Promoting E ﬀ ect of Ti Species in MnO x -FeO x / Silicalite-1 for the Low-Temperature NH 3 -SCR Reaction

: Manganese and iron oxides catalysts supported on silicalite-1 and titanium silicalite-1 (TS-1) are synthesized by the wet impregnation method for the selective catalytic reduction (SCR) of NO x with NH 3 (NH 3 -SCR), respectively. The optimized catalyst demonstrates an increased NO x conversion e ﬃ ciency of 20% below 150 ◦ C, with a space velocity of 18,000 h − 1 , which can be attributed to the incorporation of Ti species. The presence of Ti species enhances surface acidity and redox ability of the catalyst without changing the structure of supporter. Moreover, further researches based on in situ NH 3 adsorption reveal that Lewis acid sites linked to Mn 4 + on the surface have a huge inﬂuence on the improvement of denitration e ﬃ ciency of the catalyst at low temperatures.


Introduction
Nitrogen oxides (NO x ) bring out a series of environmental problems, such as photochemical pollution, acid rain and haze, which seriously threaten human health [1,2]. Nowadays, selective catalytic reduction (SCR) with NH 3 has been proved to be an effective technology for NO x emission control, which has been widely used in coal-fired power plants [3][4][5]. Commercial V 2 O 5 -WO 3 /TiO 2 catalysts are very suitable for power plants, mainly due to the flue gas there, with a high temperature (>300 • C), which can satisfy the required reaction temperature of the catalyst. However, there are still a large number of untreated NO x derived from industrial activities such as cement kilns, steel sintering and waste incinerators, where the temperature of flue gas is too low (<200 • C) to suit for these commercial catalysts [6,7]. Therefore, it is urgent to develop novel high-efficient catalysts, which have high catalytic activities at low temperatures.
Manganese-based catalysts are proved to be excellent denitration catalysts at low temperatures among the commonly used transition metal oxide catalysts [8][9][10]. Furthermore, numerous metals, like Fe, Ce and Cu, are used as active components to modify manganese-based catalysts, and for that purpose, the single manganese oxides can only maintain high denitrification activities in a narrow temperature window, accompanied by several side reactions [11][12][13][14]. In addition, proper supporters can not only provide a huge surface to disperse the active components, but can also supply abundant adsorption sites for the heterogeneous reactions [15,16]. Both Al 2 O 3 and TiO 2 have been widely investigated as common supporters in the last few decades. However, the limited specific surface area, as well as poor water and sulfur resistance of the catalysts, hinder their further developments in industrial applications [17,18]. At the same time, carbon-based materials are also The X-ray diffraction (XRD) patterns of the supporters are shown in Figure 1a. All of the supporters display obvious characteristic peaks at 7.9 • , 8.8 • , 23.1 • , 23.8 • and 24.3 • , which can be attributed to a typical Mobil Five Instructure (MFI) structure. The pattern of anatase at 25.4 • does not appear in TS-1-30, while it is found in TS-1-20, which indicates that the excessive Ti species form a small amount of anatase in TS-1-20. The existing form of Ti species in the supporters is further confirmed by Fourier transform infrared (FT-IR). As shown in Figure 2, the bands at 433, 545-546, 799-803, 1074-1075 and 1223-1225 cm −1 can be assigned to the characteristic peaks of MFI structure; the bands at 957-958 cm −1 can be the evidence for the incorporation of Ti species into frameworks [29]. Moreover, it can be seen from Figure 1b that there is no peak relevant to metal oxides in the supported catalysts, due to the extremely low metal loading of the catalysts. However, all the supported catalysts possess clear peaks that are consistent with corresponded supporters, which reveals that the supported catalysts remain the same structure with the supporters after the impregnation process [25,30].  N 2 adsorption/desorption isotherms of the samples are illustrated in Figure 3, and the corresponding textural properties of the samples are summarized in Table 2. The isotherms can be classified as type I, indicating that all the samples have microporous structures [31,32]. The specific surface area, total pore volume and average pore diameter of Silicalite-1 are 454.31 m 2 /g, 0.32 cm 3 /g and 2.85 nm, respectively. When Ti species are introduced into the structure of zeolites, both TS-1-30 and TS-1-20 show an increase of 10% in specific surface area, total pore volume and average pore diameter, which can be attributed to the difference of atom size between Ti and Si [29]. After loading MnO x and FeO x , both specific surface area and total pore volume have a decrease of 10%, while average pore diameter remains constant, indicating that most of MnO x and FeO x species coexist on the surface, without changing the textural properties of supporters, as proposed in XRD analysis [30].  The morphologies of the catalysts are further investigated by SEM and transmission electron microscopy (TEM), with the results illustrated in Figure 4. The SEM images of catalysts present a typical MFI structure with a particle size of about 150 nm. However, the incorporation of Ti species results in some nanoscale folds on the surface of the catalysts, which can be attributed to the difference of atom size between Ti and Si. The active metal oxides are highly dispersed on the surface of catalysts in the form of small particles from the TEM images. As shown in Figure 5, the element mapping by energy-disperse X-ray spectroscopy (EDS) is applied to further identify the compositions of these small particles on the surface of the catalysts (EDS mappings of the supporters are shown in Figure S1). It can be found from Figure 5 that Mn, Fe and Ti species are all uniformly dispersed around the supporters. Besides, the results of Mn and Fe species are similar, indicating that Mn and Fe species may coexist in the form of FeMnO x , due to a strong interaction between metal oxides [33][34][35]. In summary, the introduction of Ti species does not change the textural properties of catalysts, but the surface of the catalysts is wrinkled, due to the difference of atom size between Ti and Si, which may provide better active sites for the catalytic reaction.    Table 3. It can be seen from Figure 6a that the Mn 2p spectra consist of two major peaks that are assigned to Mn 2p 3/2 (peak around 642 eV) and 2p 1/2 (peak around 654 eV) . The Mn 2p 3/2 peak can be fitted by three peaks (around 642 eV, 643 eV and 645 eV), which can be assigned to Mn 2+ , Mn 3+ and Mn 4+ , respectively [36,37]. Figure 6b displays the spectra of Fe 2p, which contains two main peaks relevant to Fe 2p 3/2 (peak around 710 eV) and 2p 1/2 (peak around 725 eV). The Fe 2p 3/2 can be separated into two independent peaks (around 713 eV, 710.7 eV), which can be assigned to Fe 2+ and Fe 3+ [13,38,39]. Regarding the oxygen species, O 1s spectra are shown in Figure 6c. The spectra can be separated into two peaks that related to the lattice oxygen (peak around 530 eV, labeled as O α ) and surface adsorbed oxygen (peak around 533 eV, labeled as O β ) [40,41], respectively.  It is broadly reported that Mn 4+ species play a major role in low-temperature NH 3 -SCR reactions [37,42]. Hence, the ratios of Mn 4+ /(Mn 4+ + Mn 3+ + Mn 2+ ) over the catalysts are calculated and listed in Table 4, together with the ratios of Fe 3+ /(Fe 3+ + Fe 2+ ) and O β /(O α +O β ), which are related to the catalytic performance of catalysts [30,43,44]. The Mn 4+ /(Mn 4+ + Mn 3+ + Mn 2+ ) ratio of Mn 3 Fe 2 /TS-1-30 is 34.6%, which is a significant increase compared to that of Mn 3 Fe 2 /Silicalite-1 (25.0%). The increase of the Mn 4+ ratio can be attributed to the redox reactions between Mn and Ti species such as Mn 3+ + Ti 4+ = Mn 4+ + Ti 3+ , Mn 2+ + 2Ti 4+ = Mn 4+ + 2Ti 3+ [40]. However, the ratio of Mn 4+ remains unchanged in Mn 3 Fe 2 /TS-1-20, which can be attributed to the formation of anatase on the supporter. As mentioned above, the excessive Ti species generate anatase on the surface of the supporter, which is reported to inhibit the electron transfer between Mn species and the supporter [42]. The Fe 3+ /(Fe 3+ + Fe 2+ ) ratio displays a slight decrease when the Ti species are introduced into the supporter, which can be attributed to redox equilibrium between Fe and Ti species like Fe 3+ + Ti 3+ = Fe 2+ + Ti 4+ [45]. According to the previously reported literature, surface adsorbed oxygen species (O β ) show more reactive than lattice oxygen (O α ) in the low-temperature NH 3 -SCR reaction, due to their outstanding mobility [46,47]. As shown in Table 4, the incorporation of Ti species increases the O β /(O α + O β ) ratio, while the excessive Ti species consume the surface oxygen mainly because of the formation of anatase. In summary, Mn 3 Fe 2 /TS-1-30 tends to show the optimal catalytic performance, due to its highest ratio of Mn 4+ and surface adsorbed oxygen species.

Redox Properties of Samples
The redox properties of the catalysts are closely related to the catalytic performance in the NH 3 -SCR reaction. Therefore, H 2 temperature-programmed reduction (H 2 -TPR) is performed to characterize the redox properties of the catalysts and the results are exhibited in Figure 7 and Table 5. All the catalysts display three reduction peaks between 400 and 700 • C. The reduction peaks centered at 430 • C can be correlated with the simultaneous reduction of MnO 2 and Fe 2 O 3 (i.e., MnO 2 → Mn 2 O 3 , Mn 2 O 3 → Mn 3 O 4 and Fe 2 O 3 → FeO) [25,48]. The peaks centered at 580 • C can be assigned to the reduction of Mn 3 O 4 , while the peaks above 660 • C can be relevant to the reduction of FeO [48]. As shown in Table 5, when Ti species are introduced into the catalysts, the total area of the reduction peaks reaches 776 a.u., which has an increase of 20% compared to that in Mn 3 Fe 2 /Silicalite-1. Since the area of the reduction peak corresponds to the H 2 consumption, the redox properties of the catalysts are significantly enhanced after the incorporation of Ti species, which is beneficial to promote the NH 3 -SCR reaction at low temperatures.

Surface Acidity of Samples
The surface acidity is another important factor in low-temperature SCR reaction. Hence, NH 3 temperature-programmed desorption (NH 3 -TPD) is used to analyze the surface acidity of the catalysts, and the results are exhibited in Figure 8. For Mn 3 Fe 2 /Silicalite-1, it displays two desorption peaks labeled as I and II, which correspond to weak (<200 • C) and medium-strong (200~400 • C) acid sites, respectively [43,49]. However, more desorption peaks can be observed in Mn 3 Fe 2 /TS-1-30 and Mn 3 Fe 2 /TS-1-20, which are related to medium-strong acid sites [43,49], indicating that the new Lewis acid sites are related with the incorporation of Ti species and thus benefit the adsorption of NH 3 . The total acid amounts of these catalysts are calculated from TPD results and listed in Table 6. It shows that the acid amounts of Mn 3 Fe 2 /TS-1-30 and Mn 3 Fe 2 /TS-1-20 are larger than that of Mn 3 Fe 2 /Silicalite-1. Mn 3 Fe 2 /TS-1-30, especially, exhibits a remarkable increase (about 50%) in the acid amount, compared to that of Mn 3 Fe 2 /Silicalite-1. In summary, Ti species improve the surface acidity of the catalysts, which is beneficial for the adsorption of NH 3 , and finally enhances the catalytic performance in the low-temperature SCR reaction.   Figure 9 displays the in situ diffuse reflectance infrared Fourier transform (in situ DRIFTS) of NH 3 adsorption in a temperature range between 60 and 330 • C, to further distinguish the Brønsted acid sites (labeled as B acid) and Lewis acid sites (labeled as L acid) on the surface of the catalysts. It is widely reported that the bands at 1602-1605 cm −1 , 1225-1227 cm −1 , 1142-1163 cm −1 and 1059-1064 cm −1 can be attributed to asymmetric and symmetric bending vibrations of the coordinated NH 3 linked to L acid sites [50,51]. The bands at 1428-1448 cm −1 and 1737-1748 cm −1 can be assigned to the NH 4 + species on B acid sites [52]. The bands at 1550-1554 cm −1 and 1302-1324 cm −1 can be attributed to NH 2 species, which are transformed from the coordinated NH 3 [53,54] . Moreover, with the increase of temperature, the bands linked to B acid sites are found to decrease more noticeable than these linked to L acid sites, indicating that the bands assigned to L acid sites are more stable at a higher temperature [37]. Besides, by comparing the spectra of Mn 3 Fe 2 /TS-1-30 (20) with that of Mn 3 Fe 2 /Silicalite-1, the intensity of bands at 1602-1605 cm −1 and 1428-1448 cm −1 is enhanced after the incorporation of Ti species, which indicates that Mn 3 Fe 2 /TS-1-30 (20) can provide more adsorbed NH 3 for SCR reaction than Mn 3 Fe 2 /Silicalite-1, which can benefit the catalytic activities.  Figure 10a shows the catalytic activities of the catalysts under different reaction temperatures. It can be seen from the figure that Mn 3 Fe 2 /TS-1-30 exhibits a NO x conversion efficiency of 90% at 120 • C, which has an increase of almost 30% compared to that of Mn 3 Fe 2 /Silicalite-1 at the same temperature. In addition, both Mn 3 Fe 2 /TS-1-30 and Mn 3 Fe 2 /TS-1-20 show an obvious improvement in catalytic activities at low temperatures, especially below 150 • C. The optimized catalysts, Mn 3 Fe 2 /TS-1-30 can maintain steady NO x conversion efficiencies above 80% in the temperature range between 110 and 230 • C, with a space velocity of 18,000 h −1 . Based on the characterization of the catalysts' physical and chemical properties mentioned above, the improvement of low-temperature denitration activity is mainly attributed to the incorporation of Ti species, which enhances the redox properties of the catalysts and provides more surface acid sites for the adsorption of NH 3 . Since zeolites are reported to be sensitive to water vapor [26,28], the H 2 O resistance of the catalysts is further studied at the temperature of 180 • C. As shown in Figure 10b, when 10 vol.% H 2 O is introduced into the reaction, Mn 3 Fe 2 /Silicalite-1 displays a decrease of about 5% in NO x conversion efficiency, while Mn 3 Fe 2 /TS-1-30 remains unaffected by water vapor. It is widely accepted that the water deactivation is mainly because of the competitive adsorption between NH 3 and water vapor [16,55], and the presence of Ti species further strengthens the hydrophobicity of the catalyst [29,56], thereby improving the H 2 O resistance of the catalysts.

Preparation of Samples
Silicalite-1 and TS-1 were synthesized by a hydrothermal template method, according to the previous literature [57,58]. The tetrapropylammonium hydroxide (TPAOH, 25 wt.% aqueous solution, Beijing Innochem Technology Co., Ltd., China) was served as a microporous template, while ethyl silicate (TEOS, Tianjin Guangfu Fine Chemical Research Institute, China) and tetrabutyl titanate (TBOT, Beijing Tongguang Fine Chemical Company, China) were served as the source of silicon and titanium, respectively. The molar composition of the synthesis solution was: SiO 2 :TiO 2 :TPAOH:H 2 O = 1:(0~0.05):0.2:7. Firstly, TEOS was added dropwise into 12.8 g aqueous solution of TPAOH at 40 • C with constant magnetic stirring while TBOT was dissolved in 10 g isopropanol. Secondly, the hydrolysis solutions of TEOS and TBOT were mixed and then heated up to 80 • C to distill off isopropanol. Then, the mixture was transferred into a stainless steel autoclave and then kept in an oven at 160 • C for 48 h. Afterwards, the white precipitation was washed by deionized water and ethanol for three times, respectively. Finally, the products were dried at 80 • C overnight, followed by calcination at 550 • C in the air for 6 h. The prepared samples without titanium labeled as Silicalite-1, while others with feeding n (Si/Ti) of 30, 20 were labeled as TS-1-30 and TS-1-20, respectively. MnO x -FeO x catalysts were synthesized through the wet impregnation method. Manganese nitrate (Mn (NO 3 ) 2 , 50 wt.% aqueous solution, Saen Chemical Technology (Shanghai) Co., Ltd., China) and iron nitrate nonahydrate (Fe (NO 3 ) 3 ·9H 2 O, 99.9%, Beijing Tongguang Fine Chemical Company, China) were dissolved in deionized water as precursors and zeolites including Silicalite-1 and TS-1 were immersed into solution as supporters. The mass ratio of the mixture was: Mn:Fe:supporter (Silicalite-1 or TS-1) = 3:2:100, which is favorable for NH 3 -SCR reaction according to the previous-reported researches [25,30]. After magnetic stirring at room temperature for 2 h, the mixture was transferred into an oven at 110 • C overnight, and followed by calcination at 500 • C in the air for 4 h. The samples were labeled as Mn 3 Fe 2 /Silicalite-1, Mn 3 Fe 2 /TS-1-30 and Mn 3 Fe 2 /TS-1-20, respectively.

Characterization
X-ray diffraction (XRD) patterns of as-prepared samples were collected by a powder X-ray diffractometer (Rigaku Dmax-2400, RIGAKU, Tokyo, Japan) with Cu-Kα target. Fourier transform infrared (FT-IR) spectra were collected on a FT-IR spectrophotometer (Nicolet iS50, ThermoFisher, Waltham, MA, USA). The textural properties of as-prepared samples were investigated using a nitrogen adsorption apparatus (ASAP2020, Micromeritics, Norcross, GA, USA) at 77 K. Specific surface area, pore volume and pore size were calculated by Brunauer-Emmett-Teller (BET), single point and Barrett-Joyner-Halenda (BJH) methods, respectively. Surface morphologies of as-prepared samples were carried out on a field emission scanning electron microscope (Merlin Compact, ZEISS, Oberkochen, Germany) operating at 10 kV. Transmission electron microscopy (TEM) images of as-prepared samples were screened on a transmission electron microscope (JEM-2100F, JEOL, Tokyo, Japan) operating at 200 kV. The chemical composition of as-prepared samples was measured by an inductively coupled plasma-atomic emission spectrometer (Thermo iCAP6000 ICP-OES, ThermoFisher, Waltham, MA, USA). The surface chemical species of as-prepared samples were characterized by an X-ray photoelectron spectroscopy (AXIS Supra, Kratos Analytical Ltd., Manchester, UK) with Al Kα radiation. H 2 temperature-programmed reduction (H 2 -TPR) and NH 3 temperature-programmed desorption (NH 3 -TPD) experiments were carried out on a chemisorption analyzer (ChemBET Pulsar TPR/TPD, Quantachrome, Boynton Beach, FL, USA). In situ diffuse reflectance infrared Fourier transform (in situ DRIFTS) of NH 3 adsorption were obtained from a FT-IR spectrophotometer (Nicolet 6700, Thermo Fisher, USA), by accumulating 32 scans with a resolution of 4 cm −1 . The samples were firstly pretreated under a high purified N 2 stream at 400 • C for 1 h to remove physicsorbed water and other impurities. Then the background spectra were obtained at target temperatures during the cooling procedure. Subsequently, the samples were exposed to a stream of NH 3 /N 2 (1 vol.% NH 3 ) at room temperature, followed by purging with N 2 for 30 min. The desorption of NH 3 studies were measured by heating pre-adsorbed samples, and the spectra were recorded at stepped target temperatures by eliminating the corresponding background reference.

NH 3 -SCR Activity Measurements
The NH 3 -SCR catalytic performances were tested in a fixed-bed quartz reactor (6 mm of internal diameter) in the temperature range from 90 to 270 • C, with a GHSV of 18,000 h −1 . The reaction gas mixture was consisted of 500 ppm NO, 500 ppm NH 3 where [NO x ] in and [NO x ] out refer to inlet and outlet concentrations of NO x at steady state, respectively.

Conclusions
In this work, silicalite-1 and TS-1 were applied as supporters to load MnO x and FeO x nanoparticles as active components for the low-temperature NH 3 -SCR reaction. Mn 3 Fe 2 /TS-1-30 maintains NO x conversion efficiencies above 80% when the range of temperature is from 110 to 230 • C. Moreover, Mn 3 Fe 2 /TS-1-30 can maintain a denitration efficiency of 94% in the presence of 10 vol.% H 2 O at 180 • C for over 12 h. Its excellent catalytic performance and H 2 O resistance can be attributed to having much more surface acid sites and the stronger redox ability of its active components, which can be explained by the incorporation of Ti species. Therefore, the superior denitration ability and H 2 O resistance of Mn 3 Fe 2 /TS-1-30 will make it a practical catalyst for industrial largescale denitration at low temperatures.

Conflicts of Interest:
The authors declare no conflict of interest.