Low-temperature selective catalytic reduction of N2O by CO over Fe-ZSM-5 catalysts in the presence of O2

Low-temperature selective catalytic reduction of N2O by CO over Fe-ZSM-5 catalysts in the presence of O2

Journal of Hazardous Materials 383 (2020) 121117 Contents lists available at ScienceDirect Journal of Hazardous Materials journal homepage: www.else...

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Journal of Hazardous Materials 383 (2020) 121117

Contents lists available at ScienceDirect

Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

Low-temperature selective catalytic reduction of N2O by CO over Fe-ZSM-5 catalysts in the presence of O2 ⁎

T



Yanchen Youa, Siyu Chena, Jiayin Lia, Jie Zenga, Huazhen Changa, , Lei Mab, , Junhua Lic a

School of Environment and Natural Resources, Renmin University of China, Beijing 100872, China Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, USA c State Key Joint Laboratory of Environment Simulation and Pollution Control (SKLESPC), School of Environment, Tsinghua University, Beijing 100084, China b

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Editor: Navid B. Saleh

Nitrous oxide (N2O) is an important ozone-depletion substance and greenhouse gas. Selective catalytic reduction (SCR) of N2O by CO is considered an effective method for N2O elimination. However, O2 exhibited a significant inhibition effect on the catalytic performance of N2O reduction by CO. A series of iron-based catalysts were prepared to investigate the effect of O2 in SCR of N2O by CO. The Fe-Z-pH2 (Fe-ZSM-5 ion-exchanged under pH of 2) catalyst manifested superior activity at low temperature and excellent O2 resistance in N2O reduction process. The characterization results from UV–vis DR spectra and XPS indicated that α-sites are the main active sites in Fe-Z-pH2, and they were inert to O2 but highly active to N2O. It could be concluded that the competition effect between N2O and O2 was very important over different catalysts. O2 is more prevalent over α-Fe2O3 catalyst, while N2O dominates over Fe-Z-pH2 catalyst. Moreover, in the presence of O2, Fe-Z-pH2 exhibited better performance for N2O removal than non-noble mixed oxide catalysts, which might broaden the application of low-temperature SCR of N2O by CO.

Keywords: Selective catalytic reduction of N2O by CO Fe-ZSM-5 O2 resistance α-Sites

1. Introduction According to the United Nations Environment Programme (UNEP) report in 2013, nitrous oxide (N2O) has become the most important



ozone-depletion substance (Bouwman et al., 2013). Moreover, N2O is supposed as the third most important global greenhouse gases after CO2 and methane, with a Global Warming Potential (GWP) of 310 (Erisman et al., 2011; Li et al., 2014a). Thus, it is critical to eliminate N2O

Corresponding authors. E-mail addresses: [email protected] (H. Chang), [email protected] (L. Ma).

https://doi.org/10.1016/j.jhazmat.2019.121117 Received 15 January 2019; Received in revised form 27 August 2019; Accepted 28 August 2019 Available online 29 August 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

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University Catalyst Co., Ltd.) was treated with 0.2 mol/L NaOH solutions at 80 °C for 30 min (solution/zeolite = 75 ml/g) to utilize the fullexchange capacity of MFI zeolite (Cabrera et al., 2006). Then, the alkali-treated ZSM-5 was exchanged twice with 0.5 mol/L NH4NO3 at 80 °C for 2 h to obtain the NH4+ form (solution/zeolite = 50 ml/g). The NH4-ZSM-5 was finally exchanged with 0.005 mol/L Fe(NO3)3·9H2O solutions under pH value of 2 or 4 for 24 h (solution/zeolite = 100 ml/ g), followed by washing, filtration, drying at 110 °C and calcination at 550 °C. The obtained two Fe-ZSM-5 catalysts under different pH were abbreviated as Fe-Z-pH2 and Fe-Z-pH4. The α-Fe2O3 catalyst was prepared by precipitation method as reference samples. 1 mol/L NH3·H2O (AR, Xilong Scientific Co., Ltd. China) was used as a precipitant of 0.05 mol/L Fe(NO3)3·9H2O (AR, Xilong Scientific Co., Ltd. China) solution to adjust the pH value to 11. Then the mixture was stirred for 4 h at 80 °C, followed by washing, filtration, drying at 110 °C and calcination at 550 °C. Besides, CeCo catalyst (Ce/Co molar ratio of 1:9) was prepared by co-precipitation method following previous literature, in order to further demonstrate the inhibition effects of O2 over metal-oxide catalysts (You et al., 2018).

emission to preserve the ozone layer and protect the climate. The N2O abatement in adipic acid plants has been successfully commercialized due to the Clean Development Mechanism (CDM) projects (Schneider et al., 2010; Kollmuss and Lazarus, 2010). Another high-concentration industrial sources, i.e., nitric acid plants with 300–3000 ppm N2O in the exhaust gas, has also received increasing attention nowadays (Ramırez et al., 2003). The terminal control technologies for N2O removal mainly consisted of thermal decomposition, direct catalytic decomposition, non-selective catalytic reduction (NSCR) and selective catalytic reduction (SCR) (Ramırez et al., 2003; Konsolakis, 2015; Galle et al., 2001; Centi and Vazzana, 1999). Among them, the direct catalytic decomposition of N2O is the most effective technology due to its superiority of simplicity and energy efficiency (Konsolakis, 2015; Kapteijn et al., 1996). Cobalt-based catalysts exhibited the best catalytic performance for the direct catalytic decomposition among reported non-noble metal catalysts (Liu et al., 2016; Xue et al., 2007). However, the reaction temperature was still too high due to the limitation of high activation energy and lots of energy for preheating exhaust gas would be necessary. It was reported that the addition of a reducing agent could promote the N2O removal efficiency significantly, such as carbon monoxide (CO), which usually leads to a decrease of T50 (temperature of 50% catalytic conversion) over 100 °C (Ramırez et al., 2004; Boissel et al., 2006). Hence, the addition of CO was beneficial for removal of N2O at low temperature, promoting the application of CO-SCR of N2O: Catalyst

CO(g) + N2 O(g) ⎯⎯⎯⎯⎯⎯⎯⎯→CO2 (g) + N2 (g)

2.2. Catalytic activity measurement The steady-state direct catalytic decomposition and CO-SCR of N2O were carried out in a fixed-bed quartz reactor. At specific reaction temperature, the steady-state catalytic reaction was maintained over 30 min and the average concentration of outlet N2O was chosen for the calculation of N2O conversion. The reaction conditions maintained 1000 ppm N2O, 1000 ppm CO (when used), 3% O2 (when used), N2 balanced, 0.3 g catalysts. The concentration of N2O and CO were measured by an IR gas analyzer (Gasmet Dx-4000). The catalytic performance in catalytic decomposition and CO-SCR of N2O was characterized by N2O conversion. Besides, the CO oxidation experiments were also conducted in the same reactor. The catalytic performance in catalytic oxidation of CO was characterized by CO conversion.

(1)

However, SCR of N2O could be severely inhibited by molecular oxygen (O2) over metal-oxide catalysts (Pacultová et al., 2008; Konsolakis et al., 2012). Pacultova et al. (Pacultová et al., 2008) reported that the addition of 20% O2 severely inhibited the CO-SCR of N2O over Co–Mn–Al spinel, leading to the increase of T50 to approximately 150 °C. Konsolakis et al. (2012) also found Pt/Al–CeLa monolith was significantly inhibited by 2% O2 in the CO-SCR of N2O. Although the addition of CO remarkably promoted N2O removal efficiency without O2, the catalytic performance would be even worse than that of direct catalytic decomposition after the introduction of both O2 and CO. The inhibition effects of O2 in CO-SCR of N2O has not been studied intensively so far. In addition, it must be emphasized that O2 is prevalent in different N2O emission sources, for instance, there is 2–4% O2 existing in the exhaust of nitric acid plants (Konsolakis, 2015). Hence, the inhibition effects of O2 became the severe problem that limited the application of CO-SCR of N2O. It was reported that the α-sites (binuclear FeⅡ complexes) in Fezeolites are inert to molecular oxygen but active to N2O (Pirutko et al., 2009; Dubkov et al., 2002). The oxidation of benzene to phenol by Fezeolites was the most classic example, in which N2O was used as oxidant rather than O2. It was attributed to the unique property of αoxygen from the dissociation of N2O at α-sites, but the dissociation of O2 at α-sites was very difficult (Cabrera et al., 2006). In addition, Fezeolites were also rather active for the direct catalytic decomposition of N2O. Therefore, Fe-zeolites may be potential catalysts for the CO-SCR of N2O. In this work, three iron-based catalysts, one α-Fe2O3 and two FeZSM-5, with different iron species were synthesized. And we also demonstrated and rationalized the different O2 resistance of three ironbased catalysts in CO-SCR of N2O through this work. The inhibition effects of O2 on iron oxide and O2 resistance of Fe-zeolites were also investigated with different characterization methods, such as UV–vis DR spectra, XPS, H2-TPR, and CO oxidation.

2.3. Catalyst characterization The powder X-ray diffraction (XRD) was measured between 10 and 90° at a step of 10°/min with an X-ray diffractometer (Rigaku, D/max2200/PC). In addition, the short-range XRD pattern between 30 and 38° was recorded at a step of 1°/min. The BET specific surface area was measured by N2 physisorption at 77 K with a specific surface area analyzer (Quantachrome Nova, QuadraSorb Station 3). X-ray photoelectron spectroscopy (XPS) was conducted on an electron spectrometer (VG Scientific, ESCALab220i-XL) and the XPS spectra were calibrated by deposited carbon C1 s at 284.6 eV. Ultraviolet-visible diffuse reflection (UV–vis DR) spectroscopy was conducted on UV-2700 spectrophotometer from Shimadzu Corporation. High-resolution transmission electron microscopy (HRTEM) was performed on a JEM-2100 electron microscope. Temperature-programmed reduction of H2 (H2TPR) was recorded on a TPx chemisorption analyzer (Micromeritics, ChemiSorb 2720), the gas flow (10% H2/Ar) and the heating rate was kept at 30 mL/min and 10 °C/min, respectively. Temperature-programmed oxidation (TPO) of O2 or N2O was also performed on the chemisorption analyzer under a gas flow (10% O2/He or 10% N2O/He) of 30 mL/min. Before recording TPO data, the samples were pretreated by He at 400 °C for 30 min, reduced by 10% CO/He at 200 °C for 30 min and then cooled to room temperature. Diffuse reflectance infrared Fourier transform (DRIFT) spectra were obtained in an infrared spectrometer (Thermo Fisher Scientific, Nicolet iS50). The samples were pretreated in a N2 flow at 400 °C for 1 h before adsorption of N2O or CO at 100 °C.

2. Experimental 2.1. Catalyst synthesis The Fe-ZSM-5 catalysts were prepared by liquid ion exchange method. The parent H-ZSM-5 (Si/Al = 18, purchased from Nankai 2

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Fig. 1. N2O conversion of (a) direct catalytic decomposition and (b) CO-SCR with or without O2 over CeCo mixed oxides. Reaction conditions: 0.3 g catalysts, 1000 ppm N2O, 1000 ppm CO (when used), 3% O2, the total gas flow of 100 ml/min.

3. Results and discussion 3.1. Catalytic performance Our previous work showed that CeCo mixed oxides could perform well for N2O decomposition (You et al., 2018). The catalytic performance of CeCo mixed oxides was also investigated to examine the effects of O2 in the CO-SCR. As shown in Fig. 1, it was observed that the presence of CO could promote N2O removal efficiency on CeCo mixed oxides, which was also severely inhibited by O2. In the presence of O2, the N2O conversion of CO-SCR was even lower than the conversion of direct catalytic decomposition. The results further illustrated the inhibition effects of molecular oxygen over metal-oxide catalysts. The N2O conversion in direct catalytic decomposition with or without O2 was presented in Fig. 2. The N2O conversion of CO-SCR with or without O2 over different catalysts were also shown in Fig. 3. It should be noted that there was hardly NOx detected in the N2O decomposition or reduction, the N2 selectivity was nearly 100%. For the direct catalytic decomposition of N2O in the absence of O2, α-Fe2O3 was not active till 450 °C and only 37% N2O conversion was achieved at 550 °C. Fe-Z-pH2 showed the best catalytic performance and could completely remove N2O above 475 °C. But the temperature is still too high for the removal of N2O. In the presence of O2, the N2O decomposition activity over α-Fe2O3 and Fe-Z-pH4 were both inhibited. However, there was hardly any decline of catalytic activity over Fe-ZpH2. The addition of CO significantly promoted the N2O conversion for all three catalysts in the absence of O2. α-Fe2O3 became active below 300 °C and obtain over 90% N2O conversion above 375 °C. The T90 (temperature of 90% catalytic conversions) of Fe-Z-pH2 and Fe-Z-pH4 decreased below 300 °C. However, three catalysts exhibited different performance after the introduction of 3% O2. The catalytic performance of CO-SCR over α-Fe2O3 was severely inhibited. The catalytic performance of Fe-Z-pH4 was mildly inhibited, leading to a decline of T90 at approximately 50 °C. However, Fe-Z-pH2 was not affected by O2, exhibiting excellent O2 resistance. There was a similar trend in the direct catalyst decomposition. Generally, these three iron-based catalysts with O2 resistance was in the following sequence: Fe-Z-pH2 > Fe-ZpH4 > > α-Fe2O3. It suggested that Fe-ZSM-5 catalysts performed

Fig. 2. N2O conversion of direct catalytic decomposition with or without O2, (a) α-Fe2O3, (b) Fe-Z-pH4 and (c) Fe-Z-pH2. Reaction conditions: 0.3 g catalysts, 1000 ppm N2O, 3% O2, total gas flow of 100 ml/min.

much better O2 resistance than metal oxides catalysts for CO-SCR of N2O. Because CO-SCR of N2O catalyzed by mixed oxides catalysts would be severely inhibited by O2, Table 1 compared the N2O removal performance between Fe-Z-pH2 (CO-SCR of N2O in the presence of O2) and reported non-noble mixed oxides catalysts (direct catalytic decomposition of N2O in the presence or absence of O2) (You et al., 2018; Aneggi et al., 2006; Chang et al., 2004; Russo et al., 2007; Li et al., 2014b; Maniak et al., 2011; Grzybek et al., 2015). T50 of Fe-Z-pH2 for N2O removal was 260 °C, while the T50 of other mixed oxides were all higher than 300 °C. It was found that Fe-Z-pH2 could remove N2O at a lower temperature than the reported mixed oxides in the presence of O2. Only alkali-doped Co3O4 owned lower T50 for N2O removal under low space velocity and in the absence of O2. It indicated that Fe-Z-pH2 might be a promising candidate for application of low-temperature COSCR of N2O. It should be a promising N2O abatement technology at low temperature. It was recognized that iron species should be the active sites of αFe2O3, Fe-Z-pH4 and Fe-Z-pH2 catalysts, thus different methods were used to characterize the iron species on these three iron-based catalysts.

3.2. Characterization of iron species Because the formation of mononuclear [FeⅢO]+ in ZSM-5 is quite unlikely due to its strong tendency toward self-organization into binuclear oxygen-bridged FeⅢ species, [FeⅢ(μ-O)2FeⅢ]2+ (Li et al., 2013a, b). Hence, there are mainly three trivalent iron species: binuclear FeⅢ, oligonuclear FexOy, and Fe2O3 particles. The formation of the latter two iron species was related to the hydrolysis of Fe3+(H2O)6 (Cabrera et al., 2006). The XRD patterns of these three catalysts were 3

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Fig. 4. XRD patterns of α-Fe2O3, Fe-Z-pH4 and Fe-Z-pH2.

Fig. 3. N2O conversion of CO-SCR with or without O2, (a) α-Fe2O3, (b) Fe-ZpH4 and (c) Fe-Z-pH2. Reaction conditions: 0.3 g catalysts, 1000 ppm N2O, 1000 ppm CO, 3% O2, total gas flow of 100 ml/min.

shown in Fig. 4. The results presented standard diffraction patterns of α-Fe2O3, suggesting that the iron species in α-Fe2O3 are mainly comprised of Fe2O3 particles. Due to the strong hydrolysis of Fe3+(H2O)6 at pH value of 4, large Fe2O3 particles were likely to form in Fe-Z-pH4. But the Fe2O3 crystal phase could not be detected over both Fe-Z-pH4 and Fe-Z-pH2 through a short-range scanning with the slow rotating (scanning) rate, while only the diffraction peaks of MFI zeolite were detected. The results might indicate that the crystal structure of MFI zeolite was not affected by the alkali-treatment of NaOH solution. In addition, the BET specific surface area of Fe-Z-pH2 and Fe-Z-pH4 was 265.9 and 288.1 m2/g, respectively. It indicated that surface area was not an important factor of catalytic activity and O2 resistance. The energy of UV–vis light was corresponding to O→Fe(III) ligand to metal charge transfer (LMCT) absorptions (Moretti et al., 2014). Hence, UV–vis diffuse reflection spectroscopy was able to characterize trivalent iron species in Fe-ZSM-5. As shown in Fig. 5, the bands around 200˜300 nm, 300˜420 nm, and 420˜600 nm could be attributed to

Fig. 5. UV–vis DR spectroscopy of (a) Fe-Z-pH2 and (b) Fe-Z-pH4.

binuclear FeⅢ, oligonuclear FexOy, and Fe2O3 particles, respectively (Moretti et al., 2014; Wu et al., 2016). The UV–vis diffuse reflectance spectra (UV–vis DRS) of Fe-Z-pH4 and Fe-Z-pH2 exhibited obvious differences of trivalent iron species. With the peak deconvolutions of UV–vis spectra, the ratio of different trivalent iron species was calculated and shown in Table 2. It’s obvious that lower pH in the liquid ion

Table 1 Comparison of N2O removal performance between Fe-Z-pH2 and mixed oxides catalysts. Catalyst

T50 (°C)

Reaction conditions

O2 conditions

Ref

Fe-Z-pH2 Ce-Co-O Co3O4 Ce-Ni-O Co-Al-O MgCo2O4 spinel Ni-La-O K-Co3O4 Cs-Co3O4

260 320 400 304 350 470 360 240 110

0.3 g, 100 ml/min, 1000 ppm N2O 0.3 g, 100 ml/min, 1000 ppm N2O 0.2 g, 300 ml/min, 1000 ppm N2O 20000 h−1, 0.26% N2O 30000 h−1, 0.5% N2O 80000 h−1, 0.5% N2O 2400 h−1, 0.5% N2O 0.3 g, 30 ml/min, 5% N2O 0.3 g,30 ml/min, 5% N2O

3% O2 3% O2 5% O2 Without 10% O2 5% O2 Without Without Without

This work This work (You et al., 2018) (Aneggi et al., 2006) (Chang et al., 2004) (Russo et al., 2007) (Li et al., 2014b) (Maniak et al., 2011) (Grzybek et al., 2015)

4

O2

O2 O2 O2

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Table 2 The ratios of trivalent iron species derived from UV–vis spectra, the ratio of bivalent iron species, and lattice oxygen derived from XPS spectra. Sample

Fe-Z-pH2 Fe-Z-pH4 α-Fe2O3

The trivalent iron species ratios (%)

The surface elemental ratios (%)

FeⅢ

FexOy

Fe2O3 particles

Fe2+/(Fe2++Fe3+)

Oβ/(Oα+Oβ)

70.2 40.6 –

23.3 37.0 –

6.5 22.4 100.0

53.6 44.2 –

6.0 14.9 –

BET surface area (m2/g)

265.9 288.1 –

Fig. 6. H2-TPR profiles of Fe-Z-pH2 and Fe-Z-pH4.

exchange process resulted in less formation of oligonuclear FexOy and Fe2O3 particles. However, the inhibition of hydrolysis would lead to the formation of more binuclear FeⅢ if the pH value kept at 2 (Brandenberger et al., 2008). As shown in Fig. 6, H2-TPR tests were conducted to characterize the iron species in Fe-Z-pH4 and Fe-Z-pH2. Since FeⅡ in zeolite would not be reduced by hydrogen until 700 °C due to its high stabilization, the observed hydrogen consumption could be assigned to the reduction of trivalent iron species only. The intense peak at 400˜500 °C was observed over Fe-Z-pH2, which could be attributed to the reduction of binuclear FeⅢ in exchanged positions of ZSM-5. As for Fe-Z-pH4, two peaks were also observed low temperatures (below 400 °C) and high temperature (above 500 °C), respectively. The peak below 400 °C was attributed to the reduction of FeⅢ in Fe2O3 particles located in the channels of zeolite. These complicated reduction peaks above 500 °C were due to the hydrogen consumption by oligonuclear FexOy. The different iron species could result in different peak temperature location (Cabrera et al., 2006; Sazama et al., 2014). H2TPR profiles also indicated that the distribution of trivalent iron species in Fe-Z-pH4 and Fe-Z-pH2, which was consistent with the results of UV–vis DR spectra. The bivalent iron species in Fe-ZSM-5 were mainly binuclear oxygen-bridged [FeⅡ(μ-O)FeⅡ]2+ which could interconvert with [FeⅢ(μ-O)2FeⅢ]2+. Due to the stabilization of negative framework charge, the binuclear FeⅡ could still be present under O2-rich calcination environment. It was recognized as the most active sites for N2O dissociation and usually called as α-sites (Dubkov et al., 2002; Li et al., 2013a). As shown in Fig. 7(a), XPS was conducted to investigate the existence of bivalent iron species. The peak at around 710.7 and 724.0 eV was attributed to the Fe2+ 2p3/2 and Fe2+ 2p1/2, respectively. And the peak at approximately 712.6 and 726.0 eV could be assigned to the Fe3+ 2p3/2 and Fe3+ 2p1/2, separately. Besides, the wide peak at about 718 and 730 eV was due to the Fe 2p satellites (Janas et al., 2009). Through the deconvolution by XPS peak 4.1, the peak areas could reflect the existence of the binuclear FeⅡ species. As shown in Table 2, Fe-Z-pH2 had a large ratio of FeII species than Fe-Z-pH4. It might result from the less formation of oligonuclear FexOy and Fe2O3 particles. In addition, the O 1s XPS spectra in Fig. 7(b) exhibited a

Fig. 7. XPS spectra of Fe-Z-pH2 and Fe-Z-pH4, (a) Fe 2p, (b) O 1s.

significant difference of lattice oxygen (Oβ) between Fe-Z-pH2 and FeZ-pH4. The binding energy of oxygen in the framework of zeolites is similar to that of chemical adsorbed oxygen (Oα) (Zhang et al., 2018). But an obvious peak corresponding to the binding energy of lattice oxygen was observed over Fe-Z-pH4. Considering the iron species in FeZ-pH4, this peak was assigned to the lattice oxygen in Fe2O3 particles. Therefore, the higher ratio of lattice oxygen over Fe-Z-pH4 also indicated the formation of Fe2O3 particles. HRTEM was also conducted characterize Fe-zeolites before reaction. As shown in Fig. 8, no characteristic spacings of MFI zeolites could be observed, which indicated that the pore channels of Fe-ZSM-5 catalysts was destroyed by alkaline leaching. The HRTEM images of Fe-Z-pH2 5

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Fig. 8. HRTEM images of Fe-Z-pH2 and Fe-Z-pH4.

adsorption of N2O on both α-Fe2O3 and Fe-Z-pH2. Thus, it was considered that the dissociation of N2O only involved gaseous N2O, rather than adsorbed N2O. For the catalytic decomposition of N2O over αFe2O3, the bulk catalyst can donate an electron for the dissociation of N2O, forming gaseous N2 and adsorbed oxygen (Reaction 2). Then the active sites could be regenerated by the desorption of adsorbed oxygen toward molecular oxygen (Reaction 3) (Kapteijn et al., 1996). The activation energy for reaction 2 and 3 is 92.0 and 112.9 kJ/mol, respectively (Winter, 1970). Hence, the occurrence of both two steps was very hard and the desorption of adsorbed oxygen must be the rate-determining step (RDS).

were very clean and hardly any nanoparticles were detected. But there were many nanoparticles with size of 3˜10 nm in the HRTEM images of Fe-Z-pH4. These nanoparticles showed weak spacings and were concluded as Fe2O3 particles. In conclusion, the iron species in Fe-Z-pH2 were mainly binuclear [FeⅡ(μ-O)FeⅡ]2+ and [FeⅢ(μ-O)2FeⅢ]2+, while there were more Fe2O3 particles and oligonuclear FexOy in Fe-Z-pH4. In addition, α-Fe2O3 mainly consisted of Fe2O3 particles. Considering the existence of different iron species in these catalysts, the different resistance performance of O2 should be related with the reaction taking over different iron species.

[α-Fe2 O3] + N2 O(g) → [α-Fe2 O3]+ − O− + N2 (g)

(2)

3.3. The inhibition effects of O2 on iron oxide

[α-Fe2 O3]+ − O− → [α-Fe2 O3] + 0.5O2 (g)

(3)

The DRIFT spectra of α-Fe2O3 and Fe-Z-pH2 after N2O adsorption at 100 °C was shown in Fig. 9. After N2 purge, there was almost no

Figs. 2 and 3 illustrated that the N2O removal efficiency over αFe2O3 was greatly enhanced after the addition of CO. As a reductant,

Fig. 9. DRIFT spectra of α-Fe2O3 and Fe-Z-pH2 after N2O adsorption. 6

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Fig. 10. DRIFT spectra of α-Fe2O3 and Fe-Z-pH2 after CO adsorption.

oxidize CO. It was obvious that O2 is a better oxidant. It suggested the better competitivity of O2 than N2O for oxygen vacancies in α-Fe2O3. Furthermore, two TPO experiments were also performed to investigate the competitivity. Before temperature-programmed oxidation by N2O or O2, the α-Fe2O3 samples were pretreated in the atmosphere of CO to simulate reaction 4. As shown in Fig. 12, the peak temperature in N2OTPO and O2-TPO profiles was around 100 and 250 °C, respectively. The results indicated that O2 could oxidize CO-reduced α-Fe2O3 much easier than N2O. Since oxygen vacancies were usually considered as the main reductant species on CO-reduced α-Fe2O3, the TPO results might directly reveal the better competitivity of O2 than N2O. In summary, in the presence of O2, there was competition between N2O and O2 in the CO-SCR of N2O catalyzed by α-Fe2O3. But O2 was more competitive than N2O over α-Fe2O3. Hence, the addition of O2 would hinder the reduction of N2O, leading to the severe inhibition effects of CO-SCR.

CO could react with adsorbed oxygen. Consequently, the desorption of adsorbed oxygen (reaction 3) would be promoted and the activation energy for the desorption of adsorbed oxygen would significantly decrease. If the reaction path still started from reaction 2, reaction 2 would be the RDS. However, considering the high activation energy of reaction 2, the N2O removal efficiency could not be significantly promoted. Therefore, it was speculated that the reaction path has changed. The DRIFT spectra of α-Fe2O3 and Fe-Z-pH2 after CO adsorption at 100 °C was shown in Fig. 10. Only gaseous CO and weak-adsorbed CO were detected in the DRIFT spectra after CO adsorption. And the weakadsorbed CO was easily removed by N2 purge and no carbonate was detected. Hence, it was considered that there was also no reaction involving the absorbed CO. The oxidation of CO over α-Fe2O3 was usually recognized to follow Mars–van Krevelen (M-vK) mechanism (Zhao et al., 2015; Li et al., 2013c). Since α-Fe2O3 consisted of FeO6 octahedra, it is proposed the following reaction path. The reaction of CO with lattice oxygen in Fe2O9 (dimers of FeO6 octahedra in α-Fe2O3) generated oxygen vacancies (FeⅡ2 O8-△) followed by the regeneration of lattice oxygen through the oxidation of O2. The activation energy for reaction 4 is 39.8 kJ/mol (Reddy et al., 2004), much lower than that of N2O dissociation. The reaction between α-Fe2O3 and CO would be much easier than N2O dissociation on α-Fe2O3. Thus, reaction 4 would occur substantially after the addition of CO, leading to the formation of oxygen vacancies. It was reported that both O2 and N2O could supply oxygen for the regeneration of oxygen vacancies, as shown in reactions 5 and 6 (Pai et al., 2010; Hirabayashi and Ichihashi, 2014). Therefore, reactions 4–6 were supposed to be the reaction path of O2-involved COSCR over α-Fe2O3. Reactions 5 and 6 were a competitive reaction, where O2 and N2O were competitively adsorbed on the oxygen vacancies.

[Fe2III O9] + CO(g) → [Fe2II O8 -Δ] + CO2 (g)

(4)

[Fe2II O8 -Δ] + 0.5O2 (g) → [Fe2III O9]

(5)

[Fe2II O8 -Δ] + N2 O(g) → [Fe2III O9] + N2 (g)

(6)

3.4. The O2 resistance of Fe-ZSM-5 For the direct catalytic decomposition of N2O over Fe-ZSM-5, the most active sites are thought to be the binuclear [FeⅡ(μ-O)FeⅡ]2+ stabilized in ion-exchange sites, i.e., α-sites. Previous studies found that αsites are very active for the dissociation of N2O, forming α-oxygen with unique oxidation properties (Panov, 2000; Panov et al., 1998; Berrier et al., 2007). Considering the characterization results of iron species, the interconvertible binuclear [FeⅡ(μ-O)FeⅡ]2+ and [FeⅢ(μ-O)2FeⅢ]2+ were the main iron species in Fe-Z-pH2. Hence, the binuclear [FeⅡ(μ-O) FeⅡ]2+ could be regarded as the active sites of Fe-Z-pH2. Reactions 7 and 8 could be regarded as the reaction path of CO-SCR of N2O over FeZ-pH2 (Ramırez et al., 2003; You et al., 2018). The dissociation of N2O occurred at α-sites, leading to the formation of α-oxygen. In the presence of CO, α-oxygen could be easily removed with the formation of CO2.

Reactions 4 and 5 comprised of the catalytic cycle of CO oxidation by O2. Reactions 4 and 6 could be also thought as the catalytic cycle of CO oxidation by N2O. These two CO oxidation reactions started from reaction 4, which led to the formation of oxygen vacancies. Hence, the CO oxidation activity was able to reflect the competitivity of O2 and N2O. The CO oxidation with different oxidants was conducted. As shown in Fig. 11(a), the results indicated that both O2 and N2O could

[FeII − (μ-O)-Fe II] + N2 O(g) → [O−-Fe III − (μ-O)-Fe III] + N2 (g)

(7)

[O−-FeIII − (μ-O)-FeIII] + CO(g) → [FeII − (μ-O)-FeII] + CO2 (g)

(8)

Furthermore, reactions 9 and 10 could be regarded as the reaction path of CO oxidation by O2 (Li et al., 2013a; Starokon et al., 2015). The α-sites would be oxidized to binuclear [FeⅢ(μ-O)2FeⅢ]2+ by O2. Then, the α-sites could be regenerated through the reduction of binuclear [FeⅢ(μ-O)2FeⅢ]2+ by CO. In this catalytic cycle, the binuclear [FeⅡ(μO)FeⅡ]2+ and [FeⅢ(μ-O)2FeⅢ]2+ could interconvert. 7

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Fig. 11. Activity of CO oxidation by O2 or N2O, (a) α-Fe2O3, (b) Fe-Z-pH4 and (c) Fe-Z-pH2. Reaction conditions: 0.3 g catalysts, 1000 ppm CO, 3% N2O or 3% O2, total gas flow of 100 ml/min.

conversion also indicated the absolute predominance of N2O in the competition for α-sites. In summary, in the presence of O2, there was also competition between N2O and O2 in the CO-SCR of N2O catalyzed by Fe-Z-pH2. But N2O was dominant in the competition for α-sites. Hence, the addition of O2 would hardly inhibit the dissociation of N2O at α-sites, leading to the excellent O2 resistance of Fe-Z-pH2. In the O2-involved CO-SCR of N2O, there was competition between O2 and N2O. However, the dominant one is completely different over different iron species. As shown in Scheme 1, for α-Fe2O3, O2 exhibited better competitivity for the oxygen vacancies (FeⅡ2 O8-△) than N2O, so the introduction of O2 severely hindered CO-SCR of N2O. For Fe-Z-pH2, the α-sites were the main active sites and they might be inert to O2 but highly active to N2O, so FeZ-pH2 exhibited excellent O2 resistance. As for Fe-Z-pH4, in addition to binuclear iron species, the Fe2O3 particles and oligonuclear FexOy species also played important catalytic roles and these oxides would be inhibited by O2. Therefore, Fe-Z-pH4 was mildly inhibited by O2 but also exhibited good O2 resistance.

Fig. 12. TPO profiles of CO-reduced α-Fe2O3 with different oxidants.

[FeII − (μ-O)-FeII] + 0.5O2 (g) → [FeIII − (μ-O) 2-FeIII](g) [FeIII

− (μ-O) 2

-FeIII]

+ CO(g) →

[FeII



(μ-O)-FeII]

+ CO2 (g)

(9)

4. Conclusions

(10)

Fe-ZSM-5 owned excellent O2 resistance in CO-SCR of N2O, while αFe2O3 was severely inhibited in the presence of O2. It was found that there was competition for CO between N2O and O2 in CO-SCR of N2O. O2 was more competitive over α-Fe2O3 while N2O dominated over FeZSM-5 due to its binuclear α-sites, which are inert to O2 but highly active to N2O. The different competitivity for CO leads to the differences of O2 resistance. Moreover, Fe-Z-pH2 could remove N2O at a lower temperature than the reported mixed oxides in the presence of

Therefore, there was a competition for α-sites between O2 and N2O. But previous studies found that α-sites were inert to molecular oxygen, and highly active to N2O. Therefore, reaction 7 would become dominant. Similarly, reactions 7 and 8 made up of the catalytic cycle of CO oxidation by N2O, reactions 9 and 10 could be also regarded as the catalytic cycle of CO oxidation by O2. Fig. 11(c) showed that N2O was a much better oxidant than O2 over Fe-Z-pH2. The large differences of CO

Scheme 1. Proposed mechanism of CO-SCR of N2O in the presence of O2. 8

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O2. Hence, Fe-ZSM-5 was a very promising candidate for low-temperature CO-SCR of N2O. It might broaden the application of N2O abatement technology at low temperature.

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