Highly Uniform and Thermal Stable Paper-Structured Catalyst by Using Glass/Mullite Hybrid Fibers as a Matrix for Efficient Soot Combustion

Abstract

In the present study, glass/ceramic hybrid fibers were chosen as a paper matrix, which effectively balance toughness and high-temperature resistance for soot combustion applications. In order to address the issue of unevenness in the performance of paper-type catalysts caused by the differences in the dispersion behavior of glass fibers and ceramic fibers in water, a facile foam-forming technology was proposed. The obtained glass fiber/mullite composite paper with various mass ratios (1:1, 2:1, 3:1, 4:1, and 5:1) exhibit high evenness, and better high-temperature resistance than the pure glass fibers. After impregnating K-Mn active ingredients, 15K5Mn-GFF-3G1C (GF/CF = 3:1) demonstrates high tensile strength, excellent catalytic activity (T50 = 388 °C), reusability (five cycles), and high-temperature stability (800 °C, 12 h).

1. Introduction

In the past few decades, the substantial quantities of soot particulates emitted by diesel engines have brought significant risks to both human health and the environment [1]. Consequently, governments all over the world have implemented progressively stringent emission regulations. To ensure that pollutant emissions from internal combustion engines comply with these standards, the installation of a catalytic diesel particulate filter (DPF) represents an effective technological solution [2].

Due to their high thermal stability, chemical stability, and mechanical strength, honeycomb ceramic catalysts have been widely used as catalytic diesel particulate filters (DPFs) [3]. However, the relatively low efficiency of lateral mass transfer and thermal diffusion can lead to pore channel blockage and increase the pressure drop, which may adversely impact the normal operation of diesel engines, particularly under conditions of high flow rates [4]. In recent years, fiber-structured catalysts characterized by short diffusion lengths, large geometric surface areas, and high contact efficiencies have garnered significant attention in processes [5,6]. It is well established that ceramic fibers (such as mullite and ZrO₂), when utilized as catalyst supports, can enhance the high-temperature resistance as well as gas/thermal diffusion performance and corrosion resistance of DPFs [5,6,7,8,9]. Nevertheless, their inherent brittleness poses limitations on their long-term practical applications. As a cost-effective fibrous support alternative, glass fibers demonstrate superior toughness compared to ceramic fibers, which are often chosen as catalyst supports for applications such as NO_x reduction and VOC combustion [10,11,12]. However, a primary limitation of glass fibers is their relatively low melting point, which restricts their use in high-temperature regions.

Glass/ceramic hybrid fibers, as a matrix, can effectively balance the toughness and high-temperature resistance of fiber-structured catalysts, rendering them highly suitable for soot combustion applications. However, achieving high uniformity in non-woven glass/ceramic hybrid paper using the traditional wet forming method is challenging due to their intrinsic differences in water dispersion. Previous studies have demonstrated that foam forming is an effective approach for mitigating fiber flocculation and enhancing paper uniformity [13,14,15,16,17]. Zhang et al. found that when the fiber length is increased to 48 mm, foam-formed quartz fiber paper maintains excellent uniformity [18]. In that case, the optimization of fiber lapping behavior and the regulation of three-dimensional network structures can be effectively facilitated, provided there are no geometric scale constraints on the fibers. The contact efficiency between foam forming paper catalysts and soot particles can be substantially improved, thereby promoting the efficient removal of soot particles. Nevertheless, the interwoven behavior of glass fibers and ceramic fibers with distinct properties in the foam-forming environment, as well as the synergistic effect among various active components, remain insufficiently elucidated. These aspects have become critical challenges in the development of high-performance glass/ceramic hybrid paper catalysts.

In the present study, we successfully developed a highly uniform and thermally stable paper-structured catalyst by employing glass/mullite hybrid fibers as the matrix for effective soot combustion through foam forming technology. Considering the synergic effects of various active ingredients on the catalytic activity and stability towards soot oxidation [19,20], the K-Mn binary active components were effectively dispersed within the paper matrix utilizing the traditional impregnating method. The network of fibers interconnected through hierarchical pore structures provides an exceptionally favorable environment for the capture and combustion of soot. In this K-Mn binary catalytic system, the “synergistic effect” refers specifically to the interaction between potassium (K) species and manganese (Mn) oxides. This effect may be manifested as follows: K species promote the activation and migration of manganese oxide lattice oxygen, regulate the redox cycle of Mn (such as stabilizing Mn³⁺), and increase the concentration of surface alkaline and reactive oxygen species. The manganese oxide framework helps stabilize K species and prevent them from volatilizing at high temperatures. Together, these interactions result in better overall catalytic performance (e.g., lower ignition temperature T₅₀ and higher stability) than any single-component (K-only or Mn-only) catalyst. The effects of tunable proportions of glass and mullite fibers on the physical, mechanical, and catalytic performance of the paper-structured catalysts were investigated extensively.

2. Results and Discussion

2.1. Characterization of the Glass/Mullite Hybrid Paper Catalyst

Figure 1 presents the XRD patterns of the catalysts with varying glass fiber to mullite mass ratios. The diffraction peaks at 2θ of 12.8°, 18.2°, 28.6°, 37.4°, 50.2°, and 60.2° are in good agreement with potassium manganate (K_2−xMn₈O₁₆) (JCPDS Card No. 42-1348). The peaks at 2θ of 23.1°, 32.9°, 38.2°, 45.2°, 49.4°, 55.1°, and 65.8° correspond to Mn₂O₃ (JCPDS Card No. 65-1798) [21]. It was also observed that all the peaks were gradually intensified with increasing mass ratios of mullite fibers to glass fibers.

Figure 2 displays the SEM images of composite paper-type catalysts with different fiber ratios. It can be observed that as the fibers are more evenly dispersed, a small amount of agglomeration occurs when the ratio of glass fibers to mullite fibers is higher than 3 (Figure 2d,e, yellow dotted boxes). The distribution of potassium (K) and manganese (Mn) elements in the glass fiber composite paper was investigated by EDS elemental mapping analysis (Figure 3). The K element predominantly deposits on the surface of the glass fibers, whereas the Mn element interacts with the ceramic fibers and preferentially grows within the interstitial regions between the glass fibers. Thus, the incorporation of ceramic fibers could facilitate the formation of the potassium manganate phase by enabling Mn to immobilize free K species, thereby enhancing the structural and chemical stability of the catalyst.

2.2. Redox Properties of the Glass/Mullite Hybrid Paper Catalyst

Figure 4a illustrates the H₂-TPR curves of the glass/mullite hybrid paper catalyst with various ratios of glass to mullite fibers. As the mass fraction of glass fibers increases, the reduction peaks gradually shift toward lower-temperature regions, which can be attributed to a decrease in surface-active oxygen, indicating an enhanced redox capability of the catalysts. However, when the mass ratio reaches 5:1, the reduction peak temperature increases again, indicating a decrease in surface-active oxygen species. This observation can be attributed to the interaction between the potassium and glass fibers, whereby free potassium ions are incorporated into the lattice structure, resulting in a significant reduction in their mobility, which was consistence with the XRD results.

Figure 4b presents the CO₂-TPD curves of the glass fiber composite paper catalysts with different fiber ratios. All samples exhibit a broad desorption peak in the 500–800 °C range, corresponding to strong basic sites, which is due to the decomposition of carbonates [22]. The desorption peak temperature of CO₂ first decreases and then increases with the reduction in ceramic fibers, which may be attributed to the synergistic interaction between potassium and manganese.

The valence states of K, O, and Mn were analyzed by X-ray photoelectron spectroscopy (XPS), and the results are presented in Figure 5. The relative abundances of the three oxygen species in the O 1s spectra, along with the corresponding K_f/Kc atomic ratios derived from the K 2p spectra for catalysts with varying glass fiber ratios, are summarized in

Table 1. Atomic ratios calculated by O 1s and K 2p and Mn 2p.

Catalysts	O/(at. %)			Kf/Kc	Mn3+/Mn4+
	O ads	O surf	O latt
15K5Mn-GFF-1G1C	34.03	19.02	14.45	1.88	3.91
15K5Mn-GFF-2G1C	38.46	16.15	15.01	1.92	3.71
15K5Mn-GFF-3G1C	37.44	20.03	13.44	1.90	4.19
15K5Mn-GFF-4G1C	41.23	14.26	14.17	1.89	3.88
15K5Mn-GFF-5G1C	37.79	16.30	14.55	1.86	3.75

. In Figure 5a, the characteristic peaks of K 2p3/2 and K 2p1/2 are observed at binding energies of 292.3–292.8 eV and 295.1–295.5 eV, respectively. As shown in the table, the catalysts 15K5Mn-GFF-2G1C and 15K5Mn-GFF-3G1C exhibit relatively high K_f/Kc ratios of 1.92 and 1.90, respectively. This suggests that these specific fiber ratios promote a higher concentration of free K species in the catalysts, which are critical for enhancing catalytic activity [23].

Figure 5b presents the O 1s XPS spectrum of the 15K5Mn-GFF-xG1C catalyst. The peak area percentages corresponding to the three oxygen species are summarized in Table 1. The characteristic peaks associated with adsorbed oxygen (O ads), surface oxygen (O surf), and lattice oxygen (O latt) are located at 532 eV, 530 eV, and 529 eV, respectively [24]. As shown in Table 1, the sample 15K5Mn-GFF-3G1C exhibits the highest content of active oxygen, which facilitates the formation of a significant number of oxygen vacancies. Furthermore, this sample contains a high concentration of free potassium species, promoting enhanced synergistic interaction between potassium and manganese.

Figure 5c displays the Mn 2p X-ray photoelectron spectroscopy (XPS) spectra of the 15K5Mn-GFF-xG1C catalyst. The spectra reveal the characteristic Mn 2p peaks, with the binding energies for Mn 2p3/2 and Mn 2p1/2 observed at 641.9–642.1 eV and 653.0–653.5 eV, respectively. The asymmetry of these peaks indicates the coexistence of manganese ions in multiple oxidation states. Specifically, the characteristic peaks attributed to Mn³⁺ are located at 642 eV and 653 eV, whereas those corresponding to Mn⁴⁺ appear at 644 eV and 655 eV [25,26]. Among all samples, 15K5Mn-GFF-3G1C exhibits the highest Mn³⁺/Mn⁴⁺ ratio, which is consistent with the results from hydrogen temperature-programmed reduction (H₂-TPR) and carbon dioxide temperature-programmed desorption (CO₂-TPD). This observation suggests that the sample possesses an enhanced redox capability and a higher concentration of oxygen vacancies, both of which contribute to improved catalytic activity and stability.

2.3. Catalytic Performance for Soot Oxidation

The catalytic soot conversion performance of 15K5Mn-loaded glass fiber/mullite composite paper catalysts with varying fiber ratios was studied. Figure 6a presents the soot conversion profiles of the composite paper catalysts with different fiber ratios, and

Table 2. Catalytic performance of the paper catalysts using various kinds of active ingredients and matrix fibers.

Catalysts	Soot Oxidation/°C
	T10	T50	T90
GF	441	557	616
15K5Mn-GFF-1G1C	364	465	570
15K5Mn-GFF-2G1C	322	373	439
15K5Mn-GFF-3G1C	317	366	433
15K5Mn-GFF-4G1C	360	428	531
15K5Mn-GFF-5G1C	387	456	573

summarizes the T10, T50, and T90 values for these catalysts. As shown in the figure, the sample 15K5Mn-GFF-3G1C exhibits the lowest T50 value (366 °C), which is attributed to the presence of K_2−xMn₈O₁₆ on the catalyst surface, resulting in an increased number of basic sites. Combined with EDS analysis, it is observed that in the composite paper catalyst with a glass fiber to ceramic fiber mass ratio of 3:1, the K and Mn elements are uniformly distributed, indicating a favorable interaction between the support and the active components. From the control experiments with only potassium or only manganese shown in Figure 6b, it can be well explained that the combination of potassium and manganese significantly enhances the catalytic activity of the catalyst, achieving the desired synergistic effect between potassium and manganese.

The cyclic testing of 15K5Mn-loaded glass fiber/mullite composite paper catalysts with varying fiber ratios was conducted through five consecutive cycles of catalytic oxidation under identical conditions. As shown in Figure 6c, the 15K5Mn-GFF-3G1C exhibits the lowest T50 temperature (422 °C) among all the samples, and maintains high catalytic activity and stability throughout multiple reaction cycles. After aging at 800 °C for 12 h, the ΔT50 of 15K5Mn-GFF-3G1C and the active species (e.g., K and Mn) in 15K5Mn-GFF were almost identical, indicating that the active species were more effectively retained during repeated use, which contributes to superior structural integrity and long-term stability. Among all samples, 15K5Mn-GFF-3G1C demonstrates the highest catalytic activity (T50 = 366 °C), along with excellent reusability and thermal stability. In contrast, the glass/mullite hybrid paper catalysts with varying GF/MF ratios exhibit enhanced reusability (Figure 6d) and improved high-temperature resistance (Figure 7). Furthermore, 15K5Mn-GFF-3G1C exhibits a relatively low T50 value of 388 °C after aging at 800 °C for 12 h, indicating excellent thermal stability.

Combined with the practical application conditions, we deeply investigated the effect of the presence of NO in the reaction gas on the catalytic performance of 15K5Mn-GFF-3G1C in terms of the oxidation of soot. Figure 8 shows the soot conversion curve for 15K5Mn-GFF-3G1C in the presence of NO. It has been found that the T10 of all curves corresponding to 15K5Mn-GFF-3G1C moves towards lower temperatures, which may be the result of the reaction of NO and O₂ to produce NO₂, which induces the production of reactive oxygen species. Conversely, the transition of T90 to higher temperatures may be attributed to the decomposition of KNO₃, which leads to the formation of reactive alkaline O²⁻ groups that strongly adsorb NO_x substances The mobility of (free K) in potassium acetate can increase soot/catalyst contact and provide a favorable redox environment for soot oxidation. In addition, the formation of15K5Mn-GFF-3G1C favors the regeneration of NO_x.

The foam forming process plays a pivotal role in regulating the architecture of the fiber network, which further dominates the contact efficiency with soot particles, as illustrated in Figure 9. During foam expansion, the fibers are stretched and aligned along the bubble interfaces, forming a preliminary framework supported by the gas phase. With the rupture of unstable foam bubbles, the fibers aggregate at the interface to form interwoven nodes, resulting in a porous interconnected network with tunable porosity and pore size. This structural reconstruction directly affects the soot contact behavior through three synergistic mechanisms: (1) the matched pore size between the fiber network and soot particles promotes the diffusion and trapping of soot; (2) moderate fiber entanglement enhances physical interception without pore blockage; and (3) the foam-induced micro-roughness on the fiber surface increases the specific surface area, thus strengthening the interfacial interaction with soot particles. In contrast, the original fiber network without foam modification exhibits a randomly stacked structure with uneven pores, leading to limited soot contact sites and low capture efficiency.

3. Materials and Methods

3.1. Materials

The glass short fibers (6 mm) were procured from Taishan Fiber Glass Group Co., Ltd. (Tai’an, China). The mullite (3Al₂O₃·2SiO₂) short fiber cotton was obtained from Lu yang Energy-Saving Materials Co., Ltd. (Zibo, China). The glass fibers were calcined at 200 °C for 2 h, followed by immersion in a concentrated hydrochloric acid solution under ultrasonic stirring for 1 h. Subsequently, the surface-modified glass fibers were filtered and dried in an oven at 80 °C for 12 h.

The sodium silicate, potassium nitrate, manganese acetate, hydrochloric acid (36–38%), dodecyl dimethyl ammonium oxide (OB-2), polyvinyl alcohol (PVA), polyethylene oxide (PEO), and polyacrylamide (PAM) used in this work were purchased from Aladdin Reagents Co., Ltd. (Shanghai, China), and were all of analytical reagent grade.

3.2. The Fabrication of Glass/Mullite Hybrid Paper-Structured Catalysts

The schematic diagram of the experimental procedure is presented in Figure 10. Dodecyl dimethyl ammonium oxide (OB-2) was selected as the foaming agent, polyvinyl alcohol (PVA) as the organic binder, and sodium silicate as the inorganic binder. The foam forming process is schematically illustrated in Figure 8. A specific proportion of glass fibers and ceramic fibers was dispersed in an aqueous solution containing polyvinyl alcohol (PVA, 10 mL, 5 wt.%), sodium silicate (10 mL, 34 wt.%), and dodecyl dimethyl ammonium oxide (OB-2, 2 mL, 25 wt.%). The slurry concentration was maintained at 0.4%. Subsequently, the mixture was vigorously stirred at a speed of 2500 r/min for 10 min, poured into a funnel, and immediately subjected to vacuum filtration. Finally, the glass/mullite hybrid paper was fabricated by drying the wet paper at 90 °C and sintering it at 200 °C for 2 h in air.

The equivalent volume impregnation method was employed to prepare the binary K-Mn supported paper catalysts. Aqueous solutions of potassium nitrate (KNO₃) and manganese acetate (Mn(CH₃COO)₂), with a nominal mass ratio of active components to paper support (m_active/m_support), were used as metal salt precursors, corresponding to 15 wt.% K₂O and 5 wt.% MnO. The samples were designated as K-Mn/GFF-xG1C, where x (=1, 2, 3, 4, 5) denotes the relative mass ration of glass fiber with respect to mullite in the paper-based matrix. The impregnated ceramic paper was subsequently dried at room temperature and calcined at 600 °C for 1 h to ensure complete removal of organic constituents.

3.3. Characterization

The crystalline phase analysis was performed using powder X-ray diffraction (XRD) on a Bruker AXS D8 Advance X-ray diffractometer (Billerica, MA, USA) operated at 40 kV and 40 mA. The morphology of the K-Mn supported paper catalysts was characterized using a Hitachi FE-SEM SU8000 instrument (Tokyo, Japan) at an accelerating voltage of 5 kV. Elemental mapping was conducted via energy-dispersive X-ray spectroscopy (EDS) using an EDAX E1506-C2B system (Mahwah, NJ, USA). The surfaces’ chemical states were analyzed by X-ray photoelectron spectroscopy (XPS) with a VG Scientific spectrometer (West Sussex, UK) employing Al Kα radiation (1486.6 eV) as the X-ray source, operated at 12 kV and 10 mA. The powder specimen was introduced into the pretreatment chamber of the XPS instrument and subsequently heated to 300 °C for one hour under a continuous flow of pure O₂ (20 mL/min) to eliminate adventitious carbonates, hydroxyl groups, and other adsorbed species, thereby stabilizing the surface under oxidizing conditions.

The uniform distribution revealed by EDS (Figure 3) provides the structural foundation for the abundant surface-active sites detected by XPS (Table 1). Meanwhile, the higher surface-active oxygen content and Mn³⁺/Mn⁴⁺ ratio measured by XPS explain why the sample 15K5Mn-GFF-3G1C exhibits the optimal catalytic activity among the macroscopically uniformly dispersed samples. It is precisely the synergy between uniform bulk-phase dispersion and favorable surface chemical states that jointly contributes to its high performance.

Temperature-programmed reduction (H₂-TPR) experiments were carried out on a PCA1200 apparatus (Beijing Builder Optics Company, Beijing, China), equipped with a thermal conductivity detector (TCD). A sample (50 mg) was pretreated in an argon atmosphere (30 mL min⁻¹) at 200 °C for 1 h, then cooled to ambient temperature. Subsequently, a 5% H₂/Ar mixture (30 mL min⁻¹) was introduced, and the temperature was increased linearly at a rate of 10 °C min⁻¹. CO₂ temperature-programmed desorption (CO₂-TPD) measurements were also performed on the same PCA1200 apparatus with a TCD. A sample (100 mg) was pretreated in a helium atmosphere at 200 °C for 1 h, then cooled to 150 °C, followed by saturation with CO₂. Desorption was subsequently initiated under a helium flow (50 mL min⁻¹) at a heating rate of 10 °C min⁻¹.

3.4. Catalytic Performance

The catalytic activities for soot oxidation were evaluated using the temperature-programmed oxidation (TPO) method in a lab-made tubular quartz reactor. Printex U soot, purchased from Degussa (Düsseldorf, Germany), was employed as a model soot. The mixture of soot and catalyst, with a soot-to-catalyst weight ratio of 1:20, was dispersed in n-hexane via ultrasonic treatment. The resulting suspension was then added dropwise onto the catalyst matrix to ensure complete impregnation and subsequently dried at room temperature. Prior to the reaction, the catalyst–soot mixture was pretreated under a N₂ atmosphere at 200 °C for 1 h to remove moisture, residual n-hexane, and adsorbed CO₂, followed by heating from 200 °C to 700 °C at a ramp rate of 5 °C min⁻¹ in a gas stream consisting of 21% O₂ in N₂ with a total flow rate of 400 mL min⁻¹. The concentrations of CO and CO₂ in the outlet gas were monitored using non-dispersive infrared (NDIR) gas analyzers (Hubei Ruiyi Automatic Control System Co., Ltd., Wuhan, China). Each experiment was repeated for 3–5 cycles using the same sample, and no appreciable change in soot oxidation efficiency was observed. Catalytic performance was assessed based on T₁₀, T₅₀, and T₉₀, defined as the temperatures at which 10%, 50%, and 90% soot conversion were achieved, respectively.

The reusability tests were conducted on the sample exhibiting the highest catalytic activity over five consecutive catalytic oxidation cycles under identical conditions. The samples were sintered at 800 °C for 12 h (heating rate: 5 °C/min in static air, followed by furnace cooling) to evaluate their thermal stability. To assess the influence of NO on soot oxidation activity and stability, a gas mixture containing 500 ppm NO was introduced into the soot–TPO test using the sample with the highest catalytic performance.

4. Conclusions

K-Mn supported glass/mullite hybrid paper catalysts, varying the mass ratios of glass fibers to ceramic fibers, were fabricated through a facile foam-forming route. The effects of tunable proportions of glass and mullite fibers on the physical, mechanical, and catalytic performance of the paper-structured catalysts were evaluated extensively. At a glass fiber to ceramic fiber mass ratio of 3:1, the foam exhibited the most uniform size distribution, with the lowest standard deviation and an average bubble diameter of 52 μm indicating excellent foam stability and processability. The resulting foam-formed composite paper demonstrated superior formability, favorable mechanical strength, and thermal stability up to 800 °C. Among all catalyst samples, 15K5Mn-GFF-3G1C exhibited the highest catalytic activity, achieving a T50 value of 366 °C, along with excellent reusability and thermal durability.