Journal of Nanomaterials
Volume 2017 (2017), Article ID 7901686, 10 pages
https://doi.org/10.1155/2017/7901686

Research Article

Reaction and Characterization of Low-Temperature Effect of Transition Nanostructure Metal Codoped SCR Catalyst

Ke Yang,¹ Weiwei Xiao,¹ Quan Xu,¹ Jiaojiao Bai,² Yan Luo,³ Hao Guo,⁴ Li Cao,¹ Wei Cai,¹ Peng Pu,¹ and Lulu Cai²

¹State Key Laboratory of Heavy Oil Processing, Beijing Key Laboratory of Biogas Upgrading Utilization, China University of Petroleum, Beijing 102249, China
²Personalized Drug Therapy Key Laboratory of Sichuan Province, Hospital of the University of Electronic Science and Technology of China and Sichuan Provincial, People’s Hospital, Chengdu 610072, China
³Department of Chemical Engineering, West Virginia University, Morgantown, WV 26505, USA
⁴Chongqing Institute of Forensic Science, Chongqing 400021, China

Correspondence should be addressed to Quan Xu; xuquan@cup.edu.cn, Peng Pu; pupeng@hotmail.fr, and Lulu Cai; lzxlulu@126.com

Received 24 January 2017; Accepted 28 February 2017; Published 20 September 2017

Academic Editor: Jinwei Gao

Copyright © 2017 Ke Yang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Typical p-type semiconductor MnO codoped with n-type semiconductors such as CeO₂ and V₂O₅ was reported to achieve high efficiency in catalytic NO removal by NH₃. In this paper, we present novel Mn-Ce codoped V₂O₅/TiO₂ catalyst which exhibited an excellent NO conversion efficiency of 90% at 140°C. By using this codoped catalyst, the best low-temperature activity was greatly decreased when compared with single Mn- or Ce-doped catalyst. According to the characterization results from BET, XRD, and XPS, the codoped catalyst was composed of both CeO₂ and amorphous Mn. The electron circulation formed between doping elements is believed to promote the electron transfer, which may be one of the reasons for excellent low-temperature denitration performance.

1. Introduction

NO is mainly derived from industrial emissions, traffic emissions, and living emissions. NO gases react to form smog and acid rain as well as being central to the formation of tropospheric ozone. It especially can form small solid particles through the secondary chemical reactions that cause serious pollutions to the environment. Therefore, it is necessary to take a denitration treatment for flue gas after combustion. Selective catalytic reduction is the most widely used and effective methods for the removal of NO in industrial at present. The main two reactions are presented in the following:

NH₃ and NO almost do not react in the absence of the catalyst; therefore, the catalyst is the key for the whole reaction. V₂O₅/TiO₂ and V₂O₅-WO₃/TiO₂ (anatase) catalysts operated at 350–400°C, with less than 1% V₂O₅ loading, have been widely accepted as commercial catalysts [1–3]. Currently, other doped companions such as Mn, Cu, Fe, Ce, Wo, and F [4–8] and morphological changes in the supports can be used to modify the catalyst to achieve high catalytic activity [9–12]. W or Mo doped V₂O₅/TiO₂, considered as the most effective commercial catalyst, is widely used for denitration in power plants and nitric acid plants [13, 14]. However, its narrow activity temperature window forces the selective catalytic reduction (SCR) unit to be installed upstream of the desulfurizer and electrostatic precipitator where high concentrations of SO₂ and particle matters can make the catalyst bed layer blocked, accelerating the deactivation of the catalyst [15]. Therefore, there is a rising interest in high performance catalysts that can be used at low temperature. MnO has attracted significant attention because of its various types of labile oxygen species [16, 17]. Recently, Ce-doped catalyst has been found to reduce the reaction temperature significantly and has high catalytic activity and selectivity [18]. Mn-doped catalyst has shown excellent low-temperature activity, lower apparent active energy, and better ion dispersion than those of most previously reported SCR catalysts [17, 19]. This research committed to the development of low-temperature catalyst based on the V₂O₅/TiO₂ and V₂O₅-CeO₂/TiO₂ catalyst, which is the key of the selective catalytic reduction (SCR) to remove NO from effluent gas.

2. Materials and Methods

2.1. Materials

The low-temperature catalysts in the experiments were prepared with commercial anatase TiO₂ (Tianjin Guangfu Pharmaceutical) as carriers, with a specific surface area of 7.03 m²/g. Ammonium metavanadate (NH₄VO₃) was used as the precursor of vanadium, cerium nitrate (Ce(NO₃)₃·6H₂O) as the precursor of cerium, and oxalic acid solution as the precursor impregnation solution in the doping process. Manganese acetate (C₄H₆MnO4.4H₂O), copper nitrate (Cu(NO₃)2.3H₂O), cobalt nitrate (Co(NO₃)2.6H₂O), ferric nitrate (Fe(NO₃)3.9H₂O), and chromium nitrate (Cr(NO₃)3.9H₂O) were selected to provide Mn, Cu, Co, Fe, and Cr, respectively. All these salts precursors were purchased from Tianjin Guangfu Technology Development Co., Ltd. and Aladdin Technology Co., Ltd.

2.2. Catalyst Preparation

The catalysts with different loadings of vanadium and cerium in the experiment were prepared by a conventional incipient-wetness impregnation method. Firstly, the oxalic acid was dissolved in deionized water and heated to dissolve completely, used as the precursor impregnation solution. Then, a certain quality of ammonium metavanadate was added to the oxalic acid solution and stirred until dissolved completely. A quantitative powder of cerium nitrate was added in the same way, finally, adding the TiO₂ powder to the above solution, stirring, and impregnating for 1 hour. The water was evaporated from the solution by a rotary evaporator and dried at 80°C for 24 hours. The dried samples were calcined at 500°C under the air atmosphere for 2 hours. Then the catalysts were ground and sieved to 20–40 mesh for catalytic performance evaluation. Other metals like Mn, Fe, Cr, etc. were doped in the same way as described above. Eventually, Ce-V₂O₅/TiO₂ catalyst with a fixed amount of 5% (wt%) V₂O₅ but different Ce loadings of 5%, 10%, 15%, 20%, 25%, and 30% (wt%) and other Bimetallic-doped V₂O₅/TiO₂ catalysts were prepared by the same impregnation method [20–22]. The catalysts prepared are denoted as M-Ce-5V₂O₅/TiO₂. M represents the second metal, such as Mn, Fe, or Cu; and represent the loading of M (wt%) and Ce (wt%), respectively.

2.3. Catalytic Activity Test

The SCR activity measurement was performed on a fixed-bed stainless steel tube reactor with an inner diameter of 11 mm and the outer diameter of 14 mm. Laboratory gas distribution was used to simulate the flue gas in the measurement. The feed gas mixture consisted of NH₃ 500 ppm, NO 500 ppm, 3% O₂ (volume fraction), and N₂ as the balance gas. The total flow rate was 1000 mL/min controlled by mass flow meters and the GHSV = 10,000 h⁻¹ in each reaction. The concentrations of NO were measured at the inlet and outlet by flue gas analyzer to calculate the conversion rate by the following:

where [NO] = [NO] + [NO₂] and the in and out indicated the inlet and outlet concentration at steady state, respectively. The data was measured when the reaction reached the steady state (about 20–40 min) at each temperature, which could reduce the errors caused by instability.

2.4. Catalyst Characterization

The powder X-ray diffraction (XRD) measurements of the samples were recorded on a Bruker D8-Advance X-ray powder diffractometer using Cu K radiation ( Å) with scattering angles () of 5–85° and a 0.0197 step size. The specific surface areas and pore size were measured by nitrogen adsorption at −196°C by the BET method using Micromeritics ASAP 2020 M surface areas and porosity analyzer. The samples were degassed at 200°C for 12 hours. The X-ray photoelectron spectroscopy (XPS) experiments were carried out on a Thermo Fisher Escalab 250Xi X-ray photoelectron spectrometer system equipped with a monochromatic Al K X-ray source scanning from 0 to 5000 eV.

3. Results

3.1. Ce-Doped Effect of V₂O₅-TiO₂ Catalysts

NO conversions rate at various temperatures for the NH₃-SCR over Ce-doped V₂O₅/TiO₂ catalysts is shown in Figure 1. V₂O₅/TiO₂ shows above 80% NO conversion rate at a wide temperature range of 175°C to 375°C. The Ce doping can improve the catalytic activity effectively, especially from 160 to 450°C, due to the enhancement of electron transfer rate in catalyst. The 30Ce-V₂O₅/TiO₂ shows the highest NO conversion and widest temperature window with NO conversion above 90% from 160 to 400°C and the conversion rate could reach 99.83% at 200°C.

Figure 1: NO conversion over Ce-doped V₂O₅/TiO₂ catalysts with different Ce contents.

The catalytic activity with various Ce contents is shown in Figure 2. With the increase of Ce doping amount, the NO conversion firstly decreased and then increased at low-temperature zone (100–200°C) and high-temperature zone (350–450°C). 10Ce-5V₂O₅/TiO₂ shows the worst catalytic activity, the NO conversion even lower than the undoped 5V₂O₅/TiO₂ catalyst.

Figure 2: Effects of different Ce contents on NO conversion.

3.2. Low-Temperature Activity of X-Ce Codoped V₂O₅-TiO₂ Catalysts

Bimetal doped V₂O₅/TiO₂ catalyst was prepared on the basis of single Ce-doped catalyst. The Ce loadings were selected as 30 wt% based on the previous results and the cometal (Mn, Fe, Co, Cu, and Cr) loadings were varied ranging from 10 to 30 wt%. The catalytic activity was tested on the fixed-bed reactor at different temperatures, and the results are shown in Figures 3 and 4. From the results we can observe that the catalytic effect of Fe-Ce codoped catalyst is slightly lower than single Ce-doped catalyst at the low-temperature zone. The Cu-Ce codoped catalyst shows the worst activity, even less than the based V₂O₅/TiO₂ catalyst. Co-Ce and Cr-Ce codoped catalyst can improve the catalytic activity at the low-temperature zone but drop rapidly at high-temperatures zone. With a narrower active temperature window that cannot keep higher catalytic efficiency in a certain temperature range, Mn-Ce codoped V₂O₅/TiO₂ catalyst can improve the activity at low temperature effectively. It shows the best catalytic effect at low temperatures; the NO conversion can reach 95.69% at 140°C. The 20Mn-30Ce-V₂O₅/TiO₂ catalyst is the best as the effect of 20Mn-30Ce-V₂O₅/TiO₂ and 30Mn-30Ce-V₂O₅/TiO₂ is almost similar.

Figure 3: NO conversion over Mn-Ce-V₂O₅/TiO₂ catalysts with different Mn contents.

Figure 4: NO conversion over Co-30Ce, Cu-30Ce, Fe-30Ce, and Cr-30Ce codoped V₂O₅/TiO₂ catalysts with different Co, Cu, Fe, and Cr loading contents.

The NO conversion over X-Ce codoped V₂O₅/TiO₂ catalysts with loading contents of 20% at 160°C is shown in Figure 5. 20Mn-30Ce-V₂O₅/TiO₂ catalyst shows the best activity that the conversion rate can reach 99.58% which is nearly 30% higher than singer Ce-doped. The catalytic activity over X-Ce codoped V₂O₅/TiO₂ catalysts is in the order of Mn-Ce > Co-Ce > Cr-Ce > Ce > Fe-Ce > Cu-Ce codoped.

Figure 5: NO conversion over X-Ce codoped V₂O₅/TiO₂ catalysts with different loading metals at 160°C.

3.3. Mn-Ce Codoped Effect

Initially, the single Mn- and Ce-doped V₂O₅/TiO₂ catalysts are prepared by the impregnation method to investigate the effect of single Mn or Ce, as compared to the Mn-Ce codoped catalyst. The result is shown in Figure 6. The NO conversion over Mn-Ce codoped V₂O₅/TiO₂ catalyst can reach more than 90% at 140°C, which is much higher than single doped catalyst. Mn-Ce codoped V₂O₅/TiO₂ catalysts have the best low-temperature activity that can drop to 80°C which is lower than Mn-doped catalyst and Ce-doped catalyst. However, the catalytic activity is difficult to maintain at high temperature. Single Ce-doped catalyst has the widest temperature window, but the low-temperature effect is not obvious. Mn-doped catalyst has neither good low-temperature activity nor wide temperature window. Relevant characterizations have been taken to the three kinds of catalysts in this experiment.

Figure 6: NO conversion over 20Mn-30Ce-5V₂O₅/TiO₂, 30Ce-5V₂O₅/TiO₂, and 20Mn-5V₂O₅/TiO₂ catalysts.

3.4. XRD

The X-ray powder diffraction patterns of the Mn-Ce codoped and single Ce- and Mn-doped V₂O₅/TiO₂ catalysts are shown in Figure 7. All the reflections provide typical diffraction patterns for the TiO₂ anatase phase. The characteristic peaks of Ce and Mn oxides appear, respectively, in single Ce- and Mn-doped catalyst. In the pattern of 30Ce-V₂O₅/TiO₂, the doped Ce exists mainly in the form of CeO₂ and part of CeVO₄. It was found that CeO₂ can effectively improve the catalytic activity and make the reaction temperature window wider [18]. However, the formation of CeVO₄ has a certain suppression to the improvement of catalytic activity [23]. A variety of diffraction peaks of Mn oxides appeared in the single Mn-doped catalyst, including Mn₂O₃, Mn₃O₄, and MnO₂. With the codoping of Mn-Ce, the diffraction peaks of TiO₂ become weak significantly and CeO₂ crystal phase appears, but much weaker than single Ce-doped catalyst. The XRD results show that Mn, Ce, and TiO₂ have displayed a mutual influence by the codoping of Mn-Ce. The incorporation of Mn makes Ce exist in the form of CeO₂, but Mn is mostly in the amorphous state which can achieve a better low-temperature effect.

Figure 7: XRD profiles of (a) Ce-doped, (b) Mn-doped, and (c) Mn-Ce codoped V₂O₅/TiO₂ catalysts.

3.5. BET

The results of BET surface area, pore volume, and average pore diameter of each catalyst are shown in Table 1. Ce has a large particle diameter that the incorporation of Ce can improve the BET surface area effectively which is nearly five times more than original V₂O₅-TiO₂. Only a slight increase of the BET surface area has been achieved by the incorporation of Mn. But Mn-Ce codoped catalyst has the maximum surface area and minimum pore size. The change of BET surface area is consistent with the catalytic performance evaluation results in Figure 6.

Table 1: Comparison of BET surface area, pore volume, and average pore diameter.

Samples
BET surface area (m²/g)
Pore volume (cm³/g)
Average pore diameter (nm)

V₂O₅-TiO₂
5.5022
0.025621
186.2567

Ce-V₂O₅-TiO₂
28.7009
0.141243
196.8476

Mn-V₂O₅-TiO₂
8.9057
0.061350
271.3350

Mn-Ce-V₂O₅-TiO₂
33.5854
0.125967
150.0261

3.6. XPS

The XPS spectra of Ce 3d of Mn-Ce codoped and single Ce-doped V₂O₅/TiO₂ catalyst are shown in Figure 8. The spectrum of Ce 3d contains eight peaks, in which , , , , , and are the characteristic peaks of Ce⁴⁺ and and are the characteristic peaks of Ce³⁺. According to the intensity of the peak, Ce mainly exists as Ce⁴⁺ in both Mn-Ce codoped and single Ce-doped catalyst. The incorporation of Mn impacts the surface valence distribution of Ce that the Ce⁴⁺ increases significantly and Ce³⁺ reduces accordingly. The relative surface concentration of Ce⁴⁺ and Ce³⁺ calculated by the peak area is shown in Figure 9. The ratio of Ce⁴⁺/(Ce³⁺ + Ce⁴⁺) increased from 85% to 88.87% for Ce-V₂O₅/TiO₂ and Mn-doped Ce-V₂O₅/TiO₂ catalysts, respectively. These results suggest that the incorporation of Mn can convert part of the Ce³⁺ Ce⁴⁺ and increase the proportion of Ce⁴⁺.

Figure 8: XPS spectra of Ce 3d of (a) Mn-Ce-V₂O₅/TiO₂ (20Mn-30Ce) and (b) Ce-V₂O₅/TiO₂ (30Ce) catalysts.

(a)

(b)

Figure 9: Surface atomic concentration ratio of Ce³⁺ and Ce⁴⁺. (a) 30Ce-5V₂O₅/TiO₂ and (b) 20Mn-30Ce-5V₂O₅/TiO₂.

The XPS spectra of Mn 2p of Mn-Ce codoped and single Mn-doped V₂O₅/TiO₂ catalyst are shown in Figure 10. Mn 2p has two main peaks, Mn 2p1/2 (near 654 eV) and Mn 2p3/2 (near 642 eV), respectively. The characteristic peak of Mn 2p3/2 is superimposed from four peaks of Mn with different valence. Divide the characteristic peak into four subpeaks that Mn²⁺ (641.2–641.5 eV), Mn³⁺ (642.3–642.5 eV), Mn⁴⁺ (643.5–643.8 eV), and (645.8–646.0 eV) can be achieved. The relative surface concentration of Mn²⁺, Mn³⁺, and Mn⁴⁺ calculated by the peak area is shown in Figure 11. Mn mostly exists in the form of Mn²⁺ in both Mn-Ce codoped and single Mn-doped catalyst. The higher the valence, the lower the atomic concentration. Under the interaction in Mn-Ce codoped system, a small part of the low-valence Mn²⁺ is oxidized to Mn³⁺. The incorporation of Mn can react with Ce which has a variable valence that can promote the electron transfer between active components. Some chemical reactions may occur between the Mn²⁺ and Ce⁴⁺ as the following:

Figure 10: XPS spectra of Mn 2p of (a) Mn-Ce-V₂O₅/TiO₂ (20Mn-30Ce) and (b) Mn-V₂O₅/TiO₂ (20Mn) catalysts.

(a)

(b)

Figure 11: Surface atomic concentration ratio of of Mn²⁺, Mn³⁺, and Mn⁴⁺. (a) Mn-V₂O₅/TiO₂ and (b) Mn-Ce-V₂O₅/TiO₂.

The XPS spectra of V 2p of Mn-Ce codoped and single Ce- and Mn-doped V₂O₅/TiO₂ catalyst are shown in Figure 12. The characteristic peak of V2p3/2 appears within 515~518 eV, which can be divided into two peaks, V⁴⁺ (516.7 eV) and V⁵⁺ (517.6 eV). The relative surface concentration of V⁴⁺ and V⁵⁺ calculated by the peak area is shown in Figure 13. The concentration of V⁵⁺ in a descending order of Mn-Ce codoped > Ce-doped> Mn-doped. V⁵⁺ is the active center of the denitration catalyst and NO and NH₃ can easily adsorb on the V⁵⁺ centers that promote the oxidation and reduction of NO. Under the coeffect of Mn and Ce, a lot of V⁴⁺ convert into more active V⁵⁺; the concentration of V⁵⁺ increased more than 30%.

Figure 12: XPS spectra of V 2p of (a) 30Ce-5V₂O₅/TiO₂ (30Ce), (b) 20Mn-5V₂O₅/TiO₂ (20Mn), and (c) 20Mn-30Ce 5V₂O₅/TiO₂ (20Mn-30Ce) catalysts.

(a)

(b)

(c)

Figure 13: Surface atomic concentration ratio of V⁴⁺ and V⁵⁺. (a) 20Mn-5V₂O₅/TiO₂, (b) 30Ce-5V₂O₅/TiO₂, and (c) 20Mn-30Ce-5V₂O₅/TiO₂.

The XPS results show that the Mn-Ce codoped catalysts promote the interaction among Mn, Ce, and V. The three kinds of atoms are moving to higher valence direction, which is benefical for elcetron transformation and oxidation ability of the whole catalytic system. This is more conducive to the NOx reduced by HN3 at low temperatures.

4. Conclusions

Typical p-type semiconductor MnO codoped with n-type semiconductors such as CeO₂ and V₂O₅ achieved the excellent effect on NO removal by NH₃. Mn-Ce codoped vanadium-titanium catalyst system can effectively lower the reaction temperature and improve the efficiency. The NO conversion over Mn-Ce codoped V₂O₅/TiO₂ catalyst can reach more than 90% at 140°C that is much higher than single doped catalyst. Mn-Ce codoped V₂O₅/TiO₂ catalyst has the best low-temperature activity that can drop to 80°C which is lower than single Mn-doped catalyst and Ce-doped catalyst. The codoping of Mn-Ce makes Ce exist in the form of CeO₂, but Mn is mostly in amorphous state on the surface which can achieve better low-temperature effect. The incorporation of Mn can react with Ce which has a variable valence that can promote the electron transfer between the two active components to form an effective electron circulation in the presence of oxygen. The Mn, Ce, and V are moving to higher valence direction that the oxidation increased, which is more conducive to the reduced by NH₃. Thus even at low temperatures, it is possible to release O radical in the process of NO adsorption, which can be oxidized to NO₂ and then react with NH₃. All in all, Mn-Ce codoped V₂O₅/TiO₂ catalyst utilizes the electron transfer between Mn, Ce, and V effectively, and the denitration performance at low temperature is greatly improved. This finding may help scientists and engineers to development next generation smart surfaces [24, 25] with absorption functionality.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Authors’ Contributions

Quan Xu, Peng Pu, and Li Cao conceived and designed the experiments; Ke Yang and Weiwei Xiao performed the experiments; Jiaojiao Bai, Li Cao, Yan Luo, and Hao Guo analyzed the data; Wei Cai contributed reagents/materials/analysis tools; Peng Pu and Quan Xu wrote the paper.

Acknowledgments

The authors thank Beijing Municipal Science and Technology Project (nos. Z161100001316010 and D141100002814001), National Key Research and Development Plan (no. 2016YFC0303701), Tribology Science Fund of State Key Laboratory of Tribology (no. SKLTKF16A06), and Science Foundation of China University of Petroleum (nos. 2462014YJRC011, ZX20160056, and 201603) for the support.

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