Next Article in Journal
Sustainable and Environmental Catalysis
Next Article in Special Issue
MnOx Supported on Hierarchical SAPO-34 for the Low-Temperature Selective Catalytic Reduction of NO with NH3: Catalytic Activity and SO2 Resistance
Previous Article in Journal
Direct Dimethyl Carbonates Synthesis over CeO2 and Evaluation of Catalyst Morphology Role in Catalytic Performance
Previous Article in Special Issue
Novel Preparation of Cu and Fe Zirconia Supported Catalysts for Selective Catalytic Reduction of NO with NH3
 
 
Search for Articles:
Title / Keyword
Author / Affiliation / Email
Journal
Article Type
 
 
Advanced Search
 
Section
Special Issue
Volume
Issue
Number
Page
 
Journals
Catalysts
Volume 11
Issue 2
10.3390/catal11020224
catalysts-logo
Submit to this Journal Review for this Journal Propose a Special Issue
► Article Menu

Article Menu

share Share announcement Help format_quote Cite question_answer Discuss in SciProfiles thumb_up
...
Endorse textsms
...
Comment
first_page
settings
Order Article Reprints
Font Type:
Arial Georgia Verdana
Font Size:
Aa Aa Aa
Line Spacing:
Column Width:
Background:
Article

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

by
Wenshuo Zhang
1,2,
Xiaoyan Shi
1,2,
Meng Gao
1,2,
Jingjing Liu
1,
Zhihui Lv
1,2,
Yingjie Wang
3,
Yanlong Huo
1,2,
Chang Cui
1,2,
Yunbo Yu
1,2,3,4,* and
Hong He
1,2,4,*
1
State Key Joint Laboratory of Environment Simulation and Pollution Control, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China
2
University of Chinese Academy of Sciences, Beijing 100049, China
3
Ganjiang Innovation Academy, Chinese Academy of Sciences, Ganzhou 341000, China
4
Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China
*
Authors to whom correspondence should be addressed.
Catalysts 2021, 11(2), 224; https://doi.org/10.3390/catal11020224
Submission received: 19 January 2021 / Revised: 4 February 2021 / Accepted: 5 February 2021 / Published: 8 February 2021
(This article belongs to the Special Issue Selective Catalytic Reduction of NOx by NH3)
Browse Figures
Versions Notes

Abstract

:
Iron-based oxide catalysts for the NH3–SCR (selective catalytic reduction of NOx 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 (FeM0.3Ox-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 (FeM0.3Ox, 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 FeM0.3Ox-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 NOx 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.

Graphical Abstract

1. Introduction

Selective catalytic reduction of NOx by NH3 (NH3–SCR) is an effective means of NOx abatement. Various catalysts have been developed for NH3–SCR, including metal oxide catalysts (VOx-based, CeO2-based, Fe2O3-based, MnOx-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 NOx 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 FeM0.3Ox-C catalysts showed much better SCR performance than unassisted FeM0.3Ox 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 NOx conversion and N2 selectivity.

2. Results

2.1. NH3–SCR Performance

The NOx 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 NOx conversion over FeNb0.3Ox-C was 9%, which was slightly higher than that of FeNb0.3Ox (7%). With the rising temperature, the SCR activity of FeNb0.3Ox-C increased significantly, with NOx conversion of 73% at 300 °C, which was much higher than the value of 54% for FeNb0.3Ox. Such an increase in NOx conversion was also observed over Fe–Ti and Fe–Mo catalysts, especially at higher temperatures. Meanwhile, FeM0.3Ox-C catalysts showed higher N2 selectivity than FeM0.3Ox catalysts, as shown in Figure S1.

2.2. Kinetic Studies

The Arrhenius plots of the reaction rates for the reduction of NOx in the range of 200–320 °C are shown in Figure 2. The apparent activation energies (Ea) were then calculated from the fitted curves and are shown in Table 1. It was noted that the Ea of FeNb0.3Ox-C was 34.5 kJ mol−1, similar to that of FeNb0.3Ox (40.0 kJ mol−1). This was further confirmed by the reaction results of Fe–Ti and Fe–Mo catalysts with similar values of Ea (shown in Table 1). Figure 2 also shows that the three FeM0.3Ox-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 FeM0.3Ox-C were higher than those of FeM0.3Ox. Among the three FeM0.3Ox samples, interestingly, FeTi0.3Ox and FeMo0.3Ox 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. The FeNb0.3Ox-C catalyst showed a smaller surface area than that of FeNb0.3Ox (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 FeNb0.3Ox-C catalyst, while no peak due to Nb species was observed. For the FeNb0.3Ox catalyst, no characteristic peaks due to Fe or Nb components were observed, indicating their high dispersion or amorphous state [37]. For FeTi0.3Ox, only peaks attributed to α-Fe2O3 could be observed, while for FeTi0.3Ox-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 FeMo0.3Ox-C catalyst, whereas all the peaks were assignable to α-Fe2O3 in the case of FeMo0.3Ox. 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 FeNb0.3Ox-C, FeTi0.3Ox-C, and FeMo0.3Ox-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 FeTi0.3Ox-C, FeTi0.3Ox, and FeMo0.3Ox 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 FeTi0.3Ox-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 NOx 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. The total amount of NH3 desorbed from FeNb0.3Ox-C was around 260 μmol g−1, less than that for FeNb0.3Ox (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 FeNb0.3Ox-C, which was larger than that for FeNb0.3Ox (1.4 μmol m−2). For Fe–Ti and Fe–Mo, FeM0.3Ox-C catalysts also showed lower NH3 desorption but a higher value after normalization by surface area compared with FeM0.3Ox catalysts. In addition, the percentages of weakly adsorbed ammonia species were calculated and are shown in Table 3, with the values of FeM0.3Ox-C catalysts being slightly higher than those of the FeM0.3Ox 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 FeM0.3Ox-C catalysts than on FeM0.3Ox 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 FeM0.3Ox-C catalysts was slightly lower than that for FeM0.3Ox 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 Oβ) and the peak at around 531.4 eV assignable to surface oxygen (denoted as Oα). Compared with FeM0.3Ox catalysts, the Oβ species in FeM0.3Ox-C catalysts showed higher binding energy. Meanwhile, the ratios of Oα/(Oα + Oβ), as well as the proportion of Oα (that is, the value of Oα/(Oα + Oβ) plus the value of the O atom ratio on the surface), were calculated. As listed in Table 4, each of the FeM0.3Ox-C catalysts showed a higher proportion of Oα than the corresponding FeM0.3Ox catalysts, indicative of a higher content of surface O species on FeM0.3Ox-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 FeM0.3Ox-C catalysts showed higher binding energy than those in FeM0.3Ox 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 2p1/2, Fe 2p3/2 satellite and Fe 2p3/2, respectively, corresponding to Fe3+ [45,46]. As shown in Table 4, for a given acid component, the ratio of M/Fe in FeM0.3Ox-C catalysts was always higher than that in FeM0.3Ox 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 FeNb0.3Ox-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 FeNb0.3Ox (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 FeTi0.3Ox-C and FeMo0.3Ox-C, similarly, the reduction peaks of Fe2O3 occurred more clearly at higher temperatures compared with those of FeTi0.3Ox and FeMo0.3Ox, 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 FeM0.3Ox-C always exhibited lower activity for NH3 oxidation than FeM0.3Ox (except for FeNb0.3Ox-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, FeM0.3Ox-C always showed higher activity for NO2 formation (except for FeMo0.3Ox-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 FeM0.3Ox-C catalysts showed higher SCR activity as well as N2 selectivity than FeM0.3Ox 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 (FeM0.3Ox-C catalysts) showed similar intrinsic activity. Meanwhile, FeTi0.3Ox and FeMo0.3Ox catalysts with pure α-Fe2O3 phase exhibited almost the same reaction rate, which was lower than that of the FeM0.3Ox-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 FeM0.3Ox. As shown in Figure 3, meanwhile, the Fe phase in FeNb0.3Ox-C and FeMo0.3Ox-C detected by XRD was 100% γ-Fe2O3, while both the γ-Fe2O3 and α-Fe2O3 phases were observed in FeTi0.3Ox-C. Taking these findings into account, it would be expected that the FeNb0.3Ox-C and FeMo0.3Ox-C would be more active for NH3–SCR than FeTi0.3Ox-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 FeNb0.3Ox-C and FeTi0.3Ox-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), FeM0.3Ox-C catalysts showed higher exposure of acid sites. This was explained by the increased ratio of M/Fe on FeM0.3Ox-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 FeM0.3Ox-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 FeM0.3Ox-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 FeM0.3Ox-C catalysts, which benefited SCR activity [45,61,62].
For metal oxide NH3–SCR catalysts, generally, surface oxygen (Oα) 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 NOx conversion at high temperatures [1,13,16,63,64]. In our research, on one hand, the increased percentage of surface Oα 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 FeTi0.3Ox-C and FeMo0.3Ox-C catalysts at high temperatures (Figure S6), explaining the improved NOx conversion and N2 selectivity. As a result, the SCR performance of FeM0.3Ox-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 FeNb0.3Ox-C, FeTi0.3Ox-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 FeTi0.3Ox-C shows similar intrinsic activity to FeNb0.3Ox-C, even though the latter exhibits a 100% γ-Fe2O3 phase. As shown in Table 3, the desorption amount of NH3 over FeMo0.3Ox-C was slightly lower than that of FeTi0.3Ox-C, suggesting that a smaller amount of acid sites was available for NH3–SCR. Compared with FeTi0.3Ox-C, FeMo0.3Ox-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 FeMo0.3Ox-C shows similar intrinsic activity to FeTi0.3Ox-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 FeM0.3Ox-C (i.e., FeNb0.3Ox-C, FeTi0.3Ox-C, and FeMo0.3Ox-C, respectively). For comparative purposes, catalysts without CTAB were also prepared in the same way and labeled as FeM0.3Ox (FeNb0.3Ox, FeTi0.3Ox, and FeMo0.3Ox). 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 NOx conversion and N2 selectivity were calculated as follows [69,70]:
NO X   conversion ( % ) = ( 1 − [ NO X ] out [ NO X ] in ) × 100 %
  N 2   selectivity ( % ) = ( 1 − 2 [ N 2 O ] out [ NO X ] in − [ NO X ] out + [ NH 3 ] in − [ NH 3 ] out ) × 100 %
with [NOx] = [NO] + [NO2].

4.3. Kinetic Study

The apparent activation energy (Ea) and reaction order for NOx 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 NOx was controlled below 25%. The feed gas composition was 500 ppm NO, 500 ppm NH3, 5% O2, and N2 balance. The reaction rate of NOx conversion was calculated as follows:
− R NOx = F NOx × X NOx W × S
where FNOx is the molar flow rate of NOx, XNOx is the conversion of NOx, 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 3d5/2). 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, FeM0.3Ox-C catalysts synthesized with the assistance of CTAB exhibited higher SCR performance compared with FeM0.3Ox. 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 NOx 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, FeM0.3Ox-C catalysts achieved higher catalytic performance over the whole temperature range.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/11/2/224/s1, Figure S1: N2 selectivity over all the samples. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol %, N2 balance, GHSV = 500,000 h−1, Figure S2: Mössbauer spectra of all the samples at room temperature, Figure S3. (a) NH3-TPD results of all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples in a flow of N2 with a heating rate of 10 °C min−1. Before measurement, each sample was pretreated in 20% O2/N2 at 350 °C for 0.5 h, then exposed to 500 ppm NH3/N2 at 50 °C for 0.5 h, and purged by N2 for 0.5 h. (b) Repeated NH3-TPD results and (c) NH3 desorption amount of all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples with error bars, Figure S4: XPS spectra of (a) Nb 3d; (b) Ti 2p; and (c) Mo 3d over FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples, Figure S5: XPS spectra of Fe 2p over FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples, Figure S6: Direct oxidation of NH3 over all the samples. Reaction conditions: [NH3] = 500 ppm, [O2] = 5 vol %, N2 balance, GHSV = 250,000 h−1, Figure S7: Direct oxidation of NO over all the samples. Reaction conditions: [NO] = 500 ppm, [O2] = 5 vol %, N2 balance, GHSV = 250,000 h−1, Figure S8: NOx conversion over pure γ-Fe2O3 and pure α-Fe2O3. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol %, N2 balance, GHSV = 500,000 h−1, Table S1: Isomer shift (mm s−1), quadrupole splitting (mm s−1), internal hyperfine field (T), and area of sub-spectra from Mössbauer.

Author Contributions

Conceptualization, W.Z., Y.Y. and H.H.; methodology, X.S. and J.L.; investigation, W.Z., Z.L. and Y.H.; data curation, W.Z., M.G. and C.C.; writing—original draft preparation, W.Z. and Y.W.; writing—review and editing, X.S. and Y.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (grant number 21673277 and 21637005) and the Strategic Priority Research Program of the Chinese Academy of Sciences (grant number XDA23010200).

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Han, L.; Cai, S.; Gao, M.; Hasegawa, J.Y.; Wang, P.; Zhang, J.; Shi, L.; Zhang, D. Selective catalytic reduction of NOx with NH3 by using novel catalysts: State of the art and future prospects. Chem. Rev. 2019, 119, 10916–10976. [Google Scholar] [CrossRef]
  2. Shan, W.; Yu, Y.; Zhang, Y.; He, G.; Peng, Y.; Li, J.; He, H. Theory and practice of metal oxide catalyst design for the selective catalytic reduction of NO with NH3. Catal. Today 2020. [Google Scholar] [CrossRef]
  3. Granger, P.; Parvulescu, V.I. Catalytic NOx abatement systems for mobile sources: From three-way to lean burn after-treatment technologies. Chem. Rev. 2011, 111, 3155–3207. [Google Scholar] [CrossRef]
  4. Liu, F.; Yu, Y.; He, H. Environmentally-benign catalysts for the selective catalytic reduction of NOx from diesel engines: Structure-activity relationship and reaction mechanism aspects. Chem. Commun. 2014, 50, 8445–8463. [Google Scholar] [CrossRef] [PubMed]
  5. Marberger, A.; Ferri, D.; Elsener, M.; Krocher, O. The significance of Lewis acid sites for the selective catalytic reduction of nitric oxide on vanadium-based catalysts. Angew. Chem. Int. Ed. Engl. 2016, 55, 11989–11994. [Google Scholar] [CrossRef]
  6. Liu, F.; He, H.; Zhang, C.; Feng, Z.; Zheng, L.; Xie, Y.; Hu, T. Selective catalytic reduction of NO with NH3 over iron titanate catalyst: Catalytic performance and characterization. Appl. Catal. B Environ. 2010, 96, 408–420. [Google Scholar] [CrossRef]
  7. Yang, S.; Li, J.; Wang, C.; Chen, J.; Ma, L.; Chang, H.; Chen, L.; Peng, Y.; Yan, N. Fe–Ti spinel for the selective catalytic reduction of NO with NH3: Mechanism and structure–activity relationship. Appl. Catal. B Environ. 2012, 117–118, 73–80. [Google Scholar] [CrossRef]
  8. Liu, J.; Meeprasert, J.; Namuangruk, S.; Zha, K.; Li, H.; Huang, L.; Maitarad, P.; Shi, L.; Zhang, D. Facet–activity relationship of TiO2 in Fe2O3/TiO2 nanocatalysts for selective catalytic reduction of NO with NH3: In situ DRIFTs and DFT studies. J. Phys. Chem. C. 2017, 121, 4970–4979. [Google Scholar] [CrossRef]
  9. Xin, Y.; Zhang, N.; Li, Q.; Zhang, Z.; Cao, X.; Zheng, L.; Zeng, Y.; Anderson, J.A. Active site identification and modification of electronic states by atomic-scale doping to enhance oxide catalyst innovation. ACS Catal. 2018, 8, 1399–1404. [Google Scholar] [CrossRef] [Green Version]
  10. Chen, Y.; Li, C.; Chen, J.; Tang, X. Self-prevention of well-defined-facet Fe2O3/MoO3 against deposition of ammonium bisulfate in low-temperature NH3-SCR. Environ. Sci. Technol. 2018, 52, 11796–11802. [Google Scholar] [CrossRef]
  11. Qu, W.; Liu, X.; Chen, J.; Dong, Y.; Tang, X.; Chen, Y. Single-atom catalysts reveal the dinuclear characteristic of active sites in NO selective reduction with NH3. Nat Commun. 2020, 11, 1532. [Google Scholar] [CrossRef] [Green Version]
  12. Wang, H.; Qu, Z.; Dong, S.; Xie, H.; Tang, C. Superior performance of Fe1-xWxOδ for the selective catalytic reduction of NOx with NH3: Interaction between Fe and W. Environ. Sci. Technol. 2016, 50, 13511–13519. [Google Scholar] [CrossRef]
  13. Liu, F.; Shan, W.; Lian, Z.; Liu, J.; He, H. The smart surface modification of Fe2O3 by WOx for significantly promoting the selective catalytic reduction of NOx with NH3. Appl. Catal. B Environ. 2018, 230, 165–176. [Google Scholar] [CrossRef]
  14. Xin, Y.; Zhang, N.; Li, Q.; Zhang, Z.; Cao, X.; Zheng, L.; Zeng, Y.; Anderson, J.A. Selective catalytic reduction of NO with NH3 over short-range ordered W-O-Fe structures with high thermal stability. Appl. Catal. B Environ. 2018, 229, 81–87. [Google Scholar] [CrossRef] [Green Version]
  15. Zhang, N.; Xin, Y.; Wang, X.; Shao, M.; Li, Q.; Ma, X.; Qi, Y.; Zheng, L.; Zhang, Z. Iron-niobium composite oxides for selective catalytic reduction of NO with NH3. Catal. Commun. 2017, 97, 111–115. [Google Scholar] [CrossRef]
  16. Liu, F.; He, H.; Zhang, C. Novel iron titanate catalyst for the selective catalytic reduction of NO with NH3 in the medium temperature range. Chem. Commun. 2008, 17, 2043–2045. [Google Scholar] [CrossRef] [PubMed]
  17. Mou, X.; Zhang, B.; Li, Y.; Yao, L.; Wei, X.; Su, D.S.; Shen, W. Rod-shaped Fe2O3 as an efficient catalyst for the selective reduction of nitrogen oxide by ammonia. Angew. Chem. Int. Ed. Engl. 2012, 51, 2989–2993. [Google Scholar] [CrossRef] [PubMed]
  18. Qu, W.; Chen, Y.; Huang, Z.; Gao, J.; Zhou, M.; Chen, J.; Li, C.; Ma, Z.; Chen, J.; Tang, X. Active tetrahedral iron sites of γ-Fe2O3 catalyzing NO reduction by NH3. Environ. Sci. Technol. Let. 2017, 4, 246–250. [Google Scholar] [CrossRef]
  19. Ning, W.; Wang, T.; Chen, H.; Yang, X.; Jin, Y. The effect of Fe2O3 crystal phases on CO2 hydrogenation. PLoS ONE 2017, 12, e0182955. [Google Scholar] [CrossRef] [Green Version]
  20. Apte, S.K.; Naik, S.D.; Sonawane, R.S.; Kale, B.B.; Baeg, J.O. Synthesis of nanosize-necked structure α- and γ-Fe2O3 and its photocatalytic activity. J. Am. Ceram. Soc. 2007, 90, 412–414. [Google Scholar] [CrossRef]
  21. Shi, X.; Chu, B.; Wang, F.; Wei, X.; Teng, L.; Fan, M.; Li, B.; Dong, L.; Dong, L. Mn-modified CuO, CuFe2O4 and gamma-Fe2O3 three-phase strong synergistic coexistence catalyst system for NO reduction by CO with wider active window. ACS Appl. Mater. Interfaces 2018, 10, 40509–40522. [Google Scholar] [CrossRef] [PubMed]
  22. Lai, J.; Shafi, K.V.P.M.; Loos, K.; Ulman, A.; Lee, Y.; Vogt, T.; Estournès, C. Doping γ-Fe2O3 nanoparticles with Mn(III) suppresses the transition to the α-Fe2O3 structure. J. Am. Chem. Soc. 2003, 125, 11470–11471. [Google Scholar] [CrossRef]
  23. Machala, L.; Tucek, J.; Zboril, R. Polymorphous transformations of nanometric Iron(III) Oxide: A review. Chem. Mater. 2011, 23, 3255–3272. [Google Scholar] [CrossRef]
  24. Geng, Y.; Xiong, S.; Li, B.; Peng, Y.; Yang, S. The promotion of H3PW12O40 grafting on NOx abatement over γ-Fe2O3: Performance and reaction mechanism. Ind. Eng. Chem. Res. 2018, 57, 13661–13670. [Google Scholar] [CrossRef]
  25. Chernyshova, I.V.; Hochella, M.F., Jr.; Madden, A.S. Size-dependent structural transformations of hematite nanoparticles. 1. Phase transition. Phys. Chem. Chem. Phys. 2007, 9, 1736–1750. [Google Scholar] [CrossRef]
  26. Yang, S.; He, H.; Wu, D.; Chen, D.; Liang, X.; Qin, Z.; Fan, M.; Zhu, J.; Yuan, P. Decolorization of methylene blue by heterogeneous Fenton reaction using Fe3−xTixO4 (0 ≤ x ≤ 0.78) at neutral pH values. Appl. Catal. B Environ. 2009, 89, 527–535. [Google Scholar] [CrossRef]
  27. Lee, J.; Kwak, S.Y. Mn-doped maghemite (gamma-Fe2O3) from metal-organic framework accompanying redox reaction in a bimetallic system: The structural phase transitions and catalytic activity toward NOx removal. ACS Omega 2018, 3, 2634–2640. [Google Scholar] [CrossRef] [PubMed]
  28. Li, C.; Xiong, Z.; Du, Y.; Ning, X.; Li, Z.; He, J.; Qu, X.; Lu, W.; Wu, S.; Tan, L. Promotional effect of tungsten modification on magnetic iron oxide catalyst for selective catalytic reduction of NO with NH3. J. Energy Inst. 2020. [Google Scholar] [CrossRef]
  29. Bharathi, S.; Nataraj, D.; Seetha, M.; Mangalaraj, D.; Ponpandian, N.; Masuda, Y.; Senthil, K.; Yong, K. Controlled growth of single-crystalline, nanostructured dendrites and snowflakes of α-Fe2O3: Influence of the surfactant on the morphology and investigation of morphology dependent magnetic properties. CrystEngComm 2010, 12, 373–382. [Google Scholar] [CrossRef] [Green Version]
  30. Sun, X.; Chen, X.; Deng, Z.; Li, Y. A CTAB-assisted hydrothermal orientation growth of ZnO nanorods. Mater. Chem. Phys. 2002, 78, 99–104. [Google Scholar] [CrossRef]
  31. Wu, S.; Yao, X.; Zhang, L.; Cao, Y.; Zou, W.; Li, L.; Ma, K.; Tang, C.; Gao, F.; Dong, L. Improved low temperature NH3-SCR performance of FeMnTiOx mixed oxide with CTAB-assisted synthesis. Chem. Commun. 2015, 51, 3470–3473. [Google Scholar] [CrossRef]
  32. Li, Y.; Han, X.; Hou, Y.; Guo, Y.; Liu, Y.; Cui, Y.; Huang, Z. In situ preparation of mesoporous iron titanium catalysts by a CTAB-assisted process for NO reduction with NH3. Appl. Catal. A Gen. 2018, 559, 146–152. [Google Scholar] [CrossRef]
  33. Ding, S.; Liu, F.; Shi, X.; He, H. Promotional effect of Nb additive on the activity and hydrothermal stability for the selective catalytic reduction of NO with NH3 over CeZrO catalyst. Appl. Catal. B Environ. 2016, 180, 766–774. [Google Scholar] [CrossRef]
  34. Lian, Z.; Liu, F.; He, H.; Shi, X.; Mo, J.; Wu, Z. Manganese–niobium mixed oxide catalyst for the selective catalytic reduction of NOx with NH3 at low temperatures. Chem. Eng. J. 2014, 250, 390–398. [Google Scholar] [CrossRef]
  35. Fudong, L.; Hong, H. Structure-activity relationship of iron titanate catalysts in the selective catalytic reduction of NOx with NH3. J. Phys. Chem. C. 2010, 114, 16929–16936. [Google Scholar] [CrossRef]
  36. Xu, W.; Yu, Y.; Zhang, C.; He, H. Selective catalytic reduction of NO by NH3 over a Ce/TiO2 catalyst. Catal. Commun. 2008, 9, 1453–1457. [Google Scholar] [CrossRef]
  37. Xu, H.; Liu, J.; Zhang, Z.; Liu, S.; Lin, Q.; Wang, Y.; Dai, S.; Chen, Y. Design and synthesis of highly-dispersed WO3 catalyst with highly effective NH3–SCR activity for NOx abatement. ACS Catal. 2019, 11557–11562. [Google Scholar] [CrossRef]
  38. Yamada, Y.; Nishida, N. Iron-based nanoparticles and their Mössbauer spectra. Radioisotopes 2019, 68, 125–143. [Google Scholar] [CrossRef] [Green Version]
  39. Liu, T.; Guo, L.; Tao, Y.; Wang, Y.B.; Wang, W.D. Synthesis and interfacial structure of nanoparticles γ-Fe2O3 coated with surfactant DBS and CTAB. Nanostruct. Mater. 1999, 11, 487–492. [Google Scholar] [CrossRef]
  40. Guo, L.; Wu, Z.; Liu, T.; Yang, S. The effect of surface modification on the microstructure and properties of γ-Fe2O3 nanoparticles. Physica E 2000, 8, 199–203. [Google Scholar] [CrossRef]
  41. Zhang, X.; Niu, Y.; Meng, X.; Li, Y.; Zhao, J. Structural evolution and characteristics of the phase transformations between α-Fe2O3, Fe3O4 and γ-Fe2O3 nanoparticles under reducing and oxidizing atmospheres. CrystEngComm 2013, 15, 8166–8172. [Google Scholar] [CrossRef]
  42. Ramos Guivar, J.A.; Sanches, E.A.; Bruns, F.; Sadrollahi, E.; Morales, M.A.; López, E.O.; Litterst, F.J. Vacancy ordered γ-Fe2O3 nanoparticles functionalized with nanohydroxyapatite: XRD, FTIR, TEM, XPS and Mössbauer studies. Appl. Surf. Sci. 2016, 389, 721–734. [Google Scholar] [CrossRef]
  43. Özdemir, Ö.; Dunlop, D.J.; Berquó, T.S. Morin transition in hematite: Size dependence and thermal hysteresis. Geochem. Geophys. Geosyst. 2008, 9, 1–12. [Google Scholar] [CrossRef]
  44. Cheng, C.; Lin, C.; Chiang, R.; Lin, C.; Lyubutin, I.; Alkaev, E.; Lai, H. Synthesis of monodisperse magnetic iron oxide nanoparticles from submicrometer hematite powders. Cryst. Growth Des. 2008, 8, 877–883. [Google Scholar] [CrossRef]
  45. Yu, Y.; Tan, W.; An, D.; Wang, X.; Liu, A.; Zou, W.; Tang, C.; Ge, C.; Tong, Q.; Sun, J.; et al. Insight into the SO2 resistance mechanism on γ-Fe2O3 catalyst in NH3-SCR reaction: A collaborated experimental and DFT study. Appl. Catal. B Environ. 2021, 281. [Google Scholar] [CrossRef]
  46. Zhang, J.; Huang, Z.; Du, Y.; Wu, X.; Shen, H.; Jing, G. Atomic-scale insights into the nature of active sites in Fe2O3-supported submonolayer WO3 catalysts for selective catalytic reduction of NO with NH3. Chem. Eng. J. 2020, 381, 122668. [Google Scholar] [CrossRef]
  47. Nguyen, T.P.T.; Yang, K.H.; Kim, M.H.; Hong, Y.S. Selective catalytic reduction of NO by NH3 over Fe2O3-promoted V2O5/TiO2-based catalysts with high Fe2O3-to-V2O5 ratios. Catal. Today 2020. [Google Scholar] [CrossRef]
  48. Zhu, N.; Shan, W.; Lian, Z.; Zhang, Y.; Liu, K.; He, H. A superior Fe-V-Ti catalyst with high activity and SO2 resistance for the selective catalytic reduction of NO with NH3. J. Hazard. Mater. 2019, 382, 120970. [Google Scholar] [CrossRef]
  49. Wang, P.; Chen, S.; Gao, S.; Zhang, J.; Wang, H.; Wu, Z. Niobium oxide confined by ceria nanotubes as a novel SCR catalyst with excellent resistance to potassium, phosphorus, and lead. Appl. Catal. B Environ. 2018, 231, 299–309. [Google Scholar] [CrossRef]
  50. Lian, Z.; Shan, W.; Zhang, Y.; Wang, M.; He, H. Morphology-dependent catalytic performance of NbOx/CeO2 catalysts for selective catalytic reduction of NOx with NH3. Ind. Eng. Chem. Res. 2018, 57, 12736–12741. [Google Scholar] [CrossRef]
  51. Liu, F.; Asakura, K.; He, H.; Shan, W.; Shi, X.; Zhang, C. Influence of sulfation on iron titanate catalyst for the selective catalytic reduction of NOx with NH3. Appl. Catal. B Environ. 2011, 103, 369–377. [Google Scholar] [CrossRef]
  52. Mendiola-Alvarez, S.Y.; Hernández-Ramírez, A.; Guzmán-Mar, J.L.; Maya-Treviño, M.L.; Caballero-Quintero, A.; Hinojosa-Reyes, L. A novel P-doped Fe2O3-TiO2 mixed oxide: Synthesis, characterization and photocatalytic activity under visible radiation. Catal. Today 2019, 328, 91–98. [Google Scholar] [CrossRef]
  53. Pu, Z.; Liu, Q.; Asiri, A.M.; Luo, Y.; Sun, X.; He, Y. 3D macroporous MoS2 thin film: In situ hydrothermal preparation and application as a highly active hydrogen evolution electrocatalyst at all pH values. Electrochim. Acta. 2015, 168, 133–138. [Google Scholar] [CrossRef]
  54. Hanawa, T.; Hiromoto, S.; Asami, K. Characterization of the surface oxide film of a Co-Cr-Mo alloy after being located in quasi-biological environments using XPS. Appl. Surf. Sci. 2001, 183, 68–75. [Google Scholar] [CrossRef]
  55. Zhang, J.; Huang, Z.; Du, Y.; Wu, X.; Shen, H.; Jing, G. Alkali-poisoning-resistant Fe2O3/MoO3/TiO2 catalyst for the selective reduction of NO by NH3: The role of the MoO3 safety buffer in protecting surface active sites. Environ. Sci. Technol. 2020, 54, 595–603. [Google Scholar] [CrossRef] [PubMed]
  56. France, L.J.; Li, W.; Zhang, Y.; Mu, W.; Chen, Z.; Shi, J.; Zeng, Q.; Li, X. A superior Fe-Zr mixed oxide catalyst for the simultaneous reduction of NO and SO2 with CO. Appl. Catal. B Environ. 2020, 269, 118822. [Google Scholar] [CrossRef]
  57. Zhang, Z.; Li, R.; Wang, M.; Li, Y.; Tong, Y.; Yang, P.; Zhu, Y. Two steps synthesis of CeTiOx oxides nanotube catalyst: Enhanced activity, resistance of SO2 and H2O for low temperature NH3-SCR of NOx. Appl. Catal. B Environ. 2021, 282. [Google Scholar] [CrossRef]
  58. Xu, Q.; Fang, Z.; Chen, Y.; Guo, Y.; Guo, Y.; Wang, L.; Wang, Y.; Zhang, J.; Zhan, W. Titania-samarium-manganese composite oxide for the low-temperature selective catalytic reduction of NO with NH3. Environ. Sci. Technol. 2020, 54, 2530–2538. [Google Scholar] [CrossRef] [PubMed]
  59. Han, L.; Gao, M.; Feng, C.; Shi, L.; Zhang, D. Fe2O3-CeO2@Al2O3 nanoarrays on Al-mesh as SO2-tolerant monolith catalysts for NOx reduction by NH3. Environ. Sci. Technol. 2019, 53, 5946–5956. [Google Scholar] [CrossRef]
  60. Zhang, X.; Shen, Q.; He, C.; Ma, C.; Cheng, J.; Li, L.; Hao, Z. Investigation of selective catalytic reduction of N2O by NH3 over an Fe–mordenite catalyst: Reaction mechanism and O2 effect. ACS Catal. 2012, 2, 512–520. [Google Scholar] [CrossRef]
  61. Shu, Y.; Sun, H.; Quan, X.; Chen, S. Enhancement of catalytic activity over the iron-modified Ce/TiO2 catalyst for selective catalytic reduction of NOx with ammonia. J. Phys. Chem. C 2012, 116, 25319–25327. [Google Scholar] [CrossRef]
  62. Zhu, B.; Zi, Z.; Sun, Y.; Fang, Q.; Xu, J.; Song, W.; Yu, H.; Liu, E. Enhancing low-temperature SCR de-NOx and alkali metal poisoning resistance of a 3Mn10Fe/Ni catalyst by adding Co. Catal. Sci. Technol. 2019, 9, 3214–3225. [Google Scholar] [CrossRef]
  63. Yang, J.; Ren, S.; Zhou, Y.; Su, Z.; Yao, L.; Cao, J.; Jiang, L.; Hu, G.; Kong, M.; Yang, J.; et al. In situ IR comparative study on N2O formation pathways over different valence states manganese oxides catalysts during NH3–SCR of NO. Chem. Eng. J. 2020, 397, 125446. [Google Scholar] [CrossRef]
  64. Yang, S.; Wang, C.; Li, J.; Yan, N.; Ma, L.; Chang, H. Low temperature selective catalytic reduction of NO with NH3 over Mn–Fe spinel: Performance, mechanism and kinetic study. Appl. Catal. B Environ. 2011, 110, 71–80. [Google Scholar] [CrossRef]
  65. Lv, Y.; Yao, W.; Zong, R.; Zhu, Y. Fabrication of wide-range-visible photocatalyst Bi2WO6-x nanoplates via surface oxygen vacancies. Sci. Rep. 2016, 6, 19347. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  66. Li, K.; Chen, J.; Bai, B.; Zhao, S.; Hu, F.; Li, J. Bridging the reaction route of toluene total oxidation and the structure of ordered mesoporous Co3O4: The roles of surface sodium and adsorbed oxygen. Catal. Today 2017, 297, 173–181. [Google Scholar] [CrossRef]
  67. Xu, L.; Niu, S.; Lu, C.; Zhang, Q.; Li, J. Influence of calcination temperature on Fe0.8Mg0.2O catalyst for selective catalytic reduction of NO with NH3. Fuel 2018, 219, 248–258. [Google Scholar] [CrossRef]
  68. Wu, S.; Zhang, L.; Wang, X.; Zou, W.; Cao, Y.; Sun, J.; Tang, C.; Gao, F.; Deng, Y.; Dong, L. Synthesis, characterization and catalytic performance of FeMnTiOx mixed oxides catalyst prepared by a CTAB-assisted process for mid-low temperature NH3-SCR. Appl. Catal. A Gen. 2015, 505, 235–242. [Google Scholar] [CrossRef]
  69. Marberger, A.; Elsener, M.; Ferri, D.; Sagar, A.; Schermanz, K.; Krocher, O. Generation of NH3 selective catalytic reduction active catalysts fromdecomposition of supported FeVO4. ACS Catal. 2015, 5, 4180–4188. [Google Scholar] [CrossRef]
  70. Xin, Y.; Li, H.; Zhang, N.; Li, Q.; Zhang, Z.; Cao, X.; Hu, P.; Zheng, L.; Anderson, J.A. Molecular-level insight into selective catalytic reduction of NOx with NH3 to N2 over a highly efficient bifunctional Va-MnOx catalyst at low temperature. ACS Catal. 2018, 8, 4937–4949. [Google Scholar] [CrossRef] [Green Version]
Catalysts 11 00224 g001 550
Figure 1. NOx conversion over all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol %, N2 balance, GHSV = 500,000 h−1.
Figure 1. NOx conversion over all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol %, N2 balance, GHSV = 500,000 h−1.
Catalysts 11 00224 g001
Catalysts 11 00224 g002 550
Figure 2. Arrhenius plots of the reaction rates over all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol %, N2 balance, with the NOx conversion below 25%.
Figure 2. Arrhenius plots of the reaction rates over all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples. Reaction conditions: [NO] = [NH3] = 500 ppm, [O2] = 5 vol %, N2 balance, with the NOx conversion below 25%.
Catalysts 11 00224 g002
Catalysts 11 00224 g003 550
Figure 3. XRD patterns of (a) FeM0.3Ox-C and (b) FeM0.3Ox (M = Nb, Ti, Mo) samples.
Figure 3. XRD patterns of (a) FeM0.3Ox-C and (b) FeM0.3Ox (M = Nb, Ti, Mo) samples.
Catalysts 11 00224 g003
Catalysts 11 00224 g004 550
Figure 4. In situ DRIFTS of adsorption of NH3 species over all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples at 150 °C. Before measurement, each sample was pretreated in 500 ppm NH3/N2 for 0.5 h and then purged by N2 for 0.5 h.
Figure 4. In situ DRIFTS of adsorption of NH3 species over all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples at 150 °C. Before measurement, each sample was pretreated in 500 ppm NH3/N2 for 0.5 h and then purged by N2 for 0.5 h.
Catalysts 11 00224 g004
Catalysts 11 00224 g005 550
Figure 5. XPS spectra of O 1 s over (a) FeNb0.3Ox-C and FeNb0.3Ox; (b) FeTi0.3Ox-C and FeTi0.3Ox; (c) FeMo0.3Ox-C and FeMo0.3Ox.
Figure 5. XPS spectra of O 1 s over (a) FeNb0.3Ox-C and FeNb0.3Ox; (b) FeTi0.3Ox-C and FeTi0.3Ox; (c) FeMo0.3Ox-C and FeMo0.3Ox.
Catalysts 11 00224 g005
Catalysts 11 00224 g006 550
Figure 6. H2-TPR results of all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples in a flow of 10 vol % H2/N2 with the heating rate of 10 °C min−1. Before measurement, the sample was pretreated in N2 at 300 °C for 1 h, and purged by N2 for 0.5 h.
Figure 6. H2-TPR results of all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples in a flow of 10 vol % H2/N2 with the heating rate of 10 °C min−1. Before measurement, the sample was pretreated in N2 at 300 °C for 1 h, and purged by N2 for 0.5 h.
Catalysts 11 00224 g006
Table
Table 1. Activation energy and reaction rate at 260 °C of all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples.
Table 1. Activation energy and reaction rate at 260 °C of all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples.
Ea (kJ mol−1)R2Reaction Rate at 260 °C (mol m−2 s−1)
FeNb0.3Ox-C34.50.9919.9 × 10−9
FeNb0.3Ox40.00.9932.9 × 10−9
FeTi0.3Ox-C28.00.9939.5 × 10−9
FeTi0.3Ox22.60.9996.1 × 10−9
FeMo0.3Ox-C26.30.9939.5 × 10−9
FeMo0.3Ox26.20.9907.1 × 10−9
Table
Table 2. Structural parameters of all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples.
Table 2. Structural parameters of all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples.
Surface Area (m2 g−1)Pore Volume (cm3 g−1)
FeNb0.3Ox-C1720.25
FeNb0.3Ox2100.13
FeTi0.3Ox-C1510.21
FeTi0.3Ox1910.24
FeMo0.3Ox-C990.15
FeMo0.3Ox1670.24
Table
Table 3. Desorption amount of NH3 and percentage of weak-adsorbed NH3 over all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples.
Table 3. Desorption amount of NH3 and percentage of weak-adsorbed NH3 over all the FeM0.3Ox-C and FeM0.3Ox (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-C260 ± 7.01.5 ± 0.0575.9 ± 1.3
FeNb0.3Ox289 ± 7.61.4 ± 0.0772.8 ± 1.1
FeTi0.3Ox-C226 ± 4.71.5 ± 0.0375.9 ± 0.1
FeTi0.3Ox246 ± 12.71.3 ± 0.0874.2 ± 1.2
FeMo0.3Ox-C145 ± 8.31.5 ± 0.1279.2 ± 1.7
FeMo0.3Ox229 ± 15.21.4 ± 0.0877.9 ± 1.2
* with experiments and error bars shown in Figure S3.
Table
Table 4. Surface components of all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples from XPS results.
Table 4. Surface components of all the FeM0.3Ox-C and FeM0.3Ox (M = Nb, Ti, Mo) samples from XPS results.
Proportion of OOα/(Oα + Oβ)Proportion of OαM/Fe
FeNb0.3Ox-C49.4%33.2%16.4%0.49
FeNb0.3Ox48.5%32.6%15.8%0.43
FeTi0.3Ox-C50.7%38.7%19.6%0.72
FeTi0.3Ox48.3%36.4%17.6%0.45
FeMo0.3Ox-C52.5%33.3%17.5%0.97
FeMo0.3Ox49.6%35.0%17.4%0.55
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Share and Cite

MDPI and ACS Style

Zhang, W.; Shi, X.; Gao, M.; Liu, J.; Lv, Z.; Wang, Y.; Huo, Y.; Cui, C.; Yu, Y.; He, H. Iron-Based Composite Oxide Catalysts Tuned by CTAB Exhibit Superior NH3–SCR Performance. Catalysts 2021, 11, 224. https://doi.org/10.3390/catal11020224

AMA Style

Zhang W, Shi X, Gao M, Liu J, Lv Z, Wang Y, Huo Y, Cui C, Yu Y, He H. Iron-Based Composite Oxide Catalysts Tuned by CTAB Exhibit Superior NH3–SCR Performance. Catalysts. 2021; 11(2):224. https://doi.org/10.3390/catal11020224

Chicago/Turabian Style

Zhang, Wenshuo, Xiaoyan Shi, Meng Gao, Jingjing Liu, Zhihui Lv, Yingjie Wang, Yanlong Huo, Chang Cui, Yunbo Yu, and Hong He. 2021. "Iron-Based Composite Oxide Catalysts Tuned by CTAB Exhibit Superior NH3–SCR Performance" Catalysts 11, no. 2: 224. https://doi.org/10.3390/catal11020224

Note that from the first issue of 2016, this journal uses article numbers instead of page numbers. See further details here.

Article Metrics

Back to TopTop