Enhanced sulfur resistance by constructing MnO_x–Co₃O₄ interface on Ni foam in the removal of benzene

Abstract

The catalytic degradation of volatile organic compounds (VOCs) in the presence of SO₂ remains an urgent issue for industrial applications. Herein, we constructed an MnO_x–Co₃O₄ interface on Ni foam (Mn_xCo_y–NF catalysts) to improve SO₂ resistance for benzene degradation. The surface decoration of MnO_x on Mn_xCo_y–NF catalysts could generate a Co–Mn interface to tune the redox ability and active oxygen species. The Mn₁Co₁–NF catalyst showed high Co³⁺/Co²⁺ and Mn³⁺/Mn⁴⁺ ratios as well as a high O_latt/O_ads ratio, which are conducive to excellent low-temperature reducibility. Benefiting from abundant interfacial active sites, the Mn₁Co₁–NF catalyst exhibited superior catalytic activity with T₅₀ and T₉₀ values of 259 and 290 °C and SO₂-tolerance for benzene degradation. Results of in situ diffuse reflectance infrared Fourier transform spectroscopy and density functional theory calculation revealed that surface metal sulfate species were preferentially formed on surface Mn sites rather than Co sites, thereby retarding the poisoning of Co–Mn interfacial active sites. Correspondingly, the ring-opening of benzoquinone into maleate species on the Mn₁Co₁–NF catalyst was only slightly inhibited by the introduction of SO₂. This work provides a novel route to design SO₂-resistant catalysts for VOC degradation in practical applications.

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Environmental significance

Volatile organic compounds (VOCs) pose a serious threat to the environment and human health. Catalytic oxidation is an effective technology to eliminate VOCs. However, the presence of SO₂ can lead to the severe deactivation of catalysts due to the preferential reaction of SO₂ with active metal oxides to form inactive metal sulfates. Hence, to improve SO₂ tolerance, constructing an MnO_x–Co₃O₄ interface can not only improve the efficiency of benzene oxidation, but also effectively prevent SO₂ from poisoning the active sites, because of the preferential contact of SO₂ with MnO_x in catalysts, thereby protecting the active sites from poisoning. This work provides a novel route to design SO₂-resistant catalysts for VOC degradation in practical application.

1. Introduction

Volatile organic compounds (VOCs) are the major factors in environmental pollution, and many VOCs are known to be toxic, causing harm and even death to humans.^1–3 Therefore, legitimate control over VOC emissions is one of the top priorities to maintain the sustainable development of human society.^4,5 Among the available ways to eliminate VOCs, catalytic oxidation is an effective technology due to its high efficiency, low operating temperature and high product selectivity.^6,7 However, the key issue is to explore catalysts with high catalytic activity and superior anti-poisoning for VOC elimination.

Transition-metal oxides (TMOs) and their composites have been considered promising candidates for VOC elimination. Among TMOs, Co₃O₄ and MnO_x have received considerable attention owing to their excellent catalytic performance and low cost. Co₃O₄ has a spinel structure with Fd3̄m symmetry, containing Co³⁺ on octahedral coordination sites (Co_Oh³⁺) and Co²⁺ on tetrahedral coordination sites (Co_Td²⁺), and is widely used in photocatalysis,^8,9 electric catalysis^9,10 and thermal catalysis.^11,12 Because of the relatively low ΔH of vaporization of O₂, Co₃O₄ exhibits high activity for the removal of most VOCs.¹³ Manganese oxides, such as Mn₃O₄, Mn₂O₃, and MnO₂, are known for exhibiting relatively high activity in catalytic oxidation.^14,15 Compared with single-component catalysts, multi-component composite catalysts generally show excellent catalytic activity due to the synergistic and interfacial effects of the junction interface. Putla et al. reported that an MnO_x/CeO₂ catalyst exhibited outstanding catalytic activities toward both diesel soot oxidation and benzylamine oxidation compared to a pure CeO₂ catalyst, which was due to the catalytically favorable properties at the MnO_x/CeO₂ interface.¹⁶ Liu et al. prepared a 12CoCu–R catalyst with better catalytic activity for acetone oxidation, which was attributed to the fracturing of the Co–O bond by interfacial interaction between Co and Cu.¹⁷ Han et al. prepared a series of cobalt/manganese oxides with different Co/Mn ratios through the pyrolysis of CoMn–MOF-71, which exhibited improved catalytic activity for toluene oxidation owing to a synergistic effect.¹⁸ In this regard, the fabrication of Co–Mn double component composites is an effective method to improve the low-temperature activity of VOC elimination.

As we all know, an inevitable problem is the SO₂-induced deactivation of catalysts for VOC elimination due to the complex composition of fuel gas. SO₂ can react with active metal oxides via competitive adsorption to form inactive metal sulfates, resulting in blocking of the active sites.^19,20 To improve SO₂-tolerance, one strategy is to regulate the surface electronic properties of catalysts; the other approach is to introduce an active phase as a scavenger to preferentially capture SO₂.^21,22 Xu et al. revealed that V doping into a 13Cu/γ-Al₂O₃ catalyst exhibited acceptable SO₂ resistance of the 13Cu/γ-Al₂O₃ catalyst for toluene oxidation owing to the synergy between Cu and V oxide.²³ Deng et al. confirmed that a W-doped Pt/TiO₂ catalyst could inhibit SO₂ adsorption and activity, thus improving SO₂ resistance for acetone oxidation.²⁴ As previously reported, Mn-based catalysts suffered from severe SO₂-induced deactivation because the active phase MnO_x was preferentially sulfated by SO₂ to form MnSO₄.^25,26 Thus, it is reasonable to expect the preferential contact of SO₂ with MnO_x in catalysts, thereby protecting the active sites from poisoning.

In this study, Mn_xCo_y–NF catalysts were successfully fabricated by the pyrolysis of ZIF-67 and impregnation. The Mn₁Co₁–NF catalyst had abundant interfacial active sites and surface-exposed MnO_x, thus exhibiting excellent catalytic activity for benzene as well as SO₂ resistance. The mechanism of SO₂ resistance was unraveled through in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra and (density functional theory) DFT calculation. This work can benefit the design of catalysts with abundant interfaces and excellent SO₂ resistance for the abatement of VOCs.

2. Experimental section

2.1 Materials

All reagents were of analytical grade and used without further purification. Ethyl alcohol and methanol (>99.7% purity) were purchased from Concord Technology. Co(NO₃)₂·6H₂O (99.99% purity) was obtained from Sinopharm Chemical Regent Co., Ltd. 2-Methylimidazole (purity > 98%) was obtained from Saen Chemical Technology (Shanghai) Co., Ltd. Mn(NO₃)₂·6H₂O solution (50 wt%) was obtained from J&K Scientific Ltd.

2.2 Preparation of catalysts

Pretreatment of Ni foam. Ni foam (2 cm × 3 cm) was cleaned to remove nickel oxide on the surface with 1 M HCl solution and to remove organics with acetone solution, then rinsed with deionized (DI) water. The cleaned Ni foam was dried at 60 °C for 3 h, and denoted NF.

Synthesis of ZIF-67–NF. ZIF-67–NF was fabricated by a simple method according to previous reports.²⁷ In this process, 2 mmol (0.58 g) of Co(NO₃)₂·6H₂O and 16 mmol (1.3 g) of 2-methylimidazole (Hmim) were respectively dissolved in 40 mL of DI water by ultrasonic treatment at room temperature. Co(NO₃)₂·6H₂O solution was quickly added into the 2-methylimidazole solution, and then a piece of NF immediately sank to the bottom of the above solution for 1 h. The equation for the chemical reaction is as follows: Co(NO₃)₂·6H₂O + C₄H₆N₂ → C₄H₆N₂Co (ZIF-67). Finally, the samples were washed several times with deionized water and ethanol and dried at 60 °C for 12 h.

Synthesis of Co₃O₄–NF and Mn_xCo_y–NF. Co₃O₄–NF was prepared by calcining ZIF-67–NF in a muffle furnace at 500 °C for 2 h according to the TG result (Fig. S1†). The Mn_xCo_y–NF catalysts were synthesized by impregnating a certain amount of Mn(NO₃)₂·6H₂O solution according to the molar ratio of Mn/Co (x/y, 1 : 2, 1 : 1, 1 : 2) on the Co₃O₄–NF catalyst. Then these samples were collected and dried in a vacuum oven. Finally, the samples were calcined at 500 °C for 2 h to obtain the Mn_xCo_y–NF catalysts, which were denoted Mn₁Co₂–NF, Mn₁Co₁–NF, and Mn₂Co₁–NF, respectively, according to the molar ratio of Mn/Co.

2.3 Characterization of catalysts

The X-ray power diffraction (XRD) patterns of the samples were obtained on a Rigaku D/MAX 2500 diffractometer using monochromatized Cu Kα radiation with a scanning range from 5 to 80° and rate of 2 °C min⁻¹. The nitrogen sorption isotherms of the catalysts were conducted with an Autosorb-IQ at 77 K. The morphology and microstructure of these samples were characterized by a field-emission scanning electron microscope (SEM, Hitachi S-4800) and a transmission electron microscope (TEM, HT-7700). Raman spectra of the catalysts were performed with a Lab-RAM spectrometer (HORIBA Jobin Yvon S.A.S.) using a 532 nm laser. X-ray photoelectron spectroscopy (XPS) was performed with an ESCALAB250XI instrument equipped with Al Kα radiation. H₂ temperature-programmed reduction (H₂-TPR) measures were carried out on Micromeritics AutoChem II 2920. For H₂-TPR, the samples were pretreated at 300 °C for 30 min in He flow to remove adsorbed species and then cooled to room temperature. The TPR profiles were obtained with temperatures rising from room temperature to 600 °C at a rate of 10 °C min⁻¹ in 10% H₂/He flow. The method for calculating the initial H₂ consumption rate is as follows: firstly, the data (time, temperature and corresponding cumulative H₂ consumption), where there was less than 25% of the first reduction peak, was calculated and selected. Secondly, the H₂ consumption rate, which is designated as the ratio of cumulative H₂ consumption to time, can be calculated. Finally, the curve of initial H₂ consumption rate can be obtained which takes 1000/(T + 273.15) as the abscissa and the H₂ consumption rate as the ordinate. The GC-MS spectra were obtained with GC-MS (7890B-5977B, Agilent Technology, USA). The column used in the study was 30 m × 250 μm × 0.25 μm (19091S-433UI, Agilent, USA). In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFT) was conducted on a Nicolet iS50 spectrometer in the range of 1000–4000 cm⁻¹ with 32 scans at a resolution of 4 cm⁻¹. For the toluene adsorption experiment, the samples were pretreated at 310 °C in N₂ flow (100 mL min⁻¹) for 2 h, and then the background spectrum was recorded. The mixture flow (500 ppm benzene + 20% O₂ + N₂ balance) was let into the reaction cell at 310 °C, and the in situ DRIFTS spectra were collected. After that, the mixture flow (500 ppm benzene + 20% O₂ + 1 ppm SO₂ + N₂ balanced) was let into the cell and the in situ DRIFTS spectra were collected.

2.4 Catalytic activity test

The activity tests for the catalytic oxidation of benzene over the samples were determined in a continuous-flow fixed-bed quartz reactor (i.d. = 4 mm) with 50 mg of catalyst. The reactant feed was 100 ppm benzene, 20% (vol) of O₂ and N₂ equilibrium gas. The total gas flow rate was 100 mL min⁻¹, and the corresponding weight hourly space velocity (WHSV) was 120 000 mL g⁻¹ h⁻¹. The conversion of benzene (X_benzene) was calculated from the inlet benzene concentration (c_inlet) and outlet benzene concentration (c_outlet) according to eqn (1):

2.5 DFT calculations

DFT calculations were employed to investigate the contribution of introduced Mn to the adsorption of molecular SO₂. The density function theory (DFT) calculations were performed by using the GGA-PBE function and CASTEP module. A 1 × 1 × 1 Monkhorst–Pack k-point sampling density was employed with a plane-wave cutoff energy of 400 eV. The adsorption and desorption energy were calculated with the equation:

E_ads = E_SO2/surf − E_surf − E_SO2 (2)

Here E_ads is the adsorption energy of the SO₂ molecule; E_SO2/surf is the energy of the surface with SO₂ adsorbed; E_surf is the total energy of the surface (slab); and E_SO2 is the energy of the pure substrate.

3. Results and discussion

3.1 Catalytic activities

As shown in Fig. 1a, the benzene-catalytic activities of all the catalysts increased with an increase in temperature. For comparison, benzene oxidation was also performed over the pure support Ni foam, revealing that the temperatures of 50% (T₅₀) and 90% (T₉₀) conversion were above 400 °C. However, the Co₃O₄–NF catalyst obtained by direct calcination of ZIF-67–NF could achieve a T₅₀ of 314 °C and T₉₀ of 348 °C for benzene oxidation. Notably, the introduction of Mn species into the Co₃O₄–NF catalyst remarkably improved the catalytic activity, and the T₅₀ and T₉₀ values were further lowered to 293 °C and 318 °C (Mn₁Co₂–NF), 259 °C and 290 °C (Mn₁Co₁–NF), 274 °C and 300 °C (Mn₂Co₁–NF), respectively. It is worth noting that both Co and Mn were beneficial to lowering the temperature of benzene oxidation, which could be due to the synergistic effects between Co and Mn species. The Mn₁Co₁–NF catalyst exhibited the best catalytic performance, presumably due to the largest amount of active interface between Co and Mn. As shown in Fig. 1b, no apparent attenuation of benzene conversion occurred during the 80 h test over the Mn₁Co₁–NF catalyst, indicating its excellent stability. The apparent activation energy (Ea) of the prepared catalysts in Fig. 1c showed that Co₃O₄–NF presented an Ea of 105.4 kJ mol⁻¹, notably higher than those of Mn₁Co₂–NF (92.2 kJ mol⁻¹), Mn₁Co₁–NF (61.8 kJ mol⁻¹) or Mn₂Co₁–NF (70.5 kJ mol⁻¹), consistent with the catalytic activity. Conversion of the normalized benzene by surface area (Fig. S2†) was performed to exclude the influence of surface area, indicating that surface area is not the main factor in the catalytic performance.

Fig. 1 (a) C₆H₆ conversion over different Mn_xCo_y–NF catalysts as a function of temperature. (b) Long-term stability experiments over the Mn₁Co₁–NF catalyst. (c) Arrhenius plots of all catalysts. (d) Effect of SO₂ on C₆H₆ conversion over different Mn_xCo_y–NF catalysts (reaction conditions: 1 ppm SO₂, 20% O₂, 100 ppm C₆H₆, balanced by N₂, WHSV = 120 000 mL g⁻¹ h⁻¹).

Sulfur compounds and water vapor are general in the process of VOC oxidation. Therefore, the catalytic performance of as-prepared catalysts under 1 ppm SO₂ or water vapor was also examined. As shown in Fig. 1d, the on-stream reaction on all catalysts was performed at ca. 90% conversion through controlling the reaction temperature. When the gaseous SO₂ was let into the reaction system, the benzene conversion over as-prepared catalysts exhibited an obvious decrease with reaction time. In particular, the NF catalyst was totally deactivated for benzene oxidation within 1 h, and the benzene conversion of Co₃O₄–NF decreased to ca. 10% after 2 h. Such a large loss in catalytic activity should be due to the strong adsorption of SO₂ on the active sites of the catalysts. In contrast, for the Mn_xCo_y–NF catalysts, the decrease in the benzene conversion was retarded. The slope of the curves in Fig. 1d could represent the inactivation rate of the catalysts. Obviously, the inactivation rates of the Mn_xCo_y–NF catalysts were smaller than those of the NF catalyst or Co₃O₄–NF catalyst, and the Mn₁Co₁–NF catalyst showed outstanding sulfur resistance. Once the gaseous SO₂ had been cut off from the feed gas, the as-prepared catalysts were no longer deactivated, and retained stable benzene conversion except for the NF catalyst. For the NF catalyst, the benzene conversion could be slowly recovered, indicating that the active sites were exposed again due to the removal of adsorbed SO₂ from the active sites at a high temperature, which was possibly attributed to the weak interaction of SO₂ with Ni foam. It is known that water vapor always exists in exhaust emissions. Many studies have shown that water generally causes a deterioration in catalytic performance, and it is considered a typical poison in VOC oxidation.^13,28–30 As shown in Fig. S3,† benzene conversion by the Co₃O₄–NF and Mn₁Co₁–NF catalysts decreased with the introduction of water, especially for the Co₃O₄–NF catalyst, which exhibited inferior water tolerance. Compared with the Co₃O₄–NF catalyst, the Mn₁Co₁–NF catalyst improved water resistance for benzene oxidation under the different water vapor concentrations.

3.2 Structural characterization

XRD analysis was carried out to investigate the crystalline phase of the catalysts. As shown in Fig. 2a, the strong peaks at 44.6°, 51.9° and 76.5° could be well indexed to the (111), (200) and (220) planes of nickel metal (JCPDS no. 04-0850), and the diffraction peaks at 37.4° and 43.4° could be attributed to the (111) and (200) planes of nickel oxide (JCPDS no. 47-1049). For the Co₃O₄–NF catalyst, obvious peaks were observed at 31.1°, 36.8°, 59.2° and 65.1°, corresponding to the spinel Co₃O₄ phase (JPCDS no. 42-1467). After introduction of MnO_x, the Mn_xCo_y–NF catalysts still mainly displayed characteristic diffraction peaks of Co₃O₄, indicating that the loading a suitable amount of MnO_x on the surface of Co₃O₄ could not result in an obvious change in the crystal structure of Co₃O₄, but these peaks showed a slight shift to a low angle with the introduction of Mn species, indicating the doping of Mn into the Co₃O₄ lattice. No peaks of the MnO_x species were detected in the XRD pattern, which could be attributed to the high dispersion of MnO_x in the Mn_xCo_y–NF catalysts. Compared with the Co₃O₄–NF catalyst, the Mn_xCo_y–NF catalysts exhibited the weak and broad diffraction peaks of the Co₃O₄ phase, implying that the small crystal size was prone to the formation of lattice defects.³¹ Distinct Raman spectra for all catalysts were clearly observed, as displayed in Fig. 2b. For the Co₃O₄–NF catalyst, five distinct bands at 197, 484, 523, 620 and 691 cm⁻¹ corresponded to the F_2g, E_g and A_1g Raman-active modes of the spinel Co₃O₄ phase.^12,28,32,33 With the decoration of Mn species on the surface of the Co₃O₄–NF catalyst, the Mn_xCo_y–NF catalysts presented similar Raman peaks to the Co₃O₄–NF catalyst, but these Raman peaks were weakened and downshifted to lower wavenumbers (a red-shift), suggesting that the Mn_xCo_y–NF catalysts caused a lattice distortion or residual stress in the spinal matrix due to the incorporation of Mn ions.^34,35 For the Mn₁Co₁–NF and Mn₂Co₁–NF catalysts, peaks at 180, 321, 518 and 661 cm⁻¹ emerged gradually, which were assigned to the characteristic peaks for MnO₂.^31,36,37 Note that the Raman peaks of related Co₃O₄ disappeared in the Mn₁Co₁–NF and Mn₂Co₁–NF catalysts, indicating that the surface of Co₃O₄ should be covered by MnO_x, which was beneficial for the increase in the interface between MnO_x and Co₃O₄.

Fig. 2 (a) XRD patterns and (b) Raman spectra of the Co₃O₄–NF catalyst and Mn_xCo_y–NF catalysts.

The morphologies and structural features of Co₃O₄–NF and Mn_xCo_y–NF catalysts are characterized by SEM and TEM. Ni foam presented a three-dimensional and interconnected porous structure (Fig. S4a†), which facilitated the growth of ZIF-67 on Ni foam, and a series of nanoplates with a smooth surface were uniformly distributed in ZIF-67–NF (Fig. S4b†). After annealing treatment for ZIF-67–NF, the obtained Co₃O₄–NF sample comprised many dense leaflike nanosheets (Fig. 3a). The TEM image in Fig. 3b confirmed that the Co₃O₄ nanosheets had a size of ca. 1 μm, and lattice spacings of ca. 0.24 and 0.28 nm were observed in the HRTEM image of the Co₃O₄–NF catalyst (Fig. 3c), which are ascribed to the (311) and (220) planes of Co₃O₄, respectively. Compared with the Co₃O₄–NF catalyst, the Mn_xCo_y–NF catalyst showed loose honeycomb shapes with numerous thin nanosheets (Fig. 3d and S5†). As shown in Fig. 3e and S5c and d,† the triangular nanosheets with numerous pores consisted of small nanoparticles, and many nanoparticles had accumulated on the edge of the nanosheets, consistent with the BET result (Fig. S6 and Table S1†). Moreover, the line scanning images (Fig. S7†) confirmed the absence of a core–shell structure. The TEM results indicated that the leaf shape of Co₃O₄ became thinner and smaller with the introduction of Mn species. No lattice spacings of Mn species were observed in the HRTEM images of the Mn₁Co₁–NF catalyst (Fig. 3f) except the lattice spacings of Co₃O₄. The elemental mapping results of Mn₁Co₁–NF in Fig. 3g showed that Co and Mn species had a homogeneous distribution.

Fig. 3 SEM images of (a) the Co₃O₄–NF catalyst and (d) the Mn₁Co₁–NF catalyst, TEM images of (b) the Co₃O₄–NF catalyst and (e) the Mn₁Co₁–NF catalyst, HRTEM images of (c) Co₃O₄–NF and (f) Mn₁Co₁–NF, and (g) elemental mapping results of Mn₁Co₁–NF.

To explore the valence state of elements and active oxygen species on the surface of the prepared samples, X-ray photoelectron spectroscopy (XPS) was performed to characterize the Co₃O₄–NF and Mn_xCo_y–NF catalysts. As shown in Fig. 4a, the Co 2p spectra of all catalysts could be fitted into two main components (Co³⁺ and Co²⁺) together with two shake-up satellites.³⁸ The signals at binding energies of 780.0 and 795.1 eV were indicative of surface Co³⁺, whereas the peaks at 781.6 and 796.5 eV demonstrated the presence of surface Co²⁺.³⁹ As listed in Table 1, the Co³⁺/Co²⁺ molar ratio increased in the sequence Co₃O₄–NF < Mn₁Co₂–NF < Mn₂Co₁–NF < Mn₁Co₁–NF, consistent with the activity of the catalysts. As previously reported, the greater amount of surface Co³⁺ in the Co-based catalysts was beneficial to the catalytic activity.⁴⁰ The Mn 2p features in Fig. 4b exhibited two main peaks, where the peaks at 640.8 and 652.4 eV were assigned to Mn³⁺, and the peaks at 642.1 and 653.8 eV were attributed to Mn⁴⁺.⁴¹ The surface Mn³⁺/Mn⁴⁺ ratio was calculated and is shown in Table 1, and the Mn₁Co₁–NF catalyst displayed the highest Mn³⁺/Mn⁴⁺ ratio, followed by Mn₂Co₁–NF and Mn₁Co₂–NF catalysts. Combining the Co 2p and Mn 2p results, it was concluded that there should be interfacial interaction between Co and Mn species, resulting in electron transfer between Co and Mn species. In the case of the O 1s spectra (Fig. 4c), the broad asymmetrical O 1s XPS peaks were fitted into three peaks, corresponding to three kinds of surface oxygen species. The peak at 529.9 eV could be ascribed to the lattice oxygen species (O_latt), the peak at 531.5 eV belonged to chemisorbed oxygen species and the peak at 532.9 eV was related to hydroxyl species/adsorbed water molecules (O_H).⁴² In general, VOC oxidation on transition metal oxides follows the Mars–van Krevelen (MvK) mechanism, where the O_latt species should play a vital role in the deep oxidation of VOCs. It was noted that the incorporation of Mn species increased the O_latt/O_ads ratio in Mn_xCo_y–NF catalysts (Table 1), and the O_latt/O_ads ratio followed the order: Co₃O₄–NF < Mn₁Co₂–NF < Mn₂Co₁–NF < Mn₁Co₁–NF, revealing the activation of lattice oxygen by interfacial interaction between Co and Mn species.

Fig. 4 XPS spectra for (a) Co 2p, (b) Mn 2p and (c) O 1s of the Co₃O₄–NF and Mn_xCo_y–NF catalysts.

Table 1 Physicochemical properties and benzene oxidation activity of the as-prepared samples

Samples	T 50 (°C)	T 90 (°C)	Ea (kJ mol^−1)	Surface element ratio			Mn/Co^a	Mn/Co^b
				Co^3+/Co^2+	Mn^3+/Mn^α+	Olatt/Oads
a The data was calculated from XPS. b The data was calculated from ICP-MS.
Co3O4–NF	314	348	105.4	1.12	—	0.61	—	—
Mn1Co2–NF	293	318	92.2	1.35	1.69	0.73	0.61	0.54
Mn1Co1–NF	259	290	61.8	1.59	1.92	0.77	1.34	0.96
Mn2Co1–NF	274	300	70.5	1.49	1.81	0.71	0.98	0.95

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The redox properties of the Co₃O₄–NF and Mn_xCo_y–NF catalysts were further investigated by H₂-TPR. As illustrated in Fig. 5a, the pure Co₃O₄–NF catalyst exhibited three main broad peaks at 268, 310 and 351 °C, belonging to the stepwise reduction of Co³⁺ → Co²⁺ → Co⁰. For Mn_xCo_y–NF catalysts, the first reduction peak (Co³⁺ → Co²⁺) showed a slight shift to the low-temperature region, while the last two peaks (Co²⁺ → Co⁰) made an opposite movement. The improvement in low-temperature reducibility was attributed to the occupation by Mn³⁺/Mn⁴⁺ ions of octahedral Co³⁺ sites, which was conducive to catalytic activity. There is no reduction peak of MnO_x, presumably due to the reduction of MnO_x with small size at low temperature. The initial H₂ consumption rate in Fig. 5b exhibited an increasing order of low-temperature reducibility as follows: Co₃O₄ < Mn₁Co₂–NF < Mn₁Co₁–NF < Mn₂Co₁–NF. The results confirmed that the low-temperature reducibility was remarkably improved by the incorporation of Mn species.

Fig. 5 (a) H₂-TPR profiles and (b) initial H₂ consumption rate at low temperature over all catalysts.

3.3 Degradation and SO₂-tolerance mechanisms

In situ DRIFTS experiments were carried out to detect the possible intermediate species and further infer the reaction path of benzene oxidation. As depicted in Fig. 6a, the bands of intermediate species for the Mn_xCo_y–NF catalyst appeared during the initial 60 min, and the intensity became stronger with time. The bands at 3099, 3042 and 1054 cm⁻¹ were ascribed to the C–H stretching vibration in the benzene rings, indicating the adsorption of benzene on the surface of Mn₁Co₁–NF.^1,43 In addition, the band at 1194 cm⁻¹ was ascribed to phenolate species,⁴⁴ and the band at 1418 cm⁻¹ was attributed to C=O stretching vibrations of quinone species.⁴⁵ It is worth noting that the abundant maleate species were detected at 1556, 1314 and 1233 cm⁻¹, implying sufficient active oxygen species to accelerate the ring-opening of benzoquinone. Similar intermediate species were also observed in the Co₃O₄–NF catalyst (Fig. S8a†). However, the phenolate species was the most abundant in the Co₃O₄–NF catalyst, indicating the lack of active oxygen species. After introducing SO₂ into the reaction, it was observed that the corresponding bands of the intermediate species still existed, but the intensities became weaker. Meanwhile, the band at 1110 cm⁻¹ in the Mn_xCo_y–NF catalyst was attributed to sulfate species, possibly resulting from the reaction between SO₃/SO₃²⁻ and surface Mn species,⁴⁶ which could inhibit SO₂ poisoning to some extent.

Fig. 6 (a) In situ DRIFTS spectra of reactant adsorption at 310 °C (reaction conditions: 20% O₂ + 500 ppm C₆H₆ + N₂ within 60 min, N₂ in 60–90 min and 20% O₂ + 500 ppm C₆H₆ + 1 ppm SO₂ + N₂ in 90–150 min). Side view of the SO₂ adsorbed (b) on Co₃O₄–NF and Mn₁Co₁–NF (blue balls indicate Co element, purple balls indicate Mn element, red balls indicate O element and yellow balls indicate S element). (c) The normalized content of reaction intermediate species over the Mn₁Co₁–NF catalyst. (d) GC-MS spectra of the organic compounds extracted over the Co₃O₄–NF and Mn_xCo_y–NF catalysts at 250 °C.

To further study the change in Co₃O₄–NF and Mn₁Co₁–NF after introducing SO₂, we established the model of Co₃O₄ and MnO₂ (Fig. S9†) according to the XRD and Raman results. As two aspects of the data, the parameters for adsorption energy and bond lengths were analyzed. The adsorption energy (E_ads) of SO₂ increased from −0.66 eV on the surface Co sites over Co₃O₄–NF to −0.83 eV on the surface Mn sites over Mn₁Co₁–NF (Fig. 6b and S10†). In general, the adsorption energy is related to strong interaction. The high E_ads for SO₂ indicated that the formation of surface metal sulfate species on Mn sites was easier than on Co sites, in good agreement with the results of the DRIFTS experiments. In addition, SO₂ was adsorbed on the Mn sites with an O–Mn bond length of 0.17 nm and on the Co sites with an O–Mn bond length of 0.21 nm, indicating that SO₂ showed relatively stable chemical adsorption on the Mn₁Co₁–NF surface. Therefore, the surface MnO₂ on the Mn₁Co₁–NF catalyst could serve as a sacrificial site to slow down the poisoning of Co–Mn interfacial active sites.

To identify the influence of SO₂ on the changes in intermediate species more clearly, Fig. 6c shows the normalized variation in benzoquinone and maleate species over Mn₁Co₁–NF. It is worth noting that with an increase in time, the amounts of benzoquinone and maleate species were gradually reduced, but the decreasing rate of benzoquinone species was slower than that of maleate species. The results revealed that the introduction of SO₂ had no obvious influence on benzene oxidation into benzoquinone, but slightly inhibited the ring-opening of benzoquinone into maleate species on the Mn₁Co₁–NF catalyst. In contrast, benzoquinone and maleate species on the Co₃O₄–NF catalyst exhibited an obvious decreasing trend of benzoquinone and maleate species (Fig. S8b†). Thus, the introduction of Mn species could protect the Co–Mn interfacial active sites from SO₂ poisoning, thus maintaining continuous oxidation. As shown in Fig. 6d, the GC-MS spectra exhibited similar intermediate species over the Co₃O₄–NF and Mn_xCo_y–NF catalysts. Benzaldehyde was observed as a new intermediate species over the Mn₁Co₁–NF catalyst, due to the reaction between benzene and intermediate species.⁴⁷ Meanwhile, there was a large number of isopropanol and acetic acid species over the Mn₁Co₁–NF catalyst, indicating further oxidation by the surficial active oxygen species.

Based on characterizations and in situ DRIFTS results, the degradation mechanism for benzene oxidation is proposed in Fig. 7. The benzene and gas oxygen molecules were first adsorbed on the surface of the catalyst, and then the active oxygen reacted with the benzene rings to produce phenolate species via the interaction of the C–H bond with the interface between MnO_x and Co₃O₄. After further oxidation, the phenolate species were oxidized into benzoquinone species, and then the consumed active oxygen was supplemented via Co–Mn interfacial active sites by gaseous molecular oxygen. The benzoquinone species were deeply oxidized into maleate species, which could finally be decomposed into CO₂ and H₂O. The Mn₁Co₁–NF catalyst had abundant Co–Mn interfacial active sites, which were conducive to the replenishment of consumed active oxygen by gaseous molecular oxygen, thus exhibiting excellent performance for benzene degradation. As shown in Fig. 7, the surface MnO₂ served as a sacrificial site to capture SO₂ preferentially via the formation of manganese sulfate to retard the poisoning of Co–Mn interfacial active sites, thereby enhancing SO₂ tolerance.

Fig. 7 Schematic illustration of the degradation mechanism for benzene oxidation on the Mn₁Co₁–NF catalyst.

4. Conclusions

In conclusion, we elaborately constructed a series of Mn_xCo_y–NF catalysts with an MnO_x–Co₃O₄ interface, and the influence of SO₂ on benzene oxidation was deeply explored. The surface decoration of MnO_x on Mn_xCo_y–NF catalysts could enlarge the Co–Mn interface to enhance the redox ability and interfacial interaction. The Mn₁Co₁–NF catalyst had abundant Co–Mn interfacial active sites, resulting in a high amount of Co³⁺ and Mn³⁺. The large amount of O_latt species provided sufficient active oxygen species to trigger benzene degradation. As confirmed by in situ DRIFTS and DFT calculation, the surface Mn sites rather than Co sites on the Mn₁Co₁–NF catalyst could capture SO₂ to form metal sulfate, thereby retarding the poisoning of Co–Mn interfacial active sites. Thus, the Mn₁Co₁–NF catalyst exhibits high catalytic activity and SO₂ tolerance during benzene oxidation. This work provides a route to construct high-performance catalysts for enhancing catalytic activity and SO₂ tolerance in the degradation of VOCs in practical applications.

Author contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This project was supported by the National Natural Science Foundation of China (22076192, 21777166, 42175133 and 21806169), and Beijing National Laboratory for Molecular Sciences (BNLMS-CXXM-202011). The authors wish to thank facility support of the 4B9A beamline of Beijing Synchrotron Radiation Facility (BSRF).

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Footnote

† Electronic supplementary information (ESI) available: More detailed information of long-term stability experiments over the Mn₁Co₁–NF catalyst, Arrhenius plots of all catalysts, the conversion of benzene per unit area over all the catalysts, water effect on benzene conversion over Co₃O₄–NF and Mn₁Co₁–NF catalyst, SEM images, TEM images, N₂ adsorption–desorption isotherms and pore size distributions, XPS spectra, in situ DRIFTS spectra of reactant adsorption, side and top views of Co₃O₄–NF and Mn₁Co₁–NF, the top view of the SO₂-adsorbed on Co₃O₄–NF and Mn₁Co₁–NF, Table S1 (physicochemical properties of the samples). See DOI: https://doi.org/10.1039/d2en00893a

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