J. Phys. Chem. C 2007, 111, 6481-6487
6481
Reaction Mechanism of H2-Promoted Selective Catalytic Reduction of NO with C3H8 over Ag-MFI Zeolite Ken-ichi Shimizu,*,† Kenji Sugino,† Kazuo Kato,‡ Shigeru Yokota,§ Kazu Okumura,‡ and Atsushi Satsuma† Department of Molecular Design and Engineering, Graduate School of Engineering, Nagoya UniVersity, Chikusa-ku, Nagoya 464-8603, Japan, Department of Materials Science, Faculty of Engineering, Tottori UniVersity, Koyama-cho, Tottori 680-8552, Japan, and Japan Synchrotron Radiation Research Institute, 1-1-1 Kouto, Mikazuki-cho, Sayo-gun, Hyogo 679-5198, Japan ReceiVed: January 15, 2007; In Final Form: February 23, 2007
The mechanism of the H2-assisted selective catalytic reduction of NO with propane (H2-C3H8 SCR) over Ag+-exchanged MFI zeolite (Ag-MFI) was investigated by spectroscopic studies. Transient IR experiments at 573 K indicate that H2 addition increases the rates of C3H8 oxidation to acetate on Ag-MFI and NO oxidation to the weakly adsorbed NO2. IR spectra obtained during steady-state C3H8 SCR indicate that H2 addition increases the coverage of acetate, Ag+-NCO, and Al3+-NCO. The reaction of acetate with NO2 yields, via CH3NO2, NCO species, and their hydrolysis yields NH3 as a final precursor of N2. In situ UV-vis and EXAFS measurements at 573 K shows that, during the H2 + O2 and H2-C3H8 SCR reactions, Ag+ ions and partially reduced Ag42+ clusters coexist. ESR spectra obtained after exposing a H2 + O2 gas mixture show the formation of superoxide ions. Combined with the results in our previous studies, the following mechanism is proposed: The Ag42+ cluster and protons formed by H2 reduction of Ag+ ions are involved in the reductive activation of molecular oxygen into superoxide ion, which should act as an effective oxidant for C3H8 oxidation to oxygenates such as acetate and NO oxidation to NO2. Thus, H2 addition increases the coverage of surface intermediates (NCO species) and the gas-phase concentration of NO2 and consequently increases the rate of N2 formation.
Introduction The selective catalytic reduction of NO by hydrocarbons (HC SCR) in the presence of excess oxygen is a potential method for removing NOx from lean-burn and diesel exhausts.1-4 Following the finding by Satokawa and co-workers5,6 and our group6-15 of a dramatic positive effect of H2 addition on the HC SCR activity of Ag/Al2O3 at low temperatures, much attention has been focused on the “hydrogen effect” in SCR over silver catalysts from both fundamental and practical points of view.4,16-21 H2-assisted HC SCR was investigated in European transient cycle tests using a full-scale vehicle equipped with a Ag/Al2O3 system, and preliminary results demonstrated the effectiveness of this catalytic system.4 We reported a series of fundamental studies on the hydrogen effect in C3H8 SCR by Ag/Al2O36-11 and Ag zeolites.12-15 Interestingly, H2 acts not as a reducing agent of NO but as a promoter of HC SCR. On the role of H2 in H2-C3H8 SCR by Ag/Al2O3,6-11 we clarified that Agnδ+ clusters (n e 8) and protons formed by H2 reduction of Ag+ ions are involved in the reductive activation of molecular oxygen into superoxide ion, which can be effective for C3H8 activation to a partially oxidized intermediate.10,11 As for Ag zeolite, previous ex situ EXAFS results showed that hydrogen addition to Ag-MFI causes the partial reduction of isolated Ag+ cations to Ag42+ clusters, which could be the active species for HC SCR.14 Recently, the dynamic structural changes of Ag * Corresponding author. Fax: +81-52-789-3193. E-mail: kshimizu@ apchem.nagoya-u.ac.jp. † Nagoya University. ‡ Tottori University. § Japan Synchrotron Radiation Research Institute.
species in Ag-MFI were investigated by in situ time-resolved EXAFS, UV-vis, and IR spectroscopies. Prior to H2 reduction, Ag species in preoxidized Ag-MFI are present as Ag+ ions at cation-exchange sites in the zeolite.15 H2 reduction at 573 K yields Ag42+ clusters and protons on the zeolite. Upon reoxidation at 573 K, the silver clusters are redispersed to Ag+ ions. This redox cycle might be related to the promotion of HC SCR activity of Ag-MFI by H2 addition. A key characteristic of the hydrogen effect is that the effect is limited to Ag-based catalysts supported on only two types of oxides (alumina and MFI or BEA zeolite); the activity enhancement is not observed when TiO2, ZrO2, SiO2, and Ga2O3 are used as supports.6 So far, different proposals have been suggested on the role of hydrogen in catalysis using Ag/Al2O3, probably because several research groups used different reaction temperatures and feed conditions. If we find a similarity in the mechanistic cause of the hydrogen effect in the different catalysts Ag/Al2O3 and Ag-MFI, we can provide a more general and fundamental explanation of the hydrogen effect. Materials with clusters have interesting chemical properties unusual for bulk solids. Size-specific catalysis of supported gold clusters is a well-known example.22-24 Among numerous reports on the synthesis and properties of metal clusters, silver clusters appears to be very popular in this research field.25-33 Agn nanoclusters (2 < n < 8) can be easily synthesized in the cages and channels of zeolites by vacuum dehydration, by reduction with reducing reagents, and by γ- or X-ray irradiation. Although the formation of silver clusters has been studied extensively, reports on the specific catalysis of silver clusters are very few.
10.1021/jp070322q CCC: $37.00 © 2007 American Chemical Society Published on Web 04/05/2007
6482 J. Phys. Chem. C, Vol. 111, No. 17, 2007
Shimizu et al.
The objective of this study was to provide spectroscopic evidence on the essential role of H2 in H2-C3H8 SCR by AgMFI. First, the formation and reaction of adsorbed species was studied by IR spectroscopy to show that oxygenates, NO2, and NCO species are crucial intermediates. Second, the structure of the silver clusters and active oxygen species formed under H2-C3H8 SCR or H2 + O2 reaction conditions was determined by UV-vis, EXAFS, and ESR spectroscopies. Third, combined with the results of our previous studies, the role of H2 in H2C3H8 SCR by Ag-MFI is discussed focusing on the role of silver clusters and protons in reductive O2 activation to O2and its reaction with C3H8 and NO. Experimental Section The parent zeolite used in this study was H-MFI (TosohH-MFI supplied by Tosoh Corporation, Tokyo, Japan, Si/Al ) 22). Ag+-exchanged zeolite (Ag-MFI) with a Ag/Al ratio of 0.83 (Ag ) 0.50 mmol g-1)15 was prepared in a flask from H-MFI (10 g) by ion exchange with AgNO3 (3 equiv with respect to Al) in distilled water (250 cm3) at room temperature (24 h) three times. The prepared sample was filtered, rinsed with distilled water (200 cm3) three times, and dried at 383 K for 24 h; it was then calcined in flowing dry air at 823 K for 2 h. As the sample is partially rehydrated by contact with the atmosphere, the sample was recalcined at 823 K in O2 (10%)/ He prior to the subsequent characterization experiments. Ag K-edge Quick XAFS measurements were performed in transmission mode at the BL01B1 line in SPring-8. The storage ring was operated at 8 GeV. A Si(111) single crystal was used to obtain a monochromatic X-ray beam. A self-supported wafer form of the Ag-MFI sample (200 mg) with a diameter of ca. 10 mm was placed in a quartz in situ cell24 under an O2 (10%)/ He or H2 (0.5%)/O2 (10%)/He flow (200 cm3 min-1) at 573 K. Analysis of the extended X-ray absorption fine structure (EXAFS) data was performed using the REX version 2.5 program (Rigaku). The Fourier transformation of the k3-weighted EXAFS oscillation from k space to r space was performed over the range 20-120 nm-1 to obtain a radial distribution function. The inversely Fourier filtered data were analyzed with a common curve-fitting method in the k range of 48-120 nm-1. For the curve-fitting analysis, the empirical phase shift and amplitude functions for Ag-Ag and Ag-O shells were extracted from the data for Ag powder and Ag2SO4, respectively, measured at 573 K. The error for bond distance in each curvefitting data set was estimated to be below 0.004 nm. Diffuse-reflectance UV-vis measurements were made at 573 K with a UV-vis spectrometer (JASCO V-550) equipped with an in situ flow cell with a quartz window as described in our previous study.8 A diffuse-reflectance sample cell was connected with a gas flow system. The light source was led to the center of an integrating sphere by an optical fiber. Reflectance was converted to pseudo-absorbance using the Kubelka-Munk function. BaSO4 was used to collect a background spectrum. Then, a reference spectrum of preoxidized Ag-MFI taken at 573 K was subtracted from each spectrum. Various gas mixtures were fed at a flow rate of 100 cm3 min-1 to Ag-MFI (50 mg), which was pretreated in a flow of O2 (10%)/He at 823 K. The typical composition of the feed gas was NO/C3H8/O2/H2/He ) 0.1%/0.1%/10%/0.5%/balance. In situ IR spectra were recorded on a JASCO FT/IR-620 instrument equipped with a quartz IR cell connected to a conventional flow reaction system. The sample was pressed into a 50 mg self-supporting wafer and mounted into a quartz IR cell with CaF2 windows. Spectra were measured by accumula-
Figure 1. (A) IR spectra of adsorbed species on Ag-MFI at 573 K (t ) 240 s) and (B) time course of the IR band height at 1620 cm-1 in a flow of NO + O2 or NO + O2 + H2. Conditions: 0.1% NO, 0.5% H2, 10% O2.
tion of 10-20 scans at a resolution of 4 cm-1. A reference spectrum of the Ag-MFI wafer in He taken at the measurement temperature was subtracted from each spectrum. Prior to each experiment the catalyst disk was heated in 10% O2/He at 823 K for 0.5 h and then cooled to the desired temperature and purged for 0.5 h in He. Then, a flow of various gas mixtures was fed at a rate of 100 cm3 min-1. The typical composition of the feed gas was NO/C3H8/O2/H2/He ) 0.1%/0.1%/10%/0.5%/ balance. ESR spectra were measured with an X-band JEOL JESTE200 spectrometer at a microwave power level (1.0 mW).11 Prior to the ESR measurements, the sample was exposed to a flow of H2 (3%)/O2 (0.1%)/He (balance) at 423 K for 0.5 h and cooled to room temperature under a flow of the same gas mixture. Then, ESR spectra of the quenched sample were measured after the catalyst powder had been into the suprasil quartz tube without exposure to the air. Results and Discussion Formation and Reaction of Adsorption Complexes. The adsorption complexes formed upon exposure of the Ag-MFI catalyst to NO + O2 or NO + O2 + H2 flow at 573 K were examined by IR spectroscopy (Figure 1A). In flows of NO + O2 and NO + O2 + H2, a band at 1620 cm-1 was observed, and its intensity was much higher in the presence of H2. Chen et al.34 reported the IR spectra of adsorbed species formed by exposing Fe-MFI zeolite to NO + O2 and tentatively assigned the band at 1625 cm-1 to NO2 coordinated to an iron ion. We assign the band at 1620 cm-1 to a NO2 molecule coordinated to a Ag+ ion. Because the frequency of the band at 1620 cm-1 is very near the asymmetric stretching frequency of gaseous NO2 (1610 cm-1), the interaction of Ag-MFI with this ligand
H2-Promoted SCR of NO with C3H8 over Ag-MFI Zeolite TABLE 1: NO Reduction Rates, Rates of Elementary Steps, and IR Band Intensities of Intermediates for C3H8 SCR on Ag-MFI
rate of NO2(ad) formation in NO + O2 (au)a rate of acetate formation in C3H8 + O2 (au)b intensity of IR band of acetate at 1642 cm-1 (au)c intensity of IR band of Ag+-NCO at 2180 cm-1 (au)c intensity of IR band of Al+-NCO at 2282 cm-1 (au)c NO reduction rate (nmol g-1 s-1)d
without H2
with H2
2.0 1.3 0.028 0.004 0.008 14
27 6.7 0.057 0.017 0.052 910
a Relative rate of increase in IR band intensity of adsorbed NO2 (1620 cm-1) under transient conditions at 573 K estimated from time-resolved IR spectra (Figure 1B). b Relative rate of increase in IR band intensity of acetate (1642 cm-1) under transient conditions at 573 K estimated from time-resolved IR spectra (Figure 4B). c IR band intensity of adsorbed species during the NO + C3H8 + O2 reaction at 573 K under steady state (Figure 2d,e). d Steady-state rate of NO reduction to N2 in the NO + C3H8 + O2 reaction at 573 K.12
Figure 2. IR spectra of adsorbed species on Ag-MFI taken at 573 K (a) after introduction of CH3COOH (0.8 mmol g-1), followed by purging with He for 600 s; (b-e) after exposure to various gas mixtures for 1600 s; and (f) after introduction of CH3NO2 (0.2 mmol g-1), followed by purging with He for 180 s. (g) Spectrum recorded at 498 K after introduction of CH3NO2 (0.2 mmol g-1), followed by purging with He for 600 s.
should be very weak. As expected, this band gets weaker and vanishes after 120 s of purging with He at 573 K. Figure 1B shows the time dependence of the band height at 1620 cm-1. From the slope of the curve, the relative rates of NO2 formation were estimated as listed in Table 1. The results show that H2 addition increases the rate of NO oxidation to NO2 by a factor of 13. When Ag-MFI was exposed to C3H8 + O2 + H2 at 573 K, bands at 1676, 1642, 1574, 1534, and 1420 cm-1 were visible in the IR spectra (Figure 2). Figure 2 includes the spectrum obtained after introducing acetic acid (0.8 mmol/g of catalyst) into Ag-MFI at 573 K. The spectrum also has bands at 1676, 1642, 1574, 1534, and 1420 cm-1. The spectral features qualitatively correspond to those observed for acetic acid on MgO35 and Ba,Na-exchanged Y-zeolite (BaNa-Y).36 Thus, we assign the bands observed during C3H8 + O2 + H2 reaction on Ag-MFI to acetate adsorbed on the catalyst. These bands were also observed during C3H8 + O2 reaction on Ag-MFI at 573 K, although their intensity was less than that during
J. Phys. Chem. C, Vol. 111, No. 17, 2007 6483 C3H8+O2 +H2 reaction. From the slope of the curve in the time dependence of the band height at 1642 cm-1, relative rates of acetate formation were estimated (Table 1). The results show that H2 addition increases the rate of C3H8 oxidation to acetate by a factor of 5. Under H2-C3H8 SCR (NO + C3H8 + O2 + H2) conditions at 573 K, bands at 1676 and 1642 cm-1 that can be assigned to acetate and bands at 2312, 2282, 2180, and 2110 cm-1 were observed in the IR spectrum (Figure 2). The bands in the range 2310-2100 cm-1 are most likely to arise from NdCdO or CtN groups.36-45 The adsorption of HNCO on preoxidized Cu-MFI gives bands at 2300, 2260, and 2180 cm-1 that can be assigned to isocyanate ions adsorbed on Si4+ and Al3+ sites in MFI zeolite (Si4+-NCO, Al3+-NCO) and on Cu2+ sites (Cu2+-NCO), respectively.37 It is well-established that nitromethane is decomposed over various HC SCR catalysts, including BaNa-Y,36 Co-MFI,39 and Al2O3,40 to form adsorbed NCO or CN species. Yamaguchi40 examined the decomposition of nitromethane adsorbed on alumina and attributed a band at 2253-2261 cm-1 to Al3+-NCO species and a band at 2126 cm-1 to cyanide (CN-) species. The adsorption of nitromethane on BaNa-Y zeolite was investigated by Yeom et al.36 On the basis of isotopic shifts, they assigned the IR band at 2171 cm-1 to Ba2+sNdCdO. Satsuma et al.39 reported an IR study of nitromethane decomposition over Co-MFI and observed a band at 2273 cm-1 that is attributable to NCO bound to Si4+ or Al3+ and a band at 2194 cm-1 that is attributable to Co2+-NCO. The position of the 2110 cm-1 band that we observed (spectra d and e) is very close to those reported in the literature for adsorbed HCN. For example, the νCN vibration of adsorbed HCN was observed at 2104 cm-1 on Na-Y and Ba-Y zeolite41,42 and at 2100 cm-1 on silica.38 From the abovementioned discussion, we attribute the bands at 2312, 2282, 2180, and 2110 cm-1 to Si4+-NCO, Al3+-NCO, Ag+-NCO, and CN species (CN- ion or adsorbed HCN), respectively. The bands due to Al3+-NCO (2282 cm-1) and Ag+-NCO (2180 cm-1) appeared under H2-free C3H8 SCR (NO + C3H8 + O2) conditions, although their intensity was less than that during the H2-C3H8 SCR reaction. The relative intensities of these bands under these two sets of conditions are listed in Table 1. It is clear that the addition of H2 to C3H8 SCR reaction conditions increases the coverages of Ag+-NCO and Al3+NCO. Figure 2 includes the spectra obtained after introducing AgMFI to nitromethane (0.2 mmol/g of catalyst) at 498 and 573 K. The positions of the bands at 2312, 2270, and 2170 cm-1 observed at 498 K and the broad band at 2100-2120 cm-1 observed at 498 and 573 K are very close to the bands observed in the IR spectra obtained under H2-C3H8 SCR conditions (2312, 2282, 2180, and 2110 cm-1), indicating that the decomposition of nitromethane yields Si4+-NCO, Al3+-NCO, Ag+-NCO, and CN species, respectively. According to the literature,36,40,44 the bands at 1614 and 1585 cm-1 can be assigned to the aci anion of nitromethane. The reactivity of acetate toward NO + O2 was studied using time-resolved IR spectroscopy (Figure 3). It should be noted that Ag-MFI catalyzes NO2 formation by the NO + O2 reaction,46 and hence NO2, having a higher reactivity than NO, is produced in a flow of NO + O2. After introducing acetic acid (0.8 mmol/g of catalyst) into Ag-MFI at 573 K and then purging with He for 600 s (t ) 0 s in Figure 3), the flowing gas was switched to NO + O2. The intensities of the IR bands attributable to acetate (1676, 1642, 1574, 1534, and 1420 cm-1) decreased with time under NO + O2 flow, and bands due to
6484 J. Phys. Chem. C, Vol. 111, No. 17, 2007
Figure 3. Changes in IR spectra as a function of time of exposure of Ag-MFI to a flow of NO (0.1%) + O2 (10%) at 573 K. Before the measurements, Ag-MFI was preexposed to CH3COOH (0.8 mmol g-1) and then purged with He for 600 s at 573 K.
Figure 4. (A) Changes in IR spectra of adsorbed species on Ag-MFI at 573 K in C3H8 + O2 + H2 for 1600 s, followed by He purge for 420 s, and upon exposure to NO + O2. (B) Time course of the IR band heights. Conditions: 0.1% NO, 0.1% C3H8, 0.5% H2, 10% O2.
Al3+-NCO (2254 cm-1), Ag+-NCO (2180 cm-1), and CN species (2110 cm-1) appeared. This indicates that the reaction of acetate with NO + O2 yields the N-containing organic species Al3+-NCO, Ag+-NCO, and CN. The reactivity of acetate formed by the C3H8 + O2 + H2 reaction on Ag-MFI was also investigated in a similar manner. After exposing Ag-MFI to a flow of C3H8 + O2 + H2 at 573 K for 1600 s and then purging with He for 420 s, the flowing gas was switched to NO + O2. Changes in the IR spectrum are shown in Figure 4A, and a plot of the band heights as a function of time is shown in Figure 4B. The intensities of the bands due to acetate (1676,
Shimizu et al.
Figure 5. IR spectra recorded at 498 K (a) after exposure of AgMFI to CH3NO2 (0.2 mmol g-1), followed by purging with He for 600 s, (b) and after exposure of the sample to H2O (3%) for 180 s. (c) Spectrum recorded after exposure of fresh Ag-MFI to NH3 (0.5%) for 300 s, followed by purging with He for 600 s.
SCHEME 1
1642, 1574, 1534, and 1420 cm-1) do not markedly decrease after He purging, indicating that the acetate is fairly strongly adsorbed. These bands decreased with time of NO + O2 flow. A band attributable to Ag+-NCO (2170 cm-1) appeared, and its intensity increased sharply for 240 s and then decreased, whereas the band due to CN species (2110 cm-1) gradually increased. Yeom et al.36 showed that acetate adsorbed species on BaNa-Y zeolite reacts with NO2 to form nitromethane, which is then converted to NCO species. The same research group44 also reported IR evidence indicating the formation of NCO species by the reaction of acetate on Ag/Al2O3 with NO2 via nitromethane as an intermediate. From these facts, it can be concluded that the reaction of acetate on Ag-MFI with NO2 yields Al3+-NCO, Ag+-NCO, and CN species possibly via nitromethane as an intermediate. It is well-known that hydrolysis of HNCO results in the formation of NH3 and CO2.36,40,45 NH3 formation by the hydrolysis of NCO species on Ag-MFI was confirmed by the IR results shown in Figure 5. When the Ag-MFI catalyst, preexposed to nitromethane (0.2 mmol/g of catalyst) at 498 K and then purged with He for 600 s, was exposed to a flow of 3% H2O for 180 s, the intensity of the IR band due to Al3+-NCO (2270 cm-1) decreased, and a band at 1450 cm-1 appeared. A band in the same position (1450 cm-1) was observed in the spectrum taken after exposure of Ag-MFI to a flow of 0.5% NH3, and this band was assigned to the deformation band of NH4+ ion formed by the interaction of NH3 with Brønsted acid sites of zeolite.47 In contrast, the band due to CN species (2110 cm-1) did not markedly change after the introduction of water vapor, indicating an inert nature of CN species. It is well-established that NH4+ on zeolite is highly reactive toward NO2 to produce N2 and H2O.47 From these discussions, the mechanism of H2-assisted HC SCR consists of the following steps (Scheme 1): (1) oxidation of NO to NO2, (2) partial oxidation of propane to oxygenates such as acetate, (3) reaction of the oxygenates with NO2 to form NCO intermediates via nitromethane, (4) hydrolysis of the NCO intermediates to form NH4+ and CO2, and (5) reaction of NH4+ with NO2 to form N2
H2-Promoted SCR of NO with C3H8 over Ag-MFI Zeolite
Figure 6. Fourier transforms of Ag K-edge in situ EXAFS spectra of Ag-MFI measured at 573 K.
TABLE 2: Structural Parameters from the Fitted EXAFS Data in Figure 6 samples
atom
CNa
Rb (Å)
Rfc (%)
Ag-MFI in O2 Ag-MFI in H2 + O2
O O Ag Ag O
2.6 2.6 1.0 (12) (6)
2.51 2.56 2.74 (2.89) (2.50)
7.2 2.1
Ag foild Ag2SO4d
a Coordination number. b Shell distance. c Residual factor. d Data from X-ray crystallography.
and H2O. The proposed mechanism is essentially the same as those reported in our previous studies on Al2O3-based HC SCR catalysts.11 To determine whether the proposed mechanism (Scheme 1) is the primary route to N2 or a secondary step, the transient reaction rate of the possible intermediate, acetate, was compared to the steady-state NO reduction rate. The number of acetate molecules on Ag-MFI can be quantified using the extinction coefficient of adsorbed acetic acid (1.2 m2/mol) estimated from the IR spectrum in Figure 2a. From the slope of the curve in the time change of the acetate band (Figure 4B), the initial rates of acetate formation in the reaction of C3H8 + O2 + H2 (340 nmol g-1 s-1) and in the reaction in NO + O2 (450 nmol g-1 s-1) were estimated. These values are of the same order of magnitude as the steady-state NO reduction rate (910 nmol g-1 s-1) listed in Table 1, although they are 37-49% as low as the steady-state NO reduction. This suggests that acetate might contribute at least 37% to the total rate of NO reduction. Table 1 shows that hydrogen addition increases the propane partial oxidation rate by a factor of 5, whereas it increases the NO reduction rate by a factor of 65. These facts suggest that the proposed mechanism does not account for the complete NO reduction pathway, and other processes, not detected by IR spectroscopy, could also be occurring. A DFT study on the present subject is in progress in our research group, and the results will include an extended mechanism including spectroscopically undetectable active species. Structure of Ag Clusters during SCR. To investigate the structure of Ag species under the H2 (0.5%) + O2 (10%) feed conditions at 573 K, in situ Ag K-edge EXAFS measurements were obtained. Figure 6 shows Fourier transforms of k3-weighted EXAFS data for Ag-MFI at 573 K. Structural parameters derived from the curve-fitting analysis are listed in Table 2. The EXAFS results for preoxidized Ag-MFI show only a Ag-O contribution (Table 2). During the steady-state H2 + O2 reaction, a Ag-Ag shell with a coordination number (CN) of 1.0 and a bond distance of 2.74 Å was observed. The bond distance (R ) 2.74 Å) is smaller than those of Ag foil (2.89 Å) and is the same as that observed for H2-reduced Ag-MFI at 573 K in our previous in situ EXAFS study.15 Combined with
J. Phys. Chem. C, Vol. 111, No. 17, 2007 6485 in situ UV-vis and H2 temperature-programmed reduction (TPR) results, we concluded that a most probable candidate for the silver species with the in situ EXAFS feature of H2-reduced Ag-MFI (R ) 2.74 Å and CN ) 3.3 for the Ag-Ag shell) is Ag42+ with a tetrahedral structure.14,15 Assuming that isolated Ag+ ions and the tetrahedral Ag42+ cluster are the main Ag species in Ag-MFI during the steady-state H2 + O2 reaction, a Ag-Ag coordination number of 1.0 suggests that only a small portion (possibly ca. 30%) of Ag+ ions are reduced to Ag42+. The Ag-O coordination numbers did not increase upon exposure of Ag-MFI to H2 + O2, which supports this model. Diffuse-reflectance UV-vis spectroscopy has been conventionally used for characterizing silver clusters formed by reducing Ag+ ions. It is accepted that the bands centered in a range of 40000-52000 cm-1 correspond to the 4d10-to-4d95s1 transition of Ag+ ions, and the bands between 25000 and 40000 cm-1 are due to silver clusters with different sizes and oxidation states.11,18,19,25,32 In the UV-vis spectrum of AgMFI under a flow of 10% O2 (spectrum a in Figure 7) and during the steady-state NO + C3H8 + O2 + H2 reaction (spectrum b), a band centered above 40000 cm-1 due to Ag+ ion was observed. In difference spectra obtained during both the NO + C3H8 + O2 + H2 reaction (spectrum c) and the H2 + O2 reaction (spectrum d), a main band around 30000-34000 cm-1 that can be assigned to Agnδ+ (n e 8) clusters11,12,18,19,25,32 was observed. A shoulder around 25000 cm-1 in spectrum d is attributable to larger Agn (n > 8) clusters.12,15,19 Taking into account the EXAFS results for Ag-MFI obtained during the H2 + O2 reaction, indicating that Ag42+ clusters are produced in the sample, the main band centered around 30000-34000 cm-1 can be assigned to Ag42+ clusters. From the EXAFS results, it is considered that the larger Agn (n > 8) clusters, giving the UVvis band at 25000 cm-1, is a minor silver species in the H2 + O2 reaction. Its intensity was much lower in NO + C3H8 + O2 + H2 than in H2 + O2, indicating that the amount of Ag42+ clusters in NO + C3H8 + O2 + H2 is lower than that in H2 + O2. This difference can be explained by the relatively high rate of Ag42+ cluster consumption by NO + O2; previous in situ UV-vis experiments15 showed that the Ag42+ clusters in AgMFI are highly reactive toward NO + O2, and that the rate of Ag42+ cluster redispersion to Ag+ ions in NO + O2 is much higher than that in O2. The band due to larger Agn (n > 8) clusters is negligible in the spectrum under NO + C3H8 + O2 + H2 conditions, suggesting that they are not catalytically important. Summarizing the above EXAFS and UV-vis results, it is suggested that isolated Ag+ ions as the major Ag species and Ag42+ clusters as a minor Ag species are present in AgMFI under steady-state H2-C3H8 SCR conditions. Reaction of Reduced Ag-MFI with O2. In our previous studies14,15 on the H2 reduction of Ag-MFI, the following stoichiometry was proposed for the reduction of Ag+ ions to produce Ag42+ clusters and protons
4Ag+ + H2 f Ag42+ + 2H+ The reactivity of the Ag42+ clusters toward O2 was also investigated in our previous study using in situ EXAFS and UV-vis spectroscopies. When the flowing gas was switched from H2 to O2, the intensity of the UV-vis band due to Ag clusters decreased. The EXAFS results show that reoxidation with O2 at 573 K results in a decrease in the Ag-Ag coordination number from 3.3 to 0.9 and an increase in the Ag-O coordination number from 1.0 to 2.2. These results indicate that reoxidation of Ag42+ clusters with O2 yields Ag+ ions on the cation-exchange sites of MFI according to the
6486 J. Phys. Chem. C, Vol. 111, No. 17, 2007
Figure 7. In situ UV-vis spectra of Ag-MFI measured under different conditions at 573 K: (a) in 10% O2 after oxidation at 823 K, (b) during the NO + C3H8 + O2 + H2 reaction (1800 s), and (c) difference spectrum of spectrum b based on spectrum a, (d) difference spectrum obtained during the H2 + O2 reaction (1800 s).
Figure 8. (A) Changes in the IR spectra of Ag-MFI in 10% O2/He at 573 K and (B) time course of the area of the 2648 cm-1 band. Before the measurements, Ag-MFI was treated in a flow of 0.5% D2/He for 1000 s.
reaction 1
/2O2 + Ag42+ + 2H+ f 4Ag+ +H2O
Jacobs et al.28,29 and Baba et al.33 reported that silver cations in Ag-exchanged zeolites are reduced by hydrogen to generate protons and silver clusters. As we reported previously,15 acidic OD groups are produced in Ag-MFI after exposure to flowing D2 (Figure 8A). Note that predeuteration of OH groups on the zeolite was first carried out in a flow of D2O (3%)/He at 823 K for 0.5 h, after which the sample was purged in He for 0.5 h and then exposed to flowing D2 (0.5%) at 573 K for 0.5 h. The resulting IR spectrum shows a band at 2648 cm-1 that can be assigned to OD groups with Brønsted acidity.15 Figure 8B shows the time dependence of the IR band at 2648 cm-1 in a flow of He or O2 (10%)/He at 573 K. When the D2-reduced Ag-MFI was exposed to a flow of O2, the intensity of the band due to the acidic OD groups (2648 cm-1) decreased at a higher rate than under He purge, indicating that the acidic OD groups are consumed by their reaction with O2. Figure 9 shows ESR spectra recorded at 77 K for Ag-MFI after the H2 + O2 treatment at 423 and 573 K. The samples showed ESR spectrum with anisotropic g values of gxx ) 2.001, gyy ) 2.009, and gzz ) 2.025. This signal has been previously identified as O2- (superoxide) radical on silver-containing catalysts.48,49 ESR signals due to O2- were not observed after calcination in O2 at 823 K (results not shown). These results provide clear evidence on the reductive activation of molecular oxygen into the reactive oxygen species, O2-.
Shimizu et al.
Figure 9. ESR spectra (77 K) of Ag-MFI after H2 (3%) + O2 (0.1%) treatment at (a) 423 and (b) 573 K.
Mechanistic Cause of the H2 Effect on HC SCR. UV-vis spectra obtained during the H2-C3H8 SCR reaction suggest the presence of reduced Ag species, Ag42+, as well as Ag+ ions. Ag+ ions in Ag-MFI are reduced by hydrogen to generate acidic protons on the cation-exchange sites of MFI and Ag42+.14,15 Upon exposure to O2, the reoxidation of the Ag42+ clusters with O2 accompanying the consumption of protons should result in the formation to Ag+ and H2O, possibly via O2- as an intermediate. Time-resolved IR results (Figures 1 and 4, Table 1) shows that H2 addition promotes the oxidation of hydrocarbons to oxygenates and the oxidation of NO to NO2 adsorbed species. IR spectra obtained during steady-state C3H8 SCR (Figure 2, Table 1) indicate that H2 addition increases the coverage of acetate, Ag+-NCO, and Al3+-NCO. Taking the reaction mechanism shown in Scheme 1 into account, the following role of hydrogen on the promotion of C3H8 SCR over Ag-MFI is proposed. In the absence of hydrogen, the activation of molecular oxygen on the surface into reactive oxygen species is slow, and hence, subsequent C3H8 oxidation to oxygenates such as acetate and NO oxidation to NO2 are slow steps. Hydrogen promotes the reaction of molecular oxygen into reactive oxygen species, O2-, that are involved in the oxidative activation of C3H8 and NO. The reaction of acetate on AgMFI with NO2 yields Al3+-NCO and Ag+-NCO via nitromethane (Figures 3 and 4). NCO species are rapidly hydrolyzed to produce NH4+ (Figure 5), which will react with NO2 to produce N2 and H2O.47 Thus, H2 addition increases the coverage of intermediates (NCO species) and the gas-phase NO2 concentration and consequently increases the rate of N2 formation. One of the most important characteristic of the hydrogen effect is that it is limited to Ag-based catalysts but depends greatly on the support used; the activities of Ag supported on TiO2, ZrO2, SiO2, and Ga2O3 are not enhanced by the addition of hydrogen. However, Ag/Al2O3 and Ag-MFI catalysts show markedly improved performances after the addition of hydrogen to the feed. If we find a similarity in the mechanistic cause of the hydrogen effect on the different catalysts Ag/Al2O3 and AgMFI, we can provide a more general and fundamental explanation of the hydrogen effect. In this article, it is clarified that the reductive activation of O2 via the Ag+ T Ag42+ redox cycle and subsequent oxidations of NO and hydrocarbon are the essential cause of the hydrogen effect for Ag-MFI, which is essentially the same as the mechanism for Ag/Al2O3 proposed in our previous studies.7,10,11 This indicates that this is a general cause of the hydrogen effect in HC SCR. A difference between
H2-Promoted SCR of NO with C3H8 over Ag-MFI Zeolite Ag-MFI and Ag/Al2O3 originates from the higher stability of adsorbed NO2 species on the latter catalyst in the absence of H2, possibly because of the higher basicity of the alumina support than the zeolite. In the absence of hydrogen, nitrates show a marked poisoning effect on C3H8 SCR over Ag/Al2O3.10 Hydrogen retards the nitrates poisoning by decreasing the nitrates coverage via nitrate reduction to NO.10 In contrast, the NO2 coverage on Ag-MFI during HC SCR (in the absence of H2) is negligibly low (spectrum e in Figure 2), and hence, H2 does not play a role in retarding the NOx poisoning. Conclusions The mechanism of the H2-assisted selective catalytic reduction of NO with propane (H2-C3H8 SCR) over Ag+-exchanged MFI zeolite (Ag-MFI) is presented. The reaction consists of following steps: (1) oxidation of NO to NO2, (2) partial oxidation of propane to oxygenates such as acetate, (3) reaction of the oxygenates with NO2 to form NCO intermediates via nitromethane, (4) hydrolysis of the NCO intermediates to form NH4+ and CO2, and (5) reaction of NH4+ with NO2 to form N2 and H2O. Steps 3-5 can be catalyzed by Brønsted acid sites of the zeolite. H2 addition promotes steps 1 and 2 catalyzed by the redox cycle of Ag species in the following manner: The H2 reduction of Ag+ yields partially reduced Ag42+ clusters and protons. O2 is reductively activated with the Ag42+ clusters and H+ to yield O2-, which acts as an effective oxidant in steps 1 and 2. The role of H2 and the reaction mechanism proposed in this study are essentially the same as those reported for H2C3H8 SCR on Ag/Al2O3 in our previous study,11 suggesting that the proposed mechanism provides a comprehensive understanding of the hydrogen effect in HC SCR. Acknowledgment. X-ray absorption experiments were performed with the approval of the Japan Synchrotron Radiation Research Institute (Proposal No. 2006A1040). This work was partially supported by a Grant-in-Aid for Scientific Research (B) and a grant in Priority Area “Molecular Nano Dynamics” from the Ministry of Education, Science and Culture, Japan. References and Notes (1) Iwamoto, M.; Yahiro, H. Catal. Today 1996, 22, 5. (2) Hamada, H. Catal. Today 1996, 22, 21. (3) Burch, R.; Breen, J. P.; Meunier, F. C. Appl. Catal. B 2002, 39, 283. (4) Klingstedt, F.; Arve, K.; Eranen, K.; Murzin, D. Y. Acc. Chem. Res. 2006, 39, 273. (5) Satokawa, S. Chem. Lett. 2000, 294. (6) Satokawa, S.; Shibata, J.; Shimizu, K.; Satsuma, A.; Hattori, T. Appl. Catal. B 2003, 42, 179. (7) Shimizu, K.; Satsuma, A. Phys. Chem. Chem. Phys. 2006, 8, 2677. (8) Satsuma, A.; Shibata, J.; Wada, A.; Shinozaki, Y.; Hattori, T. Stud. Surf. Sci. Catal. 2003, 145, 235. (9) Shibata, J.; Shimizu, K.; Satokawa, S.; Satsuma, A.; Hattori, T. Phys. Chem. Chem. Phys. 2003, 5, 2154. (10) Shimizu, K.; Shibata, J.; Satsuma, A. J. Catal. 2006, 239, 402. (11) Shimizu, K.; Tsuzuki, M.; Kato, K.; Yokota, S.; Okumura, K.; Satsuma, A. J. Phys. Chem. C 2007, 111, 950.
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