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Ultra Low Loading of Silver Nanoparticles on Mn2O3 Nanowires Derived with Molten Salts: A High-Efficiency Catalyst for the Oxidative Removal of Toluene Jiguang Deng, Shengnan He, Shao Hua Xie, Huanggen Yang, Yuxi Liu, Guangsheng Guo, and Hongxing Dai Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b02350 • Publication Date (Web): 19 Aug 2015 Downloaded from http://pubs.acs.org on August 23, 2015

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Environmental Science & Technology

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Ultra Low Loading of Silver Nanoparticles on Mn2O3 Nanowires

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Derived with Molten Salts: A High-Efficiency Catalyst for the

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Oxidative Removal of Toluene

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Jiguang Deng*, Shengnan He, Shaohua Xie, Huanggen Yang, Yuxi Liu, Guangsheng

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Guo, Hongxing Dai*

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Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of

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Beijing on Regional Air Pollution Control, Key Laboratory of Advanced Functional

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Materials, Education Ministry of China, and Laboratory of Catalysis Chemistry and

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Nanoscience, Department of Chemistry and Chemical Engineering, College of

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Environmental and Energy Engineering, Beijing University of Technology, Beijing

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100124, China

14 15 16 17 18

* To whom correspondence should be addressed:

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Dr. Jiguang Deng and Prof. Hongxing Dai

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Tel. No.: +86-10-6739-6118; Fax: +86-10-6739-1983

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E-mail addresses: [email protected] (J. Deng); [email protected] (H. Dai).

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ACS Paragon Plus Environment


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ABSTRACT

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Using the mixture of NaNO3 and NaF as molten salt, MnSO4 and AgNO3 as metal

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precursor, 0.13 wt% Ag/Mn2O3 nanowires (0.13Ag/Mn2O3-ms) was fabricated after

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calcination at 420 oC for 2 h. Compared to the counterparts derived via the

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impregnation and polyvinyl alcohol-protected reduction routes as well as the bulk

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Mn2O3 supported silver catalyst, 0.13Ag/Mn2O3-ms exhibited a much higher

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catalytic activity for toluene oxidation. At a toluene/oxygen molar ratio of 1/400 and

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a space velocity of 40 000 mL/(g h), toluene could be completely oxidized into CO2

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and H2O at 220 oC over the 0.13Ag/Mn2O3-ms catalyst. Furthermore, the toluene

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consumption rate per gram of noble metal over 0.13Ag/Mn2O3-ms was dozen times

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as high as that over the supported Au or AuPd alloy catalysts reported in our

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previous works. It is concluded that the excellent catalytic activity of

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0.13Ag/Mn2O3-ms was associated with its high dispersion of silver nanoparticles on

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the surface of Mn2O3 nanowires and good low-temperature reducibility. Due to high

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efficiency, good stability, low cost, and convenient preparation, 0.13Ag/Mn2O3-ms is

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a promising catalyst for the practical removal of volatile organic compounds.

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KEYWORDS: molten salt fabrication, manganese oxide nanowire, supported Ag

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catalyst, toluene removal, volatile organic compound

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■ INTRODUCTION

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Volatile organic compounds (VOCs) not only cause a harmful effect on human

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health, but also give rise to the generation of particulate matter (e.g., PM2.5) that can

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result in a heavy foggy and hazy weather. Therefore, it is highly necessary to control

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emissions of VOCs. Due to the serious air pollution, more and more attention has

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been paid to the removal of VOCs. Up to now, many strategies, including physical,

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chemical, and/or biological methods, are used to remove VOCs.1,2 Among them,

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catalytic oxidation is believed to be one of the most efficient pathways, especially

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when the concentration of VOCs is in the range of hundreds to thousands of ppm.

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The key issue of such a catalytic technology is the development of economic

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catalysts with high performance.3,4 Recently, single, mixed or composite metal oxide

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supported noble metal nanocatalysts have been gained much attention. For example,

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HCHO could be completely oxidized into CO2 and H2O at room temperature over

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the Pt/TiO2 catalyst.5−8 The manganese oxide supported atomic silver catalysts

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exhibited excellent performance for the removal of HCHO9,10. Au supported on

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Co3O4 with the major exposed active (110) facets possessed high activity for

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ethylene oxidation11, and HCHO could be completely removed at room temperature

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over Au/Co3O4−CeO2.12

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It is well known that the morphology of support and the particle size of noble

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metal are main factors influencing the activity of a supported noble metal catalyst. In

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order to control the morphology of a metal oxide support and the particle size

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distribution of a noble metal, we can (i) prepare the metal oxide with a well-defined

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morphology (wire, rod, tube, sphere, cube, etc.) via the solution route (e.g., sol-gel,

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hydrothermal, and solvothermal methods), and (ii) load noble metal nanoparticles

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(NPs) on the metal oxide support via the impregnation, precipitation or polyvinyl

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alcohol (PVA)-protected reduction route. These methods are complicated and hard to

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be used for the large-scale preparation. The preparation method has a great impact on

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the physicochemical property and performance of a catalyst.13,14 Therefore, it is

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highly desired to develop more convenient strategies for preparation of

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high-efficiency catalysts. Furthermore, we have to reduce the cost of supported noble

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metal catalysts. In the literature, much attention was paid on supported Au, Pd, Pt,

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and their alloy catalysts, in which the loadings of noble metal NPs are usually high

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(1−5 wt%) for achieving good catalytic performance, and hence these noble metal

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catalysts are expensive. For the practical applications, however, we must generate

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catalysts with low cost and high performance. Herein, we report a facile strategy to

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fabricate 0.13 wt% Ag/Mn2O3 nanowires (0.13Ag/Mn2O3-ms) after calcination at

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420 oC for 2 h using the mixture of NaNO3 and NaF as molten salt, and MnSO4 and

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AgNO3 as metal precursor. It is found that this catalyst was high-efficient in

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catalyzing the oxidative removal of toluene.

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■ EXPERIMENTAL SECTION

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The molten salt strategy was used to fabricate the Mn2O3 nanowires supported

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Ag NPs (denoted as xAg/Mn2O3-ms, x = 0.13 wt%). A mixture of NaNO3, NaF,

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MnSO4, and AgNO3 were firstly well ground and then calcined at 420 oC for 2 h.

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The 0.13Ag/Mn2O3-ms sample was obtained after removal of the molten salt. For

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comparison purposes, we adopted Mn2O3-ms derived from the molten salt method

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without addition of AgNO3 as support to fabricate the Mn2O3 supported Ag NPs

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(denoted as Ag/Mn2O3-imp and Ag/Mn2O3-redn) using the wetness impregnation

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and PVA-protected NaBH4 reduction methods, respectively. The commercial MnO2

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was calcined at 750 oC for 2 h to obtain the bulk Mn2O3 support (denoted as

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Mn2O3-bulk). The Mn2O3-bulk supported Ag NPs (denoted as Ag/Mn2O3-bulk) was

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prepared via the wetness impregnation route. The detailed preparation procedures are

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described in the Supporting Information. The Ag contents in the samples were

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determined by the X-ray fluorescence (XRF) spectroscopic technique.

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All of the samples were characterized by means of the techniques, such as XRF,

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

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electron microscopy (TEM), element mapping, X-ray photoelectron spectroscopy

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(XPS), and hydrogen temperature-programmed reduction (H2-TPR). The detailed

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characterization procedures are described in the Supporting Information.

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Catalytic activities of the samples were evaluated in a continuous flow fixed-bed

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quartz microreactor (i.d. = 4 mm). To minimize the effect of hot spots, 50 mg of the

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sample (40−60 mesh) was diluted with 0.25 g of quartz sands (40−60 mesh). The

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total flow rate of the reactant mixture (1000 ppm toluene + O2 + N2 (balance)) was

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33.4 mL/min, giving a toluene/O2 molar ratio of 1/400 and a space velocity (SV) of

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ca. 40 000 mL/(g h). The 1000-ppm toluene was generated by passing a N2 flow

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through a bottle containing pure toluene chilled in an ice-water isothermal bath. In

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the case of water vapor introduction, 1.0 vol% water vapor was introduced by

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passing the feed stream through a water saturator at a certain temperature. Reactants

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and products are analyzed online by a gas chromatograph (GC-2010, Shimadzu)

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equipped with flame ionization and thermal conductivity detectors, using a

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stabilwax@-DA column (30 m in length) for VOCs separation and a Carboxen 1000

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column (1/8 inch in diameter and 3 m in length) for permanent gas separation. The

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balance of carbon throughout the investigation was estimated to be 99.5 %.

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■ RESULTS AND DISCUSSION

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The actual loadings of Ag NPs in the Ag/Mn2O3-ms, Ag/Mn2O3-imp,

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Ag/Mn2O3-redn, and Ag/Mn2O3-bulk samples were 0.13, 0.12, 0.12, and 0.23 wt%,

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respectively. It should be noted that the presence of Na+, NO3−, and F− ions might

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play a role in toluene oxidation. The results of XRF studies reveal that no Na+, NO3−,

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and F− ions were detected in the XRF spectra of the samples, indicating that the

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samples obtained after dissolution, filtration, and washing treatments were free of the

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Na+, NO3−, and F− ions.

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Figure 1 shows the XRD patterns of the as-fabricated samples. By comparing to

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the XRD patterns of the standard Mn2O3 (JCPDS PDF No. 41-1442) and MnO2

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(JCPDS PDF No. 72-1982) samples, one can deduce that the manganese oxides in

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the samples were mainly Mn2O3 with a cubic crystal structure. There was also a trace

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amount of tetragonal MnO2 phase in the 0.13Ag/Mn2O3-ms, 0.12Ag/Mn2O3-imp or

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0.12Ag/Mn2O3-redn sample. No diffraction signals assignable to the Ag or AgOx

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phase were not detected, which might be due to the ultra low Ag loadings and good

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dispersion of Ag NPs on the surface of Mn2O3. The co-presence of trace MnO2 and

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main Mn2O3 phases might be associated with the interaction between the silver NPs

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and manganese oxide nanowires after the loading of silver NPs on the surface of

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manganese oxide nanowires. It is noted that the driving force for such a phase

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transformation was not clear at the moment under the present preparation conditions,

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and it needs further investigations in the future.

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It is well known that the factors, such as the nature and mass ratio of the molten

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salt and the metal precursor and the calcination temperature and time, might greatly

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influence the physicochemical properties of the as-prepared samples. The Ag NPs

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might be generated via the decomposition of AgNO3 during the annealing process at

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420 oC. The Mn2O3 nanowires could be formed via the three steps15: (i) Most of the

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Mn2+ cations in the molten salt reacted with F− ions to generate MnF42− complex ions;

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(ii) the small amount of dissociated Mn2+ ions in the molten salt was oxidized by

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NO3− to form the Mn2O3 nuclei; and (iii) the Mn2O3 nanowires were gradually

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formed due to the directing role of complexing F− on the crystal surface when the

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Mn2+ ions in MnF42− ions were released. Figures 2 and S1 show the SEM and TEM

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images of the as-prepared samples. It is observed that the Mn2O3 in the

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0.13Ag/Mn2O3-ms, 0.12Ag/Mn2O3-imp, and 0.12Ag/Mn2O3-redn samples displayed

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a nanowire-like morphology. The 0.23Ag/Mn2O3-bulk sample displayed an

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irregularly nanosized morphology. The loading of Ag NPs did not lead to a

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significant alteration in Mn2O3 morphology. There were some Ag clusters (diameter

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< 1 nm) randomly distributed on the Mn2O3 nanowires (Figure 2d−f). This result

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indicates that the molten salt strategy was beneficial for fabrication of Ag NPs

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highly dispersed on the Mn2O3 nanowire support. As mentioned above, several

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factors could greatly influence the physicochemical properties (e.g., morphology of

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manganese oxide and size/dispersion of Ag NPs) of the as-prepared samples. The

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real reason why the present molten salt system was beneficial for generation of

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highly dispersed Ag NPs was not clear at the moment and it needs further

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investigations. The surface areas of the nanowire-like manganese oxide and its

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supported Ag nanocatalysts were in the range of 75−80 m2/g.

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In the preliminary investigation, we prepared the Mn2O3 nanowires supported Ag

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NPs with various loadings (0.025−10 wt%) using the molten salt method, and found

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that among the 0.025−10 wt% Ag/Mn2O3-ms samples, the 0.06 wt% Ag/Mn2O3-ms

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sample performed the best for toluene oxidation. However, the catalytic stability of

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0.06 wt% Ag/Mn2O3-ms was poor possibly due to the sintering of single silver atom.

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In addition to 0.06 wt% Ag/Mn2O3-ms, the 0.13Ag/Mn2O3-ms sample showed the

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highest activity in the total oxidation of toluene. Figures 3 and S2 show the toluene

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conversion and consumption rate as a function of reaction temperature at a SV of 40

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000 mL/(g h). In addition to carbon dioxide and water, no other products were

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detected in the oxidation of toluene over the as-prepared samples. Obviously, the

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catalytic

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0.12Ag/Mn2O3-imp > 0.12Ag/Mn2O3-redn > Mn2O3-ms > 0.23Ag/Mn2O3-bulk. The

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0.13Ag/Mn2O3-ms sample performed much better than the 0.23Ag/Mn2O3-bulk

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sample, indicating that the molten salt derived Mn2O3 nanowires were more suitable

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for generation of high-efficiency supported Ag catalyst. In addition, the present

activity

decreased

in

the

order

of

8


0.13Ag/Mn2O3-ms

>

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one-pot molten salt method was also superior to the traditional impregnation or

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PVA-protected NaBH4 reduction method for preparation of Mn2O3 nanowires

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supported Ag NPs. The T10%, T50%, and T90% (temperatures required for achieving

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toluene conversions 10, 50, and 90%) over 0.13Ag/Mn2O3-ms were 110, 185, and

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215 oC, respectively. The Mn2O3 sample showed a similar catalytic activity as

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compared to the MnO2 sample below 200 oC.16 That is to say, the trace amount of

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MnO2 or the interfaces between Mn2O3 and MnO2 would play a positive role in

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enhancing the catalytic activity of the supported silver nanocatalysts. In order to

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examine the catalytic stability, we carried out on-stream toluene oxidation over

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0.13Ag/Mn2O3-ms at 215 oC in the absence of water vapor and at 185 oC in the

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presence of 1 vol.% water vapor. No significant loss in activity was observed after

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30 h of on-stream reaction (Figure S3), demonstrating that this sample possessed

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good stability in catalyzing the complete oxidation of toluene.

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In the past years, our group have investigated a number of three dimensionally

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ordered mesop-/macroporous (3DOM) metal oxides, well morphological metal

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oxides, and their supported noble metal (Au, Pd or AuPd alloy) NPs for the complete

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oxidation of VOCs. Table 1 summarizes the catalytic activities of typical samples for

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toluene oxidation. Under similar reaction conditions, 0.13Ag/Mn2O3-ms (T90% = 215

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o

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238 oC),16 4.6 wt% Au/macroporous LaMnO3 (T90% = 226 oC),18 6.5 wt% Au/3DOM

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Co3O4 (T90% = 256 oC),21 6.55 wt% Au/Fe2O3 nanodisc (T90% = 260 oC),23 7.4 wt%

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Au/Co3O4 microspheres (T90% = 250 oC),24 5.8 wt% Au/3DOM Mn2O3 (T90% = 244

C) fabricated in the present study performed better than flower-like Mn2O3 (T90% =

9



199

o

C),25 4.7 wt% Au/Ce0.6Zr0.3Y0.1O2 nanorods (T90% = 265 oC),26 0.99 wt% Pd/3DOM

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Co3O4 (T90% = 275 oC)27, mesoporous Mn2O3 (T90% = 270 oC),28 6.4 wt%-9.6 wt%

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Mn/SBA-15 (T90% = 255 oC),29 and 0.86 wt% Au/MnOx (T90% = 255 oC).30 The

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catalytic activity over 0.13Ag/Mn2O3-ms was similar to that (T90% ≈ 210 oC) over

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4.05 wt% Au/mesoporous MnO217, but lower than that (T90% = 202 oC) over 7.63

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wt% Au/3DOM LaCoO320, that (T90% = 170

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La0.6Sr0.4MnO3,19 that (T90% = 189 oC) over 6.5 wt% Au/mesoporous Co3O4,22 and

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that (T90% = 168 oC) over 1.99 wt% AuPd/3DOM Co3O4 (Au : Pd mass ratio = 1:1).27

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Since the only comparison of the T50% and T90% is not sufficient to rank the

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catalytic performance of the samples, it is appropriate to compare the performance of

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supported noble metal catalysts using the turnover frequencies (TOFs), which are

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more persuasive than those of toluene conversion temperature and toluene

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consumption rate over the samples. Nevertheless, there are several kinds of active

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sites (e.g., noble metal, transition metal oxide, and interface between noble metal and

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transition metal oxide) in the reducible metal oxide supported noble metal catalysts.

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In other words, it is difficult to identify a single active site and is therefore hard to

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accurately calculate the TOFs. We believe that it is more appropriate to compare the

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performance of supported noble metal catalysts using the toluene consumption rate

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per gram of catalyst or noble metal in the present study. The typical samples that

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showed better catalytic activities (Table 1) are chosen to compare their activities in

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terms of toluene consumption rate per gram of catalyst and toluene consumption rate

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per gram of noble metal with that of the 0.13Ag/Mn2O3-ms sample, as shown in

o

C) over 6.4 wt% Au/3DOM

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Figures 4 and S4 and Table S1. In terms of T50% and T90% (Figure S4(A)) and toluene

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consumption rate per gram of catalyst (Figure S4(B)), 6.5 wt% Au/mesoporous

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Co3O422 outperformed 0.13Ag/Mn2O3-ms. At a low temperature (< 160 oC), the

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toluene consumption rates over 6.4 wt% Au/3DOM La0.6Sr0.4MnO319 and 1.99 wt%

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AuPd/3DOM Co3O427 were lower than that over 0.13 wt% Ag/Mn2O3, but those over

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the former two samples were higher than that over the latter at a high temperature.

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However, the loading and cost of the supported Au or AuPd nanocatalysts are much

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higher than those of the supported Ag nanocatalysts. For the practical applications, it

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is meaningful to compare the catalytic performance in terms of toluene consumption

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rate per gram of noble metal (Figures 4 and S4(C) and Table S1). Since the reaction

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rate decreases with the drop in toluene concentration along the reactor, it is better to

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compare the catalytic performance at low conversions. It can be clearly seen that

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0.13Ag/Mn2O3-ms

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La0.6Sr0.4MnO3,19 6.5 wt% Au/mesoporous Co3O4,22 and 1.99 wt% AuPd/3DOM

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Co3O4.27 The toluene consumption rate per gram of noble metal over 0.13 wt%

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Ag/Mn2O3 was 19−85 and 6−41 times as high as that over the latter three samples at

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100 and 180 oC, respectively. The difference in catalytic activity (i.e., toluene

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consumption rate per gram of noble metal) of the 0.13Ag/Mn2O3-ms, 6.4 wt%

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Au/3DOM La0.6Sr0.4MnO3,19 6.5 wt% Au/mesoporous Co3O4,22 and 1.99 wt%

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AuPd/3DOM Co3O427 samples decreased with a rise in reaction temperature from

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100 to 140 oC and further to 180 oC.

242

performed

much

better

than

6.4

wt%

Au/3DOM

Figure S5 illustrates the Mn 2p3/2, O 1s, and Ag 3d XPS spectra of the

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as-prepared samples, and the surface element compositions are summarized in Table

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2. Using the curve-fitting approach, each of the asymmetrical Mn 2p3/2 spectra could

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be decomposed into three components at binding energy (BE) = 640.6, 641.8, and

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643.1eV as well the satellite at BE = 644.5 eV (Figure S5(A)), ascribable to the

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surface Mn2+, Mn3+, and Mn4+ species,16,28,29 respectively. The asymmetrical O 1s

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spectrum of each sample could be devonvoluted into two or three components at BE

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= 529.5, 531.5, and 533.6 eV (Figure S5(B)), assignable to the surface lattice oxygen

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(Olatt), adsorbed oxygen (Oads, e.g., O2−, O22− or O−), and adsorbed molecular water

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species,31−33 respectively. Since the samples were pretreated in O2 at 450 oC before

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XPS measurement, the surface carbonate species would be minimized. In other

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words, the Oads species were mainly O2−, O22− or O− species. As shown in Figure

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S5(C), the Ag 3d spectrum of each sample could be decomposed into four

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components: the ones at BE = 367.5 and 373.5 eV were due to the surface Ag0

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species, whereas the ones at BE = 368.9 and 374.9 eV were due to the surface Ag+

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species.29 As mentioned above, the preparation method had a great influence on

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physicochemical property of the sample. The surface Mn4+/Mn3+ or Mn3+/Mn2+

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molar ratios of the as-prepared samples were different from each other, but there was

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only a small difference in surface Ag+/Ag0 molar ratio (Table 2) on the Mn2O3

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nanowires supported Ag samples derived from the molten salt, impregnation, and

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reduction methods. The Oads/Olatt molar ratio decreased in the sequence of Mn2O3 ≈

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0.12Ag/Mn2O3-redn > 0.12Ag/Mn2O3-imp > 0.13Ag/Mn2O3-ms. It is reported that

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adsorbed oxygen (e.g., O2−, O22− or O−) species were active for oxidation of

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hydrocarbons at low temperatures.32 Generally speaking, the higher the surface

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oxygen vacancy density, the easier the activation adsorption of O2 molecules, and the

267

better is the performance of a catalyst for the oxidation of VOCs.22 For the

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as-prepared samples, however, the change trend in Oads/Olatt molar ratio contradicted

269

with that in catalytic activity. That is to say, there were other factors governing the

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catalytic activity.

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Figure 5A shows the H2-TPR profiles of the samples. Compared to the

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0.23Ag/Mn2O3-bulk sample, the initial reduction peaks of the Mn2O3-ms sample

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shifted to lower temperatures significantly. There were three reduction peaks of the

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Mn2O3-ms sample at 460, 580, and 760 oC, assignable to the reduction of surface

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Mn4+ to Mn3+, Mn3+ to Mn2+, and bulk Mn3+ to Mn2+,16 respectively. When Ag NPs

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were loaded on the Mn2O3-ms surface, all of the reduction peaks shifted to lower

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temperatures due to presence of an interaction between Ag NPs and Mn2O3 support.

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For example, the reduction temperature of the 0.13Ag/Mn2O3-ms sample decreased

279

to 335 oC. This result suggests that loading of Ag NPs enhanced the reducibility of

280

the sample. The higher the initial H2 consumption rate, the better the

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low-temperature reducibility. In order to better compare the low-temperature

282

reducibility of these samples, we calculate the initial H2 consumption rate according

283

to the H2 consumption per mol of Mn per second, which corresponded to the initial

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25 % area of the first reduction peak where no phase transformation of the catalyst

285

occurred,34 and the results are shown in Figure 5B. Obviously, the initial H2

286

consumption

rate

decreased

in

the

sequence

of

13


0.13Ag/Mn2O3-ms

>


Page 14 of 29

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0.12Ag/Mn2O3-imp > 0.12Ag/Mn2O3-redn > Mn2O3-ms > 0.23Ag/Mn2O3-bulk, in

288

good agreement with that in catalytic activity. That is to say, the change trend in

289

catalytic activity coincides with that in low-temperature reducibility of the

290

as-prepared samples. The catalytic performance of a sample is associated with the

291

low-temperature reducibility. Therefore, there was an intrinsic relation between the

292

initial H2 consumption rate and catalytic performance of the sample. The high

293

dispersion of silver NPs and good low-temperature reducibility of the

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0.13Ag/Mn2O3-ms sample might be the main reasons why it outperformed the

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0.12Ag/Mn2O3-imp and 0.12Ag/Mn2O3-redn samples.

296 297

■ ASSOCIATED CONTENT

298

Supporting Information

299

Experimental details, additional SEM and TEM images, catalytic stability,

300

comparison of catalytic activity for the present catalyst and the catalysts reported in

301

previous studies, and Mn 2p, O 1s, and Ag 3d XPS spectra. This material is available

302

free of charge via the Internet at http://pubs.acs.org/.

303

■ AUTHOR INFORMATION

304

Corresponding Author

305

*Phone:

306

[email protected] (Dr. Jiguang Deng); [email protected] (Prof. Hongxing Dai)

307

Notes

308

The authors declare no competing financial interest.

86-10-67396118.

Fax:

86-10-67391983.

14


E-mail

addresses:

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■ ACKNOWLEDGMENTS

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This work was financially supported by Natural Science Foundation of China

311

(21103005, 21377008, and 21477005), Beijing Municipal Natural Science

312

Foundation (2132015), Natural Science Foundation of Beijing Municipal

313

Commission of Education (km201410005008), Foundation for the Author of

314

National Excellent Doctoral Dissertation of China (201462), Doctoral Fund of the

315

Ministry of Education of China (20111103120006), National High Technology

316

Research and Development Program ("863" Program) of China (2015AA034603),

317

and Foundation on the Creative Research Team Construction Promotion Project of

318

Beijing Municipal Institutions.

319

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436 437 438 439 440

20


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Captions of Tables and Figures

441 442 443

Table 1. Catalytic Activities of the 0.13Ag/Mn2O3-ms and Other Supported

444

Noble Metal Samples Reported in Previous Studies

445 446

Table 2. Surface Element Compositions of the As-Prepared Samples

447 448

Figure 1. XRD patterns of the as-prepared samples.

449 450

Figure 2. (a, b) SEM and (c) TEM, and (d−f) elemental mapping images of (a)

451

Mn2O3-ms and (b−f) 0.13Ag/Mn2O3-ms.

452 453

Figure 3. Catalytic activities of the as-prepared samples for the oxidation of toluene

454

at a toluene/oxygen molar ratio of 1/400 and a SV of 40 000 mL/(g h).

455 456

Figure 4. Toluene consumption rate per gram of noble metal at 80−180 oC over (a)

457

0.13Ag/Mn2O3-ms, (b) 1.99 wt% AuPd/3DOM Co3O4 (Au : Pd mass ratio = 1 : 1),27

458

(c) 6.4 wt% Au/3DOM La0.6Sr0.4MnO3,19 and (d) 6.5 wt% Au/mesoporous Co3O422.

459 460

Figure 5. (A) H2-TPR profiles and (B) initial H2 consumption rate of (a) Mn2O3-ms,

461

(b) 0.13Ag/Mn2O3-ms, (c) 0.12Ag/Mn2O3-imp, (d) 0.12Ag/Mn2O3-redn, and (e)

462

0.23Ag/Mn2O3-bulk.

21



Page 22 of 29

Table 1. Catalytic Activities of the 0.13Ag/Mn2O3-ms and Other Supported Noble Metal Samples Reported in Previous Studies Sample

Surface area (m2/g)

Toluene concentration (ppm)

Toluene/oxygen molar ratio

Space velocity (mL/(g h))

Toluene oxidation activity (oC) T10%

T50%

T90%

Ref.

0.13Ag/Mn2O3-ms

76

1000

1/400

40000

110

185

215

Flower-like Mn2O3 4.05 wt% Au/mesoporous MnO2 4.6 wt% Au/macroporous LaMnO3 6.4 wt% Au/3DOM La0.6Sr0.4MnO3 7.63 wt% Au/3DOM LaCoO3 6.5 wt% Au/3DOM Co3O4 6.5 wt% Au/mesoporous Co3O4 6.55 wt% Au/Fe2O3 nanodisc 7.4 wt% Au/Co3O4 microspheres 5.8 wt% Au/3DOM Mn2O3 4.7 wt% Au/Ce0.6Zr0.3Y0.1O2 nanorods 0.99 wt% Pd/3DOM Co3O4 1.99 wt% AuPd/3DOM Co3O4 (Au : Pd mass ratio = 1 : 1) Mesoporous Mn2O3

162.3 120 32.7

1000 2000 1000

1/400 1/100 1/400

20000 60000 20000

145 − 138

226 170 201

238 220 226

present work 16 17 18

31.1

1000

1/400

20000

−

150

170

19

24.2 22.4 91 18.9 22.4 34.5 79.2

1000 1000 1000 1000 1000 1000 1000

1/400 1/400 1/400 1/400 1/400 1/400 1/400

20000 40000 20000 20000 20000 40000 20000

136 215 − 135 155 211 142

188 244 162 200 242 230 218

202 256 189 260 250 244 265

20 21 22 23 24 25 26

33.2 33.1

1000 1000

1/400 1/400

40000 40000

244 145

268 164

275 168

27 27

43 553 124

1000 2500 266

− − −

19100 15000 53050

220 220 230

238 240 250

270 255 275

28 29 30

6.4 wt% Ag-9.6 wt% Mn/SBA-15

0.86 wt% Au/MnOx

22


Page 23 of 29


Table 2. Surface Element Compositions of the As-Prepared Samples

Sample

Mn4+/Mn3+ molar ratio

Mn3+/Mn2+ molar ratio

Oads/Olatt molar ratio

Ag+/Ag0 molar ratio

Mn2O3-ms

0.74

1.63

1.19

–

0.13Ag/Mn2O3-ms

0.89

1.55

0.65

0.09

0.12Ag /Mn2O3-imp

0.74

1.63

0.89

0.10

0.12Ag/Mn2O3-redn

0.88

1.26

1.20

0.10

23



Page 24 of 29

□ Mn2O3 △ MnO2

□ □

0.23 Ag/Mn2O3-bulk

□

Intensity (a.u.)

□

□ □

□ □ □ □□ □

□

0.12 Ag/Mn2O3-redn

0.12 Ag/Mn2O3 -imp

0.13 Ag/Mn2O3-ms

△

△

Mn2O3-ms 0

10

20

30

40

50

60

2Theta (o)

Figure 1. XRD patterns of the as-prepared samples.

24


70

80

Page 25 of 29


(a)

(c)

(b)

400 nm

(d)

200 nm

(e)

200 nm

(f)

Figure 2. (a, b) SEM and (c) TEM, and (d−f) elemental mapping images of (a) Mn2O3-ms and (b−f) 0.13Ag/Mn2O3-ms.

25



Page 26 of 29

100

Toluene conversion (%)

80

(× × ) 0.13 Ag/Mn2O3-ms (◇ ◇ ) 0.12 Ag/Mn2O3-imp (△ △ ) 0.12 Ag/Mn2O3-redn (○ ○ ) 0.23 Ag/Mn2O3-bulk (□ □ ) Mn2O3-ms

60

40

20

0 60

120

180

240

300

o

Temperature ( C)

Figure 3. Catalytic activities of the as-prepared samples for the oxidation of toluene at a toluene/oxygen molar ratio of 1/400 and a SV of 40 000 mL/(g h).

26


Page 27 of 29


4000 2000

(a) Ca (b) tal (c) yst (d) s

80

100

120

Toluene co

6000

nsumption rate (mL/(g noble met h)) al

1400 0 1200 0 1000 0 8000

180 160 o ) 140 C

e mp e T

ur ra t

e(

Figure 4. Toluene consumption rate per gram of noble metal at 80−180 oC over (a) 0.13Ag/Mn2O3-ms, (b) 1.99 wt% AuPd/3DOM Co3O4 (Au : Pd mass ratio = 1 : 1),27 (c) 6.4 wt% Au/3DOM La0.6Sr0.4MnO3,19 and (d) 6.5 wt% Au/mesoporous Co3O422.

27



Page 28 of 29

(A) 710

(e) H2 consumed (a.u.)

440

540

(d) 400

520

(c) 335

(b)

50

250

760

580

460

(a)

450

650

850

o

Temperature ( C) (B)

(e)

(b)

30

(a)

-4

Initial H2 consumption rate (10 mol/(mol Mn s)

40

20

10

(c) (d)

0 1

1.2

1.4

1.6

1.8

2

1000/T (K-1 )

Figure 5. (A) H2-TPR profiles and (B) initial H2 consumption rate of (a) Mn2O3-ms, (b) 0.13Ag/Mn2O3-ms, (c) 0.12Ag/Mn2O3-imp, (d) 0.12Ag/Mn2O3-redn, and (e) 0.23Ag/Mn2O3-bulk.

28


Page 29 of 29


Ultra Low Loading of Silver Nanoparticles on Mn2O3 Nanowires Derived with Molten Salts: A High-Efficiency Catalyst for the Oxidative Removal of Toluene

Jiguang Deng*, Shengnan He, Shaohua Xie, Huanggen Yang, Yuxi Liu, Guangsheng Guo, Hongxing Dai*

Beijing Key Laboratory for Green Catalysis and Separation, Key Laboratory of Beijing on Regional Air Pollution Control, Key Laboratory of Advanced Functional Materials, Education Ministry of China, and Laboratory of Catalysis Chemistry and Nanoscience, Department of Chemistry and Chemical Engineering, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China

O2

CO2

Ag/Mn2O3 nanowire

Ag

toluene

H2O


A High-Efficiency Catalyst for the Oxidative ... - ACS Publications

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