Precursor Effect on Mn-Fe-Ce/TiO 2 Catalysts for Selective Catalytic Reduction of NO with NH 3 at Low Temperatures

: Preparation of Mn/TiO 2 , Mn-Fe/TiO 2 , and Mn-Fe-Ce/TiO 2 by the deposition-precipitation (DP) method can afford very active catalysts for low-temperature NH 3 -SCR (selective catalytic reduction of NO with NH 3 ). The effect of precursor choice (nitrate vs. acetate) of Mn, Fe, and Ce on the physiochemical properties including thermal stability and the resulting SCR activity were investigated. The resulting materials were characterized by N 2 -Physisorption, XRD (Powder X-ray diffraction), XPS (X-ray photoelectron spectroscopy), H 2 -TPR (temperature-programmed reduction with hydrogen), and the oxidation of NO to NO 2 measured at 300 ◦ C. Among all the prepared catalysts 5Mn Ace /Ti, 25Mn 0.75Ace Fe 0.25Nit /Ti, and 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace /Ti showed superior SCR activity at low temperature. The superior activity of the latter two materials is likely attributable to the presence of amorphous active metal oxide phases (manganese-, iron- and cerium-oxide) and the ease of the reduction of metal oxides on TiO 2 . Enhanced ability to convert NO to NO 2 , which can promote fast-SCR like pathways, could be another reason. Cerium was found to stabilize amorphous manganese oxide phases when exposed to high temperatures.


Introduction
NH 3 -SCR (selective catalytic reduction of NO x with NH 3 ) with V 2 O 5 -WO 3 /TiO 2 as the catalyst is used successfully in stationary plants [1][2][3][4][5]. The support of choice is TiO 2 (anatase form) due to its higher surface area relative to rutile phase and the fact that SO 3 does not deteriorate the TiO 2 support. The commercial catalyst exhibits high selectivity and activity in the NH 3 -SCR of NO at 300-400 • C [3][4][5]. In order to operate at this temperature, the SCR unit is usually installed at a high dust position.
However, by placing the SCR unit at the high dust position, the catalyst's life is shortened due to the high content of ash with alkali metals in the flue gas [6][7][8]. Therefore, the tail-end position, which is located behind the SO 2 /SO 3 -removing unit is attractive. Decreased erosion and fouling at the low dust level also increases the catalyst's lifetime [9]. In order to avoid costly reheating of the flue gas to around 350 • C, tail-end placement necessitates the SCR catalyst to be significantly more active than the vanadia-tungsta based one.

Results and Discussion
The SCR NO conversion profiles of the 5Mn Nit and 5Mn Ace supported catalysts are shown in Figure 1. Among the catalysts studied, Mn deposited on TiO 2 showed superior catalytic activity followed by ZrO 2 and Al 2 O 3 . In particular, Mn Ace /Ti was more active compared to the Mn Nit /Ti. At 250 • C, the Mn Ace /Ti and Mn Nit /Ti catalysts displayed a NO conversion of 81 and 65%, respectively. The low temperature activity of the Mn/TiO 2 catalysts were compared with silica and alumina by Simirniotis et al. [24] and they concluded that Lewis acid sites, a high surface concentration of MnO 2 , and good redox properties were important in achieving low temperature SCR activity. For further experiments, TiO 2 was chosen as the unique support.
Catalysts 2021, 11, x FOR PEER REVIEW 2 of 13 activities and selectivities. Hence, Mn-Fe/TiO2 [12][13][14][15], MnOx-CeO2 [16], and Mn-Ce/TiO2 [17,18] have been reported to be highly active bimetallic catalysts for NH3-SCR of NO at low temperatures. Recently, we reported highly active low temperature Mn-Fe/TiO2 catalysts prepared by deposition-precipitation using ammonia carbamate as a precipitating agent [19]. The low-temperature SCR activity of the MnOx catalysts depends on the precursor, preparation method, and metal loading. Kapteijn et al. [20] reported that a Mn/Al2O3 catalyst was more active when prepared with Mn-acetate than with Mn-nitrate. Likewise, Li et al. [21] concluded that a Mn-acetate derived Mn/TiO2 catalyst had better activity than its Mn-nitrate based version. However, Peña et al. [22] showed that a Mn/TiO2 catalyst prepared from manganese nitrate and calcined at 400 °C had better activity at lower temperatures than a catalyst obtained from manganese acetate. Detailed investigation of the precursor effect on more active formulations like Mn-Fe/TiO2 and Mn-Fe-Ce/TiO2 catalysts has not been reported.
The optimum Mn loading of low temperature Mn/TiO2 catalysts was reported to be 20 wt.% [22] while the optimum loadings of Mn and Fe in Mn-Fe/TiO2 catalysts synthesized by impregnation were both 10 wt.% [14]. In our previous article [19], we reported that it was possible to further reduce the total metal loading on Mn/TiO2 catalysts from 20 wt.% to 5 wt.% with a change in the method of synthesis from conventional impregnation to deposition, while the total metal loading of the Mn-Fe/TiO2 catalysts could be reduced from 35 wt.% to 25 wt.%. Catalysts based on Mn-Fe/TiO2 contain high amounts of active metals (about 20-25 wt.%) compared to the traditional V2O5-WO3/TiO2 system (about 7-10 wt.%). Additionally, unsupported manganese oxide in hollandite form [23] and MnOx-CeO2 [16] exhibited high NH3-SCR activity at low temperatures.
The present article deals with the preparation of Mn/TiO2, Mn-Fe/TiO2, and Mn-Fe-Ce/TiO2 using several metal precursors. Various methods of characterization were employed to understand the differences in catalyst properties and activities.

Results and Discussion
The SCR NO conversion profiles of the 5MnNit and 5MnAce supported catalysts are shown in Figure 1. Among the catalysts studied, Mn deposited on TiO2 showed superior catalytic activity followed by ZrO2 and Al2O3. In particular, MnAce/Ti was more active compared to the MnNit/Ti. At 250 °C, the MnAce/Ti and MnNit/Ti catalysts displayed a NO conversion of 81 and 65%, respectively. The low temperature activity of the Mn/TiO2 catalysts were compared with silica and alumina by Simirniotis et al. [24] and they concluded that Lewis acid sites, a high surface concentration of MnO2, and good redox properties were important in achieving low temperature SCR activity. For further experiments, TiO2 was chosen as the unique support.  Mn/TiO 2 doped with transition metals (e.g., Ni, Cu, and Fe) had high resistance to sintering and more favorable Mn dispersion [12]. It is also reported that Mn/TiO 2 catalysts promoted with transition metals showed activity for NO oxidation to NO 2 [15].
In our previous publication, we reported the promotional effect of Fe on Mn/TiO 2 catalysts and the optimum formulation was found to be 25 Mn 0.75 Fe 0.25 /Ti using depositionprecipitation [19]. Figure 2a shows the NO conversion profiles of the 25 wt.% Mn 0.75 Fe 0.25 /Ti catalysts with Mn and Fe precursor combinations as a function of reaction temperature. All the catalysts, except for the 25Mn 0.75Nit Fe 0.25Ace /Ti catalyst, showed full conversion above 225 • C. The 25Mn 0.75Ace Fe 0.25Nit /Ti, 25Mn 0.75Nit Fe 0.25Nit /Ti, 25Mn 0.75Ace Fe 0.25Ace /Ti, and 25Mn 0.75Nit Fe 0.25Ace /Ti catalysts exhibited NO conversion of 69.6, 55.6, 47.0, and 43.1% at 150 • C, respectively, illustrating the importance of precursors on catalyst activity. Mn/TiO2 doped with transition metals (e.g., Ni, Cu, and Fe) had high resistance to sintering and more favorable Mn dispersion [12]. It is also reported that Mn/TiO2 catalysts promoted with transition metals showed activity for NO oxidation to NO2 [15]. In our previous publication, we reported the promotional effect of Fe on Mn/TiO2 catalysts and the optimum formulation was found to be 25 Mn0.75Fe0.25/Ti using deposition-precipitation [19]. Figure 2a shows the NO conversion profiles of the 25 wt.% Mn0.75Fe0.25/Ti catalysts with Mn and Fe precursor combinations as a function of reaction temperature. All the catalysts, except for the 25Mn0.75NitFe0.25Ace/Ti catalyst, showed full conversion above 225 °C. The 25Mn0.75AceFe0.25Nit/Ti, 25Mn0.75NitFe0.25Nit/Ti, 25Mn0.75AceFe0.25Ace/Ti, and 25Mn0.75NitFe0.25Ace/Ti catalysts exhibited NO conversion of 69.6, 55.6, 47.0, and 43.1% at 150 °C, respectively, illustrating the importance of precursors on catalyst activity. It is also well known that the presence of Ce can enhance the SCR performance and selectivity to N2 [17]. It is also known that the presence of Ce further overcomes the SO2 deactivation and water inhibition effects [18]. Figure 2b shows the effect of Ce precursor on the optimum 25Mn0.75AceFe0.25Nit/Ti catalyst. The presence of Ce in the 25Mn0.75AceFe0.20NitCe0.05Ace/Ti catalyst showed slightly better performance while 25Mn0.75AceFe0.20NitCe0.05Nit/Ti showed less performance than the previously optimized 25Mn0.75AceFe0.25Nit/Ti catalyst. This further confirms the sensitivity of SCR catalysts to the choice of precursors. The 25Mn0.75AceFe0.25Nit/Ti, 25Mn0.75AceFe0.20NitCe0.05Ace/Ti, and 25Mn0.75AceFe0.20NitCe0.05Nit/Ti catalysts displayed NO conversions of 69.6, 73.0, and 54.4% at 150 °C, respectively. Table 1 summarizes the N2O formation data obtained at 150 °C over the 25Mn0.75AceFe0.25Nit/Ti, 25Mn0.75AceFe0.20NitCe0.05Ace/Ti, and 25Mn0.75AceFe0.20NitCe0.05Nit/Ti catalysts, which respectively produced 35, 15, and 20 ppm of N2O under wet conditions (2.3 vol% H2O). Thus, the presence of Ce can increase the selectivity to N2. A moderate Ce content in Mn-Fe/TiO2 catalysts contributed to decreased N2O formation by hindering the over oxidation of NH3, the dominant step in N2O formation. N2O formation is controlled by selective reaction of NO with NH3, limiting the oxidation of NH3. However, at a higher concentration of water (≈10 vol%), no N2O was formed for all catalysts. It is also well known that the presence of Ce can enhance the SCR performance and selectivity to N 2 [17]. It is also known that the presence of Ce further overcomes the SO 2 deactivation and water inhibition effects [18]. Figure 2b shows the effect of Ce precursor on the optimum 25Mn 0.75Ace Fe 0.25Nit /Ti catalyst. The presence of Ce in the 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace /Ti catalyst showed slightly better performance while 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Nit /Ti showed less performance than the previously optimized 25Mn 0.75Ace Fe 0.25Nit/ Ti catalyst. This further confirms the sensitivity of SCR catalysts to the choice of precursors. The 25Mn 0.75Ace Fe 0.25Nit /Ti, 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace /Ti, and 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Nit /Ti catalysts displayed NO conversions of 69.6, 73.0, and 54.4% at 150 • C, respectively. 150 °C, respectively. Figure 3b shows the SCR activity of the 25 wt.% MnAceFeNitCeAce/Ti catalyst with different Mn-Fe-Ce mole fractions. The highest NO conversion was attained at a Mn mole fraction of 0.75 and the lowest activity at a mole fraction of 0.725, indicating that the minimum Mn content should be 0.75. Maximum NO conversion was obtained at a Fe mole fraction of 0.20 followed by the mole fractions 0.175 and 0.225. Maximum NO conversion was obtained at a Ce mole fraction of 0.05 followed by 0.075 and 0.025. The 25Mn0.75AceFe0.20NitCe0.05Ace/Ti, 25Mn0.75AceFe0.175NitCe0.075Ace/Ti, 25Mn0.75AceFe0.225NitCe0.025Ace/Ti, and 25Mn0.725AceFe0.225NitCe0.05Ace/Ti catalysts displayed NO conversions of 73.0, 65.2, 48.4, and 41.4 at 150 °C, respectively. The effect of space velocity (mLg −1 h −1 ) on the most active 25Mn0.75AceFe0.20NitCe0.05Ace/Ti catalyst is shown in Figure 4. Space velocity is an important factor to be considered in the catalyst design as well as to compare to catalysts in the open literature. The catalyst displayed a NO conversion of 98, 88 and 79%, respectively, at space velocities of 360,000, 450,000, and 600,000 mLg −1 h −1 at 200 °C. The fact that at the lowest space velocity, NO conversions of above 90% can be attained at temperatures above 200 C indicates that unselective oxidation of NH3 is not a major side reaction. The temperature window between 125 and 175 °C was used to determine an apparent activation energy of 38.6 ± 4.0 kJ/mol. This value clearly indicates that the present activity measurements on powdered samples were not strongly influenced by transport limitations in the pursued temperature window of the tail-end operation (125-175 °C).  The effect of space velocity (mLg −1 h −1 ) on the most active 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace /Ti catalyst is shown in Figure 4. Space velocity is an important factor to be considered in the catalyst design as well as to compare to catalysts in the open literature. The catalyst displayed a NO conversion of 98, 88 and 79%, respectively, at space velocities of 360,000, 450,000, and 600,000 mL g −1 h −1 at 200 • C. The fact that at the lowest space velocity, NO conversions of above 90% can be attained at temperatures above 200 C indicates that unselective oxidation of NH 3 is not a major side reaction. The temperature window between 125 and 175 • C was used to determine an apparent activation energy of 38.6 ± 4.0 kJ/mol. This value clearly indicates that the present activity measurements on powdered samples were not strongly influenced by transport limitations in the pursued temperature window of the tail-end operation (125-175 • C).  The BET (Brunauer-Emmett-Teller) surface area, H2-TPR, and NO oxidation results of the MnFe/Ti and MnFeCe/Ti catalysts are summarized in Table 2. The BET surface area of DT51-TiO2 was 83 m 2 /g, while those of the MnFe/Ti and MnFeCe/Ti catalysts showed an increased surface area even with 25 wt.% active metal content. Thus, pore blockage of  The BET (Brunauer-Emmett-Teller) surface area, H 2 -TPR, and NO oxidation results of the MnFe/Ti and MnFeCe/Ti catalysts are summarized in Table 2. The BET surface area of DT51-TiO 2 was 83 m 2 /g, while those of the MnFe/Ti and MnFeCe/Ti catalysts showed an increased surface area even with 25 wt.% active metal content. Thus, pore blockage of TiO 2 is unlikely and the active metal oxides are probably highly dispersed on the TiO 2 support. The increased surface area was due to increased microporosity compared to TiO 2 (see Figures S1 and S2, Supplementary Materials). Ease of reduction of metals oxides is known to be an indicator for favorable low temperature SCR activity [19]. The H 2 consumption profiles of the 25MnFe/Ti and 25Mn-FeCe/Ti catalysts are shown in Figure 5 and the integrated values (µmol/g) are summarized in Table 2. All 25MnFe/Ti catalysts showed almost similar reduction patterns. To distinguish the bimetallic reduction patterns of the MnFe/Ti catalysts, the reduction patterns of Fe/TiO 2 and Mn/TiO 2 were reported [19]. The Fe/TiO 2 catalyst reduced from Fe 2 O 3 to Fe at 338 • C. The Mn/TiO 2 catalyst showed three peaks corresponding to step-wise reduction of MnO 2 to Mn 2 O 3 , Mn 2 O 3 to Mn 3 O 4 , and Mn 3 O 4 to MnO [19]. The 25MnFe/Ti materials exhibited only two peaks with the first (maximum at ≈255-270 • C) corresponding to the MnO 2 reduction, and the second one (maximum at ≈350-390 • C) could be due to the reduction of subsequent manganese oxide phases mixed with iron oxide. The 25Mn 0.75Ace Fe 0.25Ace /Ti and 25Mn 0.75Nit Fe 0.25Ace /Ti catalysts showed visible shoulder peaks at around 230 and 340 • C, and that of the 25Mn 0.75Ace Fe 0.25Nit /Ti and 25Mn 0.75Nit Fe 0.25Nit /Ti catalysts did not display visible shoulder peaks because of the broad nature of the reduction profiles. The origin of the shoulder peak toward lower temperature is unclear, but might be because of the presence of smaller, more easily reducible manganese oxide particles. The 25Mn0.75AceFe0.25Nit/Ti, 25Mn0.75NitFe0.25Nit/Ti, 25Mn0.75AceFe0.25Ace/Ti, and 25Mn0.75NitFe0.25Ace/Ti catalysts consumed 4120, 4004, 3996, and 3972 µmol/g of H2, respectively. The difference in H2 consumption between the catalysts was small, but the 25Mn0.75AceFe0.25Nit/Ti catalyst was reduced at a relatively low temperature. Thus, the ease of reduction pattern and the dominating MnO2 phase (first peak) for the 25Mn0.75AceFe0.25Nit/Ti catalyst seem to be the main contributors to the superior low temper- The 25Mn 0.75Ace Fe 0.25Nit /Ti, 25Mn 0.75Nit Fe 0.25Nit /Ti, 25Mn 0.75Ace Fe 0.25Ace /Ti, and 25Mn 0.75Nit Fe 0.25Ace /Ti catalysts consumed 4120, 4004, 3996, and 3972 µmol/g of H 2 , respectively. The difference in H 2 consumption between the catalysts was small, but the Catalysts 2021, 11, 259 6 of 12 25Mn 0.75Ace Fe 0.25Nit /Ti catalyst was reduced at a relatively low temperature. Thus, the ease of reduction pattern and the dominating MnO 2 phase (first peak) for the 25Mn 0.75Ace Fe 0.25Nit /Ti catalyst seem to be the main contributors to the superior low temperature SCR activity.
The 25MnFeCe/Ti catalysts exhibited three reduction peaks, where the first two peaks can be assigned as similar to those of the MnFe/Ti catalysts and then the third reduction peak about 550 • C is due to the reduction of CeO 2 [25]. Most importantly, the 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace /Ti and 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Nit /Ti catalysts displayed a H 2 consumption of 5040 and 4907 µmol/g, respectively. This H 2 consumption, which is higher than for the 25MnFe/Ti catalysts, could be due to better dispersion of Mn, Fe, and Ce. The 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace /Ti catalyst was reduced at lower temperatures (≈10-15 • C) compared to the 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Nit /Ti catalyst. Thus, also in this case, the ease of reduction and the dominating MnO 2 phase (first reduction peak) in the 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace /Ti catalyst were the main reasons for the superior SCR activity at low temperature.
The X-ray powder diffraction (XRPD) patterns of the 25MnFe/Ti and 25MnFeCe/Ti catalysts are shown in Figure 6. The TiO 2 anatase phase was dominant in all catalysts and manganese oxide, iron oxide, and cerium oxides or other mixed phases of Mn, Fe, or Ce were not observed. This is a clear indication that active metal oxides are highly dispersed and/or in an amorphous state. To understand the amorphous state of the active metal oxides on the surface of the catalysts, thermal treatment at 400, 500, and 600 • C for 2 h was performed. It is anticipated that amorphous to crystalline phase transformation can happen by thermal treatment. The XRPD patterns of the 5MnAce/Ti, 25Mn0.75AceFe0.25Nit/Ti, and 25Mn0.75AceFe0.20NitCe0.05Ace/Ti samples calcined at several temperatures are shown in Figure  7. The 5MnAce/Ti catalyst showed similar diffraction patterns of anatase-TiO2 at 400 °C and there was a small deviation compared to the TiO2 patterns at 500 °C. With a further increase in temperature to 600 °C, MnO2, Mn2O3, and anatase phases were observed. Similarly, the 25Mn0.75AceFe0.25Nit/Ti catalyst showed only anatase phases of TiO2 at 400 and 500 °C, and crystalline Mn2O3 and anatase TiO2 phases were observed at 600 °C. No diffraction patterns of iron oxide were observed. Thus, we can clearly see that the presence of Fe can cause the transformation of the mixed manganese oxide phase into particles big enough for XRD detection to consist only of the Mn2O3 phase. The 25Mn0.75AceFe0.20NitCe0.05Ace/Ti catalyst showed only anatase TiO2 phases even at 600 °C, thus the combined presence of Fe and Ce on Mn/Ti can hamper the transformation of the amorphous to the crystalline phase. The XRPD patterns of the 5MnAce/Ti, 25Mn 0.75Ace Fe 0.25Nit /Ti, and 25Mn 0.75Ace Fe 0.20Nit -Ce 0.05Ace /Ti samples calcined at several temperatures are shown in Figure 7. The 5MnAce/Ti catalyst showed similar diffraction patterns of anatase-TiO 2 at 400 • C and there was a small deviation compared to the TiO 2 patterns at 500 • C. With a further increase in temperature to 600 • C, MnO 2 , Mn 2 O 3 , and anatase phases were observed. Similarly, the 25Mn 0.75Ace Fe 0.25Nit /Ti catalyst showed only anatase phases of TiO 2 at 400 and 500 • C, and crystalline Mn 2 O 3 and anatase TiO 2 phases were observed at 600 • C. No diffraction patterns of iron oxide were observed. Thus, we can clearly see that the presence of Fe can cause the transformation of the mixed manganese oxide phase into particles big enough for XRD detection to consist only of the Mn 2 O 3 phase. The 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace /Ti catalyst showed only anatase TiO 2 phases even at 600 • C, thus the combined presence of Fe and Ce on Mn/Ti can hamper the transformation of the amorphous to the crystalline phase. Figure 8 shows the NO conversion profiles of 5MnAce/Ti, 25Mn 0.75Ace Fe 0.25Nit /Ti, and 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace /Ti catalysts calcined at 400, 500, and 600 • C. The catalysts were most active when calcined at 400 • C where the catalysts calcined at 500 and 600 • C showed lower activity. When calcined at 400 • C, the catalysts were rich in the amorphous metal oxide phases (manganese, iron or cerium oxides), which are known to be SCR active. Further increase in calcination temperature resulted in the partial transformation of amorphous manganese oxide to crystalline manganese oxides (MnO 2 or Mn 2 O 3 ). Overall, the SCR activity of the catalysts was in parallel to the amorphous to crystalline transformation of the catalysts as also reported by Kang and Tang et al. [26,27].
25Mn0.75AceFe0.20NitCe0.05Ace/Ti samples calcined at several temperatures are shown in Figure  7. The 5MnAce/Ti catalyst showed similar diffraction patterns of anatase-TiO2 at 400 °C and there was a small deviation compared to the TiO2 patterns at 500 °C. With a further increase in temperature to 600 °C, MnO2, Mn2O3, and anatase phases were observed. Similarly, the 25Mn0.75AceFe0.25Nit/Ti catalyst showed only anatase phases of TiO2 at 400 and 500 °C, and crystalline Mn2O3 and anatase TiO2 phases were observed at 600 °C. No diffraction patterns of iron oxide were observed. Thus, we can clearly see that the presence of Fe can cause the transformation of the mixed manganese oxide phase into particles big enough for XRD detection to consist only of the Mn2O3 phase. The 25Mn0.75AceFe0.20NitCe0.05Ace/Ti catalyst showed only anatase TiO2 phases even at 600 °C, thus the combined presence of Fe and Ce on Mn/Ti can hamper the transformation of the amorphous to the crystalline phase.  Figure 8 shows the NO conversion profiles of 5MnAce/Ti, 25Mn0.75AceFe0.25Nit/Ti, and 25Mn0.75AceFe0.20NitCe0.05Ace/Ti catalysts calcined at 400, 500, and 600 °C. The catalysts were most active when calcined at 400 °C where the catalysts calcined at 500 and 600 °C showed lower activity. When calcined at 400 °C, the catalysts were rich in the amorphous metal oxide phases (manganese, iron or cerium oxides), which are known to be SCR active. Further increase in calcination temperature resulted in the partial transformation of amorphous manganese oxide to crystalline manganese oxides (MnO2 or Mn2O3). Overall, the SCR activity of the catalysts was in parallel to the amorphous to crystalline transformation of the catalysts as also reported by Kang and Tang et al. [26,27].  Figure 8 shows the NO conversion profiles of 5MnAce/Ti, 25Mn0.75AceFe0.25Nit/Ti, and 25Mn0.75AceFe0.20NitCe0.05Ace/Ti catalysts calcined at 400, 500, and 600 °C. The catalysts were most active when calcined at 400 °C where the catalysts calcined at 500 and 600 °C showed lower activity. When calcined at 400 °C, the catalysts were rich in the amorphous metal oxide phases (manganese, iron or cerium oxides), which are known to be SCR active. Further increase in calcination temperature resulted in the partial transformation of amorphous manganese oxide to crystalline manganese oxides (MnO2 or Mn2O3). Overall, the SCR activity of the catalysts was in parallel to the amorphous to crystalline transformation of the catalysts as also reported by Kang and Tang et al. [26,27].  The impact of the calcination temperature and transformation of active oxides can also be studied in combination with H 2 -TPR. The redox properties of the 5MnAce/Ti, 25Mn 0.75Ace Fe 0.25Nit /Ti, and 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace /Ti catalysts calcined at 400, 500, and 600 • C are shown in Figure 9. The 5MnAce/Ti catalyst showed three different reduction peaks, which correspond to stepwise reduction of MnO 2 to Mn 2 O 3 (≈260 • C), Mn 2 O 3 to Mn 3 O 4 (≈360 • C), and Mn 3 O 4 to MnO (≈460 • C) [19]. Increasing the calcination temperature from 400 to 600 • C, the 5MnAce/Ti catalyst shifted the first reduction peak to higher temperatures due to strong metal-support interactions [22], and the intensity of the second reduction peak was increased, which further indicates that the SCR active MnO 2 phase decreases and the Mn 2 O 3 phase increases. The shifting of both reduction peaks to higher temperatures might be due to particle growth (sintering).
Catalysts 2021, 11, x FOR PEER REVIEW 9 of 13 The impact of the calcination temperature and transformation of active oxides can also be studied in combination with H2-TPR. The redox properties of the 5MnAce/Ti, 25Mn0.75AceFe0.25Nit/Ti, and 25Mn0.75AceFe0.20NitCe0.05Ace/Ti catalysts calcined at 400, 500, and 600 °C are shown in Figure 9. The 5MnAce/Ti catalyst showed three different reduction peaks, which correspond to stepwise reduction of MnO2 to Mn2O3 (≈260 °C), Mn2O3 to Mn3O4 (≈360 °C), and Mn3O4 to MnO (≈460 °C) [19]. Increasing the calcination temperature from 400 to 600 °C, the 5MnAce/Ti catalyst shifted the first reduction peak to higher temperatures due to strong metal-support interactions [22], and the intensity of the second reduction peak was increased, which further indicates that the SCR active MnO2 phase decreases and the Mn2O3 phase increases. The shifting of both reduction peaks to higher temperatures might be due to particle growth (sintering). The 25Mn0.75AceFe0.25Nit/Ti and 25Mn0.75AceFe0.20NitCe0.05Ace/Ti catalysts showed almost similar reduction patterns at 400 and 500 °C of calcination temperature, further indicating that Fe and Ce are inhibiting the phase transformation of MnO2 to Mn2O3 and possibly particle growth (sintering). At 600 °C, the catalyst displayed a shift in the MnO2 reduction peak to high temperatures and the intensity of the second reduction peak was increased. Thus, the combined presence of Fe and Ce on Mn/TiO2 can increase the thermal stability.
The observed low temperature activity of Mn catalysts can also be explained by the NO to NO2 oxidation ability as reported previously [15,18]. Table 2 shows the oxidation of NO to NO2 on the 25Mn0.75Fe0.25Ti-DP and 25Mn0.75Fe0.25Ce0.05Ti-DP catalysts at 300 °C under wet conditions. All the catalysts displayed high NO to NO2 conversion of 41.6 to 66%. The 25Mn0.75AceFe0.20NitCe0.05Ace/Ti and 25Mn0.75AceFe0.25Nit/Ti catalysts displayed the The 25Mn 0.75Ace Fe 0.25Nit /Ti and 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace /Ti catalysts showed almost similar reduction patterns at 400 and 500 • C of calcination temperature, further indicating that Fe and Ce are inhibiting the phase transformation of MnO 2 to Mn 2 O 3 and possibly particle growth (sintering). At 600 • C, the catalyst displayed a shift in the MnO 2 reduction peak to high temperatures and the intensity of the second reduction peak was increased. Thus, the combined presence of Fe and Ce on Mn/TiO 2 can increase the thermal stability.
The observed low temperature activity of Mn catalysts can also be explained by the NO to NO 2 oxidation ability as reported previously [15,18]. Table 2 shows the oxidation of NO to NO 2 on the 25Mn 0.75 Fe 0.25 Ti-DP and 25Mn 0.75 Fe 0.25 Ce 0.05 Ti-DP catalysts at 300 • C under wet conditions. All the catalysts displayed high NO to NO 2 conversion of 41.6 to 66%. The 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace /Ti and 25Mn 0.75Ace Fe 0.25Nit /Ti catalysts displayed the highest NO to NO 2 conversion. The observed NO oxidation is consistent with the increased SCR activity of the catalysts, since partial conversion of NO into NO 2 is helpful to promote the fast SCR reaction, which is also known to go on at very low temperatures [28].
The surface composition as obtained by XPS (X-ray photoelectron spectroscopy) characterization is shown in Table 3. The 25Mn 0.75Ace Fe 0.25Nit /Ti and 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace /Ti showed a surface Mn/Fe molar ratio of 2.08 and 1.30, respectively. Thus, it appears that the precursor/promoter combination has an influence on forming MnFe oxide species on the surface of the support. Importantly, the 25Mn 0.75Ace Fe 0.25Nit /Ti catalyst showed an O α concentration of 50.1% of the total oxygen while the Ce promoted 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace/ Ti catalyst yielded 83.8%, respectively. High concentrations of chemisorbed oxygen have been reported to have a beneficial influence on the low-temperature SCR reaction [12,29] and is explained by an increased rate of NO to NO 2 oxidation [29]. Our XPS results showed a significantly higher concentration of more reactive surface oxygen (O α ) in the 25Mn 0.75Ace Fe 0.20Ni tCe 0.05Ace /Ti catalyst than in the 25Mn 0.75Ace Fe 0.25Nit /Ti catalyst. This is reflected in the higher NO to NO 2 oxidation activity (66% vs. 60.4% conversion).

X-ray Powder Diffraction
X-ray powder diffraction (XRPD) was conducted with a Huber G670 instrument. CuKα radiation in steps of 0.02 • was employed with a 2θ range of 2-60 • . The Debye-Scherrer equation was used to calculate the crystallite sizes.

Nitrogen Physisorption
BET surface areas were determined from N 2 physisorption measurements on about 100 mg at 77 K with a Micromeritics ASAP 2010 apparatus. The samples were pretreated at 200 • C for 1 h before the measurement.

Chemisorption
H 2 -TPR experiments were performed on a Micromeritics Autochem-II instrument with a reducing mixture (50 mL/min) consisting of 5% H 2 and balance Ar (Air Liquide) from 60 to 550 • C (10 • C/min). The H 2 concentration in the effluent stream was monitored with a thermal conductivity detector (TCD).

X-ray Photoemission Spectroscopy
XPS measurements at room temperature were carried out with a Thermo scientific instrument. Al Kα radiation (1484.6 eV) was used and Au served as the standard for the instrument calibration. To minimize surface contamination, samples were outgassed in vacuum for 1 h in vacuum prior to the data acquisition. Deconvolution of spectra was performed using the Thermo Scientific Avantage Data system software.

Catalytic Activity Measurements
The SCR activity measurements were conducted in a fixed-bed reactor loaded with 50 mg of the fractionized (180-300 µm) catalyst at a flow rate of 300 NmL min −1 (at room temperature) at atmospheric pressure. The concentrations at inlet were: NO = 1000 ppm, NH 3 = 1000 ppm, O 2 = 4%, and H 2 O = 2.3% with He as the make up gas. The temperature was increased from 125 to 300 • C in steps of 25 • C while the NO and NH 3 concentrations were measured continuously with a Thermo Electron Model 17C chemiluminescense NH 3 -NOx analyzer. The N 2 O concentration was measured by GC (Shimadzu 14 B GC, poraplot column, TCD detection). The concentrations were measured after reaching steady state conversion (approximately 45 min at each temperature).
The NO oxidation to NO 2 measurements were performed in the same set up loaded with 200 mg of the fractionized (180-300 µm) catalyst at a flow rate of 300 NmL min −1 (at room temperature). The inlet concentrations were: NO = 500 ppm, O 2 = 4.5% and H 2 O = 2.3% with He as balance gas. During the experiments the temperature was increased in steps of 50 • C from 150 to 350 • C while the NO and NO 2 concentrations were measured with a Thermo scientific UV-Vis spectrophotometer (Evolution 220).

Conclusions
A range of precursor combinations in deposition-precipitation synthesis were tested on highly active low temperature SCR of NO with NH 3 catalysts. Among the three supports (TiO 2 , ZrO 2 , and Al 2 O 3 ) and two precursor combinations (nitrate vs. acetate), a monometallic 5Mn Ace /Ti catalyst showed good low temperature SCR activity. Among the bimetallic catalysts, the 25Mn 0.75Ace Fe 0.25Nit/ Ti catalysts showed better low temperature SCR activity, and the trimetallic 25Mn 0.75Ace Fe 0.20Nit Ce 0.05Ace/ Ti catalyst showed the best low temperature SCR activity, respectively. The addition of Fe, and especially Ce, not only enhances the activity, but also the thermal stability by hindering the transformation of finely dispersed, easily reducible amorphous manganese oxide phases into larger, less easily reducible, and more crystalline particles as evidenced by H 2 -TPR and XRD. Fe and Ce promoted catalysts were shown by XPS to contain large amounts of surface active oxygen, further corroborating the H 2 -TPR and XRD results. This form of oxygen can enhance the oxidation of NO to NO 2 , as shown by NO oxidation measurements, and can promote fast-SCR and hence the overall activity. Furthermore, Ce lowers the selectivity toward N 2 O, possibly by enhancing the rate of reaction of activated NH 3 with NO to N 2 , thus making it unavailable for over-oxidation, which can lead to N 2 O formation.

Data Availability Statement:
The data generated in this study are fully disclosed in this manuscript.