DOI:
10.1039/C5RA25736K
(Paper)
RSC Adv., 2016,
6, 6300-6307
Mechanistic study of selective catalytic reduction of NO with NH3 over highly dispersed Fe2O3 loaded on Fe-ZSM-5
Received
3rd December 2015
, Accepted 5th January 2016
First published on 8th January 2016
Abstract
ZSM-5 supported highly dispersed FexOy clusters were prepared by a sol–gel method for selective catalytic reduction (SCR) of NO with NH3. XRD, SEM, UV-vis, H2-temperature-programmed reduction (H2-TPR), NH3-temperature-programmed desorption (NH3-TPD), and BET analyses all indicated that Fe species mainly existed as highly dispersed surface FexOy clusters with a Fe3+ concentration of 22 wt%. NO-temperature-programmed oxidation (NO-TPO) revealed that the FexOy clusters promoted the oxidation of NO to NO2, which promoted the low temperature NOX removal. NH3 was activated above 250 °C and over-oxidation of NH3 to NOX was not observed, as a result, a NOX removal efficiency of 91% was achieved at 400 °C. Moreover, the SCR reaction route was found to be temperature dependent, below 200 °C, the NOX reduction followed the reaction between NO2 and non-activated NH3. Fast SCR reaction dominated the NOX removal in the temperature window of 200–325 °C. At temperatures above 250 °C, the normal reaction between activated NH3 and NO compensated the thermodynamic limitation induced suppression of fast SCR.
1. Introduction
Many industrial processes use nitrogen oxide (NOX, mainly NO and NO2) containing reactants or produce NOX as by-products, especially in exhaust gases, which remain a major source for air pollution. In fact, NOX have given rise to a variety of increasingly harmful impacts on the environment, such as photochemical smog, acid rain, ozone depletion and greenhouse effects. Due to the increasing threat of NOX to the environment, a great many approaches have been developed to try to control its emission. Among them, selective catalytic reduction (SCR), using V2O5 as active catalyst and NH3 as reductant, has been proven to be the most effective technology and has been widely applied to reduce the NOX emission from power plants and waste incinerations.1–4 As a widely used catalyst, the detrimental of V2O5-based catalyst is its high toxicity, high activity for SO2 oxidation, formation of N2O at high temperature, which stimulated the continuing efforts to develop new catalysts.5–14 Recently, zeolite based catalysts have attracted much interest for the reduction of NO to N2 by NH3, because of their remarkable catalytic activity and nontoxic, lower activity for the oxidation of SO2 to SO3, and low N2O by-product.15–22 Among them, Fe-exchanged/loaded ZSM-5 showed high activities for the SCR reaction of NOX by NH3.9 It was reported that even under the condition of very low iron loading, a moderate activity could be achieved over Fe-ZSM-5.23 Especially heavy/over loaded Fe-ZSM-5 showed a high NO reduction activity and stability in SO2 and H2O, even higher than that of commercial V2O5-based catalysts.9,24,25 And the combination of the NH3 adsorption by zeolite and oxidation of NO to NO2 by Fe3+ was considered to be responsible for its high performance.26,27
From literature, the activity was found to be enhanced by increasing the amount of the Fe3+ exchanged/loaded on ZSM-5; however, the preparation of heavy exchange Fe-ZSM-5 with Fe concentration above 2% is still a challenge.9 In this study, by conventional aqueous ion-exchange technique using FeCl2 as reactant,28 we found that highly dispersed FexOy clusters with a Fe3+ concentration of 22 wt% was successfully loaded on Fe-ZSM-5, which further promoted de-NOX activity and broadened the reaction temperature window from 250 °C to 400 °C. Our results also suggest that the reaction route for SCR of NOX is reaction temperature dependent, at least over Fe-ZSM-5 supported highly dispersed FexOy clusters.
2. Experimental
2.1 Catalyst preparation
The starting zeolite was H-ZSM-5 (Nankai University, Si/Al ≈ 25, SBET = 425 m2 g−1). FeCl2 (99 wt%, Aldrich) was used as Fe precursors. Fe-ZSM-5 was prepared using a conventional aqueous ion-exchange technique.28 10 grams of H-ZSM-5 was added to 1 L of 0.05 M FeCl2 solution with constant stirring in air for 24 h, after stirring, the pH of the solution was measured to be 4, which facilitate the Fe ions exchange into ZSM-5 and FexOy dispersion.29 For normal Fe-ZSM-5, the mixture was filtered and thoroughly washed with deionized water and then dried at 90 °C for 10 h. For FexOy clusters heavy loaded zeolite (FeH-ZSM-5), the mixture was dried at 90 °C directly without washing. The obtained samples were then calcined at 500 °C for 6 h in air. For comparison, pure Fe2O3 was prepared by the same procedure without H-ZSM-5. Details of the surface element composition and structure properties of catalysts were summarized in Table 1.
Table 1 Fe loading, surface atom concentration and surface area of catalysts
Catalysts |
Fe contenta (wt%) |
Surface area BET (m2 g−1) |
Total pore volume (mL g−1) |
Pore diameter (nm) |
Surface atomic ratio was determined by EDS. |
H-ZSM-5 |
— |
425.06 |
0.2273 |
1.16237 |
Fe-ZSM-5 |
0.97 |
391.38 |
0.2154 |
1.13005 |
Fe2O3 |
77.54 |
112.13 |
— |
— |
FeH-ZSM-5 |
22.12 |
305.65 |
0.1542 |
1.0796 |
2.2 Catalyst characterization
X-ray diffraction (XRD) patterns were recorded on a Philips XD-98 X-ray diffractometer using Cu Kα radiation (k = 0.15406 nm). Brunauer–Emmett–Teller (BET) surface areas were studied using an ASAP2000 physical absorber. The morphology was characterized by scanning electron microscopy (SEM) (JEOL S-4800); energy dispersive spectrometry (EDS) was carried out in the same facility. X-ray photoelectron spectroscopic (XPS) data were obtained using a Thermo ESCALAB 250, the X-ray source was an Al Kα radiation, and all binding energies were referenced to the 284.8 eV C 1s.
The UV-vis diffuse reflection spectra were obtained for the dry-pressed disk samples using a Scan UV-vis spectrophotometer (UV-vis DRS: TU-1901, China) equipped with an integrating sphere assembly, and BaSO4 as reflectance sample. The spectra were recorded at room temperature in air ranged from 200 to 800 nm. All the samples were degassed at 200 °C prior to measurements.
H2-temperature-programmed reduction (H2-TPR) experiments were performed using 100 mg of each catalyst. The catalysts were preheated at 400 °C for 1 h in air. After they were cooled to 100 °C, the samples were heated up to 800 °C with a heating rate of 10 °C min−1 under a mixed flow of 10% H2 in helium and a flow rate of 30 mL min−1.
NH3-temperature-programmed desorption (NH3-TPD) experiments were performed using 100 mg of each catalyst to determine their NH3 adsorption. The sample was preheated in a N2 stream (30 mL min−1) at 400 °C for 1 h, and then cooled to 100 °C. The pretreated sample was then exposed to a mixed flow of 4% NH3 in argon at a flow rate of 20 mL min−1 for 3 h at 100 °C, and heated up to 800 °C at a heating rate of 10 °C min−1. The H2-TPR and NH3-TPD data were recorded using an on-line gas chromatograph equipped with a thermal conductivity detector.
NO-temperature-programmed desorption (NO-TPD) experiments were performed in a fixed-bed flow reactor. 1.5 g of each catalyst was pasted on three Al plates and inserted into the fixed-bed.30 The samples were exposed to a 800 ppm NO flow without O2 for 1 h at 400 °C and cooled to 100 °C in the same gas stream, then the samples were purged by a N2 until the NO signal returned to the baseline level. Finally, the temperature was ramped from 80 °C to 400 °C in N2 at a heating rate of 10 °C min−1. Similarly, NO2-TPD experiments were performance in 800 ppm NO2 diluted in N2 without O2. And the desorbed NO/NO2 concentrations depended on temperature were recorded using an NO–NO2-NOX analyzer (Testo AG testo 340).
NO-temperature-programmed oxidation (NO-TPO) experiments were performed in the same reactor from 100 °C to 400 °C under 800 ppm of NO, 5% O2 and a N2 balance at a total flow rate of 1000 sccm. NH3-temperature-programmed oxidation (NH3-TPO) experiments were also performed in the same fixed-bed flow reactor from under from 100 °C to 400 °C in 800 ppm NH3, 5% O2, and a N2 balance at a total flow rate of 1000 sccm. The inlet and outlet concentrations of NH3 were determined by titration with 0.001 M hydrochloric acid, and methyl red was used as the indicator.
2.3 Catalyst activity characterization
The catalytic activity tests were carried out in a fixed-bed quartz reactor with 1.5 g catalyst pasted on three Al plates (4 cm × 10 cm).30 The inlet and outlet concentrations of NO and NO2 were monitored using Testo AG testo 340. The simulated gas used for these tests contained 800 ppm NO, 800 ppm NH3 and 5 vol% O2 balanced by N2 at total flow rate of 1000 sccm and a GHSV of 100
000 h−1.
3. Results and discussion
3.1 Catalyst activity
Fig. 1 shows NOX conversion as a function of temperature over different catalysts. The H-ZSM-5 catalyst presented the lowest activity for NOX reduction in the investigated temperature window. Fe2O3 started to catalyze NOX reduction at 200 °C, and the conversion increased with temperature up to 62% at 350 °C, and then fell to 38% at 400 °C. For Fe-ZSM-5, the reduction of NOX started at 250 °C and then increased quickly and reached to 77% at 400 °C.23 The best performance was achieved over FeH-ZSM-5, which showed high performance even at temperature as low as 250 °C and a NOX conversion of 91% was achieved at 400 °C. Moreover, the activity loss at high temperature above 350 °C over normal catalysts was not observed at 400 °C.
 |
| Fig. 1 (a) NO conversion over this series of catalysts. (b) Ea calculated using the Arrhenius plot according to eqn (2) over FeH-ZSM-5, the inset shows the k calculated according to eqn (1). The inset is the k calculated according to eqn (1). Reaction conditions: 800 ppm NO, 800 ppm NH3, and 5% O2 balanced by N2 at total flow rate of 1000 sccm and a GHSV of 100 000 h−1. | |
In order to further clarify the temperature dependency of NOX conversion, the kinetic parameters were calculated based on the assumption that the reaction is first-order dependent on NO and zero-order dependent on NH3, and the kinetic parameters were calculated from NO conversion over FeH-ZSM-5 as:31–33
|
 | (1) |
|
 | (2) |
where
k is the reaction rate coefficient (mL g
−1 s
−1),
V is the total gas flow rate (mL s
−1),
M is the catalyst weight (g),
x is the conversion of NO
X (%). The obtained values of
k were used in the Arrhenius plot and then the activation energies (
Ea) of the SCR reaction were derived.
As shown in Fig. 1b, the reaction can be divided into three stages, at temperature below 200 °C, the apparent Ea was calculated as low as 8.98 kJ mol−1; the highest Ea of 27.01 kJ mol−1 was calculated at temperature between 200 and 325 °C; at temperature above 325 °C, the Ea was calculated to be 10.78 kJ mol−1. This suggests that the reaction route for NO reduction is temperature dependent.
3.2 XRD, SEM and BET study
The crystallographic structures of the ZSM-5 support and Fe-ZSM-5 zeolite were studied by XRD as shown in Fig. 2. All of the XRD patterns showed three diffraction peaks at 2θ = 23–25°, matching well with the standard phase of ZSM-5 zeolite,34 indicating that the frame structure of ZSM-5 was not destroyed during Fe3+ ions exchanging and/or FexOy clusters loaded. The spectrum of H-ZSM-5 are same as that of Fe-ZSM-5, indicating Fe3+ occupied the H+ position.35 Pure Fe2O3 existed as α-Fe2O3 which was supported by the diffraction peaks at 33.1°, 35.6°, 40.9°, 49.5°, and 54.1°. While for FeH-ZSM-5, peaks attributed to Fe2O3 were not detected obviously, indicates a high dispersion of FexOy clusters on Fe-ZSM-5.36
 |
| Fig. 2 XRD patterns of the samples. | |
Fig. 3 shows typical SEM images of samples, Fe2O3 showed typical needle like morphology, suggesting the α-Fe2O3 structure,37 three zeolite containing samples showed very similar morphology, again indicating the fact that frame structure was remained during Fe3+ ions exchanging and/or FexOy clusters loaded. The needle like Fe2O3 phase was not observed by SEM observation in FeH-ZSM-5, further proved the high dispersion of FexOy clusters. This assumption was also supported by BET measurement as shown in Table 1, after Fe3+ was exchanged into zeolite to form Fe-ZSM-5, the surface area decreased from 425 to 391 m2 g−1, in agreement with other reports.38 The surface area of FeH-ZSM-5 reduced to 305.65 m2 g−1 due to the loading of FexOy clusters. From the pore size change caused by Fe3+ exchange and FexOy clusters loading (from 1.162 nm to 1.13 nm and 1.08 nm for Fe-ZSM-5 and FeH-ZSM-5), Fe3+ substituted the H+, at the same time FexOy clusters also loaded inside the micropores of zeolite, thus reduced the pore size of FeH-ZSM-5.
 |
| Fig. 3 Typical SEM images of (a) H-ZSM-5, (b) Fe-ZSM-5, (c) Fe2O3, and (d) FeH-ZSM-5. | |
3.3 UV-vis analysis
The UV-vis spectra were shown in Fig. 4. The UV-vis spectra of Fe-ZSM-5 and FeH-ZSM-5 zeolites presented in this work have been deconvoluted into subbands to facilitate the assignment of different Fe species, detailed information was shown in Table 2. For Fe-ZSM-5, two strong peaks located at 218 nm and 270 nm could be attributed to the isolated Fe3+ sites in tetrahedral and higher coordination (five or six oxygen ligands),39–41 the peaks located at 300–400 nm (assigned to oligomeric clusters) and >400 nm (assigned to large Fe2O3 clusters) were weak.39–41 This indicated that Fe species were mainly exchanged into zeolite to form Fe-ZSM-5. While for FeH-ZSM-5, besides the peaks attributed to isolated Fe3+, peaks for FexOy clusters were also strong, which indicated that Fe3+ species mainly loaded on the zeolite surface as clusters with good dispersion, in good agreement with our XRD measurements. In fact, the XRD peaks attributed to Fe2O3 were hardly detected. As shown in Table 2, semi-quantitative analysis revealed that more than 60% of the Fe3+ species were exchanged into zeolite in Fe-ZSM-5, and more than 70% of Fe3+ species existed as FexOy clusters in FeH-ZSM-5.
 |
| Fig. 4 UV-vis spectra of Fe-ZSM-5 and FeH-ZSM-5. | |
Table 2 The area of the subbands derived by deconvolution of the TPD spectra and the UV/VIS-DRS spectra (area1 at λ < 300 nm, area2 at 300 < λ < 400 nm, and area3 at λ > 400 nm)
Catalysts |
Amount of acid |
Existing form of Fe species |
Areawa |
Areasb |
Area1c |
Area2d |
Area3e |
Weak surface acid sites. Strong surface acid sites. Isolated Fe3+ in tetrahedral and higher coordination. Small oligomeric FexOy clusters. Large FexOy clusters. |
H-ZSM-5 |
43.96 |
55.89 |
— |
— |
— |
Fe-ZSM-5 |
27.14 |
29.95 |
73.6 |
16 |
24.6 |
Fe2O3 |
18.79 |
— |
— |
— |
— |
FeH-ZSM-5 |
31.16 |
15.90 |
60.1 |
37.9 |
110.3 |
3.4 XPS analysis
The XPS results were shown in Fig. 5, for Fe2O3, the binding energies of Fe2p3/2 and Fe2p1/2 in Fig. 5 are located around 711.2 and 725.5 eV, indicating that Fe is directly bonded to O.42 Both Fe2p3/2 and Fe2p1/2 peaks are accompanied by a small satellite on the high binding energy side, which is the characteristic of Fe3+ in Fe2O3, in consistent with XRD characterization.43 According to Jin's report,42 when the calcine temperature was >500 °C, Fe3+ became the main products in Fe-ZSM-5. Here the Fe related peaks in Fe-ZSM-5 is very weak, only a small difference can be observed as the inset shown, indicating a low Fe exchange in Fe-ZSM-5 after water washing. While for FeH-ZSM-5, Fe related peaks were evident and all the iron species existed as Fe3+.
 |
| Fig. 5 Fe2p XPS spectra of H-ZSM-5, Fe-ZSM-5, Fe2O3, and FeH-ZSM-5. | |
3.5 H2-TPR and NH3-TPD analysis
H2-TPR experiments were shown in Fig. 6. H-ZSM-5 had no obvious H2 consumption peaks during the whole temperature range, indicating that the ZSM-5 zeolite support shows no oxidation ability. Fe2O3 showed an obvious reduction peak around 370 °C and another large broad peak around 660 °C, which are attributed to the reduction from Fe2O3 to Fe3O4, and the overlap of Fe3O4 to FeO and Fe, respectively.44 Due to the low Fe loading in Fe-ZSM-5, the Fe related reduction peaks could hardly detect, with very weak peak around 350 °C. In the case of FeH-ZSM-5, the peak attributed to Fe2O3 to Fe3O4 reduction disappeared, and a new peak located between 400 and 500 °C was detected, moreover, the peaks attributed to Fe3O4 to FeO and FeO to Fe was separated. Similar phenomena was also reported by Heinrich and Chen,45,46 normally, the reduction peak is expected to shift to the low temperature side due to the high dispersion of FexOy clusters, however, the delayed water removal from the surrounding microporous network structure induced the peak to shifted to the high temperature side.45,46 Also, the existence of interaction between highly dispersed FexOy clusters and zeolite could further shift the peak to higher temperature.47
 |
| Fig. 6 H2 temperature programmed reduction profiles of catalysts. | |
The NH3-TPD results are shown in Fig. 7. All the ZSM-5 profiles display two peaks centered at about 180 °C and 340 °C, ascribed to weak and strong surface acid sites.48,49 While for Fe2O3, only a strong peak around 220 °C was observed, no strong acid site in Fe2O3 indicating that it did not absorb NH3 at temperature above 300 °C. The area of the subbands of the TPD spectra were calculated and shown in Table 2. The NH3 desorption peak at 180 °C and 340 °C over Fe-ZSM-5 and FeH-ZSM-5 was less than that over H-ZSM-5, which suggested that part of the Fe3+ occupied H+ site and FexOy clusters further covered parts of surface H+ in zeolite (the surface H+ is considered to be the origin of the strong acid sites50,51), thus reduced the NH3 adsorption capacity of FeH-ZSM-5 even less than Fe-ZSM-5, in agreement with our TPR and BET analyses.
 |
| Fig. 7 NH3 temperature programmed desorption profiles of catalysts. | |
3.6 NO to NO2 oxidation and NOX TPD
According to literature,13,52 the existence of NO2 can accelerate the SCR reaction. Therefore, the NO to NO2 oxidation as a function of reaction temperature was studied and shown in Fig. 8. Both H-ZSM-5 and Fe2O3 showed very low activity for NO to NO2 oxidation. NOX conversion kept below 25% through the whole temperature range, while the Fe-ZSM-5 showed relative high activity and NO oxidation reached the maximum of 53% at 325 °C. Then the NO oxidation efficiency decreased with further increasing the temperature due to the thermodynamic limits.52 These results suggest that Fe-ZSM-5 has better catalytic activity to promote NO oxidation ability than Fe2O3, and thus accelerate the SCR reaction remarkable. The performance of FeH-ZSM-5 is very similar to that of Fe-ZSM-5, but it showed a higher activity at low temperature side, which could be attributed to the highly dispersed FexOy clusters in FeH-ZSM-5.
 |
| Fig. 8 Oxidation of NO to NO2 by O2 over the catalysts. Reaction conditions: [NO] = 800 ppm, [O2] = 5%, balanced by N2 at total flow rate of 1000 sccm and a GHSV of 100 000 h−1. | |
According to Fig. 8, the apparent Ea for NO oxidation over FeH-ZSM-5 and Fe-ZSM-5 was calculated to be 13.35 and 24.98 kJ mol−1, respectively. Note that the apparent Ea of NO oxidation over FeH-ZSM-5 was close to the Ea of NOX reduction at temperature below 200 °C (8.98 kJ mol−1), which suggests that the NO oxidation played an important role in the rate-determining step during NO reduction below 200 °C.
Adsorption of NOX species on the catalyst surface was reported to play an important role in NOX reduction.53 Fig. 9 shows the NOX desorption profiles of this series of catalysts. Fe2O3 is good adsorbent for both NO and NO2, and H-ZSM-5 is not a good adsorbent for either NO or NO2. Fe-ZSM-5 only adsorbed NO2, moreover, FeH-ZSM-5 was also found to adsorb NO2 only, which strongly suggested the existence of interaction between highly dispersed FexOy clusters and zeolite supporter.
 |
| Fig. 9 NO-TPD and NO2-TPD profiles of the catalysts. | |
3.7 NH3 oxidation analysis
In SCR reaction, ammonia activation is considered as one of the most important parameters to promote the NOX conversion, especially over traditional V2O5-based catalysts.54 Fig. 10 shows the NH3 oxidation over this series of catalysts. NH3 conversion over all the catalysts kept at almost 0% below 225 °C. Then NH3 conversion increased with temperature. H-ZSM-5 showed low activity for NH3 oxidation even at high temperature. The Fe-ZSM-5 catalyst only showed a limited activity promotion compared to H-ZSM-5 due to the low Fe3+ loading. Interestingly, the NH3 oxidation over FeH-ZSM-5 was found to be higher than that over pure Fe2O3 and reached 56% at 400 °C, indicating that highly dispersed FexOy clusters promoted the NH3 activation, since the FeH-ZSM-5 performed better than Fe-ZSM-5 which has the same amount of Fe exchanged in ZSM-5, and even better than pure Fe2O3. Furthermore, during NH3 oxidation, NO or NO2 species was not detected over all catalysts, indicating no NH3 over oxidation, which should be beneficial for SCR NOX removal at high temperature.
 |
| Fig. 10 NH3 oxidized by O2 over the catalysts. Reaction conditions: [NH3] = 800 ppm, [O2] = 5%, balanced by N2 at total flow rate of 1000 sccm and a GHSV of 100 000 h−1. | |
3.8 Discussions
Fe-ZSM-5 has been widely studied as catalyst for the ammonia SCR reaction. The Fe species in Fe-ZSM-5 were partly exchanged into zeolite, and partly loaded on the zeolite surface as clusters.39–41 On the one hand, Fe in Fe-ZSM-5 was reported to promote the oxidation of NO to NO2. On the other hand, the loaded Fe ions replaced the H+, which caused the slightly decrease of NH3 adsorption.26,55 Fortunately, the reaction between NO and O2, but not the NH3 adsorption, was considered as the rate-determining step.56 As a result, Fe-ZSM-5 promoted the NOX reduction. This is supported by another report which found that the SCR activity of Fe-ZSM-5 is tremendously improved by the addition of NO2 to the feed gas, especially at low reaction temperature.52 Further study suggested that the role of NO2 in the fast SCR reaction was to form surface nitrites and nitrates, while the role of NO is to reduce nitrates to nitrites, and then nitrates further reacted with NO to produce more nitrides, finally nitrites were removed from the surface by NH3 to produce N2, the reduction of nitrates by NO is the rate-limiting step in this fast SCR at 200 °C.57 While at high temperature above 300 °C, the NOX conversion in SCR reaction is mainly governed by ammonia activation since the NO oxidation is suppressed due to the thermodynamic limitation. However the undesirable over oxidation of NH3 to NOX occurred easily at high temperature.54,58,59
In the present study, according to NOX TPD and NO oxidation results, H-ZSM-5 showed no ability for either NO to NO2 oxidation or NO + NO2 adsorption, so NOX conversion could not happen over H-ZSM-5 at low temperature. At temperature above 350 °C, H-ZSM-5 started to activate NH3 as shown in Fig. 7 and 10, and this promoted NOX reduction and a NOX conversion up to 15% was obtained. For, Fe-ZSM-5, it started to reduce NO at 150 °C and the reduction increased quickly at temperature above 250 °C, which is in good agreement with the NO oxidation and adsorption as shown in Fig. 8 and 9.13,52 Note that the Fe3+ concentration is low in Fe-ZSM-5, which indicated that the isolated Fe3+ sites in tetrahedral and higher coordination are excellent active species for NOX reduction. At temperature above 325 °C, the NO to NO2 oxidation were reduced due to the thermodynamic limit,52 and NO2 adsorption also reduced, however, the NOX reduction still increased, which could be attributed to the NH3 activation at high temperature as shown in Fig. 7 and 10.
For Fe2O3, the oxidation of NO to NO2 was poor, even poorer than that of H-ZSM-5, but it adsorbed NO and NO2 as shown in Fig. 9, and it also activated the NH3 above 250 °C, which enhanced the NOX reduction up to 62% at 350 °C, at temperature above 350 °C, NO and NO2 adsorptions were suppressed, and NH3 was also totally desorbed (Fig. 7), which finally decreased the NOX reduction.
The detrimental of the Fe-ZSM-5 is its low Fe3+ concentration (less than 2%)9, fortunately the surface loaded FexOy clusters are also active species, and thus in the case of FeH-ZSM-5, the catalyst showed 20% of NO oxidation even at 100 °C (Fig. 8), this promoted its activity and a NOX conversion of 20% was achieved at 100 °C. In fact, as shown in Fig. 4, more than 70% of Fe species existed as surface FexOy clusters. At temperature above 350 °C, though the NO2 adsorption and NO to NO2 oxidation was suppressed, but this was compensated by the promotion of NH3 activation benefited from FexOy clusters as shown in Fig. 10, which still promoted the NOX reduction, and a NOX conversion of 91% was achieved at 400 °C.
From the view of SCR reactions, NO2 is involved in the following reactions:
|
6NO2 + 8NH3 → 7N2 + 12H2O
| (3) |
|
2NO2 + 4NH3 + O2 → 3N2 + 6H2O
| (4) |
|
2NO + 2NO2 + 4NH3 → 4N2 + 6H2O
| (5) |
Among them, reaction (5) is the well-known fast SCR, while in reactions (3) and (4), NO does not react with NH3 directly, thus the Ea would be less than that of the reaction (5) in which NO was involved. Taken the catalyst FeH-ZSM-5 as an example, from Fig. 1a and 8, we can find that the NOX removal efficiency is almost the same as the NO to NO2 oxidation below 200 °C, which suggested that NOX removal mainly follows reactions (3) and/or (4). At temperature between 200 and 325 °C, the NOX removal efficiency is approximately twice as that of the NO to NO2 oxidation, indicating that reaction (5) becomes the main route. At temperature above 250 °C, NH3 activation become notable. Therefore, the SCR reaction can be divided into three stages here: In stage-1 (<200 °C), the main reaction is between NO2 and non-activated NH3; in stage-2 (200–325 °C), the main reaction is NO + NO2 and non-activated NH3; while in stage-3 (>300 °C), the reaction between NO + NO2 and activated NH3 becomes the main route, and the reaction between NO and activated NH3 also occurred considering from the difference between NO to NO2 oxidation and the NO removal efficiency. Accordingly, the lowest Ea was needed in stage-1, and the highest Ea was needed in stage-2, in good agreement with our Ea calculation in Fig. 1b.
4. Conclusions
In summary, highly dispersed FexOy clusters with a Fe3+ concentration of 22 wt% was successfully loaded on ZSM-5, the FexOy clusters played a key role for the promotion of de-NOX activity, and a good NOX removal efficiency of 91% was achieved at 400 °C. Our results discovered that the NO to NO2 oxidation is essential for NOX removal at temperature below 200 °C. At temperature between 200 °C and 325 °C, fast SCR reaction became the main route for NOX reduction. The NH3 activation at temperature above 300 °C promoted the reaction between NO and NH3, which compensated thermodynamic limitation induced suppression of fast SCR. And the suppression of NH3 to NO over oxidation promised a high NO reduction efficiency at 400 °C.
Acknowledgements
This work was supported by the Environmentally Sustainable Management of Medical Wastes in China (Contract No. C/V/S/10/251), and the National Natural Foundation of Zhejiang Province, China (Grant No. Z4080070).
References
- F. C. Meunier, J. P. Breen, V. Zuzaniuk, M. Olsson and J. R. H. Ross, J. Catal., 1999, 187, 493–505 CrossRef CAS.
- G. Busca, L. Lietti, G. Ramis and F. Berti, Appl. Catal., B, 1998, 18, 1–36 CrossRef CAS.
- G. Busca, M. A. Larrubia, L. Arrighi and G. Ramis, Catal. Today, 2005, 107–108, 139–148 CrossRef.
- G. Ramis, L. Yi and G. Busca, Catal. Today, 1996, 28, 373–380 CrossRef.
- Z. H. Lian, F. D. Liu and H. He, Catal. Sci. Technol., 2015, 5, 389–396 CAS.
- S. W. Choi, S. K. Choi and H. K. Bae, J. Air Waste Manage. Assoc., 2015, 65, 485–491 CAS.
- A. Yamamoto, Y. Mizuno, K. Teramura, S. Hosokawa, T. Shishido and T. Tanaka, Catal. Sci. Technol., 2015, 5, 556–561 Search PubMed.
- X. Y. Fan, F. M. Qiu, H. S. Yang, W. Tian, T. F. Hou and X. B. Zhang, Catal. Commun., 2011, 12, 1298–1301 CrossRef.
- R. Q. Long and R. T. Yang, J. Catal., 1999, 188, 332–339 CrossRef CAS.
- Z. X. Ma, H. S. Yang, F. Liu and X. B. Zhang, Appl. Catal., A, 2013, 467, 450–455 CrossRef.
- Z. X. Ma, H. S. Yang, Q. Li, J. W. Zheng and X. B. Zhang, Appl. Catal., A, 2012, 427–428, 43–48 CrossRef.
- Q. Li, X. X. Hou, H. S. Yang, Z. X. Ma, J. W. Zheng, F. Liu, X. B. Zhang and Z. Y. Yuan, J. Mol. Catal. A: Chem., 2012, 356, 121–127 CrossRef.
- Q. Li, H. S. Yang, F. M. Qiu and X. B. Zhang, J. Hazard. Mater., 2011, 192, 915–921 CrossRef PubMed.
- W. Tian, H. S. Yang, X. Y. Fan and X. B. Zhang, J. Hazard. Mater., 2011, 188, 105–109 CrossRef PubMed.
- R. Q. Long, R. T. Yang and R. Chang, Chem. Commun., 2002, 5, 452–453 RSC.
- R. Q. Long and R. T. Yang, J. Catal., 2001, 201, 145–152 CrossRef.
- K. Krishna and M. Makkee, Catal. Today, 2006, 114, 23–30 CrossRef CAS.
- S. Brandenberger, O. Kröcher, A. Tissler and R. Althoff, Catal. Rev., 2008, 50, 492–531 CAS.
- P. S. Metkar, V. Balakotaiah and M. P. Harold, Catal. Today, 2012, 184, 115–128 CrossRef CAS.
- E. Kolobova, A. Pestryakov, A. Shemeryankina, Y. Kotolevich, O. Martynyuk, H. J. T. Vazquez and N. Bogdanchikova, Fuel, 2014, 138, 65–71 CrossRef CAS.
- A. Sultana, T. Nanba, M. Haneda and H. Hamada, Catal. Commun., 2009, 10, 1859–1863 CrossRef CAS.
- H. Sjövall, R. J. Blint and L. Olsson, Appl. Catal., B, 2009, 92, 138–153 CrossRef.
- A. Uddin, T. Komatsu and T. Yashima, J. Chem. Soc., Faraday Trans., 1995, 91, 3275–3279 RSC.
- A. Z. Ma and W. Grünert, Chem. Commun., 1999, 71–72 RSC.
- H. Y. Chen, T. Voskoboinikov and W. M. H. Sachtler, J. Catal., 1998, 180, 171–183 CrossRef CAS.
- R. Q. Long and R. T. Yang, J. Catal., 2002, 207, 224–231 CrossRef CAS.
- V. V. Lissianski, P. M. Maly and V. M. Zamansky, Ind. Eng. Chem. Res., 2001, 40, 3287–3293 CrossRef CAS.
- G. Qi and R. T. Yang, Catal. Lett., 2005, 100, 243–246 CrossRef CAS.
- J. A. Z. Pieterse, S. Booneveld and R. W. van den Brink, Appl. Catal., B, 2004, 51, 215–228 CrossRef CAS.
- Q. Li, H. S. Yang, A. M. Nie, X. Y. Fan and X. B. Zhang, Catal. Lett., 2011, 141, 1237–1242 CrossRef CAS.
- J. H. Goo, M. F. Irfan, S. D. Kim and S. C. Hong, Chemosphere, 2007, 67, 718–723 CrossRef CAS PubMed.
- X. Y. Guo, C. Bartholomew, W. Hecker and L. L. Baxter, Appl. Catal., B, 2009, 92, 30–40 CrossRef CAS.
- D. S. Zhou, Z. Y. Ren, B. Li, Z. X. Ma, X. B. Zhang and H. S. Yang, RSC Adv., 2015, 40, 31708–31715 RSC.
- M. M. J. Treacy, J. B. Higgins and R. von Ballmoos, Zeolites, 1996, 16, 330–802 CrossRef.
- A. Cihanoglu, G. Gündüz and M. Dükkancı, Appl. Catal., B, 2015, 165, 687–699 CrossRef CAS.
- M. Rauscher, K. Kesore, R. Monnig, W. Schwieger, A. Tissler and T. Turek, Appl. Catal., A, 1999, 184, 249–256 CrossRef CAS.
- G. Huo, X. G. Lu, Y. Huang, W. Y. Li and G. Y. Liang, J. Electrochem. Soc., 2014, 161, 1144–1148 CrossRef.
- B. M. Reddy, K. N. Rao, G. K. Reddy, A. Khan and S. E. Park, J. Phys. Chem. C, 2007, 111, 18751–18758 CAS.
- S. Bordiga, R. Buzzoni, F. Geobaldo, C. Lamberti, E. Giamello, A. Zecchina, G. Leofanti, G. Petrini, G. Tozzola and G. Vlaic, J. Catal., 1996, 158, 486–501 CrossRef CAS.
- G. Lehmann, Z. Phys. Chem., 1970, 72, 279 CrossRef CAS.
- M. Santhosh Kumar, M. Schwidder, W. Grünert and A. Brückner, J. Catal., 2004, 227, 384–397 CrossRef.
- M. M. Jin, R. G. Yang, M. F. Zhao, G. Y. Li and C. W. Hu, Ind. Eng. Chem. Res., 2014, 53, 2932–2939 CrossRef CAS.
- J. A. Botas, J. A. Melero, F. Martınez and M. I. Pariente, Catal. Today, 2010, 149, 334–340 CrossRef CAS.
- F. D. Liu, H. He, C. B. Zhang, Z. C. Feng, L. R. Zheng, Y. N. Xie and T. D. Hu, Appl. Catal., B, 2010, 96, 408–420 CrossRef CAS.
- F. Heinrich, C. Schmidt, E. Löffler, M. Menzel and W. Grünert, J. Catal., 2002, 212, 157–172 CrossRef CAS.
- H. Y. Chen and W. M. H. Sachtler, Catal. Today, 1998, 42, 73–83 CrossRef CAS.
- H. L. Huang, Y. Lan, W. P. Shan, F. H. Qi, S. C. Xiong, Y. Liao, Y. W. Fu and S. J. Yang, Catal. Lett., 2014, 144, 578–584 CrossRef CAS.
- T. Miyamoto, N. Katada, J. H. Kim and M. Niwa, J. Phys. Chem. B, 1998, 102, 6738–6745 CrossRef CAS.
- Y. M. Ni, A. M. Sun, X. L. Wu, G. L. Hai, J. L. Hu, T. Li and G. X. Li, Microporous Mesoporous Mater., 2011, 143, 435–442 CrossRef CAS.
- Y. H. Seo, E. A. Prasetyanto, N. Z. Jiang, S. M. Oh and S. E. Park, Microporous Mesoporous Mater., 2010, 128, 108–114 CrossRef CAS.
- G. Qi and R. T. Yang, Appl. Catal., B, 2005, 60, 13–22 CrossRef CAS.
- M. Devadas, O. Kröcher, M. Elsener, A. Wokaun, N. Söger, M. Pfeifer, Y. Demel and L. Mussmann, Appl. Catal., B, 2006, 67, 187–196 CrossRef CAS.
- M. Koebel, G. Madia and M. Elsener, Catal. Today, 2002, 73, 239–247 CrossRef CAS.
- S. Roy, B. Viswanath, M. S. Hegde and G. Madras, J. Phys. Chem. C, 2008, 112, 6002–6012 CAS.
- R. Q. Long and R. T. Yang, J. Catal., 2001, 198, 20–28 CrossRef CAS.
- H. Y. Huang, R. Q. Long and R. T. Yang, Appl. Catal., A, 2002, 235, 241–251 CrossRef CAS.
- A. Grossale, I. Nova, E. Tronconi, D. Chatterjee and M. Weibel, J. Catal., 2008, 256, 312–322 CrossRef CAS.
- J. H. Kwak, R. Tonkyn, D. Tran, D. H. Mei, S. J. Cho, L. Kovarik, J. H. Lee, C. H. F. Peden and J. Szanyi, ACS Catal., 2012, 2, 1432–1440 CrossRef CAS.
- S. Suárez, S. M. Jung, P. Avila, P. Grange and J. Blanco, Catal. Today, 2002, 75, 331–338 CrossRef.
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