Molecular Catalysis 442 (2017) 74–82

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Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat

Selective reduction of NO over Cu/Al2 O3 : Enhanced catalytic activity
by inﬁnitesimal loading of Rh on Cu/Al2 O3
Saburo Hosokawa a,b,∗ , Kazuya Matsuki b , Kyoko Tamaru b , Yudai Oshino b ,Hirofumi Aritani c , Hiroyuki Asakura a,b , Kentaro Teramura a,b , Tsunehiro Tanaka a,b,∗aElements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8245, Japan
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japanc
Department of Life Science and Green Chemistry, Saitama Institute of Technology, 1690 Fusaiji, Fukaya, Saitama 369-0293, Japanba r t i c l e

i n f oArticle history:
Received 24 July 2017
Received in revised form 30 August 2017
Accepted 4 September 2017
Keywords:
Selective reduction of NO
Cu/Al2 O3Precious metal loadinga b s t r a c t
This study demonstrates that an isolated Cu2+ species formed by interacting with tetrahedral Al specieson ␥-Al2 O3 shows a high catalytic activity for NO reduction with C3 H6 and CO under high oxygen concentrations (lean conditions), and that an inﬁnitesimal amount of Rh (0.01 wt%, 100 ppm) loaded on 0.5 wt%
Cu/Al2 O3 is a more effective catalyst for the reactions, under a wide range of oxygen concentrations, than
0.5 wt% Pd/Al2 O3 or 1.0 wt% Rh/Al2 O3 . Although 0.5 wt% Cu/Al2 O3 itself is effective for the reaction atlean conditions, it shows low catalytic activity under stoichiometric and low oxygen concentrations (richconditions). On the other hand, the loading of 0.01 wt% Rh on Cu/Al2 O3 dramatically improves catalytic
activities under stoichiometric and rich conditions, with maintaining the high NO reduction activity of
Cu/Al2 O3 itself under lean conditions. Therefore, a Rh species in the Rh/Cu/Al2 O3 system must play arole as an active site under stoichiometric and rich conditions. Under lean conditions using Rh/Cu/Al2 O3 ,an isolated Cu2+ species on ␥-Al2 O3 mainly contributes to the NO reduction properties of the catalyst.
No such advantage was observed for Pd/Cu/Al2 O3 ; that is, the catalytic activity of Cu/Al2 O3 under leanconditions is diminished by Pd loading. Hence, we reveal that the roles of the Rh and Cu species in the Rhloaded Cu/Al2 O3 catalyst are effectively shared according to oxygen concentration in selective reductionof NO, when loaded with an inﬁnitesimal amount of Rh. These observations provide novel guidelines forthe development of catalysts that more beneﬁcially use precious metal resources.
© 2017 Elsevier B.V. All rights reserved.

1. Introduction
Nitrogen oxides (NOx ) emitted from various sources, including
mobile sources such as automobiles and aircraft, and stationary
sources such as factories and homes, are well known to have serious
effects as air pollutants on global environmental problems. Typicalnitrogen oxides are nitric oxide (NO), nitrogen dioxide (NO2 ) andnitrous oxide (N2 O). Nitric oxide (NO) and nitrogen dioxide (NO2 )cause acid rain and photochemical smog, while N2 O deteriorates
the ozone layer [1]. Among them, a large amount of NOx has been
exhausted from automobiles [2]. Consequently, the selective reduc-∗ Corresponding authors at: Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8245,
Japan.
E-mail addresses: hosokawa@moleng.kyoto-u.ac.jp (S. Hosokawa),
tanakat@moleng.kyoto-u.ac.jp (T. Tanaka).
http://dx.doi.org/10.1016/j.mcat.2017.09.005
2468-8231/© 2017 Elsevier B.V. All rights reserved.

tion of NOx from automotive engines to N2 has been a longstanding
challenge [3].
To date, NOx -containing pollutants emitted from gasoline
engines in automobiles have been puriﬁed by a three-way catalytic
system that mainly removes the three harmful components: NO,
carbon monoxide (CO), and hydrocarbons (HCs) [4]. Since the regulations controlling exhaust gases become more restrictive fromyear to year, increasingly large amounts of platinum-group-metal
species (PGMs), such as Pd, Rh and Pt, are being used in thesecatalytic systems [5–12]. From an economic perspective, novel catalysts for the puriﬁcation of exhaust gases that use lower amounts
of precious metals, or other alternatives, are highly desired andtheir development has been actively pursued. To that end, manystudies into selective reductions of NO using base-metal catalysts
have been reported recently [13–20].
The concentration of oxygen in automobile exhaust gases is avery important factor for selective reduction of NO. HCs and CO
are oxidized by air to form CO2 and H2 O, but NO must be completely reduced to N2 despite the presence of oxygen. In otherS. Hosokawa et al. / Molecular Catalysis 442 (2017) 74–82

words, puriﬁcation efﬁciency depends on the air-to-fuel ratio (A/F)[21–23]; in low A/F ratios, HCs and CO cannot be completely oxidized, whereas in high A/F ratios the reduction of NO becomes
difﬁcult. As a consequence, all pollutants are usually puriﬁed understoichiometric conditions, where the A/F is around 14.6, over PGMcontaining catalysts. Considering the variations in A/F that areassociated with different driving modes, improved catalyst materials, that are applicable over a wide range of oxygen concentrations,
are highly desired.
With this background in mind, the development of Cu-based
systems as effective catalysts for selective reductions of NO has
been a focus for many years [24–30]. For example, Cu2+ ionexchange zeolites and Cu/Al2 O3 have been reported to show highcatalytic activities for NO reduction and are viewed as alternatives
to noble metals [31–36]. We have also shown that highly dispersed
Cu2+ species on catalyst supports contribute to the catalytic activityof NO-CO reactions under an oxygen-rich atmosphere [37]; hence,highly dispersed Cu species impacts positively on selective reduction of NO.
The present study investigated NO reductions during NO-COC3 H6 reactions, over Cu/Al2 O3 -based catalysts, under a variety ofoxygen concentrations. Herein, we reveal that an isolated Cu2+
species, which strongly interacts with the tetrahedral Al speciesin ␥-Al2 O3 , shows high catalytic activity for NO reductions underslightly lean conditions. We also demonstrate for the ﬁrst time thatan inﬁnitesimal loading (100 ppm) of Rh on Cu/Al2 O3 dramatically
improves catalytic activity over a wide range of oxygen concentrations.

75

was heated to 773 K in a stream of He for 1 h. The efﬂuent gases
from the reactor were analyzed by gas chromatography (Shimadzu
GC-8A, Porapak Q and MS-5A columns) equipped with a thermal
conductivity (TC) detector.
The ␭ value was used as an indicator of oxygen concentration
and was calculated according to the following equation:
␭ = ([CO]a + [NO]a + [O2 ]a × 2)/([CO]s + [NO]s + [O2 ]s × 2)
[CO]a , [NO]a , and [O2 ]a represent the actual gas concentrations, and[CO]s , [NO]s , and [O2 ]s are the concentrations under stoichiometriccondition.
Under the reaction condition described above, the concentrations of CO, C3 H6 and NO were ﬁxed, and [O2 ] was varied from2812.5 ppm to 0 ppm. The chemical reaction equation under stoichiometric condition is:
CO + C3 H6 + 4 NO + 9/2 O2 → 2 N2 + 7 CO2 + 3 H2 O
Therefore, the oxygen concentration corresponding to ␭ = 1,
[O2 ]s , is 1125 ppm, and the ␭ value in the catalytic reaction ranged
from 1.79 to 0.47. A ␭ value of 1 is considered to correspond to
A/F = 14.6. The outlet gases were analyzed after the reaction at eachoxygen concentration had proceeded for 30 min.
Catalytic CO oxidation experiments were also performed
in a ﬁxed-bed reactor as described above. The reaction gas(100 mL min−1 ), composed of 1000 ppm CO, 1000 ppm O2 , andHe as the balance, was introduced to the catalyst (200 mg) at
W/F = 0.12 g s mL−1 . The reaction gas for C3 H6 oxidation experiments was composed of 250 ppm C3 H6 , and 2250 ppm O2 , with He
as the balance.

2. Experimental methods
2.1. Catalyst preparation
␥-Al2 O3 (JRC-ALO-4; 177 m2 g−1 ) and CeO2 (JRC-CEO-3;
92 m2 g−1 ) were provided by the Catalysis Society of Japan. ZrO2
(z-2965; 66 m2 g−1 ) was kindly provided by the Daiichi Kigenso
Kogyo Co., Ltd. Cu-loaded catalysts were prepared using an impregnation method. In the case of 0.5 wt% Cu-loaded catalyst, a Cu
aqueous solution (1.58×10−2 mol L−1 ) was prepared by dissolving
Cu(NO3 )2 ·3H2 O. The support (0.995 g) was added to the mixture ofa deionized water (3 mL) and the Cu solution (5 mL), followed bysolvent evaporation at 80 ◦ C. The resulting powder was calcined in
air at 773 K for 5 h.
PGM/Cu/Al2 O3 catalysts were prepared by a gradual impregnation method in which 0.5 wt% Cu/Al2 O3 was added to an aqueous
solution containing Pt(NH3 )2 (NO2 )2 , or an ethyl acetate solution containing Pd(OAc)2 or Rh(acac)3 . For example, 0.01 wt%
Rh/0.5 wt% Cu/Al2 O3 catalyst was synthesized as follows. A Rh ethylacetate solution (1.00 × 10−3 mol L−1 ) was prepared by dissolving
Rh(acac)3 . 0.5 wt% Cu/Al2 O3 (0.999 g) obtained by calcination at
773 K was added to the mixture of acetone (8 mL) and the Rh solution (1 mL), followed by solvent evaporation at room temperature.
The resulting powder was calcined in air at 773 K for 5 h.
The BET surface areas of representative catalysts were summarized in Table S1.
2.2. Catalytic reactions
Catalytic experiments were carried out in a ﬁxed-bed ﬂow
reactor at atmospheric pressure. A 200 mg sample of the catalyst (25/50 mesh) was placed in a tubular reactor. The reactiongas (100 mL min−1 ), composed of 1000 ppm NO, 1000 ppm CO,
250 ppm C3 H6 , 0–2812.5 ppm O2 , and He as the balance, was introduced to the catalyst bed at W/F = 0.12 g s mL−1 (gas hourly spacevelocity, GHSV ≈ 15000 h−1 ). Before each experiment, the catalyst

2.3. Characterizations
X-ray diffraction (XRD) measurements were performed ona Rigaku Ultima IV X-ray diffractometer with Cu-K␣ radiation.
The Al K-edge X-ray absorption ﬁne structure (XAFS) spectra
were recorded in total-electron-yield mode at room temperatureby using a InSb double-crystal monochromator at the UVSORIMR BL2A beamline. Cu K-edge XAFS spectra were measured on
the SPring-8 BL01B1 beamline, and were recorded in transmission mode at room temperature using a Si(111) double-crystal
monochromator. X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Shimadzu ESCA using Mg-K␣ radiation.
Electron spin resonance (ESR) spectra were recorded at room temperature on a JEOL JES-X320 ESR spectrometer with the following
settings: center ﬁeld, 300 mT; sweep width, ±7 mT; modulation
width, 1.0 × 0.1 mT; amplitude, 5 mT; and time constant, 0.03 s.
The catalysts were treated at room temperature for 1 h undervacuum, before ESR measurements. Diffuse reﬂectance infrared
Fourier transform (DRIFT) spectra were acquired on an FT/IR-4700
spectrometer (JASCO, Japan), equipped with a mercury-cadmiumtellurium (MCT) detector cooled by liquid N2 . The catalyst was
pre-treated with He (100 mL min−1 ) at 773 K for 1 h and the spectrum of the sample treated in this manner was used as the
background. The reaction gas corresponding to a lean condition(total ﬂow rate, 100 mL min−1 : NO, 1000 ppm; C3 H6 , 250 ppm; O2 ,2250 ppm; He balance) was then introduced into the cell. DRIFT
spectra were obtained after the reaction had proceeded for 30 min
at 773 K.
3. Results and discussion
3.1. Catalyst characterizations
Cu/Al2 O3 catalysts were prepared by impregnation of ␥-Al2 O3
with an aqueous solution containing Cu(NO3 )2 ·3H2 O at 353 K, fol-

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S. Hosokawa et al. / Molecular Catalysis 442 (2017) 74–82

Fig. 1. (a) XRD patterns and (b) Cu K-edge XANES spectra of Cu/Al2 O3 with various amounts of Cu loading.

lowed by calcination at 773 K. XRD patterns of Cu/Al2 O3 catalysts
with various loading amounts of Cu were acquired (Fig. 1(a)).
Unfortunately, the peaks associated with the Cu species wereundetectable even for the 2.0 wt% Cu/Al2 O3 sample. Then, theCu K-edge X-ray absorption near edge structure (XANES) spectrum of 0.5 wt% Cu/Al2 O3 is almost identical to that of the 2.0 wt%
Cu/Al2 O3 sample (Fig. 1(b)). The features of these spectra are close
to those of Cu(OH)2 that contains an octahedral Cu2+ species. The
extended X-ray absorption ﬁne structure (EXAFS) oscillation of
0.5 wt% Cu/Al2 O3 is essentially identical to that of 2.0 wt% Cu/Al2 O3(Fig. S1(a)). In the Fourier-transform of EXAFS spectra, no peakat 2–4 Å, corresponding to Cu−Cu bonds, was observed even in
2.0 wt% Cu/Al2 O3 (Fig. S1(b)). These results indicate that octahedral Cu2+ species are highly dispersed on Al2 O3 , even at 2.0 wt%
loading.
To investigate the dispersion state of Cu2+ species on ␥-Al2 O3
in detail, we investigate Cu/Al2 O3 catalysts by using ESR spectra, in which Cu2+ (d9 ) species showing paramagnetic resonanceis well-known to be detectable for a highly-dispersed Cu2+ species
on supports [38–40]. Axially symmetric signals originating from thecopper nuclear spin of 1/2 were observed for the Cu/Al2 O3 catalysts
examined in this work (Fig. 2); in other words, Cu/Al2 O3 catalysts
have a parallel component (g// = 2.33) and a perpendicular component (g⊥ = 2.05). As was previously reported by us and other groups[38–40], the 0.1 and 0.5 wt% Cu/Al2 O3 samples clearly exhibit parallel hyperﬁne components that are divided into four signals, whilethese hyperﬁne structures become obscured for catalysts having Cu
loadings above 1.0 wt%. A signal (g// = 2.23) inﬂuenced by neighboring Cu2+ species clearly appears in the 1.0 wt% and 2.0 wt%
Cu/Al2 O3 sample. The parallel hyperﬁne components, which are
due to isolated Cu2+ species, are known to be generally broadened by magnetic dipole-dipole interactions with neighboring Cu2+
species. In other words, the distance between Cu2+ species highly
dispersed on Al2 O3 is suggested to approach by increasing Cu loading. Therefore, we conclude that 0.1 and 0.5 wt% Cu/Al2 O3 contain
only isolated Cu2+ species, and that local aggregation of Cu species
begins around 1.0 wt% Cu loading.
Since an aggregated Cu2+ solid species is scarcely detectable by
ESR techniques, the peak intensities associated with Cu loading
actually decrease for catalysts with high Cu loadings. In addition,
the ESR spectrum of 0.5 wt% Cu catalysts supported on CeO2 or ZrO2
hardly shows any parallel hyperﬁne component arising from iso-Table 1Results for deconvoluted peaks in Al K-edge XANES spectra.
Relative ratio of the peak areas (%)␥-Al2 O3
0.5 wt% Cu/␥-Al2 O3abPeak at 1565 eVa

Peak at 1568 eVb

Peak at 1570 eVb

43
27

14
25

43
48

The peak was attributed to tetrahedral Al species.
The peaks were attributed to distorted octahedral Al species.

lated Cu species (Fig. S2), suggesting that the Al2 O3 support assistsin the formation of the isolated Cu2+ species.
The Al2 O3 used in this study is ␥-Al2 O3 , which has a spinel
structure consisting of both tetrahedral and octahedral Al species(Fig. 3(a)). The relative ratio of tetrahedral to octahedral Al sites
is known to alter through the introduction of defects [41]. The
XANES spectrum of the Al K-edge of ␥-Al2 O3 sample is shown in
Fig. 3(b). The observed peaks are due to electronic transitions from1 s to 3p orbitals. Spectra of LaAlO3 , ␣-Al2 O3 and AlPO4 are shown
as reference. LaAlO3 and ␣-Al2 O3 contain distorted octahedral Al
species, while AlPO4 is only composed of tetrahedral Al species. On
the basis of these reference spectra and previous reports [42,43],
the peak at 1565 eV is assigned to tetrahedral Al species, and thepeaks at 1568 eV and 1570 eV to distorted octahedral Al species; inother words, ␥-Al2 O3 exhibits peaks belonging to both tetrahedral
and octahedral Al species. In the Al K-edge XANES spectrum of the
0.5 wt% Cu/Al2 O3 catalyst, Cu loading diminishes the peak at around
1565 eV belonging to the tetrahedral species. Then, the relative
ratio of tetrahedral Al species was estimated by a ﬁtting analysis
with Al K-edge XANES spectrum (Fig. S3). The peak area due totetrahedral Al species clearly decreased by Cu loading (Table 1),
suggesting that the tetrahedral Al species must be transformed tooctahedral Al species by the interaction between Cu species andthe tetrahedral Al species in Al2 O3 . These results allow us to conclude that the isolated Cu2+ species in 0.5 wt% Cu/Al2 O3 is formed
by interacting with tetrahedral Al species in ␥-Al2 O3 .
3.2. Selective reduction of NO over Cu/Al2 O3 catalysts
Fig. 4(a and b) shows the results of the reactions of NO-CO-C3 H6
at 773 K over 0.5 wt% and 2.0 wt% Cu/Al2 O3 catalysts under various oxygen concentration conditions. The catalytic activities for

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77

Fig. 2. ESR spectra of Cu/Al2 O3 with various amounts of Cu loading.

Fig. 3. (a) Crystal structure of ␥-Al2 O3 . (b) Al K-edge XANES spectra of Al2 O3 , 0.5 wt% Cu/Al2 O3 , and reference samples.

NO reduction over the Cu/Al2 O3 catalysts, with controlled amountsof Cu loading, were superior to that of Pd/Al2 O3 under lean (␭ > 1)
and rich (␭ < 1) conditions, but under stoichiometric oxygen condition (␭ = 1); 0.5 wt% Pd/Al2 O3 effectively reduced NO to N2 only
under stoichiometric oxygen condition. Speciﬁcally, the 0.5 wt%
Cu/Al2 O3 catalyst showed high catalytic activity under lean conditions (1.2 < ␭ < 1.79), and the 2.0 wt% Cu/Al2 O3 was effective underrich conditions (0.47 < ␭ < 1.0). CeO2 or ZrO2 -supported 0.5 wt% Cu
catalyst exhibited high catalytic activity only under stoichiometric and/or rich conditions (Fig. 4(c and d)), while NO reduction
on these catalysts hardly proceeded under lean conditions. No
N2 O formation was observed on Cu-based catalysts (Fig. S4), while
Pd/Al2 O3 catalyst slightly reduced NO to N2 O under lean conditions(␭ = 1.26−1.05). Considering the difﬁculty in achieving NO reduction under lean conditions over Pd/Al2 O3 or supported Cu catalysts,

the efﬁcient reduction of NO under lean conditions (␭ > 1) using the
0.5 wt% Cu/Al2 O3 catalyst is very interesting.
The catalytic activities for CO and C3 H6 oxidation, over thecatalysts examined in this study, increase with increasing oxygen concentration. With the exception of 0.5 wt% Cu/Al2 O3 , thesepollutants were completely oxidized to CO2 under conditions inwhich the oxygen concentrations were more than stoichiometriccondition (␭ = 1). On the other hand, the residual C3 H6 or CO wasobserved in the reactions under the lean conditions over 0.5 wt%
Cu/Al2 O3 catalyst.
The catalytic activities of the NO-CO-O2 and NO-C3 H6 -O2 reactions over 0.5 wt% Cu/Al2 O3 were also investigated (Fig. S5). NO
reduction in the presence of only CO hardly proceeds on 0.5 wt%
Cu/Al2 O3 , while the analogous reduction with C3 H6 showed highcatalytic activity even under lean conditions. The catalytic activity

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S. Hosokawa et al. / Molecular Catalysis 442 (2017) 74–82

Fig. 4. Selective reductions of NO under a variety of oxygen concentrations at 773 K; (a) and (c) conversion of NO to N2 ; (b) and (d) conversion of C3 H6 + CO to CO2 . The
reaction gas (100 mL min−1 ), composed of 1000 ppm NO, 1000 ppm CO, 250 ppm C3 H6 , 2812.5–0 ppm O2 , and He as the balance, was introduced to the catalyst bed. The
concentrations of CO, C3 H6 and NO were ﬁxed, and [O2 ] varied from 2812.5 ppm to 0 ppm. The oxygen concentration at a stoichiometric condition (␭ = 1) is 1125 ppm.

with C3 H6 alone is comparable to that with C3 H6 and CO underlean conditions, suggesting that C3 H6 is mainly used as a reductant
for the NO-CO-C3 H6 reaction in the presence of O2 over 0.5 wt%
Cu/Al2 O3 .
The effect of Cu loading on the Al2 O3 -supported catalyst, for NO
reduction activity under various oxygen concentrations at 773 K,
was investigated (Fig. 5). Increases in Cu loading under rich or
stoichiometric conditions resulted in enhanced NO reduction activity. However, under lean conditions, the 0.5 wt% Cu/Al2 O3 catalyst
exhibited the highest NO reduction activity among the examined
Cu/Al2 O3 catalysts.
Based on the characterization and the reaction results of
Cu/Al2 O3 catalysts with various Cu loading amounts, the oxidation characteristics of isolated Cu2+ species strongly interacted withtetrahedral Al species is much inferior to that of bulk Cu species.
Accordingly, we conclude that NO reduction on 0.5 wt% Cu/Al2 O3
proceeds by utilizing the residual CO and C3 H6 even in lean conditions.
3.3. Precious-group-metal loaded Cu/Al2 O3 catalysts
An inﬁnitesimal loading (0.01 wt%, 100 ppm) of precious metal
on 0.5 wt% Cu/Al2 O3 dramatically inﬂuenced its catalytic activity.

Fig. 6(a and b) shows the results of NO-CO-C3 H6 reactions at 773 K
under various oxygen concentrations over 0.01 wt% PGM/Cu/Al2 O3
catalysts. For CO2 production derived from CO and C3 H6 combustion under lean conditions, all of these modiﬁed catalysts
exhibit much higher activities than Cu/Al2 O3 alone. Pt/Cu/Al2 O3and Pd/Cu/Al2 O3 showed high catalytic activities for NO reduction
only under stoichiometric condition, and formed a small amountof N2 O under slightly lean conditions (␭ = 1.26–1.05) (Fig. S6).
Interestingly, NO reduction on Rh/Cu/Al2 O3 effectively proceeded
without N2 O formation over the entire oxygen concentration range,
although Cu/Al2 O3 itself showed low catalytic activity under stoichiometric and rich conditions.
The Rh/Al2 O3 catalysts hardly showed any NO reduction activity
under lean conditions regardless of Rh loading amount (Fig. 6(c andd)). Rh/Cu/Al2 O3 had high oxidation activity under lean conditions
with maintaining the high NO reduction activity of Cu/Al2 O3 itself,
although the oxidation activity of Cu/Al2 O3 did not reach 100%.
NO reduction under around stoichiometric conditions proceeded
poorly on 0.01 wt% Rh/Al2 O3 , while Rh loaded on Cu/Al2 O3 proceeded to completely reduce NO. Actually, for selective reduction ofNO under stoichiometric condition (Fig. 7), Rh/Cu/Al2 O3 displayedmuch higher catalytic activity at lower temperatures than 0.01 wt%
Rh/Al2 O3 or 0.5 wt% Cu/Al2 O3 itself, suggesting that a synergistic

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79

Fig. 5. Effect of Cu loading on N2 production under a variety of oxygen concentrations. (a) Low oxygen concentration (␭ = 0.79); (b) stoichiometric condition (␭ = 1.00); (c)high oxygen concentration (␭ = 1.79).

Fig. 6. Selective reductions of NO under a variety of oxygen concentrations on 0.01 wt% PGM/0.5 wt% Cu/Al2 O3 (PGM/Cu) and Rh/Al2 O3 ; (a) and (c) conversion of NO to N2 ;(b) and (d) conversion of C3 H6 + CO to CO2 .

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S. Hosokawa et al. / Molecular Catalysis 442 (2017) 74–82

Fig. 7. Selective reductions of NO under stoichiometric condition over 0.01 wt% Rh/0.5 wt% Cu/Al2 O3 ; (a) conversion of NO to N2 ; (b) conversion of C3 H6 + CO to CO2 .

effect operates as a result of Rh loading onto Cu/Al2 O3 under stoichiometric condition. Clearly, an inﬁnitesimal amount of Rh loaded
on 0.5 wt% Cu/Al2 O3 provides the following effects: (1) The oxidation activity under lean conditions using Cu/Al2 O3 is improved by
Rh loading, while the high reduction activity of Cu/Al2 O3 is maintained; and (2) catalytic activities under rich and stoichiometricconditions over Cu/Al2 O3 are dramatically enhanced.
The optimal Rh loading amount was investigated (Fig. S7).
The catalytic activities under the rich and stoichiometric conditions of 0.5 wt% Cu/Al2 O3 itself were drastically improved evenby 0.005 wt% Rh loading, while NO reduction under the stoichiometric condition was inadequate. The catalytic activities under theentire oxygen concentrations of 0.02 wt% Rh/Cu/Al2 O3 were essentially identical with those of 0.01 wt% Rh/Cu/Al2 O3 ; however, thecatalytic activities under lean conditions of the former catalyst
slightly decreased compared to those of the latter catalyst. One of
the reasons is that the excessively-loaded Rh species must inhibit
the role of 0.5 wt% Cu/Al2 O3 . From these results, the optimal Rh
loading amount for 0.5 wt% Cu/Al2 O3 is determined to be 0.01 wt%,
100 ppm. Thus, we reveal that Rh-loaded Cu/Al2 O3 catalyst caneffectively share the role of Rh and Cu species by an inﬁnitesimalloading of Rh.
To conﬁrm the state of Cu species in 0.01 wt% Rh/0.5 wt%
Cu/Al2 O3 , we carried out XPS and ESR measurements (Fig. S8). The
Cu 2p spectrum of Rh/Cu/Al2 O3 is essentially identical with that of
Cu/Al2 O3 . Interestingly, ESR analysis reveals that parallel hyperﬁnecomponents observed in 0.5 wt% Cu/Al2 O3 become a little unclear
by Rh loading, suggesting that the aggregation of isolated Cu2+
species slightly proceeds by Rh loading. However, since the majority of Cu species in Rh/Cu/Al2 O3 is preserved as the isolated Cu2+
species regardless of the Rh loading, Rh/Cu/Al2 O3 maintains thehigh catalytic activity under lean condition of Cu/Al2 O3 itself.
Fig. 8 displays the DRIFT spectra of adsorbed species during
NO-C3 H6 -O2 reactions at 773 K under high oxygen concentration
condition (lean condition) over 0.01 wt% PGM/0.5 wt% Cu/Al2 O3 .
The Cu/Al2 O3 sample displays bands at around 2116 cm−1 and2151 cm−1 , which are assigned to Cu+ -CO and nitrile species,
respectively [44–46]. In addition, Cu/Al2 O3 displays a weak bandat 2232 cm−1 assigned to NCO species bound to Al ions (Al-NCO).
These species are clearly observed on the Rh/Cu/Al2 O3 catalyst,
even under lean conditions, compared to Pd/Cu/Al2 O3 . The formation of N2 during the selective reductions has been reported tooccur via intermediates such as isocyanates and nitriles [46]. These
results reveal that small amounts of loaded Pd inhibit the formationof intermediates such as isocyanates and nitriles on Cu/Al2 O3 underlean conditions, resulting in low catalytic activity for NO reduction

Fig. 8. DRIFT spectra at 773 K under NO-C3 H6 -O2 ﬂow corresponding to a lean condition. The reaction gas condition is as follows: total ﬂow rate, 100 mL min−1 ; NO,
1000 ppm; C3 H6 , 250 ppm; O2 , 2250 ppm; He balance.

under these conditions. On the other hand, even under lean conditions, these intermediates are still observed for Rh/Cu/Al2 O3 . These
results demonstrate that the catalytic activity of Cu/Al2 O3 itself ispreserved under lean conditions, regardless of Rh loading.
Inspired by the results obtained from DRIFT spectroscopy,
the oxidation activities of Pd/Cu/Al2 O3 , Rh/Cu/Al2 O3 and 0.5 wt%
Cu/Al2 O3 toward CO or C3 H6 combustion were investigated (Fig. 9).
An inﬁnitesimal loading of Pd dramatically enhanced the oxidation activity of Cu/Al2 O3 . As expected, there was no signiﬁcant
improvement in the combustion activity of Rh-loaded Cu/Al2 O3 .
These results suggest that Rh loading hardly promotes CO andC3 H6 combustion compared to Pd loading. Considering the DRIFT
result and the oxidation activity of Rh/Cu/Al2 O3 , one of the synergetic effects of Rh and Cu species is the moderate improvement ofoxidation characteristics for 0.5 wt% Cu/Al2 O3 itself. As shown in
Fig. 6, Rh/Cu/Al2 O3 shows the higher oxidation activity under leancondition (␭ = 1.79) than 0.01 wt% Rh/Al2 O3 or 0.5 wt% Cu/Al2 O3
itself. The CO conversions (69%) on 0.5 wt% Cu/Al2 O3 was essentially identical with that (61%) on 0.01 wt% Rh/Al2 O3 . Interestingly,
C3 H6 conversion on 0.5 wt% Cu/Al2 O3 reached 100%, while 0.01 wt%
Rh/Al2 O3 remained C3 H6 (Conv. = 41%) even under the lean condition. Therefore, in the case of 0.01 wt% Rh/0.5 wt% Cu/Al2 O3 ,

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Fig. 9. Combustion data as functions of temperature. (a) CO and (b) C3 H6 combustion over 0.5 wt% Cu/Al2 O3 (Cu), and 0.01 wt% PGM/0.5 wt% Cu/Al2 O3 (PGM/Cu).

Rh species moderately helps the oxidation characteristics of Cu
species, resulting in the enhancement for CO and C3 H6 oxidation
under lean condition with maintaining the high NO reduction activity of Cu/Al2 O3 itself. Unfortunately, since the loading of Pd species
having extremely-high oxidation characteristics inhibits the formation of isocyanates and nitriles over Cu/Al2 O3 , Pd/Cu/Al2 O3
hardly shows the catalytic activity for NO reduction under leancondition.

Institute for Molecular Science (Proposal No. 2014B1371 and No.
14-801).

Appendix A. Supplementary data
Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.mcat.2017.09.
005.

4. Conclusions
Catalytic NO-CO-C3 H6 -O2 reactions over Cu/Al2 O3 are significantly inﬂuenced by the amount of Cu loading. As Cu loadingis increased, the catalytic activities under rich and stoichiometric conditions increase. In contrast, 0.5 wt% Cu/Al2 O3 showed thehighest activity under lean conditions among the examined catalysts, and the catalytic activity is signiﬁcantly superior to thatof the PGM catalyst, Pd/Al2 O3 . ESR and XAFS spectra reveal that
an isolated Cu2+ species, which strongly interacts with the tetrahedral Al species in ␥-Al2 O3 , plays a role as an active site underlean conditions. In addition, this study demonstrates the following unambiguous facts in relation to the effects of an inﬁnitesimalamount (0.01 wt%, 100 ppm) of Rh loaded onto Cu/Al2 O3 . Rh loading on 0.5 wt% Cu/Al2 O3 shows high catalytic activity over theentire oxygen concentration range, although the catalytic activity of 0.5 wt% Cu/Al2 O3 alone is low under stoichiometric or richcondition. On the other hand, Pt or Pd loading diminishes NO
reduction activity under lean conditions. DRIFT spectra indicatethat Rh/Cu/Al2 O3 forms isocyanates and nitriles under lean conditions, which are intermediates in these selective reductions of NO.
Since Pd/Cu/Al2 O3 has extremely high oxidation activity, no suchintermediate species are detected under lean conditions using thiscatalyst. These results suggest that NO reduction on Rh/Cu/Al2 O3
proceeds through the intermediates formed from CO and/or C3 H6 ,even under lean conditions. Therefore, the inﬁnitesimal loading ofRh on Cu/Al2 O3 enables us to effectively share the roles of Cu species
and Rh species according to oxygen concentration in NO-CO-C3 H6reaction. These observations provide the novel guideline for thedevelopment of catalysts that more beneﬁcially use precious metal
resources.
Acknowledgements
This work was partially supported by the Program for Elements
Strategy Initiative for Catalysts & Batteries (ESICB) and by a Grantin-Aid for Scientiﬁc Research (B) (General) (16H04567). The X-ray
absorption experiments were performed with the approval of theJapan Synchrotron Radiation Research Institute (JASRI) and theReferences[1] A. Fritz, V. Pitchon, Appl. Catal. B: Environ. 13 (1997) 1–25.
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