Electrocatalytic Oxidation for Efficient Toluene Removal with a Catalytic Cu-MnO_x/GF Electrode in a Solid-State Electrocatalytic Device

Abstract

A series of Cu-MnO_x/GF catalytic electrodes, with graphite felt (GF) pretreated via microwave modification as the catalyst carrier, were prepared under various hydrothermal conditions and characterized using X-ray Diffraction (XRD), Scanning Electron Microscope (SEM), X-ray photoelectron spectroscopy (XPS), N₂ adsorption–desorption, and Raman spectroscopy. The catalytic oxidation activity of catalytic Cu-MnO_x/GF electrodes toward toluene was evaluated in an all-solid-state electrocatalytic device under mild operating conditions. The evaluation results demonstrated that the microwave-modified catalytic electrode exhibited high electrocatalytic activity toward toluene oxidation, with Cu-MnO_x/700W-GF exhibiting significantly higher catalytic activity, indicating that an increase in catalyst loading capacity can promote the removal of toluene. Only CO₂ and CO were detected, with no other intermediates observed in the reaction process. Moreover, the catalytic effect was significantly affected by the relative humidity. The catalytic oxidation of toluene can be fully realized under a certain humidity, indicating that the conversion of H₂O to strongly oxidizing ·OH on the catalytic electrode is a key step in this reaction.

1. Introduction

Toluene is an important organic raw material with extensive applications and is also a common Volatile Organic Compound (VOC) pollutant. There is currently a large amount of research on metal oxide catalysts for the catalytic oxidation of toluene, but the widely used catalysts are often expensive, toxic, and poorly resistant to moisture [1]. In recent years, as researchers have focused more on modified and optimized transition metal oxide catalysts, an increasing number of inexpensive and highly stable catalysts have been developed [2,3], with the most notable being the modified manganese oxide catalysts dominated by MnO₂ [4]. The oxidation–reduction of Mn³⁺/Mn⁴⁺ in MnO₂ is conducive to the formation of reactive oxygen species on the catalyst surface, thereby improving the activity of the catalyst [5]. To improve the overall catalytic effect, manganese-based catalysts are usually modified in different ways to form different crystal structures and more exposed sites on the surface, resulting in significant differences in catalytic activity, water resistance, and toxicity resistance [6,7,8].

Although conventional catalytic oxidation can achieve the rapid oxidation of high amounts of VOCs, it is associated with problems such as high energy consumption, harsh reaction conditions, and limited application scenarios. Electrocatalytic technology, as a relatively mild method for organic pollutant oxidation, relies on active oxygen and the strong oxide ·OH generated by the anodic catalytic activation of O₂ and H₂O under an external voltage [9]. Compared to catalytic oxidation, it does not require harsh reaction conditions or high material costs and uses a low voltage to achieve efficient catalytic oxidation of organic pollutants at room temperature and thus it has received increasing attention from researchers [10,11,12]. However, due to frequent electrolyte replacement, complex operation, and limited effectiveness on insoluble VOCs, traditional electrocatalytic reactor structures are not suitable for treating insoluble gases such as toluene and xylene. Therefore, many researchers have started to use all-solid-state reactors and porous catalytic electrodes for the electrocatalytic oxidation of VOCs [13,14].

Compared with conventional electrocatalysis, all-solid-state reactors can fully utilize the hydroxyl radicals (·OH) generated by the electrolysis of water vapor to catalyze the oxidation of VOCs under a certain humidity through direct adsorption of gaseous VOCs and achieve higher mineralization rates. For metal oxide anode MO_x, ·OH can combine with MO_x after generation (Equation (1)) to generate MO_x+1 with a higher oxidation state (Equation (2)). Subsequently, MO_x+1 reacts with VOCs (Equation (3)), accompanied by side reactions that create oxygen (Equation (4)).

$$\mathrm{M}{\mathrm{O}}_{\mathrm{x}}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{M}{\mathrm{O}}_{\mathrm{x}}(\mathrm{H}\mathrm{O}\cdot )+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

(1)

$$\mathrm{M}{\mathrm{O}}_{\mathrm{x}}(\mathrm{H}\mathrm{O}\cdot )\to \mathrm{M}{\mathrm{O}}_{\mathrm{x}+1}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}$$

(2)

$$\mathrm{M}{\mathrm{O}}_{\mathrm{x}+1}+\mathrm{R}\to \mathrm{M}{\mathrm{O}}_{\mathrm{x}}+\mathrm{R}\mathrm{O}$$

(3)

$$\mathrm{M}{\mathrm{O}}_{\mathrm{x}+1}\to \mathrm{M}{\mathrm{O}}_{\mathrm{x}}+1/2{\mathrm{O}}_2$$

(4)

However, most existing all-solid-state reactors feature titanium sheet metal or foam titanium as the catalyst carrier. The titanium electrode itself has a small specific surface area and a limited catalyst loading capacity, resulting in a short contact time between the gas and the electrode, a low catalyst load, and other problems. Therefore, it shows some limitations when dealing with high-concentration VOC gases. Víctor et al. successfully prepared porous expanded graphite felt carriers using microwave irradiation for loading sulfonated polystyrene for seawater desalination treatment [15]. Inspired by this, we hypothesize that the use of highly conductive and loosely structured graphite felt may compensate for the defects of titanium electrodes [14].

In this study, graphite felt (GF) was used as a catalyst carrier to enable toluene to penetrate through to the interior of the catalytic electrode, thereby effectively improving the contact area between the catalytic electrode and the reaction molecules. A microwave processing method was used to induce surface porosity in the graphite felt [16], thereby increasing the number of adsorption sites of the catalyst on the graphite felt. A new catalytic electrode suitable for the electrocatalytic degradation of toluene was prepared, controlling the catalyst loading amount, microwave treatment power, and other conditions. The electrodes were installed on an all-solid-state electrocatalytic device to test the effects of different reaction conditions on the electrocatalytic oxidation of toluene, and the factors affecting the electrocatalytic effect of the catalysts were proposed through using other characterization methods, providing theoretical guidance and technical support for the design of electrocatalytic materials for toluene.

2. Results and Discussion

2.1. Structure and Characterization of Catalytic Electrodes

2.1.1. XRD { TC “5.3.1 Structure and Composition Analysis of Catalytic Electrode” \l 3 }

Figure 1 shows the XRD spectra of the Cu-MnO_x/GF catalytic electrode synthesized via a one-step hydrothermal method. It shows that the Cu-MnO_x catalyst loaded on the graphite felt catalyst is mainly Mn₃O₄. The reason for this observation may be that the doping of graphite felt leads to competitive reactions during the hydrothermal process and the reduction of high-valence Mn ions (Mn⁷⁺, Mn⁴⁺) by graphite felt, resulting in a decrease in the final valence state of Mn. X-ray diffraction peaks of Cu and other manganese oxides were not observed in the spectra, possibly because Cu may exist in an amorphous or low-crystallinity form [17] which cannot be detected by XRD.

2.1.2. SEM

Figure 2 shows the SEM spectra of the catalytic electrodes prepared under different conditions. As illustrated in the figure, the fiber comprises graphite felt fibers and exhibits a rod-shaped structure. The particles with a relatively high brightness are Cu-MnO_x grains, indicating that a certain amount of these grains are attached to the loaded graphite felt. Due to the limited pore structure of graphite felt fibers, Cu-MnO_x catalysts mostly adhere to the surface of graphite felt in the form of surface adsorption.

2.1.3. Thermogravimetric Analysis

The weight loss curves of catalytic electrodes prepared under different conditions in air atmosphere at 25~800 °C are shown in Figure 3. It can clearly be seen that microwave modification effectively increases the loading capacity of the catalyst for catalytic electrodes, and all catalytic electrodes, except for pure graphite felt, enter the rapid weight loss stage of graphite felt combustion after undergoing a slow dehydration process at low temperatures. Pure graphite felt itself does not contain Cu-MnO_x catalyst and does not undergo hydrothermal reaction, so there is no dehydration stage and intense combustion only begins at around 600 °C. In contrast, the graphite felt loaded with Cu-MnO_x enters a rapid weight-loss stage at a lower temperature, which may be due to the redox between MnO_x and graphite felt at high temperature, which accelerates the consumption of the latter. However, MnO_x still exists in the form of Mn₃O₄ after reacting with high-temperature air [18].

2.1.4. XPS

The chemical valence states of Cu, Mn, and O on the surface of Cu-MnO_x/GF catalytic electrodes were analyzed, and the XPS spectra are shown in Figure 4. As shown in Figure 4b, unlike MnO₂, the binding energy peak positions in the 640–650 eV and 650–660 eV regions correspond to Mn 2p3/2 and Mn 2p1/2 in Mn₃O₄, while the asymmetric peak of Mn 2p3/2 can be decomposed into three different components, namely Mn²⁺, Mn⁴⁺, and satellite peaks [19,20]. In the XPS spectrum of Cu 2p, the binding energy peak position is 934.7 eV, corresponding to Cu 2p3/2, indicating that the catalyst contains Cu²⁺. The peak at 932.5 eV also corresponds to Cu 2p1/2, which indicates the successful introduction of Cu into the catalytic electrode. The peak positions of surface hydroxyl oxygen (O_adsO-H), surface-adsorbed oxygen (O_ads), and lattice oxygen (O_latt) in the catalytic electrode are 530.3 eV, 531.7 eV, and 533.4 eV, respectively.

Table 1. Comparison of valence states of Mn and Cu in Cu-MnO_x/GF catalytic electrodes.

Sample	Mn4+ (% Atom)	Mn2+ (% Atom)	Mn4+/Mn2+	Cu2+ (% Atom)	Cu+ (% Atom)	Cu2+/Cu+
Cu-MnOx/GF	59.39	40.61	1.17	55.70	44.30	1.26

and

Table 2. Relative content of surface-adsorbed oxygen (O_ads) and lattice oxygen (O_latt) in Cu-MnO_x/GF catalytic electrodes.

Sample	Olatt (% Atom)	Oads (% Atom)	Oads/Olatt
Cu-MnOx/GF	66.54	33.46	1.99

show a comparison of the atomic contents of Mn, Cu, and O in different valence states and their forms existing on the surface of Cu-MnO_x/GF catalytic electrodes.

2.1.5. Raman Spectra

Figure 5 shows the Raman spectra of the Cu-MnO_x/GF catalytic electrode, with the characteristic peaks located at 304.4 cm⁻¹, 358.1 cm⁻¹, and 649.1 cm⁻¹ corresponding to the Mn-O bond. The characteristic peak at 649.1 cm⁻¹ corresponds to the ν₁(Mn-O) symmetric stretching vibration in octahedral [MnO₆]. The smaller characteristic peaks at 304.4 cm⁻¹ and 358.1 cm⁻¹ correspond to the telescopic vibration of ν₂(Mn-O) in Mn₃O₄. It is worth noting that the characteristic peaks at 304.4 cm⁻¹ and 358.1 cm⁻¹ for the Cu-MnO_x/GF catalytic electrode are pronounced. This is because the Cu-MnO_x/GF catalytic electrode itself contains a large amount of Mn₃O₄ crystals, which also indicates that the precursor of the catalyst is “robbed” of some oxidants in the co-hydrothermal reaction with graphite felt, leading to a decrease in the overall valence state of Mn.

2.1.6. Cyclic Voltammetry Curve

In order to explore the oxidation of toluene and H₂O on the surface of the catalytic electrode in the all-solid-state electrocatalytic device, a double-chamber, three-electrode electrolytic cell with a proton exchange membrane was used for cyclic voltammetry measurements at 25 °C. The anode was a catalytic electrode, the cathode was a platinum electrode, and the reference electrode was a saturated Ag/AgCl electrode. The scanning rate was 10 mV/s, and the potential window was −1 to 2 V. It can be seen from Figure 6 that the anodic peaks at 0.82 V in curves III and IV indicate the oxidation of Mn²⁺ to Mn³⁺/Mn⁴⁺, while the cathodic peak at 0.82 V indicates the reduction of Mn³⁺ to Mn²⁺. The redox peak of Mn was not observed in the CV curves of the other two pure graphite felt components [21]. In addition, it was found that the oxidation peak located at the 0.31 V anode corresponds to the oxidation of Cu⁺ to Cu²⁺ [22].

The onset potential for the oxygen evolution reaction (OER) on Cu-MnO_x/GF was observed at 1.23 V, which is higher than that of pure graphite felt. A high oxygen evolution overpotential assists in the accumulation of ·OH at the electrode, which is conducive to the electrocatalytic oxidation of organic pollutants [9,23]. In addition, no significant changes in the redox peak position were observed after the addition of toluene, indicating that toluene cannot be directly oxidized by the catalytic electrode at a voltage of 2 V.

2.2. Evaluation of Catalytic Electrode Activity { TC “5.3.3 Catalyst Activity Evaluation” \l 3 }

2.2.1. Electrocatalytic Oxidation of Toluene in a Dry Environment

To confirm whether toluene can be directly oxidized on the surface of the catalytic electrode, the electrocatalytic oxidation of toluene by the catalytic electrode was studied under an anhydrous atmosphere (0% RH). Figure 7 shows the toluene removal effect of the Cu-MnO_x/GF catalytic electrode under an anhydrous atmosphere (0% RH) and different voltage conditions.

From Figure 7, it can be seen that as hydroxyl radicals (·OH) are unable to be generated on the surface of the catalytic electrode under an anhydrous atmosphere (0% RH), the toluene adsorbed on the surface cannot be further catalytically oxidized to CO₂ or CO, and thus only the adsorption effect of the catalytic electrode on toluene can be observed. The results indicate that the Cu-MnO_x/GF catalytic electrode can adsorb toluene under anhydrous conditions, regardless of whether or not an external voltage is applied. However, as the voltage increases, the toluene adsorption effect of the catalytic electrode gradually weakens, which may be due to the increase in temperature of the catalytic electrode caused by the external voltage, leading to toluene desorption.

2.2.2. Electrocatalytic Oxidation of Toluene at 50% RH

Figure 8 shows the catalytic effect and the CO_x yield of the Cu-MnO_x/GF catalytic electrode on toluene at 50% RH and at different voltages. From the figure, it can be seen that when the applied voltage is 0 V, the catalytic electrode only has an adsorption effect on toluene, and no CO and CO₂ are generated during this process. As the electrolysis voltage of H₂O is 1.23 V, when the applied voltage is 2 V, 4 V, and 6 V, a large amount of water vapor is electrolyzed into strongly oxidizing ·OH, which quickly oxidizes the toluene adsorbed on the catalytic electrode’s surface to CO and CO₂, with the latter being the main product.

All catalytic electrodes loaded with Cu-MnO_x can achieve almost complete catalytic oxidation of toluene within 180 min. However, as the voltage increases, there is no significant change in the toluene removal rate and CO_x yield. This may be because higher voltage cannot significantly accelerate the electrolysis of H₂O, and hence toluene cannot be directly oxidized on the catalytic electrode. This indirectly indicates that the main oxidant in the process of electrocatalytic oxidation of toluene is the ·OH generated by H₂O electrolysis rather than reactive oxygen species generated after O₂ activation.

2.2.3. Electrocatalytic Toluene Oxidation at Different Catalytic Electrodes

Figure 9 shows the toluene removal rate and CO_x yield of different catalytic electrodes at a relative humidity of 50% and an applied voltage of 2 V. From the results, it can be seen that toluene can also be removed through adsorption by graphite felt without Cu-MnO_x loading, and a small amount of CO_x will be generated during the process. This is due to the partial ·OH generated by the electrolysis of H₂O at the electrode, but the effect is limited. Graphite felt catalytic electrodes loaded with Cu-MnO_x exhibited a high toluene removal rate and CO_x yield, with the catalytic effect improving with an increase in microwave pre-treatment power. This is because the Cu-MnO_x loading amount continues to increase, providing more active sites for the electrolysis of H₂O. Furthermore, Cu-MnO_x/700W-GF exhibits significantly a higher catalytic activity compared to other catalytic electrodes, indicating that an increase in loading can promote the removal of toluene.

2.2.4. Electrocatalytic Oxidation of Toluene at Different RHs

From the above analysis, it can be concluded that the presence of water vapor significantly affects the mechanism of toluene removal and the CO_x yield during the electrocatalytic process. Therefore, the electrocatalytic effect of Cu-MnO_x/GF on toluene was compared at different relative humidities. As shown in Figure 10, the applied voltage was fixed at 2 V and the relative humidity was varied from 0 to 75% RH. Due to the initial concentration of toluene being only 100 ppm, water vapor can be considered to be in excess.

From the results, it can be seen that when the relative humidity is 0%, toluene can only be adsorbed on the catalytic electrode and cannot be further oxidized to CO_x. Therefore, when the relative humidity increases, the yield of CO_x, especially CO₂, significantly increases. This further proves that ·OH generated by H₂O electrolysis is the key to the electrocatalytic oxidation of toluene. However, the toluene removal rate and CO_x yield did not change significantly with a further increase in relative humidity, which may be due to an excess of water vapor in the reactive gases resulting from an increase in relative humidity and the low loading capacity of the Cu-MnO_x catalyst on the catalytic electrode, which cannot provide more catalytic active sites.

2.2.5. Cyclic Stability of Catalytic Electrode for Toluene Oxidation { TC “5.3.4 Stability Analysis of Catalytic Electrode for Catalytic Oxidation ” \l 3 }

Figure 11 shows cyclic electrocatalytic testing of the Cu-MnO_x/GF catalytic electrode at voltage of 2 V and a relative humidity of 50%. At the end of each cycle, the air bag was replaced, 100 ppm toluene was added to the reaction system, and the experiment continued after the relative humidity stabilized. The results show that the catalytic activity of the catalytic electrode for toluene remained at a high level during multiple cyclic tests, indicating that the Cu-MnO_x catalyst has good water resistance and high catalytic activity. In the third cycle, the toluene removal rate of the Cu-MnO_x/GF catalytic electrode was slightly lower than in the first two cycles, which may be due to an accumulation of reaction products on the surface of the catalyst, occupying the active site.

2.3. Analysis of Electrocatalytic Oxidation Mechanism { TC “5.3.5 Mechanism Analysis of Electro-Catalytic Oxidation” \l 3 }

The above experimental results show that the process of toluene oxidation on the Cu-MnO_x/GF catalytic electrode is relatively thorough, and its oxidation products are CO and CO₂. This may occur because a large amount of ·OH can be generated during the electrocatalytic oxidation of toluene in an all-solid electrolytic cell, and the residual H⁺ moves to the cathode through the proton exchange membrane to further react with O₂ during the activation of H₂O into ·OH [14].

It can be seen that the electrocatalytic oxidation of toluene mainly relies on the strong oxidizing effect of ·OH generated by H₂O electrolysis. In contrast, the direct oxidation of toluene on the electrode is more difficult, differing from the mechanism of catalytic oxidation.

Due to the small impact of external voltage on the toluene removal rate, it can be inferred that toluene is first adsorbed by the catalytic electrode and then oxidized and decomposed by ·OH, which is generated by the electrolysis of H₂O on the catalyst surface. The oxidized product, CO_x, is released from the electrode surface after desorption, promoting further adsorption of the remaining toluene. A process diagram is shown in Figure 12. It can subsequently be inferred that the electrocatalytic oxidation of toluene is, to some extent, affected by the mass transfer rate of related substances, such as H₂O, toluene, O₂, and H⁺.

3. Materials and Methods

3.1. Preparation of Cu-MnO_x/GF Catalytic Electrodes

Cu-MnO_x/GF catalytic electrodes were prepared by the co-hydrothermal method, using high-conductivity graphite felt (resistance coefficient: 0.18~0.22 Ω·m) and a catalyst precursor. A specified amount of KMnO₄ powder was dissolved in 100 mL of deionized water. CuCl₂·2H₂O and MnCl₂·4H₂O were dissolved in 100 mL of deionized water, and these solutions were slowly added to the KMnO₄ solution under magnetic stirring. The resulting mixture was sealed and incubated at room temperature for 24 h. Subsequently, the mixture was stirred, and HCl was added until the mixture reached a pH of 4. To improve the loading capacity of the graphite felt, it was irradiated with different microwave powers (0 W, 300 W, 500 W, and 700 W) for 30 s.

The graphite felt and catalyst precursor mixture was then transferred to a high-pressure hydrothermal kettle in a muffle furnace to carry out the hydrothermal reaction. The mixture was heated to a specified temperature and maintained at that temperature for 48 h. Upon completion of the hydrothermal reaction, the resulting precipitate was removed from the hydrothermal kettle, washed, and dried at 110 °C for 12 h. According to the preparation conditions, the obtained catalytic electrodes were named pure GF, Cu-MnO_x/GF, Cu-MnO_x/300W-GF, Cu-MnO_x/500W-GF, and Cu-MnO_x/700W-GF.

3.2. Characterization Cu-MnO_x/GF Catalytic Electrodes

SEM was performed using a hot-stage field emission scanning electron microscope (Gemini 300, Zeiss, Oberkochen, Germany) to observe the surface morphology and dispersion of the active components in the samples. XRD was performed to characterize the phase of the catalyst’s active components using an X-ray diffractometer. The testing conditions were as follows: copper target material, tube voltage of 40 kV, tube current of 100 mA, scanning angle range of 10–80°, and scanning speed of 8°/min. XPS testing was conducted using an X-ray photoelectron spectrometer (Thermo Fisher ESCALAB Xi+, Thermo Fisher Scientific, Waltham, MA, USA) to analyze and characterize the elemental valence states before and after the material reactions. The testing conditions were as follows: 180° hemispherical energy analyzer, Al Kα monochromatic radiation, energy range of 0–5000 eV, and adjustable X-ray spot size ranging from 900 to 200 μm.

The pore structure characteristics of the samples (specific surface area, pore size distribution, total pore volume, and average pore size) were analyzed and characterized using an automated surface area and pore size distribution analyzer (Autosorb-IQ, Micromeritics, Norcross, GA, USA). Prior to testing, the samples were degassed at 250 °C for 6 h to remove impurities. Adsorption and desorption with N₂ as the adsorbate were performed at liquid nitrogen temperature (−196 °C) and a relative pressure (p/p₀) ranging from 0.001 to 1. Pore structure parameters such as specific surface area and pore size distribution were obtained by analyzing the samples’ adsorption–desorption isotherms.

Raman spectroscopy is often used to analyze the degree of surface defects in materials. A LabRam HR Evolution confocal Raman spectrometer, produced by HORIBA FRANCE, France, was used in this experiment. The test was conducted at room temperature with a laser wavelength of 785 nm, a test wavenumber range of 100–1200 cm⁻¹, and an exposure time of 60 s. Because the tested material is strongly magnetic, a high-intensity laser was first used to irradiate the surface of the material for 10 min, and data were collected after the fluorescence effect weakened to prevent it from affecting the experimental results.

A thermogravimetric analyzer (TGA/DSC 3+, METTLER TOLEDO, Greifensee, Switzerland) was used to examine the thermal degradation behavior of the samples. A 10 mg sample was placed in a 100 μL volume Al₂O₃ crucible and heated from ambient temperature to 800 °C. The nitrogen flow rate was 20 mL/min during the experiment. In order to calculate the kinetic parameters, data were recorded at different heating rates. The sample was burned on the thermogravimetric analyzer at a heating rate of 20 °C/min.

The cyclic voltammetric characteristics of the catalytic electrode were tested using cyclic voltammetry (CV). For CV testing CV, this experiment used a CH630E electrochemical workstation with a scanning potential range of −1~2 V, a forward scanning direction, a scanning rate of 0.01 V/s, 5 scanning cycles, and a scanning sensitivity of 0.001 V.

3.3. Electrocatalytic Oxidation Activity of Catalytic Electrode for Toluene

The electrocatalytic oxidation activity of the catalytic electrode for toluene was evaluated using an all-solid-state electrocatalytic oxidation reaction device. A schematic diagram of the device is provided in Figure 13. Two symmetrical stainless steel materials with grooves were used as the outer shell of the device, and the device was divided into two parts: the cathode and the anode. The anode chamber and cathode chamber were separated by a H-type cation exchange membrane (N117), and the two sides of the ion exchange membrane were close to the graphite felt so only H⁺ was permitted to flow through the proton exchange membrane between the two chambers. Neither the reactant nor product gas could pass through the proton exchange membrane, avoiding gas leakage.

The size of the catalytic electrode and counter electrode in the electrocatalytic device is 40 × 30 × 10 mm. The cathode chamber and anode chamber are equipped with gas inlet and outlet, respectively. A 15 mm long titanium electrode leads out on both sides of the cathode and anode chambers to connect the electrochemical workstation. To prevent the electrode from conducting with the reflecting shell, insulation paper and adhesive are present at each connection point. Before the experiment, a multimeter was used to eliminate leakage at each point. Compared to existing electrocatalytic device designs, this device allows the reaction gas to penetrate from inside the catalytic electrode, avoiding problems associated with a short contact time and small contact area, which are caused by the reaction gas flowing through the electrode surface. The specific experimental operations were as follows:

(1) Cu-MnO₂/GF catalytic electrodes prepared under different conditions were precut to a certain size according to the experimental conditions and placed in the anode chamber of the electrocatalytic device. Graphite felt of the same size without a loaded catalyst was placed in the cathode chamber. A proton exchange membrane was used to separate the cathode and anode and fixed in place with screws and gaskets.

(2) The reaction gases in the experiment were toluene, water vapor, and air. The gas in the anode chamber was recirculated, and both the toluene and water vapor were prepared using the ice bath bubbling method. The concentration of toluene in the mixing bottle was controlled to 100 ppm.

(3) After a stable toluene concentration was achieved, the electrocatalytic device, humidity controller, peristaltic pump, and meteorological chromatograph were connected to the system to form a circulating gas path. The airtightness of the device was confirmed to enable the toluene gas to repeatedly pass through the electrocatalytic device under certain humidity conditions. The concentration of the reactants and products was detected by GC (HF-901A) using an FID detector, with a column chamber temperature of 100 °C and a hydrogen flame temperature of 160 °C. The corresponding concentrations of toluene, CO, and CO₂ were converted based on the peak area measured by GC to obtain the toluene removal rate, CO generation rate, and CO₂ generation rate. The flow range of the peristaltic pump was 0~65 mL/min, and the humidity regulation range was 0~70% RH.

(4) The anode and cathode pins of the electrocatalytic device were connected to the positive and negative terminals of the DC power supply, and the constant voltage was adjusted according to the specific experimental conditions. During the experiment, the concentrations of toluene, CO, and CO₂ gas were monitored every 15 min to calculate the real-time conversion rate and CO_x production rate of toluene. Before the electrocatalytic experiment was initiated, the electrocatalytic device and catalyst were purged using 30 min of pure nitrogen gas flow to remove impurities.

The electrocatalytic performance of the Cu-MnO₂/GF catalytic electrode was tested for stability during cyclic operation. The duration of each cyclic experiment was 150 min, and the initial concentration of toluene was 100 ppm.

The catalytic activity of catalytic electrode was analyzed using the removal rate of toluene, CO, and CO₂ yield. The calculation method is described below.

(1) Removal rate of toluene

The calculation formula for toluene removal rate in catalyst activity evaluation is as follows (Equation (5)):

$${\eta }_{\mathrm{toluene}}(\%)={\displaystyle \frac{C_{\mathrm{in}}-C_{\mathrm{out}}}{C_{\mathrm{out}}}}\times 100\%$$

(5)

In the equation, ${\eta }_{\mathrm{toluene}}(\%)$ represents the removal rate of toluene at a certain temperature, while ${\mathrm{C}}_{\mathrm{in}}\left(\mathrm{ppm}\right)$ and $C_{\mathrm{out}}\left(\mathrm{ppm}\right)$ represent the concentrations at the inlet and outlet of the quartz fixed bed reactor, respectively.

(2) Yield of CO and CO₂

The electrocatalytic oxidation products of toluene are CO₂ and CO, and the reaction equation is as follows:

$${\mathrm{C}}_6{\mathrm{H}}_6+6{\mathrm{H}}_2\mathrm{O}\to 6\mathrm{C}\mathrm{O}+18{\mathrm{H}}^{+}+\mathrm{18}{\mathrm{e}}^{-}$$

(6)

$${\mathrm{C}}_6{\mathrm{H}}_6+12{\mathrm{H}}_2\mathrm{O}\to 6\mathrm{C}{\mathrm{O}}_2+30{\mathrm{H}}^{+}+30{\mathrm{e}}^{-}$$

(7)

Thus, the calculation formula for CO₂ and CO yields in the evaluation of catalyst electrocatalytic activity can be obtained as follows:

$$Yield(\%)={\displaystyle \frac{C_{{\mathrm{CO}}_{\mathrm{X}}}}{6C_0}}\times 100\%$$

(8)

In the equation, ${\mathrm{C}}_{\mathrm{C}{\mathrm{O}}_{\mathrm{x}}}$ represents the real-time concentration values of CO₂ and CO during the reaction process; ${\mathrm{C}}_0$ indicates the initial concentration value of toluene.

4. Conclusions

A series of Cu-MnO_x/GF catalytic electrodes were prepared using graphite felt modified under different microwave conditions as the catalyst carrier, and their catalytic oxidation activity was evaluated in an all-solid-state electrocatalytic device. Characterization and activity testing results indicate that the microwave-modified catalytic electrode had a higher electrocatalytic oxidation effect on toluene. Notably the Cu-MnO_x/700W-GF electrode exhibited a significantly higher catalytic activity and achieved a higher CO₂ yield (over 90%) during the reaction process. The catalytic effect was significantly affected by the relative humidity, and the complete catalytic oxidation of toluene can be realized at a certain humidity, indicating that the conversion of H₂O to strong oxidizing ·OH on the catalytic electrode is a key step in the catalytic oxidation of toluene.