TiO2 Catalyst for

Article pubs.acs.org/JPCC

Different Poisoning Effects of K and Mg on the Mn/TiO2 Catalyst for Selective Catalytic Reduction of NOx with NH3: A Mechanistic Study Rui-tang Guo,*,†,‡ Shu-xian Wang,†,‡ Wei-guo Pan,*,†,‡ Ming-yuan Li,†,‡ Peng Sun,†,‡ 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: It is well recognized that both alkali and alkali earth metals have a poisoning effect on selective catalytic reduction (SCR) catalyst. In this study, the different poisoning effects of K and Mg on Mn/TiO2 catalyst were investigated. It was found that the deactivation effect of K was much stronger than that of Mg. The effect of K or Mg addition on the physicochemical properties of Mn/TiO2 catalyst was investigated based on N2 adsorption, XRD, XPS, H2-TPR, NH3TPD, and NO-TPD techniques. The results indicated that the addition of K or Mg on Mn/TiO2 catalyst would decrease its specific area, promote the crystallization of TiO2, and lead to a decrease of Mn4+ and surface chemisorbed oxygen. Furthermore, the presence of K or Mg on Mn/TiO2 catalyst would lead to the drop of reducibility and the adsorption capacity of NH3 and NOx species. In addition, the adsorption behavior of NH3 and NOx and their surface reactions over the fresh and poisoned Mn/TiO2 catalysts were investigated by in situ DRIFT study. It was found that the NH3−SCR reaction over Mn/TiO2−Mg was mainly controlled by the L−H mechanism (≤150 °C) and E−R mechanism (>200 °C), while the NH3−SCR reaction over Mn/TiO2−K mainly followed the E−R pathway. The deactivation of Mn/TiO2−Mg mainly resulted from the inhibited adsorption and oxidation of NO, and the seriously suppressed adsorption of NH3 species made a great contribution to the deactivation of Mn/TiO2−K. Boningari et al.15 reviewed the NH3−SCR reaction mechanism over Mn-based catalyst and built the correlation of catalyst activity and stability with the acidity, manganese oxidation state, surface texture, and structural morphology. Li et al.16 reviewed the low-temperature NH3−SCR reaction over metal oxide and zeolite catalysts; the different SCR reaction mechanisms over Mn-based catalyst and Fe−zeolite catalyst were also described and compared. The recent progress on Mn-based catalysts for low-temperature SCR de-NOx with NH3 was reviewed by Liu et al.,17 and the effect of various factors on the SCR activity of Mn-based SCR catalyst and the reaction mechanism were summarized and discussed. It has been proven that the modification of manganese-based SCR catalyst by other transition metals and rare earth metals such as Fe, W, Ce, Nb, Sm, and Eu could further enhance its catalytic activity and SO 2 resistance. 18−24 Although the SCR reactor using manganese-based catalyst could be installed downstream of the electrostatic precipitator of coal-fired power plants, a small amount of fly ash containing alkali and alkali earth metals is still presented in the flue gas.25 As a result, the poisoning effect of alkali and alkali earth metals on Mn-based SCR catalyst should

1. INTRODUCTION NOx emitted from the combustion process of stationary and mobile sources has brought about great challenges due to its negative impact on the environment such as acid rain, photochemical smog, and ozone depletion.1−4 The selective catalytic reduction (SCR) of NOx with NH3 has been proven to be an effective and reliable process for NOx removal, and vanadium-based catalyst (including V2O5−WO3/TiO2 and V2O5−MoO3/TiO2) is the most widely used commercial catalyst for this process.5,6 However, some inevitable drawbacks of this catalyst still remain, including the toxicity of vanadium species, high conversion of SO2 to SO3, the formation of N2O at high temperature, and the deactivation by alkali metals in the fly ash.7−11 Due to the reasons mentioned above, much effort has been put into developing nonvanadia SCR catalysts with high de-NOx performance at low temperature and with environmentally friendly properties. Attracted by its excellent low-temperature SCR performance and essentially environmentally benign property, manganesebased SCR catalyst has become a focus of attention in recent years.12 Tian et al.13 found that the morphology of MnO2 catalyst had a great impact on its SCR performance. The study of Deng et al.14 reported that MnOx/TiO2 catalyst with a preferentially exposed (001) facet exhibited high NO conversion and high N2 selectivity at 80−280 °C. Recently, © XXXX American Chemical Society

Received: January 10, 2017 Revised: February 16, 2017 Published: March 27, 2017 A

DOI: 10.1021/acs.jpcc.7b00290 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

sample from 100 to 500 °C with a heating rate of 10 °C/min. The signal of H2 or NH3 was recorded by a thermal conductivity detector (TCD). Temperature-programmed desorption of NO was performed in a fixed-bed quartz reactor (id = 8 mm). After pretreating in Ar at 450 °C for 1 h, the catalyst sample was cooled to room temperature. Then the sample was exposed to a flow of 500 ppm of NO/Ar with a flow rate of 30 mL/min for 15 min to obtain a saturation state of NO adsorption. Next then, NOTPD experiments were carried out by heating the sample in Ar flow (300 mL/min) with a heating rate of 10 °C/min from room temperature to 500 °C. The concentrations of NOx were monitored with a continuous NOx analyzer (Thermo, model 42i-HL). The in situ DRIFT experiments were performed on a FTIR spectrometer (Thermo Nicolet iS 50) with an MCT/A detector cooled with liquid nitrogen. Before each experimental run, the sample was pretreated at 400 °C for 0.5 h in a flow of 20 vol % O2/N2 and then cooled to the desired reaction temperature. The background spectrum was collected in flowing N2 and subtracted from the sample spectrum automatically. The total flow rate of reactant gas was kept at 1 L/min, and the components of the feeding gas were as follows: 600 ppm of NH3 or/and 600 ppm of NO + 5% O2, balance N2. All the spectra were recorded by accumulating 100 scans with a resolution of 4 cm−1. 2.3. Activity Test. The SCR activities of the catalyst samples were tested in a fixed bed reactor with an inner diameter of 8 mm. The simulated flue gas was a mixture of 600 ppm of NO, 600 ppm of NH3, 5% O2, balance Ar, and GHSV = 162 000 h−1. The concentrations of NO, NO2, NH3, and N2O were continuously monitored by an FTIR spectrometer (Thermo Nicolet iS 50) equipped with a gas cell with 0.2 dm3 volume. After the SCR reaction reached a steady state, the NOx conversion could be calculated by

also been taken into consideration. In our previous study,26 the different poisoning effects of Na and K on Mn/TiO2 catalyst were investigated and discussed. Liu et al.27 pointed out that the decreased surface acidity and suppressed NO oxidation ability led to the deactivation of Ca-doped Mn/TiO2 catalyst. The study of Fang et al.28 indicated that the deactivation effect of K on Mn/TiO2 catalyst was greatly dependent on the precursors and preparation methods. It was well recognized that alkali metal and alkali earth metal had different poisoning effect on SCR catalyst;29−31 nevertheless, the different deactivation mechanism was still not very clear. So the aim of this work was to identify the variations of surface acidity, reducibility, and the adsorption/desorption behavior of reactants and their surface reactions over Mn/TiO2 catalyst caused by the addition of K or Mg; moreover, the relationship between these variations and the SCR performances would be built and discussed.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The fresh Mn/TiO2 catalyst sample was prepare by sol−gel method with manganese nitrate, butyl titanate, nitric acid, ethanol, and water as we had reported in our previous study.10 The molar ratio of Mn/Ti in Mn/TiO2 catalyst was set as 1:5. The K-poisoned Mn/TiO2 catalyst sample was prepared by impregnating the fresh Mn/TiO2 catalyst with an aqueous solution containing the required amount of potassium nitrate under rigorous stirring. Then the mixture was heated at 80 °C under vigorous stirring to mostly remove the water, followed by drying at 100 °C for 12 h. Next, the solid was calcined at 500 °C in air for 5 h to obtain the final poisoned catalyst sample. Similarly, the Mg-poisoned Mn/TiO2 catalyst sample was prepared, and the two poisoned catalyst samples were denoted as Mn/TiO2−K and Mn/TiO2−Mg, respectively. 2.2. Characterizations. The N2 adsorption−desorption isotherms were measured on a Quantachrome Autosorb-iQ-AG instrument at −196 °C. Prior to the test, all the samples were degassed at 300 °C for 4 h. The specific area of the sample was determined based on the Brunauer−Emmett−Teller (BET) method, and the pore size distribution was determined by the Barrett−Joyner−Halenda (BJH) method. XRD patterns were recorded on a Bruker D8 Advance X-ray powder diffractometer with Cu Kα radiation (λ = 0.154056 nm). To investigate the chemical states for all elements on the catalyst surface, X-ray photoelectron spectroscopy (XPS) was acquired on Thermal ESCALAB 250 spectrometer Al Kα X-rays (hν = 1486.6 eV). The binding energy shift was calibrated based on the C 1s level at 284.8 eV as the reference. The actual concentrations of various elements in each catalyst sample were analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES) on a Leeman Profile spec apparatus. Temperature-programmed reduction of H2 (H2-TPR) and temperature-programmed desorption of NH3 (NH3-TPD) experiments were conducted on a Quantachrome AutosorbiQ-C chemisorption analyzer using 0.1 g of catalyst sample. Prior to the TPR experiments, the catalyst sample was pretreated at 500 °C for 1 h under N2. Then the TPR runs were carried out under 5% H2/N2 with a heating rate of 10 °C/ min from room temperature to 900 °C. For NH3-TPD experiments, the catalyst sample was first pretreated in flowing He at 500 °C for 1 h, followed by saturation in anhydrous NH3 (5% in He) at a flow rate of 30 mL/min for about 30 min. Finally, the TPD operation was carried out by heating the

NOx conversion =

[NOx]in − [NOx]out × 100% [NOx]in

(1)

The value of N2 selectivity could be obtained by N2 selectivity ⎞ ⎛ 2[N2O]out = ⎜1 − ⎟ [NOx]in + [NH3]in − [NOx]out − [NH3]out ⎠ ⎝ × 100%

(2)

Due to the excess oxygen and [NH3]in/[NOx]in ≥ 1, the NO reaction rate in the NH3−SCR process could be regarded as first order with respect to NOx.29 Thus, the observed catalyst activity constant, k, could be calculated by k = −ln(1 − X ) × Fgas/mcat

(3)

where X is the fractional NOx conversion; Fgas is the flow rate of simulated flue gas, mL/s; and mcat is the mass of catalyst sample, g. The NO oxidation activity was tested basically under the similar conditions mentioned above; however, NH3 was not included in the simulated flue gas, and the GHSV was set as 108 000 h−1. The values of NO oxidation could be calculated by B


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The Journal of Physical Chemistry C NO oxidation =

[NO]in − [NO]out × 100% [NO]in

(4)

3. RESULTS AND DISCUSSION 3.1. Catalytic Activity. The SCR activities of the fresh and poisoned Mn/TiO2 catalyst samples are shown in Figure 1(A).

Figure 2. Observed catalyst activity constant k at 250 °C for Mn/TiO2 catalysts with different loading amount of K or Mg. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 5%, balance Ar, GHSV = 162 000 h−1.

Mn/TiO2−Mg catalyst samples decreased gradually with increasing Mg content. Moreover, the deactivation effect of K was distinctly stronger than that of Mg, as also reflected by Figure 1(A). 3.2. Chemical Compositions and Physical Properties. The chemical compositions of the catalyst samples were analyzed by ICP, and the results are summarized in Table 1. It Table 1. Analysis of Chemical Compositions of the Three Catalyst Samples metal concentrations from ICP (wt %) samples

Mn

Ti

K

Mg

Mn/TiO2 Mn/TiO2−K Mn/TiO2−Mg

14.07 13.04 14.33

53.70 52.13 52.03

/ 1.60 /

/ / 1.22

was clear that the concentrations of various metal elements were nearly identical with the theoretical contents based on molar ratio. The BET surface areas of the four catalyst samples were listed in Table 2. From Table 2, it was apparent that the

Figure 1. (A) NH3−SCR activities and (B) N2 selectivities of the fresh and poisoned Mn/TiO2 catalyst samples. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 5%, balance Ar, GHSV = 162 000 h−1.

Table 2. Textural Properties of the Three Catalyst Samples

As can be observed in Figure 1, both K and Mg had a poisoning effect on Mn/TiO2 catalyst, and the deactivation effect of Mg was weaker than that of K, agreeing well with the results of Chen et al.27 Moreover, it seemed that the poisoning effect of Mg decreased with increasing temperature, while the poisoning effect of K nearly kept stable in the whole experimental temperature range. Moreover, the N2 selectivities over the three catalyst samples are illustrated in Figure 1(B). It was clear that the addition of K or Mg on Mn/TiO2 catalyst would lead to the decrease of N2 selectivity. A similar trend had also been reported by Peng et al.25 Furthermore, the observed catalyst activity constant as a function of K or Mg loading amount is shown in Figure 2. It could be seen that the values of activity constant of K-doped Mn/TiO2 catalyst samples decreased sharply with the K/Mn molar ratio increased from 0 to 0.1. When the molar ratio of K/ Mn exceeded 0.1, the values of k nearly kept constant with increasing K loading amount. As a comparison, the k values of

samples

BET surface area (m2/g)

total pore volume (cm3/g)

average pore diameter (nm)

mean crystallite size of TiO2 (nm)


74.59 56.69 71.66

0.1064 0.0953 0.1289

3.820 3.818 5.620

12.4 19.6 14.5

addition of K or Mg on Mn/TiO2 catalyst caused the decrease of BET surface area, especially for the K-doped catalyst sample, which may be caused by the blocking effect of K and Mg on surface pores. The decreased specific surface area of poisoned catalyst samples was unfavorable to the mass transfer and adsorption of reactant species during SCR reaction. Moreover, the introduction of Mg led to more changes of pore size distribution for Mn/TiO2. As shown in Figure S1, both Mn/ TiO2 and Mn/TiO2−K displayed a peak at about 3.8 nm, while the peak of Mn/TiO2−Mg appeared at about 5.6 nm. Therefore, the total pore volume and the pore diameter of C


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The Journal of Physical Chemistry C Mn/TiO2−Mg were quite different from that of Mn/TiO2 and Mn/TiO2−K. The XRD patterns of the three catalyst samples are shown in Figure 3. The XRD peaks belonging to the anatase phase TiO2

Figure 3. XRD patterns of the three catalyst samples.

(PDF#21-2172, at 2θ = 25.28°, 48.05°, and 55.06°) and rutile phase TiO2 (PDF#21-1276, at 36.09°) appeared. No peaks of Mn, K, and Mg species could be detected, indicating that these species were well dispersed and kept as amorphous phase on TiO2 support. Moreover, the peak intensities in the XRD pattern of Mn/TiO2 catalyst increased after the addition of K or Mg. The results suggested that the loading or K or Mg could promote the crystallization of TiO2, as a result, leading to the decrease of lattice defects. Furthermore, mean crystallite sizes of anatase TiO2 in the three catalyst samples were calculated based on the Scherrer equation, as shown in Table 2. It can be observed that the crystallite sizes of the three samples were in the following order, Mn/TiO2−K > Mn/TiO2−Mg > Mn/ TiO2, which was in good accordance with the results of XRD intensity. 3.3. XPS Analysis. The surface concentrations and chemical state of various elements over the three catalyst samples were determined by XPS analysis, and the results are summarized in Table 3. The XPS spectra of Mn 2p and O 1s are illustrated in Figure 4. The XPS spectra of Mn 2p are illustrated in Figure 4(A). The two peaks centered at about 642.0 and 653.8 eV could be attributed to Mn 2p3/2 and Mn 2p1/2, respectively. After a peakfitting deconvolution, the Mn 2p XPS spectra could be separated into three peaks of Mn2+ (640.4 eV), Mn3+ (642.0 eV), and Mn4+ (644.0 eV).32−34 From the results of XPS analysis, the concentrations of surface Mn4+ over the three catalyst samples could be calculated, as summarized in Table 3. From Table 3, it can be seen that the surface Mn 4+ concentrations over Mn/TiO2, Mn/TiO2−K, and Mn/TiO2− Mg were 5.51 at. %, 2.90 at. %, and 4.84 at. %, respectively. Thus, the addition of K or Mg on the Mn/TiO2 catalyst

Figure 4. XPS spectra of (A) Mn 2p and (B) O 1s of the three catalyst samples.

resulted in a distinct decrease of surface Mn4+. It was wellknown that the redox process of Mn4+ species played a crucial role in the low-temperature NH3−SCR reaction.35 Furthermore, high Mn4+ concentration could promote the oxidation of NO to NO2, which was favorable to the NH3−SCR reaction through the “fast SCR” pathway.36,37 Therefore, the decreased Mn4+ concentration of Mn/TiO2−K and Mn/TiO2−Mg should be partially responsible for their bad SCR performances. Figure 4(B) presents the O 1s XPS spectra of the three catalyst samples. After a peak-fitting deconvolution, the O 1s XPS spectra could be separated into two peaks. The peak at lower binding energy (529.0−530.0 eV) could be assigned to the lattice oxygen (denoted as Oα), and the peak at higher binding energy (531.3−531.9 eV) could be attributed to chemisorbed oxygen (O− or O2− 2 belonging to defect-oxide or hydroxyl-like group or carbonate, denoted as Oβ).38−40 From Table 3, the concentration of chemisorbed oxygen over Mn/ TiO2 was obviously higher than that over Mn/TiO2−K and Mn/TiO2−Mg, indicating that the doping of K or Mg was unfavorable to the formation of chemisorbed oxygen. As an active oxygen species, the chemisorbed oxygen played an

Table 3. Surface Elemental Analysis and Relative Area Ratio Determined by XPS and TPR samples

Mn (at. %)

O (at. %)

Mn4+/Mn (%)

Oβ/O (%)

Mn4+ (at. %)

Oβ (at. %)

Mn4+/Mn (%) (TPR)


10.47 7.40 11.39

66.67 66.95 66.89

52.60 39.22 42.48

31.71 27.89 29.24

5.51 2.90 4.84

21.14 18.67 19.56

52.18 39.03 42.63

D


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The Journal of Physical Chemistry C important aspect in SCR reaction. Besides that, the chemisorbed oxygen could easily exchange with gas oxygen and adsorbed oxygen molecules due to its high mobility.29,41 Similar to Mn4+, the enrichment of chemisorbed oxygen could also facilitate the NO oxidation and the de-NOx performance via the enhanced “fast SCR” reaction. Based on the results of XPS analysis, the decreased concentrations of Mn4+ and chemisorbed oxygen over Mn/TiO2−K and Mn/TiO2−Mg should be in good correspondence with their low SCR activities. 3.4. H2-TPR Analysis. To investigate the effect of K or Mg addition on the redox behavior of Mn/TiO2 catalyst, H2-TPR analysis was performed, and the results are presented in Figure 5. As can be seen from Figure 5, each profile contained three

Figure 5. H2-TPR profiles of the three catalyst samples.

Figure 6. (A) NH3-TPD and (B) NO-TPD profiles of the three catalyst samples.

reduction peaks. For Mn/TiO2, the first peak at about 303 °C could be ascribed to the reduction of MnO2 to Mn2O3; the second peak at about 383 °C could be assigned to the reduction of Mn2O3 to Mn3O4; and the third peak located at about 463 °C belonged to the reduction of Mn3O4 to MnO.8,42,43 The same explanation was also suitable for the two peaks in the profiles of Mn/TiO2−K or Mn/TiO2−Mg. Therefore, the results of H2-TPR analysis agreed well with that of XPS analysis. From Figure 5, it was obvious that the reduction peaks in the profile of Mn/TiO2 catalyst shifted to a higher temperature after the addition of K or Mg, suggesting the sharp decrease of reducibility, especially for Mn/TiO2−K catalyst. Therefore, the loading of K or Mg could stabilize the Mn species and make it less reducible. It was well recognized that the reducibility of Mn-based catalyst played an important aspect in completing the catalytic cycle in the SCR reaction, so the decreased reducibility of Mn/TiO2−K and Mn/TiO2−Mg also made a contribution to their low SCR activities. Moreover, from the surface area ratio of peak 1 to the total reduction peaks in each profile, we can obtain the values of Mn4+/Mn for the three catalyst samples, which are 52.18%, 39.03%, and 42.63%, respectively (Table 2), agreeing well with the results of XPS analysis. 3.5. TPD Analysis. Adsorption of NH3 was regarded as a key step in NH3−SCR reaction,44 which was greatly dependent on the surface acidity of catalysts. In this section, the surface acidity of the three catalyst samples was characterized by NH3TPD analysis, and the results are shown in Figure 6(A). From Figure 6(A), a broad desorption peak lasting from 100 to 500 °C could be detected in the NH3-TPD profile of each catalyst sample, indicating the presence of adsorbed NH3 species with

different thermal stabilities. After the addition of K or Mg, the peak intensity decreased distinctly, suggesting the decrease of surface acid sites. The calculated peak areas for Mn/TiO2, Mn/ TiO2−K, and Mn/TiO2−Mg were 2600, 992, and 1733, respectively. Thus, the NH3 adsorption capacity of Mn/TiO2 catalyst decreased by 61.8% after the doping of K. Compared with K, the suppression effect of Mg on the adsorption of NH3 over Mn/TiO2 catalyst was relatively weaker, and only a decrease of about 33.3% could be found after the addition of Mg, which should be originated from the different alkalinities of K and Mg. However, the decreased surface acidity was not lineally with the drop of SCR activity, as illustrated in Figure 1(A) and Figure 6(A). Figure 6(B) shows the NO-TPD profiles of the three catalyst samples. For the fresh Mn/TiO2 catalyst, two NO2 desorption peaks at 150 and 275 °C could be observed, while the NO2 desorption peaks became much weaker after the doping of K or Mg, especially for the peak at higher temperature. As shown in the inset in Figure 6(B), a distinct decrease of NO2 desorption peak area could be observed. The results indicated that the addition of K or Mg on Mn/TiO2 catalyst had a suppression effect on NO oxidation. For the Mg-poisoned catalyst sample, the locations of the two NO desorption peaks were similar to that in the profile of Mn/TiO2. However, the two peaks moved to higher temperature after the introduction of K to Mn/TiO2 catalyst, suggesting that K could strengthen the bonding of NO with catalyst surface. It seemed that the inhibition effect of K or Mg on NO adsorption was relatively weak, and the quantity of NO adsorption on Mn/TiO2 catalyst decreased only by 11.1% E


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The Journal of Physical Chemistry C and 19.5% after the modification with K and Mg, as shown in the inset in Figure 6(B). Moreover, the intensity of the lowertemperature NO desorption peak decreased after the addition of K or Mg, accompanied by the increased intensity of the higher-temperature NO desorption peak. Thus, the addition of K or Mg would lead to the redistribution of adsorbed NOx species. 3.6. NO Oxidation. The oxidation of NO to NO2 could greatly promote the SCR reaction through the “fast SCR” reaction route: NO + NO2 + 2NH3 → 2N2 + 3H2O.36,37 Herein, the effect of K or Mg on NO oxidation over the Mn/ TiO2 catalyst was also investigated, and the results are shown in Figure 7. It could be seen that the addition of K or Mg on Mn/

Figure 7. NO oxidation over the three catalyst samples. Reaction conditions: [NO] = 600 ppm, [O2] = 5%, balance Ar, GHSV = 108 000 h−1.

TiO2 catalyst would result in a noticeable drop of the value NO oxidation over it, which was in good accordance with the results of NO-TPD analysis. According to the study of Chen et al.,45 the NO oxidation was crucial for the low-temperature SCR reaction, especially when the reaction temperature was below 250 °C. Thus, it may be concluded that the decreased lowtemperature SCR activity of Mn/TiO2−K and Mn/TiO2−Mg was greatly relative to the inhibited NO oxidation. 3.7. In Situ DRIFT Study. 3.7.1. NH3 Adsorption. The in situ DRIFT spectra of NH3 adsorption over different catalyst samples as a function of temperature are shown in Figure 8. As can be seen from Figure 8(A), several bands of adsorbed NH3 species were presented in the DRIFT spectra of Mn/TiO2, which could be assigned to NH+4 species on Brønsted acid sites (1700 cm−1) and coordinated NH3 on Lewis acid sites (1600, 1224, and 1176 cm−1), respectively.41,46−48 Besides that, another band at 1535 cm−1 could be ascribed to NH2 species.49 As an important intermediate product in NH3−SCR reaction, NH2 could easily react with NO to form N2 and H2O.50,51 It was distinct that the band intensities of NH3 species adsorbed on Lewis acid sites were much higher than that of the NH3 species adsorbed on Brønsted acid sites. So the surface acidity of Mn/TiO2 catalyst was mainly in the form of Lewis acidity. Moreover, all the bands became weaker with increasing temperature, indicating the thermal desorption of adsorbed NH3 species. However, the band at 1600 cm−1 was still present at 350 °C, while the two characteristic bands of the Brønsted acid site basically disappeared at 250 °C. The results revealed that the thermal stability of Lewis acid sites was stronger than

Figure 8. In situ DRIFT spectra of NH3 adsorption over: (A) Mn/ TiO2; (B) Mn/TiO2−Mg; and (C) Mn/TiO2−K.

that of Brønsted acid sites. From Figure 8(B) and Figure 8(C), it could be observed that the DRIFT spectra of NH3 adsorption over Mn/TiO2−Mg and Mn/TiO2−K were similar to that of Mn/TiO2, except the decreased band intensities. Therefore, the addition of Mg or K on Mn/TiO2 catalyst would inhibit the adsorption of NH3 species on it. It should be noticed that the band of NH2 species was very weak in the spectra of Mn/ TiO2−Mg and disappeared completely in the spectra of Mn/ TiO2−K. These results suggested that the suppression effect on the activation of adsorbed NH3 species of K was much stronger than that of Mg. F


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The Journal of Physical Chemistry C 3.7.2. NO + O2 Adsorption. Figure 9 exhibits the in situ DRIFT spectra of NO + O2 adsorption over the three catalyst

TiO2 catalyst, the band intensities decreased, meaning that the introduction of Mg or K was unfavorable to the adsorption of NOx species on Mn/TiO2 catalyst. Moreover, two new bands at 1270 and 1300 cm−1 could be found in the DRIFT spectra of Mn/TiO2−Mg and Mn/TiO2−K, respectively, which should be assigned to monodentate nitrate (1270 cm−1) and bidentate nitrate (1300 cm−1).53,56 Therefore, the addition of Mg or K not only inhibited the adsorption of NOx species but also generated new active sites for NOx adsorption. Similar results had also been found in our recent study57 and by other researchers.25 3.7.3. Reaction between NOx and the Preadsorbed NH3 Species. To investigate the activity of adsorbed NH3 species in the NH3−SCR reaction over different catalyst samples, the DRIFT spectra of the reaction between NOx and preadsorbed NH3 species at 200 °C were recorded, and the results are presented in Figure 10. As can be seen from Figure 10(A), several bands of adsorbed NH3 species were present in the DRIFT spectra of Mn/TiO2 catalyst after the pretreatment with NH3. After the introduction of NO + O2, all the bands of adsorbed NH3 species quickly diminished in 2 min, thus all the adsorbed NH3 species were active in the NH3−SCR reaction over Mn/TiO2 catalyst, suggesting the presence of the E−R mechanism.58,59 The reaction rate of adsorbed NH3 species was very high. Moreover, several bands of adsorbed NOx species appeared after the completion of NH3−SCR reaction. A similar trend could also be observed on the DRIFT spectra of Mn/ TiO2−Mg (Figure 10(B)) and Mn/TiO2−K (Figure 10(C)). However, the reaction rate of adsorbed NH3 species over Mn/ TiO2−Mg was lower than that over Mn/TiO2. The band at 1600 cm−1 was still visible after the introduction of NO + O2 for 2 min. Thus, the addition of Mg would not only inhibit the adsorption of NH3 species but also weaken its reactivity. For Mn/TiO2−K, the reactivity drop was not obvious due to the low adsorption amount of NH3 species over it. 3.7.4. Reaction between NH3 and the Preadsorbed NOx Species. On another aspect, the DRIFT spectra of the reaction between NH3 and the preadsorbed NOx species over the three catalyst samples are illustrated in Figure 11. As can be seen from Figure 11(A), several bands of adsorbed NOx species appeared in the DRIFT spectra of Mn/TiO2 catalyst after the exposure to NO + O2 for 30 min. After the introduction of NH3, all these bands quickly vanished in 2 min, indicating the high reactivity of adsorbed NOx species in NH3−SCR reaction. Thus, the L−H mechanism was also applicable for the NH3− SCR reaction over Mn/TiO2 catalyst.58 For the DRIFT spectra of Mn/TiO2−Mg catalyst (Figure 11(B)), the situation was a little different. Most of the bands of adsorbed NOx species quickly disappeared in 2 min, but the intensity of the band at 1270 cm−1 decreased very slowly with time. This band was still present after the introduction of NH3 for 30 min. From Figure 11(C), it could be seen that all the adsorbed NOx could participate the NH3−SCR reaction over Mn/TiO2−K catalyst with a very low reactivity. Combined with the results shown in Figure 11(B) and Figure 11(C), it seems that the presence of K nearly cut off the NH3−SCR reaction over Mn/TiO2 catalyst through the L−H pathway. 3.8. Effect of Reaction Temperature. As shown in Figure 1(A), the deactivation effect of K or Mg on Mn/TiO2 catalyst was greatly dependent on reaction temperature. It seemed that the poisoning effect of Mg decreased with increasing temperature, while the poisoning effect of K nearly kept stable in the whole experimental temperature range. Thereafter, the different

Figure 9. In situ DRIFT spectra of NO + O2 adsorption over: (A) Mn/TiO2; (B) Mn/TiO2−Mg; and (C) Mn/TiO2−K.

samples as a function of temperature. As shown in Figure 9(A), four bands of adsorbed NOx species were present in the DRIFT spectra of Mn/TiO2 catalyst. These bands belonged to adsorbed NO2 species (1610 cm−1), bidentate nitrate (1560 cm−1), linear nitrite (1490 cm−1), and bridged nitrate (1240 cm−1).52−55 Similar to the bands of adsorbed NH3 species, the band intensities of adsorbed NOx species decreased with increasing temperature. After the addition of Mg or K on Mn/ G


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Figure 10. In situ DRIFT spectra of NO + O2 reacted with preadsorbed NH3 species over: (A) Mn/TiO2; (B) Mn/TiO2−Mg; and (C) Mn/TiO2−K at 200 °C.

Figure 11. In situ DRIFT spectra of NH3 reacted with preadsorbed NOx species over: (A) Mn/TiO2; (B) Mn/TiO2−Mg; and (C) Mn/ TiO2−K at 200 °C.

E−R mechanism increased with increasing reaction temperature, which should be due to the accelerated activation of adsorbed NH3 species at higher temperature. When the reaction temperature was no more than 150 °C, the NH3− SCR reactions over Mn/TiO2 and Mn/TiO2−Mg were mainly controlled by the L−H mechanism; however, the E−R pathway became the major mechanism for the NH3−SCR reactions over them in the higher temperature range (≥200 °C). As for the Mn/TiO2−K catalyst, it could be found that the ratio of CL−H/ (CL−H + CE−R) for the NH3−SCR reaction over it was less than

effect of reaction temperature on the deactivation of Mn/ TiO2−K and Mn/TiO2−Mg should be discussed. On the basis of the method proposed by Yang et al.,60 the contributions of E−R and L−H mechanisms to the NH3−SCR reactions over the three catalyst samples could be obtained, and the results are shown in Figure 12. After normalization by SBET, the SCR reaction rate could be calculated. Then the contribution of the L−H mechanism to the NH3−SCR reaction could be expressed by the ratio of CL−H/(CL−H + CE−R). From Figure 12, it was clear that the contribution of the H


Article


(Figure 12(C)). So the suppressed NH3 adsorption and activation should be the main reasons for its deactivation.

4. CONCLUSIONS In this study, the different poisoning mechanism of K and Mg on Mn/TiO2 catalyst was investigated. It was found that the addition of K or Mg not only decreased the reducibility of Mn/ TiO2 catalyst but also inhibited the adsorption of reactants. The results of in situ DRIFT study revealed that the introduction of Mg would not change the mechanism of NH3−SCR reaction over Mn/TiO2 catalyst, which was a combination of the L−H mechanism (≤150 °C) and E−R mechanism (≥200 °C), while the NH3−SCR reaction over Mn/TiO2−K was mainly controlled by the E−R mechanism in the whole experimental range. For Mn/TiO2−Mg, the deactivation was mainly due to the inhibited adsorption and oxidation of NO; for Mn/TiO2− K, the suppressed adsorption and activation of NH3 species were the primary reasons for its deactivation. It should be noted that this study has been carried out without water vapor and SO2 present in the simulated SCR gas, despite those gases being usual components of most industrial off-gases originating from combustion of fossil fuels or waste. The suggested mechanisms of deactivation of the investigated catalysts might therefore be altered by exposure to industrial flue gases.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b00290. Pore size distributions of the three catalyst samples (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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Figure 12. Contributions of E−R and L−H mechanisms to the NH3− SCR reaction over: (A) Mn/TiO2; (B) Mn/TiO2−Mg; and (C) Mn/ TiO2−K. Reaction conditions: [NO] = [NH3] = 600 ppm, [O2] = 5%, balance Ar, GHSV = 108 000 h−1.

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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40% in the whole temperature range, suggesting the predominant role of the E−R mechanism in the NH3−SCR reaction over it. Based on the results, we could propose the different deactivation mechanism of Mn/TiO2−Mg and Mn/TiO2−K: (1) Deactivation mechanism of Mn/TiO2−Mg catalyst: In the lower temperature range (≤150 °C), the deactivation of Mn/TiO2−Mg was mainly caused by the inhibited adsorption and oxidation of NO, while in the higher temperature range (≥200 °C), the inhibition effect of Mg on the adsorption and activation of NH3 species was relatively weak; so, the deactivation of Mn/TiO2−Mg was not obvious. (2) Deactivation mechanism of Mn/TiO2−K catalyst: Due to the seriously inhibited NO adsorption and oxidation, the SCR reaction over it through the L−H pathway was very slow

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TiO2 Catalyst for

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