CNT catalysts for use in low-temperature NO reduction with NH3

    One-step synthesis of ternary MnO 2 -Fe2 O3 -CeO2 -Ce2 O3 /CNT catalysts for use in low-temperature NO reduction with NH 3 Yanbing Zhang, Yuying Zheng, Haiqiang Zou, Xiang Zhang PII: DOI: Reference:

S1566-7367(15)30044-3 doi: 10.1016/j.catcom.2015.08.011 CATCOM 4408

To appear in:

Catalysis Communications

Received date: Revised date: Accepted date:

19 May 2015 6 August 2015 7 August 2015

Please cite this article as: Yanbing Zhang, Yuying Zheng, Haiqiang Zou, Xiang Zhang, One-step synthesis of ternary MnO2 -Fe2 O3 -CeO2 -Ce2 O3 /CNT catalysts for use in low-temperature NO reduction with NH3 , Catalysis Communications (2015), doi: 10.1016/j.catcom.2015.08.011

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ACCEPTED MANUSCRIPT One-step synthesis of ternary MnO2-Fe2O3-CeO2-Ce2O3/CNT catalysts for use in

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Yanbing Zhang, Yuying Zheng, Haiqiang Zou, Xiang Zhang

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low-temperature NO reduction with NH3

College of Materials Science and Engineering, Fuzhou University, Fuzhou 350108,

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People’s Republic of China

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Corresponding author: Yuying Zheng

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College of Materials Science and Engineering Fuzhou University

People’s Republic of China

ABSTRACT

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E-mail: [email protected]

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Fuzhou 350108

In this article, a facile one-step strategy for the synthesis of ternary

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MnO2-Fe2O3-CeO2-Ce2O3/carbon nanotubes (CNT) catalysts was discussed. The as-prepared catalysts exhibited 73.6–99.4% NO conversion at 120–180°C at a weight hourly space velocity (WHSV) of 210 000 mlgcat−1h−1, which benefited from the formation of amorphous MnO2, Fe2O3, CeO2, and Ce2O3, as well as high Ce3+ and surface

oxygen

(Oε)

contents.

The

mechanism

of

formation

of

MnO2-Fe2O3-CeO2-Ce2O3/CNT catalysts was also proposed. Keywords:

MnO2-Fe2O3-CeO2-Ce2O3/CNT;

Amorphous;

Low

temperature;

NH3-SCR catalyst. 1. Introduction Nitrogen oxides (NOx) stemmed from a stationary source can induce photochemical smog, acid rain, ozone depletion, and greenhouse effect.1, 1

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As an

ACCEPTED MANUSCRIPT efficient NO-controlling technique, selective catalytic reduction of NO with NH3 (NH3-SCR) has received a great deal of attention.3 Nevertheless, the commercial V2O5 + WO3 (MoO3)/TiO2 catalysts require high working temperature window

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(300–400°C).4 Besides, in order to avoid the influence of SO2 and ash, these catalysts are more commonly introduced at the downstream of the desulfurizer and electrostatic

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precipitator, where the flue gas temperature is normally <200°C.5 Therefore, there is an increasing demand to develop a highly active NH3-SCR catalyst at temperatures

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<200°C.

Because of their unique tubular structure and salient chemical property, carbon

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nanotubes (CNT) have been widely used in NH3-SCR fields, such as MnOx/CNT,5 CeOx/CNT,6 VOx/CNT,7 and Mn–CeOx/CNT.8-10 Nevertheless, these catalysts show optimal catalytic activity only at 200–300°C. Thus, it is necessary to synthesize a

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NH3-SCR catalyst with excellent catalytic activity at low temperatures (<200°C).

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Previous studies reported that Mn-Fe-Ce mixed metal-oxide catalysts possessed

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remarkable NH3-SCR activity.11-13 However, the high-temperature treatment involved in their preparation was unsafe and expensive. In order to overcome this, a facile one-step

strategy

was

adopted

for

the

synthesis

of

ternary

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MnO2-Fe2O3-CeO2-Ce2O3/CNT catalysts to control NOx. The as-fabricated catalysts were used for the selective catalytic reduction of NO with NH3, and achieved desirable NH3-SCR activity at 80–180°C. 2. Results and discussion 2.1. Catalytic activity and H2-TPR analyses Figure 1 shows a plot of NO conversion versus temperature for the as-prepared catalysts. Evidently, the MnO2-Fe2O3-CeO2-Ce2O3/CNT catalysts presented better low-temperature NH3-SCR activity compared with Mn-Fe-CeOx/CNT-IM. In addition, the NO conversion of MnO2-Fe2O3-CeO2-Ce2O3/CNT catalysts improved noticeably with the increase of temperature, and it attained 85.5–99.4% at 140–180°C. It was noteworthy that the NO conversion of 4% MnO2-Fe2O3-CeO2-Ce2O3/CNT catalyst 2

ACCEPTED MANUSCRIPT was the optimum among all the catalysts, and could reach 90.6-99.4 % at 120°C at a weight hourly space velocity (WHSV) of 210 000 mlgcat−1h−1. Moreover, the

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generated N2O was <8 ppm, and could be almost ignored in the experiments. H2-TPR

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curves (Fig. S2) revealed that both the catalysts have three reduction peaks. It should be noted that a favorable redox property of the 4% MnO2-Fe2O3-CeO2-Ce2O3/CNT

Mn-Fe-CeOx/CNT-IM.

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2.2. X-Ray Photoelectron Spectroscopy results

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catalyst is attributed to its lower reduction temperature compared with

The binding energy and surface atomic composition in the near-surface region

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were estimated by X-ray photoelectron spectroscopy (XPS). It is evident from Figure 2A that the Mn 2p spectrum of the 4% MnO2-Fe2O3-CeO2-Ce2O3/CNT catalyst exhibited typical peaks at 642 and 653.75 eV, which were indexed to Mn 2p3/2 and Mn

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2p1/2, respectively. Furthermore, a spin-energy separation of 11.75 eV was in

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agreement with an early study of MnO2.14 It was also noteworthy that high-valence

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state MnO2 was advantageous to the catalytic activity.15 Fe 2p core-level spectrum (Fig. 2B) of the 4% MnO2-Fe2O3-CeO2-Ce2O3/CNT catalyst presented binding energies of 724.45 and 710.85 eV for Fe 2p1/2 and Fe 2p3/2, respectively,16 and the

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energy separation was 13.6 eV, which revealed the predominant formation of Fe2O3.17 Moreover, according to the weak shake-up satellite (centered at 719.15 eV), an offset of 8.3 eV from the basic photoelectron lines of Fe2p3/2 could be observed, further indicating the main generation of Fe2O3.16 On the basis of previous studies, Fe2O3 was believed to possess better low-temperature catalytic activity,18 which would be favorable for the NH3-SCR reaction. Ce 3d spectra of the as-prepared catalysts (Fig. 2C) could be separated into a series of characteristic peaks. The peaks at 916.5 (s1), 906.8 (s2), 901.0 (s4), 898.1 (t1), 888.7 (t2), and 882.5 eV (t4) represent Ce4+, whereas those at 902.7 (s3) and 885.1 eV (t3) represent Ce3+, revealing the coexistence of Ce3+ and Ce4+ states. Furthermore, Table S1 shows that the Ce3+ content of the 4% MnO2-Fe2O3-CeO2-Ce2O3/CNT catalyst (17.8%) was higher than 3

ACCEPTED MANUSCRIPT that of Mn-Fe-CeOx/CNT-IM (15.3%). It was worth noting that high Ce3+ content could increase the adsorbed oxygen content and oxygen mobility, and thereby

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enhance the catalytic activity.19, 20 Figure 2D depicts the O 1s spectra of both the

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catalysts. The binding energy at 529.5 eV was due to lattice oxygen (Oδ), and the peaks between 529.5 and 534 eV were due to surface oxygen (Oε). Table S1 shows

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that the Oε content of the 4% MnO2-Fe2O3-CeO2-Ce2O3/CNT catalyst (75.1%) was higher than that of Mn-Fe-CeOx/CNT-IM (72.3%). In addition, many studies indicated

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that the higher mobility of Oε was conducive to the oxidation of NO to NO2, and thus, could further accelerate the “fast NH3-SCR” reaction.20

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2.3. X-Ray Diffraction patterns

Figure 3 depicts the X-ray diffraction (XRD) patterns of the acid-treated CNT and as-prepared catalysts. All samples exhibited four distinct peaks at 26.1°, 42.7°, 53.6°,

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and 77.6°, which were assigned to graphite.21 Moreover, the as-prepared catalysts

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exhibited weak diffraction peaks at 30.4° corresponding to Ce2O3 (PDF#44-1086),

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whereas the peaks of manganese oxide and iron oxide could not be detected. This phenomenon revealed that the mixed metal-oxide catalysts were highly dispersed on CNT, and presented an amorphous state, which was advantageous to NH3-SCR

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reaction.22 In addition, the peak intensity of graphite gradually decreased with the increase of loading, implying an interaction between mixed metal oxides and CNT.5 2.4. Field Emission Scanning Electron Microscopy images Figure 4 exhibits the field emission scanning electron microscopy (FSEM) images and

relevant

element

mappings

of

the

acid-treated

CNT

and

4%

MnO2-Fe2O3-CeO2-Ce2O3/CNT catalyst. It is evident from the figure that the outer surface of the acid-treated CNT was initially smooth (Fig. 4a), and became rough after the introduction of mixed metal-oxide catalysts (Fig. 4b), suggesting that the mixed metal-oxide catalysts have been adsorbed on the CNT. In addition, HRFSEM (Fig. 4c) clearly displayed the adsorption of the metal-oxide catalysts on the CNT, indicating the generation of MnO2-Fe2O3-CeO2-Ce2O3/CNT catalyst. Moreover, 4

ACCEPTED MANUSCRIPT element mappings of the 4% MnO2-Fe2O3-CeO2-Ce2O3/CNT catalyst (Fig. 4d–h) showed the existence of Mn, Fe, Ce, C, and O elements, further demonstrating the

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generation of the mixed metal-oxide catalysts.

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2.5. Transmission Electron Microscopy images

Techniques such as transmission electron microscopy (TEM) and high-resolution

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transmission electron microscopy (HRTEM) were used to further analyze the morphologies of the samples. As shown in Figure 5a, the acid-treated CNT presented

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a clear outer surface. However, it became coarse and was decorated by nanoflake-like species after being supported by mixed metal-oxides catalysts, indicating that the

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mixed metal-oxide catalysts have successfully adsorbed on the CNT. This result was related to the FSEM conclusions (Fig. 4b and c). HRTEM (Fig. 5c) further demonstrated the presence of nanoflake-like mixed metal-oxide catalysts. It should be

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pointed out that the two-dimensional characteristic of TEM makes it difficult to find

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whether the mixed metal-oxide catalysts are dispersed on the outer or inner surface of

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the CNT.5 In addition, the lattice fringes corresponding to the metal oxides could not be observed, which confirmed the generation of amorphous mixed metal-oxide catalysts. This was in good agreement with the XRD patterns. Moreover,

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energy-dispersive X-ray spectrum (EDX) displayed the Mn, Fe, Ce, C, and O signals, further proving that mixed metal-oxide catalysts have been fabricated and supported on CNT.

3. The generation mechanism of MnO2-Fe2O3-CeO2-Ce2O3/CNT catalysts In order to better explain the preparation method, we proposed the formation mechanism as follows: Ce3+ and Fe3+ cations were initially adsorbed on the surface of the CNT via electrostatic force, and then hydrolyzed to Ce(OH)3, Fe(OH)3, and HCl. HCl was then oxidized to Cl2 by KMnO4, and the hydrolyzation reaction was accelerated. In the meantime, Ce2O3, CeO2, and Fe2O3 could be acquired due to the oxidization of KMnO4, and KMnO4 was reduced to MnO2. Finally, the ternary MnO2-Fe2O3-CeO2-Ce2O3/CNT catalyst was synthesized. The relevant reaction 5

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equations were concluded as follows:

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4. Conclusions

In brief, ternary MnO2-Fe2O3-CeO2-Ce2O3/CNT catalysts were successfully synthesized by a facile one-step approach, and they exhibited superior NH3-SCR

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activity at 80–180°C. Among them, the 4% MnO2-Fe2O3-CeO2-Ce2O3/CNT catalyst

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attained the optimal catalytic activity during the test period, which was attributed to

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the amorphous MnO2, Fe2O3, CeO2, and Ce2O3 catalysts, and high Oε and Ce3+ contents. Finally, the mechanism of the synthesis method was proposed, which would be

beneficial

in

the

future

for

the

preparation

of

highly

active

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MnO2-Fe2O3-CeO2-Ce2O3/CNT catalysts. Therefore, it could be regarded as a potential technology for the synthesis of Mn, Fe, and Ce mixed metal-oxide catalysts. Acknowledgment This work was supported by Scientific and Technological Program-Funded Project of Fuzhou City (Grant No. 2015H0016).

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Figures

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Fig. 1 NO conversion as a function of temperature for the as-prepared catalysts. Test conditions: [O2] = 5%, [NH3] = [NO] = 400 ppm and balanced by N2, WHSV = 210 000 mlgcat-1h-1, 200 mg catalyst.

Fig. 2 XPS spectra of the as-prepared catalysts: (A) Mn 2p and (B) Fe 2p for 4% MnO2-Fe2O3-CeO2-Ce2O3/CNT, (C) Ce 3d and (D) O 1s of (1) 4% MnO2-Fe2O3-CeO2-Ce2O3/CNT and (2) Mn-Fe-CeOx/CNT -IM.

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Fig. 3 XRD patterns for acid-treated CNT and as-prepared catalysts: (1) acid-treated CNT, (2) 1% MnO2-Fe2O3-CeO2-Ce2O3/CNT, (3) 2% MnO2-Fe2O3-CeO2-Ce2O3/CNT, (4) 4% MnO2-Fe2O3-CeO2-Ce2O3/CNT, (5) 6% MnO2-Fe2O3-CeO2-Ce2O3/CNT and (6) Mn-Fe-CeOx/CNT -IM.

Fig. 4 (a-c) FSEM images and (d-h) element mappings (from the red region of Fig. 4b): (a) acid-treated CNT; (b, c) 4% MnO2-Fe2O3-CeO2-Ce2O3/CNT.

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Fig. 5 (a, b) TEM, (c) HRTEM and (d) EDX spectrum (from the red region of Fig. 5b): (a) acid-treated CNT; (b, c) 4% MnO2-Fe2O3-CeO2-Ce2O3/CNT.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights

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1. Ternary MnO2-Fe2O3-CeO2-Ce2O3/CNT catalysts were synthesized by one-step method. 2. Ternary MnO2-Fe2O3-CeO2-Ce2O3/CNT showed 73.6-99.4% NO conversion at 120-180oC. 3. The as-prepared catalysts mainly consisted of amorphous MnO2, Fe2O3, Ce2O3 and CeO2.

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