Hexa-aluminates

One of the keys to the properties of the hexa-aluminates is the crystal structure. The hexa-aluminate structure consists of blocks of spinel-structure divided by mirror planes in which the large A ions are situated. This results in a layered structure. There are two different structure types, the magnetoplumbite and P-alumina structure where, for example. La and Sr in the A-position will yield magnetoplumbite while Ba yields the fl-alumina structure.  

Several ions have been substituted into the hexa-aluminate structure both in the A and B positions. The B ions seem to have the most pronounced effect on the catalytic activity of the hexa-aluminate materials. This catalytic effect seems to be closely correlated to the change in oxidation state of B ions between +2 and +3. Several different B ions have been studied. For BaMAlnOi9 (where M is a transition metal) the trend for methane combustion, measured as the temperature for 10% conversion, was Mn, Cu Fe Ni, Cr, Co. Similar results have been shown for LaMAlnOi9 also. The high activity for the Mn could be correlated to its ability to shift oxidation state.  

Spivey et al. have prepared a number of Sri jtLa tMnAliiO 9 samples and studied the influence of different La substitutions on the surface area and activity for methane conversion. The largest areas after calcination at 1300°C were shown for high and low La substitutions, while only high La substitutions showed an increased thermal stability after calcination at 1400 °C. The methane activity was the highest for x = 0.4. The results were in contradiction to the ones reported by Machida et al. who have reported a peak in activity for x = 0.2 which also showed the largest surface area. The cause of the discrepancy between the results is probably the difference in preparation method Spivey et al. have prepared the samples by co-precipitation, whereas Machida et al. have used a sol-gel method. 

The hexa-aluminates could be prepared by several different techniques, e.g. solid-state reactions, hydrolysis of alkoxides, precipitation as insoluble metal salts, etc. The preparation method has been shown to have an influence on the thermal stability of the resulting hexa-aluminate material. The morphology of the low temperature hexa-aluminate precursor is particularly important for the ability of preparing high surface area hexa-aluminate materials. Pores with a diameter below 50 A collapse more readily than larger pores when heated to high temperatures. Thus, it is important to minimize the formation of pores with a diameter below 50 A, e.g. through the use of modifiers in the alkoxide hydrolysis. 

The catalytic activity of substituted hexa-aluminates for methane combustion is low compared with noble metals, like Pd. For use in a fully catalytic combustor, these materials will have to be combined with a first stage with higher activity. For other fuels the differences are not so pronounced, e.g. for diesel. Hexa-aluminates also show a high activity in CO and H2 combustion and gasified biomass. For these kinds of fuels the ignition temperatures over hexa-aluminates are in the same area as the outlet temperature from the gas turbine compressor. 

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The difference in catalytic activity between the La- and the Ba-based hexa-aluminates results from the following reasons the first difference is the valence of cation in the mirror pleuie between tri-valent lanthanum ion and di-valent barium ion. The second is the crystal structure between magnetoplumbite and P-alumina, which are different in the coordination of ions and concentration of Frenkel-type defect in mirror plane. The redox cycle of transition metal in hexa-aluminate lattice, which closely related with catalytic activity, is affected sensitively with these two factors. 

Barium hexa-aluminate. BaAli20i9, being a compound of high practical importance, is usually synthesized via thermal decomposition of alkoxides. 

A significant drop in catalytic activity for catalytic combustion of methane due to the above-mentioned PdO decomposition or the inability of metallic Pd to chemisorb oxygen above 650 C, however, can be effectively avoided by using a catalytically more active Mn-substituted hexa-aluminate (X = Mn) as a catalyst support [71]. The catalytic activity of this Mn-hexa-aluminate compensates for the drop in activity of Pd so that a stable combustion reaction can be attained in a whole temperature range. Thus, the use of catalytically active support materials is one possible solution to overcome the unstable... 

Materials such as aluminum titanate and silicon carbide appear to be promising for high-temperature catalytic combustion. However, problems such as extrudability, the application of washcoats, and reaction with deposited washcoats are not solved yet. For instance, when hexa-aluminate, presented in the introduction to this section, was applied to silicon carbide monoliths, solid-state reactions occurred at 1200-1400 C [76], causing exfoliation of the coating and the formation of new phases. The application of an intermediate mullite layer was suggested as an approach to hinder these solid-state reactions. 

Figure 10 Osaka Gas catalytic combustor. A, B Pd powder (0.02 p.m) on cordierite C-G extruded cation-substituted hexa-aluminate. (From Ref. 106.)...

There are three important outstanding issues that need concerted R D. The first issue is the further development of materials with thermal stability under combustion condition at the high temperatures prevailing in a gas turbine combustion chamber. Promising materials have been developed, but none fulfills the demand of a lifespan of at least 1 year. Besides, the most promising materials, such as the family of the hexa-aluminates, must be available in a honeycomb monolith shape, either as washcoat or directly extruded. Much work still needs to be done to optimize the preparation of monolithic thermostable catalysts. 

K Eguchi. H Inoue, K Sekizawa, and H Arai, Thick film coating and fiber spinning of hexa-aluminate compounds for catalytic combustion, Proc 2nd Int Workshop on Catalytic Combustion, Tokyo. 18-20 Apnl (H Arai ed), Catalysis Society of Japan, Tokyo, 1994,... 

The approach of controlling morphology to control sintering offers several interesting possibilities, and these are actively under consideration. At least one involves heat resistant hexa-aluminate catalysts for catalytic combustion [46). 

The basic y-alumina structure was heat treated at about 1150°C, after which phase transformations start to occur. Calcination at T > 1300°C results in the formation of hexa-aluminate phases. These phases have a large resistance against sintering as has been proven by J. Kumari Kumar [66]. The pore diameter could be controlled in the range from about 4 to 8 nm (at 1150°C). 

Of particular interest here are the developments by Professor Aral and coworkers in Japan of hexa-aluminates and substituted hexa-aluminates as combustion catalysts [9, 70]. Barium has proved to be the most promising stabilizer for alumina. Indeed the use of barium hexa-aluminate Ba0.6Al203, and its substituted forms has become a real break-through in catalytic combustion. This is shown in Table 2, which gives the temperatures required for 10% and 90% methane combustion on various substituted barium hexa-aluminates, compared or contrasted to the thermal reaction. The temperature lowering, AT, for 90%... 

The addition of LaA103, is shown to increase both the dispersion and the resistance to sintering of the platinum supported alumina catalyst. Moreover, lanthanum hexa-aluminate (La-p-A C ) is present in the platinum catalyst fired at 1150°C. 

The heat resistance of the hexa-aluminate Sr, xLaxMnAlnO 9 n (x = 0.2) was also evaluated by Arai et al. (1991). The powder sample pressed into a disk underwent isothermal heating at 1573 K for 6400 h in air. No change was observed in the crystal structure or the chemical composition however, the surface area gradually decreased, staying above 10 m2 g-1 after 4200 h of heating. 

Hexa-aluminate ABAl] iOi9 A = alkali, alkaline earth or rare earth metal B = transition metal e.g. Mn, Fe, Cu, Co) 8m g (1400 C) 500-700 °C (T10-T90) Active phase/support... 

In addition, thermally stable oxides, like perovskites or hexa-aluminates, which are used as active material or washcoat in catalytic combustion, can be used to manufacture high temperature supports. Consequently, promising materials that are reported here could be used as support and active phase in one and the same material or as a washcoat on a support made of another material. 

Magnesium oxide has been reported to be active in methane complete oxidation by Berg and Jaras. The activity was somewhat lower compared with a Ba-substituted hexa-aluminate. The difference between the two catalysts decreased after calcination to 1500 °C. 

The main drawback with the use of perovskites is the poor thermal stability of the materials compared with hexa-aluminate based catalysts. Zwinkels et al. have compared the thermal stability of two different hexa-aluminates with a SrZr03-perovskite, a pyrochlore, see Section 3.2.4, and two spinels, see Section 3.2.3. The perovskite, as well as the pyrochlore and one of the spinels decreased in their surface area significantly. One of the explanations of the much lower stability of the perovskites compared with the hexa-aluminates is that the crystal growth will take place in three dimensions and thereby yield a material with a low surface area. Lowe et al. have studied several different perovskites and their thermal stability and conclude that the surface area of the perovskites is not sufficient for use in high temperature catalytic combustion. Similar results have been shown by Cristiani et al. ... 

Osaka Gas, Kobe Steel, etc. la, fully catalytic 1 MW Natural gas 3-stages PM/cordierite, 4-stages Mn-hexa-aluminate 380,10 bar + Simplistic configuration — Catalyst Stability 127... 

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