Effects of cerium and vanadium on the activity and selectivity of MnOx-TiO2 catalyst for low-temperature NH3-SCR

JOURNAL OF RARE EARTHS, Vol. 29, No. 1, Jan. 2011, p. 64

Effects of cerium and vanadium on the activity and selectivity of MnOx-TiO2 catalyst for low-temperature NH3-SCR WU Xiaodong (吴晓东), SI Zhichun (司知蠢), LI Guo (栗 国), WENG Duan (翁 端), MA Ziran (马子然) (State Key Laboratory of New Ceramics & Fine Process, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China) Received 28 June 2010; revised 23 July 2010

Abstract: MnOx-TiO2, CeO2-MnOx-TiO2 and V2O5-MnOx-TiO2 catalysts for low-temperature NH3-SCR were prepared by sol-gel method. The results showed that both cerium and vanadium prevented the transformation of anatase TiO2 to the rutile phase. The addition of vanadium oxide induced the segregation of crystalline Mn2O3, which contributed little to low-temperature SCR and ammonia oxidation, from the MnOx-TiO2 solid solutions. However, the selectivity of the V-containing catalyst was almost 100% due to the decreased ammonia consumption and enhanced adsorption capacity of ammonia on Brønsted acid sites at relatively high temperatures. The electron-donating effect of cerium reduced the Mn4+/Mn3+ ratio to some extent, resulting in a decreased activity for ammonia oxidation. This, in combination with the enhanced ammonia adsorption capacity by Cen+ as additional Lewis acid sites, endowed the Ce-doped catalyst a higher N2 selectivity than MnOx-TiO2 despite the slightly elevated light-off temperature for NO conversion. Keywords: MnOx-TiO2; modification; selective reduction of NO; acid sites; rare earths

Selective catalytic reduction (SCR) of NOx to N2 using ammonia or hydrocarbon as reductants is known as one of the most efficient and widely commercialized removal technology of NOx emitted from stationary or mobile sources[1]. The well-known industrial catalyst for this process is V2O5/TiO2 (anatase) promoted by either WO3 or MoO3, which operates typically at 300–400 ºC. However, the fuel-efficient characteristics of diesel engines results in their exhaust gas of very low temperature, especially during low-speed driving. Therefore, development of SCR catalysts active at low temperatures (<300 ºC) is very challenging. Platinum catalysts can function at relatively quite low temperatures, but they are very expensive. In recent years, transition metal (V, Cr, Mn, Fe, Co, Ni and Cu) oxide catalysts supported on titania have attracted a lot of interests due to the ability of operating at low temperatures[2]. Among the catalysts investigated, the supported MnOx/TiO2[2–4] or MnOx-TiO2 solid solution catalyst[5] shows high SCR activity at low temperature which is determined to a great extent by the oxidation state and dispersion of manganese oxide as well as the surface area and phase structure of titania[6]. However, the serious side reactions for NH3 and NO oxidation to N2O and NO2 on this catalyst lead to a secondary pollution. Some transition metals such as Fe, Cu, Ni and Cr have been chosen to improve the selectivity to N2 and thermal stability of MnOx/TiO2 catalyst, and Fe shows the most favorable effect on the catalytic activity among these transition metals[7,8]. Vanadium catalysts have a lot of advantages including

high activity, selectivity and resistance to sulfur dioxide, so they are widely used as industrial SCR catalysts[9–11]. Ceria has been extensively studied in three-way catalysts (TWCs) due to its good oxygen storage capacity. Recently, it is reported that manganese-cerium oxides exhibit superior SCR activity at low temperature[12–14]. Ceria is also selected as a promoter for MnOx/TiO2 catalyst to improve the resistance to SO2, but the selectivity for SCR reaction has rarely been analyzed[15–17]. In the present work, cerium and vanadium were added to MnOx/TiO2 catalyst by a sol-gel method, and the catalytic performance of the catalysts for NH3-SCR was evaluated. The structural, redox and acidic properties of the catalysts were investigated, and possible mechanisms of CeO2 and V2O5 modification on the catalytic performance of catalysts were explored.

1 Experimental 1.1 Catalyst preparation MnOx-TiO2 (MnTi), CeO2-MnOx-TiO2 (CeMnTi) and V2O5-MnOx-TiO2 (VMnTi) mixed oxide catalysts were prepared by sol-gel method. The tetrabutyl titanate (99%, Beijing Chemical Reagents) and ethanol were mixed by magnetic stirring according to the molar ratio of 1:8. Acetic acid was added to keep the pH value of the mixed solution at 3. Manganese nitrate (50% water solution, Beijing Yili) was added dropwise to obtain dark yellow gel according to the mass ratio of MnO2:TiO2=3:7. For the Ce- and V-doped

Foundation item: Project supported by 863 Project (2009AA06Z313, 2010CB732304) Corresponding author: WU Xiaodong (E-mail: [email protected]; Tel.: +86-10-62792375) DOI: 10.1016/S1002-0721(10)60403-6

WU Xiaodong et al., Effects of cerium and vanadium on the activity and selectivity of MnOx-TiO2 catalyst for low-temperature … 65

samples, cerous nitrate (99%, Yili) and ammonium vanadate (99%, Beijing Chemical Reagents) were added according to the mass ratios of CeO2:MnO2:TiO2 and V2O5:MnO2:TiO2= 1:3:6. The obtained gel was dried at 110 ºC overnight and then calcined at 500 ºC for 3 h in a muffle furnace. 1.2 Activity measurement The activity measurements for NH3-SCR were carried out in a fixed bed reactor with 500 mg of catalyst (diluted to 2 ml by silica) inside. The reaction gas mixture consisted of 500 ppm NO, 500 ppm NH3, 5% O2 and N2 in balance. The total flow of the gas mixture was 1 L/min at a gas hourly space velocity (GHSV) of 30 000 h–1. The NO conversion was measured from room temperature (RT) to 400 °C at a heating rate of 10 °C/min. The concentrations of nitrogen oxides (NO, N2O and NO2) and ammonia were measured by a Thermo Nicolet 380 FTIR spectrometer. The light-off temperature (T50) of NO conversion was defined as the temperature when NO conversion reached 50%. 1.3 Catalyst characterization X-ray diffraction (XRD) experiments were performed on a Japan science D/mas-RB diffractometer employing Cu Kα radiation (λ=0.15418 nm) at 40 kV and 120 mA. The diffractogram was recorded at 0.02º intervals in the range of 20º<2θ<80º. The lattice constants and mean crystallite sizes of anatase titania were calculated from Cohen’s method and Debye-Scherrer equation, respectively. Brunauer-Emmett-Teller (BET) surface areas were calculated from N2 adsorption data obtained via an F-Sorb 3400 apparatus (Gold APP Instrument) at ~196 ºC. Before the

measurement, the sample was degassed in N2 at 200 ºC for 2 h. X-ray photoelectron spectroscopy (XPS) experiments were carried out on a PHI-Quantera SXM system equipped with a monochromatic Al Ka X-ray source under UHV (6.7×10–8 Pa). The binding energy of C 1s (284.8 eV) was used as an internal standard. H2 temperature-programmed reduction (TPR) experiments were performed in a fixed-bed reactor with the effluent gases monitored by a mass spectrometer (OmniStar TM). Before the experiment, the sample was treated in 2% O2/He at 500 °C for 1 h. H2-TPR runs were executed from RT to 900 °C at a heating rate of 10 °C/min in 4% H2/He (30 ml/min). IR spectra for NH3 chemisorption were recorded using a thermo Nicolet 6700 FTIR spectrometer. The sample was pretreated at 500 °C in 5% O2/N2 for 1 h. After being cooled down to RT, the sample was flushed by N2 for 30 min for background collection. Then, a gas mixture containing 1000 ppm NH3 and N2 (100 ml/min) was passed through the sample for 1 h. The IR spectra were collected after being purged by N2 flow gas for 1 h. Ammonia temperature-programmed oxidation (NH3-TPO) experiments were carried out using a similar method to NH3-SCR measurement with 500 ppm NH3 and 5% O2 in N2. The light-off temperature (T50) of NH3 conversion was defined as the temperature when NH3 conversion reached 50%.

2 Results and discussion 2.1 SCR activity NH3-SCR performances of the catalysts are shown in Fig. 1.

Fig. 1 NO conversion (a), N2O production (b), NO2 production (c) and NH3 conversion (d) of the catalysts during NH3-SCR processes

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NO conversion starts at 75 ºC and exceeds 80% in a wide temperature window of 150–350 ºC on MnTi, indicating a superior low-temperature SCR activity. However, a lot of N2O and NO2 are also generated with increasing the temperature. The NO conversion is slightly inhibited on CeMnTi catalyst with a shift of T50 towards high temperature by ca. 20 ºC, and the N2 selectivity is enhanced to some extent. A similar and more obvious phenomenon is observed on VMnTi, where the T50 shifts to 210 ºC and the maximal NO conversion decreases to 90%. However, the selectivity to N2 is close to 100% over this catalyst. 2.2 Solid properties The XRD patterns of the catalysts are shown in Fig. 2. Anatase (tetragonal), rutile (tetragonal) and brookite (orthorhombic) coexist on MnTi. No characteristic peaks of MnOx phases are observed. According to the similar ionic radii of Mn4+ (0.053 nm), Mn3+ (0.058 nm) and Ti4+ (0.061 nm), it is possible for Mnx+ ions to be incorporated into the titania lattice to form MnOx-TiO2 solid solutions[5]. The characteristic peaks of rutile and brookie TiO2 disappear on the doped samples, indicating a restraining effect of Ce and V on the transformation of metastable anatase titania to the more thermodynamically stable rutile phase[18]. No diffraction peaks of ceria or vanadium oxides are observed, implying the form of amorphous oxides. Typical peaks of cubic Mn2O3 crystallites are found for VMnTi, indicating the segregation of manganese oxide out of the titania lattice. No such phenomenon occurs on CeMnTi. The bulk Mn2O3 contributes little to NO conversion at low temperatures[19]. The structural features of the catalysts based on XRD data and BET method are summarized in Table 1. The addition of cerium and vanadium does not result in any obvious distor-

tions of the titania lattice. The ionic radii of Ce4+ (0.087 nm, CN=6) and Ce3+ (0.102 nm) are much larger than those of Ti4+, Mn4+ and Mn3+, implying the difficulty of incorporation of ceria to the tetragonal structure. Vanadium oxide prefers to exist in amorphous state on the surface of MnTi because of its noncrystalline nature[2]. The sintering of anatase TiO2 is limited to some extent by addition of ceria to segregate the particles of manganese oxides and titania, while the introduction of vanadium oxide induces a certain expansion of titania lattice due to the segregation of manganese oxide. Correspondingly, the surface area of the catalyst follows the order of CeMnTi>MnTi>VMnTi. However, the addition of metal oxides does not lead to a drastic change in surface area. The O 1s, Mn 2p, Ti 2p, Ce 3d and V 2p XPS spectra of the catalysts were measured (not shown). Table 2 presents the atomic surface concentrations for these elements. Lower Mn/Ti atomic ratios than the nominal values are obtained for all the catalysts, indicating the diffusion of manganese oxide into the TiO2 matrix or its segregation and sintering[2]. A similar poor content of cerium is detected on CeMnTi, while the high vanadium content confirms the coverage of VMnTi by amorphous vanadium oxide. More Mn4+ cations are reduced to Mn3+ on VMnTi due to the formation of crystalline Mn2O3. This result is in accordance with the report that Mn2O3 exhibits higher selectivity for nitrogen while the MnO2 exhibits higher activity based on the study of unsupported manganese oxides for NH3-SCR[20]. 2.3 Redox property Fig. 3 shows the TPR profiles of the catalysts using H2 as the reductant. Two partially overlapped peaks at 315 and 375 ºC on MnTi are related to the successive reductions of highly dispersed MnO2/Mn2O3→Mn3O4 and Mn3O4→MnO in the Table 2 Atomic surface compositions of the catalysts Catalysts

Mn/Ti

Surface composition/mol.% Mn

Ti

Ce

V

molar ratio

MnTi

64.90

5.94

29.16

–

–

0.18

CeMnTi

65.37

6.42

27.57

0.64

–

0.20

1.34

VMnTi

65.29

5.46

24.41

–

4.84

0.20

0.53

Fig. 2 XRD patterns of the catalysts Table 1 Structural features of the catalysts Catalysts

Lattice constants of TiO2 a/b/nm

c/nm

V/nm3

Crystallite size of

SBET/

TiO2/nm

(m2/g)

MnTi

0.3795(3)

0.9454(6)

0.1362

8.7(3)

80

CeMnTi

0.3807(2)

0.9381(5)

0.1360

7.2(1)

112

VMnTi

0.3796(1)

0.9422(4)

0.1358

9.4(2)

73

Mn4+/Mn3+

O

Fig. 3 H2-TPR profiles of the catalysts

1.66

WU Xiaodong et al., Effects of cerium and vanadium on the activity and selectivity of MnOx-TiO2 catalyst for low-temperature … 67

solid solutions, respectively[3]. The small peak at 228 ºC is ascribed to the reduction of Mn4+ to Mn3+ at the MnOx-TiO2 interface because of interaction between manganese oxide and titania, while the broad peak above 470 ºC is due to the reduction of Ti4+ to Ti3+ in pure TiO2[3,5]. Two smaller lowtemperature peaks appear at the same temperatures on CeMnTi, indicating a weakened reducibility due to the decreased Mn4+/Mn3+ ratio. The peak centered at 550 ºC can be related to the reduction of Ce4+ promoted by Mnx+ [21], which merges with the peak for Ti4+ reduction. For the V-doped catalyst, the low-temperature peaks associated with the reduction of highly dispersed Mnx+ species decrease sharply in intensity due to the segregation and sintering of manganese oxides. An intense peak appears at 500 ºC which is attributed to overlapped reduction peaks of amorphous vanadium oxide and crystalline manganese sesquioxide. 2.4 NH3 oxidation It has been reported that the selectivity of catalyst in a SCR reaction is mainly governed by ammonia consumption[5]. To be a good catalyst for NOx abatement by NH3SCR, the primary requirement of a catalyst is that it should not exhibit a high NH3 oxidation activity, as shown in Fig. 1. Fig. 4(a) shows the ammonia oxidation activities of the catalysts. The T50 of MnTi, CeMnTi and VMnTi is 189, 211 and 335 ºC, respectively, which follow the same order of reducibility as indicated by the H2-TPR curves, and it suggests highly dispersed manganese oxides as main active sites for ammonia oxidation. The main products of ammonia oxidation over the catalysts are N2O (Fig. 4 (b)) and N2. Almost no NO or NO2 are observed. Again, the inhibition effect of

V-modification on the over-oxidation of ammonia to nitrous oxide is demonstrated. 2.5 NH3 adsorption The IR spectra of ammonia adsorbed on the catalysts at RT are shown in Fig. 5. The bands at 1 594 and 1 165 cm–1 are attributed to σas and σs model of NH3 coordinated to Lewis acid sites[2]. Two bands at 1 674 and 1 445 cm–1 are attributed to σs NH4+ and σas NH4+ resulting from ammonia coordinated to Brønsted acid sites [2,6]. It is indicated by the increase in intensity of band at 1 165 cm–1 that more Lewis acid sites are generated on CeMnTi by introduction of Cex+ which can also serve as Lewis acid sites. In combination with the decreased activity for ammonia oxidation via the electron-donating effect of cerium on the oxidation state of Mn cations, it endowed the Ce-doped catalyst a higher N2 selectivity than MnOx-TiO2 in despite of the slightly elevated light-off temperature for NO conversion. VMnTi possesses a decreased amount of Lewis acid sites but an enhanced Lewis acid strength. Meanwhile, a strong Brønsted acidity is obtained on this catalyst which arises from surface vanadyl centers. It can be said that Brønsted acid sites do not appear to be indispensable to carry out the SCR reaction at low temperatures[2]. However, it has been reported that ammonia adsorbed on Brønsted acid sites is more stable than Lewis-coordinated ammonia[22]. In this sense, more ammonium-derived species are available for NO reduction at relatively high temperatures (>200 ºC), which is responsible for the high N2 selectivity for VMnTi[23].

Fig. 5 IR spectra of the catalysts arising from contact of ammonia at RT

3 Conclusions

Fig. 4 NH3 conversion (a) and N2O production (b) of the catalysts during NH3-TPO processes

The low-temperature NH3-SCR performances of MnOxTiO2 based catalysts were greatly determined by the surface acidity and redox properties. The high oxidation state of Mnx+ and well dispersed manganese species were responsible for high NO conversion and low N2 selectivity of MnTi catalyst. The redox property of the V-doped catalyst was weakened arising from the formation of Mn2O3 crystallites. Meanwhile, more stable ammonium on Brønsted acid sites from surface vanadyl centers was available for NO reduction.

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Correspondingly, VMnTi catalyst showed a poor low-temperature activity but almost 100% selectivity to nitrogen. To achieve a balance between low-temperature activity and high selectivity, CeMnTi catalyst may be a better candidate due to modification of redox and acidic properties of the catalyst by ceria. Further work needs to be done for improving the selectivity of this catalyst.

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