Effects of Ce over TiO 2 supported MnO x -based Catalyst for NO x Reduction by Ammonia

. Ce modified MnO x -based catalysts have attracted much attention due to its high activity for selective catalytic reduction of NO x by NH 3 (NH 3 -SCR) at low-temperatures. However, the most important role of Ce on the NH 3 -SCR performance of MnO x -based catalysts has not been confirmed. Herein, the typical Ce-Mn/TiO 2 catalyst was synthesized through incipient-wetness impregnation method, the positive role of Ce on Ce-Mn/TiO 2 catalyst in the NH 3 -SCR process was revealed by combining different activity tests (including NO oxidation and NH 3 oxidation) and characterizations (including XRD, XPS and He-TPD-MS experiments). It was found that the introduction of Ce can promote the dispersion of MnO x on TiO 2 support. Meanwhile, the doping of Ce in MnO x can also increase the content of Mn 4+ species. The Mn 4+ species plays a crucial role in NO oxidation reaction, which can trigger the “Fast SCR” reaction and promote the conversion of NO x . This work provides insight into the catalyst design for NH 3 -SCR process at low-temperature.


Introduction
Nitrogen oxide (NO x ), as one of the primary atmospheric pollutants, has attracted much attention for its harm on humans and the environment. Selective catalytic reduction of NO x by NH 3 (NH 3 -SCR) has been considered to be one of the most effective deNO x technology, and the core of this process lies in the catalyst [1]. The traditional NH 3 -SCR catalyst used in the power plant is composed of V 2 O 5 -WO 3 /TiO 2 , and its suitable working temperature is between 300 °C and 400 °C [2]. However, the applicable temperature of the traditional catalyst is much higher than the flue gas temperature in cement, iron and steel, and other industries [3]. Developing low-temperature NH 3 -SCR catalyst (< 200 °C) is significance. MnO x -based catalysts have been widely investigated for their excellent NH 3 -SCR activity at low-temperature, especially after the modification of Ce element [4][5][6]. There are many efforts on developing effective Ce-Mn based catalysts and finding the promotion of Ce on NH 3 -SCR process [7][8][9]. However, the most important role of Ce on improving the NH 3 -SCR performance of MnO xbased catalysts has not been confirmed.
In this work, a series of Mn/TiO 2 , Ce/TiO 2 and Ce-Mn/TiO 2 catalysts were synthesized by incipient-wetness impregnation method using anatase TiO 2 as support. Ce-Mn/TiO 2 exhibits much better NH 3 -SCR activity than Mn/TiO 2 and Ce/TiO 2 . Combined with different activity tests (including NO oxidation and NH 3 oxidation) and characterizations (including XRD, XPS and He-TPD-MS experiments) data, the positive role of Ce element on Ce-Mn/TiO 2 catalyst in the NH 3 -SCR process was revealed.

Catalysts preparation
The TiO 2 (5-10 nm, anatase), 50 wt.% Mn(NO 3 ) 2 solution and Ce(NO 3 ) 3 •6H 2 O were purchased from Aladdin, and used without further purification. The Mn/TiO 2 , Ce/TiO 2 and Ce-Mn/TiO 2 catalysts were synthesized through incipient-wetness impregnation method. After impregnating 10 h at room temperature, the samples were dried at 120 °C for 10 h, followed by calcination at 500 °C for 4 h. The molar ratio of Mn/Ti in both Mn/TiO 2 and Ce-Mn/TiO 2 catalysts are 0.14, and molar ratio of Ce/Ti in both Ce/TiO 2 and Ce-Mn/TiO 2 catalysts are 0.02.

Catalytic performance test
The reactant gases of NH 3 -SCR process are listed as below: 0.06 % NO, 0.06 % NH 3 , 5% O 2 , and balanced by N 2 . The reactant gases of NO oxidation process are listed as below: 0.06 % NO, 5% O 2 , and balanced by N 2 . The reactant gases of NH 3 oxidation process are listed as below: 0.06 % NH 3 , 5% O 2 , and balanced by N 2 . The gas hourly space velocity (GHSV) of all the tests were 30000 h -1 . The NO and NO 2 contents were detected by an MRU OPTIMA 7 analyzer. The NO x conversion, NO to NO 2 conversion and NO x content in exhaust can be calculated as follows: (1)

Catalyst characterization
X-ray diffraction (XRD) was conducted on a PANlytical X'Pert PRO MPD X-ray Diffractometer. X-ray photoelectron spectroscopy (XPS) was detected by a VG ESCALABMK II spectrometer using an Al K α (1486.6 eV) photon source. Temperature-programmed desorption under He flow (He-TPD) was conducted on a Micromeritics AutoChem 2950 HP instrument, and the desorbed or composed O 2 was simultaneous detected by a mass spectrometer (MS, OMNIStar).

Catalytic activity test
The results of the NH 3 -SCR activity tests over TiO 2 support, Mn/TiO 2 , Ce/TiO 2 and Ce-Mn/TiO 2 are presented in Fig. 1. Compared with Ce-TiO 2 , Mn-TiO 2 catalyst showed good SCR deNO x activity at lowtemperatures, which can reach 95% NO x conversion at 160 °C. The introduction of Ce can further significantly improve the deNO x activity at low-temperatures, and its NO x conversion can reach 95% at 100 °C. As our previous reports [10][11][12], the NH 3 -SCR reaction (4NH 3 + 4NO + O 2 → 4N 2 + 6H 2 O) at lowtemperatures can be considered as the coupling of NO oxidation (2NO + O 2 → 2NO 2 ) and "Fast SCR" reaction (2NH 3 + NO + NO 2 → 2N 2 + 3H 2 O), where the "Fast SCR" reaction can occur rapidly. Therefore, the NO oxidation activity of catalyst could determine its NH 3 -SCR activity. To prove this point, NO oxidation activity tests were further tested. As shown in Fig. 2, the order of NO oxidation activities as follow: Ce-Mn/TiO 2 > Mn/TiO 2 > Ce/TiO 2 > TiO 2 , in consistence with the order of their NH 3 -SCR activities. These indicate that the introduction of Ce can increase the NO oxidation activity of Ce-Mn/TiO 2 , thus leading to its high NH 3 -SCR activity at low-temperatures. Although the oxidizing ability of catalysts can promote the NO oxidation activity and NH 3 -SCR activity at low-temperatures, it can also lead the NH 3 oxidation at high-temperatures. The results of NH 3 oxidation tests over the four samples are shown in Fig. 3. NH 3 began to be oxidized to NO x at 230 °C over Mn/TiO 2 and Ce-Mn/TiO 2 samples, which can explain the NH 3 -SCR deNO x activity start to decrease since 230 °C. Therefore, the oxidizing ability of low-temperature SCR catalyst is a double-edged sword, which can increase the NO x conversion by promoting NO oxidation at low-temperatures and decrease the NO x conversion by promoting the NH 3 oxidation at hightemperatures.

Structure characteristic of catalysts
To reveal the structure of catalysts and to construct the structure-activity relationship, XRD and XPS characteristics were conducted. The XRD patterns of the four samples are illustrated in Fig. 4.  Table 1. The content of Mn atom increases from 2.79% (Mn/TiO 2 ) to 5.85% (Ce-Mn/TiO 2 ) with the introduction of the Ce element, which can further prove that the dispersion of MnO x was improved with the dope of Ce. Meanwhile, the content of Ce atom in Ce-Mn/TiO 2 (2.70%) decreases compared with that in Ce/TiO 2 (2.41%), indicating that the Ce element could be doped in the MnO x species. Besides the dispersion of MnO x , the effect of Ce on Mn ion valence state was also analysed. As shown in the Mn 2p XPS spectra in Fig. 5, the peaks of Mn 2p3/2 were deconvoluted into three peaks at 642.0 eV, 641.0 eV and 644.6 eV, corresponding to Mn 4+ , Mn 3+ and Mn 2+ , respectively [4]. It can be seen that the content of Mn 4+ were significantly increased with the dope of Ce element. It has been widely reported that the Mn ion with a high oxidation state (such as Mn 4+ ) is the main active species for NO oxidation process over MnO x -based catalysts [13][14][15]. Therefore, the role of Ce over Ce-Mn/TiO 2 catalyst can be assigned to two aspects: First, the introduction of Ce can promote the dispersion of MnO x on TiO 2 support; Second, the dope of Ce in MnO x can increase the content of Mn 4+ , which possesses higher NO oxidation activity.

Study on the structure-activity relationship
In the previous study [12], it has been found that a bidentate nitrate species of MnO 2 NO 2 plays a crucial role over NO oxidation process, and the

Conclusion
In this work, a series of Mn/TiO 2 , Ce/TiO 2 and Ce-Mn/TiO 2 catalysts were synthesized through incipientwetness impregnation method, and Ce-Mn/TiO 2 exhibits robust NH 3 -SCR activity in low-temperature. Combined with XRD and XPS results, the positive role of Ce on Ce-Mn/TiO 2 catalyst in the NH 3 -SCR process was revealed. First, the introduction of Ce can promote the dispersion of MnO x on TiO 2 support; Second, the doping of Ce in MnO x can also increase the content of Mn 4+ species. It was found that Mn 4+ species plays a crucial role in NO oxidation process, which can trigger "Fast SCR" reaction and promote NH 3 -SCR process at lowtemperature. This work provides new insight into the catalyst design for NH 3 -SCR process at low-temperature.