Exploration of the interaction mechanism in the synergistic degradation of benzene and toluene over MnCoO_x catalysts

Highlights

	● A series of MnaCobO x (a:b = 1:2, 1:1, and 2:1) catalysts are prepared.
	● Effects of Mn/Co for catalytic performance are studied.
	● T90 of benzene and toluene in the mixture over MnCoO x are 290 and 248 °C.
	● The reaction mechanism of benzene and toluene synergistic oxidation is proposed.

Abstract

The catalytic degradation of single-component VOCs has been widely studied. However, several types of VOCs may be present in an actual industrial emission stream. Efficient synergistic removal of multicomponent VOCs is currently a popular research topic. Herein, Mn–Co samples with various Mn/Co ratios (1:2, 1:1, and 2:1) were successfully prepared, and the catalytic oxidation performance characteristics toward benzene and toluene over these samples under single and binary VOC oxidation conditions were studied. Compared with pure MnO_x and CoO_x, the prepared Co–Mn composite oxide samples exhibited significantly improved catalytic performance. MnCoO_x and MnCo₂O_x showed optimum catalytic performance, with 100% benzene and 100% toluene conversion in the mixtures at 300 and 350 °C, 100% CO₂ selectivity. Characterization methods were employed to elucidate the relevance of the catalytic activity to the structures, acidity, redox properties, Mn–Co valence state and oxygen species. Moreover, the interactions between benzene and toluene during their synergistic degradation, as well as the intermediates and potential reaction mechanisms of their simultaneous elimination, were investigated. Utilizing Mn–Co oxide compounds for cooperative catalytic oxidation of benzene and toluene represents a viable and effective technique for the practical and synergist elimination of multicomponent VOCs.

Graphical abstract

Keywords

Mn–Co oxides / Synergetic catalytic oxidation / Benzene and toluene / Reaction mechanism

1 Introduction

As urbanization and industrialization have increased, air pollution in China has become increasingly severe. The current air quality in China is severely affected by photochemical smog and fine particulate matter (PM_2.5) formed by secondary reactions (Chen et al., 2023d; Jiang et al., 2023). Volatile organic compounds (VOCs) contribute significantly to the generation of O₃ and PM_2.5 and have a substantial detrimental influence on air quality and human health (Zou et al., 2023). As a typical atmospheric pollutant, a single VOC stream may have a very complex variety of types, including alkanes, alkynes, olefins, aromatic compounds, halogenated hydrocarbons, and nitrogen-containing and oxygen-containing compounds (He et al., 2019, Chen et al., 2023b). China’s coke production occupies a major position in the world, and many VOCs are emitted from the coking industry during the coke production process (coal high-temperature distillation and by-product recovery process). Coking plants produce mostly benzene, propane, ethylene, ethane, and toluene—more than 70% of all chemicals released (Cheng et al., 2022; Li et al., 2022). Additionally, the primary VOCs are benzene, toluene, ethylbenzene, and xylene, and their high emissions and potential carcinogenicity issues have been raised by many researchers (Wang et al., 2021b; Zhang et al., 2023). Overall, the components of exhaust gases from industry are complex and diverse, so researching multi-component VOCs synergistic degradation is essential.

To meet the increasingly strict emission standards regarding VOC emissions, relevant VOC control technologies, such as recycling control processes (absorption, adsorption, condensation, etc.) and destruction methods (biofiltration, plasma, catalytic oxidation, photocatalysis, etc.), have been considered for VOC elimination (Wang et al., 2019a; Xing et al., 2022, Sun et al.2023b). Among these processes, catalytic oxidation is an efficient and economically feasible technology that can effectively oxidize VOCs to produce products such as CO₂ and H₂O (Guo et al., 2023; Rao et al., 2023; Sun et al., 2023a). Currently, catalytic oxidation has become the mainstream control technology in industry. Therefore, the most challenging aspect of VOC catalytic oxidation is the selection of high-activity catalysts.

Studies have been done on cooperative oxidation of several kinds of VOCs. Li et al. (2023c) using two-stage catalysts (PtSn/CeO₂ and Mn/ZSM-5) to oxidize trichloroethylene and toluene. The conversion of toluene and trichloroethylene reached 90% when heated to 384 °C ). Gao et al. prepared ternary metal nanoparticles using the two-step solvothermal method, which demonstrated effective catalysis. In the synergistic removal process of toluene and chlorobenzene, the toluene conversion and chlorobenzene conversion obtained 90% at 315 °C (Gao et al., 2023). However, the following issues in simultaneous oxidation of multi-component VOCs need improvements: 1) catalytic performance at low temperature, 2) the interaction between multi-component VOCs.

To better remove VOCs, the selection of catalysts has become a crucial part. Catalysts with noble metal have demonstrated outstanding catalytic performance in benzene and toluene degradation (Peng et al., 2016; Liu et al., 2023). However, owing to their scarcity and high cost, the actual use of catalysts based on noble metals is severely limited. Currently, catalysts based on nonnoble metals with superior catalytic performance are being explored. Co₃O₄ is a transition metal–oxide catalyst that is considered a promising alternative catalyst for replacing noble metal catalysts (Wu et al., 2020; Chen et al., 2021; Han et al., 2021). Bao et al. (2022) prepared anion-defective Co₃O₄ via N doping and tested it for toluene oxidation. N-doped Co₃O₄ exhibited high toluene conversion, which was related to the generation of active oxygen species and the increase in the oxygen reactivity of Co₃O₄ by N doping. A catalyst combination of Co₃O₄ and LaCoO₃ with excellent activity for toluene catalytic oxidation was synthesized, and the Co₃O₄–LaCoO₃ catalyst surface Co³⁺–O was enhanced by the contact interaction (Chen et al., 2023a). However, efforts should be made to increase the conversion at low temperatures and promote the practical utilization of nonnoble-metal catalytic materials. Co and Mn composite oxides are regarded as superior catalysts for benzene and toluene catalytic elimination because of their various metal oxidation states and superior redox ability. Porous coral-like CoMnO_x catalysts were synthesized, and their high redox ability and high amount of absorbed oxygen contributed significantly to their catalytic performance toward benzene (Li et al., 2019a). Luo et al. (2018) obtained nanocube-like metal–organic framework Mn–Co oxides, and toluene conversion at low temperatures was determined mostly by the surface Mn⁴⁺ and Co³⁺ species.

The development of dual CoMn active-component catalysts for benzene and toluene synergistic elimination is a promising strategy. In this study, Mn–Co catalysts were successfully prepared. The structures, chemical properties, and catalytic performances of pure MnO_x and CoO_x catalysts were compared with those of the MnCoO_x catalysts. A sequence of evaluations (benzene and toluene catalytic oxidation activities) and characterizations were used to explore the structure‒activity relationships of these catalysts, and in situ diffuse reflectance infrared Fourier transform spectra (in situ DRIFTS) was used to illustrate the intermediate products and possible mechanisms involved in the reactions. Additionally, the interaction of benzene and toluene in the collaborative degradation process was discussed.

2 Experimental section

Figure S1 in the Supplementary material shows the entire procedure for preparation. The detailed characterizations of the MnO_x, CoO_x, and Mn–Co-derived mixed oxides are provided in the Supplementary material. Furthermore, the mixed oxides of Mn and Co were designated Mn_aCo_bO_x (a:b = 1:2, 1:1, and 2:1).

Catalytic performance measurements were carried out at ordinary pressure, and each run required approximately 0.5 mL of sample in the form of 40–60 mesh. The simulated reaction gases consisted of 250 ppm benzene or 250 ppm toluene (the mixture gas was 250 ppm benzene and toluene), 5% O₂ and the balance was N₂. The reactant gaseous flow rate was 150 mL/min, yielding a gas hourly space velocity (GHSV) of 18000 h⁻¹. A gas chromatograph (GC 7960 plus, Allen Analytical Instruments, China) was used to examine the output gases. The catalytic performance was evaluated with respect to benzene and toluene conversion and product selectivity (Eqs. (1)–(4)):

(1)$\mathrm{b}\mathrm{e}\mathrm{n}\mathrm{z}\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{i}\mathrm{o}\mathrm{n}={\displaystyle \frac{\mathrm{b}\mathrm{e}\mathrm{n}\mathrm{z}\mathrm{e}\mathrm{n}\mathrm{e}(\mathrm{i}\mathrm{n})-\mathrm{b}\mathrm{e}\mathrm{n}\mathrm{z}\mathrm{e}\mathrm{n}\mathrm{e}(\mathrm{o}\mathrm{u}\mathrm{t})}{\text{benzene (in)}}}\text{×100}\text{\%}\text{,}$

(2)$\text{toluene conversion =}\frac{\mathrm{t}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{e}\mathrm{n}\mathrm{e}(\mathrm{i}\mathrm{n})-\mathrm{t}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{e}\mathrm{n}\mathrm{e}(\mathrm{o}\mathrm{u}\mathrm{t})}{\text{toluene (in)}}\text{×100}\text{\%}\text{,}$

(3)$\text{CO selectivity =}\frac{\text{CO (ppm)}}{\text{Total product C (ppm)}}\text{×100}\text{\%}\text{,}$

(4)$\text{C}{\text{O}}_{\text{2}}\text{selectivity =}\frac{\text{C}{\text{O}}_{\text{2}}\text{(ppm)}}{\text{Total product C (ppm)}}\text{×100}\text{\%}\text{.}$

3 Results and discussion

3.1 XRD and nitrogen adsorption and desorption analysis

The X-ray diffraction (XRD) results are provided in Fig.1(a). The peaks of MnO_x at 18.0°, 28.9°, 31.0°, 32.3°, 36.1°, 36.5°, 38.0°, 44.5°, 50.7°, 58.5°, 59.8°, and 64.6° correspond to the (1 0 1), (1 1 2), (2 0 0), (1 0 3), (2 1 1), (2 0 2), (0 0 4), (2 2 0), (1 0 5), (3 2 1), (2 2 4), and (4 0 0) planes of Mn₃O₄, respectively (JCPDS 24-0734) (Yao et al., 2018). For CoO_x, the XRD patterns exhibit reflection planes that match well with those of Co₃O₄ according to JCPDS 43-1003. The major diffraction peaks centered at 18.9°, 31.2°, 36.9°, 38.5°, 44.8°, 59.4°, 65.2°, and 77.4° were assigned to the (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (5 1 1), (4 4 0), and (5 3 3) planes, respectively (Shi et al., 2014). For the Mn_aCo_bO_x (a:b = 1:1, 1:2, and 2:1) samples, MnCoO_x and MnCo₂O_x display similar diffraction peaks. The characteristic signals at 18.5°, 30.5°, 36.0°, 37.4°, 43.7°, 54.2°, 57.7°, and 63.4° are ascribed to the (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) planes of CoMn₂O₄ (JCPDS 23-1237) (Wang et al., 2020). The diffraction peak of Mn₂CoO_x corresponds to (Co, Mn) (Co, Mn)₂O₄ (JCPDS 18-0408). The peaks located at 18.2°, 29.3°, 31.3°, 32.9°, 36.4°, 38.9°, 44.8°, 51.9°, 59.0°, 60.7°, and 65.5° correspond to the (1 1 1), (2 0 2), (2 2 0), (1 1 3), (3 1 1), (0 0 4), (4 0 0), (3 3 2), (5 1 1), (4 0 4), and (4 4 0) planes, respectively (Wang et al., 2021a). Compared with the Mn_aCo_bO_x catalysts, Mn₂CoO_x has a notably reduced peak intensity, which points to a substantial interaction involving the Mn and Co species.

Fig.1 Texture properties of CoOx, MnOx, and MnaCobOx catalysts (a) XRD (b) N2 adsorption/desorption isotherms and (c) pore size distribution.

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The N₂ adsorption–desorption isotherms and the pore size distributions of CoO_x, MnO_x, and Mn_aCo_bO_x are displayed in Fig.1(b) and 1(c) and listed in Table S1. The isotherms of CoO_x and MnO_x could not be closed, probably because the SSA values of CoO_x and MnO_x were relatively small. The curves of Mn_aCo_bO_x displayed type IV isotherms with type H3 hysteresis loops, revealing a mesoporous structure (Li et al., 2023a; 2023b). The Mn_aCo_bO_x catalysts had just one hysteresis loop in the P/P₀ range of 0.8–1.0. As shown in Fig.1(c), the Mn_aCo_bO_x catalysts had a particular bimodal pore size distribution. The smaller distributed peak was approximately 3 nm, whereas the other was concentrated between 50 and 60 nm. According to reports, PMMA nanospheres should be applied to template large pores ranging from 8 to 100 nm (Huang et al., 2022).

3.2 Raman analysis

Figure S2 depicts the Raman spectra of the CoO_x, MnO_x, and Mn_aCo_bO_x (a:b = 1:1, 1:2, and 2:1) catalysts. For CoO_x, the peaks at 658, 594, 506, 495, and 185 cm⁻¹ are associated with the A_1g, F²_2g, F¹_2g, E_g, and F_2g modes of Co₃O₄ (Xiao et al., 2020). This indicated that Co₃O₄ is present mainly in the CoO_x sample, which corresponds with the XRD results. For the MnO_x sample, the peak at 629 cm⁻¹ corresponds to the stretching vibrations of the Mn–O bond, and the weak band at 200–350 cm⁻¹ is related to the translational motion of the [MnO₆] octahedron, which agreed well with previous work on Mn₃O₄ (Bernardini et al., 2019). A series of Mn_aCo_bO_x catalysts exhibited a major peak at 584–607 cm⁻¹, showing the Mn–O stretching vibration of the [MnO₆] octahedron. Compared with that of MnO_x, the peak intensity of Mn_aCo_bO_x was significantly lower. The typical peaks of Co₃O₄ did not exist in Mn_aCo_bO_x, indicating that Co₃O₄ was absent in these samples and that Co species were substituted into the [MnO₆] structure (Wang et al., 2022). Additionally, the characteristic peaks of MnCoO_x and MnCo₂O_x showed significant Raman redshifts. This phenomenon might be due to lattice distortion and amorphous defects in the structure (Li et al., 2019b; 2021).

3.3 Catalytic performance

The catalytic activities of the CoO_x, MnO_x, and Mn_aCo_bO_x samples for benzene degradation are presented in Fig.2(a)–Fig.2(c). The benzene conversion clearly improved as the temperature increased. The MnCoO_x and MnCo₂O_x catalysts were effective in completely oxidizing benzene at 300 °C. At 350 °C, Mn₂CoO_x exhibited 100% conversion of benzene. However, even at 600 °C, benzene could not be completely removed via CoO_x and MnO_x catalysts. The benzene catalytic oxidation rate decreased from high to poor in the order of MnCo₂O_x > MnCoO_x > Mn₂CoO_x > MnO_x > CoO_x. The CO selectivity initially increased but then decreased in the range of 100–600 °C, as shown in Fig.2(b) and Fig.2(c). Moreover, the CO₂ selectivity increased with increasing temperature, which corresponds to the curves of benzene conversion.

Fig.2 The performance of CoOx, MnOx, MnaCobOx (a:b = 1:1, 1:2 and 2:1) catalysts:(a) benzene conversion, (b) CO selectivity, (c) CO2 selectivity, (d) conversion of toluene, (e) CO selectivity, (f) CO2 selectivity, (g) conversion of benzene in mixture, (h) conversion of benzene in mixture, (i) CO selectivity, (j) CO2 selectivity, (k) the benzene conversion in alone benzene and the mixture (benzene and toluene) of MnCoOx, (l) the toluene conversion in individual toluene and the mixture (benzene and toluene) of MnCoOx sample.

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The catalytic performance results for the separate elimination of toluene are illustrated in Fig.2(d)–Fig.2(f). The primary reaction temperature of toluene was 100 °C. Under the same conditions, benzene did not yet begun to react. The temperature recorded at 100% toluene conversion (T₁₀₀) increased in the following order: MnCoO_x = MnO_x < MnCo₂O_x = Mn₂CoO_x < CoO_x. MnO_x, and MnCoO_x exhibited the highest toluene conversion, and the temperature of overall conversion was 300 °C. Surprisingly, MnO_x exhibited good conversion in the catalytic elimination of toluene, while the complete conversion of benzene was obtained only at temperatures exceeding 600 °C. For the CoO_x sample, the toluene conversion was relatively poor, and the entire toluene conversion was obtained at approximately 600 °C. The CO selectivity first increased but then decreased. The highest CO value of approximately 10% was observed at 250 °C. The CO₂ selectivity over the CoO_x, MnO_x, and Mn_aCo_bO_x catalysts increased with increasing temperature. For the MnO_x and MnCoO_x samples, the CO₂ selectivity reached a maximum of 100% at 350 °C. When the full conversion temperature was reached, the carbon balance was greater than 98% for all the catalysts.

The benzene and toluene simultaneous degradation result over prepared samples at 100–600 °C are shown in Fig.2(g)–Fig.2(j), and the characteristic temperatures of benzene or toluene degradation performance in the synergistic catalytic oxidation process are shown in Tab.1. The temperatures recorded at 90% benzene or toluene conversion (T_90b and T_90t) increased in the order of MnCoO_x < MnCo₂O_x < Mn₂CoO_x < MnO_x < CoO_x. In the benzene and toluene mixed gas system, the CoO_x sample showed the lowest conversion for benzene and toluene simultaneous elimination (Fig.2(g) and Fig.2(h)). The MnO_x sample exhibited a higher toluene conversion at low temperatures, but the conversions of benzene and toluene were poorer than those of Mn_aCo_bO_x as the temperature increased. Following benzene and toluene synergistic oxidation, the following conversion outcomes were obtained: MnCoO_x > MnCo₂O_x > Mn₂CoO_x > MnO_x > CoO_x. The highest catalytic performance was shown over MnCoO_x. At 300 °C, the conversion values for toluene and benzene reached 100%, demonstrating excellent low-temperature performance, which indicates that the Co–Mn composite–oxide catalyst has strong synergistic catalytic oxidation performance. Fig.2(i) and Fig.2(j) shows the CO selectivity and CO₂ selectivity of these catalysts. As shown in Fig.2(i), the CO selectivities of MnCoO_x and MnCo₂O_x were relatively low, and the highest CO selectivity was less than 10%. For the formation of CO₂ (Fig.2(j)), the CO₂ selectivities of MnCo₂O_x and MnO_x were greater than those of the other samples at low temperatures. Although the conversion of benzene and toluene of MnO_x were lower, the selectivity for CO₂ was high, suggesting that the formation of byproducts was lower and that the MnO_x sample possessed a stronger oxidation–reduction ability, especially at low temperatures. At higher temperatures (> 300 °C), from high to low CO₂ selectivity is: MnCoO_x > MnCo₂O_x > Mn₂CoO_x > MnO_x > CoO_x. MnCoO_x and MnCo₂O_x had the best CO₂ selectivity, reaching 100% at 350 °C. According to the above results, MnCoO_x and MnCo₂O_x had the best catalytic activities for the multi component VOCs (benzene and toluene)

Tab.1 The characteristic temperature of benzene or toluene oxidation activity in benzene and toluene synergistic catalytic oxidation process

Samples	T10b (°C)	T50b (°C)	T90b (°C)	T10t (°C)	T50t (°C)	T90t (°C)
CoOx	479	528	585	460	511	546
MnOx	249	378	534	145	255	346
MnCoOx	219	266	290	132	223	248
MnCo2Ox	255	275	296	185	264	293
Mn2CoOx	302	327	354	250	277	313

Notes: b, The characteristic temperature of benzene oxidation activity in benzene and toluene mixed gas system synergistic catalytic oxidation process. T, The characteristic temperature of toluene oxidation activity in benzene and toluene mixed gas system synergistic catalytic oxidation process.

To examine the interaction of multi component VOCs in this synergistic degradation process, the benzene or toluene conversions with single pollutant and the benzene or toluene conversions under the mixture of pollutants (benzene and toluene) over the MnCoO_x samples were compared and are displayed in Fig.2(k) and 2(l). In the multi component VOCs synergistic catalytic oxidation process, the benzene conversion was less than 30% at 250 °C. When benzene is individually catalyzed oxidized, benzene has already begun to react at 150 °C, and the conversion of benzene is approximately 60% at 250 °C. Therefore, we believe that before reaching the complete conversion temperature, the conversion of alone-benzene catalytic oxidation was higher than those of multi component VOCs (benzene and toluene). Overall, the catalytic elimination of toluene is easier to carry out than that of benzene. Whether in benzene catalytic oxidation or the synergistic catalytic oxidation of benzene and toluene, toluene has already begun to react at 100 °C. The conversion of alone-toluene catalytic oxidation is greater than that of toluene in the mixed environment catalytic removal before 250 °C. Therefore, it could be supposed that the existence of benzene has an inhibitory effect on the toluene catalytic oxidation process. Similarly, benzene catalytic degradation is also inhibited by the presence of toluene; this could be the result of active sites competing adsorption during the multi component VOCs (benzene and toluene) synergistic catalytic degradation process, which will be described in the DRIFTS results section. The stability of the catalyst is among the principal indicators for characterizing catalytic performance. The stability of MnCoO_x catalyst under optimal conditions and were investigated while there was water vapor present, as shown in Fig. S3.

3.4 H₂-TPR and NH₃-TPD analysis

The H₂ temperature‒programmed reduction (H₂-TPR) results of CoO_x, MnO_x, and Mn_aCo_bO_x are given in Fig.3(a), and the H₂ consumption data are listed in Tab.2. For the CoO_x sample, a strong signal at 350 °C possibly related to the reduction of Co³⁺ to Co²⁺, whereas two relatively weak peaks at higher temperatures (412 °C and 453 °C) was attributed to the reduction of Co²⁺→ Co on the surface of Co₃O₄ and Co²⁺ → Co in the bulk of Co₃O₄, respectively (Chen et al., 2020). The H₂-TPR results for MnO_x peak at 356 and 413 °C, corresponding to the reduction of MnO₂ → Mn₂O₃ and Mn₂O₃ → Mn₃O₄ → MnO, respectively (Tang et al., 2019). The other three samples had four peaks, and the peak at 359–371 °C correspond to the reduction of Co³⁺ → Co²⁺ and MnO₂ → Mn₂O₃. The peaks at 453–466 °C correspond to Mn₂O₃ → Mn₃O₄ and Co²⁺ → Co on the surface of Mn_aCo_bO_x, and that at 542–566 °C is ascribed to Mn₃O₄ → MnO and Co²⁺ → Co in the bulk of Mn_aCo_bO_x. For the reduction peak at high-temperature (> 600 °C), it is possible to attribute the reduction of spinel CoMn₂O₄, which includes the bulk oxygen components of Mn and Co oxides, which have various valences (Wang et al., 2020). These findings all point to the existence of Co and Mn species with various valence states, including Co²⁺/Co³⁺ and Mn²⁺/Mn³⁺/Mn⁴⁺, which is in line with the XPS results. Tab.2 shows that the H₂ consumption of Mn_aCo_bO_x was obviously greater than that of CoO_x and MnO_x. In particular, for the MnCoO_x and MnCo₂O_x catalysts, the H₂ consumption values of MnCo₂O_x were 45.5 and 49.8 mmol/g, respectively, which are higher than those of the other samples, demonstrating the preferable redox performance of MnCoO_x and MnCo₂O_x and reflecting the outstanding catalytic activity in benzene and toluene synergistic oxidation.

Tab.2 Reducibility properties and XPS analysis results of the catalysts

Samples	Peak position (°C)	H2 Consumption (mmol/g)	Total H2 consumption (mmol/g)	Mn (at.%)	Co (at.%)	O (at.%)	Co3+/Co (%)	Mn4+/Mn (%)	Oβ/O (%)
CoOx	350	11.1	19.9	/	27.9	72.1	44.5	/	38.9
	412	2.4
	453	6.4
MnOx	356	9.4	16.2	35.0	/	65.0	/	8.7	51.4
	413	6.8
MnCoOx	365	6.4	45.5	17.7	20.4	61.9	58.8	43.8	51.1
	466	26.5
	542	9.7
	626	2.9
MnCo2Ox	371	7.0	49.8	15.8	15.6	68.6	55.5	41.1	54.4
	466	26.8
	556	13.4
	623	2.6
Mn2CoOx	359	4.8	35.4	20.8	10.5	68.7	53.2	30.1	49.6
	453	19.6
	566	6.8
	628	4.2

Fig.3 (a) H2-TPR profiles; (b) NH3-TPD profiles of CoOx, MnOx and MnaCobOx (a:b = 1:1, 1:2, and 2:1).

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NH₃ temperature-programmed desorption (NH₃-TPD) (Fig.3(b)) was conducted to examine the surface acidity. Considering the influence of water vapor, we connected mass spectrometry in series to obtain additional information. The NH₃-TPD profiles of CoO_x and MnO_x each show only one desorption peak over the entire temperature range. The CoO_x sample exhibited a low-intensity peak at ~100 °C, and the MnO_x sample presented a peak at 270 °C. The profiles of Mn_aCo_bO_x presented similar NH₃ desorption peaks from 50 to 350 °C. For all the catalysts, the low-temperature peaks (100 °C) could be ascribed to weakly acidic sites, and those at 250–270 °C were related to NH₃ desorption at moderately acidic sites (Ma et al., 2019; Zhang and Wang, 2024). Interestingly, the MnO_x catalysts only have weakly acidic sites, whereas CoO_x only has moderately acidic sites. The Mn_aCo_bO_x samples contained both weakly acidic sites and moderately acidic sites.

For individual benzene or toluene catalytic elimination and synergistic catalytic degradation of multi component VOCs (benzene and toluene), the benzene and toluene conversions of Mn_aCo_bO_x and MnO_x were better than those of CoO_x. In particular, for toluene catalytic degradation (in both toluene alone and in the case of toluene mixed with benzene), MnO_x showed good low-temperature activity, so it is speculated that the existence of weaker acidic sites may be beneficial for low-temperature activation, which may be the main reason why the Mn_aCo_bO_x and MnO_x materials had better low-temperature catalytic performance than CoO_x materials.

3.5 XPS analysis

To get obtain details of the valence states of these samples, CoO_x, MnO_x, and Mn_aCo_bO_x were identified by X-ray photoelectron spectroscopy (XPS) (Fig.4(a)–Fig.4(d)). The surface atomic ratios of Co³⁺/Co, Mn⁴⁺/Mn, and O_β/O were calculated from their corresponding XPS integral areas, which are presented in Tab.2.

Fig.4 XPS (a) full spectrum; (b) Mn 2p; (c) Co 2p; (d) O1s of CoOx, MnOx, and MnaCobOx (a:b = 1:1, 1:2, and 2:1).

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The Mn 2p spectrum is depicted in Fig.4(b). The two major peaks, which correspond to Mn 2p3/2 and 2p 1/2, located at around 641.5 and 653.2 eV. Three peaks could be obtained by deconvoluting Mn 2p3/2, locating at 640.9, 642.3, and 643.8 eV, which are attributable to Mn²⁺, Mn³⁺, and Mn⁴⁺, respectively (Wang et al., 2020; Fan et al., 2023). Deducing from the peak area, the related percentages of Mn⁴⁺ of MnCoO_x and MnCo₂O_x are 43.8% and 41.1%, which are higher than MnO_x (8.7%) and Mn₂CoO_x (30.2%). The increasing presence of Mn⁴⁺ on the surface increases the redox capacity (Tang et al., 2010; Wu et al., 2013). The samples corresponding Co 2p XPS spectra were separated into Co 2p3/2 and Co 2p1/2, and deconvolution allowed for the resolution of six peaks (Fig.4(c)). The Co 2p spectra show weak shake-up peaks at approximately 788 and 803 eV. The binding energy at 795.8 and 780.7 eV belong to Co²⁺ species, and the peaks at 794.5 and 779.5 eV are assigned to Co³⁺ (Tang et al., 2021; Chen et al., 2023c; Deng et al., 2024). The surface Co³⁺/Co ratios are listed in Tab.2. For the CoO_x, MnCoO_x, MnCo₂O_x, and Mn₂CoO_x catalysts, the values of Co³⁺/Co were 44.5%, 58.8%, 55.5%, and 53.2%, respectively. Doping with manganese increased the proportion of Co³⁺ species. It has been reported that Co³⁺ is more active in the oxidative elimination of VOCs (Zheng et al., 2023). Moreover, on MnCoO_x and MnCo₂O_x, the proportions of Co³⁺/Co and Mn⁴⁺/Mn were relatively high, resulting in the production of more Co³⁺–O²⁻–Mn⁴⁺ species at the interface. In the literature, the presence of Co³⁺–O²⁻–Mn⁴⁺ species enhances reactant adsorption and activation capabilities, improving catalyst redox performance (Wang et al., 2019b).

The O 1s peak (Fig.4(d)) could be split into three peaks, assigned to the lattice oxygen species (O_α, 529.3 eV), surface adsorbed oxygen species (O_β, 530.5 eV), and hydroxy oxygen species (O_γ, 532.2 eV) (Sun et al., 2013; Zhang et al., 2014). Among three oxygen species, the O_β species is the activated oxygen species, which results in an O defect with a lower coordination number and is the major active site for benzene and toluene catalytic elimination. The order of decreasing oxygen species adsorption content is MnCo₂O_x (54.4%) > MnO_x (51.4%) > MnCoO_x (51.1%) > Mn₂CoO_x (49.6%) > CoO_x (38.9%), which is consistent with toluene conversion at 250 °C in toluene alone as well as in the mixture of toluene and benzene. The O_β/(O_α + O_β + O_γ) value of MnCo₂O_x (54.4%) is greater than those of the other catalysts, but the differences in this value among MnCo₂O_x, MnO_x, and MnCoO_x are relatively small. Hence, the differences in the catalytic activity of the MnCo₂O_x, MnO_x, and MnCoO_x catalysts were small at lower temperatures (< 250 °C).

3.6 In situ DRIFTS analysis

To reveal more intermediate species generated via the benzene and toluene mixture catalytic oxidation reactions on the sample surface, in situ DRIFTS was conducted, and the findings are shown in Fig.5 and Fig.6.

Fig.5 In situ DRIFTS for adsorption of the MnCoOx sample at 100 °C with 250 of benzene (a), 250 of toluene (b), and 250 mixture of benzene and toluene (c) at different time.

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Fig.6 In situ DRIFTS for the MnCoOx sample exposure to 250 benzene + 5% O2 (a), 250 toluene + 5% O2 (b), and 250 benzene + 250 toluene + 5% O2 (c) at different temperature.

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Fig.5(a) illustrates the process of benzene adsorption on the MnCoO_x catalyst. The bands of adsorbed benzene are located at 3088, 3047, 1966, 1821, 1478, and 1034 cm⁻¹, and the peak intensity increased with time increasing. Two peaks at 3088 and 3048 cm⁻¹ are assigned to the C–H stretching vibrations in the benzene rings. Two bands, located at 1966 and 1821 cm⁻¹, are linked to the C–H bonds. The peak at 1478 cm⁻¹ are ascribed to C=C double bonds stretching vibration (Sivasankar and Vasudevan, 2004). For the adsorption of toluene (Fig.5(b)), after one hour, the peak of adsorbed toluene became constant as it rose over time. The peaks at 3073 and 3040 cm⁻¹ correspond to the C‒H vibrations of benzene rings. The C–H stretching vibration of the methyl (–CH₃) group in adsorbed toluene is shown by two bands at 2937 and 2882 cm⁻¹, while the C=C double bonds stretching vibration in benzene ring is indicated by the peak at 1497 cm⁻¹ (He et al., 2011). The occurrence of these adsorption peaks indicates that toluene was adsorbed onto the MnCoO_x surface. The adsorption peaks of benzene and toluene mixture on MnCoO_x are displayed in Fig.5(c). The peaks at 2800–3100 cm⁻¹ is ascribed to the C–H bonds of benzene rings and the –CH₃ of toluene, indicating that both benzene and toluene were adsorbed on the surface of MnCoO_x.

The formation of intermediate species is characterized via in situ DRIFTS (Fig.6). The peaks at 3085, 3047, 1953, 1808, 1478, and 1036 cm⁻¹ decreased with increasing temperature, indicating that benzene was gradually decomposed under the action of the MnCoO_x catalyst (Fig.6(a)). Moreover, a few characteristic peaks of intermediate species emerged. The weak band observed at 2972 cm⁻¹ corresponds to C−H stretching vibrations, and the band at 2310 cm⁻¹ is related to the characteristic signal of CO₂. The C=O bond is attributed to the band at 1740 cm⁻¹, presumably due to the generation of intermediate products of quinones or ketones, at 1585 cm⁻¹, the C=C double bond was noticed (Wang et al., 2017a). The prominent peaks at 1560 and 1406 cm⁻¹ are assigned to the typical signals of carboxylate species, which are associated with the ring vibrations and symmetric COO stretching groups (Wang et al., 2017b). The appearance of the bands is ascribed to the benzene ring breaking due to oxidation. The acetate species is associated with the peak at 1373 cm⁻¹, while maleic anhydride is responsible for the peaks at 1307 and 1238 cm⁻¹ (Shi et al., 2023). Moreover, the band related to the C–O bonds of alcohols or alkoxide species was detected at 1163 cm⁻¹ (Zhang et al., 2022). During benzene catalytic oxidation, benzene is first converted to benzoquinone and then to maleic anhydride and subsequently oxidized to form carboxylate species, ultimately generating the final products, CO₂ and H₂O.

As shown in Fig.6(b), 3076, 3040, 2934, 2879, 1953, and 1487 cm⁻¹ were the typical peaks of toluene, which decreased with increasing temperature. The adsorption of H₂O is linked to the peak at 1632 cm⁻¹, which was relatively large at 100 °C–200 °C. During the heating process, several new peaks were observed. The CO₂ bands (2312 cm⁻¹) were found when oxygen was present. Moreover, two bands, at 1590 and 1270 cm⁻¹, are ascribed to C=C double bonds. The bands at 1556, 1504, and 1411 cm⁻¹ are assigned to the asymmetrical oscillation of the –COO bonds of carboxylate species. The signal at 1543 cm⁻¹ could be ascribed to the asymmetric stretching oscillation of the –COO of benzoate (Li et al., 2020). The absorption bands at 1309 and 1216 cm⁻¹ correspond to maleic anhydrides (Yu et al., 2021). The band at 1155 cm⁻¹ correspond to the C–O bonds of alcohols or alkoxide species, indicating the formation of benzyl alcohol (Lei et al., 2023). With increasing temperature, these new peaks followed an upward and descending tendency, with the maximum peak intensity occurring at 200 °C. Based on the production of these intermediates, it could be speculated that toluene catalytic oxidation occurs through the generation of maleic anhydride, benzyl alcohol, benzoic acid, and benzoate and the generation of CO₂ and H₂O.

The in situ DRIFTS of multi component VOCs (benzene and toluene) and oxygen are shown in Fig.6(c). The characteristic bands intensities of benzene and toluene (3086, 3044, 2969, 2940, 2883, 1961, 1800, 1497, and 1032 cm⁻¹) lowered as the temperature rose, and the characteristic peaks were hardly visible above 300 °C. When benzene and toluene are catalyzed to oxidize each other synergistically, some intermediate species were identified, including benzoquinone (1740 cm⁻¹), maleic anhydride (1307 and 1236 cm⁻¹), alcohol or alkoxide species (1158 cm⁻¹), benzoate (1551 cm⁻¹), and carboxylate species (1576 and 1415 cm⁻¹). Ultimately, CO₂ and H₂O are generated.

The synergistic elimination processes of the intermediates generated in multi component VOCs (benzene and toluene) were analogous to those formed in benzene or toluene catalytic degradation alone. The catalytic elimination of toluene was not directly impacted by benzene introduction, the benzene catalytic degradation was unaffected by the addition of toluene. Therefore, it is thought that the MnCoO_x catalyst might remove benzene and toluene in a synergistic manner.

Fig.7(a) shows how the mixture (benzene and toluene) and benzene are adsorbed. Two bands at 3089 and 3045 cm⁻¹ are associated with C–H bonds in benzene rings. These characteristic vibrations of the benzene ring are reflected in benzene and benzene–toluene mixture adsorption. When the mixture of benzene and toluene was adsorbed on the MnCoO_x surface, the bands corresponding to the benzene ring were diminished compared with those corresponding to the adsorption of benzene alone. The presence of toluene limited the adsorption of benzene because the adsorption of toluene reduced the quantity of benzene adsorption sites that are available. Compared with the methyl adsorption peak in the adsorption of toluene alone on the MnCoO_x sample (Fig.7(b)), the methyl adsorption peaks (2935 and 2879 cm⁻¹) corresponding to toluene were significantly weakened during the compound of benzene and toluene adsorbing; this indicates that the presence of benzene in the compound of benzene and toluene affects the adsorption of toluene and that the competitive adsorption between benzene and toluene restrains the extending vibration of toluene methyl group. Based on the above discussion, a competitive relationship exists between benzene and toluene for the adsorption of pollutants at active sites, and the adsorption of multi component VOCs (benzene and toluene) on MnCoO_x could lead to mutual inhibition, which is consistent with relevant findings published in the literature (Moreno-Román et al., 2024).

Fig.7 Comparison of adsorption of (a) benzene and benzene–toluene, (b) toluene and toluene–benzene, (c) benzene + O2 and benzene–toluene + O2, (d) toluene + O2 and toluene–benzene + O2 at 200 °C over MnCoOx catalyst.

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Fig.7(c) and Fig.7(d) show in situ DRIFTS obtained by the catalytic elimination of benzene or toluene and the mixture (benzene and toluene) at 200 °C on the MnCoO_x catalyst. When the spectrum obtained for alone-benzene catalytic oxidation was compared with that obtained for the degradation of the benzene and toluene mixture at 200 °C, the intermediate bands of benzene were found to be lower in the existence of toluene. The bands at 1583 and 1412 cm⁻¹ are assigned to carboxylate species, which originated from the oxidation of the breaking benzene ring. The peak intensity at 1412 cm⁻¹ in the mixture (benzene and toluene) for catalytic oxidation was lower than that for benzene oxidation (Fig.7(c)). This indicates the inhibitory impact of toluene on benzene oxidation over the MnCoO_x sample, which is in line with the lower benzene conversion in the mixture (Fig.2(k)). Compared with the pattern obtained for individual toluene oxidation with that of benzene and toluene cooperative oxidation (Fig.7(d)), The peak intensity did not much weaken, indicating that the benzene has little inhibitory effects on toluene catalytic oxidation; this could be because toluene is more easily oxidized and decomposed than benzene is, leading to a weakening of the inhibitory effect.

In summary, the reaction process could be used to remove benzene and toluene in concert. Although benzene and toluene had a certain inhibitory effect on each other in this oxidation reaction, the intermediate products created during multi component VOCs (benzene and toluene) synergistic oxidation were similar to those formed during the benzene or toluene catalytic elimination, implying that this inhibitory impact is reversible.

4 Conclusions

MnCo oxide catalysts with various Mn/Co ratios (1:2, 1:1, and 2:1) were synthesized, and the catalytic degradation results of benzene and toluene oxidation under single and binary VOC conditions were efficiently increased. The Co–Mn composite oxide catalysts exhibited better performance than did pure MnO_x and CoO_x catalysts. MnCoO_x and MnCo₂O_x had significantly better catalytic activity, and benzene and toluene conversion in their mixtures were 100% at 300 °C. The characterization findings showed that the MnCoO_x and MnCo₂O_x catalysts exhibited strong redox performance and abundant Co³⁺ and Mn⁴⁺ species, which could considerably enhance the synergetic catalytic degradation activity of two compounds (benzene and toluene). The weaker acidic sites and O_β species were important factors for the adsorption and activation of pollutant molecules at low temperatures, thereby achieving benzene and toluene complete oxidation. Furthermore, in situ DRIFTS illustration of the mechanistic pathways of benzene and toluene synergetic catalytic oxidation. Benzene catalytic oxidation occurred through the production of benzoquinone, maleic anhydride, and carboxylate species, and toluene catalytic elimination occurred through benzyl alcohol, benzoic acid, benzoate, and maleic anhydride. Moreover, competitive adsorption and mutual inhibition occurred between benzene and toluene during the synergistic degradation process, and the inhibition effect was reversible. This work created highly effective materials for simultaneous benzene and toluene degradation, and new ideas have been provided for the construction of catalysts and their application in the context of synergistic elimination of multicomponent VOCs.

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Acknowledgements

This work was supported by the National Natural Science Foundation of China (Nos. 22206146, 22006079, and U21A20524), the Fundamental Research Funds for the Central Universities (China), the Youth Innovation Promotion Association of Chinese Academy of Sciences, the Fundamental Research Program of Shanxi Province (China) (No. 202103021223280).

Conflict Interests

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11783-025-1942-6 and is accessible for authorized users.

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