Efficient Purification of Auto-Exhaust Soot Particles Using Hexagonal Fe₂O₃ Nanosheets Decorated with Non-Noble Metals (Ni)

Abstract

Purification of soot particles from automobile exhaust has closely to do with the synergistic effect between catalyst metals. Here, several binary Ni-Fe oxide catalysts were elaborately prepared via a modified solvothermal method. A non-noble-metal (Ni)-modified hexagonal Fe₂O₃ nano-sheet catalyst (Ni−Fe₂O₃) was prepared. The introduced heteroatoms replace some of the Fe atoms, which take up the surface of the [FeO₆] octahedron, and the synergistic effect formed between the heteroatoms which are on the surface and the adjacent Fe atoms promotes the formation of coordination unsaturated ions of the activated reactants. The optimal performance was obtained with the Ni-Fe₂O₃-20 composition, with catalytic soot oxidation resulting in T₅₀, SCO₂^m, E_a and TOF of 366 °C, 99.1%, 72.7 kJ mol⁻¹ and 0.156 min⁻¹ (at 310 °C), respectively. The combination of Ni and Fe₂O₃ cells increases the ratio of Fe³⁺/Fe²⁺, making the interaction among electrons between the Ni, which was proved highly dispersed over the catalyst, and the Fe₂O₃ strong. Both exist on the catalyst surface in the form of NiFe₂O₄. Ni atoms and Fe₂O₃, which demonstrate a synergistic effect, promoting the formation of coordination unsaturated ions of the activated reactants and generating more oxygen vacancies, thus promoting the adsorption of NO and accelerating the ignition of soot in O₂ at a low temperature. The novel Ni-Fe₂O₃-X oxide cocatalyst is an improved noble-free catalyst that promotes the synergistic effect between heteroatoms and metal oxides through surface regulation. This is of great significance for the further development of economic and efficient catalysts for soot particle removal from automobile exhaust.

1. Introduction

Most cities in the world are suffering from the effects of environmental pollution, especially those in developing countries. Among all the problems of environmental pollution, the harm caused by haze has come into the public’s vision in recent years. This pollution mainly comes from small particles in the air. Of all particulate pollutants, particles smaller than 2.5 μm (aerodynamic diameter as the standard) in diameter have the greatest impact on people. On the one hand, haze will reduce the visibility of the environment and cause traffic accidents; on the other hand, smog also seriously affects human life and can cause health problems, which is a security hazard that we cannot ignore. Recent studies have found that diesel exhaust accounts for a large proportion of particulate matter (PM) pollution in China. In view of this situation, it is particularly necessary to regulate the emissions of these pollutants mentioned above in order to minimize the impact on all aspects of human life [1]. For decades, the current level of technology still makes us have to rely on the introduction of precious metals into the catalyst to achieve the purpose of cleaning the exhaust gas. The use of these precious metals to support platinum-group metals (PGMs) on catalysts is most common. However, the limitations of precious metals force us to further research the application of catalysis in automobiles. In other words, the development of safe, low-cost, easy to access catalysts can replace precious metals is an urgent need for such large-scale deployment [2].

Catalytic diesel particulate filter (CDPF) technology stands out among many approaches to the problem of PM emissions. It is recognized as by far the most effective technology for reducing PM emissions. The key to this technology lies in the research of catalyst. Because the temperature window of diesel exhaust gas is (200 ~ 500 °C), the catalyst developed by CDPF can completely oxidize soot, which can only be oxidized at high temperature, in this temperature window, so as to achieve the purpose of efficient cleaning. At present, catalysts based on noble metals are widely regarded as efficacious catalysts for diesel soot oxidation. However, the high cost and difficulty of obtaining noble metals limit their application. Non-noble metals, such as alkaline metal oxides [3], transition metal oxides [4], perovskite-like oxides [5] and ceria-based oxides [6], have been investigated. It is found that they have good catalytic performance for soot combustion, which is not weaker than that of noble metal catalysts. Among them, the catalyst with Fe₂O₃ as the base stands out.

In recent years, Fe-based catalysts have been studied, including Pt/Fe₂O₃ [7] and Fe₂O₃@CeO₂ [8], in which noble metals and rare earth metals are inevitably used. This has caused a lot of trouble in terms of raw material access and economics. In the process of soot catalytic oxidation, although SnO₂ and Cr2O₃ catalysts plays a significant role in the adsorption of NO [9], and promote the adsorption of CO through pre-adsorbed NO, SnO₂ is similarly more inclined to support precious metals (such as Pt, Ce) as catalysts [10]. For Mn-Fe₂O₃ catalysts [11], although the cost problem has been solved, the performance of catalytic oxidation of soot needs to be improved. Based on the existing research results of different researchers, non-noble-metal catalyst (Ni-Fe₂O₃-X)-modified hexagonal Fe₂O₃ nanosheets were successfully synthesized by the hydrothermal method [12] in this paper. Considering all aspects of soot removal performance of chemical agents, the catalyst formed by introducing heteroatom Ni into Fe₂O₃ has greatly adsorbed and activated NO and O₂, and thus greatly improved the soot removal performance [13]. Incorporated with the characterization results, the synergistic effect and catalytic mechanism of Ni-Fe₂O₃-X are described in detail, which provide a basis for the further progress of efficacious and inexpensive and clean catalysts for the elimination of automobile exhaust soot particles [14].

2. Materials and Methods

2.1. Synthesis of Fe₂O₃ and Ni-Fe₂O₃-X Oxide Systems

The Ni-Fe₂O₃-X nanosheet as the catalyst precursor was prepared by a hydrothermal method. The synthesis processes and the details are described as follows: Fe (NO₃)₃· 9H₂O and Ni (NO₃)₂·6H₂O (amounting to 5 mmol) are dissolved in 60 mL deionized water and strongly stirred for 10 min until the solution becomes a transparent and uniform clarified solution. The composition of different catalysts is shown in

Table 1. Prepared catalyst and Fe/Ni ratios.

Sample	Fe (NO3)3·9H2O (mmol)	Ni (NO3)2·6H2O (mmol)
Fe2O3	5.0	0
Ni-Fe2O3-1	4.95	0.05
Ni-Fe2O3-5	4.75	0.25
Ni-Fe2O3-10	4.5	0.5
Ni-Fe2O3-20	4.0	1.0
Ni-Fe2O3-30	3.5	1.5

. After obtaining a clarified solution, 1.6 mL ammonium hydroxide solution is slowly added to the solution and stirred thoroughly until there is no more precipitation. Then, the solid–liquid mixture described above is transferred into an autoclave (100 mL). The heating time is 12 h, and the heating temperature is 180 °C. After the mixture cools naturally, it is centrifuged and collected. The collected sediment is washed 3 times. The solvent used is deionized water. After that, the solid is washed once with centrifugal ethanol and dried in a vacuum for about 24 h. The precursor of Ni-Fe₂O₃-X is procured. Finally, before the precursor was ready for use, it needs to be calcined. The calcination temperature is 500 °C and the calcination time is about 4 h. The product is named as the Ni-Fe₂O₃-X catalyst (X refers to the total molar content of Ni in all metals).

2.2. Materials Characterization

The details of the characterization methods are shown in the Supporting Information.

2.3. Catalytic Performance Evaluation

The catalytic activity of Ni-Fe₂O₃-X catalyst for temperature programmed oxidation (TPO) of soot combustion in a fixed bed tubular quartz system was studied. The initial temperature of each test is 150 °C, the final temperature is 600 °C, and the warming rate is 2 °C min⁻¹. The soot particles are commercial soot particles purchased from Degusse (Printex-U). The soot particles are used to replace the solid components in soot exhaust. The catalyst amounting to 100 mg and soot amounting to 10 mg are added and evenly mixed in an agate bowl. The contact mode of the two is loose contact. The details can be found in the Supporting Information.

3. Results

3.1. XRD Analyses

The phase structure is an essential part of understanding the catalyst and can be visually understood through XRD patterns. Figure 1 illustrates nine main peaks in the position of 24.1, 33.2, 35.6, 40.9, 49.5, 54.1, 57.6, 62.4 and 64.0^o belonging to the cubic phase Fe₂O₃ (JCPDS: 33-0664) [15]. For Ni-Fe₂O₃-X (X refers to the total molar content of Ni in all metals) catalysts, in addition to the original diffraction peak of Fe₂O₃, three diffraction peaks at 30.3, 35.7 and 62.9^o were observed, belonging to the (220), (311)and (440) faces of trevorite NiFe₂O₄ (JCPDS: 10-0325), respectively [16]. In comparison with the weak diffraction peaks of pure NiO (JCPDS: 44-1159) and Ni₂O₃ (JCPDS: 14-0481), no diffraction peak was observed in the Ni-Fe₂O₃ catalyst, indicating that although the content of Ni atoms increased, it was not enriched in the form of NiO and Ni₂O₃ on the surface of the Fe₂O₃ nanosheets. In other words, the crystallinity of the catalyst decreased as the amount of Ni atoms introduced gradually increased, indicating that Ni was successfully introduced into the lattice of Fe₂O₃, replacing the Fe atoms in the hematite structure [17] and the NiFe₂O₄ structure was formed on the surface of the catalyst. For catalyst Ni-Fe₂O₃-20, as shown in Figure S4, XRD before and after the reaction shows that the remaining positions remain unchanged, although the intensity of some peaks decreases, indicating that the phase structure of the catalyst before and after the reaction does not alter basically. This phenomenon also reflects the stability of the catalyst [18].

3.2. Raman Spectra

In the study of the catalyst’s basic structure, Raman spectroscopy is able to be used to identify the prepared Fe₂O₃ and Ni-Fe₂O₃-X catalysts under visible light irradiation at 532 nm wavelength [19], and the results are illustrated in Figure 2. The prepared catalyst has an distinguishable Raman peak centered at 138 cm⁻¹, which can be considered to correspond to the vibration of the Fe-O bond [20]. Comparing the curves corresponding to the four catalysts (from a to d), it can be seen that no other distinguishable Raman peaks appeared after the introduction of Ni, and these Raman peaks all show an obvious blue shift, indicating the successful introduction of Ni [21]. In the Raman spectrum, the original Ni-Fe spinel shows five identifiable Raman bands, i.e., where the numbers and dotted lines were marked in the figure, working in concert with the E_g (326 cm⁻¹), A_1g (688 cm⁻¹) and T_2g (225, 480, 603 cm⁻¹) [22] vibration modes, indicating the existence of a NiFe₂O₄ spinel structure. The intensity of these Raman peaks increases gradually with the increase in Ni content. The results correspond to the presence of NiFe₂O₄ crystal faces in XRD, thus proving the existence of an Fe-Ni cooperative effect [23].

3.3. TEM and EDS Mapping Images

The catalytic performance in the heterogeneous catalysis process is mainly determined by the morphology of the catalyst and the exposed surface [24]. This property can be measured by TEM techniques. Correlation results are displayed in Figure 3A,B. Pure Fe₂O₃ presents a polyhedral structure, the side length of which is in the vicinity of 100–150 nm [25]. After introducing heteroatoms (Ni), the polyhedral structure turns into the nanoparticle shape. This phenomenon is consistent with the decrease in crystallinity obtained by XRD. Ni affects the growth of Fe₂O₃ and indirectly promotes the formation of NiFe₂O₄ on the surface [26]. As shown in Figure 3C, the lattice spacing is 2.96Å, and the exposed facet on the side of the structure is the {110} facet of the Fe₂O₃ polyhedron. After introducing Ni, as shown in Figure 3D, lattice fringes with spacing of 2.51Å and 2.96Å are revealed, consistent with the (220) plane of NiFe₂O₄. In addition, EDS elemental mapping is used to study the distribution of elements. In Figure 3G–J, the results are shown. It can be seen that O represented by red, Fe-K represented by orange, Ni represented by green and Fe-L represented by yellow are evenly distributed throughout the image, indicating that Ni is successfully and evenly dispersed in the Fe₂O₃ nanoparticles. In summary, a Ni-Fe₂O₃ catalyst with Fe atoms uniformly replaced by heteroatoms (Ni) in Fe₂O₃ was successfully prepared.

3.4. The Results of XPS Spectra

The surface elemental valence state and composition were measured by XPS measurements. It is of significance for the catalytic activity of heterogeneous catalysis [27]. The resulting Fe 2p, Ni 2p and O 1s spectra are illustrated in Figure 4. As shown in Figure 4A, Fe²⁺ (707.7, 721.1 eV) and Fe³⁺ (709.1 and 722.6 eV) of Fe ions are present on the surface of Fe₂O₃ and Ni-Fe₂O₃-X samples after deconvolving, and their peaks are filled with blue and orange, respectively. Additionally, as shown in Figure 4B and Figure S5, Ni-Fe₂O₃-X catalysts exhibit four main peaks at 852.9, 870.3, 855.0 and 872.2 eV, which are attributed to the Ni 2p_1/2 and 2p_3/2 spin orbits, respectively [28]. In addition, the presence of two strong Ni²⁺ satellites (859.3 eV and 876.9 eV) in the Ni 2p spectrum indicates that the principal valence state of Ni is +2. Table S2 summarizes the containing of Fe²⁺ and Fe³⁺ and the relative ratio of Fe²⁺ to Fe³⁺ (Ra). Compared with all samples, the Fe³⁺ ion is the main type of iron, and its proportion in Fe₂O₃ is 76.1%. After the introduction of heteroatoms, the Ra value of Fe₂O₃ catalyst is much greater than that of pure Fe₂O₃ (0.339), revealing the existence of Ni in the lattice of Fe₂O₃, replacing Fe²⁺ in the form of Ni²⁺. Associated with XRD results, Ni tends to replace Fe to form the structure of NiFe₂O₄ on the catalyst surface, which also explains why the principal valence state of Ni is +2 [29]. In general, the presence of Ni²⁺ ions are concomitant with the formation of O_Vs, which signally affects the catalytic performance. The introduced heteroatoms indeed boost the formation of O_Vs. At the same time, the composition of Ni³⁺-O_Vs-Fe²⁺ can constitute an active site to promote the oxidation of soot [9]. After the introduction of surface defects, as shown in Figure S6, the fraction of Ni²⁺ ions increased from 15% to 25%. Additionally, as shown in Figure 4C,D, the content of O_A showed a trend of decreasing after rising, and reached the maximum in Ni-Fe₂O₃-20 (0.27), indicating that the surface O_Vs increased and the oxidation capacity was enhanced. On the one hand, it was conducive to the adsorption of O₂. On the other hand, it contributed to the activation of O₂.

3.5. H₂-TPR Profiles

The oxygen vacancy generated in the catalyst satisfies the condition of changing the redox state of the catalyst so as to achieve equilibrium with the actual gas-phase oxygen. This result was confirmed by H₂-TPR [30]. The results are presented in Figure 5. Fe₂O₃ catalysts have two main peaks at 407 °C and 631 °C, due to the gradual reduction of Fe₂O₃ to Fe₃O₄ and eventually from Fe₃O₄ to FeO. H₂-TPR of pure NiO is shown in Figure S9. A clear reduction peak can be seen at 391 °C. This reduction peak curve is generally considered to be the reduction of Ni²⁺ to the metallic-state Ni. When Ni ions partially replace Fe ions at 407 °C, all the reduction peak temperatures of the catalyst shift to the low-temperature direction (388 °C). The most important reason for this is that as the addition of Ni gradually increases, due to the electron donor effect of Ni ions, the electron cloud density of A-site ions is transferred to B-site ions, so that the electron ability of Fe³⁺ is improved, and it is easier to be reduced to Fe²⁺. However, compared with Ni-Fe₂O₃-20, although the content of Ni ions in Ni-Fe₂O₃-30 is increased, the position of the reduction peak is lower than that of the former. A possible reason for this is that the Ni-Fe₂O₃-20 catalyst has a stronger Fe-Ni synergistic effect, and the formed NiFe₂O₄ structure is easier for Fe³⁺ to exchange electrons into Fe²⁺. In addition, in the soot combustion reaction, the active site of the catalyst promotes the oxidation of soot, and the actual reaction temperature is low, so the reduction peak in the low temperature range is the focus of catalytic soot oxidation reaction. It can be seen from Figure 5 that with the increase in Ni content, the reduction peak in the low-temperature section significantly widens the range of reduction temperature. Combined with the analysis of XPS results, this may be due to the charge compensation caused by partial Ni ion substitution, resulting in more chemisorbed oxygen on the catalyst surface. In addition, Ni-Fe₂O₃-20 < Ni-Fe₂O₃-30 < Ni-Fe₂O₃-10 < Ni-Fe₂O₃-5 for the reduction peak temperature at low temperature, and Ni-Fe₂O₃-20 catalyst also has the highest H₂ oxidation amount. These results indicate that an appropriate amount of Ni doping can improve the reducing ability and surface-active oxygen density of the catalyst. Therefore, the Ni-Fe₂O₃-20 catalyst has the best low-temperature reduction capacity, abundant reactive oxygen species and high oxygen mobility, which matches the XPS, TOF and activity results. Therefore, the above experiments can verify that the escalation of adsorption and activation of O₂ on the Fe₂O₃ surface is due to the synergistic effect of Fe-Ni [31]. The strong adsorption ensures the rapid renewal of surface oxygen in a relatively oxygen-rich environment, showing a strong oxidation competence. The strong adsorption caused by this synergistic effect maintained the rapid renewal of the oxygen which is on the surface of the catalyst, and the oxidation capacity was also maintained at a strong level.

3.6. Catalytic Performance

The catalytic oxidation performance of Fe₂O₃ and Ni-Fe₂O₃-X catalysts in the process of soot combustion was tested. The contact mode between catalyst and soot adopts the loose contact mode as described above. The results are shown in Figure 6A and

Table 2. All catalytic values of Fe₂O₃ and Ni−Fe₂O₃-X catalysts for soot removal by loose contact.

	T10(oC)	T50(oC)	T90(oC)	Sco2m (%)	R (μmol g−1 min−1)	O* Amount (μmol g−1)	TOF (min−1)	H2 Consumption (μmol g−1)	Ea (kJ mol−1)
Soot	461	584	648	65.2	-	-	-	-	-
Fe2O3	390	494	526	84.1	4.4	79.6	0.055	959.7	116.4
Ni-Fe2O3-1	351	414	450	98.4	12.1	89.4	0.135	1118.6	95.6
Ni-Fe2O3-5	338	398	427	96.0	17.3	121.4	0.142	979.3	94.7
Ni-Fe2O3-10	324	398	434	99.4	18.7	126.6	0.147	1417.0	94.5
Ni-Fe2O3-20	310	366	402	99.1	21.0	134.0	0.156	1144.9	72.7
Ni-Fe2O3-30	312	386	429	99.7	17.9	136.2	0.143	1692.7	88.9
Pt2/Fe2O3	297	365	418	98.8	22.5	136.8	0.164	2233.7	69.7

. When tested in pure soot without catalyst, the T₅₀ is 585 °C, and the SCO₂^m is only 65.2%. As shown in Figure S3, compared with common Fe₂O₃ (T₅₀ = 488 °C), Fe₂O₃ prepared in this experiment (T₅₀ = 474 °C) has certain advantages. After doping Ni into the Fe₂O₃ catalyst, it can be clearly observed from Figure 6A that the conversion curve of soot moves from 494 °C towards a low temperature (366 °C). The drop is 128 °C, indicating that the catalytic oxidation efficiency is significantly improved by the catalyst. As shown in Table 2, compared with a pure Fe₂O₃ or NiO catalyst, the soot catalytic activity and CO₂ selectivity of Ni-Fe₂O₃-X catalysts are significantly improved. As shown in Figure S7, the T₅₀ value of NiO is 437 °C, which exclude the contribution of the pure NiO catalyst to soot oxidation. By comparison, the T₅₀ value of Ni-Fe₂O₃ catalysts can be reduced by at least 170 °C on the basis of pure soot combustion, and the selectivity of CO₂ is always close to 100%, which indicates that the incorporation of Ni into the Fe₂O₃ catalyst as the active site is of importance in the oxidation efficiency of soot. Among all catalysts, the catalytic oxidation activity of Ni-Fe₂O₃-20 on soot is the best; T₅₀ is 366 °C, and the value of SCO₂^m is 99.1%, close to 100%. With the enhancement of Ni content, the catalytic oxidation activity of soot decreased. It can be seen from XRD that the NiFe₂O₄ crystal phase on the surface weakens with the increase in Ni incorporation, indicating that the synergistic effect of Ni-Fe also decreases gradually, which is consistent with the change in activity. As shown in Table 1, taking Pt/Fe₂O₃ as an example [7], it can be found that T₁₀, T₅₀ and T₉₀ (297, 365, 418 °C) of the catalyst are all at a low temperature after adding Pt into Fe₂O₃ by the dipping method. However, the Ni-Fe₂O₃-20 catalyst does not introduce any noble metals, and the T₅₀ value is similar to Pt/Fe₂O₃. This kind of catalyst, without blending noble metals, has higher economic benefits, and these metals are easy to obtain which brings great help to future research and exploration.

In the process of understanding the intrinsic properties of the catalyst, TOF can accurately react to it. Influenced by the kinetic regime, the test temperature of TOF was set at 300 °C. Ro is defined as the ratio of reaction rate values (μmol g^{− 1}min^{− 1}), which is calculated by fitting the slope of the cumulative CO_x quantity (μmol g⁻¹) to the reaction time (min) curve. At the same time, the O* quantity can be determined in the same way. Unlike the process for determining Ro, there is no O₂ in the process for determining the amount of O*. The Fe₂O₃ catalyst’s reaction rate demonstrates the least ideal situation at 300 °C, the value of which is 4.4 μmol g⁻¹ min⁻¹. Compared with the former, Ni-Fe₂O₃-X catalysts display a higher reaction rate during soot purification. Particularly, Ni-Fe₂O₃-20 catalysts reveal a reaction rate of 21.0 μmol g⁻¹ min⁻¹. Rate is also correlated with activity. It can be seen from the above that this catalyst also has the best activity, with a T₁₀ of 310 °C. Conversely, active oxygen (O* amount) is also an important parameter affecting the TOF value, which is determined by isothermal anaerobic titration. The O* quantity can be obtained by calculation, and the results are summarized in Table 2. By comparison, it can be found that the O* amount of the catalysts matches the result of the Ro value. In the midst of all catalysts, the O* amount of Fe₂O₃ catalyst is the lowest value at 79.6 μmol g⁻¹. From the changing trend of the lines after the introduction of Ni in the picture, it can be clearly observed that the O* density exhibits a slight increase. Among them, the Ni-Fe₂O₃-20 catalyst’s O* density is 134.0 μmol g⁻¹. It outperforms the other catalysts by a surprising margin. This result, compared to the worst Fe₂O₃ catalyst, is almost a 1.7-fold increase. Herein, the value of TOF over Fe₂O₃ and Ni-Fe₂O₃-X catalysts can be calculated via isothermal reactions, and is gathered in Table 1. The same conclusion is also shown in Figure S1. Based on the results of TOF value, it is noted that the introduced Ni provided extra active sites to enhance the adsorption–activation capacity for gaseous O₂, boosting the removal of soot particles. Among all, the Ni-Fe₂O₃-20 catalyst presents the highest TOF value (0.156 h⁻¹ at 300 °C). The conclusion can be proved by the consequence that Ni-Fe₂O₃-20 has the best intrinsic activity. As another important parameter to evaluate the intrinsic activity, the apparent activation energy (Ea) represents the ease of the reaction. This performance affects the frequency of soot conversion. The method commonly used to calculate Ea is the Ozawa method. The calculated results are shown in Figure 6C and Table 2. The Ea value of the Fe₂O₃ catalyst is 116.4 kJ mol⁻¹. With the substitution of Fe atoms by Ni atoms, the Ni-Fe₂O₃-20 catalyst presents the lowest Ea value (72.7 kJ mol⁻¹). The results correspond to the result of the trend of TOF values, and are similar to the ignition activity [32]. Thus, the Ni-Fe₂O₃-X catalyst can perform the best elimination efficiency. In practical applications, it is also necessary to consider the cyclic stability of the catalyst. This performance determines whether the catalyst can be reused to achieve cost savings. It is known that the Ni-Fe₂O₃-20 catalyst has the best catalytic performance among all catalysts, so this catalyst was selected to carry out the same soot-TPO test four times. As shown in Figure 6D, the values of T₁₀, T₅₀ and T₉₀ almost keep in a certain range. Moreover, after four cycles, the SCO₂^m also remains at a high level. The value is 99.1%. This phenomenon implies its excellent stability for catalytic soot purification. The hexagonal nanosheet retains its original shape after four cycles (Figure S2) and other particles were not observed on the surface of Fe₂O₃. By analyzing this representation, the following conclusions that Ni atoms have been successfully introduced into Fe₂O₃ to form a more stable structure can be drawn. Such non-noble-metal catalysts are cheap, readily available and highly effective in eliminating the catalytic activity of soot, which renders them promising candidates for soot removal, especially the Ni-Fe₂O₃-20 catalyst. Additionally, as shown in Figure S2, TEM and HRTEM images of the Ni-Fe₂O₃-uesd catalyst indicates that the catalyst presents a polyhedral structure after use. This also proves the stability of the catalysts.

Figure 6. (A) Soot-TPO tests; (B) cumulative conversion amounts of soot particles during isothermal oxidation reaction of 300 °C; (C) Ozawa plots for soot conversion of 50% over Fe₂O₃ and Ni−Fe₂O₃-X catalysts; (D) the cycle stability of Ni-Fe₂O₃-20 catalysts [33].

Figure 6. (A) Soot-TPO tests; (B) cumulative conversion amounts of soot particles during isothermal oxidation reaction of 300 °C; (C) Ozawa plots for soot conversion of 50% over Fe₂O₃ and Ni−Fe₂O₃-X catalysts; (D) the cycle stability of Ni-Fe₂O₃-20 catalysts [33].

3.7. Surface Chemical State of Ni-Fe₂O₃-X Catalysts

It is a prerequisite to determine the synergistic effect of Fe and Ni to clarify the oxidation reaction pathway of soot. As can be seen from Figure 7A and Figure S8, the Ni-Fe₂O₃-20 catalyst’s catalytic performance is T₅₀ = 366 °C. The air condition includes 40 ppm NO and the catalyst is in loose contact with the soot. The identical catalysts without NO have a noticeable drop in performance, and the activity is decreased by 22 °C and the CO₂ selectivity is decreased by 8.3% as well due to the strong oxidation and migration capacity of the NO₂ formed. These results indicate that NO₂-assisted oxidation exhibits a leading role in catalytic soot formation [30]. The relevant equations are as follows:

C_soot + NO₂ → C(O) + NO

(1)

C_soot + Fe^x+ − NO₃ → C(O) + Fe··x+ − O + NO

(2)

NO₂ acts as a powerful transfer agent of reactive oxygen species. The higher the NO₂ fraction yield, the higher the reaction rate. The surface intermediates formed by Ni-Fe₂O₃-X during the oxidation of NO are an indispensable part of studying the effect of nitrogen oxides on the catalyst surface. In situ DRIFT testing and NO-TPO provide insight into and analysis of this process. It can be seen from the comparison between Figure 7A and Figure 7D that the Ni-Fe₂O₃-20 catalyst is more likely to adsorb NO after the introduction of NO, and exists in the form of nitrate on the catalyst surface. However, the adsorption strength of pure Fe₂O₃ is relatively small. After the introduction of O₂, only about 1400 cm⁻¹ of the nitrate formed was retained on pure Fe₂O₃. On the Ni-Fe₂O₃-20 catalyst, the nitrite strength is higher and more stable (Figure 7B,E). Therefore, through the synergistic effect between Ni-Fe, the Ni-Fe₂O₃-20 catalyst can stably maintain a certain amount of surface nitrite, without being affected by external O₂, so as to better store NO₂ as surface nitrate. This result proves that NO₂-assisted oxidation plays a leading role in the catalytic process of soot. In situ DRIFTS were tested at a 30 °C temperature gradient between 30 °C and 450 °C. As shown in Figure 7C, the peaks of Fe₂O₃ catalyst are nitrate (1388 cm⁻¹ and 1359 cm⁻¹). With the increase in temperature, the nitrite gradually changes to monodentate nitrate (1380 cm⁻¹), and reached its maximum intensity at 400 °C, which was consistent with the highest fraction yield of NO₂ [31]. In Figure 7F, NO_x on the surface of the Ni-Fe₂O₃-20 catalyst is mainly in the form of nitrite (1386 and 1358 cm⁻¹). By comparing the two catalysts, it can be found that the peak position of the two catalysts does not change with the increase in temperature, indicating that the desorption of NO₂ is not the rate-determining step. Stable free NO₂- gradually disappears with increasing temperature and gradually merges into monodentate nitrate at 300 °C (1380 cm⁻¹). In the NO-TPO test, the Ni-Fe₂O₃-X catalyst generated NO₂ molecules more efficiently than the Fe₂O3 catalyst (Figure 8). By comparing the above results of Fe2O3 and Ni-Fe2O3-x catalysts, it is confirmed that another key role of Ni ions is to reduce the adsorption of NOx, which leads to the increase in NO₂ molecules in the NO-TPO test. Combined with DRIFTS results and catalytic performance, it can be seen that NO₂-assisted oxidation plays a leading role in the formation of catalytic soot, and Ni-Fe₂O₃-X catalyst significantly promotes the renewal of active oxygen species and the desorption of NO₂ molecules, showing the best performance in catalytic dust purification.

4. Conclusions

In summary, we have developed a highly active and stable Ni-Fe₂O₃-X catalyst. The structure of NiFe₂O₄ is formed on the surface of Fe₂O₃ nanoparticles through a Fe-Ni synergism, which enhances the interaction between the Ni and Fe₂O₃- {110} carrier, promotes the formation of coordination unsaturated ions of reactants, generates more oxygen vacancies thus promoting the adsorption of NO and accelerates the low-temperature ignition of soot in O₂ [34]. The results show that the synergic effect of Fe-Ni ensures the excellent catalytic activity of Ni-Fe₂O₃-X catalyst (T50 = 366 °C, TOF = 0.156 min⁻¹, Ea = 72.7 kJ mol⁻¹) during soot oxidation. In addition, the superior catalytic properties were maintained well after five cycles, making the catalyst a promising candidate for future applications. This study reveals the key role of intermetallic synergies in catalyst surface modification, and provides a promising strategy for further development of efficient and low-cost catalysts for automotive exhaust soot removal without precious metals.