A Highly Effective MnNdOx Catalyst for the ... - ACS Publications

Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX-XXX

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A Highly Effective MnNdOx Catalyst for the Selective Catalytic Reduction of NOx with NH3

Rui-tang Guo,*,†,‡ Peng Sun,†,‡ Wei-guo Pan,*,†,‡ Ming-yuan Li,†,‡ Shu-ming Liu,†,‡ Xiao Sun,†,‡ Shuai-wei Liu,†,‡ and Jian Liu†,‡ †

School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai, P. R. China Shanghai Engineering Research Center of Power Generation Environment Protection, Shanghai, P. R. China

‡

S Supporting Information *

ABSTRACT: Nd was used as the modifier to improve the NH3selective catalytic reduction (SCR) performance of MnOx catalyst used in NOx abatement. The results of the activity test indicated that MnNdOx-0.1 catalyst displayed remarkable catalytic activity and N2 selectivity in the temperature range of 50−300 °C. Characterization results proved the enhanced reducibility and the facilitated NH3 and NOx adsorption over MnNdOx-0.1 catalyst, accompanied by the existence of more Mn4+ and surface adsorbed oxygen species over its surface. The results of the in situ diffuse reflectance infrared Fourier transform study pointed out that the NH3-SCR reaction over the MnNdOx-0.1 catalyst was mainly under the control of the Langmuir−Hinshelwood mechanism (≤100 °C); when the reaction temperature was over 200 °C, the Eley−Rideal mechanism was dominant. More adsorbed NOx and NH3 species over the MnNdOx-0.1 catalyst, along with the activation effect of Nd on the adsorbed reactants, had a facilitation impact on the SCR reaction process over the MnNdOx-0.1 catalyst.

1. INTRODUCTION Emission of NOx has brought about a series of negative impacts on the health of the environment and humans, including acid rain deposition, and the generation of haze and smog.1−3 On the whole, boilers burning coal and the incinerators for municipal solid waste (MSW) disposal are the typical stationary sources of NOx emission,4 especially in developing countries such as China. To meet greater strict emission standards, a process based on selective catalytic reduction (SCR) technique holds a leading post in the field of NOx emission control.5,6 Nowadays, the SCR catalyst based on vanadium oxides using W or Mo as the assistant, which exhibits high SCR activity in 350−400 °C, is the preferred catalyst for this purpose.7,8 Nevertheless, some other inherent problems are attached to this catalyst, including the low selectivity at high temperature, the poisonous vanadium species, and the deactivation by SO2 and alkali metals.9−12 Therefore, exploitation of a new SCR catalyst without vanadium has drawn much attention of the researchers worldwide. Recently, an alternative SCR catalyst for utilization in a low temperature SCR reactor was developed for which the active components are mainly some transition metal oxides including Mn, Fe, Cu, Ce, and Co, etc.13−20 Among them, MnOx-based catalyst displayed the best low-temperature SCR performance. During the past several years, intensive investigations have been performed to bring to light the SCR property and reaction pathway of MnOx loaded on different supports such as TiO2, Al2O3, SiO2, ZrCeOx, carbon nanotube, etc.20−25 Moreover, the introduction of another metal (Ce, Cu, Fe, Co, Sm, Eu, Sb, Nb, © XXXX American Chemical Society

etc.) into a Mn-based SCR catalyst turns out to be an effective approach to improve the SCR performance or/and the resistance to SO2/ alkali metals.26−35 As mentioned above, rare earth metals are desirable additives of Mn-based SCR catalyst. Nd, as a kind of rare earth metal, was successfully used as the promoter of CeO2/ASC catalyst to improve its SCR activity.36 It is well-known that the pristine MnOx catalyst is unfit for the SCR reactor used in the industrial field owing to its small specific area and low hydrothermal stability. However, the utilization of Nd as the modifier to enhance the performance of the Mn-based SCR catalyst has still not been reported. On the basis of the characterization results, the impact of Nd doping on the structure and physicochemical features of MnOx was explored and discussed. Furthermore, the promotion mechanism of Nd modification on the SCR reaction over the MnOx catalyst would be proposed.

2. EXPERIMENTAL SECTION 2.1. Catalyst Synthesis. By virtue of the coprecipitation method, several MnNdOx catalyst samples were prepared. At first, we dissolved a certain amount of Mn(NO3)2 and Nd(NO3)3 in water. Then the pH value of the mixed solution was adjusted to 11 with aqueous ammonia solution to obtain a sediment. After being aged for 3 h, the sediment was separated Received: Revised: Accepted: Published: A

September 6, 2017 October 17, 2017 October 19, 2017 October 19, 2017 DOI: 10.1021/acs.iecr.7b03705 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research from the solution and rinsed with deionized water. Next then, the sediment was dehydrated at 100 °C for 24 h, and finally calcined at 450 °C in the air atmosphere for 5 h. For convenience, the obtained binary oxide catalyst sample was named as MnNdOx-y, where y was the molar ratio of Nd/Mn. On the basis of the same preparation procedure, pristine MnOx and NdOx catalysts were obtained and used as the reference sample. 2.2. Catalyst Characterization. The textural property of each catalyst sample was achieved from the N2 adsorption− desorption isotherms at 77 K, which was performed on a Quantachrome instrument (Autosorb-iQ-AG). Prior to N2 adsorption, the catalyst sample was subjected to degassing treatment at 300 °C for 4 h. By using the Brunauer−Emmett− Teller (BET) model and Barrett−Joyner−Halenda (BJH) model, the surface area and pore volume for each catalyst sample could be evaluated. The actual concentrations of various elements in each catalyst sample were obtained by inductively coupled plasma optical emission spectrometry (ICP-OES) analysis, which was performed on a Leeman Profile Spec apparatus. The XRD patterns were obtained on a X-ray powder diffractometer equipped with a Cu Kα radiation source (λ = 0.154056 nm). For the identification of the elements and their chemical states on the catalyst surface, the X-ray photoelectron spectroscopy (XPS) spectra were obtained on a thermal ESCALAB 250 spectrometer using Al Kα radiation (hν = 1486.6 eV). For binding energy shift calibration, the C 1s signal was chosen as the reference standard. H2-TPR analysis and NH3-TPD analysis were executed by using a chemisorption analyzer (Quantachrome Autosorb-iQC). In each test run, a definite mass of catalyst sample (0.1 g) was used. Before the H2-TPR test, the catalyst sample was first pretreated in the N2 atmosphere at a certain temperature (500 °C) for 1 h, then it was cooled to 25 °C. In the subsequent TPR run, the sample was heated in a 5% H2/N2 flow (30 mL/ min) from 25 to 800 °C at a steady rate (10 °C/min). During the NH3-TPD test, after a 1 h pretreatment in He at 500 °C, the sample was cooled to 100 °C, the catalyst sample underwent NH3 saturation with anhydrous 5% NH3/He (flow rate = 30 mL/min) for 30 min. At last, NH3-saturated sample experienced a heating process from 100 to 500 °C at a constant heating speed of 10 °C/min. In situ diffuse reflectance infrared Fourier transform (DRIFT) study was performed by means of a Fourier transform infrared spectrometer (Thermo Nicolet iS 50) connected with an intelligent collector and an MCT/A detector using liquid nitrogen as the cooling medium. At the beginning of each test run, the sample was preconditioned at 400 °C for 0.5 h in a 20% O2/N2 gas flow and then chilled to the predetermined temperature. The collection of background was performed in N2 flow, which was automatically eliminated from the sample spectra by a subtraction. The following reaction conditions for the DRIFT test were adopted: 600 ppm of NH3 and/or 600 ppm of NO/5% O2, N2 was used as the balance gas, with the blanket flow rate of 300 mL/min. All the spectra were collected by 100 scans accumulation, and the resolution was 4 cm−1. 2.3. Catalytic Activity Test. SCR activity test was carried out in a reactor with the fixed-bed type at atmospheric pressure using 0.55 cm3 catalyst sample; the inner diameter of this reactor was 8 mm. The simulated flue gas consisted of the following gases: NH3 (600 ppm), NO (600 ppm), O2 (5%), 5% H2O (when investigating the effect of H2O), 100 ppm of SO2

(when investigating the effect of SO2), and Ar as the balance gas. The simulated flue gas flow rate was set as 1 L/min, thus the reciprocal gas hourly space velocity (GHSV) value was about 108 000 h−1. The concentrations of NH3, NO2, NO, and N2O contained in the effluent gas were continuously under the monitor of a FTIR spectrometer (Thermo Nicolet iS 50) connected with a 0.2 dm3 gas cell. After about 1 h, the steady SCR reaction could be achieved, allowing the calculation of NOx conversion, NH3 conversion, and N2 selectivity by NOx conversion % =

[NOx ]in − [NOx ]out × 100% [NOx ]in

(1)

NH3 conversion % =

[NH3]in − [NH3]out × 100% [NH3]in

(2)

N2 selectivity % ⎛ ⎞ 2[N2O]out = ⎜1 − ⎟ [NOx ]in + [NH3]in − [NOx ]out − [NH3]out ⎠ ⎝ (3)

× 100%

In addition, NO oxidation activity over each catalyst sample was also tested under the similar experimental conditions, while NH3 was excluded in the reactant gas. The NO oxidation activity could be written as NO oxidation % =

[NO]in − [NO]out × 100% [NO]in

(4)

3. RESULTS AND DISCUSSION 3.1. Catalytic Performance. The SCR activities of different catalyst samples are presented in Figure 1. For pure NdOx catalyst, its SCR activity was very low. The MnOx catalyst exhibited superior activity over the NbOx catalyst, but its activity did not exceeded 90% under all reaction temperature conditions. Noticeably, the introduction of Nb into the MnOx catalyst could greatly improve its SCR activity, especially for the MnNdOx-0.1 catalyst, which displayed the best SCR performance among the five catalyst samples. Moreover, its functional temperature interval was also distinctly widened. From the work of Meng et al.,32 the promotion effect of Sm on the MnOx catalyst was only notable in a narrow temperature range (≤100 °C). However, the SCR reactivity would drop with a certain Nd/Mn mole ratio of 0.15. For the curves of NH3 conversion and N2 selectivity (shown in Figure 1B,C), the similar trend also appeared. Therefore, the modification of MnOx catalyst with appropriate amount of Nd could effectively improve its performance in the NH3-SCR reaction. 3.2. SO2/H2O Tolerance and Stability. As is well-known, an Mn-based SCR catalyst is easily subjected to the deactivation by some components of flue gas, such as SO2 and water vapor.37−39 So the effect of SO2 and H2O on the SCR reactions over MnOx and MnNdOx-0.1 catalyst samples were tested at 150 °C, as presented in Figure 2. From Figure 2, NOx conversion over MnOx catalyst underwent a remarkable drop after the introduction of SO2 and H2O. In marked contrast to the MnOx catalyst, the NOx conversion over MnNdOx-0.1 catalyst kept over 85% even after the introduction of SO2 and H2O for 12 h. Thus, the MnNdOx-0.1 catalyst exhibited much higher SO2 and H2O resistance than the MnOx catalyst, which might be attributed to the inhibited sulfate species deposition B

DOI: 10.1021/acs.iecr.7b03705 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 2. SO2 and H2O tolerance of MnOx and MnNdOx-0.1 catalysts at 150 °C.

Figure 3. XRD patterns of different catalyst samples.

Table 1. Textural Properties of Different Catalyst Samples samples

BET surface area (m2/g)

total pore volume (cm3/g)

average pore diameter (nm)

MnOx NdOx MnNdOx-0.05 MnNdOx-0.1 MnNdOx-0.15

47.34 39.22 63.94 72.33 60.84

3.414 6.382 3.059 3.402 3.405

0.065 1.737 0.296 0.215 0.242

Figure 1. (A) SCR activities, (B) NH3 conversions, and (C) N2 selectivities over MnOx, NdOx, and MnNdOx catalysts.

on its surface.40 Moreover, the NOx conversion over MnNdOx0.1 catalyst was nearly recovered to its initial value after the cut of SO2/H2O supply, which made it competitive in industrial application. The stability of a SCR catalyst is of great importance to its industrial application. So the stabilities of MnOx and MnNdOx0.1 catalysts were also tested at a constant temperature of 200 °C for 48 h, as presented in Figure S1. From Figure S1, it could be observed that the SCR activity of the MnOx catalyst declined from 81.7% to 71.2% after 48 h. In deep contrast, the same

Figure 4. SEM pictures of (A) MnOx and (B) MnNdOx catalysts.

indicator of MnNdOx-0.1 catalyst went through a decrease from about 100% to 96.1% under the same conditions. So the stability of MnNdOx-0.1 catalyst was much stronger than that C


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Figure 6. Profiles of H2-TPR analysis for MnOx, NdOx, and MnNdOx catalysts.

Table 3. Surface Acidities of MnOx, NdOx, and MnNdOx Catalysts samples

surface acidity (mmol/g)

MnOx NdOx MnNdOx-0.05 MnNdOx-0.1 MnNdOx-0.15

0.063 0.047 0.112 0.145 0.104

Figure 5. XPS spectra of MnOx, NdOx, and MnNdOx catalysts

of the MnOx catalyst, which should be due to the elevated heat endurance of active components in the presence of Nd. 3.3. Physicochemical Properties of the Catalyst Samples. XRD patterns of the catalyst samples are depicted in Figure 3. In the XRD pattern of MnOx catalyst, there were mainly Mn2O3 diffraction peaks. For the XRD pattern of the NdOx catalyst, only several peaks of Nd2O3 could be detected. As a remarkable difference, the XRD patterns of the MnNdOx catalysts only contain very weak diffraction peaks of Mn3O4, and no diffraction peak of the Nd species was apparent, suggesting the good dispersion of Nd species over MnNdOx catalysts and the existence of intensive interaction between Mn and Nd. The results mentioned above also indicated that the introduction of Nd into the MnOx catalyst had an inhibition effect on the crystallization of MnOx.

Figure 7. NO oxidation activities over MnOx, NdOx, and MnNdOx catalysts

The N2 adsorption−desorption isotherms of different catalyst samples are presented in Figure S2. According to the

Table 2. Surface Atomic Concentrations of MnOx, NdOx and MnNdOx Catalysts samples MnOx NdOx MnNdOx-0.05 MnNdOx-0.1 MnNdOx-0.15 a

Nd/Mna

Mn (atom %)

Nd (atom %)

O (atom %)

36.31 1.39 3.00 4.73

66.59 63.69 67.16 67.59 66.34

33.41 0.05 0.1 0.15

31.45 29.41 28.93

Nd/Mnb

0.065 0.102 0.163

Mn4+/Mn (%)

Oβ/(Oα + Oβ) (%)

Mn4+ (atom %)

Oβ (atom %)

16.52

23.03 16.84 30.42 38.58 28.84

5.52

15.33 10.72 20.43 26.08 19.13

21.94 26.42 20.26

6.90 7.77 5.86

Mole ratio of Nd/Mn determined by ICP analysis. bMole ratio of Nd/Mn determined by XPS analysis. D


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Figure 9. NOx adsorption DRIFT spectra over (A) MnOx and (B) MnNdOx-0.1 catalysts at different temperatures.

Figure 8. NH3 adsorption DRIFT spectra over (A) MnOx and (B) MnNdOx-0.1 catalysts at different temperature.

Apparently, the Nd/Mn molar ratio on the surfaces of MnNdOx samples were higher than the corresponding values calculated from the results of ICP analysis, suggesting the enrichment of Nd species on the surface. From Figure 5, the Mn 2p XPS spectra contained three peaks located at 640.4, 642.0, and 644.0 eV, which represented Mn2+, Mn3+ and Mn4+ respectively.41−43 According to the results of XPS analysis, the surface Mn4+ concentration of each sample could be obtained, as summarized in Table 2. It was remarkable that MnNdOx-0.1 catalyst exhibited the highest surface Mn4+ concentration (7.77 atom %), which might be ascribed to the interrupted interaction between Mnn+ and O2+ ions by Nd and the subsequent reduction from Mn4+ to Mn3+.44 As illustrated in Figure 5, two peaks could be obtained after a peak-fitting deconvolution process, which represented two kinds of oxygen species: lattice oxygen (Oα) (BE ≈ 529.6 eV) and surface adsorbed oxygen (Oβ) (BE = 531.2−531.7 eV), for instance, O− and O2− contained in hydroxyl-like or defect-oxide groups. From Table 2, it was worth noting that the Oβ concentration of MnNdOx-0.1 catalyst (26.08 atom %) was distinctly the highest among all the samples, thereby the introduction of moderate Nd into MnOx catalyst was conducive to the formation of surface adsorbed oxygen on it. As mentioned above, the MnNdOx-0.1 catalyst possessed the highest Mn4+ and Oβ concentrations among the five catalyst

definition of IUPAC, all the isotherms could be ascribed to type-IV, and the hysteresis loops belonged to type-H3, suggesting the existence of mesopores. The specific surface area of each catalyst sample is given in Table 1. The MnNdOx0.1 catalyst possessed larger specific surface area (72.33 m2/g) than that of other catalyst samples. On one hand, the presence of Nd in the MnNdOx catalyst could restrain the growth of MnOx crystals; on the other hand, the forceful interplay between Mn and Nd could promote their dispersion. All these features were beneficial to the formation of the MnNdOx-0.1 catalyst with high surface area. Figure 4 depicts the SEM images of MnOx and MnNdOx-0.1 catalysts. From Figure 4, the MnOx catalyst was composed of agglomerated near-spherical particles with a diameter of about 80 nm, while the shape of the MnNdOx-0.1 catalyst was present in a nonflaky structure, agreeing well with its relatively larger specific surface area. Moreover, the surface structure of the MnNdOx-0.1 catalyst was also propitious to the contact between reactant species and catalyst surface, consequently, facilitating the NH3−SCR reaction over it. The Mn 2p and O 1s XPS spectra of the five catalysts are present in Figure 5, and the surface element concentrations determined by the XPS technique are listed in Table 2. E


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Figure 10. DRIFT spectra recorded at 50 °C for feeding 600 ppm of NO + 5% O2 over NH3-pretreated (A) MnOx and (B) MnNdOx-0.1.

Figure 11. DRIFT spectra recorded at 50 °C for feeding 600 ppm of NH3 over NOx-pretreated (A) MnOx and (B) MnNdOx-0.1.

samples, which had a distinct intensifying effect on NO oxidation to NO2.32,42 Correspondingly, the low-temperature SCR activity over it could be enhanced through the “fast SCR” route:42 NO + NO2 + 2NH3 = N2 + H2O, as also proven by the results shown in Figure 1. 3.4. H2-TPR Analysis. The reducibility of Mn-based catalyst was conducive to the redox cycle in the SCR reaction.45 By the H2-TPR technique, we investigated the redox ability of each sample, as presented in Figure 6. As seen in Figure 6, two or three H2 consumption peaks appeared in each TPR profile. In the TPR profile of the MnOx catalyst, three peaks located at about 357, 463, and 572 °C were visible and could be assigned to the continuous reduction process of Mn4+ → Mn3+ → Mn2+.46−49 For the profile of the NdOx catalyst, two reduction peaks could be found at about 560 and 683 °C, the two peaks should stem from the reduction of surface and bulk Nd oxides.50 After the addition of Nd on the MnOx catalyst, a reduction peak shifting to lower temperature was seen as compared with the corresponding peak in the profile of pure MnOx catalyst. Clearly, MnNdOx-0.1catalyst displayed the lowest reduction temperature, along with the largest reduction peak area among the five catalyst samples. These features suggested that the Mn species in the MnNdOx-0.1 catalyst were

more reducible than that contained in other catalyst samples, which was favorable to the redox reaction in the NH3-SCR process. 3.5. NH3-TPD Analysis. NH3 adsorption has a significant impact on the NH3-SCR reaction, which is closely related to the nature of surface acid sites over the SCR catalyst, and the NH3-desorption behavior from different catalysts was investigated by a NH3-TPD study, as displayed in Figure S3. Obviously, there was a desorption peak lasting from about 100−350 °C, thus the adsorbed NH3 species were of different heat endurance (confirmed by the following in situ DRIFT results).51−53 The peaks in the MnOx and NdOx profiles were very low, thus the two catalyst samples only possessed weak surface acid sites. For the MnNdOx catalysts, the desorption peaks in their profiles were much stronger, thus the doping of Nd on MnOx catalyst could effectively enhance its surface acidity, as also listed in Table 3. The surface acidity of MnNdOx-0.1 catalyst was about 2.3 times of that of MnOx catalyst, as a consequence, greatly facilitating NH3 adsorption and the following SCR reaction. 3.6. NO Oxidation. The oxidation of NO to NO2 plays a crucial aspect in the NH3-SCR reaction below 250 °C owing to the “fast SCR” reaction route.54 So the activities of NO F


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Figure 14. Contributions of E-R and L-H mechanisms to the SCR reactions taken place over (A) MnOx and (B) MnNdOx-0.1 catalysts.

that the NO oxidation activity over the NdOx catalyst was very low. Evidently, the introduction of Nd could enhance the NO oxidation activity of the MnOx catalyst, especially for MnNdOx0.1 catalyst, which should be caused by the enrichment of surface Mn4+ and active oxygen species. 3.7. In Situ DRIFT Study. The adsorption behavior of reactant species and their surface reactions were investigated based on an in situ DRIFT study, which was useful in understanding the promotion mechanism of Nd modification on MnNdOx-0.1 catalyst. 3.7.1. NH3 Adsorption. To further identify the nature of adsorbed NH3 species over MnOx and MnNdOx-0.1 catalysts, the NH3 adsorption DRIFT spectra over them at different temperature were recorded, and the results could be seen in Figure 8. Two bands at 1605 and 1182 cm−1 could be observed in Figure 8A, which represented the N−H bending vibration (symmetric or asymmetric mode) in the NH3 species over Lewis acid sites, for the DRIFT spectra of MnNdOx-0.1 catalyst, a new band appeared at 1436 cm−1, which indicated NH+4 species on Brønsted acid sites.56−58 Thereafter, most of the surface acid sites over pure MnOx catalyst belonged to Lewis acid sites, and the modification of MnOx catalyst with Nd could generate new Brønsted acid sites on it. Because of the desorption of adsorbed NH3 species, the bands became weaker with temperature. It seemed that the adsorbed NH3 species on Lewis acid sites were more stable, the two bands (1605, 1182 cm−1) were still visible at 300 °C, along with the disappearance of the band at 1436 cm−1. From the different band intensities shown in Figure 8A and Figure 8B, it could be concluded that

Figure 12. SCR reaction rates for (A) MnOx and (B) MnNdOx-0.1 catalysts by varying NO and NH3 inlet concentrations.

Figure 13. Arrhenius plots of SCR reaction rates for MnOx and MnNdOx-0.1 catalysts.

oxidation for the five catalysts were also evaluated, as shown in Figure 7. Due to the conversion from kinetics-controlled regime to thermodynamics-controlled regime,55 a parabolic trend could be detected in the activity curves. It was noticeable G


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Figure 15. Promotion mechanism of Nd on the SCR reaction over MnNdOx-0.1.

introduction of 600 ppm of NO + 5% O2/N2 into the IR cell to study the reaction process by DRIFT technique. From Figure 10, the NH3 preadsorption treatment formed two NH3 adsorption bands (1605 and 1182 cm−1). The feed of NO + O2 made the two bands gradually become weaker and disappear completely after 10 min. Then the adsorbed NOx species came into being, as seen from the bands at 1618, 1550, 1277, and 1216 cm−1, respectively, implying that the adsorbed NH3 species over the MnOx catalyst surface were replaced by the adsorbed NOx species. As shown in Figure 10B, although more adsorbed NH3 species were present over MnNdOx-0.1 catalyst, they quickly vanished after the introduction of NO + O2 for 2 min, accompanied by the appearance of some adsorbed NOx species. Therefore, the existence of Nd in the MnNdOx-0.1 catalyst could make the adsorbed NH3 species more active. In addition, the high reactivity of the adsorbed NH3 species over MnOx and MnNdOx-0.1 revealed the existence of the Eley− Rideal (E-R) mechanism.64,65 For the same reaction process over MnOx and MnNdOx-0.1 catalyst at 250 °C (Figure S4), the preadsorbed NH3 species exhibited similar reactivities. All the adsorbed NH3 species disappeared quickly after the addition of NO + O2 for 2 min, along with the emergence and growth of the bands belonging to the adsorbed NOx species. 3.7.4. Introduction of NH3 after NO+O2 Preadsorption. From another aspect, the introduction of NH3 and NOx was performed in an opposite order. First, the catalyst sample was pretreated under the atmosphere of 600 ppm of NO + 5% O2/ N2 at 50 °C for half an hour, then it was purged by N2 for 15 min. After that, 600 ppm of NH3/N2 was introduced into the IR cell to start the SCR reaction. The DRIFT spectra for this process are presented in Figure 11. Noticeably, several kinds of

more acid sites were present on the surface of the MnNdOx-0.1 catalyst, as also revealed by the results of NH3-TPD study. The oxidation of NO could also profit from the presence of more Lewis acid sites.59 In the present study, the greater NO conversion to NO2 and the adsorption of more NH3 species should all contribute to the strengthened “fast SCR” reaction over the MnNdOx-0.1 catalyst. 3.7.2. NOx Adsorption. Figure 9 shows the NOx adsorption DRIFT spectra over MnOx and MnNdOx-0.1 catalysts at different temperature. Several kinds of adsorbed NOx species were visible in Figure 9, which included adsorbed NO2 species (1618 and 1616 cm−1), bidentate nitrate (1550 cm−1), monodentate nitrate (1277 and 1268 cm−1), and bridged nitrate (1226 and 1216 cm−1).60−62 Compared with Figure 9A, Figure 9B shows a different band at 1472 cm−1 belonging to linear nitrite.63 Thus the introduction of Nd into the MnOx catalyst could provide new NOx adsorption sites. Moreover, higher band intensities could be detected in the spectra of the MnNdOx-0.1 catalyst, indicating the promoted NOx adsorption over it. Noticeably, the formation of more adsorbed NO2 over the MnNdOx-0.1 catalyst was evident, as could be seen from the stronger band at 1616 cm−1 (Figure 9B), which further confirmed the results of the NO oxidation test. Similar to the phenonmenon in Figure 8, the bands decreased with increasing temperature. It seemed that bidentate nitrate and bridged nitrate were of high thermal stability; the corresponding bands were still present at 350 °C. 3.7.3. Introduction of NOx after NH3 Preadsorption. To investigate the behavior of preadsorbed NH3 species during the SCR reaction process, the catalyst sample was subjected to 600 ppm of NH3/N2 pretreatment at 50 °C for 30 min. Then the sample was purged with N2 for 15 min, followed by the H


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energies could be calculated from the slope of the plots, which were 9.22 kJ/mol (for MnNdOx-0.1 catalyst) and 18.22 kJ/ mol(for MnOx catalyst), respectively. The lower active energy value for the MnNdOx-0.1 catalyst might be due to the generation of more active sites over it. 3.8.2. Promotion Mechanism. By virtue of Yang’s method,67 the roles of E-R and L-H mechanisms in the NH3-SCR reactions over pure MnOx and MnNdOx-0.1 could be determined. The SCR activities with different NO and NH3 inlet concentrations were measured in differential regime, then they were normalized by the SBET to obtain the reaction rates, as presented in Figure 14. The ratio of RL‑H/(RE‑R + RL‑H) represented the L-H mechanism percentage in the NH3-SCR reaction. From Figure 14, it could be seen that reaction temperature had a crucial impact on the percentages of E-R and L-H mechanisms in the NH3-SCR reaction, and the L-H mechanism contribution decreased with increasing temperature, while the trend of the E-R mechanism was just the opposite. For the SCR reaction over pure MnOx, the E-R mechanism played a predominant role. A similar conclusion had also been proposed by Meng et al.32 in their study. The NH3-SCR reaction over MnNdOx-0.1 catalyst was basically under the joint control of the L-H mechanism ( 0.985), further confirming that both E-R and L-H mechanisms were applicable. In kinetics study, the reaction rate of a SCR catalyst could be obtained after the normalization based on its BET surface area as follows:

R=

XNOQCf VmWSBET

NO + O2 (g) → NO2 (a) MnOx NH3(g) ⎯⎯⎯⎯⎯⎯⎯⎯→ NH3(a)

(9)

(Lewis acid sites)

(10)

NO2 (a) + 2NH3(a) + NO(g) → 2N2 + 3H 2O (fast SCR reaction)

(11)

4. CONCLUSIONS The modification of MnOx catalyst with a proper amount of Nd (Nd/Mn molar ratio = 0.1) could evidently enhance its NH3SCR performance. Characterization techniques showed that the introduction of Nd into the MnOx catalyst could enhance its reducibility and promote reactants adsorption. Furthermore, Nd in the MnNdOx-0.1 catalyst had a notable activation effect on the adsorbed NH3 and NOx species, especially at low temperature. Accordingly, the NH3-SCR reaction over the MnNdOx-0.1 catalyst could be facilitated through both E-R and L-H routes, which brought about the outstanding SCR performance of the MnNdOx-0.1 catalyst.

(8)

where XNO is the NO conversion in the SCR reaction; Q is the simulated flue gas flow rate (mL/h); Cf is the inlet concentration of NO (600 ppm); Vm is the gas molar volume (22.4 mL/mmol); W is catalyst weight (g), and SBET is the catalyst specific surface area (m2/g). The Arrhenius plots of NO conversion over MnOx and MnNdOx-0.1 catalysts are shown in Figure 13. The activation I


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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.7b03705. Effect of reaction time on NOx conversion, nitrogen adsorption isotherms, NH3-TPD profiles, DRIFT spectra recorded at 250 °C for feeding 600 ppm of NO+5% O2 over NH3-pretreated MnOx and MnNdOx-0.1, DRIFT spectra recorded at 250 °C for feeding 600 ppm of NH3 over NOx-pretreated MnOx and MnNdOx-0.1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Rui-tang Guo: 0000-0002-5646-5099 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of China (21546014) and the Natural Science Foundation of Shanghai, China (14ZR1417800).

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REFERENCES

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