Enhanced Oxygen Vacancies in Ce-Doped SnO₂ Nanofibers for Highly Efficient Soot Catalytic Combustion

Abstract

In this paper, cerium was incorporated into polyhydroxyltriacetictin (PHTES) using the sol-gel method combined with electrospinning technology to prepare a series of composite oxide fiber catalysts Sn_xCe_1−xO₂ in different proportions. The structures and soot catalytic activities of Sn_xCe_1−xO₂ fibers were studied under loose contact conditions. When Ce entered the crystal lattice of SnO₂, the structural symmetry of the SnO₂ was destroyed, which inhibited the crystallization and grain growth of the fiber, and fiber catalysts with a larger specific surface area were obtained. Moreover, the introduction of Ce improved the number of oxygen vacancies and redox ability of the catalyst, thus promoting the catalytic activity of the catalyst for soot particles. In particular, among them, the Sn_0.7Ce_0.3O₂ fiber catalysts had the strongest catalytic oxidation ability regarding soot particles and could oxidize soot particles at a lower temperature and faster catalytic rate. The results of the temperature-programmed oxidation of Sn_0.7Ce_0.3O₂ fiber catalyst, conducted three times under the same conditions, were basically consistent, indicating that the experimental results are reliable and repeatable. In addition, the Sn_0.7Ce_0.3O₂ fiber catalyst showed good cycle stability.

1. Introduction

Soot is generated due to the incomplete combustion of the fuel in the cylinder of a diesel engine, which causes respiratory problems, cardiovascular diseases, and skin cell mutation [1,2]. This has led to the development of various exhaust after-processing technologies. The most common technology for impeding soot discharge is the use of diesel particulate filters (DPFs) or a gasoline particulate filter (GPF) to trap particulates. The typical diesel exhaust temperature is in the range of 200–500 °C, and the combustion temperature of exhaust soot needs to exceed 600 °C. Hence, oxidation catalysts need to catalyze the reaction at a lower temperature.

Among various kinds of soot oxidation catalysts, CeO₂-based materials have attracted the attention of many researchers. The surface oxygen vacancy (O_v) can be tuned by doping foreign cations in the crystal lattice of CeO₂, including transition metals, rare earth metals, alkali metals, and alkaline earth metals. Sarli et al. investigated the effect of Ag and Cu in CeO₂-based DPFs on soot oxidation, and both elements improved the catalytic performance of CeO₂ despite the different mechanisms [3]. The researchers found that CePr had the possibility of replacing the precious metal content of Pt by loading CePr and Pt catalysts in DPF to simulate the catalysis of soot found in practical applications [4]. Dutta et al. doped CeO₂ with metals, such as Zr, Ti, Hf, and Sn, to replace Ce⁴⁺, which can activate Ce³⁺ and Ce⁴⁺ ion pairs and promote the reversible reaction. Due to the Sn⁴⁺ + 2Ce³⁺ ↔ Sn²⁺ + 2Ce⁴⁺ redox equilibrium, electron exchange can occur, which greatly increases the adsorption sites of oxygen species, so CeO₂ doped with Sn shows the highest oxygen storage capacity [5,6,7,8]. Chen et al. doped Sn⁴⁺ into CeO₂ to form the Ce_xSn_1−xO₂ mixed oxide for the selective oxidation of CO, which can also improve the redox performance of Sn⁴⁺ and Ce⁴⁺, increase the mobility of lattice oxygen, and enhance the selective oxidation of CO activity [9]. Pijolat et al. applied several metal ions, such as Mg²⁺, Ca²⁺, Al³⁺, Sc³⁺, Pr³⁺, Y³⁺, La³⁺, Hf⁴⁺, Si⁴⁺, Zr⁴⁺, and Th⁴⁺ into CeO₂ for modification, to activate Ce³⁺/Ce⁴⁺ ion pairs and promote the flow of lattice oxygen. Among them, the doping of the Zr element is beneficial for improving its oxidation-reduction capacity and oxygen storage capacity. The formation of a cerium-zirconium solid solution can significantly improve thermal stability and inhibit the high-temperature sintering of CeO₂ [9,10,11,12,13,14,15,16,17,18,19,20,21]. In addition, the observed catalytic reaction is part of gas-solid-solid interaction. Since soot oxidation is largely determined by the mutual contact between soot and catalyst, to maximize the utilization of the contact points between the two substances, catalysts with different morphologies, such as nanospheres, nanorods, nanotubes, nanosheets, etc., have been studied extensively [22,23,24,25,26,27]. The ceramic fibers are arranged in a net-like structure after high-temperature treatment, which not only increases the contact points of soot but also facilitates gas diffusion, also enabling higher soot catalytic activity when in loose contact [28,29,30,31,32,33]. Cerium and cerium-based materials exhibit excellent properties (Ce⁴⁺ ⇌ Ce³⁺ + O_v) and high oxygen storage capacity due to their unique physicochemical properties. In the results published by many researchers, the material has shown good catalytic activity, and with the study of its various forms, it has also shown different catalytic performance results. In order to further study the influence of the synergistic effect of fiber structure and tin-cerium composite on catalytic performance, we aim to explore a material that can solve the soot catalysis problem in practical applications and will study the improvement of the soot catalytic performance of tin-cerium composite oxide fibers.

In the present work, Sn_xCe_1−xO₂ (x = 0.1, 0.2, 0.3, 0.4, 0.5) composite oxide fibers were prepared using the sol-gel method and electrospinning technology for soot oxidation. The reaction mechanism is shown in Figure 1. Under loose contact conditions, the effect of different tin-cerium ratios on the catalytic activity of the catalysts was explored. NO (1000 ppm) was introduced into the atmosphere to study the effect of NO on catalyst activity. The structure and physicochemical properties of catalysts were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer–Emmett–Teller (BET) surface areas and pore size analyses, H₂ temperature-programmed reduction (H₂-TPR), O₂ temperature-programmed desorption (O₂-TPD) and X-ray photoelectron spectroscopy (XPS).

2. Results and Discussion

Figure 2 shows the XRD patterns of fiber catalysts with different molar contents of Ce. For the SnO₂, there were three strong diffraction peaks at 26.6°, 33.9°, and 51.8°, corresponding to the (110), (101), and (211) crystal planes (JCPDS no.41-1145), respectively. In the Sn_0.8Ce_0.2O₂, the characteristic weak peaks of CeO₂ could be observed at 28.5° and 47.5°. Moreover, with the increase in Ce content, the characteristic diffraction peak intensity of CeO₂ gradually increased. Strong characteristic peaks could be observed at 28.5°, 33.1°, 47.5°, and 56.3° in the Sn_0.5Ce_0.5O₂, which can be assigned to the cubic fluorite structures of the (111), (200), (220) and (311) planes, respectively (JCPDS no. 43-1002).

In Figure 2, compared with pure SnO₂, the diffraction peak of SnO₂ in the Sn_xCe_1−xO₂ catalyst shifted to a lower angle, and the intensity of the diffraction peak weakened, indicating that Ce entered the crystal lattice of SnO₂, destroyed the structural symmetry of the catalyst, and reduced the crystallinity. It can be observed from

Table 1. Phase composition, lattice parameters, crystallite size, and specific surface area of the SnO₂ and Sn_xCe_1−xO₂ fiber catalysts.

Catalyst	Lattice Parameter (nm)	Crystallite Size (nm)	SBET (m2g−1)
SnO2	0.4730 (SnO2)	6.65	23.52
Sn0.9Ce0.1O2	0.4739 (SnO2)	3.66	32.65
Sn0.8Ce0.2O2	0.4744 (SnO2)	2.55	40.46
Sn0.7Ce0.3O2	0.4754 (SnO2)	1.54	45.32
	0.5392 (CeO2)	7.97
Sn0.6Ce0.4O2	0.4740 (SnO2)	3.44	38.97
	0.5385 (CeO2)	7.21
Sn0.5Ce0.5O2	0.4740 (SnO2)	4.01	45.10
	0.5386 (CeO2)	6.38

that the lattice constant of the Sn_xCe_1−xO₂ catalyst was larger than that of pure SnO₂ because the ionic radius of Ce (Ce³⁺: 0.103 nm; Ce⁴⁺: 0.092 nm) was larger than that of Sn (Sn²⁺: 0.093 nm; Sn⁴⁺: 0.071 nm). With the doping of Ce, part of the Ce entered the crystal lattice of SnO₂. As is seen in Figure 2, the phase of CeO₂ could be observed, where Ce coexisted with SnO₂ in the form of CeO₂. The existence of CeO₂ and SnO₂ phases could clearly be observed in Sn_0.7Ce_0.3O₂, which had the largest lattice constant, and the crystal grain size of SnO₂ was the smallest. Therefore, when Ce entered the crystal lattice of SnO₂, the growth of the crystal grain was inhibited and a composite metal oxide catalyst with a larger specific surface area was obtained. The CeO₂ peaks of Sn_0.6Ce_0.4O₂ and Sn_0.5Ce_0.5O₂ catalysts were stronger and the lattice constants of Sn_0.6Ce_0.4O₂ and Sn_0.5Ce_0.5O₂ catalysts were smaller than that of the Sn_0.7Ce_0.3O₂ (see Table 1). It can be inferred that the entry of Sn with a smaller ion radius into the crystal lattice of CeO₂ caused a decrease in the lattice constant.

The SEM images of the Sn_xCe_1−xO₂ fibers are shown in Figure 3; Figure 3f shows a partially enlarged view of Figure 3e. It can be seen from Figure 3 that all Sn_xCe_1−xO₂ fibers were arranged in a network structure, showing a high porosity. The beaded structure of the fibers in Figure 3a,e could provide more contact points for the soot particles during the catalysis process. In Figure 3f, it can be seen that there were some potholes and wrinkles in the fibers’ surfaces that made the contact between the soot particles and the catalyst firmer and more complete. Figure 4 shows the element distribution of the Sn_0.7Ce_0.3O₂ composite fiber. From the mapping results, it can be observed that Sn and Ce were uniformly distributed on the surface of the fiber catalyst.

Figure 5 and Figure 6 showed the absorption-desorption curves and pore size distribution diagrams of pure SnO₂ and Sn_xCe_1−xO₂ fiber catalysts, respectively. In Figure 5, one can see that all the fiber catalysts were type IV isotherms, SnO₂ had an obvious saturated adsorption platform, which belongs to the H2-type hysteresis loop, and the Sn_xCe_1−xO₂ fibers did not exhibit adsorption saturation in the relatively high-pressure area, so all loops appeared as H3-type hysteresis loops. The pore size was mainly concentrated between 2 and 15 nm (see Figure 6), and all the prepared fibers were of mesoporous materials. It can be seen from Table 1 that all the specific surface areas of Sn_xCe_1−xO₂ were much larger than that of pure SnO₂. Since the specific surface area is an important factor affecting catalytic activity, Sn_0.7Ce_0.3O₂ had the largest specific surface area, which could provide good contact efficiency for soot particles and expose more active sites; therefore, it exhibited the best catalytic activity.

The H₂-TPR profiles of the SnO₂ and Sn_xCe_1−xO₂ fiber catalysts are shown in Figure 7. The curve of the SnO₂ fiber presented a reduction peak in the range of 400–600 °C, which was the reduction in bulk Sn⁴⁺ to Sn²⁺ and the reduction in surface Sn²⁺ to Sn⁰ [8,34,35]. Compared with the SnO₂ fiber, all the reduction peaks of Sn_xCe_1−xO₂ fibers moved in a lower temperature direction. The redox balance Sn⁴⁺ + 2Ce³⁺ ↔ Sn²⁺ + 2Ce⁴⁺ occurred in the Sn_xCe_1−xO₂ fibers, which could improve the oxidation ability of Sn⁴⁺ and Ce⁴⁺, making them more prone to reduction. The low-temperature reduction peak at 200–300 °C was due to the reaction of the Sn⁴⁺ reduction to Sn²⁺ on the fiber surface. The high-temperature reduction peak in the range of 400–600 °C corresponded to the reduction of Ce⁴⁺ on the surface in addition to the reduction of Sn⁴⁺. When the molar content of tin increased, the stronger reduction peak indicated that the tin element is beneficial to the migration of lattice oxygen [9]. Since the grain size affects the oxidation-reduction ability of the catalyst, the lowest low-temperature reduction peak of Sn_0.7Ce_0.3O₂ may relate to the smallest SnO₂ grain size [10].

To study the active oxygen properties of the Sn_xCe_1−xO₂ fiber catalysts, the O₂-TPD experiment was used for characterization, and the results are shown in Figure 8. The low-temperature desorption peak of the catalyst below 400 °C represented the adsorbed oxygen species on the surface of the catalyst, which was prone to desorption under low-temperature conditions and was the main oxygen species component involved in the catalytic oxidation reaction of soot. The high-temperature desorption peaks above 400 °C mainly belonged to the desorption of lattice oxygen on the catalyst surface. When there are oxygen vacancies in the catalyst, part of the bulk lattice oxygen can also migrate to the surface through the oxygen vacancies and desorb at high temperatures [36,37]. As shown in Figure 8, the diffraction peaks in the curve could be attributed to three types of desorption peaks: the diffraction peaks in the range of 100–200 °C were from the desorption of O₂ adsorbed on the catalyst surface and were recorded as O-α. The diffraction peaks appearing in the range of 200–400 °C were caused by the desorption of O²⁻ and O⁻ adsorbed on the oxygen vacancies and were recorded as O-β. The desorption peak that appeared above 400 °C could be attributed to the desorption of O²⁻ in the catalyst lattice and was recorded as O-γ. In the curve of SnO₂, the peaks of O-α and O-β oxygen species were both small, and there were fewer active oxygen species on the surface. At 552 °C, there was a larger diffraction peak that was attributed to the O-γ species. Since O-α and O-β are the main components involved in the catalytic oxidation of soot, it is necessary to study their assigned desorption peaks further.

For the catalyst Sn_xCe_1−xO₂, compared to O-β, the peak intensity of O-α was basically negligible. With the incorporation of Ce, the number of oxygen vacancies in the catalyst increased. O-β could be clearly observed, as seen in Figure 8, and the peak temperature of oxygen desorption was shifted in the low-temperature direction. Obviously, compared with SnO₂, the amount of adsorbed oxygen on the surface of the catalyst Sn_xCe_1−xO₂ was much larger, indicating that the co-existence of Sn and Ce interacted on the surface of the catalyst to produce active oxygen components that participated in the catalytic reaction. The O-β desorption temperature of Sn_xCe_1−xO₂ and SnO₂ gradually decreased in the following order: SnO₂ > Sn_0.6Ce_0.4O₂ > Sn_0.5Ce_0.5O₂ > Sn_0.8Ce_0.2O₂ > Sn_0.9Ce_0.1O₂ > Sn_0.7Ce_0.3O₂. The Sn_0.7Ce_0.3O₂ catalyst had the lowest desorption temperature, indicating that the binding force of oxygen vacancies and adsorbed oxygen was weak, but it had better oxygen mobility, and active oxygen was more prone to desorption so as to participate in the catalytic reaction.

The surface of the catalyst is the main site for the catalytic reaction of soot. XPS was used to study the chemical valence and composition of the catalyst surface. Figure 9 shows the XPS spectrum of Sn 3d_5/2 of the Sn_xCe_1−xO₂ fiber catalyst. According to the XPS database, the binding energy range of Sn²⁺ is 485.6–487.0 eV and the binding energy range of Sn⁴⁺ is 486.0–487.1 eV [38]. With the addition of Ce, the ratio of Sn²⁺/(Sn²⁺ + Sn⁴⁺) increased, compared with that of SnO₂. For quantitative calculation, peak fitting was performed, and the relevant results are listed in

Table 2. XPS parameters for Sn, Ce, and O of the SnO₂ and Sn_xCe_1−xO₂ fiber catalysts.

Fiber Catalysts	Sn2+/(Sn2+ + Sn4+) (%)	Ce3+/(Ce3+ + Ce4+) (%)	(OII + OIII)/OI (%)	OII/(OI + OII + OIII) (%)
SnO2	13.08	/	0.29	16.39
Sn0.9Ce0.1O2	21.36	22.44	2.40	30.59
Sn0.8Ce0.2O2	23.51	24.26	2.81	30.92
Sn0.7Ce0.3O2	54.72	27.74	3.42	33.31
Sn0.6Ce0.4O2	37.92	21.84	2.02	29.53
Sn0.5Ce0.5O2	44.95	20.77	1.38	27.44

. The peaks of Sn_0.7Ce_0.3O₂ at 485.7 eV and 486.6 eV were attributed to Sn²⁺ and Sn⁴⁺, respectively, and the proportion of Sn²⁺ was the highest, reaching 54.72%. Figure 10 shows the XPS spectrum of Ce 3d with peak fitting, where V’ and U’ represent the Ce³⁺ peaks, and V, V”, V’’’, U, U” and U’’’ represent the Ce⁴⁺ peak [39]. As shown in Table 2, the Ce³⁺/(Ce³⁺ + Ce⁴⁺) ratio of Sn_0.7Ce_0.3O₂ was the highest, at about 27.74%. This is because the interaction of Sn and Ce converts CeO₂ to Ce₂O₃. The presence of Ce³⁺ led to an imbalance in the internal charge of the catalyst and the formation of oxygen vacancies on the surface, thereby improving the content of adsorbed oxygen on the surface [40]. The change trend of the surface oxygen species was quantitatively calculated by performing peak separation processing on the O 1s spectrum of the catalyst (see Table 2 and Figure 11). O_I was attributed to lattice oxygen, O_II was the oxygen species adsorbed on the oxygen vacancies on the catalyst surface, and O_III was assigned to the oxygen species adsorbed on the surface of the fiber [41]. According to the calculation results of (O_II + O_III)/O_I, it can be seen that the content of adsorbed oxygen on the surface of Sn_xCe_1−xO₂ is higher than that of SnO₂, indicating that the coexistence of the two elements increased the content of O_II and O_III on the catalyst surface. Since the Sn_0.7Ce_0.3O₂ fiber catalyst had the highest content of Sn²⁺ and Ce³⁺, it should have the most oxygen vacancies among them. From the calculation results in Table 2, it can be seen that the O_II ratio in the catalyst was the highest, and the relative concentrations of adsorbed oxygen on the surface and oxygen species on the oxygen vacancies matched the catalytic activity of soot. The adsorbed oxygen has high activity and migrates to the surface of the catalyst easily, so it plays a vital role in the catalytic reaction [42].

Figure 12 shows the TPO curves of Sn_xCe_1−xO₂ and SnO₂ in a 21% O₂/N₂ atmosphere for the catalytic oxidation of soot under loose contact conditions. Without a catalyst, there was basically no conversion of soot particles below 450 °C, and oxidation began to occur at about 492 °C. The oxidation of soot particles was basically concentrated in the temperature range of 500–600 °C and reached a complete oxidation state when approaching 600 °C. SnO₂ fibers greatly improved the oxidation activity of soot particles, and significantly reduced the characteristic temperature of the soot particles catalyzed. When the Sn_xCe_1−xO₂ fiber catalysts were added, the catalytic activity of the soot was further improved, and with the doping of Ce, the activity of the catalyst oxidizing the soot changed in a mountain-like pattern. The order of the catalytic oxidizing ability of the soot was as follows: Sn_0.7Ce_0.3O₂ > Sn_0.8Ce_0.2O₂ > Sn_0.9Ce_0.1O₂ > Sn_0.6Ce_0.4O₂ > Sn_0.5Ce_0.5O₂. In the range of 400–440 °C, the Sn_xCe_1−xO₂ catalysts started to oxidize soot; basically, they could convert the soot particles completely before the temperature reached 500 °C.

According to the TPO of the soot conversion rate (Figure 12), the indicators for evaluating the catalytic oxidation of soot particles by the catalyst are listed in

Table 3. Soot combustion activity of the SnO₂ and Sn_xCe_1−xO₂ fiber catalysts.

Catalyst	T10 (°C) a	T50 (°C) b	T90 (°C) c	T50–10 (°C) d	Tm (°C) e	SCO2 (%) f	Reference
Soot	492	554	581	62	575	53.86	This work
SnO2	445	487	533	42	483	98.92	This work
Sn0.9Ce0.1O2	419	452	491	33	448	91.38	This work
Sn0.8Ce0.2O2	414	448	473	34	453	92.57	This work
Sn0.7Ce0.3O2	411	437	462	26	440	95.18	This work
Sn0.6Ce0.4O2	421	459	496	38	463	95.84	This work
Sn0.5Ce0.5O2	439	479	503	40	486	98.68	This work
CeO2 fibers g	395	533	-	138	553	-	[30]
CeO2 flakes g	450	552	-	102	567	-	[30]
CeO2 nanorods h	356	500	554	144	-	-	[43]
CeO2 flakes h	433	554	622	121	-	-	[43]
CeO2 nanocubes i	420	465	575	45	-	-	[44]
CeO2 nanofibers j	480	555	560	75	-	-	[45]

^a T₁₀ (10% soot conversion temperature). ^b T₅₀ (50% soot conversion temperature). ^c T₉₀ (90% soot conversion temperature). ^d T_50–10 (T₅₀–T₁₀). ^e T_m (maximum combustion rate temperature). ^f S_CO2 (CO₂ selectivity) = $\frac{{\mathrm{Total} \mathrm{concentration} \mathrm{of} \mathrm{CO}}_2}{{\mathrm{Total} \mathrm{concentration} \mathrm{of} \mathrm{CO}}_2+\mathrm{Total} \mathrm{concentration} \mathrm{of} \mathrm{CO}}\times 100\%.$ ^g Reaction condition: 10 vol% O₂/N₂ 100 N ml min⁻¹. ^h Reaction condition: 10 vol% O₂/N₂. ⁱ Reaction condition: 50% air/50% N₂ 100mL min⁻¹. ^j Reaction condition: 10 vol% O₂/N_2.

; the intention of the evaluation was to characterize more intuitively the ability of the Sn_xCe_1−xO₂ fiber catalysts to oxidize soot. The SnO₂ fiber reduced the temperature at which the soot reached the maximum combustion rate by 92 °C, and the T_50–10 dropped to 42 °C. Compared with SnO₂, the Sn_xCe_1−xO₂ catalysts significantly improved the catalytic activity of the soot, especially the Sn_0.7Ce_0.3O₂ fiber catalyst, which had the strongest catalytic oxidation capacity for soot. It reduced T₁₀ to 411 °C, T_m to 440 °C, T₉₀ to 462 °C, and T_50–10 to 26 °C, indicating that the Sn_0.7Ce_0.3O₂ fiber catalyst could complete the oxidation of soot particles at a lower temperature and faster catalytic rate. The synergistic effect of the Sn_xCe_1−xO₂ composite oxide and fiber structure significantly improved the catalytic activity with soot and could effectively remove soot from exhaust gas. The selectivity of the catalyst to CO₂ is listed in Table 3. Under the conditions of having no catalyst, the S_CO2 of the soot particles was only 53.86%. When the SnO₂ fiber was added, the selectivity was increased to 98.92%, and the soot was almost completely burned and converted into CO₂. The Sn_0.7Ce_0.3O₂ fiber with the best catalytic activity had a S_CO2 of 95.18%. When comparing T_50–10, this may be due to the excessively fast combustion process, which reduced the CO₂ selectivity.

Under the same conditions, the Sn_0.7Ce_0.3O₂ fiber catalyst was subjected to temperature-programmed oxidation tests, conducted in triplicate (see Figure 13). The TPO curves almost overlapped, indicating that the catalytic oxidation efficiency was almost the same, the characteristic temperatures T₁₀, T₅₀ and T₉₀ had no obvious changes, and the experimental results were reliable and repeatable. In order to evaluate the catalytic stability, the Sn_0.7Ce_0.3O₂ fiber catalyst with the highest catalytic activity was tested for 5 consecutive cycles. The TPO files are shown in Figure 14, and the specific indicators are listed in

Table 4. Soot combustion activity of the Sn_0.7Ce_0.3O₂ fiber catalyst recycling performance.

Fiber Catalyst	T10 (°C)	T50 (°C)	T90 (°C)	T50–10 (°C)	Tm (°C)	SCO2 (%)
Sn0.7Ce0.3O2-cycle1	411	437	462	26	440	95.18
Sn0.7Ce0.3O2-cycle2	404	443	477	39	453	88.28
Sn0.7Ce0.3O2-cycle3	406	453	486	47	464	85.85
Sn0.7Ce0.3O2-cycle4	408	452	488	44	463	84.29
Sn0.7Ce0.3O2-cycle5	410	457	494	47	466	82.68

. It can be observed that the T₅₀ increased slightly. T₅₀ increased by 6 °C for 2 cycles, and T₅₀ increased by 20 °C after 5 cycles. After 3 to 5 cycles, T_m basically no longer rose. Although the activity of the catalyst decreased slightly in the high-temperature zone, T₉₀ was also guaranteed to be within 500 °C. As the cycle increased, the selectivity of CO₂ decreased. After 5 cycles, S_CO2 still remained at 82.68%, indicating that the prepared Sn_0.7Ce_0.3O₂ fiber had good cycle stability.

The TPO file of the soot oxidized by Sn_0.7Ce_0.3O₂ under 1000 ppm NO/21% O₂/N₂ atmosphere is shown in Figure 15. The catalytic oxidation reaction of soot was essentially a redox reaction, and NO was also a component that needed to be eliminated in the exhaust gas treatment. In the low-temperature range, NO could easily come into contact with the adsorbed oxygen on the surface of the catalyst and be oxidized to NO₂, which had a stronger oxidizing ability than O₂. It could then directly react with soot to generate CO_x and reduce the initial combustion temperature. It can be seen from Figure 15 that T₁₀ was reduced by 27 °C. The catalytic oxidation of soot was a reaction that occurred at the contact point of oxygen–catalyst–soot particles. When NO participated in the reaction, they became O₂–NO₂–soot particles. Since the gas was easier to transport and make contact with soot particles, compared with a 21% O₂/N₂ atmosphere, the presence of NO further promoted the ability of the catalyst Sn_0.7Ce_0.3O₂ to catalyze the soot particles. After the atmosphere contained NO, the T₁₀, T₅₀, and T₉₀ of the Sn_0.7Ce_0.3O₂ catalyst were 384 °C, 434 °C, and 459 °C, respectively.

3. Experimental

3.1. Materials

Tin chloride (SnCl₄, 99%, Chengdu Xiya Chemical Regent Co., Ltd., Chengdu, China), potassium acetate (CH₃COOK, ≥98%, Tianjin Kermel Chemical Reagent Co., Ltd., Tianjin, China), methanol anhydrous (CH₃OH, ≥99.5%, Tianjin Fuyu Chemical Reagent Co., Ltd., Tianjin, China), acetic acid (CH₃COOH, ≥99.5%, Tianjin Fuyu Chemical Reagent Co., Ltd., Tianjin, China), polyoxyethylene (PEO, Mw~1,000,000, Aladdin, Shanghai, China) and cerium nitrate hexahydrate (Ce(NO)₃·6H₂O, ≥99.5%, Aladdin, Shanghai, China) were used as starting materials without further purification. Deionized water (DIW) with a resistivity higher than 18.2 MΩ·cm was used in the entire experimental section.

3.2. Preparation of the Sn_xCe_1−xO₂ Fiber Catalyst

(1)

Preparation of the polyhydroxyltriacetictin (PHTES) precursor:

Tin chloride was dissolved in methanol anhydrous, with stirring, until the solution became clear, then a methanol solution of potassium acetate was added to the above solution in ice-bath conditions. The supernatant was concentrated to dryness, then dissolved in acetone to further remove the KCl. Finally, the solution was condensed to create the PHTES precursor.

(2)

Preparation of the spinning sol:

Firstly, the PHTES precursor was dissolved in a mixed solution of CH₃OH and CH₃COOH. Secondly, Ce(NO)₃·6H₂O was dissolved in DIW; finally, the PEO/DIW/CH₃COOH solution was added dropwise according to the required Sn/Ce molar ratio.

(3)

Preparation of fibers:

The DC high-voltage power supply provided a working voltage of 11 kV, the microsyringe pump provided a propulsion rate of 2.0 mL h⁻¹, and the distance to the receiving device was 13 cm. The ambient temperature and humidity were controlled at about 25 °C and 20%, respectively. The collected precursor fibers were placed in a muffle furnace, then the temperature was raised according to the set heat-treatment program. The heat-treated process was set at being heated from room temperature (RT) to 650 °C at a heating rate of 1 °C min⁻¹, then held for 1 h, and, finally, cooled along with the furnace.

3.3. Characterization

X-ray diffraction (XRD, Bruker D8 Advance diffractometer, equipped with Cu-Kα (λ = 1.540598Å) radiation) was used to record the structure data for fibers at 40 kV and 40 mA. A scanning electron microscope (SEM, S-4800, Hitachi, Japan) was used to observe the morphologies of the fibers at an accelerating voltage of 7 kV. A BET (JW-BK112, JWGB Sci & Tech, Beijing, China) using the BJH (Barret-Joyner-Halenda) method was employed to analyze the surface areas and pore size at 77 K. H₂-TPR was carried out on a PCA-1200 instrument (Builder Electronic Technology Co., Ltd., Beijing, China), monitored by a thermal conductivity detector (TCD). First, 50 mg of the fibers were purged with Ar at 200 °C and cooled down to RT with an Ar flow, then the temperature increased to 850 °C at the rate of 10 °C min⁻¹ (10% H₂/Ar, 30 mL min⁻¹). O₂-TPD was also performed on a PCA-1200 instrument (Builder Electronic Technology Co., Ltd., Beijing, China), monitored with a thermal conductivity detector (TCD). Then, 100 mg of the fibers were pretreated in He (30 mL min⁻¹) at 200 °C, then they were exposed to O₂ at 80 °C and flushed with He. Finally, the profiles were carried out by increasing the temperature to 850 °C at 10 °C min⁻¹ (He, 30 mL min⁻¹). XPS (ESCALAB 250 ThermoFisher SCIENTIFIC, Waltham, MA, USA) was used to detect the surface chemical compositions, using Al-Kα as an X-ray source.

3.4. Activity Evaluation

The activity of the fiber catalysts was conducted using a Gasboard-3100 (Wuhan, China) infrared spectrometer, using the TPO method. Printex-U (Degussa) was used as the model for soot. First, 20 mg of soot and 200 mg of catalysts were accurately weighed and placed in a beaker with slight shaking to enter a loose contact state, then the soot-catalyst sample was simply mixed with quartz sand (660 mg) before loading it into the reactor. Before measurement, the mixture was treated in ultra-high purity N₂ (200 mL min⁻¹) at 200 °C for 40 min. To measure the reaction behaviors of the catalysts, all data were collected with the increase in the temperature to 750 °C at a rate of 5 °C min⁻¹. The volume composition of the feed gas was 21% O₂ and was balanced by ultra-high-purity N₂, with a gas space velocity of 80,400 mL h⁻¹g⁻¹. These experiments were repeated 3 times for the same sample. No obvious difference was found in the soot oxidation efficiency. The characteristic temperatures (T₁₀, T₅₀, T₉₀) and the CO₂ selectivity (S_CO2) were used to evaluate the catalytic performance of the fibers [46]. Meanwhile, the maximum combustion rate temperature (T_m) was also used as an evaluation index.

4. Conclusions

The fibers were arranged in a three-dimensional network structure, which could improve contact efficiency between the catalyst and the soot particles. Compared with nanoparticles in the catalytic process, which tended to agglomerate and reduce the catalytic effect, the catalytic performance of the fibers was better. Part of the Ce entered the SnO₂ fiber of the tetragonal rutile phase to form a solid solution structure, which showed a uniform distribution of Sn and Ce elements. However, with the increase in the Ce doping ratio, CeO₂ with a cubic fluorite structure gradually appeared. The introduction of Ce made the surface of the catalyst possess more abundant and more active oxygen species. Among them, the Sn_0.7Ce_0.3O₂ fiber had the largest specific surface area and showed better redox ability, so it exhibited the highest soot catalytic performance among the Sn_xCe_1−xO₂ fiber catalysts. The T₁₀, T₅₀, T_90, and T_50–10 of the Sn_0.7Ce_0.3O₂ fiber were 411 °C, 437 °C, 462 °C, and 26 °C, respectively. The temperature corresponding to the maximum burning rate was 440 °C, which was 135 °C lower than that of the pure soot particles, indicating that the Sn_0.7Ce_0.3O₂ fiber catalyst could oxidize soot particles at a lower temperature and at a faster rate. Since NO came in contact easily with the adsorbed oxygen on the surface of the catalyst, it was oxidized to NO_2, which could directly react with the soot particles to form CO_x, reducing the initial combustion temperature, and the oxidation ability of Sn_0.7Ce_0.3O₂ on soot was further improved in an atmosphere of 1000 ppm NO/21% O₂/N₂. Our research work provides an insight into the practical application of soot catalysis. It also provides a foundation for future research into fabricating fibrous materials into devices with practical applications.