Iron-Based Composite Oxide Catalysts Tuned by CTAB Exhibit Superior NH₃–SCR Performance

Abstract

Iron-based oxide catalysts for the NH₃–SCR (selective catalytic reduction of NO_x by NH₃) reaction have gained attention due to their high catalytic activity and structural adjustability. In this work, iron–niobium, iron–titanate and iron–molybdenum composite oxides were synthesized by a co-precipitation method with or without the assistance of hexadecyl trimethyl ammonium bromide (CTAB). The catalysts synthesized with the assistance of CTAB (FeM_0.3O_x-C, M = Nb, Ti, Mo) showed superior SCR performance in an operating temperature range from 150 °C to 400 °C compared to those without CTAB addition (FeM_0.3O_x, M = Nb, Ti, Mo). To reveal such enhancement, the catalysts were characterized by N₂-physisorption, XRD (Powder X-ray diffraction), NH₃-TPD (temperature-programmed desorption of ammonia), DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy), XPS (X-ray Photoelectron Spectroscopy), and H₂-TPR (H₂-Total Physical Response). It was found that the crystalline phase of Fe₂O₃ formed was influenced by the presence of CTAB in the preparation process, which favored the formation of crystalline γ-Fe₂O₃. Owing to the changed structure, the redox-acid properties of FeM_0.3O_x-C catalysts were modified, with higher exposure of acid sites and improved ability of NO oxidation to NO₂ at low-temperature, both of which also contributed to the improvement of NO_x conversion. In addition, the weakened redox ability of Fe prevented the over-oxidation of NH₃, thus accounting for the greatly improved high-temperature activity as well as N₂ selectivity.

1. Introduction

Selective catalytic reduction of NO_x by NH₃ (NH₃–SCR) is an effective means of NO_x abatement. Various catalysts have been developed for NH₃–SCR, including metal oxide catalysts (VO_x-based, CeO₂-based, Fe₂O₃-based, MnO_x-based catalysts, etc.) and zeolite catalysts (Cu- and Fe–exchanged zeolite catalysts, etc.) [1,2]. Among them, iron-based oxide catalysts have received much attention due to their high catalytic activity as well as non-toxicity, low cost, and accessibility [1,3,4]. As for NH₃–SCR catalysts, both acid sites and redox sites are necessary to guarantee efficient NO_x reduction [1,5]. High dispersion of sites having the same function and close coupling of redox-acid sites are important for the design of catalysts with superior NH₃–SCR performance [2]. Taking this principle into account, large numbers of iron-based composite oxide catalysts have been prepared and investigated for NH₃–SCR, including Fe–Ti [6,7,8], Fe–Mo [9,10,11], Fe–W [12,13,14], Fe–Nb [15], and so on, in which the Fe component provides redox sites while a second component supplies acid sites. For instance, Liu et al. [6,16] reported a novel iron titanate catalyst with outstanding performance in the medium-temperature range (200–400 °C), in which a specific Fe–O–Ti structure acted as the main active phase. Recently, Qu et al. [11] identified a di-nuclear entity (an isolated Mo ion and one adjacent surface Fe ion) as the active site in a Mo₁/Fe₂O₃ single-atom catalyst, thus giving an explanation for the improved SCR reaction at acid-redox interfaces.

For iron-based oxides, generally, the γ-Fe₂O₃ crystalline phase shows better activity than α-Fe₂O₃ in NH₃–SCR [7,17,18] and other catalytic reactions, such as CO₂ hydrogenation, photodecomposition of H₂S, and NO reduction by CO [19,20,21]. However, the γ-Fe₂O₃ active phase is thermally unstable and undergoes irreversible transformation to α-Fe₂O₃ at elevated temperatures. Such transformation generally occurs between 300 °C and 400 °C [18,22,23,24], related to several factors, including the particle size, morphology of γ-Fe₂O₃ and presence of a coating layer or dopants [23,25]. As a result, maintaining the γ-Fe₂O₃ active phase in iron-based oxide catalysts is of great importance to the durability of catalysts in the NH₃–SCR reaction. To restrain such phase transformation, previous researchers found it effective to introduce another metal into iron oxides, such as Mn, Ti or W, or to fabricate coating layers [21,22,24,26,27,28].

Hexadecyl trimethyl ammonium bromide (CTAB) is a kind of amphiphilic organic compound containing both hydrophobic and hydrophilic ends [29], which are often used as template-directing agents that can function as both “structural” and “chemical” promoters. It was reported that CTAB could adjust the crystalline phase of metal oxides [30,31,32]; for instance, the steric hindrance of CTAB was found to prevent the transition of anatase to rutile for TiO₂ [31]. In this study, Nb, Ti and Mo were chosen to couple with Fe in order to provide sufficient acidity for NH₃–SCR [9,18,33,34,35,36]. During the preparation process of Fe–M (M = Nb, Ti, Mo) catalysts meanwhile, CTAB was added into the precursor solution. With the assistance of CTAB, the obtained FeM_0.3O_x-C catalysts showed much better SCR performance than unassisted FeM_0.3O_x catalysts. It was identified that CTAB adjusted the structural properties, contributing to the formation of the γ-Fe₂O₃ phase. Further investigation revealed that the modified catalysts showed high exposure of acid sites and an enhanced ability for low-temperature NO oxidation, but restrained behavior for NH₃ oxidation at high temperatures, thus contributing to improved NO_x conversion and N₂ selectivity.

2. Results

2.1. NH₃–SCR Performance

The NO_x conversion in the NH₃–SCR reaction over all the prepared catalysts under the high GHSV (gaseous hourly space velocity) of 500,000 h⁻¹ is shown in Figure 1. At 150 °C, the NO_x conversion over FeNb_0.3O_x-C was 9%, which was slightly higher than that of FeNb_0.3O_x (7%). With the rising temperature, the SCR activity of FeNb_0.3O_x-C increased significantly, with NO_x conversion of 73% at 300 °C, which was much higher than the value of 54% for FeNb_0.3O_x. Such an increase in NO_x conversion was also observed over Fe–Ti and Fe–Mo catalysts, especially at higher temperatures. Meanwhile, FeM_0.3O_x-C catalysts showed higher N₂ selectivity than FeM_0.3O_x catalysts, as shown in Figure S1.

2.2. Kinetic Studies

The Arrhenius plots of the reaction rates for the reduction of NO_x in the range of 200–320 °C are shown in Figure 2. The apparent activation energies (E_a) were then calculated from the fitted curves and are shown in

Table 1. Activation energy and reaction rate at 260 °C of all the FeM_0.3O_x-C and FeM_0.3O_x (M = Nb, Ti, Mo) samples.

	Ea (kJ mol−1)	R2	Reaction Rate at 260 °C (mol m−2 s−1)
FeNb0.3Ox-C	34.5	0.991	9.9 × 10−9
FeNb0.3Ox	40.0	0.993	2.9 × 10−9
FeTi0.3Ox-C	28.0	0.993	9.5 × 10−9
FeTi0.3Ox	22.6	0.999	6.1 × 10−9
FeMo0.3Ox-C	26.3	0.993	9.5 × 10−9
FeMo0.3Ox	26.2	0.990	7.1 × 10−9

. It was noted that the E_a of FeNb_0.3O_x-C was 34.5 kJ mol⁻¹, similar to that of FeNb_0.3O_x (40.0 kJ mol⁻¹). This was further confirmed by the reaction results of Fe–Ti and Fe–Mo catalysts with similar values of E_a (shown in Table 1). Figure 2 also shows that the three FeM_0.3O_x-C catalysts exhibited almost the same reaction rate per square meter of surface area, which is further listed in Table 1 (reaction rate at 260 °C). In agreement with Figure 1, the activities of FeM_0.3O_x-C were higher than those of FeM_0.3O_x. Among the three FeM_0.3O_x samples, interestingly, FeTi_0.3O_x and FeMo_0.3O_x exhibited similar reaction rates.

2.3. Structural Properties

2.3.1. N₂-Physisorption Analysis

The surface areas and pore volumes of all the samples are summarized and displayed in

Table 2. Structural parameters of all the FeM_0.3O_x-C and FeM_0.3O_x (M = Nb, Ti, Mo) samples.

	Surface Area (m2 g−1)	Pore Volume (cm3 g−1)
FeNb0.3Ox-C	172	0.25
FeNb0.3Ox	210	0.13
FeTi0.3Ox-C	151	0.21
FeTi0.3Ox	191	0.24
FeMo0.3Ox-C	99	0.15
FeMo0.3Ox	167	0.24

. The FeNb_0.3O_x-C catalyst showed a smaller surface area than that of FeNb_0.3O_x (172 m² g⁻¹ vs. 210 m² g⁻¹) while exhibiting a larger average pore size. A decrease in surface area induced by the assistance of CTAB in the preparation process was also observed for Fe–Ti and Fe–Mo, with detailed information shown in Table 2.

2.3.2. XRD Analysis

The XRD patterns in Figure 3 illustrate the crystalline phases of the synthesized samples. Diffraction peaks associated with γ-Fe₂O₃ appeared for the FeNb_0.3O_x-C catalyst, while no peak due to Nb species was observed. For the FeNb_0.3O_x catalyst, no characteristic peaks due to Fe or Nb components were observed, indicating their high dispersion or amorphous state [37]. For FeTi_0.3O_x, only peaks attributed to α-Fe₂O₃ could be observed, while for FeTi_0.3O_x-C, besides the peaks of α-Fe₂O₃ with lower intensity, peaks of γ-Fe₂O₃ appeared. As for the Fe–Mo catalysts, interestingly, only peaks due to γ-Fe₂O₃ were observed for the FeMo_0.3O_x-C catalyst, whereas all the peaks were assignable to α-Fe₂O₃ in the case of FeMo_0.3O_x. As γ-Fe₂O₃ and other iron oxides (such as Fe₃O₄) show very similar diffraction patterns [38], ⁵⁷Fe Mössbauer spectroscopy measurements were carried out to confirm the crystalline phase of the iron oxides present, with results shown in Figure S2. The data were fitted using Moss Winn with parameters including isomer shift (mm s⁻¹), quadrupole splitting (mm s⁻¹), internal hyperfine field (T), and area listed in Table S1. According to previous research [39,40,41,42,43,44], the isomer shift values of doublets indicated the signals from Fe³⁺ for each sample. In other words, no signal of Fe²⁺ was observed, which ruled out the possibility of Fe₃O₄ in Fe–M–C catalysts. The values of isomer shift around 0.32 mm s^-1 and internal hyperfine field around 49.8 T of sextets confirmed the presence of γ-Fe₂O₃ in FeNb_0.3O_x-C, FeTi_0.3O_x-C, and FeMo_0.3O_x-C. In addition, the values of isomer shift around 0.38 mm s^-1 and internal hyperfine field around 51.2 T of sextets for FeTi_0.3O_x-C, FeTi_0.3O_x, and FeMo_0.3O_x were ascribed to α-Fe₂O₃. These results were in good agreement with XRD results. Based on the results, the ratio of γ-Fe₂O₃ and α-Fe₂O₃ components in FeTi_0.3O_x-C was further calculated from the areas in Table S1 with the value of about 3:1 [44].

2.4. Acidity and Redox Ability

2.4.1. Acidic Properties

In the NH₃–SCR reaction, acid sites on the catalysts are responsible for the adsorption of ammonia and play an important role in NO_x reduction. To investigate the surface acidity, NH₃-TPD experiments were carried out, and the amount of NH₃ desorption that occurred was calculated. For all the catalysts, one peak at around 150 °C and another peak around 300 °C were observed, which correspond to weakly adsorbed and strongly adsorbed ammonia species, respectively [45]. To reduce errors in the determination of the NH₃ desorption amount, the NH₃-TPD experiments were repeated two times (Figure S3a,b), with results shown in Figure S3c and

Table 3. Desorption amount of NH₃ and percentage of weak-adsorbed NH₃ over all the FeM_0.3O_x-C and FeM_0.3O_x (M = Nb, Ti, Mo) samples.

	NH3 Desorption Amount (μmol g−1) *	NH3 Desorption Amount Normalized by Surface Area (μmol m−2)	Percentage of Weak-Adsorbed NH3 (%)
FeNb0.3Ox-C	260 ± 7.0	1.5 ± 0.05	75.9 ± 1.3
FeNb0.3Ox	289 ± 7.6	1.4 ± 0.07	72.8 ± 1.1
FeTi0.3Ox-C	226 ± 4.7	1.5 ± 0.03	75.9 ± 0.1
FeTi0.3Ox	246 ± 12.7	1.3 ± 0.08	74.2 ± 1.2
FeMo0.3Ox-C	145 ± 8.3	1.5 ± 0.12	79.2 ± 1.7
FeMo0.3Ox	229 ± 15.2	1.4 ± 0.08	77.9 ± 1.2

* with experiments and error bars shown in Figure S3.

. The total amount of NH₃ desorbed from FeNb_0.3O_x-C was around 260 μmol g⁻¹, less than that for FeNb_0.3O_x (around 289 μmol g⁻¹). After normalization by surface area (that is, the NH₃ adsorption amount divided by the surface area), the calculated NH₃ desorption value was 1.5 μmol m⁻² for FeNb_0.3O_x-C, which was larger than that for FeNb_0.3O_x (1.4 μmol m⁻²). For Fe–Ti and Fe–Mo, FeM_0.3O_x-C catalysts also showed lower NH₃ desorption but a higher value after normalization by surface area compared with FeM_0.3O_x catalysts. In addition, the percentages of weakly adsorbed ammonia species were calculated and are shown in Table 3, with the values of FeM_0.3O_x-C catalysts being slightly higher than those of the FeM_0.3O_x catalysts, respectively.

To identify the types of acid sites, DRIFT studies of NH₃ adsorption at 150 °C were performed and shown in Figure 4. The bands at 1672 cm⁻¹ and 1430 cm⁻¹ were attributed to NH₄⁺ species adsorbed on Brønsted acid sites, and bands at 1606 cm⁻¹ and 1209 cm⁻¹ were ascribed to NH₃ species adsorbed on Lewis acid sites [46,47,48]. By comparing the intensity of bands at 1430 cm⁻¹, one can easily observe that more NH₄⁺ species adsorbed on Brønsted acid sites on FeM_0.3O_x-C catalysts than on FeM_0.3O_x catalysts. In contrast, as indicated by the intensity of the band at 1606 cm^-1, the amount of NH₃ species adsorbed on Lewis acid sites on FeM_0.3O_x-C catalysts was slightly lower than that for FeM_0.3O_x catalysts.

2.4.2. XPS Analysis

To explore the electronic states and atomic concentrations of surface atoms, XPS analysis was carried out for all the samples. As shown in Figure 5, the O 1 s spectra could be fitted into two peaks, with the peak at around 530.1 eV corresponding to lattice oxygen (denoted as O_β) and the peak at around 531.4 eV assignable to surface oxygen (denoted as O_α). Compared with FeM_0.3O_x catalysts, the O_β species in FeM_0.3O_x-C catalysts showed higher binding energy. Meanwhile, the ratios of O_α/(O_α + O_β), as well as the proportion of O_α (that is, the value of O_α/(O_α + O_β) plus the value of the O atom ratio on the surface), were calculated. As listed in

Table 4. Surface components of all the FeM_0.3O_x-C and FeM_0.3O_x (M = Nb, Ti, Mo) samples from XPS results.

	Proportion of O	Oα/(Oα + Oβ)	Proportion of Oα	M/Fe
FeNb0.3Ox-C	49.4%	33.2%	16.4%	0.49
FeNb0.3Ox	48.5%	32.6%	15.8%	0.43
FeTi0.3Ox-C	50.7%	38.7%	19.6%	0.72
FeTi0.3Ox	48.3%	36.4%	17.6%	0.45
FeMo0.3Ox-C	52.5%	33.3%	17.5%	0.97
FeMo0.3Ox	49.6%	35.0%	17.4%	0.55

, each of the FeM_0.3O_x-C catalysts showed a higher proportion of O_α than the corresponding FeM_0.3O_x catalysts, indicative of a higher content of surface O species on FeM_0.3O_x-C.

The XPS Spectra of Nb 3d, Ti 2p, and Mo 3d are shown in Figure S4, showing Nb, Ti, and Mo elements in their highest valence state [49,50,51,52,53,54]. In addition, it was observed that the acid components (Nb, Ti, and Mo) in the FeM_0.3O_x-C catalysts showed higher binding energy than those in FeM_0.3O_x catalysts. The spectra of Fe 2p are shown in Figure S5, with the peaks located around 724.5 eV, 718.9 eV and 710.7 eV, which correspond to Fe 2p_1/2, Fe 2p_3/2 satellite and Fe 2p_3/2, respectively, corresponding to Fe³⁺ [45,46]. As shown in Table 4, for a given acid component, the ratio of M/Fe in FeM_0.3O_x-C catalysts was always higher than that in FeM_0.3O_x catalysts. This result indicated that a surface enrichment of acid components was induced by CTAB addition during the process of catalyst preparation, which was consistent with the results of NH₃-TPD.

2.4.3. H₂-TPR Analysis

To further investigate the reducibility of the prepared catalysts, H₂-TPR experiments were carried out. As shown in Figure 6, the peaks below 450 °C correspond to the reduction of Fe₂O₃ to Fe₃O₄. Further reduction of Fe₃O₄ to FeO and Fe occurred at higher temperatures (above 450 °C) [13,55,56]. Over the FeNb_0.3O_x-C catalyst, it was observed that the peaks due to the reduction of Fe₂O₃ to Fe₃O₄ were centered at higher temperatures (348 °C and 413 °C) compared with FeNb_0.3O_x (centered at 312 °C and 397 °C). Such weakening of the redox ability of Fe by CTAB addition was also observed during the reduction of Fe₃O₄ to FeO and Fe. As for FeTi_0.3O_x-C and FeMo_0.3O_x-C, similarly, the reduction peaks of Fe₂O₃ occurred more clearly at higher temperatures compared with those of FeTi_0.3O_x and FeMo_0.3O_x, respectively.

2.4.4. Direct Oxidation of NH₃ and NO

Direct oxidation reactions of NH₃ and NO were also conducted, with results shown in Figures S6 and S7, respectively. At temperatures below 250 °C, the direct oxidation of NH₃ hardly occurred over any of the samples, benefiting the NH₃–SCR reaction. At temperatures above 250 °C, NH₃ conversion increased with rising temperature, during which the FeM_0.3O_x-C always exhibited lower activity for NH₃ oxidation than FeM_0.3O_x (except for FeNb_0.3O_x-C at the temperature of 400 °C). This was possibly due to the weakened redox ability induced by CTAB addition. By contrast, the direct oxidation of NO occurred over the whole temperature range, during which the NO conversion increased with rising temperature, reached a maximum value, and then decreased. Interestingly, FeM_0.3O_x-C always showed higher activity for NO₂ formation (except for FeMo_0.3O_x-C at temperatures above 250 °C). These results suggested that the intrinsic properties governing the direct oxidation of NO were different from that of NH₃ oxidation. Combined with the results listed in Table 4, it can be deduced that surface oxygen plays a crucial role in NO oxidation to NO₂ over the Fe–M catalysts [6,57,58].

3. Discussion

The SCR performance results (Figure 1 and Figure S1) and kinetic studies (Figure 2) showed that FeM_0.3O_x-C catalysts showed higher SCR activity as well as N₂ selectivity than FeM_0.3O_x catalysts over the whole temperature range. It can be concluded from XRD (Figure 3) and Mössbauer spectra (Figure S2 and Table S1) that the addition of CTAB into solutions of precursors promoted the formation of γ-Fe₂O₃, which was more active in terms of SCR activity than α-Fe₂O₃ (Figure S8). Specifically, combining the XRD results with the kinetic studies, it was clear that samples containing the γ-Fe₂O₃ phase (FeM_0.3O_x-C catalysts) showed similar intrinsic activity. Meanwhile, FeTi_0.3O_x and FeMo_0.3O_x catalysts with pure α-Fe₂O₃ phase exhibited almost the same reaction rate, which was lower than that of the FeM_0.3O_x-C catalysts. This illustrated that the formation of the γ-Fe₂O₃ phase induced by CTAB addition during the catalyst preparation process improved the catalytic activity of FeM_0.3O_x. As shown in Figure 3, meanwhile, the Fe phase in FeNb_0.3O_x-C and FeMo_0.3O_x-C detected by XRD was 100% γ-Fe₂O₃, while both the γ-Fe₂O₃ and α-Fe₂O₃ phases were observed in FeTi_0.3O_x-C. Taking these findings into account, it would be expected that the FeNb_0.3O_x-C and FeMo_0.3O_x-C would be more active for NH₃–SCR than FeTi_0.3O_x-C. By comparing the results of Figure 1 with those of Figure S8, it can be easily found that, at a given temperature, the activity of the pure γ-Fe₂O₃ sample was much lower than FeNb_0.3O_x-C and FeTi_0.3O_x-C. These results, in turn, indicate that other factors also have a great influence on the catalytic performance of the Fe–M system.

It is well-accepted that optimizing the acid-redox properties is crucial to designing SCR catalysts with high catalytic performance. In our research, the alteration of the crystalline phase modified both the acidity and reducibility of the Fe–M catalysts, which played a significant role in the improved SCR performance. As for acidity, despite the smaller surface area (Table 2) and lower total desorption amount of NH₃ (Table 3), FeM_0.3O_x-C catalysts showed higher exposure of acid sites. This was explained by the increased ratio of M/Fe on FeM_0.3O_x-C catalysts revealed by XPS (Table 4), consistent with higher exposure of acid components. The NH₃-TPD results (Figure 4) indicated more weakly adsorbed ammonia on FeM_0.3O_x-C catalysts. As previous research indicated the weaker stability of NH₄⁺ species adsorbed on Brønsted acid sites compared to NH₃ species adsorbed on Lewis acid sites [59,60], this suggested that more NH₄⁺ species adsorbed on Brønsted acid sites existed on FeM_0.3O_x-C catalysts. This was further confirmed by the DRIFT results (Figure 5). In conclusion, increased exposure of acid components, especially Brønsted acid sites, was observed over FeM_0.3O_x-C catalysts, which benefited SCR activity [45,61,62].

For metal oxide NH₃–SCR catalysts, generally, surface oxygen (O_α) is important for the oxidation of NO to NO₂, thus promoting the “fast–SCR” reaction at low-temperature [6,57,58]. In addition, the over-oxidation of NH₃ accounted for the decreased NO_x conversion at high temperatures [1,13,16,63,64]. In our research, on one hand, the increased percentage of surface O_α boosted the oxidation of NO to NO₂ (as shown in Table 4 and Figure S7), thus benefiting the activity at low temperatures. On the other hand, the weaker reducibility of Fe species revealed by H₂-TPR (Figure 6) suppressed the over-oxidation of NH₃ over FeTi_0.3O_x-C and FeMo_0.3O_x-C catalysts at high temperatures (Figure S6), explaining the improved NO_x conversion and N₂ selectivity. As a result, the SCR performance of FeM_0.3O_x-C catalysts over the whole temperature range was improved. In addition, the shift of binding energy for O 1 s, Nb 3d, Ti 2p, and Mo 3d in XPS results associated with the addition of CTAB was possibly due to deviation of the electron cloud [65,66,67], which indicated enhanced interaction between Fe and acid components and was related to the observed change in the crystalline phase.

Compared with FeNb_0.3O_x-C, FeTi_0.3O_x-C exhibited a higher proportion of surface oxygen (Table 4) and a higher ability for NO oxidation to NO₂ (Figure S7). As discussed above, the surface oxygen species were active for the oxidation of NO to NO₂, thus promoting the “fast–SCR” reaction at low-temperature. With this in mind, it is reasonable that FeTi_0.3O_x-C shows similar intrinsic activity to FeNb_0.3O_x-C, even though the latter exhibits a 100% γ-Fe₂O₃ phase. As shown in Table 3, the desorption amount of NH₃ over FeMo_0.3O_x-C was slightly lower than that of FeTi_0.3O_x-C, suggesting that a smaller amount of acid sites was available for NH₃–SCR. Compared with FeTi_0.3O_x-C, FeMo_0.3O_x-C also exhibited a lower proportion of surface oxygen (Table 4), resulting in lower activity for NO oxidation to NO₂ (Figure S7), which is not beneficial for the occurrence of fast–SCR. Taking these facts into account, it is reasonable that FeMo_0.3O_x-C shows similar intrinsic activity to FeTi_0.3O_x-C, even though the former contains 100% γ-Fe₂O₃.

4. Materials and Methods

4.1. Catalyst Preparation

The CTAB-assisted co-precipitation process was inspired by previous research on FeMnTi catalysts by Wu et al. [68]. Typically, CTAB was first dissolved into deionized water, and then precursors containing Fe(NO₃)₃·9H₂O, C₁₀H₅NbO₂₀ (or Ti(SO₄)₂, and (NH₄)₆Mo₇O₂₄·4H₂O) in the required molar ratios were added to the CTAB solution. The molar ratio of M:Fe (M = Nb, Ti and Mo) was set at 3:10. After stirring for 1 h, the solution was heated to 90 °C and kept for 10 h. The obtained precipitates were filtered, washed and dried overnight at 105 °C. Then the samples were calcined at 400 °C for 5 h. The obtained catalysts were labeled as FeM_0.3O_x-C (i.e., FeNb_0.3O_x-C, FeTi_0.3O_x-C, and FeMo_0.3O_x-C, respectively). For comparative purposes, catalysts without CTAB were also prepared in the same way and labeled as FeM_0.3O_x (FeNb_0.3O_x, FeTi_0.3O_x, and FeMo_0.3O_x). Commercial pure γ-Fe₂O₃ and pure α-Fe₂O₃ were also used for comparison. Before NH₃–SCR activity tests, all the samples were pressed, crushed, and sieved to 40–60 mesh.

4.2. Activity Test

The NH₃–SCR activities were tested in a fixed-bed quartz tube flow reactor with an inner diameter of 4 mm (at GHSV of 500,000 h⁻¹, the mass of each catalyst was around 60 mg). The feed gas consisted of 500 ppm of NO, 500 ppm of NH₃, 5 vol % O₂, balanced by N₂, with a gas flow rate of 500 mL min⁻¹. The concentrations of NH₃, NO, NO₂, and N₂O were continually monitored by FTIR spectrometer (IS10 Nicolet) (City, State, Abbr(if has), Country)(Thermo, Waltham, MA USA), which was equipped with a multiple path gas cell (2 m).

The NO_x conversion and N₂ selectivity were calculated as follows [69,70]:

$${\mathrm{NO}}_{\mathrm{X}} \mathrm{conversion}(\%)=\left(1-\frac{{\left[{\mathrm{NO}}_{\mathrm{X}}\right]}_{\mathrm{out}}}{{\left[{\mathrm{NO}}_{\mathrm{X}}\right]}_{\mathrm{in}}}\right)\times 100\%$$

(1)

$${ \mathrm{N}}_2 \mathrm{selectivity}(\%)=\left(1-\frac{2{[{\mathrm{N}}_2\mathrm{O}]}_{\mathrm{out}}}{{[{\mathrm{NO}}_{\mathrm{X}}]}_{\mathrm{in}}-{[{\mathrm{NO}}_{\mathrm{X}}]}_{\mathrm{out}}+\left[{\mathrm{NH}}_3{]}_{\mathrm{in}}-\right[{\mathrm{NH}}_3{]}_{\mathrm{out}}}\right)\times 100\%$$

(2)

with [NO_x] = [NO] + [NO₂].

4.3. Kinetic Study

The apparent activation energy (E_a) and reaction order for NO_x reduction were measured in a fixed-bed quartz tube flow reactor with an inner diameter of 4 mm. In this case, the mass of each catalyst was 20 mg, and the conversion of NO_x was controlled below 25%. The feed gas composition was 500 ppm NO, 500 ppm NH₃, 5% O₂, and N₂ balance. The reaction rate of NO_x conversion was calculated as follows:

$$-{\mathrm{R}}_{\mathrm{NOx}}=\frac{{\mathrm{F}}_{\mathrm{NOx}}\times {\mathrm{X}}_{\mathrm{NOx}}}{\mathrm{W}\times \mathrm{S}}$$

(3)

where F_NOx is the molar flow rate of NO_x, X_NOx is the conversion of NO_x, W is the weight of the catalyst, and S is the BET (Brunauer−Emmett−Teller) surface area.

4.4. Catalyst Characterization

N₂-physisorption analysis was obtained at 77 K using a Quantachrome Autosorb-1C instrument (Anton Paar, Graz, Austria) at liquid nitrogen temperature. The specific surface areas were calculated by the BET equation in the 0.05–0.30 partial pressure range. The pore volumes and average pore diameters were determined by the BJH method from the desorption branches of the isotherms. Prior to each N₂-physisorption analysis, the samples were degassed at 300 °C for 3 h. Powder X-ray diffraction (XRD) patterns of the samples were conducted on a Brucker D8 diffractometer (Brucker, Karlsruhe, Germany) with Cu Kα (λ = 0.15406 nm) radiation. The scan range of 2θ range was from 20° to 90° with a step size of 0.02°.

The temperature-programmed desorption of ammonia (NH₃-TPD) experiments were carried out on a fixed-bed quartz tube flow reactor with an inner diameter of 4 mm. The concentration of NH₃ was continually monitored by an FTIR spectrometer (IS10 Nicolet), which was equipped with a multiple path gas cell (2 m). Prior to each TPD experiment, the 100 mg samples were pretreated in 20% O₂/N₂ at a flow rate of 300 mL min^-1 at 350 °C for 0.5 h, and then cooled down to 50 °C and purged by N₂ for 0.5 h. The samples were then exposed to a flow of 500 ppm NH₃/N₂ (500 mL min⁻¹) at 50 °C for 0.5 h, followed by N₂ purging for 0.5 h. Finally, the temperature was raised to 800 °C in N₂ with the rate of 10 °C min⁻¹. In situ DRIFTS experiments were performed on an FTIR spectrometer (Nicolet IS50) equipped with an MCT/A detector cooled by liquid nitrogen. Each catalyst was pretreated in 20 vol % O₂/N₂ at 400 °C for 0.5 h and then cooled down to 150 °C. The samples were exposed to a flow of 500 ppm NH₃ with an N₂ balance for 0.5 h and purged by N₂ for another 0.5 h. XPS measurements were carried out on an X-ray photoelectron spectrometer (Thermo Fisher Scientific K-Alpha, Thermo, Waltham, MA, USA) with Al Kα radiation (1486.8 eV) at an energy resolution of 0.05 eV (Ag 3d_5/2). The binding energies of Fe 2p, Nb 3d, Ti 2p, Mo 3d, and O 1 s were calibrated using the C 1 s peak (BE = 284.8 eV) as standard. The temperature-programmed reduction of hydrogen (H₂-TPR) experiments were carried out on a Auto Chem 2920 chemisorption analyzer (Micromeritics, Aachen, Germany). In a typical measurement, 150 mg of the sample was first, preprocessed in a flow of N₂ with the total flow rate of 50 mL min⁻¹ at 300 °C for 1 h, and then cooled to 50 °C, followed by N₂ purging for another 0.5 h. Then the temperature was linearly increased from 50 to 900 °C at the heating rate of 10 °C min⁻¹ in a flow of 10 vol % H₂/N₂ (50 mL min⁻¹), during which the H₂ consumption was continuously recorded by a thermal conductivity detector (TCD).

5. Conclusions

In our research, FeM_0.3O_x-C catalysts synthesized with the assistance of CTAB exhibited higher SCR performance compared with FeM_0.3O_x. Characterization revealed that the presence of CTAB in the preparation process of Fe–M (M= Nb, Ti, Mo) composite oxides adjusted the crystalline phase of iron oxides and modified both the reducibility and acidity. The optimized catalysts exposed more surface acid sites, which was beneficial to the SCR activity. In addition, surface oxygen was increased, which benefited the NO oxidation and the “fast SCR” reaction. In addition, the redox ability of Fe was weakened by the addition of CTAB, thus restraining the over-oxidation of NH₃ and improved NO_x conversion as well as N₂ selectivity at high temperatures. Overall, both the acidity and reducibility were tuned by CTAB addition during the process of catalyst preparation; thus, FeM_0.3O_x-C catalysts achieved higher catalytic performance over the whole temperature range.