Metal-substituted Hexaaluminate Catalysts

Hexaaluminate (HA) materials containing transition metal ions in the structure have been extensively investigated for GT applications, in view of their excellent thermal stability and catalytic activity [9, 99-101]. 

These materials have been prepared both by hydrolysis of the alkoxides [100] and by coprecipitation from soluble salts of the constituents by using N H4O H or (N H4)2CO3 as precipitating agent [106]. Monophasic samples with surface areas in the range 10-15 m g have been obtained upon calcination at 1300 °C [100, 106]. 

Recently, the synthesis of nano-sized HA has been proposed via reverse-micro-emulsion preparation, which is reported to be effective for controlling the hydrolysis and polycondensation of the alkoxides of the constituents. Using this preparation route, the nanoparticles crystallize directly to the desired phase at the relatively low temperature of 1050 °C and maintain surface areas higher than 100 m g after calcination at 1300 °C for 2h [107-109]. 

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In the following, a review of the traditional and novel concepts of catalytic combustion for GTs is addressed, with emphasis on the requirements and challenges that the different applications open to catalysis. The most relevant characteristics of PdO-supported catalysts and of transition metal-substituted hexaaluminates (which have been most extensively considered for lean combustion applications) are described, along with those of noble metal catalysts adopted in rich combustion systems. 

The active layer must provide the required activity, selectivity and thermochemical stability properties. Different active phases can be adopted depending on the operating constraints and the fuel type. In the following we will mainly focus on CH4 (i.e. the main constituent of natural gas) as the reference fuel for GT applications. In this respect, the combustion catalysts that have been most extensively investigated for configurations based on lean combustion concepts are PdO-based systems and metal-substituted hexaaluminates. 

Because of the close similarity of ionic radii, transition metal ions (M = Mn, Cu, Fe, Cr, Co, Ni) can be partially substituted for A1 ions. These transition metal ions can provide significant activity in combustion reaction.5 At low metal concentrations, the structural and morphological properties are not significantly affected by substitution, so that high thermal stability is maintained. Substituted hexaaluminates can be directly shaped in the monolith form required by the combustor, providing bulk active catalysts without need of ceramic supports. 

In line with general indications on the reactivity order over both mixed oxide and noble metal catalysts, CO and H2 were found to be much more reactive than CH4 over all the investigated hexaaluminate catalysts. Specifically, the following reactivity order was determined over Mn-substituted samples CO>H2 CH4. Tio% values of 230 °C and 320 °C were obtained over BaMnAlnOj9 for CO and H2 respectively, to be compared with 540 °C required by CH4 combustion under similar experimental conditions. Apparent activation energies for CO and H2 combustion were calculated to be 10 kcal/mol lower than that of CH4 combustion (13-15 kcal/mol vs. 21-23 kcal mol), in line with the marked activity differences. 

It was shown [2] that substituted hexaaluminates is applied for CH4 oxidation. These catalysts are prepared using precipitation of soluble nitrates of metals with NH4(C03)2 at constant T (60°C) and pH ( 7,5). For Mn(Fe)-substituted hexaaluminates the temperatures of 50% CH4 conversion are in the range of 560 - 776°C. Obtaining BaFenOig hexaferrite is accompanied by decrease in T50 to 533°C [3]. It can be believed that further T50 decrease can be achieved by using Mn-substituted hexaferrites. 

Hexaaluminates represent a class of materials that are highly resistant to sintering at high temperature. Therefore, these materials have attracted the attention of researchers who are involved in developing catalysts for high temperature applications. Hexaaluminates have been used as supports as well as the active material in catalytic combustion reactions [121-142]. Hexaaluminates can be represented by the formula AAI12O19 where A is an alkaline or alkaline-earth metal. They consist of a lamellar structure and both A cation and A1 can be partly substituted by other cations. Incorporation of other cations drastically modify the catalytic activity of these materials. However, such modifications are rather limited compared to the possibilities existing in perovskite type oxides. 

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