Catalytic Combustion Properties of M-substituted Hexaaluminates

2 Catalytic Combustion Properties of M-substituted Hexaaluminates - Most of the catalytic studies performed over hexaaluminate materials deal with the combustion of CH4 as the main component of natural gas, i.e., the typical fuel of gas turbines. Arai and co-workers were the first to investigate the CH4 combustion activity of BaMAlnOjg with M=Cr, Mn, Fe, Co, Ni prepared via the alkoxide route.5 Activity tests were performed over powder catalysts using a conventional quartz microreactor fed with a diluted CH4-air mixture (1% CH4) at high-space velocity (GHSV=48000 h 1). The results are summarized in Table 3 in terms of T10% (i.e., the temperature required to achieve 10% conversion). 

La-hexaaluminates showed higher catalytic activity for all the M-substituted systems with respect to Ba-hexaaluminates.15 27 A similar promoting effect was also found for the composition Sro.gLao.aMn lnO, 15,16,26 which also exhibits the maximum surface area, but only when prepared via hydrolysis of alkoxides. 

However, the extent of the activity enhancement cannot be related to the higher surface area of this material. Two possible explanations were proposed to account for the effect of mirror plane composition on combustion activity one is related to the different oxidation state of the cation in the mirror plane the other is associated with the crystal structure of layered-alumina materials (i.e., magne-toplumbite and (3-Al203) which have different population and co-ordination of the ions in the mirror planes. Both these electronic and structural factors can, in principle, affect the redox properties. 

No limits on the content of transition metal ion were found for Fe-substituted systems. As mentioned above, and in line with crystallographic literature,45 a monophasic sample with magnetoplumbite structure was obtained for the completely substituted BaFe O. This sample showed excellent CH4 combustion activity upon calcination at 700 °C, but it markedly deactivated either upon increasing the calcination temperature or upon treatment at high tempera- 

Groppi and co-workers46,26 investigated the catalytic activity of Mn- and Fe-substituted hexaaluminates prepared via the carbonate route for CO and H2 combustion. These species are the main components of fuels from gasification of carbon and biomasses that represent an alternative to natural gas in gas turbine applications. 

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