Low-temperaturecatalyticcombustionofmethaneoverMnOx-CeO2mixedoxidecatalysts:Effectofpreparationmethod资源-CSDN文库

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http://www.paper.edu.cn

-1-

Low-temperature catalytic combustion of methane over

MnO

-CeO

mixed oxide catalysts: Effect of preparation

method

Shi Limin

, Chu Wei

, Qu Fenfen

, LuoShizhong

1 School of Chemical Engineering, Sichuan University, Chengdu (610065)

2 Department of Environment, Sichuan University, Chengdu (610065)

E-mail：chuwei65@yahoo.com.cn

Abstract

The effect of preparation method on MnO

-CeO

mixed oxide catalysts for methane combustion at low

temperature was investigated by means of BET, XRD, XPS, H

-TPR techniques and methane oxidation

reaction. The catalysts were prepared by the conventional coprecipitation, plasma and modified

coprecipitation methods, respectively. It was found that the catalyst prepared by modified

coprecipitation was the most active, over which methane conversion reached 90% at a temperature as

low as 390℃. The XRD results showed the preparation methods had no effect on the solid solution

structure of MnO

-CeO

catalysts. More Mn

and richer lattice oxygen were found on the surface of

the modified coprecipitation prepared catalyst with the help of XPS analysis, and its reduction and BET

surface area were remarkably promoted. These factors could be responsible for its higher activity for

methane combustion at low temperature.

Keywords: MnO

-CeO

mixed oxide; solid solution; methane combustion; low-temperature activity.

1. Introduction

For the environment and energy consideration, complete oxidation of methane into harmless

and H

O has been paid much attention in catalytic combustion field. Among the

heterogeneous catalysts, the supported noble metal Pd-based catalysts show excellent activity at

low temperature [1-3]. The temperature corresponding to 90% methane conversion (T

) over

Pd/SnO

catalyst is 440 ℃ [2]. Pd/Sn

1-x

catalyst has been also reported to possess high

activity at low temperature for methane complete oxidation, over which T

is 378 ℃ [3]. However,

poor thermal stability and expensive cost of noble metals prevent the widespread application of

these catalysts. Recently, the transition metal mixed oxides are attractive due to the relatively low

price and as high as or slightly higher catalytic activity toward methane combustion at low

temperature than supported noble metals [4, 5].

Among the transition metal mixed oxides of interest for oxidation reactions, MnO

-bases

mixed oxide catalysts exhibit great potential. It is generally believed that MnO

are compounds

with a typical berthollide structure and contain labile lattice oxygen. Their catalytic properties are

attributed to the ability for manganese to form oxides with variable oxidation states (MnO

, Mn

, or MnO) and to their oxygen storage capacity in the crystalline lattice [6, 7]. For

methane oxidation reaction, the Mn

＋

sites are stronger catalytic active sites than the Mn

＋

sites [6,

8]. The active site on supported MnO

is mainly identified as being Mn

of MnO

in other

oxidation processes [7, 9]. In addition, CeO

has been widely used as a promoter and an oxidation

catalyst owing to its unique redox properties and high oxygen storage capacity [10, 11]. Compared

with pure MnO

and CeO

, MnO

-CeO

mixed oxides showed higher catalytic activities because

manganese and cerium oxides formed solid solution in which oxygen reservoir of CeO

and the

mobility of oxygen species were greatly enhanced [12, 13].

The catalytic performance of MnO

-CeO

mixed oxides is notably affected by the preparation

This work was supported by the National Natural Science Foundation of China (205903603) and by the 973

project of the Ministry of Science and Technology of China (2005CB221406).

http://www.paper.edu.cn

-2-

methods and conditions.The MnO

-CeO

catalyst modified with Ag exhibited much higher activity

at low temperature for oxidation of formaldehyde because the content of lattice oxygen was

apparently increased [14]. Previous studies have proved that the modified coprecipitation prepared

MnO

-CeO

catalyst was more active than those prepared by coprecipitation and sol-gel methods

[13]. Considering the same catalytic mechanism (Mars-Van-Krevelen redox mechanism) as total

oxidation of formaldehyde, modified coprecipitation method was used to prepare MnO

-CeO

mixed oxide catalysts for methane combustion in this contribution. Meanwhile, as effective

molecule activation and surface modification approaches, the plasma technique was also

considered to prepare MnO

-CeO

catalysts in this work. It was reported that the plasma prepared

catalysts showed higher dispersion and enrichment on surface of active components, decrease of

reduction temperature, etc. [15-17]. Since these favorable effects, the low-temperature activities of

catalysts prepared by plasma method could be remarkably improved.

With the aforementioned background, the aim of the present work was to investigate the

effect of preparation method on the properties of MnO

-CeO

mixed oxide catalysts for methane

combustion at low temperature. MnO

-CeO

catalysts were prepared by coprecipitation, plasma

and modified coprecipitation methods, respectively, and were characterized with BET、XRD、XPS

and TPR techniques.

2. Experimental procedure

2.1. Catalyst preparation

MnO

-CeO

mixed oxides (Mn/ (Mn+Ce)=0.5，molar ratio) were prepared by three different

methods. (1) Coprecipitation method: 50 %Mn(NO

)

solution and (NH

)

Ce(NO

)

with a molar

ratio of 5:5 was mixed in dissolved distilled water. 2 M NaOH solution was added to the mixing

solution at 50℃ drop by drop until the pH value reached 10.5 with stirring. The mixtures were

further aged at 50℃ for 2 h in the mother liquid. After filtration and washing with distilled water,

the obtained solid was dried at 110℃ overnight and calcined at 500℃ for 6 h in air. The catalyst

was designed as CP. (2) Plasma method: the catalyst was prepared using the same process as the

above. Furthermore, the catalyst precursor was treated 90 min by glow discharge plasma before

calcination. The treatment approach was similar to that in Ref [15]. The precursor was put into the

discharging tube and decomposed in vacuum. The discharge parameters are as follows: frequency

13.56 MHz, discharge voltage 100V, anodic current 100 mA. The prepared catalyst was denoted

as PP. (3) Modified coprecipitation method: the coprecipitation procedure was the same as CP, but

the metal oxide precursors contained KMnO

. The molar ratio of Mn(NO

)

, KMnO

and

(NH

)

Ce(NO

)

was 1:4:5. The catalyst was nominated as MP. The precipitates were filtered and

washed with distilled water many times during the preparation as Ref [13]. There should be no or

considerable little potassium in the MP catalyst and the effect of traces of potassium on catalytic

activity was not considered in this work. For a comparison purpose, pure MnO

was prepared by

coprecipitation of Mn(NO

)

and KMnO

with the ratio of 1:4.

2.2. Catalyst characterization

The BET surface area of catalysts were determined by N

adsorption experiments at -196℃

on a micromeritics NOVA 1000e. Before each measurement, the sample was degassed in vacuum

at 300℃ for 3 h.

X-ray powder diffraction (XRD) patterns were recorded on the DX-2000 diffractometer using

Cu K

(l = 1.54056 Å) radiation between 10°and 80°. The voltage and anode current were 40 kV

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