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Influence of zeolite support on activity enhancement by addition of hydrogen for SCR of NO by propane over Ag-zeolites
Applied Catalysis B: Environmental 54 (2004) 137–144
Influence of zeolite support on activity enhancement by addition of hydrogen for SCR of NO by propane over Ag-zeolites Junji Shibata a,∗ , Yuu Takada a , Akira Shichi a , Shigeo Satokawa b , Atsushi Satsuma a,1 , Tadashi Hattori a a
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b Tokyo Gas Co. Ltd., 1-16-25 Shibaura, Minato-ku, Tokyo 105-0023, Japan Received 25 November 2003; received in revised form 9 March 2004; accepted 18 March 2004 Available online 4 August 2004
Abstract Effect of addition of H2 on SCR of NO by C3 H8 below 673 K was studied over various types of Ag-containing zeolites (MOR, MFI, BEA and Y). By the addition of 0.5% H2 , NO conversion below 673 K was increased on Ag-MFI and Ag-BEA, while it was not on Ag-MOR and Ag-Y. The dependence of NO conversion on H2 concentration also differed greatly by zeolite types. The C3 H8 -SCR activity was enhanced by the addition of H2 in the following order; Ag-MFI > Ag-BEA > Ag-MOR Ag-Y. The increment of NO conversion on Ag-MFI by the addition of H2 became large with an increase in Si/Al ratio of MFI. UV-Vis spectroscopic study exhibited that the state of Ag species differed by zeolite types. From comparison between the C3 H8 -SCR activity and UV-Vis spectra, it is confirmed that moderately agglomerated Agn δ+ cluster (2 ≤ n ≤ 4) is highly active species for NO reduction independently on zeolite types except Ag-Y. Balance of Ag species that governed the C3 H8 -SCR activity was shifted to more cationic Ag side with an increase of Si/Al ratio and in the order of acid strength (MOR > MFI > BEA). It is suggested that oxidative dispersion of agglomerated Ag species is accelerated by the increase in acid amount and strength. Control of the balance of Ag species by acid amount and strength would be a novel concept for design of Ag catalysts that exhibit high HC-SCR activity. © 2004 Elsevier B.V. All rights reserved. Keywords: HC-SCR; Ag catalyst; H2 ; Zeolite; Cluster; Acidity
1. Introduction A selective catalytic reduction of NO by hydrocarbons (HC-SCR) in the presence of excess oxygen is a potential method to remove NOx from lean-burn and diesel exhausts. Since the pioneering works by Iwamoto et al. [1] and Held et al. [2], extensive research has been done on development of de-NOx catalysts and many types of zeolite-based catalyst have been reported [3–5]. Ag has attracted much attention since Miyadera [6] reported the high activity of the SCR by ethanol over Ag/Al2 O3 . Ag-containing zeolites are known as active catalysts above 673 K for SCR of NO by light hydrocarbons including CH4 and by (CH3 )2 O [4,5,7–12]. It
is believed that Ag+ ion is active species for HC-SCR on Ag catalysts among many reports. On the other hand, we recently reported that the formation of cationic Ag cluster drastically enhances NO reduction activity at lower temperatures by addition of small amount of H2 for SCR with light hydrocarbons over Ag/Al2 O3 [13] and Ag ion-exchanged MFI zeolites [14]. Moreover, we proposed that Ag species (Ag+ ion, Agn δ+ cluster, metallic Agm cluster and Ag metal) are balanced during the SCR by C3 H8 on Ag/Al2 O3 and Ag-MFI as described in Eq. (1) [13,14]. H2 ,C3 H8
H2 ,C3 H8
NO,O2
NO,O2
Ag+ ion Agn δ+ cluster Agm cluster, Ag metal (1)
∗
Corresponding author. Tel.: +81-527893192; fax: +81-527893193. E-mail address: [email protected] (J. Shibata). 1 Co-corresponding author.
0926-3373/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.03.005
Formation of Agn δ+ cluster (2 ≤ n ≤ 4) results in remarkable enhancement of the C3 H8 -SCR activity. Ag+ ion
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is less active for the C3 H8 -SCR and further agglomerated Ag species (metallic Agm cluster (3 ≤ m ≤ 5) and Ag metal) are responsible for not NO reduction but C3 H8 combustion. The high HC-SCR activity of cationic Ag cluster is also reported by Sato et al. for SCR by n-decane over 0.05% Rh doped 4 wt.% Ag/Al2 O3 [15,16]. For practical use, it is necessary to form Ag cationic cluster under very low H2 concentration of a few hundred ppm. Use of supports that lead to easier Ag agglomeration is one of the strategies for design of Ag catalysts with high performance. In this paper, the influence of zeolite support on the C3 H8 -SCR activity over Ag-containing zeolites was investigated by using different zeolite supports in Si/Al ratio and topology (MOR, MFI, BEA, Y) to show that the balance of Ag species can be controlled by acid amount and strength.
2. Experimental Zeolites shown in Table 1 were used as parent zeolites in the present study. Na+ ion in Tosoh Na-MFI and Tosoh Na-MOR and H+ ion in JRC-Z-HB25 were exchanged to NH4 + ion with an aqueous NH4 NO3 solution at 353 K for 18 h. H-MOR(13) was prepared by dealumination treatment of JRC-Z-HM20(3) (Si/Al = 10) in an aqueous solution of 0.2 M HCl at 341 K for 24 h followed by filtration, rinse with distilled water, drying at 383 K and calcination in flowing dried air at 773 K for 6 h. Ag ion-exchanged zeolites were prepared from H or NH4 -form zeolites by ion-exchange with an aqueous AgNO3 solution at room temperature [7,8,11]. After filtration, the samples were calcined in the same manner as H-MOR(13). The calcined samples were analyzed by inductively coupled plasma emission spectroscopy (ICP, Jarrel-Ash MODEL 975) to determine their elemental composition. Table 2 shows the list of calcined Ag-zeolites employed in the present study. Ag amount was fixed at 3.5 ± 0.3 wt.% or 5.0 ± 0.5 wt.%. Hereafter, the catalysts will be designated as Ag-MFI(Si/Al ratio)-Ag exchange level, e.g. Ag-MFI(20)-83. Ag-MFI(22)-58 is the catalyst used in
Denotation
Si/Al ratio
Parent zeolite
Treatment
MFI(13) MFI(20) MFI(22) BEA(13)
13 20 22 13
JRC-Z5-25Ha Tosoh-H-MFIb Tosoh-H-MFIb JRC-Z-HB25a
No No No Ion-exchange in an aqueous solution of NH4 NO3 Ion-exchange in an aqueous solution of NH4 NO3 Dealumination in an aqueous solution of HCl No
MOR(13) Y(2.4) a b
7.7 13 2.4
Tosoh-Na-MORb JRC-Z-HM20(3)a JRC-Z-HY4.8a
Catalyst
Si/Al ratio
Ag/Al ratio
Ag content (wt.%)
Ag-MFI(13)-30 Ag-MFI(20)-47 Ag-MFI(20)-83 Ag-MFI(22)-58 Ag-MFI(35)-90 Ag-BEA(17)-62a Ag-MOR(7.7)-33 Ag-MOR(13)-34 Ag-Y(2.4)-13
13 20 20 22 35 17 7.7 15 2.4
0.30 0.47 0.83 0.58 0.90 0.62 0.33 0.34 0.13
3.5 3.3 5.4 3.5 3.5 5.5 5.4 3.8 4.7
a
This sample was prepared with H-BEA(13).
our previous study [14]. The Si/Al ratio in Ag-BEA was increased from 13 to 17 by dealumination in the preparation of Ag-BEA, and this sample will be designated as Ag-BEA(17)-62. The catalytic test was performed with a fixed-bed flow reactor by passing a mixture of 0.1% (1000 ppm) NO, 0.1% (1000 ppm) C3 H8 , 10% O2 , and 0.5% (5000 ppm) H2 in He at a rate of 100 cm3 min−1 over 0.2 g catalyst (GHSV = 19,000 h−1 ). Prior to the experiment the catalyst was heated in 10% O2 /He at 773 K for 1 h. After reaching steady state, the effluent gas was analyzed by gas chromatography and NOx analyzer (Best BCL-100 uH). The reaction results were described in terms of NO conversion to N2 , C3 H8 conversion to COx , H2 conversion and ratio of NO conversion to C3 H8 conversion as an index of efficiency of C3 H8 to NO reduction. N2 O was little detected (below 5%). Diffuse reflectance UV-Vis spectra of catalysts were measured with JASCO V-570. The sample was treated with exposing various gas mixtures at 573 K and quenched at room temperature. Then, UV-Vis spectra of the quenched sample were measured after moving into an optical quartz cell without exposure to the air.
3. Results 3.1. Effect of 0.5% H2 addition on C3 H8 -SCR over various Ag ion-exchanged zeolites
Table 1 List of the zeolites used in the study
MOR(7.7)
Table 2 List of the Ag ion-exchanged zeolites used in the study
Supplied from Catalysis Society of Japan. Supplied from Tosoh Co., Japan.
Fig. 1 shows the conversions of NO for the C3 H8 -SCR in the absence and presence of 0.5% H2 over Ag ion-exchanged zeolite catalysts as a function of temperature. The catalysts employed here were Ag-MFI(20)-83, Ag-BEA(17)-62, Ag-MOR(7.7)-33 and Ag-Y(2.4)-13 containing Ag contents around 5 wt.%. Clearly, the effect of adding 0.5% H2 on NO reduction activity differed by zeolite types. On Ag-MFI(20)-83 and Ag-BEA(17)-62, NO conversions were increased by the addition of 0.5% H2 . The increment in NO conversion by adding H2 was 49% for Ag-MFI(20)-83 and 29% for Ag-BEA(17)-62 at 573 K, and the enhancement effect of H2 for Ag-MFI(20)-83 was larger than that for Ag-BEA(17)-62. On Ag-MOR(7.7)-33, NO conversion was
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Fig. 1. NO conversion to N2 in the (䊉) presence and (䊊) absence of H2 over: (A) Ag-MFI(20)-83, (B) Ag-BEA(17)-62, (C) Ag-MOR(7.7)-33 and (D) Ag-Y(2.4)-13.
not increased by the addition of 0.5% H2 . NO reduction activity of Ag-Y(2.4)-13 was enhanced by the addition of H2 only above 673 K, and NO conversion was low (16% at 773 K). Table 3 shows that NO and C3 H8 conversions on Ag ion-exchanged zeolites used in Fig. 1 for the C3 H8 -SCR in the absence and presence of 0.5% H2 at 573 K. In the absence of H2 , not only NO but also C3 H8 conversions were below 10%. C3 H8 conversions on Ag-zeolites except Ag-Y(2.4)-13 were increased by the addition of H2 similarly to NO conversions. Here, a ratio of NO conversion to C3 H8 conversion on Ag-BEA(17)-62 in the presence of 0.5% H2 was 1.1 and was lower than that on Ag-MFI(20)-83. This result indicates that Ag-BEA(17)-62 is less selective in NO reduction than Ag-MFI(20)-83 because contribution of non-selective C3 H8 combustion becomes large. Table 3 Conversions over Ag ion-exchanged zeolites for C3 H8 -SCR in the absence and presence of 0.5% H2 at 573 K Catalyst
Ag-MFI(20)-83 Ag-BEA(17)-62 Ag-MOR(7.7)-33 Ag-Y(2.4)-13
Conversions at 0% H2 (%)
Conversions at 0.5% H2 (%)
NO
C3 H8
NO
C3 H8
7 9 9 0
6 7 7 0
56 38 11 2
40 35 9 0
(NO/C3 H8 ) conversion at 0.5% H2 1.4 1.1 1.2 –
3.2. Dependence of C3 H8 -SCR activity on H2 concentration Fig. 2 shows the NO and C3 H8 conversions for the C3 H8 -SCR as a function of H2 concentration at 573 K. Clearly, the dependence of both conversions on H2 concentration differed by zeolite types. On Ag-MFI(20)-83, NO conversion was increased by the addition of 0.25% H2 . Subsequently, NO conversion increased with an increase in H2 concentration until 1%, and decreased gradually with further increase in H2 concentration. Behavior of NO conversion toward H2 concentration on Ag-BEA(17)-62 was similar to that on Ag-MFI(20)-83 although NO conversion on the former was lower than that on the latter. On Ag-MOR(7.7)-33, NO conversion was increased by the addition of 1% H2 , and continued to increase with further increase in H2 concentration. NO reduction activity of Ag-Y(2.4)-13 was very low independently on H2 concentration. The order of NO conversion in the presence of H2 was Ag-MFI(20)-83 > Ag-BEA(17)-62 > Ag-MOR(7.7)-33 Ag-Y(2.4)-13. C3 H8 conversion increased monotonously with the increase in H2 concentration, except Ag-Y that exhibited little C3 H8 oxidation activity independently on H2 concentration. The order of C3 H8 conversion was Ag-BEA(17)-62 = Ag-MFI(20)-83 > Ag-MOR(7.7)-33 Ag-Y(2.4)-13. UV-Vis measurements were conducted to investigate Ag species existing in Ag ion-exchanged zeolites under the
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Fig. 2. Dependence of concentration of H2 on NO conversion to N2 and C3 H8 conversion to COx in the presence of H2 over: (䊉) Ag-MFI(20)-83, (䉱) Ag-BEA(17)-62, (䊏) Ag-MOR(7.7)-33 and (䉲) Ag-Y(2.4)-13 at 573 K. Conditions: NO = 1000 ppm, C3 H8 = 1000 ppm and O2 = 10%.
C3 H8 -SCR. The four Ag-zeolites employed in Figs. 1 and 2 were treated under the C3 H8 -SCR in the presence of various H2 concentrations at 573 K before measurement of UV-Vis spectra. Fig. 3 shows the UV-Vis spectra of these four Ag-zeolites. For Ag-MFI(20)-83 (Fig. 3A), an absorption band at 212 nm was observed after the SCR in the absence of H2 . This band was also observed for Ag/Al2 O3 [17,18] and other Ag ion-exchanged zeolite catalysts [8,14,19], and was assigned to the 4d10 to 4d9 s1 transition of Ag+ ion. This result indicates that Ag+ ion is predominant Ag species under the C3 H8 -SCR in the absence of H2 . On the other hand, new bands at 260 and 285 nm appeared by the addition of 0.25% H2 . These bands were in agreement with the bands due to Agn δ+ cluster (2 ≤ n ≤ 4), e.g. Ag4 2+ and Ag4 3+ [14,19]. The band intensities of Agn δ+ cluster increased with an increase in H2 concentration until 1%. Further increase in H2 concentration led to the appearance of the new bands at 250 and 312 nm. These bands were assigned to metallic Agm cluster (3 ≤ m ≤ 5), e.g. Ag3 and Ag5 [14,20], although the assignment of these bands to cationic cluster which is larger or more neutral than Agn δ+ cluster is not excluded.
This behavior of Ag species on Ag-MFI(20)-83 was very similar to that on Ag-MFI(22)-58 in our previous report [14]. Similarly to Ag-MFI, Ag+ ion (208–212, 230–235 nm) was main Ag species under the C3 H8 -SCR atmosphere in the absence of H2 on the other Ag ion-exchanged zeolites (Fig. 3B–D). However, dependence of change in Ag species on H2 concentration differed by the zeolite types as described below. For Ag-BEA(17)-62 (Fig. 3B), the bands of Agn δ+ cluster (260 nm) and metallic Agm cluster (320 nm) appeared above 0.25% H2 . Agn δ+ cluster and metallic Agm cluster simultaneously increased with the increase in H2 until 2.5%. Above 2.5% H2 , the band intensity of metallic Agm cluster further increased and broad absorption band above 350 nm was observed, which was assigned to Ag metal [8,13,14,17,18]. For Ag-MOR(7.7)-33 (Fig. 3C), Ag+ ion was main Ag species under the C3 H8 -SCR in the presence of H2 below 0.5% as well as in the absence of H2 . A band due to Agn δ+ cluster (258 nm) appeared by the addition of above 1% H2 , and its intensity of Agn δ+ cluster increased with an increase in H2 concentration above 1%. For Ag-Y(2.4)-13
Fig. 3. UV-Vis spectra of: (A) Ag-MFI(20)-83, (B) Ag-BEA(17)-62, (C) Ag-MOR(7.7)-33 and (D) Ag-Y(2.4)-13 after a flow of NO + C3 H8 + O2 + H2 at 573 K. H2 concentration was varied from 0 to 5%. Conditions: NO = 1000 ppm, C3 H8 = 1000 ppm and O2 = 10%.
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(Fig. 3D), both Agn δ+ cluster (280 nm) and metallic Agm cluster (310 nm) were formed above 0.25% H2 accompanying with disappearance of the band due to Ag+ ion, and UV-Vis spectra were hardly changed even higher H2 concentration. Dominant Ag species under the C3 H8 -SCR in the presence of 0.5% H2 also differed by the zeolite types. For Ag-MOR(7.7)-33 (Fig. 3C), Ag+ ion was predominant Ag species. For Ag-MFI(20)-83, Agn δ+ cluster was observed, though Ag+ ion was predominant. For Ag-BEA(17)-62, Agn δ+ cluster became more abundant than Ag+ ion, and even metallic Agm cluster was formed. For Ag-Y(2.4)-13, metallic Agm cluster and Agn δ+ cluster were main Ag species and the band due to Ag+ ion was not observed. The ratio of band intensity of metallic Agm cluster to that of the Agn δ+ cluster was higher for Ag-Y(2.4)-13 than for Ag-BEA(17)-62. From the above observation, the extent of reductive agglomeration of Ag species by the addition of 0.5% H2 was in order of Ag-Y(2.4)-13 > Ag-BEA(17)-62 > Ag-MFI(20)-83 > Ag-MOR(7.7)-33. 3.3. Effect of Si/Al ratio on C3 H8 -SCR activity of Ag catalysts Influence of Si/Al ratio on enhancement effect of C3 H8 -SCR activity by the addition of H2 was investigated by using 3.5 wt.% Ag-MFI zeolites with various Si/Al ratios. NO conversions on Ag-MFI with various Si/Al ratios in the absence and presence 0.5% H2 at 573 K are shown as circle symbols in Fig. 4A. In the absence of H2 , NO conversion decreased gradually with an increase in Si/Al ratio. On the other hand, NO conversion in the presence of H2 increased with an increase in Si/Al ratio until 20, and then reached a plateau. The increment of NO conversion by the addition of 0.5% H2 was calculated by subtracting NO conversion in the absence of H2 from that in the presence of 0.5% H2 . As shown in Fig. 4B, the increment of NO conversion by H2 became gradually large with an increase in Si/Al ratio. The C3 H8 -SCR in the absence and presence of 0.5% H2 was also carried out over Ag-MOR(13)-34 (square symbols) with about 3.5 wt.% Ag content. NO conversion on Ag-MOR(13)-34 in the presence of 0.5% H2 was lower than that on Ag-MFI(13)-30. Here, it should be noted that the increment of NO conversion by the addition of 0.5% H2 over Ag-MOR was lower than that over Ag-MFI with the same Si/Al ratio, as shown in Fig. 4B. The influence of Si/Al ratio on Ag species in MFI zeolites was investigated with UV-Vis spectroscopy. In this measurement, we used two 3.5 wt.% Ag-MFI which differ in Si/Al ratio. Fig. 5 shows UV-Vis spectra of Ag-MFI(13)-30 and Ag-MFI(20)-47 after the treatment under the C3 H8 -SCR in the presence of 0.5% H2 at 573 K. Clearly, the bands due to Agn δ+ cluster (260 and 285 nm) were stronger for Ag-MFI(20)-47 than for Ag-MFI(13)-30.
Fig. 4. (A) NO conversion in the (䊊, 䊐) absence and (䊉, 䊏) presence of 0.5% H2 at 573 K as a function of Si/Al ratio in (䊊, 䊉) Ag-MFI and (䊐, 䊏) Ag-MOR containing about 3.5wt% Ag. (B) Increment of NO conversion by the addition of 0.5% H2 at 573 K as a function of Si/Al ratio in (䊉) Ag-MFI and (䊏) Ag-MOR.
Fig. 5. UV-Vis spectra of: (a) Ag-MFI(13)-30 and (b) Ag-MFI(20)-47 after a flow of NO+C3 H8 +O2 in the presence of H2 at 573 K. Conditions: NO = 1000 ppm, C3 H8 = 1000 ppm, O2 = 10%, and H2 = 0.5%.
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4. Discussion 4.1. Active Ag species of various Ag ion-exchanged zeolites for C3 H8 -SCR Previously, we have reported the relationship of NO reduction activity with the state of Ag species for C3 H8 -SCR in the presence of H2 over Ag-MFI(22)-58 [14]. The C3 H8 -SCR activity of Ag+ ion is lower at lower temperatures than acid site, while activity of Agn δ+ cluster is remarkably higher. Metallic Agm cluster and Ag metal particle are responsible for C3 H8 combustion rather than NO reduction by C3 H8 . Zeolite types influence strongly NO reduction activity (Figs. 1, 2 and 4) and Ag species under the C3 H8 -SCR in the presence of H2 (Figs. 3 and 5). If our proposal that NO reduction activity is related to Ag species for Ag-MFI(22)-58 is extended to other Ag-zeolites, the variation in the C3 H8 -SCR activity will be attributed mainly to difference of the state of Ag species by zeolite types. Extension of our proposal to other zeolites is demonstrated by the following results. Ag+ ion was dominant Ag species on all Ag-zeolites in the absence of H2 (Fig. 3) and on Ag-MOR (Fig. 2) even in the presence of H2 below 0.5%. In this case, NO reduction activity was very low (Figs. 1 and 2). Under the conditions in the presence of H2 of 1–5% for Ag-MOR and 0.25–1% for Ag-MFI and Ag-BEA, NO conversion increased with the increase in the band intensity of Agn δ+ cluster (Fig. 3), which indicates high NO reduction activity of Agn δ+ cluster. It was also found that NO reduction activity and formation of Agn δ+ cluster were promoted by the addition of 0.5% H2 over Ag-MFI with higher Si/Al ratio (Figs. 4 and 5). Metallic Agm cluster formed on Ag-BEA in the presence of H2 (Fig. 3) led to lower NO reduction activity and selectivity over Ag-BEA than over Ag-MFI (Table 3). The decrease of NO conversion on Ag-MFI and Ag-BEA with the increase in H2 above 2.5% (Fig. 2) can be ascribed to excessive Ag agglomeration to metallic Agm cluster and Ag metal particle. At first glance, however, the results on Ag-Y are not agreement with this: NO and C3 H8 conversions at 573 K were nearly zero independently on H2 concentration (Fig. 2) though Agn δ+ cluster was formed in the presence of H2 (Fig. 3D). This contradiction can be explained as follows. It was reported that Ag2 + and Ag3 2+ , formed through reduction by H2 , were located at site I (hexagonal prism) and I (sodalite cage) in Y zeolite [21,22]. Contact of molecules with site I and I’ requires passage of molecules in six-ring window (2.8 × 2.8 Å) between sodalite cage and supercage. Size of the six-ring window is smaller than kinetic diameter of C3 H8 (4.2 Å) [23]. Therefore, C3 H8 is difficult to be activated on Ag cluster, which results in little C3 H8 -SCR activity of Ag-Y at low temperature. The increase of NO reduction activity in the presence of H2 above 723 K may be attributed to migration of Ag cluster into sites to which C3 H8 is accessible.
The results on all Ag-zeolites except for Ag-Y in the present study have corroborated our previous proposal for the relationship between C3 H8 -SCR activity and Ag species. This allows us to focus on the difference of the state of Ag species for the support effect on the C3 H8 -SCR over Ag-zeolites. 4.2. Effect of zeolite types on the state of Ag species under C3 H8 -SCR The employed Ag-zeolites differ from each other in Si/Al ratio, acid strength and pore diameter. Extent of agglomeration of Ag species may be influenced by a pore diameter of a zeolite since size of Ag species is restricted in a pore of zeolites. Largest pore diameters of MFI, BEA and MOR are 5.3 × 5.6, 6.6 × 6.7 and 6.5 × 7.0 Å, respectively [24]. The order of pore diameter does not agree with the order of the extent of Ag agglomeration (BEA > MFI > MOR), and hence the state of Ag is controlled by factors other than pore diameter of the zeolites. Here, the C3 H8 -SCR activity of Ag-zeolites differs by zeolite types in the absence of H2 as can be seen in Fig. 1. We confirmed that NO reaction rate decreased linearly with the increase of Ag/Al ratio for C3 H8 -SCR in the absence of H2 on Ag-MFI at lower temperature [14]. In addition, Satsuma et al. [25] reported that the acid amount and strength were the controlling factor for CH4 -SCR over H-MFI and H-MOR. The above reports imply that the result in the absence of H2 in Fig. 1 is attributed possibly to difference of amount and strength of acid site more active than Ag+ ion. The C3 H8 -SCR activity in the presence of H2 is the sum of activity of Ag species and acid site. Hence, the difference of NO conversions between in the absence of H2 and in the presence of 0.5% H2 , in other words, the increment of NO conversion is more appropriate to evaluation of activity of Ag species itself. Agn δ+ cluster, highly active species for the C3 H8 -SCR, increased with increase in Si/Al ratio under the presence of 0.5% H2 for the Ag-MFI zeolites with the same Ag amount (Fig. 5). In general, increase in Si/Al ratio leads to decrease in acid amount. Therefore, acid site in MFI zeolites suppresses Agn δ+ cluster formation, which results in the enhancement effect of NO reduction activity of Ag-MFI by the addition of 0.5% H2 (Fig. 4). Difference of acid amount in zeolites is one of the reasons why the reductive agglomeration of Ag species by the addition of H2 differed by zeolite types. However, the enhancement effect of NO reduction activity of Ag-MOR by the addition of 0.5% H2 was smaller than that of Ag-MFI with the same Si/Al ratio as shown in Fig. 4. Moreover, the order of agglomeration of Ag species (BEA > MFI > MOR) disagreed with that of Si/Al ratio (MOR > BEA > MFI). These results mean that the extent of the agglomeration of Ag species governing NO reduction activity depends strongly on factors other than acid amount in zeolites. It is well-known that acid strength differs by
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topology of zeolites [26–29]. Katada et al. [27–29] reported that NH3 adsorption heat, estimated from NH3 -TPD profiles of various H-zeolites, was dependent on zeolite topologies and the order of this heat was MOR (145 kJ mol−1 ) > MFI (130 kJ mol−1 ) > BEA (120 kJ mol−1 ) > Y (110 kJ mol−1 ). In addition, acid strength of HNa-zeolites is equal to acid strength of H-zeolites [27]. Therefore, it is reasonable that partial ion-exchange of Ag in H-zeolites does not affect acid strength of zeolites themselves. We used NH3 adsorption heat of zeolites reported by Katada et al. as acid strength of zeolites. The relationship of Ag species with acid strength is discussed from the information on Ag species obtained from UV-Vis spectra of 5 wt.% Ag-zeolites in the presence of 0.5% H2 (Fig. 3). For Ag-MOR with the highest acid strength (Fig. 3C), Ag+ ion was predominant Ag species in the presence of 0.5% H2 . For Ag-MFI with medium acid strength (Fig. 3A), Agn δ+ cluster was in existence. Metallic Agm cluster and Ag metal particle were formed by further
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the Ag species is moderately agglomerated to Agn δ+ cluster, highly active species for the C3 H8 -SCR. Acid strength of zeolites is the important factor controlling the reductive agglomeration of Ag species by the addition of H2 . From UV-Vis spectra under the C3 H8 -SCR in the presence of 0.5% H2 , Ag species on Ag-Y was more agglomerated than on Ag-BEA. This phenomenon at lower H2 concentration may be explained by lower acid strength of Y (110 kJ mol−1 ) than BEA (120 kJ mol−1 ). However, even at high H2 concentration, UV-Vis spectra was hardly changed. Ag cluster in Y may be more stable than in other zeolites. Support effect was not expressed by Eq. (1) that we reported previously [14]. Our present study reveals that acid amount and strength in zeolites govern this balance among Ag species. We have proposed the following scheme (Eq. (2)) including acidic hydroxyl group (HOZ) for an explanation of the influence of acid amount and strength on the agglomeration of Ag species.
(2) decrease in acid strength of zeolites (Ag-BEA: Fig. 3B). The extent of reductive agglomeration of Ag species (Ag-BEA > Ag-MFI > Ag-MOR) agreed with the reverse order of acid strength of H-zeolites, which strongly suggests that an increase in acid strength of H-zeolites led to the suppression of Ag agglomeration by the addition of H2 . This relationship is reflected in the dependence of the acid strength of zeolites on the enhancement effect of C3 H8 -SCR activity by the addition of 0.5% H2 as shown in Fig. 6. MFI support with medium acid strength gave the largest increment of NO conversion defined as the difference of NO conversions between in the absence of H2 and in the presence of 0.5% H2 , because
Fig. 6. Increment of NO conversion by the addition of 0.5% H2 at (䊊) 573 and (䊉) 523 K and as a function of adsorption heat of ammonia on zeolite supports, for C3 H8 -SCR over: (a) Ag-MOR(7.7)-33, (b) Ag-MFI(20)-83, (c) Ag-BEA(17)-62 and (d) Ag-Y(2.4)-13.
Here, Agn p+ and Agm represent Agn δ+ cluster and further agglomerated Ag species (metallic Agm cluster and Ag metal particle), respectively. Agn p+ cluster is anchored at ion-exchanged sites (OZ− ) in a zeolite, while Agm cluster might be interacted with acidic hydroxyl group as a metal-proton adduct [30]. H2 and C3 H8 agglomerate reductively Ag species accompanying with formation of acidic hydroxyl group by hydrogen atom derived from the reductants. The existence of each step is also confirmed by H2 treatment of AgNa-zeolites [31–33]. Formation of acidic hydroxyl group by H2 is revealed with IR [31] and 1 H MAS NMR [33]. We have inferred that Ag species is agglomerated through Eq. (2) in the present system, although we do not check the formation of HOZ in the presence of H2 . On the other hand, NO and O2 oxidatively disperse agglomerated Ag species accompanying with consumption of acidic hydroxyl group through formation of H2 O. Dispersed Ag species is anchored at ion-exchanged sites as Ag+ OZ− and Agn p+ (OZ− )p . The existence of oxidation steps of cationic Ag cluster to Ag+ ion and Ag metal to more dispersed Ag species is confirmed under O2 treatment [31,32,34] and heat treatment in vacuo [35] on reduced Ag-zeolites. The participation of acidic hydroxyl group in the oxidation reaction is supported by the report on oxidation of Ag0 over AgH-MOR under the H2 atmosphere in the temperature programmed mode [36]. In our present study, the effect of acid amount and strength of zeolite supports would be explained with Eq. (2) as follows. According to [Eq. (2)], acidic hydroxyl group is included in the reactants for oxidative dispersion reactions whereas it is not for reductive agglomeration reactions. Thus, it is reasonable that the increase in acid amount increases oxidative dispersion rate, although oxidative dis-
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persion and reductive agglomeration are not checked yet. On the other hand, the increase in acid strength remarkably accelerates oxidative dispersion of Ag species due to an increase in the ability to donate proton in HOZ itself. Consequently, the increase in acid amount and strength results in the shift of the balance of Eq. (2) to the left. The balance in Eq. (2) is shifted to the left under the lean-burn condition in the presence of very low H2 concentration of a few hundred ppm, compared with the case in the presence of 0.5% H2 . Support with small amount of weak acid site shifts to the right of this balance, which would result in remarkable enhancement of HC-SCR activity by the addition of very small amount of H2 into lean-burn exhausts.
5. Conclusions The C3 H8 -SCR activity of Ag-zeolites in the presence of H2 varied with Si/Al ratio and topology, in other words, acid amount and strength of zeolites. This variation in catalytic activity comes from the support effect on the balance of Ag species under the C3 H8 -SCR in the presence of H2 ; the balance was shifted to more cationic Ag side with the increase of acid amount and strength in zeolites. It is suggested that this phenomenon is caused by acceleration of dispersion of agglomerated Ag species through the reaction with oxidants and proton on zeolites. Use of support with small amount of weak acid site would be a promising design of Ag catalysts that exhibit high HC-SCR activity by the addition of very small amount of H2 into lean-burn exhausts.
Acknowledgements This work was partly supported by a Grant-in-Aid from the Ministry of Education, Science and Culture, Japan. The authors wish to thank Dr. K. Itabashi of Tosoh Co. for providing zeolite samples.
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Influence of zeolite support on activity enhancement by addition of hydrogen for SCR of NO by propane over Ag-zeolites Junji Shibata a,∗ , Yuu Takada a , Akira Shichi a , Shigeo Satokawa b , Atsushi Satsuma a,1 , Tadashi Hattori a a
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan b Tokyo Gas Co. Ltd., 1-16-25 Shibaura, Minato-ku, Tokyo 105-0023, Japan Received 25 November 2003; received in revised form 9 March 2004; accepted 18 March 2004 Available online 4 August 2004
Abstract Effect of addition of H2 on SCR of NO by C3 H8 below 673 K was studied over various types of Ag-containing zeolites (MOR, MFI, BEA and Y). By the addition of 0.5% H2 , NO conversion below 673 K was increased on Ag-MFI and Ag-BEA, while it was not on Ag-MOR and Ag-Y. The dependence of NO conversion on H2 concentration also differed greatly by zeolite types. The C3 H8 -SCR activity was enhanced by the addition of H2 in the following order; Ag-MFI > Ag-BEA > Ag-MOR Ag-Y. The increment of NO conversion on Ag-MFI by the addition of H2 became large with an increase in Si/Al ratio of MFI. UV-Vis spectroscopic study exhibited that the state of Ag species differed by zeolite types. From comparison between the C3 H8 -SCR activity and UV-Vis spectra, it is confirmed that moderately agglomerated Agn δ+ cluster (2 ≤ n ≤ 4) is highly active species for NO reduction independently on zeolite types except Ag-Y. Balance of Ag species that governed the C3 H8 -SCR activity was shifted to more cationic Ag side with an increase of Si/Al ratio and in the order of acid strength (MOR > MFI > BEA). It is suggested that oxidative dispersion of agglomerated Ag species is accelerated by the increase in acid amount and strength. Control of the balance of Ag species by acid amount and strength would be a novel concept for design of Ag catalysts that exhibit high HC-SCR activity. © 2004 Elsevier B.V. All rights reserved. Keywords: HC-SCR; Ag catalyst; H2 ; Zeolite; Cluster; Acidity
1. Introduction A selective catalytic reduction of NO by hydrocarbons (HC-SCR) in the presence of excess oxygen is a potential method to remove NOx from lean-burn and diesel exhausts. Since the pioneering works by Iwamoto et al. [1] and Held et al. [2], extensive research has been done on development of de-NOx catalysts and many types of zeolite-based catalyst have been reported [3–5]. Ag has attracted much attention since Miyadera [6] reported the high activity of the SCR by ethanol over Ag/Al2 O3 . Ag-containing zeolites are known as active catalysts above 673 K for SCR of NO by light hydrocarbons including CH4 and by (CH3 )2 O [4,5,7–12]. It
is believed that Ag+ ion is active species for HC-SCR on Ag catalysts among many reports. On the other hand, we recently reported that the formation of cationic Ag cluster drastically enhances NO reduction activity at lower temperatures by addition of small amount of H2 for SCR with light hydrocarbons over Ag/Al2 O3 [13] and Ag ion-exchanged MFI zeolites [14]. Moreover, we proposed that Ag species (Ag+ ion, Agn δ+ cluster, metallic Agm cluster and Ag metal) are balanced during the SCR by C3 H8 on Ag/Al2 O3 and Ag-MFI as described in Eq. (1) [13,14]. H2 ,C3 H8
H2 ,C3 H8
NO,O2
NO,O2
Ag+ ion Agn δ+ cluster Agm cluster, Ag metal (1)
∗
Corresponding author. Tel.: +81-527893192; fax: +81-527893193. E-mail address: [email protected] (J. Shibata). 1 Co-corresponding author.
0926-3373/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2004.03.005
Formation of Agn δ+ cluster (2 ≤ n ≤ 4) results in remarkable enhancement of the C3 H8 -SCR activity. Ag+ ion
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is less active for the C3 H8 -SCR and further agglomerated Ag species (metallic Agm cluster (3 ≤ m ≤ 5) and Ag metal) are responsible for not NO reduction but C3 H8 combustion. The high HC-SCR activity of cationic Ag cluster is also reported by Sato et al. for SCR by n-decane over 0.05% Rh doped 4 wt.% Ag/Al2 O3 [15,16]. For practical use, it is necessary to form Ag cationic cluster under very low H2 concentration of a few hundred ppm. Use of supports that lead to easier Ag agglomeration is one of the strategies for design of Ag catalysts with high performance. In this paper, the influence of zeolite support on the C3 H8 -SCR activity over Ag-containing zeolites was investigated by using different zeolite supports in Si/Al ratio and topology (MOR, MFI, BEA, Y) to show that the balance of Ag species can be controlled by acid amount and strength.
2. Experimental Zeolites shown in Table 1 were used as parent zeolites in the present study. Na+ ion in Tosoh Na-MFI and Tosoh Na-MOR and H+ ion in JRC-Z-HB25 were exchanged to NH4 + ion with an aqueous NH4 NO3 solution at 353 K for 18 h. H-MOR(13) was prepared by dealumination treatment of JRC-Z-HM20(3) (Si/Al = 10) in an aqueous solution of 0.2 M HCl at 341 K for 24 h followed by filtration, rinse with distilled water, drying at 383 K and calcination in flowing dried air at 773 K for 6 h. Ag ion-exchanged zeolites were prepared from H or NH4 -form zeolites by ion-exchange with an aqueous AgNO3 solution at room temperature [7,8,11]. After filtration, the samples were calcined in the same manner as H-MOR(13). The calcined samples were analyzed by inductively coupled plasma emission spectroscopy (ICP, Jarrel-Ash MODEL 975) to determine their elemental composition. Table 2 shows the list of calcined Ag-zeolites employed in the present study. Ag amount was fixed at 3.5 ± 0.3 wt.% or 5.0 ± 0.5 wt.%. Hereafter, the catalysts will be designated as Ag-MFI(Si/Al ratio)-Ag exchange level, e.g. Ag-MFI(20)-83. Ag-MFI(22)-58 is the catalyst used in
Denotation
Si/Al ratio
Parent zeolite
Treatment
MFI(13) MFI(20) MFI(22) BEA(13)
13 20 22 13
JRC-Z5-25Ha Tosoh-H-MFIb Tosoh-H-MFIb JRC-Z-HB25a
No No No Ion-exchange in an aqueous solution of NH4 NO3 Ion-exchange in an aqueous solution of NH4 NO3 Dealumination in an aqueous solution of HCl No
MOR(13) Y(2.4) a b
7.7 13 2.4
Tosoh-Na-MORb JRC-Z-HM20(3)a JRC-Z-HY4.8a
Catalyst
Si/Al ratio
Ag/Al ratio
Ag content (wt.%)
Ag-MFI(13)-30 Ag-MFI(20)-47 Ag-MFI(20)-83 Ag-MFI(22)-58 Ag-MFI(35)-90 Ag-BEA(17)-62a Ag-MOR(7.7)-33 Ag-MOR(13)-34 Ag-Y(2.4)-13
13 20 20 22 35 17 7.7 15 2.4
0.30 0.47 0.83 0.58 0.90 0.62 0.33 0.34 0.13
3.5 3.3 5.4 3.5 3.5 5.5 5.4 3.8 4.7
a
This sample was prepared with H-BEA(13).
our previous study [14]. The Si/Al ratio in Ag-BEA was increased from 13 to 17 by dealumination in the preparation of Ag-BEA, and this sample will be designated as Ag-BEA(17)-62. The catalytic test was performed with a fixed-bed flow reactor by passing a mixture of 0.1% (1000 ppm) NO, 0.1% (1000 ppm) C3 H8 , 10% O2 , and 0.5% (5000 ppm) H2 in He at a rate of 100 cm3 min−1 over 0.2 g catalyst (GHSV = 19,000 h−1 ). Prior to the experiment the catalyst was heated in 10% O2 /He at 773 K for 1 h. After reaching steady state, the effluent gas was analyzed by gas chromatography and NOx analyzer (Best BCL-100 uH). The reaction results were described in terms of NO conversion to N2 , C3 H8 conversion to COx , H2 conversion and ratio of NO conversion to C3 H8 conversion as an index of efficiency of C3 H8 to NO reduction. N2 O was little detected (below 5%). Diffuse reflectance UV-Vis spectra of catalysts were measured with JASCO V-570. The sample was treated with exposing various gas mixtures at 573 K and quenched at room temperature. Then, UV-Vis spectra of the quenched sample were measured after moving into an optical quartz cell without exposure to the air.
3. Results 3.1. Effect of 0.5% H2 addition on C3 H8 -SCR over various Ag ion-exchanged zeolites
Table 1 List of the zeolites used in the study
MOR(7.7)
Table 2 List of the Ag ion-exchanged zeolites used in the study
Supplied from Catalysis Society of Japan. Supplied from Tosoh Co., Japan.
Fig. 1 shows the conversions of NO for the C3 H8 -SCR in the absence and presence of 0.5% H2 over Ag ion-exchanged zeolite catalysts as a function of temperature. The catalysts employed here were Ag-MFI(20)-83, Ag-BEA(17)-62, Ag-MOR(7.7)-33 and Ag-Y(2.4)-13 containing Ag contents around 5 wt.%. Clearly, the effect of adding 0.5% H2 on NO reduction activity differed by zeolite types. On Ag-MFI(20)-83 and Ag-BEA(17)-62, NO conversions were increased by the addition of 0.5% H2 . The increment in NO conversion by adding H2 was 49% for Ag-MFI(20)-83 and 29% for Ag-BEA(17)-62 at 573 K, and the enhancement effect of H2 for Ag-MFI(20)-83 was larger than that for Ag-BEA(17)-62. On Ag-MOR(7.7)-33, NO conversion was
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Fig. 1. NO conversion to N2 in the (䊉) presence and (䊊) absence of H2 over: (A) Ag-MFI(20)-83, (B) Ag-BEA(17)-62, (C) Ag-MOR(7.7)-33 and (D) Ag-Y(2.4)-13.
not increased by the addition of 0.5% H2 . NO reduction activity of Ag-Y(2.4)-13 was enhanced by the addition of H2 only above 673 K, and NO conversion was low (16% at 773 K). Table 3 shows that NO and C3 H8 conversions on Ag ion-exchanged zeolites used in Fig. 1 for the C3 H8 -SCR in the absence and presence of 0.5% H2 at 573 K. In the absence of H2 , not only NO but also C3 H8 conversions were below 10%. C3 H8 conversions on Ag-zeolites except Ag-Y(2.4)-13 were increased by the addition of H2 similarly to NO conversions. Here, a ratio of NO conversion to C3 H8 conversion on Ag-BEA(17)-62 in the presence of 0.5% H2 was 1.1 and was lower than that on Ag-MFI(20)-83. This result indicates that Ag-BEA(17)-62 is less selective in NO reduction than Ag-MFI(20)-83 because contribution of non-selective C3 H8 combustion becomes large. Table 3 Conversions over Ag ion-exchanged zeolites for C3 H8 -SCR in the absence and presence of 0.5% H2 at 573 K Catalyst
Ag-MFI(20)-83 Ag-BEA(17)-62 Ag-MOR(7.7)-33 Ag-Y(2.4)-13
Conversions at 0% H2 (%)
Conversions at 0.5% H2 (%)
NO
C3 H8
NO
C3 H8
7 9 9 0
6 7 7 0
56 38 11 2
40 35 9 0
(NO/C3 H8 ) conversion at 0.5% H2 1.4 1.1 1.2 –
3.2. Dependence of C3 H8 -SCR activity on H2 concentration Fig. 2 shows the NO and C3 H8 conversions for the C3 H8 -SCR as a function of H2 concentration at 573 K. Clearly, the dependence of both conversions on H2 concentration differed by zeolite types. On Ag-MFI(20)-83, NO conversion was increased by the addition of 0.25% H2 . Subsequently, NO conversion increased with an increase in H2 concentration until 1%, and decreased gradually with further increase in H2 concentration. Behavior of NO conversion toward H2 concentration on Ag-BEA(17)-62 was similar to that on Ag-MFI(20)-83 although NO conversion on the former was lower than that on the latter. On Ag-MOR(7.7)-33, NO conversion was increased by the addition of 1% H2 , and continued to increase with further increase in H2 concentration. NO reduction activity of Ag-Y(2.4)-13 was very low independently on H2 concentration. The order of NO conversion in the presence of H2 was Ag-MFI(20)-83 > Ag-BEA(17)-62 > Ag-MOR(7.7)-33 Ag-Y(2.4)-13. C3 H8 conversion increased monotonously with the increase in H2 concentration, except Ag-Y that exhibited little C3 H8 oxidation activity independently on H2 concentration. The order of C3 H8 conversion was Ag-BEA(17)-62 = Ag-MFI(20)-83 > Ag-MOR(7.7)-33 Ag-Y(2.4)-13. UV-Vis measurements were conducted to investigate Ag species existing in Ag ion-exchanged zeolites under the
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Fig. 2. Dependence of concentration of H2 on NO conversion to N2 and C3 H8 conversion to COx in the presence of H2 over: (䊉) Ag-MFI(20)-83, (䉱) Ag-BEA(17)-62, (䊏) Ag-MOR(7.7)-33 and (䉲) Ag-Y(2.4)-13 at 573 K. Conditions: NO = 1000 ppm, C3 H8 = 1000 ppm and O2 = 10%.
C3 H8 -SCR. The four Ag-zeolites employed in Figs. 1 and 2 were treated under the C3 H8 -SCR in the presence of various H2 concentrations at 573 K before measurement of UV-Vis spectra. Fig. 3 shows the UV-Vis spectra of these four Ag-zeolites. For Ag-MFI(20)-83 (Fig. 3A), an absorption band at 212 nm was observed after the SCR in the absence of H2 . This band was also observed for Ag/Al2 O3 [17,18] and other Ag ion-exchanged zeolite catalysts [8,14,19], and was assigned to the 4d10 to 4d9 s1 transition of Ag+ ion. This result indicates that Ag+ ion is predominant Ag species under the C3 H8 -SCR in the absence of H2 . On the other hand, new bands at 260 and 285 nm appeared by the addition of 0.25% H2 . These bands were in agreement with the bands due to Agn δ+ cluster (2 ≤ n ≤ 4), e.g. Ag4 2+ and Ag4 3+ [14,19]. The band intensities of Agn δ+ cluster increased with an increase in H2 concentration until 1%. Further increase in H2 concentration led to the appearance of the new bands at 250 and 312 nm. These bands were assigned to metallic Agm cluster (3 ≤ m ≤ 5), e.g. Ag3 and Ag5 [14,20], although the assignment of these bands to cationic cluster which is larger or more neutral than Agn δ+ cluster is not excluded.
This behavior of Ag species on Ag-MFI(20)-83 was very similar to that on Ag-MFI(22)-58 in our previous report [14]. Similarly to Ag-MFI, Ag+ ion (208–212, 230–235 nm) was main Ag species under the C3 H8 -SCR atmosphere in the absence of H2 on the other Ag ion-exchanged zeolites (Fig. 3B–D). However, dependence of change in Ag species on H2 concentration differed by the zeolite types as described below. For Ag-BEA(17)-62 (Fig. 3B), the bands of Agn δ+ cluster (260 nm) and metallic Agm cluster (320 nm) appeared above 0.25% H2 . Agn δ+ cluster and metallic Agm cluster simultaneously increased with the increase in H2 until 2.5%. Above 2.5% H2 , the band intensity of metallic Agm cluster further increased and broad absorption band above 350 nm was observed, which was assigned to Ag metal [8,13,14,17,18]. For Ag-MOR(7.7)-33 (Fig. 3C), Ag+ ion was main Ag species under the C3 H8 -SCR in the presence of H2 below 0.5% as well as in the absence of H2 . A band due to Agn δ+ cluster (258 nm) appeared by the addition of above 1% H2 , and its intensity of Agn δ+ cluster increased with an increase in H2 concentration above 1%. For Ag-Y(2.4)-13
Fig. 3. UV-Vis spectra of: (A) Ag-MFI(20)-83, (B) Ag-BEA(17)-62, (C) Ag-MOR(7.7)-33 and (D) Ag-Y(2.4)-13 after a flow of NO + C3 H8 + O2 + H2 at 573 K. H2 concentration was varied from 0 to 5%. Conditions: NO = 1000 ppm, C3 H8 = 1000 ppm and O2 = 10%.
J. Shibata et al. / Applied Catalysis B: Environmental 54 (2004) 137–144
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(Fig. 3D), both Agn δ+ cluster (280 nm) and metallic Agm cluster (310 nm) were formed above 0.25% H2 accompanying with disappearance of the band due to Ag+ ion, and UV-Vis spectra were hardly changed even higher H2 concentration. Dominant Ag species under the C3 H8 -SCR in the presence of 0.5% H2 also differed by the zeolite types. For Ag-MOR(7.7)-33 (Fig. 3C), Ag+ ion was predominant Ag species. For Ag-MFI(20)-83, Agn δ+ cluster was observed, though Ag+ ion was predominant. For Ag-BEA(17)-62, Agn δ+ cluster became more abundant than Ag+ ion, and even metallic Agm cluster was formed. For Ag-Y(2.4)-13, metallic Agm cluster and Agn δ+ cluster were main Ag species and the band due to Ag+ ion was not observed. The ratio of band intensity of metallic Agm cluster to that of the Agn δ+ cluster was higher for Ag-Y(2.4)-13 than for Ag-BEA(17)-62. From the above observation, the extent of reductive agglomeration of Ag species by the addition of 0.5% H2 was in order of Ag-Y(2.4)-13 > Ag-BEA(17)-62 > Ag-MFI(20)-83 > Ag-MOR(7.7)-33. 3.3. Effect of Si/Al ratio on C3 H8 -SCR activity of Ag catalysts Influence of Si/Al ratio on enhancement effect of C3 H8 -SCR activity by the addition of H2 was investigated by using 3.5 wt.% Ag-MFI zeolites with various Si/Al ratios. NO conversions on Ag-MFI with various Si/Al ratios in the absence and presence 0.5% H2 at 573 K are shown as circle symbols in Fig. 4A. In the absence of H2 , NO conversion decreased gradually with an increase in Si/Al ratio. On the other hand, NO conversion in the presence of H2 increased with an increase in Si/Al ratio until 20, and then reached a plateau. The increment of NO conversion by the addition of 0.5% H2 was calculated by subtracting NO conversion in the absence of H2 from that in the presence of 0.5% H2 . As shown in Fig. 4B, the increment of NO conversion by H2 became gradually large with an increase in Si/Al ratio. The C3 H8 -SCR in the absence and presence of 0.5% H2 was also carried out over Ag-MOR(13)-34 (square symbols) with about 3.5 wt.% Ag content. NO conversion on Ag-MOR(13)-34 in the presence of 0.5% H2 was lower than that on Ag-MFI(13)-30. Here, it should be noted that the increment of NO conversion by the addition of 0.5% H2 over Ag-MOR was lower than that over Ag-MFI with the same Si/Al ratio, as shown in Fig. 4B. The influence of Si/Al ratio on Ag species in MFI zeolites was investigated with UV-Vis spectroscopy. In this measurement, we used two 3.5 wt.% Ag-MFI which differ in Si/Al ratio. Fig. 5 shows UV-Vis spectra of Ag-MFI(13)-30 and Ag-MFI(20)-47 after the treatment under the C3 H8 -SCR in the presence of 0.5% H2 at 573 K. Clearly, the bands due to Agn δ+ cluster (260 and 285 nm) were stronger for Ag-MFI(20)-47 than for Ag-MFI(13)-30.
Fig. 4. (A) NO conversion in the (䊊, 䊐) absence and (䊉, 䊏) presence of 0.5% H2 at 573 K as a function of Si/Al ratio in (䊊, 䊉) Ag-MFI and (䊐, 䊏) Ag-MOR containing about 3.5wt% Ag. (B) Increment of NO conversion by the addition of 0.5% H2 at 573 K as a function of Si/Al ratio in (䊉) Ag-MFI and (䊏) Ag-MOR.
Fig. 5. UV-Vis spectra of: (a) Ag-MFI(13)-30 and (b) Ag-MFI(20)-47 after a flow of NO+C3 H8 +O2 in the presence of H2 at 573 K. Conditions: NO = 1000 ppm, C3 H8 = 1000 ppm, O2 = 10%, and H2 = 0.5%.
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4. Discussion 4.1. Active Ag species of various Ag ion-exchanged zeolites for C3 H8 -SCR Previously, we have reported the relationship of NO reduction activity with the state of Ag species for C3 H8 -SCR in the presence of H2 over Ag-MFI(22)-58 [14]. The C3 H8 -SCR activity of Ag+ ion is lower at lower temperatures than acid site, while activity of Agn δ+ cluster is remarkably higher. Metallic Agm cluster and Ag metal particle are responsible for C3 H8 combustion rather than NO reduction by C3 H8 . Zeolite types influence strongly NO reduction activity (Figs. 1, 2 and 4) and Ag species under the C3 H8 -SCR in the presence of H2 (Figs. 3 and 5). If our proposal that NO reduction activity is related to Ag species for Ag-MFI(22)-58 is extended to other Ag-zeolites, the variation in the C3 H8 -SCR activity will be attributed mainly to difference of the state of Ag species by zeolite types. Extension of our proposal to other zeolites is demonstrated by the following results. Ag+ ion was dominant Ag species on all Ag-zeolites in the absence of H2 (Fig. 3) and on Ag-MOR (Fig. 2) even in the presence of H2 below 0.5%. In this case, NO reduction activity was very low (Figs. 1 and 2). Under the conditions in the presence of H2 of 1–5% for Ag-MOR and 0.25–1% for Ag-MFI and Ag-BEA, NO conversion increased with the increase in the band intensity of Agn δ+ cluster (Fig. 3), which indicates high NO reduction activity of Agn δ+ cluster. It was also found that NO reduction activity and formation of Agn δ+ cluster were promoted by the addition of 0.5% H2 over Ag-MFI with higher Si/Al ratio (Figs. 4 and 5). Metallic Agm cluster formed on Ag-BEA in the presence of H2 (Fig. 3) led to lower NO reduction activity and selectivity over Ag-BEA than over Ag-MFI (Table 3). The decrease of NO conversion on Ag-MFI and Ag-BEA with the increase in H2 above 2.5% (Fig. 2) can be ascribed to excessive Ag agglomeration to metallic Agm cluster and Ag metal particle. At first glance, however, the results on Ag-Y are not agreement with this: NO and C3 H8 conversions at 573 K were nearly zero independently on H2 concentration (Fig. 2) though Agn δ+ cluster was formed in the presence of H2 (Fig. 3D). This contradiction can be explained as follows. It was reported that Ag2 + and Ag3 2+ , formed through reduction by H2 , were located at site I (hexagonal prism) and I (sodalite cage) in Y zeolite [21,22]. Contact of molecules with site I and I’ requires passage of molecules in six-ring window (2.8 × 2.8 Å) between sodalite cage and supercage. Size of the six-ring window is smaller than kinetic diameter of C3 H8 (4.2 Å) [23]. Therefore, C3 H8 is difficult to be activated on Ag cluster, which results in little C3 H8 -SCR activity of Ag-Y at low temperature. The increase of NO reduction activity in the presence of H2 above 723 K may be attributed to migration of Ag cluster into sites to which C3 H8 is accessible.
The results on all Ag-zeolites except for Ag-Y in the present study have corroborated our previous proposal for the relationship between C3 H8 -SCR activity and Ag species. This allows us to focus on the difference of the state of Ag species for the support effect on the C3 H8 -SCR over Ag-zeolites. 4.2. Effect of zeolite types on the state of Ag species under C3 H8 -SCR The employed Ag-zeolites differ from each other in Si/Al ratio, acid strength and pore diameter. Extent of agglomeration of Ag species may be influenced by a pore diameter of a zeolite since size of Ag species is restricted in a pore of zeolites. Largest pore diameters of MFI, BEA and MOR are 5.3 × 5.6, 6.6 × 6.7 and 6.5 × 7.0 Å, respectively [24]. The order of pore diameter does not agree with the order of the extent of Ag agglomeration (BEA > MFI > MOR), and hence the state of Ag is controlled by factors other than pore diameter of the zeolites. Here, the C3 H8 -SCR activity of Ag-zeolites differs by zeolite types in the absence of H2 as can be seen in Fig. 1. We confirmed that NO reaction rate decreased linearly with the increase of Ag/Al ratio for C3 H8 -SCR in the absence of H2 on Ag-MFI at lower temperature [14]. In addition, Satsuma et al. [25] reported that the acid amount and strength were the controlling factor for CH4 -SCR over H-MFI and H-MOR. The above reports imply that the result in the absence of H2 in Fig. 1 is attributed possibly to difference of amount and strength of acid site more active than Ag+ ion. The C3 H8 -SCR activity in the presence of H2 is the sum of activity of Ag species and acid site. Hence, the difference of NO conversions between in the absence of H2 and in the presence of 0.5% H2 , in other words, the increment of NO conversion is more appropriate to evaluation of activity of Ag species itself. Agn δ+ cluster, highly active species for the C3 H8 -SCR, increased with increase in Si/Al ratio under the presence of 0.5% H2 for the Ag-MFI zeolites with the same Ag amount (Fig. 5). In general, increase in Si/Al ratio leads to decrease in acid amount. Therefore, acid site in MFI zeolites suppresses Agn δ+ cluster formation, which results in the enhancement effect of NO reduction activity of Ag-MFI by the addition of 0.5% H2 (Fig. 4). Difference of acid amount in zeolites is one of the reasons why the reductive agglomeration of Ag species by the addition of H2 differed by zeolite types. However, the enhancement effect of NO reduction activity of Ag-MOR by the addition of 0.5% H2 was smaller than that of Ag-MFI with the same Si/Al ratio as shown in Fig. 4. Moreover, the order of agglomeration of Ag species (BEA > MFI > MOR) disagreed with that of Si/Al ratio (MOR > BEA > MFI). These results mean that the extent of the agglomeration of Ag species governing NO reduction activity depends strongly on factors other than acid amount in zeolites. It is well-known that acid strength differs by
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topology of zeolites [26–29]. Katada et al. [27–29] reported that NH3 adsorption heat, estimated from NH3 -TPD profiles of various H-zeolites, was dependent on zeolite topologies and the order of this heat was MOR (145 kJ mol−1 ) > MFI (130 kJ mol−1 ) > BEA (120 kJ mol−1 ) > Y (110 kJ mol−1 ). In addition, acid strength of HNa-zeolites is equal to acid strength of H-zeolites [27]. Therefore, it is reasonable that partial ion-exchange of Ag in H-zeolites does not affect acid strength of zeolites themselves. We used NH3 adsorption heat of zeolites reported by Katada et al. as acid strength of zeolites. The relationship of Ag species with acid strength is discussed from the information on Ag species obtained from UV-Vis spectra of 5 wt.% Ag-zeolites in the presence of 0.5% H2 (Fig. 3). For Ag-MOR with the highest acid strength (Fig. 3C), Ag+ ion was predominant Ag species in the presence of 0.5% H2 . For Ag-MFI with medium acid strength (Fig. 3A), Agn δ+ cluster was in existence. Metallic Agm cluster and Ag metal particle were formed by further
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the Ag species is moderately agglomerated to Agn δ+ cluster, highly active species for the C3 H8 -SCR. Acid strength of zeolites is the important factor controlling the reductive agglomeration of Ag species by the addition of H2 . From UV-Vis spectra under the C3 H8 -SCR in the presence of 0.5% H2 , Ag species on Ag-Y was more agglomerated than on Ag-BEA. This phenomenon at lower H2 concentration may be explained by lower acid strength of Y (110 kJ mol−1 ) than BEA (120 kJ mol−1 ). However, even at high H2 concentration, UV-Vis spectra was hardly changed. Ag cluster in Y may be more stable than in other zeolites. Support effect was not expressed by Eq. (1) that we reported previously [14]. Our present study reveals that acid amount and strength in zeolites govern this balance among Ag species. We have proposed the following scheme (Eq. (2)) including acidic hydroxyl group (HOZ) for an explanation of the influence of acid amount and strength on the agglomeration of Ag species.
(2) decrease in acid strength of zeolites (Ag-BEA: Fig. 3B). The extent of reductive agglomeration of Ag species (Ag-BEA > Ag-MFI > Ag-MOR) agreed with the reverse order of acid strength of H-zeolites, which strongly suggests that an increase in acid strength of H-zeolites led to the suppression of Ag agglomeration by the addition of H2 . This relationship is reflected in the dependence of the acid strength of zeolites on the enhancement effect of C3 H8 -SCR activity by the addition of 0.5% H2 as shown in Fig. 6. MFI support with medium acid strength gave the largest increment of NO conversion defined as the difference of NO conversions between in the absence of H2 and in the presence of 0.5% H2 , because
Fig. 6. Increment of NO conversion by the addition of 0.5% H2 at (䊊) 573 and (䊉) 523 K and as a function of adsorption heat of ammonia on zeolite supports, for C3 H8 -SCR over: (a) Ag-MOR(7.7)-33, (b) Ag-MFI(20)-83, (c) Ag-BEA(17)-62 and (d) Ag-Y(2.4)-13.
Here, Agn p+ and Agm represent Agn δ+ cluster and further agglomerated Ag species (metallic Agm cluster and Ag metal particle), respectively. Agn p+ cluster is anchored at ion-exchanged sites (OZ− ) in a zeolite, while Agm cluster might be interacted with acidic hydroxyl group as a metal-proton adduct [30]. H2 and C3 H8 agglomerate reductively Ag species accompanying with formation of acidic hydroxyl group by hydrogen atom derived from the reductants. The existence of each step is also confirmed by H2 treatment of AgNa-zeolites [31–33]. Formation of acidic hydroxyl group by H2 is revealed with IR [31] and 1 H MAS NMR [33]. We have inferred that Ag species is agglomerated through Eq. (2) in the present system, although we do not check the formation of HOZ in the presence of H2 . On the other hand, NO and O2 oxidatively disperse agglomerated Ag species accompanying with consumption of acidic hydroxyl group through formation of H2 O. Dispersed Ag species is anchored at ion-exchanged sites as Ag+ OZ− and Agn p+ (OZ− )p . The existence of oxidation steps of cationic Ag cluster to Ag+ ion and Ag metal to more dispersed Ag species is confirmed under O2 treatment [31,32,34] and heat treatment in vacuo [35] on reduced Ag-zeolites. The participation of acidic hydroxyl group in the oxidation reaction is supported by the report on oxidation of Ag0 over AgH-MOR under the H2 atmosphere in the temperature programmed mode [36]. In our present study, the effect of acid amount and strength of zeolite supports would be explained with Eq. (2) as follows. According to [Eq. (2)], acidic hydroxyl group is included in the reactants for oxidative dispersion reactions whereas it is not for reductive agglomeration reactions. Thus, it is reasonable that the increase in acid amount increases oxidative dispersion rate, although oxidative dis-
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persion and reductive agglomeration are not checked yet. On the other hand, the increase in acid strength remarkably accelerates oxidative dispersion of Ag species due to an increase in the ability to donate proton in HOZ itself. Consequently, the increase in acid amount and strength results in the shift of the balance of Eq. (2) to the left. The balance in Eq. (2) is shifted to the left under the lean-burn condition in the presence of very low H2 concentration of a few hundred ppm, compared with the case in the presence of 0.5% H2 . Support with small amount of weak acid site shifts to the right of this balance, which would result in remarkable enhancement of HC-SCR activity by the addition of very small amount of H2 into lean-burn exhausts.
5. Conclusions The C3 H8 -SCR activity of Ag-zeolites in the presence of H2 varied with Si/Al ratio and topology, in other words, acid amount and strength of zeolites. This variation in catalytic activity comes from the support effect on the balance of Ag species under the C3 H8 -SCR in the presence of H2 ; the balance was shifted to more cationic Ag side with the increase of acid amount and strength in zeolites. It is suggested that this phenomenon is caused by acceleration of dispersion of agglomerated Ag species through the reaction with oxidants and proton on zeolites. Use of support with small amount of weak acid site would be a promising design of Ag catalysts that exhibit high HC-SCR activity by the addition of very small amount of H2 into lean-burn exhausts.
Acknowledgements This work was partly supported by a Grant-in-Aid from the Ministry of Education, Science and Culture, Japan. The authors wish to thank Dr. K. Itabashi of Tosoh Co. for providing zeolite samples.
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