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Journal Pre-proofs Full Length Article The promoting effect of S-doping on the NH3-SCR performance of MnO x /TiO2 catalyst Xuesong Liu, Qifan Yu, Hongfeng Chen, Peng Jiang, Jianfa Li, Zhongyun Shen PII: DOI: Reference:
S0169-4332(19)33510-X https://doi.org/10.1016/j.apsusc.2019.144694 APSUSC 144694
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Applied Surface Science
Received Date: Revised Date: Accepted Date:
29 May 2019 5 November 2019 13 November 2019
Please cite this article as: X. Liu, Q. Yu, H. Chen, P. Jiang, J. Li, Z. Shen, The promoting effect of S-doping on the NH3-SCR performance of MnO x /TiO2 catalyst, Applied Surface Science (2019), doi: https://doi.org/10.1016/ j.apsusc.2019.144694
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The promoting effect of S-doping on the NH3-SCR performance of MnOx/TiO2 catalyst Xuesong Liua,c,*,[email protected], Qifan Yua, Hongfeng Chenb, Peng Jianga, Jianfa Lia, Zhongyun Shenb aCollege
of Chemistry and Chemical Engineering, Shaoxing University, Shaoxing, 312000, Chnia
bShaoxing
Testing Institute of Quality and Technical Supervision, Shaoxing, 312000, Chnia
State Key Laboratory of Green Building Materials, China Building Materials Academy, Beijing, 100024, P. R. China Graphical abstract *Corresponding author. Highlights S is doped into TiO2 via the replacement of Ti4+ by S4+ to form Ti-O-S bond. Brønsted acid sites and redox property are enhanced due to more active species. Reactivity and amount of ad-NH3 and ad-NOx species are improved. L-H route occurred on the S-doped catalyst is more significant than E-R route. MnTiS0.05 exhibits the best SCR performance and widest operating window.
Abstract A series of S-doped MnOx/TiO2 catalysts were prepared by sol-gel method for the purpose of enhancing SCR performance. The obtained catalyst with mole ratio of S:Ti = 0.05:1 exhibited the best low temperature SCR performance and the widest active temperature window. According to the results of various analyses, the proper S was doped into TiO2 lattice via the replacement of Ti4+ by S4+ to generate Ti-O-S bond, along with increasing surface area, more surface Mn4+ species and more active oxygen species (O2-). These enhanced properties improved the Brønsted acid sites and redox property of the properly S-doped MnOx/TiO2 catalyst. These advantages of S doping can essentially benefit the de-NOx ability of MnOx/TiO2 catalysts. Meanwhile, the in situ DRIFT experiments revealed that the adsorbed NH4+ and gas-phase NO2 species were greatly increased after proper S-doping, along with the enhanced 1
reactivity of the adsorbed NH3 and NOx species. Both E-R and L-H mechanisms occurred on the S-doped catalysts, while the promoting effect of S-doping was performed by facilitating the L-H route during SCR reaction. Keyword: MnOx/TiO2 catalyst, S-doping, In-situ DRIFT, NH3-SCR 1. Introduction Selective catalytic reduction (SCR) of NOx with NH3 is regarded as the most efficient strategy to control NOx emission from industries. A variety of catalysts are the core of this technology, such as Mn-based catalysts [1, 2]. As early as 1990s, Kapteijn have found that pure MnO2 catalyst exhibited the most excellent SCR performance among the MnOx species [3]. In the following decades, numerous researches of MnOx-based catalysts with or without support have been reported. Compared with the commercial V2O5-WO3(MoO3)/TiO2, catalysts of this type are identified as one of the most promising alternatives for practical utilization due to its excellent low temperature SCR performance [4, 5]. Among the supported MnOx catalysts, anatase TiO2 is usually chosen as the support due to its relatively high thermal stability and SO2 resistance [6]. However, its poor surface acidity (being a weak Lewis acid) may hinder the SCR performance of MnOx/TiO2 catalyst [7, 8]. Therefore, numerous efforts such as elemental doping have been performed to improve surface acidity and catalytic activity of MnOx/TiO2 catalyst [9, 10]. For instance, the surface acidity and surface area of MnOx/TiO2 catalysts were greatly improved after introducing Ce, resulting in the higher SCR performance [11, 12, 13, 14]. The addition of Fe also increased the total surface acidity and acid strength of MnOx/TiO2 catalysts, and leaded to the enhancement of catalytic activity [15, 16, 17]. Nowadays, many elements, such as W [18], Nd [4, 19], Nb [20], Ni [21, 22], Sn [23, 24], Sm/Zr [5, 9], Eu [25] and Ho [26], have been employed to improve the catalytic performance of MnOx/TiO2 catalyst by increasing the surface acidity and redox property. But all the investigations about modified MnOx/TiO2 catalysts focus on the doping with metal elements, and dope with non-metals has rarely been reported in literature. However, it is found that the 2
non-metal ions doping into the lattice can change the surface physics and chemistry of metal oxides by forming the surface defects, such as oxygen vacancies [6, 27, 28, 29]. To our knowledge, little is known about the modification effects of non-mentals on surface properties and SCR activity of MnOx/TiO2 catalysts. The study herein bridges the gap between the knowledge of proposing a novel S-doping method, and the objective of further enhancing the SCR performance of MnOx/TiO2 catalysts. The S-doped MnOx/TiO2 catalysts were synthesized by sol-gel method, and characterized with various analytical methods, such as XRD, BET and XPS, to investigate the changes of structure and texture properties after S-doping. The effects of S-doping on the acidity, redox property and reaction mechanism were also investigated by NH3-TPD, H2-TPR and in situ DRIFT experiments. NO(g) + O2(a)→NO2(a) 2. Experiments 2.1 Catalyst preparation All the reagents were used as received without further purification. The S-doped MnOx/TiO2 catalysts were synthesized by sol-gel method. Typically, 30 mL of tetrabutyltianate (C16H36O4Ti) were dissolved in 60 mL of absolute ethylalcohol (C2H6O) containing 1 mL of nitric acid (HNO3). Meanwhile, solution (50 wt%) of 12.5469 g Mn(NO3)2 solution (50 wt%) and a certain amount of thiourea (CH4N2S) were added to the mixed solution of 30 mL of deionized water, 30 mL of C2H6O and 2 mL of HNO3. Then, the solution containing Mn and S was dropwise added to the Ti solution under continuous stirring to form the sol. After aging for 24 h, the gel was formed and dried at 80 oC for 24 h. The S-doped MnOx/TiO2 catalysts were obtained after calcining at 450 oC for 2 h with the heating rate of 1 oC min-1. In this way, the mole ratios of S:Mn:Ti were theoretically 0.05/0.1/0.15:0.4:1 in the catalysts, respectively according to the amount of CH4N2S added. The corresponding catalysts were denoted as MnTiS0.05, MnTiS0.1 and MnTiS0.15, respectively. Undoped catalyst was prepared without addition of CH4N2S, which was named as MnTi. 2.2 Catalyst characterization 3
The chemical compositions of the catalysts were analyzed by using X-ray fluorescence spectrometer on a Shimadzu XRF-1800 with Rh radiation. X-ray diffraction (XRD) was performed on a Shimadzu X-ray diffractometer XRD-6100 (Cu Kα radiation, λ=1.5418 Å) with 2θ = 10-90°. N2 adsorption/desorption isotherms at 77 K was used to determine the specific surface area and pore size distribution by the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods (Tristar II 3020 analyzer, Micromeitiecs, USA). Raman spectra were recorded on a Raman spectrometer (InVia, Renishaw) with an argon laser as the excitation source operating at 532 nm under atmospheric pressure. The surface oxidation states of the catalysts were determined by X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250Xi analyzer). The XPS peak fitting process was performed on XPS Peak Fit v4.1 software based on Gaussian-Lorentz principle. The binding energies (BE) of various elements were calibrated using a C 1s peak (284.6 eV). H2-TPR and NH3-TPD experiments were conducted on a TP-5080 automated chemisorption analyzer (Xianquan, China). 50 mg of sample was pre-treated in a flow of N2 (30 mL min-1) at 250 oC for 20 min and then cooled to room temperature. After stabilization, TPR experiments were performed from 50 to 850 oC at 10 oC min-1. NH3-TPD experiments were carried out as the follows: 50 mg of sample was pre-treated in He flow (30 mL min-1) at 250 oC for 30 min and then cooled to 50 oC. After NH3 adsorption for 30 min, the sample was purged with He for 1 h at 100 oC. Then the TPD operation was executed from 100 to 500 oC at 10 oC min-1 under He flow.
2.3 SCR activity tests NH3-SCR activity tests of the catalysts were performed by using a fixed-bed quartz reactor with 0.8 mL of catalysts (pressed, crushed and sieved to 50-80 mesh) at 150-350 oC under atmospheric pressure. The feed steam with a GHSV of 50000 h-1 consisted of 500 ppm NO, 500 ppm NH3, 5% O2, 10% H2O and N2 as the balance gas. Before each test, the catalyst was pre-treated under a flow of 5% O2/N2 at 400 oC for 30 min. The outlet gas was monitored using a flue gas analyzer (Thermo Nicolet 380). 4
The concentrations of NOx (NO, N2O, and NO2) were detected at the ppm level. The NOx conversion and N2 selectivity were calculated according to Eqs. 1 and 2, respectively. Error! Objects cannot be created from editing field codes.(1) Error! Objects cannot be created from editing field codes.(2)
The NH3 oxidation tests were performed for all catalysts using a feed gas composition of 500 ppm NH3, 5% O2 and N2 in balance. The NH3 conversion was calculated by Eq. (3): Error! Objects cannot be created from editing field codes.(3) The NO oxidation tests were performed for all catalysts using a feed gas composition of 500 ppm NO, 5% O2 and N2 in balance. The NO conversion to NO2 was calculated by Eq. (4): Error! Objects cannot be created from editing field codes. (4)
2.4 In-situ DRIFT tests The adsorption behavior of reactants and their surface reactions were investigated by in situ DRIFT study, which was performed on a FTIR spectrometer (Thermo Nicolet iS 50) equipped with a smart collector and an MCT detector cooled by liquid nitrogen. Firstly, the catalyst was pretreated in a gas flow of 20% O2/N2 at 450 oC for 0.5 h to remove traces of organic residues. Next then, the sample was cooled down to the desired temperature, and subsequently purged by N2 (100 mL min-1) for 30 min for background collection. Afterwards, the gas mixture was flowed through the sample at 100 oC for 30 min to obtain the pre-adsorbed samples. In the experiments of in situ DRIFT study, the following conditions were adopted: the feed steam consisted of 500 ppm NH3, or/and 500 ppm NO + 5% O2, the total flow rate was kept at 100 mL min-1. 5
2.5 Ex-situ DRIFT tests The ex situ spectra of the catalysts were recorded at 30 oC and the spectrum of KBr under the same condition was used as the background. 3. Results and discussion 3.1 Chemical compositions, phase structures and textural properties of the catalysts The chemical compositions of all catalysts and their weight percentages are listed in Table 1. The mole ratios of Ti:Mn:S in the catalysts were calculated based on the weight percentages and the molar mass of each element. As shown in Table 1, the mole ratios of Ti:S in each catalyst are close to the theoretical values (1:0.05, 1:0.10 and 1:0.15, respectively), while the ratios of Ti:Mn are slightly lower than the theoretical value (1:0.4). Nevertheless, the values of Ti:Mn in each catalyst are very close to each other. XRD and Raman spectra were used to study the effect of S-doping on the phase structures of catalysts. The XRD patterns of S-doped and undoped MnOx/TiO2 catalysts are depicted in Fig. 1(a). All the diffraction peaks of four catalysts emerge at about 25.35°, 38.02°, 48.21°, 54.83° and 62.62°, matching with the (101), (004), (200), (211) and (204) planes of anatase TiO2 (PDF-ICDD. 21-1272), respectively. Meanwhile, no characteristic peaks assigned to MnOx and SOx can be observed in all catalysts. The crystallite sizes of the catalysts decrease slightly with the increasing S content (Table 1). Moreover, as shown in Table 1, the position of (101) plane of MnTi catalyst locates at 25.35° and the value of full-width-half-maximum (FWHM) is 1.579. After S-doping, the positions of the same plane of MnTiS0.05, MnTiS0.1 and MnTiS0.15 shift to 25.31°, 25.31°, 25.30°, respectively. The corresponding FWHMs broaden to 1.625, 1.689 and 1.704, respectively. The decreased crystallite sizes and broadened FWHMs are usually found on the S-doped TiO2-based photocatalysts. This phenomenon means the S is successfully doped into the TiO2 lattice, leading to suppress the TiO2 crystal growth by increasing the grain stress [6, 10, 30]. Fig. 1(b) exhibits the Raman spectra of the doped and undoped MnTi catalysts. The peaks at 151, 400, 520 and 647 cm-1 assigned to the representative anatase TiO2 can be 6
observed on all the catalysts. Figs.1(c) and (d) display the N2 adsorption/desorption isotherms and pore size distributions of S-doped and undoped catalysts. The results of BET surface area, pore volume and pore size are summarized in Table 2. From Fig. 1(c), all the catalysts exhibit the type IV isotherm with H3 hysteresis loops, indicating the mesoporous structure (2-50 nm) with narrow slit-like shapes [27]. The pore size distributions in Fig. 1(d) suggest that all the four catalysts consist of small mesopores (3-8 nm) with the average pore diameter of 4-5 nm (Table 2). The MnTi catalyst possess BET surface area of 123.1 m2 g-1 and pore volume of 0.19 cm3 g-1. After doping with a small amount of S, the surface area of MnTiS0.05 increases to 131.9 m2 g-1. However, the surface area and pore volume of MnTiS0.1 and MnTiS0.15 catalysts decrease slightly with further increasing S content. A larger surface area and pore volume of MnTiS0.05 may be beneficial to adsorb more reactants and disperse the active components, which can lead to an improvement of SCR performance [1, 27, 30].
3.2 SCR activity of the catalysts The NOx conversions as a function of temperature at 150-350 oC by using various catalysts are shown in Fig. 2(a). Evidently, MnTi catalyst exhibits the minimum NOx conversion of 44.2% (at 150 oC) and maximum conversion of 98.2% (at 270 oC), respectively. Meanwhile, > 85% NOx conversion was obtained in the temperature range of 200 to 300 oC on MnTi catalyst. In contrast, the NOx conversion at the whole temperature range is promoted by using MnTiS0.05 catalyst instead. It exhibits > 77% NOx conversion at 150-190oC and > 85% NOx conversion at 190-350 oC,
respectively, which is wider than that of MnTi in SCR temperature operating
range. However, when the S content increases further in MnTiS0.1 and MnTiS0.15 catalysts, the NOx conversion and temperature range using these catalysts become lower and narrower than that using MnTi. So, it is concluded that the appropriate 7
S-doping can improve the SCR performance and broaden the temperature operating range of MnOx/TiO2 catalyst. Fig. 2(b) shows the N2 selectivity of the catalysts for NH3-SCR reaction. The decrease in N2 selectivity at high temperatures is observed on all the catalysts with the formation of NO2 and especially N2O (Figs. S1a and S1b). Such a decrease is more significant for MnTi catalyst. Obviously, S modification succeeds in inhibiting the generation of N2O and NO2.
As shown in Fig. S2, the MnTiS0.05 shows excellent durability in the long-time SCR test. Fig. S3 and Table S1 show the characterization results of fresh and used MnTiS0.05, from which it can be known that the S-doped MnOx/TiO2 catalyst and doping S were stable after the long-time running. These features make it a promising SCR catalyst which can be applied in de-NOx of low-temperature emissions. The catalytic activities of the catalysts for NH3 oxidation and NO oxidation are shown in Fig. 3(a) and Fig. 3(b), respectively. The order of the catalyst activity turns to be MnTi > MnTiS0.05 > MnTiS0.1> MnTiS0.15 for NH3 oxidation and MnTiS0.05 > MnTiS0.1> MnTi> MnTiS0.15 for NO oxidation, respectively. The oxidation of NH3 by O2 to consume the reductant and the oxidation of NO to produce NO2 are important for the high- and low-temperature SCR activities of the catalysts. S addition inhibits NH3 oxidation over the modified catalysts to some extent, which may facilitate the NH3-SCR performance at high temperatures with more available reductant gas. Meanwhile, MnTiS0.05 and MnTiS0.1 show higher catalytic activity for NO oxidation than MnTi, improving the low-temperature SCR activity via the so-called fast SCR reaction [18].
3.3 Surface chemical states of the catalysts The surface chemical states of the S-doped and undoped catalysts were analyzed by XPS analysis, as shown in Fig. 4. Two peaks centered at 458.8 and 464.6 eV are observed in Fig. 4(a) over MnTi and MnTiS0.05 catalysts, attributing to the Ti 2p3/2 8
and Ti 2p1/2 of Ti4+ [31]. With the increasing S content, the peaks of Ti 2p shift to higher positions (458.6 and 464.6 eV) and the peak intensity become lower. The higher binding energy and the lower peak intensity of the S doped TiO2 indicate an influence of S addition on the electronic state of Ti element; probably some of the Ti4+ ions get substituted with S4+ ions in the lattices forming Ti-O-S structure [10]. The decrease in electron density around Ti atom is due to the greater electronegativity of S via O attracting Ti's electrons [32]. From Fig. 4(b), the Mn 2p3/2 of all the catalysts can be fitted into three peaks assigned to Mn2+ (641.2-641.6 eV), Mn3+ (642.0-641.6 eV) and Mn4+ (643.0-643.7 eV), respectively [18, 33]. The amounts of Mn4+ of four catalysts were calculated and listed in Table 2. Obviously, the amount of Mn4+ for MnTiS0.05 (54.48%) is much larger than that of MnTi, MnTiS0.1 and MnTiS0.15. It is well known that more Mn4+ species is beneficial to the low temperature SCR activity owing to its excellent redox ability [9, 20], which is proved by the results of NOx conversion (Fig. 2a). Additionally, the binding energies of Mn4+ for MnTiS0.1 and MnTiS0.15 are 0.6-0.7 eV higher than that of MnTi and MnTiS0.05 (Table 2), indicating the active Mn species may be sulfated after introducing too much S [12, 34]. The O 1s spectra of four catalysts are displayed in Fig. 4(c) and can be deconvoluted into two categories of “Oα” and “Oβ”. The Oα located at 529.6-530.4 eV with three sub-peaks correspond to lattice oxygen species (O2-). The Oβ centered at 531.5-532.0 eV are matched with the chemisorbed oxygen species (O2- from oxygen vacancies and adsorbed OH groups) [6, 27, 35]. Moreover, it is apparently that the binding energies of Oβ shift to higher position with increasing the doped amount of S, together with an increase in the ratios of Oβ (calculated by Oβ/O and listed in Table 2). These increased Oβ on MnTiS0.1 and MnTiS0.15 catalysts mainly originated from the extra S-OH groups from the hydration of SO42-, which has been proved to improve the acidity of catalysts [13, 32]. The oxygen vacancies may be related to the formation of Ti-O-S bond, which is thought to be beneficial to SCR activity [27]. As that shown in Fig. 4(d), the S 2p peaks of the S-doped catalysts are 9
overlapped in four sub-peaks with two chemical states of S4+ (168.2-168.4 eV and 169.3-169.4 eV) and S6+ (168.7-169.0 eV and 169.9-170.3 eV) [36]. However, no peaks attributed to S2- can be observed in Fig. 4(d), suggesting no replacement of O2with S2- to form Ti-S-Ti bond [37]. Combined with the XRD results, it implies that the substitution of Ti4+ by S4+ to generate Ti-O-S bond may occur, which is beneficial to generate more oxygen vacancies to form active oxygen species (O2-) after adsorbing O2 molecules [6, 28, 38].
3.4 Surface acidity and redox property of the catalysts Surface acidity and redox property are two important factors to affect the catalytic performance during NH3-SCR reaction, which were characterized by NH3-TPD and H2-TPR experiments. As shown in Fig. 5(a), all the catalysts present a broad NH3 desorption peak at 100-500 oC. The low-temperature NH3 desorption peak at 100-250 °C is ascribed to desorption of weak acid sites, while the high-temperature desorption peak at 250-500 °C is assigned to desorption of medium strong and strong acid sites [17, 18]. Table 3 lists the amounts of the two acid sites calculated from the NH3-TPD results. It is noticeably that the amounts of acid sites for all the S-doped catalysts are increased gradually. These increased acid sites may originate from the enhancement of surface Mn4+ species, oxygen vacancies caused by Ti-O-S bond and extra S-OH groups from the hydration of SO42- [20]. The amounts of weak acid sites for S-doped catalysts increases with S doping, while the amounts of medium and strong acid sites decreases on MnTiS0.15, indicating the less medium and strong acids due to the possibly formed MnSO4 [38, 39]. Combining the ratio of Mn4+/Mnx+ and Oβ/O from XPS results, it can be seen that the increase of extra S-OH groups from MnSO4 is closely related to the continuous increase of the weak acid sites, while the extra S-OH groups is not significantly related to the medium and strong acid sites. Fig. 5(b) displays the H2-TPR profiles of four catalysts. The MnTi catalyst exhibits three reduction peaks. Table 3 lists the amounts of different reducible species 10
on the catalysts based on the TPR curves. The peaks centered at 346 oC and 529 oC can be assigned to the reduction of MnOx, such as from MnO2 to Mn2O3 and from Mn3O4 to MnO, respectively [19, 39]. The peak at 607 oC may be corresponded to the reduction of active surface TiO2 species [40]. After the addition of S, the TPR profile of MnTiS0.05 catalyst is close to the MnTi, while the H2 consumption attributed to the slightly increased reduction of MnO2 to Mn2O3. It means that the easily reduced species (Mn4+) increase after proper S-doping, which is consistent with the XPS results (Table 2) and may be related to the well dispersion of MnOx species because of the high surface area of MnTiS0.05 catalyst [2, 6]. However, with the further increasing of S content, the lower reduction peaks of MnOx for MnTiS0.1 (392oC) and MnTiS0.15 (428 oC) shift to higher temperature together with the gradual decrease of H2 consumption, indicating the loss of redox capacity due to the decline of surface Mn4+ species. Meanwhile, the higher reduction peaks of MnOx move to the higher temperature, which is located at 564 oC for MnTiS0.1 and 626 oC for MnTiS0.15 with more H2 consumption. This may be caused by the reduction of hardly decomposed sulfates (MnSO4), which are inactive or harmful for SCR reaction [41]. The H2-TPR experiments reveal that the redox property is enhanced slightly after proper S-doping of MnOx/TiO2 catalyst owing to the increasement of the easily reduced surface Mn4+ species. Nevertheless, excess S may result in sulfur poisoning because of the formation of hardly decomposed sulfates.
3.5 Ex situ DRIFT study Infrared spectrum is an effective technique to detect the formation of Sulfur species. Fig. 6 shows the IR spectra of MnTi and MnTiS0.05 catalysts. The bands at 3330 and 1610 cm-1 are assigned to the OH-bending vibration [6], and that at 530 cm-1 is caused by Ti-O stretching vibration. S-doping has no obvious effect on the whole infrared spectrum, but a new band appears in the 1035 cm-1. According to previous literature [42], the band of 1035 cm-1 can be attributed to the formation of Ti-O-S. 11
3.6 In situ DRIFT study 3.6.1 NH3 adsorption and NO+O2 adsorption over the catalysts at different temperatures The DRIFT spectra of NH3 adsorption recorded from 100 oC to 450 oC for MnTi and MnTiS0.05 catalysts are exhibited in Figs. 7(a) and (b), respectively. For MnTi in Fig. 7(a), the band at 1608 cm-1 is assigned to coordinated NH3 adsorbed on Lewis acid sites. The bands at 1640-1800, 1448 and 1388 cm-1 are ascribed to NH4+ species bound to Brønsted acid sites [5, 9, 20 ,25]. The spectra of NH3 adsorption for MnTiS0.05 are close to that of MnTi, as shown in Fig. 7(b). The bands attributed to coordinated NH3 (1602 cm-1) and NH4+ species (1640-1800, 1445 and 1385 cm-1) are observed, meaning the existence of adsorbed NH3 species (ad-NH3) on the two acid sites. Noticeably, compared with MnTi catalyst, the intensities of bands belonging to Brønsted acid sites on MnTiS0.05 enhance significantly, while that belonging to Lewis acid sites decreases slightly. The decline of Lewis acid sites may due to the decrease in the amounts of active surface TiO2 species as proved by the H2-TPR results. So, it is concluded that S-doping mainly increased the number of Brønsted acid sites on MnTiS0.05 catalyst. Combined with the NH3-TPD results, it can be seen that the increased acid sites after S doping are mainly attributed to the Brønsted acid sites. Additionally, all the peaks of two catalysts gradually become weaker with increasing temperature and absolutely vanish at 450 oC, suggesting the strength of both acid sites may be unchanged obviously after S-doping. Figs. 7(c) and (d) display the NOx adsorption DRIFT spectra of MnTi and MnTiS0.05 catalysts at 100-450 oC. As shown, several characteristic peaks of adsorbed NOx species can be observed on MnTi catalyst, representing adsorbed NO2 (1625 cm-1), bridged nitrate species (1597 cm-1) and bidentate nitrate species (1572 cm-1) [43, 44, 45]. MnTiS0.05 catalyst exhibits the similar DRIFT spectra of NOx adsorption, except the emergence of a new band at 1316 cm-1 that may be attributed to 12
bridged nitrate species [45]. However, the enhanced band intensities and areas can be observed on the MnTiS0.05 catalyst, implying the adsorption of more NOx species (ad-NOx) to form new active sites [20, 25]. Especially, the band intensity of the adsorbed NO2 molecules (1620 cm-1) for MnTiS0.05 is much higher than that of MnTi, indicating the generation of more NO2 species to improve the low-temperature SCR activity via its promotion of a “fast SCR” reaction [18]. With the increasing of temperature from 100-450 oC, the intensities of all bands for the two catalysts gradually decrease and completely disappear at 350 oC. 3.6.2 The behaviors of adsorbed NH3 and NOx species for the catalysts during SCR reaction The behaviors of NO+O2 reacted with pre-adsorbed NH3 and NH3 reacted with pre-adsorbed NO+O2 for MnTi and MnTiS0.05 catalysts were investigated by the time-dependent in situ DRIFT experiments at 150 oC. The results are depicted in Fig. 8. As seen in Fig. 8(a) for MnTi catalyst, the characteristic bands (1640-1800, 1610, 1448 and 1385 cm-1) of adsorbed NH3 species connected with Lewis and Brønsted acid sites emerge after the pre-treatment of 500 ppm NH3 for 30 min. Subsequently, all the adsorbed NH3 species are quickly consumed after the addition of NO+O2 and completely vanish in 20 min, followed by generation of some adsorbed NOx species (e.g. 1625 cm-1). The similar tendency of NO+O2 reacted with pre-adsorbed NH3 species can be seen in Fig. 8(b) for MnTiS0.05 catalyst. However, the band linked to Lewis acid sites at 1610 cm-1 quickly vanishes in 5 min in Fig. 7(b), while it is still present for MnTi at the same time. Although more Brønsted acid sites are generated on the MnTiS0.05 surface (Fig. 5(a) and 6(b)), the bands’ intensities are slightly lower than that of MnTi, indicating the quick consumption of the adsorbed NH3 species. Therefore, it is likely that the reactivity of adsorbed NH3 species was improved after the proper S-doing [20, 25, 47]. More importantly, from Figs. 8(a) and (b), the NH3 species can firstly adsorb on the Lewis and Brønsted acid sites and then react with the gas-phase NOx species, suggesting the existence of Eley-Rideal (E-R) mechanism 13
during the NH3-SCR reaction [9, 25]. Figs. 8(c) and (d) demonstrate the transient reaction of NH3 and pre-adsorbed NOx species for MnTi and MnTiS0.05 catalysts. Apparently, several adsorbed NOx species generate on the surface of two catalysts after the NO+O2 pretreatment for 30 min, which is consistent with the results of Figs. 7(c) and (d). All these adsorbed NOx species were expended rapidly after the introduction of NH3 within 5 min. Meanwhile, the bands belonging to adsorbed NH3 species begin to grow and finally replace the adsorbed NOx species on the two catalysts surface subsequently. Nevertheless, the bands at 1623 and 1597 cm-1 for MnTiS0.05 completely disappear in 3 min, while they are still observed at the same time for MnTi. Combined with the NOx adsorption results in Figs. 7(c) and (d), it is concluded that the proper S-doping can increase the amount of adsorbed NOx species and enhance its reactivity. Additionally, the new band at 1537 cm-1 can be observed, attributing to the NH2 intermediate originated from oxidation of NH3 by oxygen [48]. Therefore, these evidences demonstrate that all the adsorbed NOx species can react with the gas-phase NH3 to participate in the SCR reaction, which follows the Langmuir-Hinshelwood (L-H) mechanism [9, 20, 25, 49].
Above all, the proper amount of S can be doped into TiO2 lattice via the replacement of Ti4+ by S4+ to form Ti-O-S bond, leading to the increased surface area and generation of more oxygen vacancies, which is proved by the EPR, PL and Ex situ DRIFT tests from the Zhong’s and Liu’s group [6, 27, 28, 42]. The increasement of surface area can disperse the MnOx species to form more active Mn4+ species, resulting in the enhanced surface acidity and redox property. Additionally, it is also favorable to adsorb more reactants (NH3 and NO). Meanwhile, the generated oxygen vacancies are readily to adsorb oxygen molecules and generate active oxygen species (O2-). The presence of O2- and Mn4+ are in favor of oxidizing adsorbed NO to form ad-NO2 and activating NH3 to generate ad-NH4+, respectively. All these evidences are beneficial to open the “fast SCR” route during the SCR reaction, leading to the 14
improved low-temperature SCR performance and broadened active temperature window of MnTiS0.05 catalyst. Furthermore, in situ DRIFT experiments demonstrate that the increased surface acidity mainly originated from the NH4+ linked to Brønsted acid sites (proved by NH3-TPD). The oxidation of NO is facilitated to form more gas-phase NO2 molecules on the surface of MnTiS0.05 sample, agreeing with the XPS results. Meanwhile, the reactivity of the ad-NH3 and ad-NOx species was also improved by the proper S-doping. Then, compared with the transient reactions of (NO+O2+ad-NH3) and (NH3+ad-NOx), it seems that both the E-R and L-H mechanisms existed during the NH3-SCR reactions for MnTi and MnTiS0.05 catalysts. However, the reaction rate of (NH3+ad-NOx) is much faster than that of (NO+O2+ad-NH3), although the proper S-doping can simultaneously promote the above two processes. The experimental evidences suggest that the L-H mechanism may be more significant on basis of the larger amount of ad-NH3 (NH4+), ad-NOx (NO2) and NH2 intermediate. Therefore, the promoting effect of the proper S-doping on the NH3-SCR performance of MnTiS0.05 catalyst can be described by eqns. (2)-(7) [48, 49]:
NH 3 (g) NH +4 (a)
on Brønsted acid sites (2)
NH +4 (a) NH 2 (a)+2H + +e- (3)
O 2 (g) O 2 (a) (4) NO (g) + O 2 (a) NO 2 (a) (5) NH 2 (a) + NO (g) NH 2 NO (a) N 2 +H 2 O (6) NH +4 (a) + NO 2 (a)+ e- NH 4 NO 2 (a) N 2 +H 2 O (7) However, excess S-doping into MnTi catalyst will decrease the surface area and Mn4+ species on MnTiS0.1 and MnTiS0.15 catalysts, which is caused by the generation of Mn-sulfated species or sulphates on the catalyst surface. These sulphates seem to weaken the redox property of catalysts, leading to the decline of SCR activity. 15
4. Conclusions This study reveals that proper S-doping can enhance the SCR performance and broaden the active temperature range of MnOx/TiO2 catalyst, while too much S will weaken the redox property and SCR activity. According to the experimental and characterization results, the main conclusions are summarized as the follows: (1) S was doped into TiO2 lattice via the replacement of Ti4+ by S4+ to generate Ti-O-S bond, along with increasing surface area and generating more oxygen vacancies. (2) The Brønsted acid sites and redox property were enhanced after proper S-doping owing to the more surface Mn4+ species and active oxygen species (O2-). (3) The proper S-doping can enhance the reactivity and amount of ad-NH3 and ad-NOx species by increasing the Brønsted acid sites and by oxidizing NO to form more gas-phase NO2, which are beneficial to improve the low temperature SCR performance. (4) Both E-R and L-H mechanisms occurred on the S-doped MnOx/TiO2 catalyst. The promoting effect of the proper S-doping on NH3-SCR performance was mainly performed by the L-H route. (5) MnTiS0.05 catalyst exhibited the best low temperature SCR performance and widest active temperature window. Acknowledgements The authors acknowledge financial support from the National Natural Science Foundation of China (No. 51702215), the Science and Technology Department of Zhejiang Province (LGG19B070002), the Science and Technology Program of Shaoxing (2017B70051), Shaoxing University (No. 2016LG1001), National Training Programs of Innovation and Entrepreneurship for Undergraduates (201810349011), Xinmiao Talent Program of Zhejiang Province(2018R432003) and the National Key R&D Program of China (No. 2017YFC0210700). References [1] B. H. Zhao, R. Rui, X. G. Guo, L. Gao, T. F. Xu, Z. Chen, X. D. Wu, Z. C. Si, D. 16
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Mole ratios
Crystallite
2θ, FWHM of (101) plane
Ti
Mn
S
of Ti:Mn:S
size (nm)
2θ (°)
FWHM
45.4
17.5
0.00
1:0.34:0
5.21
25.35
1.579
22
MnTiS0.05
44.8
17.3
1.20
1:0.34:0.04
5.04
25.31
1.625
MnTiS0.1
42.6
16.0
3.27
1:0.33:0.11
4.83
25.31
1.689
MnTiS0.15
41.1
15.5
4.53
1:0.33:0.16
4.82
25.30
1.704
Table 2 The results of BET surface area, pore volume, pore size and XPS for all catalysts. Mn4+/Mnx+
Oβ/O
Oβ
(%)
(%)
643.0
531.5
49.54
13.59
4.34
643.0
531.7
54.48
14.74
0.16
4.71
643.6
532.0
35.58
25.92
0.15
4.27
643.7
532.0
26.69
28.51
Binding energy (eV)
SBET
Pore volume
Pore size
(m2g-1)
(cm3g-1)
(nm)
Mn4+
MnTi
123.1
0.19
4.98
MnTiS0.05
131.9
0.19
MnTiS0.1
118.9
MnTiS0.15
110.1
Catalysts
Table 3 Acidity of the all catalysts determined by NH3-TPD. Catalyst MnTi MnTiS0.0 5 MnTiS0.1 MnTiS0.1 5
Surface acidity / μmol NH3 g−1 cat. Weak Medium strong and strong Total 18.82 24.04 42.86 22.63 54.07 76.7
H2 consumption / μmol /g cat. 1 2 3 Total 14.73 8.43 0.96 24.12 15.92 9.42 25.34
28.02 30.55
9.57 8.03
79.96 75.84
107.98 106.39
19.30 23.19
0.32 0.03
29.19 31.25
Table 3 Acidity of the all catalysts determined by NH3-TPD. Catalyst MnTi MnTiS0.0 5 MnTiS0.1 MnTiS0.1 5
Surface acidity / μmol NH3 g−1 cat. Weak Medium strong and strong Total 18.82 24.04 42.86 22.63 54.07 76.7
H2 consumption / μmol /g cat. 1 2 3 Total 14.73 8.43 0.96 24.12 15.92 9.42 25.34
28.02 30.55
9.57 8.03
79.96 75.84
107.98 106.39
23
19.30 23.19
0.32 0.03
29.19 31.25
S0169-4332(19)33510-X https://doi.org/10.1016/j.apsusc.2019.144694 APSUSC 144694
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
29 May 2019 5 November 2019 13 November 2019
Please cite this article as: X. Liu, Q. Yu, H. Chen, P. Jiang, J. Li, Z. Shen, The promoting effect of S-doping on the NH3-SCR performance of MnO x /TiO2 catalyst, Applied Surface Science (2019), doi: https://doi.org/10.1016/ j.apsusc.2019.144694
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The promoting effect of S-doping on the NH3-SCR performance of MnOx/TiO2 catalyst Xuesong Liua,c,*,[email protected], Qifan Yua, Hongfeng Chenb, Peng Jianga, Jianfa Lia, Zhongyun Shenb aCollege
of Chemistry and Chemical Engineering, Shaoxing University, Shaoxing, 312000, Chnia
bShaoxing
Testing Institute of Quality and Technical Supervision, Shaoxing, 312000, Chnia
State Key Laboratory of Green Building Materials, China Building Materials Academy, Beijing, 100024, P. R. China Graphical abstract *Corresponding author. Highlights S is doped into TiO2 via the replacement of Ti4+ by S4+ to form Ti-O-S bond. Brønsted acid sites and redox property are enhanced due to more active species. Reactivity and amount of ad-NH3 and ad-NOx species are improved. L-H route occurred on the S-doped catalyst is more significant than E-R route. MnTiS0.05 exhibits the best SCR performance and widest operating window.
Abstract A series of S-doped MnOx/TiO2 catalysts were prepared by sol-gel method for the purpose of enhancing SCR performance. The obtained catalyst with mole ratio of S:Ti = 0.05:1 exhibited the best low temperature SCR performance and the widest active temperature window. According to the results of various analyses, the proper S was doped into TiO2 lattice via the replacement of Ti4+ by S4+ to generate Ti-O-S bond, along with increasing surface area, more surface Mn4+ species and more active oxygen species (O2-). These enhanced properties improved the Brønsted acid sites and redox property of the properly S-doped MnOx/TiO2 catalyst. These advantages of S doping can essentially benefit the de-NOx ability of MnOx/TiO2 catalysts. Meanwhile, the in situ DRIFT experiments revealed that the adsorbed NH4+ and gas-phase NO2 species were greatly increased after proper S-doping, along with the enhanced 1
reactivity of the adsorbed NH3 and NOx species. Both E-R and L-H mechanisms occurred on the S-doped catalysts, while the promoting effect of S-doping was performed by facilitating the L-H route during SCR reaction. Keyword: MnOx/TiO2 catalyst, S-doping, In-situ DRIFT, NH3-SCR 1. Introduction Selective catalytic reduction (SCR) of NOx with NH3 is regarded as the most efficient strategy to control NOx emission from industries. A variety of catalysts are the core of this technology, such as Mn-based catalysts [1, 2]. As early as 1990s, Kapteijn have found that pure MnO2 catalyst exhibited the most excellent SCR performance among the MnOx species [3]. In the following decades, numerous researches of MnOx-based catalysts with or without support have been reported. Compared with the commercial V2O5-WO3(MoO3)/TiO2, catalysts of this type are identified as one of the most promising alternatives for practical utilization due to its excellent low temperature SCR performance [4, 5]. Among the supported MnOx catalysts, anatase TiO2 is usually chosen as the support due to its relatively high thermal stability and SO2 resistance [6]. However, its poor surface acidity (being a weak Lewis acid) may hinder the SCR performance of MnOx/TiO2 catalyst [7, 8]. Therefore, numerous efforts such as elemental doping have been performed to improve surface acidity and catalytic activity of MnOx/TiO2 catalyst [9, 10]. For instance, the surface acidity and surface area of MnOx/TiO2 catalysts were greatly improved after introducing Ce, resulting in the higher SCR performance [11, 12, 13, 14]. The addition of Fe also increased the total surface acidity and acid strength of MnOx/TiO2 catalysts, and leaded to the enhancement of catalytic activity [15, 16, 17]. Nowadays, many elements, such as W [18], Nd [4, 19], Nb [20], Ni [21, 22], Sn [23, 24], Sm/Zr [5, 9], Eu [25] and Ho [26], have been employed to improve the catalytic performance of MnOx/TiO2 catalyst by increasing the surface acidity and redox property. But all the investigations about modified MnOx/TiO2 catalysts focus on the doping with metal elements, and dope with non-metals has rarely been reported in literature. However, it is found that the 2
non-metal ions doping into the lattice can change the surface physics and chemistry of metal oxides by forming the surface defects, such as oxygen vacancies [6, 27, 28, 29]. To our knowledge, little is known about the modification effects of non-mentals on surface properties and SCR activity of MnOx/TiO2 catalysts. The study herein bridges the gap between the knowledge of proposing a novel S-doping method, and the objective of further enhancing the SCR performance of MnOx/TiO2 catalysts. The S-doped MnOx/TiO2 catalysts were synthesized by sol-gel method, and characterized with various analytical methods, such as XRD, BET and XPS, to investigate the changes of structure and texture properties after S-doping. The effects of S-doping on the acidity, redox property and reaction mechanism were also investigated by NH3-TPD, H2-TPR and in situ DRIFT experiments. NO(g) + O2(a)→NO2(a) 2. Experiments 2.1 Catalyst preparation All the reagents were used as received without further purification. The S-doped MnOx/TiO2 catalysts were synthesized by sol-gel method. Typically, 30 mL of tetrabutyltianate (C16H36O4Ti) were dissolved in 60 mL of absolute ethylalcohol (C2H6O) containing 1 mL of nitric acid (HNO3). Meanwhile, solution (50 wt%) of 12.5469 g Mn(NO3)2 solution (50 wt%) and a certain amount of thiourea (CH4N2S) were added to the mixed solution of 30 mL of deionized water, 30 mL of C2H6O and 2 mL of HNO3. Then, the solution containing Mn and S was dropwise added to the Ti solution under continuous stirring to form the sol. After aging for 24 h, the gel was formed and dried at 80 oC for 24 h. The S-doped MnOx/TiO2 catalysts were obtained after calcining at 450 oC for 2 h with the heating rate of 1 oC min-1. In this way, the mole ratios of S:Mn:Ti were theoretically 0.05/0.1/0.15:0.4:1 in the catalysts, respectively according to the amount of CH4N2S added. The corresponding catalysts were denoted as MnTiS0.05, MnTiS0.1 and MnTiS0.15, respectively. Undoped catalyst was prepared without addition of CH4N2S, which was named as MnTi. 2.2 Catalyst characterization 3
The chemical compositions of the catalysts were analyzed by using X-ray fluorescence spectrometer on a Shimadzu XRF-1800 with Rh radiation. X-ray diffraction (XRD) was performed on a Shimadzu X-ray diffractometer XRD-6100 (Cu Kα radiation, λ=1.5418 Å) with 2θ = 10-90°. N2 adsorption/desorption isotherms at 77 K was used to determine the specific surface area and pore size distribution by the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods (Tristar II 3020 analyzer, Micromeitiecs, USA). Raman spectra were recorded on a Raman spectrometer (InVia, Renishaw) with an argon laser as the excitation source operating at 532 nm under atmospheric pressure. The surface oxidation states of the catalysts were determined by X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCALAB 250Xi analyzer). The XPS peak fitting process was performed on XPS Peak Fit v4.1 software based on Gaussian-Lorentz principle. The binding energies (BE) of various elements were calibrated using a C 1s peak (284.6 eV). H2-TPR and NH3-TPD experiments were conducted on a TP-5080 automated chemisorption analyzer (Xianquan, China). 50 mg of sample was pre-treated in a flow of N2 (30 mL min-1) at 250 oC for 20 min and then cooled to room temperature. After stabilization, TPR experiments were performed from 50 to 850 oC at 10 oC min-1. NH3-TPD experiments were carried out as the follows: 50 mg of sample was pre-treated in He flow (30 mL min-1) at 250 oC for 30 min and then cooled to 50 oC. After NH3 adsorption for 30 min, the sample was purged with He for 1 h at 100 oC. Then the TPD operation was executed from 100 to 500 oC at 10 oC min-1 under He flow.
2.3 SCR activity tests NH3-SCR activity tests of the catalysts were performed by using a fixed-bed quartz reactor with 0.8 mL of catalysts (pressed, crushed and sieved to 50-80 mesh) at 150-350 oC under atmospheric pressure. The feed steam with a GHSV of 50000 h-1 consisted of 500 ppm NO, 500 ppm NH3, 5% O2, 10% H2O and N2 as the balance gas. Before each test, the catalyst was pre-treated under a flow of 5% O2/N2 at 400 oC for 30 min. The outlet gas was monitored using a flue gas analyzer (Thermo Nicolet 380). 4
The concentrations of NOx (NO, N2O, and NO2) were detected at the ppm level. The NOx conversion and N2 selectivity were calculated according to Eqs. 1 and 2, respectively. Error! Objects cannot be created from editing field codes.(1) Error! Objects cannot be created from editing field codes.(2)
The NH3 oxidation tests were performed for all catalysts using a feed gas composition of 500 ppm NH3, 5% O2 and N2 in balance. The NH3 conversion was calculated by Eq. (3): Error! Objects cannot be created from editing field codes.(3) The NO oxidation tests were performed for all catalysts using a feed gas composition of 500 ppm NO, 5% O2 and N2 in balance. The NO conversion to NO2 was calculated by Eq. (4): Error! Objects cannot be created from editing field codes. (4)
2.4 In-situ DRIFT tests The adsorption behavior of reactants and their surface reactions were investigated by in situ DRIFT study, which was performed on a FTIR spectrometer (Thermo Nicolet iS 50) equipped with a smart collector and an MCT detector cooled by liquid nitrogen. Firstly, the catalyst was pretreated in a gas flow of 20% O2/N2 at 450 oC for 0.5 h to remove traces of organic residues. Next then, the sample was cooled down to the desired temperature, and subsequently purged by N2 (100 mL min-1) for 30 min for background collection. Afterwards, the gas mixture was flowed through the sample at 100 oC for 30 min to obtain the pre-adsorbed samples. In the experiments of in situ DRIFT study, the following conditions were adopted: the feed steam consisted of 500 ppm NH3, or/and 500 ppm NO + 5% O2, the total flow rate was kept at 100 mL min-1. 5
2.5 Ex-situ DRIFT tests The ex situ spectra of the catalysts were recorded at 30 oC and the spectrum of KBr under the same condition was used as the background. 3. Results and discussion 3.1 Chemical compositions, phase structures and textural properties of the catalysts The chemical compositions of all catalysts and their weight percentages are listed in Table 1. The mole ratios of Ti:Mn:S in the catalysts were calculated based on the weight percentages and the molar mass of each element. As shown in Table 1, the mole ratios of Ti:S in each catalyst are close to the theoretical values (1:0.05, 1:0.10 and 1:0.15, respectively), while the ratios of Ti:Mn are slightly lower than the theoretical value (1:0.4). Nevertheless, the values of Ti:Mn in each catalyst are very close to each other. XRD and Raman spectra were used to study the effect of S-doping on the phase structures of catalysts. The XRD patterns of S-doped and undoped MnOx/TiO2 catalysts are depicted in Fig. 1(a). All the diffraction peaks of four catalysts emerge at about 25.35°, 38.02°, 48.21°, 54.83° and 62.62°, matching with the (101), (004), (200), (211) and (204) planes of anatase TiO2 (PDF-ICDD. 21-1272), respectively. Meanwhile, no characteristic peaks assigned to MnOx and SOx can be observed in all catalysts. The crystallite sizes of the catalysts decrease slightly with the increasing S content (Table 1). Moreover, as shown in Table 1, the position of (101) plane of MnTi catalyst locates at 25.35° and the value of full-width-half-maximum (FWHM) is 1.579. After S-doping, the positions of the same plane of MnTiS0.05, MnTiS0.1 and MnTiS0.15 shift to 25.31°, 25.31°, 25.30°, respectively. The corresponding FWHMs broaden to 1.625, 1.689 and 1.704, respectively. The decreased crystallite sizes and broadened FWHMs are usually found on the S-doped TiO2-based photocatalysts. This phenomenon means the S is successfully doped into the TiO2 lattice, leading to suppress the TiO2 crystal growth by increasing the grain stress [6, 10, 30]. Fig. 1(b) exhibits the Raman spectra of the doped and undoped MnTi catalysts. The peaks at 151, 400, 520 and 647 cm-1 assigned to the representative anatase TiO2 can be 6
observed on all the catalysts. Figs.1(c) and (d) display the N2 adsorption/desorption isotherms and pore size distributions of S-doped and undoped catalysts. The results of BET surface area, pore volume and pore size are summarized in Table 2. From Fig. 1(c), all the catalysts exhibit the type IV isotherm with H3 hysteresis loops, indicating the mesoporous structure (2-50 nm) with narrow slit-like shapes [27]. The pore size distributions in Fig. 1(d) suggest that all the four catalysts consist of small mesopores (3-8 nm) with the average pore diameter of 4-5 nm (Table 2). The MnTi catalyst possess BET surface area of 123.1 m2 g-1 and pore volume of 0.19 cm3 g-1. After doping with a small amount of S, the surface area of MnTiS0.05 increases to 131.9 m2 g-1. However, the surface area and pore volume of MnTiS0.1 and MnTiS0.15 catalysts decrease slightly with further increasing S content. A larger surface area and pore volume of MnTiS0.05 may be beneficial to adsorb more reactants and disperse the active components, which can lead to an improvement of SCR performance [1, 27, 30].
3.2 SCR activity of the catalysts The NOx conversions as a function of temperature at 150-350 oC by using various catalysts are shown in Fig. 2(a). Evidently, MnTi catalyst exhibits the minimum NOx conversion of 44.2% (at 150 oC) and maximum conversion of 98.2% (at 270 oC), respectively. Meanwhile, > 85% NOx conversion was obtained in the temperature range of 200 to 300 oC on MnTi catalyst. In contrast, the NOx conversion at the whole temperature range is promoted by using MnTiS0.05 catalyst instead. It exhibits > 77% NOx conversion at 150-190oC and > 85% NOx conversion at 190-350 oC,
respectively, which is wider than that of MnTi in SCR temperature operating
range. However, when the S content increases further in MnTiS0.1 and MnTiS0.15 catalysts, the NOx conversion and temperature range using these catalysts become lower and narrower than that using MnTi. So, it is concluded that the appropriate 7
S-doping can improve the SCR performance and broaden the temperature operating range of MnOx/TiO2 catalyst. Fig. 2(b) shows the N2 selectivity of the catalysts for NH3-SCR reaction. The decrease in N2 selectivity at high temperatures is observed on all the catalysts with the formation of NO2 and especially N2O (Figs. S1a and S1b). Such a decrease is more significant for MnTi catalyst. Obviously, S modification succeeds in inhibiting the generation of N2O and NO2.
As shown in Fig. S2, the MnTiS0.05 shows excellent durability in the long-time SCR test. Fig. S3 and Table S1 show the characterization results of fresh and used MnTiS0.05, from which it can be known that the S-doped MnOx/TiO2 catalyst and doping S were stable after the long-time running. These features make it a promising SCR catalyst which can be applied in de-NOx of low-temperature emissions. The catalytic activities of the catalysts for NH3 oxidation and NO oxidation are shown in Fig. 3(a) and Fig. 3(b), respectively. The order of the catalyst activity turns to be MnTi > MnTiS0.05 > MnTiS0.1> MnTiS0.15 for NH3 oxidation and MnTiS0.05 > MnTiS0.1> MnTi> MnTiS0.15 for NO oxidation, respectively. The oxidation of NH3 by O2 to consume the reductant and the oxidation of NO to produce NO2 are important for the high- and low-temperature SCR activities of the catalysts. S addition inhibits NH3 oxidation over the modified catalysts to some extent, which may facilitate the NH3-SCR performance at high temperatures with more available reductant gas. Meanwhile, MnTiS0.05 and MnTiS0.1 show higher catalytic activity for NO oxidation than MnTi, improving the low-temperature SCR activity via the so-called fast SCR reaction [18].
3.3 Surface chemical states of the catalysts The surface chemical states of the S-doped and undoped catalysts were analyzed by XPS analysis, as shown in Fig. 4. Two peaks centered at 458.8 and 464.6 eV are observed in Fig. 4(a) over MnTi and MnTiS0.05 catalysts, attributing to the Ti 2p3/2 8
and Ti 2p1/2 of Ti4+ [31]. With the increasing S content, the peaks of Ti 2p shift to higher positions (458.6 and 464.6 eV) and the peak intensity become lower. The higher binding energy and the lower peak intensity of the S doped TiO2 indicate an influence of S addition on the electronic state of Ti element; probably some of the Ti4+ ions get substituted with S4+ ions in the lattices forming Ti-O-S structure [10]. The decrease in electron density around Ti atom is due to the greater electronegativity of S via O attracting Ti's electrons [32]. From Fig. 4(b), the Mn 2p3/2 of all the catalysts can be fitted into three peaks assigned to Mn2+ (641.2-641.6 eV), Mn3+ (642.0-641.6 eV) and Mn4+ (643.0-643.7 eV), respectively [18, 33]. The amounts of Mn4+ of four catalysts were calculated and listed in Table 2. Obviously, the amount of Mn4+ for MnTiS0.05 (54.48%) is much larger than that of MnTi, MnTiS0.1 and MnTiS0.15. It is well known that more Mn4+ species is beneficial to the low temperature SCR activity owing to its excellent redox ability [9, 20], which is proved by the results of NOx conversion (Fig. 2a). Additionally, the binding energies of Mn4+ for MnTiS0.1 and MnTiS0.15 are 0.6-0.7 eV higher than that of MnTi and MnTiS0.05 (Table 2), indicating the active Mn species may be sulfated after introducing too much S [12, 34]. The O 1s spectra of four catalysts are displayed in Fig. 4(c) and can be deconvoluted into two categories of “Oα” and “Oβ”. The Oα located at 529.6-530.4 eV with three sub-peaks correspond to lattice oxygen species (O2-). The Oβ centered at 531.5-532.0 eV are matched with the chemisorbed oxygen species (O2- from oxygen vacancies and adsorbed OH groups) [6, 27, 35]. Moreover, it is apparently that the binding energies of Oβ shift to higher position with increasing the doped amount of S, together with an increase in the ratios of Oβ (calculated by Oβ/O and listed in Table 2). These increased Oβ on MnTiS0.1 and MnTiS0.15 catalysts mainly originated from the extra S-OH groups from the hydration of SO42-, which has been proved to improve the acidity of catalysts [13, 32]. The oxygen vacancies may be related to the formation of Ti-O-S bond, which is thought to be beneficial to SCR activity [27]. As that shown in Fig. 4(d), the S 2p peaks of the S-doped catalysts are 9
overlapped in four sub-peaks with two chemical states of S4+ (168.2-168.4 eV and 169.3-169.4 eV) and S6+ (168.7-169.0 eV and 169.9-170.3 eV) [36]. However, no peaks attributed to S2- can be observed in Fig. 4(d), suggesting no replacement of O2with S2- to form Ti-S-Ti bond [37]. Combined with the XRD results, it implies that the substitution of Ti4+ by S4+ to generate Ti-O-S bond may occur, which is beneficial to generate more oxygen vacancies to form active oxygen species (O2-) after adsorbing O2 molecules [6, 28, 38].
3.4 Surface acidity and redox property of the catalysts Surface acidity and redox property are two important factors to affect the catalytic performance during NH3-SCR reaction, which were characterized by NH3-TPD and H2-TPR experiments. As shown in Fig. 5(a), all the catalysts present a broad NH3 desorption peak at 100-500 oC. The low-temperature NH3 desorption peak at 100-250 °C is ascribed to desorption of weak acid sites, while the high-temperature desorption peak at 250-500 °C is assigned to desorption of medium strong and strong acid sites [17, 18]. Table 3 lists the amounts of the two acid sites calculated from the NH3-TPD results. It is noticeably that the amounts of acid sites for all the S-doped catalysts are increased gradually. These increased acid sites may originate from the enhancement of surface Mn4+ species, oxygen vacancies caused by Ti-O-S bond and extra S-OH groups from the hydration of SO42- [20]. The amounts of weak acid sites for S-doped catalysts increases with S doping, while the amounts of medium and strong acid sites decreases on MnTiS0.15, indicating the less medium and strong acids due to the possibly formed MnSO4 [38, 39]. Combining the ratio of Mn4+/Mnx+ and Oβ/O from XPS results, it can be seen that the increase of extra S-OH groups from MnSO4 is closely related to the continuous increase of the weak acid sites, while the extra S-OH groups is not significantly related to the medium and strong acid sites. Fig. 5(b) displays the H2-TPR profiles of four catalysts. The MnTi catalyst exhibits three reduction peaks. Table 3 lists the amounts of different reducible species 10
on the catalysts based on the TPR curves. The peaks centered at 346 oC and 529 oC can be assigned to the reduction of MnOx, such as from MnO2 to Mn2O3 and from Mn3O4 to MnO, respectively [19, 39]. The peak at 607 oC may be corresponded to the reduction of active surface TiO2 species [40]. After the addition of S, the TPR profile of MnTiS0.05 catalyst is close to the MnTi, while the H2 consumption attributed to the slightly increased reduction of MnO2 to Mn2O3. It means that the easily reduced species (Mn4+) increase after proper S-doping, which is consistent with the XPS results (Table 2) and may be related to the well dispersion of MnOx species because of the high surface area of MnTiS0.05 catalyst [2, 6]. However, with the further increasing of S content, the lower reduction peaks of MnOx for MnTiS0.1 (392oC) and MnTiS0.15 (428 oC) shift to higher temperature together with the gradual decrease of H2 consumption, indicating the loss of redox capacity due to the decline of surface Mn4+ species. Meanwhile, the higher reduction peaks of MnOx move to the higher temperature, which is located at 564 oC for MnTiS0.1 and 626 oC for MnTiS0.15 with more H2 consumption. This may be caused by the reduction of hardly decomposed sulfates (MnSO4), which are inactive or harmful for SCR reaction [41]. The H2-TPR experiments reveal that the redox property is enhanced slightly after proper S-doping of MnOx/TiO2 catalyst owing to the increasement of the easily reduced surface Mn4+ species. Nevertheless, excess S may result in sulfur poisoning because of the formation of hardly decomposed sulfates.
3.5 Ex situ DRIFT study Infrared spectrum is an effective technique to detect the formation of Sulfur species. Fig. 6 shows the IR spectra of MnTi and MnTiS0.05 catalysts. The bands at 3330 and 1610 cm-1 are assigned to the OH-bending vibration [6], and that at 530 cm-1 is caused by Ti-O stretching vibration. S-doping has no obvious effect on the whole infrared spectrum, but a new band appears in the 1035 cm-1. According to previous literature [42], the band of 1035 cm-1 can be attributed to the formation of Ti-O-S. 11
3.6 In situ DRIFT study 3.6.1 NH3 adsorption and NO+O2 adsorption over the catalysts at different temperatures The DRIFT spectra of NH3 adsorption recorded from 100 oC to 450 oC for MnTi and MnTiS0.05 catalysts are exhibited in Figs. 7(a) and (b), respectively. For MnTi in Fig. 7(a), the band at 1608 cm-1 is assigned to coordinated NH3 adsorbed on Lewis acid sites. The bands at 1640-1800, 1448 and 1388 cm-1 are ascribed to NH4+ species bound to Brønsted acid sites [5, 9, 20 ,25]. The spectra of NH3 adsorption for MnTiS0.05 are close to that of MnTi, as shown in Fig. 7(b). The bands attributed to coordinated NH3 (1602 cm-1) and NH4+ species (1640-1800, 1445 and 1385 cm-1) are observed, meaning the existence of adsorbed NH3 species (ad-NH3) on the two acid sites. Noticeably, compared with MnTi catalyst, the intensities of bands belonging to Brønsted acid sites on MnTiS0.05 enhance significantly, while that belonging to Lewis acid sites decreases slightly. The decline of Lewis acid sites may due to the decrease in the amounts of active surface TiO2 species as proved by the H2-TPR results. So, it is concluded that S-doping mainly increased the number of Brønsted acid sites on MnTiS0.05 catalyst. Combined with the NH3-TPD results, it can be seen that the increased acid sites after S doping are mainly attributed to the Brønsted acid sites. Additionally, all the peaks of two catalysts gradually become weaker with increasing temperature and absolutely vanish at 450 oC, suggesting the strength of both acid sites may be unchanged obviously after S-doping. Figs. 7(c) and (d) display the NOx adsorption DRIFT spectra of MnTi and MnTiS0.05 catalysts at 100-450 oC. As shown, several characteristic peaks of adsorbed NOx species can be observed on MnTi catalyst, representing adsorbed NO2 (1625 cm-1), bridged nitrate species (1597 cm-1) and bidentate nitrate species (1572 cm-1) [43, 44, 45]. MnTiS0.05 catalyst exhibits the similar DRIFT spectra of NOx adsorption, except the emergence of a new band at 1316 cm-1 that may be attributed to 12
bridged nitrate species [45]. However, the enhanced band intensities and areas can be observed on the MnTiS0.05 catalyst, implying the adsorption of more NOx species (ad-NOx) to form new active sites [20, 25]. Especially, the band intensity of the adsorbed NO2 molecules (1620 cm-1) for MnTiS0.05 is much higher than that of MnTi, indicating the generation of more NO2 species to improve the low-temperature SCR activity via its promotion of a “fast SCR” reaction [18]. With the increasing of temperature from 100-450 oC, the intensities of all bands for the two catalysts gradually decrease and completely disappear at 350 oC. 3.6.2 The behaviors of adsorbed NH3 and NOx species for the catalysts during SCR reaction The behaviors of NO+O2 reacted with pre-adsorbed NH3 and NH3 reacted with pre-adsorbed NO+O2 for MnTi and MnTiS0.05 catalysts were investigated by the time-dependent in situ DRIFT experiments at 150 oC. The results are depicted in Fig. 8. As seen in Fig. 8(a) for MnTi catalyst, the characteristic bands (1640-1800, 1610, 1448 and 1385 cm-1) of adsorbed NH3 species connected with Lewis and Brønsted acid sites emerge after the pre-treatment of 500 ppm NH3 for 30 min. Subsequently, all the adsorbed NH3 species are quickly consumed after the addition of NO+O2 and completely vanish in 20 min, followed by generation of some adsorbed NOx species (e.g. 1625 cm-1). The similar tendency of NO+O2 reacted with pre-adsorbed NH3 species can be seen in Fig. 8(b) for MnTiS0.05 catalyst. However, the band linked to Lewis acid sites at 1610 cm-1 quickly vanishes in 5 min in Fig. 7(b), while it is still present for MnTi at the same time. Although more Brønsted acid sites are generated on the MnTiS0.05 surface (Fig. 5(a) and 6(b)), the bands’ intensities are slightly lower than that of MnTi, indicating the quick consumption of the adsorbed NH3 species. Therefore, it is likely that the reactivity of adsorbed NH3 species was improved after the proper S-doing [20, 25, 47]. More importantly, from Figs. 8(a) and (b), the NH3 species can firstly adsorb on the Lewis and Brønsted acid sites and then react with the gas-phase NOx species, suggesting the existence of Eley-Rideal (E-R) mechanism 13
during the NH3-SCR reaction [9, 25]. Figs. 8(c) and (d) demonstrate the transient reaction of NH3 and pre-adsorbed NOx species for MnTi and MnTiS0.05 catalysts. Apparently, several adsorbed NOx species generate on the surface of two catalysts after the NO+O2 pretreatment for 30 min, which is consistent with the results of Figs. 7(c) and (d). All these adsorbed NOx species were expended rapidly after the introduction of NH3 within 5 min. Meanwhile, the bands belonging to adsorbed NH3 species begin to grow and finally replace the adsorbed NOx species on the two catalysts surface subsequently. Nevertheless, the bands at 1623 and 1597 cm-1 for MnTiS0.05 completely disappear in 3 min, while they are still observed at the same time for MnTi. Combined with the NOx adsorption results in Figs. 7(c) and (d), it is concluded that the proper S-doping can increase the amount of adsorbed NOx species and enhance its reactivity. Additionally, the new band at 1537 cm-1 can be observed, attributing to the NH2 intermediate originated from oxidation of NH3 by oxygen [48]. Therefore, these evidences demonstrate that all the adsorbed NOx species can react with the gas-phase NH3 to participate in the SCR reaction, which follows the Langmuir-Hinshelwood (L-H) mechanism [9, 20, 25, 49].
Above all, the proper amount of S can be doped into TiO2 lattice via the replacement of Ti4+ by S4+ to form Ti-O-S bond, leading to the increased surface area and generation of more oxygen vacancies, which is proved by the EPR, PL and Ex situ DRIFT tests from the Zhong’s and Liu’s group [6, 27, 28, 42]. The increasement of surface area can disperse the MnOx species to form more active Mn4+ species, resulting in the enhanced surface acidity and redox property. Additionally, it is also favorable to adsorb more reactants (NH3 and NO). Meanwhile, the generated oxygen vacancies are readily to adsorb oxygen molecules and generate active oxygen species (O2-). The presence of O2- and Mn4+ are in favor of oxidizing adsorbed NO to form ad-NO2 and activating NH3 to generate ad-NH4+, respectively. All these evidences are beneficial to open the “fast SCR” route during the SCR reaction, leading to the 14
improved low-temperature SCR performance and broadened active temperature window of MnTiS0.05 catalyst. Furthermore, in situ DRIFT experiments demonstrate that the increased surface acidity mainly originated from the NH4+ linked to Brønsted acid sites (proved by NH3-TPD). The oxidation of NO is facilitated to form more gas-phase NO2 molecules on the surface of MnTiS0.05 sample, agreeing with the XPS results. Meanwhile, the reactivity of the ad-NH3 and ad-NOx species was also improved by the proper S-doping. Then, compared with the transient reactions of (NO+O2+ad-NH3) and (NH3+ad-NOx), it seems that both the E-R and L-H mechanisms existed during the NH3-SCR reactions for MnTi and MnTiS0.05 catalysts. However, the reaction rate of (NH3+ad-NOx) is much faster than that of (NO+O2+ad-NH3), although the proper S-doping can simultaneously promote the above two processes. The experimental evidences suggest that the L-H mechanism may be more significant on basis of the larger amount of ad-NH3 (NH4+), ad-NOx (NO2) and NH2 intermediate. Therefore, the promoting effect of the proper S-doping on the NH3-SCR performance of MnTiS0.05 catalyst can be described by eqns. (2)-(7) [48, 49]:
NH 3 (g) NH +4 (a)
on Brønsted acid sites (2)
NH +4 (a) NH 2 (a)+2H + +e- (3)
O 2 (g) O 2 (a) (4) NO (g) + O 2 (a) NO 2 (a) (5) NH 2 (a) + NO (g) NH 2 NO (a) N 2 +H 2 O (6) NH +4 (a) + NO 2 (a)+ e- NH 4 NO 2 (a) N 2 +H 2 O (7) However, excess S-doping into MnTi catalyst will decrease the surface area and Mn4+ species on MnTiS0.1 and MnTiS0.15 catalysts, which is caused by the generation of Mn-sulfated species or sulphates on the catalyst surface. These sulphates seem to weaken the redox property of catalysts, leading to the decline of SCR activity. 15
4. Conclusions This study reveals that proper S-doping can enhance the SCR performance and broaden the active temperature range of MnOx/TiO2 catalyst, while too much S will weaken the redox property and SCR activity. According to the experimental and characterization results, the main conclusions are summarized as the follows: (1) S was doped into TiO2 lattice via the replacement of Ti4+ by S4+ to generate Ti-O-S bond, along with increasing surface area and generating more oxygen vacancies. (2) The Brønsted acid sites and redox property were enhanced after proper S-doping owing to the more surface Mn4+ species and active oxygen species (O2-). (3) The proper S-doping can enhance the reactivity and amount of ad-NH3 and ad-NOx species by increasing the Brønsted acid sites and by oxidizing NO to form more gas-phase NO2, which are beneficial to improve the low temperature SCR performance. (4) Both E-R and L-H mechanisms occurred on the S-doped MnOx/TiO2 catalyst. The promoting effect of the proper S-doping on NH3-SCR performance was mainly performed by the L-H route. (5) MnTiS0.05 catalyst exhibited the best low temperature SCR performance and widest active temperature window. Acknowledgements The authors acknowledge financial support from the National Natural Science Foundation of China (No. 51702215), the Science and Technology Department of Zhejiang Province (LGG19B070002), the Science and Technology Program of Shaoxing (2017B70051), Shaoxing University (No. 2016LG1001), National Training Programs of Innovation and Entrepreneurship for Undergraduates (201810349011), Xinmiao Talent Program of Zhejiang Province(2018R432003) and the National Key R&D Program of China (No. 2017YFC0210700). References [1] B. H. Zhao, R. Rui, X. G. Guo, L. Gao, T. F. Xu, Z. Chen, X. D. Wu, Z. C. Si, D. 16
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Mole ratios
Crystallite
2θ, FWHM of (101) plane
Ti
Mn
S
of Ti:Mn:S
size (nm)
2θ (°)
FWHM
45.4
17.5
0.00
1:0.34:0
5.21
25.35
1.579
22
MnTiS0.05
44.8
17.3
1.20
1:0.34:0.04
5.04
25.31
1.625
MnTiS0.1
42.6
16.0
3.27
1:0.33:0.11
4.83
25.31
1.689
MnTiS0.15
41.1
15.5
4.53
1:0.33:0.16
4.82
25.30
1.704
Table 2 The results of BET surface area, pore volume, pore size and XPS for all catalysts. Mn4+/Mnx+
Oβ/O
Oβ
(%)
(%)
643.0
531.5
49.54
13.59
4.34
643.0
531.7
54.48
14.74
0.16
4.71
643.6
532.0
35.58
25.92
0.15
4.27
643.7
532.0
26.69
28.51
Binding energy (eV)
SBET
Pore volume
Pore size
(m2g-1)
(cm3g-1)
(nm)
Mn4+
MnTi
123.1
0.19
4.98
MnTiS0.05
131.9
0.19
MnTiS0.1
118.9
MnTiS0.15
110.1
Catalysts
Table 3 Acidity of the all catalysts determined by NH3-TPD. Catalyst MnTi MnTiS0.0 5 MnTiS0.1 MnTiS0.1 5
Surface acidity / μmol NH3 g−1 cat. Weak Medium strong and strong Total 18.82 24.04 42.86 22.63 54.07 76.7
H2 consumption / μmol /g cat. 1 2 3 Total 14.73 8.43 0.96 24.12 15.92 9.42 25.34
28.02 30.55
9.57 8.03
79.96 75.84
107.98 106.39
19.30 23.19
0.32 0.03
29.19 31.25
Table 3 Acidity of the all catalysts determined by NH3-TPD. Catalyst MnTi MnTiS0.0 5 MnTiS0.1 MnTiS0.1 5
Surface acidity / μmol NH3 g−1 cat. Weak Medium strong and strong Total 18.82 24.04 42.86 22.63 54.07 76.7
H2 consumption / μmol /g cat. 1 2 3 Total 14.73 8.43 0.96 24.12 15.92 9.42 25.34
28.02 30.55
9.57 8.03
79.96 75.84
107.98 106.39
23
19.30 23.19
0.32 0.03
29.19 31.25