The catalytic activity and surface characterization of Ln2B2O7 (Ln=Sm, Eu, Gd and Tb; B=Ti or Zr) with pyrochlore structure as novel CH4 combustion catalyst
Introduction
It is well known that combustion catalyst can reduce the temperature of operation from 1500 to 1300 °C and suppress the NOx formation [1]. There have been numerous studies on the application of high-temperature oxidation catalysts in gas-turbine power generation [2], [3]. Among the most important properties are high combustion activity at typical combustor inlet conditions, thermal stability and high thermal shock resistance [4]. High activity can be obtained with catalysts that have a high catalyst surface area. Various promising materials, stable at high temperatures, have been reported in the literature [5]. A series of hexaaluminate were developed by Arai and coworkers [6]. Their sol–gel synthesized hexaaluminate (BaAl12O19 and Sr0.8La0.2MnAl11O19) exhibited a high thermal stability and catalyst activity at high temperature. Groppi et al. [7] achieved a similar performance when preparing the Mn-substituted barium hexaaluminate using a coprecipitation method. Pyrochlore (A2B2O7) exhibits high chemical stability and catalytic activity at high temperatures in oxidative coupling of methane [8], [9]. The application of pyrochlore as combustion catalysts was recently suggested since reasonable surface area and catalytic activity were observed in spite of the high-temperature treatment for the formation of pyrochlore structure [10].
In the present study, we prepared Ln2B2O7 pyrochlore material for high-temperature combustion by the sol–gel method. Methane combustion and surface characterization were carried out in order to study the relation between catalytic activities and surface properties. The catalysts were characterized with XRD, N2 BET method and XPS.
Section snippets
Catalyst preparation
Pyrochlore materials (Ln2B2O7; Ln=Sm, Eu, Gd and Tb; B=Zr or Ti) were prepared by the hydrolysis method of metal alkoxide. Calculated weight of zirconium isopropoxide (Aldrich, zirconium isopropoxide in isopropanol) or titanium isopropoxide (Aldrich Co., USA, 99.99%) was dissolved in 150 ml of isopropanol at 80 °C for 5 h under N2 atmosphere. The stoichiometric amount of lanthanide metal (Ln=Sm, Eu, Gd and Tb) nitrate and water were dissolved in another 50 ml isopropanol. The amount of water was
Crystal structure analysis and surface area
Fig. 1 shows the XRD profiles of Ln2Ti2O7 and Ln2Zr2O7 calcined at 1200 °C for 2 h. Also, all samples have the pyrochlore structure similar to the reference materials without the formation of any impurities in the pyrochlore oxide. The change of the XRD intensity and the position of 2θ values were also examined according to the calcination temperature for Sm2Zr2O7 (not shown). With the increase of the calcination temperature, the position of 2θ indicating pyrochlore structure was not changed and
Conclusions
The pyrochlore structure was successfully formed by the sol–gel method. The temperature to form the crystal structure was lowered by 300 °C and its surface area was increased compared to typical solid state reactions. The crystal structure was observed after calcination at 800 °C. Ln2Zr2O7 showed the catalytic activity for methane combustion, but Ln2Ti2O7 did not. The activity of methane combustion of Ln2Zr2O7 seems to be due to low binding energy of Ln species and Zr element on the surface of
Acknowledgements
This study is funded by Center for Ultramicrochemical Process Systems (CUPS) sponsored by KOSEF (2001–2002).
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