Supported binary CuOx-Pt catalysts with high activity and thermal stability for the combustion of NH3 as a carbon-free energy source

Article abstract of DOI:10.1039/c8ra07969b

Recently, NH₃ has been thought to be a renewable and carbon-free energy source. The use of NH₃ fuel, however, is hindered by its high ignition temperature and N₂O/NO production. To overcome these issues, in this study, the combustion of NH₃ over copper oxide (CuO_x) and platinum (Pt) catalysts supported on aluminium silicates (3Al₂O₃·2SiO₂), aluminium oxides (Al₂O₃), and silicon oxides (SiO₂) were compared. To achieve high catalytic activity for the combustion of NH₃ and high selectivity for N₂ (or low selectively for N₂O/NO), conditions for the preparation of impregnated binary catalysts were optimised. With respect to the binary catalysts, sequentially impregnated CuO_x/Pt/Al₂O₃ exhibited relatively higher activity, N₂ selectivity, and thermal stability. From XRD and XAFS analyses, CuO_x and Pt in CuO_x/Pt/Al₂O₃ were present as CuAl₂O₄ and metallic Pt, respectively. Given that the combustion activity was closely associated with the Pt nanoparticle size, which was estimated from the Scherrer equation and the pulsed CO technique, highly dispersed Pt nanoparticles were crucial for the low-temperature light-off of NH₃. For single and binary catalysts, although NH (imide) deformation modes as a key species for N₂O production were detected by in situ FTIR spectral analysis, the band intensity of CuO_x/Pt/Al₂O₃ was less than those of CuO_x/Al₂O₃ and Pt/Al₂O₃. Therefore, CuO_x/Pt/Al₂O₃ exhibits high selectivity for N₂ in NH₃ combustion.

Full text of DOI:10.1039/c8ra07969b

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PAPER
Supported binary CuOx–Pt catalysts with high
activity and thermal stability for the combustion of
NH3 as a carbon-free energy source†
b
Cite this: RSC Adv., 2018, 8, 41491
Saaya Kiritoshi,a Takeshi Iwasa,
Kento Araki,a Yusuke Kawabata,a
b
ac
a
*
Tetsuya Taketsugu,
Satoshi Hinokuma
and Masato Machida
Recently, NH3 has been thought to be a renewable and carbon-free energy source. The use of NH3 fuel,
however, is hindered by its high ignition temperature and N2O/NO production. To overcome these
issues, in this study, the combustion of NH3 over copper oxide (CuOx) and platinum (Pt) catalysts
supported on aluminium silicates (3Al2O3$2SiO2), aluminium oxides (Al2O3), and silicon oxides (SiO2)
were compared. To achieve high catalytic activity for the combustion of NH3 and high selectivity for N2
(or low selectively for N2O/NO), conditions for the preparation of impregnated binary catalysts were
optimised. With respect to the binary catalysts, sequentially impregnated CuOx/Pt/Al2O3 exhibited
relatively higher activity, N2 selectivity, and thermal stability. From XRD and XAFS analyses, CuOx and Pt
in CuOx/Pt/Al2O3 were present as CuAl2O4 and metallic Pt, respectively. Given that the combustion
activity was closely associated with the Pt nanoparticle size, which was estimated from the Scherrer
equation and the pulsed CO technique, highly dispersed Pt nanoparticles were crucial for the low-
temperature light-off of NH3. For single and binary catalysts, although NH (imide) deformation modes as
a key species for N2O production were detected by in situ FTIR spectral analysis, the band intensity of
CuOx/Pt/Al2O3 was less than those of CuOx/Al2O3 and Pt/Al2O3. Therefore, CuOx/Pt/Al2O3 exhibits high
selectivity for N2 in NH3 combustion.
Received 25th September 2018
Accepted 27th November 2018
DOI: 10.1039/c8ra07969b
rsc.li/rsc-advances
production along with the fact that it is a renewable energy
source should be emphasised to ensure that the society utilises
Introduction
H2 along with NH3. As a proof-of-concept for using NH3 as a fuel,
an NH3-fuelled micro-gas turbine has been employed to
demonstrate the potential of NH3-red power plants at the
Fukushima Renewable Energy Institute in Japan, as well as an
NH3-fuelled industrial furnace.3 Compared to fossil fuels,
however, NH3 exhibits disadvantages of a high ignition temper-
ature, low combustion rate, and N2O/NOx production. Therefore,
to overcome these issues, it is imperative to develop a novel NH3
combustion system. First, a catalytic NH3 combustion system,
which decreases the ignition temperature and N2O/NOx emis-
sions, was developed. Previously, our group has successfully
developed a catalytic NH3 combustion system and novel catalysts
exhibiting high activity, N2 selectivity, and thermal stability.4 For
example, copper oxides (CuOx) supported on Al2O3-based
composite oxide materials (such as 10Al2O3$2B2O3 and 3Al2O3-
$2SiO2) exhibit high N2 selectivity and thermal stability, whereas
binary CuOx and silver (Ag) supported on Al2O3 exhibit high
activity and N2 selectivity.4 However, the former catalyst exhibits
Recent studies have reportedly considered NH3 to serve not only
as a H2 carrier material but also as a renewable, carbon-free
energy source because of its high energy density (3160 W h
Lꢀ1) and negligible thermal NOx emission.1 Currently, NH3 is
industrially produced at a high pressure using N2 (produced from
air) and H2 (produced from natural gas) by employing the Haber–
Bosch process, which consumes a large amount of energy.
However, the energy and costs associated with H2 production and
natural gas is 70–90% of that required for NH3 production.2
Therefore, the low energy and costs that are associated with H2
aField of Environmental Chemistry and Materials, Division of Materials Science and
Chemistry, Faculty of Advanced Science and Technology, Kumamoto University,
2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan. E-mail: hinokuma@
kumamoto-u.ac.jp; Tel: +81-96-342-3653
bDepartment of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810,
Japan
cInternational Research Organization for Advanced Science and Technology,
Kumamoto University, Japan
ꢂ
a low activity (ignition temperature at ꢁ300 C), while the latter
† Electronic supplementary information (ESI) available: Calculation formulae,
calculated thermodynamic Pt–PtOx phase equilibrium, XRD patterns,
HAADF-STEM/EDX mapping images, product selectivities for the NH3–O2 and
NH3–NO–O2 reactions, relationship between NH3 combustion activity and Pt
particle size, NH3- and NO-TPD proles. See DOI: 10.1039/c8ra07969b
catalyst exhibits low thermal stability (deactivation temperature
at ꢁ900 ꢂC likely because of the low melting point of Ag).4
Therefore, it is imperative to develop catalysts to exhibit high
activities and thermal stabilities for NH3 combustion.
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On the other hand, binary systems comprising supported subsequent Pt (Pt/CuOx), were synthesised in a similar fashion.
8
8
CuOx–Ag,5 CuOx–Pt,6 CuOx–Au,7 CuOx–CeO2 and –Li2O3 cata- As CuOx/Pt/Al2O3 exhibited a high catalytic activity for the
lysts exhibiting a high performance for the selective catalytic combustion of NH3, physically mixed catalysts (CuO + Pt/Al2O3,
oxidation of NH3 (NH3-SCO) have been previously reported. In CuAl2O4 + Pt/Al2O3, and CuOx/Al2O3 + Pt/Al2O3) with the same
particular, CuOx–Pt systems are expected to exhibit high composition ratios were also prepared. To evaluate their
thermal stability because of the high melting point of Pt (ꢁ1770 thermal stabilities and catalytic properties, the as-prepared
ꢂC). Recently, Sun et al. have synthesised impregnated CuOx–Pt/ catalysts were subjected to thermal aging at 900 ꢂC for 100 h
ꢂ
ZSM-5 catalysts and reported that CuOx on Pt species can lead to in air and/or at 1000 C for 5 h in air.
higher NH3-SCO performance compared to single catalysts.6 On
´
the other hand, Jabłonska has synthesised Al2O3-, TiO2-, and
Characterisation
ZrO2-supported CuOx–Pt, –Pd and –Rh catalysts6 and found that
the supported CuOx–Pt catalyst exhibits high activity and N2
selectivity for NH3-SCO. However, in these studies, NH3 is
considered to cause air pollution; therefore, the thermal
Powder X-ray diffraction (XRD) patterns were recorded using
monochromatic Cu Ka radiation (30 kV, 20 mA, Multiex,
Rigaku). The average size of the Pt particles was calculated by
the XRD line broadening method by using the Scherrer equa-
tion.10 The chemical composition was determined by X-ray
uorescence (XRF; EDXL-300, Rigaku) measurements. High-
angle annular dark-eld scanning transmission electron
microscopy (HAADF-STEM) and energy-dispersive X-ray spec-
troscopy (EDS) mapping images were recorded on a JEM-
ARM200CF (JEOL) system. Brunauer–Emmett–Teller (BET)
surface area (SBET) values were calculated from the N2 adsorp-
ꢂ
stability is investigated at temperatures of less than 900 C.
In this study, supported binary CuOx–Pt catalysts were syn-
thesised, and their catalytic properties for the combustion of
NH3 (as an energy source) at high reaction temperatures were
examined. To achieve high catalytic activity for the combustion
of NH3, N2 (low N2O/NO) selectivity and thermal stability,
preparation conditions for impregnated binary catalysts were
optimised. Moreover, the relationship between the local struc-
ture and catalytic properties for the combustion of NH3 over
supported CuOx–Pt catalysts was discussed.
ꢂ
tion isotherms, which were recorded at ꢀ196 C (Belsorp, Bel
Japan, Inc.). Pt dispersion and particle size were determined by
ꢂ
the pulsed CO technique (BELCAT-B, BEL Japan, Inc.) at 50 C
aer O2 oxidation and subsequent H2 reduction at 400 ꢂC
according to previous studies.11 Pt dispersion and particle size
were calculated by assuming hemispherical particles with the
stoichiometric adsorption of CO on Pt at a CO : Pt ratio of 1 : 1.
Cu K-edge and Pt L3-edge X-ray absorption ne structure (XAFS)
spectral analyses were carried out at BL9A of PF and BL01B1 of
SPring-8. XAFS spectra were recorded in the transmission mode
at RT using an ionisation chamber lled with N2 for the incident
beam, another chamber lled with 75% N2/Ar for the Cu K-edge
and 50% N2/Ar for Pt L3-edge for the transmitted beam, and
a Si(111) double-crystal monochromator. Reference samples
(e.g. Cu2O, CuO, CuAl2O4, and PtO2) were mixed with boron
nitride (BN) powder to achieve an appropriate absorbance at the
edge energy, whereas the catalysts were used without mixing
with BN. XAFS data were processed using the IFEFFIT soware
package (Athena and Artemis).
Experimental
Catalyst preparation
3Al2O3$2SiO2 (3A2S) of the catalyst support material was
prepared by using an alkoxide method. According to previous
studies,4,9 Si(OC2H5)4 (TEOS, Wako Pure Chemicals) was rst
dissolved in C2H5OH (Wako Pure Chemicals) at a concentration
of 1 M at room temperature (RT). Second, aer the addition of
H2O and HCl (Wako Pure Chemicals), the solution was stirred at
70 ꢂC for 5 h, and a Si solution was prepared. Third, the alcohol
was dehydrated. The H2O and HCl concentrations were H2O/
TEOS ¼ 2 (mol molꢀ1) and HCl/TEOS ¼ 0.1 (mol molꢀ1),
respectively. Next, Al[OCH(CH3)2]3 (AIP, Sigma-Aldrich) was
subjected to reux conditions and dissolved in (CH3)2-
CHCH2OH (Nacalai Tesque) to prepare an Al solution. The
precursor solution was prepared by mixing the Al and Si solu-
tions in a volume ratio of 3 : 2 at RT. H2O was added, and the
solution was stirred for 1 h to prepare the precursor solution.
Catalytic NH3 combustion tests
The precursor sols were dried at 110 ꢂC for 48 h, affording The catalytic combustion of NH3 was performed in a ow
xerogel precursor powders. These powders were pulverised and reactor at atmospheric pressure. Catalysts (10–20 mesh,
calcined at 600 ꢂC for 3 h and nally at 1200 ꢂC for 5 h in air. In <0.3 mm thickness, 50 mg) were xed in a quartz tube (OD: 6
addition, a-Al2O3 (Wako Pure Chemicals), g-Al2O3 (JRC-ALO-8, mm, ID: 4 mm) with quartz wool at both ends of the catalyst
Catalysis Society of Japan), SiO2 (JRC-SIO-10, Catalysis Society bed. The temperature dependence of the catalytic activity was
of Japan), and CuO (Wako Pure Chemicals) were used as evaluated by heating the caꢀta1lyst bed from RT to 900 ꢂC at
ꢂ
support materials.
a constant rate of 10 C min while a gas mixture containing
Single and binary supported CuOx (CuO loading of 6 wt%) 1.0% NH3, 1.5% O2 and He balance at 100 cm3 minꢀ1 (W/F ¼ 5.0
and/or Pt (Pt loading of 2 wt%) catalysts were prepared by the ꢃ 10ꢀ4 g min cmꢀ3) was supplied. The O2 excess ratio for NH3
co-impregnation (CuOx–Pt) of an aqueous solution of Cu(NO3)2 combustion was expressed as l ¼ (pO2/pNH3)exp./(pO2/pNH3)-
(Wako Pure Chemicals) and [Pt(NH3)2(NO3)2] (Tanaka Kikin- stoichiom., where p denotes pressure. The NH3/N2O/NO, N2 and
zoku Kogyo), followed by drying and calcination at 600 ꢂC for 3 h NO2 gas concentrations were analysed using a nondispersive
in air. In addition, the sequentially impregnated binary cata- infrared gas analyser (VA-3011, Horiba), chromatography (GC-
lysts, i.e. Pt and subsequent CuOx (CuOx/Pt) and/or CuOx and 8A,
Shimadzu)
and
chemiluminescence
(NOA-7000,
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Shimadzu) measurements. The ESI† provides the calculation higher activity, N2 (lower N2O/NO) selectivity, and thermal
formulae used to determine the concentration ratios. stability for the combustion of NH3 compared to all binary
In situ Fourier transform infrared (FTIR) spectra were catalysts before and aer thermal ageing at 900 ꢂC (Table 1).
recorded on a Nicolet 6700 spectrometer using a diffuse- ESI† shows the XRD patterns of the other catalysts. The XRD
reectance reaction cell with a BaF2 window connected to patterns of as-prepared CuOx/Pt/3A2S and CuOx/Pt/SiO2 before
a gas supply and a heating system to enable measurements at and aer aging revealed peaks corresponding to CuO, whereas
atmospheric pressure.ꢂFirst, the catalysts were preheated in situ peaks corresponding to CuAl2O4 were observed for CuOx/Pt/
ꢂ
in owing He at 400 C for 30 min prior to each experiment. 3A2S and CuOx/Pt/Al2O3 (900 C), possibly related to the solid-
Aer pre-treatment, the temperature of the catalyst was state reaction between g-Al2O3 and CuOx. Diffraction peaks
decreased to 200 ꢂC, followed by the subsequent purging of the for Cu species were not observed for as-prepared CuOx/Pt/Al2O3,
cell with He and then lling with 0.3% NH3/He mixed gas. possibly related to the high dispersion of CuOx. The Al2O3 phase
Finally, FTIR spectra were recorded while the catalysts were for the binary catalysts was transformed from g to a, and the
maintained under a stream of NH3/He.
diffraction peaks for Pt became sharper aer thermal aging at
temperatures of higher than 900 C. Therefore, the growth of
ꢂ
particles and sintering of Al2O3 and Pt are induced by thermal
aging. Indeed, the Pt particle sizes calculated by the Scherrer
equation increased with the aging temperature (Table 1). On the
other hand, as peaks corresponding to CuO were observed for
the binary catalysts prepared using a-Al2O3 (CuOx/Pt/a-Al2O3)
before and aer aging, it was possibly difficult to form CuAl2O4
from the solid-state reaction between a-Al2O3 and CuOx (ESI†).
Fig. 2 shows the normalised Cu K-edge X-ray absorption
near-edge structure (XANES) spectra and k-space extended X-ray
absorption ne structure (EXAFS) oscillations of the catalysts
and the three reference compounds (Cu2O, CuO and CuAl2O4,
respectively). Cu K-edge XANES spectra and k-space EXAFS
oscillations of CuOx/Pt/3A2S (900 ꢂC) and CuOx/Pt/Al2O3 before
and aer thermal aging at 900 ꢂC were similar to those of
CuAl2O4, whereas those of CuOx/Pt/3A2S and CuOx/Pt/SiO2
before and aer thermal aging at 900 ꢂC were similar to those of
CuO. In the XANES spectra of CuOx/Pt/3A2S and CuOx/Pt/SiO2
before and aer aging, the pre-edge (at ꢁ8985 eV, correspond-
ing to Cu2+ 1s / 4p + ligand and Cu2+ 1s / Cu2+ charge-
transfer excitation)13 for CuO was observed. XAFS analyses
were consistent with the XRD patterns of the catalysts. The Pt
L3-edge XAFS proles of the catalysts and two references (i.e. Pt
foil and PtO2, respectively) are also depicted in Fig. 3. For as-
prepared CuOx/Pt/Al2O3, the XANES prole was slightly
similar to that observed for PtO2. According to the calculated
thermodynamic Pt–PtOx phase equilibrium (ESI†), PtO2 and
metallic Pt exhibited thermodynamic stability at RT and 600 ꢂC,
respectively. Therefore, Pt nanoparticles (especially, their
surfaces) in CuOx/Pt/Al2O3 are thought to be oxidised during the
cooling process of calcination. By contrast, the XANES proles
of the other catalysts were similar to that of Pt foil (metallic Pt)
as against the calculated phase equilibrium. This disagreement
is supposedly caused by the calcination and/or thermal aging-
induced sintering of metallic Pt nanoparticles in these cata-
lysts; hence, the bulk metallic Pt particles are stable at RT.
Next, HAADF-STEM and EDS mapping images of sequen-
tially impregnated CuOx/Pt/Al2O3 (900 ꢂC) were recorded
(Fig. 4). From the HAADF-STEM image, dispersed nanoparticles
with a bright contrast supported on Al2O3 particles (blue for Al)
were observed, and the EDS mapping images revealed the
presence of Pt (red).ꢂOn the other hand, for the CuOx species of
CuOx/Pt/Al2O3 (900 C), the EDS mapping images revealed the
presence of CuOx aggregates (green). However, because the
Computational details
Spin-restricted density functional theory (DFT) computations
were carried out for NH3/Ptx (x ¼ 13 and/or 20) at the M06 level
using the def-SV(P) basis sets, with the 60-electron relativistic
effective core potential for Pt, as implemented in TURBOMOLE
under the resolution of the identity approximation.12 The model
clusters of Pt13 with Ih symmetry and Pt20 with Td symmetry
having four (111) faces were locally optimised until imaginary
frequencies were removed. One atop geometry for NH3–Pt13 and
three initial geometries for NH3–Pt20 were considered, i.e. atop
the (111) face; bridge-to-edge site; and atop the vertex site,
respectively. These geometries were optimised until no imagi-
nary frequencies were observed. The adsorption energy of NH3
on Ptx was estimated as Eads ¼ E(NH3) + E(Ptx) ꢀ E(NH3–Ptx),
where E(A) denotes the electronic energy of species A.
Results & discussion
Local structures of supported CuOx and Pt catalysts
Fig. 1 shows the XRD patterns of sequentially impregnated
binary (CuOx/Pt) catalysts, because CuOx/Pt/Al2O3 exhibited
Fig. 1 XRD patterns of sequentially impregnated catalysts before and
after thermal aging 900 ꢂC for 100 h in air.
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Table 1 Catalytic properties of supported catalysts before and after thermal aging at 900 ꢂC for 100 h in air
Desorbed
gas d/mmol
mꢀ2
Selectivity
at T90 a/%
Pt particle size/nm
SBET /m2 gꢀ1
b XRD
Pulsed CO
NH3
NO
a
a
c
Catalyst
Phase
T10 /ꢂC
T90 /ꢂC
N2
N2O
NO
CuOx/Pt/3A2S
CuO/Pt/3A2S
210
203
303
295
188
213
189
220
234
301
356
344
476
450
289
266
278
339
316
456
90
89
92
91
81
87
86
86
93
96
8
10
6
2
1
2
32
27
149
102
156
99
139
14
182
81
18
50
CuOx/Pt/3A2S (900 ꢂC)
CuOx/Al2O3
CuAl2O4/Pt/3A2S
CuAl2O4/g-Al2O3
CuAl2O4/a, g-Al2O3
Pt/g-Al2O3
1.3
0.4
0.6
2.0
1.6
1.5
0.050
0.063
0.051
0.140
0.064
0.444
CuOx/Al2O3 (900 ꢂC)
Pt/Al2O3
8
1
19
12
13
13
7
<1
<1
<1
1
<1
3
11
31
15
33
13
24
11
Pt/Al2O3 (900 ꢂC)
CuOx/Pt/Al2O3
Pt/g, q-Al2O3
e n. d.
5
CuAl2O4/Pt/g-Al2O3
CuAl2O4/Pt/a-Al2O3
CuO/Pt/SiO2
CuOx/Pt/Al2O3 (900 ꢂC)
CuOx/Pt/SiO2
176
e n. d.
e n. d.
CuOx/Pt/SiO2 (900 ꢂC)
CuO/Pt/SiO2
<1
a
b
c
Temperature at which NH3 conversion reached 10% and 90%. Calculated from XRD line broadening method. Calculated from pulsed CO
chemisorption. d Estimated by NH3- and NO-TPD ranging from 50 ꢂC to 500 ꢂC. e The amount of CO chemisorption was not detected (n. d.).
overlap of the uorescence lines of Pt-L and Cu-K, as denoted by combustion of NH3 (Fig. 5), indicating that the N2O is produced
the solid arrows in the overlaid image, a physical proximity from the NH3–NO reaction.
between Pt and CuOx nanoparticles in CuOx/Pt/Al2O3 was
For N2O production, on the other hand, according to the
supposed aer aging at 900 ꢂC. On the other hand, Pt particles kinetic model for NH3 oxidation, the elementary reaction of NH
with sizes larger than 200 nm were clearly observed for CuOx/Pt/ (imide) + NO / N2O + H exhibited high sensitivity for N2O
SiO2 (900 C) (ESI†).
production.14 Moreover, for catalytic NH3 oxidation and NH3–
NO reactions, NH was also regarded as a key species for N2O
production.15 Therefore, to verify the adsorption of NH on the
catalysts, in situ FTIR spectra of NH3 adsorbed on the catalysts
before and aer aging were recorded at 200 ꢂC (Fig. 6); this
temperature is the approximate initiation temperature for NH3
combustion (Fig. 5). As has been reported previously,15 FTIR
bands were observed at 1250 and 1625 cmꢀ1, corresponding to
the deformation modes of NH3 adsorbed on Lewis acid sites, for
all catalysts. Moreover, for Pt/Al2O3 and CuOx/Pt/Al2O3, a set of
bands were slightly observed at 1395 and 1695 cmꢀ1, corre-
sponding to the asymmetric and symmetric bending vibrations
of NH3 species on the Brønsted acid sites. Although the single
band at 1458 cmꢀ1 corresponding to the NH (considered as
a species for N2O production) deformation modes was observed
for all catalysts, the band intensity for CuOx/Pt/Al2O3 was less
than that for Pt/Al2O3. As CuOx/Pt/Al2O3 prevented the produc-
tion of NO from the combustion of NH3 and the dissociative
adsorption of NH3, the catalyst nally achieved high N2
selectivity.
ꢂ
Combustion properties of supported CuOx and Pt catalysts
Fig. 5 shows the comparison of the temperature dependence on
the selectivities of products obtained over supported CuOx/Pt,
CuOx/Al2O3 and Pt/Al2O3 before and aer thermal aging at
900 ꢂC under O2 excess conditions (l ¼ 2). ESI† shows those of
the other catalysts. With respect to the supported CuOx/Pt
catalysts before and aer aging, CuOx/Pt/Al2O3 exhibited
a comparatively higher catalytic NH3 combustion activity and N2
(lower N2O/NO) selectivity. For CuOx/Pt/Al2O3 and Pt/Al2O3
before and aer aging, the light-off curves for NH3 were ob-
tained at ꢁ200 ꢂC, whereas CuOx/Al2O3 exhibited lower catalytic
activity (T10: ꢁ300 ꢂC). On the other hand, before and aer
aging, CuOx/Pt/Al2O3 and CuOx/Al2O3 exhibited lower NO
selectivity compared to Pt/Al2O3 although NO selectivities for all
catalysts were observed aer the NH3 conversion reached
ꢁ90%. In contrast, the selectivity of N2O over the catalysts
before and aer aging increased in the order of CuOx/Al2O3 <
CuOx/Pt/Al2O3 < Pt/Al2O3. Finally, CuOx/Pt/Al2O3 exhibited
higher N2 selectivity. Therefore, binary CuOx/Pt/Al2O3 renders
Catalytic properties of supported CuOx and/or Pt
synergistic effects of Pt (enhancing the NH3–O2 reaction) and Table 1 summarises the catalytic properties of supported CuOx/
CuOx (increasing N2 selectivity). To investigate the low NO Pt, CuOx/Al2O3 and Pt/Al2O3 before and aer thermal aging at
ꢂ
selectivity of CuOx/Pt/Al2O3 and CuOx/Al2O3, an NH3–NO–O2 900 C, and ESI† shows those of the other catalysts. Catalytic
reaction test (0.8% NH3, 0.2% NO, 1.4% O2, He balance) was activity was expressed in terms of the light-off temperature at
also performed (shown in ESI†). For the NH3–NO reaction, which 10% conversion of NH3 was achieved (T10), and the
CuOx/Pt/Al2O3 also exhibited higher activity compared to CuOx/ product selectivities were evaluated at the reaction temperature
Al2O3 and Pt/Al2O3. NH3 was supposedly consumed not only by at which 90% NH3 conversion was achieved (T90). ESI† shows
O2 but also by NO, which was formed as an intermediate during the temperature dependence of the product selectivities for the
the light-off of NH3. However, the selectivity of N2O for the NH3– combustion of NH3 over the catalysts. For single catalysts sup-
NO–O2 reaction over CuOx/Pt/Al2O3 was higher than that for the ported on each material (i.e. 3A2S, Al2O3 and SiO2), supported
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Fig. 2 Cu K-edge (upper) normalised XANES spectra and (lower)
EXAFS oscillations of sequentially impregnated catalysts before and
after thermal aging and three references (i.e. Cu2O, CuO, and CuAl2O4,
respectively).
Fig. 3 Pt L3-edge (upper) normalised XANES spectra and (lower)
EXAFS oscillations of sequentially impregnated catalysts before and
after thermal aging and two references (i.e. Pt foil and PtO2,
respectively).
CuOx exhibited low activity and N2O/NO selectivities, whereas
supported Pt exhibited high activity and N2O/NO selectivity
(ESI†). On the other hand, binary supported catalysts exhibited
the same high activity as the single supported Pt, albeit
a slightly lower N2O/NO selectivity. As the combustion activity
was closely associated with the Pt particle size, which was
calculated by the Scherrer equation (plot shown in the ESI†),
highly dispersed Pt nanoparticles supposedly played a key role
in the low-temperature light-off of NH3. In this study, for pulsed
CO chemisorption, it is expected that CO is adsorbed not only
on Pt but also on CuOx in case of binary system catalysts.16
Therefore, the correlation between the NH3 combustion
activity (T10) and the Pt particle size can be estimated using the
XRD line broadening method by employing the Scherrer
equation.
To elucidate the correlation between the activity and size of
Pt nanoparticles, the adsorption energy (Eads) between NH3 and
Pt13 and/or Pt20 clusters (Pt–N) were estimated by DFT compu-
tations (Fig. 7). Compared with the Pt–N Eads of the vertex for the
Pt20 (larger) cluster, that of the vertex for the Pt13 (smaller)
cluster was higher, indicating that NH3 preferentially adsorbs
on highly dispersed Pt nanoparticles as well as coordinatively
unsaturated (cus) Pt atoms. Indeed, for the Pt20 cluster, the Pt–N
Eads of three initial geometries (see inset in Fig. 7) increased in
the order of atop < edge < vertex. Moreover, the bond distance of
N–H in NH3 adsorbed on Pt13 was longer than that on Pt20
,
suggesting that cus Pt exhibits high activity for the dissociation
of the N–H bond in adsorbed NH3. Compared with the activity
of binary CuOx–Pt catalysts prepared by co-impregnation, those
of the sequentially impregnated CuOx/Pt catalysts were higher
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Fig. 4 HAADF-STEM image and EDS mapping analysis of CuOx/Pt/
Al2O3 (900 ꢂC). Red, green, and blue points denote the Pt-L, Cu-K, and
Al-K fluorescence lines, respectively.
probably because Pt nanoparticles can be highly dispersed onto
each support material without inhibition from the presence of
CuOx.
Among the CuOx/Pt catalysts supported on various support
materials, CuOx/Pt/Al2O3 exhibited a relatively higher activity,
N2 selectivity, and thermal stability (Table 1). However,
a signicant decrease in SBET was observed for CuOx/Pt/Al2O3
2
(900 C) (139 / 14 m gꢀ1) in comparison with that observed
ꢂ
ꢂ
ꢂ
for CuOx/Al2O3 (900 C) and Pt/Al2O3 (900 C). For this reason,
rst, it is considered that the signicant decrease in specic
surface area can be attributed to the phase transition of (g / a)
ꢂ
Al2O3 as well as thermal aging at 900 C. However, in case of
only g-Al2O3, the phase transformation to a-Al2O3 was observed
at approximately 1200 ꢂC.17 Next, however, according to
a previous report related to the effects of divalent cation addi-
tives on the phase transition of (g / a) Al2O3, the additive Cu2+
species that coexist with g-Al2O3 accelerates its phase transition
to a-Al2O3 at a low temperature of 1200 ꢂC, which can be
attributed to the fact that CuAl2O4 is formed at a temperature of
lower than 1200 ꢂC.18 Indeed, in this study, a-Al2O3 was
observed in CuOx/Al2O3 aer thermal aging at 900 ꢂC (Table 1).
Finally, it is probable that the additive and co-existing Pt also
accelerates the formation of CuAl2O4 as well as the phase
transition to a-Al2O3. Although no concrete data are available,
a similar signicant decrease of the specic surface area could
be observed in both Cu–Al–O and Cu–Pt–Al–O system catalysts
aer thermal aging occurred at 1000 ꢂC.19
Fig. 5 Product selectivities for the catalytic combustion of NH3 (NH3–
O2) over catalysts before and after thermal aging in air at 900 ꢂC for
100 h. Reaction conditions: 1.0% NH3, 1.5% O2, l ¼ 2, He balance, W/F
¼ 5.0 ꢃ 10ꢀ4 g min cmꢀ3
.
selectivities compared to CuOx/Pt/Al2O3 (using g-Al2O3), the
results of which can be explained from the difference in the CuOx
phase of CuO in CuOx/Pt/a-Al2O3 and CuAl2O4 in CuOx/Pt/g-
Al2O3. Previously, our group has reported that supported CuAl2O4
exhibits a higher NH3–NO–O2 reaction activity to N2 compared to
supported CuO because supported CuAl2O4 exhibits a higher
fraction of the Cu2+ active species for the reaction.4 Therefore, the
trend for N2O/NO selectivities over CuOx/Pt/a-Al2O3 and CuOx/Pt/
Al2O3 can be explained in a similar manner. Moreover, physically
mixed catalysts (CuO + Pt/Al2O3, CuAl2O4 + Pt/Al2O3 and CuOx/
Al2O3 + Pt/Al2O3) exhibited lower N2 selectivity at high reaction
Notably, the high performance for CuOx/Pt/Al2O3 was main-
tained aer thermal aging at 1000 ꢂC despite the phase transition
(g / a) and decrease in the surface area. On the other hand,
CuOx/Pt supported on a-Al2O3 exhibited slightly higher N2O/NO
ꢂ
temperatures (ca. > 600 C) compared to binary catalysts (ESI†),
indicating that highly dispersed Pt and proximate CuOx particles
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the comparison of the temperature dependence on the selectiv-
ities of the products that were obtained when compared to those
of the commercially available CuO, Cu2O and metallic Cu. The
ꢂ
metallic Cu was prepared from CuO that was reduced at 400 C
for 30 min in 5% H2/He because CuO can be reduced to metallic
ꢂ
Cu at an approximate temperature of 300 C, as demonstrated
from the H2-TPR measurement.4 For observing the NH3
combustion activity, the light-off curve for CuO was obtained at
ꢂ
approximately 300 C; further, Cu2O and metallic Cu exhibited
lower activity when compared to that exhibited by CuO. There-
fore, it can be considered that CuO (Cu2+ oxidation state) acts as
an active site during the reaction. However, while considering the
selectivities of the products, Cu2O and metallic Cu exhibited low
N2O/NO selectivities because their activities (NH3 conversions)
were observed to be low. In the O2-excess condition for NH3
combustion (l ¼ 2.0), it can be considered that the combustion
over CuOx catalysts proceeds because of redox cycling between
CuO and Cu2O.4 Further, the XRD patterns of CuO and Cu2O
before and aer the reaction (l ¼ 2.0) also indicate redox cycling
because the patterns of Cu2O aer the reaction revealed diffrac-
tion peaks corresponding to those of CuO (ESI†). In addition,
based on our previous studies related to operando XAFS, similar
redox behaviours could be observed in the supported CuO cata-
lysts (CuOx/SiO2).4 Therefore, it can be assumed that the effect of
metallic Cu on combustion is negligible.
Fig. 6 In situ FTIR spectra of NH3 adsorbed on supports and catalysts
before and after thermal aging in air at 900 ꢂC for 100 h. The spectra
were recorded at 200 ꢂC in gas feeds of 0.3% NH3 with He balance.
Conclusions
In this study, binary CuOx and Pt catalysts supported on 3A2S,
Al2O3 and SiO2 suppressed the production of N2O/NOx via the
catalytic combustion of NH3 as a carbon-free energy source.
Among the binary supported CuOx and Pt catalysts, sequentially
impregnated CuOx/Pt catalysts tended to exhibit higher activity
because Pt nanoparticles were highly dispersed onto each
support material without inhibition due to the presence of
CuOx. Compared to sequentially impregnated CuOx/Pt/3A2S
and CuOx/Pt/SiO2, CuOx/Pt/Al2O3 exhibited high NH3 combus-
tion activity, N2 selectivity, and thermal stability. Before and
aer aging, CuOx/Pt/Al2O3 was characterised as proximate
CuAl2O4 and dispersed metallic Pt particles. As the combustion
activity was closely associated with the Pt particle size, highly
dispersed Pt nanoparticles were thought to play a key role in the
low-temperature light-off of NH3. For selectivity, the proximate
CuAl2O4 in CuOx/Pt/Al2O3 enhanced the NH3–NO to N2 reaction,
and the catalyst prevented the dissociative adsorption of NH3 to
NH, which was regarded as an intermediate species obtained
during the N2O production. Therefore, CuOx/Pt/Al2O3 exhibits
high N2 selectivity.
Fig. 7 Adsorption energy (Eads) between NH3 and Pt13 and/or Pt20
clusters (Pt–N) and bond distances of N–H in NH3 adsorbed on Pt13
and/or Pt20 obtained by the DFT computations.
are required to achieve high N2 selectivity for catalytic NH3
combustion. On the other hand, Pt/CuO exhibited high N2
selectivity, albeit low catalytic activity (ESI†), probably because of
low Pt dispersion. To examine the acid and base properties of
these catalysts, the amount of desorbed gas per surface area of
the catalyst was estimated using NH3- and NO-temperature-
programmed desorption (TPD) over a range of 50–500 ꢂC
(ESI†). Table 1 summarises this data. For all catalysts aer aging,
the amount of desorbed NO increased noticeably, likely because
of decreased surface area. However, before and aer aging, CuOx/
Pt/Al2O3 exhibited relatively higher NH3 and NO adsorption
properties compared to CuO/Al2O3 and Pt/Al2O3. These results
indicated that CuOx/Pt/Al2O3 exhibits higher acidity and basicity
compared to single supported catalysts; therefore, the reactivity
of NH3–NO to N2 is enhanced on the CuOx/Pt/Al2O3 surface.
Finally, the effects of CuO, Cu2O and metallic Cu on the
catalytic NH3 combustion properties are discussed. ESI† denotes
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This research was supported by JST, PRESTO (JPMJPR1344) and
JSPS (18K14326). This work was partly supported by JST-Mirai
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BL01B1 of SPring-8 with the approval of JASRI (Proposal No.
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