Promoting Effects of Acid Treatment on Catalytic Performance of K-Sepiolite Clay Fibers for Soot Oxidation

Abstract

In this study, sepiolite clay fibers were activated through hydrochloric acid acidification at various concentrations. The effects of different acid environments on the phase structure, morphology, and physicochemical properties of the activated sepiolite fibers were studied extensively. It was found that calcite impurities can be effectively removed when the acid concentration exceeds 1 M. Furthermore, the specific surface area of K-supported sepiolite fibers increases continuously with rising acid concentration, reaching 107.9 m²/g when the hydrochloric acid concentration is 7 M. The soot temperature-programmed oxidation (TPO) results demonstrated that K-supported sepiolite fibers acidified with 3 M HCl exhibited the highest catalytic activity, with T₁₀ and T₅₀ values of 323 °C and 348 °C, respectively. The 10 wt% K-supported sepiolite paper catalyst, using 3 M HCl-activated sepiolite fibers as the matrix, exhibited the lowest T₅₀ value of 436 °C and showed excellent stability compared to all other paper catalyst samples. This study on the activation of sepiolite-based catalysts under various acidic conditions advances the development of highly active and stable mineral catalytic materials and facilitates their practical application.

1. Introduction

With the improvement of living standards, the usage of automobiles has significantly increased. Diesel engines are extensively employed in trucks, long-distance buses, ships, and other transportation systems due to their high efficiency and power output. However, traditional diesel engines emit substantial amounts of toxic exhaust gases, particularly fine particulate matter (PM), which poses serious risks to both the environment and human health [1]. Diesel particulate filters (DPFs) are exhaust purification devices that remove PM through physical trapping mechanisms. Nevertheless, excessive accumulation of PM on the filter surface can lead to increased backpressure, reduced engine performance, and ultimately engine failure [2]. Therefore, the ability to completely oxidize the PM deposited within the DPF is critical for maintaining the filtration efficiency and operational longevity of the device [3].

To achieve the regeneration of the DPF device by completely burning the deposited particles without requiring additional heating, the most commonly employed approach is to load a catalyst onto the filter body surface of the DPF. It is well known that the introduction of potassium (K) into the DPF exhibits a favorable catalytic oxidation effect on soot particles [4]. Potassium nitrate (KNO₃) is often used as the precursor of a potassium-containing catalyst for burning diesel soot or particulate matter [5,6,7]. Sui et al. [5] prepared a K-V-CA catalyst and found that the catalytic activity of KNO₃ was higher than KCl, and the K-V-CA catalyst with the molar ratio of 6:1:1 had the lowest soot combustion temperature.

The catalytic oxidation activity of soot is influenced by several factors, including the degree of contact between the soot and the catalyst, the efficiency of heat transfer, and the diffusion extent of the reactants. These factors are closely related to the structural characteristics of the catalyst support. Paper-structured catalyst (PSC), as a novel type of structured catalyst, features a porous structure along with excellent geometric adaptability and thermal stability, enabling efficient diffusion of heat and reactants within the catalytic material [8,9,10]. Banus et al. [11] fabricated a catalytic paper using ceramic fibers, cerium oxide, and potassium nitrate, and reported that it exhibited high catalytic performance. In our previous studies, paper catalysts using CeO₂-ZrO₂ fibers and TiO₂-ZrO₂ fibers as matrix have been developed, which displayed high catalytic activity, thermal stability, and reusability [12,13].

Sepiolite (named α-sepiolite, Si₁₂Mg₈O₃₀(OH)₄(H₂O)₄·8H₂O) is a magnesium-rich silicate clay mineral characterized by a naturally occurring fibrous and layered structure. This unique structural feature endows sepiolite with a large specific surface area and strong adsorption capacity. Moreover, sepiolite fibers are cost-effective, readily available, and environmentally benign, as they do not cause secondary pollution [14,15]. Zhang et al. [16] investigated the phase transitions and microstructural evolution of sepiolite fibers upon heating from 300 °C to 1300 °C. They found that sepiolite fibers began to lose crystalline water at approximately 300 °C, which led to a reduction in specific surface area. However, significant phase transformation and morphological changes occurred only at temperatures of 852 °C and above 1000 °C, respectively. He et al. [17] reported that higher calcination temperatures had minimal impact on the overall structure of sepiolite. As the calcination temperature increased, the channel grooves within the sepiolite structure gradually closed, resulting in a progressive decrease in the BET specific surface area. In our previous study [18], a K-supported PSC was developed using sepiolite fibers as the matrix, which exhibited excellent catalytic performance for soot oxidation due to the synergistic interaction between the matrix and the active component.

However, to address the issue of tunnel collapse following exposure to high temperatures, natural sepiolite requires modification. Currently, various modification techniques have been applied to sepiolite, including quaternary ammonium salt treatment [19,20], β-FeOOH modification [21], KOH activation [22], and organic functionalization [23]. Among these approaches, acid modification is considered the simplest and most effective method. F. Franco et al. [24] treated sepiolite with HCl and HNO₃ solutions under microwave radiation, successfully obtaining sepiolite fibers with a significantly increased specific surface area in a relatively short time. Wang et al. used methyltrichlorosilane (MTS) as a modifier, and HCl in MTS reacted with interlayer magnesium ions in the layered magnesium silicate crystal structure of sepiolite, which led to the delamination of sepiolite and increased the pore volume and area of sepiolite [25]. Acid-modified sepiolite can effectively remove magnesium, calcite, and other pore-blocking impurities, and may even disrupt Si–O–Si bonds to form silanol groups, thereby opening up the pore structure, enhancing adsorption capacity, and increasing the specific surface area [26]. For the application of sepiolite-based paper catalysts in DPF devices, it is essential not only to achieve suitable pore size and adsorption properties but also to ensure good thermal stability. Nevertheless, the effects of acidification on the sepiolite structure vary depending on the treatment conditions. To the best of our knowledge, the effects of acidification conditions on the thermal stability of sepiolite have not yet been thoroughly investigated.

In this study, sepiolite clay fibers were activated through hydrochloric acid acidification at various concentrations. The effects of different acid environments on the phase structure, morphology, and physicochemical properties of the activated sepiolite fibers were studied extensively. Furthermore, the soot catalytic performance of K-supported activated sepiolite fibers, as well as that of the K-supported paper catalyst using activated sepiolite fibers as the matrix, was evaluated to highlight the promoting effects of different acidic conditions.

2. Results and Discussion

2.1. Effects of Different HCl Concentrations on the Structural Properties for Bare and K-Supported Activated Sepiolite Fibers

The XRD patterns of sepiolite fibers modified with different concentrations of hydrochloric acid are shown in Figure 1a. The characteristic peak of calcite was also found at 2θ = 29.404°, 47.487°, and 48.510° in the sample sepiolite fibers (RS). The characteristic peak of calcite decreased at 0.5HS and completely disappeared at 1HS, 3HS, 5HS, and 7HS, indicating that with the increase in hydrochloric acid concentration, the characteristic peak of calcite decreased at 1HS, 3HS, 5HS, and 7HS. Calcite in RS will react with hydrochloric acid. Under the acidification condition of the hydrochloric acid solution with a concentration greater than 1 mol/L, calcite will be completely removed. In addition, the characteristic peaks of the six samples showed at 2θ = 7.306°, 19.728°, 20.607°, 23.726°, 26.447°, and 34.264°, and the characteristic peaks did not change significantly with the change in hydrochloric acid concentration. The results indicated that acidification had little effect on the main phase of sepiolite fibers. The XRD pattern of K-supported acidified sepiolite fiber is shown in Figure 1b. The six samples still had obvious sepiolite characteristic peaks after loading K, indicating that loading K will not destroy the main phase structure of sepiolite fibers. The phase of KNO₃ can be found in K/0.5HS, K/1HS, K/3HS, K/5HS, and K/7HS, and the peak of KNO₃ is the most obvious in K/3HS.

The specific surface area and pore volume of modified sepiolite fibers with different concentrations of hydrochloric acid and loaded with K are shown in

Table 1. Specific surface area and pore volume of sepiolite fibers were acidified differently.

Sample	BET Surface Area (m2/g)	Pore Diameter (nm)
RS	38.4	9.45
0.5HS	52.1	11.91
1HS	77.1	11.43
3HS	67.6	12.60
5HS	80.9	10.46
7HS	64.2	11.89
K/RS	43.1	7.39
K/0.5HS	66.2	8.43
K/1HS	62.3	8.38
K/3HS	70.5	9.62
K/5HS	85.4	7.17
K/7HS	107.9	7.40

. After acidification, the specific surface area and average pore diameter of the samples increased, and the specific surface area of the 5HS sample was the largest, reaching 80.9 m²/g. The average pore diameter of the 3HS sample was the largest, reaching 12.60 nm, indicating that acidification treatment could remove impurities in sepiolite fibers, optimize its surface structure, and form a larger pore structure. K/HS, 0.5K/1HS, K/3HS, and K/5HS, and the specific surface area of no less than the K/7HS sample K/RS also proves this point. However, this change did not increase or decrease regularly with the increase in hydrochloric acid concentration, which may be caused by the uneven contact between RS and acid during the acidification process.

Figure 2 shows the SEM photos of RS and 3HS. It can be seen that the surface of the clay fibers of the sepiolite is smooth, with an average diameter of about 0.2 μm. Some impurities can be seen in the RS sample (Figure 2a, white circle region), while there are no impurities in the SEM photos of the 3HS sample, indicating that acidification with 3 mol/L hydrochloric acid solution can remove the impurities in the sepiolite. In addition, the interconnected microfibers in the 3HS sample can provide many uniform, large pores, which facilitate adequate contact between the catalyst and the soot particles.

According to the thermogravimetric curve (Figure 3a), the sepiolite fiber exhibits four distinct weight-loss stages, which correspond to the removal of adsorbed water, zeolitic water, coordinated water, and hydroxyl groups, respectively [16]. It was reported that the dehydration of coordinated water was responsible for the reduction in porosity and the increase in the average pore size of sepiolite paper [18]. Combined with the data presented in

Table 2. Parameters of acidified sepiolite fibers of different concentrations obtained from TG curves.

Sample	Mass Loss Percentage for Each Temperature Range
	50–150 °C	150–350 °C	350–650 °C	650–850 °C
RS	1.42	2.65	18.15	1.30
0.5HS	4.26	2.34	8.34	1.65
1HS	4.43	2.36	2.24	2.38
3HS	3.73	2.16	2.06	2.34
5HS	4.16	2.00	2.16	2.30
7HS	4.62	2.22	2.50	2.05

, it can be observed that although the acid-treated sample exhibited a higher weight loss within the temperature range of 50–150 °C, the overall mass loss percentage decreased between 50 °C and 850 °C, particularly in the range of 350–650 °C. These findings indicate that acidification can effectively reduce the loss of coordinated water from the sepiolite fiber and thereby enhance its thermal stability. Among all the samples, 3HS exhibited the lowest mass loss percentage. In the range of 500 °C~700 °C, the mass loss of K/3HS (Figure 3b) is more obvious than that of 3HS (Figure 3a), which is caused by the decomposition of KNO₃.

As shown in the UV-visible diffuse reflectance spectra (Figure 4), the K/RS sample exhibits two absorption peaks, α and β. Upon potassium loading of the acidified samples, the α peak disappeared and the β peak exhibited a redshift, which may be attributed to a change in substituents, potentially due to the partial replacement of Mg by K within the sepiolite fiber structure. The more pronounced β peak observed after acidification indicates that this treatment facilitates a greater degree of substitution between K and Mg. Consequently, acidification enhances the formation of a larger specific surface area and improved pore structure in sepiolite fibers, thereby promoting the loading of active species. Among the samples, K/3HS displayed the strongest β peak, suggesting the most effective substitution.

Figure 5 presents the desorption peaks of K-loaded sepiolite fibers acidified with varying concentrations of hydrochloric acid in the high-temperature range (500–800 °C), which correspond to the strong basic sites of the samples. RS and K/RS exhibit a broad peak near 740 °C, attributed to the decomposition of the associated mineral calcite, resulting in the release of CO₂. In contrast, K/1HS, K/3HS, K/5HS, and K/7HS do not display this peak, indicating that calcite is effectively removed from RS following acidification. Furthermore, as the hydrochloric acid concentration increases, the desorption peak temperature in the high-temperature region gradually decreases. The lowest peak temperature, observed at 578 °C for K/3HS, suggests a reduction in sample alkalinity, corresponding to potassium-containing carbonate species. These carbonates are primarily formed through the high-temperature reaction between K₂O and CO₂. However, with further increases in hydrochloric acid concentration, the desorption peak temperatures of K/5HS and K/7HS rise, which may be attributed to an increase in K₂O content, leading to a shift in the peak to higher temperatures.

2.2. Catalytic Activities of K-HS and K-HSP for Soot Oxidation

Figure 6a,b presents the soot TPO profiles of K-loaded sepiolite fibers acidified with varying concentrations of hydrochloric acid. As shown in Figure 6a, K/RS and K/0.5HS exhibit a distinct peak within the temperature range of 600–700 °C, whereas K/1HS, K/3HS, K/5HS, and K/7HS do not display such a peak in this range. This peak is attributed to the decomposition of calcite present in the sepiolite ore, resulting in the release of CO₂. The absence of this peak in samples acidified with higher HCl concentrations indicates that carbon soot is completely oxidized before 500 °C. The presence of this peak leads to lower T₅₀ and Tm values as well as higher T₉₀ values for K/RS and K/0.5HS, suggesting that 0.5 mol/L hydrochloric acid was insufficient to fully remove calcite from the raw sepiolite, which is consistent with the XRD analysis results. Figure 6b illustrates the relationship between soot conversion rate and temperature during the catalytic combustion process. Combined with the data in

Table 3. Catalytic performance of potassium-supported acidified sepiolite fiber and paper catalyst for soot formation.

Sample	Element Content/wt%			T10/°C	T50/°C	T90/°C	Tm/°C
	K	Ca	Mg
RS	0.31	11.13	10.37	-	-	-	-
Soot	-	-	-	458	561	616	586
K/RS	4.38	9.25	11.35	342	365	644	354
K/0.5HS	4.92	6.11	13.04	329	353	573	351
K/1HS	6.51	0.49	13.58	345	368	400	364
K/3HS	13.43	0.25	11.24	323	348	364	346
K/5HS	16.74	0.57	7.61	332	357	375	357
K/7HS	14.42	0.35	8.69	345	362	379	362
K/RSP	-	-	-	396	422	670	407
K/1HSP	-	-	-	385	404	483	400
K/3HSP [18]	-	-	-	400	412	426	410
K/5HSP	-	-	-	388	401	415	401
K/7HSP	-	-	-	390	402	411	403

, it can be observed that increasing the hydrochloric acid concentration significantly enhances the catalytic activity of the samples. The T₅₀ values initially decrease and then increase with increasing acid concentration, with K/3HS exhibiting the highest catalytic performance (T₁₀ and T₅₀ at 323 °C and 348 °C, respectively).

A small peak was observed in both K/RS and K/RSP within the temperature range of 500–700 °C, as illustrated in Figure 6a,c, indicating that the paper-forming process of sepiolite did not effectively remove impurities from the raw sepiolite material. This kind of peak was caused by the decomposition of calcite in sepiolite. The T₅₀ values of K/RSP, K/1HSP, K/3HSP, K/5HSP, and K/7HSP were measured at 422 °C, 404 °C, 412 °C, 401 °C, and 402 °C, respectively, showing a general decreasing trend with increasing acidification concentration. In comparison, the T₅₀ values of K/RS, K/1HS, K/3HS, K/5HS, and K/7HS were all higher than those of K/RSP, K/3HSP, K/5HSP, and K/7HSP, which may be attributed to the complexity of the paper-forming process that limits the full manifestation of the intrinsic properties of sepiolite fibers. The T₉₀ values of K/RSP, K/1HSP, K/3HSP, K/5HSP, and K/7HSP exhibited a similar trend to T₅₀, also decreasing with increased acidification. According to the data summarized in Table 3, the ΔT values (ΔT = T₅₀ − T₁₀) for K/RSP, K/1HSP, K/3HSP, K/5HSP, and K/7HSP were 26 °C, 19 °C, 12 °C, 13 °C, and 12 °C, respectively. These ΔT values are significantly lower than those of K/RS, K/1HS, K/3HS, K/5HS, and K/7HS, indicating a faster catalytic reaction rate for the paper catalysts. This improvement can be attributed to the structural characteristics of the paper catalysts, which enhance the contact efficiency between the catalyst and carbon soot, thereby promoting the catalytic reaction rate.

2.3. Catalytic Stability of K-HS and K-HSP for Soot Oxidation

Before the TPO test, the original mass of the sample was weighed and recorded as m₀. In every TPO test, the sample was taken out and weighed, and recorded as m_x (x = 1, 2, 3, 4, 5). The mass loss was the difference between m₀ and m_x. The calculation formula was as follows:

$$\mathrm{Relative} \mathrm{Mass} (\%)={\displaystyle \frac{({\mathrm{m}}_0-{\mathrm{m}}_x)}{{\mathrm{m}}_0}}\times 100\mathrm{\%}$$

The mass of the catalyst decreases with increasing recycling cycles, and the T₅₀ value also rises with the number of cycles, indicating a decline in catalytic performance. Among all samples, the most significant mass loss occurs during the first soot–TPO test. Under identical experimental conditions, five consecutive soot–TPO cycles were conducted on three samples—K/3HS, K/5HS, and K/7HS. In Figure 7b, the catalyst obviously lost weight after the first cycle, mainly due to the decomposition of potassium nitrate. After the fourth and fifth cycles, the mass of the catalyst increased, which may be due to the adsorption of potassium carbonate and other substances. The results revealed that K/3HS exhibited the smallest T₅₀ variation (23 °C) and the least mass loss, demonstrating superior cyclic stability, as illustrated in Figure 7a,b.

As shown in Figure 7c, the T₅₀ value of the sepiolite fiber paper catalyst is higher than the temperature corresponding to the first soot–TPO run. This indicates that both the sepiolite paper catalyst and the sepiolite fiber catalyst exhibit reduced catalytic activity during use. The primary reason for this decline is the loss of the active component K after the first soot–TPO cycle. In the second soot–TPO cycle, the T₅₀ values of K/RSP, K/1HSP, K/3HSP, K/5HSP, and K/7HSP were measured at 569 °C, 517 °C, 436 °C, 547 °C, and 530 °C, respectively. With increasing acidification concentration, the T₅₀ of K/3HSP first decreases and then increases. Acidification with 3 M hydrochloric acid significantly improved the internal pore structure of sepiolite and increased its specific surface area. In contrast, acidification with 5 M and 7 M hydrochloric acid caused substantial structural damage to the sepiolite.

2.4. Effect of NO and H₂O on Catalytic Activity of Sepiolite Fiber Paper Catalyst

In practical operating conditions, soot catalysts are typically exposed to a gas mixture containing NO, H₂O, and O₂. To simulate the practical application environments, NO or H₂O was often introduced into the reaction gas stream. When NO is mixed into the reaction gas, the conversion rate of CO₂ generated by all samples is above 90%, the T_m value is kept at about 400 °C, and T₁₀ moves to a lower temperature, which may be due to NO₂-induced reactive oxygen species (2NO₂ + C→2CO) generated by the reaction of NO with O₂, and the amount of reactive oxygen species increases, which makes soot react to the catalyst [18]. As a result, the ignition temperature (T₁₀) of all the samples moves to a lower temperature (Figure 8a,b). However, the T₅₀ and T₉₀ values of K/5HSP and K/7HSP move to a higher temperature in comparison to the TPO results without NO. This may be due to the strong NO-adsorption of K/5HSP and K/7HSP, which could inhibit the reaction of NO with O₂.

When H₂O was introduced into the reaction gas, the T₁₀, T₅₀, and T₉₀ values for all samples shifted toward higher temperatures, while the CO₂ conversion rate remained above 90%. This phenomenon may be attributed to the loss of free K species. Additionally, some active potassium species could react with hydroxyl groups from H₂O, thereby reducing the availability of potassium for electron donation.

3. Materials and Methods

3.1. Preparation of Acidified Sepiolite Fibers

The sepiolite fibers (RS) acidified by hydrochloric acid at different concentrations were prepared as follows. A total of 10 g sepiolite fibers were impregnated in hydrochloric acid solution (100 mL) of different concentrations (0.5, 1, 3, 5, and 7 mol/L) for 8 h. After being drained, filtered, washed to neutral, and then dried in a drying oven at 60 °C for 4 h, acidified sepiolite fibers were obtained and labeled with 0.5HS, 1HS, 3HS, 5HS, and 7HS, respectively.

The K-supported sepiolite fibers were prepared through a traditional impregnating method. The potassium nitrate solution with a nominal mass ratio of 10 wt% (${\displaystyle \frac{{\mathrm{m}}_{\mathrm{K}\mathrm{N}{\mathrm{O}}_3}}{{\mathrm{m}}_{\mathrm{S}\mathrm{e}\mathrm{p}\mathrm{i}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{e} \mathrm{f}\mathrm{i}\mathrm{b}\mathrm{e}\mathrm{r}\mathrm{s}}}}\times 100\%=10\%$) was used as an active ingredient. After drying at 60 °C and sintering at 400 °C for 3 h, K-supported raw and activated sepiolite fibers were obtained and denoted as K/RS, K/0.5HS, K/1HS, K/3HS, K/5HS, and K/7HS, respectively.

3.2. Design of the Paper-Structured Catalysts

Sepiolite fiber paper was obtained by the wet forming method, using sepiolite fiber treated with hydrochloric acid with different concentrations as a matrix, and through beating, forming, drying, and calcination. All the chemical reagents were analytical grade. In a typical preparation process, 0.75 g sodium hexametaphosphate was dissolved in 1200 g of water, then 5 g acidified sepiolite fiber was added with different concentrations of HCl (RS, 1HS, 3HS, 5HS, and 7HS), 0.5 g of water glass (10 wt%), and 4 g polyvinyl alcohol were added to the above solution. Then, a high-speed disperser was used to stir the above mixed pulp at a speed of 500 rpm. Subsequently, the uniformly dispersed slurry was carefully poured into the paper molding machine, where a circular paper sheet with a diameter of 20 cm was obtained via vacuum filtration. The formed paper was dried using a rapid paper dryer (AT-GZQ, Shandong Animet Instrument Co. Ltd., Shandong, China) and then calcined at 400 °C for 4 h to yield the sepiolite paper supports. The sepiolite paper was cut into small pieces with a diameter of 3 cm for TPO tests. The equal volume impregnation method was used to obtain the paper catalysts. The potassium nitrate solution with a nominal mass ratio of 10 wt% was used as an active ingredient (${\displaystyle \frac{{\mathrm{m}}_{\mathrm{K}\mathrm{N}{\mathrm{O}}_3}}{{\mathrm{m}}_{\mathrm{S}\mathrm{e}\mathrm{p}\mathrm{i}\mathrm{o}\mathrm{l}\mathrm{i}\mathrm{t}\mathrm{e} \mathrm{p}\mathrm{a}\mathrm{p}\mathrm{e}\mathrm{r}}}}\times 100\mathrm{\%}=10\%$). After drying and calcination at 400 °C for 4 h, the heating rate was 5 °C/min, and the K-supported acidified sepiolite fiber paper catalyst was obtained by natural cooling to room temperature. It is expressed as K/RSP, K/1HSP, K/3HSP, K/5HSP, and K/7HSP.

3.3. Characterization

The crystal structure of the sample was characterized by an X-ray powder diffractometer (XRD) from Bruker D8 ADVANCE A25X of AXS (Karlsruhe, Germany). The radiation source is Cu Kα, incident wavelength λ = 0.15418 nm, working under the condition of 40 kV tube voltage and 40 mA tube current, and a Ni filter was adopted; scanning range was 5–85° and scanning step was 4°/min.

The specific surface area and pore size of sepiolite fibers before and after acidification were measured with ASAP 2460 (BET, Micromeritics Instrument Co. Ltd., Norcross, GA, USA) automatic specific surface area and micropore pore analyzer. The samples were first degassed in a N₂ atmosphere at 200 °C for 12 h.

The thermal stability of sepiolite fibers treated with hydrochloric acid at different concentrations was analyzed by a differential scanning calorimeter (TG-DSC) from the Perkin Elmer Company in the Waltham, MA, USA. The flow rate was 50 mL/min in a mixed atmosphere of 20% O₂/N₂, and the temperature was raised from 10 °C/min to 1000 °C at room temperature.

The elemental composition of K-supported sepiolite fibers was determined using an inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 720, Santa Clara, CA, USA). The sample was characterized by a SU8010 scanning electron microscope (SEM) produced by Hitachi, Tokyo, Japan. The working voltage was 5 KV, and the gold spraying time was 180 s.

The UV-visible diffuse reflectance spectrum (UV-Vis DRS) of the samples was measured by a Hitachi U-4100 spectrograph (Tokyo, Japan). The measured spectral range was 200–1200 nm.

The redox properties of sample catalysts were determined by temperature-programmed adsorption (CO₂-TPD) in a fixed-bed flow reactor. The sample mass of 50 mg was treated at 200 °C under He gas for 1 h, cooled to below 50 °C, and saturated and adsorbed with CO₂ (50 mL/min) for 1 h. The temperature was raised to 850 °C in a He atmosphere at a rate of 10 °C/min.

The catalytic oxidation activity of soot catalysts was evaluated by the soot programmed temperature rise oxidation (soot–TPO) method. CO₂ and CO concentrations were monitored using a gas chromatograph type 3420A (Beijing Rayleigh Analytical Instruments Co. Ltd., Beijing, China). Printex U simulated soot particles produced by Degussa AG in Frankfort, Germany, were used for testing. Before the test, a certain proportion (carbon black: 1:8 catalyst) was ground for 10 min to make it fully contacted and evenly mixed. The mixed sample was placed in a glass tube, which was sealed with quartz cotton at both ends. N₂ was injected into the tube and heated to 200 °C for 60 min to remove CO₂ from the tube and the catalyst surface. After that, the test was carried out in synthetic air (21% O₂, 79% N₂) with a flow rate of 120 mL/min, a test temperature range of 200 °C–700 °C, and a heating rate of 2 °C/min to monitor the concentration of CO₂ and CO.

The catalytic performance evaluation process of K-supported sepiolite paper catalyst: a certain amount of carbon black was ultrasonically dispersed with n-hexane, and the positive and negative sides were uniformly added to the sepiolite fiber paper, where the weight ratio of carbon black to the catalyst was 1:20. After drying at 80 °C (volatilization of n-hexane), the samples were placed in a fixed bed reactor to evaluate the catalytic performance. First, N₂ was injected and heated to 200 °C for 60 min, and ambient and adsorbed CO₂ on the surface of the catalysts was removed. After that, the test was carried out in synthetic air (21% O₂, 79% N₂) with a flow rate of 400 mL/min. In order to evaluate the presence of NO and H₂O on soot oxidation activity and stability, a gas mixture containing 500 ppm of NO, as well as 6% of H₂O, was used in the experiment with a soot oxidation temperature range of 200 °C–700 °C, and a heating rate of 5 °C/min. An infrared analyzer (Xi’an Poly Energy Instrument Co. Ltd., Xi’an, China, JNYQ1-41) was used to monitor the concentrations of CO₂ and CO.

The evaluation indices of catalytic oxidation activity of soot are as follows: soot particle ignition temperature (T₁₀), maximum combustion rate temperature (T_m), temperature at 50% combustion (T₅₀), and soot burnout temperature (T₉₀).

4. Conclusions

Acid activation of sepiolite fibers using hydrochloric acid at varying concentrations effectively removes associated mineral impurities, such as calcite, opens the internal pore structure, and increases the specific surface area of the fibers. The specific surface area of sepiolite fibers acidified with 7 mol/L hydrochloric acid reached 107.9 m²/g after being loaded with potassium-based active components. With increasing hydrochloric acid concentration, the catalytic activity of the samples improved significantly. The T₅₀ value initially decreased and subsequently increased, indicating an optimal acidification level. Among the samples, K/3HS exhibited the highest catalytic performance, with T₁₀ and T₅₀ values of 323 °C and 348 °C, respectively. The catalytic combustion performance of the K-supported paper catalysts using acidified sepiolite fibers under different acidification conditions as the matrix were evaluated. The lowest T₅₀ value was observed for K/3HSP at 436 °C. Furthermore, the presence of NO was found to enhance the light-off activity of the K-supported sepiolite paper catalyst, as indicated by a lower T₁₀ value. In contrast, the presence of H₂O led to the leaching of active components, resulting in a reduction in catalytic activity.