Cu–Pd Single-Atom Alloy Catalyst for Highly Efficient NO reduction

Abstract

A series of Cu-Pd alloy nanoparticles supported on Al2O3 were prepared and tested as catalysts for deNO
x
reactions. XRD, HAADF-STEM, XAFS, and FT-IR analyses revealed that a single-atom alloy structure was formed when the Cu/Pd ratio was 5, where Pd atoms were well isolated by Cu atoms. Compared with Pd/Al2O3, Cu5Pd/Al2O3 exhibited outstanding catalytic activity and N2 selectivity in the reduction of NO by CO: for the first time, the complete conversion of NO to N2 was achieved even at 175 °C, with long-term stability for at least 30 h. High catalytic performance was also obtained in the presence of O2 and C3H6 (model exhaust gas), where a 90% decrease in Pd use was achieved with minimum evolution of N2O. Kinetic and DFT studies demonstrated that N-O bond breaking of the (NO)2 dimer was the rate-determining step and was kinetically promoted by the isolated Pd.

Full text

Chemical
Science
rsc.li/chemical-science
ISSN 2041-6539
Volume 10 Number 36 28 September 2019 Pages 8275–8492
EDGE ARTICLE
Shinya Furukawa et al.
A Cu–Pd single-atom alloy catalyst for highly e cient
NO reduction

ACu–Pd single-atom alloy catalyst for highly
efficient NO reduction†
Feilong Xing,
a
Jaewan Jeon,
a
Takashi Toyao,
ab
Ken-ichi Shimizu
ab
and Shinya Furukawa *
ab
A series of Cu–Pd alloy nanoparticles supported on Al
2
O
3
were prepared and tested as catalysts for deNO
x
reactions. XRD, HAADF-STEM, XAFS, and FT-IR analyses revealed that a single-atom alloy structure was
formed when the Cu/Pd ratio was 5, where Pd atoms were well isolated by Cu atoms. Compared with
Pd/Al
2
O
3
,Cu
5
Pd/Al
2
O
3
exhibited outstanding catalytic activity and N
2
selectivity in the reduction of NO
by CO: for the first time, the complete conversion of NO to N
2
was achieved even at 175 C, with long-
term stability for at least 30 h. High catalytic performance was also obtained in the presence of O
2
and
C
3
H
6
(model exhaust gas), where a 90% decrease in Pd use was achieved with minimum evolution of
N
2
O. Kinetic and DFT studies demonstrated that N–O bond breaking of the (NO)
2
dimer was the rate-
determining step and was kinetically promoted by the isolated Pd.
Introduction
The reactions of nitric oxide (NO) have garnered intense interest
from researchers in the human health,
1
and bioinorganic,
2
industrial,
3
and environmental chemistry elds.
4
Specically,
NO removal has long been studied as an indispensable process
for exhaust-gas purication.
5
Platinum-group metals (PGMs)
such as Pt, Pd, and Rh are known to be efficient catalysts for the
reduction of NO using CO,
6,7
H
2
,
8
NH
3
,
9
and hydrocarbons
10
as
reductants. The recent challenges in this eld involve devel-
oping catalytic systems that function (1) at low temperatures
under cold-start conditions,
11
(2) with minimum use of
PGMs,
12,13
and (3) without emitting N
2
O,
14–16
which is a potent
greenhouse gas.
17
These issues have been individually studied
using different materials. The development of a single material
that enables (1)–(3) is therefore highly desirable. To the best of
our knowledge, no such material has been reported. In partic-
ular, achieving both (1) and (3) is difficult because N
2
O reduc-
tion to N
2
on PGMs requires relatively high temperatures (>300
C).
18
Therefore, an appropriate catalyst design is needed to
obtain not only outstanding catalytic activity toward NO
reduction but also high selectivity to N
2
with minimal incor-
poration of PGMs.
A promising approach that overcomes these challenges is the
single-atom alloying concept,
19
which is relevant to single-atom
chemistry.
20,21
The dilution of PGM atoms with less active metal
atoms not only substantially reduces the use of PGMs but also
enables drastic modication of the electronic and geometric
structures for enhanced catalysis.
22
For example, the isolation of
Pt or Pd with group 11 metals (Au, Ag, and Cu) enables molec-
ular transformations that hardly proceed in the absence of
single-atom alloying, such as selective hydrogenation,
23–27
for-
mic acid dehydrogenation,
28
and hydrosilylation.
29
In these
systems, the group 11 metals act as inert elements but modify
the electronic and geometric factors of the PGM and, thus, its
catalytic properties. Conversely, for NO reduction, the group 11
elements are known to be capable of NO activation.
30–32
There-
fore, applying the single-atom alloying concept to NO reduction
systems should provide an unprecedented synergistic effect for
efficient NO conversion.
In this study, we focused on Cu as a main component
because of its intrinsic activity toward NO reduction and its
high earth abundance. We found that Cu–Pd/Al
2
O
3
(Cu/Pd ¼5)
acts as a highly efficient catalyst for NO reduction at low
temperatures (>150 C), without generating N
2
O emissions.
Herein, we report both an innovative catalytic system for effi-
cient NO reduction and novel catalytic chemistry of single-atom
alloys.
Experimental details
Catalyst preparation
Boehmite (g-AlOOH) was supplied by SASOL Chemicals. g-Al
2
O
3
was prepared by the calcination of boehmite at 900 C for 3 h.
Pd/Al
2
O
3
(Pd: 2 wt%) and Cu–Pd/Al
2
O
3
(Cu: 6 wt%, Cu/Pd ¼1)
were prepared by a conventional impregnation method. The g-
Al
2
O
3
support was added to a vigorously stirred aqueous
a
Institute for Catalysis, Hokkaido University, N-21, W-10, Sapporo 001-0021, Japan.
E-mail: furukawa@cat.hokudai.ac.jp; Fax: +81-11-706-9163
b
Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, Katsura,
Kyoto 615-8520, Japan
†Electronic supplementary information (ESI) available: Details of
characterization, kinetic analysis, and DFT calculations. See DOI:
10.1039/c9sc03172c
Cite this: Chem. Sci.,2019,10,8292
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Received 27th June 2019
Accepted 5th August 2019
DOI: 10.1039/c9sc03172c
rsc.li/chemical-science
8292 |Chem. Sci.,2019,10,8292–8298 This journal is © The Royal Society of Chemistry 2019
Chemical
Science
EDGE ARTICLE

solution containing Pd(NH
3
)
2
(NO
2
)
2
(Kojima Chemicals,
4.765 wt% in HNO
3
) and/or Cu(NO
3
)
2
$3H
2
O (Sigma-Aldrich,
99%), followed by stirring for 3 h at room temperature (50 ml
H
2
O per gram of Al
2
O
3
). The mixture was dried under a reduced
pressure at 50 C, followed by reduction under owing H
2
(30
ml min
1
) at 400 C (Pd) or 800 C (CuPd) for 1 h. The Cu/Al
2
O
3
(Cu: 6 wt%) and Cu–Pd/Al
2
O
3
(Cu: 6 wt%, Cu/Pd ¼3 and 5)
catalysts were prepared by a deposition–precipitation method
using urea. The g-Al
2
O
3
support was added to a vigorously
stirred aqueous solution of Cu(NO
3
)
2
$3H
2
O (50 ml H
2
O per
gram of Al
2
O
3
). Then, an aqueous solution of urea (Kanto, 99%)
was added dropwise to the stirred mixture at room temperature
(urea/Cu ¼30). The mixture was sealed with a plastic lm and
kept with stirring at 70 C for 10 h. Aer completing the
precipitation of Cu(OH)
2
, the supernatant was decanted and the
resulting crude product was washed with deionized water three
times, followed by drying under a reduced pressure at 50 C and
calcination at 500 C for 1 h. For Cu/Al
2
O
3
, the resulting CuO/
Al
2
O
3
was reduced under owing H
2
at 400 C for 1 h. For Cu–
Pd/Al
2
O
3
(Cu/Pd ¼3 and 5), the resulting CuO/Al
2
O
3
was used
for successive impregnation of Pd in a similar fashion to that
mentioned above. The resulting Pd–CuO/Al
2
O
3
was reduced
under owing H
2
at 400 C for 1 h.
Reaction conditions
The catalyst (0.05 g) diluted with quartz sand (1.95 g, Miyazaki
Chemical, 99.9%) was treated under owing hydrogen (50
ml min
1
) at 400 C for 0.5 h prior to the catalytic reactions. NO
reduction by CO was performed in a xed-bed continuous ow
system by feeding NO (5000 ppm), CO (5000 ppm), and He
(balance) with a total ow rate of 96 ml min
1
(GHSV ¼80 000
h
1
). The gas phase was analyzed using an online thermal
conductivity detection gas chromatograph (Shimadzu GC-8A,
column: SHINWA SHINCARBON ST) located downstream. A
stability test was done using twice the amount of catalyst (0.10
g) under similar conditions (GHSV ¼40 000 h
1
). Aer a time-
on-stream of 24 h, the catalyst was regenerated by owing
hydrogen (50 ml min
1
) at 400 C for 0.5 h, followed by
continuing the catalytic run. A kinetic study was performed by
changing the concentration of NO and CO between 0.3 and
0.6% with that of the counterpart xed at 0.5%. The reaction
temperature was maintained at 150 C so that NO conversion
did not exceed 30%, and the reaction rate (mol s
1
mol
Pd
1
) was
calculated on the basis of NO conversion. NO + CO + O
2
and NO
+CO+O
2
+C
3
H
6
reactions were performed under stoichio-
metric conditions as follows: NO (5000 ppm), CO (10 000 ppm),
O
2
(2500 ppm), He (balance) with a total ow rate of 96
ml min
1
(GHSV ¼80 000 h
1
), and NO (5000 ppm), CO (5000
ppm), O
2
(5625 ppm), C
3
H
6
(1250 ppm), and He (balance) with
a total ow rate of 96 ml min
1
(GHSV ¼80 000 h
1
),
respectively.
Characterization
The crystal structure of the prepared catalyst was examined by
powder X-ray diffraction (XRD) using a Rigaku MiniFlex II/AP
diffractometer with Cu Karadiation. High-angle annular dark
eld scanning transmission electron microscopy (HAADF-
STEM) was carried out using a JEOL JEM-ARM200 M micro-
scope equipped with an energy dispersive X-ray (EDX) analyzer
(EX24221M1G5T). STEM analysis was performed at an acceler-
ating voltage of 200 kV. To prepare the TEM specimen, all
samples were sonicated in ethanol and then dispersed on a Mo
grid supported by an ultrathin carbon lm.
The Fourier-transformed infrared (FT-IR) spectra of adsor-
bed CO were obtained with a JASCO FTIR-4200 spectrometer
equipped with an MCT detector in transmission mode (reso-
lution 4 cm
1
). The samples were prepared as self-supporting
wafers (2.0 cm diameter, <0.5 mm thickness) and were placed
inside an IR cell with CaF
2
windows. A custom glass manifold
was connected to the cell to control the gas for pretreatment and
the amount of CO introduced. The cell was rst purged with He,
and the sample was reduced under owing hydrogen (50
ml min
1
) at 400 C for 30 min. Aer reduction, the wafer was
cooled to 40 C under owing He. The wafer was exposed to CO
(0.5%) and He (balance) with a total ow rate of 50 ml min
1
for
20 min. Aer the CO exposure, He was owed for 5 min to
remove the gas phase and weakly adsorbed CO, followed by IR
spectral measurements.
X-ray absorption ne structure (XAFS) spectra were recorded
on the BL14B2 station at SPring-8 of the Japan Synchrotron
Radiation Research Institute. A Si(311) double-crystal mono-
chromator was used. Energy calibration was performed using
Pd foil. The spectra were recorded at the edges of Pd K in
a transmission mode at room temperature. The pelletized
sample was pre-reduced with H
2
at 400 C for 0.5 h, and then
sealed in a plastic pack under a N
2
atmosphere together with an
ISO A500-HS oxygen absorber (Fe powder). The obtained XAFS
spectra were analyzed using Athena and Artemis soware ver.
0.9.25 included in the Demeter package. The Fourier trans-
formation of the k
3
-weighted EXAFS from kspace to Rspace was
performed over a krange of 3.0–15 ˚
A
1
. Some of the Fourier-
transformed EXAFS spectra in the Rrange of 1.2–3.0 ˚
A were
inversely Fourier transformed, followed by an analysis using
a usual curve tting method in a krange of 3–15 ˚
A
1
. The back-
scattering amplitude or phase shiparameters were simulated
with FEFF 6L and used to perform the curve tting procedure.
For Pd–Cu scattering, intermetallic Cu
3
Pd with a Pm

3mstruc-
ture was considered for the FEFF simulation. The amplitude
reduction factor (S
02
) of Pd was determined to be 0.775 by tting
the spectra of Pd black and then used for tting of other EXAFS
spectra.
Computational details
Periodic DFT calculations were performed using the CASTEP
code
33
with Vanderbilt-type ultrasopseudopotentials
34
and the
Perdew–Burke–Ernzerhof exchange–correlation functional
based on the generalized gradient approximation.
35
The plane-
wave basis set was truncated at a kinetic energy of 350 eV,
and a Fermi smearing of 0.1 eV was utilized. Dispersion corre-
lations were considered using the Tkatchenko–Scheffler
method with a scaling coefficient of s
R
¼0.94 and a damping
parameter of d¼20.
36
The reciprocal space was sampled using
This journal is © The Royal Society of Chemistry 2019 Chem. Sci.,2019,10,8292–8298 | 8293
Edge Article Chemical Science

ak-point mesh with a spacing of typically 0.04 ˚
A
1
, as generated
by the Monkhorst–Pack scheme.
37
Geometry optimization was
performed on supercell structures using periodic boundary
conditions. The surface was modeled based on Cu(211)-(2 3)
(for NO and N
2
O related reactions), Cu(111)-(2 2) (for CO
oxidation), and Cu(111)-(3 3) (for N
2
O decomposition) slabs
that were six atomic layers thick with 13 ˚
A of vacuum spacing.
The convergence criteria for structural optimization and energy
calculation were set to (a) an SCF tolerance of 1.0 10
6
eV per
atom, (b) an energy tolerance of 1.0 10
5
eV per atom, (c)
a maximum force tolerance of 0.05 eV ˚
A
1
, and (d) a maximum
displacement tolerance of 1.0 10
3
˚
A. The transition state
search was performed using the complete linear synchronous
transit/quadratic synchronous transit (LST/QST) method.
38,39
Linear synchronous transit maximization was performed, fol-
lowed by energy minimization in the directions conjugate to the
reaction pathway. The approximated TS was used to perform
QST maximization with conjugate gradient minimization
renements. This cycle was repeated until a stationary point
was found. Convergence criteria for the TS calculations were set
to root-mean-square forces on an atom tolerance of 0.10 eV ˚
A
1
.
Results and discussion
The monometallic Cu (6 wt%) and Pd (2 wt%) and the bimetallic
Cu–Pd (Cu: 6 wt%, Cu/Pd ¼1, 3, or 5; hereaer, Cu
x
Pd, x¼1, 3,
or 5) catalysts were prepared using g-Al
2
O
3
as a support by
deposition–precipitation and/or impregnation methods. X-ray
diffraction (XRD) patterns of the prepared catalysts revealed
that Cu–Pd solid-solution alloy phases with bimetallic compo-
sitions similar to the metal ratio in the feed were formed
(Fig. S1†and Table 1).
The crystallite sizes estimated using Scherrer's equation
were 3–4 nm for all of the catalysts. Fig. 1a and b show a high-
angle annular dark eld scanning transmission electron
microscopy (HAADF-STEM) image of Cu
5
Pd/Al
2
O
3
and the size
distribution of the nanoparticles, respectively. A relatively
narrow size distribution between 2 and 6 nm with an area-
weighted mean diameter of 4.2 nm was obtained, consistent
with the crystallite size estimated by XRD (Table 1).
The energy-dispersive X-ray spectroscopy (EDS) analysis of
a single nanoparticle revealed that the Cu and Pd atoms
comprising the nanoparticle were homogeneously dispersed
(Fig. 1c). The high-resolution HAADF-STEM image shows an fcc
crystal structure viewed along the [100] direction, consistent
with the formation of a solid-solution alloy (Fig. 1d). Moreover,
the isolation of Pd atoms by Cu was indicated by the presence of
atoms with distinct Z contrasts. Note that the corresponding
HAADF-STEM image of Cu/Al
2
O
3
showed a weak and uniform Z
contrast compared with that of Cu
5
Pd/Al
2
O
3
(Fig. S2†).
The degree of Pd isolation was further investigated by
Fourier-transform infrared (FT-IR) and extended X-ray absorp-
tion ne structure (EXAFS) analyses (Fig. 2). As shown in Fig. 2a,
the FT-IR spectra of CO adsorbed onto Pd/Al
2
O
3
exhibited
absorption peaks assigned to the stretching vibration of C]O
Table 1 Detailed information on the catalyst prepared in this study
Pd CuPd Cu
3
Pd Cu
5
Pd Cu
Pd loading (wt%) 2.0 10.1 3.4 2.0 0
Cu loading (wt%) 0 6.0 6.0 6.0 6.0
Cu fraction in the catalyst 0 0.50 0.75 0.83 1.0
Cu fraction in particles 0 0.44 0.72 0.89 1.0
Crystallite size/nm
a
3.2 3.3 3.8 3.8 (4.2)
b
4.0
a
Estimated from Scherrer's equation using a Scherrer constant of 0.477
for the area-weighted mean diameter.
b
Area-weighted mean diameter
obtained from TEM images.
Fig. 1 (a) HAADF-STEM image of Cu
5
Pd/Al
2
O
3
and (b) size distribution
of the nanoparticles. (c) Elemental map of the Pd + Cu overlayer, as
acquired by EDS. (d) High-resolution image of a single nanoparticle.
Fig. 2 (a) FT-IR spectra of CO adsorbed on the prepared catalysts. (b)
Fourier transforms of the Pd K-edge EXAFS spectra of the Pd-based
catalysts.
8294 |Chem. Sci.,2019,10,8292–8298 This journal is © The Royal Society of Chemistry 2019
Chemical Science Edge Article

adsorbed on top (2086 cm
1
), bridge (1975 cm
1
), and hollow
sites (1880 cm
1
).
40
Similar absorption peaks were observed for
CuPd/Al
2
O
3
, suggesting that the Pd–Pd ensembles largely
remain even aer 1 : 1 alloying. For Cu-rich samples, an
absorption band assignable to CO adsorbed on metallic Cu was
also observed at 2100 nm
1
.
41
The peak intensities for the bridge
and hollow CO substantially decreased and disappeared in the
spectra of Cu
3
Pd/Al
2
O
3
and Cu
5
Pd/Al
2
O
3
, respectively, indicating
that Pd atoms at the surface were isolated upon 5 : 1 alloying.
There remained a weak absorption band for linear CO on Pd
in the spectrum for Cu
5
Pd/Al
2
O
3
, suggesting that the isolated Pd
atoms are also present at the surface. Fig. 2b shows the Fourier
transforms of the Pd K-edge EXAFS spectra of the Pd-based
catalysts (the X-ray absorption near edge structure spectra,
raw EXAFS oscillations, curve-tting, and summary of EXAFS
curve tting are shown in Fig. S3–S5 and Table S1,†respec-
tively). CuPd/Al
2
O
3
showed both Pd–Pd and Pd–Cu bonds, while
Cu
5
Pd/Al
2
O
3
exclusively showed Pd–Cu bonds, suggesting that
the Pd atoms in the bulk were also isolated by Cu upon 5 : 1
alloying. Thus, small Cu–Pd nanoparticles with a single-atom
alloy structure were successfully formed on the Al
2
O
3
support.
Considering the limited sensitivity of EXAFS and FT-IR, we
cannot completely exclude the presence of Pd–Pd interaction.
However, only a small number of Pd–Pd sites, if any, seem not to
contribute to the overall catalytic performance.
We next tested the catalytic activity of Cu
x
Pd/Al
2
O
3
in NO
reduction by CO (GHSV ¼80 000 h
1
), as a model reaction for
exhaust-gas purication. Fig. 3a shows the NO conversion to N
2
(C
N
2
) for the prepared catalysts as a function of reaction
temperature. Here, C
N
2
was obtained by multiplying the NO
conversion and the N
2
selectivity (Fig. S6†). Pd/Al
2
O
3
gave the
lowest C
N
2
, because of the poor N
2
selectivity <40% (Fig. S6b†).
Cu/Al
2
O
3
exhibited a higher C
N
2
than Pd/Al
2
O
3
because of its
much higher N
2
selectivity (Fig. S6b†). CuPd/Al
2
O
3
showed a C
N
2
trend similar to that of Cu/Al
2
O
3
because of the consequence of
increased NO conversion and decreased N
2
selectivity (Fig. S6†).
Thus, the 1 : 1 alloy of Cu and Pd gave an insufficient catalytic
performance for selective NO reduction. Interestingly, however,
both NO conversion and N
2
selectivity increased when the
alloying ratio was increased to 3 : 1 and 5 : 1 (Fig. S6†), which
resulted in great enhancements in C
N
2
(Fig. 3a). NO was
completely converted to N
2
over Cu
5
Pd/Al
2
O
3
without gener-
ating N
2
O emissions even at 200 C, where Pd showed a C
N
2
of
only 5%. Notably, on going from CuPd to Cu
5
Pd, the catalytic
activity increased even though the Pd content was decreased to
1/5 (Table 1). Therefore, a specic synergistic effect between Cu
and Pd likely contributed to the unique properties of the single-
atom alloy catalyst. We emphasize that using an excess amount
of Pd/Al
2
O
3
(0.50 g) with 10 times the equimolar Pd included in
Cu
5
Pd/Al
2
O
3
(labeled as Pd 10) still resulted in poor perfor-
mance (Fig. 3b), highlighting the outstanding performance of
the single-atom alloy catalyst. We also tested the long-term
stability of Cu
5
Pd/Al
2
O
3
in NO reduction by CO under stan-
dard conditions (GHSV ¼40 000 h
1
), where 100% C
N
2
was
maintained at 175 C. Although a number of bimetallic catalysts
for NO reduction have been reported thus far,
12–16,42–48
to the best
of our knowledge, the present work represents the rst success
in complete NO
x
removal at a temperature less than 200 C. At
150 C, although C
N
2
decreased slightly at the initial stage
because of N
2
O formation, it recovered aer a short H
2
treat-
ment. This result implies that the accumulation of oxygen
species at the catalyst surface triggers the loss of N
2
selectivity
and that the catalytic performance could be recovered under
rich conditions. We next examined the catalytic performance of
Cu
5
Pd/Al
2
O in NO reduction in the presence of O
2
and O
2
+
C
3
H
6
; these conditions more closely resemble those encoun-
tered in practical use. Cu/Al
2
O
3
delivered poor performance
under NO + CO + O
2
conditions. By contrast, Cu
5
Pd/Al
2
O
exhibited much higher performance than Cu/Al
2
O and Pd/Al
2
O.
Notably, Cu
5
Pd/Al
2
O still exhibited a performance better than or
comparable to “Pd 10”even in the presence of O
2
or O
2
+
C
3
H
6
, respectively (Fig. 3b and S7;†a comparison with
temperature dependence and T
50
is presented in Fig. S7†).
Furthermore, N
2
O evolution was sufficiently suppressed over
Cu
5
Pd, where the C
N
2
O
(NO conversion to N
2
O, Fig. 3b and S7†)
was much lower than those for Pd. Thus, the single-atom alloy
catalyst enabled not only a decrease in the noble metal use to 1/
10 but also highly selective NO
x
removal. In the reactions con-
ducted in the presence of O
2
and O
2
+C
3
H
6
, reaction temper-
atures greater than 200 C were needed to achieve sufficient
catalytic performance (Fig. 3 and S7†). A possible explanation is
that the number of active sites for NO reduction decreased
because of the involvement of other reactions such as CO and/or
C
3
H
6
oxidation.
Fig. 3 (a) NO conversion to N
2
during the NO reduction by CO over
Pd, Cu, and Cu–Pd catalysts as a function of reaction temperature
(NO, CO: 0.5%, GHSV ¼80 000 h
1
). (b) Comparison between C
N
2
and
C
N
2
O
in NO reduction in the presence of O
2
and C
3
H
6
. (c) Stability test
for Cu
5
Pd/Al
2
O
3
in the NO + CO reaction at low temperatures (NO,
CO: 0.5%, GHSV ¼40 000 h
1
).
This journal is © The Royal Society of Chemistry 2019 Chem. Sci.,2019,10,8292–8298 | 8295
Edge Article Chemical Science

Next, to clarify the nature of the synergistic effect, we con-
ducted a mechanistic study based on kinetic analysis and
density functional theory (DFT) calculations. First, the apparent
activation energy (E
A
) for NO reduction by CO was estimated for
the representative catalysts. The corresponding Arrhenius-type
plots and the resulting E
A
values are shown in Fig. 4 and
Table 2, respectively. Cu
5
Pd gave an E
A
value lower than those of
Pd and Cu, which is consistent with the observed catalytic
activity. In addition, we estimated the reaction orders for NO
and CO pressures (P
NO
and P
CO
, respectively) to consider the
rate-determining step (RDS). Both Cu and Cu
5
Pd showed
negative and positive orders for P
NO
and P
CO
, respectively (see
Table 2 and Fig. S8†for details). Unlike the case for Pd-based
catalysts,
49
bond dissociation of N–O has been speculated to
occur via (NO)
2
dimer formation on Cu surfaces.
50–52
Therefore,
in the present study, an extended Langmuir–Hinshelwood
model that includes (NO)
2
dimer formation, the subsequent
N–O scission (N
2
O formation), and N
2
O decomposition (N
2
formation) was considered. We solved the rate equation of each
step regarded as the RDS by using the overall site balance and
equilibrium constants for the other steps (see the ESI,†kinetic
analysis). In most cases, the reaction order for P
NO
is positive,
which is inconsistent with the observed experimental results.
Conversely, when the N–O scission of the (NO)
2
is considered as
the RDS, the orders for P
NO
and P
CO
range from 2 to 0 and
from 0 to 2, respectively, in agreement with the experimental
values. Thus, our kinetic study suggests that the N–O scission
was the RDS in NO reduction by CO. Upon incorporation of Pd
atoms into pure Cu, the order for P
NO
becomes less negative,
while that for P
CO
decreases substantially. This result indicates
that the strong adsorption of NO inhibits CO adsorption onto
Cu, while the latter is enhanced in the presence of Pd. We
performed DFT calculations for the relevant elemental steps on
pure and Pd-doped Cu surfaces. On the basis of the literature,
53
the step site of the (211) surface was considered the active site
for N–O scission (Fig. S9†). The corresponding energy diagrams
are shown in Fig. 5.
NO adsorption was weakened by the addition of Pd, which
is consistent with the change in the reaction orders (Table 2).
Dimerization occurs at the terrace site adjacent to the step site,
with a negligible energy barrier. The subsequent N–O scission
is triggered by capture of an oxygen atom by the edge Cu
atoms, resulting in the formation of an on-top N
2
OwithE
A
values of 59.8 and 47.9 kJ mol
1
for pure and Pd-doped Cu,
respectively. The calculated E
A
values agree with the experi-
mental values (Table 2). The lower E
A
for the Pd-doped Cu
appears to originate from the destabilized adsorption of the
(NO)
2
dimer by Pd.
We also considered the CO oxidation process (CO + O /
CO
2
), which is necessary for the catalytic cycle (Fig. S10†). The
CO + O reaction over pure and Pd-doped Cu(111) surfaces gave
E
A
values of 60.9 kJ mol
1
and 34.1 kJ mol
1
, respectively (Table
2 and Fig. S10†). These values are very similar to or lower than
those for N–O scission, which is consistent with the RDS being
the scission of N–O. We also simulated the N
2
O decomposition
process (N
2
O/N
2
+ O) on Cu(211) and (111) surfaces to
understand the intrinsic high N
2
selectivity of Cu (Fig. 6 and
S11; see S12†for the pictures of the optimized structures).
The monodentate linear N
2
O was bent to form a bidentate
N
2
O at the edge site of the Cu(211) plane without an energy
barrier. The bidentate N
2
O was subsequently decomposed into
N
2
and O with a low E
A
of 33.4 kJ mol
1
, indicating that the N
2
O
once formed could be smoothly decomposed into N
2
to afford
high N
2
selectivity. Although the Cu(111) surface was also active
Fig. 4 Arrhenius-type plots obtained in the NO + CO reaction over
Cu
5
Pd/Al
2
O
3
, Cu/Al
2
O
3
, and Pd/Al
2
O
3
catalysts.
Table 2 Activation energies estimated from experiments and from
DFT calculations, along with reaction orders
E
A
/kJ mol
1
(/eV)
Pd Cu Cu
5
Pd
Experiment 99.5 60.8 42.7
DFT: N–O 100.2
a
59.8 47.9
DFT: CO + O 100.1
b
60.9 34.1
Reaction order
P
NO
—0.27 0.02
P
CO
—1.93 0.44
a
NO dissociation at the step of Pd(511).
16
b
CO oxidation on Pd(111).
16
Fig. 5 Energy diagrams of NO adsorption, dimerization, and the
dimer's decomposition over pure and Pd-doped Cu(211) surfaces. The
total energy of the slab and free NO was set to zero.
8296 |Chem. Sci.,2019,10,8292–8298 This journal is © The Royal Society of Chemistry 2019
Chemical Science Edge Article

for N
2
O decomposition in a similar fashion, the energy barrier
was higher than that of Cu(211) (51.6 kJ mol
1
, Fig. S11†).
Because large Cu–Cu ensembles are present on the surface of
the Cu and Cu-rich catalysts (Cu
5
Pd and Cu
3
Pd), N
2
O decom-
position could also be enhanced on these catalysts. However,
for CuPd, this effect is limited because of the dilution of Cu–Cu
ensembles and the increase of Pd–Pd ensembles. Thus, our
calculation rationalized the substantial enhancement in cata-
lytic activity on the basis of the formation of the Cu–Pd single-
atom alloy and the origin of the excellent selectivity for N
2
formation. The elucidated mechanism differs completely from
those proposed for other bimetallic alloy systems. For example,
for the Pt–Co system, Sato et al. reported that alloying with Co
makes Pt electron-rich, which enhances back donation to
adsorbed NO, inducing bond breaking.
13
Therefore, Co likely
acts as a promoter for Pt. By contrast, in our system, the isolated
Pd acts as an efficient promotor for Cu.
Conclusion
We prepared a series of Cu–Pd/Al
2
O
3
catalysts for selective NO
reduction at low temperatures. Alloying of Pd with a large
amount of Cu (Cu/Pd ¼5) isolates Pd and drastically improves
both the catalytic activity and N
2
selectivity, affording
outstanding catalytic performance. In the NO reduction by CO,
NO is completely converted to N
2
even at 175 C, with long-term
stability for at least 30 h. The high catalytic performance is also
achieved in the presence of O
2
and C
3
H
6
, where the amount of
Pd needed for a comparable performance can be reduced to 1/
10, with minimum evolution of N
2
O. For Cu/Al
2
O
3
and Cu
5
Pd/
Al
2
O
3
, the N–O bond scission of the (NO)
2
dimer is the RDS in
NO reduction by CO. This step is kinetically facilitated by the
isolated Pd atoms. N
2
O decomposition to N
2
smoothly proceeds
on the Cu surface, which contributes to the excellent N
2
selec-
tivity observed for Cu and Cu-rich catalysts. The key to this
efficient catalysis is the sufficient isolation of Pd atoms by Cu,
highlighting the importance of catalyst design based on single-
atom alloy structures. The insights gained in this study provide
not only a highly efficient deNO
x
system with substantially
reduced noble-metal content, but also open a new path for the
chemistry of single-atom alloys.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by JSPS KAKENHI (Grant Numbers
17H01341 and 17H04965) and by MEXT within the projects
“Integrated Research Consortium on Chemical Sciences
(IRCCS)”and “Elements Strategy Initiative to Form Core
Research Center”, as well as by the JST CREST project
JPMJCR17J3. We deeply appreciate Dr H. Asakura of Kyoto
University for help with XAFS measurement. We thank the
technical staffof the Research Institute for Electronic Science,
Hokkaido University for help with HAADF-STEM observation.
Computation time was provided by the supercomputer systems
at the Institute for Chemical Research, Kyoto University. X-ray
absorption measurements were carried out at the BL-14B2
beamline of SPring-8 at the Japan Synchrotron Radiation
Research Institute (JASRI; 2018A1757).
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