Methane Combustion Over Ni/CexZr1–xO2 Catalysts: Impact of Ceria/Zirconia Ratio

Article abstract of DOI:10.1002/cctc.202000947

A series of Ce_xZr_1–xO₂ (x=1, 0.83, 0.17, 0) supports and 2 wt.% Ni/Ce_xZr_1–xO₂ catalysts were synthesized and evaluated for CH₄ oxidation. The Ce_xZr_1–xO₂ supports showed moderate activity (T₅₀=519-638 °C) with CO by-product formation. Nickel incorporation onto Ce_xZr_1–xO₂ lowered the T₅₀ to 438–477 °C and eliminated the formation of CO. The results indicated that moderate Zr doping improved the support oxygen storage capacity (OSC) and reducibility of the catalyst at low temperatures (H₂-TPR) leading to an enhanced CH₄ combustion turnover frequency: Ni/ZrO₂0.17Zr_0.83O₂20.83Zr_0.17O₂. The apparent activation energies of Ni/Ce_xZr_1–xO₂ catalysts (87-92 kJ/mol) were much lower than the ones of Ce_xZr_1–xO₂ supports and Ni/quartz sand (111-120 kJ/mol) indicating that CH₄ oxidation is facilitated when Ni is deposited on Ce_xZr_1–xO₂. Moreover, stability tests with H₂O and CO₂ showed that Zr doping can improve the catalyst stability, while H₂O had a stronger reversible inhibition effect than CO₂ for CH₄ combustion.

Full text of DOI:10.1002/cctc.202000947

The European Society Journal for Catalysis
Supported by
Accepted Article
Title: Methane combustion over Ni/CexZr1-xO2 catalysts: impact of
ceria/zirconia ratio
Authors: Junjie Chen, Benjamin D. Carlson, Todd J. Toops, Zhenglong
Li, Michael J. Lance, Stavros Karakalos, Jae-Soon Choi, and
Eleni Kyriakidou
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To be cited as: ChemCatChem 10.1002/cctc.202000947
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01/2020

10.1002/cctc.202000947
ChemCatChem
FULL PAPER
Methane combustion over Ni/CexZr1-xO2 catalysts: impact of
ceria/zirconia ratio
Junjie Chen,[a] Benjamin D. Carlson,[a] Todd J. Toops,[b] Zhenglong Li,[b] Michael J. Lance,[c] Stavros G.
Karakalos,[d] Jae-Soon Choi,[b,1] and Eleni A. Kyriakidou*[a]
[a]
J. Chen, B. D. Carlson, Prof. E. A. Kyriakidou
Department of Chemical and Biological Engineering
University at Buffalo, The State University of New York
Buffalo, NY 14260, USA
E-mail: elenikyr@buffalo.edu
[b]
[c]
[d]
[1]
Dr. T. J. Toops, Dr. Z. Li, Dr. J.-S. Choi
Energy and Transportation Sciences Division
Oak Ridge National Laboratory
Oak Ridge, TN 37831, USA
Dr. M. J. Lance
Materials Science and Technology Division
Oak Ridge National Laboratory
Oak Ridge, TN 37831, USA
Dr. S. G. Karakalos
Department of Chemical Engineering
University of South Carolina
Columbia, SC 29208, USA
Present address: Catalyst Development Center, Basic Materials and Chemicals R&D, LG Chem, Daejeon, Korea.
Supporting information for this article is given via a link at the end of the document.
Abstract: A series of CexZr1-xO2 (x = 1, 0.83, 0.17, 0) supports and 2
CH4 is the most difficult hydrocarbon to activate due to its
four highly stable and symmetric C-H bonds; therefore, its
complete combustion has always been a challenge.[6] Platinum
group metals (PGMs) are widely regarded to have the best activity
for activating this C-H bond and completely oxidizing CH4.[7]
Supported palladium (Pd) catalysts, in particular Pd/Al2O3,[8]
wt.% Ni/CexZr1-xO2 catalysts were synthesized and evaluated for CH4
oxidation. The CexZr1-xO2 supports showed moderate activity (T50
=
519-638 °C) with CO by-product formation. Nickel incorporation onto
CexZr1-xO2 lowered the T50 to 438-477 °C and eliminated the formation
of CO. The results indicated that moderate Zr doping improved the
support oxygen storage capacity (OSC) and reducibility of the catalyst
at low temperatures (H2-TPR) leading to an enhanced CH4
combustion turnover frequency: Ni/ZrO2 < Ni/Ce0.17Zr0.83O2 < Ni/CeO2
< Ni/Ce0.83Zr0.17O2. The apparent activation energies of Ni/CexZr1-xO2
catalysts (87-92 kJ/mol) were much lower than the ones of CexZr1-xO2
supports and Ni/quartz sand (111-120 kJ/mol) indicating that CH4
oxidation is facilitated when Ni is deposited on CexZr1-xO2. Moreover,
stability tests with H2O and CO2 showed that Zr doping can improve
the catalyst stability, while H2O had a stronger reversible inhibition
effect than CO2 for CH4 combustion.
[9]
Pd/CeO2 and Pd/ZrO2,[10] are considered to be highly active
catalysts for the complete CH4 combustion at temperatures as low
as 400 °C.[11]
Despite the promising CH4 combustion
performance of supported Pd catalysts, alternatives are actively
being sought due to its high cost. Nickel (Ni), an earth abundant
and cost-effective metal, has shown high activity towards CH4
activation, as evidenced by its use in CH4 reforming reactions,
including dry,[12] steam[13] and autothermal reforming.[14] However,
one of the main challenges when using Ni supported on SiO2 and
Al2O3 catalysts is that they can easily be deactivated by the
deposition of carbonaceous materials, e.g. coke.[15] Coke
deposition on metallic Ni commonly occurs due to lack of readily
available oxygen species that highlights the need for efficient
oxygen activation at the catalytic sites.[16] Recent studies showed
that combining nickel with reducible oxide supports with high
oxygen mobility can facilitate the oxidation and removal of
Introduction
As the push for alternative energy sources grows, natural gas has
become a promising resource due to its abundance and since it
is a cleaner burning fuel compared to traditional fossil fuels.
Recent discoveries of shale gas reserves,[1] advances in hydraulic
drilling[2] and a subsequent decline in natural gas prices[3] have
led to wide utilization of natural gas in the power generation and
transportation industries.[4] However, the main component of
natural gas, methane (CH4), has 32 times higher 100-year global
warming potential than that of CO2.[5] Thus, unburnt CH4
emissions from combustion engines or turbines have the potential
to offset any positive environmental impacts gained from utilizing
natural gas. Therefore, minimizing CH4 emissions has recently
attracted significant attention.
carbonaceous
intermediate
species,
preventing
coke
formation.[17] Ceria (CeO2), as a metal oxide support, is able to
readily store and release oxygen species because it contains
oxygen vacancies in its lattice structure formed by the Ce4+/Ce3+
[19]
redox pair.[18] However, the weak thermal stability of CeO2
drastically limits its practical application in CH4 combustion
reactions, that require temperatures up to 400-700 °C. Doping
tetravalent elements into the CeO2 lattice, e.g. Ti4+, Hf4+, Rf4+, Zr4+,
has been reported to improve its thermal stability as well as
increase its oxygen storage capacity (OSC) by creating more
oxygen vacancies in the CeO2 lattice.[20] Tetravalent zirconium
has been reported to be one of the most promising additives for
1
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10.1002/cctc.202000947
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ceria structures, since its ionic radius is slightly smaller than Ce4+ Results and Discussion
(Ce4+: 0.97 Å; Zr4+: 0.84 Å ), which can create additional defects
both in the surface and bulk phase of CeO2.[21]
Textural and structural characterization
Although Ni/CexZr1-xO2 catalysts have been used in various
CH4 reforming reactions,[22] limited research has been conducted
on their use in the complete combustion of CH4. Pengpanich et
al.[23] used CeO2-ZrO2 mixed oxides as catalysts and showed that
the CH4 oxidation performance depended on the ceria/zirconia
ratio and the catalyst reducibility. However, there was no specific
pattern observed between the catalyst redox properties and the
CH4 oxidation performance. Combining NiO and ceria-zirconia
supports improved the support catalytic activity. For example,
Thaicharoensutcharittham et al.[24] showed that loading Ni onto
Ce0.75Zr0.25O2 reduced the T50 (temperature that 50% CH4
conversion is achieved) of the catalyst by 110 °C for the CH4
combustion reaction. This behavior was attributed to the
enhanced reducibility induced by incorporation of Ni onto
Ce0.75Zr0.25O2. Moreover, a recent study showed that the T50 of
NiO/Ce0.75Zr0.25O2 was ~100 °C lower than the T50 of
MgO/Ce0.75Zr0.25O2, attributed to the higher oxygen mobility of the
NiO/Ce0.75Zr0.25O2 catalyst.[25] However, there is no detailed study
on the effect of the support ceria/zirconia ratios on the catalyst
redox properties (OSC and reducibility) and thus the CH4
combustion activity.
The N2 adsorption/desorption isotherms of the as-synthesized
CexZr1-xO2 (x = 1, 0.83, 0.17, 0) supports calcined at 450 C are
shown in Fig. 1. Both pure CeO2 and ZrO2 have a type IV
isotherm with an H1-type hysteresis loop, which can be attributed
to the presence of large inter-particle mesopores.[27] Ceria-
zirconia solid solutions (Ce0.83Zr0.17O2, Ce0.17Zr0.83O2) showed a
type IV isotherm with an H2-type hysteresis loop, indicating the
existence of a mesoporous texture and ink-bottle shaped
pores.[28] The BET surface area and BJH pore volume of the as-
synthesized CexZr1-xO2 supports calcined at 450 C and 600 C
are listed in Table 1. After calcination at 450 C, CeO2,
Ce0.83Zr0.17O2, Ce0.17Zr0.83O2 and ZrO2 had similar surface areas
(89, 88, 88 and 102 m2/g, respectively). The pore volume of CeO2
was 0.19 cm3/g, whereas Ce0.83Zr0.17O2, Ce0.17Zr0.83O2 and ZrO2
had similar pore volumes of about 0.08 cm3/g. The CexZr1-xO2
supports were also calcined at 600 C for 5 h to investigate their
thermal stability. The BET surface areas and BJH pore volumes
of all CexZr1-xO2 supports decreased after calcination at 600 C,
which can be attributed to the increase of their crystal sizes at
high temperatures.[29] However, the decrease in surface area
observed over ceria-zirconia solid solutions (31-45%) was lower
compared to the decrease in surface area over pure CeO2 (76%)
and ZrO2 (65%). Moreover, Ce0.83Zr0.17O2 showed the smallest
decrease in surface area (31%) after calcination at 600 °C
compared to all studied supports. This behavior indicates that
zirconia doping can improve the thermal stability of ceria supports
by preventing the growth of ceria crystallites at 600 C.[30]
Another challenge for CH4 combustion reactions is catalyst
deactivation via coke formation or metal sintering due to exposure
to high temperatures (~400-700 °C).[26] Deactivation of Ni/CeO2
has been previously observed after CH4 combustion reaction.[26a,
26b]
For example, Zhang et al.[26a] showed that CeO2 nanorods in
Ni/CeO2 catalysts significantly sintered after reacting at 600 °C,
which was attributed to the low thermal stability of the CeO2
nanorods. Sintering of CeO2 after exposure to high temperatures
was also reported by Pakulska et al.[26b] In this study, the T50 of
Ni/CeO2 increased by 250 °C after calcination at 900 °C/4 h,
attributed to loss of the CeO2 surface area by 86%. Thus,
developing catalysts that are thermally stable is crucial for
avoiding catalyst deactivation.
While the Ni/CexZr1-xO2 catalytic system has shown
promising performance, a detailed study is needed to more
completely describe the reactivity and the mechanism. In this
study we establish the key kinetic measurements and material
effects which we will follow-up with a detailed mechanistic
discussion and model proposal. A series of CexZr1-xO2 (x = 0,
0.17, 0.83, 1) supports and Ni/CexZr1-xO2 catalysts were
evaluated towards the combustion of CH4 in order to determine
the effect of the catalyst redox properties on the CH4 combustion
performance. Moreover, the impact of Zr doping on the thermal
stability of the Ni/CexZr1-xO2 catalysts was examined in three
consecutive cycling light-off tests. The inhibition effect of H2O and
CO2 as well as the stability of Ni/CexZr1-xO2 catalysts at 450 C
were also evaluated.
Figure 1. N2 adsorption (solid) /desorption (open) isotherms of CexZr1-xO2
(x = 0, 0.17, 0.83, 1) supports.
Table 1. BET surface area and BJH pore volume of CexZr1-xO2 (x = 0, 0.17, 0.83, 1) supports after calcination at 450, 600 C for 5 h.
Calcination at 450 C (5 h) Calcination at 600 C (5 h)
Surface area Pore volume Surface area Pore volume
(m2/g) (cm3/g) (m2/g) (cm3/g)
Sample
CeO2
Lattice parameters
(Å)
Surface
area
decrease (%)
89
0.19
21
0.045
76
5.41
5.39
5.19
5.13
Ce0.83Zr0.17O2
Ce0.17Zr0.83O2
88
88
0.078
0.071
61
48
0.070
0.068
31
45
ZrO2
102
0.088
36
0.084
65
2
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CexZr1-xO2 samples decreased with increasing Zr content as
shown in Table 1. Considering that Zr4+ (0.84 Å) has a smaller
ionic radius than Ce4+ (0.97 Å), the decrease of the lattice
parameters with increasing Zr content can be attributed to the
successful incorporation of Zr into the CeO2 lattice.[32] The 2 wt.%
Ni/CexZr1-xO2 catalysts showed the same XRD pattern with the
corresponding CexZr1-xO2 supports. Peaks attributed to Ni oxides
or Ni0 formation were not observed, indicating that either Ni is well
dispersed or its size is smaller than the detection limit of the XRD
instrument used.[33]
The incorporation of Zr into the Ce lattice was also
evidenced by the Raman spectra of CexZr1-xO2 supports (Fig. S1).
The main peak at 463 cm-1 of pure CeO2 is attributed to the F2g
stretching mode of the fluorite structure. The peaks of pure ZrO2
centered at 335, 380, 476, 536, 558, 619 and 634 cm-1 indicated
the existence of monoclinic phase of ZrO2.[34] In the Ce0.83Zr0.17O2
sample, the F2g peak shifted to 466 cm-1 and its intensity
decreased compared to pure CeO2, attributed to the disruption of
the lattice structure due to incorporation of Zr4+.[35] The intensity
of the peak at 604 cm-1 increased slightly compared to the same
peak in pure CeO2 indicating the generation of oxygen vacancies
by Zr4+ doping. Four main peaks appeared at 248, 315, 458, 623
cm-1 over Ce0.17Zr0.83O2 indicating that it has a cubic phase.[36]
Further insights into the structure and morphology of
Ni/CexZr1-xO2 were obtained by HAADF STEM imaging and EDS
mapping (Fig. 3). EDS mapping of Ce and Zr containing samples
(Figs. 3 (a.2, b.2, c.2, b.3, c.3, d.2)) shows that ceria and zirconia
are well dispersed. Phase separation was not observed that
further proved that zirconia was doped into the ceria lattice
homogeneously. Moreover, Figs. 3 (a.3, b.4, c.4, d.3) show that
Ni is well dispersed over all synthesized samples, in agreement
with the XRD results. Formation of well dispersed Ni species can
Figure 2. XRD patterns of 2 wt.% Ni/CexZr1-xO2 (x = 0, 0.17, 0.83, 1) and CexZr1-
xO2 samples.
be attributed to their stabilization by CexZr1-xO2 supports.
A
similar stabilization effect was observed when Pt and Au were
supported on ceria based supports for the water-gas shift
reaction.[37]
Redox properties of CexZr1-xO2 supports and Ni/CexZr1-xO2
catalysts
The redox properties CexZr1-xO2 and 2 wt.% Ni/CexZr1-xO2
samples were studied by H2-TPR (Fig. 4) and O2 pulse
chemisorption (Table 2). The H2-TPR profile of pure CeO2 (Fig.
4(a)) has two main peaks centered at 467 C and 718 C. The
low temperature peak (467 °C) is assigned to the reduction of
Ce4+ on the surface and sublayer which is more accessible to gas
phase H2, while the high temperature peak (718 C) is attributed
to the reduction of Ce4+ in the bulk phase.[38] Pure ZrO2 showed
no H2 consumption at temperatures below 500 C and a low
intensity H2 consumption peak centered at ~665 C. Doping
zirconia into the ceria lattice resulted in significant changes in the
H2 consumption profiles. Specifically, the high temperature
reduction peak of Ce0.83Zr0.17O2 (689 °C) shifted to lower
temperatures compared to pure CeO2 (718 °C), while the low
temperature reduction peak (493 °C) shifted to higher
temperatures compared to CeO2 (467 C). These results indicate
the improved reducibility of bulk Ce4+ in the Ce0.83Zr0.17O2 support,
which leads the surface/sublayer Ce4+ and bulk Ce4+ reduction
peaks to move towards each other as a result of Zr4+ doping. A
Figure 3. HAADF STEM images and EDS elemental maps of (a.1-3) 2 wt.%
Ni/CeO2, (b.1-4) 2 wt.% Ni/Ce0.83Zr0.17O2, (c.1-4) 2 wt.% Ni/Ce0.17Zr0.83O2 and
(d.1-3) 2 wt.% Ni/ZrO2 catalysts.
X-ray powder diffraction spectroscopy was used to examine
the crystallinity of CexZr1-xO2 supports and 2 wt.% Ni/CexZr1-xO2
catalysts (Fig. 2). The lattice parameters of the CexZr1-xO2
supports are listed in Table 1. The X-ray diffraction pattern of
CeO2 contained four prominent diffraction peaks at 2θ = 28.5°,
33.1°, 47.5°, 56.3° that correspond to (111), (200), (220), (311)
planes, respectively. These XRD diffraction peaks demonstrate
that the as-synthesized CeO2 has the same cubic fluorite
structure with space group FM-3m.[31] Pure ZrO2 contains both
monoclinic and tetragonal phases as denoted by the peaks at 2θ
= 24.3°, 28.2°, 31.4°, 40.8°, 50.1° (monoclinic phase) and 2θ =
30.3°, 34.7°, 44.8° (tetragonal phase).[23] The XRD patterns of the
mixed oxides Ce0.83Zr0.17O2 and Ce0.17Zr0.83O2 do not have the
same peaks as pure ZrO2, while their peaks are slightly shifted to
higher angles compared to those of pure CeO2. This behavior
indicates that single phase ceria-zirconia solid solutions were
formed during synthesis. Moreover, the lattice parameters of the
3
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Table 2. OSC and OSCC (μmol/g) of CexZr1-xO2 (x = 0, 0.17, 0.83, 1) supports
at 450 C.
Sample
OSC
OSCC
CeO2
160.8
231.1
139.3
0.7
162.6
302.1
163.3
3.0
Ce0.83Zr0.17O2
Ce0.17Zr0.83O2
ZrO2
improve the mobility of oxygen species, resulting in an increased
OSC, OSCC.[43] Further increasing the Zr concentration from
Ce0.83Zr0.17O2 to Ce0.17Zr0.83O2 and pure ZrO2 resulted in a
decrease in Ce4+/Ce3+ reduction pairs and a decrease in OSC at
450 C from 231 to 139 to 0.7 μmol/g, respectively. Similar to the
OSC, the OSCC increased with Zr doping, followed by a decrease
for higher Zr concentrations. The OSCC values were always
higher than the OSC values that can be explained by the diffusion
of oxygen species from the surface to the bulk support phase
leaving oxygen vacancies on the surface that can be further
titrated during OSCC measurements.[39]
Figure 4. H2-TPR of (a) CexZr1-xO2 (x = 0, 0.17, 0.83, 1) supports, (b) 2 wt.%
Ni/CexZr1-xO2 (x = 0, 0.17, 0.83, 1) catalysts and 2 wt.% Ni/quartz sand.
small shoulder at 324 C can be assigned to the most reactive
surface oxygen species on the Ce0.83Zr0.17O2 surface.[39] A single
reduction peak at 558 C is present over Ce0.17Zr0.83O2, which is
attributed to the merging of the surface and bulk phase reduction
peaks.[40]
The H2-TPR profiles of Ni/CexZr1-xO2 catalysts are shown in
Fig. 4(b). The reducibility of pure NiO was also evaluated over a
2 wt.% Ni/quartz sand sample, since quartz sand is non-reducible.
Thus, the peak at 293 C can be assigned to NiO reduction. The
reduction of all Ni supported on Ce containing catalysts started at
lower temperatures compared to the Ni/quartz sand sample. For
instance, 2 wt.% Ni/CeO2 started consuming H2 at ~130 C, that
is 137 C lower than pure CeO2. The sharp peak at 214 C can
be attributed to the reduction of well dispersed NiO
nanoparticles.[26a] The shoulder peak at 253 C can be attributed
to the reduction of active surface or sublayer oxygen species. The
high temperature peak at 724 C is similar to the high temperature
peak present in pure CeO2, and thus attributed to the reduction of
bulk Ce4+. The first reduction peak of all Zr doped catalysts was
at higher temperatures compared to 2 wt.% Ni/CeO2. The low
temperature reduction peak of 2 wt.% Ni/Ce0.83Zr0.17O2 (235 C) is
assigned to NiO reduction, while the higher temperature reduction
peak (329 C) can be assigned to the reduction of sublayer Ce4+.
A single H2 consumption peak at 293 C present in 2 wt.%
Ni/Ce0.17Zr0.83O2 catalyst corresponds to the reduction of NiO and
the surface/bulk Ce4+. A different reduction profile compared to
the Ce containing catalysts was observed for 2 wt.% Ni/ZrO2,
where two broad reduction peaks appeared at ~401 and 585 C.
The reduction peak of Ni oxide was shifted to a higher
temperature (401 °C) compared to Ni supported on Ce containing
catalysts, attributed to the strong Ni-ZrO2 interactions.[41] The high
temperature reduction peak (585 C) corresponds to the 665 C
peak in the pure ZrO2 support. The improved reducibility of ZrO2
in the Ni/ZrO2 catalyst compared to the pure ZrO2 support is
attributed to Ni loading. Comparison of the reducibility of
Ni/CexZr1-xO2 catalysts with their corresponding supports shows
that the Ni/CexZr1-xO2 catalysts are reduced at lower temperatures,
indicating that Ni loading can significantly improve the surface
oxygen reactivity of the support.
Methane combustion
Fig. 5(a) shows the CH4 conversion as a function of the inlet feed
gas temperature over CexZr1-xO2 supports. The activity of quartz
sand is also reported as a reference. The CH4 conversion of
quartz sand was negligible at temperatures lower than 650 C,
indicating that quartz sand cannot activate CH4 and O2. A low
(4%) CH4 conversion observed at ~700 C over the quartz sand
sample can be the result of homogeneous gas phase reactions.[44]
Moreover, the main product of CH4 combustion over quartz sand
was CO instead of CO2 (Fig. 6(a)) and radical reaction products
(Fig. S2), such as ethane, ethylene and formaldehyde, indicating
that gas phase reactions can occur at elevated temperatures. In
contrast to quartz sand, CexZr1-xO2 samples can convert CH4 to
CO2 at high temperatures with the reactivity shown in Fig. 5(b).
Pure ZrO2 had the highest T50 and T90 (average temperature that
50% and 90% CH4 conversion is achieved) at ~638 and 714 C,
respectively, which can be attributed to the low reactivity of
surface oxygen. The T50 of pure CeO2 was 524 C, about 114 C
lower than the T50 of pure ZrO2. The improved performance of
pure CeO2 compared to pure ZrO2 in the CH4 combustion reaction
suggests the importance of active oxygen species, as indicated
by the improved reducibility of CeO2 compared to ZrO2 (Fig. 4).
Doping moderate amounts of zirconia into the ceria lattice
improved the CH4 conversion of pure CeO2. Specifically, the T50
and T90 of Ce0.83Zr0.17O2 were 519 and 588 C, respectively, lower
than the T50 (524 °C) and T90 (635 °C) of pure CeO2. The
improved performance of Ce0.83Zr0.17O2 can be attributed to the
reactive surface oxygen species generated by Zr doping, shown
as a shoulder peak at 324 C in H2-TPR (Fig. 4). The CH4
oxidation T50,90’s and the amount of CO formed (Fig. 6(a)) over
CexZr1-xO2 followed the opposite pattern as the OSCs, with an
improvement in OSC leading to lower T50,90’s and lower amounts
of CO formed. This indicates that the reactivity of surface oxygen
on CexZr1-xO2 support plays an important role in CH4 oxidation.
However, there is no clear relationship between the support
OSCC and the CH4 combustion performance, indicating that the
OSCC might not be a key parameter for the lean combustion of
CH4. The OSCC measurements take into account both the
surface and bulk phase oxygen and the former has been assumed
The OSC and OSCC of CexZr1-xO2 supports calculated from
the O2 consumption during O2 pulse chemisorption are listed in
Table 2. Ceria oxides are known to have the ability to store and
release oxygen due to the Ce4+/Ce3+ reduction pairs,[42] whereas
zirconia is almost non-reducible below 500 C (Fig. 4(a)). The
OSC of pure CeO2 was 161 μmol/g at 450 °C, whereas ZrO2 had
a negligible OSC of ~0.7 μmol/g. The Ce0.83Zr0.17O2 solid
solutions showed higher OSC and OSCC compared to both pure
CeO2 and ZrO2 at 450 °C. A possible explanation for this behavior
is that doping of Zr into the Ce lattice can increase the defects and
4
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10.1002/cctc.202000947
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to be the active site for the CH4 combustion reaction.[23] However,
the bulk phase oxygen species may not participate in the
combustion reaction under lean burn conditions resulting in
disagreement between the OSCC values and the CH4 combustion
performance trend. It is worth noting that pure CeO2 was not able
to reach 100% CH4 conversion even at temperatures as high as
700 C, due to its weak thermal stability at elevated
temperatures.[26b] However, ceria-zirconia solid solutions can fully
convert CH4 at temperatures ~700 C. A potential explanation for
this behavior is that the thermal stability of ceria-zirconia supports
was improved by zirconia doping. This observation is also
consistent with the BET and BJH results after calcination at 450
and 600 C (Table 1).
The catalytic performance of 2 wt.% Ni/CexZr1-xO2 catalysts
is shown in Fig. 5(c, d). The T10 of 2 wt.% Ni/CeO2 and 2 wt.%
Ni/Ce0.83Zr0.17O2 was ~375 C, that was ~50 C lower than the
T10’s of 2 wt.% Ni/Ce0.17Zr0.83O2 and 2 wt.% Ni/ZrO2. All studied
catalysts reached ~100% CH4 conversion at temperatures
between 650-700 °C. The T50,90’s of the 2 wt.% Ni/CexZr1-xO2
catalysts and the 2 wt.% Ni/quartz sand sample are shown in Fig.
5(d). The 2 wt.% Ni/CeO2 and 2 wt.% Ni/Ce0.83Zr0.17O2 catalysts
had the lowest T50’s at around 438 and 452 °C, respectively,
whereas the T50’s of 2 wt.% Ni/Ce0.17Zr0.83O2 and 2 wt.% Ni/ZrO2
were observed at a higher temperature (~476 °C). The T90’s of 2
wt.% Ni/CeO2 and 2 wt.% Ni/Ce0.83Zr0.17O2 were similar (523-532
C), about 120 C lower than the T90 of 2 wt.% Ni/ZrO2. The T90
of 2 wt.% Ni/Ce0.17Zr0.83O2 (545 °C) was in between the T90’s of
ceria rich and zirconia-only catalysts. A benchmark 0.5 wt.%
Pd/Al2O3 was also evaluated under the same reaction conditions
as the Ni/CexZr1-xO2 catalysts. The performance of 0.5 wt.%
Pd/Al2O3 is shown in Fig. 5(c, d). Compared to Ni/CexZr1-xO2
catalysts, 0.5 wt.% Pd/Al2O3 showed an improved performance,
with T50 and T90 of 403 and 445 °C, respectively. Moreover, Fig.
6(b) shows that CO formation was negligible over all Ni/CexZr1-
xO2 samples (100% CO2 selectivity; data not shown here),
whereas CO formation was observed over Ni/quartz sand
emphasizing the importance of reducible supports. The state of
supported Ni over CexZr1-xO2 was determined by X-ray
photoelectron spectroscopy (XPS). Ni 2p3/2 spectra of both fresh
(Fig. 7(a)) and used (Fig. 7(b)) samples were collected. The
peaks at ~855.8 and ~855.5 eV over fresh and used samples,
respectively, are attributed to NiO.[26a, 45] These results indicated
that nickel was mainly present as NiO after the CH4 oxidation
reaction.
Figure 5. (a, c) CH4 combustion performance as a function of inlet temperature
and (b, d) temperature that 50% and 90% CH4 conversion is achieved over (a,
b) CexZr1-xO2 (x = 0, 0.17, 0.83, 1) supports and quartz sand and (c, d) 2 wt. %
Ni/CexZr1-xO2 catalysts, 2 wt.% Ni/quartz sand and 0.5 wt.% Pd/Al2O3 sample.
Figure 6. CO formation during CH4 combustion (a) over CexZr1-xO2 supports
and quartz sand, (b) 2 wt.% Ni/CexZr1-xO2 catalysts and 2 wt.% Ni/quartz sand
sample.
Fig. 8 compares the reaction rates of CexZr1-xO2, Ni/CexZr1-
xO2 and Ni/quartz sand at temperatures ranging from 340-440 C
with 6.8 kPa CH4 and 33.8 kPa O2 and conversions XCH < 15%.
₄
The apparent activation energies of CexZr1-xO2 varied from 112 to
119.5 kJ/mol and they are close to the apparent activation energy
of Ni/quartz sand (110.6 kJ/mol) (Table 3). The Ea of Ni/CexZr1-
xO2 catalysts were much smaller (87.3 to 94.8 kJ/mol) than the
ones of Ni/quartz sand and CexZr1-xO2 supports. The significant
decrease in the Ea of Ni/CexZr1-xO2 catalysts indicates that the
reaction mechanism over Ni/CexZr1-xO2 catalysts is different from
the reaction mechanism over Ni/quartz sand and CexZr1-xO2
supports. The improvement in the CH4 combustion performance
and the decrease in Ea over Ni/CexZr1-xO2 catalysts indicated that
CH4 oxidation is facilitated when Ni is deposited on CexZr1-xO2
Figure 7. Ni 2p3/2 XPS spectra of (a) fresh Ni/CexZr1-xO2 and (b) used
Ni/CexZr1-xO2 catalysts.
Figure 8. Reaction rates for methane combustion over (a) CexZr1-xO2 and (b)
Ni/CexZr1-xO2, Ni/quartz sand catalysts.
5
This article is protected by copyright. All rights reserved.

10.1002/cctc.202000947
ChemCatChem
FULL PAPER
Table 4. CO uptake and TOF at 400 C of Ni/CexZr1-x catalysts.
Sample
CO uptake (µmol/g)[a] TOF at 400 oC (s-1)
Table 3. Apparent activation energies of CexZr1-xO2 supports, Ni/CexZr1-
xO2 catalysts and Ni/quartz sand.
Ni/CeO2
85.0
36.1
36.2
59.6
0.245
0.463
0.154
0.0887
Sample
Ea (kJ/mol) Sample
Ea (kJ/mol)
87.3
Ni/Ce0.83Zr0.17O2
Ni/Ce0.17Zr0.83O2
Ni/ZrO2
CeO2
119.5
117.0
117.0
112.0
Ni/CeO2
Ce0.83Zr0.17O2
Ce0.17Zr0.83O2
ZrO2
Ni/Ce0.83Zr0.17O2
Ni/Ce0.17Zr0.83O2
Ni/ZrO2
93.1
94.8
[a] measured by CO pulse chemisorption at 40 oC.
92.3
Ni/quartz sand
110.6
supports. Incorporation of Ni onto CexZr1-xO2 allowed efficient
CH4 activation, since CexZr1-xO2 supports lack active sites for CH4
activation. Additionally, Ni loading promoted the reactivity of the
support surface oxygen species (H2-TPR) that further improved
the CH4 combustion performance. As reported previously, CH4
reduces Ni oxides to Ni0 in the absence of oxygen while the
support oxygen species re-oxidize Ni0 to Ni oxides, resulting in the
reduction of the support.[46] Thus, Ni/CexZr1-xO2 catalysts with
increased reducibility can facilitate the transfer of the support
lattice oxygen to reduced Ni and further oxidize the surface
carbon-containing intermediates formed after CH4 activation. The
improved reducibility of the catalysts can result in higher CH4
combustion performance under lean conditions. Based on the H2-
TPR results (Fig. 4(b)), the reduction temperature of Ni oxide
increased in the following order: 214 C (Ni/CeO2) < 235 C
(Ni/Ce0.83Zr0.17O2) < 293 C (Ni/Ce0.17Zr0.83O2) < 401 C (Ni/ZrO2)
and the T50 for CH4 conversion increased in a similar fashion: 438
Figure 9. CH4 oxidation TOF at 400 C over Ni/CexZr1-xO2 catalysts and OSC
of CexZr1-xO2 supports.
with the BET measurements of the supports (Table 1) that
showed an increased thermal stability of ceria-zirconia solid
solutions compared to pure CeO2 and ZrO2 supports.
C (Ni/CeO2)
<
452 C (Ni/Ce0.83Zr0.17O2)
<
476 C
(Ni/Ce0.17Zr0.83O2) ~ 477 C (Ni/ZrO2). The impact of the support
on the catalyst reactivity was determined by calculating the TOF
at 400 C where the reaction rate was normalized by the active
surface nickel determined by CO pulse chemisorption (Table 4).
Although the same method (dry impregnation) was used for
loading nickel onto CexZr1-xO2 supports, the samples had different
amounts of active surface nickel, attributed to the difference in
their pore volume (Table 1). The TOFs of Ni/CexZr1-xO2 catalysts
The stability test (450 C, CH4:O2:Ar = 1:5:9, GHSV ~
18,000 mLg-1h-1) and the impact of 5% H2O (5.1 kPa) and 5% CO2
(5.1 kPa) over Ni/CexZr1-xO2 are shown in Fig. S5. Ni/CeO2 lost
about 16% of its initial activity under dry conditions for the first 8
h of the experiment, while less deactivation was observed over
ceria-zirconia solid solution samples. Activity inhibition over all
catalysts was observed when 5% H2O (5.1 kPa) was introduced
to the feed, which can be attributed to H2O and CH4 competition
for active sites resulting to a decrease in the amount of active sites
available for CH4 activation.[47] The performance of the catalysts
can be recovered when H2O was removed from the feed,
indicating that the inhibition effect of H2O is reversible. Persson
et al. showed a continuous activity loss over Pd/Al2O3 when 5%
H2O vapor was introduced in the reaction feed.[48] The activity of
Pd/Al2O3 was not recovered after H2O removal, indicating that
Ni/CexZr1-xO2 catalysts showed an enhanced H2O resistance
compared to Pd/Al2O3 catalysts. A minor inhibition effect was
observed over all studied catalysts when 5% CO2 (5.1 kPa) was
introduced in the reactor feed, indicating that CO2 desorption from
the active sites is much quicker than H2O. Removing CO2 from
the feed gas led to recovery of the catalytic performance.
at 400 C (Table 4) (Ni/ZrO2 < Ni/Ce0.17Zr0.83O2 < Ni/CeO2
<
Ni/Ce0.83Zr0.17O2) increased in exactly the same pattern as the
OSCs of CexZr1-xO2 supports (Table 2) (ZrO2 < Ce0.17Zr0.83O2 <
CeO2 < Ce0.83Zr0.17O2) (Fig. 9). It is worth noting that even though
Ni/Ce0.83Zr0.17O2 and Ni/CeO2 had a similar CH4 oxidation
performance (Fig. 5(c, d)), the TOF of Ni/Ce0.83Zr0.17O2 (0.46 s-1)
was 1.9 times greater than the TOF of Ni/CeO2 (0.245 s-1). This
behavior can be attributed to the lower amount of active surface
nickel over Ce0.83Zr0.17O2 compared to CeO2 (Table 4). These
results further emphasize the contribution of oxygen species from
CexZr1-xO2 to the Ni/CexZr1-xO2 activity of CH4 oxidation. Thus,
improving the support OSC and the catalyst, support reducibility
by doping moderate amounts of Zr can result to an improved CH4
oxidation TOF.
Stability tests
CH4 lean combustion cycling tests were conducted in order to Conclusion
compare the catalysts thermal stability. Fig. S3 shows the 2 wt.%
Ni/CexZr1-xO2 catalysts light-off cycling tests (3 cycles) up to 700
C. The T50 and T90 for each catalyst is summarized in Fig. S4.
The T50 of 2 wt.% Ni/CeO2, 2 wt.% Ni/Ce0.83Zr0.17O2, 2 wt.%
Ni/Ce0.17Zr0.83O2 and 2 wt.% Ni/ZrO2 increased by 78, 53, 69 and
73 C in the third cycle compared to the first cycle (Fig. S4(a)),
indicating the improved thermal stability of Zr containing catalysts
compared to the pure CeO2 one. These results are in agreement
Bare CexZr1-xO2 supports are active for the CH4 combustion
reaction and a moderate Zr doping (Ce0.83Zr0.17O2) generates
surface oxygen vacancies, improves the OSC, reducibility,
thermal stability, and thus combustion activity.
However,
formation of CO as a product of partial oxidation can be observed
over CexZr1-xO2 supports due to lack of active sites for CH4
activation. Incorporation of Ni onto CexZr1-xO2 enhanced the
6
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10.1002/cctc.202000947
ChemCatChem
FULL PAPER
The catalysts crystal structure was studied by X-ray diffraction (XRD).
XRD profiles were collected for 2θ from 5 to 90° with a step size of 1.0167°
using the Rigaku Ultima IV with Cu Kα X-ray source.
surface reducibility, facilitated the activation of CH4, improved the
CH4 combustion activity and compressed CO formation to zero.
Ni/CeO2 catalyst showed promising CH4 oxidation performance
with T50 (438 C) only 35 C higher than that of 0.5 wt.% Pd/Al2O3.
An increase in the CexZr1-xO2 support OSC and the reducibility of
Ni/CexZr1-xO2 catalysts resulted in an increase of the CH4
oxidation TOF in the following order: Ni/ZrO2 < Ni/Ce0.17Zr0.83O2 <
Ni/CeO2 < Ni/Ce0.83Zr0.17O2. Moreover, cycling CH4 oxidation
tests showed that Zr doping can improve the thermal stability of
catalysts. Long term stability tests with H2O and CO2 showed that
H2O has a severe inhibition effect over Ni/CexZr1-xO2 catalysts.
This work illustrates the importance of the catalyst reducibility and
the support OSC in the lean combustion of CH4 in Ni/CexZr1-xO2
catalysts and provide insights for the rational design of future non-
precious metal catalysts. A follow-up work will focus on a detailed
mechanistic discussion and kinetic model proposal.
Zirconia incorporation into the ceria lattice was also verified by Raman
spectroscopy. Raman spectra for CexZr1-xO2 supports were collected with
a Renishaw inVia Raman Microscope with excitation wavelength of 514
nm. All the samples were dried at 110 C for 8 h in air before experiments.
Scanning transmission electron microscopy (STEM) images and energy
dispersive X-ray spectroscopy (EDS) elemental maps were collected using
a FEI Talos F200X STEM. The 2 wt.% Ni/CexZr1-xO2 samples were
prepared for imaging by dusting the sample on a Cu grid.
H2-temperature programmed reduction (H2-TPR), O2 and CO pulse
chemisorption experiments were conducted on a Micromeritics AutoChem
II 2920 equipment with a thermal conductivity detector (TCD). In all these
tests, around 50 mg of sample was loaded in a U-shaped quartz tube
reactor. H2-TPR was used for evaluating the redox properties of CexZr1-
xO2 supports and 2 wt.% Ni/CexZr1-xO2 catalysts. Before the H2-TPR tests,
each sample was pretreated with 3% O2/Ar (50 cm3/min at STP (sccm)) at
450 C for 2 h, followed by purging with Ar (50 sccm) at 450 C for 1h for
the purpose of removing physisorbed oxygen. Afterwards, the samples
were cooled down to room temperature. Once room temperature was
reached the reactor was fed with 10% H2/Ar while the samples were
heated up from room temperature to 1000 C with a ramp rate of 10 C/min.
Experimental Section
Catalyst synthesis
The catalyst supports CexZr1-xO2 (x = 1, 0.83, 0.17, 0) were synthesized
by co-precipitation as reported previously.[46, 49] Briefly, 0.1 M precursor
stock solutions were prepared by dissolving 10.9 g of cerium nitrate
hexahydrate (Ce(NO3)3∙6H2O) and 8.5 g of zirconyl nitrate hydrate
(ZrO(NO3)2∙xH2O, x~6) in 250 mL of deionized (D.I.) water. The ceria and
zirconia stock solutions were mixed in appropriate molar ratios
(ceria:zirconia = 100:0; 83:17; 17:83; 0:100) such that 80 ml ceria/zirconia
precursor solutions were prepared. The ceria/zirconia precursor solutions
were then added drop-wise into 48 mL of ammonium hydroxide (NH4OH)
under continuous stirring. The precursors precipitated instantly. The
precipitants were collected by vacuum filtration, washed with around 500
mL of D.I. water and dried at 110 C for 8 h. The synthesized supports
were then calcined at 450 C and 600 C for 5 h with a temperature ramp
rate of 5 C/min under static air.
O2 chemisorption tests were conducted for determining the oxygen storage
and release ability of CexZr1-xO2 supports at 450 °C. Specifically, the
samples were first reduced by 10% H2/Ar at 450 °C for 1 h followed by
purging with Ar (50 sccm) for 1 h under the same temperature to remove
physisorbed H2. Finally, 20 pulses of 100% O2 were injected in the sample
until no further consumption of O2 was observed. The amount of O2
consumed in the first pulse was used to calculate the OSC of the support,
also known as dynamic OSC, corresponding to the most reactive oxygen
vacancies.[26a] The cumulative O2 consumption was used for calculating
the oxygen storage capacity complete (OSCC).
The CO uptake by the active surface of nickel was measured by CO pulse
chemisorption. In CO pulse chemisorption experiments, the catalysts
were first reduced by 10% H2/Ar at 450 C for 1 h and then purged by Ar
at 450 C for another 0.5 h to remove physisorbed H2. After cooling down
to 40 C, a series of CO pulses were injected into the samples until the
catalyst surface was saturated by CO molecules.
Nickel was deposited on the synthesized supports, calcined at 450 C for
5 h, by dry impregnation.[46] Specifically, for the synthesis of a 2 wt.%
Ni/Ce0.83Zr0.17O2 catalyst, 0.099
g of nickel nitrate hexahydrate
(Ni(NO3)2∙6H2O) was dissolved in 0.076 mL D.I. water that had equal
volume with the pore volume of 0.98 g Ce0.83Zr0.17O2 support. The nickel
precursor solution was then added drop wisely onto the Ce0.83Zr0.17O2
support under vigorous stirring and kept stirring for another 2 h. The
formed paste was subsequently dried at 110 C for 8 h and calcined at 450
C for 5 h with a temperature ramp rate of 10 C/min under static air. A
similar procedure was followed for the synthesis of all 2 wt.% Ni/CexZr1-
xO2 samples. A 2 wt.% Ni/quartz sand sample was synthesized by wet
impregnation. In all cases the Ni weight loading was maintained to 2 wt.%.
A 0.5 wt.% Pd/Al2O3 catalyst was synthesized by incipient wetness using
0.1 M Pd(NO3)2•4 NH3 stock precursor solution. Water purified to 18.2
MΩ-cm was used for the preparation of all aqueous solutions. Ceria nitrate
hexahydrate (99% purity (Sigma-Aldrich)), zirconyl oxynitrate hydrate
(99% purity, (Sigma-Aldrich)), nickel nitrate hexahydrate (99.9+% purity
(Strem chemicals)), Pd(NO3)2•4 NH3 solution (10 wt.% in H2O, Sigma
Aldrich), ammonium hydroxide solution (ACS reagent, 28.0~30.0% NH3
basis (Sigma-Aldrich)) and γ-Al2O3 (Sasol) were used as-received.
The state of supported Ni over fresh (catalyst after calcination) and used
(catalyst after one light-off test) Ni/CexZr1-xO2 catalysts was determined by
XPS. XPS experiments were conducted using a Kratos AXIS Ultra DLD
XPS (Kratos Analytical), which is equipped with a monochromatic Al Kα
source and operated at 15 keV, 150 W. Ni 2p3/2 spectra were collected
with a pass energy of 40 eV.
Catalyst evaluation
CH4 combustion experiments were performed in a packed bed reactor as
reported previously.[50] A U-shaped quartz tube reactor (I.D. = 8 mm) was
loaded with 100 mg catalyst diluted with 100 mg quartz sand. The feed
gas ratio used was CH4:O2:Ar = 1:5:9, the total flow was 30 sccm and
GHSV was ~ 18,000 mLg-1h-1.[26a] The reactor gas lines were heated
above 100 C to prevent water condensation. The gases were mixed
thoroughly before introduction to the reactor. The inlet feed gas and
catalyst bed temperatures were measured with one K-type thermocouple
placed above the catalyst bed and a second thermocouple placed in the
center of the catalyst bed. CH4 combustion light-off curves were collected
in a temperature range of 200 to 700 C with a temperature ramp rate of
10 C/min. The stability of the catalysts was also evaluated over three
consecutive light-off cycles under the same reaction conditions. The
Catalyst characterization
The BET (Brunauer-Emmett-Teller) surface area and BJH (Barrett-Joyner-
Halenda) pore volume of as-synthesized CexZr1-xO2 supports calcined at
450 and 600 C for 5 h were determined by N2 physisorption with a
Micromeritics Tri-Star II surface area and porosity analyzer. The CexZr1-
xO2 supports were degassed at 150 C for 3 h before the test. BJH pore
volumes were calculated based on the desorption branch of the isotherms.
7
This article is protected by copyright. All rights reserved.

10.1002/cctc.202000947
ChemCatChem
FULL PAPER
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4
푁
푐표푛푣푒푟푡푒푑 퐶퐻 푚표푙푒푐푢푙푒푠
4
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(2)
푁
푠푢푟푓푎푐푒 푁푖 푎푡표푚푠
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푁푐표푛푣푒푟푡푒푑 퐶퐻
=
푚표푙푒푐푢푙푒푠
퐿
22.4
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Acknowledgements
The access to the FEI Talos F200X STEM was provided by the
Department of Energy, Office of Nuclear Energy, Fuel Cycle R&D
Program and the Nuclear Science Facilities. The UB authors
were partially supported by start-up funding from the UB
Department of Chemical and Biological Engineering. A portion of
this research was sponsored by the Laboratory Directed
Research and Development Program of Oak Ridge National
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Entry for the Table of Contents
The performance of non-precious metal CexZr1-xO2 (x = 1, 0.83, 0.17, 0) and 2 wt.% Ni/CexZr1-xO2 catalysts was evaluated for the CH4
oxidation reaction. The results showed that CH4 oxidation turnover frequency (TOF) over Ni/CexZr1-xO2 catalysts was improved by
moderate Zr doping in CeO2 (redox properties/oxygen storage capacity) in the following order: Ni/ZrO2 < Ni/Ce0.17Zr0.83O2 < Ni/CeO2 <
Ni/Ce0.83Zr0.17O2.
Researcher’s Twitter username: Kyriakidou’s Catalysis Research Group
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