Intermetallic compound PtMny-derived Pt−MnOx supported on mesoporous CeO2: Highly efficient catalysts for the combustion of toluene

https://doi.org/10.1016/j.apcata.2020.117509Get rights and content

Highlights

  • PtMny intermetallic nanocrystals are fabricated using the PVP-assisted EG reduction method.

  • mPt−nMnOx/meso-CeO2 is prepared via adsorption of PtMny NCs on meso-CeO2.

  • 0.37Pt−0.16MnOx/meso-CeO2 perform the best for toluene combustion.

  • 0.37Pt−0.16MnOx/meso-CeO2 exhibits good water-resistant performance.

  • Oads and Mn2+ concentration, low-temp. reducibility, and toluene adsorption ability govern activity.

Abstract

Intermetallic compounds are a kind of important materials in heterogeneous catalysis. In this work, we first synthesized the PtMny intermetallic nanocrystals using the polyvinyl pyrrolidone-assisted ethylene glycol reduction method, and then loaded them on the surface of mesoporous CeO2 (meso-CeO2) derived from a KIT-6-templating route, generating the mPt−nMnOx/meso-CeO2 (m = 0−0.39 wt%, n = 0−1.21 wt%) catalysts after calcination at 500 °C in air. It is found that the as-obtained catalysts displayed an ordered mesoporous architecture with surface areas of 95−108 m2/g. The 0.37Pt−0.16MnOx/meso-CeO2 sample exhibited the best catalytic performance for toluene combustion (T50 % =162 °C and T90 % =171 °C at space velocity = 40,000 mL/(g h)). Kinetic analysis reveals that the apparent activation energy (57 kJ/mol) obtained over the best-performing 0.37Pt−0.16MnOx/meso-CeO2 sample was lower than those (63−75 kJ/mol) obtained over the other samples. Furthermore, the 0.37Pt−0.16MnOx/meso-CeO2 sample possessed good thermal stability and water-resistant performance. Benzyl alcohol, benzoic acid, and maleic anhydride were proven to be the main intermediates of toluene combustion, hence, toluene combustion might take place through a sequence of toluene → benzyl alcohol and benzoic acid → maleic anhydride → carbon dioxide and water, which might obey the Eley−Rideal reaction mechanism. It is concluded that loading of Pt and MnOx enhanced the adsorbed oxygen and Mn2+ species concentration and low-temperature reducibility, thus promoting toluene combustion over 0.37Pt−0.16MnOx/meso-CeO2.

Graphical abstract

Loading the PtMny intermetallic nanocrystals derived from the PVP-assisted EG reduction route on meso-CeO2 and after calcination in air generates the Pt−MnOx/meso-CeO2 catalysts. The good performance for toluene combustion of 0.37Pt−0.16MnOx/meso-CeO2 is related to its high adsorbed oxygen and Mn2+ species concentration and good low-temperature reducibility.

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Introduction

Volatile organic compounds (VOCs) are the important precursors of photochemical smog and fine particulate matter (PM2.5), which are harmful to the atmosphere and human being's health [1]. Accordingly, it is highly required to abate VOCs emissions. Among the methods of VOCs removal, catalytic combustion is considered to be the most effective because of its low cost and high efficiency [2,3]. The key to catalytic combustion of VOCs is generation of the catalysts that are cheap in cost, high in activity, and good in stability.

Although supported Pt catalysts exhibit excellent activity for VOCs combustion at low temperatures [4], they are expensive and easily sintered at high temperatures, which greatly limit their wide applications. To solve these problems, it is a better choice to dope a base metal in Pt nanoparticles (NPs) to reduce the Pt amount and improve the thermal stability. As we know, support plays a remarkable role in improving catalytic activity [5,6]. Owing to excellent oxygen storage and release ability, ceria-based materials have attracted much attention in recent years [7,8]. A higher surface area is favorable for enhancement in activity of a material, and the good strategy to increase surface area is to fabricate the material with an ordered mesoporous architecture.

Intermetallic compounds are the materials comprising two or more elements located left and around the Zintl line in the periodic table [9], and their crystal structures are completely or at least partly ordered and different from those of the constituent elements. Heterogeneous catalysis of intermetallic compounds has been regarded as a quickly developing field [10]. The peculiar combination of the crystal and electronic structures of intermetallic compounds results in their unique adsorption and thus catalytic properties, in which covalent bonding is necessary to stabilize the specific crystal and electronic structure of an intermetallic compound. Such a site preference can provide the catalytic stability under reaction conditions. This makes intermetallic compounds be highly interesting catalytic materials. For example, Armbrüster et al. [11] adopted a co-reduction of the palladium and gallium precursors in organic solvents to prepare the GaPd or GaPd2 NPs, and observed that the activity per Pd atom of these materials reached those of the commercially available supported Pd catalysts while preserving the excellent selectivity of the unsupported materials. Osswald et al. [12] reported that nearly no deactivation or loss in selectivity was observed over the Ga7Pd3, GaPd or GaPd2 intermetallic compound as compared with the 5 wt% Pd/Al2O3 sample within 20 h of reaction (hydrogenation of acetylene). The high selectivity of the Ga−Pd compounds prevented formation of the carbonaceous deposits and hence these intermetallic compounds showed good stability. To the best of our knowledge, there have been rare works related to the utilization of intermetallic compounds in the application of VOCs combustion. For example, Willis et al. [13] studied promotional effect of the transition metal on methane oxidation over the palladium catalysts, and found that some metals (Fe, Co, and Sn) inhibited sintering of the active Pd metal phase, while the others (Ni and Zn) increased its intrinsic activity as compared with a monometallic Pd catalyst. Therefore, intermetallic compounds are promising materials in heterogeneous catalysis. In recent years, VOCs combustion over the supported bimetallic catalysts has been studied intensively and extensively. For instance, we previously [14] synthesized Au−Pd alloys supported on three-dimensionally ordered macroporous (3DOM) Co3O4, and found that the bimetallic catalysts were more active and stable than the single metal counterparts for toluene oxidation. After investigating the one-step seeding growth-derived core-shell Au@Co NPs, Yan et al. [15] pointed out that the Au@Co catalyst was highly active for hydrolytic dehydrogenation of ammonia borane. Ho et al. [16] used the borane−amine reduction method to obtain the bimetallic MPd (M = Co or Cu) NPs, and claimed that the MPd NPs showed good catalytic performance for formic acid electrooxidation.

In the past several years, our group has prepared 1.67 wt% Mn3O4−2 wt% Au/3DOM La0.6Sr0.4CoO3 [17], 0.25 wt% Pt1/meso-Fe2O3 [18], and 8.5 wt% Co3O4/MnO2 [19] via the polyvinyl alcohol-protecting reduction route, and observed that most of the materials performed well in catalyzing combustion of the typical VOCs (e.g., toluene, benzene, and o-xylene). Herein, we first synthesized the PtMny intermetallic nanocrystals (NCs) using the polyvinyl pyrrolidone (PVP)-assisted ethylene glycol (EG) reduction method, and then loaded them on the surface of ordered mesoporous CeO2 (meso-CeO2), characterized their physicochemical properties of the as-prepared mPt−nMnOx/meso-CeO2 (m = 0−0.39 wt%, n = 0−1.21 wt%), and evaluated their catalytic activities for toluene combustion.

Section snippets

Catalyst preparation

The ordered mesoporous silica (KIT-6) template was fabricated according to the procedures reported in the literature [20]. Ordered mesoporous CeO2 (meso-CeO2) was fabricated by the KIT-6-templating strategy. 2.0 g of the KIT-6 template was added to 40 ml of ethanol containing 4.0 g of Ce(NO3)3⋅6H2O. The mixture was first dried at room temperature (RT) and calcined at a ramp of 1 °C/min from RT to 600 °C and kept at 600 °C for 6 h. The obtained powders were twice treated in a hot (60 °C) NaOH

Crystal phase composition, pore structure, and surface area

Fig. 1A shows XRD patterns of the meso-CeO2, 0.39 Pt/meso-CeO2, 0.16MnOx/meso-CeO2, and mPt−nMnOx/meso-CeO2 samples. After referring to XRD pattern (JCPDS PDF# 34-0394) of the standard ceria sample, we can know that CeO2 in all of the samples possessed a cubic crystal structure, and their crystal planes are shown in Fig. 1A(e). No diffraction signals assigned to the MnOx and/or Pt phases were observed, a result possibly owing to their low loadings and good dispersion on the surface of meso-CeO2

Conclusions

The PtMny intermetallic NCs were synthesized via the PVP-assisted EG reduction route. Loading of PtMny NCs on the surface of meso-CeO2 (derived by the KIT-6-templating method) and after calcination at 500 °C in air led to generation of the mPt−nMnOx/meso-CeO2 catalysts. All of the samples were cubic in crystal structure and displayed an ordered mesoporous architecture with surface areas of 95−108 m2/g. Loading of Pt and MnOx resulted in improved low-temperature reducibility and increased Oads

Credit author statement

Mr. Xiaohang Fu prepared the catalysts, evaluated the activities, and made characterization of XRD, TEM, HAADF−STEM, BET, O2-TPD, toluene-TPD, and H2-TPR; Prof. Jiguang Deng and Lin Jing examined the effect of treatment temperature on activity of the typical samples; Mr. Xing Zhang, Kunfeng Zhang, Zhuo Han, and Xiyun Jiang did the XPS and in situ DRIFTS characterization and examined the effects of SV, H2O, CO2, and SO2; Prof. Hongxing Dai and Dr. Yuxi Liu were responsible for the whole work.

Acknowledgements

This work was supported by the NSF of China (21677004, 21876006, and 21976009), National Natural Science Committee of China−Liaoning Provincial People's Government Joint Fund (U1908204), and Foundation on the Creative Research Team Construction Promotion Project of Beijing Municipal Institutions (IDHT20190503), and Natural Science Foundation of Beijing Municipal Commission of Education (SQKM201710005004).

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