/Al2O3 OTM. It seems that the use of NiAl2O4 as
support stabilizes the surface metallic nickel species. During the
reduction of the OTM by H2, two types of metallic nickel can be
formed: “free” metallic nickel by the reduction of NiO and fixed
monodispersed nickel species produced by the partial reduction of
NiAl2O4. These latter species consist of nickel particles located in
the nickel aluminate structure that are stabilized by the surround-
ing environment, preventing sintering and reoxidation that could
lead to the loss of activity [43].
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NiO/Al2O3
NiO/ZrO2
NiO/SiO2
NiO/TiO2
NiO/NiAl2O4
100 200 300 400 500 600 700 800 900
Temperature (°C)
Fig. 6. Temperature-programmed oxidation profiles of spent catalysts.
time-on-stream throughout the entire experiment for all supported
NiO-OTMs. On the other hand methane concentration increases
with time for NiO/ZrO2 and NiO/Al2O3, reflecting the slow decrease
of methane conversion with time-on-stream, whereas it remains
stable for NiO/NiAl2O4 which exhibited the highest stability. The
opposite trend is observed for H2 concentration; as methane
conversion decreases due to catalyst deactivation, hydrogen pro-
duction decreases as well.
3.2.2. Post-reaction characterization
The pure reforming experiments demonstrated that the sup-
port has a strong influence on the performance of NiO OTMs. The
use of ZrO2, Al2O3 and NiAl2O4 as supports led to the synthesis
of OTMs with high catalytic activity and stability in conventional
methane reforming to hydrogen with satisfactory stability, while
on the other hand TiO2 and SiO2 supports demonstrated low initial
methane conversion and very high deactivation rate. Possible rea-
sons for the high deactivation could be either partial re-oxidation
of the metallic Ni phase to NiO by steam, extensive coke depo-
sition [44,45] or significant sintering of the Ni particles on the
carrier surface [46,47]. To investigate the above possibilities, post-
characterization of the NiO supported samples after the reforming
reaction was performed with TPO (for coke determination), XRD
(for determining the Ni crystal size) and XPS (for Ni oxidation state
determination).
TPO profiles of the spent catalysts are presented in Fig. 6, while
the amount of deposited coke and the rate of carbon production
expressed in moles of C per mol of CH4 in the inlet of the reactor
are given in Table 2. All materials exhibited two CO2 peaks, indi-
cating the formation of two types of carbon: one oxidized at low
temperatures (300 ◦C), probably corresponding to absorbed CHx or
oligomeric carbon that can be easily removed by H2 or steam under
reaction conditions, and one at higher temperatures correspond-
ing to the oxidation of polymeric carbon [47–49]. NiO/TiO2 and
Fig. 5. Product concentration as a function of time for (a) NiO/ZrO2, (b) NiO/Al2O3
and (c) NiO/NiAl2O4 (T = 650 ◦C, S/C ratio = 3, GHSV = 100,000 h−1).
Analysis of the reaction products as a function of time on stream
is presented only for the three most active catalytic materials.
The composition of the outlet dry product stream from methane
reforming over NiO/ZrO2, NiO/Al2O3 and NiO/NiAl2O4 is shown
in Fig. 5(a)–(c) respectively. High concentration of hydrogen, in
the order of 75–80% and near the equilibrium, is observed with
all materials. The concentration of CO remains in all cases at
low levels, while CO2 concentration is in 10–12% range. The low
levels of CO, much lower than the equilibrium predicted concen-
tration, indicate that the materials are also active in the water
gas shift reaction. As shown in Fig. 5(a)–(c), the concentration
of CO and CO2 in the outlet stream of the reactor is stable with
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Table 3
7
Table 2
Carbon deposition and rate of carbon production for the five spent NiO-OTMs.
Crystal size of reduced fresh and spent NiO/SiO2 and NiO/TiO2 calculated from XRD
patterns.
Time on
wt% C on
catalyst
Rate of C production [mol
C(s)/mol CH4 in] (%)
stream (h)
Sample
Crystal size (nm)
40% NiO/SiO2
40% NiO/TiO2
40% NiO/ZrO2
40% NiO/Al2O3
40% NiO/NiAl2O4
2
2
10
10
20
0.003
0.025
0.020
0.087
0.231
0.49
0.56
0.07
0.29
0.39
40% NiO/SiO2
Fresh
Used
40% NiO/TiO2
Fresh
Used
28.0
41.4
38.7
48.1
NiO/ZrO2 also had small peaks at very low temperature (<200 ◦C),
corresponding to loosely bound carbon, however they are much
smaller than the higher temperature peaks. The relative amount of
each type of carbon varies greatly depending on the support used.
In the case of NiO/Al2O3, NiO/ZrO2 and NiO/NiAl2O4 the main CO2
peak occurs at temperature around 600 ◦C. On the other hand, the
amount of easily removed carbon is much higher over NiO/TiO2,
while over NiO/SiO2 the two peaks are comparable.
Comparing the total amount of carbon deposited on the catalyst
surface (Table 2), very low values were observed for all samples,
with NiO/NiAl2O4 demonstrating the highest amount of formed
coke (0.231 wt%). As the exposure of each material to reaction con-
ditions (CH4/steam atmosphere) was different, carbon deposition
values are also presented as the rate of coke formation per unit
of CH4 feed. The corrected carbon formation rates show that Ni
supported on SiO2 and TiO2 exhibits the highest rates of carbon
production, followed by Ni on NiAl2O4, Al2O3 and finally ZrO2.
Even though the use of SiO2 and TiO2 support seems to enhance
the deposition of carbon on the catalyst surface, the differences
are not significant enough to provide explanation for their poor
performance in the reforming reaction.
reaction, was calculated form XRD patterns. Comparing the average
crystal size of fresh and used catalysts (Table 3), a small increase
is observed probably due to the agglomeration of nickel crystal-
lites during the reforming reaction. However the extent is very low
to justify the differences in performance among the nickel OTMs
supported on different oxides.
Reoxidation of Ni in the presence of excess steam at rela-
tively low temperature reforming conditions has been previously
reported [50,51]. X-ray Photoelectron Spectroscopy was employed
to determine whether the origin of fast deactivation for the mate-
rials supported on titania and silica is the re-oxidation of metallic
Ni to inactive-to-reforming NiO by steam during the reaction. XPS
characterization was performed on a representative low stability
material, NiO/TiO2, fresh pre-reduced in H2 and after the reform-
ing experiments. The same characterization was also performed
on NiO/ZrO2 samples, reduced and after reaction, to be used as
benchmark, considering that this material exhibited high activ-
ity and stability in the reforming reaction. The Ni 2p spectra of
fresh NiO/TiO2 and NiO/ZrO2 in reduced form are presented in
Fig. 7a and b respectively. Two main peaks centered on 852.2 eV and
854.7 eV are observed in the 2p3/2 transition of nickel on NiO/TiO2,
corresponding to metallic Ni and NiO respectively. The satellite
peak of NiO also appears at ∼659.5 eV [52,53]. Similar peaks are
In order to detect whether Ni agglomeration occurs during
reforming, the crystal size of metallic nickel on NiO/SiO2 and
NiO/TiO2, fresh after reduction in H2 and used after the reforming
2+ Ni0
Ni
2+ Ni0
Ni
(b) NiO/ZrO2_ fresh-reduced
(a) NiO/TiO2_ fresh-reduced
884 880 876 872 868 864 860 856 852 848
Binding Energy(eV)
884 880 876 872 868 864 860 856 852 848
Binding Energy(eV)
2+ Ni0
Ni
2+ Ni0
Ni
(c) NiO/TiO2_ used
(d) NiO/ZrO2_ used
884 880 876 872 868 864 860 856 852 848
Binding Energy(eV)
884 880 876 872 868 864 860 856 852 848
Binding Energy(eV)
Fig. 7. X-ray photoelectron spectra of Ni 2p transition of the fresh reduced NiO/TiO2 (a) and NiO/ZrO2 (b) and the NiO/TiO2 (c) and NiO/ZrO2 (d) after conventional reforming
experiments.
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100
values are very similar to what was attained during the conven-
CH equilibrium conversion
tional reforming experiments, proving that nickel oxide can indeed
be reduced in CH4/steam atmosphere, yielding a metallic nickel
surface of similar reforming activity with that obtained with hydro-
gen pre-reduction. However, the stability of OTMs was different
under chemical looping conditions. Whereas NiO/NiAl2O4 was the
most stable during conventional reforming, NiO/ZrO2 exhibited by
far higher stability (less than 2% relative conversion decrease) than
either NiO/Al2O3 or NiO/NiAl2O4 in the chemical looping steam
methane reforming tests. Deactivation of the two latter materials
is similar and equal to ∼23% activity loss after 20 consecutive redox
cycles, corresponding to 20 h of exposure to CH4/steam. This loss
of activity could be attributed to low thermal stability of alumina
and nickel aluminate supports when exposed to high temperature
of re-oxidation (850 ◦C) for extended periods of time [55].
The reformate gas concentration during a representative redox
cycle (5th cycle) are presented in Fig. 9(a), (b) and (c) for NiO/ZrO2,
NiO/Al2O3 and NiO/NiAl2O4 respectively. Symbols refer to data
acquired from the gas chromatograph. CO and CO2 concentrations
recorded on-line by the gas analyzer are also presented in the same
figures with continuous lines. A sharp evolution of the reformate
gas directly after start-up indicates that the NiO readily reduces
with a very fast rate, even in the presence of steam, over all sup-
ports. It also shows that the reforming reactions are catalyzed
immediately as soon as a small amount of metallic nickel forms. The
hydrogen that is produced from the reforming reactions helps fur-
ther in completing the NiO reduction and maintaining the oxygen
carriers in reduced form for the entire duration of the reform-
ing cycle. Steady state conditions in the reactor are achieved after
∼20 min of reaction time. High hydrogen concentrations (70–76%)
are observed for all tested OTMs. Moreover, it is important to note
that no CO2 evolution was observed during the re-oxidation step
(in any cycle) for the ZrO2, Al2O3 or NiAl2O4 supported NiO, indi-
cating that the use of high S/C ratio employed prevents carbon
formation. Based on studies by Cho et al. [57] carbon formation
on Ni-based particles can be hampered by adding 50 vol% steam to
the fuel.
Comparison of our results with literature is difficult since most
of the studies in chemical looping reforming employed very low S/C
ratios [11,27,30,58] or used only CH4 as feed. Dupont et al. [58] per-
formed chemical looping steam methane reforming experiments at
800 ◦C with a S/C ratio of 1.8 using NiO-based OTMs with Ni loading
ranging from 12.6 to 39.2% and observed high CH4 conversion lev-
els (99%) during the whole duration of the 10 min reforming step.
These conversion levels are higher than the ones recorded in this
study (80–85%), mainly due to thermodynamic limitations at the
lower reforming temperature employed in our study.
The results from the evaluation of NiO/ZrO2, NiO/Al2O3 and
NiO/NiAl2O4 in cyclic chemical looping steam methane reform-
ing experiments indicate that these oxygen carriers have good
potential for use in sorption enhanced chemical looping reform-
ing, especially NiO/ZrO2 OTM. Similar performance as that attained
during conventional reforming can be achieved with addition-
ally good stability in alternating reforming/oxidation conditions.
Process wise, the implementation of the sorption enhanced chem-
ical looping reforming with these materials will lead to a process
with significantly lower thermal demands than the conventional
reforming process (up to 55% [20]), as the reforming reaction is
performed at lower temperature and part of the required heat is
generated by the OTM’s reoxidation. Moreover, CO2 is captured
and separated in one-step, significantly simplifying the reforming
process which is composed of the reformer, plus water gas shift
reactors and separation processes. The actual impact of replacing a
conventional reforming process with a sorption enhanced chemical
looping reforming process in terms of process complexity and eco-
nomic figures can only be assessed via a detailed techno-economic
4
90
80
70
60
50
40
30
20
10
0
NiO/Al2O3
NiO/ZrO2
NiO/NiAl2O4
0
2
4
6
8
10
12
14
16
18
20
Number of cycle (-)
Fig. 8. Methane conversion during 20 reduction-reforming/oxidation cycles for NiO
supported on Al2O3, ZrO2 and NiAl2O4 (Reduction/reforming: T = 650 ◦C, S/C ratio = 3,
GHSV = 100,000 h−1, duration: 1 h, Oxidation: T = 850 ◦C, air flow, duration: 15 min).
also detected in the Ni 2p spectrum of NiO/ZrO2, shifted however
to slightly higher binding energies (853 eV, 855.3 eV and 860.1 eV
respectively), implying a different electronic density of nickel on
the surface of zirconia. Even though these samples were reduced
prior to XPS measurements, peaks corresponding to NiO appear in
the spectra with 54% and 36% contribution to the total nickel con-
tent in NiO/TiO2 and NiO/ZrO2 respectively. This indicates that part
of surface Ni is re-oxidized by ambient air, as the XPS measurements
were performed ex situ.
The corresponding XPS spectra of the used NiO/TiO2 and
NiO/ZrO2 after the conventional reforming experiments are pre-
sented in Fig. 7c and d. For NiO/TiO2, the comparison with the
spectrum of the fresh catalysts in reduced form clearly portrays
that in the used materials more than 90% of surface nickel appears
in its oxidized state. This shows that metallic Ni is almost com-
pletely re-oxidized by steam during the reaction, leading to poor
performance and fast deactivation. On the other hand, nickel on
ZrO2 seems to remain in its reduced state after reaction, with only
∼45% of Ni being re-oxidized which is comparable to the part of
Ni in the oxidized state in the fresh reduced sample (36%). The re-
oxidation of nickel to nickel oxide by steam could also be a reason of
deactivation of the NiO/SiO2 sample [54,55]. In addition, it is known
that silica is very unstable in the presence of steam which leads in
hydrolysis and total restructuring of the support, resulting in large
decrease of pore volume and surface area [56]. Unfortunately, it
was not possible to measure the BET surface area and pore volume
of the used samples due to the low quantity of sample employed in
the conventional reforming experiments.
3.3. Chemical looping reforming experiments
Based on the evaluation of the OTMs under conventional reform-
ing conditions, NiO/Al2O3, NiO/ZrO2 and NiO/NiAl2O4 in reduced
form appear suitable for reforming, exhibiting high methane con-
version and satisfactory stability. These materials were therefore
selected for further testing under chemical looping steam methane
reforming conditions, starting with the oxygen carriers in oxidized
form. The OTMs chemical looping activity was evaluated during 20
redox cycles in a bench scale fixed bed flow unit.
Fig. 8 presents the conversion of CH4 as a function of redox cycles
for the three supported NiO oxygen carriers. All three OTMs exhibit
high initial CH4 conversion, ranging between 78 and 87%. These
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9
100
30
27
24
21
18
15
12
9
Post-characterization of the used materials with XPS showed that
the most probable reason for deactivation is oxidation of metal-
lic nickel by steam to nickel oxide on the titania support, whereas
nickel on NiO/ZrO2 remains in reduced state.
CH4/ steam
He
He
Air
90
80
70
60
50
40
30
20
10
0
H2
NiO/ZrO2, NiO/Al2O3 and NiO/NiAl2O4 were further tested
under chemical looping steam methane reforming conditions for
20 consecutive cycles in a fixed bed flow unit. All three OTMs
exhibited high initial CH4 conversion, ranging between 78 and
87%, similar to that attained during conventional reforming. This
result is important as it proves that nickel oxide can indeed be
reduced in CH4/steam atmosphere, yielding a metallic nickel sur-
face of similar reforming activity as that obtained with hydrogen
pre-reduction. Moreover, the products’ concentration profile dur-
ing the reforming step of the cycle showed that the reformate
gas evolves almost simultaneously with the introduction of the
CH4/steam feed, indicating that reforming reactions occur as soon
as some metallic nickel is formed on the surface. In terms of sta-
bility, NiO/ZrO2 exhibited an excellent stability with less than 2%
relative decrease in conversion after 20 consecutive cycles. Deac-
tivation of NiO/Al2O3 and NiO/NiAl2O4 was higher and equal to
∼23% decrease in conversion for both OTMs. This loss of activity
can be possibly attributed to low thermal stability of alumina and
nickel aluminate supports when exposed to the high temperature
of re-oxidation (850 ◦C) for extended periods of time.
CO2
a
CO
CH4
6
3
0
0
0
0
10 20 30 40 50 60 70 80 90 100 110 120
Time (min)
100
90
80
70
60
50
40
30
20
10
0
30
27
24
21
18
15
12
9
CH4/ steam
H2
He
He
Air
CO2
b
CH4
6
Acknowledgments
CO
3
This research was made possible by a grant (NPRP Grant No.
5-420-2-166) from the Qatar National Research Fund (a member
of Qatar Foundation). The statements made herein are solely the
responsibility of the authors. Dr. P. Patsalas and Mr. N. Pliatsikas
from the Physics Department of the Aristotle University of Thessa-
loniki are gratefully acknowledged for the XPS measurements.
0
10 20 30 40 50 60 70 80 90 100 110 120
Time (min)
100
90
80
70
60
50
40
30
20
10
0
30
27
24
21
18
15
12
9
CH4/ steam
H2
He
He
Air
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In this work, NiO-based oxygen carriers supported on ZrO2,
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Please cite this article in press as: A. Antzara, et al., Activity study of NiO-based oxygen carriers in chemical looping steam methane
reforming, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.10.027