The effect of non-selective oxidation on the Mn2Co1Ox catalysts for NH3-SCR: Positive and non-positive

Chemical Engineering Journal 385 (2020) 123797

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The effect of non-selective oxidation on the Mn2Co1Ox catalysts for NH3SCR: Positive and non-positive

T

⁎

Wenjuan Zhua,b, Xiaolong Tanga,b, , Fengyu Gaoa, Honghong Yia,b, Runcao Zhanga, Jiangen Wanga, Chen Yanga, Shuquan Nia a b

College of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, PR China Beijing Key Laboratory of Resource-oriented Treatment of Industrial Pollutants, Beijing 100083, PR China

H I GH L IG H T S

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

effectively active site and non• SCR selective oxidation site are coexisting on Mn-Co catalysts.

O acted as non-selective site has • Co positive influences for the “fast SCR” 3

4

and activation of NH3.

Co O phase also has negative influ• ences, which can add N O and NO by 3

4

2

•

2

O2 oxidation. Both oxidation of NH3 by O2 and the redox reaction of NH3 with NOx can produce N2O.

A R T I C LE I N FO

A B S T R A C T

Keywords: Mn2Co1Ox NH3-SCR Non-selective oxidation Co3O4 N2O reaction mechanism

In order to explore the generation of NO2 and N2O on the Mn-Co catalysts in the NH3-SCR process, Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts, prepared by two different complexing agents, i.e., polyethylene glycol (PEG) and urea as double complexing agents, and ammonia (PA), were further investigated by NO + O2 and NH3 + O2 experiments, besides NH3-SCR test. Catalytic activity and physicochemical properties of the catalysts were compared and analyzed through NH3-TPD, O2-TPD, H2-TPR, XRD, Raman and XPS. The NH3-SCR results show that the Mn2Co1Ox (PEG) exhibits higher NO conversion than Mn2Co1Ox (PA) due to larger amount of lattice oxygen accompanied with more Con+, and surface adsorbed oxygen species, as well as the improvement of surface acidity, proved by O2-TPD, XPS and NH3-TPD, respectively. Furthermore, the separate oxidation experiments show that Mn2Co1Ox (PEG) effectively exhibits the oxidation of NH3 to N2O and also the oxidation of NO to NO2 by O2 respectively, which can influence the catalytic performance. The XRD, XPS and Raman characterization results indicate that Mn2Co1Ox (PEG) contains more Co3O4 than Mn2Co1Ox (PA), that can produce non-selective pure oxidation reaction by the NO + O2 and NH3 + O2 experiments of Co3O4 phase. Information from in situ infrared experiments is that the strongly adsorbed NH3 species on the Mn2Co1Ox (PEG) is susceptible to deep dehydrogenation and react with NOx to produce N2O in the presence of O2. Meanwhile, the oxidation site (Co3O4) can also oxidize NO to more NO2, thereby improving the “fast SCR” reaction. However, if the proportion of oxidation sites is high, NO and NH3 can be oxidized to more N2O will be produced by the reaction between NO and NH3.

⁎

Corresponding author at: College of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, PR China. E-mail address: [email protected] (X. Tang).

https://doi.org/10.1016/j.cej.2019.123797 Received 30 September 2019; Received in revised form 25 November 2019; Accepted 10 December 2019 Available online 13 December 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 385 (2020) 123797

W. Zhu, et al.

1. Introduction

(NO3)2·4H2O (2.45 g) were dissolved in 150 mL of Polyethylene glycol (PEG 1000) to form a pink solution. Then 0.6 g Urea were added into the solution. After stirring for 3 h at 80℃, the solution was put into a 200 mL Teflon-lined stainless steel autoclave and kept in a muffle furnace at 180 °C for 12 h. Cooling down to room temperature naturally, the precipitate was separated by centrifugation, washed several times with ethanol and dried in an oven at 100 °C for 10 h. Finally, the precipitate was calcined at 450 °C for 2 h to obtain product. Mn2Co1Ox (PA) was prepared by co-precipitation, 0.005 mol of Co (NO3)2·6H2O (1.46 g) and 0.01 mmol of Mn (NO3)2·4H2O (2.45 g) were dissolved in 150 mL of water to form a pink solution. Then add ammonia until the pH reaches 10. After stirring for 3 h at 80℃, the precipitate was separated by centrifugation, washed several times with ethanol and dried in an oven at 100 °C for 10 h. Finally, the precipitate was calcined at 450 °C for 2 h to obtain product. Standard Co3O4 catalyst was prepared by a hydrothermal process as described in the followings. In a typical procedure, 0.015 mol of Co (NO3)2·6H2O were dissolved in 150 mL of Polyethylene glycol (PEG 1000) to form a pink solution. Then 0.6 g Urea were added into the solution. After stirring for 3 h at 80℃, the solution was put into a 200 mL Teflon-lined stainless steel autoclave and kept in a muffle furnace at 180 °C for 12 h. Cooling down to room temperature naturally, the precipitate was separated by centrifugation, washed several times with ethanol and dried in an oven at 100 °C for 10 h. Finally, the precipitate was calcined at 450 °C for 2 h to obtain product.

In the past decades, many investigations have been performed for SCR catalysts with low-temperature activity [1–8]. Among various catalysts, MnOx-based catalysts have shown the improved NH3-SCR activities compared to other metal oxides due to the existence of unstable oxygen and various manganese valence states [9–11]. The results of the study show that the SCR activity of manganese oxide decreases according to MnO2 > Mn5O8 > Mn2O3 > Mn3O4 > MnO. In particular, MnO2 can display highest unit surface area activity, Mn2O3 with strong surface oxygen mobility has better N2 selectivity in the lowtemperature range (100–300 °C) [12,13]. Although the MnOx catalysts have the advantages of high catalytic efficiency and low reaction temperature, pure MnOx catalysts exhibit narrow temperature window [14–17]. Compared to the MnOx catalysts, the Co-modified catalysts, including Mn2Co1Ox, MnCo2O4 and CoaMnbOx catalysts, were proposed and investigated for low-temperature NH3-SCR of NOx [8,18–20]. The introduction of Co can increase the number of acid and redox sites due to the presence of abundant Mn4+, Co3+ and surface active oxygen, which are responsible for NO oxidation [6,20]. The presence of NO2 produced by NO oxidation can promote “fast SCR”, which improves catalytic activity and broadens temperature window of NH3-SCR [9,21,22]. These research has revealed that synergistic interaction between metals caused by the new crystalline phase or solid solution can influence NO conversion rate and N2 selectivity [5,18,23,24]. At present, researchers have used precipitation [26,26], hydrothermal synthesis [5,27,28] and template self-assembly methods [18,19,29] to obtain Mn-Co catalysts with better physicochemical properties. Mn-Co catalysts prepared with different complexing precipitants exhibit different NH3-SCR properties [30]. With in-depth research, Mn-Co catalysts will generate more N2O and NO2 in the process of NH3-SCR and thus reduce the N2 selectivity, which has not been effectively solved [20,23,31]. In the NH3-SCR process, the amount of N2O and NO2 may be produced by the oxidation of ammonia, the pyrolysis of nitrate nitrogen, the formation of ammonium nitrogen and stable nitrate species [32–36]. Some researchers believe that there is more highly reactive oxygen in the crystal phase, which oxidizes part of NH3 to N2O [37–39]. In order to effectively reduce the amount of N2O and NO2, it is necessary to further investigate the formation path of by-products on different catalyst surfaces. In this study, we used two complexing agents to prepare Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) respectively. The NH3-SCR study show that the Mn2Co1Ox metal oxide catalysts prepared by different complexing agents have significantly catalytic performance differences, which Mn2Co1Ox (PEG) displays superior NO catalytic activity, while has a lower N2 selectivity. In order to further explore the difference in catalytic paths between the two Mn2Co1Ox catalysts, the NH3 oxidation and NO oxidation experiments were performed. Comparative analyses with X ray photoelectron spectroscopy (XPS), NH3 temperature-programmed desorption (NH3-TPD), O2 temperature-programmed desorption (O2-TPD), H2 temperature-programmed reduction (H2-TPR), Raman spectroscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) are conducted to investigate the reaction mechanisms and pathways of the LT-SCR process for this Mn-Co catalyst, which is significant to the design and synthesis of highperformance LT-SCR catalysts.

2.2. Materials characterization X-ray diffraction (XRD) patterns were acquired (Bruker:D8 Advance Diffractometer) in the 2θ range of 5–80° at a scan rate of 0.02° s−1 with Cu Kα radiation. Barrett–Joyner–Hallend (BJH) and Brunauer − Emmett − Teller (BET) measurements were used to characterize the pore volume, average pore size, and specific surface area of the nanocomposites. X-ray photoelectron spectroscopy (XPS) was used to monitor the surface compositions and chemical states of the constituent elements and performed on a Thermo Fisher Scientific Escalab 250XI XPS System with Al K a radiation. NH3-TPD experiments were carried out in a fixed-bed quartz reactor and the concentration of NH3 was continuously analyzed by an online Nicolet IS10 IR spectrometer, and the measure temperature in the gas pool was 150 °C. Each sample usage was 75 mg. First, the sample was pretreated in high purity He at 400 °C for 1 h. Second, the sample was saturated with NH3/He mixture (500 ppm NH3) at room temperature for 2 h. Then the sample was pretreated again in high-purity He at room temperature for 1 h to remove gaseous and weakly physically adsorbed NH3. Finally, the sample was heated from 50 to 500 °C at a rate of 10 °C·min−1 in flowing high-purity He (flow rate was 100 mL·min−1). O2-TPD experiments were operated on ASAP 2920 instrument. The catalyst was pretreated in 10%O2/He flow (50 mL min−1) at 450 °C for 1 h and cooled down to 50 ℃, which was followed by purging with He for 2 h. Then the sample was heated up from 50 ℃ to 800 °C at a rate of 10 °C min−1. H2 temperature programmed reduction (H2-TPR) experiments were carried out on a Quanta Chrome Instruments. The samples (100 mg) were pretreated at 400 °C in N2 for 1 h and cooled down to room temperature (50 °C). Then the temperature was raised to 800 °C at a rate of 10℃·min−1. The Raman spectra of the catalysts were performed at room temperature by a Thermo Fisher Scientific DXR2 at 532 nm with the exposure time of 50 s. In situ DRIFTS experiments were carried out on an Nicolet IS50 spectrometer equipped with an MCT/A detector cooled by liquid nitrogen and a ZnSe window. Prior to each experiment, the sample was pretreated at 400 °C for 1 h under N2 purging at a total flow rate of 100 mL/min. The background spectrum was collected in flowing N2 and automatically subtracted from the sample spectrum. All spectra were recorded from 600 to 4000 cm−1 by collecting 100 scans with a resolution of 4 cm−1.

2. Experimental 2.1. Materials preparation All chemicals used in the experiments were of analytical grade, and used without further purification. Mn2Co1Ox (PEG) was prepared by a hydrothermal process as described in the followings. In a typical procedure, 0.005 mol of Co (NO3)2·6H2O (1.46 g) and 0.01 mmol of Mn 2

Chemical Engineering Journal 385 (2020) 123797

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Fig. 1. Sample test system diagram.

The NH3-SCR activity tests were carried out in a fixed-bed quartz reactor (i.d. = 8 mm) using 0.1 g catalyst with 40–60 mesh at atmospheric pressure. The gas mixture was made up of 500 ppm of NO, 550 ppm NH3, 5 vol% O2 and N2 acted as balance gas, and the total flow rate of feed gas was 100 mL/min, corresponding to the GHSV of 45000 h−1. The inlet and outlet concentrations of NOx was measured using Thermo Fisher Scientific Nicolet iS50 FT-IR spectrometer. The test system is shown in Fig. 1. Test time for each temperature was 30 min. NO conversion and N2 selectivity were calculated as follows:

NO Conversion (%)

60

40

40 Mn2Co1Ox (PEG) Mn2Co1Ox (PA) Mn2Co1Ox (PEG) Mn2Co1Ox (PA)

60

80

20

⎟

0 50

2[N2 O]out ⎞ × 100 N2 selectivity (%) = ⎛1 − [NO]in + [NH3 ]in − [NO]out − [NH3 ]out ⎠ ⎝ ⎜

20

80

[NO]outlet ⎞ NO conversion (%) = ⎛1 − × 100 [NO]inlet ⎠ ⎝ ⎜

0

100

N2 selectivity(%)

2.3. Catalytic performance tests

⎟

The NO + O2 and NH3 + O2 oxidation experimental conditions are the same as the NH3-SCR test conditions, the NO + O2 experiment was made up of 550 pm NO and 5 vol% O2, and the NH3 + O2 experiment was made up of 550 ppm NH3 and 5 vol% O2. Test time for each temperature was 30 min, which was same as NH3-SCR experiment.

The catalytic performance results of Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts for NO conversion with NH3 in the range of 75–250 °C are shown in Fig. 2. The NO conversion increases with the reaction temperature increase, while the N2 selectivity decreases over Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts. Mn2Co1Ox (PEG) catalyst has a higher NO conversion than Mn2Co1Ox (PA), but has a lower N2 selectivity. It is well known that byproducts such as NO2 and N2O will be generated in the NH3-SCR reaction, resulting in the reduction of catalytic performance. The key SCR catalyst reactions are shown as:

2NH3 + NO + NO2 → 2N2 + 3H2 O

(1.2)

8NH3 + 6NO2 → 7N2 + 12H2 O

(1.3)

200 Reaction Temperature (oC)

250

100

presence of 5 vol% O2). Reaction (1.2) is always present and favorable to SCR reaction. It is useful to form more NO2 over catalyst to promote this ‘‘fast SCR reaction” by the following reaction. However, if too much NO2 is produced (more than 1:1 = NO2:NO), Reaction (1.4) will become operative. This is undesirable because the ‘‘excess’’ NO2 can yield N2O which is a strong greenhouse gas [21,39]:

3.1. Mn2Co1Ox activity test

(1.1)

150

Fig. 2. NO conversion (left) and N2 selectivity (right) in the NH3-SCR reaction as a function of the temperature over Mn2Co1Ox catalysts. Reaction conditions: [NH3] = 550 ppm, [NO] = 500 ppm, [O2] = 5 vol%, N2 balance, GHSV of 45,000 h−1.

3. Results and discussion

4NH3 + 4NO + O2 → 4N2 + 6H2 O

100

2NH3 + 2NO2 → N2 + N2 O + 3H2 O

(1.4)

In addition to the redox reaction as mentioned above, the oxidation reactions between NO and O2, NH3 and O2 may coexist in the process of NH3-SCR, which can produce NO2, NO and N2O. In order to explore the effect of oxidation reaction by O2 in the Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts, NO and NH3 oxidation experiments in the presence of O2 were carried out, respectively. The oxidation reaction experiments between NO and O2 were conducted, the NO2 outlet concentration is shown in Fig. 3. Test time for different temperature was 30 min, which was same as to NH3-SCR experiments. The NO2 concentration increases with the reaction temperature increasing over Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts. From the beginning of temperature (75℃), the amount of NO2 generated by Mn2Co1Ox (PEG) catalyst is higher than Mn2Co1Ox (PA)

Reaction (1.1) is generally the ‘‘standard SCR reaction’’. As NO2 is always present in the exhaust to some extent (about 10% of NOx in the 3

Chemical Engineering Journal 385 (2020) 123797

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Fig. 3. NO2 concentration in the NO + O2 oxidation as a function of the temperature over Mn2Co1Ox catalysts. Reaction conditions: [NO] = 550 ppm, [O2] = 5 vol%, N2 balance, GHSV of 45,000 h−1.

catalyst. As mentioned, the generation of NO2 is conducive to the“fast SCR reaction”, while side reaction (Reaction (1.4)) tend to occur when more NO2 is generated in the system. Consequently, the N2 selectivity of Mn2Co1Ox catalysts were reduced. The oxidation reaction experiments between NH3 and O2 were performed, the N2O outlet concentration is shown in Fig. 4. The N2O concentration increases with the reaction temperature increase over Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts. And the N2O concentration produced by Mn2Co1Ox (PEG) catalyst increases dramatically at 125 ℃ and then increases stability with the rise of temperature, eventually reaches 145 ppm at 275℃. While the N2O production of Mn2Co1Ox (PA) catalyst increases gradually with the increase of temperature, and reaches 140 ppm at last (275℃), which is similar to the amount of Mn2Co1Ox (PEG) catalyst. Although the amount of N2O ultimately produced by the two catalysts is similar, it is easily produced by the reaction between NH3 and O2 at low temperature on the Mn2Co1Ox (PEG) catalyst. Furthermore, the storage rate of NH3 can reach 100% on the Mn2Co1Ox (PEG) at 125℃, which is easier than Mn2Co1Ox (PA) as shown in Fig. 4. It’s known that adsorption and activation of NH3 can improve the NO conversion in the NH3-SCR process [40].

Fig. 5. Comparison chart of N2O concentration between NH3 + O2 oxidation and NH3-SCR experiments as a function of the temperature over (A) Mn2Co1Ox (PEG) and (B) Mn2Co1Ox (PA) catalysts.

In order to obtain the main sources of N2O in the process of NH3SCR, the comparison chart of N2O concentration between NH3 + O2 oxidation and NH3-SCR for the two catalysts were shown in Fig. 5. The amount of N2O produced in the NH3 + O2 experiment is less than NH3SCR in the range of 75-250℃, which indicates that the formation of N2O is not solely from NH3 oxidation. Although the oxidation degree of NH3 by O2 is difficult to explore in the whole process of NH3-SCR due to the interaction between multiple reactions, the amount of N2O in the NH3SCR experiment, produced by the oxidation between NH3 and O2, will not exceed that of NH3 + O2 experiment. It can be clearly seen from Fig. 5 (A) and (B) that a large part of the N2O produced by the Mn2Co1Ox (PEG) catalyst is derived from other reactions besides NH3 + O2 oxidation. In response to this phenomenon, Ciardelli et al. proposed a mechanism involving nitrate species as an intermediate for the redox reaction between NOx and NH3. First of all, NO2 react with NH3 to form NH4NO3 and N2 in a fast reaction (Reaction (1.5)) [32]

Storage rate of NH3 ¢%£

Mn2Co1Ox (PEG) Mn2Co1Ox (PA)

250

Mn2Co1Ox (PEG) Mn2Co1Ox (PA)

80 200 60

150

40

100

20 0 50

N2O concentration ¢ppm£

300 100

50

100

150

200

250

0

2NH3 + 2NO2 → NH4 NO3 + N2 + H2 O

Reaction Temperature (oC)

(1.5)

And then NH4NO3 begins to decompose gradually at 100℃, thus generating N2O. In order to further strengthen this opinion, experiments of Mn2Co1Ox (PEG) at 250℃ were carried out. As shown in Fig. 6, the concentration of N2O produced by Mn2Co1Ox (PEG) at 250℃ decreased from 320 ppm to 140 ppm, which are same as the N2O

Fig. 4. The storage of NH3 and N2O concentration in the NH3 + O2 oxidation as a function of the temperature over Mn2Co1Ox catalysts. Reaction conditions: [NH3] = 550 ppm, [O2] = 5 vol%, N2 balance, GHSV of 45,000 h−1. 4

Chemical Engineering Journal 385 (2020) 123797

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550ppm NO

550ppm NO

Concentration (ppm)

700

to strong acid sites. Whether strong or weak acid sites, the acid strength and amount of Mn2Co1Ox (PEG) catalyst were higher than Mn2Co1Ox (PA) catalyst, particularly Mn2Co1Ox (PEG) has about three times as much strong acid as Mn2Co1Ox (PA), as shown in Table 1. In Fig. 7, the temperature exceeds 400℃ is the location that NH3 is released at the maximum extent, which is consistent with the desorption temperature of Olatt in the picture of O2-TPD (Fig. 8). Based on this, we can speculate that lattice oxygen is closely related to the adsorption and activation of gaseous NH3. According to the NH3 + O2 oxidation experiment (Fig. 4), Mn2Co1Ox (PA) and Mn2Co1Ox (PEG) catalysts eventually generate the same amount of N2O. However, the NH3 storage temperature of Mn2Co1Ox (PEG) at about 150℃ is lower than that of Mn2Co1Ox (PA) (200℃). This may be due to the strong surface oxidation property of Mn2Co1Ox (PEG).

NO NO2 NH3 N2O

800 550ppm NH3

NH3

600 500 NO

550ppm NH3

400 300 180ppm

200 100 0 0

1000

2000

3000

4000

3.2.2. O2-TPD results Oxygen adsorption and then desorption (O2-TPD) was used to study the interaction and activation of oxygen molecules with catalyst surface, with the results are shown in Fig. 8. Generally, the oxygen desorption peaks that appeared at low temperature (< 400 °C) and high temperature (> 400 °C) are assigned to the Oads and Olatt, respectively. As observed in Fig. 8, there are two desorption peaks of O2 can be observed for Mn2Co1Ox (PEG). The peak in the range of 100–400 °C could be assigned to the atomic state oxygen adsorbed on the surface of metal oxides species (O2−, O−). The last peak in the range of 400–700 °C could be assigned to the release of lattice oxygen (O2−) [41]. It is well known that lattice oxygen and surface adsorbed oxygen can be converted to each other [42,43]. We speculated that the lattice oxygen is continuously converted to surface chemical adsorption oxygen with the increase of temperature, which finally escapes from the catalyst surface during the desorption process of O2. Based on this point, Mn2Co1Ox (PEG) catalyst has significantly more Olatt that can convert to the more surface chemical adsorption oxygen, improves oxidation capacity of the catalyst than Mn2Co1Ox (PA). This is also consistent with the results for XPS and NH3-TPD. At the same time, gaseous O2 (noted as O2(g)) plays a key role in supplementing the chemical adsorption of oxygen and lattice oxygen, proving by the results of blank O2-TPD (Fig. 8). The following figure shows the blank O2TPD comparison figure of Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts. It can be seen that the gas electrical signal can hardly be detected when O2(g) is not adsorbed, and the catalytic activity of the two catalysts decreased significantly in the experiment of NH3-SCR without O2, which indirectly proved that O2(g) participated in the redox cycle in the whole SCR system.

5000

Reaction Time (s) Fig. 6. The reaction of Mn2Co1Ox (PEG) in 550 ppm NO + 5 vol O2, 550 ppm NO + 550 ppm NH3 + 5 vol% O2, and then 550 ppm NH3 + 5 vol% O2 at 250 ℃.

concentration produced by NH3-SCR and NH3 + O2 experiments respectively, when NO was removed. Based on this, we speculate that a large part of N2O may come from the side reaction (reaction (1.4)) between NO2 and NH3 in Mn2Co1Ox catalysts, expect for NH3 oxidation by O2. Compared with Mn2Co1Ox (PA), Mn2Co1Ox (PEG) has the higher activity in NO + O2 oxidation, NH3 + O2 oxidation and NH3-SCR through the above analysis, while also has the higher NO2 and N2O generation rate and concentration. These differences may relate to the oxidation properties of catalysts. 3.2. Chemical property 3.2.1. NH3-TPD and BET results NH3-TPD can be used to investigate the acid properties of catalyst surface including the strength and amount of acid. As shown in Fig. 7, the spectra of two catalysts showed similar patterns, with three desorption peaks centered at around 110, 260 and 530 ℃, respectively. According to the literature [23], the peaks below 400 ℃ could be divided into the weak and medium acid sites, while the last desorption peak in the higher temperature region above 400℃ could be attributed

3.2.3. H2-TPR results In Fig. 9, for Mn2Co1Ox (PEG) catalyst, the weak peak observed at 249 and 261℃ can be assigned to the reduction of surface oxygen species and MnO2 to Mn2O3, respectively. The peaks at 320 and 352℃ are associated with the reduction of Co3+ to Co2+ and Mn3+ to Mn2+, respectively. And the peaks at 458℃ and 623℃ are associated with the reduction of Co2+ to Co0 and bulk oxygen species in Fig. 9. Compared with Mn2Co1Ox (PEG) catalyst, the two peaks of Mn2Co1Ox (PA) attribute to reduction of Co2+ to Co0 and bulk oxygen species are offset to the high temperature, indicates that Co2+ and bulk oxygen species are reduced difficultly in the Mn2Co1Ox (PA). This may be due to the lower contents of these species. While the peaks area in Fig. 9 follows the order: Mn2Co1Ox (PEG) > Mn2Co1Ox (PA), implying that Mn2Co1Ox (PEG) possesses superior oxidizing properties. XRD, XPS and Raman characterization can prove that the oxidizing activities may be related to the structure and composition. 3.3. Crystal phase 3.3.1. XRD results The as-synthesized products are first characterized by XRD to

Fig. 7. NH3-TPD profiles of Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts. 5

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Table 1 Summary of textural parameters of Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts. sample

sp surf. area/m2·g−1

pore diam/nm

weak acidity/mmol·g−1

Strong acidity/mmol·g−1

Mn2Co1Ox (PEG) Mn2Co1Ox (PA)

79.8 70.5

3.7 18.8

0.7 0.6

2.4 0.8

Fig. 8. O2-TPD and Blank O2-TPD profiles of Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts.

Fig. 10. XRD pattern of Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts.

the same method, respectively [43]. The band at 628 cm−1 in Mn2Co1Ox (PA) catalyst is attributed to Mn-O vibration of MnOx, and the band at 678 cm−1 is attributed to Co-O vibration; The 690 cm−1, 537 cm−1, 497 cm−1, and 207 cm−1 of Mn2Co1Ox (PEG) obviously coincides with the Co3O4 peak position, so it is classified as Co-O bond vibration, and 628 cm−1 is the Mn–O bond vibration in MnCoO3. It can be seen from Fig. 10 that Mn2Co1Ox (PEG) have more obvious Co–O bonds of Co3O4 phase, which can prove that Mn2Co1Ox (PEG) may contain more Co3O4 phase structures. 3.3.3. XPS analysis XPS analyses are carried out to investigate the more detailed elemental composition and oxidation state of the Mn2Co1Ox (PA) and Mn2Co1Ox (PEG) composite. The survey spectrum of the Mn2Co1Ox (PEG) composite reveals the existence of Mn, Co and O from the reference. The detailed spectrum of Mn 2p features two main spin–orbit lines of Mn 2p3/2 and Mn 2p1/2 at binding energies of 641.8 and 653.2 eV with a separation of 11.4 eV by the Gaussian fitting method, as shown in Fig. 12(A). After refined fitting, the spectrum of Mn 2p3/2 could be divided into three peaks. Among them, the peak located at 643.1 eV is characteristic of Mn4+, 641.8 eV for Mn3+ and 640.7 eV for Mn2+. By utilizing a Gaussian fitting method, the Co 2p emission spectra are all fitted with two spin orbit doublets, corresponding to Co 2p3/2 and Co 2p1/2 spin orbit coupling. In the Co 2p spectrum in Fig. 12(B), the two Co 2p3/2 and Co 2p1/2 peaks located at 780.2 and 795.2 eV are accompanied by two prominent shake-up satellite peaks (786.6 and 803.3 eV), which distinctly proves the dominating presence of the Co2+ and Co3+. Moreover, the deconvolved O1s spectrum in Fig. 11(C) displays two peaks: a large peak at 529.7 eV and the other peak at 531.5 eV, which can be indicative of the oxygen in the structure lattice and the oxygen in surface-adsorbed O, respectively [45]. The relative distribution of elemental valence states of Mn2Co1Ox (PA) and Mn2Co1Ox (PEG) catalysts calculated by normalization method are shown in the Table 2, from which it can be concluded that Mn2Co1Ox (PEG) has a similar content of Mn, but relatively high Co2+, Co3+ contents and Oβ that correspond to the results of XRD and Raman characterizations.

Fig. 9. H2-TPR profiles of Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts.

identify its crystallographic structure. The wide-angle XRD patterns of Mn2Co1Ox (PA) and Mn2Co1Ox (PEG) catalysts are shown in Fig. 10. The Mn2Co1Ox (PA) and Mn2Co1Ox (PEG) catalysts basically had the same structure as Co3O4 (JCPDS 80-1535) and CoMnO3 (JCPDS 752090), respectively, as confirmed by XRD analysis. Some strong Co3O4 diffraction peaks appear in the XRD pattern of Mn2Co1Ox (PEG) may indicate that it can contain more Co3O4 phase than Mn2Co1Ox (PA).

3.3.2. Raman spectra It is well known from the literature that the bands of MnOx are generally 186 cm−1, 271 cm−1, 378 cm−1, 474 cm−1, 507 cm−1, 579 cm−1 and 632 cm−1 [44]. The bands at 709 cm−1, 639 cm−1, 542 cm−1, 499 cm−1, 214 cm−1 are assigned to the A1g, F2g, F2g, Eg and F2g symmetric vibration in standard Co3O4 phase that prepared by 6

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678

628 Mn2Co1Ox (PA)

Intensity (a.u)

690 665 537

497

Mn2Co1Ox (PEG) 207

709

Standard Co3O4 542

499 214

639

800

700

600

500

400

300

200

Wave number (cm-1) Fig. 11. Raman spectra of Mn2Co1Ox catalysts and Co3O4 catalyst.

NH3-TPD and O2-TPD demonstrate that lattice oxygen in the catalyst contributes to the adsorption and activation of gas NH3(noted as NH3 (g)). According to this, it can speculate that the high Co and O contents on the surface of Mn2Co1Ox (PEG) was related to the strong surface oxidation property, which promote the adsorption and activation of NH3(g). Based on the above analysis, the Co3O4 phase exists in the Mn2Co1Ox catalysts may improve t the adsorption and activation of NH3, further can change catalytic properties of catalysts. 3.4. Co3O4 activity test In order to prove above statements, the NH3-SCR, NO + O2 oxidation and NH3 + O2 oxidation experimental diagrams of the Co3O4 structure were supplemented. As shown in Fig. 13. It can be seen from the NO + O2 oxidation (Fig. 13(B)) and NH3 + O2 oxidation (Fig. 13(C)) diagrams of Co3O4 catalyst that the amount of NH3 and NO have no change respectively before 100 °C. While NH3 and NOx concentration decreased significantly, NO2 and N2O were not formed in the NH3-SCR (Fig. 13(A)). It can conclude that NH3 reacts with NOx to form N2 before 100 °C. Combined with the oxidation activity diagram of NH3, almost all of N2O generated by the NH3-SCR is derived from the oxidation reaction of NH3. Because the maximum amount (200 ppm) and temperature (175 °C) of N2O produced in the NH3-SCR are the same as the NH3 + O2 oxidation in the temperature range of 100–250 °C. We speculate that NH3 + O2 oxidation may become the dominant reaction after 100 °C. And in the temperature range of 75–175 °C, NO of NH3-SCR showed a steady downward trend on the standard Co3O4, while NO has almost no change in the NO + O2 oxidation experiment, it further explained that NO reacted with NH3 in this temperature range. At 175–225 ℃, NO produced by NH3-SCR (Fig. 13(A)) gradually increased, comparing the NH3 + O2 oxidation experiment (Fig. 13(C)), it can be speculated that this part of NO mainly comes from the oxidation of NH3 and O2. After 225℃, NO in the process of NH3-SCR showed a significant downward trend, because NO oxidation by O2 dominated in this temperature range. To sum up, NO2 is derived from NO and NH3 oxidation with O2, respectively. NO oxidation predominating among them. From the above analysis results, it can speculate that a non-selective complete oxidation reaction may tend to occur in the standard Co3O4 that can increase the generation amount of NO2 and N2O.

Fig. 12. XPS spectra for Mn 2p (A), Co 2p (B), and O 1s (C) of the catalysts: (a) Mn2Co1Ox (PEG) and (b) Mn2Co1Ox (PA).

3.5. Reaction path In order to investigate the SCR reaction path on the surface of Mn2Co1Ox (PEG) at 100℃ that showed the best NH3-SCR activity, we 7

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Table 2 Relative atomic percentages of different valence elements. Sample

Mn2+

Mn3+

Mn4+

Co2+

Co3+

Oα

Oβ

Mn-Co (PEG) Mn-Co (PA)

5.0% 6.4%

7.0% 6.9%

6.0% 6.6%

9.2% 6.5%

8.5% 8.1%

22.5% 34.7%

41.8% 30.8%

conducted in-situ infrared experiments of different gas adsorption. The FTIR spectra of NO adsorption on the surface of Mn2Co1Ox catalysts in the presence of O2 are shown in Fig. 14. The broad bands centered at 1625, 1547, 1530, 1276, 1240, 1033, 998 and 808 cm−1 can be observed on the surface of Mn2Co1Ox (PEG) at 100℃. The broad band with a center at 1625 cm−1 can be assigned to bridged-NO3−, the bands at 1547, 1276, 1033 cm−1 can be attributed to bidentate-NO3−, and 1530, 1240 and 998 cm−1 are assigned to monodentate-NO3− [37,46,47]. For Mn2Co1Ox (PA) catalyst, 808 cm−1 band attributed to the stable N-O are not founded. From Fig. 14, the Mn2Co1Ox (PEG) catalyst has more and stronger peaks attributed to the stable N-O than Mn2Co1Ox (PA) catalyst. In other words, Mn2Co1Ox (PEG) is more prone to form stable nitrate species, which is related to the improved oxidizing ability by Co3O4 phase. As shown in Fig. 15(A), the bands at 1605 and 1461 cm−1 do not change after NO and O2 introduced, and the bands at 1624, 1547, 1528, 1272, 1241, 1033, 996, 816 cm−1 that can be attributed to stable nitrate species gradually appear and increase with the time. The strong band at 1189 cm−1 due to NH3 species adsorbed at L acid sites gradually decreases, and the nitrate species increase with the introduction of NO (g). Meanwhile, the band at 1500 cm−1 assigned to –NH2 specie does not appear, this may be due to the strong surface oxidation of the catalyst under oxygen conditions. It is well known that N2O can be produced by the reaction between NOx species and –NH/-N dehydrogenized from –NH3. However, it is difficult to determine the strongly adsorbed NH3 species at L acid sites react with NOx (g) or adsorbed nitrate species to produce N2O. In summary, the adsorbed NH3 species are easily dehydrogenated and oxygenated to generate intermediate species of -NxOy in the presence of O2, then can desorb to release N2O, NO and NO2, which may be due to the exists of Co3O4. In the Fig. 15 (B), the bands at 1627, 1553, 1521, 998 cm−1 gradually disappear due to the reaction of bridged nitrates with NH3 species when NH3 and O2 were introduced. And the asymmetric NH3 species adsorbed by L acid sites (1607 cm−1) gradually appear. However, the band at 1553, 1276, 1241, 1033, and 808 cm−1 assigned to the stable nitrate species do not disappear after passed NH3 + O2, while the NH3 species adsorbed at the L acid sites gradually emerge, this further indicates that these stable nitrate species do not react with NH3. Furthermore, the band at 1501 cm−1 attributed to the –NH2 exists, it can indicate that NH3(g) reacts with nitrate species to form N2. Combined with the above analysis, the appearance of this specie represents that the nitrate species can react with the NH3(g) to form N2, and react with the strongly adsorbed –NH3 to form a part of N2O. In the Fig. 16, the bands at 1620, 1547, 1528, 1241, 996 cm−1 gradually disappear due to the reaction of bridged nitrates with NH3 species when NH3 were introduced. And the asymmetric NH3 species adsorbed by L acid sites (1603 cm−1) gradually appear. The 1565 cm−1 appears in the presence of NH3 and O2, which is classified as –NH/–N specie. Compared to Fig. 15(B), the 1565 cm−1 do not appear, while the 1503 cm−1 appear in the Fig. 16. Based on this, it can speculate that the nitrate species react with NH3(g) to form N2 easily in the absence of O2. In the presence of O2, a part of the NH3 (g) is adsorbed on the surface of the catalyst to form adsorbed NH3 specie, which is further dehydrogenated into the –NH/-N specie and -NxOy specie (Fig. 15 (B)). The monodentate nitrate specie centered at 1244 cm−1 still exists on the surface of Mn2Co1Ox (PEG) pre-adsorbed with NO and O2 after the introduction of NH3 and O2, which indirectly proved that the strong adsorption of NH3 specie is prone to deep oxidation. This is related to

Fig. 13. NO, NH3, NO2 and N2O concentration in the NH3-SCR (A), NO and NO2 concentration in the NO oxidation (B), NO, NH3, NO2 and N2O concentration in the NH3 oxidation (C) as a function of the temperature over Co3O4 catalyst.

the presence of Co3O4 phase in the Mn2Co1Ox (PEG) catalyst. As is mentioned above (Fig. 6), Co3O4 phase acted as non-selective oxide site can improve the NH3 adsorption and activation, which increase NO conversion, but generate N2O easily at the same time. Fig. 17 shows the FTIR spectra of co-adsorption of NO and NH3 in 8

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W. Zhu, et al.

Mn2Co1Ox (PEG) Mn2Co1Ox (PA)

Mn2Co1Ox (PEG)

1276 1240

1547 1530

1033 998

1625

808

1548 1526

1500

1250

1000

1750

750

1274 1241

1547 1528

1500

Wavenumber (cm )

1250

750

1282 1201 1246

1033 996

Intensity (a.u)

1272 1241

1461

808

1000

Mn2Co1Ox (PEG) Mn2Co1Ox (PA)

20min 30min 8min 10min 4min 6min NO+O2 2min NH3+O2 60min

Mn2Co1Ox (PEG)

1528 1547

996

Fig. 16. In situ DRIFTS of NH3 adsorption on Mn2Co1Ox (PEG) pre-adsorbing NO + O2 at 100 ℃.

Fig. 14. In situ DRIFT spectra of NO adsorption for 30 min on Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts at 100 ℃.

Intensity (a.u)

816

Wavenumber (cm-1)

-1

1624 1605

1033

1000 1620

(A)

1197

1503

1281 1339

1750

1281

Intensity (a.u)

Intensity (a.u)

1603

20min 30min 10min 15min 4min 5min 1min 3min NH3-1min NO+O2-30min

816

1033

1507 1607 1635

816

1234 1287

1464

1200

1750

1500

1250

1000

750

1600

Wavenumber (cm-1)

20min 8min 4min NH3+O2 NO+O2

1287 1244

Intensity (a.u)

1201

1501

10min 6min 2min 30min

1033

1276 1241

1627

808

1500

1250

800

the presence of O2 over the Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts at 100℃. Mn2Co1Ox (PA) has a distinct peak at 1464 cm−1, which was generally classified as NH4+ species adsorbed by B acid sites, indicating that there are more –OH groups. It can be clearly seen from Fig. 17 that NH3 adsorbed species and nitrate species exist simultaneously. Based on the above analysis, it can conclude that the adsorbed species on some active sites tend to undergo non-selective oxidation reactions, which may be related to the presence of Co3O4. Mn2Co1Ox (PA) and Mn2Co1Ox (PEG) catalysts have the similar degree of N2 formation, because they have the same strength of –NH2 species (1507 cm−1). It is obvious found that there are strong nitrate and NH3 adsorbed species in Mn2Co1Ox (PEG) catalyst compared with Mn2Co1Ox (PA). It is undoubtedly beneficial to the adsorption and conversion of NO to some extent, but a part of NO and NH3 are converted to the byproducts, rather than N2. Based on the above analysis, the presence of the Co3O4 crystal phase is the main cause of this difference. Due to its strong oxidation properties, Co3O4 promotes the adsorption of NO and NH3 to generate stable nitrates and strongly adsorbed NH3 species. But stable nitrates and adsorbed NH3 species tend to produce non-selective oxidation reactions, thus aggravating the by-products formation. Therefore, Mn2Co1Ox (PEG) catalyst has a high NO conversion rate and poor N2 selectivity.

998

1750

1000

Fig. 17. In situ DRIFT spectra of NH3-SCR for 30 min on Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts at 100℃.

816

1553 1521

1200

Wavenumber (cm )

Mn2Co1Ox (PEG)

1565

1400

-1

(B)

1607

1030

1342

1189

1000

750

-1

Wavenumber (cm ) Fig. 15. In situ DRIFTS of NO + O2 adsorption on Mn2Co1Ox (PEG) pre-adsorbing NH3 + O2 (A) and at NH3 + O2 adsorption on Mn2Co1Ox (PEG) preadsorbing NO + O2 (B) at 100 ℃.

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4. Conclusion

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The comparison of Mn2Co1Ox (PEG) and Mn2Co1Ox (PA) catalysts, prepared by different complexing agents, has showed that Mn2Co1Ox (PEG) catalyst due to the stronger oxidation property, caused by the existence of more Co3O4 phases, has superior NO conversion and the lower N2 selectivity than Mn2Co1Ox (PA) catalyst. The Co3O4 is prone to non-selective pure oxidation reaction, which can easily oxidize NO to NO2 via the reaction between NO and O2, and oxidize NH3 to N2O, NO and NO2 via other reaction between NH3 and O2, respectively. A certain degree of oxidation site (Co3O4) promotes the NO conversion of the catalyst by oxidizing NO to more NO2, which contributes to the “fast SCR” reaction. However, when the proportion of oxidation sites is high, NO can be oxidized to more NO2 produced by the stable nitrate, which can react with NH3 adsorbed species to release N2O easily at high temperature. At the same time, Co3O4 phase acted as pure oxidation sites can improve the NH3 adsorption and activation, which increase NO conversion, but generate N2O easily with O2. We have concluded that not only did pure oxidation sites promote the “fast SCR” reaction but also added the formation of NO2 and N2O by non-selective oxidation reactions. However, it was not clearly whether the existence of effective SCR active site is related to the MnCoO3. To further explore the mechanism of NH3-SCR reaction, we should focus on the exploration of various single crystal catalysts. At the same time, we also considered that the certain proportion of oxidation sites can increase the SCR activity, but it is not simply controlled by the proportion of metal added in the preparation process. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was financially supported by the National Key R&D Program of China (2017YFC0210303), National Natural Science Foundation of China (21806009, 21677010), China Postdoctoral Science Foundation (2018M631344, 2019T120049) and Fundamental Research Funds for the Central Universities (FRF-TP-18-019A1). References [1] L.J. Liu, S. Su, K. Xu, H.F. Li, M.X. Qing, S. Hu, Y. Wang, J. Xiang, Fuel 255 (2019) 115798. [2] Z.F. Hao, Z.R. Shen, Y. Li, H.T. Wang, L.R. Zheng, R.H. Wang, G.Q. Liu, Angew. Chem. Int. Ed. 58 (2019) 6351. [3] Y.R. Bai, J.P. Dong, Y.Q. Hou, Y.P. Guo, Y.J. Liu, Y.L. Li, X.J. Han, Z.G. Huang, Chem. Eng. J. 361 (2019) 703. [4] H.X. Jiang, Q.Y. Wang, H.Q. Wang, Y.F. Chen, M.H. Zhang, ACS Appl. Mater. Interfaces 8 (2016) 26817. [5] J.W. Shi, G. Gao, Z.Y. Fan, C. Gao, B.R. Wang, Y. Wang, Z.H. Li, C. He, C.M. Niu,

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