Low temperature SCR reaction over Nano-Structured Fe-Mn Oxides: Characterization, performance, and kinetic study

Applied Surface Science 457 (2018) 1116–1125

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Full Length Article

Low temperature SCR reaction over Nano-Structured Fe-Mn Oxides: Characterization, performance, and kinetic study

T

⁎

Changai Zhanga,b, Tianhu Chena, Haibo Liua,b, , Dong Chena, Bin Xua, Chengsong Qinga a b

Lab for Nano-minerals and Mineral Materials, School of Resources & Environmental Engineering, Hefei University of Technology, Hefei 230009, China Institute of Atmospheric Environment & Pollution Control, School of Resources & Environmental Engineering, Hefei University of Technology, Hefei 230009, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Limonite Thermal activation Fe-Mn oxides Low temperature SCR Kinetic analysis

Fe-Mn oxides was prepared by thermal activation of Mn-rich limonite at different temperatures, and then developed as catalysts for low-temperature NH3-SCR. The obtained samples were characterized by XRD, XRF, TEM, XPS, BET, H2-TPR, NH3-TPD, NH3-SCR model reaction and in situ DRIFTS. The results indicated that Fe-Mn oxides catalyst derived from the thermal treatment at 300 °C (H300) exhibited an excellent NO conversion which was higher than 90% at a temperature range from 130 to 300 °C. The large surface area (95.9 m2/g), large amount of acid sites, suitable reduction behavior, and abundant surface hydroxyl positively responsed to the excellent performance of H300 for NH3-SCR. The results of in situ DRIFTS and kinetic study demonstrated that when the SCR reaction temperature reached to 100 °C, both the Eley–Rideal mechanism and the LangmuirHinshelwood mechanism might happen over H300. However, the Langmuir-Hinshelwood mechanism could be approximately neglected at higher reaction temperatures. This study favors the understanding of NH3-SCR mechanism of the prepared provides Fe-Mn oxides and explores new application field for Mn-rich limonite in low-temperature NH3-SCR.

1. Introduction Nitrogen oxides (NOx) from coal-fired power plants and automobiles have been major pollutant for air pollution [1–4]. They contribute to acid rain, photochemical smog, ozone depletion and greenhouse effect, which further injured the human health and the growth of plants significantly [5,6]. Selective catalytic reduction (SCR) of NO with NH3 is the major technology for the control of NOx for several decades [7–9]. V2O5−WO3/TiO2 as the SCR catalyst has been widely used to control the emission of NOx from automobile exhaust gas and industrial combustion of fossil fuels, and exhibits excellent catalytic performance between 300 and 400 °C [10–14]. The SCR unit is usually placed in the upstream of the desulfurizer and electrostatic precipitator in order to avoid reheating of the flue gas [15,16]. However, in order to conquer the negative influence of SO2 and dusts on the catalytic performance of NH3-SCR catalyst to extend its lifetime [16,17], the SCR units can be located downstream desulfurizer and electrostatic precipitator, which could improve the N2 selectivity at low temperature concurrently [16,18]. Therefore, a novel and cost-efficient NH3-SCR catalyst with excellent selective catalytic performance at low temperature (> 300 °C) should be developed.

In recent years, there has been great interest in the transition metal oxide catalysts like MnOx-CeO2, Fe2O3/TiO2, Mn-Fe/TiO2 and Ce-Cu/ TiO2 [6,7,12,18–20]. Previous researches have demonstrated that Fe2O3 and Fe containing mixed nanoparticle-oxides, especially Fe/Mn oxide, exhibit excellent SCR performance and high N2 selectivity at a wide temperature window of 100–300 °C [6,11,21–24]. The way of low temperature SCR reaction follows the Eley-Rideal mechanism (the reaction of gaseous NO with adsorbed NH3 to an activated transition state and decomposition to H2O and N2), and the Langmuir-Hinshelwood mechanism (the reaction of adsorbed NO and adsorbed NH3 on the adjacent sites to H2O and N2) [3,25]. As for Fe/Mn oxide catalysts, the main reaction mechanism depend on the reaction temperature and the type of Fe/Mn oxide spinel [26], focusing on the devotion of acid sites (Brönsted acid site and Lewis acid sites). It is reported that Fe2O3 can be obtained by thermally treated limonite [27]. Natural limonite widely spread on the earth and the crystal iron is usually substituted by manganese, aluminum, zinc, etc [28]. The heated products showed a topotactic relationship to the original mineral, which had high surface area and abundant micropores [26,27]. Furthermore, natural limonite is a low-cost mineral, and the heated products are eco-friendly and highly active mineral material.

⁎ Corresponding author at: Lab for Nano-minerals and Mineral Materials, School of Resources & Environmental Engineering, Hefei University of Technology, Hefei 230009, China. E-mail address: [email protected] (H. Liu).

https://doi.org/10.1016/j.apsusc.2018.07.019 Received 16 April 2018; Received in revised form 27 June 2018; Accepted 3 July 2018 Available online 05 July 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.

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Therefore, the Fe-Mn oxides with abundant micropores and high surface area are speculated to be obtained by the thermal decomposition of limonite. In this work, natural Mn-rich limonite was used to prepare Fe-Mn oxide catalysts by thermal treatment. The obtained samples were characterized by XRD, XRF, TEM, XPS, BET, H2-TPR and NH3-TPD, the SCR activity of the catalysts was investigated through the laboratoryscale catalytic system. Furthermore, the mechanism of the SCR reaction at low temperature was investigated using in situ DRIFTS study and kinetic analysis.

NH3, 3 vol% of O2, and balance of Ar, the gas flow rate of 300 mL min−1 was controlled by mass flow controllers (Sevenstar D08, Beijing). Catalytic activity testing was carried out at a temperature range of 100–300 °C with a gas hourly space velocity (GHSV) of 72,000 h−1. The temperature was regulated through temperature-programmed controller. The concentration of NO, NO2, NH3, and N2O were monitored by a Fournier transform IR (FT-IR) spectrometer. As the SCR reaction reached the steady state, the ratio of NO conversion and N2 selectivity were calculated in terms of the following equations:

2. Experimental

NOconversion (%) =

[NO]in −[NO]out × 100% [NO]in

N2 Oselecticity (%) =

2[N2 O]out × 100% [NH3 ]in −[NH3 ]out + [NOx ]in −[NOx ]out

2.1. Catalyst preparation Natural limonite was collected from Tongling city, Anhui province, China. The natural limonite was crushed and the particle size was controlled between 0.25 mm (60 mesh) and 0.38 mm (40 mesh) by sieving. The obtained limonite particles were annealed in air for 1 h at different temperatures (300, 400, 500, 600, 700, 800 °C). Thermal treatment was carried out in a tube furnace with a temperature-programmed controller, designing 10 °C min−1 in this work. To simplify the sample labeling, abbreviation was used. H300 means catalyst obtained by thermal treatment of natural limonite at 300 °C.

[NOx ] = [NO] + [NO2] 2.4. In situ DRIFTS study In situ DRIFT spectra were recorded on a Fourier transform infrared spectrometer (FTIR, Nicolet NEXUS 870) equipped with a smart collector and an MCT detector cooled by liquid N2. The fine catalyst was placed in a ceramic crucible and manually pressed. In the test, the catalyst was pretreated with high purified N2 at 120 °C for 30 min to remove the physisorbed water, then, the catalyst was exposed to NH3N2 mixture (500 ppm NH3, 30 mL min−1) or NO-O2-N2 mixture (500 ppm NO and 3.0% O2, 30 mL min−1) for 30 min to be saturated. After that, the catalyst was purged by high purified N2 (30 mL min−1) for 1 h again. And then, In situ DRIFTS were recorded at different time (1min, 3 min, 5 min, 10 min, 20 min, 30 min) after the N2 flow was switched to a flow of NO-O2 (500 ppm NO and 3.0% O2, 30 mL min−1) or 500 ppm NH3 (30 mL min−1).

2.2. Catalyst characterization X-ray diffraction (XRD) measurements were carried out on a Rigaku D/max diffractometer equipped with Cu Kα radiation (50 KV and 100 mA). The diffraction patterns were recorded between 15° and 70° with a step of 4° min−1. Chemical composition was measured on an X-ray fluorescence (XRF) spectrometer (Shimadzu XRF-1800) with Rh radiation. High resolution transmission electron microscope (HRTEM, JEM2100) was used to characterize the crystal, morphology and pore structure of the catalysts. The specific surface area, pore volume and pore size distribution were measured by N2 adsorption-desorption at liquid nitrogen temperature with a Quantachrome NOVA 3000e analyzer. The samples were first degassed at 110 °C for 24 h. X-ray photoelectron spectroscopy (XPS) data were obtained with ESCALAB250Xi electron spectrometer from Thermo Fisher Scientific using Al Kα radiation. The C1s line at 284.6 eV was taken as a reference for the binding energy calibration. NH3-temperature programmed desorption (NH3-TPD) experiments were carried out in a quartz reactor. About 200 mg catalyst was pretreated by high purified Ar (40 mL min−1) at 100 °C for 1 h. After that, the catalyst was saturated with NH3-Ar mixture (5000 ppm NH3, 40 mL min−1) at 50 °C for 1 h and subsequently flushed with high purified Ar (40 mL·min-1) at the same temperature for 1 h to remove the gaseous NH3. Finally, the catalyst was heated from 50 °C to 500 °C at a rate of 10 °C·min-1 in high purified Ar (40 mL min−1). Molecule from the outlet of the quartz reactor was monitored using a quadrupole mass spectrometer (Hiden QIC-20, m/z = 15).H2-temperature programmed reduction (H2-TPR) experiments were performed in a quartz reactor connected to a quadrupole mass spectrometer (Hiden QIC-20, m/z = 2), with H2-Ar mixture (5.0% H2 by volume, 50 mL min−1) as a reductant. Prior to the reduction, the catalyst (100 mg) was pretreated in high purified Ar at 100 °C for 1 h. After that, the H2-TPR started from 100 to 600 °C at a rate of 10 °C min−1.

3. Results and discussion 3.1. Characterization 3.1.1. XRD and XRF The XRD patterns of thermally treated limonite were displayed in Fig. 2. The reflections at 2θ = 21.3, 33.2, 35.6, 36.6° (PDF-JCPDS 722234) can be found and identified as goethite (α-FeOOH) with low crystallinity and quartz at 2θ = 21, 26.7° (PDF-JCPDS 85-794) can be found. These reflections of α-FeOOH completely disappeared and were replaced by hematite (α-Fe2O3) when the thermal treatment temperature was not less than 300 °C. The decomposition of α-FeOOH is responsible for the appearance of α-Fe2O3. The thermal decomposition of α-FeOOH forming nano-porous α-Fe2O3 was previously reported [26,27]. Furthermore, an increase in thermal treatment temperature significantly improved the crystallinity of α-Fe2O3. Particularly, the particle size calculated by Scherrer equation based on the reflection of (1 1 0) plane was determined to be 9.0, 12.2, 16.7, 20.1 nm for the H500, H600, H700 and H800 catalysts, respectively. On the other hand, the results of XRF indicate the limonite contained Fe2O3 63.10 wt%, MnO 12.88 wt%, SiO2 3.91 wt%, and loss on ignition wt 16.79%. The absence of Mn oxides in the XRD pattern should be contributed to the special existing formation [12]. 3.1.2. TEM TEM images of limonite and catalysts were displayed in Figs. 3(1) and 3(2). As seen in the TEM mapping of the raw limonite, the distribution of Mn element was partially overlapped with that of Fe element. The substitution of Mn for Fe in the crystal structure of α-FeOOH and the adsorption of α-FeOOH to Mn have been reported [26]. The overlap of both Mn and Fe elements was ascribed to the existence of

2.3. Catalytic test The SCR of NO by NH3 was carried out in a fixed-bed quartz tube reactor with an inner size of 6 mm, as shown in Fig. 1. The typical reactant gas composition was as follows: 1000 ppm of NO, 1000 ppm of 1117

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Fig. 1. Schematic diagram of SCR experiments.

Hematite

Goethite

712.8 eV should be attributed to the Fe3+ derived from the crystal surface, likely Fe-OH species [26]. It is worth noting an increase in thermal treatment temperature decreased the intensity, indicating an decrease of the amount of Fe3+ cation and Fe-OH. The decrease of surface area and further dehydroxylation were taken account for this phenomenon. Fig. 4b shows the XPS spectra over the spectral region of Mn2p, two main peaks of Mn 2p3/2 and Mn 2p1/2 can be observed. Furthermore, the Mn 2p3/2 spectra can be fitted with three components for each sample by performing peak-fitting deconvolution: 640.8641.3 eV, 642.1–642.3 eV and 643.7–644.2 eV, as shown in Fig. 5. These deconvoluted peaks corresponded to the Mn2+, Mn3+ and Mn4+, respectively [9,29–33], which indicated that Mn2+, Mn3+, and Mn4+ co-exist on the surface of these catalysts. As summarized in Table 1, the concentration of Mn2+ on the surface of catalysts decreased with the increase of thermal treatment temperature, and close to 0% at 800 °C. The concentration of Mn4+ increased from 25.0% to the maximum of 35.7% when the temperature increased to 600 °C, which was ascribed to the oxidation of Mn2+ and Mn3+, and then decreased to 17.6% after heating at 800 °C, because of the decomposition of MnO2 at higher temperature.

quartz H800 H700 H600 H500 H400 H300 raw

20

30

50

40

2

60

70

o

Fig. 2. XRD patterns of natural limonite and H300-H800.

Mn-substituted α-FeOOH or adsorbed Mn on the surface of α-FeOOH. Besides, amorphous Mn oxides should also be the other existing formation of Mn due to the incomplete overlap. Hence, it was concluded the limonite mainly contained iron oxides, manganese oxides and silicon oxides on the basis of the aforementioned results of XRD and XRF. To discuss the intuitionistic images of catalysts and the effect of thermal treatment temperature, TEM and HRTEM was utilized to characterize H300, H500 and H700, shown in Fig. 3(2). The spacing of crystal plane of (1 1 0), (1 1 3) were calculated and identified as the crystal plane of Fe2O3, The spacing of crystal plane of (2 2 2) was identified as the crystal plane of Mn2O3. Combining with the TEM images, FeOOH had transformed into Fe2O3 and the newly formed Fe2O3 kept the original shape of FeOOH, Mn oxides should be the main existing formation of Mn in those catalysts. On the other hand, much pore can be observed in the images of H300 and H500 in contrast to the extremely few pore in H700. The results of TEM shown the Nano-Structured Fe-Mn Oxides was obtained by thermal treatment of limonite.

3.1.4. NH3-TPD Surface acid property of catalysts can affect the adsorption and desorption of NH3 efficiently, which further results in different catalytic performance for NH3-SCR reaction [34]. To characterize the adsorption property of NH3 on the surface of the newly formed α-Fe2O3 catalysts, NH3TPD was performed and the curves were displayed in Fig. 6. It can be seen that catalysts exhibit three desorption peaks from 50 °C to 500 °C. It is well known that the temperature of desorption peak is related to the acid strength [35]. Therefore, two main desorption peak at 119–124 °C and 215–225 °C originated from the Bronsted acid sites adsorbed NH3 and the Lewis acid sites adsorbed NH3, respectively [20,30]. Besides, H600, H700 and H800 had a desorption peak over 310 °C, which was attributed to the desorption of NH3 from strong Brönsted acid sites. Combining with the results of XRD and XPS, the relatively stronger Brönsted acid sites in this work was ascribed to the Fe-OH derived from the incomplete decomposition of α-FeOOH. The calculated adsorbed NH3 amount decreased as an order of H300 (299.9 μmol·g−1) > H400 (275.9 μmol g−1) > H500 (263.9 μmol g−1) > H600 (229.1 μmol g−1) > H700 (216.1 μmol g−1) > H800 (204.6 μmol g−1). Particularly, H300 shown the maximum NH3 desorption amount implying the maximum adsorption amount.

3.1.3. XPS The surface information of various catalysts was illustrated by XPS technique, especially for the analysis of oxidation state of Mn element. XPS spectra of catalysts involving Fe 2P and Mn 2P are given in the Fig. 4, Fig. 5 and Table 1. As presented in Fig. 4, the binding energies located at approximately 709.8 and 711.1 eV are attributed to the Fe3+ cations in the crystal structure of α-Fe2O3. Besides, the binding energy centered at

3.1.5. H2-TPR H2-TPR was used to investigate the oxidation states of the newly formed α-Fe2O3 catalysts, as shown in Fig. 7. The profiles recorded from 1118

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Fig. 3(1). TEM and EDS-Mapping images of natural limonite.

NO below 200 °C. Under the same reaction temperature, the lower the thermal treatment temperature was, the higher the NO conversion was. In particular, the NO conversion of H300 and H400 reached 99% at 150 °C and remained under 93% below 300 °C, which was related to its large surface area and NH3 adsorption amount and suitable reduction behavior. Meanwhile, a small amount of N2O formed over H300-H800 in this reaction, N2O selectivity of NO reduction generally increased with the increase of reaction temperature over these catalysts (shown in Fig. 8b).

H300, H400, H500, H600 showed three reduction peaks. The peak at about 180 °C was attributed to the reduction of amorphous MnO2 on the surface of catalysts. The peak at about 289 °C was assigned to the reduction of Mn2O3 to Mn3O4, while the broad peaks at higher temperature were probably attributed to the reduction of Mn3O4 to MnO and Fe2O3 to FeO [22,29,36]. TPR profiles of H700 and H800 showed only one broad peak at about 464 °C which was related to the reduction of Mn3O4 and Fe2O3 [22]. It was obvious that the reduction peaks temperature of H300, H400, H500 catalysts were lower than that of H600, H700, H800, and the peaks area of H300-H800 decreased with the increase of thermal treatment temperature, which indicated that the oxidation ability of these α-Fe2O3 catalysts was ranked by H300 > H400 > H500 > H600 > H700 > H800.

3.3. Reaction mechanism and reaction kinetic study 3.3.1. In situ DRIFTS For the case of reaction between NO + O2 and adsorbed NH3, H300 was first exposed to NH3 at 120 °C until it was saturated, and purged with N2 for 30 min. Then, NO + O2 was introduced into the IR cell. The results are shown in Fig. 9(a). After the adsorption of NH3, the IR bands appeared at 1609 and 1188 cm−1 were assigned to coordinated NH3 on Lewis acid sites. These the bands at 1680, 1432 and 1304 cm−1 were assigned to ionic NH4+ on Brønsted acid sites [23,33]. The band at 1230 cm−1 may be attributed to the oxidization specie of adsorbed ammonia(eNH2) [37]. After injecting NO + O2 over NH3 pretreated H300, the intensities of bands at 1609, 1432, 1304 and 1188 cm−1, assigned to adsorbed ammonia species, diminished gradually with time until complete disappearence. Meanwhile, bridging nitrate (1602 cm−1), monodentate nitrite (1235 cm−1), monodentate nitrate (1540 cm−1) and bidentate nitrate (1580 cm−1) [24,38] appeared after 20 min of reaction. These results suggest that both Brønsted and Lewis acid sites can take part in the SCR reaction. Then, the reactants were introduced to H300 in the reversed order. The catalyst was first treated with NO + O2 at 120 °C until it was saturated, followed by N2 purging for 30 min. Then, the treated sample was exposed to NH3. As shown in Fig. 9(b), after the adsorption of NO + O2, H300 was covered by monodentate nitrite (1235 cm−1),

3.1.6. BET To investigate the specific surface area and confirm the pore size data of thermally treated limonite, nitrogen-adsorption-desorption (BET) was carried out for each sample. As presented in Table 2, the surface area and pore volume of series of catalysts experienced an evident decrease with an increasing temperature, while the average pore size increased to 23.7 nm at 800 °C. H300 had the maximum surface area of 95.9 m2/g, which was attributed to the formation of slitshaped micropores due to the dehydroxylation of goethite, and then substantially decreased to 31.2 m2/g after heating at 800 °C, where internal and interparticles sintering occurs [27]. As well known, catalyst with a large surface area is propitious to a catalytic reaction due to the more active sites [7,20]. It is believed that H300 will exhibit a better catalytic performance than other catalytics. 3.2. Catalytic performance Fig. 8 (a) shows the NO conversions over H300-H800 catalysts with different reaction temperatures of 100, 150, 200, 250, 300 °C. It can be seen that the increase of reaction temperature favored the conversion of 1119

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Fig. 3(2). TEM and HRTEM images of H300 (a, b), H500 (c, d), H700 (e, f).

monodentate nitrate (1540 cm−1), bridging nitrate (1602 cm−1), and bidentate nitrate (1580 cm−1). After the introduction of NH3 into the cell, the band intensity for the monodentate nitrite at 1235 cm−1 decreased quickly and disappeared within 5 min. Meanwhile, the band at 1432 cm−1 corresponding to the adsorbed ammonia species appeared. However, the intensities of bands at 1540, 1602 and 1580 cm−1, assigned to surface nitrate species, decreased slightly slower, and they could still be observed after 30 min. These variation of intensity suggested that the monodentate nitrite had high reactivity with NH3, while the nitrate species seem to be bound with adsorbed ammonia species at 120 °C, so the SCR reaction through the nitrite route was faster than that through the nitrate route at low reaction temperature.

NO(g) → NO(ad) NO(ad) + Mn+]O → M(n−1)+eOeNO(ad) (n−1)+

eOeNO(ad) + ½O2(g) → M

(n−1)+

eOeNO(ad) + NH3(ad) → M

(n−1)+

eOeNOeNH3(ad) → M

(n−1)+

eOeNO2(ad) + NH3(ad) → M

(n−1)+

eOeNO2eNH3(ad) → M

(n−1)+

eOH + ¼O2(g) → M

M M M M

M

M

3.3.2. Mechanism and kinetic study In situ DRIFTS study demonstrated that both the Langmuir–Hinshelwood mechanism (i.e. reaction of adsorbed ammonia species with adsorbed NOx species) and the Eley–Rideal mechanism (i.e. reaction of activated ammonia with gaseous NO) may contribute to the SCR reaction over H300 [38,39]. The SCR reaction through the Langmuir-Hinshelwood mechanism can be approximately described as [40,41]: NH3(g) → NH3(ad)

(2) (3) eOeNO2(ad)

(4)

eOeNOeNH3(ad)

(5)

(n−1)+

(n−1)+

eOH + N2 + H2O

(n−1)+

(n−1)+

eOeNO2eNH3(ad)

eOH + N2O + H2O

(n−1)+

]O + ½H2O

n+

(6) (7) (8) (9)

Reaction (1) was the adsorption of gaseous ammonia on the Brønsted acid sites and Lewis acid sites to form adsorbed ammonia species including coordinated NH3 and ionic NH4+. Reaction (2) was the adsorption of gaseous NO. Then, adsorbed NO was oxidized by Mn2+, Mn3+, Mn4+ and Fe3+ on the surface to form adsorbed NO2− (i.e., reaction (3)), which then reacted with adsorbed ionic NH4+ to NH4NO2 (i.e., reaction (5)). At last, the formed NH4NO2 was decomposed to N2 and H2O (i.e., reaction (6)). Meanwhile, NO2− on the surface can be further oxidized to NO3−

(1) 1120

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Fe 2p

2p3/2

2p1/2

Mn 2p

2p3/2

2p1/2

H300

H300

H400

Intensity (a.u.)

Intensity (a.u.)

H400

H500

H600

H500

H600

H700

H700 709.8

712.8

711.1 H800

735

730

725

720

715

710

H800

705

665

660

655

650

645

640

635

Binding energy (eV)

Binding energy (eV)

Fig. 4. Fe2p and Mn2p XPS spectra of H300-H800.

H300

641.1 644.2

640

642

644

646

648

641.1 644.0

640

Binding energy(eV)

640

644

646

648

648

644.2

H600

644

646

642.3

H800

Binding energy(eV)

Intensity(a.u.)

Intensity(a.u.)

642

H700

641.0

640

646

643.9

641.1

640

Binding energy(eV) 642.3

644

642.1

H500

644.1

642

642

Binding energy(eV)

Intensity(a.u.)

Intensity(a.u.)

642.3

641.2

H400

642.3

Intensity(a.u.)

Intensity(a.u.)

642.3

648

644.2

641.1

642 644 646 Binding energy(eV)

648

640

642 644 646 Binding energy(eV)

Fig. 5. Mn2p XPS spectra of H300-H8i00 fitted by peakfit. 1121

648

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Table 1 Analysis results of the valence state of manganese elementary of H300-H800. Mn

Binding energy/eV

Percents

641.1 641.1 641.2 641.1 641.0 641.1

20.90% 16.15% 10.88% 7.34% 8.20% 7.83%

H800

4+

Mn

Binding energy/eV

Percents

642.3 642.3 642.3 642.1 642.3 642.3

54.11% 54.31% 54.90% 57.01% 72.60% 74.14%

Binding energy/eV

Percents

644.2 644.0 644.1 643.9 644.2 644.2

24.99% 29.54% 34.22% 35.65% 19.20% 18.03%

NH3(g) → NH3(ad) NH3(ad) + M

]O → M

eOH + NH2(ad)

(11)

NH2(ad) + Mn+]O → M(n−1)+eOH + NH(ad)

(12)

NH(ad) + Mn+]O + NO(g) → M(n−1)+eOH + N2O

(13)

NH(ad) + Mn+]O + ½O2(g) → M(n−1)+eOH + NO

(14)

M

eOH+¼O2(g) → M

]O + ½H2O

n+

H300

Volume (cc/g)

Pore size (nm)

H300 H400 H500 H600 H700 H800

95.9 87.3 80.6 66.7 34.6 31.2

0.205 0.174 0.173 0.131 0.105 0.176

7.96 8.56 8.57 13.8 22.5 23.7

and the formation of N2 and N2O from the oxidation of NH3 was mainly related to Reactions (11) and (13). According to reaction (6) and (11), the kinetic equations of N2 formation rates through Langmuir-Hinshelwood mechanism (i.e., δSCR/ L–H) and the Eley-Rideal mechanism (i.e., δSCR/E–R) can be described as [25,42]

400

d[N2 ] ́̂ ISCR/LH = =k 6 [M(n-1)+-O-NO-NH3 ]ad dt H400

o

100

500

200

300

400 o

o

o

319 C

300

400 o

Temperature ( C)

500

H500

o

500

o

217 C

100

200

300

400

o

o

123 C o

217 C

100

200

o

319 C

300

400

o

500

Temperature ( C) Fig. 6. NH3-TPD curves of H300-H800. 1122

500

Temperature ( C)

H700

Intensity(a.u.)

Intensity(a.u.)

o

(15)

119 C

Temperature ( C)

H600

200

600

BET (m2/g)

215 C

o

100

500

Sample

o

Temperature ( C)

216 C

300 400 Temperature (°C)

Table 2 Specific surface area, pore volume and average pore size of H300-H800.

119 C

Intensity(a.u.)

Intensity(a.u.)

o

225 C

123 C

200

Intensity(a.u.)

o

300

o

388 C

(9)

120 C

200

o

289 C

Fig. 7. H2-TPR curves of H300-H800 catalysts.

Reaction (1) represents the adsorption of gaseous ammonia on H300, which was then activated by Mnn+ and Fe3+ to form NH2 (i.e., reaction (10)). Then, gaseous NO was reduced by NH2 on H300 to N2 and H2O (i.e., reaction (11)). Meanwhile, the activated NH2 in reaction (10) could be further oxidized to NH (i.e., reaction (12)), which then reacted with gaseous NO to form N2O (i.e., reaction (13)). Reaction (13) was the deep oxidation of NH to NO, which was the key step of the CeO reaction. As is shown, NH3 cannot be directly oxidized to N2 or N2O,

100

H500

100

(10)

NH2(ad) + NO(g) → N2 + H2O

(n−1)+

o

402 C

H600

H300

(1) (n−1)+

464 C

H400

(i.e., reaction (4). Then, the formed NO3− was reduced by adsorbed ammonia species to form N2O and H2O (i.e., reactions (7) and (8), which was the key step of the NSCR reaction. Reaction (9) was the reoxidization of formed M(n−1)+ on H300. The SCR reaction through the Eley-Rideal mechanism, the NSCR reaction, and the CeO reaction over H300 can be approximately described as [23,41]

n+

o

H700

H800

Intensity(a.u.)

H300 H400 H500 H600 H700 H800

Mn

3+

Intensity (a.u.)

Sample

2+

o

124 C

100

o

219 C 318oC

200

300

400 o

Temperature ( C)

500

Applied Surface Science 457 (2018) 1116–1125

100

100

80

80

60 H300 H400 H500 H600 H700 H800

40 20 0

100

N2O selectivity/%

NO conversion/%

C. Zhang et al.

60 40 20 0

150 200 250 300 (a) Reaction temperature/ oC

H300 H400 H500 H600 H700 H800

100

150

200

250

300

(b) Reaction temperature/oC

Fig. 8. De-NOx activity of H300-H800 catalysts (Reaction condition: [NO] = 1000μL · L−1, [NH3] = 1000 μL · tL−1, [O2] = 3%, total flow rate = 300 mL · min−1, GHSV = 72,000 h−1).

d[NO]g d[N2 ] d[NH2]ad ́̂ ISCR/ER = =− =− =k11 [NH2 ]ad [NO]g dt dt dt

(i.e., kSCR/ER) can be obtained after the linear regression of Fig. 10a, the intercept and the slope can be used to describe kSCR/LH and kSCR/ER, respectively [25]. Table 3 shows that δSCR and gaseous NO concentration can be satisfactorily fitted by linear relationships due to the high values of correlation coefficients (R2 > 0.994). The intercept (i.e., kSCR/LH) was slightly higher than zero when the reaction temperature was 100 °C, while the intercepts were close to zero at 150–300 °C. These indicate that the Langmuir-Hinshelwood mechanism partly contributed to the reaction at 100 °C and which can be approximately neglected with the increase of reaction temperature. As a result, the SCR reaction over H300 mainly conformed to the Eley-Rideal mechanism and the rate of the SCR reaction was approximately directly proportional to the gaseous NO concentration. Previous studies demonstrated that when the reaction reached the steady state, NH concentration over H300 can be described as [25]

(16)

where k6, k11, [M(n-1)+-O-NO-NH3 ]ad, [NH2 ]ad and [NO]g were the kinetic constants of Reactions (6) and (11), the concentration of NH4NO2, NH2 on H300, and gaseous NO concentration, respectively. As the reactions reached the steady state, the concentrations of NH4NO2 and NH2 on the surface of H300 can be approximately regarded as constants, which were not related to gaseous NH3 or NO concentrations [43,44]. Therefore, the SCR reaction rate (i.e., δSCR) on H300 can be described as [25]

dN2 dN2 ́̂ (ER) + (LH) = k11 [NH2 ]ad [NO]g = ISCR dt dt + k 6 [M(n − 1) +-O-NO-NH3 ]ad

= kSCR/ER [NO]g + kSCR/LH

(17)

kSCR/ER = k11 [NH2 ]ad

(18)

[NH]= kSCR/LH =

k 6 [M(n − 1) +-O-NO-NH3 ]ad

(19)

1580 1602 1540

(20)

where k12, k13 and k14 were the kinetic constants of Reactions (12), 13 and 14, respectively. Then, the NSCR reaction rate (i.e., δNSCR) over H300 can be described as

As hinted by Eq. (17), there would be a linear relationship between the SCR reaction rate and gaseous NO concentration. Therefore, the reaction kinetic constant of SCR reaction through the Langmuir-Hinshelwood mechanism (i.e., kSCR/LH) and the Eley-Rideal mechanism

0.1

k12 [NH2 ] k13[NO]g +k14

1286 1235

0.1

1602

1432 1540

1286

30min

30min

1230 20min

20min

10min

10min

1188 5min

5min

3min

3min

1min 1680 1609 2000

1800

1600

1432

1400

1235 1min

1304 Sample with NH 3 1200

B

1580

1000

2000

(a) Wavenumber /cm-1

1800

1600

Sample with NO+O 2 1400

1200

1000

(b) Wavenumber/cm-1

Fig. 9. (a) DRIFT spectra taken at 120 °C upon passing NO + O2 over NH3 presorbed H300 (b) DRIFT spectra taken at 120 °C upon passing NH3 over NO + O2 presorbed H300. 1123

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500

100

100

800

mol g-1 min-1

400

200 250

300

300

200

NSCR /

SCR/

mol g-1 min-1

150

100

200

400

400 200

(a)

300

400 500 600 700 NO concentration / ( L L-1)

1000

600

700

150 200

200

mol g-1 min-1

mol g-1 min-1 /

500

100

100

250 300

NO /

200

0 300

400

(b) NO concentration / ( L L-1)

150

CO

300

0

300

100

250

600

0

300

150

400

500

600

(c) NO concentration /(

800

300

600 400 200 0

700

250

300

400

500

600

700

(d) NO concentration / ( L L-1)

L L-1)

Fig. 10. Effect of the gaseous NO concentration on (a) δSCR, (b) δNSCR, (c) δCO and (d) δNO. Reaction conditions: [NO] = 300–700 μL·L−1, [NH3] = 500 μL · L−1, [O2] = 3%, total flow rate = 300 mL min−1, WHSV = 720,000 cm−3·g−1·h−1 (100 °C)/1,200,000 cm−3·g−1·h−1 (150 °C)/3,000,000 cm−3·g−1·h−1 (200 °C, 250 °C)/ 9,000,000 cm−3·g−1·h−1 (300 °C), conversion of NO: 10–20%.

d[NO] d[NH] ́̂ ICO = =− dt dt

Table 3 The rate constants of the SCR reaction through the E-R mechanism (kSCR/ER) and the rate constants of the SCR reaction through the L-H mechanism (kSCR/LH) (μmol·g−1·min−1). Sample

Temperature (°C)

= k14 [NH]ad [Mn+=O]=k14

́̂ ISCR = kSCR/ER [NO]g +kSCR/LH

k12 [NH2 ] k [NH2 ][Mn+=O] [Mn+=O] = 12 k [NO] 13 g k13[NO]g +k14 +1 k14

(23) H300

100 150 200 250 300

kSCR/ER/106

kSCR/LH

R2

kNSCR

0.054 0.131 0.216 0.315 0.792

1.72 0 0 0 0

0.999 0.999 0.994 0.999 0.996

4.87 24.79 88.17 171.36 527.90

The relationships between δNO, δSCR, δNSCR, δCO should conform to the following equation

dNO ́ ̂ ́̂ ́̂ ́̂ I NO == I SCR+ I NSCRI CO =kSCR/ER dt

=kSCR/ER [NO]g +kSCR/LH+kNSCR-k CO

Fig. 10c shows that the rate of the CeO reaction approximately closed to zero at 100–200 °C. Hinted by Eq. (22), the NSCR reaction rate would not vary notably with the increase of gaseous NO concentration. This deduction was demonstrated in Fig. 10b. Fig. 10c also shows that the rate of the CeO reaction of H300 was higher than zero at 250–300 °C, which indicated that the catalytic oxidization of NH3 to NO simultaneously occured at relatively high temperatures. The CeO reaction rate obviously decreased with the increase of gaseous NO concentration when gaseous NO concentration was low. This result was in line with the hint of Eq. (23). Therefore, the NSCR reaction rate would increase with the increase of gaseous NO concentration (hinted by Eq. (21), which was demonstrated in Fig. 10b.

dN2 O dN O ́̂ (E-R)+ 2 (L-H) INSCR = dt dt = k13 [NH]ad [NO]g [Mn+=O]+k 8 [M(n-1)+-O-NO2-NH3 ]ad =

k12 [NH2 ][Mn+=O] 1+ k

k14

+k 8 [M(n-1)+-O-NO2-NH3 ]ad (21)

13[NO]g

where k8, [Mn+=O] and [M(n-1)+-O-NO2-NH3 ]ad were the kinetic constant of Reactions (8), the concentrations of Fen+/Mnn+ and NH4NO2 on H300, respectively. Meanwhile, if the CeO reaction did not happen (i.e., k14 was very low) or gaseous NO concentration was very high, the value of k14/ [NO]g was close to zero. Thereby, Eq. (21) can be transformed as following

́̂

INSCR =k12 [NH2

][Mn+=O]+k

(n-1)+-O-NO -NH ] 8 [M 2 3

(24)

4. Conclusions Fe/Mn oxide catalysts containing nano-porous α-Fe2O3 and amorphous Mn oxides were obtained by thermal treatment of limonite·NH3SCR model reaction results showed that H300 exhibited the best catalytic performance compared with other catalysts in this work, which was related to its maximum surface area of 95.9 m2/g, the best

(22)

Furthermore, the rate of the CeO reaction (i.e., δC−O) on H300 can be described as [25] 1124

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oxidation ability, and the largest amount of Bronsted acid sites and Lewis acid sites. The results of mechanism and kinetic study showed that the SCR reaction on H300 mainly conformed to the Eley-Rideal mechanism, the Langmuir-Hinshelwood mechanism partly contributed to the reaction at 100 °C and which can be approximately neglected with the temperature increasing above 100 °C. This study implied that Mn-rich limonite after thermal treatment can be used as low cost and environmental catalysis in low-temperature NH3-SCR.

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