Promoting N2 Selectivity of CeMnOx Catalyst by Supporting TiO2 in

Subscriber access provided by UNIV AUTONOMA DE COAHUILA UADEC

Kinetics, Catalysis, and Reaction Engineering

Promoting N2 Selectivity of CeMnOx Catalyst by Supporting TiO2 in NH3-SCR Reaction Lei Zhang, Lulu Li, Chengyan Ge, Tingzhen Li, Chuanjiang Li, Shuxin Li, Feng Xiong, and Lin Dong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00650 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Promoting N2 Selectivity of CeMnOx Catalyst by Supporting TiO2 in NH3−SCR Reaction Lei Zhang*,†, Lulu Li‡, Chengyan Ge§, Tingzhen Li†, Chuanjiang Li†, Shuxin Li†, Feng Xiong†, Lin Dong*,‡ †School

of Environmental and Chemical Engineering, Chongqing Three Gorges University,

Chongqing 404000, People’s Republic of China ‡Key

Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical

Engineering, Nanjing University, Nanjing 210093, People’s Republic of China §School

of Chemistry and Chemical Engineering, Yancheng Institute of Technology, Yancheng

224051, People’s Republic of China

* To whom correspondence should be addressed. Corresponding author: E-mail: [email protected]; E-mail: [email protected]

1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: CeMnOx catalyst was synthesized by combustion method and the supported Ti/CeMnOx catalyst was subsequently prepared by wetness impregnation. Ti/CeMnOx catalyst exhibited higher N2 selectivity than CeMnOx catalyst in the selective catalytic reduction NO by NH3 (NH3−SCR), and the generation of side products including in N2O and NO2 were discussed. N2 adsorption/desorption, X−ray diffraction (XRD), X−ray photoelectron spectroscope (XPS), H2 temperature programmed reduction (TPR), NH3 temperature programmed desorption (TPD) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were carried out to reveal the promotion effect of TiO2 on the N2 selectivity of CeMnOx catalyst. Results indicated that loading TiO2 weakened the redox ability and enhanced the NH3 adsorption ability of CeMnOx catalyst, thus inhibited NO/NH3 oxidation and facilitated the Eley−Rideal reaction pathways even at higher temperature.

Keywords: low temperature denitrification; NH3−SCR; N2 selectivity; Ti/CeMnOx catalyst

2


Page 2 of 21

Page 3 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction The selective catalytic reduction NOx by NH3 technique is the most widely used for NOx degradation (deNOx) in coal−fired power statio.1 Commercial V2O5−WO3/TiO2 catalyst works well in the range from 300 to 400 °C, thus the catalyst need be installed in the upstream of the electrostatic precipitator and desulfurization.2,3 However, the ash and SO2 in the flue gas result in channel block and catalyst poisoning. In order to solve these problems, one way is installing the catalyst in the downstream of the electrostatic precipitator and desulfurization. In this installation mode condition, the temperature of flue gas is usually below 200 °C without extra heating device, V2O5−WO3/TiO2 catalyst is no more satisfactory, and thus the catalysts with good catalytic activity at low temperature must be developed. Manganese−based (Mn−based) catalysts are one of the most potential catalysts applied in the low temperature NH−SCR reaction. It is reported that the catalysts consisted of manganese oxide and other metal oxides can exhibit high activity at low temperature. For example, CeMnOx and TiMnOx are the most reported Mn−based metal oxide catalysts,4-12 and the others including in CrMnOx,2 WMnOx,13 FeMnOx,14,15 NiMnOx,16 ZrMnOx,17 NdMnOx,18 etc. which show good catalytic activity in the low temperature range from 100 to 200 °C. The formation of new phase, the variety of Mn valence states, the increase of surface acid sites and specific surface area are considered as main reasons for improving the low temperature activity of Mn−based catalysts.2-18 Though Mn−based catalysts have received increasing attention for utilization in the low temperature NH3−SCR reaction, some problems including the poisoning effects by H2O or/and SO2, poisoning effect of alkali metal and poor N2 selectivity must be solved to satisfy requirements in actual working condition.8, 19-22 According to literature, it can be found that N2 selectivity is sensitive to reaction temperature over Mn−based catalysts in NH3−SCR reaction.3,5,13,15,18,23 Though NO conversion over Mn−based catalysts is very high in the activity window, more side products including N2O or/and NO2 form when reaction temperature increases, which can’t meet the stringent legislations and regulations for limiting NOx emissions. In this report, CeMnOx catalysts were prepared by combustion method, and the supported Ti/CeMnOx catalyst was prepared by impregnation method. The obtained catalysts were characterized by XRD, XPS, N2 adsorption/desorption, H2−TPR, NH3−TPD, and in situ DRIFTS. We focus on studying the effect of supporting TiO2 on the N2 3



selectivity of CeMnOx catalysts.

2. Experimental Section 2.1. Sample Preparation A series of CeMnOx catalysts were prepared by combustion method. Cerium nitrate, manganese nitrate, critic acid and distilled water were added into a round flask. After stirring for 1 h at room temperature, the flask was heated by oil bath at 110 °C, until the distilled water was completely evaporated. Subsequently the oil bath temperature rose to 180 ºC and the mixtures combusted to obtain powder samples. The powder samples were calcined at 500 °C for 4 h in flowing air. For simplicity, the optimized sample (the molar ratio Ce/Mn=3/1) is denoted as CeMn. The catalytic activity of CeMn catalysts with different Ce/Mn ratios was given in Figure S1 in the Supporting Information. The supported Ti/CeMnOx catalyst was prepared by wetness impregnation. According to our previous study about Ti/CeO2 supported catalyst, the molar ratio of Ti/(Ce + Mn) was decided to 0.15.24 The catalytic activity of Ti/CeMn catalysts with different Ti/(Ce + Mn) ratios was also given in Figure S2 in the Supporting Information. The preparation of sample with 0.15 of Ti/(Ce + Mn) ratio was given as follow. 0.5076 g tetrabutyl titanate and 1.5 g CeMn sample were added into 10 mL ethanol under stirring for 1 h. After that, 2 mL water was added. The solvent was evaporated at 80 °C, subsequently dried at 110 ºC for 3 h and calcined at 500 ºC for 4 h in flowing air. For simplicity, the sample with 0.15 of Ti/(Ce + Mn) ratio is denoted as Ti/CeMn.

2.2. Characterization N2 physisorption isotherms were measured at –196 °C on a Micrometrics ASAP−2020 analyzer. The sample was degassed under vacuum at 300 °C for 4 h before each measurement. XRD patterns were obtained by a Philips X’pert Pro diffractometer with Ni−filtered Cu Kα1 radiation (0.15408 nm). The X−ray tube was operated at 40 kV and 40 mA. XPS experiments were implemented on a PHI 5000 VersaProbe system with monochromatic Al Kα radiation (1486.6 eV). The binding energy of C1s at 284.6 eV was used for calibrating the charging effect of samples. H2−TPR measurements were conducted in a quartz U−tube reactor connected to a thermal conductivity detector (TCD). H2/Ar (7% H2 by volume) mixture was used as reaction gas keeping 4


Page 4 of 21

Page 5 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


gas rate at 40 mL·min-1, and each sample usage was 50 mg. Firstly, the sample was pretreated in high−purity N2 at 200 °C for 1 h, and then reduced by H2/Ar mixture from room temperature to 850 °C (heating rate was 10 °C·min-1). 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 oC. Each sample usage was 200 mg. Firstly, the sample was pretreated in high−purity He at 450 °C for 1 h. Secondly, 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 oC to 500 °C at a rate of 10 °C·min-1 in flowing high−purity He (flow rate was 100 mL·min-1). In situ DRIFTS spectra were collected on a Nicolet Nexus 5700 FTIR spectrometer equipped with a high−sensitive MCT detector. Powder samples were pretreated in high−purity N2 at 400 °C for 1 h prior to the collection of background spectra and adsorption experiments.

2.3. Catalytic Activity Tests NH3−SCR catalytic performance of sample catalysts was evaluated in a fixed−bed quartz reactor under steady state, and the experimental device schematic has been given in our previous work.25 The reaction gases were composed of 500 ppm NO, 500 ppm NH3 and 5 vol.% O2 with N2 as rest gas, and the total gas flow rate was 100 mL·min-1. Each sample (150 mg) was pretreated in N2 at 200 °C for 1 h before testing catalytic performance. The inlet and outlet concentration of NO, NH3, NO2, and N2O were continuously analyzed by an online Nicolet IS10 IR spectrometer. NO conversion and N2 selectivity were calculated using the following equations: NO conversion (%) = 100  ([NO]in – [NO]out)/ NOin N2 selectivity (%) = 100  ([NO]in – [NO]out + [NH3]in– [NH3]out – [NO2]out – 2[N2O]out)/ ([NO]in – [NO]out + [NH3]in – [NH3]out)

3. Results and Discussion 3.1. Catalytic Activity Results

5



Figure 1. Catalytic performance results of CeMn and Ti/CeMn samples (a); the outlet concentration of N2O and NO2 in NH3−SCR reaction over CeMn and Ti/CeMn samples (b). Catalytic performance results of CeMn and Ti/CeMn samples in the range of 50−250 oC were shown in Figure 1a. CeMn had little higher NO conversion than Ti/CeMn below 100 ºC, while when the reaction temperature was above 100 ºC, both samples exhibited nearly 100% NO conversion. However, the N2 selectivity of Ti/CeMn was much higher than that of CeMn in the whole temperature range. Catalytic reaction test show that supporting TiO2 could improve the N2 selectivity of CeMn. The content of side product including N2O and/or NO2 was recorded and shown in Figure 1b. It can be found that only N2O formed below 175 ºC, while N2O and NO2 formed above 175 ºC over both CeMn and Ti/CeMn, and the content of side products increased with the reaction temperature rising. More importantly, CeMn produced more N2O and NO2 than Ti/CeMn at the same reaction temperature.

Figure 2. N2O outlet concentration in NH3 oxidation reaction, the reaction conditions: 150 mg catalyst, 500 ppm NH3 + 5% O2 with the total gas flow velocity of 100 ml·min-1 (a); NO2 outlet concentration in NO oxidation reaction, the reaction conditions: 150 mg catalyst, 500 ppm NO + 5% O2 with the total gas flow velocity of 100 ml·min-1 (b). 6


Page 6 of 21

Page 7 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NH3 oxidation reaction was conducted. As shown in Figure S3 in the Supporting Information, CeMn had higher NH3 conversion than Ti/CeMn, which indicates that TiO2 modification could inhibit NH3 oxidation. Figure 2a shows N2O outlet concentration in NH3 oxidation reaction. It is worth noting that no NO or NO2 was detected in the whole reaction temperature range. Yang et.al5 reported that only N2O produced when NH3 was introduced to MnOx−CeO2 catalyst at 180 oC and Jurng et.al26 also found similar results over Mn/TiO2 catalyst when the temperature was below 250 oC. When the reaction temperature was below 100 ºC, more than 20% NH3 was oxidized without N2O product, which indicates NH3 oxidation reaction was: 4NH3 + 3O2 = 2N2 + 6H2O

(1)

N2O produced when the temperature was above 100 ºC, which indicates that reaction (2) happened except for reaction (1): 2NH3 + O2 = N2O + 3H2O

(2)

Figure 2a shows that N2O concentration increased with the reaction temperature rising over CeMn and Ti/CeMn, while CeMn had higher N2O concentration than Ti/CeMn. The results of NH3 oxidation reaction illustrate that TiO2 modification could inhibit N2O formation in the NH3 oxidation process. NO oxidation reaction was conducted and NO2 outlet concentration was shown in Figure 2b. NO2 concentration increased with the reaction temperature increasing over CeMn and Ti/CeMn, and NO2 concentration obviously enhanced above 200 ºC. Meanwhile, CeMn had higher NO2 concentration than Ti/CeMn, which indicates that TiO2 modification could inhibit the NO oxidation reaction: 2NO + O2 = 2NO2

(3)

Combined with the above catalytic reaction results, for SCR reaction over CeMn, when the temperature was below 175 ºC, N2 selectivity decreased at a relatively slow rate with the increase of reaction temperature. The major reactions were the so called “standard SCR” reaction (4) and “fast SCR” reaction (5).27 Combined with the N2O concentration in Figure 1b and Figure 2a, N2O concentration in Figure 1b for NH3−SCR reaction was higher than N2O concentration in Figure 2a for NH3 oxidation reaction, and the difference value of N2O concentration in Figure 1b and in Figure 2a for the same catalysts was larger and larger with the reaction temperature increasing. It can be explained by the side reaction (6), which was sensitive to temperature, and the generation 7



of N2O via the side reaction (6) was more and more at higher temperature. 4NH3 + 4NO + O2 = 4N2 + 6H2O

(4)

2NH3 + NO + NO2 = 2N2 + 3H2O

(5)

4NH3 + 4NO + 3O2 = 4N2O + 6H2O

(6)

When the reaction temperature was above 175 ºC, N2 selectivity obviously dropped. On one hand, the side reaction (2) and (6) happened and were sensitive to temperature. On the other hand, Figure 2b has shown that NO2 concentration drastically increased above 175 ºC as the existence of side reaction (3), although the generation of NO2 was partly good for the reaction (5), the side reaction (3) of NO oxidation could produce excessive NO2 during SCR process, which couldn’t be completely consumed by reaction (5). For NH3−SCR reaction over Ti/CeMn, the concentrations of N2O and NO2 were inhibited, thus Ti/CeMn exhibited higher N2 selectivity than CeMn. We will further discuss the effect of supporting TiO2 on improving the N2 selectivity of CeMn.

3.2 XRD and N2 Adsorption/Desorption Results

Figure 3. XRD patterns (a) and N2 adsorption−desorption isotherm curves (b) of CeMn and Ti/CeMn samples. As shown in Figure 3a, all XRD peaks could be ascribed to the cubic fluorite−type phase CeO2 (PDF−ICDD 34−0394) over CeMn and Ti/CeMn, which suggests that TiO2 species might be either dispersive or amorphous over Ti/CeMn. N2 adsorption−desorption isotherms (Figure 3b) and BJH pore distribution curves (Figure S4 in the Supporting Information) of CeMn and Ti/CeMn are also displayed. It can be seen from Figure 3b that both samples exhibited the IV−type isotherm with Type H3 hysteresis loop as defined by IUPAC, which indicates that their mesoporous structure formed by the agglomeration of plate−like particles.28,29 Textural data of 8


Page 8 of 21

Page 9 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


these samples are listed in Table 1. CeMn and Ti/CeMn had similar specific surface area with ca. 20 m2·g–1 and mean pore diameter with ca. 11 nm. Thus XRD and BET results indicate that textural properties were not the main factors to influence on the catalytic performance of CeMn by supporting TiO2. Table 1 Textural data and surface acid content of CeMn and Ti/CeMn Samples

Specific surface area/m2·g-1

Pore diameter/nm

Acidity/mmol·g-1

CeMn

18.8

11.6

19.1

Ti/CeMn

22.7

10.9

25.3

3.3 H2−TPR and NH3−TPD Results Redox properties were investigated by H2−TPR. As shown in Figure 4a, both CeMn and Ti/CeMn samples had three reduction peaks, α, β and γ. According to literatures30-32, the peaks α and β should be mainly attributed to the consequential reductions of MnO2 to Mn3O4 and Mn3O4 to MnO, respectively. In addition, the reduction of surface Ce4+ was also likely to be included in the peaks α and β. The peak γ was mainly resulted from the reduction of bulk CeO2,24 and loading TiO2 had slightly effect on the peak temperature of γ. It is worth noting that loading TiO2 could obviously influence on the reduction temperature of peaks α and β. The α and β peak temperature of Ti/CeMn shifted ca. 60 oC to higher temperature compared with CeMn, which indicates that loading TiO2 could weaken the redox ability of CeMn. Previous catalytic reaction testing results indicate that the side reactions (2) and (3) were inhibited and the N2 selectivity of NH3−SCR reaction increased over Ti/CeMn compared with CeMn. Thus, properly weakening the redox ability of CeMn by supporting TiO2 could reduce the amount of side products and was beneficial to enhancing N2 selectivity in NH3−SCR reaction.

Figure 4. H2−TPR (a) and NH3−TPD (b) results of CeMn and Ti/CeMn samples. 9



NH3−TPD was conducted to investigate the surface acid properties including in the acid strength and amount. As shown in Figure 4b, both CeMn and Ti/CeMn had a broad desorption peak, the desorption peak temperature of Ti/CeMn was higher than that of CeMn, and the total desorption temperature of Ti/CeMn at ca. 430 oC was higher than that of CeMn at ca.330 oC, which indicates that loading TiO2 could enhance the NH3 adsorptive strength on the surface of CeMn. In addition, the acid amounts of CeMn and Ti/CeMn were calculated by integrating NH3−TPD peaks and the results were listed in Table 1. The acid amount of Ti/CeMn was higher than that of CeMn, which indicates that supporting TiO2 could also promote the surface acid amount of CeMn. Thus, for CeMn, both surface acid strength and surface acid amount could be improved by loading TiO2. It is possible that the change of surface acid properties resulting from the surface modification of TiO2 reduced the generation of N2O via the side reaction (2) and (6), and improved N2 selectivity in NH3−SCR reaction.

3.4 XPS Results XPS experiments were conducted to investigate the surface elemental valence state of catalysts. As shown in Figure 5, for Ti 2p spectra, the binding energy of Ti 2p3/2 was 458.1 eV, which indicates that the value state of Ti was +4 on the surface of Ti/CeMn sample. In addition, as shown in Table S1, the molar ratio of Ti measured by XPS was higher than that measured by XRF, which indicates that TiO2 was mainly dispersive state on the surface of CeMn, which is accordance with XRD results. For O 1s spectra, the bands O′ and O′′ could be ascribed to surface oxygen species and bulk oxygen species, respectively.4,33 For Ce 3d spectra, both Ce3+ (the bands of u′ and v′) and Ce4+ (the bands of u, u′′, u′′′, v, v′′, v′′′) could be found over CeMn and Ti/CeMn samples.4 In addition, Mn2+ (640.5−640.8 eV for 2p3/2), Mn3+ (641.8−642.3 eV for 2p3/2) and Mn4+ (ca. 643.5 eV for 2p3/2) were mainly existed over CeMn and Ti/CeMn samples as shown in Mn 2p spectra.33,34 Table 2 shows that CeMn had more surface oxygen species than Ti/CeMn, and it is probably because that supporting TiO2 could inhibit active oxygen species on the surface of CeMn, which resulted in the decrease of redox ability of Ti/CeMn as proved by H2-TPR results. In addition, Table 2 shows that the ratios of Ce3+/( Ce3+ + Ce4+) and Mn4+/∑Mnn+ of CeMn were nearly the same as those of Ti/CeMn, which illustrates that supporting TiO2 had no obvious influence on the surface value state of Ce and Mn elements. 10


Page 10 of 21

Page 11 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. XPS results of CeMn and Ti/CeMn samples. Table 2 Surface element content of CeMn and Ti/CeMn measured by XPS. Concentration/mol.%

Atomic ratio/%

Samples CeMn

Ce

Ti

Mn

O

Ce3+/(Ce3+ + Ce4+)

O'/( O' + O'')

Mn4+/∑Mnn+

14.6

0.0

4.1

81.3

16.2

53.8

17.0

10.5

7.1

4.5

16.6

42.7

16.4

Ti/CeM

77.9

n

3.5 In situ DRIFTS Adsorption−Desorption Results

11



Figure 6. NH3 adsorption−desorption DRIFTS of CeMn and Ti/CeMn samples. The reaction conditions were: 1000 ppm NH3 and N2 balance. Samples were pre−adsorbed to saturation by introduction of NH3 for 1 h, and purged with N2 for 15 min at 100 oC, and then the corresponding spectra were collected in flowing N2 as the function of temperature. NH3 adsorption−desorption DRIFTS experiments were conducted to further investigate the influence of supporting TiO2 on the surface acid properties of CeMn. As shown in Figure 6, the bands in the range of 3200−3400 cm-1 could be assigned to N−H vibrations of adsorbed NH3.10 For CeMn, the bands at 1603, 1298, 1192 and 1124 cm-1 could be ascribed to the adsorption of NH3 on Lewis acid sites, and previous study found that the bands due to the adsorption of NH3 on Lewis acid sites over pure CeO2 were at 1574, 1291, 1143 and 1061 cm-1, this is possibly due to the interaction between CeO2 and MnOx.35,36 In addition, the band at 1437 cm-1 could be attributed to the adsorption of NH3 on Brönsted acid sites.10,15,37 For Ti/CeMn, there were only two bands in 1000−1800 cm-1 at 1614 and 1189 cm-1 which could be ascribed to NH3 on Lewis acid sites. According to our previous report,38 the single band at ca. 1180 cm-1 was the Lewis acid sites provided by TiO2, and that is to say, NH3 adsorption and activation mainly happened on the surface dispersed TiO2 over Ti/CeMn. It can be seen from Figure 6 that the intensity of bands decreased with the increase of temperature both over CeMn and Ti/CeMn. For CeMn, the bands totally disappeared when the temperature was above 325 oC. However, the bands still existed at 400 oC for Ti/CeMn. Thus the NH3 adsorption strength of CeMn could be enhanced by supporting TiO2, which was in accord with the results of NH3−TPD. NO adsorption feature was investigated by NO + O2 adsorption−desorption DRIFTS and the results were shown in Figure 7. The bands at 1608−1624 and 1230−1209 cm-1 could be ascribed to 12


Page 12 of 21

Page 13 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


bridging nitrate, the bands at 1572−1547 and 1294−1263 cm-1 could be attributed to bidentate nitrate, and the band at 1031−1015 cm-1 was the NO2 symmetric vibrational band.18,39 For the same nitrate species, the difference of adsorption peak location between CeMn and Ti/CeMn was due to the surface modification of TiO2. All adsorption species were nitrate species for CeMn. However, except for nitrate species, nitrite species (the bands at 1500 and 1448 cm-1)11,39 could be found for Ti/CeMn when the temperature was below 175 oC, which was possibly due to weakening the redox property of Ti/CeMn by loading TiO2 as evidenced by H2−TPR. Some studies also show that nitrite species was a viable reaction route and might be beneficial for the low temperature NH3−SCR activity.27,40 In addition, both CeMn and Ti/CeMn exhibited the nearly same desorption behavior for absorbed nitrate species, indicating that supporting TiO2 had less influence on the adsorption strength of nitrate species.

Figure 7. NO + O2 adsorption−desorption DRIFTS of CeMn and Ti/CeMn samples. The reaction conditions were: 1000 ppm NO, 5% O2 and N2 balance. Samples were pre−adsorbed to saturation by introduction of NO + O2 for 1 h, and purged with N2 for 15 min at 100 oC, and then the corresponding spectra were collected in flowing N2 as the function of temperature. To further investigate the influence of supporting TiO2 on the reaction mechanism of CeMn, NH3 + NO + O2 in situ DRIFTS reaction experiments were conducted. As shown in Figure 8, for CeMn, when temperature was below 200 oC, the bands at 1609, 1280 and 1189 cm-1 due to Lewis acids and 1432 cm-1 due to Brönsted acid could be found, indicating that mainly NH3 adsorption happened in the low temperature reaction process. When temperature was higher than 200 oC, nitrate species (the bands at 1524 and 1255 cm-1) existed and the intensity enhanced with the increase of temperature, while NH3 adsorption decreased and totally disappeared above 250 oC, 13



indicating NO adsorption mainly happened in the high temperature reaction process. It is well known that NH3 adsorption and activation was an essential step in the NH3−SCR process.10,13,40,41 According to in situ DRIFTS reaction results, it can be inferred that NH3−SCR reaction happened via the Eley−Rideal mechanism (the reaction between the adsorbed NH3 species and the gaseous NO/O2) at low temperature (below 200 oC) and mainly the Langmuir−Hinshelwood mechanism (the reaction between the adsorbed NH3 species and the adsorbed NOx species) at high temperature (above 200 oC) over CeMn. For Ti/CeMn, in situ DRIFTS reaction shows that the NH3 adsorption on Lewis acid sites (the bands at 1606, 1196−1176 cm-1) and on Brönsted acid sites (the band at 1440 cm-1). Similar to CeMn, only NH3 adsorption could be detected when the temperature was below 200 oC and the adsorption intensity of NH3 decreased with the temperature increasing over Ti/CeMn. Different from CeMn, NH3 adsorption could still exist at 300 oC and only a weak band at 1254 cm-1 due to nitrate species was found when temperature was above 200 oC

over Ti/CeMn. Thus, it can be speculated that NH3−SCR reaction was dominated via the

Eley−Rideal mechanism in the range of 100−300 oC over Ti/CeMn.

Figure 8. NH3 + NO + O2 in situ DRIFTS of CeMn and Ti/CeMn samples as the function of temperature. The reaction conditions were: 500 ppm NO, 500 ppm NH3, 5% O2 and N2 balance. The samples were pre−adsorbed in the flue of NO + NH3 + O2 for 1 hour at 100 oC and the spectra were collected as the function of temperature.

4. Conclusions In this study, CeMn and Ti/CeMn catalysts were prepared and their NH3−SCR performance was investigated. Catalytic reaction results indicate that N2 selectivity was sensitive to reaction 14


Page 14 of 21

Page 15 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperature, N2O production was originated from the side reaction (2) and (6), and NO2 production was related to the side reaction (3). Combined with multi−characterizations, the promotion effect of N2 selectivity of CeMn catalyst by loading TiO2 could be explained, and the reasons were summarized. On one hand, TiO2 modification could properly weaken the redox ability of CeMn catalyst, which could inhibit NH3 nonselective oxidation and NO oxidation reaction to some extent. On the other hand, the adsorption ability of NH3 was enhanced by supporting TiO2, and NH3 was preferential to be adsorbed on Ti/CeMn in the competition adsorption between NH3 and NO, thus NH3−SCR reaction over Ti/CeMn catalyst was dominated by the Eley−Rideal mechanism even at above 200 oC, and the generation of NO2 and N2O could be inhibited. This work provides a simple strategy for improving N2 selectivity of CeMnOx catalyst.

Authors Information *Corresponding

Authors

(L. Z.)* Phone:+86-023-58102261. E-mail: [email protected]; (L. D.) Phone:+86-25-83592290. E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgement The financial supports of the National Natural Science Foundation of China (Nos. 21607019, 21806077, 21607122), the Chongqing Science &Technology Commission (cstc2013jcyjA20007) and the Open Project Program of Jiangsu Key Laboratory of Vehicle Emissions Control (No. OVEC045) are gratefully acknowledged.

Supporting Information Further additional information about NO conversion and N2 selectivity results of CeMn samples with different Ce/Mn molar ratios (Figure S1), NO conversion and N2 selectivity of CeMn and xTi/CeMn samples (Figure S2), NH3 conversion results of CeMn and Ti/CeMn samples in NH3 oxidation reaction (Figure S3), Pore diameter distribution curves of CeMn and Ti/CeMn samples (Figure S4), The normalized element amount of 15Ti/CeMn measured by XRF and XPS (Table 15



S1).

References (1) Wang, J.; Zhao, H.; Haller, G.; Li, Y. Recent advances in the selective catalytic reduction of NOx with NH3 on Cu–Chabazite catalysts. Appl. Catal. B: Environ. 2017, 202, 346–354. (2) Chen, Z,; Yang, Q.; Li, H.; Li, X.; Wang, L.; Tsang, S. C. Cr–MnOx mixed–oxide catalysts for selective catalytic reduction of NOx with NH3 at low temperature. J. Catal. 2010, 276, 56–65. (3) Roy, S.; Viswanath, B.; Hegde, M. S.; Madras, G. Low–temperature selective catalytic reduction of NO with NH3 over Ti0.9M0.1O2-δ (M = Cr, Mn, Fe, Co, Cu). J. Phys. Chem. C 2008, 112, 6002–6012. (4) Yao, X.; Kong, T.; Yu, S.; Li, L.; Yang, F.; Dong, L. Influence of different supports on the physicochemical properties and denitration performance of the supported Mn–based catalysts for NH3–SCR at low temperature. Appl. Surf. Sci. 2017, 402, 208–217. (5) Yang, S.; Liao, Y.; Xiong, S.; Qi, F.; Dang, H.; Xiao, X.; Li, J. N2 selectivity of NO reduction by NH3 over MnOx–CeO2: Mechanism and key factors. J. Phys. Chem. C 2014, 118, 21500– 21508. (6) Andreoli, S.; Deorsola, F. A.; Pirone, R. MnOx–CeO2 catalysts synthesized by solution combustion synthesis for the low–temperature NH3–SCR. Catal. Today 2015, 253, 199–206. (7) Chen, L.; Yao, X.; Cao, J.; Yang, F.; Tang, C.; Dong, L. Effect of Ti4+ and Sn4+ co– incorporation on the catalytic performance of CeO2–MnOx catalyst for low temperature NH3– SCR. Appl. Surf. Sci. 2019, 476, 283–292. (8) Chang, H.; Chen, X.; Li, J.; Ma, L.; Wang, C.; Liu, C.; Schwank, J. W.; Hao, J. Improvement of activity and SO2 tolerance of Sn–modified MnOx–CeO2 catalysts for NH3–SCR at low temperatures. Environ. Sci. Technol. 2013, 47, 5294–5301. (9) Wei, Y; Liu, J.; Su, W.; Sun, Y.; Zhao, Y. Controllable synthesis of Ce–doped α–MnO2 for low–temperature selective catalytic reduction of NO. Catal. Sci. Technol. 2017, 7, 1565–1572. (10) Wu, Z.; Jiang, B.; Liu, Y.; Wang, H.; Jin, R. DRIFT study of manganese titania–based catalysts for low–temperature selective catalytic reduction of NO with NH3. Environ. Sci. Technol. 2007, 41, 5812–5817. (11) Hu, H.; Cai, S.; Li, H.; Huang, L.; Shi, L.; Zhang, D. Mechanistic aspects of deNOx 16


Page 16 of 21

Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


processing over TiO2 supported Co–Mn oxide catalysts: Structure–activity relationships and in situ DRIFTs analysis. ACS Catal. 2015, 5, 6069–6077. (12) Sun, C.; Liu, H.; Chen, W.; Chen, D.; Yu, S.; Liu, A.; Dong, L.; Feng, S. Insights into the Sm/Zr co–doping effects on N2 selectivity and SO2 resistance of a MnOx–TiO2 catalyst for the NH3–SCR reaction. Chem. Eng. J. 2018, 347, 27–40. (13) Liu, F.; Shan, W.; Lian, Z.; Xie, L.; Yang, W.; He, H. Novel MnWOx catalyst with remarkable performance for low temperature NH3–SCR of NOx. Catal. Sci. Technol. 2013, 3, 2699−2707. (14) Chen, Z.; Wang, F.; Li, H.; Yang, Q.; Wang, L.; Li, X. Low–temperature selective catalytic reduction of NOx with NH3 over Fe–Mn Mixed–Oxide catalysts containing Fe3Mn3O8 phase. Ind. Eng. Chem. Res. 2012, 51, 202–212. (15) Yang, S.; Wang, C.; Li, J.; Yan, N.; Ma, L.; Chang, H. Low temperature selective catalytic reduction of NO with NH3 over Mn–Fe spinel: Performance, mechanism and kinetic study. Appl. Catal. B: Environ. 2011, 110, 71–80. (16) Wan, Y.; Zhao, W.; Tang, Y.; Li, L.; Wang, H.; Cui, Y.; Gu, J.; Li, Y.; Shi, J. Ni–Mn bi– metal oxide catalysts for the low temperature SCR removal of NO with NH3. Appl. Catal. B: Environ. 2014, 148–149, 114–122. (17) Zuo, J.; Chen, Z.; Wang, F.; Yu, Y.; Wang, L.; Li, X. Low–temperature selective catalytic reduction of NOx with NH3 over novel Mn–Zr mixed oxide catalysts. Ind. Eng. Chem. Res. 2014, 53, 2647–2655. (18) Guo, R.; Sun, P.; Pan, W.; Li, M.; Liu, S.; Sun, X.; Liu, S.; Liu, J. A highly effective MnNdOx catalyst for the selective catalytic reduction of NOx with NH3. Ind. Eng. Chem. Res. 2017, 56, 12566–12577. (19) Guo, R.; Wang, Q.; Pan, W.; Zhen, W.; Chen, Q.; Ding, H.; Yang, N.; Lu, C. The poisoning effect of Na and K on Mn/TiO2 catalyst for selective catalytic reduction of NO with NH3: A comparative study. Appl. Surf. Sci. 2014, 317, 111–116. (20) Jin, R.; Liu, Y.; Wang, Y.; Cen, W.; Wu, Z.; Wang, H.; Weng, X. The role of cerium in the improved SO2 tolerance for NO reduction with NH3 over Mn–Ce/TiO2 catalyst at low temperature. Appl. Catal. B: Environ. 2014, 148–149, 582–588. (21) Yu, S.; Jiang, N.; Zou, W.; Li, L.; Tang, C.; Dong, L. A general and inherent strategy to 17



improve the water tolerance of low temperature NH3–SCR catalysts via trace SiO2 deposition. Catal. Commun. 2016, 84, 75–79. (22) Cimino, S.; Totarella, G.; Tortorelli, M.; Lisi, L.. Combined poisoning effect of K+ and its counter–ion (Cl− or NO3−) on MnOx/TiO2 catalyst during the low temperature NH3–SCR of NO. Chem. Eng. J. 2017, 330, 92–101. (23) Stanciulescu, M.; Caravaggio, G.; Dobri, A.; Moir, J.; Burich, R.; Charland, J. P.; Bulsink, P. Low–temperature selective catalytic reduction of NOx with NH3 over Mn–containing catalysts. Appl. Catal. B: Environ. 2012, 123–124, 229–240. (24) Zhang, L.; Li, L.; Cao, Y.; Yao, X.; Ge, C.; Gao, F.; Deng, Y.; Tang, C.; Dong, L. Getting insight into the influence of SO2 on TiO2/CeO2 for the selective catalytic reduction of NO by NH3. Appl. Catal. B: Environ. 2015, 165, 589–598. (25) Zhang, L.; Sun, J.; Li, L.; Ran, S.; Li, G.; Li, C.; Ge, C.; Dong, L. Selective catalytic reduction of NO by NH3 on CeO2–MOx (M = Ti, Si, and Al) dual composite catalysts: Impact of surface acidity. Ind. Eng. Chem. Res. 2018, 57, 490–497. (26) Park, E.; Kim, M.; Jung, H.; Chin, S.; Jurng, J. Effect of sulfur on Mn/Ti catalysts prepared using chemical vapor condensation (CVC) for Low–temperature NO reduction. ACS Catal. 2013, 3, 1518–1525. (27) Ciardelli, C.; Nova, I.; Tronconi, E.; Chatterjee, D.; Bandl–Konrad, B.; Weibel, M.; Krutzsch, B. Reactivity of NO/NO2–NH3 SCR system for diesel exhaust aftertreatment: Identification of the reaction network as a function of temperature and NO2 feed content. Appl. Catal. B: Environ. 2007, 70, 80–90. (28) Thommes, M.; Kaneko, K.; Neimark, A. V.; Olivier, J. P.; Rodriguez–Reinoso, F.; Rouquerol, J.; Sing, K. S. W. Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report). Pure Appl. Chem. 2015, 87, 1051−1069. (29) Fang, C.; Zhang, D.; Cai, S.; Zhang, L.; Huang, L.; Li, H.; Maitarad, P.; Shi, L.; Gao, R.; Zhang, J. Low–temperature selective catalytic reduction of NO with NH3 over nanoflaky MnOx on carbon nanotubes in situ prepared via a chemical bath deposition route. Nanoscale 2013, 5, 9199– 9207. (30) Yao, X.; Ma, K.; Zou, W.; He, S.; An, J.; Yang, F.; Dong, L. Influence of preparation 18


Page 18 of 21

Page 19 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


methods on the physicochemical properties and catalytic performance of MnOx–CeO2 catalysts for NH3–SCR at low temperature. Chin. J. Catal. 2017, 38, 146–159. (31) Li, H.; Qi, G.; Tana; Zhang, X.; Huang, X.; Li, W.; Shen, W. Low–temperature oxidation of ethanol over a Mn0.6Ce0.4O2 mixed oxide. Appl. Catal. B: Environ. 2011, 103, 54–61. (32) Fan, Z.; Wang, Z.; Shi, W.; Gao, C.; Gao, G; Wang, B.; Wang, Y.; Chen, X.; He, C.; Niu, C. Charge–redistribution–induced new active sites on (0 0 1) facets of α–Mn2O3 for significantly enhanced selective catalytic reduction of NO by NH3. J. Catal. 2019, 370, 30–37. (33) Fan, Z.; Shi, W.; Gao, C.; Gao, G.; Wang, B.; Niu, C. Rationally designed porous MnOx– FeOx nanoneedles for low–temperature selective catalytic reduction of NOx by NH3. ACS Appl. Mater. Interfaces 2017, 9, 16117–16217. (34) Fan, Z.; Shi, W.; Gao, C.; Gao, G.; Wang, B.; Wang, Y.; He, C.; Niu, C. Gd–modified MnOx for the selective catalytic reduction of NO by NH3: The promoting effect of Gd on the catalytic performance and sulfur resistance. Chem. Eng. J. 2018, 348, 820–830. (35) Zhang, L.; Zou, W.; Ma, K.; Cao, Y.; Xiong, Y.; Wu, S.; Tang, C.; Gao, F.; Dong, L. Sulfated temperature effects on the catalytic activity of CeO2 in NH3–selective catalytic reduction conditions. J. Phys. Chem. C 2015, 119, 1155–1163. (36) Zhang, L.; Pierce, J.; Leung, V.; Wang, D.; Epling, W. Characterization of ceria’s interaction with NOx and NH3. J. Phys. Chem. C 2013, 117, 8282–8289. (37) Gao, C.; Shi, W.; Fan, Z.; Wang, B.; Wang, Y.; He, C.; Wang, X.; Li, J.; Niu, C. "Fast SCR" reaction over Sm–modified MnOx–TiO2 for promoting reduction of NOx with NH3. Appl. Catal. A: Gen. 2018, 564, 102–112. (38) Zhang, L.; Li, L.; Cao, Y.; Xiong, Y.; Wu, S.; Sun, J.; Tang, C.; Gao, F.; Dong, L. Promotional effect of doping SnO2 into TiO2 over a CeO2/TiO2 catalyst for selective catalytic reduction of NO by NH3. Catal. Sci. Technol. 2015, 5, 2188–2196. (39) Hadjiivanov, K. Identification of neutral and charged NxOy surface species by IR spectroscopy. Catal. Rev. 2000, 42, 71–144. (40) Yuan, R.; Fu, G.; Xu, X.; Wan, H. Bronsted–NH4+ mechanism versus nitrite mechanism: New insight into the selective catalytic reduction of NO by NH3. Phys. Chem. Chem. Phys. 2011, 13, 453–460. (41) Chen, L.; Si, Z.; Wu, X.; Weng, D. DRIFT study of CuO–CeO2–TiO2 mixed oxides for NOx 19



reduction with NH3 at low temperatures. ACS Appl. Mater. Interfaces 2014, 6, 8134–8145.

20


Page 20 of 21

Page 21 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Only

21


Promoting N2 Selectivity of CeMnOx Catalyst by Supporting TiO2 in

Recommend Documents