Infrared Study of the NO Reduction by Hydrocarbons over Iron Sites

J. Phys. Chem. C 2010, 114, 15713–15727

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Infrared Study of the NO Reduction by Hydrocarbons over Iron Sites with Low Nuclearity: Some New Insight into the Reaction Pathway Jinlin Long, Zizhong Zhang, Zhengxin Ding, Rusheng Ruan, Zhaohui Li, and Xuxu Wang* Research Institute of Photocatalysis, State Key Laboratory Breeding Base of Photocatalysis, Fuzhou UniVersity, Fuzhou, China, 350002 ReceiVed: June 1, 2010; ReVised Manuscript ReceiVed: August 9, 2010

To understand at a molecular level the nature of iron-catalyzed selective reduction of NO by hydrocarbons (HC-SCR), binuclear µ-hydroxo-bridged iron clusters with a well-defined structure were constructed in cavities of Y zeolite by surface organometallic chemistry of ferrocene and were characterized by extended X-ray absorption fine structure spectroscopy and NO adsorption monitored by infrared (IR) spectroscopy. IR spectroscopy was used to study in detail the SCR of NO by propylene, ethylene, and methane over these well-defined iron sites. The results reveal that the NO reduction undergoes different reaction pathways, depending on the type of hydrocarbon reductants and the reaction temperature. In the absence of O2, propylene can reduce NO coadsorbed over Fe sites by a NO adduct mechanism at room temperature, while at high reaction temperatures greater than 150 °C, NO2 produced by the disproportionation reaction of NO contributes in major part to the SCR reaction. NH3 produced by the sequential reactions of olefins, acetaldehyde, acetic acid, and nitro-compounds with NO2 was found to be a key intermediate for the formation of N2. The nitrocompounds formed by coadsorption of reductant and oxidant over Fe sites may be the key intermediates initiating the reduction process at high temperature. Methane as a reductant represents a NO2 reduction pathway different from propylene and ethylene. It is oxidized to CO2, CO, and H2O probably via the nitromethane pathway, while NO2 is directly reduced to N2. The whole reaction network of the iron-catalyzed HC-SCR in the absence of O2 was proposed and discussed by a combination of literature with some important intermediates observed such as R-CN, HNCO, -COO-, and NH3. The catalytic results show that the presence of O2 greatly aids in the iron-catalyzed reduction of NO to N2, but the intrinsic reduction pathway is not changed. 1. Introduction Catalytic abatement of nitrogen oxides emitted from stationary power plants, waste combustion plants, and automobiles is of great concern in the field of environmental catalysis, because of the fact that they are causing serious damage to the environment, climate, and human health. Among the methods being investigated to clean exhaust gas streams of NOx, selective catalytic reduction (SCR) of NOx by hydrocarbons including CH4, C3H6 and C4H10, CO, H2, and ammonia has been wellestablished to be a very promising and prospective approach to the solution.1-3 Many transitional and noble metals such as Cu, V, Fe, Co, Ga, In, Ag, Pt, and Pd have been well-known to be catalytically active for the SCR process.1,3 However, the studies also showed that the SCR process depends not only on the chemical states of catalytically active components, but the type of reductant. Only a few elements such as Co, Pd, Ga, Ni, and In can activate methane to reduce NO into N2, while Fe and Cu are catalytically active using C2+ hydrocarbon or ammonia as reducing agents.4-6 The origin causing such a difference remains unclear until now; therefore, understanding completely the SCR chemistry over these active components is a key to designing highly efficient NOx removal catalysts. Since Fe/ZSM-5 was found to be an effective catalyst for NOx removal by Feng and Hall in 1997,7 zeolite-supported iron has received durative attention due to remarkable catalytic activity and selectivity for the decomposition of N2O and the SCR of nitrogen oxides by hydrocarbons or ammonia in the * To whom correspondence should be addressed. Tel: +86-59183779251. Fax: +86-591-83779251. E-mail: [email protected].

presence of H2O and SO2.8-10 Much effort was devoted to the identification of active Fe sites and the preparation of catalysts with high activity and selectivity,11-13 but the debate concerning the nature of Fe sites active in the HC-SCR reaction is still alive. Binuclear oxygen-bridged Fe sites have been claimed to be active sites for the HC-SCR reaction thanks to extended X-ray absorption fine structure (EXAFS) studies.14 On the other hand, Joyner and Stockenhuber proposed that Fe4O4 nanoclusters are responsible for the NOx reduction with propene,15 whereas Schwidder et al.16 suggested that isolated iron is the active site of the HC-SCR of NO. Which one type of iron site (isolated iron, binuclear iron species, or iron oxide clusters) is catalytically active for the HC-SCR reaction? The main reason of the debate rests with the facts that no method [e.g., UV/vis, infrared (IR), electron paramagnetic resonance (EPR), EXAFS, X-ray photoelectron spectoscopy (XPS), and X-ray diffraction (XRD)] can definitively distinguish between isolated iron species and iron oxide clusters of different nuclearity in Fe-contained zeolites.17 Moreover, the HC-SCR chemistry of NOx over iron-containing zeolite catalyst has been widely investigated with IR spectroscopy18-20 and yet has not been fully unveiled to date. The role of nitrogen-containing organic intermediates and nitrate species formed in the reduction process of NOx was wellinvestigated,21,22 but two confusions about the HC-SCR chemistry need to be further clarified; one concerns the adsorption states and activation process of NO and hydrocarbons over Fe sites. Which oxidant (NO, NO+, or NO2) takes part in the reaction? In addition, why does Fe need C2+ hydrocarbon? The

10.1021/jp104998d  2010 American Chemical Society Published on Web 08/25/2010

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definitive elucidation of all of these scientific issues is strongly dependent on the identification and construction of well-defined iron centers. The distribution of iron sites in channels of zeolite has been well-known to be strongly dependent on iron content and preparation methods. The more homogeneous distribution of one Fe site (e.g., isolated iron cations) could be ideally obtained by careful thermal activation of isomorphously substituted samples with low Fe amount, minimizing the tendency of iron to form clusters. However, the high Fe amount generally leads to the most heterogeneous distribution of iron species with different nuclearity (from isolated iron cations to oxide clusters, passing through dimers and small oligomers in ion exchange positions).23 Chemical vapor deposition of FeCl3 is presently the most investigated method to prepare zeolite catalysts with high Fe amount, but it cannot also ensure the uniform dispersion of iron oxide species in channels of zeolite in the case of high Fe content. In previous work, we developed a surface organometallic chemistry route (SOMC) to successfully construct binuclear iron-oxo clusters predominant absolutely in supercages of Y zeolite.24 These relatively uniform binuclear iron species with a well-defined chemical state can offer a platform to explore the actual HC-SCR chemistry of NO. The main objective of the present study is to unravel the selective reduction mechanism of NO with olefins and alkanes over these well-defined binuclear iron sites. In the present work, we first constructed these binuclear iron clusters in supercages of Y zeolite by the SOMC method and then investigated in detail the adsorption states and surface reaction processes of NO and hydrocarbons over these binuclear iron sites with Fourier transform infrared (FT-IR) spectroscopy. The structure of these binuclear iron clusters was characterized by EXAFS and IR spectroscopies. Propylene, ethylene, and methane were selected as reducing agents to comprehensively examine the difference between the SCR mechanism of NO with olefin and alkane. This work brings us some new insights into the iron-catalyzed HC-SCR reaction mechanism. 2. Experimental Section 2.1. Materials. The NH4Y zeolite with a SiO2/Al2O3 ratio of 5.1 was purchased from Aldrich. Ferrocene (C10H10Fe, 98%) was purchased from Acros and purified by sublimation under vacuum prior to use. 2.2. Preparation of Catalysts. The sample denoted as Fe/ HY-Gx (x: iron content) was prepared by surface organometallic chemistry of ferrocene (SOMC method). The preparation was performed in glassware equipment connected to a vacuum line. NH4Y zeolite (ca. 1.0 g) was treated at 673 K for 24 h under flowing oxygen to transform completely it into H type Y zeolite, accompanied by treatment under vacuum (4 × 10-2 Pa) at 673 K for 3 h. After it was cooled to room temperature, a certain amount of ferrocene was introduced onto the zeolite by sublimation. The system was then kept at 423 K for ca. 50 h to ensure a complete reaction. More experimental details were described in ref 25. The resulting solid was treated at 773 K for 4 h in flowing oxygen and subsequently exposed to air. 2.3. Catalyst Characterization. Inductively coupled plasma atomic emission spectrometry (ICP) was used for the determination of Fe content in the samples. The X-ray absorption experiments were performed at the XAFS station (Beamline 4W1B) of the Beijing Synchrotron Irradiation Facility (BSRF) with stored electron energy of 2.2 GeV and ring currents between 160 and 250 mA. Data were collected in a transmission mode at room temperature with a sampling step of ∼1.0 eV

Long et al. for X-ray absorption near edge structure (XANES) and ∼3.0 eV for EXAFS. In all cases, the irradiation was monochromatized using a Si(311) double-crystal monochromator. The intensities of the incident and transmitted beams were monitored using two N2 and 50% argon-doped N2-filled ionization chambers, respectively. Energies of samples were calibrated using a Fe metal foil standard, assigning the starting point of the K-edge to 7111.2 eV. The energy resolution is 0.3 eV. The data treatment was carried out using the software package WinXAS2.0. For background subtraction and XANES normalization, a linear polynomial was fitted to the pre-edge region and a third-order polynomial to the postedge region. The radial structural function (RSF) FT [k3x(k)] was obtained by Fourier transformation of the k3-weighted experimental function [x(k)][I(k) - I0(k)]/I0(k) multiplied by a Gauss window in the range 2.4-13.9 Å-1. The Fourier-filtered data were fitted in R-space in the range 1-4 Å for R-Fe2O3 and in the range 1-3 Å for other samples. To obtain the structural parameters including interatomic distances (R), coordination numbers (CN), Debye-Waller factors (∆σ2), and edge energy shifts (∆E0), phase shifts and backscattering amplitudes extracted from a reference material R-Fe2O3 (Fe-O) were used to fit the EXAFS data. The validity of a fit was checked by the fitting of k3-weighted spectra in k-space. The quality of a fit was estimated from the values of the variances of the imaginary and absolute parts of the FT. 2.4. IR Experiment. The IR experiments were carried out on a Nicolet 670 FT-IR spectrometer with a DTGS detector. For the gas-phase IR spectra taken, the IR spectrum collected before gas dosage was used as background. For the IR spectra of surface species adsorbed on the catalyst, the gas-phase IR spectrum collected was used as background. Each spectrum consists of 32 scans taken at 4 cm-1 resolution. Typically, prior to introducing adsorbents, the catalyst must be pretreated in vacuum at 673 K for 3 h. When the catalyst was cooled to room temperature (residual pressure in the reaction cell is equal to 10-4 Torr), pure NO (99.5 wt %) and hydrocarbons (99.9 wt %) were added by a gas syringe. IR spectra of NO adsorption on ca. 20 mg of self-supporting wafer were recorded at the given partial pressures of NO with a homemade static in situ IR cell equipped with CaF2 transparent windows at room temperature. All reported spectra of adsorption and reaction of NO and hydrocarbons are backgroundsubtracted. The reaction temperature is variable from room temperature to 673 K. Fitting of IR spectra was carried out with a mixed function composed of 80% Gaussian and 20% Lorentzian. Prior to fit, all spectra must be treated by baseline correction and Fourier self-deconvolution with the same parameters (triangular apodization function, half width at half height σ ) 2 cm-1, resolution enhancement factor κ ) 2.5). 2.5. Catalytic Testing. Steady-state catalytic activity tests for the selective reduction of NO with hydrocarbons were carried out using a fixed-bed continuous-flow reactor. About 0.1 g of catalyst was held in a quartz glass tube (i.d. 4.0 mm), equipped with a temperature programming controller. Prior to measurement, the catalyst was pretreated in situ in flowing He at 500 °C for 0.5 h. The reaction gas was a stoichiometric mixture of 2000 ppm NO, 1000 ppm hydrocarbons (propylene, ethylene, and methane), and 500 ppm oxygen. In the experiments without O2, O2 was removed from the reaction gas while keeping the concentration of the other gases constant with He as balance gas. The total flow rate of the reaction gas was kept at 100 mL min-1 (GHSV ) 32000 h-1) by use of mass flow controllers. The gas compositions were monitored by a GC (Agilent 6820)

Infrared Study of the NO Reduction by Hydrocarbons

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Figure 1. EXAFS spectrum of the catalyst Fe/HY-G4.9.

TABLE 1: Fit Results from EXAFS Reported in Figure 1 sample

shell

CN ((0.2)

R (Å) ((0.02)

σ2 (×103 Å2) ((0.1)

∆E0 (eV) ((0.2)

Fe/HY-G4.9

Fe-O1 Fe-O2 Fe-Fe

3.0 1.8 1.5

1.94 2.04 3.11

6.5 5.3 9.7

2.5 2.2 8.0

equipped with a thermal conduction detector (TCD). Two packed columns were used to separate the products: Porapak Q column for separating N2O and CO2, and 5 Å molecular sieve column for separating N2, NO, O2, and CO. 3. Results 3.1. Geometric Structure of Grafted Iron Species. Element analysis shows that the Fe/HY zeolite catalyst prepared by the SOMC method contains 4.9 wt % of iron (denoted as Fe/HYG4.9). It is ideally expected that the Fe/HY-G4.9 catalyst possesses the same iron species as the series of catalysts reported in the previous work,24 because the grafted iron content (4.9 wt %) is just controlled in the range of 2.1-8.1 wt % (where the majority of iron is present in a binuclear state). The geometric environment of iron species in the Fe/HY-G4.9 untreated at 673 K in vacuum was characterized by X-ray absorption spectroscopy. The K3-weighted EXAFS Fourier transformation spectrum for the sample is displayed in Figure 1. The Fourier transform (absolute part) of the EXAFS spectrum has two maxima between 1 and 2 Å and between 2 and 3 Å. The former is assigned to the oxygen atoms surrounding to the iron absorber in the first coordination shell; the latter mainly originates from Fe-Fe backscattering due to the presence of a weak oscillation. A model consisting of a single Fe absorber coordinated to two shells of oxygen atoms and one shell of Fe atoms was used to fit the EXAFS spectrum. Fitted results are given in Table 1 and point to the presence of oligonuclear clusters composed of two or three iron atoms for the Fe/HY-G4.9. The sum of the O neighbors and the Fe neighbors is found to be ca. 5 and ca. 1.5, respectively. The Fe-Fe distance of 3.11 Å and the Fe-O2 distance of 2.04 Å are consistent with the Fe-Fe distance (3.08-3.16 Å) and the Fe-O distance (1.96-2.02 Å) obtained for the FeIII2(µ-OH)2 core, respectively.26-28 The absence of the Fe-Fe coordination shell at longer distances (4-6 Å) indicates that the Fe/HY-G4.9 catalyst is free of iron oxide nanoparticles. The Fe-O1 distance of 1.94 Å is in good agreement with the average distance found for Fe-O-Al bridging oxygen,14 suggesting that iron is indeed anchored on acidic bridging oxygen sites. According to these results reported above, one can conclude that iron in the Fe/HY-G4.9 has a very low nuclearity from monomer to trimer. A minor amount of isolated iron and iron trimer is present in the catalyst as shown by the

Figure 2. FT-IR spectra of NO adsorbed at room temperature. (a) Fe/HY-G4.9 treated in vacuum at 673 K for 3 h, (b) after addition of 2.53 Torr NO, and (c) after addition of 5.06 Torr. The inset shows deconvoluted sub-bands of the IR spectrum (c) in the region of 1950-1750 cm-1.

EPR line at g ) 4.3 (see Figure S1 in the Supporting Information). Considering (0.2 error of coordination number, it was estimated that 80% of iron is present in the binuclear µ-hydroxo-bridged iron state in the Fe/HY-G4.9. The local structure and formation mechanism of the binuclear iron clusters bound on the cavity surface of Y zeolite were discussed in detail in the previous work.24 The material containing the binuclear iron species with relatively uniform distribution can be used as a model catalyst to study the SCR mechanism of NO by olefins and alkanes. 3.2. IR Observation of Adsorption and Activation of NO on Iron Sites. Adsorption states of NO on these binuclear iron sites at room temperature were observed by in situ IR spectroscopy as shown in Figure 2. Prior to introduction of NO, the Fe/HY-G4.9 sample was treated by evacuation at 673 K for 3 h so as to clear up the catalyst surface and thus expose completely active iron sites in reactive atmosphere (Figure 2a). After introduction of NO (2.53 Torr), a strong and broad band centered at 1870 cm-1 with a very weak shoulder at 1845 cm-1 and two weak bands at 1918 and 1815 cm-1 are observed in the IR spectrum (Figure 2b). These bands of NO adsorption are very consistent with those observed on the reduced Fe-Y zeolite referenced in the literature.29 The pair of bands at 1918 and 1815 cm-1 is unambiguously assigned to Fe2+(NO)2 according to the literature.15,24,29,30 The bands at 1870 and 1841 cm-1 are very similar to those observed for Fe-HZSM-11, FeBEA, sublimed Fe/ZSM-5 and Fe/ZSM-5 prepared by solidstate ion exchange after oxidative pretreatment at 823 K.15,20,30,31 It is evident that the band at 1870 cm-1 is a broad and asymmetric absorption. The Gaussian fit result reveals that this band can be divided into two deconvoluted sub-bands located at 1880 and 1860 cm-1, as shown in the inset of Figure 2. The bands in this region are quite sensitive to the location and nature of iron species. Bell et al.20 assigned the doublet at 1876 and 1856 cm-1 to mononitrosyl species on iron sites located at different positions, that is, the five- and six-membered rings of the zeolite crystals, respectively. Joyner and Stockenhuber15 ascribed the bands at 1880 and 1841 cm-1 to mononitrosyl coordinated to oligomeric Fe2+ ions and isolated Fe2+ ions, respectively, and Mul et al.30 also assigned the NO absorption band at 1874 cm-1 to oligonuclear FexOy in the zeolite channels in light of a correlation between the NO absorption intensity and the activity of N2O decomposition, whereas Sun et al.32 suggested that the band at 1874 cm-1 is associated with extraframework Fe-O-Al species. Considering the above-mentioned

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Figure 3. IR spectra of C3H6 adsorbed on Fe-NO sites at room temperature (after introducing 5.06 Torr of NO into the reactor loading, 20 mg of catalyst self-supporting wafer for ca. 10 min, and then dosing C3H6).

characterization results that the binuclear iron species bound on acidic sites are predominant in the Fe/HY-G4.9, we believe that the bands at 1880 and 1860 cm-1 should belong to mononitrosyl species-adsorbed Fe2+ cations of binuclear clusters located at different positions and that the shoulder at 1841 cm-1 should belong to mononitrosyl species adsorbed on isolated iron sites. The IR results indicate that the evacuation treatment leads to the reduction of a part of Fe3+ to Fe2+. This is confirmed by the XPS results shown in Figure S2 (see the Supporting Information). Upon increasing the equilibrium pressure of NO to 5.06 Torr, a visible change is observed in the IR spectrum (Figure 2c): A new band occurs at 2241 cm-1, which is attributed to N2O adsorbed on Fe2+ centers.23 Moreover, it is clearly observed that two bands at 1609 and 1583 cm-1, corresponding, respectively, to Fen+(NO2) and Fen+(NO3) (n ) 2, 3),20 increase remarkably in intensity. A shoulder, which might belong to some forms of adsorbed NO220 or H2O formed probably by the reaction of HNO3 with Fe-OH, occurs at 1635 cm-1. The co-occurrence of the three bands at 2241, 1609, and 1583 cm-1 indicates that the binuclear iron clusters can directly activate NO in the absence of O2 and therefore lead to the disproportionation of a fraction of NO into N2O and NO2, subsequently adsorbed on iron sites. Partial NO2 reacts with hydroxyls to form HNO3, which might react with Fe-OH to produce Fen+(NO3) and H2O. Impurities of NO2 in the NO, from trace amounts of oxygen,33 could be responsible for some of the surface species formation. However, NO2 impurities could not explain the N2O observed, and as will be discussed below, NO2 impurities would not be able to explain the steady-state reactivity data. The intensity of the main absorption band at 1866 cm-1 is nearly unchanged upon increasing the equilibrium pressure of NO, showing that NO adsorption on iron sites reaches saturation. Unexpectedly, we do not observe the formation of NO+ species, which commonly show a broad characteristic band at ca. 2133 cm-1 in the IR spectra. The lack of the NO+ species might originate from the reaction with water (NO+ + H2O ) HNO2 + H+).34 3.3. IR Observation of Coadsorption and Activation of NO + C3H6 on Iron Sites. After adsorption of NO (5.06 Torr) on the catalyst, the obtained IR spectrum (Figure 3) is basically identical with Figure 2c. A small amount of NO2 is formed as a result of the disproportionation of NO, as indicated by the two bands at 1616 and 1580 cm-1. We introduce a certain amount of propylene at room temperature to observe their coadsorption states. It appears that the addition of propylene affects the chemical states of NO adsorbed on iron sites and consequently results in a distinct change in the IR spectrum of the Fe/HY-G4.9. As shown in Figure 3, in the C-H stretching

Long et al. vibration region, eight new IR bands at 3073, 3058, 3001, 2979, 2950, 2926, 2894, and 2858 cm-1 due to the stretching vibrations of alkyl and olefinic C-H bonds are observed. They are different remarkably from the stretching vibrations of gaseous propylene (see Figure S3 in the Supporting Information). For the dC-H stretching vibrations of adsorbed propylene, not only does the position of the main vibration band shift from 3105 to 3073 cm-1 but also the number of IR band decreases from four (3105, 3091, 3081, and 3033 cm-1) to two (3073 and 3050 cm-1). The large shift toward low wavenumbers (∆ν ) 32 cm-1) and the decrease in the number of absorption bands are an obvious indication of the strong interaction between the CdC bond of propylene and the adsorbed NO molecules. Moreover, it is interesting to note that three C-H bonds of the methyl group of adsorbed propylene have almost the same stretching vibrations as those of gaseous propylene, except that the most intense band shifts from 2952 to 2979 cm-1. The lack of change in the position of six bands at 3001, 2979, 2950, 2926, 2894, and 2858 cm-1 suggests that the methyl group of adsorbed propylene does not interact with iron ions or hydroxyl groups and has a high degree of freedom. In the range of 1650-1350 cm-1 due to the C-H bending vibrations and CdC and C-C stretching vibrations of propylene, six bands at 1635, 1455, 1441, 1433, 1417, and 1380 cm-1 are observed. The band at 1635 cm-1, belonging to the stretching vibration of CdC bond, has a strongest absorption. By comparison to the IR spectrum of gaseous propylene (see Figure S3 in the Supporting Information), the band shifts by ca. 30 cm-1 toward low wavenumbers due to the interaction between propylene and NO adsorbed on iron sites (the interaction will be further discussed in section 4.2). This is in parallel with the shift of the dC-H stretching bands. The 1380 cm-1 band is a characteristic IR absorption of methyl group. The absence of the characteristic band of C-(CH3)2 species (appearing commonly at ca. 1370 cm-1) indicates that no propylene is adsorbed on acidic hydroxyl groups to form carbonium ions.35 In the NO absorption range of 2000-1750 cm-1, the addition of propylene results in a more drastic change in the IR spectra. It appears that the band belonging to mononitrosyl adsorbed on binuclear iron sites shifts gradually from 1870 to 1860 cm-1 with increasing dosage of propylene along with the gradual decrease in intensity of the doublet at 1918 and 1815 cm-1 (Figure 3). Detailed fit analysis with a mixed function composed of 80% Gaussian and 20% Lorentzian further reveals an interesting phenomenon; that is, the intensity of the sub-band at 1880 cm-1 decreases gradually with increasing dosage of propylene, while the sub-band at 1860 cm-1 increases correspondingly (Figure 4A-F). The good linear relation between the integrated intensity of the sub-bands at 1880 and 1860 cm-1 (Figure 4G) demonstrates that such an increase in intensity of the sub-band at 1860 cm-1 results from the shift of the subband at 1880 cm-1. It can be thus deduced that the decrease of vibration frequency of the band at 1880 cm-1 maybe suffers from the perturbation of adsorbed propylene molecules, which will be further discussed below. To further confirm the adsorption site, we plotted the integrated intensity of the sub-band at 1880 cm-1 versus the integrated intensity of the bands in the region of 3120-3040 cm-1, as shown in Figure 4H. The near linear relation indicates that propylene is chemically adsorbed on binuclear iron sites. These results suggest that the iron sites corresponding to the 1880 cm-1 band are accessible preferably to propylene molecules, while the iron sites corresponding to the 1860 cm-1 band are inaccessible to propylene molecules.



Figure 4. (A-F) Fitting IR spectra in the 1950-1750 cm-1 region of Figure 3 with a mixed function of Gaussian and Lorentzian (G) relationship between the bands at 1880 and 1860 cm-1. (H and I) Relationship between the bands at 1880, 1918, and 1841 cm-1 and adsorbed propylene (the integrated area of the 3120-3040 cm-1 region serves as a measure of the adsorbed propylene).

For the doublet at 1918 and 1815 cm-1 representing dinitrosyl species, their intensity tends to decrease with propylene dosage, whereas the intensity of the band at 1841 cm-1 representing mononitrosyl species adsorbed on isolated iron sites displays a trend of increase with propylene dosage. This suggests that adsorption of propylene results in a redistribution of the NO species by displacement of NO from its initial adsorption sites. Because the integrated intensity of the three bands is significantly influenced by the choice of baseline and half bandwidth (denoted as HBW) as seen in Table S1 in the Supporting Information, we analyzed the change in peak height of the bands at 1918 and 1841 cm-1 with propylene dosage. As shown in

Figure 4I, the 1918 cm-1 band decreases gradually in peak height, while the peak height of the 1841 cm-1 sub-band increases with increasing propylene pressure. This implies that the increase of the band at 1841 cm-1 results from the displacement of dinitrosyl species by propylene. Thus, it can be concluded that a small amount of propylene is also adsorbed on isolated iron sites. Moreover, it is interesting to note that three new weak bands occur at 1725, 1691, and 1563 cm-1. Simultaneously, a new weak band, corresponding to the N-H vibration of amide species,36 appears at 3290 cm-1. The intensity of the four bands increases with increasing dosage of propylene. They result from

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Figure 5. IR spectra of adsorbed species on the catalyst surface during C3H6-NO catalytic reduction in the absence of O2: (a) 2.53 Torr C3H6, (b) 2.53 Torr C3H6 + 2.53 Torr NO, (c) after heating at 423 K for 30 min, (d) after heating at 473 K for 30 min, (e) after heating at 523 K for 30 min, (f) after heating at 573 K for 30 min, and (g) after heating at 573 K for 10 h and evacuation at 573 K for 60 min.

neither adsorbed propylene nor adsorbed NO and NO2 but belong to products or intermediates from the NO + C3H6 reaction. This indicates that a minor fraction of adsorbed propylene reacts with coadsorbed NO at room temperature. NO is reduced to amide species containing NH2 or NH groups as indicated by the bands at 3290, 1691, and 1563 cm-1, while propylene may be oxidized to ketone or aldehyde species as indicated by the band at 1725 cm-1. It is evident that the SCR reaction of NO with C3H6 starts at room temperature in the absence of O2. The key factor initiating the reaction under O2free conditions will be analyzed and discussed in detail below. 3.4. IR Observation of the NO Reduction by C3H6 over Iron Sites. Upon increasing the temperature to 423 K under the oxygen-free conditions, the tremendous modification of IR spectra is observed for the catalyst (Figure 5c): First of all, the intensity of the υ(C-H) bands in the 3100-2800 cm-1 range decreases, while the band at 3582 cm-1 attributed to acidic hydroxyl groups in the cavity of the Y zeolite remarkably increases in intensity. Three new weak bands corresponding to the υ(N-H) stretching vibrations of ammonium37 occur synchronously at 3366, 3274, and 3185 cm-1 and increase distinctly in intensity with increasing reaction temperature. It shows that NO is reduced to NH3 at 423 K. Second, some important intermediates from NO are detected during the reaction, as characterized by the IR spectra in the 2350-2100 cm-1 range. Four new bands appear at 2313, 2282, 2258, and 2235 cm-1. According to the literature,38-40 the first two bands at 2313 and 2282 cm-1 belong to acetonitrile species, and the two bands at 2258 and 2235 cm-1 are assigned unambiguously to HNCO species and the CtN stretching vibration of adsorbed nitrile (HCtN), respectively. With increasing reaction temperature, these bands increase visibly in intensity. Especially the 2235 cm-1 band, it is dominant in the IR spectrum at 573 K (Figure 5B-f). After evacuation at 573 K for 60 min (Figure 5B-g), the

Long et al. three bands at 2313, 2282, and 2258 cm-1 disappear completely, whereas the band at 2235 cm-1 becomes stronger and broader because of the strong bonding between the formed HCN species and the iron sites. In addition, in the 1750-1300 cm-1 range, some oxidation products or intermediates from propylene are discerned. Five new bands occur at 1766, 1712, 1675, 1590, and 1485 cm-1 and increase in intensity with reaction temperature. The pair of bands at 1590 and 1485 cm-1 is attributed to -COO- species.41 The band at 1712 cm-1 belongs to some organic products containing CdO groups, while the band at 1675 cm-1 and the weak shoulder at ca. 1560 cm-1 should be ascribed to amide or organic nitro-compounds.41,42 As for the 1766 cm-1 band, some studies tentatively ascribed it to the asymmetric N-O stretching band of trans-(NO)2 dimers or the ν1 + ν2 combination band of NO3-.43,44 However, the 1766 cm-1 band disappears in company with the 1870 cm-1 band representing NO adsorbed on iron sites after evacuation (Figure 5C-g), and no NO2 is observed in the gas phase. These results suggest that the 1766 cm-1 band should be attributed to lowspin Fe(I)-NO+ complex.15,45 Assignments of all infrared bands observed as a result of coadsorption and reaction of NO with propylene are summarized in detail in Table 2. The SCR chemistry of NO over the binuclear iron sites in the presence of low amount of O2 (1.27 Torr) was examined too, as indicated by the IR spectra shown in Figure 6. The addition of O2 affects the adsorption states of NO and C3H6 over iron sites at room temperature. The pair of bands at 1918 and 1816 cm-1 representing Fe(NO)2 species decreases remarkably in intensity upon introducing O2, indicating that a fraction of NO adsorbed on iron sites reacts with O2 molecules to form NO2, leading to the decrease of Fe(NO)2 and Fe(NO) species. Simultaneously, the main absorbance band at 1857 cm-1 increases visibly in intensity. A more distinct change can be observed from the difference spectrum after addition of O2, as shown in Figure 7. The intensity of the 1882 cm-1 band corresponding to mononitrosyl adsorbed on oligomeric iron centers and the 1813 cm-1 band corresponding to nitrosyl adsorbed on isolated iron centers decreases along with the increase in intensity of the 1845 cm-1 band and the 1635 band belonging to adsorbed NO2 or formed H2O. Although the increase of NO2 is remarkable upon addition of O2, the propylene oxidation seems to be not accelerated. It suggests that NO2 is not responsible for the propylene oxidation at room temperature. This will be further discussed below. The IR spectra of the catalyst surface during the reaction in the presence of O2 are shown in Figure 8. Beyond 423 K, the collected IR spectra are almost the same as those in the absence of O2, suggesting that the presence of O2 does not alter the selective reduction pathway of the NO + C3H6 system at high reaction temperatures greater than 423 K. 3.5. IR Observation of Adsorption and Reaction of NO and C2H4 over Iron Sites. To understand the role of reductant containing π-electron in the formation of NH3 and amide species, we chose ethylene as the reductant to further examine the adsorption and catalytic reduction of NO over iron sites under the oxygen-free conditions with IR spectroscopy. As shown in Figure 9, the intensity of a few bands at the 3100-2800 cm-1 range attributed to ethylene adsorbed species is very weak, showing that the amount of ethylene adsorbed on iron sites is extremely less. This is presumably due to the weaker coordination ability resulting from the lack of a methyl electron donor. A drastic difference from propylene is that there is no occurrence of the redox reaction of NO with ethylene at room temperature.



TABLE 2: Assignments of Infrared Bands Observed as a Result of Coadsorption and Reaction of NO with Propylene wavenumber (cm-1) 3500-3150 3010-3050 3000-2800 2320-2310 2290-2280 2260-2250 2240-2220 1910-1920 1860-1880 1840-1860

assigned species +

NH4 or R-NH2 υ(dC-H) υr(-C-H) R-CtON R-CtON HNCO HNCO Fe2+(NO)2 NO on iron clusters NO on isolated iron site

references

wavenumber (cm-1)

37 this work this work 38-40 38-40 38-40 38-40 15, 24, 29, 30 15, 20, 30, 31 15, 20, 30, 31

Upon increasing the reaction temperature, the IR spectra of the catalyst are changed largely. As shown in Figure 10, with increasing temperature from 373 to 573 K, in the 3400-3100 cm-1 range, the υ(N-H) stretching bands of NH4+ species formed increase gradually in intensity. Simultaneously, in the 2340-2100 cm-1 range, four bands belonging to HNCO and -CN species occur at 2310, 2282, 2256, and 2225 cm-1. In addition, the oxidized products of ethylene including carboxylic acid and amide species, which are indicated by the bands at 1674, 1591, 1562, and 1480 cm-1, are discerned obviously in the IR spectra. These observations are perfectly consistent with the results of the propylene reduction, suggesting that ethylene has the same oxidation pathway as propylene at high temperature. According to the intensity of the characteristic IR bands of these intermediates or products, one can conclude that it is better to apply propylene than ethylene to reduce NO, as the result of the stronger coordination of the former than the latter to divalent iron ions. In the presence of a low amount of O2 (1.27 Torr), NO and C2H4 undergo the same redox process judging from the IR

Figure 6. Effect of the low amount of O2 on the adsorption states of NO and C3H6 on iron sites: (a) 1.27 Torr NO, (b) 1.27 Torr C3H6 + 1.27 Torr NO, and (c) 1.27 Torr C3H6 + 1.27 Torr NO + 0.63 Torr O 2.

1810-1820 1770-1750 1730-1720 1700-1690 1680-1670 1640-1630 1620-1610 1600-1580 1570-1560 1490-1440

assigned species

references

Fe (NO)2 low-spin Fe(I)-NO+ υr(R-CdO) υr(R-CdO) amide I or R-ONO species υr(-CdC-), H2O, or NO2 adsorbed NO2 adsorbed iron sites R-COO-, Fen+(NO3) amide II or organic nitro species R-COO- and NH4+

15, 24, 29, 30 15, 45 this work this work 41, 42 this work 20 20, 41 41, 42 37, 41

2+

spectra of the catalyst surface unshown here. However, in the gas-phase IR spectra shown in Figure 11, NO2, which is the product from the reaction of NO with O2, is formed as heating at 423 K. This is different from the propylene reduction system where no NO2 is formed in the gas phase (data are not shown herein). Beyond 423 K, NO2 quickly reacts with ethylene, as indicated by the sharp decrease in intensity of the band at ca. 1610 cm-1 (Figure 11d,e). It demonstrates that NO2 is the crucial intermediate initiating the reduction process at a high reaction temperature. 3.6. IR Observation of the NO Reduction by CH4 over Iron Sites. Figure 12A displays the collected IR spectra of the catalyst surface during the reduction of NO by CH4 in the absence of O2. Upon addition of NO (5.06 Torr), a strong absorbance band ascribed to mononitrisyl adsorbed on oligomeric iron clusters appears at 1871 cm-1 and two bands belonging to NO2 and NO3- adsorbed on iron sites occur at 1615 and 1579 cm-1, respectively. This is in line with the foregoing results in section 3.2. Introduction of methane does not affect the chemical adsorption states of NO and formed NO2 on iron sites, as indicated by the almost unchanged IR spectrum (Figure 12A-b). No vibration bands are observed in the range of 3200-2800 cm-1, indicating that the adsorption amount of

Figure 7. Difference spectra of Figure 6.

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Long et al.

Figure 10. IR spectra of adsorbed species on the catalyst surface during the C2H4 + NO reaction in the absence of O2: (a) after heating at 373 K for 30 min, (b) after heating at 423 K for 30 min, (c) after heating at 473 K for 30 min, (d) after heating at 523 K for 30 min, (e) after heating at 573 K for 30 min, and (f) after heating at 623 K for 30 min.

Figure 8. IR spectra of adsorbed species on the catalyst surface during the C3H6 + NO reaction in the presence of O2: (a) after heating at 373 K for 30 min, (b) after heating at 423 K for 30 min, (c) after heating at 473 K for 30 min, (d) after heating at 523 K for 30 min, and (e) after heating at 573 K for 10 h and evacuation at 573 K for 60 min.

Figure 11. IR spectra of gaseous products from the reaction of NO with ethylene in the presence of O2: (a) after addition of 2.53 Torr NO and 2.53 Torr C2H4, (b) after addition of 2.53 Torr O2, (c) after heating at 423 K for 30 min, (d) after heating at 473 K for 30 min, and (e) after heating at 573 K for 30 min.

Figure 9. IR difference spectra of C2H4 adsorbed on Fe-NO sites at room temperature (part pressure of NO, 2.53 Torr; part pressure of C2H4, 5.06 Torr).

methane in cavities of the Fe/Y zeolite should be extremely less, even negligible. After heating above 473 K (Figure 12Ac-e), the occurrence of the two bands attributed to organic species containing CdO groups at 1734 and 1689 cm-1 and the complete disappearance of the bands at 1615 and 1579 cm-1

demonstrate that methane is oxidized to formaldehyde or formic acid, and the adsorbed NO2 is synchronously reduced to N2. Figure 12B shows the gas-phase IR spectra during the NO + CH4 reaction in the absence of O2. It is clearly observed that two negative NO2 bands occurs at ca. 1630 and 1600 cm-1 upon dosage of CH4, illuminating that the amount of gaseous NO2 decreases, probably forming a NO2-CH4 adduct complex. However, the NO2-CH4 adduct is not directly observed in both the gas-phase spectrum and the IR spectrum of the catalyst (Figure 12A-b), perhaps because it is adsorbed on the wall of the reactor, outside of the IR beam path. As the temperature is increased to 473 K, the NO2 bands at ca. 1630 and 1600 cm-1 further decrease in intensity (Figure 12B-b,c), followed by the


Figure 12. (A) IR spectra of adsorbed species on the catalyst surface during the CH4 + NO reaction in the absence of O2: (a) after addition of 5.06 Torr NO, (b) after addition of 5.06 Torr CH4, (c) after heating at 473 K for 30 min, (d) after heating at 573 K for 30 min, and (e) after heating at 673 K for 120 min. (B) Gas-phase IR difference spectra during the CH4 + NO reaction in the absence of O2: (a) after addition of 5.06 Torr CH4, (b) after heating at 473 K for 30 min, and (c) after heating at 573 K for 30 min.

appearance of two characteristic IR bands of CO at the 2300-2100 cm-1 region. It indicates that NO2 formed by the disproportionation of NO on binuclear iron sites is further reduced to N2 (GC evidence for N2 formation will be shown later) and CH4 is oxidized to CO and CO2. CO should be formed from decomposition of formic acid. The intensities of both the NO band centered at ca.1817 cm-1 and the N2O band at 2237 cm-1 are unchanged during the reaction, indicating that CH4 can not reduce NO and N2O below 573 K. Obviously, the formation of NO2 and the NO2-CH4 adduct is the key step in the CH4-SCR reaction. To further prove the foregoing conclusion, we also monitor the SCR process with IR spectroscopy in the presence of O2. Figure 13A shows the IR spectra of the catalyst during the adsorption and reaction of NO and CH4 on iron sites. The addition of O2 (2.53 Torr) does not affect the IR spectrum (Figure 13A-c), but the subsequent heating at 423 K causes a remarkable change (Figure 13A-d). The two bands attributed to NO2 and NO3- species adsorbed on iron sites occur, respectively, at 1615 and 1578 cm-1 along with the sharp decrease in intensity of the band at 1869 cm-1, indicating that NO adsorbed on iron sites is oxidized to NO2 via the stoichiometric reaction of 2NO + O2 ) 2NO2. In addition, a new band appears at 1786 cm-1 besides the characteristic absorption bands of the products and intermediates containing CdO groups at 1738 and 1684 cm-1. This is different from the foregoing results in the absence of O2. According to the literature,15 the band at 1786 cm-1 should be ascribed to low-spin Fe(I)-NO+ complex. With increasing reaction temperature, the two bands at 1615 and 1578 cm-1 decrease gradually in intensity and even


Figure 13. (A) IR spectra of adsorbed species on the catalyst surface during the CH4 + NO reaction in the presence of O2: (a) after addition of 2.53 Torr NO, (b) after addition of 2.53 Torr CH4, (c) after addition of 2.53 Torr O2, (d) after heating at 423 K for 30 min, (e) after heating at 473 K for 30 min, (f) after heating at 523 K for 30 min, and (g) after heating at 573 K for 300 min. (B) Gas-phase IR difference spectra during the CH4 + NO reaction in the presence of O2: (a) after addition of 2.53 Torr CH4, (b) after addition of 2.53 Torr O2, (c) after heating at 423 K for 30 min, (d) after heating at 473 K for 30 min, (e) after heating at 523 K for 30 min, and (f) after heating at 573 K for 30 min.

disappear (Figure 13A-e-g), showing that NO2 and NO3adsorbed on iron sites are consumed via the overall reaction of 2NO2 + CH4 ) N2 + CO2 + 2H2O. The gas-phase IR spectra reflect a more distinct change in the formation and consumption of NO2, as shown in Figure 13B. Upon addition of O2, only a minor fraction of NO is oxidized to NO2 (Figure 13B-b). Heating at high temperature accelerates the reaction of 2NO + O2 ) 2NO2, as indicated by the strong absorbance band at ca. 1620 cm-1. Upon increasing the reaction temperature to 573 K, the intensity of the band at ca. 1620 cm-1 decreases sharply along with the gradual increase in intensity of the characteristic absorption bands of CO, indicating that a great amount of NO2 is reduced to N2 (GC evidence for N2 formation will be shown later), and accordingly, CH4 is oxidized to CO or CO2 and H2O. It can, therefore, be concluded that NO must be first transformed into NO2 and then can be reduced by CH4. The reduction of NO by methane is different obviously from those by propylene and ethylene, which will be discussed in detail below. 3.7. Steady-State Catalytic Activity. It can be expected that different reductants will result in different overall activities for the iron-catalyzed HC-SCR reaction. The emphasis was placed on the roles of O2 and the type of reductant in the reaction in the study, and thus, we examine first the steady-state conversion of NO by propylene, ethylene, and methane over the Fe/HYG4.9 catalyst in the absence of O2, as shown in Figure 14.

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Figure 14. Catalytic activity of the Fe/HY-G4.9 sample for the HCSCR reaction with (a) propylene, (b) ethylene, and (c) methane as a reductant in the absence of O2.

Propylene represents the highest NO conversion and the most amount of detected products, N2, CO2, and CO at 600 °C, showing that propylene has a better performance for NO reduction than ethylene and methane. Remarkably, the SCR of NO with hydrocarbons can proceed in the absence of O2. It is in basic consistency with the above-reported IR results. However, the on-set temperature of the NO reduction by propylene and ethylene is at 350 °C, while methane shows a higher onset temperature of NO conversion (500 °C). This is much higher than the initial reaction temperature achieved by IR study (298 K for propylene, 423 K for ethylene, and 473 K for methane). It originates mainly from the difference in reaction system and detection method. IR spectroscopy is very sensitive to the change in molecular configuration of reactants or products adsorbed on the catalyst surface, whereas GC analysis mainly detects gaseous molecules, irrespective of adsorbed reactants and products. Moreover, for the IR system, the products/intermediates formed on the catalyst surface are accumulative and thus more easily discerned with IR spectroscopy, while in the steadystate reaction system they must be diffused into the gas phase and then detected by GC. Obviously, the difference between detection object of IR and GC determines the different apparent on-set reaction temperatures. Figure 15 displays the steady-state conversion of NO by propylene, ethylene, and methane over the Fe/HY-G4.9 catalyst in the presence of O2. The presence of O2 has a distinct effect on the HC-SCR reaction. The on-set reaction temperature is decreased to 200 °C for propylene, 300 °C for ethylene, and 400 °C for methane, based on N2 production. Moreover, the concentration of detected products, N2, CO2, and CO, is significantly higher than those in the absence of O2. This indicates that the presence of O2 promotes the reaction. Especially for the C3H6-SCR reaction, the presence of O2 leads to a large enhancement of N2 selectivity (see Figure S6 in the Supporting Information). The highest N2 selectivity (ca. 65%) in the C3H6-SCR system occurs at 400 °C, which is lower than

Long et al.

Figure 15. Catalytic activity of the Fe/HY-G4.9 sample for the HCSCR reaction with (a) propylene, (b) ethylene, and (c) methane as a reductant in the presence of O2.

those in the C2H4-SCR (ca. 520 °C) and CH4-SCR (ca. 580 °C) systems. Propylene still shows a better performance for the NO conversion as compared to ethylene and methane. These results are also in agreement with the IR results reported above. It is interesting to note that in the presence of O2 the conversion of NO by propylene decreases gradually with increasing reaction temperature and then increases quickly, while ethylene and methane show a trend of decrease in NO conversion with reaction temperature. In the presence of O2, the lowest conversion for propylene is achieved at ca. 400 °C. The main origin remains unclear and needs to be further studied. In addition, ethylene as a reductant is in favor of the formation of more amount of CO as compared to propylene. It can be seen from Figure 15D that in the presence of O2 the yield of CO in the C2H4-SCR reaction is much higher than that in the C3H6-SCR reaction. This is different from the result obtained in the absence of O2. Taking into account the environmental repercussion together with the catalytic efficiency, we propose that propylene among the selected three reductants is more suitable for the SCR reaction of NO than ethylene and methane. 4. Discussion 4.1. Adsorption and Reactivity of NO on Binuclear µ-Hydroxo-Bridged Iron Clusters. Much work regarding NO adsorption on iron sites was only focused on the identification of highly dispersed iron species by nitric oxide probe based on its strong affinity toward Fe2+ ions.46 In this work, the study of NO adsorption is not only to characterize nuclearity of iron sites but also to wonder if NO can be activated by binuclear iron sites. The aforementioned results show that NO adsorption on the Fe/HY-G4.9 catalyst dehydrated in vacuum at 673 K commonly represents five IR absorptions located at 1918, 1880, 1860, 1841, and 1815 cm-1, respectively. We tentatively ascribed the two sub-bands at 1880 and 1860 cm-1 to mononitrosyl species on binuclear iron sites. However, it is interesting to note that adsorption of propylene only causes the vibration frequency shift of the sub-band at 1880 cm-1, while the sub-

Infrared Study of the NO Reduction by Hydrocarbons SCHEME 1: Adsorption Models of NO and Propylene on the Binuclear Iron Sites with Two Nonequivalent Iron Species

band at 1860 cm-1 seems to be unaffected. The unexpected phenomenon originates presumably from the unique location of both iron cations in the binuclear iron clusters. One iron cation representing the 1880 cm-1 IR band has been definitely shown to be anchored by the grafting reaction of ferrocene with acidic protons on the O1 site that lies in a plane of a 12-membered ring connecting the supercages,24 and another iron representing the 1860 cm-1 IR band may be bound on the O2 site that lies in the six-membered ring of sodalite units according to the Fe-Fe distance of 3.11 Å. We believe that the appearance of two absorption bands in the IR spectra for mononitrosyl adsorbed on binuclear iron clusters should originate from this difference between geometrical environments of both iron cations. The adsorption model of NO on binuclear iron sites with two nonequivalent iron species is shown in Scheme 1A. Upon increasing the dosage of NO, the disproportionation of NO into N2O and NO2 is clearly discerned. Earlier studies considered that the disproportionation of nitric oxide on zeolites is a metal-catalyzed reaction.47 Rivallan et al.48 also observed a similar phenomenon that NO titration of oxidized Fe-ZSM-5 results in the formation of a complex network of interplaying neutral (NO, NO2, and N2O4) and ionic species (NO+, NO2-, and NO3-). We can, therefore, make sure that the appearance of NO disproportionation originates from iron sites. To elucidate the fact that propylene can reduce NO at room temperature, a key question that we must consider is derived from the products or intermediates formed: What can oxidize propylene molecules at room temperature without the addition of gaseous oxygen? The precise answer to this question depends on understanding the activation and decomposition of NO over the binuclear iron sites. There exist six possible oxygen sources: NO, N2O, NO2, lattice oxygen of active sites, activated NO molecule (NO adsorbed on active sites), and atomic oxygen. Among these oxygen sources, stable N2O molecule, NO gaseous molecules, and the lattice oxygen of active iron sites lack the ability to oxidize adsorbed propylene at room temperature. As

J. Phys. Chem. C, Vol. 114, No. 37, 2010 15723 for atomic oxygen, we chose methane to titrate it by the reaction of O + CH4 ) CH3OH at room temperature.49 No oxidized products, such as methanol, formaldehyde, or formic acid, of methane are formed as shown by the results in section 3.6, suggesting that no atomic oxygen is formed as an addition of NO. It can, therefore, be concluded that the oxidation of propylene at room temperature under the oxygen-free conditions is not due to atomic oxygen but ascribed to either NO2 produced mainly by a disproportionation reaction of NO or activated NO molecules. NO2 has been well-established to be a key intermediate in the metal-catalyzed HC-SCR reaction.50-52 In this work, combined with the literature,47,48 we established that it is derived mainly from a disproportionation reaction of NO on the zeolite in light of the quick increase of NO2 with increasing dosage of NO as shown in Figure 2. The impurities of NO2 in the NO source (purity of NO is equal to 99.5 wt %) are minor. The contribution of the impurities of NO2 to the catalytic reaction should be negligible. The amount of NO2 formed by the disproportionation reaction is considerable, and it is detected with IR spectroscopy in the gas phase at room temperature as shown in Figure 12B. Partial NO2 produced is directly adsorbed on iron sites to form Fe(NO2)n+ complex indicated by the IR band in the 1630-1600 cm-1 region, and partial NO2 reacts with H2O or hydroxyls to form NO3- and H+,53-55 which adsorbed on iron sites to form Fe(NO3)n+ indicated by the IR band in the 1600-1570 cm-1 region. NO2 increases remarkably upon addition of O2, but the propylene oxidation seems not to be accelerated at room temperature as shown in Figures 6 and 7. It suggests that NO2 is not responsible for the propylene oxidation at room temperature. As far as adsorbed NO molecules are concerned, their NtO bonding is weakened due to the coordination to Fe2+, as reflected by the red shift of the IR characteristic band from 1890 to ca.1880 cm-1 for NO adsorbed on binuclear iron sites and to ca. 1841 cm-1 for NO adsorbed on isolated Fe2+ sites. As a consequence, these activated NO molecules become unstable and are easily disproportionated to NO2 and N2O in the presence of excess NO. This is quite identical with the previously reported results that the NO disproportionation reaction commonly takes place on coordinatively unsaturated Fe complexes and other transition metal complexes.56,57 When propylene and NO are coadsorbed on iron sites, propylene molecules are unavoidably oxidized by these activated NO molecules that are correspondingly reduced as described in section 3.3. Thus, we proposed that adsorbed NO on iron sites contributes mainly to the propylene oxidation observed at room temperature. At a high reaction temperature greater than 150 °C, adsorbed NO2 contributes mainly to the HC-SCR reaction. Under the oxygenfree conditions, the disproportionation is a main transformation channel of NO to NO2 (4NO ) 2NO2 + N2).15 O2 is not very important for SCR with C3H6 and C2H4, because both can interact directly with NO, but it greatly aids in the SCR with CH4 because it enhances the oxidation of NO to NO2, which is necessary for the oxidation of CH4. 4.2. Adsorption States of Hydrocarbons and Formation of Nitro-Complex. Among the selected reducing agents, both propylene and ethylene have a CdC double bond and thus can be adsorbed on iron centers by a coordinative interaction with Fe2+ sites or on NO adsorbed on Fe sites to form a Fe-NO-C3H6 complex. Because of the donor-electron effect of methyl group, the π electron density of propylene is higher than that of ethylene, together with the lower volatility of propylene as compared to ethylene and methane. As a result,

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the adsorption amount of propylene on iron active sites is far more than ethylene and methane. It is estimated from the absorbance intensity of the C-H bands that the adsorption amount of reductants decreases in the order of C3H6 . C2H4 > CH4. Especially CH4, its adsorption amount on the catalyst surface is negligible. Perhaps this is one of reasons why the iron-contained zeolite catalyst has a lack of activity upon using CH4 and C2H4 as reducing agents. There are two ways to adsorb propylene on binuclear Fe2+ sites as following: One is that propylene replaces NO to coordinate with Fe2+ sites. If so, such an adsorption will result in the decrease in intensity of the IR bands. Another is that propylene is adsorbed on NO species coordinated to Fe2+ sites to form a Fe-NO-C3H6 complex. Such an adsorption will result in the shift of the IR bands. Apparently, according to the results shown in Figure 4, the band representing NO species adsorbed on oligomeric iron sites shifts from 1880 to 1860 cm-1 (∆ ) ca. 20 cm-1) due to the perturbation from adsorbed propylene molecules, and it can be concluded that the main adsorption model of propylene proceeding on the surface of Fe/HY-G4.9 catalyst is in line with the second case as illuminated in Scheme 1C. The first adsorption model should be excluded for the adsorption of propylene on binuclear iron sites because it is not consistent with the IR results in section 3.3; that is, introduction of propylene leads to the increase in intensity of the band at 1860 cm-1 (Figure 4). As for the adsorption of propylene on isolated iron sites, we can deduce that it is in line with the first adsorption model illuminated in Scheme 1E, as the result of the decrease in intensity of the bands at ca.1918 and 1815 cm-1 upon addition of propylene. Moreover, we conducted a validating experiment using reverse order of adsorption: first propylene and then NO. Because of the stronger affinity of NO to Fe2+ ions as compared to olefin molecules, the obtained IR results are consistent with the above-reported spectra. Therefore, it can be concluded that a nitro-complex is formed by an adduct way on iron sites at room temperature, which is a crucial step to the HC-SCR reaction. 4.3. Possible Reaction Pathways of the Iron-Catalyzed HC-SCR Reaction. The type of reductant is a pivotal factor affecting the catalytic behavior. Chen et al.19 studied NOx reduction over FeZSM-5 catalyst with different hydrocarbons, such as CH4, C3H6, C3H8, and iso-C4H10. It was found that N2 yield is dependent on the type of hydrocarbon reductants. Iwamoto and Hamada58 classified the hydrocarbon reductants over Cu-ZSM-5 as selective (i.e., C2H4, C3H6, C3H8, and C4H8) and nonselective (i.e., CH4 and C2H6). Such a classification seems also to be suitable for the iron-contained Y zeolite catalysts. Different hydrocarbon reducing agents lead to different reduction products of NO. With propylene and ethylene, NO is mainly reduced to NH3 and amide species in the absence of O2 below 573 K, but the amount of NO reduced is less. Methane can but reduce NO2, whereas olefins with a CdC bond can reduce NO in the absence of O2. Especially propylene, it can react with NO even at room temperature. According to the intermediates or products identified by IR spectroscopy, it can be concluded that the NO reduction undergoes multiple reaction pathways dependent on hydrocarbon reductants. The proposed SCR pathways of NO with C3H6, C2H4, and CH4 in the absence and presence of O2 are illuminated in Scheme 2. It provides a relatively comprehensive description of the SCR chemistry of NO with hydrocarbons and can account for all important reactive intermediates and products. The nontrivial part of this reaction mechanism is the formation of NH3 from NO, which is a great emphasis placed on the discussion of the reaction pathway.

Long et al. According to these IR observations, at room temperature, NO is reduced by propylene to organic amine (as indicated by the bands at 3290, 1691, and 1563 cm-1), probably via a HCN intermediate instead of NH3 because the amine formed cannot be further hydrolyzed at such a low temperature. Propylene may be oxidized accordingly to ketone or aldehyde species as indicated by the 1725 cm-1 band. Therefore, we proposed that the propylene-SCR reaction may proceed by a NO adduct mechanism. The NO disproportionation and the C3H6-SCR reaction can proceed simultaneously on the catalyst. NO participates in the C3H6-SCR reaction, while NO2 produced by the disproportionation is not responsible for the reaction, which has been already discussed in detail above. Coadsorption of propylene and NO on iron sites is a key step, because it facilitates the formation of the C3H6-NO adduct complex that is a crucial transition state. It is expected that other olefins, such as C4H8, as reductants should have the same reaction pathway as propylene. C2H4 is an exceptional example, because of the extremely low adsorption on Fe2+ sites. However, at a high reaction temperature, in terms of some of the depicted intermediates including RCN, HNCO, RCOO-, and NH3, the proposed reaction pathway for the C3H6-SCR in the absence of O2 is different from the pathway at room temperature as shown in Scheme 2. NO2 contributes in major part to the HC-SCR reaction. The whole reaction proceeds via the formation of R-NO2 intermediates, which are finally decomposed into CO2, CO, H2O, and NH3 by the sequential reaction equations (1-10) suggested in the literature.19,38,41,59-61 The nitro-compound intermediates were not clearly identified because their characteristic absorption bands in the 1620-1550 cm-1 region overlap greatly with the vibrations of -COO- species formed, but they played undoubtedly the same role in the iron-catalyzed HCSCR reaction as in the SCR of NOx catalyzed by Cu and Ag.59,60 The catalytic redox chemistry of C2H4 over iron sites should be the same as propylene due to the similar chemical properties. Methane as reductant represents a different NO2 reduction pathway from propylene and ethylene. It is oxidized to be CO2, CO, and H2O via the nitromethane pathway, while NO2 is directly reduced to N2. For both the C3H6-SCR and the C2H4-SCR in the absence of O2, the formation of N2 as indicated by GC maybe involves the fast NH3-SCR reaction of 2NH3 + NO2 + NO ) 2N2 + 3H2O occurring on oxo-Fe3+ sites.62,63 The presence of O2 is necessary to the HC-SCR reactions, as indicated by the great improvement of catalytic activity. It is undisputed that the presence of O2 enhances remarkably the rate of NO reduction by a few pathways as follows: (1) The NH3 intermediates are deepoxidized by the two overall reactions of 4NH3 + 4NO + O2 ) 4N2 + 6H2O and 2NH3 + NO2 + NO ) 2N2 + 3H2O,64,65 which may be a key step to the final conversion of NO to N2. In this case, O2 aids the SCR by converting NO to NO2, which can then form HNO2 and HNO3, finally leading to NH4NO3.66,67 The NH4NO3 intermediate can be reduced by NO to NH4NO2, which then decomposes to N2 and H2O along with the production of N2O above about 250 °C.68 This can explain the strange behavior, that is, the increase of N2O above about 150 °C shown in Figures 11 and 13B. (2) Alkane reductants can be oxidized to hydrocarbon oxygenates. The organic oxygenates are more reactive than alkanes and alkenes for SCR.60,61,69 (3) Promoting the formation of NO2 crucial species by the overall reaction of 2NO + O2 ) NO2. NO2 is more reactive than NO with respect to these oxygenates of CH4, C2H4, and C3H6.60,61,69-71 Within the whole reaction, although no NO+ is formed by the interaction of NO with acidic protons in the absence and presence of O2,



SCHEME 2: Proposed Reaction Pathways for the Iron-Catalyzed HC-SCR Reaction

we think that Bro¨nsted acidic sites of the zeolite still are necessary to the iron-catalyzed HC-SCR reaction. It is certain that the presence of Bro¨nsted acidic protons favors the reduction of NOx to N2, as shown in Scheme 2. The IR spectrum of NO adsorption after the reaction, shown in Figure 16, still displays a main band at ca. 1870 cm-1 and a weak shoulder centered at 1843 cm-1, indicating that a majority of iron are still present in the form of binuclear iron clusters after the reaction. However, the absorbance intensity of the two bands decreases greatly perhaps due to the coverage of products generated during the reaction. It implies that iron species with low nuclearity are likely responsible for the HC-SCR reaction.

5. Conclusions The following conclusions may be drawn from the present work: (1) NO can be disproportionated to N2O and NO2 in cages of the Fe/Y zeolite catalyst. The formed NO2 as an oxidant plays a crucial role in the HC-SCR reaction. The disproportionation, in major part, contributes to the selective reduction of NO with olefins in the absence of O2. (2) Adsorption of reductants on iron active sites is the pivotal step involving the formation of nitrogen-containing complex intermediates, strongly affecting the catalytic activity. Among the selected hydrocarbons, propylene exhibits the highest adsorption amount on the catalyst

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Figure 16. Comparison of IR spectra of NO adsorption before (a) and after (b) the reaction.

surface and has the strongest reactivity to NOx adsorbed species. Methane is less active for the reduction of NO in both the absence and the presence of O2. (3) After the HC-SCR reaction, the characteristic IR bands at 1870 and 1845 cm-1, representing respectively mononitrosyl adsorbed on oligomeric Fe2+ species and isolated iron sites, reappear (Figure 16), suggesting that iron species with low nuclearity are likely responsible for the HC-SCR reaction. (4) NO adsorbed on iron sites can directly take part in the SCR reaction with propylene at room temperature, because the NtO bonding is greatly weakened by the coordination with coordinatively unsaturated Fe2+ sites. Unlike binuclear copper sites, binuclear iron clusters cannot catalytically decompose NO to N2 and O2. (5) The intermediates and products formed with olefins as reductants are completely different from those with methane, suggesting the drastic difference between the reaction mechanism of olefin and alkane. The formation of N2 occurs via the oxidation of NH3 intermediate for the olefinSCR. (6) The presence of O2 greatly aids in the iron-catalyzed reduction of NO to N2 by the formation of large amounts of NO2, which is generally more reactive than NO for the HCSCR, but the intrinsic reduction pathway is not changed. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 20873022 and 20701008), National Basic Research Program of China (973 Program, No. 2007CB613306), the National High Tech R&D Program of China (863 Program, 2008AA06Z326), the Natural Science Foundation of Fujian Province of P. R. China (2010J05024), and Programs for Changjiang Scholars and Innovative Research Team in University (PCSIRT0818) and New Century Excellent Talents in University (XSJRC2007-19) of Fujian Province of P. R. China. We thank Professor Yaning Xie, Beijing Photo Factory, for the XAFS experiments and Professor Kemei Wei for catalytic experiments. Supporting Information Available: Figures of the EPR spectrum of the catalyst Fe/HY-G4.9, XPS spectra of the catalyst Fe/HY-G4.9, comparison of the IR spectrum of gaseous and adsorbed propylene, difference IR spectra, relationship between the bands, and N2 selectivity as a function of reaction temperature and table of Gaussian fitting analysis results. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Amiridis, M. D.; Zhang, T.; Farrauto, R. J. Appl. Catal., B 1996, 10, 203. (2) Heck, R. M. Catal. Today 1999, 53, 519. (3) Traa, Y.; Burger, B.; Weitkamp, J. Microporous Mesoporous Mater. 1999, 30, 3.

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JP104998D

Infrared Study of the NO Reduction by Hydrocarbons over Iron Sites

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