Revealing the Highly Catalytic Performance of Spinel CoMn2O4 for

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Research Article Cite This: ACS Catal. 2019, 9, 6698−6710

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Revealing the Highly Catalytic Performance of Spinel CoMn2O4 for Toluene Oxidation: Involvement and Replenishment of Oxygen Species Using In Situ Designed-TP Techniques Cui Dong,† Zhenping Qu,*,† Yuan Qin,† Qiang Fu,‡ Hongchun Sun,† and Xiaoxiao Duan†

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†

Key Laboratory of Industrial Ecology and Environmental Engineering, School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, People’s Republic of China ‡ State Key Lab of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, People’s Republic of China S Supporting Information *

ABSTRACT: The catalytic oxidation of toluene to CO2 and H2O over nanoflower spinel CoMn2O4 synthesized by the oxalic acid sol−gel method has been investigated, and it demonstrates lower activation energy (35.5 kJ/mol) for toluene oxidation compared with that using the metal oxides (Co3O4, MnOx, and Co3O4/MnOx), which shows nearly 100% conversion of toluene at 220 °C in the presence or absence of water vapor (2.0 vol %). Compared with the metal oxides (Co3O4/MnOx, MnOx, and Co3O4), the obtained spinel CoMn2O4 has a larger surface area, rich cationic vacancy, and high mobility of oxygen species, which are the reasons for its high activity for toluene oxidation. The different oxygen species shows the different role in VOCs oxidation, and the in situ designed-TP techniques are conducted to investigate the involvement of surface lattice oxygen, bulk lattice oxygen, and gaseous oxygen in catalytic oxidation of toluene over the spinel CoMn2O4 and Co3O4/MnOx catalysts. For spinel CoMn2O4, the surface lattice oxygen is the reactive oxygen species, which first induces the catalytic reaction. Furthermore, the gaseous oxygen moves to the bulk phase lattice and then migrates to the surface to form the surface lattice oxygen, which is different from the mixed-metal oxides Co3O4/MnOx that dissociates and activates gaseous oxygen only on the surface of the catalyst and requires a higher temperature. In addition, it is found that the toluene oxidation occurs via the benzyl alcohol−benzoate−anhydride−acetate reaction pathway over spinel CoMn2O4, and the conversion of the surface anhydride is the rate-controlling step, especially at 200−210 °C, which is also different from the mixed-metal oxides Co3O4/MnOx. These results could provide a considerable experimental basis for understanding the mechanism by which oxygen species participate in toluene oxidation. KEYWORDS: toluene oxidation, spinel CoMn2O4, oxygen species, in situ designed-TP techniques, in situ DRIFTS

1. INTRODUCTION Driven by increasing environmental pollution, especially the emission of volatile organic compounds (VOCs) from various sources, abatement technologies are urged for maintaining a clean air atmosphere. Catalytic oxidation, as one of the most effective approaches and strategies, has attracted a great deal of attention.1−5 Transition-metal oxides are potential alternative catalysts because of their low cost and promising activity. In addition, researchers have found that the spinel-type catalyst enjoys high stability compared with mixed-metal oxides and perovskites in the catalytic oxidation.6,7 In spinel (AB2O4), metals A and B are in tetrahedral and octahedral positions, and the adjacent cations (A and B) can optimize the catalytic activity via electron exchange.8−11 To obtain excellent catalytic performance, a larger surface area and abundant defects are required in the spinel. Chen et al.12 found that spinel © XXXX American Chemical Society

CoxMn3−xO4 with a larger surface area exhibited a considerable activity toward the oxygen reduction/evolution reactions (ORR/OER) at room temperature. Zou et al.13 constructed the metal defects (manganese/cobalt vacancy) by defect engineering to increase the active site for the oxygen reduction reaction (ORR). Attempts have been made to apply spineltype catalysts into the oxidation of VOCs. Finocchio et al.14 reported that the surface site Crn+ on MgCr2O4 could weaken C−H bond by hydrogen abstraction of hydrocarbons and the formation of a C−O bond (alkoxy groups). Spinel CoMn2O4 prepared by the sol−gel method can achieve 80% toluene conversion at 320 °C,15 which is lower than that of other MnReceived: March 31, 2019 Revised: May 18, 2019 Published: June 17, 2019 6698

DOI: 10.1021/acscatal.9b01324 ACS Catal. 2019, 9, 6698−6710

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ACS Catalysis based metal oxides (T100 = 210−230 °C).2,16 Also, compared with single-metal oxides and mixed -metal oxides, spinel is less explored in toluene oxidation, and its reaction mechanism is still unclear. Despite that the catalytic mechanism of VOCs oxidation over spinel sample is still ambiguous, the Mars−van Krevelen mechanism involving the reduction and oxidation of the catalytically active species has been proposed.17,18 The reactant molecules (VOCs) can be oxidized by the lattice oxygen of the catalyst, which is then reoxidized by gaseous oxygen.19 The oxygen species of the catalyst have been considered to play a key role in their activity and have been extensively studied. Chen et al.20 reported that the surface adsorbed species (by Xray photoelectron spectroscopy) were reactive oxygen in the oxidation of VOCs (ethanol, toluene, acetone, and acetic ether) on spinel NiCo2O4. The high concentration of surface adsorbed oxygen species on the active interface of Co3O4− MnCo2O4.5 shows superior catalytic activity in toluene oxidation (by hydrogen-temperature-programmed reduction, O2-temperature-programmed desorption and X-ray photoelectron spectroscopy).21 Jiang et al.17 had correlated the content of surface adsorbed oxygen species (by X-ray photoelectron spectroscopy) to the benzene oxidation activity of the spinel MnCo2O4. In addition, Giraudon et al.22 conducted the catalytic experiments in the absence of gaseous oxygen and found that the surface (subsurface lattice oxygen) and lattice oxygen of spinel CuMn2O4 can participate in the toluene oxidation (determined by gas chromatography). Wang et al. and co-workers23 proposed that the accessibility of lattice oxygen (by thermal gravimetric analysis) should be the main determinant factor of the activity on the octahedral molecular sieve (OMS-2) catalyst toward VOCs (ethanol and acetaldehyde) oxidation, instead of the content of lattice oxygen. All these studies have determined that the oxygen species in the spinel can significantly affect the catalytic oxidation activity, but how these oxygen species participate in the reaction and the subsequent reoxidation step of the catalyst has not been elucidated and remains unknown. The precipitants used to synthesize spinel are widely varied (such as NaOH, KOH, Na2CO3, NH3, H2C2O4, Na2HCO3, (NH4)2CO3, and NH4HCO3),24 and the physicochemical properties (surface area and particle size) of the spinel will be different when we choose different precipitants.25 In addition, oxalic acid precipitants can make the oxide form a porous structure during the synthesis, which is beneficial to the distribution of manganese oxidation states and the formation of rich lattice oxygen.26 Thus, in this work, the spinel CoMn2O4 catalyst with a large specific surface area and an abundance of cationic defects is successful synthesized by the oxalic acid sol−gel method to exclusively study the activity for toluene catalytic oxidation. In addition, we evaluate the oxidation of toluene on a spinel sample from another perspective, where the pathway of the reoxidation from gaseous oxygen and how the oxygen species participate in the reaction are measured by the in situ designed-TP techniques. The mixed-metal oxides Co3O4/MnOx are also used for comparison to further assist in elucidating the mechanism of reoxidation and circulation of oxygen species for toluene oxidation in spinel sample. Moreover, the surface intermediates species over spinel CoMn2O4 during toluene oxidation are also investigated via in situ DFIFTS. Spinel CoMn2O4 is more easily reoxidized to generate a large amount of bulk-phase lattice oxygen species which then migrate to the

surface to form surface lattice oxygen. In addition, the spinel shows a high ability to destroy the aromatic ring, particularly for the breakage of benzoate to generate an anhydride species, thereby resulting in the high catalytic activity for toluene oxidation at low temperatures.

2. EXPERIMENTAL SECTION 2.1. Catalysts Preparation. Synthesis of Spinel CoMn2O4. Manganous acetate (0.02 mol) and cobalt acetate (0.01 mol) were added to 10 mL of ethanol under magnetic stirring at 80 °C to form a solution. Another oxalic acid solution (0.24 mol/L) was added quickly to the above solution. It was then stirred at 80 °C for 0.5 h, and the resulting precipitate was centrifuged and washed three times with deionized water and once with alcohol. The precipitate was dried at 80 °C for 12 h and was calcined from room temperature to 400 °C in air for 3 h. Synthesis of Mixed-Metal Oxides Co3O4/MnOx. The mixed-metal oxide was prepared by impregnation of the MnOx support using a solution of Co(NO3)2 with a Mn/Co molar ratio of 2:1. Following impregnation, the sample was dried at room temperature for 24 h, and then it was dried at 80 °C for another 12 h. The as-prepared sample was calcined from room temperature to 400 °C in air for 3 h. Synthesis of Unitary MnOx and Co3O4. First, 0.03 mol of manganous acetate or cobalt acetate was added to 10 mL of ethanol under magnetic stirring at 80 °C, and then oxalic acid solution (0.24 mol/L) was added quickly to the above solution. It was then stirred at 80 °C for 0.5 h, and the resulting precipitate was centrifuged and washed three times with deionized water and once with alcohol. The precipitate was dried at 80 °C for 12 h and was calcined from room temperature to 400 °C in air for 3 h. 2.2. Catalyst Characterization. The nitrogen physisorption measurements of samples were conducted on a Quantachrome NOVA 4200e by the Brunauer−Emmett− Teller (BET) method. X-ray diffraction (XRD) patterns were carried out on a Rigaku D/MAX-RB X-ray diffractometer with Cu Kα radiation. The XRD data were scanned from 10 to 90° with a rate of 8°/min. Scanning electron microscope (SEM) studies were conducted on instrument of JSM-5600LV. X-ray photoelectron spectroscopy (XPS) measurements were tested on an instrument of Thermo VG ESCALAB250, equipped with an Al Kα excitation source. Hydrogen-temperature-programmed reduction (H2-TPR) was carried out with a TCD detector on Quantachrom Automated Chemisorption. First, 0.05 g of catalyst was pretreated in a He stream at 200 °C for 30 min to remove the surface weakly adsorbed materials and then cooled to room temperature. Then the TPR experiments were programmed to heat the catalysts to 900 °C at a rate of 10 °C/min in a flow of 10 vol % H2/He. 2.3. Catalytic Tests. The catalytic activity of toluene oxidation was evaluated in a tubular fixed-bed reactor system, and 0.2 g (40−60 mesh) of catalyst was placed in a fixed-bed rector. The component of the reaction gas mixture contains 500 ppm toluene and 20%O2/Ar, and the total flow was 75 mL/min. The equivalent of gas hourly space velocity (GHSV) was approximately 22 500 mL g−1 h−1. The 500 ppm toluene in the gas mixture was generated by using an Ar bubbler through a bottle with pure toluene liquid and chilled in an ice−water isothermal bath at 0 °C. All reaction gas lines were heated to 100 °C by a heating band in order to prevent toluene 6699

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2), respectively. The experiment was performed as follows: (i) After the t-TPD experiment, the catalyst was purged with helium to room temperature. (ii) The used catalyst was purged with 5% O2/He at different temperatures (T = 25, 200, and 250 °C) for 40 min, respectively, and then the reactor was purged with He to room temperature to clean the weakly physical adsorption gaseous oxygen. (iii) Then the adsorption and desorption of toluene was carried out following the above t-TPD experiment (t-TPD → O2 (T) → t-TPD) (Step 2-2-1). To more clearly compare the changes in oxygen species after oxygen supplementation, the oxygen signal of the sample was also given in the He-TPD experiment (t-TPD → O2 (T) → HeTPD) (Step 2-2-2). Further, after the adsorption of toluene was saturated, the spinel CoMn2O4 system was purged with helium and then heated in a flow of 5% O2/He to investigate the temperatureprogrammed surface reaction of toluene (t-TPSR) (Step 3). In addition, after the first t-TPD experiment, toluene was adsorbed on the used catalyst again and then heated to 500 °C under a flow of 5% O2/He to detect the formation of CO2 (t-TPD → t-TPSR) (Step 2-3). In situ diffuse reflectance infrared fourier transform spectroscopy (in situ DRIFTS) was conducted on a Thermos Nicolet iS 10 equipped with ZnSe windows. The powder catalyst was pretreated in N2 (25 mL/min) at 300 °C for 30 min and then cooled to room temperature. A background spectrum was gathered at the corresponding temperature (200, 210, and 220 °C). The gaseous toluene (6 mL/min) was introduced into the reaction cell at 200 °C for 30 min and then purged with helium for another 30 min to remove the weakly adsorbed toluene. Thereafter, a mixture reaction gas of 20 vol % O2 + 80 vol % N2 (25 mL/min) was introduced into the cell to react with toluene. Then, the sample was further heated to 210 and 220 °C, respectively. The IR spectra were collected continuously for 30 min at each step, whereas the background spectra were also subtracted from each spectrum.

condensation on the tube walls. The quantitative analysis of reactants and products were analyzed online at a specific temperature by a gas chromatograph (GC-2014, Shimadu) equipped with flame ionization detector (FID) and thermal conductivity detector (TCD). The toluene conversion was calculated according to the following equations: X toluene =

[toluene]in − [toluene]out × 100% [toluene]in

where [toluene]in and [toluene]out represents the toluene inlet and outlet concentrations, respectively. 2.4. Kinetic Measurement. The specific reaction rate was normalized on a per-catalyst weight basis, as shown below: R (μmol m−2 h−1) =

F×X × 10−6 W×S

where R is the specific reaction rate, F (mol s−1) is the flow of gaseous molecules, X is toluene conversion, W (g) is the weight of the catalyst, and S (m2 g−1) is the specific surface area of the catalyst. To calculate the apparent activation Ea RT

( )), the

energies (Ea) using the Arrhenius law (k = A exp

toluene conversion was controlled to below 15%, and the gas hourly space velocity (GHSV) was changed from 22 500 mL g−1 h−1 to 90 000 mL g−1 h−1. In addition, by checking the Weistz−Prater parameter, it was confirmed that there is no internal pore diffusion resistance.27,28 C Wp =

r′ × R p2 Deff × Cs

where r′ is the reaction rate (kmol kg−1s−1), Rp represents the catalyst particle radius (m), Deff represents the effective gasphase diffusivity (m 2 s −1 ), and C s represents the gas concentration at the catalyst external surface (kmol m−3). The value of CWP was 3.74 × 10−4 < 0.3, indicating no significant mass transfer limitations in this catalytic systems. 2.5. In Situ Experiments. In situ designed temperatureprogrammed (TP) experiments were conducted in a tubular microreactor connected with a computer-interfaced quadruple mass spectrometer (MS) detector. The complete framework of in situ designed-TP experiments is illustrated in Scheme S1 in the Supporting Information. Here, the sample was pretreated in situ under a helium flow at 150 °C for 1 h to remove the surface weakly adsorbed H2O and CO2 and then purged to room temperature. For the He-TPD experiment, the spinel CoMn2O4 was programmed to 900 °C at a rate of 5 °C/min under helium atmosphere (Step 1), and the oxygen (m/z = 32) was detected. The adsorption of toluene was conducted at room temperature. After saturation, the system was purged with helium for 1 h to remove the physically adsorbed toluene on the surface, and then the experiment was started. Toluene desorption and CO2 formation were tested by raising the temperature from 25 to 500 °C at a rate of 5 °C/min under helium atmosphere (t-TPD) (Step 2). After the t-TPD experiment, the used catalysts were purged with He to room temperature and then heated in helium flow (t-TPD → HeTPD) (Step 2-1) to detect the residual oxygen species and CO2 in catalyst. Then an additional oxygen compensation experiment after the t-TPD was performed on spinel CoMn2O4 with the activation temperature varying at 25, 200, and 250 °C (Step 2-

3. RESULTS AND DISCUSSION 3.1. Structure and Texture of Catalysts. The crystallographic structure of the samples is measured by XRD, as shown in Figure 1a. The pattern of CoMn2O4 shows well-resolved characteristic diffraction peaks, which correspond to the CoMn2O4 spinel-type structure (JCPDS card no. 77-0471). Nevertheless, the Co3O4/MnOx sample prepared by the impregnation method mainly exists in the form of Mn5O8, Mn2O3, and Co3O4 phases, which are identical to pure MnOx and Co3O4. The morphology information on the as-prepared samples is obtained by SEM (Figure 1b). The nanoflower morphology of the spinel CoMn2O4 is basically observed. In comparison, the cotton-like and particle-like morphology of composite metal oxide Co3O4/MnOx catalyst are all observed to be identical to pure MnOx and Co3O4. Spinel CoMn2O4 shows an IV isotherm with a H3 hysteresis loop lying in the range of 0.4−0.99 (Figure S1a), indicating the formation of the uniform mesoporous structure,29 and the pore diameter is in the range of 3−5 nm (Figure S1b). The spinel CoMn2O4 sample prepared by oxalic acid sol−gel method shows the highest pore volume (0.24 m3 g−1), the largest specific surface area (124.4 m2 g−1) and the smallest mean pore diameter (3.7 nm) compared with those of single- or mixed-metal oxides (Table S1). In our work, the decomposition of oxalic acid precipitant at high temperature during the synthesis process for spinel CoMn2O4 sample can release a large amount of CO and 6700

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CO2, which is beneficial to the formation of mesoporous structure and the increase of specific surface area. That is why that the specific surface area of spinel CoMn2O4 is higher than that of samples with similar spinel-like structure in the literature (45−49 m2 g−1).30 Large surface area of spinel can generate more oxygen vacancy and surface defects, which will introduce the formation of abundant surface active sites31 and thus the enhancement of the toluene adsorption and activation ability.32 The surface cobalt and manganese oxidation state are probed using the XPS technique. The XPS peaks in the Mn 2p region of all samples (Figure 2a) possess the same binding energy, indicating the identical chemical manganese composition on the surface. Furthermore, the binding energy (BE) at 641.1−641.4 eV and 643.0−643.7 eV corresponds to Mn3+ and Mn4+,33−36 respectively. Note that the relative intensity of Mn4+ in comparison with Mn3+ in the Mn 2p region of spinel CoMn2O4 is 0.64 (Table S1), which should be due to the part conversion of surface Mn3+ to Mn4+. In the Co 2p spectra (Figure 2b), a satellite peak at 785.8 eV for CoMn2O4 indicates that the surface Co exists in the form of Co2+ species.37,38 Furthermore, the ΔECo 2p (BE gap of Co 2p between the main peak and satellite peak) value of 15.46 eV also confirms that only the Co2+ exists in CoMn2O4.37 For Co3O4, the peak at 781.5 eV with a low intensity shakeup satellite peak at 791.1 eV and the ΔECo 2p value of 15.05 eV indicates the coexistence of the surface Co2+ and Co3+ species, which is consistent with the results given in previous research.38 For the mixed-metal oxide Co3O4/MnOx, the ΔECo 2p value of 15.03 eV is very similar to that of the single-metal oxides Co3O4, indicating that the element Co in the mixed-metal oxides is mainly in the form of Co3O4. The higher average cation valence of 2.93 (>8/3 ≈ 2.67) for CoMn2O4 is considered to be the existence of cations with an oxidation state larger than the stoichiometric oxides, which means that a cationic vacancy is formed to maintain charge balance.39 Furthermore, because of the Schottky defects in the spinel,24 an oxygen vacancy is formed together with cationic vacancy and can be studied via O 1s, as shown in Figure 2c. The O 1s spectra are divided into three peaks of 529.8, 531.0, and 533.6 eV, which can be attributed to surface

Figure 1. XRD patterns (a) and SEM images (b) of spinel CoMn2O4, Co3O4/MnOx, MnOx, and Co3O4.

Figure 2. Mn 2p (a), Co 2p (b), and O 1s (c) of XPS spectra of spinel CoMn2O4, Co3O4/MnOx, MnOx, and Co3O4. 6701

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than the mixed-metal oxides Co3O4/MnOx (1.91 μmol m−2 h−1). To further better evaluate the catalytic performance, Table S2 shows the catalytic activity of different catalysts for toluene oxidation, including CoMn2O4 in this work, other Mn-based catalysts, perovskite, spinel, and noble metal catalysts reported in the references. Under similar conditions, the catalytic activity (T90 = 210 °C) over spinel CoMn2O4 catalyst is much higher than those over rod-like MnO2 (T90 = 225 °C), perovskite La0.8Ce0.2MnO3/CeO2 (T90 = 240 °C), perovskite LaNiO3 (T90 = 250 °C), MnAl/Na-L(II) (T90 = 246 °C), spinel CoMn2O4 (T80 = 320 °C), noble metal 1.67Mn3O42Au/3DOM LSCO (T90 = 230 °C), and noble metal Pd/ Co3AlO (T90 = 230 °C) but lower than that 1.99AuPd/3DOM Co3O4 catalyst (T90 = 168 °C). The apparent activation energy is evaluated and shown in Figure 4b. Compared with the single-metal oxides Co3O4 (Ea = 63.7 kJ/mol), MnOx (Ea = 51.8 kJ/mol), and mixed-metal oxides Co3O4/MnOx (Ea = 53.1 kJ/mol), the spinel CoMn2O4 shows an apparent decrease of activation energy (Ea = 35.5 kJ/mol). Inspired by the literature,41 the stability of catalysts in a continuous reaction is checked at 200 °C (39% of toluene conversion) and 220 °C (100% of toluene conversion), as shown in Figure 4c. During the stability test in 700 min, the spinel shows a change in toluene conversion from 39% to 26% at 200 °C and can maintain above 98% of toluene conversion at 220 °C in the presence or absence of water vapor (2.0 vol %). When GHSV is further increased from 22 500 to 45 000 mL g−1 h−1, the toluene conversion still reaches 90% at 227 °C on spinel CoMn2O4. 3.3. Exploration of Reaction and Recovery Mechanism of Oxygen Species. From the relationship between structure and activity, it has been found that spinel CoMn2O4 with a high mobility of oxygen species exhibits high activity and excellent stability for toluene oxidation, which indicates that the oxygen species in the catalyst should play an important role in the reaction. Oxygen species in the catalyst involved in the oxidation of VOCs are excited by the Mars−van Krevelen mechanism (MVK), where the reactant is oxidized by lattice oxygen species of the catalyst, and then it is reoxidized by gaseous oxygen.19,42−44 It is necessary to study how oxygen species (including surface lattice oxygen and bulk lattice oxygen) participate in the reaction and the pathway of oxygen species consumed by gaseous oxygen supplementation. 3.3.1. Determination of Lattice Oxygen and Its Reaction with Toluene. The on-stream reaction at 220 °C on spinel CoMn2O4 sample is performed, as shown in Figure 5a. When the gaseous oxygen is turned off, the toluene conversion of spinel CoMn2O4 decreases with time, but still remains above 80% for 14 min, and reaches 16% while continuing the reaction for another 33 min. Differently, for the mixed-metal oxides Co3O4/MnOx sample, the toluene conversion immediately decreases to 16% in 14 min at 240 °C when gaseous oxygen is cut off. After the gaseous oxygen is again introduced into the feed gas, the toluene conversion nearly achieves 100% and recoveries for both samples. Toluene oxidation still occurs gradually with time in the absence of gaseous oxygen for both samples, which means that the oxygen species in the catalyst can react with toluene. However, the decrease in toluene conversion rate for the two samples is dramatically different. Thus, the composition and content of the oxygen species are further studied by temperature-programmed desorption (HeTPD) experiments, as shown in Figure 5b. On the basis of the

lattice oxygen (Olatt), O2− in surface oxygen vacancy and chemisorbed oxygen, respectively. The concentration of oxygen vacancies (OV) on the spinel catalyst can be evaluated by the following equation:40 A OV OV % = × 100% A OL + A OV + A OC where A OV , A OL and A OC represents the peak area of O2− in surface oxygen vacancy, lattice oxygen, and chemisorbed oxygen, respectively. The concentration of oxygen vacancies reaches 51.6% for the spinel CoMn2O4, which can provide the sufficient active centers or an oxygen adsorption sites for the catalysis reaction. The H2-TPR profile of spinel CoMn2O4 sample (Figure 3) shows three reduction peaks. The weak peak viewed at 302 °C

Figure 3. H2-TPR profiles of spinel CoMn2O4, Co3O4/MnOx, MnOx, and Co3O4.

can be attributed to the reduction of Mn4+ to Mn3+. The second peak, at 405 °C, is ascribed to the reduction of Mn3+ to Mn2+. The high-temperature peak at 556 °C corresponds to the reduction of spinel-like environment Co2+ to metallic Co0.15 Compared with other single-metal oxides MnOx or mixed-metal oxides Co3O4/MnOx, the reduction of Mn4+ in spinel CoMn2O4 turns significantly to the lower temperature. It has been known that the metal−oxygen bond in the Schottky defect will be shortened,39 which results in the asymmetric metal−oxygen distortion or lengthening in the neighborhood of periodic crystal framework, and thus, the high mobility of oxygen species in spinel catalyst. 3.2. Catalytic Performance and Kinetic Studies of Catalysts. The toluene oxidation measurement was conducted to assess the catalytic performance of the samples. Figure 4a exhibits the temperature dependence of the toluene conversion of the samples, and the toluene conversion rises with an increase in temperature. The spinel CoMn2O4 sample, which reaches 50% conversion at 202 °C and 90% conversion at 210 °C, shows higher catalytic activity than those of single-metal oxides (Co3O4 and MnOx) and mixed-metal oxides (Co3O4/ MnOx). Furthermore, no other gas-phase byproducts were detected, and only CO2 and H2O were observed by gas chromatography and mass spectrometry (Figure S2), which demonstrates that toluene can be completely converted to the final products CO2 and H2O. The reaction rate per unit weight and unit surface area is measured at 210 °C, and the spinel reaction rate is 3.3 μmol m−2 h−1, which is 1.73 times higher 6702

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Figure 4. (a) Toluene conversion of spinel CoMn2O4, Co3O4/MnOx, MnOx, and Co3O4 catalysts. (b) Arrhenius plots of toluene reaction rates of spinel CoMn2O4, Co3O4/MnOx, MnOx, and Co3O4 catalysts. (c) On-stream toluene oxidation (200 and 220 °C) and the effect of water vapor on catalytic oxidation at 220 °C over spinel CoMn2O4 at GHSV = 22 500 mL g−1 h−1.

literature,45−47 in general, the low-temperature peak (300−500 °C) represents the surface lattice oxygen species, and the hightemperature peak (T > 500 °C) is attributed to the releasing of the bulk lattice oxygen species of crystal structure.47,48 For spinel CoMn2O4 sample, the lower desorption temperature of the surface lattice oxygen species means the higher activity of oxygen species.48 A toluene-adsorbed spinel CoMn2O4 is measured for temperature-programmed desorption (t-TPD) in helium gas flow (Figure 5c). The amount of toluene desorbed from spinel CoMn2O4 is obviously higher than that of mixed-metal oxides Co3O4/MnOx, and the adsorption capacity of spinel CoMn2O4 is higher. In addition to the observation of toluene desorption from spinel CoMn2O4, the oxidation of adsorbed toluene by the active oxygen species in the catalyst is also observed during the t-TPD experiment, and CO2 product (m/z = 44) is generated (Figure 5d). Compared with the Co3O4/MnOx catalyst, a larger amount of CO2 formation suggests that more toluene can be activated and more reactive oxygen species in CoMn2O4 can participate in the toluene oxidation. The broad CO2 generation peak in the range of 46−460 °C for the CoMn2O4 spinel catalyst also indicates that the active oxygen species gradually participate in the oxidation reaction and the oxygen species are diverse and own the good activity at low temperatures. At the same time, it is found that the reactive oxygen species in the spinel CoMn2O4 can fully participate in the t-TPD reaction (Figure S3) and that no

desorption of reactive oxygen species is observed during our experiment range after the spinel CoMn2O4 catalyst undergoes t-TPD. Importantly, the spinel CoMn2O4 still keeps the good spinel structure after the t-TPD experiment (Figure S4). Differently, for mixed-metal oxides Co3O4/MnOx, an oxygen desorption peak at 638 °C is still retained after the t-TPD experiment (Figure S3), indicating that the oxygen species in the catalyst cannot all participate in the reaction. 3.3.2. Determination of Oxygen Replenishment Mechanism by In Situ Designed-TP Techniques. It has been known that most of the reactive oxygen species in the CoMn2O4 catalyst can be consumed during the toluene temperatureprogrammed reaction (t-TPD → He-TPD, Figure S3). Then it is essential to well understand whether the gaseous oxygen can be directly dissociated and activated on the surface or move into the lattice of the catalyst to form the oxygen cycle during the real reaction process. The oxygen replenishment experiments for spinel CoMn2O4 sample at different temperatures (T = 25, 200, and 250 °C) are executed, as shown in Figure 6a. When the gaseous oxygen is added into the used catalyst (after t-TPD) at 25 °C, and then the temperature-programmed desorption experiment (t-TPD → O2 (25°C) → He-TPD) is performed, a small amount of bulk lattice oxygen species is desorbed at high temperature (T = 664 °C). Interestingly, when the temperature for supplying gaseous oxygen increases to 200 °C, the intensity of oxygen desorption peaks at high temperature (T = 664 °C) is significantly enhanced, and more 6703

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ACS Catalysis

Figure 5. (a) Toluene conversion in O2-free reaction as a function of time of spinel CoMn2O4 and Co3O4/MnOx. (b) He-TPD profiles of spinel CoMn2O4 and Co3O4/MnOx. (c) Toluene desorption and (d) CO2 production profiles in t-TPD of spinel CoMn2O4 and Co3O4/MnOx.

TPD) was also performed (Figure 6b). The CO2 signal is detected to demonstrate the reaction between the oxygen species and toluene. To clearly compare the toluene reaction performance of the reoxidation catalyst, the results of the toluene reaction with oxygen species in fresh catalyst are also given. When the used catalyst (after t-TPD) was supplemented with gaseous oxygen at 25 °C, two CO2 peaks are observed at 301 and 388 °C (t-TPD →O2 (25°C) → t-TPD), respectively. This further suggests that the oxygen species in the catalyst can be replenished by the gaseous oxygen at 25 °C and is also an active oxygen species for toluene oxidation. As the temperature for oxygen supplemental rises to 200 °C, the amount of CO2 production is increased, particularly at low temperature, and the CO2 peaks move significantly toward low temperature (279 and 361 °C). More importantly, it is found that when the temperature is increased to 250 °C, three CO2 peaks can be obtained, which is similar to the profile of CO2 production during the t-TPD. Therefore, the oxygen species consumed in spinel catalyst are fully supplemented by gaseous oxygen during the reaction. At relatively higher temperature (200−250 °C), gaseous oxygen is more easily replenished into the catalyst, while gaseous oxygen accelerates the rate of oxygen migration from bulk to surface, thereby improving the surface oxidation reaction with toluene. Therefore, surface lattice oxygen can react with the toluene at low temperature (