604
Chemistry Letters 2001
Simultaneous Catalytic Removal of Nitrogen Oxides and Soot by Copper-Loaded MFI Zeolites
Yasutake Teraoka,* Kazunori Kanada, Hiroshi Furukawa, Isamu Moriguchi, and Shuichi Kagawa
Department of Applied Chemistry, Faculty of Engineering, Nagasaki University, Nagasaki 852-8521
(Received March 12, 2001; CL-010217)
Cu-loaded MFI zeolites showed the catalytic activity for
the oxidation of soot and reduction of NOx simultaneously in
the soot–NOx–O2 reaction system. Ion-exchanged and impreg-
nated catalysts showed the comparable activity, but the latter
was decidedly superior to the former with respect to the selec-
tivity to NO reduction into N2.
Nitrogen oxides (NOx) and soot particulates emitted from
diesel exhaust have been greatly contributing to the environ-
mental pollution, and currently the regulation of diesel emis-
sions becomes tightened with respect to nitrogen oxides (NOx)
and particulate matters (PM): PM consists mainly of soot and
soluble organic fraction (SOF). Apart from the catalytic after-
treatments so far investigated actively, such as the selective cat-
alytic reduction of NOx by hydrocarbons and the oxidation of
CO, gaseous hydrocarbons and SOF,1 another possible option is
the simultaneous catalytic removal of NOx and PM (soot).2 We
have been studying the simultaneous NOx–soot removal reac-
tion with respect to catalyst development3–8 and reaction mech-
anism7,9,10 and revealed that mixed metal oxides with per-
ovskite-related3,4,7,8 and spinel5–7 structures are promising cata-
lysts for this reaction. This paper reports the catalytic property
of Cu-loaded MFI for the simultaneous NOx–soot removal
reaction. It has turned out that Cu-loaded MFI catalysts pre-
pared by an impregnation method are good candidates showing
high activity and selectivity to N2 formation.
Na-MFI (SiO2/Al2O3=23.3, MFI2) and NH4-MFI (39.5,
MFI4) were kindly supplied by Tosoh Corporation. Cu ion-
exchanged MFI (Cu-MFI) was prepared as follows. The parent
zeolite was treated with 0.1 M aq NaNO3 at 60 °C for 1 day,
followed by a conventional ion-exchange procedure using aq
Cu(II) acetate at 60 °C for 1 day. In the preparation of Cu-
impregnated catalyst (Cu/Na-MFI2), Na-MFI2 powder was put
in an aq solution of Cu(II) acetate, and immediately the suspen-
sion was evaporated to dryness. 11 Both Cu-MFI and Cu/Na-
MFI2 were finally air-calcined at 550 °C for 1 h. The Cu load-
ing was expressed by wt% of Cu: 1 wt% Cu loading corre-
sponds to 27% and 42% ion-exchange levels for MFI2 and
MFI4, respectively.
The catalytic activity for the simultaneous NOx–soot
removal was evaluated by a technique of the temperature pro-
grammed reaction (TPR).3–10 A catalyst and activated carbon12
(ca. 5 wt%) was well mixed by mortar and pestle. The tight
mixture (0.33 g) thus obtained was packed in a reactor and
heated at a rate of 1 °C min–1 under flowing NO(0.5%)–
O2(5%)–He(balance) at 20 cm3 min–1 and the outlet gas was
analyzed with intervals of about 15 min by a TCD gas chro-
matograph (Shimadzu GC-8A).
rence of the simultaneous NOx–soot removal reaction: the soot
pre-mixed with the catalyst was oxidized by either NOx or O2 to
produce CO2, and NOx was reduced by the soot into N2 and
N2O. The CO2 formation over Na-MFI2 took place at higher
temperature region than that over 3.1 wt% Cu-MFI2 (Figure 1).
In addition, the soot was completely oxidized into CO2 over all
the Cu-loaded catalysts, while CO amounting about 18% of
CO2 was formed over Na-MFI (not shown in Figure 1). These
results indicate that Cu introduced in MFI zeolites effectively
works as a catalyst for the simultaneous NOx–soot removal
reaction. From the TPR result, ignition temperature (Tig),
which was used as a measure of activity, was obtained by
extrapolating the steeply ascending portion of the CO2 forma-
tion curve to zero CO2 concentration (estimation error; ±5 °C),
and total amounts of CO2, N2 and N2O formed throughout the
TPR run, V[CO2], V[N2] and V[N2O], were obtained by inte-
grating the respective curves. The selectivity to N2 formation
(S[N2]) was defined by V[N2]/V[CO2], which corresponds to a
fraction of soot used for the reduction of NO into N2.
In Figure 2, Tig value (A) and the selectivity to N2 forma-
tion (B) are plotted as a function of the Cu loading. For all the
catalyst systems, the Tig values decreased, or the soot ignition
activity increased, with increasing the Cu loading and reached
the constant activity above ca. 1 wt%. These results indicate
that the soot ignition activity of Cu-loaded MFI in the simulta-
neous NOx–soot removal reaction is almost exclusively deter-
mined by the Cu loading (wt%) irrespective of the Si/Al ratio of
MFI, preparation methods and existing state of Cu. As for the
selectivity to N2 formation, both the ion-exchanged catalysts,
Cu-MFI2 and Cu-MFI4, showed nearly the same tendency that
the S[N2] gradually decreased with increasing the Cu loading
and reached the steady value above ca. 2.3 wt%. On the other
hand, impregnated Cu/Na-MFI2 showed higher S[N2] values
TPR result over 3.1 wt% Cu-MFI2 in the NO–O2–He
atmosphere is shown in Figure 1. The formation of CO2, N2
and N2O at the same temperature range evidenced the occur-
Copyright © 2001 The Chemical Society of Japan
Teraoka, Yasutake
