LaMnO3 Catalyst

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Article

A Facile Method for in-situ Preparation of the MnO/LaMnO Catalyst for the Removal of Toluene 2

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Wenzhe Si, Yu Wang, Shen Zhao, Fang-yun Hu, and Junhua Li Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b06255 • Publication Date (Web): 17 Feb 2016 Downloaded from http://pubs.acs.org on February 29, 2016

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Environmental Science & Technology

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A Facile Method for in-situ Preparation of the

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MnO2/LaMnO3 Catalyst for the Removal of Toluene

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Wenzhe Si, a Yu Wang,a Shen Zhao, a Fangyun Hu, a and Junhua Li a,*

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a

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Environment, Tsinghua University, Beijing, 100084, P. R. China

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ABSTRACT

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MOx/ABO3 is a promising catalyst for the high-efficiency removal of volatile organic

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compounds. However, this catalyst is limited on practical applications due to its complex

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synthesis procedure and high cost. In this work, the MnO2/LaMnO3 catalyst was prepared in-situ

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using a facile one-step method for the first time, in which partial La cations were selectively

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removed from three dimensionally chain-like ordered macroporous (3DOM) LaMnO3 material.

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After selective removal, the obtained MnO2/LaMnO3 sample expressed an excellent catalytic

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performance on toluene oxidation. Toluene could be completely oxidized into CO2 and H2O at

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290 °C over the MnO2/LaMnO3 catalyst with a toluene/oxygen molar ratio of 1/100 and a space

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velocity of 120,000 mL/(g h). In addition, the apparent activation energy value of

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MnO2/LaMnO3 was 57 kJ/mol, which was lower than those of other metal oxides catalysts.

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According to O2-TPD and XPS results, it is concluded that the high catalytic performance of

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MnO2/LaMnO3 was mainly associated with the large amount of oxygen species and the excellent

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of

ACS Paragon Plus Environment

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lattice oxygen mobility. MnO2/LaMnO3 is a promising catalyst for the practical removal of

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volatile organic compounds due to its high efficiency, good stability, low cost, and convenient

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preparation.

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1. INTRODUCTION

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The emissions of volatile organic compounds (VOCs) have become a notable problem during

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the high development of industry and transportation because they cause harm to both the

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environment and human health.1-3 Among the present technologies for the elimination of VOCs,

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catalytic oxidation is an effective option due to its high efficiency and low operating

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temperature.4-7 The research on the development of high-performance catalysts is the key point

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for catalytic oxidation. The noble-metal catalysts have already exhibited high catalytic activity in

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the previous studies.8,

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sintering and a poisoning tendency, prevent the noble metals from wide industrial applications.5,

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6, 10

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thermal stability and low cost in some practical applications.11, 12 Therefore, it is necessary and

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important to elucidate the properties of transition metals oxides and improve their catalytic

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activity.

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However, many problems, such as high cost, low thermal stability,

In contrast, transition metal oxides are considered as a promising candidate due to their high

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ABO3 perovskites, as one of the common transition metals oxides, have been extensively

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applied as catalysts for selective catalytic reduction of NOx from automotive exhausts, CO2

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reforming of CH4 and catalytic oxidation of VOCs.13-15 Nevertheless, the VOCs oxidation

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activity of perovskites is not satisfactory because it is limited by their particularly low surface

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area (< 10 m2/g).15, 16 One of the solutions is to disperse ABO3 onto a metal oxides support

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(MOx) with a large surface area and thermal stability.15, 17, 18 Such ABO3/MOx materials inherit

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both superior catalytic activity from ABO3 and high stability from MOx. Kustov et al. capsulated


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LaCoO3 (LCO) catalysts into the mesoporous ZrO2, and the LCO/ZrO2 catalyst showed better

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performance than the LCO without ZrO2 support.17 Parvulescu et al. prepared a LCO perovskite

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catalyst with Ce1-xZrxO2 supports to study its performances on benzene and toluene oxidation.15

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The LCO/Ce1-xZrxO2 showed better catalytic activity than the bulk LCO.

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Another solution is to load MOx (e.g., MgO, MnO2, and Co3O4) on ABO3.16,

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In the

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MOx/ABO3 structure, the ABO3 perovskites play roles not only as supports but also as an active

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compose. Weng et al. explored a route for the syntheses of MgO/LCO catalyst.16 MgO was

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highly dispersed and the catalyst exhibited good performance on toluene and methane

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oxidations. Dai and coworkers prepared three dimensionally structured LaMnO3 (LMO) and its

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supported MnOx catalyst, which showed better performance than bulk LMO on toluene and

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methanol removal.19 Above all, preparation of MOx/ABO3 (or ABO3/MOx) is obviously

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beneficial for the increase of catalytic activity regarding the removal of VOCs. However, the

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synthesis of ABO3 and MOx must be performed independently, which is visibly complicated and

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of high-cost. In addition, accurate regulation and control of the synthesis conditions are required

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to ensure the high dispersion of active species (ABO3 or MOx) on the supports (MOx or ABO3),

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increasing the difficulty of the synthesis technique and decreasing the qualified rate of products.

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Both of these issues restrict the practical applications for MOx/ABO3 (or ABO3/MOx) regarding

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the elimination VOCs. It is necessary to design a facile and low-cost method to prepare such

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high-performance MOx/ABO3 (or ABO3/MOx) catalysts.

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To tackle the aforementioned challenges, we illustrate a one-step approach to produce in-situ a

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MnO2/LMO catalyst by selectively removing partial La element from LMO using acid treatment

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(Scheme 1). During this process, MnO2 can be directly formed on the surface of the LMO,

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requiring no extra procedure. To increase the contacting surface areas of LMO with acid, a three


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dimensionally chain-like ordered macroporous (3DOM) LMO material was first prepared. After

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selective removal of partial La cations, the MnO2/LMO catalyst was obtained with a large

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surface area and good dispersion. Furthermore, the amount and mobility of surface oxygen

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species were significantly improved after the acid treatment, resulting in the production of the

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MnO2/LMO catalyst with excellent catalytic activity on toluene oxidation.

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2. EXPERIMENTAL SECTION

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2.1 Chemicals and Materials

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The chemicals used for the preparation were all of A.R. grade. Potassium peroxydisulfate

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(K2S2O8) and methyl methacrylate were purchased from Aladdin (China, Shanghai).

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Lanthanum(Ⅲ) nitrate hydrate (La(NO3)3·6H2O), manganese( ) nitrate (Mn(NO3)2, 50 wt%

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aqueous solution) and nitric acid (HNO3) were obtained from Sinopharm Chemical Reagent Co.,

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Ltd. (China, Shanghai). All chemicals were used without further purification.

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2.2 Synthesis of monodisperse PMMA microspheres

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The hard templates (monodisperse PMMA microspheres) were first synthesized. 3.00 mmol

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K2S2O8 was mixed with 1500 mL deionized water in a three-necked flask, stirred at 400 rpm and

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heated at 70 ºC to obtain a transparent solution. Next, the resulting solution was degassed with

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continuous flowing N2. 115 mL of methyl methacrylate was added into the above-described

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solution when the temperature maintained at 70 ºC. The mixture was then stirred at 70 ºC for 1 h

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until a white suspension was obtained. After cooling down, the suspension was centrifuged for

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75 min. The resulting precipitate was washed with deionized water and then dried in a water bath

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at 80 ºC. The wet solid was continued to dry at room temperature (RT) for 48 h and then ground

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well.

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2.3 Preparation of bulk LMO, 3DOM-LMO and MnO2/LMO samples


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13 g of La(NO3)3·6H2O, 6.97 mL Mn(NO3)2 and 14 mL deionized water were mixed together

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by stirring for 1 h. Next, approximately 2.00 g of PMMA template was impregnated in the mixed

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solution. After the PMMA microspheres were thoroughly wetted, the excessive liquid was

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filtered, and then the solid was dried at RT for 24 h. Next, the resulting solid was calcined as

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follows: the first calcination step was performed under N2 flow. The precursor was heated at a

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rate of 1 °C/min up to 300 ºC and then kept at this temperature for 3 h. After cooling down to RT,

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the second calcination step was performed under air flow. The solid was heated up to 300 ºC and

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then held for 2 h; subsequently, the solid was heated up to 750 ºC and then held for 4 h. During

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the whole process, the heating rate was 1 ºC/min. The obtained material was denoted as 3DOM-

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LMO. Bulk LMO sample was prepared using a traditional sol-gel method.

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The MnO2/LMO sample was prepared by a selective removal method in which the 3DOM-

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LMO powder was immersed in 3M diluted HNO3 solution. After approximately 10 minutes, the

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solid was filtered and washed for 3 times, and then dried at 60 ºC overnight. The final product

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was denoted as MnO2/LMO. The detailed synthesis procedures are described in the Supporting

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Information.

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2.4 Characterization of the LMO, 3DOM-LMO and MnO2/LMO samples

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All of the samples were characterized using multiple techniques, which included: X-ray

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diffraction (XRD), scanning electron microscopy (SEM), elemental mapping, X-ray

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photoelectron spectroscopy (XPS), hydrogen temperature-programmed reduction (H2-TPR) and

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oxygen temperature-programmed desorption (O2-TPD). The detailed characterization procedures

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are described in the Supporting Information.

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2.5 Catalytic evaluation


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The catalytic activities for the toluene oxidation were measured by a continuous flow fixed-

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bed quartz microreactor (i.d. = 6.0 mm) using 0.05 g sample (40–60 mesh). The gaseous toluene

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was produced by a nitrogen-blowing method, passing N2 flow through a bottle containing pure

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toluene chilled in an ice-water isothermal bath. The gas mixture contained 2000 ppm toluene,

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20% O2 and N2 (balance gas). The total flow rate was 100 mL/min, giving a GHSV of 120,000

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mL/(g h). The concentrations of toluene were monitored on line by a gas chromatograph (Agilent

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7890A) equipped with a flame ionization detector (FID, using the column of Porapack-

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Q/molecular sieve 5A, 2 m in length) and a thermal conductivity detector (TCD, using the

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column of RT-QPlot divinylbenzene PLOT, 30 m in length). The details are described in the

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Supporting Information.

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3. RESULTS AND DISCUSSION

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3.1 Crystal structure

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Figure 1 shows the XRD results of the three samples. It is found that the XRD peaks of the

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bulk LMO and 3DOM-LMO samples in the 2θ range of 20–80o could be well indexed as the

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XRD pattern of the standard LMO sample (JCPDS No. 50-0299).20, 21Both of the LMO samples

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were single-phase, and the crystal structures showed a rhombohedral phase. Generally, the

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crystal phase (orthorhombic, rhombohedral, or monoclinic) of LMO is strongly dependent on the

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synthesis method. By adopting various preparation approaches with or without a template,

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rhombohedral and hexagonal LMO could be generated. After the acid treatment, the crystallinity

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of the diffraction peaks was weakened, and a new peak (located at 36.7o) assigned to MnO2

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appeared. This suggests that the dilute HNO3 can selectively dissolute partial La cations in LMO

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perovskites, and the remaining part tended to form MnO2 crystals. According to the position of

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the new peak (36.7o) and the previous research,19 it can be assumed that the crystal phase of the


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newly created MnO2 is gamma phase (JCPDS No. 14-0644). The peak at 36.7o was the main

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peak of gamma-MnO2.22, 23 SEM mapping revealed that the La elements were gradually removed

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from surface to bulk (Supporting Information, Figure S4). This suggests that the acid attack

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mainly occurred on the surface of 3DOM-LMO. Although the Mn elements were slightly lost,

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most of the Mn elements remained after the acid attack. The reaction of acid attack occurred

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selectively, and the La elements were easier to remove. As the acid treatment is uniform, the

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newly created MnO2 was highly dispersed on the LMO surface. We could not find any

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agglomeration of MnO2 from the SEM mapping results.

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3.2 Morphology, surface area and pore structure

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Figure 2 illustrates the morphological changes of the samples. It can be observed that the LMO

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sample exhibited an irregular compact surface, leading to low surface area and small amount of

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active sites. The SEM images of 3DOM-LMO catalyst were given in Figures 2(b) and 2(c),

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which showed a high-quality chain-like ordered macroporous morphology. The average pore size

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was approximately 47 nm, and the average wall thickness was approximately 22 nm. Figure 2(d)

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revealed the destruction of the 3DOM structure after the acid treatment. The original smooth

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surface of the macropores became rumpled and the well-ordered structure became disordered,

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and many nanopores appeared, which resulted from the increase of the surface area of

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MnO2/LMO. The surface area and pore structure results (Table 1) from the N2 adsorption–

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desorption isotherms and pore-size distributions of these samples (Supporting Information,

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Figure S1) were consistent with the information from the SEM images. The surface area was

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increased more than 6 times after the acid treatment, from 22.3 m2/g (3DOM-LMO) to 144.7

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m2/g (MnO2/LMO). It is suggested that the shrinkage in the volume of the treated material could

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be responsible for the morphological and surface area changes after the acid treatment. After


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calculation, there is theoretically 23.9 % shrinkage in volume from LMO to γ-MnO2.22 Because a

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number of γ-MnO2 was formed on the surface of the 3DOM-LMO after the acid treatment,

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several gaps emerged with it as a result of the shrinkage occurring in the MnO2/LMO sample,

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thereby significantly increasing the surface area. Normally, a porous structure and high surface

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area are beneficial for the improvement on the catalytic performance by facilitating the

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accessibility of the reactant molecules to the catalysts.23

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3.3 Reducibility

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The H2-TPR experiment was performed to determine the reducibility of the catalysts. The H2-

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TPR profiles of the three samples are exhibited in Figure 3, and their quantitative analysis results

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are listed in Table 1.The reduction reaction occurred over the chain-like LMO sample with the

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following characteristics: the reduction peak at 399 ºC is attributed to the reduction of Mn4+ to

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Mn3+, as well as the removal of over-stoichiometric oxygen, the peak at 485 ºC is assigned to the

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single-electron reduction of Mn3+ located in a coordination-unsaturated microenvironment, and

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the reduction peak above 833 ºC was due to the reduction of the left Mn3+ to Mn2+.20, 21 The

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temperatures of the two reduction peaks (407 and 852 ºC) of the bulk LMO sample were much

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higher than those (399 and 833 ºC) of the porous counterpart, indicating that the formation of the

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chain-like 3DOM structure could facilitate the reduction of LMO. After the acid treatment, all of

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the reduction peaks shifted to lower temperatures. The sharp reduction peaks at 292 and 388 ºC

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of MnO2/LMO sample were ascribed to the reduction of MnO2 for Mn4+ to Mn3+ and Mn3+ to

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Mn2+, respectively.19 The reduction peak at 773 ºC is assigned to the reduction of LMO, which

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appeared much earlier than those of bulk LMO and 3DOM-LMO sample. It is demonstrated

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from the change of the reduction peaks that the MnO2 species did exist and the MnO2/LMO

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structure was formed after the acid treatment, which was in accord with the XRD results.


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Moreover, the positions of the reduction peaks for MnO2/LMO sample (292 and 773ºC) were

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lower than those for the other two LMO catalysts, proving that the MnO2/LMO sample

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possessed the best low-temperature reducibility among the three samples. To further compare the

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low-temperature reducibility, we calculated the total H2 consumptions of the three samples. For

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LMO perovskites, the theoretical H2 consumption for the reduction of Mn3+ to Mn2+ is 2.06

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mmol/g if all the Mn species in LMO perovskites are Mn3+. For the 3DOM-LMO sample, the

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total H2 consumption of the three main peaks at 399, 485 and 833ºC was approximately 2.04

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mmol/g, which was larger than that of the bulk LMO sample (1.92 mmol/g). By means of the

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acid treatment, the total H2 consumption of three main peaks at 292, 388 and 773ºC for

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MnO2/LMO sample was 8.79 mmol/g. This value is observably larger than those for the other

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two catalysts, which indicated that parts of La elements were selectively removed after the acid

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treatment. The MnO2/LMO sample possessed the highest amount of total H2 consumption among

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the three catalysts. The excellent low-temperature reducibility of the MnO2/LMO sample was

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confirmed by the early reduction peak position, and the large amount of the total H2 consumption

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may result in the improvement on the catalytic performance over the toluene oxidation.

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3.4 Surface composition, metal oxidation state and oxygen species

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The information related to surface element compositions, metal oxidation states, and adsorbed

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species of a solid material can be detected accurately by XPS tests. Figure 4 shows the Mn 2p3/2

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and O 1s XPS spectra of the three samples. It can be found that there was the co-presence of

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surface Mn4+ and Mn3+ species, as well as surface lattice oxygen (Olatt), adsorbed oxygen (Oads,

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e.g., O2−, O22−, or O−), and surface carbonates (CO32−) on the surfaces of these three samples.24, 25

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The surface Mn4+/Mn3+ and Olatt/Oads atomic ratios can markedly influence the VOCs catalytic

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oxidation performance of manganite. According to the quantitative analyses on the XPS spectra,


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the surface element compositions and surface Mn4+/Mn3+ and Olatt/Oads atomic ratios could be

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estimated over these manganite samples, as summarized in Table 1. It could be found that the

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surface La/Mn atomic ratios for bulk LMO (1.38) and 3DOM-LMO (1.21) were obviously

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higher than the theoretical value (1.00). This result suggested that the native surface of LMO

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perovskites was preferentially occupied by La cations, which was in accord with our previous

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research.22 After selective removal of La cations, the surface La/Mn atomic ratio for MnO2/LMO

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sample decreased to 0.21, proving that the La element was indeed partly removed. In addition,

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the La/Mn atomic ratio in the whole MnO2/LMO was 0.44 (Supporting Information, Table S1),

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higher than the surface value, thus proving that the removal of La elements with the acid attack

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mainly occurred on the surface of the sample. The differences in the surface La/Mn atomic ratios

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may result indifferent performances between LMO perovskites and the MnO2/LMO sample on

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catalytic activity because the toluene catalytic reaction at low temperatures usually occurred on

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the surface transition metal ion-sites (such as Mn cations).27 From Figure 4(a), it can be observed

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that the asymmetrical Mn 2p3/2 XPS spectrum of each sample could be decomposed to three

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components at BE = 641.3, 642.8, and 644.7 eV, assignable to the surface Mn3+ and Mn4+

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species and the satellite of the Mn3+ species, respectively.20-23 The surface Mn4+/Mn3+ molar ratio

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increased from 0.39 (bulk LMO) to 1.35 (MnO2/LMO). This result may be caused by the

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appearance of MnO2 species. For LMO perovskites, the main valence state of Mn cations is +3.

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After the acid treatment, some MnO2 species appeared in the LMO perovskite, especially on the

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surface of the sample. The main valence state of Mn cations in MnO2 is +4, which resulted in the

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increase of the surface Mn4+/Mn3+ molar ratio in MnO2/LMO. This may be beneficial for the

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enhancement on toluene oxidation. For each sample, the O 1s spectrum (Figure 4(b)) could be

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decomposed into three components at BE = 529.1, 530.6, and 531.8 eV, attributed to the Olatt,


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Oads and carbonate species, respectively.22,

23

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smaller than that of the bulk LMO sample (2.50). After the acid treatment, the surface Olatt/Oads

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ratio of MnO2/LMO sample increased to 2.78, which was larger than the other two samples. The

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rise in surface active oxygen species concentration, especially in Olatt concentration, gives rise to

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enhanced catalytic performance of MnO2/LMO sample for the total oxidation of toluene.

The Olatt/Oads ratio of 3DOM-LMO (1.41) was

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The O2-TPD tests were performed to further investigate the differences of oxygen species for

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the three samples (Figure 5). Generally, the oxygen desorption peaks that appeared at low

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temperature (400 °C) are assigned to the release of Oads and

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Olatt, respectively.19 For bulk LMO and 3DOM-LMO samples, the peaks at low temperature were

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very weak, illustrating that the LMO perovskites possessed a small amount of Oads. The intensity

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of Olatt peaks for 3DOM-LMO was stronger than that for bulk LMO. It is indicated that the

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3DOM-LMO could release more Olatt than the bulk LMO during the toluene oxidation process.

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Among the three samples, the O2 desorption for MnO2/LMO catalyst started at the lowest

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temperature (372 °C), suggesting that the active oxygen species in MnO2/LMO could be released

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more readily.23 It was also found that the areas of the Olatt desorption peaks for the other two

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samples were obviously smaller than that for the MnO2/LMO sample, which suggested that the

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MnO2/LMO owned the largest amount of active oxygen species. It is well known that the

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oxidation of VOCs over transition metal oxide catalysts occur via the Mars–van Krevelen type

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redox cycle, and the nucleophilic attack of the Olatt lead to the occurrence of this reaction.26 Thus,

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the Olatt species play a vital role in the VOCs catalytic oxidation. The large amount of Olatt and

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the low Olatt desorption temperature may improve the toluene catalytic activity for MnO2/LMO

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sample.

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3.5 Catalytic performance


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When no catalysts were added into the toluene catalytic reaction, no significant conversion of

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toluene could be observed below 400 °C, indicating that no homogeneous reactions occurred

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under the adopted reaction conditions. The evaluation of the catalytic activities on the oxidation

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of toluene is shown in Figure 6(a). The toluene oxidation activity increased with an increase in

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reaction temperature, and the porous 3DOM-LMO catalyst performed better than the bulk

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counterpart. After the acid treatment, the MnO2/LMO showed the best performance among the

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three samples. Notably, toluene was completely oxidized to CO2 and H2O over the three

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manganese oxides catalysts, and no partial oxidation products were detected, as substantiated by

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the good carbon balance (ca. 99.5%) in each run. T50 and T90 (the reaction temperature

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corresponding to the toluene conversion of 50% and 90%, respectively) were used to compare

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the catalytic activities of the samples. For the bulk LMO, 3DOM-LMO and MnO2/LMO

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samples, the values of T50 are 336, 323, and 263 °C, respectively, and T90 are 381, 357 and 279

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°C, respectively (Table 1). Therefore, it can be deduced that the activities of the three samples

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enhanced in the order of bulk LMO < 3DOM-LMO < MnO2/LMO. The oxidation of toluene at

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low temperatures usually occurs on the surface transition metal ion-sites (such as Mn

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cations).27The native surface of LMO perovskites was preferentially occupied by La cations, as

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proved by the XPS results. After the acid treatment, the MnO2/LMO structure was formed, and

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the surface of the sample was occupied by Mn cations, which resulted in the better reactive

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performances on the MnO2/LMO sample than those on the other two LMO samples. As is known,

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the surface oxygen species, low-temperature reducibility and surface area are the mainly factors

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influencing the catalytic performance of the manganite materials.19,

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catalytic activity and characterization results presented in this work, it can be found that there

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was an obvious correlation of surface area, surface oxygen species, and low-temperature

26, 28

By comparing the


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reducibility with the toluene oxidation activity. Therefore, the good catalytic performance of

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MnO2/LMO for toluene oxidation could be resulted from its higher surface area, higher surface

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oxygen species concentration, and better low-temperature reducibility, as well as the special

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MnO2/LMO structure. The MnO2/LMO with large surface area and many nanopores could not

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only facilitate the toluene access into the inside of catalyst but also increase the contact area of

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the sample with toluene, thereby accelerating the reaction of toluene oxidation.

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It is not sufficient to evaluate the catalytic activity of the samples just by the comparison of the

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T50 and T90. It is well accepted that the surface areas can significantly influence the catalytic

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activities. As the surface areas of the three samples varied substantially, the apparent activation

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energy (Ea) was introduced to exclude the effect of the surface area. The catalyst with lower Ea

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value can cause the toluene to be oxidized more readily.23 The Ea values were calculated using

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the Arrhenius plots in Figure 6(b), and the Ea value of MnO2/LMO was found to be the lowest

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among the three samples. The differences in Ea are most likely generated by the surface oxygen

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species. The MnO2/LMO sample possessed the largest amount of Olatt and made the Olatt release

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more easily, thereby enhancing the Olatt mobility on the catalyst, bringing an improvement on the

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toluene oxidation. As a result, the Ea value of MnO2/LMO was the lowest among these samples.

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Note that, under similar reaction conditions, the Ea value (57 kJ/mol) over our MnO2/LMO

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catalyst was much lower than those (Ea value = 126 kJ/mol) over 30 wt% MnOx/Al2O3,29 (Ea

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value = 124 kJ/mol) over Ni0.5Zn0.5Fe2O4,30 (Ea value = 94 kJ/mol) over 15 wt% CuO/Al2O3,29

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and (Ea value = 58 kJ/mol) over LaMnO3-PL-1,21 but higher than those (Ea value = 56 kJ/mol)

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over 20 wt% LaCoO3/CeO2 and (Ea value = 48 kJ/mol) over 4.9 Au/LaMnO3.31, 32

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Furthermore, the influence of different GHSV values on the toluene conversion were also

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investigated (Figure 6(c)). In the case of MnO2/LMO sample, the T50 and T90 values were 263


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294

and 279 °C at GHSV =120,000 mL/(g h), respectively. These T50 and T90 values were 13 and 10

295

°C lower, respectively, than those achieved at GHSV= 240,000 mL/(g h). With the GHSV value

296

decreased from 120,000 to 60,000 mL/(g h), the activity increased. That is, the catalytic

297

performance on toluene oxidation is highly influenced by the GHSV values. The toluene

298

oxidation was improved with the decrease of the GHSV values. Next, we calculated the Ea

299

values over the MnO2/LMO sample on different GHSV values, as shown in Figure S5. It could

300

be found that the Ea values on different GHSV were similar. This result means that the SV did

301

not influence the Ea values. Above all, the catalytic activity on toluene oxidation for manganese

302

oxides catalysts is primarily decided by surface oxygen species, low-temperature reducibility,

303

surface area and SV values. However, for the Ea values, the surface area and GHSV values do

304

not play a decisive role. As the oxidation of toluene over catalysts manganese oxides catalysts

305

occur on the basis of the Mars–van Krevelen type redox cycle, the Olatt play the key role on the

306

Ea value.

307

To test the catalytic stability of the MnO2/LMO sample, an uninterrupted reaction experiment

308

was performed (Figure 6(d)). Two runs of catalytic tests (heating up and cooling down) were

309

first performed, and then 12 h of on-stream reaction at the reaction temperature of 275 °C and

310

GHSV of 120,000 mL/(g h); finally, two runs of catalytic tests (heating up and cooling down)

311

were performed. The running time of the whole progress was 36 h, and the result of this

312

continuous test illustrated that the activity of MnO2/LMO depressed little within 36 h of reaction.

313

The good stability provides the possibility for the practical elimination of toluene.

314 315

ASSOCIATED CONTENT


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316

Supporting Information. Preparation and characterization method and additional results are shown.

317

“This material is available free of charge via the Internet at http://pubs.acs.org.”

318

AUTHOR INFORMATION

319

Corresponding Author

320

*Tel.: +86 10 62771093; Fax.: +86 10 62771093. E-mail: [email protected].

321

Notes

322

The authors declare no competing financial interest.

323

ACKNOWLEDGMENT

324

This work is supported by the National Natural Science Foundation of China (21325731,

325

51478241, and 21221004), the National High-Tech Research and Development (863) Program of

326

China (2013AA065304) and China Postdoctoral Science Foundation (2015M581115).

327

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Table 1 Surface element compositions, BET surface areas, H2 consumption, toluene oxidation

424

activity and apparent activation energies (Ea) of the three samples.

XPS SBET Mn4+/Mn3+

Olatt/Oads

molar ratio

molar ratio

molar ratio

bulk LMO

1.38

0.39

2.50

2.9

3DOMLMO

1.21

0.50

1.41

MnO2/ LMO

0.21

1.35

2.78

La/Mn

H2 consumption

toluene oxidation activity and apparent activation energy T50

T90

Ea

(°C)

(°C)

(kJ/mol)

1.92

336

381

71

22.3

2.04

323

357

66

144.7

8.79

263

279

57

(m2/g)

(mmol/g)

425 426


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427 428

Scheme 1. Synthesis route of the MnO2/LMO sample.

429


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Page 23 of 29


430 431

Figure 1. XRD patterns of the three samples. From bottom to top: bulk LMO (JCPDS No. 50-

432

0299), 3DOM-LMO (JCPDS No. 50-0299), and MnO2/LMO.

433


23


434

Page 24 of 29

.

435

Figure 2. SEM images of the three samples. (a) bulk LMO, (b-c) 3DOM-LMO and (d)

436

MnO2/LMO.

437


24

Page 25 of 29


438 439

Figure 3. H2-TPR profiles of the three samples.

440 441


25


Page 26 of 29

442 443

Figure 4. (a) Mn 2p3/2 and (b) O 1s XPS spectra of the three samples.

444


26

Page 27 of 29


445 446

Figure 5. O2-TPD profiles of the three samples.

447


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Page 28 of 29

448 449

Figure 6. (a) Activity profiles and (b) Arrhenius plots of the three samples, (c) effect of GHSV

450

on the catalytic activity over MnO2/LMO sample, (d) catalytic stability of MnO2/LMO at GHSV

451

= 120,000 ml/(g h).

452


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TOC/Abstract art


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LaMnO3 Catalyst

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