Cu/ZrO2 catalysts for NO-CH4 reaction

Article abstract of DOI:10.1246/cl.2000.532

Cu(2wt%)/ZrO₂ shows a high activity for catalytic NO decomposition with CH₄ at 873 K. Cu K-edge XANES results show that Cu²⁺ ions were reduced to form Cu⁺ after the reaction, and, metallic Cu⁰ was scarcely seen. It is concluded that the redox of the Cu ions (Cu²⁺ ? Cu⁺) plays an important role for NO-CH₄ reaction.

Full text of DOI:10.1246/cl.2000.532

5
32
Chemistry Letters 2000
Cu/ZrO Catalysts for NO-CH Reaction
2
4
†
†
††
Hirofumi Aritani,* Shuji Kawaguchi, Takashi Yamamoto, Tsunehiro Tanaka, Yasuaki Okamoto, and Seiichiro Imamura
Faculty of Engineering and Design, Kyoto Institute of Technology, Sakyo-ku, Kyoto 606-8585
†
Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyoto, Sakyo-ku, Kyoto 606-8501
††
Department of Material Science, Faculty of Science and Engineering, Shimane University, Matsue 690-0823
(Received February 17, 2000; CL-000169)
Cu(2wt%)/ZrO shows a high activity for catalytic NO
for 1 h in a He atmosphere before the reaction. For the reaction,
2
decomposition with CH at 873 K. Cu K-edge XANES results
a gas mixture containing NO (5300ppm)/CH (2700ppm)/He was
4
4
show that Cu2+ ions were reduced to form Cu after the reac-
+
fed to the reactor. The total flow-rate was 33.5 cm min (SV =
3
-1
0
3
-1 -1
tion, and, metallic Cu was scarcely seen. It is concluded that
8040 cm g-cat h ) in both the pretreatment and reaction. In
the case of rising temperature reaction, the product gas was ana-
lyzed at 50 K intervals (kept on this temperature for 10 min)
while the temperature was raised from 423 to 873 K at a rate of
2
+ →
+
the redox of the Cu ions (Cu
Cu ) plays an important role
←
for NO- CH reaction.
4
-
1
5
K min . In the steady state reaction case, the reaction was
Since the appearance of the reports on Cu/MFI,1,2 many
workers have reported the selective catalytic reduction (SCR) of
carried out at 773 K with the same composition and flow rate as
in the non-steady state case. For each reaction mode, the prod-
uct gas was analyzed periodically by means of a gas-chromato-
graph (Shimadzu GC-4B, TCD) with two columns packed with
Porapak-Q and molecular-sieve 5A. After the reaction, the cata-
lyst sample was treated with He for more than 5 min at the reac-
tion temperature, and cooled to room temperature overnight in a
He atmosphere. The catalyst sample was then collected and was
subjected to the XANES measurement. The XANES spectra
were measured at the BL-7C station of the Photon Factory in the
High Energy Accelerator Research Organization (KEK-PF) with
a Si(111) double crystal (d = 3.13551 /Å) monochromator. The
spectra were measured in a transmission mode at room tempera-
ture. Data analysis of XANES was carried out by the method
NO with hydrocarbons on copper-containing heterogeneous
x
catalysts. In particular, SCR of NO with CH is one of the note-
4
worthy reactions because of the low reactivity of CH . In the
4
presence of O , many Cu ion-exchanged zeolites such as Cu-
2
3
MFI catalyze rather CH combustion than SCR of NO. In this
reaction case, In/MFI-based catalysts or Pd-based ones are
reported to exhibit high activity. In the absence of O , Cu-MFI
and Pd-MFI catalysts have been reported as the active ones.
4
4
5,6
7
2
6
For other catalyst systems except for zeolite-based ones, Cu/ZrO2
has reported recently to exhibit the SCR activity with ammonia
8,9
or hydrocarbons in the oxidizing atmosphere.
The reactivity
of NO decomposition with CO and the details of the active
10,11
12
species of Cu/ZrO for NO decomposition were reported.
In
described elsewhere.
2
this case, highly dispersed Cu metal species were formed during
the reaction at above 473 K, and deactivation was brought about
by aggregation of Cu metal species at high temperature of
around 773 K. Since CO shows strong reducibility, Cu on ZrO2
can be reduced easily. In this study, we employed the Cu/ ZrO2
catalyst for NO decomposition with CH . It has been found that
4
ZrO -supported Cu shows higher activity for this reaction than
2
Cu catalysts on other supports. The relationship between the
reactivity and the formation of the active Cu species was also
discussed by using Cu K-edge XANES spectroscopy.
Supported Cu catalysts (2 wt% as Cu) were prepared by
impregnation of each oxide support with an aqueous solution of
Cu(NO ) ·3H O (Nacalai Tesque). The oxide supports used in
3
2
2
this study were SiO (Aerosil), γ-Al O , ZnO (Nacalai Tesque),
2
2
3
TiO (Wako; mixture of rutile and anatase), ZrO (Nacalai
Table 1 shows the reactivity of the supported Cu catalysts
2
2
Tesque; monoclinic), and SiO -Al O (JRC-SAH-1; containing
for NO decomposition with CH . The products on all the cata-
2
2
3
4
2
8.6 wt% of Al O ). These supports were calcined at 873 K for
lysts were N , H O, and CO only, and N O or CO could not
2
3
2
2
2
2
3
h prior to the impregnation. The MgO support was obtained
detected, indicating that the reaction, 4 NO + CH → 2 N + 2
4 2
by calcination of Mg(OH) (Kojundo-Kagaku) at 873 K for 6 h.
H O + CO , proceeds predominantly. As the composition ratio
2 2
2
The impregnating solution was stirred at room temperature, and
then evaporated at 343 K for ca. 6 h to form a paste. The paste
was then dried overnight and calcined at 773 K for 6 h. A refer-
of NO/CH in the reactant gas is about 2, more than 60 % of
4
CH remained when NO decomposed completely. The reactivi-
4
ty at 873 K is summarized as follows; Cu/ZrO > Cu/ZnO >
2
ence compound, CuZrO (Soekawa Rikagaku), was also
Cu/MgO > Cu/Al O > Cu/TiO > others. In addition, copper-
3
2
3
2
employed for the reaction The NO decomposition with CH4
was carried out using a fixed-bed flow reactor with an on-line
gas chromatograph. The catalyst sample (0.25 g) was set in a
ceramic tube reactor. Each catalyst sample was treated at 873 K
zirconate (CuZrO ) and ZrO showed no reactivity. ZrO -sup-
3
2
2
ported Cu catalyst exhibits the highest activity for supported
catalysts, and this result is in accordance with the case of NO-
1
0
CO reaction. However, the reactive temperature ranges are
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
533
different between NO-CO and NO-CH reactions. NO reduc-
Cu K-edge XANES was applied in order to reveal the state
4
tion to produce N O and N at low temperature of around 453 K
took place in the case of NO-CO reaction, although it does not
of active Cu species, because the XANES spectrum reflects the
2
2
1
0
13,14
valence of Cu ions and their structure sensitively.
The
proceed at this temperature in the NO-CH reaction case. It is
XANES spectra of Cu/ZrO with various Cu contents are
4
2
likely that the difference in the reactivity between these two
reactions possibly relates to the reducibility of the reductants
shown in Figure 1. For the Cu/ZrO samples before the reac-
tion, the spectra almost resemble one another, and they can be
reproduced roughly by the summation of XANES spectra of
2
(
CO and CH ). From the result of successive (steady-state)
4
reaction at 873 K, CH conversion was 37.1 and 37.2 % for 30
CuO and Cu(OH) . After the reaction at 873 K, the spectra of
4
2
and 180 min, respectively, and NO conversion is 100 % during
the whole reaction time. It indicates that the NO decomposition
all the samples are changed; the edge energy of XANES
becomes low, indicating that Cu ions were partly reduced dur-
ing the reaction. From the value of edge energy, the reduced
2+
takes place catalytically without deactivation of Cu/ZrO . The
2
relationship between the Cu content in Cu/ZrO and the activity
Cu ions in the reacted Cu/ZrO samples can not be assigned as
2
2
+
(
temperature-rising reaction) is shown in Table 2. In all the
Cu metal but can be interpreted as Cu . Thus, coexistence of
2+
+
Cu and Cu ions on ZrO are suggested about after the NO-
2
CH reaction. In the case of NO-CO reaction, highly dispersed
4
0
Cu species are formed in Cu/ZrO during the reaction at less
2
1
1
than 773 K, and they may be the catalytically active species.
On the other hand, Cu ion is formed in the NO-CH reaction.
+
4
These results show that Cu on ZrO can be reduced easily dur-
2
ing the NO decomposition with the reductant (CO or CH ), and
4
the reduction level of Cu ions strongly relates to the reducibility
2
+
of the reductant. Since CH has weak reducibility, Cu is
4
+
reduced partly to form Cu when the decomposition of NO and
CH proceeds. Thus, the Cu ions on ZrO act as the active
Cu ) plays an impor-
tant role for NO-CH reaction. For the NO-CO reaction, Cu
+
samples, NO decomposed completely at 873 K. The reactivities
4
2
2
+ →
+
of 1–4 wt% Cu/ZrO samples are almost the same, although 2
species, and redox of the Cu ions (Cu
2
←
2
+
wt% Cu/ZrO exhibits the slightly higher conversion of NO at
2
4
+
0
7
73 K. For 2 wt% Cu/ZrO catalyst, the molar ratio of converted
species on ZrO are reduced stepwise to Cu , dispersed Cu ,
2
2
NO/CH (at 873 K) was 5.3. In the case of steady-state reaction
at 873 K, the ratio was 5.4 for 30 and 180 min. If the NO-CH4
and finally to metal particles, and the formation of the metal
particle causes the deactivation. On the other hand, the metal-
4
1
5
reaction proceeds stoichiometrically (4 NO + CH → 2 N + 2
lic species is not formed for NO-CH reaction, and therefore,
4
2
4
+
H O + CO ), the ratio should be 4. These results may suggest
the catalytically active Cu species on ZrO is stable for NO-
2
2
2
that non-stoichiometric (2 NO → N + O and/or 4 NO → N +
CH reaction at 873 K.
2
2
2
4
2
NO ) reaction independent on CH proceeds partly.
The XANES experiments at the Cu K-edge were per-
formed under the approval of Photon Factory (KEK-PF)
Program Advisory Committee (Proposal No. 94G205).
2
4
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