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 ofApplied Chemistry, Faculty ofEngineering, 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 NO, simultaneously in
`the soot—NOx®2 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 (N02) 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 (N02)
`and particulate matters (PM): PM consists mainly of soot and
`soluble organic fraction (SOF). Apart from the catalytic after-
`treatrnents so far investigated actively, such as the selective cat-
`alytic reduction of N02 by hydrocarbons and the oxidation of
`CO, gaseous hydrocarbons and SOF,' another possible option is
`the simultaneous catalytic removal of N02 and PM (soot)? We
`have been studying the simultaneous NOx—soot removal reac-
`tion with respect to catalyst development3‘8 and reaction mech-
`anism7’9’1° and revealed that mixed metal oxides with per-
`ovskite-related3=4=7=8 and spine15’7 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/A12O3=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(ll) acetate, and immediately the suspen-
`sion was evaporated to dryness. ” Both Cu-MFI and Cu/Na-
`MFI2 were finally air-calcined at 550 °C for l 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).3’1° A catalyst and activated carbon”
`(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(ba1ance) 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).
`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 N20 at the same temperature range evidenced the occur-
`
`25
`
`§ 20
`_,
`ON
`E 15
`X
`
`"E 10
`%
`Z 5
`g
`
`0
`
`
`
`100
`
`200
`
`300
`
`400
`
`500
`
`800
`
`Temperature I “C
`
`15
`
`§
`Q
`10 B
`:
`N
`O
`0
`5 5
`3
`0
`
`0
`
`of
`reaction
`programmed
`Temperature
`1.
`Figure
`simultaneous NOX-soot
`removal over 3. 1 wt% Cu-MFI2
`(closed symbols) and Na—MFl2 (open symbol). Only the
`CO2 formation curve is shown for Na-MFI2. X[N3] and
`X[N2O] are conversions of NO into N; and N30, respectively.
`
`rence of the simultaneous NOx—soot removal reaction: the soot
`pre-mixed with the catalyst was oxidized by either N02 or 02 to
`produce CO2, and NO,‘ was reduced by the soot into N2 and
`N20. 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 N a-MF1 (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 N20 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 NO,,—soot removal reaction is almost exclusively deter-
`mined by the Cu loading (Wt%) irrespective of the Si/A1 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
`
`Copyright © 2001 The Chemical Society of Japan
`
`JM 1009
`
`1
`
`JM 1009
`
`

`
`605
`
`investigated, the most active catalyst was the Cu-exchanged
`catalyst, 3.5 wt% Cu-MFI2. The impregnated 3.8 Wt% Cu/Na-
`MFI2 was more selective to the N2 formation than Cu/A1203,
`CuO and any of the metal exchanged catalysts. As compared
`with mixed metal oxide catalysts so far reported, 3.8 wt%
`Cu/Na-MF12 is medium in activity but the most selective to
`the N2 formation: the highest selectivity was about 6%
`attained by La0_9K(,_1Cu0_7V0_3O,,,3 La1_9K0_1Cu0_95V0_05O4,4 and
`Cu0_951(0_05Fe2O46 (see Figure 3). Moreover, the Cu-loaded
`MFI catalysts have an advantage over the other NOx—soot
`removal catalysts that they catalyze the HC-SCR reaction: it
`was experimentally confirmed that when the mixture of soot
`and 4.6 wt% Cu-MFI2 was heated in an O2—NO—C2H4 stream,
`C2H4—SCR reaction proceeded in addition to the NOx—soot
`removal.
`
`In conclusion, it has turned out that Cu/Na-MFI2 prepared
`by impregnation method is a promising N Ox—soot removal cata-
`lyst showing medium soot ignition activity and high selectivity
`to N2 formation. This letter reports only the evaluation of the
`catalytic performance. There remain many subjects to be inves-
`tigated in future studies such as reasons why impregnation
`method or CuO on the outer surface of MFI affords high selec-
`tivity to N2 formation and why Cu ions exchanged in zeolite
`pores catalyze the reaction involving soot, which is present on
`the outer surface.
`
`References and Notes
`
`1
`
`2
`
`3
`
`4
`
`7
`
`8
`
`P. Zelenka, W. Cartellieri, and P. Herzog, Appl. Catal. B,
`10, 3 (1996).
`K. Yoshida, S. Makino, S. Sumiya, G. Muramatsu, and R.
`Helferich, SAE Paper, 1989, 892046.
`Y. Teraoka, K. Nakano, S. Kagawa, and W.F. Shangguan,
`Appl. Catal. B, 5, L181 (1995).
`Y. Teraoka, K. Nakano, W. F. Shangguan, and S. Kagawa,
`Catal. Today, 27, 107 (1996).
`5 W. F. Shangguan, Y. Teraoka, and S. KagaWa,Appl. Catal.
`B, 8, 217 (1996).
`6 W. F. Shangguan, Y. Teraoka, and S. Kagawa, Appl. Catal.
`B,16, 149 (1998).
`Y. Teraoka and S. Kagawa, Catal. Surveys from Jpn., 2,
`155 (1998).
`Y. Teraoka, W. F. Shangguan, K. Jansson, M. Nygren, and
`S. Kagawa, Bull. Chem. Soc. Jpn., 72, 133 (1999).
`9 W. F. Shangguan, Y. Teraoka, and S. KagaWa,Appl. Catal.
`B, 12, 237 (1997).
`10 Y. Teraoka, W.F. Shangguan, and S. Kagawa, Res. Chem.
`Intermed, 26, 201 (2000).
`11 During the evaporation-to-dryness process, a small portion
`of Cu ions might be ion-exchanged. However, a majority
`of them were deposited on the outer surface of MFI and
`were present as CuO after calcination, which was con-
`firmed by H2—TPR as well as color of the catalysts.
`12 Activated carbon (Nakalai Chemicals) was used as a sub-
`stitute for the diesel soot, because under the present experi-
`mental condition its reactivity was almost the same as that
`of the dry soot obtained by the incomplete combustion of
`diesel oil and so far used in our 1aboratory.3‘1°
`
`Chemistry Letters 2001
`
`500
`
`400
`
`o Cu/NaMFl2
`
`Egfiimili
`
`E] Cu-MFI4
`
`O Cu/NaMF|2
`O Cu-MFI2
`
`0
`
`1
`
`2
`
`5
`4
`3
`Cu loading I vvt%
`
`5
`
`7
`
`8
`
`ignition temperature. T,g, and (B)
`(A) Soot
`Figure 2.
`selectivity to N2 formation. S[N2], of Cu-loaded MFI
`catalysts as a function of Cu loading.
`
`than the ion-exchanged catalysts in the whole Cu-loading range
`examined, with a moderate maximum at 1.2 Wt% loading.
`Furthermore, Cu/Na-MFl2 were superior to Cu-MFl2 with
`respect to less amount of N20 formation. The selectivity to
`N20, V[N2O]/(V[N2]+V[N2O]), over catalysts with >1 Wt% Cu
`loadings were 7.1—l2.2% and 14.9—23.9% for Cu/Na-MFI2 and
`Cu-MFI2, respectively. As a result, impregnated Cu/Na-MFI2
`with the Cu loadings higher than ca. 1 Wt% are good catalysts
`showing both high activity and selectivity to N2 formation.
`Figure 3 shows the relation between Tig and S[N2] in order
`to compare the catalytic performance for the simultaneous
`NOx—soot removal. Among ion-exchanged MFI catalysts
`
`10
`*§"aP+5
`11
`
`KH
`4
`
`0,
`7
`,0,
`5
`
`1
`300
`
`I
`
`..J._
`350
`
`I
`
`1
`400
`
`Tig/°C
`
`1K
`
`M
`
`SF.
`
`310+
`2101
`
`D1
`12
`
`I
`
`1
`250
`
`8
`
`6
`
`3
`£4
`W
`
`2
`
`Q
`
`200
`
`Figure 3. Relation between soot ignition temperature, Tig,
`and selectivity to N2 formation. S[I\.";].
`1. 3.8 Wt% Cu/Na-M1712
`7. Na-MFI2
`2. 3.5 Wt% Cu-MFl2
`8. Curr 95Kg_g5Fe2O4
`3.
`1.2 W’[% Cu-MFI2
`9. La0_gK01C1lo 7V()_30,;
`4.
`12 Wt%
`l.a1_9Ko_|Cu;;_:;5Vn (1504
`5. 28 Wt% Co-MF12
`3.5 Wt% Cu/A1203
`11.
`6
`33 Wt% Pd-MFI2
`12. CuO
`
`2