In Situ DRIFTs Investigation of Promotional Effects of Tungsten on

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In Situ DRIFTs Investigation of Promotional Effects of Tungsten on MnO-CeO/meso-TiO Catalysts for NO Reduction x

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Kaiwen Zha, Sixiang Cai, Hang Hu, Hongrui Li, Tingting Yan, Liyi Shi, and Dengsong Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08600 • Publication Date (Web): 01 Nov 2017 Downloaded from http://pubs.acs.org on November 1, 2017

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The Journal of Physical Chemistry

In Situ DRIFTs Investigation of Promotional Effects of Tungsten on MnOx-CeO2/meso-TiO2 Catalysts for NOx Reduction Kaiwen Zha †,§ Sixiang Cai, ‡,§ Hang Hu, † Hongrui Li, †Tingting Yan, † Liyi Shi, † and Dengsong Zhang*,†,‡ †

Research Center of Nano Science and Technology, Shanghai University, Shanghai

200444, P. R. China. ‡

State Key Laboratory of Marine Resource Utilization in South China Sea, Hainan

University, Haikou 570228, P. R. China. §

These authors contributed equally to this work.

Abstract: In this work, mesoporous TiO2 spheres supported MnCeW mixed oxide catalysts (MnCeW/m-TiO2) for selective catalytic reduction of NOx with NH3 were prepared by a wet impregnation method. It is interesting that the MnCeW/m-TiO2 catalysts exhibited excellent SCR activity and N2 selectivity in a wide temperature range, even under the high gas hourly space velocity. From in situ diffuse reflectance infrared transform spectroscopy (in situ DRIFTs) studies of desorption, it could be concluded that the addition of tungsten brought about more Brønsted acid sites and reduced the energy barrier of NOx species adsorbed on the surface. At high temperature range, there were still some Brønsted acid sites and NOx species including bidentate nitrate and nitro compounds in MnCeW/m-TiO2, therefore more intermediates could take 1

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part in the SCR reactions as well as better catalytic performance. Besides, the in situ DRIFTs of transient reactions indicated that the formed NH3 species and NOx species of MnCeW/m-TiO2 were more reactive due to the promotional effects of tungsten. A series of traditional characterizations also revealed the promotional effects of tungsten for surface active elements, catalytic redox properties and acid sites of NH3 adsorption. In a word, all the results confirmed that the introduction of W could enhance active NH3 and NOx species as well as surface active elements, thus contributed to the catalytic performance. The present investigations may open a path for design and application of catalysts with outstanding catalytic activity and selectivity.

1. INTRODUCTION Over the past few decades, nitrogen oxides (NOx) emitted from automobiles and stationary sources have been considered to be a major cause of atmospheric environment pollution across the world.1-5 These NOx pollutants can result in many environmental problems such as greenhouse effects, ozone depletion and haze which could threaten human health and economic development.6-8 In recent years, NOx pollution has become more and more serious in some developing countries, especially in China. As for the control measures, selective catalytic reduction (SCR) with NH3 has been regarded as the most effective and widely employed technique for the control of NOx.9-14 As the key elements of SCR technology, the V2O5-WO3/TiO2 and V2O5-MoO3/TiO2 catalysts have been commercially used in the industry.15,16 However, 2


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unignorable problems still exist. For instance, the toxicity of vanadium oxides, low N2 selectivity at relative high temperatures, narrow operating temperature window and the easy deactivation limit the utilization potential of V2O5-WO3 (MoO3)/TiO2 catalysts.17-21 Therefore, it is essential to develop highly efficient vanadium-free catalysts. Recently, Mn-based catalysts have been identified as one of the most promising catalysts which could avoid the problems in relation to the existing industrial catalytic systems.22-24 Manganese oxides demonstrate high catalytic activities at low temperatures, owing to their excellent redox properties. Besides, the interaction of Mn species with co-dopant species or promoters also obviously influences the overall performance of composite catalysts.25-27 Among all Mn-based supported catalysts, MnOx-CeO2/TiO2 might be the candidate for practical application in NH3-SCR of NOx. It is well known that cerium as one of the most abundant rare-earth elements is of great interest because of its remarkable oxygen storage capacity and reducibility.28 Wu et al. have reported that the SO2 tolerance of Mn/TiO2 could be improved by Ce modification, owing to to the inhibition of the (NH4)2SO4 and NH4HSO4 as well as active species sulfation.29 Furthermore, the MnxCe1-xO2/HZSM-5 catalysts were applied to the catalytic oxidation of chlorobenzene which also presented good performance.30,31 Liu et al. developed a novel Mn-Ce-Ti catalyst for NH3-SCR, which showed a great synergistic effect between Mn and Ce, and thus the excellent deNOx performance. The dual redox cycles (Mn4+ + Ce3+ ↔ Mn3+ + Ce4+, Mn4+ + Ti3+ ↔ Mn3+ + Ti4+) might play a key role in the whole catalytic process.32 Besides, Liu et al. 3



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found that different preparation methods could affect catalytic performance and the Mn-Ce mixed oxide catalysts prepared by the surfactant-template method showed high NOx conversion.33 However, despite the high catalytic activity during low-temperature SCR reaction, too much N2O is still formed with the increase of reaction temperature and the low N2 selectivity poses an obstacle to its further application.5,34 So it’s necessary to improve the performance of the MnOx-CeO2/TiO2 catalysts for the practical application. Tungsten has been widely used as an efficient additive which could enhance the SCR activity and N2 selectivity.18,35 The main functions of WO3 are to increase the acidity which is crucial for ammonia adsorption and to decrease nonselective oxidation of ammonia, thus widening the SCR temperature window. In Ce-W/TiO2 catalysts, the reducibility of Ce is significantly improved by WO3, which is conducive to the Ce3+ formation as well as the better catalytic performance.28 In addition, the structure and concentration of WOx species on TiO2 supporter are also important to activity and stability. Optimal surface coverage of intermediate WOx species is more favorable for the deNOx efficiency and hydrothermal stability.36,37 Liu et al. developed WO3 modified Mn-Zr mixed oxides and WO3 promoted Fe2O3 as efficient NH3-SCR catalysts.38,39 Therefore, tungsten would be the preferable promoter for Mn-Ce/TiO2 catalysts. On the other hand, there were several disadvantages in traditional TiO2 nanoparticle carriers, such as low specific surface area and easy sintering. The pore structure of mesoporous TiO2 spheres could provide high specific surface area and restrain the migration of active components in the SCR process. Therefore, compared 4


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with traditional TiO2 nanoparticles, the mesoporous TiO2 spheres could make the active component disperse more homogeneously rather than aggregation. In the present study, we developed a novel mesoporous TiO2 spheres supported Mn-Ce-W mixed oxide catalyst. The employment of mesoporous TiO2 spheres as supporters could make the interaction of all compositions and the promotional effects of W doped MnOx-CeO2/TiO2 would be more distinct and convictive. The catalytic performance, structure, surface properties and redox ability of the catalysts were characterized. The role of W additives and the enhancement mechanism over the catalysts were fully clarified. 2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts A solvothermal method was used for preparation of mesoporous TiO2 spheres. 1 mL of tetraisopropyl titanate and 60 mL of acetone were mixed thoroughly with stirring for 30 min. Then the mixture was transferred to a Teflon-lined stainless steel autoclave and sealed. After heating at 200 °C for 12 h, the products were collected and washed with acetone and deionized water for several times. The white precipitate was dried at 60 °C overnight. Next, the powders were calcined at 400 °C for 2 h with heating rate of 2 °C min-1 in air. The obtained specimen was denoted as m-TiO2. Mesoporous TiO2 spheres supported mixed oxide catalysts were prepared by a wet impregnation method. At first, we should calculate according to the mass fraction of different metal oxides in a catalyst (MnO2: 9 wt%, CeO2: 9 wt%, WO3: 8 wt%). In a typical process, take mesoporous TiO2 spheres supported MnCeW metal oxides 5



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catalyst as an example, 0.4229 g of Mn(Ac)2·4H2O, 0.3784 g of Ce(NO3)3·6H2O and 0.1406 g of P2O5·24(WO3)·44(H2O) were then dissolved in 30 mL of H2O containing 1.17 g of m-TiO2. After vigorously stirring for a few minutes, the water was evaporated in a vacuum rotary evaporator at 45 °C. Finally, after drying at 110 °C in an oven and grinding, the resultant powders were calcined at 500 °C for 2 h in air. The catalysts were labeled as MnCeW/m-TiO2. As a comparison, the TiO2 nanoparticles supported MnCeW mixed oxide samples were also fabricated by using the same method and the various processing parameters were almost unchanged. The obtained samples were denoted as MnCeW/TiO2. 2.2. Catalytic Activity Measurements The SCR activity measurements were carried out using 0.3 g catalysts (40-60 mesh) in a fixed-bed quartz reactor. The following gas composition was used: 500 ppm NO, 500 ppm NH3, 5 vol% O2, 8 vol% H2O (when used), 100 ppm SO2 (when used) and N2 in balance. The total flow rate was 260 mL/min, and the corresponding gas hourly space velocity (GHSV) was 40000 h-1. In the high GHSV test, the catalyst dosage was 0.2 g and the total flow rate was 500 ml·min-1, thus the GHSV was 100000 h-1. The concentrations of NOx and N2O were analyzed by a VM4000 flue gas analyzer and a G200 N2O detector, respectively. The concentration of NH3 was monitored by a DR95C ammonia detector. The NO conversion percentage and N2 selectivity were calculated as the following formulas:

NO conversion

NOin - NOout ×100% (1)

NOin

! "1 − $&

$% &'()

* +, - $./ +, 0 $&* '() 0 $./ '()

1 × 100%

6


(2)

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Where the NOx stands for the total concentration of NO and NO2. The [2]in, [ 2 ]out, [N2O]out, [NOx]in, [NOx]out, [NH3]in and [NH3]out correspond to the concentration of inlet and outlet under steady state. The GHSV values were obtained using the following equation: 3456

78

(3)

9:; %

Where the MnCeW/m-TiO2 > Mn/m-TiO2 > MnW/m-TiO2 > MnCeW/TiO2. The MnCe/m-TiO2 had such an outstanding low-temperature activity due to the good redox ability of Mn and remarkable oxygen storage capacity of Ce. But this was a double-edged sword, as the NO conversion of MnCe/m-TiO2 started to decrease at only 220 °C. In the meantime, the temperature window above 95% of MnCeW/m-TiO2 was broadened to 140-340 °C by the introduction of W and the NO conversion was still kept at 93% even at 360 °C. As a comparison, the MnCe/m-TiO2, Mn/m-TiO2, MnW/m-TiO2 and MnCeW/TiO2 catalysts exhibited narrow temperature windows above 95% of 123-278 °C, 150-290 °C, 165-360 °C and 185-360 °C respectively. But it is worth noting that all the catalysts containing W showed laudable high-temperature activity. These results demonstrated that the W promoter could enhance the catalytic activity at high temperature and widen the temperature window, without too much attenuation of the low-temperature activity. Since the MnCeW/m-TiO2 catalysts exhibited outstanding NO conversion in a broad temperature range and the MnCe/m-TiO2 catalysts showed the highest NO conversion at low temperature, they were chosen to study the performance under high GHSV condition. The results of NO conversion and N2 selectivity over 9



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MnCeW/m-TiO2 and MnCe/m-TiO2 under a GHSV of 100000 h-1 were illustrated in Figure 1b. It was found that the MnCeW/m-TiO2 catalysts still showed a wide temperature window above 70% of 225-390 °C and its maximum NO conversion was about 80%. As for the MnCe/m-TiO2 catalysts, its advantage at low temperature was no longer apparent and its best NO conversion of nearly 70% was reached in a scanty temperature range. More seriously, the N2 selectivity of MnCe/m-TiO2 started to decrease at about 185 °C, and down to 80% at 390 °C. As mentioned above, the remarkable redox ability of Mn and Ce would give rise to the nonselective oxidation of NH3, thus the decline of N2 selectivity and deactivation of catalyst. Surprisingly, the N2 selectivity of MnCeW/m-TiO2 was kept above 90% in the whole temperature range, revealing that even at relative high temperature the introduction of W could effectively improve the N2 selectivity. The stability of MnCeW/m-TiO2 catalysts was also tested under 40000 h-1 GHSV (Figure S2a). As the time elapsed, the catalytic activity of MnCeW/m-TiO2 catalyst was found to be maintained during the test period. The NO conversion only decreased about 1% after the SCR reaction at 200 °C for 24 h. The phenomenon evidently suggested that MnCeW/m-TiO2 catalysts present a preeminent stability. Obviously, there was no influence for the activity of MnCeW/m-TiO2 catalysts by the addition of H2O (Figure S2b), manifesting that the MnCeW/m-TiO2 catalysts had a good ability for water resistance. The NO conversion of MnCeW/m-TiO2 catalysts decreased dramatically when the SO2 was introduced and then leveled off after 5h (Figure S2c). The activity had a slight recovery after cutting SO2 off. The combined introduction of 10


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SO2 and H2O into the MnCeW/m-TiO2 catalysts resulted in a similar phenomenon (Figure S2d). Likewise, the NO conversion could also be recovered slightly after the H2O and SO2 was removed. According to the previous reports, the inactivation of catalyst is commonly resulted from the blocked active sites which are caused by the sulfation of active phases and appearance of ammonium sulfate species.40,41 Generally speaking, the MnCeW/m-TiO2 catalysts exhibited excellent SCR activity and N2 selectivity in a wide temperature range, even under high GHSV, as well as decent stability and H2O durability. The above results demonstrated that the promotional effect of catalyst was achieved by the addition of tungsten. 3.2. Structure Properties The morphology of MnCeW/m-TiO2 catalysts was investigated by HRTEM. As revealed in Figure 2a, the distinct globular TiO2 with a diameter of about 330 nm and the asperous surface with some mesopores were observed. Meanwhile, Figure 2b confirmed that the mesoporous TiO2 spheres were formed in the anatase phase, as manifested by the lattice fringes of 0.35 nm which were indexed to (101) crystalline planes. As shown in Figure 2c, after the loading of MnCeW mixed oxides, the shape of spherical catalyst was well maintained. The spatial distribution of different elements over the MnCeW/m-TiO2 catalysts was further illustrated by the EDX mapping (Figure 2d-i). The elemental mapping images verified the uniform distribution of Mn, Ce, W and Ti species of the MnCeW/m-TiO2 catalysts. The well-distributed active metal oxides might profit from the confinement effect of mesoporous channel in TiO2 spheres. It is well known that the good dispersion of 11



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active compositions is crucial to the catalytic activity of catalysts. Therefore, the mesoporous TiO2 spheres supported MnCeW mixed oxide catalysts achieved the expected design purpose. The XRD was performed to ascertain the crystal structure of all catalysts. For both mesoporous TiO2 spheres supported mixed oxides (Figure S3) and TiO2 nanoparticles supported MnCeW mixed oxide samples (Figure S5a), the typical diffraction peaks assigned to the anatase phase (JCPDs No.21-1272) were observed, proving the same crystal form of mesoporous TiO2 spheres and TiO2 nanoparticles. This result was consistent with the above HRTEM analysis. In addition, for all the mesoporous TiO2 spheres supported catalysts, no characteristic peaks of other phase were detected. This result suggested that the active species were in a homodisperse state or the crystallites size was less than 5 nm in the mesoporous TiO2 spheres supported catalysts, thus to the benefit of interactions among active sites, promoters and supporters as well as the catalytic performance. The N2 adsorption-desorption analysis was carried out to study and compare the differences of surface area, pore volume and diameter over various catalysts. It can be found that the N2 adsorption-desorption isotherms of mesoporous TiO2 spheres supported catalysts revealed typical type IV curves with distinct type H3 hysteresis loops in the relative pressure of 0.4-0.8, attesting the mesoporous structure based on the definition of IUPAC (Figure S4). On the other hand, for TiO2 nanoparticles supported MnCeW mixed oxide catalysts, the isotherm showed a type II curve with obvious rise of N2 adsorption capacity in the relative pressure of 0.8-1.0, which was 12


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the representative response of TiO2 nanoparticles to the N2 adsorption-desorption characterization (Figure S5b). The textural parameters of all catalysts were summarized in Table 1. The specific surface area of MnCeW/TiO2 was only 65 m2·g-1, while those of mesoporous TiO2 spheres supported catalysts were 96-117 m2·g-1. Therefore, the mesoporous TiO2 spheres could afford more active sites and motion space for the reactant gas as the catalyst carrier. It is worthy of noting that the MnCeW/m-TiO2 had the smallest specific surface area of 96 m2·g-1 but the best catalytic performance due to the promotional effects of W. 3.3. Surface Elements Analysis The oxidation state of each useful element on the different catalysts surface was investigated by XPS. Figure 3a showed that the O 1s bands were divided into two characteristic peaks centered around 529.7 eV and 531.2 eV. The former one could be ascribed to the lattice oxygen species (such as O2-, denoted as Oβ) and the latter one was attributable to the surface adsorbed oxygen species (e.g. O- and O22- corresponded to hydroxyl-like groups and defect oxides, denoted as Oα).14,42 As is well-known, the Oα species are much more active than the Oβ species due to their high mobility and reactivity.43,44 So the relative concentration ratios of Oα species of the different samples were calculated and the results were listed in Table 2. The Oα/ (Oα+ Oβ) ratios of the MnCeW/m-TiO2, MnCe/m-TiO2, MnW/m-TiO2 and Mn/m-TiO2 catalysts were 53.85%, 45.85%, 38.46% and 31.92%, respectively. The Oα species percentage of the MnCeW/m-TiO2 catalysts was higher than those of the other catalysts, suggesting that the tungsten played an important role in the increase of active oxygen species. 13



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As shown in Figure 3b, the Mn 2p spectra could be deconvoluted into three peaks belonged to Mn2+ (640.4 eV), Mn3+ (642 eV) and Mn4+ (644.1 eV).23,26 It could be found that the corresponding ratio of Mn4+/Mn over MnCeW/m-TiO2 catalysts surface (31.41%) was higher than that over MnCe/m-TiO2 (22.98%), MnW/m-TiO2 (17.67%) and Mn/m-TiO2 (14.32%). The Mn4+ phase and their favourable redox cycle had been considered to be beneficial for the SCR performance. It was clear that most Mn4+ species existed on the surface of MnCeW/m-TiO2 catalysts, thus improved the redox properties as well as the catalytic performance. The W 4f spectra of the catalysts containing tungsten were shown in Figure 3c. All the catalysts had a broad band which could be fitted into two pairs of peaks corresponding to W5+ species (orange curves) and W6+ species (cyan curves). The data suggested that the amount of W5+ species over the MnCeW/m-TiO2 catalysts (46.97%) was the largest among all samples. It had been reported that the W5+ species could provide oxygen vacancies which were beneficial to the oxygen adsorption.45 The production of more W5+ species resulted from the interactions between W and other active components. As for Ce 3d spectra in Figure 3d, the complicated bands of Ce 3d mainly contained ten components. The sub-peaks labelled v0, v′, u0 and u′ represented the 3d104f1 electronic state belonging to Ce3+ species and those labelled v, v′′, v′′′, u, u′′ and u′′′ represented the 3d104f0 electronic state corresponding to Ce4+ species.46-48 Similar to the W5+ species, the Ce3+ phase could also give rise to charge imbalance and vacancies which would be in favor of the generation of hydroxyl-like groups and 14


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oxide defects.20 The percentage of Ce3+ species over MnCeW/m-TiO2 (47.94%) was appreciably higher than that over MnCe/m-TiO2 (43.82%). This result meant that the NH3 adsorption and the chemisorbed oxygen species could be enhanced owing to the abundant hydroxyl-like groups and oxide defects, respectively, which was in keeping with the above results of O 1s bands. It could be concluded that the MnCeW/m-TiO2 catalysts had more useful elements with specific chemical valence, thus contributed to the outstanding catalytic performance, thanks to the promotional effects of tungsten. 3.4. Redox Properties Analysis It is widely accepted that the catalysis is a redox process in essence and therefore the redox property of catalyst plays a key role in the whole catalytic cycle. Figure 4a showed the H2-TPR profiles of all catalysts. For the mesoporous TiO2 spheres supported mixed oxide catalysts, all their reduction peaks of surface species appeared in the temperature range of 270-380 °C which were related to the reduction of high dispersion of oxides species.49 Furthermore, with the addition of Ce and W, the reduction peaks increasingly moved to higher temperature. This skewing trend might be caused by the interaction of catalytic activity species which suggested that the redox properties of catalysts were changed. More importantly, the reduction peak area and temperature of MnCeW/m-TiO2 catalysts were the biggest and highest, indicating that there were the most reduction species in the MnCeW/m-TiO2 catalysts thus to the benefit of catalytic reaction.50 In contrast, the peak around 531 °C was observed in MnCeW/TiO2 catalysts which could belong to the reduction of bulk tungsten oxide species (Figure S6).51 This showed that tungsten oxides aggregated in the TiO2 15



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nanoparticles supported MnCeW mixed oxide samples, which was consistent with the XRD results. On the other hand, the reduction peak area of MnCeW/TiO2 was also very large, implying the promotional effect of W for the catalytic redox properties indirectly. The above results demonstrated that the synergistic interactions between W and other active components could significantly improve the redox properties of catalysts by the addition of tungsten. 3.5. NH3-TPD Analysis It has been widely realized that the adsorption and activation of NH3 on the catalyst surface are crucial steps in the NH3-SCR reactions.52 The strength of surface acid sites is an important factor in this process. So the acid sites of all catalysts were determined using NH3-TPD technique. As shown in Figure 4b, two NH3 desorption peaks were observed around 166 and 348 °C in Mn/m-TiO2. The former peak was assigned to ionic NH4+ belonging to Brønsted acid sites and the latter one was assigned to coordinated NH3 bounded to Lewis acid sites.53,54 After the introduction of Ce, the location of NH3 desorption had a little change while the peak area slightly increased. However, in the curve of MnW/m-TiO2 catalysts, the two peaks of different acid sites moved to higher temperature range accompanied by a remarkable area increase, indicating that both the strength and amount of acid sites could be enhanced by the incorporation of W. Moreover, as shown in the NH3-TPD curves of MnCeW/m-TiO2, those NH3 desorption peaks became much stronger and the region became much broader. It can be concluded that the introduction of W could dramatically enhance the Brønsted and Lewis acid sites, thereafter facilitate the SCR reactions. 16


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3.6. In Situ DRIFTs Studies for Adsorption of NH3 or NO + O2 In order to further identify the surface adsorption species, the in situ DRIFTs of NH3 or NO + O2 adsorption at 30 °C over all catalysts was implemented and the results are shown in Figure 5. Obviously, the bands around 1607 cm-1 and 1177cm-1 were assigned to the asymmetric and symmetric bending vibrations of N-H bond in coordinated NH3 adsorbed on Lewis acid sites.55 Several bands around 1690 cm-1 which were attributed to the asymmetric bending vibration of N-H bond in NH4+ species on Brønsted acid sites were observed.56 As a result, almost all the samples were mainly covered by Lewis acid sites except for MnCeW/m-TiO2. On the other hand, after NO + O2 adsorption, some NOx species were formed on the surfaces of catalysts. Normally, the bands at 1698, 1626, 1523, 1334-1400, 1198 and 1096 cm-1 could belong to N2O4, gaseous NO2, monodentate nitrate, nitro compounds, asymmetric and symmetric vibrations of monodentate nitrite, respectively.7,28,50 Compared with MnCeW/TiO2 (Figure S7), the catalysts with mesoporous TiO2 sphere carriers exhibited stronger ability to lead the NOx species adsorbed and formed. This improvement benefited from the mesoporous channels as well as the high specific surface areas. Furthermore, the in situ DRIFTs of NH3 desorption at different temperatures were carried out to examine the thermostability and change of acid sites on MnCe/m-TiO2 after the W addition. As shown in Figure 6, the peaks of coordinated NH3 and NH4+ species appeared after NH3 adsorption. In other words, Lewis and Brønsted acid sites were observed in both samples, but the ratios were different. It was easy to see that 17



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the ratio of Brønsted to Lewis acid sites of MnCeW/m-TiO2 was higher than that of MnCe/m-TiO2. It means that the introduction of tungsten resulted in more Brønsted acid sites which were beneficial to SCR reactivity. In addition, as the temperature increased, the intensity of acid sites in MnCeW/m-TiO2 decreased more slowly. Especially, there were still a certain number of Brønsted acid sites even at 350 °C in MnCeW/m-TiO2 while those in MnCe/m-TiO2 diminished before 200 °C. It was one of the reasons for the greater high-temperature activity and broader temperature window above 90%. These results were in good agreement with the above NH3-TPD analysis. The in situ DRIFTs of NO + O2 desorption on both samples were also applied to reveal the status of NOx species adsorbed over the catalyst surface, and the results were shown in Figure 7. After exposure to NO + O2, the samples were covered by a series of NOx species whose bands were ascribed to N2O4 (1700 cm-1), weekly adsorbed and gas phase NO2 (1667 and 1622 cm-1), monodentate nitrate (1520 and 1517 cm-1), nitro compounds (1338-1400 cm-1), monodentate nitrite (1191 and 1097 cm-1).7,57,58 More NOx species could be observed on the surface of MnCeW/m-TiO2 due to the incorporation of W. It can be deduced that the addition of W could reduce the energy barrier of NOx species formed and adsorbed on the catalyst surface. With increasing temperature, the bands of NOx species on MnCe/m-TiO2 vanished soon and all the bands on MnCeW/m-TiO2 decreased gradually. In particular, the monodentate nitrate and nitro compounds on MnCeW/m-TiO2 showed the strongest stability and completely disappeared even at the temperature of 270 and 330 °C, respectively. At 18


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high temperature, the remaining NOx species on the surface of MnCeW/m-TiO2 might participate in the SCR reaction and contribute to the catalytic activity. This should be another reason why the MnCeW/m-TiO2 catalysts exhibit remarkable catalytic performance at a relative high temperature range. 3.7. In Situ DRIFTs Studies of Transient Reactions To identify the reactivity of adsorbed species in the SCR reaction, the transient reaction studies by in situ DRIFTs were performed. Here, we chose 200 °C as the test temperature where MnCeW/m-TiO2 and MnCe/m-TiO2 catalysts simultaneously showed sufficient high catalytic activity and distinct differences could be observed. The reactions between NO+O2 and pre-adsorbed NH3 species as a function of time over MnCeW/m-TiO2 and MnCe/m-TiO2 catalysts were proceeded (Figure 8a-b). After the adsorption of NH3 for 1h, the bands around 1196, 1610 cm-1 and 1690 cm-1 could be observed, which were respectively assigned to symmetric and asymmetric bending

vibrations

of

coordinated

NH3 and

ionic

NH4+

appeared

over

MnCeW/m-TiO2.27,50 As for MnCe/m-TiO2, the bands of NH3 species bound to Lewis and Brønsted acid sites also emerged. When NO + O2 were introduced, all the peaks of different NH3 species decreased in intensity. Meanwhile, several bands were attributed to NOx species including nitro compounds (1353 and 1339 cm-1) and bidentate nitrate (1542 cm-1) started to accumulate.32,59 It was found that the peaks of NH3 species over MnCeW/m-TiO2 almost disappeared after 15 min when the NH3 species over MnCe/m-TiO2 visibly existed even after 20 min. The result indicated that the NH3 species adsorbed on the catalysts surface became more active due to the 19



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tungsten doping. To further compare the reactivity of adsorbed NH3 species on different samples, the attenuation of band at 1521 cm-1 over MnCe/m-TiO2 and band at 1196 cm-1 over MnCeW/m-TiO2 with normalized intensities at 200 °C was recorded as a function of time as shown in Figure 8c. It can be seen that the intensity of band at 1196 cm-1 over MnCeW/m-TiO2 decreased about 70% within 10 min, however, the intensity of band at 1521 cm-1 over MnCe/m-TiO2 merely decreased about 35%. This result provided the compelling evidence that the formed NH3 species on MnCeW/m-TiO2 were highly reactive due to the incorporation of tungsten while the species on MnCe/m-TiO2 were not active enough. The in situ DRIFTs of reactions between NH3 and pre-adsorbed NOx species as a function of time over both catalysts were also studied (Figure 9a-b). The gas reactants were introduced to samples in the reverse order. After the exposure to NO + O2 for 1h, the surface of MnCeW/m-TiO2 was principally covered by nitro compounds (1356 cm-1) and bidentate nitrate (1541 cm-1) while the nitro compounds (1360 cm-1) were dominated over MnCe/m-TiO2. Two minutes later, the bands around 1196 cm-1 were ascribed to coordinated NH3 on Lewis acid sites appeared over MnCeW/m-TiO2 as well as MnCe/m-TiO2.35,58,60 The inset of 1300-1600 cm-1 which only contained the bands of NOx species was provided to minimize the effect that the NH3 species adsorbed too intensively to clearly estimate the NOx species variation. Similar to the previous results, the nitro compounds in MnCe/m-TiO2 still existed after introduction of NH3 for 20 min, whereas the bidentate nitrate rapidly vanished during 5 min and the nitro compounds almost disappeared after 20 min in MnCeW/m-TiO2. Similarly, 20


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the consumption of band at 1361 cm-1 over MnCe/m-TiO2 and band at 1355 cm-1 over MnCeW/m-TiO2 with normalized intensities at 200 °C was recorded as a function of time as shown in Figure 9c. It was apparent that the intensity of band at 1355 cm-1 decreased about 75% in 10 min over MnCeW/m-TiO2 while only 30% for band at 1361 cm-1 over MnCe/m-TiO2, indicating that the NOx species on MnCeW/m-TiO2 were much easier to react with NH3 species compared with those on MnCe/m-TiO2. The above cases in Figure 8 and Figure 9 manifest that all the NH3 and NOx species could be adsorbed and formed on the surfaces of MnCeW/m-TiO2 and MnCe/m-TiO2, but there was so much different in reactivity of these species. In a word, MnCeW/m-TiO2 catalysts have more outstanding ability of NH3 and NOx species adsorption and the formed species were more reactive, thus contributing to the great catalytic performance. It can be concluded that the MnCe/m-TiO2 catalyst was significantly promoted by the incorporation of tungsten. Based on the presented results and discussion, the promoted SCR performance was achieved by the addition of W and the corresponding mechanism was proposed in Scheme 1. The SCR activity and N2 selectivity of the catalysts would be notably improved due to the special role of W. In a broad temperature range, the MnCeW/m-TiO2 catalysts had strong capabilities to catalyze NOx into N2 instead of N2O. Besides, the MnCeW/m-TiO2 catalysts had the smallest specific surface area but the best catalytic performance among all the prepared catalysts. We could owe them to abundant surface active elements, splendid redox properties, more acid sites for active NH3 species adsorption and more NOx species taking part in the SCR reactions. 21



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Even at relative high temperature, there were still some Brønsted acid sites and NOx species in MnCeW/m-TiO2, hence resulted in remarkable high-temperature activity and wide operating window. At 200 °C, the NOx species were mainly nitro compounds and bidentate nitrate over MnCeW/m-TiO2, while those over MnCe/m-TiO2 were mainly nitro compounds. Furthermore, both the NH3 and NOx species on the surface of MnCeW/m-TiO2 were more reactive as the reaction intermediates due to the promotional effects of W. In addition, the specific porous structure of MnCeW/m-TiO2 catalysts could promote the dispersity of tungsten as well as the interactions between W and other active species, and afford diffusion paths for the reactant gas thus the confinement effect. 4. CONCLUSIONS In summary, the mesoporous TiO2 spheres supported Mn-Ce mixed oxide catalysts was successfully modified by the introduction of W. The MnCeW/m-TiO2 catalysts exhibited outstanding SCR activity in a wide temperature range (temperature window of NO conversion above 95% was 140-340 °C under the GHSV of 40000 h-1) and excellent N2 selectivity (above 90% in the whole temperature range of 90-390 °C under the GHSV of 100000 h-1). The representative MnCeW/m-TiO2 catalysts also showed remarkable stability and decent H2O durability. The characterization results of all the prepared catalysts demonstrated the promotional effects of W for surface active elements, catalytic redox cycles as well as acid sites of NH3 adsorption. In addition, the evidence from in situ DRIFTs indicated that the addition of tungsten led to the generation of more Brønsted acid sites and weakened the energy barrier of NOx 22


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species adsorbed on the surface. More importantly, the formed NH3 species and NOx species of MnCeW/m-TiO2 were more active due to the promotional effects of tungsten. The present results may guide the design and application of catalysts with outstanding catalytic activity and selectivity.

ASSOCIATED CONTENT Supporting Information The N2 selectivity of catalysts; The stability test, H2O durability test, SO2 durability test and H2O/SO2 durability test of the MnCeW/m-TiO2 catalyst; XRD patterns of catalysts; N2 adsorption-desorption isotherms of catalysts; XPS spectra of MnCeW/TiO2 catalyst; H2-TPR and NH3-TPD profiles of MnCeW/TiO2 catalyst; In situ DRIFTs of NH3 desorption and NO+O2 desorption over MnCeW/TiO2 catalyst are available in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Tel: +86-21-66137152. Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors acknowledge the support of the National Natural Science Foundation of China (21722704; U1462110).

REFERENCES

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32


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Table 1. Textural properties of various catalysts. Specific surface area

Pore volume

Average pore diameter

(m2 g-1)

(cm3 g-1)

(nm)

MnCeW/TiO2

65

0.34

5.61

MnCeW/m-TiO2

96

0.25

6.53

MnCe/m-TiO2

101

0.24

6.53

MnW/m-TiO2

117

0.34

6.55

Mn/m-TiO2

101

0.29

6.55

Catalysts

Table 2. The corresponding concentration ratios over various catalysts. Oα/O

Mn4+/Mn

W5+/W

Ce3+/Ce

(%)

(%)

(%)

(%)

MnCeW/m-TiO2

53.85

31.41

46.97

47.94

MnCe/m-TiO2

45.85

22.98

-

43.82

MnW/m-TiO2

38.46

17.67

31.12

-

Mn/m-TiO2

31.92

14.32

-

-

MnCeW/TiO2

50.19

24.36

42.19

45.22

Catalysts

33



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Figure 1. (a) NO conversion over MnCeW/m-TiO2, MnCeW/TiO2, Mn/m-TiO2, MnCe/m-TiO2 and MnW/m-TiO2 catalysts during NH3-SCR reaction. Reaction conditions: total flow rate is 260 mL/min, [NO] = [NH3] = 500 ppm, 5 vol% O2, GHSV = 40000 h-1. (b) NO conversion and N2 selectivity over MnCeW/m-TiO2 and MnCe/m-TiO2 catalysts as a function of temperature under a high GHSV. Reaction conditions: total flow rate is 500 mL/min, [NO] = [NH3] = 500 ppm, 5 vol% O2, GHSV = 100000 h-1.

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Figure 2. TEM images of mesoporous TiO2 spheres (a and b) and MnCeW/m-TiO2 catalysts (c); EDX mapping of MnCeW/m-TiO2 catalysts (d: Ti, e: O, f: Mn, g: Ce, h: W, i: combine).

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Figure 3. XPS spectra for (a) O 1s, (b) Mn 2p, (c) W 4f, and (d) Ce 3d of mesoporous TiO2 spheres supported mixed oxide catalysts.

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Figure 4. (a) H2-TPR and (b) NH3-TPD profiles of MnCeW/m-TiO2, MnCe/m-TiO2, MnW/m-TiO2 and Mn/m-TiO2 catalysts.

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Figure 5. In situ DRIFTs of (a) NH3 desorption and (b) NO+O2 desorption over MnCeW/m-TiO2, MnCe/m-TiO2, MnW/m-TiO2 and Mn/m-TiO2 catalysts after exposure to a flow of 500 ppm NH3 or 500 ppm NO + 5% O2 for 1 h at 30 oC.

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Figure 6. In situ DRIFTs of NH3 desorption over (a) MnCeW/m-TiO2 and (b) MnCe/m-TiO2 catalysts as a function of temperature.

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Figure 7. In situ DRIFTs of NO+O2 desorption over (a) MnCeW/m-TiO2 and (b) MnCe/m-TiO2 catalysts as a function of temperature.

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Figure 8. In situ DRIFTs of the transient reactions at 200 °C over (a) MnCeW/m-TiO2 and (b) MnCe/m-TiO2 catalysts between NO+O2 and pre-adsorbed NH3 as a function of time. (c) Consumption of the bands at 1521 cm-1 over MnCe/m-TiO2 and 1196 cm-1 over MnCeW/m-TiO2 catalysts.

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Figure 9. In situ DRIFTs of the transient reactions at 200 °C over (a) MnCeW/m-TiO2 and (b) MnCe/m-TiO2 catalysts between NH3 and pre-adsorbed NO+O2 as a function of time. (c) Consumption of the bands at 1361 cm-1 over MnCe/m-TiO2 and 1355 cm-1 over MnCeW/m-TiO2 catalysts.

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Scheme 1. Promoted SCR performance and the corresponding mechanism of the catalysts.

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TOC Graphic

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In Situ DRIFTs Investigation of Promotional Effects of Tungsten on

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