Hexaaluminate metal-substituted

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. 

With respect to CO oxidation an activity order similar to that described above for CH4 combustion has been obtained. A specific activity enhancement is observed for Lai Co 1-973 that has provided a 10% conversion of CO already at 393 K, 60 K below the temperature required by LalMnl-973. This behavior is in line with literature reports on CO oxidation over lanthanum metallates with perovskite structures [17] indicating LaCoOs as the most active system. As in the case of CH4 combustion, calcination at 1373 K of LalMnl has resulted in a significant decrease of the catalytic activity. Indeed the activity of LalMnl-1373 is similar to those of Mn-substituted hexaaluminates calcined at 1573 K. Dififerently from the results of CH4 combustion tests no stability problems have been evidenced under reaction conditions for LalMnl-1373 possibly due to the low temperature range of CO oxidation experiments. Similar apparent activation energies have been calculated for all the investigated systems, ranging from 13 to 15 Kcal/mole, i.e almost 10 Kcal/mole lower than those calculated for CH4 oxidation. 

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. 

Groppi and co-workers23,24 investigated the thermal evolution of Mn- and Fe-substituted hexaaluminates with different amounts of transition metal ions prepared via the co-precipation route. The dried precursors of BaMnxAl 2.xOJ9... 

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. 

Nature and Role of the Transition Metal Ion in the Structure - Catalytic combustion over M-substituted hexaaluminates occurs via a redox mechanism involving reversible variation of the oxidation state of the transition metal ion in the structure. Because of this, the nature and the role of the transition metal ion in the structure have been extensively investigated. 

The alkaline earth hexaaluminate supports mentioned in Section 3.5.2 have been made catalytically active by the addition of transition metal ions [25]. Manganese substitution produced the highest catalytic activity. The manganese-substituted aluminates maintain high surface areas at 1000 °C (see Table 1), but their activity per unit area is so low that the activity on a per gram basis remains much lower than that of the noble metals. The same applies to the Cu/La-substituted alumina included in Table 1. Note that surface areas at 1300°C will be significantly lower... 

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. 

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. 

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