Significant enhancement of catalytic activities of manganese oxide octahedral molecular sieve by marginal amount of doping vanadium

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Abstract

Vanadium doped hollandite-type manganese oxide octahedral molecular sieve (OMS-2) was synthesized by a simple hydrothermal route. The results of catalytic tests showed that marginal amount of doping vanadium had greatly enhanced catalytic activities of OMS-2 not only in low-temperature complete oxidation of formaldehyde at 140 °C but also in high-temperature complete oxidation of methane at 450 °C. The results of XPS, HRTEM and H2-TPR measurements revealed that the enhancements of the catalytic activities primarily resulted from the increase of surface defect sites regardless of reaction temperatures and nature of reactants, rather than reducibility and average oxidation state of manganese.

Introduction

Hollandite-type manganese oxide octahedral molecular sieve (OMS-2) has attracted a great deal of attention because of their potential applications as ion or molecular sieves [1], [2], [3] and oxidation catalysts [4], [5], [6]. Luo et al. [7] found that OMS-2 with 0.46 × 0.46 nm2 tunnel size had higher catalytic activity towards complete oxidation of benzene than todorokite manganese oxide with 0.69 × 0.69 nm2 tunnel size. They believed that special tunnel structure of OMS-2 principally accounted for strong affinity towards organic compounds. Recently, we comparatively studied tunnel structure effect of manganese oxides in complete oxidation of HCHO, and found that OMS-2 exhibited higher catalytic activity than pyrolusite or todorokite [8].

The OMS-2 materials doped by metal cations greatly influenced physical and chemical properties, and the nature, the amount and the location of the doping ions had key roles in altering the properties [9], [10], [11]. Some metal cations such as Cu2+, Ni2+, Co2+ or Fe3+ have been successfully incorporated into the framework of OMS-2 substituting manganese cations [10]. Especially, Suib et al. [11] reported a new route for higher valency ion substitution into OMS-2 framework. The insertion of vanadium in the framework of OMS-2 may create vacancy sites in the manganese oxide structure, which enhances the ionic properties of the material. This feature makes this material highly suitable for water sensoring and catalytic applications.

In this paper, effect of marginal amount of doping vanadium on catalytic activities of OMS-2 in complete oxidations of HCHO and CH4 was investigated. Physical and chemical properties of the catalysts were characterized by various techniques to establish relationship between surface structure and activity.

Section snippets

Synthesis

The catalysts were synthesized by hydrothermally treating a suspension (70 cm3) containing MnSO4·H2O (2.48 g), KMnO4 (1.66 g) and desired Na3VO4·12H2O in a Teflon-lined autoclave (100 cm3) at 160 °C for 24 h. The amount of Na3VO4·12H2O was adjusted to obtain 0 or 2 mol.% of V/Mn. The resulting black slurry was filtered, washed and dried at 110 °C overnight, followed by calcined at 500 °C in air for 6 h for characterizations and evaluations of catalytic activities. The catalysts are denoted as “0%V-OMS-2”

Bulk and texture properties

Fig. 1 gives the XRD patterns of the catalysts. 0%V-OMS-2 shows the intense and sharp diffraction peaks, which can be attributed to a tetragonal structure of cryptomelane (KMn8O16, JCPDS 29-1020). With the addition of 2% vanadium, the crystal structure of OMS-2 materials remains unchanged. No additional peaks due to vanadium species appear in 2%V-OMS-2, indicating that vanadium ions are well-dispersed and have been probably substituted for the framework manganese cations of OMS-2 [11], and no

Conclusions

Vanadium doped OMS-2 was synthesized by the simple hydrothermal route and investigated for both low-temperature complete oxidation of formaldehyde at 140 °C and high-temperature complete oxidation of methane at 450 °C. The results of catalytic tests showed that only 2%V doping amount had greatly enhanced catalytic rates of OMS-2 by ca. 1.5 times for both reactions regardless of reaction temperatures and nature of reactants. Accordingly to the results of XPS, HRTEM and H2-TPR measurements, the

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (20977018), Natural Science Foundation of Shanghai (09ZR1402500) and National High-Tech Research and Development (863) Program of China (2007AA061802).

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