A Promising Monolithic Catalyst for Advanced VOCs Oxidation by Graphene-Doped α-MnO₂ Loaded on Cordierite Honeycomb

Abstract

A high-activity, low-cost, and easy-to-prepare monolithic catalyst is crucial for the industrial catalytic combustion of volatile organic compounds (VOCs) in a cost-effective manner. In this study, a highly efficient monolithic catalyst, designated as 4GM/COR, was developed by loading 4% graphene-doped α-MnO₂ (4GM) catalyst onto pre-etched cordierite (COR) blocks using a straightforward “ball-milling-assisted impregnation” method. The anchoring force of the cordierite pores, generated through oxalic acid etching, enables the uniform and robust loading of powdered 4GM onto COR, preventing detachment under high temperatures or high gas flow rates. The loading rate, specific surface area, and concentrations of Mn³⁺ and surface-lattice and absorbed oxygen species in the monolithic catalyst increase with impregnation times from 2 to 4, indicating that catalytic activity is optimized through repeated impregnation. Catalytic performance tests demonstrated that the 4-4GM/COR exhibited the highest activity, achieving 90% degradation of toluene at 200 °C under both dry and humid (relative humidity is 85%) conditions. Furthermore, the 4-4GM/COR maintains high catalytic stability and activity even at a large GHSV of 6000 h⁻¹. To conclude, the 4-4GM/COR monolithic catalyst developed in this study not only represents a promising option for industrial applications but also serves as an important reference for the synthesis of monolithic catalysts.

1. Introduction

The rapid advancement of industrial processes has intensified atmospheric challenges posed by volatile organic compounds (VOCs), which act as critical precursors in photochemical ozone production and secondary fine particulate matter (PM2.5) formation, while concurrently triggering severe public health risks—from respiratory impairments to carcinogenic effects [1,2]. This dual environmental and epidemiological impact has elevated VOCs emission control to a paramount priority in sustainable development agendas worldwide.

Catalytic combustion stands as a pivotal technology for VOCs abatement, yet its industrial scalability hinges critically on the economic viability and catalytic performance of employed materials, which is intrinsically governed by two interdependent factors: (1) the catalyst’s intrinsic properties (activity, durability, cost) and (2) its structural configuration. Developing low-cost catalysts with high intrinsic activity and enhanced durability directly reduces operational expenditures (OPEX) by minimizing both catalyst consumption and energy demands. Concurrently, optimizing catalyst architecture to maximize active site accessibility—through engineered pore structures or surface functionalization—improves both catalyst utilization efficiency and VOCs treatment efficiency by enhancing reactant-catalyst contact and mass transfer dynamics [3,4]. Compared with the powdered catalysts, the strategic deposition of active catalytic coatings onto honeycomb monolithic substrates can achieve this effect by mitigating flow resistance, preventing channel blockage, and maximizing active site accessibility [5]. Therefore, the integration of a monolithic structured substrate and an active component to fabricate monolithic integrated catalyst aligns with industrial requirements for scalable, energy-efficient VOCs treatment systems.

Noble metal catalysts, while renowned for their superior catalytic activity in combustion processes, face limitations in industrial applications due to their high cost and scarcity [6,7]. This inherent vulnerability has driven substantial research interest toward transition metal oxides as alternative catalysts. Among various transition metal oxides, manganese-based catalysts have emerged as particularly promising candidates for VOCs oxidation, owing to their structural versatility and multivalent redox characteristics [8,9,10]. In particular, α-MnO₂ has been proven to have higher activity among the various crystal types (i.e., α-, β-, γ-, and δ-) [9,10,11]. The catalytic mechanism of manganese oxides primarily involves oxygen activation and transfer processes mediated by reversible Mn³⁺/Mn⁴⁺ redox cycles, where surface electron transfer plays a critical role [8]. This fundamental understanding suggests that enhancing electrical conductivity could improve catalytic efficiency by accelerating charge transfer during redox reactions. Recent advancements in catalyst engineering have found the catalytic activity can be further optimized through strategic modifications including heteroatom doping (e.g., (i.e., Cu [12], Co [13])) and graphene hybridization [8]. In particular, graphene hybridization can achieve multifaceted synergistic effects: (1) The two-dimensional carbon matrix prevents nanoparticle aggregation, significantly increasing accessible catalytic surfaces for VOCs adsorption/desorption; (2) Graphene’s superlative conductivity establishes efficient electron transport pathways, facilitating rapid redox cycling; (3) Structural integration promotes oxygen vacancy regeneration through interfacial charge redistribution [14]. These combined advantages position graphene-incorporated α-MnO₂ (GM) composites as ideal active components for developing monolithic integrated catalyst systems with industrial-scale applicability.

Cordierite honeycomb ceramic, distinguished by its exceptional thermal stability, mechanical robustness, and cost-effectiveness, has become the substrate of choice for monolithic catalysts in various domains such as industrial flue gas treatment, vehicle exhaust purification, chemical synthesis, and biochemistry [15,16,17]. Since the catalytic performance is directly related to the active component, how to firmly fix the active component onto the cordierite substrate without affecting its catalytic activity is of great importance [18]. In the last decades, various kinds of loading methods have been developed, including impregnation [19], coating [20], hydrothermal synthesis [21], and in-situ synthesis [16,22], among others. Despite the widespread research and utilization of the cordierite monolithic integrated catalyst, challenges persist, including the non-uniform distribution of active components (notably in the coating method), the propensity of coatings to peel under the influence of flue gas flow and thermal shocks, and the complexity and stringency of preparation processes (as observed in in-situ/hydrothermal synthesis) that hinder scalability [23]. Therefore, despite a low-temperature active component, a facile, efficient, and easy-to-operate loading method also plays an important role in fabricating monolithic integrated catalyst.

In this work, a simple ball-milling-assisted impregnation method for loading active graphene-doped α-MnO₂ (GM) catalyst onto cordierite blocks was developed for preparation of GM/cordierite monolithic catalyst. First, the powdered 4GM (α-MnO₂ doped with 4 wt.% graphene) catalyst was prepared through a hydrothermal synthesis, followed by being ground into nanosized particles using a planetary ball mill. Second, the honeycomb cordierite blocks were etched with oxalic acid to create more micropore structures on the surface. Finally, the treated cordierite blocks were impregnated in the powdered GM suspension to complete the active component loading. This methodology demonstrates three distinctive technical merits: (1) Mechanical anchoring enhancement: oxalic acid etching induces porous surface topographies on cordierite, providing physical interlocking sites for GM nanoparticle immobilization; (2) Interfacial bonding reinforcement: π-π interactions between graphene and the silicate framework generate covalent-like interfacial adhesion, significantly improving peel resistance; (3) Nanostructural precision: ball milling enables controlled particle size reduction, while the graphene matrix suppresses nanoparticle aggregation during impregnation, collectively optimizing active surface accessibility and uniformity. Finally, the catalytic performance of the GM/cordierite monolithic catalyst for toluene oxidation under both dry and humid atmospheres were comprehensively evaluated by temperature-programmed testing, catalytic cycling testing, and stability testing. Meanwhile, structural and electronic characterizations (XRD, BET, SEM, XPS, H2-TPR) were conducted to reveal the underpinning mechanisms of catalytic performance. Furthermore, the catalytic oxidation performance of toluene at varied Gas Hourly Space Velocity (GHSV) from 750 h⁻¹ to 6000 h⁻¹, as well as the catalytic oxidation performance of different kinds of VOCs (i.e., toluene and acetone), were investigated to demonstrate its application potentials.

2. Results and Discussions

2.1. Catalyst Characterization

The active component 4GM was loaded onto cordierite (COR) supports to fabricate monolithic catalysts through different methodologies. As evidenced by the macroscopic observations of prepared catalysts (Figure S1), the ball-milling-assisted impregnation method yielded x-4GM/COR samples (Figure S1a–c) with superior catalyst distribution uniformity. Systematic optimization revealed a positive correlation between impregnation cycles and loading efficiency, achieving progressively enhanced loading contents of 4.7 wt.%, 6.0 wt.%, and 6.8 wt.% for 2-4GM/COR, 3-4GM/COR, and 4-4GM/COR, respectively. In contrast, the 4GM/COR prepared by both the coating method (Figure S1d,e) and the hydrothermal treatment (Figure S1f) not only presents uneven distribution of catalyst particles on the surface of cordierite block with partial channel blockage, but also poor adhesion stability, as evidenced by visible catalyst detachment from the cordierite substrates. This comparative analysis confirms the ball-milling-assisted impregnation method as an effective strategy for achieving both homogeneous catalyst distribution and stable immobilization on cordierite supports, addressing the critical challenges of particle aggregation and interfacial adhesion encountered in conventional preparation approaches.

XRD analysis was performed to discern the crystalline structural differences between COR and x-4GM/COR, as depicted in Figure S2. The diffractograms for both COR and x-4GM/COR displayed characteristic cordierite diffraction peaks (PDF#84-1897), with no notable phase alterations in the cordierite structure after loading the 4GM catalyst. Moreover, the XRD patterns for x-4GM/COR did not reveal any distinct peaks ascribable to MnO₂ species, which was mainly attributed to the low loading quantity and/or the dispersion of 4GM particles across the cordierite surface. Importantly, as the number of impregnation cycles increased, a gradual diminishment in the intensity of primary diffraction peaks corresponding to crystal planes such as (200) and (312) was observed. This trend likely results from the increased loading rate and dispersion of 4GM catalyst particles on the cordierite substrate.

To investigate the morphological evolution of cordierite during pretreatment and catalyst loading, SEM analysis was conducted on pristine cordierite, 4GM particles, and impregnated monolithic catalysts (Figure 1a–f). The untreated cordierite substrate (Figure 1a) exhibits a continuous, smooth surface with inherent structural integrity. Acid etching pretreatment (Figure 1b) generates a porous architecture characterized by uniformly distributed submicron-scale cavities (3–5 μm in length), creating an enhanced surface area for catalyst anchoring. Ball-milled 4GM precursor particles (Figure 1c) demonstrate a well-defined coral-like morphology with hierarchical nano-features. Progressive catalyst loading through multiple impregnation cycles is evidenced by SEM characterization. The 2-4GM/COR sample (Figure 1d) shows partial pore occupation with isolated 4GM clusters, confirming initial stage deposition. Subsequent impregnation cycles (3-4GM/COR, Figure 1e) yield increased surface coverage but the 4GM were unevenly distributed on the cordierite surface with obvious particle agglomeration. Optimal dispersion is achieved in 4-4GM/COR (Figure 1f), where the 4GM particles were present in reduced size and uniformly distributed on the subsurface pore networks. Complementary EDS mapping (Figure 1g,h) verifies the homogeneous spatial distribution of MnO₂ and graphene components within the composite architecture. This systematic investigation demonstrates that iterative impregnation cycles not only enhance catalyst loading capacity (from 4.7 wt.% to 6.8 wt.%) but critically improve deposition uniformity. Quadruple impregnation achieves optimal catalyst dispersion, balancing pore accessibility with maximal active site exposure while preventing particle over-accumulation.

The textural properties of 4GM, pre-etched cordierite (COR), and x-4GM/COR composites were systematically analyzed using nitrogen adsorption/desorption isotherms, with key parameters summarized in

Table 1. Table of pore structure parameters of 4GM/COR monolithic catalyst.

Catalyst	Surface Area (m2/g)	Average Pore Size (nm)	Total Pore Volume (cm3/g)	Mesoporous Pore Volume (cm3/g)
Cordierite	12	5.6	0.017	0.015
2-4GM/COR	5	6.2	0.008	0.006
3-4GM/COR	7	6.8	0.011	0.010
4-4GM/COR	8	6.9	0.013	0.011
4GM micron particle	112	7.4	0.210	0.180

and the isotherm profiles shown in Figure S3. The 4GM catalyst exhibited the highest specific surface area among the tested materials, displaying characteristic type IV adsorption isotherms indicative of its well-defined mesoporous structure (Figure S3a,b). In contrast, the pre-etched cordierite substrate demonstrated a significantly lower surface area, primarily due to its macroporous architecture featuring submicron-scale cavities (3–5 μm in length), as revealed by SEM. Notably, while macropores are easily visible, structural analysis reveals the coexistence of residual mesopores (Figure S3c). The integration of 4GM particles into the cordierite matrix induced distinct textural modifications: the hysteresis loop magnitude decreased progressively, accompanied by reductions in both surface area and pore volume. These observations suggest partial infiltration of 4GM particles into the cordierite’s pore network, coupled with potential pore-blocking effects that render some mesopores inaccessible (“dead pores”). Intriguingly, the composite materials exhibited progressive increases in specific surface area with successive impregnation cycles. This phenomenon is mainly attributable to two synergistic factors: (1) the inherently superior surface area of 4GM relative to cordierite, and (2) enhanced loading rate and dispersion uniformity of 4GM particles across the COR substrate with repeated loading, maximizing the exposure of 4GM’s high-surface-area architecture. This trend underscores the critical role of multi-cycle impregnation in achieving both optimal particle distribution and maximized exposure of active sites. The 4-4GM/COR catalyst, characterized by an optimal loading capacity, surface area, and pore volume, is anticipated to exhibit enhanced catalytic performance in VOCs oxidation.

X-Ray Photoelectron Spectroscopy (XPS) enables systematic investigation of surface elemental composition and chemical states in monolithic catalysts. Figure S4 presents the Mn 2p and O 1s spectra for 4GM and three monolithic catalysts, with corresponding quantitative data summarized in

Table 2. XPS analyses of catalysts.

Catalyst	Mn 2p			O 1s
	Mn3+, %	Mn4+, %	Mn3+/Mn4+	Olatt, %	Oabs, %	Ofunc, %
4GM micron particle	68.56	31.44	2.18	43.85	52.83	3.32
2-4GM/COR	48.04	51.96	0.92	4.74	57.35	37.91
3-4GM/COR	58.68	41.42	1.42	5.23	69.33	25.44
4-4GM/COR	66.40	33.60	1.98	5.54	60.12	34.34

. Analysis of the Mn 2p spectra (Figure S4a) reveals three characteristic peaks through deconvolution: surface Mn³⁺ at 639.89 eV, Mn⁴⁺ at 641.70 eV, and a satellite peak at 651.54 eV. Notably, the 2-4GM/COR catalyst exhibits pronounced oscillations in its Mn 2p spectrum, indicative of heterogeneous distribution and relatively low concentration of active components. In contrast, the 4-4GM/COR catalyst demonstrates minimal spectral fluctuations, suggesting improved homogeneity in the dispersion of active components across the cordierite substrate. The proportion of Mn³⁺ is observed to increase progressively with more maceration cycles, approaching the Mn³⁺ level of the 4GM particles (refer to Figure S4a and Table 2). Since the weaker bond energy of Mn³⁺-O compared to Mn⁴⁺-O bonds facilitates the dissociation and activation of oxygen molecules to participate in oxidation reaction, a higher proportion of Mn³⁺ is pivotal for the catalytic oxidation of VOCs at lower temperatures, which is extensively confirmed in previous literature [10]. Moreover, the large existence of surface low valence Mn would promote dissociation and activation of circumambient oxygen atoms since Mn³⁺ ions induce localized charge imbalance (Mn⁴⁺ → Mn³⁺ reduction), compensated by the formation of surface oxygen vacancies, which act as active sites for O₂ adsorption and dissociation, lowering the activation energy for molecular oxygen activation [9].

The deconvoluted O 1s XPS spectra in Figure S4b resolve into three oxygen species: surface lattice oxygen (O_latt) at 527.33 eV, surface adsorbed oxygen species (O_abs) at 529.41 eV, and oxygen containing functional groups (O_func) at 531.37 eV. Quantitative analysis (Table 2) reveals a significant redistribution of oxygen species upon loading 4GM onto the cordierite (COR) support. Specifically, the O_latt content undergoes a sharp reduction from 43.85% in pristine 4GM to 4.74–5.54% in supported catalysts, while O_func increases dramatically from 3.32% to 25.44–37.91%. This inversion is attributed to the dominance of oxygen-rich surface bonds inherent to the cordierite substrate (e.g., Al–O, Si–O, and Ca–O bonds), which weaken the lattice oxygen signal from the active phase. Notably, despite this overall suppression of surface lattice oxygen, its relative proportion systematically increases with successive impregnation cycles. This trend suggests progressive reconstruction of the active phase’s oxygen framework, likely due to improved dispersion and stronger interfacial interactions between 4GM nanoparticles and the COR substrate, indicating the 4-4GM/COR possesses an optimized ability to storage and release oxygen species. Concurrently, the O_ads fraction generally increases with impregnation cycles (e.g., 4-4GM/COR vs. 2-4GM/COR), directly correlating with oxygen vacancies that arise from charge compensation (Mn⁴⁺ → Mn³⁺ reduction) and serve as critical adsorption/activation sites for molecular O₂, and thereby play a pivotal role in catalytic oxidation of VOCs.

The synergistic interplay of these factors in 4-4GM/COR—elevated Mn³⁺ content, abundant O_latt, O_ads, and oxygen vacancies—creates an optimized micro-environment for oxygen activation. Lattice oxygen facilitates bulk redox cycles, while adsorbed oxygen and vacancies lower the activation energy for O₂ dissociation, collectively enhancing VOCs oxidation efficiency at low temperatures.

2.2. Catalyst Performance Analysis

2.2.1. Evaluation of Catalytic Activity

The catalytic activity of cordierite, x-4GM/COR, and monolithic catalyst prepared by the in-situ hydrothermal method were evaluated by using 300 ppm toluene/air as simulated VOC gas at a GHSV of 750 h⁻¹. The T₉₀ is an important parameter to evaluate catalytic performance, which corresponds to toluene degradation at 90%. As depicted in Figure S5, the bare cordierite support demonstrated limited catalytic activity within the temperature range of 150 to 330 °C, indicating that 4GM catalyst particles are the predominant active components. As expected, catalytic activity of x-4GM/COR is better than that of monolithic catalyst prepared by the in-situ hydrothermal method.

According to the analyses above, the x-4GM/COR monolithic catalysts prepared by the ball-milling assisted impregnation technique exhibited superior catalytic performance, and the effect of impregnation times on catalytic activity was further investigated in both dry and humid environments (RH = 85%), as shown in Figure 2. In the dry environment (Figure 2a), increasing the impregnation cycles is beneficial for the monolithic catalyst to improve catalytic activity. The 2-4GM/COR monolithic catalyst has poor performance with T₉₀ of 287 °C, while the 3-4GM/COR presents similar catalytic performance as the 4-4GM/COR, arriving T₉₀ at 200 °C. The loading amounts of 4GM catalysts on 3-4GM/COR and 4-4GM/COR are higher than that of 2-4GM/COR, leading to the enhanced catalytic performances. Furthermore, the excellent catalytic performances of 3-4GM/COR and 4-4GM/COR may be attributed to the synergy effect of abundant O_latt, O_ads, and Mn³⁺ species (Table 2), which implies abundant oxygen vacancies and excellent mobility of oxygen species [11,24,25], playing a key role in the catalytic processes.

The catalytic activities of x-4GM/COR in the humid environment were shown in Figure 2b. The catalytic activities of the x-4GM/COR catalysts at an RH of 85% increased with increasing impregnation cycles. As shown in Figure S6, the 4GM catalyst particles exhibited excellent water resistance, with a T₉₀ of approximately 155 °C (RH = 85%) [14]. As expected, the activity of the 2-4GM/COR catalyst was significantly reduced with increasing humidity, which may be directly related to the low loading amount of active components. In contrast, both the 3-4GM/COR and 4-4GM/COR monolithic catalysts exhibited excellent catalytic activity in the humid atmosphere (Figure 2b), achieving 95% toluene degradation at around 210 °C. It is worth noting that the 4-4GM/COR monolithic catalyst exhibited superior catalytic activity in the low-temperature region (<200 °C) in the humid (RH = 85%) environment, with T₅₀ and T₉₀ values of 160 °C and 198 °C, respectively. This phenomenon may be attributed to the reaction between partial O_ads and water molecules, resulting in the generation of OH* species, which can participate in the process of toluene degradation. In summary, it is believed that the high loading amount of the ball-milled 4GM particles retains the characteristic of high moisture resistance, contributing to the excellent catalytic performance of the 4GM/COR monolithic catalyst in a humid atmosphere.

The catalytic performances of the 4-4GM@COR monolithic catalyst were investigated over five cycles to ensure the repeatability of the catalytic reaction and the stability of the catalyst, as shown in Figure 2c,d. Whether in dry or humid (RH = 85%) environments, the 4-4GM/COR monolithic catalyst maintained a T₉₀ of approximately 200 °C, confirming its high cycling stability. It can be concluded that the uniformly dispersed active components and large specific surface area are conducive to the adsorption of VOCs and the dissociation of products, thereby accelerating the mass transfer efficiency of the catalytic process and improving the catalytic cycling performance. In addition, the high concentration of surface-adsorbed species (Table 2) ensures the excellent cycling performance, as it represents abundant oxygen vacancies. Overall, the high cycling stability of the 4-4GM/COR monolithic catalyst indicates that catalyst deactivation caused by the repeated start and stop of equipment can be effectively avoided in industrial scenarios.

2.2.2. Evaluation of Catalyst Applicability

After exploring the cycling performance of 4-4GM/COR on toluene degradation, the catalytic stability of the 4-4GM/COR monolithic catalyst was further investigated at 270 °C, as depicted in Figure 3. It is found that the catalyst was able to maintain almost 100% toluene degradation with CO₂ selectivity, amounting to 75–80% in the dry atmosphere during the 10 h test (Figure 3a), demonstrating that most of the toluene is effectively degraded into the final product CO₂ with only a small amount of by-products generated. Similarly, in a humid (RH = 85%) environment, the 4-4GM/COR monolithic catalyst maintained almost 100% toluene degradation, but there is a slight decrease in CO₂ selectivity (65~75%). The presence of water vapor introduces hydroxyl radicals (OH*), which seem to participate in the oxidation process, leading to the formation of small amounts of by-products like benzyl alcohol and formic acid [26]. The reduction in CO₂ selectivity in the presence of humidity is likely due to these alternate reaction pathways facilitated by the hydroxyl radicals. Overall, the high CO₂ selectivity demonstrates the high proportion of surface adsorbed oxygen species provides sufficient active species for the sustained catalytic reactions to achieve efficient mineralization of toluene. The efficient mineralization of toluene to CO₂ also indicates that the catalyst could be an excellent candidate for applications in air purification or industrial exhaust treatment, where VOCs like toluene need to be removed to mitigate environmental and health impacts.

In-situ DRIFTS was further conducted to elucidate the catalytic mechanism of toluene oxidation and product generation pathway. As shown in Figure 4, the obvious main characteristic peak detected at 2348 cm⁻¹ can be attributed to CO₂ [27]. The characteristic peak of CO₂ gradually increases with the increase of temperature, suggesting that the continuous increase in temperature is conducive to accelerate the catalytic oxidation of toluene. The peak at 1695 cm⁻¹ can be attributed to the tensile vibration of the benzene ring skeleton ν(C=C), which confirmed that the benzene intermediate was attached to the catalyst surface during the catalytic process. The characteristic peaks of the in-plane and out-plane bending vibration of methyl ν(C-H) detected at 821, 3035, and 3139 cm⁻¹ [28] indicates that toluene gas molecules first adsorb on the catalyst surface and then undergo oxidation reaction. Additional peaks at 1172 and 1236 cm⁻¹, assigned to ν(-OH), and those at 2800 and 2913 cm⁻¹, linked to benzyl species [29], emerge with increasing temperature, signifying benzyl alcohol as an intermediate in the toluene oxidation pathway. The characteristic peak of ν(-COO) at 1409 and 1535 cm⁻¹ is attributable to carboxylate species [29]. The characteristic peak of ν(-C-O) at 920, 1041 cm⁻¹ is ascribed to the generation of alkoxide species [30,31]. Combined with the XPS results, toluene molecules that adsorbed on the catalyst surface were oxidized by adsorbed oxygen species to generate CO₂ as major product with minor intermediate products (such as benzyl alcohol and benzoic acid).

After exploring the catalytic stability, the effect of gas hourly space velocity (GHSV) on catalytic performance was further investigated to evaluate the adaptability of 4-4GM/COR in industrial processes (Figure 5a). As the GHSV increased from 750 h⁻¹ to 6000 h⁻¹ (equivalent to an airspeed of approximately 1 to 8 m/s and a retention time of approximately 0.2 to 0.025s), the 4-4GM/COR monolithic catalyst maintained a stable T₉₀ at 200 °C, signifying the monolithic catalyst exhibited superior adaptability to high wind speeds. This can be attributed to the parallel arrangement of honeycomb pores in the cordierite, which is beneficial for catalytic processes via providing a high surface area with a low pressure drop. Furthermore, by loading 4GM onto the cordierite, the effective contact area between the catalyst and the VOCs was increased and the reaction path length within the same volume was extended, contributing to a more thorough interaction between VOCs and the catalyst, which leads to efficient degradation. In summary, the 4-4GM/COR monolithic catalyst demonstrated great potentials for industrial VOC degradation applications due to its stability and adaptability to different GHSV.

In order to comprehensively evaluate the applicability of 4-4GM/COR, the catalytic performance for different kinds of VOCs degradation was investigated by introducing acetone as another kind of VOCs. The performances for catalytic oxidation of toluene (alone), acetone (alone), and a mixture of toluene and acetone were investigated by setting the concentration of each kind of VOC at 300 ppm and the total GHSV at 750 h⁻¹, as illustrated in Figure 5b. With regard to single-type VOC, the catalytic performance of acetone over 4-4GM/COR is similar to that of toluene, achieving 98% conversion at 230 °C. When toluene and acetone were catalyzed together, it is interesting to find the toluene conversion was greatly promoted while the acetone conversion was slightly inhibited at the low-temperature region (<165 °C), which is speculated to be attributed to the preferential adsorption of toluene owing to the π-π accumulation effect between benzene rings of graphene and toluene. With the increase of temperature, the toluene conversion curve shifted to a higher temperature, mainly because of competition between toluene and acetone on the active sites. Even though the presence of acetone raises the T₉₀ for toluene to 250 °C, this performance is deemed equivalent to some noble metal catalysts, highlighting the potential of 4-4GM/COR as a cost-effective alternative to expensive noble metal catalysts for VOC degradation.

2.2.3. Discussion on Application Potentials

Whether a catalyst can be used in industrial processes depends on many factors, including activity, stability, cost, scalability and availability, toxicity and environmental impact, and so on.

Table 3. Performance comparison of 4-4GM/COR with other related monolithic catalysts.

Monolithic Catalyst	T90 (°C)	WHSV/GHSV h−1/ mL/(gh)	Concentration (ppm)	Loading Method	Application Potential Analysis	Ref.
4-4GM/COR	200	6000 h−1	300	Ball-milling assisted impregnation method	Low material cost; No precious metal; Easy preparation	This work
0.07Pd/δ-MnO2-NFA	219	10,000 h−1	250	A low-temperature hydrothermal route	Pd; High cost	[32]
Ag/MnO2-COR	275	10,000 h−1	1000	Wet impregnation method with aluminum sol binder	Ag; High cost	[33]
Co0.67Mn0.33/COR	220	45,000 mL/(g·h)	500	Ultrasonic impregnation	Precious Co; High cost	[34]
La0.7Ce0.3CoO3/Ce0.75Zr0.25O2-Al2O3/COR	218	6000 mL/(g·h)	1000	Coating method with PMMA binder	Precious Co, La, Ce; High cost; Complex preparation	[35]
LaCoO3/ZrO2/ COR	216	-	1000	Coating method with PVA binder	Precious Co, La; High cost	[36]
La0.8Ce0.2MnO3/COR	217	6000 h−1	500	Coating method with PMMA binder	Precious La, Ce; High cost	[37]
Pt/CuMnCe/CH	260	5000 h−1	2000	Immersion method	Precious Pt, Cu, Ce; High cost	[38]

compares the 4-4GM/COR monolithic catalyst extensively with other representative monolithic catalysts used for toluene oxidation in the literature. First, the 4-4GM/COR catalyst developed in this study demonstrates an attractive option for industrial applications since it has superior catalytic activity, which can reduce energy consumption and costs. Additionally, the 4-4GM/COR catalyst exhibits excellent water resistance, durability, and stability, rendering it suitable for a wider range of applications. Second, compared with commercial noble metal catalysts, the 4-4GM/COR catalyst has the advantage of low cost, as the raw materials (MnOx and graphite) required for the monolithic catalyst are readily available and inexpensive. Furthermore, the ball-milling assisted impregnation method employed for the preparation of the monolithic catalysts not only ensures strong adherence of the catalyst particles to the support material but also eliminates the need for additional binders or high-temperature/high-pressure synthesis processes, thereby further reducing preparation costs. Last, the 4-4GM/COR monolithic catalyst features easy scalability and excellent adaptability to high gas hourly space velocities (GHSV) due to its robust mechanical strength and low pressure drop. Clearly, the 4-4GM/COR monolithic catalyst proposed in this study exhibits outstanding overall performance in terms of activity, cost-effectiveness, availability, and scalability.

3. Materials and Methodologies

3.1. Materials

Cordierite, purchased from Haichuan Chemical Co., Ltd., Shanghai, China, is a cylindrical honeycomb block with a diameter of 34 mm, a height of 8 mm, and an internal square hole side length of 1mm. Oxalic acid dihydrate (H₂C₂O₄·2H₂O, AR, ≥99.5%) and absolute ethanol (C₂H₅OH, AR, ≥99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China.

3.2. Monolithic Catalyst Preparation

(1)

Cordierite pretreatment

Cordierite blocks were repeatedly cleaned using deionized water and anhydrous ethanol to remove surface impurities, then dried in an oven at 110 °C for 12 h. Subsequently, cordierite blocks were placed in the 200 mL oxalic acid solution (10 wt.%) for 12 h to etch the surface of the support. After soaking, the cordierite block was picked out and washed with deionized water, and then dried for subsequent treatment.

(2)

Preparation of monolithic catalysts

Graphene-doped α-MnO₂ was synthesized by the in-situ hydrothermal method, which is described in detail in the Supplementary Materials. To prepare the monolithic catalysts, 3 g of the 4GM powder were initially ground for 2 h at a 200 r/min rate in a planetary mill. Subsequent to milling, the 3 g 4GM catalyst was dispersed in 200 mL of deionized water to form a nano-sized particle suspension. The cordierite was then immersed in this suspension for 6 h, after which it was removed, and the excess solution was expelled from the honeycomb structure using a blower. The sample was subsequently dried at 110 °C for 12 h. The resulting monolithic catalyst was designated as 1-4GM/COR. The impregnation process described above was repeated to synthesize x-4GM/COR catalysts (where x = 2, 3, or 4, indicating the number of impregnation cycles). The weights of the x-4GM/COR catalysts were recorded before and after impregnation to calculate loading rate. In comparison, in-situ hydrothermal synthesis and a coating method using binders were also investigated for the preparation of the monolithic catalyst, with comprehensive details provided in the Supplementary Materials.

3.3. Catalyst Characterization Method

The crystal phase of the samples was characterized by an X-Ray diffractometer (XRD, PANalyticalAeris, Almelo, Holland), in which the Cu-Kα radiation voltage was 40 kV, the current was 15 mA, the scanning range was 8~65°, and the step size was 0.02°. N₂ adsorption-desorption isotherms were performed on Autosorb-iQ (Quantachrome Instruments Co., Ltd., Boynton Beach, FL, USA) using N₂ as the analytical gas at -196 °C. Before the measurement, the catalyst was degassed at 200 °C for 6 h to remove physically adsorbed water and impurities. The specific surface area was determined by the Brunauer Emmett Teller (BET) equation, and the pore size distribution was analyzed by the quenched solid density functional theory (QSDFT) method. Thermal field emission scanning electron microscopy (SEM, TESCAN MIRA LMS, Beijing, China) was used to characterize the surface morphology of the samples. The surface chemistry was investigated by X-Ray photoelectron spectroscopy (XPS) using a 150 W Al Kα probe beam on a KRATOS AXIS Ultra DLD instrument (XPS, SHIMADZU, Kyoto, Japan). H₂ temperature-programmed reduction (H₂-TPR) was performed on ChemBET Pulsar TPR/TPD (Quantachrome Instruments Co., Ltd., Boynton Beach, FL, USA) to characterize the redox ability of the catalysts. In the TPR experiment, the sample mass was uniformly set to 5 mg, He was used as the carrier gas, 5% H₂/He was used as the reaction gas, and the flow rate of the reaction gas was controlled at 110–120 mL/min. The in-situ DRIFTS experiment was performed on a Fourier transform infrared spectrometer (DRIFTS, Thermo Nicolet Co., Ltd., Waltham, MA, USA) to investigate the catalytic mechanism of toluene.

3.4. Catalyst Performance Testing

In this study, a VOC generator (FD-PG02, Flender Testing Equipment Co., Ltd., Suzhou, China) was used to simulate the VOC gas. Synthetic air was used as the carrier gas, and the flow rate was stabilized at 250 mL/min, 500 mL/min, or 1000 mL/min by a mass flow meter (FL-802, Flowmethod Co., Ltd., Shenzhen, China). Liquid toluene (acetone when needed) was injected into the carrier gas using a micro-syringe pump (SPLab 01, Shenzhen Pump Industry Co., Ltd., Shenzhen, China.) and vaporized at 110 °C to generate a VOC gas stream.

The catalytic reaction was carried out in a quartz reactor (530 mm in length, 40 mm in outer diameter, 35 mm in inner diameter), and the temperature could be adjusted from room temperature to 1200 °C. For each experiment, three pieces of cordierite catalyst blocks were filled into the reactor with the inlet toluene concentration set to 300 ppm. The outlet of the reaction device was connected to a GC/MS gas chromatograph (SP-3420A, BFRL Co., Ltd., Beijing, China) to analyze the concentrations of toluene (acetone when acetone was used as simulated VOC gas), CO₂, and CO. In order to study the effect of humidity on the catalytic performance, deionized water was injected into the VOC generator through another micro-syringe pump, and a humidity detector was equipped to measure the atmospheric humidity in real-time. The experimental setup was illustrated in Figure S7 in the Supplementary Materials.

The main parameters to evaluate the catalytic performance of catalysts for toluene oxidation are the toluene degradation efficiency (X_toluene) and the CO₂ selectivity (S_CO2). The calculation formulas are as follows:

$$X_{toluene}(\%)={\displaystyle \frac{{[Toluene]}_{in}-{[Toluene]}_{out}}{{[Toluene]}_{in}}}\times 100$$

(1)

$$S_{{CO}_2}(\%)={\displaystyle \frac{[{CO}_2]}{7\times ({[Toluene]}_{\mathrm{in}}-{[Toluene]}_{\mathrm{out}})}}\times 100$$

(2)

where [Toluene]_in and [Toluene]_out are the concentrations of toluene at the reactor inlet and outlet, respectively, and [CO₂] is the concentration of CO₂ at the reactor outlet. The number 7 in Formula (2) represents that 1 mole of toluene can be completely degraded into 7 moles of CO₂.

4. Conclusions

The monolithic catalyst was fabricated via a facile ball-milling-assisted impregnation strategy. A cordierite honeycomb substrate, pre-etched with oxalic acid to enhance surface roughness and area for catalyst anchoring, served as the structured support. Nano-sized 4GM particles, obtained by mechanical ball milling, were uniformly and firmly anchored onto the cordierite’s submicron-scale cavities (3–5 μm in length) without blocking the pristine COR channels. Structural analyses revealed that iterative impregnation cycles (up to four cycles) progressively enhanced the loading rate and distribution uniformity of the active components, specific surface area, and pore volume. Concurrently, the optimized surface chemistry—characterized by elevated Mn³⁺ content, abundant lattice oxygen species, and oxygen vacancy density—synergistically facilitated the activation and mobility of reactive oxygen species, thereby boosting catalytic performance.

The 4-4GM/COR catalyst demonstrated exceptional toluene oxidation activity, achieving 90% conversion (T90) at 200 °C under dry conditions and 198 °C in humid atmospheres (RH = 85%), outperforming most reported monolithic catalysts. Remarkably, it maintained a stable T90 of 200 °C across five consecutive cycles and under varying gas hourly space velocities (GHSV: 750–6000 h⁻¹), confirming its robustness in practical applications. Additionally, the 4-4GM/COR monolithic catalyst demonstrated excellent catalysis stability with 100% toluene conversion at 270 °C in both dry and humid environments. In summary, the monolithic catalyst prepared using the “ball-milling assisted impregnation method” exhibits high activity, stability, and adaptability to varying GHSV. The preparation method is simple, easy-to-operate, and holds great potential for industrial applications.