Formation of a Novel Type of Reverse Microemulsion System and Its Application in Synthesis of the Nanostructured La_0.95Ba_0.05MnAl₁₁O₁₉ Catalyst.

Abstract

In the study, a novel microemulsion system, consisting of water, iso-propanol and n-butanol, was developed to synthesize the nanostructured La 0.95 Ba 0.05 MnAl 11 O 19 catalyst with high surface area and catalytic activity for methane combustion.Recently, catalytic combustion has attracted considerable attention due to its potential application for burning natural gas in gas turbines with extremely low levels of NO x emissions. 1 The main challenge in the area is to select and synthesize catalytic materials with high thermal stability and high activity. 4 However, nanostructured hexaaluminate catalysts are very difficult to synthesize, due to serious agglomeration and growth of the particles. In this work, a novel reverse microemulsion system was developed, which consisted of water, hydrophilic iso-propanol and half-hydrophobic n-butanol (W/iP/ nB). The nanostructured La 0.95 Ba 0.05 MnAl 11 O 19 catalyst was also synthesized in the system. The novel reverse microemulsion system could be potentially reused for preparation of the catalyst, and the preparation procedure is also simplified.Typically, the system was composed of 5-20 vol.% water, 5-20 vol.% iso-propanol and 60-90 vol.% n-butanol. To prepare the microemulsion, the appropriate amounts of water, iso-propanol and n-butanol were mixed and then stirred until the emulsion became transparent. The La 0.95 Ba 0.05 MnAl 11 O 19 catalysts were synthesized by hydrolysis of Ba(iso-OPr) 2 and Al(iso-OPr) 3 in the reverse microemulsion system, which consisted of an aqueous solution of stoichiometric La(NO 3 ) 3 and Mn(NO 3 ) 2 /iso-propanol/n-butanol. ‡ The stoichiometric amounts of Al(iso-OPr) 3 and Ba(iso-OPr) 2 were dissolved in iso-propanol, and the alkoxide solution was added to the ternary solvent system to hydrolyse and polycondense. The obtained sample was directly transferred to an autoclave for removing the solvent with supercritical drying (290 °C, 6.0 MPa). After drying, the sample was calcined at 1200 °C for 2 h. The structure property and phase behavior of the water in the system were investigated by means of FT-IR §, NMR ¶, electric conductivity (EC) ∑, laser light scatting granulometer **. The morphology, crystal structure and texture property of the catalyst were characterized by TEM, XRD and N 2 -adsorption etc. † †. The catalytic activity over the catalyst for methane combustion was measured in a fixed reactor which is described elsewhere. 5 As shown in Fig. 2, d H2O at V = 0.02 is ca. 4.49, and d H2O gradually shifts to low field with increasing V. d H2O is electron density of H 1 in H 2 O, which denotes the polarization behavior of water molecules. The larger the value of d H2O , the stronger the polarization and the hydrogen-bonding interaction of the water molecules are. 6 This also indicates that the hydrogen bonds of water become stronger with increasing of water content in the ternary system. It has been reported that ethanol molecules exist as selfassociated aggregates through hydrogen bonds between ethanol molecules in nonpolar solvents...

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