In situ DRIFTS investigation on the SCR of NO with NH3 over V2O5 catalyst supported by activated semi-coke

Applied Surface Science 313 (2014) 660–669

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In situ DRIFTS investigation on the SCR of NO with NH3 over V2 O5 catalyst supported by activated semi-coke Jinping Wang a , Zheng Yan b , Lili Liu a , Yan Chen a , Zuotai Zhang a , Xidong Wang a,∗ a

Beijing Key Laboratory for Solid Waste Utilization and Management, College of Engineering, Peking University, Beijing 100871, China Liaoning Key Laboratory of Clean Energy and Institute of Clean Energy and Environmental Engineering, College of Energy and Environment, Shenyang Aerospace University, Liaoning 110034, China b

a r t i c l e

i n f o

Article history: Received 26 April 2014 Received in revised form 5 June 2014 Accepted 7 June 2014 Available online 13 June 2014 Keywords: V2 O5 Activated semi-coke Selective catalytic reduction (SCR) Mechanism

a b s t r a c t 3 wt.% of V2 O5 is loaded onto the activated semi-coke (V2 O5 /ASC) via impregnation method and used for low temperature selective catalytic reduction (SCR) of NOx with NH3 . The prepared V2 O5 /ASC catalyst yields an over 90% NO conversion rate with excellent N2 selectivity at 250 ◦ C with a space velocity of 12,000 h−1 . The adsorption state of different species and reaction behaviors under various conditions are systematically examined with in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS). It is evidenced that NH3 and NO are absorbed on different active sites over the V2 O5 /ASC catalyst. At the reaction temperature (200 ◦ C) in this study, NH3 is mainly absorbed on the Lewis acid sites provided by V2 O5 , and NO is mainly absorbed on the active sites originated from the support ASC. In addition, the NH3 -SCR process takes place according to two pathways, including reaction between the coordinated NH3 and gaseous NO (E–R mechanism), and reaction between the absorbed NO2 and coordinated NH3 (L–H mechanism). The latter one plays a primary role for the improved low-temperature SCR performance of V2 O5 /ASC catalyst. © 2014 Elsevier B.V. All rights reserved.

1. Introduction In the last decades, selective catalytic reduction (SCR) process with NH3 is proved to be one of the most outstanding methods for the removal of nitrogen oxides (NOx ) from the flue gas emitted from stationary polluting sources, such as thermal power plants [1,2]. V2 O5 based on TiO2 promoted by WO3 or MoO3 catalysts have been widely used in the commercial SCR systems within the optimum temperature range of 300–400 ◦ C [3–6]. These costly catalysts are usually placed upstream of the electrostatic precipitator, which makes them to be easily poisoned by the dust and SO2 in high concentration [1,7]. For these reasons, extensive studies have been conducted to develop catalysts active at lower temperature. Among the potential novel catalysts [8–21], the ones with V2 O5 supported on carbonaceous materials (V2 O5 /CM) have exhibited high efficiencies at lower temperature (usually 180–250 ◦ C) [17,18,20–23], which hence provide a promising alternative to the TiO2 -based ones. In addition, V2 O5 /CM is reported to be less

∗ Corresponding author at: Peking University, Department of Energy and Resources Engineering, Haidian Distric, Beijing 100871, China. Tel.: +86 10 8252 9083; fax: +86 10 8252 9010. E-mail address: [email protected] (X. Wang). http://dx.doi.org/10.1016/j.apsusc.2014.06.043 0169-4332/© 2014 Elsevier B.V. All rights reserved.

sensible to SO2 deactivation [22,23], thus promoting its potential of industrial application in the near future. In order to further improve the activity of V2 O5 /CM, the SCR reaction mechanisms over V2 O5 /CM catalysts are investigated by different technologies, including the transient response analysis [24,25], temperature-programmed desorption (TPD) [26,27] and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) [26,28,30]. In spite of the intensive efforts dedicated, several controversies still exist. By means of transient response analysis, TPD and DRIFTS, Galvez et al. [29] demonstrate that the SCR reaction over activated carbon supported V2 O5 catalysts (V2 O5 /AC) takes place between the adsorbed species of NH3 on the Brønsted acid sites and NO molecules in gaseous phase, following an Eley–Rideal (E–R) mechanism, which is consistent with the results obtained by Zhu et al. [22]. However, Sun et al. [30] suggest a different SCR mechanism over the activated coke supported V2 O5 catalyst, involving the adsorbed NO2 and adsorbed NH3 on both Lewis and Brønsted acid sites (L–H mechanism). Besides, another hypothesis proposed by Huang et al. [21] according to the TPD results suggests that chemisorbed NH3 species on Lewis acid sites are vital for carbon nanotubes supported V2 O5 catalysts (V2 O5 /CNTs), and the catalytic process depends on the adsorbed NH3 species reacting with gaseous NO (E–R mechanism) as well as the adsorbed NO2 species (L–H mechanism). Obviously, controversies still exist on

J. Wang et al. / Applied Surface Science 313 (2014) 660–669

the behaviors of NO and NH3 in the reaction over V2 O5 /CM catalysts, among which the properties of the support should not be neglected. Further detailed information on the reaction process and the roles of carbonaceous support and V2 O5 are extremely essential for better understanding of the reaction mechanism. In our previous work [32,33], the cheap solid waste material semi-coke (SC), a by-product of lignite coking, has been employed in the preparation of adsorbent and catalysts to remove SO2 and NOx from simulated flue gas. It has been shown that, with specific treatments, activated semi-coke (ASC) is able to offer suitable physical structure and high surface activity for utilization as support or catalyst. In this contribution, the SC is employed as raw material to prepare V-containing composite catalyst for low-temperature SCR of NO with NH3 . Physical and chemical properties of both the ASC and V-containing catalyst (V2 O5 /ASC) are widely detected by characterization of nitrogen physisorption, SEM, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and laser Raman spectra. Combined with the in situ DRIFTS results conducted at different conditions, catalytic behaviors of NO and NH3 in SCR reaction over the V2 O5 /ASC catalyst are explored systematically and then two possible reaction pathways are proposed. In addition, the role of ASC in the catalytic performance is discussed. 2. Experimental 2.1. Activated semi-coke supports For the preparation of the activated semi-coke supports, a kind of cheap commercial carbon material (Shenmu Semi-coke Co., Ltd., China) was used. With the main aim of increasing the concentration of oxygen functionalities on the support, the raw material was ground and sieved into granules ranging 10–20 mesh (0.85–2 mm, marked as SC) and then submitted to the concentrated HNO3 (40 wt.%) at 60 ◦ C for 2 h. After being rinsed with distilled water till pH ∼ 7, the semi-coke was dried at 120 ◦ C for 5 h, followed by calcination in Ar at 700 ◦ C for 4 h, marked as ASC. 2.2. Catalyst preparation Vanadium oxide (VOx ) was loaded onto the ASC by excessive impregnation methods. The ASC was immerged into an aqueous solution containing NH4 VO3 and oxalic acid at 60 ◦ C in water bath with rotary mixing for 5 h. After the impregnation, the catalyst was dried at 105 ◦ C for 9 h, followed by a calcination in Ar stream at 500 ◦ C for 5 h and a pre-oxidization in air at 200 ◦ C for 2 h. Loading amount of the active components was ascertained accurately by the method mentioned in the previous work [31]. The catalyst containing about 3 wt.% V2 O5 was used for the following study, marked as V2 O5 /ASC.

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Table 1 Textual properties of the supports and the catalysts. Samples

Specific surface area (m2 /g)

Total pore volume (cm3 /g)

Average pore diameter (nm)

SC ASC V2 O5 /ASC

19 230 220

0.013 0.034 0.027

5.12 4.00 3.95

field emission scanning electron microscopy (SEM, Hitachi S-4800) system. Molecular structures of the products were detected by a laser Raman micoscpoe system (Renishaw RM-1000) at room temperature with the accuracy in the Raman shift estimated to be ∼0.5 cm−1 . 2.4. SCR activity measurements The SCR reactions were carried out in a fixed bed continuous flow quartz reactor (25 mm of internal diameter) in the reaction temperature range of 100–300 ◦ C, containing 1 g of catalyst. The typical reactant gas mixture contained 1000 ppm NO, 1000 ppm NH3 , 2% O2 and Ar as balance at a total flow rate of 400 mL/min, corresponding to a space velocity of about 12,000 h−1 . The concentrations of NO, NO2 , N2 O and NH3 at inlet and outlet were continuously monitored by an on-line FT-IR spectrophotometer (Nicolet 670). The data was collected when the catalytic reaction substantially reached a steady state condition for an hour at each temperature. NO conversion and N2 selectivity were respectively calculated as follows: NO conversion (%) =

N2 selectivity (%) =





1−

1−

[NOX ]out [NOX ]in



× 100%

2[N2 O]out [NOX ]in − [NOX ]out



× 100%

2.5. In Situ DRIFTS studies The DRIFTS studies were performed on a fourier tranform infarared spectrometer (FT-IR, Nicolet 670) equipped with an in situ Harrick DRIFT cell containing ZnSe window and MCT detector cooled by liquid N2 . The catalysts were ground into powder, diluted with KBr and placed in the cell, and then heated to 300 ◦ C in Ar with a total flow rate of 100 mL/min for 30 min to remove the adsorbed impurities. A spectrum of pure KBr was collected in flowing Ar in order to perform a background correction at each testing temperature. The DRIFTS spectra were recorded by accumulating 100 scans with a resolution of 4 cm−1 . 3. Results and discussions

2.3. Catalyst characterization 3.1. Characterizations Textural properties of as-prepared samples were determined using a nitrogen adsorption apparatus (Micromeritics ASAP2010) at 77 K. Before measurement, the samples were outgassed at 250 ◦ C overnight under vacuum. The specific surface areas were calculated with Brunauer–Emmett–Teller (BET) theory [34] and the pore volume was obtained from the N2 desorption isotherm using Barrett–Joyner–Halenda (BJH) equations [35]. Crystal structures of the samples were determined by using an X-ray diffractometer (XRD, Rigaku Dmax/2400, Cu-K␣ radiation) at a scanning rate of 8◦ /min over the 2 range of 10–80◦ . The surface atomic states of the catalysts were analyzed by using X-ray photoelectron spectroscopy (XPS, AXIS ULTRADLD ) with Al K␣ radiation (h = 1486.6 eV) at 150 W. Surface morphologies of the samples were imaged with a

As a typical gas–solid reaction, the removal of NO is directly affected by the textural properties of the catalysts. A relatively high BET surface area (SBET ) is proved to be favored in the SCR process. It has been evidenced in our previous study [32] that SBET of the support can be greatly enhanced by the activation process, with an increased pore volume and decreased average pore size as listed in Table 1. Also, the nitric acid modification induces a rise in acid groups and oxygen-containing groups on the surface of the support [32]. With the loading of V2 O5 , part of the pores is occupied, leading to a slight decrease of surface area and pore volumes (Table 1). Remarkably, the enriched oxygen-containing groups on ASC surface enable the catalyst to have a high dispersion and anti-sintering

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Fig. 1. (a) SEM image of the ASC, (b) SEM image of the V2 O5 /ASC, (c) enlarged image of VOx particles from panel (b), and (d) EDX spectra of the VOx particles in panel (c).

ability, which hence guarantee the relatively high surface area after loading of V2 O5 [32,36]. Surface morphologies of the ASC and V2 O5 /ASC catalyst are respectively displayed in Fig. 1(a) and (b). It can be seen that the image of SC shows an apparent porous structure (Fig. 1(a)), which is favored for the loading process. As to V2 O5 /ASC, a large amount of particles in nanosize (100–200 nm) are formed on the ASC surface (Fig. 1(b)). Noteworthily, a self-assembled nano-network structure can be observed in the magnified SEM image (Fig. 1(c)). It has been acknowledged that the catalyst particles in nanoscale can exhibit high activity even with a relatively low loading amount [31]. The obtained nanostructure therefore indicates a high dispersion and utilization of the precious V2 O5 catalyst. Meanwhile, the highly dispersed nanoparticles are beneficial for the improvement of catalyst textural properties, which is consistent with the nitrogen adsorption results mentioned above. Compositions of these particles are further investigated using EDX analysis, the results of which demonstrate that the main component of the particles is VOx (Fig. 1(d)). X-ray diffraction patterns of the ASC and V2 O5 /ASC catalyst are illustrated in Fig. 2. Two broad diffraction peaks of (0 0 2) and (1 0 0) crystal faces are observed in the ASC [37,38]. The (0 0 2) diffraction peak located at 20–30◦ reflects the existence of graphite crystallite with layer structure, while the (1 0 0) diffraction peak located at 40–45◦ represents the turbostratic graphite-like structure [37–39]. Mochida et al. [40] have proposed that the ␲-bonds contained in graphite crystallite on the ASC surface are beneficial for the oxidation of NO to NO2 at low temperature in the presence of O2 . This unique property allows the carbonaceous material to exhibit an intrinsic denitration capacity, which also can be enhanced by the abundant oxygen functional groups, particularly with the HNO3 modification [32,41]. After loading of VOx particles, typical peaks corresponding to the graphite crystalline still can be detected. However, the diffraction scans of vanadium oxides, such as V2 O5 , VO2 , V2 O4 or V2 O3 , are not observed. In consideration of the calcination temperature (500 ◦ C) as well as the studies conducted by Chary et al. [42] and Li and Zhong [43], the absence of VOx characteristic XRD peaks should be caused by the low loading amount of VOx (about 3 wt.%).

Composition of the VOx particles on the ASC is identified by using XPS technique. Fig. 3(a) shows the survey-scan spectrum, in which peaks corresponding to vanadium, oxygen and carbon are seen. The peak at 284.6 eV in the C (1s) core-level spectrum (Fig. 3(b)) is resulted from the sp2 -hybridized carbon atoms of the graphene sheets [44], which confirm the existence of graphite crystallite with layer structure in the catalyst. Fig. 3(c) illustrates the core level binding energies for V (2p) peaks. The binding energies for V 2p3/2 and 2p1/2 observed at 517.1 and 524.7 eV agree well with those of V5+ in V2 O5 , respectively [45]. The broad and asymmetric spectrum for O (1s) (Fig. 3(d)) is deconvoluted into three peaks, which respectively represent the lattice oxygen O␣ (peak at 530.0 eV), chemisorbed oxygen O␤ (peak at 531.3 eV) and hydroxyl groups O␥ (peak at 533.1 eV) [46]. The oxygen O␥ is probably from the adsorbed H2 O over the catalyst surface and the oxygen O␤ is ascribed to the –OH on the surface [46,47]. As to the oxygen O␣ , it is commonly attributed to the O (1s) of V2 O5 [47].

Fig. 2. XRD patterns of ASC and V2 O5 /ASC.

J. Wang et al. / Applied Surface Science 313 (2014) 660–669

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Fig. 3. XPS spectra for V2 O5 /ASC surface: (a) survey spectrum, (b) core-level spectrum of C (1s), (c) core-level spectrum of V (2p), (d) core-level spectrum of O (1s).

The characteristic spectra of V2 O5 revealed in Fig. 3 evidence that the VOx on the catalyst surface is V2 O5 . Laser Raman spectroscopy is conducted to further identify the phase of V2 O5 on the ASC surface and the spectrum profile is illustrated in Fig. 4, where spectrum of the ASC is also listed for comparison. Two bands located at 1601 and 1360 cm−1 are detected in the ASC spectrum. The former one (1601 cm−1 ) corresponds to the E2g species of single-crystal graphite, while the latter one (1360 cm−1 ) is assigned to be the vibration mode of the graphite

lattice of which the Raman activity is originated from the loss of translational symmetry at the borders of the crystalline areas [48]. Herein, the Raman results further strengthen the conclusions drawn from the XRD analysis of ASC. When the ASC is loaded with V2 O5 particles, a typical Raman spectrum for crystalline V2 O5 can be detected [49], in which the characteristic peaks are located at 145, 195, 282, 303, 403, 483, 529, 700 and 992 cm−1 . The bands located at 992 cm−1 and 403, 282 cm−1 are respectively assigned to the stretching and the bend˚ as a result of an unshared ing vibration of the V O bonds (1.54 A) oxygen [50,51]. Commonly, the V O bond in V-containing catalysts is considered to be an important active site for SCR reaction in V-containing catalysts [20,26,27,52]. The band at 700 cm−1 is assigned to the doubly coordinated oxygen (V2 –O) in stretching mode, as a result of the corner-unshared oxygens in common to two pyramids [51]. The triply coordinated oxygen (V3 –O) bond in stretching mode due to the edge-shared oxygens in common to three pyramids reveals at 529 cm−1 [51,53]. The bands located at 483 and 303 cm−1 are assigned to the bending vibrations of the bridging V–O–V (doubly coordinated oxygen), and two more low-frequency Raman bands at 195 and 145 cm−1 are assigned to the stretching mode of (V2 O2 )n in the lattice vibration. These two peaks are strongly associated with the layered structure of V2 O5 [53,54]. Hence, information from the Raman spectra can definitely determine the particles on ASC surface to be crystalline V2 O5 . 3.2. SCR activities

Fig. 4. Raman spectra of the ASC and V2 O5 /ASC.

NO conversion rate over the ASC and V2 O5 /ASC catalyst are measured with the variation of temperature (100–300 ◦ C, 50 ◦ C for each step) as shown in Fig. 5(a). It is found that the ASC sample

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Fig. 5. NH3 -SCR activities of ASC and V2 O5 /ASC catalyst with the variation of temperature: (a) NO conversion rate, (b) N2 selectivity and N2 O formation. Reaction conditions: 1000 ppm NO, 1000 ppm NH3 , 2% O2 , Ar as balance gas, GHSV: 12,000 h−1 .

exhibits an expected intrinsical denitration capacity which drops at first and then grows up with the increase of temperature from about 20% at 200 ◦ C to about 40% at 300 ◦ C. It has been evidenced that oxygenated functional groups and graphite crystalline are regarded as the important active sites that can effectively promote the chemisorption of NH3 and NO. Higher temperature further facilitates the following catalytic reduction, leading to an increase in the SCR activity along with the increase of temperature (200–300 ◦ C). In the case of V2 O5 /ASC, an obviously enhanced NO conversion rate is obtained. The profile in Fig. 5(a) illustrates a sharp increase of NO conversion rate over the V2 O5 /ASC catalyst from about 25% at 100 ◦ C to over 80% at 200 ◦ C. When the temperature is further elevated beyond 250 ◦ C, the efficiency attains over 90%. N2 selectivity and N2 O formation over the as-prepared samples are illustrated in Fig. 5(b). It can be seen that few differences exist in the N2 O formations between the ASC and V2 O5 /ASC catalyst. The N2 O concentration slightly increases with an increase in the temperature and the values are all lower than 30 ppm, which lead to the relatively good N2 selectivities of more that 80%. However, due to the higher NO conversion rate of V2 O5 /ASC, its N2 selectivity (over 90%) is significantly higher than that of the ASC. The different activity patterns of the two samples imply that the V2 O5 /ASC catalyst follows a different SCR mechanism from that of the ASC in the SCR process. It is reasonable to believe that the loading of V2 O5 is responsible for the transformation in the reaction behaviors.

acid sites (ıs (NH4 + )) [55,56], whereas the band at 1458 cm−1 corresponds to the asymmetric bending vibrations of NH4 + (ıas (NH4 + )) [55,56]. For the coordinately adsorbed NH3 on Lewis acid sites, bands at 1627 and 1233 cm−1 represent the asymmetric bending vibrations of N–H bonds (ıas (NH)) [20,57], bands in the range of 1000 ∼ 1200 cm−1 represent the symmetric N–H bending

3.3. In situ DRIFTS studies on V2 O5 /ASC In situ DRIFTS studies are conducted to investigate the mechanism on molecular level, mainly through identification of the active sites, the adsorbed species and the intermediates on the surface under different conditions. It should be noted that DRIFT spectra of the original samples with no treatment have been subtracted from the measured ones as below for better identification. 3.3.1. Adsorption of NH3 Surface properties of the V2 O5 /ASC catalyst after being exposed to 1000 ppm NH3 /Ar for different time at 50 ◦ C and 200 ◦ C (Fig. 6) are recorded to distinguish the adsorption sites for NH3 . As illustrated in Fig. 6(a), when the temperature is controlled to be 50 ◦ C, several bands immediately appear at 1741, 1689, 1627, 1545, 1521 and 1458 cm−1 in 3 min. However, new bands located at 3324, 1233 cm−1 and in the range of 1000 ∼ 1200 cm−1 are detected in 5 min. Then the whole spectra keep relatively stable to 30 min. The bands at 1741 and 1689 cm−1 are attributed to the symmetric bending vibrations of NH4 + species on Brønsted

Fig. 6. DRIFT spectra of V2 O5 /ASC exposed to 1000 ppm NH3 at the temperature of 50 ◦ C and 200 ◦ C for various time: (a) 50 ◦ C, (b) 200 ◦ C.

J. Wang et al. / Applied Surface Science 313 (2014) 660–669

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vibrations (ıs (NH)) [58,59] and the band at 3324 cm−1 is assigned to the asymmetric N–H stretching vibrations (as (NH)) [60]. For the bands appearing at 1545, 1521 cm−1 , Kijlstra et al. [61] suggest that the band at 1510 cm−1 is attributed to an amide species, and Ramis et al. [28] propose that the bands at 1550 cm−1 may be related to the intermediate from oxidation of NH3 . Accordingly, the bands at 1545 and 1521 cm−1 are respectively assigned to the intermediate of oxidation of NH3 and amide species (–NH2 ). When the temperature is raised to 200 ◦ C, different adsorption states for NH3 on the V2 O5 /ASC catalyst surface are presented in Fig. 6(b). The bands assigned to ıas (NH) (1627 and 1239 cm−1 ), ıs (NH) (1000 ∼ 1200 cm−1 ), as (NH) (3330 cm−1 ) and amide species (–NH2 ) (1557 and 1513 cm−1 ) remain stable with increasing intensities as time increases. Nevertheless, the bands corresponding to the ıs (NH4 + ) and ıas (NH4 + ) are hardly detectable at this temperature. In this process, two new bands located at 3152 and 1397 cm−1 emerge after 5 min, which are assigned to the symmetric N–H stretching vibrations in the coordinately adsorbed NH3 on Lewis acid (s (NH)) [60] and the NH4 + species adsorbed on the support [62]. All the changes on the adsorption states of NH3 suggest that the Lewis acid sites are much more stable than the Brønsted acid sites over the V2 O5 /ASC catalyst and the quantity of coordinated NH3 is larger than that of the NH4 + ions at the reaction temperature (200 ◦ C). 3.3.2. Co-adsorption of NO and O2 As to the co-adsorption of NO and O2 , characterization is conducted with 1000 ppm NO + 2% O2 /Ar. Fig. 7(a) displays the DRIFT spectra at the temperature of 50 ◦ C. After the initial 3 min, two distinct bands appear at 1634 and 1596 cm−1 , and several weak bands appear at 1914, 1850, 1369 and 1011 cm−1 . Along with the increase of time, intensities of the weak bands at 3 min gradually increase and the spectra afterwards keep relatively high consistency. The bands located at 1369 and 1011 cm−1 should be ascribed to the nitrate species and cis-N2 O2 2− respectively with relatively good stability [63,64]. The bands at 1914 and 1850 cm−1 are widely viewed as the gaseous or weakly adsorbed NO [64,65]. According to the studies conducted by Chen et al. [66] and Qi et al. [67], the bands located at 1634 and 1596 cm−1 can be assigned to the gaseous or weakly adsorbed NO2 . At the temperature of 200 ◦ C, similar DRIFT spectra are obtained at different time during the co-adsorption of NO + O2 (Fig. 7(b)). By comparison, bands corresponding to the gaseous or weakly adsorbed NO (1914 and 1842 cm−1 ), gaseous or weakly adsorbed NO2 (1633 and 1604 cm−1 ), nitrate species (1374 cm−1 ) and cis-N2 O2 2− (1011 cm−1 ) which have already been detected at 50 ◦ C can be found as well, indicating the existence of these groups at higher temperature. However, the nitrate species (1374 cm−1 ) are not so distinct as they are at 50 ◦ C. The results strongly suggest that different nitrate species, gas phase or weekly adsorbed NO and NO2 simultaneously appear over the V2 O5 /ASC catalyst and the temperature only affects their intensities without difference on the species. 3.3.3. Comparison of NH3 and NO + O2 adsorption on the ASC and V2 O5 /ASC During the low temperature SCR reactions, multiple intermediates can be generated due to the complex proceedings of adsorption and catalysis, particularly when it comes to a composite catalyst. The intrinsical denitration capacity of the ASC revealed in this study is likely to influence the catalytic behaviors of V2 O5 /ASC in a conducive way. The adspecies of NH3 and NO + O2 on the surface of ASC and V2 O5 /ASC at a typical low temperature of 200 ◦ C are then recorded (Fig. 8). For the adsorption of NH3 , it can be observed in Fig. 8(a) that the species formed and their intensities on the surface of V2 O5 /ASC catalyst bear little resemblance to those on the

Fig. 7. DRIFT spectra of V2 O5 /ASC exposed to 1000 ppm NO + 2% O2 at the temperature of 50 ◦ C and 200 ◦ C for various time: (a) 50 ◦ C, (b) 200 ◦ C.

ASC surface. Massive coordinately adsorbed NH3 species on Lewis acid sites with relatively high intensities (3330, 3162, 1662, 1239 and 1000 ∼ 1200 cm−1 ) are detected on the V2 O5 /ASC catalyst surface. For the ASC, only several weak peaks corresponding to the NH4 + (1739 and 1685 cm−1 ) and the intermediate from oxidation of NH3 (1557 cm−1 ) are found. Thus, it can be deduced that NH3 adsorption occurs on both ASC and V2 O5 /ASC catalyst surface but at different active sites. Furthermore, the Lewis acid sites with strong NH3 adsorption ability on the surface of V2 O5 /ASC are mostly provided by the V2 O5 at 200 ◦ C instead of the ASC support. Besides, the peaks corresponding to the NH species (1436 ∼ 1460 cm−1 ) [55], which are originated from the excessive hydrogen abstraction of the coordinated NH3 , are hardly detected in the DRIFT spectra of NH3 adsorption over the ASC and V2 O5 /ASC catalyst. According to the studies conducted by Liu et al. [68] and Kapteijn et al. [76], the NH species are unable to react with NO to form N2 instead of N2 O specie and the reaction paths are described as follows: NO

NH(ads) −→NHNO(ads) → N2 O Based on the DRIFTS results, it can be deduced that the NH species are not so easy to form on both the ASC and V2 O5 /ASC catalyst, resulting in the low amount of N2 O productivity in the SCR process illustrated in Section 3.2. In Fig. 8(b), little difference between the spectra of the two samples can be noticed. Specifically, it refers to the high similarity

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Fig. 8. DRIFT spectra of the ASC and V2 O5 /ASC exposed to different gas flowing at 200 ◦ C for 10 min: (a) 1000 ppm NH3 , (b) 1000 ppm NO + 2% O2 .

on the adsorbed NOx species formed on both ASC and V2 O5 /ASC catalyst surface and the corresponding band intensities, including the gaseous NO (1914 cm−1 ), gaseous NO2 molecules (1633, 1604 cm−1 ) and nitrate species (1374 cm−1 ). As previous studies have evidenced, the adsorption of NO is very weak over the vanadiatitania catalysts [52,63,69–72]. By comparison, it can be deduced that the ASC should play a major role in the formation of adsorbed NOx species over the V2 O5 /ASC catalyst, resulting from the graphite crystalline and the oxygen functional groups as illustrated in Section 3.1.

bands due to NH4 + species adsorbed on the ASC and the vibrations of coordinately NH3 adsorbed on Lewis acid diminish. Meanwhile, several new bands located at 1904, 1843, 1637, 1604, 1366 and 1018 cm−1 arise and then remain stable, which are attributed to the adsorbed NOx species. Thus, it can be concluded from the results above that NOx can react with the adsorbed NH3 species easily, especially with the intermediate from oxidation of NH3 and amide species.

3.3.4. Reaction between nitrogen oxides and NH3 adspecies In this experiment, V2 O5 /ASC catalyst is pre-exposed to a flow containing 1000 ppm NH3 /Ar for 30 min followed by Ar purging at 200 ◦ C for 15 min. Then, 1000 ppm NO + 2% O2 /Ar is introduced into the cell and the DRIFT spectra are recorded along with the variation of time (Fig. 9). As noted above, the DRIFT spectra of the sample treated with NH3 -containing flow at 200 ◦ C reveals coordinately adsorbed NH3 on Lewis acid sites (3330, 3152, 1622, 1239 and 1000 ∼ 1200 cm−1 ), intermediate from oxidation of NH3 (1557 cm−1 ), amide species (1513 cm−1 ) and NH4 + species (1397 cm−1 ). It can be seen in Fig. 9 that admission of the NO + O2 flow gives rise to an immediate vanish of the intermediate and the amide species originating from oxidation of NH3 . After 5 min, the

3.3.5. Reaction between NH3 and adsorbed nitrogen oxides species By reversing the sequence of the inlet flow gas in Section 3.3.4, the sample is pre-exposed to a flow containing 1000 ppm NO + O2 /Ar for 30 min followed by Ar purging for 15 min at 200 ◦ C. Then, 1000 ppm NH3 /Ar is introduced into the cell and the DRIFT spectra are recorded along with the variation of time. As shown in Fig. 10, bands for the as (Cis-N2 O2 2− ) (1011 cm−1 ), nitrate species (1373 cm−1 ), the gaseous or weakly adsorbed NO (1913 and 1842 cm−1 ) and NO2 (1634 and 1604 cm−1 ) appear in the DRIFT spectrum at initial period (3 min). During the admission of NH3 /Ar over the catalysts until 30 min, the bands assigned to the as (Cis-N2 O2 2− ) and the gaseous or weakly adsorbed NO and NO2 gradually decrease. In this case, it can be concluded that both the NO and NO2 are considered to be involved in the SCR process.

Fig. 9. DRIFT spectra of V2 O5 /ASC pretreated in flowing 1000 ppm NH3 followed by exposure to 1000 ppm NO + 2% O2 at 200 ◦ C for various times.

Fig. 10. DRIFT spectra of V2 O5 /ASC pretreated in flowing 1000 ppm NO + 2% O2 followed by exposure to 1000 ppm NH3 at 200 ◦ C for various time.

J. Wang et al. / Applied Surface Science 313 (2014) 660–669

However, the band at 1373 cm−1 has not disappeared, but shifts to 1391 cm−1 . It suggests the nitrated species are not active in the SCR process, but affected by the existence of NH3 in SCR process. The shift is also observed in the research conducted by Qi et al. [67] ˜ et al. [73]. They propose that the shift is resulted from and Pena the electron transfer between the NH3 and nitro-compounds. Furthermore, strong bands attributed to the vibrations of adsorbed NH3 species, especially the NH4 + species on Brønsted acid sites (1458 and 1850 ∼ 1640 cm−1 ) are observed. Meanwhile, intensities of these peaks are significantly higher than those form by the direct adsorption of NH3 on the V2 O5 /ASC catalyst without being pretreated in the flow of NO + O2 (Fig. 6(b)). The variation on the intensity of adsorbed NH4 + species is ascribed to the formation of nitric acid species originated from the reaction between adsorbed NO2 and H2 O (product), and the nitric acid species can further serve as Brønsted acid sites on the catalyst surface [67,73]. Besides, bands for the intermediate from oxidation and amide species are also strengthened, which indicates the oxidation ability of the catalyst will be enhanced by the formation of nitric acid species.

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NO(g) + NH2 (a) → NH2 NO(a)

(3)

NH2 NO(a) → N2 (g) + H2 O

(4)

(2) Reactions between the absorbed NO2 and coordinated NH3 (L–H mechanism), where the active sites are mainly provided by the ASC: ASC active sites

2NO + O2 (g)

−→

Lewis acid sites

NH3 (g)

−→

2NO2 (a)

NH3 (a)

(7)

NO(g) + NO2 (NH3 )2 → . . . . . . → 2N2 + H2 O

(8)

Actually, these two pathways mentioned above can be summarized to the overall reactions separately denoted as “standard SCR” (Reaction (9)) and “fast SCR” (Reaction (10)) [74,75].

2NO2 (a) + 2NO + 4NH3 → 4N2 + 6H2 O

For NH3 -SCR process over the V2 O5 /CM catalysts, the mechanism has been studied extensively in the past decades. Different hypotheses have been proposed for the mechanism including the reaction between the adsorbed NH3 species and gaseous NO over Lewis or Brønsted acid sites (E–R mechanism) [22,29], the reaction between the adsorbed NH3 species and the adsorbed NO2 (L–H mechanism) [30], and the co-existence of E–R and L–H mechanism [21]. In this study, analyses with DRIFTS have demonstrated the relatively high ability of V2 O5 /ASC catalyst on NH3 adsorption and oxidation. Both Lewis and Brønsted acid sites can be provided by the V2 O5 , leading to the appearance of coordinated NH3 , NH4 + , –NH2 and other intermediates from NH3 oxidation on the V2 O5 /ASC catalyst. At the reaction temperature (200 ◦ C), the Lewis acid sites are much more stable than the Brønsted acid sites and the quantity of coordinated NH3 is larger than that of NH4 + ions. As described in Section 3.3.4, after the NO + O2 /Ar stream is introduced to the reaction cell, the bands corresponding to the absorbed NH3 species diminish quickly. Thereby, it is reasonable to deduce that adsorption of NH3 on the Lewis acid sites predominates the reaction between NOx and coordinated NH3 . Owing to the ␲-bonds in the graphite crystallite and the oxygen functional groups on the ASC support, NO can be also adsorbed onto the V2 O5 /ASC catalyst. NO is commonly oxidized to NO2 , N2 O2 2− and NO3 − . Afterwards, with the admission of NH3 at the reaction temperature (200 ◦ C), bands assigned to the gaseous or weakly adsorbed NO and NO2 molecules decrease, while the nitrate species keep stable. In addition, the co-existence of NH3 and nitrate adspecies (Fig. 8(b)) evidences that NH3 and NOx can be adsorbed over different active sites on the surface of the as-prepared composite catalyst. These results indicate that it is NO2 and NO rather than nitrate species that participate in the SCR reaction. Combining the results obtained in this study and the literature [23–25,30–35], the SCR of NO with NH3 over the V2 O5 /ASC catalyst most probably takes place according to two pathways and the simplified reactions are illustrated as following: (1) Reactions between the coordinated NH3 and gaseous NO over the Lewis acid sites (E–R mechanism), where the ASC mainly serves as the support: Lewis acid sites

NH3 (g)

−→

NH3 (a)

NH3 (a) + O(a) → NH2 (a) + OH(a)

(1) (2)

(6)

NO2 (a) + 2NH3 (a) → NO2 (NH3 )2

O2 + 4NO + 4NH3 → 4N2 + 6H2 O 3.4. Mechanism discussion

(5)

(9) (10)

It is found that in the “standard SCR” process, O2 and NO can take part in the reaction directly, whereas in the “fast SCR”, NO must be oxidized into NO2 in the first place. Nevertheless, the “fast SCR reaction” exhibits a reaction rate at least 10 times higher than that of the well-known “standard SCR” pathway [74]. As far as this study is concerned, the ASC is not only a support, but also provides active sites to accelerate the oxidation of NO (Reaction (5)) in the “fast SCR”. Thus, the proceeding of “fast SCR” on the V2 O5 /ASC plays a significant role in promoting the whole SCR performance, giving rise to a high catalytic activity. In this case, capacities of the ASC on adsorbing and oxidizing of NO to NO2 should be a key factor to determine the performance of V2 O5 /ASC catalyst in low-temperature SCR. 4. Conclusion In this work, nanostructured V2 O5 is loaded onto ASC and exhibits a good SCR catalytic performance at low temperature with a loading amount of 3 wt.%. The prepared V2 O5 /ASC catalyst can achieve a NO conversion rate of over 90% with excellent N2 selectivity at the reaction condition of 250 ◦ C and 12,000 h−1 . The ASC support shows high ability for oxidation of NO, but the V2 O5 catalyst mainly works on adsorption and oxidation of NH3 . Both the Lewis and Brønsted acid sites exist on the surface of V2 O5 /ASC composite catalyst. At the fixed reaction temperature of 200 ◦ C, the coordinated NH3 species adsorbed on the Lewis acid sites predominate and the NH3 -SCR process takes place according to two pathways, including the reaction of coordinated ammonia with gaseous NO, in which the ASC serves as the support (E–R mechanism), and the reaction of absorbed NO2 with coordinated ammonia (L–H mechanism), in which the ASC provides active sites. The latter pathway can be effectively promoted by increasing oxidation property of the ASC, which consequently improves the overall catalytic performance of the V2 O5 /ASC at low temperature. Acknowledgments The authors gratefully acknowledge financial support by the Common Development Fund of Beijing and the National Natural Science Foundation of China (51272005, 51172001 and 51172003). Supports by the National High Technology Research and Development Program of China (863 Program, 2012AA06A114) and Key Projects in the National Science & Technology Pillar Program (2011BAB02B05) are also acknowledged.

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